DSM 15 CRO KMO 5 april 1985

Development of a 'clean' phosphoric acid process

Author: K. Weterings, Central Laboratory DSM, Geleen, The Netherlands

Summary

The current 'wet' phosphoric acid processes are based on the digestion of phosphate rock with sulphuric acid. Disadvantages of these processes are that usually concentrated.phosporic acid can be obtained only by evaporation, and that the phosphoric acid and the by-product, gypsum, are contaminated.with,Ra, As, Cd and other heavy metals. To avoid these drawbacks, a new process with a so-called pre-digestion route has been proposed. In this process, the phosphate rock is first digested with phosphoric acid, in this phase radium can be precipitated. Next, A s , Cd and possibly other heavy metals are separated out, and only then Ca is removed with sulphuric acid. During a gypsum colloquium in 1982, organized by the 14inistry for Physical Planning, and Environment of the Netherlands, DSM pointed out that this is the only way to obtain both clean phosphoric acid and clean gypsum. In September 1982, a government-sponsored-project for the deve- lopment of this process on laboratory scale was started, with agreement from the other producers of phosphate fertilizers. The project is carried out by three doctoral candidates, under the respon- sibility and supervision of UKF and DSM. The work on Cd-removal concentrates on binding Cd to an immobilized organic complex, but other methods are also i nve s t i g a t ed . With regard to the removal of Ca, the final step in the production of phosphoric acid, attention focusses on crystallization of a clean calcium- sulphate-hemihydrate, that is, a hemihydrate containing the smallest possible amount of co-crystallized phosphate. In this report, the several aspects of the new phosphoric acid process will be discussed in detail, and particular

hemihydrate can be further processing or, now that it is Cd-free, disposal as a waste product.

attention is paid to hemihydrate crystallization. The destiny of the clean -

Table of contents

1. Introduction

2. Reasons for a clean phosphoric acid process

3 . How to obtain clean gypsum

4 . Dissolution of phosphate in phosphoric acid: pre-digestion

5 . Purification

6. Hemihydrate crystallization

7. Literature

,

/* evapors- - / t ion /

H3P04 / + DH/ /

b . c

1. Introduc tionl,

acid - 0.58 e

52 Z T205

Phosphoric acid is an important chemical intermediate, which is used mainly in the fertilizer industry. Phosphate-containing fertilizers are produced from calcium phosphate rock. The rock usually is converted first to phosphoric acid, because in this way readily soluble fertilizers with a high nutrient content can be obtained. Glo- bal phosphoric acid production i s directly related to the steadily growing demand for phosphate fertilizers, to meet the food needs of increasing world population. Phosphoric acid is produced either by the wet process or by the electric furnace process. In the electric furnace process, phosphate rock is reduced, with the aid of coke, in an electric furnace. Si02 (gravel) is added to bind the CaO released to form CaSi03. The overall reaction is given by

1500 OC 2 Ca3(P04)2 + 6 Si02 + 10 C --) P4 + 6 (CaO, SiO2) + 10 CaO (OH = +730 kcal)

In this reaction scheme, the presence of other impurities (F, Cog) is ignored. Since virtually all of the impurities in the rock remain behind in the by- product (calcium silicate slags and a ferrophosphorus alloy), the electric furnace process yields a very pure product acid, which is used in detergents and as nutrient phosphate. The energy demand of the electric furnace process however is enormous. Therefore, the world demand for phosphoric acid for use in the fertilizer industry is met by wet-process phosphoric acid plant; the impurities in the phosphoric acid are usually not detrimental and may even be essential as micronutrients and for granulating the fertilizers. Total wet-process phosphoric acid production is enormous: ca. 32 lo6 tonnes (World-1983),. In the Netherlands, the output is 0.6 . 10 6 tonnes. These quantities are expressed as 100 X H3PO4. Conversion of phosphate rock to phosphoric acid usually takes place by reaction of the rock with sulphuric acid; calcium pre- cipitates as calcium-sulphate-hydrate, Cas04 . 2 H20 or Cas04 35. H20, which is subsequently filtered off.

Figure 1: Schematic representation of the production of phosphoric acid and gypsum (DH) from phosphate rock and sulphuric- acid according to the classical dihydrate (DH) process.

75-80 "C ____*

water [Ca3(P04)2]3.CaF2 + 10 H2SO4 6 H3PO4 -I- 10 Cas04 2 H20 J. + 2 HF 3.

0

phosphate-1 ~ sulphuric acid41 '+ water

Most of the wet-process phosphoric acid produced in the world is obtained by the classical DH-process. Drawbacks of this process are: - the gypsum (DH) contains lattice P2O5, causing a low phosphate efficiency. The gypsum is not suitable for the building industry and cannot be used as a cement retarder.

wash water

- the gypsum as well a1 the phosphoric acid contain Ra, As, Cd and other heavy metals.

/ acid - 0.58 / ev apo r a- ,HH -WDH . /

- , + HH 1 * 1 complex

H3P04 t ion *

5 2 27 P205 /

- In the first instance, a dilute acid is obtained ( ca. 30 % P2O5). Con- centrated acid (ca. 52 % PzO5) is obtained by evaporation. -

In the fertilizer industry, the concentration of phosphoric acid is usually expressed in % P2O5. To obtaind the H3PO4 concentration, this figure should be multiplied by 1.38. Thus, a P2O5 concentration of 30 % is equivalent to ca. 41 X H3PO4 and 52 % P2O5 to ca. 72 % H3PO4. A separate gypsum purification step *in the current DH-processes (and in HH-DH-processes) is perhaps technically feasible, but not economically. Moreover, the phosphoric acid produced would stil contain Ra and Cd, and it would still have to be concentrated by evaporation. The so-called phosphogypsum can, in principle, be used instead of mineral gyp- sum in the building industry, after urification. In spite of this ca. 90 % of

into rivers or seas or impounded on land, world gypsum consumption being 6 96 . 10 tonnes.

Phosphogypsum can hardly compete with mineral gypsum3), since it comes available as a contaminated aqueous slurry, requiring extra purification and drying operations. Gypsum obtained in the DH-process contains too much co-crystallized P2O5, and the slight radioactivity of phosphogypsum is a major reason for not using it in the building,industry. No applications have been found for Dutch phospho- gypsum. The quantity of phosphogypsum produced in the Netherlands (ca. 2,4 . lo6 tonnes/yrb is much greater than the quantity of mineral gypsum con- sumed (ca. 0.7 . 10 tonnesjyr).

the phosphogypsum produced (115 10 % tonnes, world, 1983) is disposed off

50 B 60 "C 10 Cas04 . f H20 I 10 Cas04 2 H20

H2S04/H3POq/water

-4-

A higher phosphate efficiency, and a dihydrate suitable f o r building products and as a cement retarder, is obtained by modifying the classical DH-process into a HH-DH process (e.g. the Nissan I phosphoric acid plant of UKF in Pernis, the Netherlands). In this process, the calcium sulphate lattice is recrystallized from hemihydrate to dihydrate (DH) or the other way around; in this way, co-crystallized phosphate is largely removed, but radium and other heavy metals remain present in virtually the samen quantities.

- 2. Reasons for a clean phosphoric acid process4)

In the production of phosphoric acid, the impurities originally present in the phosphate rock will be distributed between the phosphoric acid and the gypsum as indicated in Table 1. The distribution slightly varies with the process used; Table I reflects the situation at the UKF plant in Pernis, the Netherlands.

Tabel 1. Average composition of sedimentary phosphate, phosphori'c acid and phosphogypsum

Sedimentary Phosphoric Phosphogypsum acid phosphate -

~205' wt. % F wt. %

org. C. wt. % V-PPm Cr-ppm U-PPm Ra-10'6 ppm As PPm Cu ppm Pb PPm Cd PPm Mg PPm

s i 0 2 wt. %

32 4 3 .O 0.30 240 170 120 38 20 33 10 8-100 (28)* 0.1-0.5

52 (HH-DH) 1.2 0.6 0.12 290 310 170 2 - 3 18 47 2 12-130 (36)* 0.1-0.3

_ _

: 0.4; l .O (DH) 1 .o 1.1 0.15 ca. 20 5 5 22 3 4 4 1-15 (2 - 3 ) * 4; 0.1-0.3

* UKF-Pernis

The table shows that phosphogypsum is slightly contaminated with heavy metals (ppm-level). In Western Europe there are international skeletal agreements __

aiming at progressive reduction of gypsum dumping. Since 1975, the Dutch National Working Party on Phosphogypsum, in which the

. governement and the phosphate fertilizer industry are represented, has been working on the problem. It has become clear that, in terms of environmental hygiene and economics, fill utilization of the available phosphate gypsum is hardly feasible. This holds especially for the current, Ra- and Cd-containing, gypsum. Recentiy, severai phosphoric acid plants have been built which involve a direct route to strong acid by modified conditions (no sulphuric-acid

~

,

96 Z sulphu- phosphate-] riC acid-1

dilution; higher temperature). These are the so-called HH-processes (e.g. Fisons) and the HH-DH-process with intermediate filtration of HH followed by HH- td DH-recrystallization (Nissan 11). These processes are more energy-efficient. The drawbacks that remain are a low phosphate efficiency because of incorporation of P2O5 in the HH (Fisons) and the presence of impurities in the HH or DH and in the phosphoric acid (Fisons, Nissan 11).

0.25 X lattice P205

wash water

Figure 3. Schematic representation of the HH-process (Fisons-process)

- 0 /

/ - H3P04 + HH / 0

L A

wash water phosphate-1 96 Z sulphuric acid-1

I J I . 1 T;: lattice P205 hemihydrate 1.3

4

/ H3P0 / - 0

/ HH 1 , /

product acid '

ca. 42 ,"d P205 . return acid - -c

/ - , /

HH - DH - /

/ /

Figure 4. Schematic representation of the HH-DH-process (Nissan 11)

ca. 100 "C [Ca3(P04)2]3 e CaF2 + 10 H2SO4 -6 H3PO4 + 10 Cas04 e f H20 + + 2 HF .t

- less water than with DH and Nissan I

-b-

The newly proposed, clean phosphoric acid process, which is yet to be developed, aims at producing clean phosphoric acid and gypsum free from heavy metals and P2O5, so that disposal and/or further processing of the gypsum is less of a problem.

3 . How to obtain clean gypsum4)

In principle, there are three ways to obiain clean gypsum, possibly in com- bination with clean'phosphoric acid: - using clean phosphate rock; - gypsum purification - a clean phosphoric acid process.

Almost all phosphate rocks are sedimentary deposits. Igneous rocks are much purer, especially with regard to heavy metals (Cd 8 ppm; Ra 1-2.10'6 ppm). However, their lead content is the same or higher. The world reserves of igneous rock are very limited, so that the use of igneous phosphate as raw material is no solution of the purity problem.

As pointed out in Chapter 2 , much research has been done, on an international scale, on methods torpurify phosphogypsum by re-crystallization during the manufacture of phosphoric acid, to obtain a grade suitable for use as a building material and as a cement retarder. However, this does not solve the problem of gypsum contaminated with Ra, As and heavy metals. A separate gypsum purification is not economically feasible, while in addition the phosphoric acid produced still contains Cd.

In current phosphoric acid processes, phosphate rock is directly digested with H2SO4. In the first reactor, the gypsum or HH will precipitate, and many impurities will co-crystallize. Up to now a considerable reduction of these impurities by re-crystallization during the process has been found t o be impossible, except for P20.j. This problem can be solved by a process with a pre-digestion step in which Cas04 phosphoric acid.

precipitation is preceded by digestion of the phosphate rock with

Figure 5. Schematic representation of the pre-digestion route

phosphate rock-l sulphuric acid- 1 7

c

wash water

After the pre-digestion step, during which special measures are taken to pre- cipitate radium, first As, Cd and possibly other heavy metals are removed; after this Ca is removed by HH-precipitation with H2SO4: - Pre-digestion: [Ca3(P04)2]3 . CaF2 + excessive amount of phosphoric acid + Ca(H2P04)~ in phosphoric acid. - Purification: measures for removal of undes€rable elements. Possibly, measures are taken to prevent incorporation of Cd in HH. In that case, only the product acid still needs to be purified. - Calcium sulphate precipitation: 2 Ca(H~P04)2 + 2 H2SO4 + H20 -+ 2 Cas04 . f H20 + + 4 H3PO4. Since the current phosphoric acid processes (e.&, UKF Pernis) involve a high energy cost for evaporation of product acid (30 X P2O5) to concentrated phosphoric acid (52 % P2O5>, the variable production costst of phosphoric acid production according to the pre-digestion route, in which concentrated acid is obtained directly, may well be lower than the variable costs of current processes, so that production of clean phosphoric acid and gypsum according td the pre-digestion route may be possible at not too high additional costs. In cooperation with DSM and UKF, the Delft Institute of Technology has been doing exploratory work on pre-digestion and on removal of Ra and heavy metals. In particular, the precipitation of Cd as a Cd complex has been studied. During a gypsum colloquium in March 1982, which was organized by the then Mihistry of Health and Environment of the Netherlands, DSM pointed out that production of clean phosphoric acid and clean gypsum required new technology, and in September 1982 a government-sponsored development project was started at the Delft Institute of Technology, with agreement from the Dutch Phosphate Fertilizer Industry and under the responsibility and supervision of UKF and DSM, Since late 1983 early 1984 and early 1985 three doctoral candidates are working on the project.

-

pre-

t ion diges-

I

4. Dissolution of phosphate in phosphoric acid: pre-digestion

product acid -

- 9 . 6 4 purif i- HH- cation precipi- L / L-

/

tation 47 % P*05 ,/ / -

/

When phosphate rock is digested with phosphoric acid, unwanted impurities in the phosphate rock, such as Fe, A I and Si, dissolve to a lesser extent than with digestion by sulphuric acid. undissolved impurities can be removed as inerts after digestion.

I I impurities

I I 11

. Phosphate rock contains about 5 0 % CaO, whereas only 3-5 .5 % CaO dissolves in phosphoric acid, depending on temperature ( 8 0 - 1 2 0 "C) and phosphoric-acid con- centration ( 4 2 - 4 7 % PzO5). Therefore, very large recycle streams are necessary, compared with the current processes (see Figure 5 ) . This disadvan- tage is offset by the fact that the dissolution of phosphate, in the presence of free H2SO4, can be a fast reaction and also because the evaporation step is largely or totally dispensed with.

CaO (5.6 %) dissolves only at high temperatures ( 1 2 0 "C), as indicated in Figure 6 . At low temperatures, a high percentage of CaO can dissolve ( 5 . 7 X), but then the phosphoric acid concentration should be much lower. In order to have favourable laboratory operating conditions, it was decided to use a digestion temperature of ca. 90 "C, at which temperature about 3 . 5 % CaO dissolves. The phosphoric acid concentration being 47 % P2O5.

At a high phosphoric acid concentration ( 4 7 % P 2 O 5 ) , the maximum percentage of -

-

Figure 6 . The relation between the phosphoric acid concentration and the tem- perature at which the maximum percentage of CaO dissolves

% P O

2t5

l o 1

5 . 5 ./ 2 47 % P,O,

/- dissolved CaO /

constant at about 5 . 7 0

I o 120 (y boiling point)

I I 1 I I I I I I

0 20 4 0 60 80 100 120 140 160 -t

Temp. in OC __

The dissolution of phosphate in phosphoric acid and the precipitation of hemi- hydrate from the solution constitute an essential element of the new ~

phosphoric acid technology which is being developed. If too many sulphate ions are present in the recycled acid, precipitation of calcium sulphate on the surface of the phosphate rock particles may present a major problem, because the resultant impermeable coating around the particles blocks further digestion. This problem may also arise in current processes with also a large amount of HH already being produced in the predigestion step. When the con-

-7-

centration of sulphate ions increases, the calcium sulphate nucleation front moves through the liquid film towards the particle surface. When the sulphuric acid concentration is sufficiently high, the fron will reach the particle surface, completely blocking the digestion reaction. Obviously, this should be prevented by keeping the excess amount of H2SO4 sufficiently low (< 2 %).

5. Purification

5.1. Ra-removal --------------- In one version of the process (DSM), the pre-digestion of the phosphate with phosphoric acid (return acid) is combined with simultaneous precipitation of ca. 10 % of the calcium ions by addition of a suitable quantity of sulphuric acid to the return acid. By, for example, flocculation, followed by f.iltration, the fine RaS04 particles are removed together with the hemihydrate* obtained. Next, after removal of Cd etc., the actual hemihydrate precipiation step takes place. In another process (Giulini),.BaC03 is added to precipitate RaS04 *together with Bas04 and hemihydrate: Ca3(P04)2 + excessive amount of phosphoric acide + very deficient amount of sulphuric acid + (possibly) BaC03 + Ca(H2P04)2 in phosphoric acid + Cas04 . f H20 J. f Ba(Ra)SOq + . Other methods of removing Ra are being studied by the DelEt Institute of Technology.

5.2. Cd-removal

The problem of Cd storage and disposal is least when cadmium and other unde- sirable elements are removed after the contaminated HH and inerts formed during digestion have been separated from the monocalcium-sulphate-in- phosphoric-acid-solution. In principle, there are many ways to remove Cd, for example precipitation, ion exchange, absorption, adsorption, electrodialysis etc. Some methods are not economically feasible, e.g. extraction. Others are chemically or technically impossible (zeolites cannot be used for example, because of the pH of the solution and the presence of Ca in the digestion liquor). In practice, the choice of Cd-removal method is restricted to (co)-precipitation, adsorption (whether or not preceded by precipitation) and ion exchange. The most obvious technique, precipitation as CdS (with Na2S or H2S), will result in the for- mation of colloidally dispersed CdS due to the small quantities involved, while H2S will inevitably also be formed, which is undesirable. H2S formation is also a drawback of co-precipitation of CdS with, for example, CuS or MCP. A possibility which has been researched extensively, is precipitation of Cd as a Cd-complex. For this purpose a selection was made out of. organic S-compounds. A problem is the stability demands under the prevailing con- ditions (95 "C, 47 % P2O5). The indicated stability demands are important as well in the method of ion-exchange. - Next to the above-mentioned methods, studies are in progress to prevent Cd- incorporation in HH. In that case, only the product acid still needs to be purified, which, because-of the smaller quantity to be treated and the possibly l e a s stringent c o n d i t i o n s , could offer an attractive alternative.

---------------

-1u-

3.2 % CaO; 45 X P 3 0 5

(45-47 %> P,O, I d

17 2 H2S04

6 . Hemihydrate crystallization5)

this s t e p 4 left 01

.”

H3P0

with 1

After pre-digestion, inerts filtration and specific procedures for either removal of undesirable elements or for prevention of incorporation of such elements in HH, actual phosphoric-acid production takes place by Ca- precipitation as hemihydrate, the Cas04 modification stable at the prevailing temperature and phosphoric acid concentration. The current status of the work on continuous HH-crystallization is represented by the schematic diagram of the continuous laboratory set-up below (Figure 7 ) .

7 - 7 b

- Figure 7. Continuous HH-precipitation by combination of the digestion liquor and the H2SOq/H3P04 mixture

O X H- . b

digestion liquor may be

I It a -1 % H, 0

1 product a$d lhemih ydrat e

The amount of free H2SO4 to be applied is arbitrary, except in the final

In order to achieve a high P2O5 efficiency, but also to make the gypsum suitable for building applications or to facilitate disposal of the gypsum, it is essential that the phosphate content of the gypsum is as low as possible. Phosphate is mainly present in the gypsum as lattice P2O5, which is phosphate incorporated in the Cas04 crystal lattice by replacement of SO4- ions by HPO4= crystal structure, with the same unit cell dimensions. The substitution results in a solid solution of Cas04 . nH20 and CaHP04 . nH20. The undesirable substitution is promoted by a high phosphate concentration, and by an increased viscosity, which slows down the reaction to CaSOq-hydrate by diffusi‘on inhibition. The substitution is reduced by a temperature increase, which results in a lower viscosity, and by an excess of free H2SO4, so-that more protons are available to desolvate the phosphate-ligand of Ca2+ and more Cas04 Note: by free H2SO4 is meant the amount of sulphuric acid which is con- tinuously present in excess of the stoichiometric amount of Ca* and SO4= ions required for precipitation of CaSOq-hydrate. There are limits to the amount of free H2SO4 that can be applied, in connection with undesirable secondary nucleation (see below). The calcium sulphate precipitate should meet the following requirements: - it should be easy to filter, i.e. the crystals should be large and not too reguiar (no dendrites!); - it should be easy to wash, i.e. the crystals should be firm and smooth;

. crystallizer.

- ions. This can hardly be prevented: Cas04 and CaHP04 have the same

and hence less Ca-phosphate is present. __

__

- -

- the crystals should be maximally pure. A compromise must be struck between the first two requirements.

Also in HH-crystallization, the sulphate-ion concentration plays a dominant role. When this concentration is-high, the rate of secondary nucleation is also high, resulting in a very large amount of small crystals and in defects and dislocations, because of the high degree of supersaturation. The extent to which the sulphate-ion concentration deviates Erom the stoichiometric concentration greatly influences the shape and dimensions of the CaSOq-hydrate crystals formed, especially in industrial crystallizers (in a medium of pure monocalciumphosphate and H2SO4 this influence is much smaller) : 1) a large excess of free sulphuric acid: long thin needles; readily filterable but less readily washable.

2) a moderate excess of sulphuric acid: larger crystals, in the shape of short, thick needles, which are readily filterable and washable.

3) a small excess or even a deficiency of sulphuric acid: relatively small crystals.

Besides S0q'-ions, impurities also have a large influence. In view of the poor solubility of a salt like hemihydrate, there should be sufficient crystal sur- face area (more than 10 wt % crystals present). In order for the crystals to be maximally pure, they should not grow too East; otherwise, co-precipitation and inclusion of impurities are likely to occur. Phosphate losses, by imperfect digestion do not occur in the pre-digestion route. Phosphate losses by inclusion of phosphoric acid and lattice P2O5, can be prevented by quick levelling of-local high calcium sulphate supersaturation in the medium and a well-controlled, slow crystallization of the HH, that i s , an optimum concentration of free SOq2' at a constant Ca-supply. At a par- ticular percentage of free acid, high supply rat-es of monocalcium phosphate solution and, hence, of H2SOq/phosphoric acid mixture, which are both factors determ-ining the degree of supersaturation, always yield higher amounts of lat- tice P2O5 than low supply rates'). Further, at a particular supply rate the amount of lattice P2O5 first decreases, as the concentration of free acid increases from 1 X to about 2.2 % but then increases. The minimum is about 0.35 % lattice P2O5 (relative to DH) at a phosphate rock throughput of 100 kg/hr .m3.

In the classical DH-process and its process variations (e.g. HH .+ DH and HH), the percentage of lattice P2O5 keeps decreasing as the concentration of free sulphuric acid increases (see Fig. 8). In the pre-digestion route, however, there is an optimum in the concentration of free acid. The initial decrease in the inclusion of lattice P2O5 with increasing sulphuric-acid concentration has the same explanation as in the classical DH process: more protons are available to desolve phosphate ligand, and more Cas04 is present than Ca-phosphate.

9

Figure 8. Lattice P2O5 in DH (classical route) and HH (pre-digestion) as a function of the concentration of free sulphuric acid

Z lattice P 0 I

(calculated re- I lative to DH) I

2 5

1 e is present

k 250 pre-digestion (47 2 '205) * 100

classical DH-proces (30 Z P205) 1

-1 0 1 2 3 4 I I I a

Z free H2S04

3 Phosphate-rock throughput in kg/m .hr

The'increase in the concentration of lattice P2O5 in the predigestion route ~

which occurs at free sulphuric acid concentrations of more than about 2.2 % is probably due to the fact that HH is instantaneously formed from Ca2+ and S042', whereas in the classical phosphate digestion with H2S04, HH and DH are formed from S042' and phosphate rock. Furthermore if the concentration of free sulphuric acid becomes too high, the increased degree of.secondary nucleation that may result from this will cause the amount of phosphate

As indicated in Figure 7 , first HH is precipitated to yield a free-sulphuric- acid concentration of 2 %. The excess sulphuric acid is compensated in the second crystallizer. Fig. 9 below shows that at 0 % H2SO4 much HH i s dissolved in the phosphoric acid. If this phosphoric acid is used for dissolving (digesting) the phosphate rock, a large amount of Ca-ions will be returned together with the phosphoric acid, so that during pre-digestion the dissolved HH will crystallize (again see Fig. 9). This can be prevented by adding mono- calciumphosphate to the return acid in a third crystallizer, for example until the free-sulphuric-acid conentration is -1 %, so that most of the HH will crystallize there.

' included in crystals to increase.

__

-

Figure 9. percentage of free S042', at 90 "C and 45 % P2O5

The solubility of hemihydrate, as % CaS04, as a function of the

0.94.

7, CaS04

1 .o

\

-3 -2 -1 1 2 3

.

? * 0

7. Literature

1. Fertilizer Science and Technology series Volume I, part I and 11, Phosphoric acid edited by A.V. Slack; Marcel Delcker inc., New York-1968

2. Fertilizer Science and Technology series Volume 111, Phosphates and phosphoric acid Pierre Becker, Marcel Dekker inc., New York and Basel-1983

3. K. Weterings Verwerking van afvalgips uit de fosforzuurbereiding (phosphogypsum processing) PT-procestechniek - 35, 1980, no. 2, pp. 79 - 86 Chemisch Magazine, April 1980, M 233 and 234.

4. K. Weterings The Utilisation of phosphogypsum The Fertilizer Society, Londen 21110182, Proceedings no. 208

5. M. Jansen, A. Waller, J. Verbiest, R.C. van Landschoot, G.M. v. Rosmalen Incorporation of phosphoric acid in calciumsulphate-hemihydrate crystallized from a monocalciumphosphate solution. 9th symposium om Industrial Crystallization, 25-28 Sept. 1984 Process Technology, Proceedings 2, Elsevier, dated by S.J. Jancic and E.J. de Jong, pg. 171-176. .

6 . S . E . Dahlgren Physico-chemical aspects of phosphoric acid by-produced calciumsulphate as a raw material. Chemie-1ng.-Techn. MS 104174.

7. A . B . Amin and M.A. Larson i & EC, Process Design and Development 7, 1968, 135.