According to Karmrodt, the following proportions of the nitrogen contained in various animal substances are actually converted into cyanogen during the manufacture of yellow prussiate of potash:—
| Per cent. | |
| Woollen rags | 16 |
| Horn | 20 |
| Leather cuttings | 33 |
| Cow hair | 14 |
| Dried blood | 16 |
| Horn charcoal | 56 |
| Rag charcoal | 33 |
As is well known, human excreta contain a considerable proportion of nitrogen, and there seems no reason why this should not be employed in the manufacture of yellow prussiate. It is quite possible that municipal bodies might find this a convenient and profitable plan of disposing of a portion of the sewage with which they have to deal. It is obvious to all persons who have given this subject much consideration, that the nitrogen required in the manufacture of yellow prussiate of potash might be obtained with comparative ease from the surrounding atmosphere. Indeed, from a theoretical point of view this seems a charming process. About fifty years ago the Society of Arts awarded Lewis Thompson a medal in connection with this very process. Thompson ignited a mixture of 2 parts pearlash, 2 parts coke, and 1 part iron turnings in an open crucible for a considerable time at a full red heat. The resulting black mass was found to contain a large quantity of ferrocyanide, together with excess of carbonate of potash, &c. This process, or a similar one, in which a current of air was passed over a mixture of charcoal and iron saturated with carbonate of potash, was tried on a large scale for two years at Bramwell’s works at Newcastle. About 1 ton of yellow prussiate was made daily by this process; but it was not found to work profitably, and was eventually abandoned, chiefly, it is said, owing to the large amount of fuel required, and because the cylinders, whether of iron or fireclay, were not able to stand for any length of time the intense heat to which they were subjected.
The annexed illustrations, [Figs. 14 to 17], show the arrangement of a prussiate of potash furnace at Sir E. Buckley’s works, at Clayton, Manchester, which are well designed to prevent nuisance: A, iron pot; B, fire-place; a, cover of pot; b, stirrer; c, hinged pipe conveying vapours to the flues; d, flues surrounding the pot, and leading to the chimney-shaft; e, chain to lift up cast-iron vapour hood.
Brunquell, a German manufacturer, has criticised the present method of conducting operations, and proposes that it is necessary as far as practicable to aid the secondary formation of cyanogen by ammonia and incandescent charcoal, and to avoid loss of potash by using pure animal substances, and preventing contact with the solid products of combustion from the furnace. With this view he adopts a horizontal reverberatory furnace, the hearth of which is a cast-iron tray about 4½ ft. long, 4 ft. wide, and 3½ in. deep. The crown of the furnace is built as flat as possible, the working space is limited, and the charge is kept from contamination by the fire. Such a furnace, despite certain drawbacks, presents important advantages. Fuel is economised; the process is hastened so that seven or eight charges can be dealt with in a day, instead of only four; and the furnaces cost less and endure longer. The charge consists of 220 lb. potash, of which two-thirds is from evaporated mother-liquors, and one-third fresh; 44 lb. animal charcoal from the carbonisation of substances poor in nitrogen; 140-150 lb. of pure animal matters as dry as practicable; and 17½ lb. iron. The firing is urged and the charge is stirred till all the potash is fused, when the ash-pit is closed, and the damper turned on for charging half the animal charcoal. The firing and stirring are again pushed on till the proper consistency is attained, and potassium vapour begins to burn off. In this state the mass is ready to receive the animal
Figs. 14, 15, 16, 17.—Yellow Prussiate Furnace.
substances, those rich in nitrogen being first added in small portions at a time. Their effect is to render the mass hard, dry, and difficult of fusion, whereupon the remainder of the animal charcoal should be promptly introduced. After thorough agitation, the working door is closed for a short time, and the contents of the furnace are rapidly discharged into a covered iron pan.