February 2012

Flash of brilliance

Kinross Gold’s Paracatu operation tests out flash flotation

By Eavan Moore

Kinross Gold’s Paracatu Mine in Minas Gerais, Brazil, had budgeted for 80 per cent gold recovery in its second concentration plant, but one year after commissioning in 2008, the operation had topped out at 77 to 79 per cent. In 2009, the mine called in Blue Coast Metallurgy to audit the plant. “They approached us to go down there and really help them understand why the gold recovery was lower than the projected numbers,” says Chris Martin, principal metallurgist at Blue Coast.

Over-grinding leads to losses

Originally, the plant’s flowsheet began with a semi-autogenous grinding mill, which then fed into two parallel ball mill circuits. Before material entered each ball mill, it passed through a set of cyclone density separators that diverted the finer particles to be recovered through conventional flotation and cyanide leaching. The remaining coarser material was ground in the ball mill, and sent to a jig for recovery. Anything not recovered by the jig was returned to the cyclone and ball mill circuit.

To make their assessment, Blue Coast personnel and Paracatu’s metallurgical team collected samples from three different days and 24 different points in the plant. “With samples from all the streams, we can kind of paint a picture of how the plant is working at a given moment in time,” explains Martin. The Blue Coast team assayed the material by mass and found high gold grades at midsize fractions of +38 and +75 microns. However, these composed less than 10 per cent of the concentrate produced by the jig. Of the weight in the jig concentrate, 70 per cent was in the +300 micron fraction, containing only five to 10 per cent of the gold.

While building the plant originally, Kinross had assumed that the gold would be associated with coarse arsenopyrite and had chosen radial Pan-American jigs designed to recover coarser particles. But the majority of gold, as it turned out, liberated at finer fractions. “We found that there’s very little supercoarse gold,” explains Martin. “It really only starts to liberate at 150 microns.”

The jigs had been dismissing these smaller particles back to the plant’s milling circuit again and again, where repeated grinding damaged their recoverability. “It was getting very over-ground,” says Martin, “and there’s quite a lot of evidence to suggest that the gold surfaces were being degraded in some way that rendered them less susceptible to the downstream flotation process.”

The samples taken from the flotation circuit showed that flotation had been working, as long as liberated gold was coarse enough. All of the gold coarser than 12 microns had made it into the concentrate. But only 15 per cent of finer gold floated, suggesting that over-grinding had serious consequences for recoverability.

As a temporary fix to over-grinding, the Paracatu team decided to increase the cutoff size for material sorted into the ball mill by the cyclones from about 90 microns to about 110. Coarsening the grind led to increased throughputs without reducing gold recovery. Recoveries actually increased slightly, presumably because fewer gold particles were being over-ground.

Flash flotation prevents over-grinding

Blue Coast’s plant survey showed that the main gravity recovery circuit was failing to catch gold that was liberated at medium particle sizes. The company suggested that if coarser particles could be exposed to flotation much earlier, then recovery rates would improve.

Flash flotation cells, used for base and precious metals since the 1980s, are designed to do this. So named because they work more rapidly than conventional flotation, flash methods recover liberated particles that respond easily to flotation but are too coarse to enter mainstream flotation tanks. In contrast to typical flotation cells, flash cells sit within the grinding circuit. By using flotation early in the process, gold that is liberated at that stage can be recovered to avoid over-grinding, while coarser particles return to the mill.

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