Two other plans of economizing these waste gases have originated with the same inventor. In one he employs such an arrange ment as is shown in Fig. 164, of which a a is the decomposing vessel, made of slate, or of wood lined with thick lead, and provided inside with coils of leaden pipe for conducting the steam necessary to heat the contents ; b b is a large eductor dipping into the solu tion, and by means of which the gases are carried away ; c is the hole for charging the cistern ; d forms the connection of a pump for forcing in air oxygen or other gases ; at e communication is made with the nitric acid tank, and f is a tap for emptying the decomposer of its contents.
Into the decomposer are introduced 100 lb. of sugar or a proportional weight of starch or treacle, and then 600 lb. of nitric acid of 1.220 sp. gr. are added gradually during a space of twelve to fifteen hours. Steam is turned on to heat the mixture, and when the conversion into oxalic acid is complete, which point may be known by the absence of coloured fumes on the admission of air, the contents are drawn off at the top f into water baths for concentration, and thence are carried into suitable coolers for crystallization. Whilst the nitric acid is reacting, air is pumped in through d; or the gases having been collected and mixed with air, are pumped in through the saccharine liquid.
In the second process the object aimed at is the production of cyanogen compounds, for which purpose the gases are taken over a mixture of carbonaceous aubstances, iron and an alkali, heated to a high degree, and are passed thence throngh water, and after being collected are forced through a pipe comiected with the faces of some retorts or tubes arranged in furnaces. Similar retorts to those used in the manufacture of coal gas may be used, but double ones are preferoble, that is, two retorts connected so as to form one of about 10 to 12 ft. in length, and fixed in two furnaces ; they may be of irou, and severs' pairs may be placed in the same oven.
The proportions recommended are : Commercial potash . .. 100 parts.
Coal dust, coke, or soot .. 100 Iron filings .. .. .. 30 or a strong solution of alkali (preferably potash) may be made, and absorbed by sawdust, and a mixture formed consisting of 30 parts of iron borings or filings and 100 parts of small coal for every 100 parts potash used.
Fig. 165 is a section of one pair of retorts ; a a' are the retorts furnished with movable plates b b' c is the cock connecting the pipe through which the gases are forced, d is the exit pipe for un condensable gases, e is a safety valve, and f f are the furnaces.
Having heated the retorts, a charge of the mixture is introduced, nearly sufficient to fill one retort, a', and when it has attained a dull red beat the nitrogenous gases are passed over it with occasional stirring for about three or four hours, at the end of which time the plate b is removed. The half finished oharge is then forced into a, where it is heated t,o a bright red degree, a new charge is inserted into a', and the same treattnent continued. The charges in both retorts must be stirred occasionally, which may be dono by means of an iron rake or stirrer introduced through the cover-plates. After three or four hours the plates b b' are removed, when the charge in a is drawn into a suitable vessel and covered, the charge in a' is transferred to a, and a new oharge introduoed. By this armngement all the gases from the fresh charge, together with the uncombined gases from the pipe c, have to pass over the half-finished charge. The charge drawn from a having been allowed to cool, is ground and treated in the usual manner for obtaining the cyanides. Of the uncombined gases we shall speak presently.
Instead of the arrangement shown in Fig. 163, a reverberatory furnace may bo used, into which is introduced au& a charge as that already described, or a mixture of 100 parts sulphate of potash. 84 chalk.
100 small coal.
30 ,, iron filings or borings.
It is preferred to arrange the furnaecs so as to compel the nitrogenous gases, together with tho uncombined gases from the ordinary combustion of coal, to pass over the charges placed in commu nication with each other. Fig. 166 is an arrangement which meets this requirement ; a a a are reverberatory furnaces, b b is a main flue trona ordinary furnaies, c is the pipe through which nitro genous gases are forced from the gas-holder into the flue ; d d d are fires in the reverberatory furnaces. The furnaces are so provided with dampers that any one of the series may be stopped without interfering with the working of the others. Into each furnace a charge is introduced, and when it has attained a red heat the gases are passed over it, with occasional stirring, for six or eight hours, when the charge is withdrawn, and treated as before mentioned.