Ordinary sulphuric acid results from the oxidation of sulphurous oxide, the hydration of sulphuric oxide (anhydride), as we have already seen, and from other chemical reactions; but the method commonly adopted, and among manufacturers universally, is the first named. • The sul phurous oxide is derived from burning brimstone or pyrites, and is oxidized by the intervention of nitric acid, supplied either in the liquid form, or as a salt undergoing decomposition. The principal supply of oxygen is derived, however, not from the nitric acid, but from the atmospheric air, in whose presence (as in that of steam) the reaction is allowed to proceed.
Sulphuric acid may be justly described as the most important of the chemical products, since in consequence of its numerous applications, it has become the "mainstay of our present industrial chemistry." The production of the acid is achieved in two separate ways : (a) by the burning of sulphur or sulphides to produce sulphurous acid, which is subsequently oxidized to form sulphuric acid; and (b) the decomposition of sulphates by heat. By the first-named process, which is by far the most general, only a hydrated acid of about 1.558 sp. gr. is commonly produced, and from which it is impossible to evaporate all the water, but only such proportion as to leave the monohydrate HOSO,. By the last-named process, however, an anhydrous acid is liberated, which may either be used as such, or may be dissolved in monohyd•ated acid to produce Nordhausen. But the cost of production by this method is so much greater than by the other, that it is adopted for the manufacture of acid to be used for purposes which demand a stronger acid than can be made by the combustion process.
The production of acid by the combustion process is always assisted by nitric acid gas, derived either from the decomposition of nitrates by the action of sulphuric acid (conducted in a portion of the kilns), or from liquid nitric acid introduced into some part of the chamber system.
The apparatus and manipulation necessary in the production of acid by combustion of sulphur (brimstone), aided by nitric acid gas from the decomposition of nitrates (of potash, or soda, but almost exclusively the latter), will now be considered.
Figs, 37 (plan), 38 (longitudinal section), and 39 (vertical section taken through the chambers), show a small manufactory capable of producing daily about 30 to 35 cwt. of acid, at sp. gr. The same letters of reference are used in each of these three figures.
A is the brimstone kiln, made of well-burnt bricks, and bound together by iron bars a. In front of the retaining walls the strong iron plates b are fixed, and held by the fonr tie-bars a. The
nuts visible at a are for screwing the plates tight np to the wall. The front plate is pierced by a door through which the charge is introduek d, and in which is a hole for the admission of a constant cm rent of air. These openings should not be made larger than requirements demand, and they must be fitted with iron closures.
. The eharge.of sulphur is introduced regularly every hour on the cast-iron plate r, under which is a fire-place, z, for the purpose of heating the plate at the commencement of the operation, or whenever it may become necessary to do so,• but it is not by any means indispensable, and is wa n ling in many works. The joints are carefully plastered up after admission of the charge.
Th e decomposition of tbe nitre with sulphuric acid is conducted in the cast-iron pots, d. These are filled with the materials outside the kiln, and are introduced immediately after the brimsteme.
The gases formed by the combustion of the brimstone and the decomposition of the nitre, mixed with atmospheric air,' stream through the vertical channel B to the first leaden chamber. This channel is built of brick and strengthened by iron bolts, e. The connection between this channel and the first leaden chamber is made by the cast-iron tube C. Atf the lead of the chamber wall is drawn over the tubo and, after the joint has heir] well stopped with putty, an iron clamp is tightly bolted round. At g the tube is fitted with MI iron man-hole plate, and the joints similarly made tight. This plate is removed froin time to time, in order that the foreign matters, carried with the gases in the form of dust and here deposited, may be cleared out. All joints must be made with the greatest care and stability, for to obtain good working results it is essential that there be not the slightest communication between the atmosphere and the interior of the chamber, save through the kilns.
39.
From the tube C the gases pass into the first leaden chamber D at f. In this chamber, measuring 7l_ ft. long, 22 ft. 9 in. wide, and 17 ft. 10 in. high, the greater part of the acid is condensed. The gases which do not condense here escape by the opening at the end and near the floor of the chamber into the leaden connection pipe E, which conducts them into the upper part of the second chamber F. This chamber is only 22 ft. 9 in. long and 10 ft. 6 in. in width and height. The still remaining gases flow hence by the exit opening near the floor, through the leaden pipe G, into the third chamber H, made of the same width and height as the second chamber, but only 16 ft. 3 in. in length.