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Chlorine : Electrolytic Chlorine And Alkali
The electrolysis of brine, i.e., a solution of sodium chloride, is applied on a large scale for the production of caustic soda and chlorine. The salt in solution is decomposed by an electric current, chlorine being evolved from the positive pole (anode) and a solution of caustic soda and hydrogen gas being formed at the negative pole (cathode):
The reaction may be considered as taking place in two stages, in the first of which sodium is deposited at the cathode and chlorine at the anode; the sodium then reacts with the water, forming a solution of caustic soda and evolving hydrogen:
2NaCl = 2Na + Cl2;
2Na + 2H2O = 2NaOH + H2.
Various types of electrolytic cells are used in the process.
The Castner-Kellner cell consisted
of a shallow slate tank divided into three compartments by slate partitions not quite touching the floor. The floor is covered by a pool of mercury, thus separating the three compartments. Each end compartment is filled with strong brine, the middle one with water. Anodes of carbon are placed in the end compartments, whilst the cathode consists of mercury and a bundle of iron rods in the central compartment. Ninety per cent, of the current passes from the mercury in the middle compartment to the iron cathode, and ten per cent, through a resistance in parallel. Chlorine is evolved in the end compartments, and is led off by earthenware pipes. Sodium discharged on the mercury in the end compartments, acting as a cathode, dissolves, forming sodium amalgam. The cell is given a slow rocking motion by an eccentric, and the amalgam is brought from the end compartments to the middle compartment, where it acts as an anode and decomposes the water, forming a solution of caustic soda. Hydrogen is evolved from the iron cathode. In the new type of cell the tank is stationary, and the mercury is moved by an archimedean screw, finally dropping over a cascade into water to free it from sodium, after which it re-enters the cell.
In the Gibbs cell,
used by the United Alkali Co. at Widnes, the anodes are carbon rods separated from the cylindrical iron cathode by a diaphragm of asbestos paper. The solution of caustic soda then obtained is not so pure as that obtained in the Castner-Kellner cell, and the sodium chloride contained in it must be separated by crystallisation.
Chlorine is liquefied by compression to 6 atm. at 15°, or by cooling at ordinary pressure in iron pipes. It is sent out as liquid chlorine in steel tanks or cylinders. The electrolytic chlorine is purer than that made by chemical methods. Some of it is used in making stannic chloride, or chlorinated acetylenes. Pure hydrochloric acid is prepared by combining electrolytic hydrogen and chlorine by combustion, and absorption in water. Chlorine is also used in the preparation of bleaching powder and of hypochlorite solutions for bleaching and for petroleum refining.
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