UNIT-1

SODA ASH:

washing soda or soda crystals or Sal Soda. and is a sodium salt of carbonic acid.

Soda ash has a high pH in concentrated solutions. It can irritate the eyes, respiratory tract and skin. It should not be ingested, because it can corrode the stomach lining.

The accepted commercial standard for soda ash is expressed in terms of the equivalent sodium oxide (Na2O) content. A 99.5% soda ash is equivalent to 58.2% Na2O (the conversion equation is: % Na2CO3 x 0.585 = % Na2O).

• Properties • Molecular formula Na2CO3• Molar mass 105.9884 g/mol (anhydrous)

124.00 g/mol (monohydrate) 286.14 g/mol (decahydrate)

• Appearance White solid, hygroscopic • Density 2.54 g/cm3 (anhydrous)

2.25 g/cm3 (monohydrate) 1.46 g/cm3 (decahydrate)

• Melting point 851 °C (anhydrous) 100 °C (decompose, monohydrate) 34 °C (decomp, decahydrate)

• Boiling point 1600 °C (anhydrous)• Solubility in water 22 g/100 ml (20 °C) • Insoluble in alcohol, ethanol

Physical properties

Sodium carbonate is a white crystalline solid, which can exists as anhydrous salt (Na2CO3), monohydrate salt (Na2CO3.H2O), heptahydrate salt (Na2CO3.7H2O) and decahydrate (Na2CO3.10H2O - washing soda).

Sodium carbonate is readily soluble in water.

On heating, decahydrate salt gradually loses water to finally give anhydrous salt (Na2CO3 - soda ash)

Chemical PropertiesAction of water Sodium carbonate dissolves in water with the evolution of heat

and the aqueous solution is alkaline in character due to hydrolysis.

Action of acids Sodium carbonate is readily decomposed by acids when

carbon dioxide is evolved.

Action of carbon dioxideWhen carbon dioxide gas is bubbled into the aqueous solution of sodium carbonate, sparingly soluble sodium bicarbonate is precipitated.

Action of limeSodium carbonate is causticised by lime, giving caustic soda.

Action of sulphur dioxide When SO2 gas is passed into the aqueous solution of sodium

carbonate, sodium sulphate or bisulphate is produced.

With metal salt solutionsWith aqueous solutions of certain metal salts, sodium carbonate gives carbonates, basic carbonates or hydroxides

Reaction with sand (SiO2)When sodium carbonate is fused with sand, water glass or sodium silicate is formed.

Uses For the manufacture of glass. For washing purposes in laundries. For the manufacture of other sodium compounds like

sodium silicates, sodium hydroxide, borax, hypo etc. As a household cleansing agent. In paper and soap/detergent industries. For the softening of water. A mixture of Na2CO3 and K2CO3 is used as a fusion

mixture. In textile industry and petroleum refining fire extinguisher powder drilling mud to improve fluidity

USES IN INDUSTRIAL SECTORS : Glass industry: Soda ash is used in the manufacturing of flat and container

glass. Acting as fluxing agent, it allows lowering the melting temperature of sand and therefore reduces the energy consumption.

Detergent industry: Soda ash is used in a large number of prepared domestic

products: soaps, scouring powders, soaking and washing powders containing varying proportions of sodium carbonate, where the soda ash acts primarily as a builder or water softener.

Steel industry: Soda ash is used as a flux, a desulfurizer, dephosphorizer and

denitrider.

Manufacturing processes:

1)Leblanc process

2)Solvay’s process (Ammonia-soda process)

3)Dual process (modified Solvay's process)

Leblanc process:

In 1791, the French chemist Nicolas Leblanc patented a process for producing sodium carbonate from salt, sulfuric acid, limestone, and coal.

First, sea salt (sodium chloride) was boiled in sulfuric acid to yield sodium sulfate and hydrogen chloride gas, according to the chemical equation

2 NaCl + H2SO4 → Na2SO4 + 2 HCl

Next, the sodium sulfate was blended with crushed limestone (calcium carbonate) and coal, and the mixture was burnt in a furnace , producing calcium sulfide.

Na2SO4 + CaCO3 + 2 C → Na2CO3 + 2 CO2 + CaS

The sodium carbonate was extracted from the ashes with water, and then collected by allowing the water to evaporate.

Draw backs:

The hydrochloric acid produced by the Leblanc process was a major source of air pollution, and the calcium sulfide by product also presented waste disposal issues. However, it remained the major production method for sodium carbonate until the late 1880s.

Hydrochloric acid gas, was industrially useless in the early 1800s, it was simply vented into the atmosphere.

Also, an insoluble, smelly solid waste (Calcium sulphide ) was produced.

For every 8 tons of soda ash, the process produced 5.5 tons of hydrogen chloride and 7 tons of calcium sulfide waste. This solid waste (known as galligu) had no economic value, and was piled in heaps and spread on fields near the soda works, where it weathered to release hydrogen sulfide, the toxic gas responsible for the odor of rotten eggs.

Solvay’s process:

In 1861, the Belgian chemist Ernest Solvay developed a more direct process for producing soda ash from salt and limestone through the use of ammonia. The only waste product of this Solvay process was calcium chloride, and so it was both more economical and less polluting than the Leblanc method.

Solvay’s process flow sheet

Principle reactions:

When carbon dioxide is passed into a concentrated solution of brine saturated with ammonia, ammonium bicarbonate is produced,

The ammonium bicarbonate then reacts with common salt forming sodium bicarbonate,

Sodium bicarbonate being slightly soluble (in presence of sodium ions) gets precipitated. The precipitated sodium bicarbonate is removed by filtration and changed into sodium carbonate by heating.

The mother liquor remaining after the precipitation of sodium bicarbonate contains ammonium chloride. This is used to regenerate ammonia (one of the raw materials) by steam heating with milk of lime.

Lime is obtained by heating limestone.

Ammonia and carbon dioxide liberated are utilized in making the whole process cyclic and continuous. The only by-product in the process is calcium chloride.

Process description:Saturating or ammoniating tank:

• In this tank, impurities of calcium and magnesium salts present in brine are precipitated as carbonates or hydroxides. This occurs when a little carbon dioxide gas (from ammonia recovery tower) is bubbled through a 20% sodium chloride solution (brine).

Filter:

The precipitated carbonates and hydroxides are removed after pumping the ammoniacal brine through filter press.

Cooler:

The clear ammoniacal brine is passed through coolers to bring it to room temperature

Carbonating tower:

Carbonation of ammoniacal brine is carried out on the principle of counter-current. The clear ammoniacal brine solution is pumped to the top of the tower. This tower is partitioned using horizontal plates that have a central hole and covered with a perforated plate.This flows downwards and meets a current of CO2 introduced from the bottom of the tower at a pressure of 1-2 atmosphere. As a result of the reactions therein, ammonium chloride and crystals of sodium bicarbonate are formed.

These crystals remain suspended in the mother liquor forming thick milky liquid.

Rotary suction filter This thick milky liquid is then filtered by means of

the rotary suction filter. Sodium bicarbonate deposits on the filter cloth. After washing with a spray of cold water it is scrapped off, and dried. The filtrate containing ammonium carbonate, ammonium chloride and some sodium chloride is pumped to the top of the ammonia recovery tower.

Lime kilnHere, limestone is burnt to produce carbon

dioxide and lime.

Carbon dioxide is sent to the carbonating tower, while the lime is slaked in a large quantity of water to form milk of lime. This is pumped to the middle of the ammonia recovery tower.

Ammonia recovery tower: Here, the filtrate from the rotary suction filter

containing ammonium bicarbonate and ammonium chloride is introduced at the top of the tower. Milk of lime enters at the middle part, and steam is introduced from the base. Ammonium bicarbonate is decomposed by heat while ammonium chloride is decomposed by milk of lime.

The gaseous ammonia along with a little carbon dioxide is sent to the saturation tank, while the liquid flowing out of the tower contains calcium chloride , the only waste.

Calcination Sodium bicarbonate obtained from the rotary suction

filter is calcined in closed tubular calcining pans fitted with scrapers. On calcination, sodium carbonate is formed and carbon dioxide is evolved.

The soda ash obtained from the pans is dissolved in water and crystals of hydrated sodium carbonate Na2CO3.10H2O, are recovered from the solution by crystallization.

Merits of the Solvay's process:This process gives soda of much higher purity

(99.5%). It is less expensive as the raw materials are

cheap and CO2 and NH3 are recovered to be used again.

Calcium chloride is the only waste and so no pollution problems.

Formation of sodium bicarbonate as an intermediate.

Can use low-grade brine Less electric powerLess corrosion problemsDoes not require ammonia plant investment

Disadvantages of Solvay processHigher salt consumption Higher investment in ammonia recovery unit

verses crystallization units for ammonium chloride

Waste disposal of calcium chloride More steam consumptionHigher capacity plant for economic break-even

operation With current fertilizer shortage, all of the

ammonium chloride will be used as a mixed chemical fertilizer ingredient, so co-product disposal no problem

Dual process

• In this process ammonium chloride is produced as a co product and

differs from Solvay process ,in that it does not recycle ammonia.

Process description:

• The mother liquor from the carbonating system, containing ammonium chloride, un reacted salt and traces of carbonate is passed through a bed of salt in a salt dissolver to help in precipitation of NH4Cl which is crystallized in a refrigeration tank operating at 0 C .

• The slurry is centrifuged and NH4Cl crystalls are dried in rotary drier and packed for shipping.

• The product is 98% pure and is marked as ammonium chloride fertilizer with nitrogen content of 25% nitrogen content.

• The mother liquor obtained after the separation of ammonium chloride crystals is recycled to the carbonation vessels placed in series.

Chlorine:

• ColorChlorine has a greenish yellow color.• OdorIts odor is pungent and suffocating.• Vapor densityThe vapor density of chlorine = 35.5, which is nearly 2.5 times denser

than air (Vapor density of air = 14.4).• SolubilityChlorine is only moderately soluble in water. A 100 ml of water can

dissolve about 46 ml of chlorine at 0oC. The solution smells of chlorine, and is called 'Chlorine water'.

• LiquefactionWhen chlorine is cooled and compressed it gets liquefied to an amber

colored liquid. It boils at -34.5oC

• Chlorine is used for bleaching cotton, linen, wood pulp, etc. • It is used in the sterilization of drinking water, and the water in

swimming pools. • The strong oxidizing action of chlorine destroys organic matter like

bacteria, fungus and other micro-organisms. In most of the advanced countries chlorine has now been mostly replaced by ozone, which is more environment friendly disinfectant.

• It is used in the extraction of gold, platinum, etc. • Chlorine is used in the manufacture of bleaching powder,

hydrochloric acid, chlorides, chlorates, chloroform, carbon tetrachloride, etc.

• It is used in the production of poisonous gases like phosgene (carbonyl) chloride - COCl2), tear gas (chloropicrin - Cl3.NO2).

• In public sanitation chlorine is used as a disinfectant the form of bleaching powder.

Caustic soda (Sodium hydroxide):

• Sodium hydroxide, NaOH, is also known as lye. • Molecular formula :NaOH• Molecular weight : 40

Properties: • Physiacl state: White, deliquescent ,avialable in pellets or flakes

form. • M.P : 318 C• B.P :1390 C • Specific Gravity : 2.13 • strong base • water soluble • dissolving NaOH in water is an exothermic reaction • absorbs carbon dioxide from the air

Uses: Soap and detergent production Pulp manufacturing for producing paper

(Sodium hydroxide was also widely used in making paper. Along with sodium sulfide, NaOH is a key component of the white liquor solution used to separate lignin from cellulose fibers in the Kraft process. And also in bleaching the brown pulp resulting from the pulping process.)

Domestic uses

(Sodium hydroxide is used in the home as a drain cleaning agent for clearing clogged drains. It is distributed as a dry crystal or as a thick liquid gel. The chemical mechanism employed is the conversion of grease to a form of soap. )

Extraction of alumina from bauxite (Bayer Process) (In the Bayer process, sodium hydroxide is used in the

refining of alumina containing ore (bauxite) to produce alumina (aluminium oxide) which is the raw material used to produce aluminium metal via the electrolytic Hall-Héroult process.)

Removal of sulfur dioxide and hydrogen sulfide in petroleum refining.

(Sodium hydroxide can be added to poor quality crude oil to remove sulfurous impurities in a process known as caustic washing. The process forms a highly toxic waste which containing hydrogen sulfide, organic disulfides and mercaptans. It is banned.)

For production of Biodiesel:(For the manufacture of biodiesel, sodium hydroxide is used as a

catalyst for the transesterification of methanol and triglycerides.)

Oil drilling:(Sodium hydroxide is used as an additive in drilling mud to to

neutralise any acid gas (such as hydrogen sulfide and carbon dioxide) which may be encountered in the geological formation as drilling progresses.)

Cleaning agent:(Sodium hydroxide is frequently used as an industrial

cleaning agent where it is often called "caustic". It is added to water, heated, and then used to clean the process equipment, storage tanks, etc. It can dissolve grease, oils, fats and protein based deposits.)

Use in chemical analysis:

In analytical chemistry, sodium hydroxide solutions are often used to measure the concentration of acids by titration.

Safety• Solid sodium hydroxide and solutions of more than 2%

by weight (0.5 M) should be labeled as corrosive.• Solid sodium hydroxide or solutions of sodium hydroxide will cause chemical burns, permanent injury or scarring, and blindness if it contacts unprotected human or animal tissue. • Protective equipment such as rubber gloves, safety

clothing and eye protection should always be used when handling the material or its solutions.

• Dissolution of sodium hydroxide is highly exothermic, and the resulting heat may cause heat burns or ignite flammables.

•

Production of Caustic soda and chlorine :

• Electrolysis of concentrated sodium chloride solutions (brine) produces chlorine gas, hydrogen gas and aqueous sodium hydroxide.

• 2NaCl(aq) + 2H2O(l) -----> H2(g) + Cl2(g) + 2NaOH(aq)

• Cl2(g) is produced at the anode (positive electrode).

• H2(g) and NaOH(aq) are produced at the cathode (negative electrode).

• Three types of electrolytic cell are used to produce sodium hydroxide from brine:

• Castner-Kellner Cell (Mercury Process)

• Nelson Diaphragm Cell

• Membrane Cell

Castner-Kellner Cell (Mercury Process)

Anode (positive electrode): Titanium

Anode reaction (oxidation): 2Cl-

(aq) -----> Cl2(g) + 2e

Cathode (negative electrode): mercury flowing along bottom of cell

Cathode reaction (reduction): Na+

(aq) + e -----> Na(s)

Na(s) dissolves in the liquid mercury to form an amalgam which is moved to the decomposer.

In the decomposer the amalgam reacts with water to form sodium hydroxide, hydrogen gas and mercury.

2Na/Hg + 2H2O(l) -----> 2Na+ + 2OH- + H2(g) + 2Hg(l)

Sodium hydroxide solution and hydrogen gas are collected.

Mercury is recycled through the electrolytic cell.

Mercury is toxic so care must be taken to prevent mercury losses.

Process:• Saturated sodium chloride solution is fed

continuously into a container fitted with titanium anodes.

• The bottom of the cell consists of a layer of mercury, circulated by means of a pump. This layer serves as cathode.

• Under the conditions that prevail, sodium ions are preferentially discharged. The sodium metal dissolves in the mercury to form an amalgam. This amalgam is reacted with water in a "decomposer", producing hydrogen gas and sodium hydroxide solution:

• The mercury is cooled and recycled.

• Chlorine gas is formed at the anodes, and is collected.

• A disadvantage of this process is the use of mercury, which is poisonous and which may have harmful effects, not only for the workers, but also for the environment.

Advantages :1) Mercury as solvent remains in the from of liquid over

the whole temp.range.(75 to 85 oC)2) It has a good solvent power to form sodium amalgam.3) Pure 50 % NaOH (with out evaporation )4) Pure chlorine gas.Disadvantages:1) High electric energy consumption.2) Requires pure brine to avoid risk of explosion

through hydrogen generation cell.3) Hg is toxic in nature.

Nelson Diaphragm Cell

• The diaphragm separates the anode and cathode, but allows ions to pass in both directions. Anodes are titanium coated, and cathodes are made from steel.

• In order to prevent hydroxyl ions passing through the diaphragm the level of liquid is kept higher in the anode compartment, maintaining a flow from anode to cathode. Unfortunately this results in some chloride ion migration into the cathode compartment, giving a sodium hydroxide/sodium chloride mixture.

• One of the main disadvantages of the diaphragm cell is that it produces low quality sodium hydroxide (containing sodium chloride) at only 12% concentration, and so additional energy is needed to purify and concentrate it before it can be sold.

Anode (positive electrode): carbon (graphite) or titanium coated with Ru-Ti oxide.

Anode reaction (oxidation):2Cl-

(aq) -----> Cl2(g) + 2e

Cathode (negative electrode): Steel mesh

Cathode reaction (reduction):2H2O(l) + 2e -----> H2(g) + 2OH-

(aq)

Na+ migrates across diaphragm to cathode compartment combining with OH- to form NaOH.

Overall cell reaction

2H2O(l) + 2Cl-(aq) + 2Na+

(aq) -----> 2Na+(aq) + 2OH-

(aq) + H2(g) + Cl2(g)

Advantages :

1)Utilization of low purity brine

2) Low electricity consumption.

Disadvantages:1) NaOH produced is dilute and chlorine contaminated.

and requires evaporative concentration.

2) Chlorine gas contains O2

3) High cost of asbestos emission avoidance

Membrane Cell

Ion-exchange membrane selectively allows Na+ and water to flow to the cathode compartment but prevents products from moving between compartments.

Saturated brine enters the anode compartment where chlorine gas is formed.

Anode (positive electrode): titanium

Anode reaction (oxidation):2Cl-

(aq) -----> Cl2(g) + 2e

Cathode (negative electrode):nickel

Cathode reaction (reduction):2H2O(l) + 2e -----> H2(g) + 2OH-

(aq)

– Na+ migrates across the membrane to cathode compartment combining with OH- to form NaOH.

– Overall cell reaction 2H2O(l) + 2Cl-

(aq) + 2Na+(aq) -----> 2Na+

(aq) + 2OH-(aq) + H2(g) +

Cl2(g)

– Product is concentrated sodium hydroxide.

• Favoured method for producing sodium hydroxide as there is no hazardous waste, very pure sodium hydroxide is produced and it has the lowest energy requirements

Advantages:

1) Produces pure NaOH solution

2) Lowest electricity consumption than other two.

3) No utilization of mercury and asbestoes.

Disadvantages:

1) NaOH produced is only about 35%

2) High purity brine is required.

3) High cost and limited lifetime of membranes.