This disposition possesses the following defects:— 1. If the glass or stoneware pipe, through which the acid is introduced to the ehamber from without, should break during working operations, and thus render it necessary to insert another, it is very difficult to make sure that the new one shall exactly reach the uppermost saucer.
2. It may easily happen that in sneh a case the unstable pile may be disarranged or even altogether upset.
3. The topmost saucers are so small that in a very short time they become filled with mud, principally composed of solid matters introduced with the kiln gases, and this is especially the ease when using pyrites.
On this account it has become more general to use stoneware or pipe-clay saucers all of one size, viz., about 2 ft. 6 in. diameter, and 2* in. deep. Twelve of three are placed terrace-like, ono behind another, in one of clambers, in such manner that the lip of each higher one reaches over the ono immediately below so as to deliver the acid into it. The saucers are arranged on loosely stacked piles of fire-brioks, as shown in Fig. 46.
Whilst the nitric acid is slowly passing along the apparatus, it is acted upon by the sulphurous acid gas present and reduced to hyponitrie, or even nitrous acid, which enters into the volume of the gases ; the sulphuric acid then gradually formed in the upper part of the chamber atmosphere finally reaches the floor of the chamber free from nitric acid.
Formerly it was common to devote two small chambers, called drums, to the nitric acid. These were 22 ft 9 in. long, 9 ft. 9 in. wide, and 11 ft. G in. high, or were cylindrical and about llf to 12 ft. diameter, and were placed between the sulphur-burner and the large chamber. In the second of these the bcfore-mentioned apparatus was placed.
They stood at a somewhat higher level than the first small chamber into which tbe stilphuric acid flowed as fast as it formed, and whose only purpose was to give the sulphurous acid full opportunity to act upon it and free it from nitrogen com pounds. Into this chamber only so much steam was admitted as was necessary to prevent the formation of chamber crystals or to decompose such.crystals when formed. The acid collected in this first small chamber flowed into the large chamber.
With properly conducted working, the first "drum" is un necessary, for it is quite easy to produco sulphuric acid free front nitrogen compounds in the same chamber where the nitric acid is decomposed, if only the sulphurous acid is allowed time enough, and the amount of steam so regulated that the acid produced contains more than 4 equivalents of water to 1 equi valent of acid. More recently, these small chambers for the decomposition of the nitric acid have
been done away with in many works, and the process allowed to go on in the first large working chamber. lt must be remarked, however, that in this arrangement it is impossible to get a perfect combination of the nitric acid with the sulphurous acid, which is a great defect.
We have already shown the part played theoretically by the nitric acid in the manufacture, and have uow described the construction and general arrangement of the apparatus employed in its introduction to the working process. There now remain to be considered the conditions necessary to ensure the moat perfect results.
Though the conduct of the process of the manufacture of sulphuric acid looks rernarkably simple, it depends uPon so many chemical and physical conditions that it is often difficult when things go wrong to find out the true cause of the mischief.
The principal paints needing attention are :— The temperature.
The draught and inlet of air.
The steam.
Tbe way in which the gases are taken through the chambers.
Temperature.—,-The temperature exercises an important influence on the conduct of the process and acts differently in the various parts of the apparatus.
In the sulphur-burner too high a temperature will volatilize the sulphur, aud form 't flowers of sulphur," which collect in the first chamber. To avoid this evil, the burner is built in the form of a high vault, with thin walls, and of larger dimensions than necessary, and in many works the roof and the front and back walls are made of iron plates. The iron bed-plate should be at least four square feet for every one cwt. of brinnstoue burnt per twenty-four hours, and often twice or thrice that size is given to it. The greatest degree of heat is reached immediately before the insertion of a new charge, and thus it may easily occur that a part of the new charge be volatilized. This may be prevented by damping the new charge beforehand. While it is necessary to take care that the temperature of the bed-plate does not rise too high, it is equally itaportant to watch that other parts of the apparatus do not become too cool.