Other chemical agents calculated to arrest the sulphurous acid may be suggested, such, for instance, as a soluble sulphide (e.g., the yellow liquor from alkali waste heaps, if available), or hydrated peroxide of iron. In the one case an alkaline hyposulphite, and in the other sulphate of iron would be produced, both of which have a commercial value. But it remains yet to be seen by actual working whether the condenser is equally applicable to the ordinary processes of lead smelting.
As respects vapours proceeding from smelting - houses where the ore - hearth is used, some advantage appears to be gained by the use of a sliding shutter, which can be let down in front of the opening to the hearth during part at least of the operation. At several works a hood is in use to catch the offensive fume proceeding from the slags when drawn forward upon the forestone, a pipe above conveying the fume to a chimney outside. With the same object, the ore-hearth is at some works set back within an arched recess capacious above and similarly communicating with the outside air. But even this is sometimes inefficacious. A suggestion made by Norman Cookson ap - pears valuable, namely, to bring the hood well forward and to draw the air required for the blast from the upper part of the interior of it.
Where iron is used to remove sulphur, whether in the Cornish method of smelting or in smelting with the blast furnace, there is of course less sulphurous acid made, and therefore less to discharge into the air, than when iron is not used. The iron fixes the sulphur in forming a protosulphide. But unless the matte be thrown away, which would be a wasteful proceeding, there is still a liability of nuisance being created in the subsequent dealing with it. If the matte be burned or "calcined," sulphurous acid is again formed, and must be got rid of somehow, although it is more easy to utilize it without creating nuisance when it is formed in this way, inasmuch as it need not be inconveniently diluted with air, being capable of burning by itself without any fuel. (Dr. Ballard's Report.)
Wet methods have been tried for treating lead ores. They offer notable advantages with argentiferous ores, more especially those containing also an appreciable quantity of zinc. It is easy to see that the zinc increases the loss of lead in the dry method of treatment, either by rendering the slags pasty (when they mechanically retain minute grains of lead), or by augmenting the volume of metallic vapours carried out of the apparatus.
In the Roswag process, the ore, finely ground, and calcined if necessary, is treated with hydrochloric acid, more or less concentrated according to the ore, but usually consisting of a mixture of equal volumes of water and the commercial acid. After formation of the various chlorides, the mass is allowed to settle, and decanted hot. The cooling favours the deposition of the major part of the lead chloride; the liquor is again applied to more ore, and takes up a further quantity of lead chloride, which it deposits under similar conditions. The number of successive decantations required to exhaust the ore is generally 3 or 4.
The removal of the chlorides left in contact with the ore is effected by washing with hot water. The wash - waters and solutions are mixed, and concentrated if necessary. The remaining lead is precipitated by metallic zinc -old zinc from galvanization, for instance; as the amount of lead to be precipitated is approximately known, the consumption of zinc as a precipitant is calculated at the rate of 1/3 to 2/3.
The metallic lead sponge, which contains the silver of the ore, is washed with a little water and pressed into bricks to remove the liquid which it holds; these bricks are put into a rever - beratory furnace with an alkaline flux and a little charcoal to prevent oxidation.
The liquors containing the zinc chloride are heated to boiling; the zinc is precipitated by milk of lime, and the zinc oxide thus obtained is washed and compressed into bricks constituting a material assaying 55 per. cent. and more of metallic zinc.
Maxwell-Lyte's process is for the separation of the metals lead, zinc, silver, copper, and iron, in mixed ores, by a wet chemical treatment, having in view the employment of the cheapest possible re - agents, such as brine, hydrochloric acid, old scrap zinc, hard spelter, chalk, and quicklime, thus rendering it possible to utilize economically on a large scale otherwise relatively valueless ores often hitherto treated as refuse, and to present them in marketable forms, either directly applicable in the arts, or treatable by the ordinary modes of smelting for the production of the pure metals. Following is a summary of the principal reactions on which it is based, taking for. example a mixed ore containing blende with argentiferous "galena and copper pyrites, such as the so - catted bluestone of the Parry's Mountain and Mona mines of Anglesea, North Wales.
By calcining this ore, most of its sulphur is burned off, but part - is oxidized into sulphuric acid, variable quantities of which remain combined with the metallic oxides produced The gases resulting from this calcination, strongly charged with sulphurous acid, may be utilized for the manufacture of sulphuric acid. There generally remains in the calcined ores 3 to 5 Per cent. of sulphur, in one form or another, but this does not interfere with the treatment of the ore.
When this calcined ore is finely pulverized and then treated for a certain length of time with hot or boiling hydrochloric acid of about 15 to 17 per cent. HC1 in proper quantity, ie. the quantity requisite to bring into solution the zinc and other more soluble bases, the zinc and the copper can be for the most part dissolved, and if. this solution be now decanted off. and cooled, most of the lead and silver remain insoluble, either with the gangue in the attacking - tub, or separate out of the decanted liquor as it cools. (For the most perfect working of the process, the acid should be used in such quantity as to leave an excess of about 5 or 6 Per cent. of free HC1 still in. solution, together with the chlorides produced, but some latitude is allowable in this respect.) On adding another like quantity of hydrochloric acid, the lead, as well as that part of the silver which may have before escaped attack, becomes completely chloridized, and largely dissolved by this second acid, while more or less of the iron ses - qui - oxide, always present in such ores after calcination, is taken into solution as sesqui - chloride. This iron sesqui - chlo - ride seems materially to assist in completing the chlorination of the lead and stiver.