Gold Leaching Time Requirements

The leaching is usually accomplished in four or six stages in agitated tanks to allow for the required cyanidation reaction. The ore ground to less tan 100 me sh is leached with cyanide solution as a thick (45-40% solids) slurry. Lime is added to maintain a protective alkalinity (ph more than 10), the pulp is vigorously aerated and agitated to promote the solid-liquid reaction. The required time depends on the maximum size of the gold grains, and may vary from 8 to 24 hours. Deep tanks with turbine agitators of low tip speed are preferred. Pachuca tanks have also been successful in leaching auriferous ores. The dissolution of precious metals must be completed before the leached pulp flows to the CIP circuit.

The gold recovery circuit may include counter current cyclone washing or thickening in order to have a pulp with 40-45 solids pulp de the carbon circuit. The underflow flows pulp flows through trash screens (e.g. DSM screen) and is fed to the CIP circuit. The absorption circuit is a cascade of four to six tanks; each of one is equipped with a number of equalized pressure air cleaned internal launder screens. The screens retain the granular carbon while allowing the fine ore slurry to advance to the next stage. Carbon is moved counter-currently to the flow of the ore pulp, from stage to stage, by vertical recessed impeller pumps. See Fig. 4 and Fig. 5.

Carbon concentration in each tank is maintained, roughly constant, at 15 to 40 g/ l, depending of the ore and type of operation. Carbon can be usually loaded with 3,000 to 4,000 g Au/ton when removed from the screen of the first tank absorption. This is the last tank for the carbon flow.

There are several types of interstage screening devices for the separation of the carbon from pulp. One type is a 20 mesh vibrating screen above each tank; the pulp and the entrained carbon are raised by means of an outside airlift onto the vibrating screen. The pulp flows through the screen of the next tank in line; the coarse granular carbon is retained on the screen and is discharged into the tank from which it came.

The device used by Mintek, 20 mesh, air cleaned screens are installed in the periphery launder of the absorption tanks. The screens retain carbon within the tank while allowing the overflow slurry to advance from stage to stage. Some times, a prescreening can reduce or eliminate screening problems between CIP stages. Some advantages obtained are: eliminate the woodchips present into the pulp, the volume of pulp passing through the vibrating screen is small, the pulp passing through a 28 mesh vertical screen passes very readily through similar 20 mesh screen on the periphery of the absorption tanks.

The optimum parameters for CIP are: pH 10-11; pulp density 40-45% solids; and free cyanide 0.05% into circuit, more than 0.015% out of circuit. The pulps density limits are important for the intermixing of the carbon with the ore suspension. If solids make up less than 40% of pulp, the carbon trends to sink; if the pulp density is more than 45% solids, the carbon floats.

With the proper conditions above, gold will load in preference to copper. If there is mercury in the ore, it will follow the gold and must be reported from the final product. Lead, zinc, arsenic, antimony, cobalt and nickel will also be absorbed, but they usually don not create problems. Pachucas can be used as CIP tanks and can handle somewhat coarser particles than agitated tanks. The impeller in agitated CIP tanks must be large and sweeping and have low speed. There always some loss of gold with the carbon, but this can be minimizing screening the carbon after regeneration.

The optimum loading of the carbon has to be determined taking the economics of the operation into account. Frequent stripping and handling is costly, generates losses through fines and may lower the capacity of the carbon. It is recommended that the gold loading on the carbon be kept moderately low, 150-300 oz/ton, to maintain a low inventory and control of losses through uncollected carbon fines.

The carbon must be pre-abraded before use. Use of low tip speed agitators, plastic pipes with long radius sweeping curves, and a minimum number of fittings is helpful in maintaining the integrity of the carbon grains. Some gold mills use dual impellers in the CIP tanks to lower the impeller tip speed. Coconut carbon must be used in CIP since is resistant to abrasion. Depending on the relevant economics, a number of CIP plants use sodium carbonate rather than lime to avoid lime scale building on both carbon and screens.