the higher carbon treatment rates. In many cases the inorganic fouling is minimal and the requirement for acid treatment of the carbon is reduced to as low as 20 per cent of the overall elution rate. Elution. The old debate of Zadra versus AARL is hopefully over. The use of Zadra systems was a significant contributor to the negative view of using carbon for silver recovery. The AARL elution process has the advantage of a short elution time (3 hours) compared with the 12-24 hours for Zadra. Clearly the AARL can easily accommodate much higher treatment rates with a smaller column compared to Zadra. Further advances, such as the split elution, have significantly reduced operating costs, thereby making the carbon route more competitive for silver recovery. Electrowinning. This has always been labour intensive and any increase in the size of the electrowinning plant was not favored, especially the size of plant required for high silver recovery. In practice, however, the high silver level actually becomes an advantage as it produces a non-coherent deposit that can simply be washed off the stainless-steel wool, filtered, dried and smelted. The usual problems of acid treating steel wool or calcining large quantities of iron are overcome. Modern electro-winning plant designs incorporate low cost, single pass, high efficiency cells with in-situ cleaning. The new designs are no longer labour intensive. Smelting. The use of stainless-steel cathodes to produce a silver/gold slime results in far less flux requirement and therefore reduced metal loss. The smelting furnace size is also reduced because of the smaller volumetric requirements. The bullion product gives a high quality silver/gold bar. Regeneration. It is generally assumed that carbon regeneration is a high capital and operating cost area of the carbon plant. Again, no cognisance has been taken of the process fundamentals of this step. Comparative cost estimates are frequently made on the basis of regeneration of 100 percent of eluted carbon. What is so often forgotten is that the organic fouling is more a function of ore treatment rate and not the contained metal values. Typically, a 1mtpa gold plant would treat 1000tpa of carbon. If silver were present, say at 50g/t (soluble), then the carbon treatment rate through elution would increase to say, 3000tpa ie three times. However, the
carbon regeneration capacity could stay close to the original 1000tpa because this is tied to the ore treatment rate, not the silver levels, with some minor adjustment because of a slightly higher carbon inventory.
Cost Considerations
The economic model takes into account the various factors considered above to give more realistic values to the capital and operating costs for the carbon circuit. In a conventional gold plant, the process selected is usually CIP for reasons of lower capital and operating costs and reduced soluble losses. Additional advantages apply when treating ores with difficult liquid:solid separation steps. eg. Clays. When treating ores with high silver content a substantial increase in the size of the carbon plant is usually predicted by some designers. This results in a substantial increase in capital and operating costs which can reverse the economic advantages of the CIP flowsheet. Other concerns are the displacement of gold by silver on the carbon, causing soluble gold losses and poor silver adsorption kinetics. In well designed plants these factors are not material. For a gold ore, the size of the carbon circuit increases more or less proportionally to the ore treatment rate, not the metal grade. A plant treating 1mtpa of ore would typically treat 1000tpa of carbon through the acid wash, elution and regeneration stages. The indicative capital cost relationship for varying plant sizes is shown graphically in Figure 1 . The range of capital costs for a particular size are indicated to reflect differing project-specific detail requirements such as for various climatic conditions. The treatment of a high silver gold ore results in an increase in the size of the carbon circuit when compared to a 'gold' plant with the same ore treatment rate. This is due to the lower loading ratio of silver compared to gold. Using the above example of 1mtpa ore treatment rate the increase in carbon treatment rate with a 30glt soluble silver ore would typically be three times that of the gold ore i.e. 3000tpa. However, it is not the total recovery plant that increases in proportion, only the elution stage increases to this level. As previously noted, the acid wash and Capital Costs
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