The process is similar to that employed for the production of bicarbonate of soda, chlorine gas being generated in leaden vessels, or in the ordinary stone stills, by the action of hydrochloric acid upon peroxide of manganese, and passed through powdered ferrocyanide spread upon wooden shelves or trays iv a close chamber. The ferrocyanide should be dried before use. The result of this first part of the operation is a deep orange-coloured powder, which is dissolved in hot water, and run into coolers to crystallize. Ferricyanide separates out from the chloride of potassium, crystallization being assisted by small rods or pieces of string. The mother liquors are evaporated and dissolved, and a second crop of inferior fcrricyanide is obtained. Occasionally the first, powdered, product, without crystallization, is sold as a commercial article.
Ferricyanide of potassium, or red prussiate, as it is more frequently called, is largely used in dyeing and printing operations, to produce peculiar shades of blue, and as a " discharge " of indigo colour—chiefly for the former purpose. Its discharging powers depend upon the process of rapid oxidation already alluded to. If a piece of " dip " blue be soaked in red prussiate and dried, and then passed through a bath of caustic potash, the colour is immediately oxidized and destroyed. The process is, however, expensive, and certain difficulties arise in " thickening " the cloth. It has been proposed to use calcined magnesia in place of caustic potash, but the element of expense still forms a serious obstacle.
The beat rough teat for red prussiate is the appearance and size of the crystals. They should lose no weight when dried, and dissolve readily and completely in water.
Ferrocyanide of Potassium. (Fa., Prussiate joule de Potasse ; GER., Ferrocyanhalium). Synonym, yellow prussiate of potash. Formula, useful salt, when pure, occurs in the form of large, transparent, amber-coloured + from an octahedron with a square base. They have a strong saline taste and are permanent in the air. At 100°, the three equivalents of water are driven off, leaving the anhydrous salt, and this at a little over red heat splits up into cyanide of potassium, carbide of iron, and various gaseous products. Heated, with free admission of air, the cyanide is converted into cyanate. Ferrocyanide is soluble in about four parts of cold, and two and a half parts of hot water. The following table gives the composition of the aqueous solution at different densities :— Tho ery,tals become gradually decomposed in a strong light, giving off hydrocyanic acid and becoming ry slightly alkaline.
Crystallized ferroeyanide was first manufactured by Macquer, about one hundred years ag,o, by dhsolving prussian blue in caustic potash and concentrating the solution. Pru,sian blue and allied salts had been known for fifty years before Maequer's time. Since then endless patents have been taken out for its manufacture and improvement, of which may be mentioned those of Bennie', Gentile, Naumann, K.ulilmann, Spence, Laming, Krafft, Swindell, and Bramwell. Nearly all these have been abandoned or not carried at all to a successful issue. For full details, the reader is referred to books of scientific) research and the patent records.
The process of manufacture, as usually carried on, consists in the mutual decomposition of nitrogenous animal matter, an alkali, usually carbonate of potassium, and iron. The first part of the process is devoted to the fusion, or calcining, of the raw materials. Many forms of apparatus have been devised for the purpose. That set forth in Fig. 205 is perhaps the ono in commonest use. A cast-iron vessel, shaped like an egg, with a narrow neck, is built into a brickwork furnace, resting upon the neck at the one end and a strong projecting k-uob at the other. It is also secured by a pair of cross-wise arms near tho mouth, which run up into the brick work. Heat is applied from underneath, only a small passage round the pot being left.
The products of combustion finally pass off to the chimney through a bole just above the neck, at the end ,Ir,sitc to the fireplace.
Another form of pot is cylindrical in shape, about 2 ft. in diameter and 2 ft. 6 in. deep.
A series of suoh pots is arranged over suitable furnaces and through the cover of each passes a vcrtioal shaft with revolving arms or blades upon it to agitate the contents of the pot and assist in tho decomposition. In France, the apparatus employed resembles an ordinary gas retort. On the Continont, it is usual fo carbonize the nitrogenous matter before mixing it with the potassiurn salt, for which purpose a variety of stills aro used. In this way, a great part of the nuisance arising from the evil odours whioh eseape when the raw materials are mixed and stirred up with the potash, is prevented. The animal matter is exposed to a low red heat, until the escape pipe from the still or retort begins to cool down. The gases which escape are ig,nited. The products of such previous distillation are about as follows:— Animal charcoal .. 75 parts Liquid, containing carbonate of ammonia .. .. 125 „ Animal oil .. .. 40 „ the original charge being 250 parts.