But the item that caught my attention in your December 2010 issue was the retreatment by Mine Waste Solutions of residues at Buffelsfontein. Buffelsfontein was first and foremost a gold mine in Jack Scott’s Strathmore Group. Production of uranium from the gold plant tailings was added in the late 1950s using sulphuric acid with manganese dioxide to provide the acid oxidising conditions at elevated temperature to bring the uraninite in the ore into solution as uranyl sulphate.
At this time West Rand Consolidated Mines Ltd, the first mine to undertake uranium production in South Africa, was augmenting its uranium production by hosing down the stopes on a monthly basis, neutralising the “acid mine drainage” with lime then settling and pumping the uranium rich sludge to the uranium plant. We are now familiar with what was happening. Exposed to air and water, pyrite on the stope walls was being oxidised by a species of bacterium -thiobacillus ferro-oxidans- to ferric sulphate and sulphuric acid. This led to the replacement of MnO2 in the uranium leach at Buffelsfontein by an integrated circuit in which ferrous ions in the barren liquor were bacterially reoxidised to ferric ions and fed back to the leach circuit.
It was also common knowledge that long crushed gold ore lying at the edges and bottom of ore stockpiles tended to develop coatings of complex iron compounds which rendered the gold somewhat refractory to the alkaline cyanide leach process. Building silos for the in-line storage of crushed ore provided a means of ensuring first in first out to the mills. This led to the prediction that it would be better to leach the milled ore for uranium first followed by gold in a reverse leach operation that would polish any iron compounds from gold particles in the acid conditions of the uranium leach, allowing a better extraction of gold in the subsequent cyanidation process. This was confirmed first in pilot operations but it was still a brave decision in the late 1970s to stop the Buffelsfontein plants for a weekend while the piping was switched for the reverse leach. It was certainly vindicated by improved gold recovery.
Back to Page 21 of your December 2010 issue and the statement that “it makes sense for us to bed down the TSF and the new gold module before embarking on the commissioning of the uranium plant”. Agreed, but I would expect that gold particles in the material brought back from the slimes dams would be coated to some extent with iron compounds, somewhat refractory to alkaline cyanidation and would be more amenable to dissolution in the reverse leach situation. I hope this is the intention.
John Freer. Hon. Life FSAIMM