Carhonate of Potash from the Lees of Wine and Tartars.—After fermentation, the lees or " yeast " is placed in bags and pressed into hard cakes. These are then dried and burnt over a slow fire, yielding a light porous product called "cend-es grareko," which consista of a mixture of salts, chiefly carbonate of potash ; lixiviated and calcined in the manner already described, the cendres gravelees give a remarkably pure carbonate. Tho refuse from the mauufucture of brandy is treated in similar fashion.
The well-known cream of tartar, or acid tartrate of potash, crystallized out from a solution of the crust which forms upon the casks during fermentation, is incinerated, and yields a black mass, called "black flux," which consists of various potash salts—oldefly carbonate—mixed with charcoal. This "black flux," deflagratcd with nitre, yields a fairly good potassium carbonate, sometimes called "white flux." These processes are carried on only to a small extent, and chiefly in France and Spain.
Carbonate of Potash from the Sulphate, Britie Springs, &c.—Next to the ashes of plants, the greatest amount of carbonate of potassium is obtained from the sulphate. The process is almost identical with the Le Blanc soda process, the raw material (sulphate) being obtained either from chloride of potash and sulphuric acid by the ordinary sulphate process, or by decomposing the former with sulphate of magnesium, or " kicserit." Equal weights of sulphate, and finely divided limestone, or chalk, together with varying quantities of small cost, are furnaced together in a reverberatory furnace, the product being an exceedingly impure carbonate of potassium. When the decomposition is complete, the molten mass is raked out, broken up when cool, and lixiviated in tanks. The soluble salts of potash and soda are thereby dissolved out, evaporated, and calcined iu a small reverberatory furnace. Further details of this process will be given when treating of the soda compounds.
A carbonate of better quality is produced by following more closely the carbonating operation of the sodu process. The liquors from the tanks are evaporated, the chloride and sulphate of potas sium which separate out during the concentration being skimmed off, aud sawdust is thrown in. Tho dried salts are then removed to the carbonator and exposed to a heat at first gentle, but finally urged to dull redness. By this process, the sulphur compounds are oxidized into sulphate
and the caustic potash converted into carbonate. The chief object of the sawdust is to keep the mass of salt open. A carbonate carefully made in this manner should give about the following composition : Carbonate of potash .. 89.59 Sulphate „ Chloride „ 2.49 Carbonate of soda .. 2.33 Soluble silica arid alumina .. •I9 Insoluble matter .. •13 1.27 99 98 Upon a small scale, a hydrated carbonate is manufactured containing about 15 per cent. of water and having the formula + This is made by dissolving the crude carbonate, boiling down not quite to dryness, and allowing the mass to cool, stirring it vigorously in the meantime. The product is a fine white granular salt.
The manufacture of potassium carbonate from the sulphate by the methods described is a rapidly increasing industry. Something like 16,000 tons per annum are now produced, and, all things con sidered, it seems likely to supersede the other processes. It is much more amenal le to the altered conditions of the labour market; a pure article is more readily obtained ; and the soil is not im poverished by a too rapid withdrawal of the potash compounds, which must be the case where beet-root and other plants are the material operated upon.
It is of great importance to free the potassium carbonate as much as possible from soda com pounds, as these considerably destroy the fine character which the potash salts give to the articles manufactured from them. In the preparation of biebrome, ferrocyanide of potassium and soft soaps, for example, the potash compound should not contain above 3 per cent. of soda.
Preparation from " Suint."—From this remarkable source about 100,000 kilns. of carbonate of potash of very good qu,dity are annually obtained. .Saint is the term given to the sweat exuded from the akin of the sheep, and retained in the wool. The residue left after Ilxiviation of the skins in water contains earthy matter mixed with charcoal en very finely divided that it may be used as black paint. According to MM. Maumend and Rogelet, a fleece weighing 4 kilos. contains 600 grm. of suint, capable of yielding 198 grm. of pure carbonate of potash. According to Fuchs, however, the quantity of suint amounts to only about 300 grm., made up as follows :— Sulphate of potash .. 7.5 grm. = 2.5 per cent.