Selenium is also a frequent constituent in the flue dust. Some ores, after being subjected to roasting, yield iron capable of being worked. This is more particularly the case with the Spanish and Portuguese pyrites.
The following is an outline of the process by which sulphuric acid is obtained, and of the chemical changes which occur during its manufacture:
The sulphur or sulphide being placed on the hearth of the furnace, shown at A in the accompanying cut, when heated from below, soon takes fire, and combining with the oxygen of the atmospheric air, the admission of which into the furnace is regulated by an experienced workman, by the door shown in the plate, forms sulphurous anhydride. An iron pot, standing on the hearth of the furnace, contains a mixture of nitrate of soda and oil of vitriol, and this becoming heated by the burning sulphur, decomposition of the salt ensues, and fumes of nitric acid are given off. The sulphurous anhydride and nitric acid gases thus formed together with air are carried into large leaden chambers, standing on, and supported by, massive frameworks of stout timber. Steam is admitted continuously by several jets (see plate) into these chambers, which are covered at the bottom with water to a depth of about three inches.
As soon as the mixed gases enter the chamber and come into contact with the steam, the sulphurous anhydride acts on the nitric acid, forming sulphuric acid, which falls into and is absorbed by the water on the floor of the chamber, and nitric oxide, which is liberated in the chamber.
The following equation will illustrate the reaction:
2HNO3 + 3SO2 + 2H2O = 3H2SO4 + 2NO.
170 parts by weight of nitrate of soda are required to oxidise to sulphuric acid 96 parts of sulphur, whereas rarely more, and frequently less, than 5 parts of soda are required by the vitriol maker. This saving of material is effected by the function performed in the chamber by the nitric oxide resulting from the decomposition of the nitric acid.
The nitric oxide reacting upon the air in the chamber abstracts oxygen from it and becomes converted into nitric peroxide, thus:
2NO + O2 = 2NO2.
Nitric peroxide is a very unstable compound, and directly it comes into contact with the fresh sulphurous anhydride entering the chamber, it oxidises it in the presence of water to sulphuric acid, thus: