Lenntech Water treatment & purification


By passing a current of electricity through a sodium chloride solution the salt is decomposed into chlorine at the anode and sodium at the cathode. But the latter at once decomposes a molecule of water of the solution, forming caustic soda and setting free hydrogen. Hence the products of electrolysis are chlorine, caustic soda, and hydrogen, of which the last mentioned is of no practical value at present.
There are serious mechanical difficulties encountered in all electrolytic processes for decomposing salt. The chlorine set free at the anode must not be permitted to diffuse through the whole solution, since it causes secondary reactions. To prevent this diffusion, various devices have been proposed, most of them being porous diaphragms between the anode and cathode. But no material is yet known which, while offering no resistance to the passage of the electrical current, still prevents the diffusion of the sodium hydroxide used for the diaphragms, because of the destructive action of the chlorine. The nascent chlorine is also very destructive to the anode and practically only platinum, 01' slabs cut from magnetite (Fea04), . have proved efficient in withstanding its action. These are expensive, and magnetite slabs are very fragile. If the hydrogen liberated at the cathode is permitted to escape through the solution, it stirs the liquid, aiding the diffusion of the chlorine, and the consequent formation of chlorates and hypochlorites, thus: -

1) NaCl = Na + C1.
2) Na + H"O = NaOn + H.
3) 2 NaOH + 2 Cl = NaClO + NaCl + H20.
4) 3 NaCIO = NaClOa + 2 NaC1.
5) NaCIOa + 6 H = NaCl + 3 H20.

Thus reactions 3, 4 and 5 cause a loss, since they regenerate salt from the chlorine set free.
LeSueur's process;; formerly employed the apparatus described in Lunge's "SnlphUl'ic Acid and Alkali," VoL III, p. (j(H. The cathode, of iron wire gauze, was placed in a slanting position. On it resterl the diaphragm, consisting of two parts, -a sheet of parchllIent paper and It double sheet of asbestos cemented together by blood albumin, coagulated and hardened by treatment with potassium bichromate. An earthenware bell enclosed the anode, which was made of lead, carrying carbon rods dipping into the salt solution. Caustic soda was formed in the solution outside the bell, and owing to the inclined position of the cathode, the hydrogen was expected to escape readily, thus preventing polarization. But it proved in practice that the earthenware bells were disintegrated by the caustic soda solution, while the hydrogen set free Oll the lower side of the cathode did not ascend along the sloping diaphragm and escape, but diffused through it and found its way into the interior of the cell.'rhis resulted in forming a dangerous mixtUl'e of hydrogen and chlorine, and it is said that several serious explosions occurred. Consequently the form of the cell has been altered; but no facts regarding the improved cell can be given, owing to the secrecy maintained about its construction and working. The diaphragms are rapidly destroyed, lasting only from 24 to 48 hours. The anodes are consumed more slowly, lasting about six weeks. The process yields a solution of caustic containing 10 per cent NaOH.
In Carmichael's apparatus," an asbestos diaphragm, impregnated with Portland cement, is used. The diaphragm rests horizontally on the cathode at the bottom of the cell; above it is a bell to collect the hydrogen given off. '1'he anode is a grating of copper rods, covered with hard rubber, through which numerOllS platinum points project into the brine. This anode is suspended in the top of the cell, and the chlorine set free is thus only momentarily in contact with the liquid. The salt solution is fed into the cell at the top, in a rapid stream of drops while the mixture of caustic soda and salt flows continuously from the bottom. The supply of brine is so regulated that the caustic formed at the cathode is drawn off before it has time to diffuse through the liquid. The solution drawn from the cell contains about 20 per cent of caustic soda, and about 75 per cent of the salt is decomposed. The reaction is carried on at a temperature of about 80° C. in the top of the cell near the anode, while the region around the cathode is kept as cool as possible.
Being removed from the immediate action of the chlorine, the diaphragms are very durable.
Greenwood's apparatus t consists of an iron vessel, coated with electrolytically deposited copper; this is made the cathode, and in it is placed a circular anode coated with carbon. Between the anode and the vessel walls is a diaphragm made up of a series of V-shaped circular troughs of glass or porcelain, fitted together, the spaces between them being packed with asbestos. The chlorine from the anode chamber is led away by suitable pipes, and the caustic-salt solution passes into another similar cell, where more of the salt is decomposed. The cells are placed en cascade, the brine flowing from the top one, down through the series. The solution obtained in this process contains about 2.2 per cent NaOH. In the Holland and Richardson process: the cathode is covered with cupric oxide. The hydrogen liberated here reduces the oxide to metallic copper, and polarization is prevented. If caustic soda is desired, the cathode is placed horizontally at the bottom of the cell. The caustic solution formed, being heavy, remains on the cathode, while the chlorine escapes from the anode at the top. When "bleaching liquors" or hypochlorites are desired, the anode is put at the bottom of the cell and the cathode at the top. In this case the chlorine rises through the caustic solution and is absorbed:-
2 NaOH +012 =NaOCl + NaCl + H20.

'rhe Hargreaves-Bird process * employs an asbestos diaphragm, impregnated with Portland cement, or with clay and sodium silicate; it is fastened on a wire gauze and placed in a horizontal position. To avoid the use of a diaphragm, nnmerous processes have been proposed in which mercury is used as the cathode or is placed between the anode and cathode.
The Hermite process t has attracted much attention as a method of making bleaching and disinfecting liquors from magnesium or sodium chloride solutions; but it is not used for the production of free chlorine or caustic.
The Castner process:l: appears to be the most promising of the methods using mercury between the anode and cathode.
The cell (Fig. 48) is divided into three compartments, the two outside ones containing brine and the carbon anodes (A), while the middle one contains the caustic solution and the iron cathode (C). The sodium set free is taken up by the mercury, forming an amalgam. The cell is made to rock slightly by the cam (E), and the motion carries the mercury and amalgam into the centre compartment, where the amalgam acts as the anode during the passage of the current to the cathode, the sodium being liberated. A regulated supply of water flows into the centre compartment continuously, while a corresponding amount of caustic solution overflows into a collecting tank, the process being thus uninterrupted. Each cell is about 6 feet by 3 feet by 6 inches, and will decompose about 56.5 pounds of salt daily, producing 38.5 pounds of caustic and 34.5 pounds of chlorine per each 3.5 horse-power. The electrodes being near together, there is but little resistance, and the voltage is only about 4, with a current of 550 amperes. The wear and tear is said to be small. The process is claimed to yield a 20 per cent solution of caustic, free from hypochlorites, while the chlorine gas is very pure, containing only a little hydrogen. 'rhe mercury seldom contains more than 0.02 per cent of sodium, which is removed electrolytically. No hypochlorites are produced, and the electrical efficiency is claimed to be over 88 per cent.
Although electrolytic processes have been much elaborated within the last decade, the difficulties encountered in preventing the formation of hypochlorites and regeneration of the salt, the destructive action of the chlorine and caustic on the diaphragms and other parts of the apparatus, and the large size of plant needed for a comparatively small output, have deterred most manufacturers from engaging on a large scale in such an uncertain enterprise. Then, too, except in those favored places where water power can be had cheap, the electricity must be generated by means of boiler, engine, and dynamo, a method which consumes much fuel with low efficiency. The electromotive force needed to decompose sodium chloride is a little over two volts, but the resistance of the bath, together with polarization, increases the tension to from 3.5 to 4 volts. A current of one ampere at 4t volts will yield, theoretically, 0.00292 pounds of chlorine and 0.0033 pounds of caustic soda per hour. Cross and Bevan * calculate that with an efficiency of 80 per cent, caustic soda costs £12 10s. per ton, and bleaching powder £7 10s. per ton, when produced by electrolysis. According to Raussermann,t one ampere, with 80 per cent efficiency, yields 28.56 grams of N aOR and 25.:~ grams Cl in 24 hours, the voltage being 3.5. Thus 35 amperes are needed to produce one kilo of Na'on in 24 hours.
If a theoretical yield were obtained, the chlorine evolved would make about 100 pounds of bleaching powder for each 40 pounds of caustic soda produced. But the latter, which is in much greater demand than bleaching powder, can be made more cheaply from ammonia soda; therefore it would seem that if electrolytic methods prove successful in the future their expansion would be limited to supplying bleaching powder and chlorates, and the caustic be regarded as a by-product. Moreover, the caustic liquors produced by electrolysis are dilute, necessitating much evaporation, and the product is contaminated with much chloride and chlorate.

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