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previous operation. The solution of sodium fluoride, NaF, is boiled with the requisite quantity of burnt lime, which converts it into caustic soda, NaOH, which is separated from the precipitated CaF2 by decantation and washing. The solution is evaporated down to a concentrated solution of NaOH, or to dryness, as desirable. The lime used should be as pure and free from iron as possible, to avoid contaminating the alumina.

Alumina may also be obtained from alum stone or alum shales; by converting these into alums or into Al2(SO4)3Aq. by any of the well-known methods of alum-makers, and then the alumina made by calcining this salt, as given on p. 144.

PART VIII.

MANUFACTURE OF THE DOUBLE CHLORIDE OF ALUMINIUM AND SODIUM.

SINCE the cost of the aluminium chloride is equal to the cost of the sodium used, in making aluminium, many attempts have been made to cheapen its manufacture from alumina, but without much success. Mr. Frishmuth, of Philadelphia, claims that he has lowered the cost of aluminium principally by making the AlC16 much cheaper than before. Mr. Webster's process, which has been applied on such a large scale, is altogether concerned with producing the Al2C16 cheaply, not a word being said about improvements in making the sodium. However, with these exceptions, and possibly one or two others which will be found further on, the making of these chlorides has remained much as Deville left it. The description of their manufacture as conducted at Salindres is the only account given by Mierzinski, and so that may be taken as the process now in general use, especially in Europe. It will be found on p. 166. I add here just a few words to Fremy's description of the process, which may serve to render his description more exact:

Mierzinski says that when the double chloride is to be made, special importance is to be attached to using materials free from iron in preparing the alumina, as iron cannot be removed from Al2Clo.-• 2NaCl as easily as from Al'C16. Mierzinski also devotes some space to descriptions of chlorine generators, but that is a separate subject, full descriptions of which can be found in any good work on practical chemistry..

There have been a few attempts to make these chlorides in different ways from that used in Deville's process. I find two such processes,

which, however, cannot have been much of an improvement on Deville's, or else they would have supplanted it.

M. Dullo makes the following observations on the production of Al2C16 direct from clay:-*

This

"Up to the present time the AlCl necessary to the production of aluminium has been prepared by treating cryolite or beauxite, calcining them with carbonate of soda, and neutralizing directly with HCl or CO2 the aluminate of soda formed. process may be simplified, and the AlCl obtained much more easily, by direct treatment of clay. For this purpose a good clay, free from iron and sand, is mixed with enough water to make a thick pulp, to which is added NaCl and pulverized carbon. For every 100 parts of dry clay there are

* Bull. de la Soc. Chem. 1860, vol. v. p. 472.

taken 120 parts NaCl and 30 of carbon. The mixture is dried and broken up into small fragments, which are then introduced into a red-hot retort traversed by a current of chlorine. Carbonic oxide is disengaged, while at the same time Al2C16 and a little SiCl are formed. It is not necessary that the chlorine should be absolutely dry, it may be employed just as it comes from the generator. The gas is absorbed very rapidly, because between the aluminium and silicon there are reciprocal actions under the influence of which the chemical actions are more prompt and energetic. The aluminium having for chlorine a greater affinity than silicon has, AlCl is first formed, and it is only when all the aluminium is thus transformed that any SiCl is formed. When SiCl begins to form the operation is stopped, the incandescent mixture is taken out of the retort and treated with water. The solution is evaporated to dryness to separate out a small quantity of silica, SiO2, which is in it, the residue is taken up with water, and the Al C16.2NaCl remains when the filtered solution is evaporated to dryness. These successive solutions and evaporations might probably be suppressed, especially if only enough chlorine is passed over the incandescent clay to just convert all the aluminium into Al2C1, in which case no SiCl4 will be formed, and therefore no soluble silica can exist in the solution to contaminate the AlCl or

impede its reduction. M. Dullo recommends reducing the Al'C16.2NaCl by zinc. See Part X.

'Chemical News,' 1873, p. 307, contains a short account of an improved method of producing Al2C1, which consists essentially in passing vapors of hydrochloric acid, HCl, and carbon disulphide, CS2, simultaneously over heated alumina or clay. The CS2 changes it into aluminium sulphide, Al'S, and the HCl converts this into APC16, which distils.

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