i84 



NATURE 



\yune 20, 1889 



they are quite full, the fronts are then sealed up, and the charge 

 allowed to remain undisturbed for about four hours, during which 

 time the water of the alumina hydrate is completely expelled. At 

 the end of this time the valves on the chlorine main are opened, 

 and the gas is allowed to pass into the charged retorts. In the 

 rear of each retort, and connected therewith by means of an 

 earthenware pipe, are the condenser boxes, which are built in 

 brick. These boxes are provided with openings or doors, and 

 also with earthenware pipes, connected with a small flue for 

 carrying off the uncondensed vapours to the large chimney. At 

 first the chlorine passed into each retort is all absorbed by the 

 charge, and only carbonic oxide escapes into the open boxes, 

 where it burns. After a certain time, however, dense fumes are 

 evolved, and the boxes are then closed, while the connecting pipe 

 between the box and the small flue serves to carry off the 

 uncondensed vapours to the chimney. 



The reaction which takes place is as follows : — • 



AI2O3 + 2NaCl + 3C + 6C1 = 2AlCl3NaCl + 3CO. 



The chlorine is passed in for about seventy-two hours in vary- 

 ing quantity, the boxes at the back being opened from time to 

 time by the workmen to ascertain the progress of the distillation. 

 At the end of the time mentioned the chlorine valves are closed, 

 and the boxes at the back of the furnace are all thrown open. 

 The crude double chloride as distilled from the retorts, condenses 

 in the connecting pipe and trickles down into the boxes, where 

 it solidifies in large irregular masses. The yield from a bench 

 of five retorts will average from 1600 to 1800 pounds, which is 

 not far from the theoretical quantity. After the removal of the 

 crude chloride from the condenser boxes the retorts are opened 

 at their charging end, and the residue, which consists of a small 

 quantity of alumina, charcoal, and salt, is raked out and remixed 

 in certain proportions with fresh material, to be used over again. 

 The furnace is immediately re-charged and the same operations 

 repeated, so that from each furnace upwards of 3500 pounds of 

 chloride are obtained weekly. With ten of the twelve furnaces 

 always at work the plant is easily capable of producing 30,000 

 pounds of chloride per week, or 1,500,000 pounds per annum. 



Owing to the presence of iron, both in the materials used (viz. 

 charcoal, alumina, &c.) and in the fire-clay composing the retorts, 

 the distilled chloride always contains a varying proportion of 

 this metal in the form of ferrous and ferric chloride. When it 

 is remembered that it requires 10 pounds of this chloride to pro- 

 duce I pound of aluminium by reduction, it will be quite ap- 

 parent how materially a very small percentage of iron in the 

 chloride will influence the quality of the resulting metal. I 

 may say that, exercising the utmost care as to the purity of the 

 alumina and the charcoal used, and after having the retorts made 

 of special fire-clay containing only a very small percentage of 

 iron, it was found almost impossible to produce upon a large scale 

 a chloride containing le^s than 0*3 per cent of iron. 



This crude double chloride, as it is now called at the works, is 

 highly deliquescent, and varies in colour from a light yellow to 

 a dark red. The variation in colour is not so much due to the 

 varying percentage of iron contained as to the relative proportion 

 of ferric or ferrous chlorides present, and although a sample may 

 be either very dark or quite light, it may still contain only a 

 small percentage of iron if it be present as ferric salt, or a very 

 large percentage if it is in the ferrous condition. Even when 

 exercising all possible precautions, the average analysis of the 

 crude double chloride shows about o'4 per cent, of iron. The 

 metal subsequently made from this chloride therefore never 

 contained much less than about 5 per cent, of iron, and, as this 

 quantity great injures the capacity of aluminium for drawing 

 into wire, rolling, &c., the metal thus obtained required to be 

 refined. This was successfully accomplished by Mr. Castner and 

 his able assistant, Mr. Cullen, and for some time all the metal 

 made was refined, the iron being lowered to about 2 per cent. 



The process, however, was difficult to carry out, and required 

 careful manipulation, but as it then seemed the only remedy for 

 effectively removing the iron, it was adopted and carried on for 

 some time quite successfully, until another invention of Mr. 

 Castner rendered it totally unnecessary. This consisted in 

 purifying the double chloride before reduction. I cannot now 

 explain this process, but I am able to show some of the product. 

 This purified chloride, or pure double chloride, is, as you see, 

 quite white, and is far less deliquescent than the crude, so that 

 it is quite reasonable to infer that this most undesirable property 

 is greatly due to the former presence of iron chlorides. I have 

 seen large quantities containing upwards of \\ per cent, of iron, 



or 150 pounds to io,oco of the chloride, completely purified 

 from iron in a few minutes, so that, whilst the substance before 

 treatment was wholly unfit for the preparation of aluminium, 

 owing to the presence of iron, the result was, like the sample 

 exhibited, a mass containing only i pound of iron in 10,000, or 

 0"oi per cent. The process is extremely simple, and adds little 

 or no appreciable cost to the final product. After treatment, 

 this pure chloride is melted in large iron pots and run into drums 

 similar to those used for storing caustic soda. As far as I am 

 aware, it was generally believed to be an impossibility to remove 

 the iron from anhydrous double chloride of aluminium and 

 sodium, and few, if any, chemists have ever seen a pure white 

 double chloride. 



Aluminhim Manufactu7-e. 



I now come to the final stage of the process, viz. the reduction, 

 of the pure double chloride by sodium. This is effected, not in 

 a tube of Bohemian glass, as shown in Mr. Barlow's lecture in 

 1856, but in a large reveiberatory furnace, having an inclined 

 hearth about 6 feet square, the inclination being towards the 

 front of the furnace, through which are several openings at 

 different heights. The pure chloride is ground together with 

 cryolite in about the proportions of two to one, and is then 

 carried to a staging erected above the reducing furnace. The 

 sodium, in large slabs or blocks, is run through a machine similar 

 to an ordinary tobacco-cutting machine, where it is cut into small 

 thin slices ; it is then also transferred to the staging above the 

 reducing furnace. 



Both materials are now thrown into a large revolving drum, 

 when they become thoroughly mixed. The drum being opened 

 and partially turned, the contents drop out into a car on a tram- 

 way directly below. The furnace having been raised to the 

 desired temperature, the dampers of the furnace are all closed to 

 prevent the access of air, the heating gas also being shut off. The 

 car is then moved out on the roof of the furnace until it stands 

 directly over the centre of the hearth. The furnace roof is pro- 

 vided with large hoppers, and through these openings the charge 

 is introduced as quickly as possible. The reaction takes place 

 almost immediately, and the whole charge quickly liquefies. At 

 the end of a certain time the heating gas is again introduced and 

 the charge kept at a moderate temperature fjr about two hours. 

 At the end of this period the furnace is tapped by driving a bar 

 through the lower openintr, which has previously been stopped 

 with a fire-clay plug, and the liquid metal run out in a silver 

 stream into moulds placed below the opening. When the metal 

 has all been drawn off, the slag is allowed to run out into 

 small iron waggons and removed. The openings being again 

 plugged up, the furnace is ready for another charge. From 

 each charge, composed of about 1200 pounds of pure chloride, 

 600 pounds of cryolite, and 350 pounds of sodium, about 115 to 

 120 pounds of aluminium are obtained. 



The purity of the metal entirely depends upon the purity of the 

 chloride used, and without exercising more than ordinary care 

 the metal tests usually indicate a purity of metal above 99 per 

 cent. On the table is the metal run from a single charge, its 

 weight is 116 pounds, and its composition, as shown by analysis, 

 is 99"2 aluminium, 03 silicon, and o'5 iron. This I believe to 

 be the largest and the purest mass of metal ever made in one 

 operation. 



The result of eight or nine charges are laid on one side, and 

 then melted down in the furnace to make a uniform quality, the 

 liquid metal, after a good stirring, being drawn off into moulds. 

 These large ingots, weighing about 60 pounds each, are sent to 

 the casting shop, there to be melted and cast into the ordinary 

 pigs, or other shapes, as may be required for the making of 

 tubes, sheets, or wire, or else used directly for making alloys of 

 either copper or iron. 



■ The following table shows approximately the quantity of each 

 material used in the production of one ton of aluminium : — 



Metallic sodium 6,300 pounds 



Double chloride ... ... ... 22,400 ,, 



Cryolite 8,000 ,, 



Coal ... ... ... ... ... 8 tons. 



To produce 6,300 pounds of sodium is required : — 



