Silver Coconut

Mr Malcolm R Paterson

GreenGold Group, Sahid Sudirman Center 39th Floor, Jl. Jend Sudirman Kav. 86, Jakarta 10220, E-mail: malcolm@greengoldengineering.com, Telephone: +62 8118 627 194, The Silver Coconut Carbon Versus Precipitation For High Silver Ores

content. In nearly all cases this was caused by an under-sized elution plant. The operations that were more successful such as the Martha Hill Project in New Zealand were those using the AARL elution system, which is more flexible than other elution systems owing to its ability to treat higher carbon flows. However, the publicity generated by the unsuccessful projects overshadowed the successful ones. The resultant confusion concerning CIP and silver has persisted to the present day. The basic flowsheet choice is between zinc precipitation (Merrill-Crowe) and carbon adsorption (CIP). Various hybrid combinations of these two options are also considered as a compromise. A detailed description of the processes is not provided here as this is well described elsewhere. (Ref 1). The hybrid options are worthy of some discussion as they attempt to overcome some of the perceived problems with the two basic flowsheets when applied to high silver ores. The first hybrid option is to treat thickener overflow by Merrill- Crowe to reduce the grade of solution in the thickener underflow reporting to a subsequent CIP plant. It is believed that this proportionally reduces the carbon treatment rate. In fact, as the solution grade decreases, due to thickener feed dilution so does the carbon loading. Halving the solution grade does not halve the carbon treatment rate. Gold and Silver loading onto carbon is an equilibrium process, therefore, the lower the solution grade, the lower the carbon loading. The second hybrid option is the replacement of the electrowinning step by zinc precipitation. However, there are hidden cost implications in this route, which are not always considered. Treatment of high silver grade ores usually involves very large masses of recovered metal, up to several hundred times the weight of the gold recovered. FLOWSHEET OPTIONS

Whenever flowsheets are considered for treatment of silver ores, the conventional approach is in favor of a Merrill-Crowe processing circuit rather than CIL/CIP. This is understandable when the historical development of CIP is considered. What has been disregarded is the fact that for the last 30 years there have been ClP plants successfully treating ore with high silver/gold ratios. The paper presented addresses the continuing debate on the use of carbon for silver recovery and seeks to explain the origin of the Silver Coconut 'myth'. During the 1970’s and 1980's significant advances in gold processing technology centered around CIP. Many of these developments occurred almost in isolation on three different continents. Cross-pollination of ideas was slow despite the excellent work done by international conferences. Prior to the commercial application of CIP both North America and South Africa had well proven Merrill-Crowe processes for most gold operations. CIP was initially developed in North America but further refined in South Africa where it became the major process for gold recovery. Its popularity quickly grew internationally, in particular for the treatment of low grade oxide deposits. The high clay content in many oxide ores proved a challenge for the liquid/solid separation step with Merrill Crowe. Although South Africa made major advances in CIP technology, there was no need to address the treatment of silver ores since South African ores do not contain any significant amount of silver. Australasia saw the development of several operations having high silver levels designed with CIP circuits. Several of these plants encountered processing problems due to the high silver Since many major gold projects contain large amounts of silver, this discussion is of particular interest to the region.

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