The gases passing off from the retorts or furnaces, together with a jet of steam (if ammonia be required) are forced by means of a pump or fan through a furnace containing spongy platinum, iron, or clay burnt and broken in pieces to render it permeable, or any mixture of these substanees, which are heated to about 370° (700° F.), and are conducted thence into a suitable condenser for collecting the cyanides of potash and ammoniacal salts which are formed. These may be separated in the usual manner, or may be applied to the manufacture of artificial manures, thus:— The residue of the ground charges from the retorts or furnaces, after having washed out the cyanides, consists of some cyanide of potash, iron, sulphate of iron, sulphate of lime, carbon and some undecomposed sulphate of potash. This mixture is treated with ammoniacal liquors in such quantities as to present sufficient free ammonia to decompose the sulphate of lime, or it may be mixed with bone dust, superphosphate of lime, or other salts, to form a suitable fertilizer. Fig. 167 shows the arrangement of the apparatus employed by Jullion with the object of recovering and utilizing the liberated nitrogen compounds resulting from the manufacture of oxalic acid by the action of nitric acid on vegetable substances.
In the first place, instead of using the ordinary jars or open vessels for the manufacture of oxalic acid, he places the mother liquid, together with the organic substance to be acted upon, in closed or covered vessels formed of earthenware capable of containing about one hundred gallons each, having the necessary pipes and openings, as shown in the accompanying figure, where a is a decomposing vessel (of which there may be several if required), set in a water bath b, which is heated by steam or in any other convenient manner. The materials to be operated upon are supplied to the decomposing vessel through the aperture c, which is stoppered, and when the materials are decomposed the residuum or products are withdrawn from the vessel through the same aperture by means of a syphon. A small stoppered opening d is made in the ground air tight cover f for the purpose of applying a thermometer to ascertain the temperature of the contents of the vessel, and e is a pipe for supplying the vessel with atmospheric air or oxygen. The gases evolved from the decomposition of the materials in the vessel a pass off through the pipe g to the main h. The pipe e for the supply of atmospheric air or oxygen gas is connected with a gas holder, so that a proper supply of air or gas may always be maintained in the vessel a, or it may be drawn in by an exhausting apparatus, arranged suitably for the purpose. The main h is connected with a vessel or tube i, filled with platinum in the state of sponge, or with asbestos, coated or covered with platinum, and the tube is kept at an elevated temperature by means of the furnace k.
It is preferred to heat the vessel 1 to from 315° (600° F.) to 482° (900° F.). The apparatus being properly connected and arranged, and a portion of nitric acid having been added, in the usual way for making oxalic acid to the contents of the decomposing vessel a, heat is applied to it. As soon as decomposition of the materials commences and the gases or oxides of nitrogen begin to be liberated, there is blown or forced through the pipe e into the decomposing vessel, and directly upon the surface of its contents, a regular stream of oxygen gas or atmospheric air, either at common temperatures or in a heated state ; by which means the oxides of nitrogen are converted into higher states of oxidation, one portion remaining in the liquor, and another portion, by condensing with aqueous vapour on the cooler parts of the vessel, being returned to the said liquor and there performing the same part as a new addition of nitric acid, and thus diminishing the quantity of nitric acid required to completo the operation ; while the remainder of the gases which have escsped condensation pass off in conjunction with the excess of oxygen or atmospheric air through the exit pipe g into the main h, by which they are conducted into the tube 1, containing the platinum or platinized asbestos, where a combination takes place with the formation of nitrous and nitric acids. The acids so produced, together with the excess of oxygen or atmospheric air, must be passed through a pipe 1 into a condensing apparatus, which should be formed of a series of vessels n 7n, of a description similar to those used in the ordinary process of manufacturing or distilling nitric acid, in which water or dilute nitric acid should be placed. Tho delivering or exit pipe n from the last of these receivers should be made to dip about an inch or an inch and a half into water or dilute nitric acid in another vessel m, so as to place the condensing acids under alight pressure, and tho uncondensed vapours may pass off through the exit pipe o. Care should be taken in this process that such a quantity of atmosphene air or oxygen gas be supplied to the decomposing vessel, that a portion of oxygen shall still he mingled with the gases that pass off through the water after the conversion of the whole of the oxides of nitrogen into nitrous and nitric acids. In all cases where the oxides of nitrogen are c,omparatively dry, as in the early part of the process for manufa,cturing oxalic acid, when the heat employed for the decomposition of the materials is insufficient to raise any great amount of water in the state of vapour, it is recommended that u jet of steam should be passed, along with the mixed gases, through the tube containing the platinum or platinized asbestos, the presence of aqueouB vapour greatly assisting the formation of nitric acid.