NA TURE 



[June 2, \ 



The electrolytic removal of tin from tin-plate is said to be 

 carried on to a considerable extent. The tinned scrap is sus- 

 pended in iron baskets which form the anode, and the tin de- 

 posited in the spongy form on sheet-iron kathodes, the electro- 

 lyte being a solution containing 12 to 15 per cent, of sodium 

 chloride, to which a little caustic soda is added from time to 

 time to prevent the precipitation of stannous oxide. The solu- 

 tion is warmed to 40° or 50° C, and since tin dissolves under 

 these circumstances independently of the electric current, it 

 is necessary occasionally to evaporate the solution and work up 

 the residue for sodium stannate. Sodium is now made ex- 

 clusively by the electrolysis of fused caustic soda. In Mr. 

 Castner's process the heat generated by the passage of the 

 current is utilised to keep the bath in the fused state. Mag- 

 nesium is also a product of electrolysis. The small quantity 

 produced is made at Hemelingen, near Bremen, by the electro- 

 lysis of perfectly dry, fused carnalite (magnesium potassium 

 chloride). The iron crucible in which the salt is melted serves 

 as kathode, the central carbon anode being surrounded by a 

 perforated porcelain or stoneware cylinder which retains the 

 chlorine. 



The application of electrolysis to the precipitation of gold 

 from cyanide liquors, marks an advance of some importance in 

 the metallurgy of gold. Gold is not completely precipitated 

 in a reasonable time by zinc from solutions containing less than 

 O'l or 0'2 per cent, of free potassium cyanide, whereas with the 

 electrolytic process the concentration of the solution is a matter of 

 indifference. It thus becomes possible, by the employment of 

 very dilute cyanide solutions, to extract economically the small 

 quantities of gold contained in slimes and tailings which would 

 otherwise have 'been thrown away. A further advantage of the 

 electrolytic precipitation is that the gold obtained contains some 

 89 per cent, of gold, instead of the 70 per cent, contained in the 

 zinc bullion. The solutions to be electrolysed contain from 

 o'Oi to 0*05 per cent, of potassium cyanide, according to the 

 nature of the ore treated, together with from i to 4 dwts. of 

 gold per ton of solution, in the form of potassium aurocyanide. 

 They have, therefore, a very high resistance. Owing to the very 

 small quantity of gold to be deposited, however, a very small 

 current is sufficient (o"6 ampere per square metre), and the 

 baths can be worked with the moderate E. M.F. of 4 volts. 

 The quantity of electric energy required is thus small, and its 

 cost is almost negligible compared with that of the rest of the 

 process. The solution flows into the electrolytic tank at one 

 end, and passes alternately over and under the electrodes until 

 it flows out of the tank deprived of 80 to 90 per cent, of its 

 gold. The kathodes consist of thin sheets of lead, and the 

 anodes of iron enclosed in canvas bags to retain the precipitate 

 of Prussian blue which forms on them. They are placed about 

 l^ inches apart. The gold remaining in the liquors flowing 

 from the electrolytic tanks is not lost, these liquors being made 

 up to strength with fresh cyanide and used again. After remain- 

 ing in the tanks some months the lead kathodes are sufficiently 

 rich in gold to be removed and submitted to cupellation. Owing 

 to the important advantages already mentioned, the employment 

 of this process is rapidly extending ; in 1896, two years after 

 the first installation of the process, over 46,000 ounces of gold 

 were obtained in the Transvaal by means of it, and at present 

 it is much more extensively used. 



Turning, now, to the application of electrolysis to the pro- 

 duction of substances other than metals, there is an important 

 group of industries engaged in the electrolysis of potassium and 

 sodium chlorides, producing, according to the conditions em- 

 ployed, caustic alkalis and chlorine, hypochlorites or chlorates. 



In the first case it is necessary to keep the primary products 

 of the decomposition separate, and this is accomplished in two 

 ways : (i) by the use of a porous diaphragm ; (2) by means of 

 mercury. The manufacture of a diaphragm which shall be 

 sufficiently durable with a solution of caustic soda on one side 

 of it, and of chlorine on the other, is by no means easy. The 

 fact that diaphragms are being successfully used proves, how- 

 ever, that the difficulties are not insuperable. A more serious 

 drawback is the impossibility of separating the caustic alkali 

 from the chloride. As soon as the solution at the kathode con- 

 tains hydroxyl ions, these begin to migrate, under the influence 

 of the current, toward the anode, in time passing into the anode 

 compartment and giving rise to the formation of hypochlorites 

 and chlorates and to the evolution of oxygen, and so diminishing 

 the efficiency of the cell. It is therefore necessary to draw off 

 the solution from the kathode compartment of the cell while it 



still contains much undecomposed chloride, and to separate this 

 from the caustic alkali as far as possible during the process of 

 concentration. 



The process of Hargreaves and Bird avoids this to a great 

 extent in a very ingenious way. The kathode in this process 

 consists of a sheet of copper gauze, upon which the diaphragm 

 is built up of asbestos mixed with some cementing material. 

 The diaphragm, with the copper gauze outside, forms the outer 

 wall of the cell which contains the solution of salt and the 

 carbon anode. The caustic soda is thus formed outside the cell, 

 and is washed down and converted into carl^onate by a mixture 

 of steam and carbon dioxide. The diaphragm is made so 

 impervious that, when in good condition, no liquid will run out 

 of the cell, and only three molecules of salt to 100 molecules of 

 sodium carbonate are obtained. The diaphragms last about 

 thirty days, but, according to Kershaw, yield less favourable 

 results toward the end of the time. Hulin's process is somewhat 

 similar to that of Hargreaves and Bird, the diaphragm-kathode 

 consisting of a sheet of porous carbon through which the caustic 

 soda solution is forced as quickly as it is formed by hydrostatic 

 pressure inside the cell. 



When mercury is employed as the kathode, the diaphragm 

 becomes unnecessary, the mercury taking up sodium in contact 

 with the salt solution and giving it up to pure water in another 

 vessel. A great many devices have been contrived for causing 

 the mercury to alternately perform these functions. The simplest 

 and most effective is undoubtedly the rocking cell of Mr. 

 Castner. This consists of a shallow oblong tank divided into 

 three compartments by means of partitions which do not quite 

 reach the bottom. A thin layer of mercury lying on the bottom 

 of the cell lutes the spaces below the partitions, thus dividing 

 the cell into three separate compartments. The two end ones 

 contain strong brine and carbon anodes, the central one pure 

 water and an iron kathode which is connected electrically with 

 the mercury. The cell is tilted slightly from side to side, so 

 that the mercury flows from one end compartment to the other, 

 always covering the floor of the central compartment, however. 

 In this way the sodium taken up at the ends is conveyed to the 

 water in the centre. The central compartment form.s really a 

 galvanic element, consisting of sodium amalgam and iron in a 

 solution of caustic soda ; the connection of the iron to the 

 mercury short circuits this cell, and therefore hastens the dis- 

 solution of the sodium. The caustic soda obtained in this way 

 is practically pure, and the current efficiency over 90 per cent, of 

 the theoretical value ; whilst the electromotive force required is 

 4 volts for each cell. 



If instead of keeping the products of the electrolysis of a salt 

 solution separate they are mixed together in the cold, a solution 

 of hypochlorite is formed. A limit to the concentration attain- 

 able is, however, quickly reached, partly owing to the electro- 

 lysis of the hypochlorite, partly to its reduction by the hydrogen 

 evolved. Hermite employs rotating zinc kathodes, between 

 each pair of which a platinum gauze anode is fixed, the electro- 

 lyte, consisting of a solution of salt and magnesium chloride, 

 flows through the apparatus, yielding a weak bleaching liquor 

 suitable for bleaching paper pulp or for deodorising sewage, in 

 which latter case sea water may be used. Kellner attains the 

 same result by using a long tank in which a large number of 

 carbon plates are fixed in such a way that the solution flowing 

 in at one end of the tank must circulate between each pair of 

 plates before passing out of it. Only the two end plates are 

 attached to the terminals of the dynamo, so that each intermediate 

 plate acts on one side as anode, on the other as kathode. 



The electrolytic preparation of potassium chlorate was patented 

 by Charles Watt as early as 1851, but the idea was not put into 

 practice until 1889, when Gall and Montlaur started the first 

 electrolytic potassium chlorate plant at Villers-sur-Hermes in 

 Switzerland. They employ thin platinum-iridium anodes and 

 iron kathodes, and maintain the solution at a temperature of 

 50° to 60° C. by the heat evolved by the passage of the current. 

 The electrolytic cell is divided by a diaphragm of porous 

 earthenware into a smaller kathode, and a larger anode com- 

 partment, in order to prevent as far as possible the reduction of 

 the chlorate by the hydrogen evolved at the kathode. A current 

 of 10 amperes per sq. dcm., and an E.M.F. of 5 volts are used, 

 and the caustic potash formed at the kathode transferred to the 

 anode compartment sufficiently fast to absorb all the chlorine 

 evolved. The potassium chlorate crystallises out in the anode 

 compartment, its solubility being diminished by the employment 

 of a saturated solution of potassium chloride as electrolyte. The 



NO. 1492, VOL. 58] 



