The background to the flotation process was similar to that for the cyanide process. The reserves of ores containing coarse-grained minerals were dwindling, and ores containing fine-grained minerals—which were difficult to extract by the processes of the time— needed to be mined. The cyanide process was unsuitable for ores containing lead, zinc, and copper, all of which occurred as sulfide minerals. Gravity-separation processes gave mediocre results with ores containing these minerals in the form of fine grains. By 1900, the problem of mineral losses in tailings had become serious.
The rich and large lead-silver-zinc ore deposit at Broken Hill in Australia focused attention on this problem. This deposit had the form of a giant vertical boomerang with its ends pointing down and its apex breaking the surface. The oxidized ores at the surface were coarse grained and the minerals could be extracted easily. But the deeper primary ores were a serious problem, because the lead, silver, and zinc sulfides in them were fine grained, and finer grinding was needed to liberate them for concentration. Mining began at Broken Hill in 1885, and by 1893 the high-grade oxidized ores were nearing exhaustion. The future of the town of 20,000 people depended entirely on the discovery of processes to extract sulfide minerals from the fine-grained primary ores.
After years of expensive effort, flotation proved to be the solution. It worked because sulfide minerals have different surface properties from oxides, and, if the pulp chemistry is right, air bubbles will attach to sulfides and float them to the surface while oxides are not affected. Tube mills, which had been used successfully in the gold industry, were adapted to grinding sulfide ores to liberate the fine minerals and to make small particles that air bubbles could lift.
The cyanide and flotation processes remove metallic minerals from the ore. These minerals have been liberated from the ore by the fineness of grind, which is determined by the size of the mineral particles in the ore. The types of rocks that contain minerals vary widely in grindability and abrasive characteristics. The size of the mineral particles varies with the type of minerals in the ore body. Testing is required for each ore type and deposit to determine its grindability and abrasive characteristics; no rules of thumb or comparisons are available, as there are for grinding cement raw material and clinker. In the manufacture of portland cement, size reduction is an integral part of the process, particularly in the grinding of the clinker. For the cyanide, flotation, and other mineral concentration processes, size reduction is part of the feed preparation stage, not part of the process. The need to properly prepare the feed led to the development of wet closed — circuit grinding in the tumbling mills discussed in this chapter.