The process adopted iu this country is somewhat rougher, no previous carbonization of the animal niatter being effected. A charge of good commercial potassium carbonate, usually about 80 lb., is fused in a pot of the description set forth above. A varying quantity of animal matter, dependent upon the quality and conetitution, is then introduced, together with certain amount of iron clippings or borings, and the whole thoroughly stirred up, the heat being kept at low redness. It is not absolutely necessary to add the metal, as the iron of the pot will yield a sufficient quantity, but it is preferable to do so. The animal matter should be carefully introduced in small quantities at a time, so as to effect thorough decomposition, the stirring going on the whole time. Something like I00 lb. of nitrogenous material will he required by 80 lb. of potassium carbonate, but the charge, of course, varies with the percentage of nitrogen, and requires careful judgment. Dried blood, feathers, hoofs and horns are about the best material, containing from 14 to 17 per eent. of nitrogen. NVool and hair form a very fair material, with about 12 per cent. of nitrogen. Leather parings are often used, bnt only contain about 8 per cent. An excess of animal matter has to be added, because an enormous loss of nitrogen is sustained by evolution in the free state and by the formation of ammonia. In this part of the process, the organic matter is decomposed. The carbonate of potassium is reduced by the carbon to potassium, while another portion of carbon seizes upon the nitrogen to form cyanogen (CV) which promptly con stitutes cyanide of potassium with the alkaline metal. Towards the end of the process, when a smell of ammonia is perceived, the pot should be tightly closed up and the fire urged away. After about two hours the mouth, or lid, is opened and the mixture, now of a thick pasty consistence, stirred up. If no tongues of flame make their appearance, the calcination is complete. The contents of the pot are then shovelled out and allowed to cool and harden into what is technically called "metal" or "prussiate cake." In place of potassium carbonate in this calcining operation, it has been proposed to use sulphate of potassium with small coal, or sulphide of potassium in an already reduced state. Besides protecting the pot from too rapid corrosion the addition of a little metallic iron greatly facilitates the process. The following are some recipes for charging :— Prussiate cake of good quality will contain about 10 per cent. of cyanide of potassium, 4 per cent. of sulphocyanide, 3 per cent. of cyanate, 3 per ceut. of sulphate, 57 per cent. of carbonate, 22 per cent. of silica and insoluble matter, together with traces of charcoal, lime salts, &c.
The " metal " or " cake" is broken up when cooled into small pieces, thrown into vats, and lixiviated with water, or weak liquor from previous operations. Sometimes the pieces are digested with cold water first, and the heat is gradually raised; sometimes the water, or liquor, is run on at once, hot. After lixiviation, the whole is allowed to settle, and the clear supernatant liquor drawn off by a leaden pipe, run into an evaporating pan, concentrated about 10°, and run into crystallizing cones. Here a first crop of impure crystals, a mixture of prussiate and chloride of potassium, separates out. These are removed, drained, redis.olved in hot water, concentrated up to 1.27, and again crystallized. Prussiate of potash now separates in large and nearly pure crystals.
To make the best crystals, the solution of the first crop should be filtered through cloth after concentration. The crystallizing vessels should be set in not too cold a place, or surrounded with mats, &c., to prevent too rapid deposit of the crystals. The mother liquors are either used to lixiviate fresh cake or, when not too full of impurities, are concentrated to specific gravity, and crystallized. A somewhat impure ferrocyanide is thereby obtained which may be purified by
recrystallization. The mother liquors, when they are too impure to yield these secondary crystals, are evaporated and calcined, yielding a product termed "blue salts," or "return alkali." This contains up to 70 per cent. of carbonate of potassium, with various other salts and insoluble matter, and is used in the melting and calcining operation, along with fresh salt.
It is unnecessary to enter into details of the great variety of patents that have been from time to time taken out in connection with the manufacture of ferrocyanide. A great number of materials have been proposed as subjects of treatment—gas-lime, guano, coal, &c. Two patents, or improve ments of the process just described, deserve mention. The first is Berry's, for the formation of cyanogen from animal matter, and is set forth in Figs. 206 to 209. He proposed to break coke or charcoal into pieces about the size of a walnut and to dissolve the potash salt in water, or, preferably, urine, and the iron in nitric or acetic acid. The whole of these materials are thoroughly mixed together until they form a thick paste, dried, and pulverized. They are then put into a series of iron pipes or retorts, similar to those used in the manufacture of coal gas, only placed vertically. The animal matter—dried blood, &c.—is placed in a separate compartment, but connected with the retorts mentioned. In Fig. 208, ABCD is a horizontal section of the whole furnace, in which are placed four elliptical pipes, about 6 ft. long and 18 in. in diameter. The arch given to the furnace serves to drive the heat back upon the pipes W, W, W, W. The fire-bars, or grates, are shown at a bcd, Fig. 208. I I, Fig. 207, is the retort, placed in a separate com partment. K K' is a pipe connecting the retort with the elliptical pipes. In Fig. 206 is shown the pipe K K', connecting the retort with the elliptical pipes. This connecting tube enters (Fig. 206) at S into the pipe W, and at S' into the pipe Will. In Fig. 209, the tube K K', with its cocks ts and u', is shown in detail, C being a safety valve to prevent any acoident arising from a possible obstruction of gases in tie pipes. S is the cover of the retort, L the ash-pit, a and c the door of the furnace. The arrows indicate the direction of the current of heat, which passes off from the pipes through j. The pipes must be thoroughly heated before any fire is introduced to the retort, then tiro decomposition of the gases may be readily accomplished. The smoke finally escapee to the chimney f, g being an opening to expose the retort to the direct action of the heat. In Fig. 20C arc shown the junctions connecting the four pipes with their gas burners Z Z through the cocks mm. r r' rlf arc covers closing the pipes with bolas in them, and stoppers c e' e" c". In this way, the current of the gases can be changed, and the otherwise necessary stirring up of the contents of the pipes avoided. About half through the process the cocks u m' should be closed, and u' m opened. The gases then pass into the branch K' and enter W", then through g into W, through p into u, 0 and W, finally escaping by the burner Z. By this regulation of the cocks u m' and u' m the current can be reversed at will. It is advisable, however, to have holes in the pipes so arranged that the contents can be loosened if any obstruction occurs. The inflammable gases evolved by the decomposition show by the colour of the flame at the burners bow the operation is progressing. When the jet becomes small and clear, with a pinkish colour, the reaction is complete. The animal matter is thoroughly carbonized, and the nitrogen, ammonia and other gases acting upon the mixture in the pipes have formed ferrocyanide of potassium of the quality known as prussiate cake, which is lixiviated and treated in the usual way.