The impure carbonate of ammonia is converted into sulphate by heating with gypsum (sulphate of lime). By sublimation sulphate of ammonia passes over, leaving carbonate of lime in the retorts. The solution of the impure carbonate is sometimes converted into sulphate by percolation through gypsum, the lime salt being converted into carbonate by an exchange of acid.
The manufacture of this salt is much more economically carried out by conveying the ammonia gas direct into a vessel containing strong sulphuric acid. If precautions be taken to pass the ammonia into the acid, free from tarry contaminations, it is possible, by a recrystallization, to obtain the salt with a remarkable degree of purity. When the ammonia passes over into the acid it is accompanied with steam and certain volatile matters contained in the gas liquor, most of which, immediately coming in contact with the strong and hot acid, are carbonized, and so effectually prevented from contaminating the product.
Fig. 201 shows the arrangement for this method of production. A cylindrical boiler A is placed so that the liquid or vapours produced in it can be conveniently received in another boiler B, and which may be either above or below A. These boilers may be heated by the direct heat of a furnace or an inlet of steam. A better plan is to heat the contents of these boilers by passing steam through a series of circular tubes or pipes, which prevents the further dilution of the ammoniacal liquor, the waste steam from the evaporating pans may be used for this purpose. From B the ammonia is conveyed by a pipe P into vats or tuna V V' V", containing strong sulphuric acid. Branch pipes p reaching nearly to the bottom of the vats are supplied with taps so that the ammonia can be made to pass into either of them. When the acid in any one of the vats is saturated, the resulting solution of sulphate is drawn off into a large tank C, and allowed to stand until all the impurities have subsided. The clear liquid is then drawn off into shallow pans E, and evaporated. The sulphate which crystallizes out, acenrnulates on the bottom of these pans, and is removed into other pans D to drain. The salt is finally dried by being placed in wicker baskets, and is now ready for the market.
201.
The vats are lined with stout sheet lead, and are made perfectly air-tight, an outlet is made in the top, so that by means of a waste pipe W the noxious vapours arising from the decomposition in the vats can be led away to be burnt iu the furnace.
So far wo deal only with the uncoinbined or free ammonia; to obtain the ammonia which is not free, milk of limo is added to the contents of the retorts, and the process conducted in the same way. At one time the sulphate was obtained by adding sulphuric acid direct to the ammonia liquor, the gases which were evolved were burnt in the furnace.
In localities where tho limo waste is likely to become troublesome, and large supplies of gas liquor can be easily obtained, it is questionable whether it is not more economical to run away tho combined ammonia than to take the trouble of extracting it.
The sulphate of ammonium is employed extensively as a manure and for the production of ammonia alum.
The manufacture of ammoniacal salts is not at present confined to any particular locality. London, Liverpool, and • ilnagow, are the principal seats of its manufacture. The conditions necessary for successfully carrying on this manufacture are, large supplies of ammoniacal liquor without much expense for conveying it to the factory, and the vicinity of large acid works. The demand for artificial manures has stimulated the development of this manufacture, and so keenly is the competition felt in tho oast of production, that one of the mot important considerations is to economize fuel by employing waste heat wherever practicable.
Estimation of Ammoniaral Preparations.—For manufacturing purposes, the volumetric methods are the simplest, and yield results sufficiently exact for any commercial purpose. For solutions of ammonia, liquor ammonia, &e.; an accurately marked pipette, holding, say 10 c.c., is dipped into the liquid, so as to take out a definite quantity without risking any loss of the gas, which could scarcely be avoided by pouring out. The 10 c.c. is poured into a narrow stoppered bottle, previously tared; from the weight of the 10 c.c. the sp. gr. may be obtained by simply shifting the decimal point one place nearer to the left, thus, if the 10 c.c. weigh grammes the sp. gr. of the sample is •965, as 1 c.c. of water weighs 1 gramme.