As a gravity concentration man in the midst of so many eminent flotation experts I feel like Daniel. However, I would respectfully disagree with the opening remarks by Mr Viljoen.
First, he suggested that gravity concentration of cassiterite was not effective in sizes below 30 microns. In a paper to the Institution of Mining and Metallurgy in 19621, I gave some figures for gravity concentration of declaimed material both for cassiterite and for wolfram which indicated that careful shaking table concentration of fine declaimed material could give recoveries of 75 to 90 percent in the size ranges above 13 micron. These were figures obtained from normal plant operation. It would be interesting to know if any tests were carried out on gravity concentration of the efficiently declaimed feed to this flotation plant to see what the recovery would be.
Secondly, Mr Viljoen suggested that gravity concentration plants were expensive. He gives the cost of a flotation plant at R300 000 to treat a feed of 400 t.p.d. to the declaiming section or 160 t.p.d. to the flotation cells. From his operating cost figures it would appear that the flotation cost is approximately R2 to R4 per ton of feed to flotation and the total operating cost about R2 per ton of feed. A table plant to treat this tonnage of declaimed feed would consist of about 30 tables and should cost considerably less to install and very much less to operate than the flotation plant.
Third, Mr. Viljoen suggests that the concentrate grade from a gravity concentration operation would be unacceptably low. The paper quoted above showed that once a primary fine concentrate had been made, giving a suitably declaimed material, further gravity concentration could give concentrate of over 40 per cent Sn with high recoveries. Tailing from this dressing stage was naturally returned to the feed. To give these high grades, sulphide had been removed by flotation but this necessary flotation step is simple and cheap to operate and could be restricted to the final stage when the quantities involved are small.
I would agree that flotation has a part to play in fine tin recovery but this role would only seem to become fully effective at sizes below 10 micron and would seem to require, as Mr. Viljoen has suggested, a mechanism for sizing which is effective down to, say, 1 micron. In the meantime it would be interesting to try tabling the flotation tailing to see if it was possible to recover the plus 20 micron cassiterite which is not recovered well by flotation.
*Consulting Metallurgist, Ang1o-American Corporation of S.A. By E. B. VILJOEN One point which has been made during discussion is the suggestion that cassiterite particles have a fully hydrated surface at normal pH. I wonder if this layer could materially affect the overall specific gravity of a very small cassiterite particle and, if so, whether gravity concentration of the very fine cassiterite would be improved if carried out in acid conditions, which would inhibit the formation of this hydrated surface.
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