Or, the sulphur may be determined by dissolving 1 grm. finely powdered ore with 4 grin. chlorate of potash in nitric acid. When the solution is completed, the liquid is diluted and filtered. The filtrate contains the sulphur as sulphuric acid, which may be determined by titrated chloride of barium solution.
The amount of sulphur contained in the burnt pyrites must also be frequently tested in order to check the perfeotiou of the burning in the kilns. The same processes may be adopted for that as we have already indicated for the raw ore, but the sample must be larger on account of the lessened amount of sulphur. For instance, ,5 grin. roasted ore may be mixed with 5 grm. carbonate of soda and 5 grm. chlorate of potash, without salt.
The total amount of sulphur contained in an ore is never utilized, even with the best possible working, but a portion always remains in the cinders unburnt. The proportion of loss varies greatly and depends upon the natural quality of the ore, the arrangement of the kiln, and the care bestowed on the roasting. In hard, close-grained iron pyrites, and in copper pyrites, more sulphur is lost than from the porous, gritty iron pyrites found in the coal measures. With the former, the air finds so much difficulty in piercing into the interior of the ore that towards the end of the roasting the further oxidation proceeds very slowly, and thus portion of the sulphur must be allowed to remain. Under the most favourable conditions the amount of the sulphur left in the ore may not exceed 2 per cent., but as a rule 4 per cent. is wasted. When the sulphuric acid is only formed as a bye-product in the reduction of metallic sulphides, a very much larger proportion of the sulphurous acid is often allowed to escape up the chimney.
In the roasting of pyrites a slight, variable amount of sulphuric acid is formed, free from water, and this finds its way to the chambers with the kiln gases. Besides the sulphur the metals also will be oxidized, and the oxygen thus required will be admitted to the kiln as air, and the proportion required has a considerable effect upon the process.
We shall now consider the points to be observed in working, in the same order as we have already done for using native sulphur.
The Temperature.—Through the oxidation of the metals contained in the pyrites, such an amount of heat is generated that the gases will be much hotter than when sulphur is used, rising even to 200°. For this reason, instead of it being necessary to check any tendency to cooling in the connection flues, it is absolutely necessary to cool the gases somewhat before admitting them to the chambers. This is sometimes effected by passing them up a double-lined shaft formed of thick sheet lead, 30 feet high and 2 feet in diameter, into the outer casing of which cold water constantly flows at the bottom and escapes in a heated state at the top. An arrangement such as we have shown in Fig. 65 is more common, however, and nothing of the sort is needed when Glover's towers are used.
The Draught and Admission of Air.—The proportions of air required for the oxidation of the sulphur contained in pyrites vary considerably. We will see the proportion needed by bisulphide of iron or pure iron pyrites. This consists of 1 Equivalent Iron .. .. Fe = 28, and 2 „ Sulphur .. 321 Equivalent bisulphide of iron .. 60, or 44 per cent. of iron and 531 per cent, of sulphur.
Though when hurning bard iron pyrites all the iron is not oxidized to seeing that sometimes magnetic iron (FeO, is formed, still we must calculate upon the complete oxida tion of the iron normally, which is a point endeavoured to be attained in order that all the sulphur may be utilized. Then 2 equivs. or 120 parts of bisulphide require 3 equivs. or 24 pees of oxygen for the oxidation of the iron, and a further 8 equivs. or 64 parts of oxygen for the conversion of their 4 equivs. or 64 parts of sulphur to sulphurous acid. In all 11 equivs. or 88 parts of oxygen are thus needful for the roasting. Besides this, 4 equivs. or 32 parts of oxygen must be introduced for the conversion of the 4 equivs. or 128 parts of sulphurous acid formed into sulphuric acid.
From the preceding it follows that 1. For every 1000 parts bisulphide of iron are needed :— 200 parts oxygen for oxidation of the iron, 5331 „ „ formation of sulphurous acid, and 2661 „ „ conversion of sulphurous to sulphuric acid.