The condensing batteries may of course consist of any number of vessels, but as a rule only a sufficient number to retain all the ammonia is employed, five being in most cases sufficient for this purpose. The last condenser is connected with an exhaust-pump, p p, which is kept in motion by a motor, P.
The pump removes all the bodies remaining in the last condenser and forces them, according to the position of the cock back of the pump, either into a glass-bell or through the pipe H and the nozzles a into the fire-place where they are burned.
To obtain the various products of distillation, the condensing vessels have to be filled with acid, and should therefore be constructed of lead or at least of sheet-iron lined with lead. The products to be obtained depend on the fluid used for filling the vessels; if filled with dilute sulphuric acid, ammonium sulphate is obtained, which may be utilized in the preparation of fertilizers. If hydrochloric acid is employed for the absorption of ammonia, solution of ammonium chloride is obtained, which may be crystallized by evaporation.
The products evolved in the destructive distillation of bones consist of various hydrocarbons and appear either as badly-smelling brown liquors—bone-tar—or as illuminating gas. The vapors contain further considerable quantities of ammonium carbonate and cyanide of ammonium. To obtain the latter, the last condensing vessel is filled with green vitriol solution, the cyanogen compounds remaining behind in the solution. If the gas escaping from the last condenser is to be used for illuminating purposes, it is freed from the greater portion of carbonic acid contained in it by purification with lime.
The mode of operation with Belgian retort furnaces is as follows: The retorts having been filled with comminuted degreased bones, the doors are closed perfectly gas-tight and firing is commenced. The pump is set in motion until a jet of gas blows through the nozzles. When this gas-jet burns with a luminous flame, destructive distillation of the bones is in full blast. The pump is then run with such velocity that the pressure in the interior of the retorts, as indicated by the manometer, is slightly greater than the external air-pressure, and the operation is thus continued so long as inflammable gases escape from the pipe H. The pump is then stopped, and one-half of the charcoal contained in the retorts is withdrawn to the canisters placed ready to receive it. The lids of the canisters are then luted down with a paste of char-dust and water, making an air-tight joint, and the charcoal is allowed to cool.
The retorts having been partially emptied, are at once refilled to the brim with crushed bones and closed gas-tight. There is but little or no loss of heat between withdrawing and charging, and distillation of the freshly-introduced bones commences immediately after charging, and is finished in a much shorter time than in the beginning of the operation.
In making animal charcoal on a large scale there are obtained from 2000 lbs. of raw material:
Animal charcoal, 1180 to 1220 lbs.
Ammoniacal liquor, 178 to 180 lbs.
Gas, 222 to 248 cubic yards.
However, these figures refer only to bones degreased by steaming, whereby a considerable portion of the cartilaginous substance passes into solution in the form of glue. In working bones degreased with benzine, larger yields than those indicated above are as a rule obtained. The ammoniacal liquor contains on an average 10 per cent. of ammonia. The gas freed from carbonic acid yields 2.7 times more light than good coal-gas.