Caustic potash is usually prepared by dissolving one part of commercial carbonate in 12 parts of water, boiling the solution and gradually adding one part of thoroughly burnt lime—preferably made into milk of lime. The boiling is continued for about twenty minutes after the whole of the lime is added, when a dense carbonate of lime is formed, and falls to the bottom as a, heavy powder. Caustic potash remains in solution together with an admixture of various other salts of potassium, the sulphate and chloride, &c. The clear liquor is run off and evaporated to dryness in an iron—or, if required of particularly good quality, a silver, vessel. It is then fused at a strong heat, whereby the water is as far as possible expelled, and run into moulds. The lime must be added until a small portion of the filtered liquid gives no effervescence when treated with an acid. The moulds are of iron usually, and are in two halves, screwed together to make a tight joint. The fused caustic potash is poured in at the top, and, when cool, the halves are unscrewed and the solid product, iu the form of sticks removed. This is the ordinary "stick," or " lump " potash of commerce. A still better article may be made by treating the stick potash with methylated spirit, or alcohol, evaporating the solution to dryness, and fusing in au iron or silver vessel. It is never, however, free from a certain admixture with other salts of potassium. The " liquor potassce" is usually prepared in this way, the process being carefully conducted so as to leave only the smallest possible quantity of carbonate. The quantity of water must not be less than that stated, and it is well to have a slight excess of lime. Liquor potassre and lump potash have usually about the following composition :— Liquor potassm should stand about 1.05 specific gravity.
The well known " potash " and " pearl ash " of America and Russia are very extensively used in this manufacture. It will be noted that the process is similar to the ordinary production of caustic soda, and may be carried out from the commencement, employing chloride of potassium in place of the corresponding chloride of sodium. This substance heated with sulphuric acid, gives a sulphate of potash with the evolution of hydrochloric acid. The potash salt furnaced with chalk and small coal in a reverberatory furnace gives an impure carbonate, which is drawn out of the furnace in large lumps answering to the "black ash" of the Le Blanc soda process. These lumps are broken up and lixiviated in tanks, the dissolved pure carbonate run off into pans, boiled down to dryness and furnaced.
The best carbonate to use in this process, is that obtained by igniting cream of tartar. The crude potash, or pearl ash, gives only an impure product, and requires further purification by the treatment with alcohol as above mentioned. This is Berthollet's plan. The first caustic solution should be concentrated until it attains a thick consistency, and well agitated with one-third its volume of alcohol. The mixture is then allowed to settle, the impurities—chloride, carbonate, aud
sulphate of potash, &c.—subside with whatever lime or oxide of iron may be present, and the lighter caustic potash solution remains clear upon the top. This is decanted off, freed by distillation from the alcohol, and evaporated to dryness in a silver vessel. Whatever impurities are left float on the surface during concentration, and are removed. The pure hydrate is then run into moulds, or poured out on plates to cool.
Other methods of preparation are the following :— (a) by the action of the metal potassium upon water ; hydrogen gas is evolved, and hydrate of potash formed. The liquid is evaporated and fused in silver apparatus as already described. In this way the purest caustic potash is prepared. (b) By the action of the monoxide upon water, thus : + = 2 (KHO).
) By decomposing, nitrate of potassium with metallic copper at a r( d heat, thus :— 2 KNO, + 3 Cn, = 6 CnO + N, + K,O.
One part of tho potassium salt, and 2 or 3 parts of copper clippings RTC arranged in layers in a crucible and exposed to a doll red heat for hal f an hour. The mass is then allowed to cool, lixiv iated with water and settled. Oxide of copper separates out, and the clear caubtic potash in solution is decanted off, evaporated and fused. ei; By decomposing sulphate of potash by hydrate of barium, thus :— K2S0, + Ba11,0, = BaSO, + 2 KHO.
The baryta water is added in slight excess, and the sulphate of barium settled out, leaving caustic potash to be treated in the manner already described.
The uses of caustic potash are many and varied. Its chief application, of course, is in tho manufacture of soap. It is also used in the refining of saltpetre, in the manufacture of oxalic acid, and is a chemical reagent, absorbing carbon dioxide in orzanie and inorganic analyses, drying gases, &c. It is a powerful poison, and is used to some considerable extent in medicinal prepara tions as an antacid.
The chief impurities are carbonate of lime, oxide of iron, peroxide of potassium, carbonate of potassium, sulphate of potassium, and nitrate of potassium. The presence of most of these bodies depends upon the care with which the preparation is conducted.
The following analyses by Tatlock represent the composition of various kinds of commercial potash.
The manufacture of potash is carried on to a large extent in France and Germany. In England very excellent qualities axe made upon a considerable scale in Lancashire and at Glasgow, the Green bank Alkali Company and the North British Chemical Company being the largest manufacturers.
Carbonate of Potassium. (Fn., Carbonate de Potasse; GER., Kohlensaures Kalt).—Synonynas, carbonate of potash, potashes, pearl ash, salt of tartar. Formula K,CO3. Combining proportion 138.2. Specific gravity 2.2.