The heat should be carefully regulated during this operation, as the colour of the salt may be spoilt by the evaporation of the tarry matter. The subliming pots vary in size, according to the extent of the other portions of the plant. They are constructed of cast-iron circular pots set in brickwork, the flues are arranged so that the heat from the furnace circulates around them. The pots are closed with a heavy dome, which is kept in its position either by its own weight or wedges. It is safe to have a small hole in these domes so. as to allow the escape of any water or non-con densable matter ; the hole is kept clear by thrusting through the salt, which accumulates over it, a short iron rod. These holes allow also the operation to be watched.
A sublimer holding a charge of 2 to 2i tons may have a diameter and depth of about 8 or 9 ft. The domes fit air-tight, by being luted outside with clay and canvas. The above charge will require about seven to nine days to work off. If the temperature has been too low, the mass, instead of having a finely-grained structure, will present a somewhat cloudy or effloresced appear ance, though still fibrous. If the whole of the water has not been removed before placing the crude crystals in the sublimers, the portions in contact with the covers will be contaminated with the iron, and stained a brown colour ; this is cut away with axes before being packed for the market.
Another method of treating the crude ammonia liquor for the production of chloride is to convey the ammonia from a still or boiler direct into a vat containing strong hydrochloric acid. The noxious gases are conducted away to the furnace and burnt. The ammonia and a portion of the steam together entering the acid make the whole very warm, which helps to destroy any empy reumatic matters which may be carried over. The liquid is drawn off into shallow tanks, to allow the impurities to subside, and is then concentrated in the evaporators and set by to crystallize ; the solution may be almost enough concentrated on the evaporators as to deposit the salt on cooling, but there would probably be more trouble in getting rid of the water before it could be sent to the sublimers, if in this state. Where, however, the salt is to be converted into liquid ammonia, this would be a matter of no consequence.
The manufacture of the chloride by this method would no doubt be far preferable to the former, even if we allow the salt to crystallize in both cases the same way.
By the first method, cnormons quantities of water have to be got rid of by evaporation; four or five days have to be allowed for the impurities to subside after the acid is added, and the prcduet itself is a very impure article, and requires care to prevent the tarry impurities passing into the sublimate.
By the ,,cond method, 20,000 gallons of crude liquor could be worked up with a compamtively small plant in a single day euitable for the crystallizers, and the salt so obtained would, when dry, be sufficiently pure for a great many purposes, in fact, for the more important uses to which it is applied.
The addition of milk of lime to the crude liquor when placed in the stills or boilers will liberate tho combined ammonia, and thue make the yield from the same gas liquor equal to that obtained by the first method.
The ammonia gss may be passed into a solution of chloride of iron or manganese instead of hydrochloric acid, and in manufacturing sulphate of ammonium, the sulphates of iron or manganese may bo employed; the gases which are generated form insoluble compounds with these metals, whit:di are deposited before the solution passes to the evaporators.
Tho manufacture of the salts of ammonia is best carried out on a large scale; the sublimation of the chloride is much more economically conducted by working on large quantities of the crude salt, from the fact that the cost of labour and fuel remaine nearly the same for working off large or small charges.
The manufacture of the chloride of ammonium from the carbontde is carlied out in Glasgow by utiliziug the chlorine refuse from bleach-works and bleaching-powder manufaetories. The carbonate of manganese, which settles down by standing, is regenemted into peroxide, and utilized again fer chlorine. The mother liquor, or bittern, from eea-water is also employed. The chloride is obtaiucd by evaporating, with occasional stirring so as to avoid tho formation. of largo masses of crystals; it is well dried and mixed with a little animal or vegetable charco.d, which, by acting ou the chloride of iron present, prevents the sublimate from being coloured.
The Scotch chloride is white, contains but little iron, and is free in most cases from lead. It eometimes shows the presence of manganese. In Liverpool, the chloride is sublimed in iron domes ; coneequently, the salt is stained with the chloride of this metal, and in addition is generally contaminated with lead. In the sublimation of the chloride, it is fouud that a quantity of tho ealt collects in the centre of these domes in. a conical heap, wbieh is called the yoke. To avoid this, it is usual to build up in these dornes a brickwork form of the yoke, Fig. 199, in which D is the iron dome and Y the yoke. The heat necessary to sublime this portion of tho salt must be raised so high that it is deeomposed ; hence, in practice, it is used in the recharging of the subliming veesele.