ACID FROM SULPHIDES.—We have already remarked that in comparatively recent times sulphur has been to a very great extent displaced by various metallic sulphides in the manufacture of sulphuric acid. Principally iron pyrites is used ; but in many places copper pyrites also, and even zinc-blende is so roasted that the sulphurous acid evolved may be utilized for acid-making.
In the manufacture from iron pyrites the acid is often the only product of value, and even when the resulting oxide of iron is economized the acid remains the chief product. Iron pyrites is now mined in many places simply for acid-making, where formerly it was altogether neglected. But there are many pyrites beds, especially in Spain, Portugal, and Norway, which contain a considerable proportion of copper. In many works this pyrites is used first as a source of sulphur for acid making, and the copper is afterwards recovered from the cinders by the wet process.
For the modification, or more properly the extension, of sulphuric acid making by the use of pyrites, we have to thank a king of Naples, who in 1838 gave the monopoly of the trade in Sicilian brimstone to a French company at Marseilles. Through the rapacity of the king and the company the price of brimstone was put at such an exorbitant figure that consumers immediately sought a means of relieving themselves of the burden. In consequence of this, the use of pyrites, which had already been inaugurated both in England and several continental countries, came to be very quickly and generally adopted in the manufacture. When the threats of England had caused the withdrawal of the monopoly, and brimstone had returned to its normal price, the pyrites was in many cases given up again ; but in other places where the brimstone cost much on account of transport the use of pyrites was continued. The utilization of the sulphurous acid liberated in copper-smelting was not attempted till some years later.
The sulphurous acid generated in the roasting of zinc-blende is utilized in few places for the manufacture of sulphuric acid, principally because the zinc-blende burns with considerable difficulty, and the heat generated by its combustion is not sufficient to roast it completely. Hence a con
siderable additional heat must be supplied, and the roasting must be carried on in kilns which do not admit of such convenient economy of the liberated sulphurous acid as the ordinary form of pyrites kilns.
The sulphur in pyrites costs so much leas than native sulphur that it would probably have become the only source of sulphuric acid making were it not that all pyrites contains a certain proportion of arsenic, which finds its way into the sulphuric acid as arsenious acid. The elimination of this arsenic from the acid is very difficult, and therefore acid which is required to be free from arsenic is made in large quantities from native brimstone. Probably about 4t of the total product of sulphuric acid is from pyrites. The same apparatus may be used when pyrites are employed as with brimstone, except the kilns, which need to be especially constructed.
In the mining of metallic sulphides, besides the large pieces, a great deal of dust is formed, and also in wet workings a large quantity of mud. These different grades require various forms of kilns for their treatment, or the dust ore may be burnt in the same kiln with the lump ore, if it be first made into balls or cubes about 2-1 in. in diameter. Sometimes it is necessary to separate all the dust from the lump ore by sifting, and the former is then worked up with soft clay. The plastic mass is formed into balls or cubes in the hand or in moulds, and these are then dried by the waste heat of the kilns. Oocasionally the dust is moistened with weak sulphuric acid and a less proportion of clay used. The balls may be dried on iron plates placed on the top of the kiln flue. In this way they are rendered so hard that they crumble little more than the lump ore. The ad mixture of clay has a great drawback, however, inasmuch as the decomposition of the clay towards the end of the process retards the burning so much that some of the sulphur is necessarily lost.