2. Persoz’s method to oxidise sulphurous acid by means of nitric acid, and to regenerate the nitric oxide resulting from the reduction of the acid by the oxygen of the air in the presence of steam.
3. a, by the decomposition of gypsum by superheated steam at a red heat; or b, by decomposing the gypsum by chloride of lead.
The failure of the above and other efforts has led to the chemist turning his attention to the elaboration and perfection of the old process, in the working of which considerable improvements have been introduced within the last ten or fifteen years; improvements resulting not only in a diminished cost of production, but in the manufacture of a purer, and therefore better acid.
The proper construction of the furnaces, ovens, and grates on which the firing of the sulphur or pyrites takes place, together with the flues, is an important condition in the manufacture of the acid; and to this end a great deal of scientific knowledge and experience have lately been applied with excellent effect. Of the many improvements in this direction for burning poor ores of pyrites is a contrivance much used in Germany, where the furnace on which it is carried out is known as Gerstenhöfer’s oven. It is shown in the accompanying drawing.
The furnace is fitted inside with a number of little fire-clay projections, arranged as shown in the plate, in banks or terraces, the function of which is to prolong the exposure of the pyrites to heat. The furnace having been previously raised to a red heat, by means of a coal or wood fire (which is then extinguished), the pyrites are admitted into it through the hoppers (a). At the base of the hoppers are grooved iron rollers, which crush the lumps of ore as they enter the chambers, and by thus reducing their size, expose a larger amount of surface to the action of heat. The greater part of the sulphur of the pyrites is thus burnt off, as the lumps pass from terrace to terrace, the heat at the same time generated by their combustion being sufficient to keep up that of the furnace. A moderate blast of air is admitted at c, whilst the sulphurous acid formed ascends through d into
the leaden chambers, the spent pyrites falling out through the apertures at c.
Another improved furnace is Perret’s, which is largely used in France. In this, small lumps of pyrites are placed on horizontal plates, and exposed to the hot gases generated in kilns below. The gases, on their way to the chambers, sweep over the pyrites and rob them of their sulphur.
The most important and noticeable improvement, however, of late years in sulphuric acid manufacture is that resulting from the addition to the plant of a contrivance of Gay-Lussac. Previous to this invention, the sulphuric acid of commerce, amongst other impurities, always contained appreciable quantities of certain oxides of nitrogen, the results of which were not only the contamination of the acid, but a waste of substances, which, properly utilised, are essential for the conversion of the sulphurous and sulphuric acid, and the loss of which leads to an increased consumption of nitrate of soda. Under the old method, these valuable oxides of nitrogen, which, with a large amount of nitrogen and a small quantity of oxygen, constituted the spent air of the last leaden chamber, were carried off into the air, and consequently lost. Now, instead of being allowed to diffuse into the atmosphere, they are made to pass through a tower or chamber (shown at C in the plate below) filled with coke, through which a thin stream of sulphuric acid is made to trickle. In passing through the coke, therefore, the expiring spent gases come into contact with the sulphuric acid, to which they give up their oxides of nitrogen. From the tower (C) the acid flows into a cistern (D), whence it is pumped up to the top of another tower (E), either filled with coke, or arranged with inclined shelves, as shown in the plate. In this tower the acid meets with a current of hot sulphurous acid and air coming up from the furnace, which deprive it of the oxides of nitrogen, and the gaseous mixture enters the chambers, whilst the denitrafied acid flows off into a suitable reservoir.
