The flotation of gold, uranium, and pyrite from Witwatersrand ores

03 Jul 2018

It is apt that simultaneous consideration should be given to the flotation of gold, uranium, and pyrite because the association between these three constituents of the Witwatersrand reefs makes it difficult to consider them in isolation. Gold is probably the most floatable of all the constituents of the reef, but the close association between as much as 30 per cent of the gold and radioactive minerals (both uraninite and thucholite), and between as much as 20 per cent of the gold and the various sulphide minerals (pyrite) present in the reef, means that, in general, high recoveries of gold are associated with the simultaneous recovery of both uranium and sulphides. Similarly, the recovery of uranium is likely to result in the simultaneous recovery of gold and sulphides, and the recovery of sulphides to result in the simultaneous recovery of gold and uranium. However, the reefs have an average sulphur content of about 1,7 per cent so that there has been considerable experience in the production of a relatively high-grade sulphide concentrate containing comparatively small amounts of gold and uranium. The gold is usually present in economic quantities in these concentrates, which therefore require cyanidation before being delivered to the end-user. The uranium is mainly present as thucholite in the concentrates, and is accordingly not generally present in economic quantities. The concentration of gold in the reefs is about 0,001 per cent, so that there has been very little experience of the production of a gold concentrate by direct flotation from the ore. Instead, there has been some work on the use of flotation to produce a gold concentrate from a material prepared by some alternative technique of primary concentration to a gold content of about 0,1 per cent. The uranium in the reefs is present at about 0,02 per cent on average, but there have been problems in the development of a flotation technique that will give a uranium concentrate because (i) much of the uranium is present as uraninite, which is inherently difficult to float; (ii) the uraninite tends to be very finely ground during milling, which makes its selective flotation even more difficult; (iii) a small but significant fraction of the uranium is carried by siliceous minerals such as zircon, leucoxene, titanite, monazite, and various phyllosilicates, all of which are difficult to float selectively; and (iv) thucholite is floated very readily, but tends to be rather coarse because its low density permits it to escape from milling circuits before being milled. Accordingly, in this paper flotation to produce a 'pyrite' (sulphide) concentrate will first be discussed in detail; some work on the production of gold concentrates will be reviewed; there will be a short discussion of the attempts to produce uranium concentrates; and the paper will close with a review of processes involving the simultaneous flotation of all three species. The discussion will centre on the Witwatersrand ores; while flotation is important in the treatment of the arsenopyritic gold ores of the Barberton area, the economic importance of that area is too slight to justify detailed treatment. Similarly, the role of flotation in the production of uranium at Palabora is ignored.