The principle of this process is that when a mixture of iron, copper, lead, and zinc sulphides is roasted, the surfaces of the particles of the three former sulphides are changed to oxide and sulphate at a comparatively low temperature, whereas the zinc sulphide is practically unaltered. The partly roasted material is then subjected to a heated-acid oil-flotation process, such as the Minerals Separation process, when the zinc sulphides will float away, and the deadened or sulphated compounds of iron and lead will sink with the gangue.

Preliminary tests must be made on ores which are to be treated by this process in order to ascertain in just what combination the metals really exist. While the ores at Broken Hill are somewhat granular, the grains of galena and blende being, to a great extent, separable by comparatively coarse crushing, there are many steely-grained admixtures of blende, galena, and pyrite which have to be reduced to a slime before the individual grains are separated.

A photograph of the small Minerals Separation process unit used in extended trials at Bendigo is shown in Fig. 54.

On one lot of Broken Hill slime concentrate the laboratory test showed the concentrate to contain 32.4% zinc and 22.3% lead. This was roasted and treated in the testing laboratory, and two products were made : No. I, a zinc concentrate, containing 50.8% zinc and 6.3% lead ; No. 2, a lead residue, containing 2.6% zinc and 49% lead. About 53i% of the original concentrate was recovered as a zinc concentrate, and about 38% reported as a lead residue. About 85% of the total zinc and 85% of the total lead were recovered in this test.

In actual operation there were obtained from this ore a zinc concentrate, containing 50.4% zinc and 7.3% lead, and a lead residue, containing 12% zinc and 41% lead. The lead residue may be improved by further agitation, raising the content of the lead and reducing that of the zinc.

A complex sulphide from Tasmania, containing an intimate mixture of blende, galena, and pyrite, was also tested. This ore had to be crushed dry to a slime product to insure separation of the blende and galena. The ore assayed 30.9% zinc and 9.9% lead, and yielded in actual operation in the experimental plant a zinc concentrate containing 57% zinc and 5% lead, and a lead residue containing 5.8% zinc and 20% lead. These Tasmanian ores were so intermixed that some mineralogists considered the zinc-lead sulphide to be an isomorphous compound.

Preliminary laboratory tests must be made on ores to be treated by this method to determine the percentage of flotation obtainable, the amount of lead required to be converted to sulphate, acid consumption, crushing, etc. The ore is roasted in an Edwards furnace, the temperature being kept below the ignition point of zinc sulphide ; usually the temperature of the heated gases passing over the ore, as tested by a pyrometer, varies between 300 and 400 C. The end point of roasting is determined by prior laboratory experiment, and varies with the composition and fineness of the ore. Tests are made from time to time on the furnace product to find the amount of lead present as sulphate, and as soon as the predetermined amount is reached the ore is discharged from the furnace.

In the first test the ore was roasted in charges until sufficiently sulphated ; but latterly, and with more satisfactory results, a continuous feed and discharge and a slow rabble-motion were used. The first to oxidize is the pyrite, and next the galena, the zinc sulphide being almost unaffected. The time required for Broken Hill slime-concentrate, when roasted in charges, was about three hours.

The partly roasted ore is fed into the boot of a bucket elevator with about five times its weight of a dilute solution of sulphuric acid, heated to near the boiling point. The amount of acid required is determined by laboratory experiment, and is as high as loolb. per ton treated.

The zinc, being unaltered by the slight roast, floats at the surface as mineral froth in the same way as was described in Chap. IX., while the iron and lead particles, being altered on the surface to soluble sulphates, are wetted by the water and sink. It may be noted here that the extreme limit of water-wetting is the case of solubility.

Kenneth A. Mickle* proposes to get an analogous alteration of the galena particles by treating the crushed material with ferric chloride, and then subsequently treating by a flotation process.

HYDE PROCESS.

James M. Hyde, of Basin, Montana, has developed a flotation process for treating the ores of the Butte & Superior Company. The experiments at Basin were so successful that the Butte & Superior Company has erected a mill at Butte on lines laid down by Mr. Hyde.

The description of the process is given in U.S. Patent No. 1,022,085, as follows : The ore, after crushing, is mixed with acid before thickening in a tank A (Fig. 55). It has been found that adding the acid in advance cuts down the amount required, and promotes the settlement of colloidal material, which may be further aided, if necessary, by adding copperas, alum, or some other coagulant. From the thickener A the pulp is pumped to the first treatment tank B, where oil is added and the entire mass agitated. A float concentrate is then taken off, and the gangue goes to the tank B', where the process is repeated, and thence to B", where the pulp is agitated and a third concentrate taken off. From B" the gangue is discharged as a final tailing.

There is considerable silica mixed with the first concentrate, so it is re-cleaned in a" second device CC', the concentrate from which is the final product, while the tailing is all sent back to B'.

As an example of actual work with this machine, the following description is given : The material treated was the slimy portion of an ore, consisting principally of quartz, blende, and various amounts of decomposed granite, in which the felspars were largely kaolinized. About 92% of this material was fine enough to pass a i5o-mesh screen. Without the use of a coagulant, part of the slime, containing a notable percentage of zinc, would remain in suspension over 24 hours.

The pulp, as produced, contained from 29 to 100 or more parts of water by weight to one part of ore, to which was added, before thickening, from one-quarter to one-half pound of sulphuric acid per ton of ore. The thickened pulp was heated in the flotation machine by live steam to expand the gas bubbles, and facilitate coating the sulphide particles with the oil used, which, in this case, was ' candle-maker's red oil.' The oil consumption cannot be calculated beforehand, but must be gauged by experience. The ore fed carried 23.3% of zinc, the concentrate 51.4%, and the tailing 3.2%, indicating a 91% recovery. The concentrate from the primary section of the machine ran 38 to 42% of zinc. The Butte & Superior ore contains rhodochrosite, quartz, and sulphides and is an ideal ore for flotation. The ore has been treated for some time at the Basin reduction plant, rented from the United Copper Company by the Butte & Superior Co. The mill was too small and the rental heavy, so plans were drawn for a i,ooo-ton mill, one 500- ton unit of which was in operation by the end of May, the other about a month later. The installation consists of a crushing plant, jigs, hydraulic classifiers, a slime-plant for oil-flotation, and Wilfley tables, which are used only to separate the lead from the zinc. The ore is first reduced by rolls to 6-mesh, and, after jigging, the tailing is re-ground to 40-mesh, and the tailing from the fine jigs ground to loo-mesh, and treated by oil-flotation. The pulp is thickened to three parts water and one part ore, and is mixed with a small quantity of oil and sulphuric acid and agitated.