Introduction:

The chlor-alkali industry is basically the industry which generates or manufactures chlorine and alkali, sodium hydroxide (that is, caustic soda), sodium carbonate (that is, soda ash) and its derivatives and compounds based on the calcium oxide (lime). These are heavy inorganic chemicals. This is so as they are formed and employed on a large scale. The essential raw material for the manufacture of the chlor-alkali chemicals is salt (that is, common salt).

The Chlor-Alkali Processes:

Caustic soda and chlorine are prepared nearly completely as co-products via the electrolysis of brine. There are three fundamental methods for the electrolysis production of chlorine with the nature of the cathode reaction based on the particular process. Each and every process represents a different process of keeping the chlorine produced at the anode separate from the caustic soda and hydrogen formed at the cathode. The three methods are the diaphragm cell method, the mercury cell method and the membrane cell method. The fundamental principle in the electrolysis of a sodium chloride solution is as shown:

A) At anode, chloride ions are oxidized and chlorine (Cl₂) is made.

B) At cathode:

i) In the mercury method, a sodium or mercury amalgam is made by hydrogen and hydroxide ions and is formed via the reaction of the sodium in the amalgam by water in the denuder.

ii) In the membrane and diaphragm cells, water decomposes to make hydrogen and hydroxide ions at the cathode.

For all the methods, the dissolving of salt is:

NaCl → Na⁺ + Cl^-

The anode reaction for all the methods is:

2Cl^-_(aq) → Cl_2(g) + 2e^-

The cathode reaction is:

2Na⁺_(aq) + 2H₂O +2e^- → H_2(g) + 2Na⁺_(aq) + 2OH^-_(aq)

The overall reaction is: 

2Na⁺_(aq) + 2Cl_(aq) + 2H₂O → 2Na⁺_(aq) +2OH^-_(aq) + Cl_2(g) + H_2(g)

The chlor-alkali electrolytic processes:

We are familiar that there are three electrolytic cell methods comprised in the production of chlorine, caustic soda all along with the hydrogen. We shall now take them as one after the other and possibly compare them.

The mercury cell method:

The mercury cell method which has been in use in Europe since the year 1892 comprises two 'cells' (that is, the electrolyzer and the decomposer or denuder). In the primary electrolyzer (or brine cell), purified and saturated brine comprising around 25% sodium chloride flows via an elongated trough which is slightly inclined from the horizontal. At the bottom of this trough, a shallow film of mercury flows all along the brine cell concurrently by the brine. Closely spaced above the cathode assembly is suspended. 

By the passage of electric current via the cell, the brine passing via the narrow space between the electrodes decomposes thus discharging chlorine gas at the anode and metallic sodium at the cathode.

The chlorine gas is mounted up above the anode assembly and released to the purification method.

As the sodium metal is discharged at the surface of the mercury cathode, it immediately forms an amalgam. The liquid amalgam flows from the electrolytic cell to a separate reactor, termed as the decomposer, where it reacts by water in the presence of a graphite catalyst to make sodium hydroxide and hydrogen gas. The sodium -free mercury is fed back to the electrolyzer and re-used.

The overall summary of the reactions which occur in mercury cell is as illustrated below:

Reaction in the electrolyzer:

2Na⁺ + 2Cl^- + 2Hg → 2Na-Hg + Cl_2(g)

                                   Amalgam

Reaction in the decomposer:

2Na-Hg + 2H₂O → 2Na⁺ + 2OH^- + H_2(g) +  2Hg

The mercury cell method has a merit over diaphragm and membrane cell which is that it generates a chlorine gas with almost no oxygen, and a 50% caustic soda solution. Though, this method gives rise to the environmental discharges of mercury. The cells as well operate at a higher voltage as compare to diaphragm and membrane cells and as such more energy is employed.

The diaphragm electrolytic cell method:

The diaphragm cell was the first commercial method employed to produce chlorine and caustic soda from brine. It varies from the mercury cell method in that all reactions occur within one cell and the cell effluent includes both salt and caustic soda. A diaphragm is used to separate the chlorine discharged at the anode and the hydrogen and caustic soda produced directly at the cathode. Devoid of the diaphragm to isolate them, the hydrogen and chlorine would react to make sodium hypochlorite (NaClO), which in turn reacts further to make sodium chlorate (NaClO₃).

The diaphragm is generally made up of asbestos and separates the feed brine (that is, anolyte) from the caustic-comprising catholyte. Purified brine that enters the anode compartment percolates to the cathode chamber via the diaphragm.

How is the diaphragm electrolytic cell employed in the formation of caustic soda and chlorine?

At first, all the brine is purified as commercial sodium chloride generally comprises impurities like calcium, magnesium and iron compounds. Such impunities are eliminated by adding lime and soda ash that precipitates the impurities as insoluble carbonates and hydroxides. The clear brine is neutralized by hydrochloric acid and the purified brine is allowed to settle.

Subsequently, the brine is electrolyzed in the diaphragm cells. Throughout electrolysis, sodium ions move to the cathode, where H⁺ ions and OH^-ions are as well formed as a result of reduction of water. The chloride ions on the other hand move towards the anode where they are released as chlorine gas. The reaction is as illustrated below:

NaCl → Na⁺ + Cl^-

H₂O → H⁺ + OH^-

At cathode:

2H₂O + 2e^- → H₂ + 2OH^-

Na⁺ + OH^- → NaOH_(aq)

At anode:

Cl^- - e^- → Cl

Cl + Cl → Cl_2(g)

The caustic soda solution acquired from the cell includes around 10 to 15% caustic soda and some unconverted NaCl. Thus to separate the two, the weak caustic soda is first concentrated to 50% in a double or triple effect evaporator ad the NaCl that is less soluble specifically in the presence of caustic soda is therefore separated and employed again to make more brine. 

There are some merits related by the use of diaphragm cells:

1) Operating at a lower voltage as compare to mercury cells.

2) Operating by pure brine as compare to need by membrane cells.

The membrane cell process:

The membrane electrolytic method is the most promising and fast developing method for the production of chlor-alkali. Though, the replacement of existing mercury and diaphragm cells having membrane cells is occurring at a much slower rate due to the long lifetime of the former and the high capital costs of replacement. 

In this method, the anode and cathode are separated via a water impermeable ion-conducting membrane. The brine solution flows via the anode compartment where chloride ions are oxidized to chlorine gas.

The sodium ions migrate via the membrane to the cathode compartment that consists of flowing caustic soda solution. The de-mineralized water added to the catholyte circuit is hydrolyzed, discharging hydrogen gas and hydroxide ions. The sodium and hydroxide ions join to produce caustic soda that is usually brought to a concentration of 32 to 35% by re-circulating the solution before it is discharged from the cell.

The membrane prevents the migration of chloride ions from the anode compartment to the cathode compartment; thus, the caustic soda solution produced doesn't have sodium chloride as in the diaphragm cell procedure. Whenever the caustic soda is needed to reach a concentration of 50%, the caustic liquor formed has to be concentrated through evaporation by using steam.

The membranes employed in the chlor-alkali industry are generally made up of perfluorinated polymers. The cathodes material employed in the membrane cells is either stainless steel or nickel. The cathodes are often coated by a catalyst which is more stable as compare to the substrate and that raises surface area and decreases over-voltage. The anodes employed are metals.

In comparison by the other methods, membrane cells have the benefit of producing an extremely pure caustic soda solution and of employing less electricity. The membrane procedure doesn't utilize highly toxic materials such as mercury and asbestos.

The membrane procedures have the following drawbacks:

1) The caustic soda generated might require to be evaporated to be evaporated to increase the concentration.

2) For several applications, the chlorine gas produced requires to be processed to take away oxygen.

3) The brine entering the membrane cell should be of very high purity; therefore it often needs costly additional purification steps prior to the electrolysis.

A schematic representation of a membrane cell method is illustrated in the figure shown below:

Fig: Membrane cell process

Manufacture of sodium carbonate:

The Sodium carbonate is either found naturally or is made up from common salt (NaCl) and limestone.

There are two major sources of sodium carbonate:

1) From salt and calcium carbonate by the ammonia soda (Solvay) procedure.

2) Form sodium carbonate and hydrogen carbonate ores (that is, trona and nacholite)

On industrial scale, the sodium carbonate is manufactured by the Solvay method. This method comprises various phases that are as follows:

Stage 1: Ammoniation of purified brine:

In this phase, saturated or concentrated brine is allowed to flow down the ammoniating tower. This tower is fitted by mushroom shaped baffles. Such baffles control the flow of brine and make sure proper mixing and saturation of ammonia. Ammonia gas is absorbed in the concentrated brine to provide a solution having both sodium chloride and ammonia.

Stage 2: Carbonation of ammoniated brine:

In this, ammoniated brine is allowed to trickle down a carbonating tower termed as the Solvay tower. The tower is as well fitted by baffle plates. Brine is mixed by carbon-dioxide gas, produced via heating limestone in a separate chamber known as 'kiln'.

CaCO₃ → CaO + CO₂

The baffle plates make sure the flow of solution and breaks up carbon-dioxide to small bubbles to generate good conditions for reaction.

What chemical reactions occur in the Solvay tower?

A) CO₂ reacts by ammonia to form ammonium carbonate.

2NH₃ + CO₂ + H₂O → (NH₄)₂CO₃

B) Ammonium carbonate further reacts by CO_2n to prepare ammonium bicarbonate.

(NH₄)₂CO₃ + CO₂ + H₂O → 2NH₄HCO₃

C) Ammonium bicarbonate then reacts by NaCl to prepare sodium bicarbonate.

NH₄HCO₃ + NaCl → NaHCO₃ + NH₄Cl

Such reactions are exothermic in nature, thus the solubility of NaHCO₃ increases. To counteract this, lower portion of the Solvay tower is cooled and the precipitate of NaHCO₃ is separated through vacuum filtration and washed to take away ammonium salts.

Stage 3: Conversion of NaHCO₃ to Na₂CO₃:

In this phase, dry sodium bicarbonate is heated in the rotary furnace known as 'Calciner' to provide anhydrous sodium carbonate. The carbon-dioxide is re-circulated to the carbonation tower.

2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O

Stage 4: Ammonia recovery or regeneration process:

Whenever limestone is heated, CaO is obtained all along with CO₂. CaO is then treated by water to form quicklime, Ca(OH)₂. Quicklime is then heated by NH₄Cl to form NH₃ and a by-product, calcium chloride. Ammonia therefore produced is utilized again in this method.

CaO + H₂O → Ca(OH)₂

2NH₄Cl + Ca(OH)₂ → CaCl₂ + 2NH₃ + 2H₂O

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