Phosphoric Acid Manufacturing: using raw materials and salfuric acid

“Manufacturing of Phosphoric Acid using Rock Phosphate and Sulfuric Acid”

Department of Chemical Engineering, SSJCOE, Dombivli (E), [2014‐2015] Page | 1

Chapter No 1: Introduction

A] History of Development

Hennig Brandt (1630-1710) an Alchemist in Hamburg, Germany discovered

Phosphorous in 1969. Phosphorous burns in air and exist in nature as phosphate.

Carl Wilbelm Scheel(1742-1786) and Johan Gottlieb Gahn (1745-1818) from Sweden

studied the nature of bone , they dissolved bone ash [Ca3(PO4)2 in the form of

hydroxypatite] in Sulfuric acid (H2SO4) so making phosphoric acid. In late 18th

century M.M. Coignet of Lyan, France improved the peltier process of making ash,

acidifying it with sulfuric acid (H2SO4) to produce phosphoric acid.

Ca3(PO4)2 + 3H2SO4 → 3CaSO4 + 2H3PO4

H3PO4 ∆→ HPO3 + H2O

4HPO3 + 12C → P4 + 2H2 + 12CO

In the year 1870-1872 wet process phosphoric acid for fertilizer use was first

produced commercially in Germany, United States, Baltimore, Maryland for short

period. Strength of phosphoric acid was increased as high grade of rock become

available after 1900.

The Israel mining industries (IMI) institute of research and development invented the

hydrochloric acid route for making phosphoric acid in1950,which of incorporated

solvent extraction. Hydrochloric acid (HCl) was chosen because of near by source of

chloride from dead sea operation.

Ca3(PO4)2 + 6HCl → 2H3PO4 + 3CaCl2

For summarizing (WPA) Wet Phosphoric Acid Process Technology took a big leaps

forward with development of strong acid process in 1970.

Between1927-1932 group such as Swiss with Dorr and other worked on the higher

P2O5 concentration process. Their aim was to produce 40%-50% P2O5 acid directly at

the filter. In 1932 Dorr built a tree - train plant at Trail, with the capacity of 40-50

tons per day, this process of producing 30-32% P2O5 acid at filter exist was known as

strong acid process. After 1930, Nordendreen took out patent for the manufacturing of

more concentrated (40% P2O5) Phosphoric Acid by means of formation of

hemihydrates or anhydride but it was not until 1970.

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B] Properties of Phosphoric Acid

1. Physical Properties:

1 Formula H3PO4

2 Molecular weight 98.04

3 Appearance

At normal temperature it is a

colourless liquid or rhombic

crystals

4 Physical state Solid crystalline

5 Melting point / Freezing point 42.40C (1080F)

6 . Boiling point 2600C

7 Vapour pressure 0.0285 mm Hg at 200C

8 Density at 15.50C 1.583 gm/cc (75%),

1.694 gm/cc (85%)

9 Viscosity at 200C

15 centistokes (75%)




10 Specific gravity 1.710 at 600F

11 Odour Inodorous at ordinary

temperature

12 Solubility Soluble in water and ethanol

13 Refractive index n20 /D 1.433

14 Non toxic in nature

15 Phosphoric acid is quite corrosive in nature. Its corrosive nature increases with

increase temperature.

16 Vapour density 3.4(Air = 1)

17 Flash point Not flammable

18 Auto ignition temperature Not applicable



2. Chemical Properties :

1. Molten Phosphoric acid slowly undergoes auto dehydration

Auto dehydration of phosphoric acid

2H3PO4 → H4P2O7 + H2O

2. Acidic Properties:

a) It is medium strong tribasic acid and this forms three series of salts

i) Primary Phosphates (M1H2PO4)

ii) Secondary Phosphates (M2HPO4)

iii)Tertiary Phosphate (M3PO4)

b) Dissociation of Phosphoric acid

H3PO4 H2PO4- HPO4

-2 PO4-3

c) Phosphoric acid (H3PO4) is moderately acidic, Primary phosphates are

weakly acidic, Secondary phosphates (HPO4-2) are weakly basic, Tertiary

phosphates (PO4-3) are strongly basic. The last specie (PO4

-3) largely

hydrolyzed in water

PO4-3 + HOH → HPO4

-2 + OH-

Alpha plot for phosphoric acid and its conjugate

(basehttp://ion.chem.usu.edu/sbialkow/Classes/3600/Overheads/H3A/H3A.html,

11/09/2014)



3. Basic Properties:

a. Molten anhydrous phosphoric acid is a good conductor of electricity,

because of self ionization and formation of phosphotacidium ion P(OH)4+

2H3PO4 → H4PO4+ + H2PO4

-

b. Due to slow auto dehydration of phosphoric acid in melts causes reaction

such as

2H3PO4 ↔ H4P2O7 + H2O

H4P2O7 + H2O ↔ H3O+ + H3P2O7

-1

H3P2O7- + H3PO4 ↔ H2P2O7

-2 + H4PO4-

Hence molten phosphoric acid has high content of ions.

4. Redox properties:

a) Phosphoric acid in aqueous solution is very poor oxidizing agents

b) Phosphoric acid is good reducing agent

5. Esterification of Phosphoric acid:

a. Phosphoric acid forms mono ester with alkenes.

H3PO4 + C3H6 ↔ H3PO4C3H8

b. Reaction of phosphoric acid with alcohols also gives phosphoric acid

ester

H3PO4 + R-CH2OH ↔ R-H2PO4CH2

c. Reaction of carbonate mineral with 100% phosphoric acid

CaCO3 + H3PO4 → CaHPO4 + H2CO3

H2CO3 → H2O + CO2 (liq)

CO2 (liq) → CO2 (gas)



C] Industrial Importance and Uses

The following industries has the application of phosphoric acid as an intermediate or

raw material in manufacturing of their products or sometimes refining of the products

1. Fertilizer industry : The important fertilizer products made from the

phosphoric acid are:

a. Triple super phosphate [CaH4(PO4)2.H2O]

b. Ammonium phosphate [(NH4)2HPO4]

c. Mono ammonium phosphate [(NH4)H2PO4]

d. Merchant acid

e. Super phosphoric acid

f. Liquid fertilizers

2. Industrial phosphates : The principle industrial phosphates made from

phosphoric acid are:

a. Mono sodium phosphate [NaH2PO4]

b. Sodium acid phosphate [Na2H2P2O7]

c. Sodium meta phosphate [NaPO3]

d. Disodium phosphate [Na2HPO4]

e. Tetra pyrophosphate [Na4P2O7]

f. Tri sodium phosphate [Na3PO4]

g. Sodium tripolyphosphate[Na5P3O10]

h. Dicalcium phosphate[CaHPO4]

i. Tetra potassium pyrophosphate [K4P2O7]

3. Beverage Industry :

Phosphoric acid is added to soft drinks as an acidifying agent which imparts

desired tangy taste acid sourness to soft drinks. The pH value is maintained

between 2-4. It is chiefly used in core type beverages but has also found

application in preparation of phosphate beverages such as orange, lemon and

cherry phosphates. The acidity furnished in the form of ortho phosphoric acid is

beneficial to health.



4. Textile industry :

Here the sults of phosphoric acid such as trisodium phosphates, disodium

phosphates and sodium metaphosphates are generally used for performing the

following purposes:

a. Removal of grease and oil from cotton and wool

b. Dyeing

c. Degumming of silk [Removal of sericin & silk glue]

d. Weighing of silk [Increasing the weight of silk fiber]

5. Sugar refining industry:

A small amount of dil. Phosphoric acid solution is sprayed on the sugar in the

centrifugal, which results in brightening and improvement of the colour of the

product. The acid exists in several way in colour improvement

a. Retains the pit of the syrup on the crystals

b. Weak up slits of organic acids and release the volatile acids

c. Precipitates dark colour organic salts as colourless phosphates

6. As a catalyst and oil refining agent :

The three main processes where phosphoric acid used as catalyst to alter

composition of hydrocarbons are:

a. Dehydrogenation

b. Polymerization

c. Alkylation

a. Dehydrogenation:

Dehydrogenation is brought about mainly by decomposition or cracking of

petroleum products of very high elevated temperature and pressure with

the development of the catalyst. The desired results could be accomplished

at lower temperatures.

b. Polymerization :

It is the reaction opposite to hydrogenation where phosphoric acid is again

used as catalyst for reducing both temperature and pressure.



c. Alkylation :

Alkylation is the reaction of dissimilar hydrocarbons. It is favoured by

high pressure but can be carried out at reasonably low temperature with the

help of the phosphoric acid and catalyst.

7. Photography: Free phosphoric acid is employed in the aniline process for:

a. Reproduction of line subjects

b. To regulate the acidity or alkalinity of the developer both.

c. Developing out papers by emulsion.

8. Rust removal (metal surface cleaning):

Phosphoric acid is used to remove rust by direct application to rusted iron, steel

tools, on other surfaces which changes the reddish brown iron oxide (rust) to ferric

phosphate

2H3PO4 + Fe2O3 → 2FePO4 + 3H2O

Liquid phosphoric acid is used for electroplating and often formulated as thick

gel. The rust may also be removed via phosphate conversion coating. This coating

provides the desired corrosion resistance also.

9. Water treatment : The main phosphate derivatives uses for this purpose are:

a. Trisodium phosphate

b. Tetra sodium pyrophosphate

c. Mono sodium phosphate

d. Di sodium phosphate

All these phosphates leads to eutrophication of the water.

10. Fire retardants:

Ammonium hydrogen phosphate decomposes on heating loses ammonia and

produces phosphoric acid which slow down the combustion cellulose. The other

phosphates used as fire retardants are urea phosphatestetrasis (hydroxyl methyi)

phosphonium chloride and ammonium polyphosphate.



11. Detergents and soaps :

Phosphoric acid salts such as trisodium phosphates and super phosphoric acid are

the main constituents used in manufacturing of detergents and soaps

12. Dental cements:

Phosphoric acid is widely used in dental cements giving good dental properties as

follows:

a. Hardness and high crushing strength

b. Quick setting and strong adhesive properties

c. Resistance to solvent effect of saliva

d. Germicidal properties

Two type of phosphate dental cements are:

a. Zinc phosphate cements

b. Silicate elements

13. Glasses:

Phosphoric acid is used for making glasses with some modified properties as

follows:

a. Optical glasses having desired refractive index dispersion ratio.

b. Glasses having high ultraviolet transmissions

c. Fluorescent glasses

d. Heat absorbing glasses

e. Hydro fluoric acid resistance glasses

MISCELLANEOUS USES

Phosphoric acid is used:

1. As a leavening agent

2. In the preparation of albumin derivatives

3. In the preparation of animal feed supplements

4. As a buffer agent for e.g buffer for high performance liquid

chromatography

5. As the electrolyte in phosphoric acid fuel cell

6. As a pH adjusters in cosmetics and skin care product



7. As a catalyst in asphalt binding, liquefaction of wood, hydration of alkenes

to produce alcohols.

8. In compound semiconductor processing, phosphoric acid is a common wet

etching agent: for e.g. in combination with hydrogen peroxide and water it

is used to etch in gas selective to lnP.



D] Economic Aspects

1. Demand-Supply position:

Phosphoric acid has many applications as fertilizer & non-fertilizer products.

Therefore the demand for phosphoric acid is a derived demand & the rate of growth in

demand is largely dependent on the rate of growth in the sectors that use it as an input.

The total global consumption of phosphoric acid increases from 37.1 Million Metric

Ton in 2005/06 to 43.7 MMT in 2014/15. The table below shows the increase in

phosphoric acid consumption [P2O5 basic] from 2005 to 2014 on calendar year basis.

Table (1) World Fertilizer Consumption

Year Phosphoric Acid Consumption

(MMT)

% Growth in Consumption

2005 37.1

2006 39.0 +5.1%

2007 40.5 +3.8%

2008 41.8 +3.3%

2009 37.6 -5.4%

2010 40.6 +6.3%

2011 39.8 -3.3%

2012 40.3 -0.9%

2013 41.2 +2.2%

2014 43.7 +2.9%

[P. Heffer, IFA, June, 2005/06/07/08/09/10/11/12/13/14]

The increase in the consumption of phosphoric acid leads to the increase in the

demand of the same world demand of the phosphoric acid thus grow from 34.6 MMT

in year 2006 to 43.6MMT in 2014

These increases in demand of phosphoric acid pressurize the industry to operate at

higher capacity. World phosphoric acid capacity in 2007 decreased to 43.2 MMT, but

rebound to 45 MMT of P2O5 in 2008 due to new projects in China & Morocco which



is further increased to 55.5 MMT of P2O5 in2014 because of new plants set-up during

this year in countries like Jordan, Tunisia, Saudi-Arabia & Morocco.

The increasing capacity impacts on the global potential supply of phosphoric acid

which was observed to have a marginal surplus of more than 15% over the year of

2006-2004. Global phosphoric acid supply/demand projections show a stable potential

balance which detailed in the table below.

Table (2) World Phosphoric Acid Potential Supply/Demand Balance (Million

Metric Tonnes, P2O5)

Year 2010 2011 2012 2013 2014

Capacity* 47.8 51.0 52.3 53.8 55.5

Total Supply* 39.6 41.5 43.3 45.3 47.1

Fertilizer Demand* 31.3 32.8 34.2 35.5 36.6

Non-Fertilizer use* 5.5 5.6 5.6 6.0 6.2

Distribution Losses 0.7 0.8 0.8 0.8 0.9

Total Demand* 37.6 39.2 40.6 42.3 43.6

Balance* 2.0 2.3 2.7 3.0 3.4

% of Supply

Increase

5% 6% 6% 7% 7%

[M. prud’homme, IFA, June 2010]

*Definitions of various terms in context with the table: *Capacity: Here capacity is the effective/ Theoretical capacity, representing the maximum achievable production. *Supply: Supply is computed from the ‘effective capacity’, multiplied by highest operating rate achieved in the respective year. *Demand: There are two types of demand

a. Fertilizer Demand b. Non-Fertilizer Demand

a. Fertilizer Demand: It is the ability or willingness of farmers to buy fertilizer at a given probable consumption in one calendar year b. Non-Fertilizer Demand: Consumption as non-fertilizer use, referred to industrial use. Net non –fertilizer demand excludes the use of products that are recovered as a by-product from industrial process and then used as fertilizers.

Total Demand = Fertilizer Demand + Non-fertilizer Demand + Distribution losses

*Potential Balance: It is the difference between supply & total demand.

Potential Balance = Supply – Total Demand



2. Import Export Data:

Out of the total trade of approximately 5 million tonnes of phosphoric acid, India

imports more than 2.5 million tonnes every year. However phosphoric acid is not

freely traded & more than 50% of Indian transaction are by way of long term supply

arrangements between producers & importers.

About 1.2 million tonnes of phosphoric acid is imported by India from Morocco,

which is about 50% of India’s total import. According to the Indian bureau of Mines,

import of phosphoric acid decreased to 2 Metric Ton in 2010-2011 from 2.69MT in

the previous year. Imports of acid considerably increased to 2.32 MT in 2011-2012.

Imports are mainly from Morocco (47%), Senegal (17%), Tunisia (14%) & South

Africa (18%).

Table (1) Import of Phosphoric Acid during [2009-2012]

Countries 2009-2010

Qty(Tonnes)

2010-2011

Qty(Tonnes)

2011-2012

Qty(Tonnes)

All countries 2692899 2008376 2324532

Morocco 1273174 860313 1084630

Senegal 332198 212676 392742

USA 222308 235231 323771

Tunisia 298101 227292 98371

South Africa 332770 353897 208469

Israel 142757 31322 66330

Saudi Arabia 5620 30381 -

UAE - 13892 -

China 3998 9173 12074

Lebanon - 7682 -

Indonesia - - 46929

Malaysia - 549 34632

Ghana - - 14957

Other Countries 81973 77923 41627

[IMYB, 2009-10, 2010-11, 50th & 51th edition]

India still depends on agricultural field which increases the demand of fertilizer.

Hence 90-95% Phosphoric Acid produced in India is used for production of

fertilizers. Therefore India never exports phosphoric acid on large scale.



The export of phosphoric acid decreases drastically to 11798 tonnes in 2010-11 from

143195tonnes in 2009-2010 which is further increased to 18674 tonnes in 2011-12.

Table (2) Export of Phosphoric Acid during [2009-2012]

Countries 2009-2010 Qty

(tonnes)

2010-2011 Qty

(tonnes)

2011-2012 Qty

(tonnes)

All countries 143195 11798 18674

Bangladesh 46675 5229 13

Indonesia 56221 5405 18411

Saudi Arabia 8392 1001 -

UAE 23 24 23

Mozambique 18 36 20

Sri Lanka 3 19 10

Taiwan 21815 9 16

Oman - 12 -

Kenya - 52 -

Nepal 4 4 86

Japan - - 7

Nigeria - 1 40

Sudan - - 1

Other Countries 10044 117 38

[IMYB, 2009-10, 2010-11, 2011-12]

If India wants to ensure phosphoric acid, availability through imports,

companies need to participate in more production joint ventures in countries rich in

resource like Morocco, Senegal etc. and for long term supply arrangements. [Press

Information bureau, 27 March 2008]

3. Manufacturing Facilities: (In India & in World)

1. Phosphoric acid is produced either by acidulation of rock phosphate by a mineral

acid in wet process (i.e. by using H2SO4 or HCL) or by burning of phosphorous

produced through electro-thermal process. [Dryden & M. G. Rao, 2010]



2. Process using H2SO4 is the most & most important and may be classified

according to the hydrates form in which calcium sulfates crystallizes, viz,

anhydrates (CaSO4), hemihydrates (CaSO4. ½ H2SO4) and dihydrates

(CaSO4.2H2O). The hydrate form is controlled mainly by temperature and acid

concentration. Table (1) gives the silent features of contemporary process

technology (using H2SO4) in commercial use.

Table (1) Silent Features of Contemporary Process Technologies for fertilizer

grade Phosphoric Acid (H2SO4 route)

Sr

No

Name of the

Process

No. of

Separation

Steps

Data Furnished by Process Licensors

Capitalized

cost Rs.

(Crores)

P2O5 recovery

in%

Energy

Requirement

per ton P2O5

(KWH)

Gypsum

Quality

1 Dihydrate 1 29 95-96 125 Not good

2 Hemihydrate

-Dihydrates

2 31 98-98.5 110 Excellent

3. Dihydrate-

Hemihydrate

2 31 98-99 110 Excellent

4 Hemihydrate 1 25 93-94 100 Reported

poor

[Executive summary, 2003]

3. The features listed include P2O5 recovery efficiencies, specific energy

consumption, temperature conditions required to be maintained in reactors & re-

crystallizer and quality of by-product gypsum from various process.

4. The conventional dihydrate process remains the most predominant because of its

low capital cost, low operating temperatures and flexibility of operation. But the

process suffers from relatively low P2O5 recovery & low strength of acid. Newer

process which claims to overcome these limitations of the dihydrate process are

hemihydrates-dihydrate (double filtration stage) and dihydrate-hemihydrate.

5. HCl acidulation process produces technical grade acid. After acidulation, the acid

has to concentrated & purified before used in detergents or food industries. This

process gives CaCl2 as a by-product which is difficult to dispose. Capital cost and



cost of production are higher than those sulfuric acidulation plants though the cost

difference is reducing gradually with improving HCl process.

6. Electro-thermal reduction of phosphate rock produces very pure phosphoric acid,

but the cost of production is extremely high because of high cost of power in India

capital cost is highest for the plant based on this technology.

Table (2) Technology Status of Indian Industry (capacity in tones of P2O5/year)

A. Plants Based on Conventional Dihydrate Process

Sr.

No

Manufacturer Installed

Capacity

Process Licensor Engineering

Conductor

1 FACT Limited,

Udyogmandal

33,000 Dorr Oliver, U.S.A. Hindustan

Dorr Oliver

2 EID parry, India

Limited

10,696 Societe-de-Prayon

Belgium

Simon Carves

Ltd. U. K.

3 GDFC Limited 52,500 Chemico, U.S.A. through

Hitachi zosen, Japan

Dorr Oliver, U.S.A.

Hitachi Zosan

Japan

4 Coromandel

Fertilizers Ltd

91,000 Dorr Oliver, U.S.A. Dorr Oliver

Ltd, U.S.A.

5 Albright Morarji &

Pandit Ltd.


Belgium through

Albright & Wilson U.K.

Charamsi

Morarji

Chemical Co.

Ltd.

6 Fact Limited,

Cochin

1,18,800 Societe-de-Prayon

Belgium through FEDO

FEDO

7 Hindustan Zinc 26,800 Not Available Not Available

8 Hindustan Lever

Ltd.

41,850 Mac-him/Simchem FEDO

9 Paradeep

Phosphate Ltd.

2,25,000 Jacobs International Inc.

with Indian associate

Hindustan Dorr oliver

Ltd.

Hindustan

Dorr oliver Ltd



B. Plants Based on Hemihydrate-Dihydrate (Nissan Process)

Sr

No


Capacity


Conductor

1 RCF Ltd. 30,000 Nissan, Japan PDIL

2 SPIC Ltd. 52,800 Nissan, Japan Hitachi zosen

Japan

3 HFC Ltd. 27,600 Nissan, Japan through,

PDIL

PDIL

C. Plants Based on Dihydrate-hemihydrate (Central Glass Prayon Process)

Sr

No Manufacturer

Installed

Capacity Process Licensor

Engineering

Conductor

1 Hindustan Copper

Ltd 68,000

Societe-de-Prayon

Belgium FEDO

2 FCI Ltd 1,19,000 Societe-de-Prayon

Belgium FEDO

D. Plants based on Hydrochloric Acid Process

Sr

No


Capacity


Conductor

1 Ballarpur Industries 24,000 AEA France (IMI

Process)

Krebs & Cie

Pvt. Ltd.

E. Plants Based on Thermal processes

Sr.

No


Capacity


Conductor

1 Star Chemicals (Bombay)

Pvt. Ltd

6000 TVA, U.S.A. -

2 Excel Industries NA TVA, U.S.A. -

3 Transport Industry Ltd NA TVA, U.S.A. -



Table (3) Technology Status of World in Manufacturing Phosphoric Acid

(Capacity in tones of P2O5/year)

A. Plants Based on Conventional Dihydrate process

Sr.

No


Capacity


Conductor

1 Dorr Oliver, Ltd,

U.S.A.

70,000 Self -

2 Simon Carves

Ltd. , U.K.


Belgium

NA

3 Hitachi Zosen,

Japan

65,000 Dorr Oliver Ltd,

U.S.A.

Dorr Oliver Ltd,

U.S.A.

4 Chemico, U.S.A. NA Dorr Oliver Ltd,

U.S.A.

Dorr Oliver Ltd,

U.S.A.

5 Jacobs

International Inc.

14,000 Michim/Simchem NA

B. Plants Based on hemihydrates-Dihydrate Process

Sr.

No


Capacity


Conductor

1 Nissan, Japan 1,10,000 Self -

2 PDIL, for two Plants 54,000 Nissan, Japan Nissan, Japan

3 Hitachi zosen for one

Plants

32,000 Nissan, Japan Nissan, Japan

C. Plants Based on Hydrochloric Acid (IMI) process

Sr.

No


Capacity


Conductor

1 AEA, France 56,000 Self Krebs & Cie Pvt.

Ltd.

D. Plants Based on Thermal Process

Sr.

No


Capacity


Conductor

1 TVA’s, U.S.A. NA Self -

[Exectutive summary, 2009]



7. Research & development has been made during the last 30 years and modification

in the process has been achieved. Some have modified effluent disposal system

and have also developed methods for better utilization of gypsum.

8. FEDO, PDIL, GSFC Ltd, RCF and Ballapur Industries Ltd. Are the companies

who have reported doing some R & D work pertaining to phosphoric acid.

Unfortunately, there has been very little commercial application of in-house R &

D work done in the country.

9. There are many Phosphoric acid producing Industries across the world. Here are

some of those Industries with their capacities mentioned below in Table (3)

10. These Industries across the world had made lots of advancement in the

manufacturing process, purification process of phosphoric acid. They had

discovered the technology for the extraction of fluorine and uranium from the

phosphor-gypsum. Some of these technologies are also being imported to India.



E] Brief Details about Raw Materials Used

A. Rock Phosphate:

1. Introduction

Finely ground rock phosphate is used as a source of P2O5 for Phosphoric acid

production. The major part of world rock phosphate about 30 million tonnes per year

is converted to phosphoric acid. The majority of the product phosphoric acid is further

converted to fertilizer. A small proportion of rock phosphate is sold as cheap

fertilizer. Rock phosphate is generally found in two forms: 1] Igneous Phosphate2]

Sedimentary Phosphate.

Rock phosphates in the apatite group are preferred such as fluorapatite (CaF),

chlorapatite (CaCl), and Hydroxypatite (CaOH). Fluorapatite variants are mostly used

i.e. Ca10(PO4)6F2. The phosphate rock is always complex. It contains several

impurities. Total world reserves rock phosphate estimated to be over 65 billion tones,

are sufficient to supply the world for almost 375 years at the current rate of

consumption.

In India Rajasthan is principle producing state, contributing 90% of total production

followed by Madhya Pradesh with 10%. About 52% of the total production of rock

phosphate is of grade 30-35% P2O5, 6% of 25-30% P2O5 grade, 1% of 20-25% P2O5

grade and 40% of 15-20% P2O5 grade. Only 25-30% requirement of rock phosphate is

met through indigenous sources. The remaining requirement is met through import of

rock phosphate.


a. Formula :Ca10(PO4)6F2

b. Physical state & appearance :Powder

c. Color :Grey & Yellow

d. Odor :Odorless

e. Melting Point :71400⁰C

f. Solubility :Insoluble

g. Relative Density :3.2 Kg/lit

h. Bulk Density(1% moisture) :1.75Kg/lit



Following are the general specification of rock phosphate recommended for use in

manufacturing of phosphoric acid:

Component % in Rock phosphate

P2O5 32-34%

CaO 1.5% (max)

Fe2O3 + Al2O3 2.0% (max)

CO2 3.0% (max)

SO3 Should not be in sulfide form

SiO2 2.5-5% (max)

F 4% (max)

MgO 0.5% (max)

Cl 0.015% (max)

Organic Matter 1.5% (max)

3. Chemical Properties:

i. Reaction of rock phosphate with sulfuric acid:

Ca10(PO4)6F2 + 10H2SO4 + 20H2O → 6H3PO4 + 10(CaSO4.2H2O)

ii. Reaction of rock phosphate with hydrochloric acid:

Ca10(PO4)6F2 + 6HCL + 6H2O → 2H3PO4 + 3CaCl2

iii. Reaction of rock phosphate with phosphoric acid

2Ca10(PO4)6F2 + 14H3PO4 → 10Ca(H2PO4)2 + 2HF

iv. Reaction of rock phosphate with silica gives elemental phosphorous

2Ca3(PO4)2 + 10C + 6SiO2 → P4 (Yellow Grade) + 6CaSiO3 + 10CO

4. Components of Rock phosphate:

For production of phosphoric acid main criteria for rock phosphate are:

a. P2O5 content: 33-38% P2O5 rock phosphate is considered as high grade.

For production of phosphoric acid high grade rock phosphate is required.

b. CaO content: It affects the sulfuric acid consumption. Each process of

CaO needs an equivalent of 17.5 Kg sulfuric acid per ton of rock



phosphate. Relatively pure acid can be produced from rock phosphate

containing very large amount of calcite.

c. Fluorine: Usually occurs sedimentary rock as 10% of the P2O5 weight.

Fluorine can be corrosive component if not enough silica is also not

present. With high sodium content most of it will precipitate during

phosphoric acid reaction.

d. Sulfates: Existing sulfates in rock phosphate will save the corresponding

amount of sulfuric acid during acidulation.

e. SiO2: Reactive silica is needed to combine with fluorides to prevent

corrosion. However highly reactive silica will depress the filtration rate.

f. Al2O3, Fe2O3: Not a problem during manufacturing of phosphoric acid but

afterward when using the phosphoric acid. Sludge formation with

concentrated acid, builds water insoluble components in the phosphate

fertilizer.

g. MgO: Stays with acid phase. It increases viscosity strongly.

h. Na2O: Precipitates as Na2SiF6 from acid.

i. Organics: It forms during reaction. Dark cloudy solids suspended in

product acid.

Rock phosphate vary in composition around the world & even within a local

mine,hence the need to analyze the rock on a regular basis as a part of phosphoric acid

production plant control stratergy.

5. Manufacturing Processes:

The phosphate rock having maximum P2O5 content & a minimum of impurities is

suitable for the production of phosphoric acid. However rock phosphate contains

different types of impurities & non-phosphate materials. It is important to remove or

minimize all the contents of these impurities in rock phosphate.

The separation of phosphate rock from impurities & non-phosphate materials for use

in manufacturing of phosphoric acid consist of beneficiation, Drying and calcining at

some operation and grinding. Rock phosphate from the mine is first sent to separate

sand & clay and to remove impurities. The wet beneficiated rock phosphate may be

dried or calcined depending on its organic content. Dried or calcined rock is ground in

roll or ball mills to a fine powder.



In most of the cases to reach commercial grade of some 30% of P2O5, only screening

or drying is required. For sedimentary rock suitable, suitable techniques for economic

ore concentration re-crushing & screening or grinding followed by pneumatic particle

size selection and washing and disliming by hydrocyclones or classifiers.

6. Uses and applications:

a. It is used in production of phosphoric acid as a raw material.

b. It is used in production of fertilizer such as single superphosphate, triple

superphosphates and ammonium phosphate as a raw material.

c. It is used in production of elemental phosphorous as raw material.

d. In some cases it is directly used as a fertilizer.

e. Rock phosphate is used in animal feed supplements, food preservatives,

anticorrosion agent, cosmetics, fungicides, ceramics, water treatment and

metallurgy.

B. Sulfuric Acid:

1. Introduction

Beside rock phosphate sulfuric acid is second raw material needed for phosphoric acid

production. It is preferred to utilize strong phosphoric acid in order to obtain high

P2O5 content or washing efficiency. Sulfuric acid is the first choice for commercial

processes, because it produces insoluble calcium sulphate (Gypsum) which can be

easily filtered. Wet process phosphoric acid is the major world sulfuric acid

consumer, with nearly 50% of the total production.

Sulfuric acid is the highly corrosive strong mineral acid. For wet process phosphoric

acid production generally 72-75% concentrated sulfuric acid is used. This

concentration of sulfuric acid is obtained by mixing strong sulfuric acid(97%

minimum conc.) and weak sulfuric acid(67% min conc.) in the mixing vessel.




a. Molecular Formula : H2SO4

b. Molecular Weight : 98

c. Appearance : Colourless viscous liquid

d. Odour : Odourless

e. pH : 0.3 (1 N solution)

f. Vapour Pressure : <0.0012mmHg

g. Vapour Density : 1.2 Kg/m3

h. Boiling Point : 290⁰C

i. Melting Point : 10.33⁰C

j. Decomposition Temperature : 340⁰C

k. Solubility : Soluble in water

l. Specific Gravity :1.841

m. It dissolves most of the metals.

n. It is conductor of heat & electricity

Following general specification of sulfuric acid is recommended for use in

manufacturing of phosphoric acid.

Concentration 97% (min.)

Residue on ignition 0.054%

Iron (as Fe) 0.01%

Chloride (as Cl2) 0.0003%

Arsenic (as As2O3) <1%

3. Chemical Properties:

a. Reaction of sulfuric acid with water: It is highly exothermic reaction as of

formation of hydronium ions.

H2SO4 + H2O → H3O+ + HSO-

4

b. Acid-Base reaction: Sulfuric acid reacts with most bases to give

corresponding sulphate. Consider a reaction of sulfuric acid with copper oxide.

CuO + H2SO4 → CuSO4 + H2O

Sulfuric acid can also be used to dispose weaker acid from their salts.

Consider reaction of sulfuric acid with sodium acetate.

H2SO4 + CH3COONa → NaHSO4 + CH3COOH



c. Reaction with metals: Sulfuric acid reacts with metals producing hydrogen

gas and salts. It attracts reactive metals such as iron, aluminum, zinc,

manganese and nickel.

Fe + H2SO4 → H2 + FeSO4

d. Reaction with sodium chloride: Sulfuric acid reacts with sodium chloride

and gives hydrogen chloride gas and sodium bisulfate

NaCl + H2SO4 → NaHSO4 + HCl

e. Reaction with non-metals: Sulfuric acid oxidizes non-metals such as carbon

& sulfur.

C + 2H2SO4 → CO2 + 2SO2 + 2H2O

S + 2H2SO4 → 3SO2 + 2H2O

f. Electrophilic aromatic substitution: Benzene undergoes electrophilic

aromatic substitution with sulfuric acid to give the corresponding sulfonic

acid.

4. Manufacturing process

a. Contact process or DCDA process: Sulfur is burned to produce sulfur dioxides

S + O2 → SO2

This is then oxidized to sulfur trioxides in the presence of vanadium oxide catalyst.

2SO2 + O2 ↔ 2SO3

The sulfur trioxide is absorbed into 97-98% H2SO4 to form oleum (H2S2O7) also

known as fuming sulfuric acid.

H2SO4 + SO3 → H2S2O7

H2S2O7 + H2O → 2H2SO4

b. Wet sulfuric acid process: Sulfuric acid is burned to produce sulfur dioxides

S + O2 → SO2

This is oxidized to sulfur trioxides using oxygen and vanadium oxide as catalyst.

2SO2 + O2 ↔ 2SO3

‐H2O

H2SO4, SO3 OH

O

O

S



Sulfur trioxide is hydrated into sulfuric acid

SO3 + H2O → H2SO4 (g)

Then it is condensed to get liquid 97-98% H2SO4

c. Sulfuric acid can be produced by burning sulfur in air and dissolving the gas

produced in the hydrogen peroxide solution.

SO2 + H2O2 → H2SO4

4. Uses & application

a. It is sued as the main raw material in the phosphoric acid production.

b. It is used in production of fertilizers such as ammonium sulfate,

superphosphate and ammonium phosphate.

c. It is used to produce various Chemicals such as zinc sulfate, alum etc.

d. It is used in paper pulp & detergent Industries.

e. It is used in the textile Industries to produce rayon and the artificial fiber.

f. It is used in textile finishing.

g. It is used in petroleum industries for petroleum refining.

h. It is used in paint & pigment industries.



F] Justification

1. Syllabus requirement: The manufacturing of phosphoric acid by wet process

using sulfuric acid and rock phosphate as raw material has been previously

studied as part of syllabus in subject “Chemical Process-I” (6th Semester,

Mumbai University). This study provided information about raw materials,

process detail, flow sheet and engineering problems of manufacturing process.

2. Significance of phosphoric acid: Phosphoric acid serves as an intermediate

product for most of the fertilizers used in India. India is agro based country. It

requires large production of phosphoric acid based fertilizers which are

ammonium phosphate, triple phosphate, and liquid mixed fertilizers.

Phosphoric acid also used in dental science.

3. Uses and application: Phosphoric acid serves large application in fertilizers,

pharmaceutical beverages, textile, oil refinery, sugar, soap and detergents and

glass industry. Phosphoric acid used as catalyst to alter the composition of

hydrocarbons in dehydration, polymerization and alkylation process. It also

used in photography, rust removal, water treatment and fire retardant as main

component.

4. Import and export requirement: Demand of phosphate fertilizers has grown

rapidly in India. India has limited resource of rock phosphate therefore relied

heavily on import of both phosphate rock and phosphoric acid for production

of fertilizers. India is a great importer of phosphoric acid. Out of total trade of

approximately 5 million tons. India imports more than 2.5 million tons per

year because of great demand of fertilizers. India exports phosphoric acid to a

very small extent

5. Storage, Handling, Transportation, Loading and unloading: Phosphoric

acid is non-flammable, non-toxic and less corrosive in nature. Therefore it is

easy to storage, handling and transportation. Storage facilities required for

phosphoric acid are relatively more expensive than that of the solid fertilizer.

Phosphoric acid is transported in rubber lined steel tankers or stainless steel

tankers. The clarified acid is transported over long distance in special ocean

going ships. In India phosphoric acid is imported in such ships.



6. Special significance in fertilizer industry: Phosphoric acid has find its ample

use in fertilizer industry. It has a special significance in manufacturing of

complex fertilizers like nitrophosphate, ammonium nitrophosphate,

superphosphoric acid etc. also its serves a important raw material in the

production of some industrial phosphate like sodium phosphate and calcium

phosphate.

7. Importance of byproduct produced along with phosphoric acid: In

manufacturing of phosphoric acid, important byproduct i.e gypsum and

fluosilicic acid is obtained. This byproduct also has many industrial

importances. Gypsum is soft sulfate mineral, it can be used as main

constituents in many forms plasters, fluosilicic acid is used as fluoridation

agents for drinking water.

By considering all above points it is very much important to study the manufacturing

of phosphoric acid using rock phosphate and sulfuric acid as a raw material in

detailed.



Chapter No 2: Literature Survey

In this literature survey the review of various research articles published on

international level and the information collected from patent papers are presented.

Paul and Colin presented the method of production of phosphoric acid using two stage

crystallization and filtration process employing a feed acid tank assembly and

recovery solution tank assembly for production of high strength phosphoric acid with

high recovery of P2O5. In this process Phosphoric acid and phosphate rock is

dissolved in a reaction vessel to form the slurry. The slurry is then reacted in a first

stage crystallization with sulfuric acid to produce calcium sulfate hemihydrates. The

product acid is separated from the hemihydrates via filtration and the filter cake is

then reacted with additional sulfuric acid to produce dihydrate calcium sulfate

(Gypsum) and recovery solution. The gypsum is separated from the recovery solution

via filtration and removed as a byproduct. Thus this process is producing high

strength acid having concentrations of 39% P2O5 or higher and high P2O5 yields from

the rock phosphate of 99% or greater commercial grade phosphoric acid has a P2O5

concentration or purity of about 50-54% where as food grade P2O5 has a

concentration or purity of about 54-62%. [Kucera P. , Weyrauch C. G. ,2014]

Macharro, Olveza and Larios studied the purification of industrial grade phosphoric

acid (P2O5) by conventional electrodialysis. High concentrated phosphoric acid

solution containing sulphates and chlorides as impurities is produced at anode. All

other impurities are removed at cathode. Experiment was conducted using three

compartment cell with anion and cation exchange membranes and industrial acid

solution was introduced into the central compartment. The elemental analysis of

diluted solution shows that the composition of magnesium, phosphate and sodium

reduced in the central compartment. The ratios of the concentration of ions and the

phosphate essentially unchanged by the process, consequently electrodialysis could

not purify the acid in the central compartment. Migration of phosphate ion to the

anolyte produced highly concentrated phosphoric acid solution containing sulphates

and chloride impurities. Migration of phosphate ions across the membranes consumed



[Kucera P. , Weyrauch C. G. ,2014]

large amount of energy. The Three compartment cell produced a highly concentrated

phosphoric acid solution with ionic impurities in the anolyte. Such migration process

consumes a large amount of energy and would therefore be extremely costly. Hence

electrodialysis process is not commercially viable for purifying phosphoric acid.

[Macharro J.J. , et. al, 2013]



[Macharro J.J. , et. al, 2013]

Dr. Megy J.A. gives the outlines of the economics, environmental and quality

advantages of the improved hard process over the wet acid process for the

manufacture of phosphoric acid. The improved hard process will permit the use of

leaner rock phosphate because it has greater tolerance of some common and

magnesium. it will also reduce the environmental action of gypsum stacks because

inert solid aggregate is the by product of the process not gypsum. This process

produces super phosphoric grade acid at a significantly reduced cost compared with

the wet acid process. In this process treated rock phosphate, silica and green

petroleum coke are mixed in required proportions, dried and ground. Then aqueous

slurry of wash waste clay or bentonite is added to the mixture to supply the moisture

and clay to needed strong agglomarates. Product scrubber is used to produce

phosphoric acid from stream processed from the kiln. The remaining gases from the

product scrubber is fed to flue gas scrubber, where limestone is used to remove

impurities from the flue gases. As long as the amount of water vapour in kiln gases is

sufficiently low, the acid strength of 76% P2O5 is readily obtained. The hot kiln solids



are cooled with air, which is then used to dry the materials prior to grinding and

agglomerates prior to entering the kiln.

It utilises resources that are unsuitable or difficult to process by wet acid methods.

Improved hard process also has several environmental advantageous. Regardless of

the ore use, process produces solid waste in the form of amorphous agglomerates

suitable for aggregate uses rather than gypsum piles. The improved Hard process is a

dry process so threats to ground or surface water reduced. It also makes use of green

sulphur petroleum coke. Improved hard process phosphoric acid is produced

relatively pure, high concentration acid suitable for use of fertilizers. [Dr. Megy J. A.,

2002]

[Dr. Megy J. A., 2002]

Hotta and Kubota provides purification method of phosphoric acid, which includes

bringing phosphoric acid containing arsenic contact with hydrogen halide, which is

expected to be applicable to a broader range of use in the fields of food, medical and

electronic material. The method given in the paper includes simple treatment step and

required no special apparatus. The arsenic content of phosphoric acid drastically falls

during an organic synthetic reaction accompanying production of hydrogen chloride

in high concentration phosphoric acid. By bringing phosphoric acid into contact with

hydrogen halide, the arsenic content of phosphoric acid can be reduced to 1ppm.

When the contact is performed in the presence of a compound capable of generating

hydrogen halide under acidic conditions, the arsenic removal effect is enhanced.

Hydrogen halide causes arsenic in the phosphoric acid to convert to an arsenic



compound having a high volatility, such as arsenic halide and arsenic hydride and the

compound evaporates at treatment temperature. The compound capable of generating

hydrogen halide under the acidic condition is a halide of iron(II), copper(I) or tin(II),

which is used to remove arsenic from the phosphoric acid.

The arsenic content of 10 to 100 ppm of general industrial phosphoric acid can be

reduced to about 1 ppm by treating phosphoric acid with hydrogen halide. The total

cost for treatment is low and since sodium compound is not used, unlike the

conventional method sodium does not remain in large amount in product phosphoric

acid. [Hotta K. & Kubota F., 2005]

[Hotta K. & Kubota F., 2005]

Smith & Jackson explains filter support having a surface with plurality of

perforations. At least one spray bar may be substantially fixed adjacent to the surface

of the filter support. A spray bar may include a plurality of nozzles for directing a

pressure fluid toward the filter support to move in a cylindrical while the spray bar

directs pressurized fluid towards the filter support. This method include drawing a

vacuum on one side of the movable filter support, opposite the mixture draw the

phosphoric acid through the filter. Drawing vacuum on one side of the movable filter



support result in calcium sulphate accumulation in the opening of the movable filter

support. Which include removing filter from the filter support and causing the filter

support to move in cyclical manner and the filter is removed from the filter support.

During cyclical motion, a plurality of nozzles in the spray bar, may spray pressurized

fluid towards the surface of the filter support. This occurs through multiple cycle

movement such that the spraying results in a sweeping motion across the surface of

the filter support. After a majority of openings in the filter support are substantially

free of calcium sulphate blockage spraying may be terminated.

Fluid is sprayed at a pressure greater than 5000 psi to clean the filter support. Wherein

the fluid is sprayed at a flow rate between 450-900 gallons per minute at an angle

perpendicular to the surface of the filter support, which substantially cleans all the

openings of filter support and makes filter support free of calcium sulphate. [Smith G.

L. & Jackson J. D., 2010]

Other literature shows how continuous melt suspension crystallization process is used

for the purification of phosphoric acid, which consist of a. cascade of mixed product

removal (MSMPR) crystallizer and a gravity wash column for subsequent solid liquid

separation. The crystallization equipment consist of MSMPR crystallizer and a wash

column (length 1200mm). Seed crystals were prepared from a solution of reagent

grade acid (85% H3PO4 & specific gravity of 1.7) & cooled to -5oC to -10oC & size is

maintain to 0.1 mm. After 1 hour of crystallization, magma is entrained to the top of

wash column by screw propeller stirrer.

Because of density difference between crystal and liquid, crystals in the magma

settled down to from a loosely packed bed, which was melted to the bottom of column

having temperature equal to melt temperature of phosphoric acid hemihydrate. Part of

the melt is withdrawn while part of it returned as wash liquid & crystals is enabled.

The melt containing impurities was extracted as residual at the top of column. Thus,

feeding in the crystallizer and withdrawing of the product and the residual at the

bottom & top of the column respectively allows the continuous operations of the

crystallization equipment. Purity of the crude crystals descending in the wash column

increases due to:-

I. displacement of the mother liquor

II. washing of the liquid layer adhering to the crystals



III. Crystallization of pure melt on cold crystals.

IV. Sweating of the crystals in contact with the hot melt or adiabatic re-

crystallization of the crystals.

Continuous suspension crystallizer to purify phosphoric acid ensures full suspension

of crystals and without secondary nucleation. Also impurity concentration of the

product is lowered. [Chen A., 2012]

1- mixed suspension mixed product removal crystallizer

2- scraped surface crystallizer to produce the crystals in the mother liquor

3- optional addxitional mixing vessel

4- wash column for efficient removal of crystal

Process flow diagram for melt crystallization process

[Chen A., 2012]

According to Mollere P. and other collegues crystallization starts by adding relatively

large amount if fine, relatively pure seed crystals to acid which has been cooled to

supersatured conditions. Thereafter crystallization proceeds under conditions which

substantially favor growth on the seeds and disfavor secondary nucleation. The

process consists of a helical cooling coil device, agitated crystallization vessel,

alternate acid cooler. Fig shows schematic outline with present invention of

crystallization process. First the acid at a concentration of ( 62 - 65% P2O5) and a

temperature between 30 0C and 60 0C is supply over the helical cooling coil inside



which passes the coolant having temperature ranges from -20 to -15 0C. Thus the

temperature of the acid between -9 and -13 0C is achieved.

The overflow is then passed to the agitated crystallizer comprising of agitator shaft

carrying impeller blades. The residence time within crystallizer should be less than 1

minute. For commercially crystallizer, residence time as short as 10 seconds is

prefered. The acid is again passed through the acid cooler which is operated with

respect to first cooler. Because agitation provides higher heat transfer rates due to

which process becomes thermodynamically unstable and there may be the risk of

increase in unwanted crystallization occurring in the device. It provides a continuous

process, since the seed crystals always present in the crystallization vessel to cause

continued crystallization of seed. Once the process setup it is self- sustaining.

Using this method of seed preparation the process becomes continuous by only 1st

seed supply (extraneous), also the acid of concentration (45 - 50% P2O5) is obtainable

from the process. [Mollera P., et. al, apr 14, 2001]

1- helical cooling coil device

2- agitated crystallizer 3- alternate cooling coil

Seed Crystallization Process

[Mollera P., et. al, apr 14, 2001]



Carr J. M. proposed that filtration of wet process superphosphoric acid by vacuum

filtration is greatly enhanced by the use of a filter aid having a statistically selected

distribution of particle sizes. Super phosphoric acid is applied to the precoat by

rotating drum through an acid container, with an supplement application being made

by means of a distribution box. The applied acid thereafter passes through the

particles of filter aid with the solid impurities being filtered out or entrapped at or

very near the surface of the precoat.

A doctor knife rests against the outer surface of the precoat and serves to

continuously remove a thin outer layer of the precoat and entrapped solid impuries.

Since drum rotates in a counterclockwise manner, the knife continuously removes the

material just before the precoat is re- exposed to the superphosphoric acid in acid

container. In this matter, a fresh surface of precoat is continually presented to the

super phosphoric acid. The filtered acid, meanwhile is drawn through the vacuum

lines, the filter product line, the receiver and collected in the filter product tank.

Filtration using filter aid in vacuum ensures 90% acid recovery from the slurry which

is economically profitable with less input. [Cary J. M., Jr. et. al., 1995]

filtration using rotary filter

[Cary J. M., Jr. et. al., 1995]



Pillai J. K explains how a digested slurry from a wet process phosphoric acid

production process is dewatered by gravity assisted vacuum filtration using as a

filtration aid an aqueous solution of a polymeric filtration and that has been subjected

to high shear mixing to enhance the performance of the filtration aid and enhance the

filtration. The yield of WPA process is encountered both an extreme need for

filtration efficiency and extreme filtration conditions. Filtration aids, particularly

polymeric filtration are often used to enhance the filtration process. Enhancement

generally sought from filtration aids is:-

i. Increase in filtration rate

ii. Minimization or elimination of solids in the filtrate

iii. Minimization of liquid in the filter cake

In WPA process the product obtained is desired to be in concentrated form,

enhancing the filtration process and reducing the amount of acid associated with the

filter cake are very important. The present invention provides a process for the

dewatering of digested slurry from a wet process phosphoric production digestion

process, in which an aqueous solution of polymeric filtration acid is added to the

digested slurry. The dewatering is accomplished with gravity assisted vacuum

filtration while the temperature of slurry remains in the range from 63 0C – 135 0C the

liquid phase contains about 28 to 42 wt % soluble P2O5. [Pillai K. J., et. al., 2003]

Purification using gravity assisted vacuum filter

[Pillai K. J., et. al., 2003]



Amin M. I. along with other collegues proposed the solvent Extraction of iron from

Egyptian Wet process phosphoric acid is investigated. This process is important

because of the presence of iron in phosphoric acid results in viscosity & consequently

decreasing the filtration rate. Before iron extraction pretreatment is given to the

phosphoric acid using diethyl-hexyl phosphoric acid. Crude acid is mixed with

bentonite for approx 30mins & left for settle down. Flocculating agents such as

Polycrylamide type is added to enhance the settlement of suspends.

Extraction experiment is carried out in mechanically agitated thermostatic beaker

containing iron in 50 ml phosphoric acid & 100 ml of D2EHPA. This mixture is

agitated at 400 rpm speed. After 5 mins Organic & aqueous phase are separated. Iron

conc. In aqueous phase is determined by adsorption spectrometer. Conc. of iron in

organic phase is calculated by difference of iron in aqueous phase before & after

extraction. Concentrated phosphoric acid (44% P2O5) was purified by solvent

extraction using mixture of butanol & octanol in 3 steps. These trials are expensive

because of applying scrubbing process.

Removal of iron from wet process phosphoric acid is investigated by solvent

extraction using solvent diethyl hexyl phosphoric acid. The % removal of Fe was

69.3- 99.9 % for Egyptian phosphoric acid & diluted phosphoric acid is a obtained

without scrubbing process. These trails are expensive because of applying scrubbing

process so the main goal of this work is extraction of iron from Egyptian wet process

phosphoric acid without applying scrubbing during purification process.[Amin M. I.,

2014]

One of the journal gives us the basic overview of central prayon process for

manufacturing/ production of phosphoric acid by Dihydrate-hemihydrate process.

Overviews of major equipments are given along with process flow diagram. In his

IFA symposium presentation, paul smith reviewed the processes presently available &

examined the features of each process. He observed prayon enjoys considerable

expertise in licensing Dihydrate plants, enjoying market leadership. Prayon pioneered

the use of flash cooler to remove the heat of reaction. The flash cooler was originally

designed in 1950’s & has progressively upgraded. Prayon currently offers the 4



Egyptian wet process phosphoric acid using organic solvent extraction

[Amin M. I., 2014]

Dihydrate process & 20 units have been installed capable of producing over 1000

Tonn/Day P2O5 acid. The traditional prayon reactor consist of a multicompartmented

monolithic concrete reactor & a separate de-super saturation (Digestion section)

which can be concrete or rubber & carbon brick lined, Carbon steel tanks. This

process produces merchant grade self drying gypsum & a slightly stronger phosphoric



acid. Phosphoric acid production by dihydrate-hemihydrate 32-35% P2O5 process

(Central prayon process) produces merchant grade gypsum & slightly stronger

phosphoric acid. [Fertilizer International, 2011]

Central prayon process

[Fertilizer International, 2011]

Awwad N. S. proposed an article which gives successive process for purification of

phosphoric acid produced by wet process are given & the extraction of organic such

as iron, silica, fluorine, Uranium are also included. Materials & measurements:

Commercial wet process phosphoric acid containing P2O5 57%, Fe 2.6%, F 0.7% &

Uranium 50ppm. Determination of P2O5 & Fluorine: P2O5 was determined

spectrochemically by a solution containing yellow molybdovanadate phosphoric acid

complex. Extraction of silica from rice husk: Raw rice husk (RH) was stirred with the

solution of 5% KOH at a weight ratio 1:12 (g/ml) of rice husk to solution respectively

& heated to boiling for 30minutes mixture is kept overnight. Filtered & washed twice

with distilled water until filtrate becomes neutral. To maintain the pH in range 5-7

10%HCl is added in filtrate. The precipitate formed was filtered dried & weighed.



Hydrogen peroxide was used as oxidizing agent to ensure that all Fe(2) & U(4) ions

are oxidized to Fe(3) & U(6). The advantages of hydrogen peroxides are that it forms

no precipitates with other reactants. The clays of iliminite were used to remove humic

acid & suspended materials from crude phosphoric acid. Flocculating agents was

added & the mixture was left for 10 minutes for settling. Extraction process was

carried out by shaking 5ml of organic solvent with 5ml of crude acid in separating

funnel for 15 min at 25⁰C. Maximum removal of organic matter such as Fe, Fluorine

and Uranium is achieved by this process. Extraction of P2O5 can also be done by

using various alcohols. [Awwad N. S., et. al., 2013]

One of the arab journal gives short overview of the phosphoric acid production

process (wet process) and the causes of corrosion in the various equipment of plant.

The corrosion resistant materials to be used for all major equipments of plant are

explained briefly. Corrosion prevention or resistance for

a. Reactor: The first stage for which special material are required is the reactor for

digesting the ore. The reactor vessel is made of cement or carbon steel with non-

metallic linings and sometimes augmented with acid resisting bricks.

b. Mixers: The mixers are made up of stainless steels such as VDMR alloy 904

L(1.4539) or VDMR alloy 926 (1.4529). The application of this alloys are however

limited by the increasing impurity in phosphate ores and higher temperature for

digesting the ore. Material VDMR alloy 31 (1.4962) is particularly resistant to

corrosion in phosphoric acid slurring.

c. Filter: The filtration equipment also requires special materials. In particular good

resistance to pitting and crevice corrosion is a criterion here. The material used for

this plant section are for example 300 series stainless steels VDMR alloy 904

L(1.4539) or VDMR alloy 926 (1.4529). Even the higher resistance for pitting and

crevice corrosion can be achieved with VDMR alloy 31(1.4560).

d. Evaporator and heat exchanger: It is either equipped with metal tubes or is mode of

graphite. It has been shown that metallic heat exchanger operates satisfactorily when

hot water is used as heating medium. The solids settling on the tube walls, mainly

gypsum are easily removed by routine cleaning, which prevents corrosion after



deposits. VDMR alloy 28 (1.4563) and VDMR alloy 31 (1.4562) are used for the heat

exchangers.

Purification and extraction of phosphoric acid

[Awwad N. S., et. al., 2013]

e. Other equipments (plate HE , pumps and fans, equipments for the removal of

gaseous fluorine compounds): Special stainless steel and special nickel alloys are

used. VDMR alloy 31(1.4562), which combines a high chromium content with



adequately high molybdenum content, demonstrates high resistance to uniform and

localized corrosion in phosphoric acid under diverse conditions and is in fact superior

to higher alloy materials in this respect.

According to the causes of corrosion in the equipments of phosphoric acid production

plant various corrosion preventable or resistance materials are suggested for all the

major equipments. [Fertilizer Focus, 2014]

Wet phosphoric acid production process flow diagram

[Fertilizer Focus, 2014]



Chapter no 3: Various Process Descriptions

We have the following processes for the manufacturing of phosphoric acid along with

their detailed description.

1] Acidulation of rock phosphate using H2SO4:

There are five process for producing phosphoric acid using rock phosphate and

H2SO4. They are:

a. Di-hydrate process [26%-32% P2O5]

b. Hemihydrate process [40%-52% P2O5]

c. Di-hydrate – Hemihydrate process [32%-36% P2O5]

d. Hemihydrate – Di-hydrate process [40%-52% P2O5]

e. Hemihydrates – Recrystallization process [30%-32% P2O5]

Reaction :

A] Di-hydrate Process

Process Description:

The description process comprises four stages: grinding, reaction, filtration and

concentration. For di-hydrate process particle size acceptable is (60%-70% less than

150 m), which are attained either by ball mill or rod mill. The tricalcium phosphate

by reaction with conc. Sulphuric acid is converted into phosphoric acid and insoluble

calcium sulphate. The operating conditions for di-hydrate precipitation are 26%-32%

P2O5 and 70 0C- 80 0C. This temperature is controlled by passing the slurry through, a

flash cooler or by air circulating cooler. Filtration separates the phosphoric acid from

the calcium sulphate di-hydrate. On filtration, the gypsum is given at least two stages

of washing to ensure a satisfactory recovery of soluble P2O5.

3Ca3(PO4)2CaF2 + 10H2SO4 + 5H2O 6H3PO4 +10CaSO4.1/2H2O + 2HF

2CaSO4.1/2H2O + 3H2O 2CaSO4.2H2O



Process Block Diagram:

Di-hydrate Process

Advantages:

1. Single stage filtration, simple in design.

2. Flexibility of rock phosphate.

3. Ease of operation.

4. Required lower grades of material of construction.

5. Low maintenance cost and high operating factors.

6. Easy transport of gypsum slurry.

Disadvantages:

1. Produces acid at 26%-32% P2O5.

2. Normally requires steam for evaporation.

3. Acid has high level of Al and F.

4. P2O5 efficiency 94%-96%.

5. Requires weak acid storage and evaporation.



B] Hemihydrate Process

Process description:

The storage is similar to those of the di-hydrate process but grinding may be

unnecessary. Reaction occurs and operating conditions are selected in this process so

that the calcium sulphate is precipitated in the hemihydrates form. It is possible to

produce 40%-52% P2O5 acid directly, with consequent valuable savings in energy

requirements. Hemihydrate crystals tend to small and less well formed thus

hemihydrates slurries tend to be more difficult to filter.

Process block diagram:

Hemihydrate Process

Advantages:

1. Single stage filtration.

2. Produces strong acid directly 40%-48% P2O5.

3. No intermediate storage if acid is produced at user strength.

4. Uses coarse rock.

5. Ease of operation.



Disadvantages:

1. Limited number of rocks process industrially.

2. Large filter area required for 48% P2O5 acid.

3. High lattice loss, low P2O5 efficiency [90%-94%]

4. Produces impure hemihydrates.

5. Tight water balance.

6. Requires higher grade alloys.

7. Care required in designing and shut down.

C] Hemihydrate – Di-hydrate Process


It is possible to obtain 40%-52% P2O5 acid directly, by acidulating under hemihydrate

conditions and separating hemihydrate before recrystallization, in this process. Thus

the sequence is as follows:

i. Acidulate under hemihydrate conditions.

ii. Separate product.

iii. Recrystallize hemihydrates to di-hydrate.

iv. Filter and return liquors to process.

The additional filter and the other equipment required, add to the typical cost of the

plant enable savings to be made on evaporation equipment.

Advantages:

1. It produces strong acid directly [40%-52% P2O5]

2. It produces pure acid (low SO4, Al, F)

3. Limited post precipitation.

4. Uses coarse rock.

5. Low sulphuric acid consumption.

6. High P2O5 efficiency [98.5%]

7. Produces a pure gypsum.



Disadvantages:

1. Two stage filtration, lower utilization.

2. Limited number of rock processed industrially.

3. Care required in design and shut down.

4. High recrystallization volume.

5. High capital cost investment.

D] Di-hydrate – Hemihydrate Process


In this process, although the reaction runs under di-hydrate conditions, it is not

desirable to affect a very high degree of P2O5 recovery, during the separation of the

acid from di-hydrate. The succeeding dehydration stage requires around 20%-30%

P2O5 and 10%-20% sulphuric acid. The strength of product acid is 32%-35% P2O5.

The sequence is as follows:

i. Acidulate under hemihydrate conditions.

ii. Separate product.

iii. Recrystallize hemihydrates to di-hydrate.

iv. Filter and return liquors to process.

Advantages:

1. Flexible as to rock source.

2. Proven process.

3. Produces a pure hemihydrates.

4. High P2O5 efficiency (98%)

5. Higher acid strength [32%-36% P2O5]

6. Lower sulfuric acid consumption.

7. Gypsum may be used directly for plaster board or as a cement retarder, after

the additional of lime and rehydration in a storage pile.

Disadvantages:

1. Two stage filtration, lower utilization.

2. High capital cost investment.



3. Requires steam for conversion.

4. Requires 35% acid storage and evaporator.

5. Rock slurry feed is unacceptable.

6. Requires final rehydration of hemihydrate to di-hydrate gypsum.

7. Normally, requires rock grinding.

8. Care required in design and shut down.

9. Requires sophisticated material of construction.

F] Hemihydrate Recrystallization Process


The acidulation occurs at hemihydrate conditions, the hemihydrates are then directly

recrystallize to di-hydrate without intermediate hemihydrate separation as separate

product. The flow diagram of this process resembles that of the multiple reactor di-

hydrate process with the exception that the attack reactor operates under hemihydrate

conditions, while succeeding reactors operate under conditions favouring the

rehydration of hemihydrate to di-hydrate gypsum. This is encouraged by seed di-

hydrate crystals recycled in the slurry form the filter feed. The product is no more

concentrated than that obtained from di-hydrate gypsum but the gypsum is much

purer.

Advantages:

1. Single stage filtration.

2. Proven process with sedimentary rock.

3. Produces a pure gypsum.

4. Higher P2O5 efficiency (97%).

5. Slightly higher acid strength [30%-32% P2O5]

6. Lower sulfuric acid consumption.

7. Lower filter area.

Disadvantages:

1. Requires a fine rock grind.

2. Requires sulfuric acid dilution.

3. Post precipitation before and after evaporation.



2] Acidulation of rock phosphate using HCl (IMI process):

Reaction:

Ca3(PO4)2CaF2 + 20HCl 6H3PO4 +10CaCl2 + 2HF


Phosphate rock is ground to 20 mesh screen and fed into a dissolver where an acid

stream of conc. HCl plus make up wash water from the counter current decantation

system is added. Fumes of CO2, HF, HCl are scrubbed for acid recovery.

The mixture is fed to series of decantation units with overflow from the first settler

moving to the counter current solvent extraction operations. The solid underflow goes

to 2-3 washing thickners.

Extraction of H3PO4 plus some free HCl is done in a battery of mixer settlers (a) with

CaCl2 + CaF2 retained in aqueous phase. The extract is again passed through several

more mixer settlers (b) for removal of trace impurities of Ca++ which are co-extracted.

The extractant is aqueous reflux from the next unit transfer extractors (c) where water

extracts H3PO4 and HCl from solvent phase.

The washed solvent is recycled to a final series of mixer settlers (d) where trhe

balance of HCl is extracted from raffinate phase of extractors (a). the acid free brine is

send to steam stripping for solvent recovery. The aqueous acid raffinate from (c) is

separated and concentrated in a triple effect evaporator to give three different

overhead streams:

i. Alcohol-water overhead flash from the first effect which is condensed and

returned to evaporators (a).

ii. Dilute HCl from the second effect.

iii. Conc. HCl from the third effect.

Advantages:

1. Advantage of this method is it produces less impurity level in phosphoric acid.

2. Phosphoric acid obtain from this process is >50 wt% P2O5.

Disadvantages:

1. Due to the use fo HCl corrosion problem occurs.



3] Thermal Process:

A] Using phosphate rock and blast furnace

Reaction:

Ca3(PO4)2 + 3SiO2 + 5C 2P + 5CO + 3CaSiO3

2P + 5CO + 5O2 P2O5 + 5CO2

P2O5 + 3H2O 2H3PO4 85-90% yield


Phosphate rock is pulverized and mixed with coke powder and binder is compressed

to 5000 psi resulting into the briquettes. Briquettes are dried and charged along with

sand and additional coke powder from top of the blast furnace. The preheated air

(1000 – 11000C) is charged from bottom of the blast furnace via tuyere. A tuyere is

cooled copper conical pipe numbering 12 in small furnace and up to 42 in large

furnace through which hot air is blown in to the furnace. Preheated air leads to

burning of briquettes giving temperature rise up to 13700C. The coke acts as reducing

agent as well as fuels. About 760kg of coke is consumed in reduction of phosphate

rock to phosphorous and remaining generates heat by combustion with air. Reaction is

completed in the furnace itself producing P2O5 and calcium silicates as slag. The

product gases also contain carbon monoxide and nitrogen along with dust particles.

For purification, it is passed through cyclone separator and phosphorous condenser.

Thus, P2O5 and elemental phosphorous are separated out. Hot P2O5 gases are cooled

in the heat exchanger. Therefore, superheated steam is produced and a part of gas is

taken into regenerative blast furnace. As a result the entire phosphorous and

phosphorous pentoxide is cooled and purified before taken into hydrating towers.

Purification of phosphoric acid includes removal of arsenic by hydrogen sulfide

treatment followed by filtration.



Process flow diagram:

Manufacturing of phosphoric acid using Blast furnace

Advantages:

1. Rock phosphate varying in impurity like sand is acceptable

2. Purity of phosphoric acid obtained by this process is over 85%

3. The yield of phosphate is about 85%-90%

4. There is no formation of gypsum as in acidulation process

5. Resulting phosphoric acid can be used in manufacturing of insecticides,

pesticides, detergents

Disadvantages:

1. Maintenance cost of blast furnace is much more than installation cost

2. It is difficult to maintain temperature over 1000 0C in blast furnace using

preheated air

3. About 750 kg of coke is consumed per 1000 kg of phosphoric acid produced

wich is much more than electric furnace process.

4. Briquette binder can affect the product purity and its rate of formation

5. Product phosphoric acid can not be used for fertilizer production



B] Using phosphate rock and electric furnace

Reaction:

Ca3(PO4)2 + 3SiO2 + 5C 2P + 5CO + 3CaSiO3 ΔH= -364.8 kcals

2P + 5CO + 5O2 P2O5 + 5CO2

P2O5 + 3H2O 2H3PO4 85-90% yield ΔH = - 44.9 kcals


The phosphate rock is reduced to elemental phosphorous by the action of coke and

heat in the presence of sand in electric arc furnace subsequent oxidation of

phosphorous gives phosphorous pentoxide which on hydration gives the product

phosphoric acid.

Phosphate rock after proper grinding and primary purification is taken into sintering

oven where it is nodulized and granulized so that fast oxidation of the separated

phosphorous takes place. Temperature of 1095 0C is maintained in electric furnace so

that maximum amount of elemental phosphorous extracted out and oxidation takes

place. Since fluoride of phosphorous and calcium are the common impurity which

reacts with sand giving flourosilicates as the slag.

The gases from the furnace, phosphorous and carbon monoxide are removed by the

suction process and the oxidation product P2O5 is taken into hydration column which

gives P2O5 to H3PO4 at about 85 0C. Purification of phosphoric acid is carried out by

H2S to remove Arsenic, H2SO4 to remove calcium salts and Silica to remove

fluorides. All the byproducts are removed before concentrating the acid and filtering it

as final product.




Manufacturing of phosphoric acid using electric furnace

Advantages:

1. Low grade phosphate rock can be used for this process

2. Iron and aluminium oxides are not objectionable as in wet process

3. The by product carbon monoxide is used as a fuel for calcinations

4. Purity of phosphoric acid obtained is over 85%

5. Yield of phosphate is about 87%-92%

Disadvantages:

1. It is difficult to maintain 2400 0F temperature in electric furnace

2. Process requires large electricity to maintain high temperature in electric

furnace

3. Electrode consumption for this process is also too high

4. Water is kept over phosphorous to avoid direct contact with the air because

phosphorous oxidizes vigorously

C] Oxidation and Hydration of phosphorous

Reaction:

2P + 2½O2 P2O5

P2O5 + 3H2O 2H3PO4 (94 – 97% yield)



Process description

At the locations away from phosphate rocks mines from purified elemental

phosphorous is oxidized and hydrated to give phosphoric acid. In the manufacturing

process molten phosphorous is sprayed into combustion chamber along with

preheated air and superheated steam. Combustion of phosphorous increases the

temperature up to 1980 0C. Furnace design depends on the requirement with respect to

quantity and quality. They are made of acid proof structural bricks, graphite, carbon

and stainless steel.


Manufacturing of phosphoric acid by oxidation and hydration of phosphorous

Advantages:

1. Phosphoric acid concentration is over 85%

2. Yield of the phosphate is about 94%-97%



3. Resulting of phosphoric acid can be used for manufacturing of insecticides,

pesticides & detergents

Disadvantages:

1. Process utilizes large amount of steam, which causes to increase the capital

cost investment for the process

2. Glass wool filter is so costly and delicate equipment

3. Product phosphoric acid can not be used for fertilizer production

4] Acidulation of rock phosphate using nitric acid

Reaction:

Ca10(PO4)6F2 + 20 HNO3 6 H3PO4 + 10 Ca(NO3)2 + 2HF


When phosphate rock is treated with nitric acid, it forms phosphoric acid and soluble

calcium nitrate. Rock phosphate is crushed in ball mill. Acidulation of crushed rock

phosphate is processed in reaction vessel by using 40%-70% concentrated nitric acid.

The reaction temperature is 120 0C- 130 0C and the reaction time ranges from 1-2.5

hrs crystal of calcium nitrate are form along with phosphoric acid. These crystals are

separated from phosphoric acid by using filtration. Phosphoric acid is concentrated in

evaporator. The product phosphoric acid contains 55%-72% P2O5.

Advantages:

1. For manufacturing of phosphoric acid wet process are more popular due to

increase demand for higher grade fertilizer

2. It is energy saving process as comparison with thermal process

3. Less expensive process as comparison with thermal process

Disadvantages:

Though thermal process is expensive but it gives extremely pure phosphoric acid, but

this process produces phosphoric acid of 55%-72% P2O5.



COMPARATIVE STUDY OF ALL THE PROCESSES

Sr

No

Name of the

process

Capital

investment

Phosphoric

acid conc.

P2O5

efficiency

Energy

requirement

(/ton P2O5)

By-product

information

1 di-hydrate 29 crore 26%-32% 94% 125 KWH 60% gypsum

2 Hemihydrate

- Di-hydrate

31 crore 40%-52% 98.5% 110 KWH 95% gypsum

3 Di-hydrate -

Hemihydrate

31 crore 32%-36% 98% 110 KWH 95% gypsum

4 Hemihydrate 25 crore 40%-52% 90%-94% 100 KWH 40% gypsum

5 Hemihydrate

recrystalli-

zation

30 crore 30%-32% 97% 115 KWH 80% gypsum

6 Using rock

phosphate

and HCl

35 crore 70%-72% 95% 250 KWH Insoluble

CaCl2

7 Using rock

phosphate

and electric

furnce

45 crore 85% 87%-97% 4070 KWH NA

8 Using rock

phosphate

and blast

furnce

42 crore Over 85% 85%-90% 3000 KWH NA

9 By oxidation

and

hydration of

phosphorous

45 crore 85%-87% 94%-97% 150 KWH NA

10 Using rock

phosphate

HNO3

35 crore 65%-72% 92%-96% 270 KWH Ca(NO3)2 as

waste



Chapter no 4: Justification for Selection of Process

The choice among the processes available can be made only after intensive study, but

influence mainly by the inter-related factors:

A] Process Reliability:

This is the most important factor influencing the selection. The process reliability

depends upon the efficiency of plant, ease of operation, severity of operating

conditions and maintenance down time after the reliability of a process. The

hemihydrate – di-hydrate process for production of phosphoric acid using rock

phosphate and sulfuric acid operates at less severe condition and has ease of operation

and lower cost of maintenance, thus increasing the reliability.

B] Adaptability for Various Rock Phosphates:

The hemihydrate – di-hydrate process has been shown flexibility to use different types

of rock phosphates without affecting the plant efficiency and the plant capacity. With

the increasing price and decreasing quality of rock phosphate, the hemihydrate – di-

hydrate process give an easily filterable calcium sulphate with different types of rock

phosphates.

C] By-product Quality and Disposal:

The hemihydrate – di-hydrate process produces excellent quality of gypsum as a by-

product which can further used in cement production and other uses. Other

acidulation processes using H2SO4 produces relatively poor quality of gypsum which

creates problem for filtration. Acidulation process using HCl (IMI process) produces

calcium chloride as by-product which creates disposal problem. A poor quality of

gypsum means cost of effluent treatment is high.

D] P2O5 Recovery:

P2O5 recovery and gypsum quality are inter-related terms. A higher recovery results in

better quality of gypsum. A higher P2O5 results in better quality of gypsum produced.

Di-hydrate process yields a lower recovery as compared with hemihydrate – di-

hydrate process. The hemihydrate – di-hydrate process has high recovery of P2O5 like

electric furnace process.



E] Capital Investment and Maintenance Cost:

The hemihydrate – di-hydrate process poses near about same capital investment and

maintenance cost as other acidulation process, but this process has very low capital

investment and maintenance cost as compared to thermal acid process. This process

has ease of control system hence the maintenance cost is also lower than electric

furnace process.

F] Energy Requirement:

The hemihydrate – di-hydrate process has same energy requirement as other

acidulation process using sulfuric acid, but it has shown very less energy requirement

as compared to thermal acid process. The hemihydrate – di-hydrate process shown

that 110 KWH per ton of P2O5 production, where thermal acid shown that 4070 KWH

required per ton of P2O5 production.

G] Raw Material Cost:

Rock phosphate is used as main raw material in the hemihydrate – di-hydrate process

is very much cheaper than the elemental phosphorous used in manufacturing of

phosphoric acid using oxidation and hydration process. Sulfuric acid is also compared

to hydrochloric acid and nitric acid.

By considering all the above points along with advantages and disadvantages of each

and every process we have selected the hemihydrate – di-hydrate process for our

project work



Chapter no 5: Details of Selected Process

A] Process Description

I] Phosphate Rock Grinding System:

Unground rock from rectangular unground concrete silo in the rock phosphate silo

area is fed to vibratory feeder. From the vibratory feeder the belt conveyor feds to

bucket elevator which discharges unground rock phosphate into the main service

unground rock phosphate bunker in the grinding unit building.

The unground rock phosphate from unground rock phosphate bunker is fed by disc

feeder to the ball mill. The capacity of this ball mill is 40 MT/hr. the crushed rock

phosphate from the ball mill enters the classifier, via classifier feeder and the riser

pipe. The coarser particles are fed back to the ball mill from classifier, while dust–

laden air enters the cyclone, for the separation of proper size of ground rock

phosphate. The fine dust from the cyclone is recycled back to the suction of main

blower to atmosphere. A constant purge is maintained from the discharge of main

blower through dust collector.

The fine dust collected from the dust collector is mixed with the main stream of

ground rock by screw conveyor and rotary air lock. For the purpose of drying rock hot

air is introduced into the ball mill by air heater. Nominal capacity of ground rock

phosphate silo is 350 tones of ground rock.

II] Ground Phosphate Rock Handling System:

Ground rock is transferred from the silo to the ground rock service bunker by

pneumatic pump. Ground rock phosphate service bunker is fitted with the dust

collector at the top with the high and low level alarm. The bunker has hold up

capacity of 90 MT of ground rock phosphate. Process rock from the service bunker is

drawn at constant rate by a continuous weight feeder and is fed to premixer through

screw conveyor.

The smooth flow of ground rock phosphate is kept by the aeration unit in the ground

rock phosphate service bunker. It is desirable to check the accuracy of integrating

mechanism of the weight feeder periodically.



Operating condition for ground rock feeding:

a) Particle size of ground rock:

minimum 90% thru 100 tyler mesh

minimum 70% thru 200 tyler mesh

b) Rock Analysis:

prior to the feeding of rock phosphate analysis of following component should be

made:

moisture : 1.5% (max)

CaO : 52% (max)

P2O5 : 33% (min)

SO3 : 1.4% (max)

c) Rock feeding method:

Feeding of the rock should be continuous and at constant rate consistent with the

feeding of the acid. This is necessary to prevent imbalance of rock/acid ratio taking

place in the reactor

III] Sulfuric Acid Dilution and Return Acid System:

a) Sulfuric acid system:

The sulfuric acid dilution system can be operated at 98% sulfuric acid and dilution

water or mixture of 98% sulfuric acid and 64% sulfuric acid available from nitric acid

concentration plant.

Case 1: when 98% sulfuric acid exclusively used, it is pumped from storage to the

head tank. 98% sulfuric acid from the head tank and deminerlized water and dilution

water are fed at predetermined rates through flow controller to the graphite mixing

block to ensure that diluted acid is of 75% strength. The temperature of diluted acid is

controlled by recirculation of process water. The diluted acid is mostly passed through

mixing vessel.



Case2: when 64% sulfuric acid is available from HNO3 concentration plant, it is fed

to head tank. Acid would be drawn at constant rate by the flow controller and fed to

sulfuric acid mixing vessel. Where in it get mixed with the 98% sulfuric acid. Amount

of dilution water required to obtain 75% sulfuric acid are mixed and cooled in dilution

cooler, similar to case 1.

b) Return acid system:

The temperature of return acid in the return acid calibration tank is regulated by the

flow ratio of steam by premixer temperature recording controller.

The return phosphoric acid is adjusted to a designated level of P2O5 concentration i.e.

18%-21% P2O5 and metered within a accuracy of ±0.5% by flow controller before

feeding to the premixer through mixing vessel.

The return acid contains dissolved or supersaturated SiO2, CaSO4.2H2O and the

temperature of acids is about 450C – 500C under the normal conditions when this

return acid is mixed with the hot sulfuric acid (80 0C). the temperature of resultant

mixed acid is between 60 0C – 75 0C. The solubility of gypsum in the consequently

solid gypsum is separated from the mixed acid.

Operating conditions for sulfuric acid & return acid:

i) Concentration of sulfuric acid fed:

75% sulfuric acid in case 1

72.5% sulfuric acid in case 2

ii) Concentration of return acid:

18% -21% with permissible deviation in the concentration from the actual operating

design 0.5% range.

iii) Temperature control:

In case 1, the temperature of diluted sulfuric acid is controlled by adjusting flow rate

of cooling water to the sulfuric acid dilution cooler automatically and set of

temperature control adjusted so that to maintain the temperature of premixer at

designated level.



In case 2, the temperature of premixer controlled by adjusting flow rate of steam into

return acid calibration tank automatically and the index of temperature controller

should be set at the designated temperature.

IV] Premixer and Digester Section:

Dilute sulfuric acid and the return acid enter the mixing vessel with agitator which

ensures adequate mixing of acids. The mixed acids enter the premixer along with rock

phosphate through screw conveyor and whole mass is agitated by the agitator. The

phosphoric acid and the hemihydrates gypsum is formed in premixer and digestor

under carefully controlled conditions of reaction.

This condition associated with the following factors, individually and in combination:

a) Reaction temperature

b) Combination of sulfuric acid and P2O5 concentration of acid mixture

c) % Decomposition of phosphate rock attained under these stage

d) SO3 / CaO ratio of the slurry

e) The total retention time in this stage i.e. the reaction time

Factors a and b individually or in combination have significant effect on the proper

formation of hemihydrates calcium sulfates. The temperature condition of 90 0C -

920C (±2 0C) is the optimum temperature in the premixer and digestor. Factor c under

the decomposition stage, a certain minimum % of P2O5 in the rock phosphate must

become water soluble. This % defined by the formula,

Where A = total P2O5 in Gypsum cake

B = water soluble P2O5 in gypsum cake

C = total CaO in gypsum cake

X = total P2O5 in phosphate rock

Y = total CaO in Phosphate rock

% decomposition = [ 1- { [(A-B)/C] / [X/Y] } ] * 100



The objective of the factor of the process is for high P2O5recoveryand the production

of quality gypsum.

Factor d, the SO3/ CaO ratio of the slurry also influences the formation of large, well

shaped crystal of gypsum.

Factor e, the total retention time in the decomposition stage is associated with the

stability of hemihydrate gypsum. The Nissan process requires hemihydrates gypsum

should be formed in this stage. The hemihydrate stage should be transient stage which

is followed by formation of dehydrate gypsum through hydration and

recrystallization.

Operating conditions:

a) Temperature of slurry in premixer: 90 0C (accuracy of temperature control ± 2 0C)

b) Temperature of slurry in digestor: 90 0C (accuracy of temperature control ± 2 0C)

c) Decomposition % of Rock phosphate: 80%-90%

d) Hydration ratio of CaSO4 : 0.5 – 0.7

e) Temperature controller:

The temperature of the premixer controlled by the aid of the temperature recording

controller by varying the temperature of the feed sulfuric acid in case 1.

In case 2, the temperature of premixer is controlled by the aid of temperature

recording controller by varying the temperature of the feed return acid. The

temperature of the digestor should be kept at 90 0C- 98 0C by regulating the volume of

draught air effecting surface cooling of the slurry in these tanks.

V] Crystallization Section:

The slurry through the last digestor processed through the three crystallizers in series.

These vessels are agitated and connected by launders and here the hydration and

crystallization of the gypsum is complete. The slurry is air cooled by an air blower,

duct work and dispatches into each crystallizer. The slurry from the third crystallizer

is collected in slurry pumping tank by slurry transfer pump to the filter feed splitter

box, back to number 1 crystalleizer where it helps cooling and crystallization.



Slurry is also metered to filter and the excess over flows back to number 3

crystallizer. The total volume and the distribution of air to each crystallizers are

controlled by dampers on air lines. Fumes are removed from the crystallizers by two

numbers of exhausts fans and passed to atmosphere through the exhausts stack. The

pressure in each crystallizer is kept slightly negative. The temperature in the

crystallizers is controlled to maintain optimum declining temperature gradient for re-

crystallization and hydration. It is removed by temperature recorder.

The solubility of calcium sulfate in its hemihydrate state rises rapidly with the drop in

the temperature. The solubility of the di-hydrate calcium sulfate with maximum at

about 50 0C is for less than hemihydrate in the crystallization stage, the slurry of

relative high temperature containing hemihydrate calcium sulfate is cooled to a

temperature below the hemihydrate – di-hydrate transition point. Solubility of

hemihydrate increases considerably.

In the actual operation of Nissan process, however, the solubility difference does not

act directly as a motive force. The recrystallization of the hemihydrate to di-hydrate is

dependent not only on the temperature but also on the ratio of the recycled feed slurry

and the ratio of the recycled feed slurry and the ratio of new slurry feed respectively

to slurry retained on the crystallizer. The actual motive force promoting the

recrystallization is subject to the degree of super saturation of di-hydrate gypsum. The

hydration and crystallization are frequently influenced by minor constituents of the

rock phosphate, such as metallic sesquioxides, SiO2, fluorine and organic matter. As it

is apparent considerable hydration heat is generated in this stage, necessiting a fairly

large quantity of heat to be removed. This is accomplished in practice by air cooling,

in the crystallization stage, the following factors are to be considered:

1. Retention time

2. Hydration temperature

3. Recycle ratio of feed slurry

4. Free sulfuric acid content in filtrate acid

Most important factor in the Nissan process can be said to be the retention time. The

term retention time is defined as the average retained time of the slurry in series of

reaction vessels ignoring the effects of such recycle. This is expressed as:



Q = V / F

Where, Q = Retention time

V = Effective volume of crystallizer

F = feed rate of slurry from the decomposition

The necessary retention time in the crystallization is determined based on the

reactivity of rock to be used.

As previously mentioned, at low temperature di-hydrate gypsum would be formed

under the some concentration is held constant. The temperature required for the

formation of di-hydrate gypsum from the hemihydrate calcium sulfate can be found

on the transition temperature curve.

This temperature taken from reffering calcium sulfate phase diagram is somewhat

higher than actual temperature applied in Nissan process. This difference is due to the

fact that the rate of hydration of hemihydrate calcium sulfate is accelerated at the

lower temperature than the conversion point. As it is well known, when the degree of

super saturation is too high numerous crystals of very fine size are formed, a tendency

of undesirable for manufacturing of phosphoric acid. The actual process conditions

applied to crystallization stage has been established on consideration of the above two

factors.

That is crystallizer number 1 in order to minimize the hemihydrate supersaturation

and to do prevent the formation of fine crystal nuclei, a higher temperature than the

optimum is applied. In crystallizer number 3 a temperature necessary for quik and

complete hydration is selected. With respect to factor 3, the ratio of the recycle slurry

is between 1:1 and 2:1. The slurry recycled from the crystallizer number 3 accelerates

crystallization by supplying seed crystals. When a proper recycle ratio is selected

hydration can be easily promoted two times or quicker as compared with the

conditions of no feed recycle or improper recycle.


a) Temperature of Crystallizers: 65 0C – 70 0C



Typical temperatures of crystallizers:

i. Crystallizer no 1: 63 0C

ii. Crystallizer no 2: 58 0C

iii. Crystallizer no 3: 53 0C

iv. Crystallizer no 4: 53 0C

b) Decomposition %:

Minimum: 98% (for Algeria and Tunisian rock)

98.5% (for Morocco, Jordan, Florida rock)

c) Hydration ratio: 1.90

d) Slurry recycle ratio: 1:1 to 2:1

VI] Filtration Section:

The phosphoric acid and the gypsum are separated by filtration using horizontal

vacuum belt filter. The slurry from the crystallizer containing phosphoric acid along

with the di-hydrate gypsum is passed to slurry pumping tank. The tank is cylindrical

with the agitation, completely lined vessel of the size 4500 mm *4000 mm height

having hold up capacity 56 m3. Slurry pumping tank pumps the slurry to filter feed

splitter box. The filter feed splitter box is a vertical rectangular type of size 1500mm *

1200 mm * 1100 mm (l*b*h) having hold up capacity of 0.4 m3.

The purpose of the splitter box is to maintain uniform flow rate of hydrated slurry to

the filtration unit and excess supply of the slurry is fed back to the crystallizer. The

filter used is horizontal vacuum belt filter, having effective filtration area is 29 m2.

The slope of filtration unit is 0.6%, so adjusted in such way that the discharge end of

the filter is slightly higher than the feed end. The vacuum is created by barometric

condenser in the downstream which results into suction of phosphoric acid from

gypsum is collected into three receivers.

The phosphoric acid from the first receiver having 30% P2O5 acid is pumped to the

clarifier and the phosphoric acid from the second receiver having 18% - 20% P2O5 is

sent to the return acid tank. Phosphoric acid from third receiver is having 5% P2O5 is

sent to 1st acid wash of the gypsum. The liquor passed from all the sections of the



filter under vacuum i.e. slurry feed acid wash, water wash, cake drying, cloth drying

section all have entrained in them. The air from the filtrate separator on the slurry feed

acid wash and the water wash sections of the filter passes to primary spray condenser

which removes any acidic vapours that may be in the air by scrubbing with process

water.

The decontaminated air then enters the primary vacuum pump. A similar arrangement

applies for the cake drying section of the filter which has its own separator and

secondary spray condenser and it is connected to secondary vacuum pump. The

process water leaving the scrubber and passes to the sealed tank respectively for

disposal.

Principal of washing of filter cake:

1) The washing effect is dependent mainly on two factors below under the conditions

of cake thickness of the vacuum applied being considered to be constant. These

factors are E, displacement efficiency and ‘n’ is washing ratio. Washing ratio is

defined as :

n = (volume of washing fluid) / (volume of liquor cake)

Displacement efficiency E is defined as % of soluble matter washed of the filter cake

to the total soluble matter before washing ratio n = 1, is assumed to be applied and

this factor E is characteristic to the filter cake itself in gypsum cake washing, this

depending on crystal size, uniformity of crystals size distribution etc.

Taking the remaining ratio of the soluble matter in the cake as ‘R’ is given as follows:

R= { 1 – [E/100] }^n

Where R = remaining ratio of soluble matter in the cake

E = displacement efficiency

n = washing ratio



Thickness of filter cake:

Cake thickness has significant meaning on washing effect. Cake washing time is

required period for the time of disappearing of washed liquor from the surface of the

cake. Cake drying time which is characteristic of filter cake and co-relating with the

thickness of such cake is also necessary subsequent to the cake washing time and

prior to the further washing of it. Minimizing remaining water soluble P2O5 is subject

to the lowering the liquor content in the filter cake prior to its washing. Under the

production requirement certain amount of dry solid must be separated within certain

limited time.

Operating conditions for filtration:

a) Temperature of wash water: 50 0C – 65 0C

b) Accuracy of flow meter slurry, wash water and wash acid: ±2%

c) Vacuum required: 160 mmHg abs (maximum)

d) Thickness of filter cake : 30 mm (minimum)

Reaction:

3Ca3(PO4)2CaF2 + 10H2SO4 + 5H2O 6H3PO4 +10CaSO4.1/2H2O + 2HF




B] Process Block Diagram



CHAPTER NO 6:

THERMODYNAMICS AND REACTION KINETICS

A] THERMODYNAMICS

Introduction

Thermodynamics is a branch of physics concerned with heat and temperature and

their relation to energy and work. Thermodynamics checks the feasibility of the given

reaction by using Gibb’s Free energy. Gibbs free energy can be calculated by using

following formula:

∆G = ∆H - T∆S ……….[A]

Significance:

1. If ∆G is negative then, the proportion of products present will be large.

2. If ∆G is positive then, the proportion of products present will be small.

3. If ∆G is zero then, there will be equal proportion of products and reactants.

4. If ∆G is more negative than -40kJ/mol then, the reaction is regarded as having

gone to completion.

5. If ∆G is more positive than about +40kJ/mol then, the reaction is regarded as

not occurred at all.

The formation reaction of phosphoric acid by hemihydrate- dihydrate process (Nissan

Process) using sulfuric acid and rock phosphate is as follows:-

3Ca3(PO4)2CaF2 + 10H2SO4 + 5H2O 6H3PO4 + 10CaSO4.1/2H2O + HF ………(1)

2CaSO4.1/2H2O + 3H2O 2CaSO4.2H2O ………(2)

Where reaction (1) is carried at 920C in premixer and digester and reaction (2) is

carried out at 570C in crystallizers.



Calculations

1. Standard values (at 250C) of heat of reaction(∆Ho) and entropy (∆So) for all

component presents in the given reactions is given below:

Sr.

No.

Components ∆Hfo(kJ/mol) ∆So(kJ/mol)

1. Ca3(PO4)2CaF2 -1506.88 0.19056

2. H2SO4 -814 0.1569

3. H2O -258.83 0.06991

4. H3PO4 -1254.36 0.15062

5. CaSO4.1/2H2O -1576.74 0.13054

6. HF -271.12 0.173.68

7. CaSO4.2H2O -2022.63 0.19414

2. Data of specific heat capacity (Cp) for all the component present in the

reaction is as follows:

Components Cp. dT .dT

Temperatures 920C 570C 920C 570C

Ca3(PO4)2CaF2 -0.1873 0.1665 0.028 0.0164

H2SO4 0.1315 0.0628 2.55×10-3 1.617×10-3

H2O 2.3092 1.1022 0.0107 0.0067

H3PO4 0.00221 0.00105 0.043×10-3 0.027×10-3

CaSO4.1/2H2O -0.0167×1010 -0.00371×1010 -614×103 -205×103

HF 2.967×103 -1.067×103 -12.4 -6.25

CaSO4.2H2O 13.063 6.239 0.254 0.16

3. Heat of reaction (∆Hf) can be calculated at any temperature using the formula:

∆Hf = ∆Hfo+ ∆Cp. dT ………(3)

For reaction (1)

Cp. dT = ƩCp.dTproduct - ƩCp.dTreactants

= -1.6699×109 – 12.2991 = -1.67×109 kJ/mol.K



∆Hfo = ƩHf

oproduct- ƩHf

oreactant

= -20612 +14089.79

= -6522.21 kJ/mol

From equation (3),

∆Hf = -6522.21 + -1.67×109

∆Hf= -1.67×109kJ/mol

Similarly, for reaction (2)

∆Hf= 73.98×106 kJ/mol

Also,

∆S = ∆S0 + ∆

.dT ………(4)

∆S0 = ƩS0product - ƩS0

reactant

= -8.62×10-3 kJ/mol

∆.dT = Ʃ .dTproduct -Ʃ .dTreactant

= -6.14×106 kJ/mol

From equation (4),

∆S = -8.62×10-3 + -6.14×106

∆S = -6.14×106 kJ/mol

Similarly, for reaction (2)

∆S = 0.409×106 kJ/mol



From equation [A],

∆G = ∆H - T∆S

1. For reaction carried out at temperature of 920C (365 K)

∆G1 = ∆H - T∆S

= (-1.67×109) – 365(-6.14×106)

∆G1 = -3.91×109kJ/mol

2. For reaction carried out at temperature of 570C (3300C)

∆G2 = ∆H - T∆S

= (73.98×106) – 330(0.409×106)

∆G2 = -60.99×106 kJ/mol

From the value of Gibb’s Free energy of both the reactions, we conclude that both

reactions are feasible and goes to completion.

B] REACTION KINETICS

Introduction

The formation reaction of phosphoric acid is completed in two steps. The first step is

the first peak to dissolution of Fap and neutralization H2PO4- into H3PO4, and the

second step to precipitation of CaSO4.1/2H2O according to the following reaction:

For First Peak Reaction

Ca10(PO4)6F2(s) +20{H+}sol → {10Ca+2 + 6H3PO4 + 2HF}sol 1st step

{Ca2+}sol + {SO42-}sol + 1/2 {H2O}sol → CaSO4·1/2H2O(s) 2nd step

With mean iteration enthalpy values as -15.0 and --12.0 kJ per mole of Ca-sulfate,

respectively.

For Second Peak

CaSO4.0.5H2O(s) + 3/2 H2O → CaSO4.2H2O(s)



The dissolution rate of HH, rdis, is proportional to under saturation (Ce –[Ca]) where

Ce is the Ca concentration corresponding to solubility of HH, whereas precipitation

rate of DH, rpp, is of order rate 2 for Ca concentration, and so,

rdis = k1(Ce − [Ca2+]) and ………..(1)

rpp = k2 [Ca2+]2 .……….(2)

Where k1 and k2 are constant rate.

The global rate is controlled by the Ca2+concentration, so

= rdis– rpp……….(3)

d[Ca2+]/dt = k1(Ce - [Ca2+]) – k2 [Ca2+]2

Giving d[Ca2+]/dt + k2[Ca2+]2 + k1 [Ca2+] = k1Ce ………..(4)

On differentiating we get the following two solutions,

[Ca2]=[1/2k2][−k1+C1th{C1t/2+lnC2}] and ………..(5)

[Ca2+]=[1/2k2][−k1 + C1th{C1t/2 + ln(−C2)}] ………(6)

With C1 and C2 two constants depending both on k1, k2 and Ce.

C1 = [4k1Cek2 + k12]1/2 and ………..(7)

C2 = [1/2k2Ce][4k1C1−2k12 - 8k1k2 Ce −4C1Cek2−4Ce2 k2

2]1/2 ………..(8)

Let Q1 and Q2 denote the heat quantities released by hemihydrate dissolution and

dehydrateprecipitation reactions respectively. Variation of each of these quantities is

proportional tothe corresponding reaction rate as,

dQ1/dt = VΔdisHk1(Ce − [Ca2+]) and ………..(9)

dQ2/dt = VΔppHk2[Ca2+]2 ……...(10)

With ΔdisH and ΔppH the molar enthalpies of dissolution of HH and precipitation of

DH, respectively.



The total energy is Q = Q1 + Q2 and is variation is expressed as,

dQ/dt = VΔdisk1 (Ce − [Ca2+]) + VΔppΗk2[Ca2+]2 ………(11)

The final heat flow expression is expressed as,

dQ/dt = VΔdisHk1 [Ce −(1/2k2)(− k1 + C1th{C1t/2 + lnC2})]

+ VΔppHk2[(1/2k2)(−k1 + C1th{C1t/2 +lnC2})]2 …….....(12)

After performing the iterations on these expressions, we obtain the following values

of six parameters,

ΔHdis = 12.30 kJ/mol, Ce = 3.27×10-4mol/L

ΔHpp= -24.90 kJ/mol, C2 = 59.33

We obtain,

k1 = 5.00×10-5 s-1 ………{reaction (1)}

k2= 1.99×10-3L/mol.s ………{reaction (2)}



Chapter No 7: Mass and Energy Balance

A] Mass Balance

Introduction

A material balance is the accounting of all materials enters, leaves, accumulated or is

depleted in a process or in processes unit in a given time. All material balance

calculation are based on the law of conservation of mass which states that matter can

neither be created nor be destroyed during a process. The law of conservation of mass

can also be stated as,

1. The total mass of all substances taking part in a process remains constant.

2. Within a given isolated system, the mass of a system remains constant

regardless of the changes taking place within the system.

3. The total mass of various compartments remains constant during an unit

operation or a chemical reaction.

Hence, according to law of conservation of mass

INPUT = OUTPUT + ACCUMULATION

For steady state, accumulation=0

Therefore, INPUT = OUTPUT

Material balance further classified as,

1. Material balance without chemical reaction.

2. Material balance with chemical reaction.

General procedure for material balance:

1. Assume suitable basis for calculation.

2. Adopt weight such as gram or kgs in case of no chemical reaction.

3. Draw a block diagram for each equipment with inlet and outlet streams.

4. Search the unknown quantities or composition, flow rates.

5. In case of problem involving chemical reaction, search out a limiting

component.



6. The quantity of reacting component appearing in the product stream is the

quantity of that material remains unreacted.

PLANT CAPACITY: 150 MTPD

1MTPD = 1.1022 TPD = 1102.2kg/day = 45.925kg/hr

Therefore, 150MTPD = 70.3 kmol/hr

Basis: 70.3 kmol/hr of phosphoric acid leaving as product

The reaction takes place in premixer and crystallizer respectively, therefore we carried

the material balance across the following two units only.

A] Across the premixer/ digester:

Reaction:

3Ca3(PO4)2CaF2 + 10H2SO4 + 5H2O 6H3PO4 + 10CaSO4.1/2H2O + HF

Diagram:

F M

P

R

Where, F = Flowrate of total feed to the premixer

R = Flowrate of recycle feed

M = Flowrate of mixed feed

P = Flowrate of desired product

Premixer/

digestor Separation

by filter



Overall material balance:

M = F + R

To calculate the quantity of mixed feed, we use the stoichiometry principle to the

above equation,

i. For rock phosphate [Ca3(PO4)2CaF2]:

6 kmoles of H3PO4 ≡ 3 kmoles of [Ca3(PO4)2CaF2]

70.3 kmoles/hr of H3PO4 ≡ ? kmoles of [Ca3(PO4)2CaF2]

Therefore, kmoles of [Ca3(PO4)2CaF2] required = .

= 35.15 kmoles/hr

ii. For sulfuric acid [H2SO4]:

6 kmoles of H3PO4 ≡ 10 kmoles of H2SO4

70.3 kmoles/hr of H3PO4 ≡ ? kmoles of H2SO4

Therefore, kmoles of H2SO4 required = .

= 117.167 kmoles/hr

iii. For water [H2O]:

6 kmoles of H3PO4 ≡ 5 kmoles of H2O

70.3 kmoles/hr of H3PO4 ≡ ? kmoles of H2O

Therefore, kmoles of H2O required = .

= 58.583kmoles/hr

iv. For hemihydrate gypsum [CaSO4.1/2H2O]:

6 moles of H3PO4 ≡ 10 moles of CaSO4.1/2H2O

70.3 kmoles/hr of H3PO4 ≡ ? kmoles of CaSO4.1/2H2O

Therefore, kmoles of CaSO4.1/2H2O required = .

= 117.167kmoles/hr

v. For hydrogen fluoride [HF]:

6 moles of H3PO4 ≡ 2 moles of HF

70.3 kmoles/hr of H3PO4 ≡ ? kmoles of HF

Therefore, kmoles of HF required = .

= 23.433kmoles/hr



Therefore,

Total Mixed Feed (M) = kmoles of [Ca3(PO4)2CaF2] + kmoles of H2SO4

+ kmoles of H2O

= 35.15 + 117.167 + 58.583

= 210.9 kmol/hr

Now, recycle ratio ( ) = 1 : 1

Therefore, R= F

Overall material balance,

M = R+ F

Therefore, M = 2F

F = = 210.9/2 = 105.45kmol/hr

Therefore, R = F = 105.45kmol/hr

Actual amount of fresh feed required:

Basis:- 105.45kmol/hr of fresh feed,

The proportion of feed is,

Ca3(PO4)2CaF2 : H2SO4 : H2O = 3: 10: 5

Amount of [Ca3(PO4)2CaF2] = .

= 31.635 kmol/hr

Amount of H2SO4 = .

= 52.725 kmol/hr

Amount of H2O = 105.45 – (31.635 + 52.725)

= 21.09 kmol/hr



UNIT MATERIALS INPUT

(kmol/hr)

OUTPUT

(kmol/hr)

PREMIXER

Ca3(PO4)2CaF2 31.635 --

H2SO4 52.725 --

H2O 21.09 --

RECYCLE FEED 105.45 --

H3PO4 -- 70.3

CaSO4.1/2H2O -- 117.167

HF -- 23.433

TOTAL 210.9 210.9

B] Across the crystallizers:

Reaction:


Diagram:

F R

Where, F = Flowrate of feed in

R = Flowrate of product out

Basis: 117.167 kmol/hr of hemihydrate gypsum entering

To calculate the quantity of other substance by applying stoichiometry to above

equation.

i. For water [H2O]:

2 moles of CaSO4.1/2H2O ≡ 3 moles of H2O

117.167 kmol of CaSO4.1/2H2O ≡ ? kmol of H2O

Therefore, kmols of H2O = .

= 175.75kmol/hr

CRYSTALLIZER



ii. For dihydrate gypsum [2CaSO4.2H2O]:

2 moles of CaSO4.1/2H2O ≡ 2 moles of 2CaSO4.2H2O

117.167 kmol of CaSO4.1/2H2O ≡ ? kmol of 2CaSO4.2H2O

Therefore, kmols of 2CaSO4.2H2O = .

= 117.167kmol/hr

Overall material balance:

Flowrate of HH in + rate of accumulation of H2O = flowrate of DH out

UNIT MATERIALS INPUT (kmol/hr) OUTPUT (kmol/hr)

CRYSTALLIZER

CaSO4.1/2H2O 117.167 --

H2O 175.75 175.75

CaSO4.2H2O -- 117.167

TOTAL 292.917 292.917

For material balance in kg/hr

Basis: 6890kg/hr of H3PO4 leaving

UNIT MATERIALS INPUT (kmol/hr) OUTPUT(kmol/hr)

PREMIXER/

DIGESTOR

Ca3(PO4)2CaF2 15564.42 ---

H2SO4 5167.05 ---

H2O 379.62 ---

RECYCLE FEED 2954.989 ---

H3PO4 --- 6889.4

CaSO4.1/2H2O --- 16989.215

HF --- 187.464

TOTAL 24066.079 24066.08

CRYSTALLIZER

CaSO4.1/2H2O 16989.215 ---

H2O 3163.5 2108.997

CaSO4.2H2O --- 18043.718

TOTAL 20152.715 20152.715



B] HEAT BALANCE

Introduction

The basis of energy balance of the system is the low of conservation of energy, which

states that energy can neither be created nor be destroyed during a process, although it

can be converted from one form to another and the total energy of an isolated system

remains constant. This law is also called as first law of thermodynamics.

For closed system, the energy balance equation is,

[Final system energy] – [Initial system energy]= [Net energy transferred by system]

General procedure for energy balance:

1. Assume a suitable basis of calculation.

2. Draw a block diagram of the process and lable the stream.

3. Determine the quantity of flowrates of all components with the help of

material balance.

4. Determine the enthalpy of each stream components entering or leaving.

5. If a chemical reaction is involved where in heat is evolved or absorbed, it must

be included in the energy balance equation.

6. If the heat capacity data is provided for component involved, choose a

reference temperature on which they are based for the convenience of

calculation.

Energy Balance Calculation:

Heat capacity Cp (J/mol.K) is calculated at average temperature using these formulas.

Sr. No. COMPONENTS Cp value

1. Ca3(PO4)2CaF2 15.72 + 0.1092T cal/mol.K

2. H2SO4 0.4628 cal/g.K

3. H2O 267370-2.0901T + 8.125T2 – 0.014116T3 J/kmol.K

4. H3PO4 0.7735 cal/g.0C

5. CaSO4.1/2H2O 18.52 + 0.002197T – 156800/T2cal/mol.K

6. HF 62.520 – 223.02T + 0.6297T2

7. CaSO4.2H2O 46.8 cal/mol.K



Temperature of different equipments:

1. Premixer = 900C = 363K

2. Digester = 920C = 365K

3. Crystallizer(1st) = 530C = 326K

4. Crystallizer(2nd & 3rd) = 580C = 331K

5. Crystallizer(4th) = 630C = 336K

Heat capacity data for different temperatures in (J/mol/K)

Component 363K 365K 326K 331K 336K

Ca3(PO4)2CaF2 55.3536 55.578 51.3192 51.8652 52.4112

H2SO4 4.722×10-3 4.722×10-3 4.722×10-3 4.722×10-3 4.722×10-3

H2O 671.035 671.689 650.118 653.948 657.484

H3PO4 7.89×10-3 7.89×10-3 7.89×10-3 7.89×10-3 7.89×10-3

CaSO4.1/2H2O 75.7728 75.7708 74.2708 74.4708 74.6957

HF 2.081 2.552 -5.720 -4.766 -3.781

CaSO4.2H2O 195.624 195.624 195.624 195.624 195.624

I. Premixer:

The energy balance around a premixer is based on the following chemical reaction:

3Ca3(PO4)2CaF2 + 10H2SO4 + 5H2O 6H3PO4 + 10CaSO4.1/2H2O + HF

To calculate Q:

Q = [∑Hproduct - ∑Hreactant]

∑Hproduct = HH3PO4 + HCaSO4.1/2H2O + HHF + Hrecycle feed

∑Hreactant = HCa3(PO4)2CaF2 + HH2SO4 + HH2O

H (enthalpy) = mCpdT = mCp(365 – 298) = mCp(67)



Component Rate In

(kg/hr)

Rate out

(kg/hr) Cp (363 K) H(enthalpy)

Ca3(PO4)2CaF2 15564.42 --- 55.3536 57.72×106

H2SO4 5167.05 --- 4.722×10-3 1634.720

H2O 379.62 --- 671.035 17.067×106

H3PO4 --- 6889.4 7.89×10-3 3641.94

CaSO4.1/2H2O --- 16989.215 75.7728 86.247×106

HF --- 187.464 2.081 26137.54

Recycle feed 2954.989 --- 1.23 0.243×106

∑Hproduct = 86.276×106 J/kg

∑Hreactant = 57.72×106 + 1634.720 + 17.067×106 +0.243×106

= 75.061×106 J/kg

Q = (86.276×106) – (75.061×106)

Qheat = 11.215×103 kJ/kg

The amount of heat supplied to the premixer is 11.215×103 kJ/kg.

∑Hreactant =75.061×106 J/kg ∑Hproduct= 86.276×106 J/kg

II. Digester: H (enthalpy) = mCpdT = mCp(365-363) = mCp(2)

Component Rate In

(kg/hr)

Rate Out

(kg/hr)

Cp (365K)

J/mol.K H(enthalpy)

Ca3(PO4)2CaF2 15564.42 --- 55.578 1.73×106

H2SO4 5167.05 --- 4.722×10-3 48.7976

H2O 379.62 --- 671.689 0.5099×106

H3PO4 --- 6889.4 7.89×10-3 7151.073

CaSO4.1/2H2O --- 16989.215 75.7708 108.7147

HF --- 187.464 2.552 2.574×106

Recycle feed 2954.989 --- 1.21 956.816

PREMIXER




∑Hreactant = 1.73×106 + 48.7976 + 0.5099×106 + 7151.073

= 2.247×106 J/kg

Q = (2.575×106) – (2.247×106)

Qheat = 0.328×103 kJ/kg

The amount of heat supplied to the digester is 0.328×103 kJ/kg.

∑Hreactant = 2.247×106 J/kg ∑Hproduct= 2.575×106 J/kg

III. 1st Crystallizer: H (enthalpy) = mCpdT = mCp(326-365) = mCp(-39)

Component Rate in

(kg/hr)

Rate out

(kg/hr) Cp (326K) H(enthalpy)

CaSO4.1/2H2O 16989.215 --- 74.2708 -49.21×106

H2O 3163.5 --- 650.118 -80.209×106

CaSO4.2H2O --- 18043.718 195.624 -137×106

∑Hproduct = -137×106 J/kg

∑Hreactant = -49.21×106 + -80.209×106

= -129.419×106 J/kg

Q = (-137×106) – (-129.419×106)

Qcool = -7.581×103 kJ/kg

The amount of heat to be removed from 1st crystallizer is 7.581×103 kJ/kg.

∑Hreactant =-129.419×106 J/kg ∑Hproduct= -137×106 J/kg

1st crystallizer

DIGESTER



VI. 2nd& 3rd Crystallizer: H (enthalpy) = mCpdT = mCp(331-326) = mCp(5)

Component Rate in

(kg/hr)

Rate out

(kg/hr) Cp (326K) H (enthalpy)

CaSO4.1/2H2O 16989.215 --- 74.4708 6.326×106

H2O 3163.5 --- 653.948 10.343×106

CaSO4.2H2O --- 18043.718 195.624 15.64×106


∑Hreactant = 6.326×106 + 10.343×106

= 16.669×106 J/kg

Q = (15.64×106) – (16.669×106)

Qcool = -1.029×103 kJ/kg

The amount of heat to be removed from 2nd& 3rd crystallizer is 1.029×103 kJ/kg.

∑Hreactant= 16.669×106 J/kg ∑Hproduct= 15.64×106 J/kg

V. 4th Crystallizer: H (enthalpy) = mCpdT = mCp(336-331) = mCp(5)

Component Rate in

(kg/hr)

Rate out

(kg/hr) Cp (326K) H (enthalpy)

CaSO4.1/2H2O 16989.215 --- 74.6957 6.345×106

H2O 3163.5 --- 657.484 10.39×106

CaSO4.2H2O --- 18043.718 195.624 15.648×106


∑Hreactant = 6.345×106 + 10.39×106

= 16.735×106 J/kg

Q = (15.64×106) – (16.735×106)

Qcool = -1.087×103 kJ/kg

2nd& 3rd crystallizer



The amount of heat to be removed from 4th crystallizer is 1.087×103 kJ/kg.

∑Hreactant= 16.735×106 J/kg ∑Hproduct=15.648×106 J/kg

OVERALL HEAT BALANCE

Qtotal = Qpremixer + Qdigester – Qcrystallizers

Qtotal = 11.215×103 + 0.328×103 – 7.581×103 -2(1.029×103)

– 1.087×103

Therefore, Qtotal = 0.817×103 kJ/kg

Total heat supplied to the process is 0.817×103 kJ/kg

4th crystallizer



Chapter No 9: Design of Critical Equipment

A] DESIGN OF BALL MILL:

Function of ball mill is to ground the rock phosphate. Unground rock phosphate

varies from 10cm to 0.05cm in size. For design of ball mill, we consider that

unground rock phosphate has feed size of 0.08cm to 2cm. We have to crush this feed

upto 0.007cm.

Data:

Feed size : 20mm (20000 microns) to 2 mesh(841 micron)

Product size : 0.007cm (200 mesh, 70%product passes through it)

Sp. Gravity of rock phosphate: 1.1710 at 160C & 2.74 at 300C

Work index of rock phosphate: 9092

Feed rate: 44tonnes/hr

1. Length & diameter of ball mill

To determine length of ball mill and also diameter of ball mill we assume that

25 tonnes of unground rock phosphate is present in the mill at any time (56%

of total feed rate)

Now,

D = 124.0 × C + 485.7

L = 85.71 × C + 1854

Where C = tonnes of feed

D = diameter in mm

L = Length in mm

D = 124.2 × 25 + 485.7

= 3590.7 mm = 3.590m

= 11.4829ft

For designing purpose we consider diameter of ball mill is 12ft.

D =12ft



L = 85.71 × 25 × 1854

= 3996.75mm = 3.99675m = 13.11ft ≈ 14ft

For designing purpose we consider 10% excess length for suitable grinding

condition

L = 1.1 × 14 = 15.4ft

Consider length of ball mill

L = 16ft

2. Diameter of Ball

Db = √ √√

Where,

Db = diameter

Pf = size in µ of 80% passing in the fresh feed

C = 200 for ball mill

Wi = work index

Vcr = percentage of critical speed

Sgs = specific gravity of feed

D = internal diameter of mill in m

By substituting all the values, we get,

Db = 42.72mm

Ball mill run at 65% of critical speed

For designing purpose we consider ball diameter is 50mm and 40mm

Db = 50mm and 40 mm

3. Number of Balls:

Volume of ball mill = × 122 × 16

= 1809.55ft

Bulk volume of balls charge ratio to the volume of mills is known as filling

ratio (F) and its range is 30- 45%. Assuming filling ratio to be 35%.

Therefore, F =

Where, bulk volume of balls = F × volume of ball mill

= 0.95 × 1809.55

= 633.34ft3



Volume of single ball for 50 mm dia= 4/3 × π × r3 =4/3 × π × 0.0823

= 230.9×10-3 ft3

Volume of single ball for 40 mm dia= 4/3 × π × r3 =4/3 × π × 0.0683

= 131.7×10-3 ft3

No. of balls for 50 mm dia=

= .

.

No. of balls = 1985

No. of balls for 40 mm dia=

= .

.

No. of balls = 2715

4. Critical speed and operating speed of ball mill and is given by,

ηc= √

ηc= √.

. .

ηc= 0.3709 rps

ηc= 22.25 rpm

Operating speed is (65%) of critical speed

η = 0.65 × ηc

η = 0.65 ×22.25

η = 14.46 rpm

For balls we take alloy steel is IS-6079 as material of construction. Therefore total

weight of balls is 61.7tonnes.



5. Power requirement of ball mill

Using bond crushing law

= 0.3162× Wi × (√

√

)

Where, m = feed rate of rock phosphate

Wi = work index

P = power required to ball mill

Dp = diameter of product particle

Df = diameter of feed particle

P = 44000 × 0.3162 × 9.92 × (√ .

√

)

P = 478530.10kW

P =641708.86 HP

6. Thickness of ball mill;

t =

Where, P = design pressure

Di = internal diameter

J = joint efficiency

f = design or permissible stress at design temperature

Assuming internal pressure P = 0.3 N/mm2

Therefore, design pressure, p = 0.3 × 1.1 = 0.33N/mm2

Allowable stress at 300C = 30N/mm2

t = . .

. .

t = 23.82mm

t ≈ 25 mm



B] DESIGN OF PREMIXER:

Data:

Type – cylindrical with agitation, completely lined vessel

Agitator type – anchor agitation

Size of vessel – 2275mm ID × 1465mm liquid height

Hold up capacity – 5.95m3

Agitator rpm(N) – 120


Temperature – 900C

Pressure – hydrostatic

M.O.C – carbon steel with butyl rubber lining + carbon brick lining

Function – to mix up completely rock phosphate with sulfuric acid and phosphoric

acid

Diagram:

Typical configuration and dimensions of an agitated vessel

Where, Da = diameter of agitator

D = diameter of tank



HA = height of agitator from base of tank

H = depth of liquid

WB = width of baffles

N = speed of agitator

i. For anchor agitator

= 0.9

Da = 0.9× D = 0.9 × 2275

Da = 2047.5mm

ii. Power number (Np) = 215(Re)-0.955 , Re≤ 100

Where Re = Reynolds number

= ……..(1)

To calculate,

ρf =

………(2)

where, m = mass of the material

v = volume of the material

Sr. No. Component m (kg/hr) v (kg/hr)

1. H2O 379.62 0.3796

2. H2SO4 5167.05 2.808

3. Return acid 2954.989 8.2351

4. Rock phosphate 15564.42 8.833

From equation (2),

ρf = 1188.11 kg/m3

Assuming the viscosity (µ) to be 1 kg/m.s

From eqn (1) we have,

Re = 1188.11 × 120/60 × 2.0475

Re = 4865.31045

Re ≈ 4870



Thus the flow inside the premixer is turbulent

iii. Pressure inside premixer:

P = hydrostatic pressure ……..{given}

= ρgh

= 1188.11 × 9.81 × h

Height of liquid is assumed to be 75% of the height of tank

h = 0.75 × 1.465 = 1.09875m

Therefore, P = 1188.11× 9.81× 1.09875

P = 12806.325 N/m2

iv. Power required (Np):

Np =

= . .

. .

Np = power required = 2.722hP

v. Shaft Design:

Rated motor torque Tr,

Tr = ×10 N-m

Tr = 162.45 N-m

Maximum torque for design, Tm

Tm = 1.5 × Tr

= 1.5 × 162.45

Tm = 243.686 N-m

Design of shaft based on pure torsion,

Tm = × d3 × fs

Note: Shaft material: steel

Permissible shear stress fs: 52N/mm2

Elastic limit in tension (fb): 240N/mm2

Modulus of elasticity (E): 1.95 × 105N/mm2



d3 = = 23866.973

Therefore, d= 28.79×10-3m

Design of shaft based on pure bending

a) Fm = Tm/(0.75×Rb)

Where Fm = bending force

Rb = radius of impeller

Therefore, Fm = 243.686/(0.75×1.02375)

Fm = 317.376 N-m

b) Mm = Fm× l

Where, Mm= maximum bending moment

l = length of shaft

= 730mm ……{assumed}

Mm = 317.376× 0.73

Mm = 231.684N-m

c) Mm = × fb × d3

d3 = Mm×32/(π× fb)

d = 21.42×10-3m

Design based on equivalent twisting moment,

Te = √ 2 2

Te = 336.244 N-m

Te = × 52 × d3

Therefore, d = 32×10-3m

Design based on equivalent bending moment

Me = ½(Mm + √ 2 2

= ½(231.684 + 336.244)



Me = × 240 × d3

d = 22.927× 10-3m

selecting the maximum diameter of shaft as 32mm

Design of shaft based on critical speed,

Actual speed should not be between 66.6% to 130% of critical speed Nc

Nc = N×100/66.6

Nc = 120×100/66.6or

Nc = 180.180rpm

Now, Nc = 946/δ1/2

Where, δ = appropriate deflection in mm

δ = 27.565mm

The maximum deflection due to each load acting independently is given by,

δ =

Weight(W) = mass × 9.81

= 0.236 × 106 N

δ = . .

. = 27.565×10-3

d = 21.44×10-3m

Hence selected shaft size is 22mm



vi. Hub and Key Design.

Hub diameter of agitator = 2d

= 2 × 22 = 44mm

Length of hub = 2.5d = 55mm

Length of key = 1.5d = 33mm

/ = lb.fs = l.(t/2).fc

.

/= 33 × b × 65 = 33×(t/2) × 130

b = t = 0.0103mm

vii. Blade Design:

Carbon steel can be used as construction material for blade.

Stress, f = .

/

Therefore on substituting the values, f = 2362.86N/mm2

viii. Uffing Design of stuffing box and gland

Internal diameter of stuffing box from eqn

d1 = d + 2×( clearance between shaft and stuffing box)

c = 0.2d + 5mm

= 0.2(22) +5 = 9.4mm say 10mm

d1 = 22 + (2×10) = 42mm

Thickness of the gland flange

= (d/8)× 12.5 = (22/8) × 12.5 = 16mm



Thickness of stuffing box body,

t = + 6mm

= .

+ 6

Therefore t = 6.09mm

Load on glands,

F = ×P(d12 –d2

2) = db2 n ×ft

Therefore, db = 1.6mm

Use minimum stud diameter as 16mm and 4 no. of studs.

Thickness of stuffing box flange = 1.75 db

= 1.75× 1.6

Therefore, ts= 28mm



C] Design of Horizontal Belt Filter

Thickness Of Filter Cake:

For continuous filtration application the rate at which the cake is formed normally

determines type of vacuum filter.

Sr No Process Type Rate of cake built- up Suitable equipment

1 Rapid filtering 0.1-10 cm/sec Gravity pan, Horizontal table

or top feed drum, continuous

pusher type centrifuge

2 Medium Filtering 0.1-10 cm/min Vacuum Drum or disc or pan

or Vacuum belt filter, peeler

type centrifuge

3 Slow filtering 0.1-10 cm/hr Pressure filter, disc or tubular

centrifuge

4 Clarification Negligible cake Precoat drum, filter aid system,

sand deep bed filters

Mass of solids in filter cake = (1-Ɛ)*A*Ɩ*ρ

Where, Ɛ=porosity of cake= 0.2 (for given cake)

Ɩ =cake thickness

ρ = solid density = 2350 Kg/M3

A = cross section area of filter cake= cloth width*cake thickness (Ɩ)

Clothe width= 3.4 M

Mass of solids in filter cake = 0.2986 Kg/sec

0.2986 = (1-0.2)*3.4*Ɩ*Ɩ*2350

0.2986 = 6392*Ɩ2

Ɩ = 6.8348*10-3 M/sec

Ɩ = 4.1 mm/min OR 0.41 cm/min

Hence for this thickness of cake horizontal belt vacuum filter is effective.



Area Of Filtration:

C = It is solid concentration per unit volume of filtrate = wt. of solid/volume of liq

C = 9754.33/(62500/1638)

C = 255.64 Kg/m3

Cx = kg of solids per kg of slurry = 0.2986/202123

Cx = 0.135

Feed rate = 2.2123 Kg/sec

Flow rate of filtrate

V/tc = Feed rate*(Cx/C) = 2.2123*(0.135/255.817)

V/tc = 101685*10-3 m3filtrate/sec

V/Atc = (2*ΔP/Ctcµα)0.5

ΔP = pressure drop = 80,000 Pa

µ = filtrate viscosity = 0.02 Ns/m2

α = (4.37*109)*(ΔP)0.3 = (4.37*109)*(80,000)0.3

α = 1.2924*1011m/kg

sp. surface area of cake (So) =16077.31 m2

tc = filter cycle time = 90sec

V/Atc = (2*ΔP/Ctcµα)0.5

101685*10-3/A = (2*80,000/90*0.02*1.2924*1011*255.5817)0.5

A= 25.6317 m2

Area for filtration should be 25-30 m2

Rate Of Filtration

Q=A*ΔP/µ*[Rf+(C*α*V/A)]

Rf = cake thickness/permeability for fine filter sheet

Rf = 6.833*10-6/0.15*10-12 = 4.555*107

Q = 25.6317*80,000/0.02*[(4.555*107)+(255.5817*1.2924*1011*100.94/25.6317)]

Q = 8.1099*10-7 m3/sec

Q = 2.9195 lit/hr



Length Of Filter Cloth

Head pulley dia (D) = 1400mm

Tail pulley dia (d) = 1400mm

Assuming Centre to Centre distance (C) = 15m

Assuming quarter turn drive

L = (π/2)*(D+d) + (C2+D2)0.5+(C2+d2)0.5

L = (π/2)* (1.4+1.4) + (1.42+152)0.5 + (1.42+152)0.5

L = 32.329 M

Peripheral Velocity of pulley

V = velocity

N = max. speed of motor = 12RPM

V = π* Dia* N/60

V= π*1.4*12/60

V = 0.879 m/sec



Chapter No 9: Materials of Construction

Material of Construction for Different Units:

Rubber or brick lined digesters are used, with high alloy metallic components,

(e.g. N08028, N06985, N06030, N0803). After filtration to remove precipitated

saltsthe product acid concentration is roughly 35-44%. Stronger concentration of 58-

75% is obtained by vacuum evaporation. Evaporators are generally shell & tube

exchangers with tubes of alloys 31(NJ08031), G-30(N06030) or G-3 (N06985).

Materials:

1. Aluminum Alloys:

Aluminum is resistant to phosphoric acid up to 20% conc. to 65ºC. Hot

phosphoric acid is used to bright dip aluminum before anodizing.

2. Steel & cast iron:

Steel forms a moderately protective phosphate film in the acid above 70%

concentration & resistance may be further enhanced by arsenic salt.

Unalloysed cast irons have the similar resistance in strong acids. Austentic

nickel iron are resistant to slightly above room temperature but even the 14%

silicon iron normally resistant to all concentrations at atm boiling point can be

attacked by fluoride concentration.

3. Stainless steel:

The straight chromium (ferritic or martensitic) grades of stainless steel find no

of practical applications in phosphoric acid. Types 316L(S31603) &

317L(S31703) resist uncontaminated acid in all concentration to about 80ºC.

However velocities above 1m/sec can cause erosion corrosion in valves &

piping. It precisely because of contaminants in wet process acid that certain

high alloy, high performance stainless steel were developed. Alloy

28(N08028), 31(N08031), & N08904 or N08700 have been successfully used

in lieu of S31603 but N06985 & N06030 are usually preferred.

4. Copper & its alloy:

Copper & its high strength alloys as well as C70600 heat exchangers tubes

have been successfully used in unaerated acid in the absence of other



oxidizing agents (iron & copper). They have been the materials of choice in

some high temperature organic synthesis under reducing conditions.

5. Nickel & its alloy:

Chromium free alloys N02200 &N04400 are of small use in phosphoric acid,

being less resistant than copper alloys under reducing condition &attacked

under oxidizing conditions. The Molybdenum grade N10665 will resist pure

acid of all concentrations to about 65ºC & up to 50% in atm boiling points. It

is however rapidly corroded by oxidizing contaminants. Alloys of primary

interest are the specialty alloy N06007 & 6% molybdenum stainless steel alloy

31(N08031). These were developed specially for wet process phosphoric acid

6. Reactive Metals:

Titanium: because it is not a oxidizing acid phosphoric acid tends to attak

titanium zirconium: zirconium resist pure acid to about 50% concentration,

and above the atmospheric boiling temperature, but corrosion rates increases

sharply above about 60% at 100ºC fluorides, zirconium equipment has been

used to process a phosphoric acid-containing medium.

Tantalu : It will resist all concentrations of phosphoric acid to about 190ºCif

there is not more than a small fluoride concentration

Niobium: Niobium resists phosphoric acid up to 95% & up to 60ºC

7. Noble Metals:

Gold and platinum will resist all concentration of pure acid to the atmospheric

boiling point silver corrosion rate increase from less than 1mpy in 15%acid at

the atmospheric boiling point to 2mpy in 60% and 7mpy is 85% up to 250ºC

(480 F)

8. Non-Metals:

Organic: In the absence of solvent contamination conventional plastic, PVC,

polyethylene, epoxyphenolic resist all concentration to the inherent

temperature limitof the plastic. Troweled –epoxy coatings are used to protect

concrete tank padsfromacid spillage. The fluorinated plastics are resistant to

higher temperature.

Inorganic: Carbon & graphite are inert in all concentrations of acidwith or

without contaminants to at least 350ºC. A film of phosphate protects against

oxidation. Some impervious graphite products are limited to about 170ºC by



the constituents of cemented connections. Glass is useful only at ambient

temperature even without fluoride contamination.

9. Storage facilities:

for product phosphoric acid

Tanks FRP or N08700 or N08904

Piping’s FRP or S31603

Valves CF3M

Pumps CF3M

Gaskets Elastomeric, filled polytetra-fluoethylene (PTFE) or flexible graphite.



Chapter No 10: Cost Estimation

Total operating cost (TOC) is the sum of:

1. Fixed Capital (FCC)

2. Working Capital (WC)

Fixed capital cost is again comprised of:

1. Direct cost

2. Indirect cost

1. Direct Cost: Cost of material &labor involved in actual installation of complete

facility complete under this category. This includes

a) Purchased equipment’s cost (PEC)

b) Installation including insulation & painting(IC)

c) Instrumentation & controls installed(ICC)

d) Piping installed (PI)

e) Electrical Installed (EI)

2. Indirect Cost: Expenses which are not directly involved with material & actual

installation of complete facility comes under this category.

a) Engineering & supervision cost (E&SC)

b) Construction expenses & contractors fee (CE & CC)

Working Capital: It is the sum of manufacturing cost & general expenses.

1. Manufacturing Cost: It is the sum of Direct production cost, Fixed charges &

plant overhead cost.

a) Direct production cost: comprise of

i. Raw Material Cost(RMC)

ii. Operating labor Cost(OLC)

iii. Utilities Cost(UC)

iv. Maintenance & repair cost (M&RC)



b) Fixed charges: comprise of

i. Depreciation cost(DC)

ii. Local Taxes (LT)

c) Plant overhead cost

2. General Expenses: General expense is the sum of administrative cost,

Distribution & selling cost, Research & Development cost.

GE=20% of (Total Capital Investments)

FORMULAE FOR CALCULATIONS:

1. For Total Capital Investments (TCI):

WC = 20% of (TCI)

And TCI = FCI+WC

TCI = FCI+ 0.2TCI

Therefore FCI = (1-0.2) TCI

TCI = FCI/0.8

2. TOC = FCC+WC

3. NET PROFIT = (Gross Income -Taxes)

Gross Income = Total Income –Total Product Cost

Where,

Total Income = (Selling price × Quantity of product manufacturing)

4. RATE OF RETURN (ROR):

ROR = (NET PROFIT × 100)/Total Operating cost

5. PAY BACK PERIOD = TOC /(Profit+Depreciation)



CALCULATIONS:

Direct cost:

a) To calculate purchased equipment cost (PEC):

Sr No. EQUIPMENT COST (in `)

1 Ball mill (Carbon steel, 10ft Dia×16ft length) 22,39,060

2 Mill Fan (44,000 M3/hr) 3,09,506

3 Dust collector (150mm×3150mm long) 3,28,760

4 Vent Fan (15,000 M3/hr) 6,12,600

5 Acid Mixing Tank 62,000

6 Premixer (2275mmID×1465mm ht) 64,680

7 Digester (2Nos,2820mm ID×3620mm ht) 3,61,136

8 Crystallizer (4 Nos, 6700mmID×7700mm ht) 29,70,680

9 Slurry Pumping Tank (4500mm ID×4000mm ht) 4,14,120

10 Filter feed splitter box (1500×1200×1000)mm 48,980

11 Horizontal Vacuum Belt Filter 36,95,870

12 Clarifier (7m Dia×4m ht) 1,01,522

13 Reaction fume scrubber 77,380

14 Storage Tank 42,340

PURCHASED EQUIPMENT COST (PEC) ` 11.28×106 /-

b) IC = 30% OF PEC

IC = 0.3×11.28×106

IC = `30384×106 /-

c) ICC = 15% OF PEC

ICC = 0.15×11.28×106

ICC = ` 1.692×106 /-

d) PC = 20% OF PEC

PC = 0.2×11.28×106

PC = ` 2.252×106 /-



e) EC = 20% OF PEC

EC = 0.2×11.28×106

EC = ` 2.252×106 /-

DIRECT COST = PEC+IC+ICC+PC+EC

=(11.28+3.384+1.692+2.256+2.256)×106

DIRECT COST = ` 20.868×106/-

Indirect Cost:

a) E&SC = 20% OF Direct cost

E&SC = 0.2×20.868×106

E&SC = ` 4.1736×106/-

b) CE&CC = 30% of E&SC

CE&CC = 0.3×4.1736×106

CE&CC = ` 1.252×106/-

INDIRECT COST = E&SC + CF&CC = (4.1736+1.252)×106

INDIRECT COST = ` 5.4256×106 /-

FIXED CAPITAL COST = DIRECT COST+INDIRECT COST

= (20.868+5.4256)×106

FCC = ` 26.2936×106/-

Direct Production Cost:

a) Raw Material Cost (RMC)

=(8.3×215293.75)H2SO4+(648517.5×16)ROCK PHOSPHATE

RMC = ` 12.163×106/-

b) Operating Labor Cost (OLC) = 10% of Total Product cost(TPC)

TPC = Fixed charges/0.15



Fixed Charges(FC) = Depreciation cost (DC) + Local Taxes (LT)

DC=15% of FCC = 0.15×26.2936×106

DC = ` 3.944×106/-

LT = 20% of FCC = 0.2× 26.2936×106

LT = ` 5.2588×106/-

FC = DC+LT = (3.944+5.2588)×106

FC = ` 9.2028×106/-

Total Product Cost = FC/0.15

TPC = ` 61.325×106/-

OLC= 10% of TPC = 0.1×61.325×106

OLC= ` 6.1325×106/-

c) Utilities cost (UC):10% of TPC

UC = ` 6.1325×106 /-

d) Maintenance & Repair Cost (M&RC):5% of FCC

M&RC = 0.5×26.2936×106

M&RC = ` 1.3147×106/-

Direct Production cost= RMC+OLC+UC+M&RC

DPC = (12.136+6.1352+6.1352+1.3147)×106

DPC = 25.7480×106/-

Plant Overhead Cost (POHC) = 10% of TPC

POHC= 0.1×61.325×106

POHC= ` 6.1325×106 /-



Manufacturing cost (MC) = DPC+FC+POHC

= (25.7481+9.2028+6.1352)× 106

MC = ` 41.0861×106 /-

GENERAL EXPENSES (GE) = 20% of TOC

Therefore, TOC = FCC+WC = FCC+ (0.2×TOC)

FCC = TOC×(1-0.2)

TOC = FCC/0.8 = (26.2936×106)/0.8

TOC = ` 36.617×106/-

GE = 0.2×36.617×106

GE = ` 7.3234×106/-

Working Capital (WC):

WC = MC+GE = (41.0861+7.3234)× 106

Now,

TOC= FCC+WC = (26.2936+48.4095) 106

TOC = ` 74.7031×106/-

Total Income:

Total Income = Selling price×Quantity of product Manufactured

= 12.6×688904×1000/24

Total Income = ` 83.835×106/-

Gross Income:

Gross Income = Total Income – Total Product cost = (83.835 – 61.325)× 106

Gross Income = ` 22.483×106/-



NET PROFIT:

NET PROFIT = Gross Income – Taxes = (22.483 -5.2588)× 106

NET PROFIT = ` 17.2242×106/-

Rate of Return (ROR) = (NET PROFIT/TOC) ×100

ROR = (17.2242/74.7031)×100

ROR = 43.0568%

Pay Back Period(PBP) = TOC/(Profit+ Depreciation)

PBP = (74.7031×106)/(17.2242+3.944)×106

PBP = 2.529 Year



Chapter No 11: PLANT LOCATION/SITE

The location of the plant can have a crucial effect on the profitability of a project, and

the scope for future expansion. Many factors must be considered when selecting a

suitable site. The principal factors to consider are:

1. Location, with respect to the marketing area.

2. Raw material supply.

3. Transport facilities.

4. Availability of labour.

5. Availability of utilities: water, fuel, power.

6. Availability of suitable land.

7. Environmental impact, and effluent disposal.

8. Local community considerations.

9. Climate.

10. Political and strategic considerations.

A] Marketing area

Phosphoric acid produced in bulk quantities; therefore the cost of the product per

tonne is relatively low and the cost of transport a significant fraction of the sales price.

The plant should be located close to the primary market. In an international market,

there may be an advantage to be gained by locating the plant within an area with

preferential tariff agreements.

B] Raw materials

The availability and price of suitable raw materials will often determine the site

location. Plants producing bulk chemicals are best located close to the source of the

major raw material; where this is also close to the marketing area.

C] Transport

The transport of materials and products to and from the plant will be an overriding

consideration in site selection. If practicable, a site should be selected that is close to

at least two major forms of transport: road, rail, waterway (canal or river), or a sea

port. Road transport is being increasingly used, and is suitable for local distribution

from a central warehouse. Rail transport will be cheaper for the long-distance



transport of bulk chemicals. Air transport is convenient and efficient for the

movement of personnel and essential equipment and supplies, and the proximity of

the site to a major airport should be considered.

D] Availability of labour

Labour will be needed for construction of the plant and its operation. Skilled

construction workers will usually be brought in from outside the site area, but there

should be an adequate pool of unskilled labour available locally; and labour suitable

for training to operate the plant. Skilled tradesmen will be needed for plant

maintenance. Local trade union customs and restrictive practices will have to be

considered when assessing the availability and suitability of the local labour for

recruitment and training.

E] Utilities

Chemical processes invariably require large quantities of water for cooling and

general process use, and the plant must be located near a source of water of suitable

quality. Process water may be drawn from a river, from wells, or purchased from a

local authority. At some sites, the cooling water required can be taken from a river or

lake, or from the sea; at other locations cooling towers will be needed. Electrical

power will be needed at all sites.

Electrochemical processes that require large quantities of power; for example,

aluminium smelters, need to be located close to a cheap source of power. A

competitively priced fuel must be available on site for steam and power generation.

F] Environmental impact and effluent disposal

All industrial processes produce waste products, and full consideration must be given

to the difficulties and cost of their disposal. The disposal of toxic and harmful

effluents will be covered by local regulations, and the appropriate authorities must be

consulted during the initial site survey to determine the standards that must be met.

An environmental impact assessment should be made for each new project, or major

modification or addition to an existing process.



G] Local community considerations

The proposed plant must fit in with and be acceptable to the local community. Full

consideration must be given to the safe location of the plant so that it does not impose

a significant additional risk to the community.

On a new site, the local community must be able to provide adequate facilities for the

plant personnel: schools, banks, housing, and recreational and cultural facilities.

H] Land (site considerations)

Sufficient suitable land must be available for the proposed plant and for future

expansion. The land should ideally be flat, well drained and have suitable load-

bearing characteristics. A full site evaluation should be made to determine the need

for piling or other special foundations.

I] Climate

Adverse climatic conditions at a site will increase costs. Abnormally low temperatures

will require the provision of additional insulation and special heating for equipment

and pipe runs. Stronger structures will be needed at locations subject to high winds

(cyclone/hurricane areas) or earthquakes.

J] Political and strategic considerations

Capital grants, tax concessions, and other inducements are often given by

governments to direct new investment to preferred locations; such as areas of high

unemployment. The availability of such grants can be the overriding consideration in

site selection.



Chapter No 12: PLANT LAYOUT

The economic construction and efficient operation of a process unit will depend on

how well the plant and equipment specified on the process flow-sheet is laid out.

The principal factors to be considered are:

1. Economic considerations: construction and operating costs.

2. The process requirements.

3. Convenience of operation.

4. Convenience of maintenance.

5. Safety.

6. Future expansion.

7. Modular construction.

A] Costs

The cost of construction can be minimised by adopting a layout that gives the shortest

run of connecting pipe between equipment, and the least amount of structural steel

work. However, this will not necessarily be the best arrangement for operation and

maintenance.

B] Process requirements

An example of the need to take into account process considerations is the need to

elevate the base of columns to provide the necessary net positive suction head to a

pump or the operating head for a thermosyphon reboiler.

C] Operation

Equipment that needs to have frequent operator attention should be located

convenient to the control room. Valves, sample points, and instruments should be

located at convenient positions and heights. Sufficient working space and headroom

must be provided to allow easy access.



D] Maintenance

Heat exchangers need to be sited so that the tube bundles can be easily withdrawn for

cleaning and tube replacement. Vessels that require frequent replacement of catalyst

or packing should be located on the outside of buildings. Equipment that requires

dismantling for maintenance, such as compressors and large pumps, should be placed

under cover.

E] Safety

Blast walls may be needed to isolate potentially hazardous equipment, and confine the

effects of an explosion. At least two escape routes for operators must be provided

from each level in process buildings.

F] Plant expansion

Equipment should be located so that it can be conveniently tied in with any future

expansion of the process. Space should be left on pipe alleys for future needs, and

service pipes over-sized to allow for future requirements.

G] Modular construction

In recent years there has been a move to assemble sections of plant at the plant

manufacturer’s site. These modules will include the equipment, structural steel, piping

and instrumentation. The modules are then transported to the plant site, by road or

sea. The advantages of modular construction are:

1. Improved quality control.

2. Reduced construction cost.

3. Less need for skilled labour on site.

4. Less need for skilled personnel on overseas sites.

Some of the disadvantages are:

1. Higher design costs.

2. More structural steel work.

3. More flanged connections.

4. Possible problems with assembly, on site.



H] General considerations

Open, structural steelwork, buildings are normally used for process equipment; closed

buildings are only used for process operations that require protection from the

weather. The arrangement of the major items of equipment will usually follow the

sequence given on the process flow-sheet: with the columns and vessels arranged in

rows and the ancillary equipment, such as heat exchangers and pumps, positioned

along the outside.

SITE SELECTION

The process units and ancillary buildings should be laid out to give the most

economical flow of materials and personnel around the site. Hazardous processes

must be located at a safe distance from other buildings. Consideration must also be

given to the future expansion of the site. The ancillary buildings and services required

on a site, in addition to the main processing units (buildings), will include:

1. Storages for raw materials and products: tank farms and warehouses.

2. Maintenance workshops.

3. Stores, for maintenance and operating supplies.

4. Laboratories for process control.

5. Fire stations and other emergency services.

6. Utilities: steam boilers, compressed air, power generation, refrigeration,

transformer stations.

7. Effluent disposal plant.

8. Offices for general administration.

9. Canteens and other amenity buildings, such as medical centres.

10. Car parks.

General Consideration:

A] When roughing out the preliminary site layout, the process units will normally be

sited first and arranged to give a smooth flow of materials through the various

processing steps, from raw material to final product storage. Process units are

normally spaced at least 30 m apart; greater spacing may be needed for hazardous

processes.



B] The location of the principal ancillary buildings should then be decided. They

should be arranged so as to minimise the time spent by personnel in travelling

between buildings. Administration offices and laboratories, in which a relatively large

number of people will be working, should be located well away from potentially

hazardous processes. Control rooms will normally be located adjacent to the

processing units, but with potentially hazardous processes may have to be sited at a

safer distance.

C] The siting of the main process units will determine the layout of the plant roads,

pipe alleys and drains. Access roads will be needed to each building for construction,

and for operation and maintenance.

D] Utility buildings should be sited to give the most economical run of pipes to and

from the process units.

E] Cooling towers should be sited so that under the prevailing wind the plume of

condensate spray drifts away from the plant area and adjacent properties.

F] The main storage areas should be placed between the loading and unloading

facilities and the process units they serve. Storage tanks containing hazardous

materials should be sited at least 70 m (200 ft) from the site boundary.



Chapter No 13: Health and Safety Aspects

A] Rock Phosphate: [Ca10(PO4)6CaF2] :

1) General Information:

Boiling Point : N/A

Flash Point (Method Used) : N/A

Fighting Procedure :Rock Phosphate is an inorganic material. It is non

flammable and non-hazardous.

2) Health Hazard Data

Route(s) of Entry :Inhalation, Skin, Ingestion

Health Hazards :Acute and Chronic

Short term :no effect other than nuisance dust.

Long term :due to fluoride in the rock, large amounts over a long

period of time may cause weight loss.

Emergency and First Aid Procedures:

Eyes : flush with water

Skin : wash with water.

Call doctor if irritation persists.

3) Precautions for Safe Handling and Use

1. Steps to Be Taken in Case Material Is Released or Spilled: If uncontaminated,

sweep up or collect and reuse as product. Use conventional housekeeping

methods.

2. Waste Disposal Method: Use conventional housekeeping methods. Stay in

accordance with State and Federal regulations.

3. Precautions to Be Taken in Handling and Storing



B] 98% SULFURIC ACID [H2SO4]:


a) Causes severe burns.

b) Reacts violently with water.

c) Contents may be under pressure of explosive,

d) Flammable hydrogen gas.

e) Highly reactive and capable of igniting combustible material on contact.

f) Corrosive. Causes burns, tissue destruction, Can cause blindness.

g) Harmful if inhaled. Causes upper respiratory tract irritation, lung irritation,

chest pain, wheezing, shortness of breath, a burning

h) sensation, tickling of the nose and throat, sneezing, Repeated exposure to high

levels of sulfuric acid mist may cause etching

i) tooth enamel in persons who breathe through their mouths.

j) Harmful if ingested. Can cause irritation, abdominal pain, corrosion, burns to

mouth and esophagus, death.

k) When mists are released from this product they are considered to be probable

or suspected human carcinogens

2) First aid measures for accidental:

a) Hold eyelids open and flush with a steady, gentle stream of water for at least

15 minutes. Seek immediate medical attention.

b) In case of contact, immediately wash with plenty of water for at least 15

minutes. Seek medical attention if irritation developes or persists. Remove

contaminated clothing and shoes. Clean contaminated clothing and shoes

before re-use.

c) Remove victim from immediate source of exposure and assure that the victim

is breathing. If breathing is difficult, administer oxygen, if available. If victim

is not breathing, administer CPR (cardio-pulmonary resuscitation). Seek

medical attention.

d) Do not induce vomiting. If the person is conscious and has no trouble

breathing a small (no more than one glass) amount of water may be given. Do

not leave victim unattended. To prevent aspiration of the swallowed product,



lay victim on side with head lower than waist. If vomiting occurs do not re-

administer water. Do not give anything by mouth to an unconscious person.

e) Immediately obtain medical attention.

3) Fire hazard data:

Flash Point: Not Applicable

Not combustible. Strong oxidizers can react with reducing agents or combustibles

producing heat and causing ignition. Reacts violently with water releasing heat and

corrosive material.

a) Not combustible. Use extinguishing method suitable for surrounding fire.

Recommended (small fires): dry chemical.

b) Firefighters should wear NIOSH/MSHA approved positive pressure breathing

apparatus with full face-piece and full acidresistant protective clothing. Fight

fire from maximum distance.

4) Evacuation Procedures and Safety:

a) Personnel handling this material should be thoroughly trained to handle spills

and releases. Do not direct hose streams into an unignited transportation spill

(tank truck or tank car).

b) Stop leak if it can be done without risk. Dike spill using absorbent or

impervious materials such as earth, sand or clay. Dike or retain dilution water

or water from firefighting for later disposal.

c) Pump any free liquid into an appropriate closed container. Exercise caution

during neutralization as considerable heat may be generated. Carefully

neutralize spill with soda ash. Absorb neutralized spill with an inert absorbent.

Scrape up and place in appropriate closed container.

5) Environmental and Regulatory Reporting:

Do not flush to drain. Runoff from fire control or dilution water may cause pollution.

Dispose of as a hazardous waste. Spills may be reportable to the National Response

Center (800-424-8802) and to state and/or local agencies. Large spills should be

handled according to a predetermined plan.



6) Handling and storage:

a) Do not breathe vapors and mists. Do not get on skin or in eyes. This product

reacts violently with bases liberating heat and causing spattering.

b) When diluting an acid, ALWAYS add the acid slowly to water and stir well to

avoid spattering. NEVER ADD WATER TO ACID.

c) Store in tightly closed containers. Store in an area that is dry, well-ventilated,

diked with impermeable material, Freezing point varies with concentration.

Maximum recommended storage temperature = 104F (40C). Corrosion rates

increase at elevated temperatures.

C] PHOSPHORIC ACID (H3PO4):


a) Clear, colorless solution with caustic odor.

b) Routes of Entry: Skin, eyes, inhalation and ingestion.

2) First aid information:

a) Inhalation of mists can cause corrosive action on mucous membranes.

Symptoms include burning, choking, coughing, wheezing, laryngitis, shortness

of breath, headache or nausea. Move casualty to fresh air and keep at rest. Get

medical attention if symptoms persist.

b) Symptoms include eye burns, watering eyes. Rinse with plenty of water for a

minimum of 15 minutes and seek medical attention immediately.

c) Symptoms include burning, itching, redness, inflammation and/or swelling of

exposed tissues. Immediately flush with plenty of water for at least 15 minutes

while removing contaminated clothing and wash using soap. Get medical

attention if necessary.

d) Do Not Induce Vomiting. Causes corrosive burns of the mouth, gullet and

gastrointestinal tract if swallowed. Symptoms include burning, choking,

nausea, vomiting and severe pain. Wash out mouth with water and give a glass

of water or milk. Get medical attention immediately.



3) Fire-fighting measures

Product is not flammable.

a) Use appropriate media for adjacent fire. Cool containers with water, keep

away from common metals.

b) Wear self-contained, approved breathing apparatus and full protective

clothing, including eye protection and boots.

c) Emits toxic fumes under fire conditions.

d) Material can react with metals to produce flammable hydrogen gas. Forms

flammable gases with aldehydes, cyanides, mercaptins, and sulfides.

4) Accidental release measures

a) use of personal protective equipment.

b) Cleanup personnel need personal protection from inhalation and skin/eye

contact. Evacuate and ventilate the area. Prevent spillage from entering drains.

Cautiously add water to spill, taking care to avoid splashing and spattering.

Neutralize diluted spill with soda ash or lime. Absorb neutralized spill with

vermiculite or other inert absorbent material, then place in a suitable

c) Created onrelease to the environment may be subject to federal/national or

local reporting requirements. Dispose of all waste or cleanup materials in

accordance with local regulations. Containers, even when empty, will retain

residue and vapors.

5) Handling and storage

a) Use with adequate ventilation. Wash thoroughly after using. Keep container

closed when not in use.

b) Store in cool, dry well ventilated area. Keep away from incompatible

materials. Drains for storage or use areas for this material should have

retention basins for pH adjustment and dilution of spills.



c) Provide local exhaust, preferably mechanical.

d) If necessary use an approved respirator with acid vapor cartridges.

e) Wear chemical safety glasses with a face shield for splash protection.

f) Wear neoprene or rubber gloves, apron and other protective clothing

appropriate to the risk of exposure.

g) Provide eyewash stations, quick-drench showers and washing facilities

accessible to areas of use and handling. Have supplies and equipment for

neutralization and running water available.

D] Gypsum (CaSO4.2H2O):

1) General information:

a) Exposure to airborne dust may cause immediate or delayed irritation or

inflammation. Eye contact by large amounts of dry powder or splashes of wet

gypsum dust may cause eye irritation.

b) Direct contact may cause irritation by mechanical abrasion.

c) Gypsum may contain trace amounts of free crystalline silica. Prolonged

exposure to respirable free silica can aggravate other lung conditions and

cause silicosis, a disabling and potentially fatal lung disease.

d) Exposure to gypsum dust may cause irritation to the moist mucous membranes

of the nose, throat, and upper respiratory system.

2) First aid measures:

a) Immediately flush eyes thoroughly with water. Continue flushing eye for at

least 15 minutes, including under lids, to remove all particles.

b) Wash skin with cool water and pH-neutral soap or a mild detergent. Seek

medical treatment if irritation persists or later develops.

c) Remove to fresh air. Seek medical help if coughing and other symptoms do

not subside.

d) Do not induce vomiting. If conscious, have the victim drink plenty of water

and call a physician immediately.



3) Accidental release measures

a) Collect dry material using a scoop. Avoid actions that cause dust to become

airborne. Avoid inhalation of dust and contact with skin.

b) Wetting of spilled materials may be beneficial to minimize generation of

airborne dusts.

4) Handling and Storage:

Respirable crystalline silica-containing dust may be generated during processing,

handling and storage. Use well equipped ventilation, masks and goggles.



Chapter No 14: Pollution Control Measures

1) Control measures for dust emission from grinding section:

Maharashtra Pollution Control Board limit: 150 Mg/Nm3

Measures:

a) Vent fan damper should be fully open

b) Pulse air pressure should be OK (about 7 Kg/cm2)

c) If pulse sir pressure is below 6 Kg/cm2 then instrument and mechanical

maintenance department should be informed for corrosive action.

d) Differential pressure across dust collector should be less than 150 mmwc

e) Checking dust emission from central laboratory

f) If dust emission is more than 150 Mg/Nm3 then dust collector bags should be

inspected and replaced damaged bags.

Control emission of fluorine fumes at work place:

Maharashtra Pollution Control Board limit: 25 ppm at stack and below 3 ppm at work

place

Measures:

a) Premixer, digester duct inspection cover should be in closed position

b) Closing ducting and launder covers, if covers are open

c) Ensure cleaning of exhaust duct daily to avoid choking in exhaust duct

d) If ducting is in choked condition, then mechanical maintenance department

should be informed to drop the ducting for cleaning.

e) Venturi water valve to venturi scrubber should be open



Control measures for minimizing generation of large quantity and bad quality

effluent:

Measures:

To improve effluent quality:

a) Isolate equipment to be cleaned

b) Recycle slurry to process

c) Dilute the balance slurry with water before draining

d) Checking that foaming is not taking place in filtrate compartments etc. to

prevent acid spillage

e) Ensure antifoam dosing in filtrate compartments etc. in case of foaming

f) Check no leakage is taking place from phosphoric acid, slurry warman pumps,

etc.

g) Isolate pump in case of leakage and inform mechanical maintenance

department for attending leakage

h) Check filter wash system is working in auto mode to prevent acid overflow

from filtrate compartments, return acid tank

i) If filtrate wash system is not working in auto mode then instrumentation

department should be informed

j) Analyzing effluent pH, total phosphate and maintain records

k) Plant effluent is recycled back in process

To reduce in effluent generation quality:

a) Ensure dry cleaning of equipment, floors to minimize effluent generation.

Water is not to be used for cleaning

b) Use only minimum water for final cleaning of equipments if required

c) Transferring the sludge collected from equipment after dry cleaning to

crystallizer to recover P2O5 content from gypsum

Analyze effluent quantity trend and maintain records and take appropriate actions in

case of deviations in quality and quantity of effluents.



Chapter No 15: Any Other Related Topic

1. HYDROFLUOSILISIC ACID (BYPRODUCT) SEPARATION:

i. This is the ultimate product of hydrogen fluoride (HF) which is obtained

from first reaction as follows

ii. For fluorine recovery the hydrogen fluorine evolved in fluorine in reaction

as above may react with excess silica present in rock to form silicon tetra

fluoride which when hydrolyzed to form fluosilisic acid. These reactions

are

iii. These hydrofluosilisic acid was found to have the following specifications.

a. H2SiF6 : 14% min

b. P2O5 : 1-1.5% max

c. Suspended Solids : 2-3% max

iv. H2SiF6 has found various applications in numerous field as follows

a. In water fluoridation

b. In ceramics to increase hardness

c. As Disinfectant in copper & brass vessel in breweries

d. In building works for hardening cement, plaster of paris, concreate

flooring, preserving masonry.

e. As general Disinfectant

f. In technical plants

g. As wood preservative & impregnating compounds

h. In electroplating

i. In the manufacture of sodium, Ammonium, Magnesium, Zinc, Copper,

Barium, Lead & other fluosilicates

It serves the following industries: Sodium, silica, glass, cement, fluoride

industries.

3Ca3(PO4)2CaF2 + 10H2SO4 + 5H2O → 6H3PO4 + 10CaSO4 + 1/2H2O + 2HF

4HF + SiO2 → SiF4 + 2H2O 3SiO4 + 2H2O → 2H2SiF6 + SiO2



v. SEPARATION TECHNIQUE:

a. EVAPORATION UNIT:

i. The product acid from storage tank is pumped to vacuum

evaporator which is associated with catch all separator on top

ii. Here the actual separation of phosphoric acid & hydrogen

fluorine takes place

iii. The phosphoric acid is separated from bottom ,HF gasses are

obtained from top which reacts with SiO2 to produce silicon

tetrafluoride SiF4

iv. These fumes then passed to next unit

b. FUMES SCRUBBING UNIT:

i. The fumes of SiO4 so obtained is scrubbed here with the help of

make-up water which is carries from the condensate tank

ii. In the scrubbing process SiO4 reacts with water to form H2SiF6,

Hydrofluosilisic acid

c. BAROMETRIC CONDENSOR UNIT:

i. Fumes of hydrofluosilisic acid are condensed & pumped to

storage tank.

vi. Material of Construction: The H2SiF6 acid storage tank is made up of

carbon steel with inner of the tank is rubber lined

vii. From here it taken up for transportation to desired destination

2. URANIUM EXTRACTION FROM WET PROCESS PHOSPHORIC ACID:

(LIQUID MEMBRANE APPROCH)

i. Uranium occurrence is usually less therefore extraction of uranium is done

from WPPA(Wet Process Phosphoric Acid)

ii. Earlier it was done by solvent extraction process. In 1970’s research on

liquid membrane approach for separation of uranium from wet process

phosphoric acid was initiated at exxon research &Engg. co.



iii. Liquid membrane technology utilizes water-in-oil emulsion as vehicle to

simultaneous removal of uranium from WPPA feed & concentrate it

iv. The simultaneous extraction & stripping operation of LM process results

in kinetically controlled uranium separation , removing some of limitations

of the thermodynamically dominated solvent extraction process

v. In contrast to solvent extraction LM extraction exhibited an inverse

temperature response, insensitivity to phosphoric acid strength of 5-8 m &

insensitivity to concentration of complexing agent in the range of 0.07-

0.14 m

WORKING:

i. The process sequence involved emulsifying the aqueous strip phase into an

organic phase containing a surfactant &complexing agents

ii. The emulsion was then dispersed in aqueous feed phase, which contained

the species to be removed. During this complexing step, the desired

species was transferred & concentrate in emulsified aqueous internal phase

iii. Upon completion of contracting, the emulsion &raffinate phases are

disengaged in gravity settler.

iv. The last step in LM process is the isolation of the concentrated aqueous

internal phase

ADVANTAGES:

The unique aspects of this technique was that extraction & stripping were affected

simultaneously than sequentially in conventional solvent extraction

3. ENVIRONMENTAL IMPACT OF PHOSPHOGYPSUM:

i. Usually the phosphogypsum obtained from WPPA is disposed on

agricultural land

ii. This phosphogypsum is laden with heavy metals compounds etc.

which are capable of detoriating the land quality & capacity

iii. Heavy metals includes Arsenic, cadmium, chromium, etc. which are

very poisonous if intake will done by plants

iv. In order to reduce the toxic contents from the phosphogypsum, they are

treated with various solvent extraction process or many different

technologies



Chapter no. 16: Conclusion

Our project at “Rashtriya Chemical and Fertilizers Ltd.” Is “Manufacturing of

phosphoric acid using rock phosphate and sulfuric acid as a raw materials” of capacity

150 MTPD. The process is called wet process which gives phosphoric acid of

fertilizer grade i.e. of 27 – 30 %. This acid acts as an intermediate raw material in

manufacturing of other fertilizers like suphala, ANP, DAP etc.

This project has provided us with the opportunity of implementing the knowledge of

chemical engineering aspects whichever we had studied through our academic courses

into practical considerations. Thus helps us in improving our concepts related to the

same. And throughout we cover the topics such as mass balance, energy balance,

design of critical equipment, thermodynamics etc. and realize that whatever covered

theoretically varied a lot with what happens practically.

As this project was guided to us by two of our guides (internal and external) which

were bounded with abundance of knowledge had helped us in every way in improving

our skills and thought us new things like habit of reading journals, magazines, books,

art of making presentation, writing bibliography etc, which is a great present to us and

will be helpful throughout our life. We show our gratitude towards them for guiding

us and shaping and moulding us to the best.

Here we conclude our project by experiencing lots of industrial knowledge and

enthusiasm.



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27. William H. Waggman, “Phosphoric Acid and Phosphatic Fertilizers”,

Reinhold Publishing Corporation, 2nd edition, 1962

28. R. K. Sinnot, “Chemical Engineering Design”, Volume 6, 4th edition, 2005,

Page no. 34-130, 194-240, 892-906

29. www.livestrong.com, 20/9/2014



30. http://www.wikipedia.org/wiki/phosphoricacid.html, 20/9/2012

31. http://www.aerofiltri.it/phosphate.html, 20/9/2014

Documents

Phosphoric Acid Manufacturing: using raw materials and salfuric acid