INTRODUCTION
Many gold projects in the world have a nuisance level of copper in the ore. The copper grade is too low to be considered an economic copper project but has sufficiently high cyanide soluble copper (CNsolCu) levels to be problematic as a gold project. The measurement of CNsolCu becomes a complex task as there is often no simple relationship between total copper and CNsolCu in different parts of an ore body. Estimating CNsolCu involves a leaching process and is sometimes carried out on each 1m interval of the core, a long and expensive addition to the exploration program.
The Resource estimation process imposes a cost on ore containing CNsolCu and the subsequent impact on Ore Reserves can be considerable (1).
Grade control in mining becomes more complex as CNsolCu has to be considered as well as gold grades. Even close space drilling does not ensure ore selectability due to minimum mining volumes.
Process plant operation becomes more difficult as control of free cyanide levels in the leach requires greater attention to the plant ore feed composition. Sudden increases in CNsolCu can reduce free cyanide levels, impact gold recovery and cause excessive copper loading onto the carbon.
Plant throughput can be limited in periods of high copper input due to detox circuit limitations.
From the above, it is clear that the nuisance value of CNsolCu can be high and often a material distraction when developing a gold project. The inclusion of the ReCYN process into the flowsheet allows the CNsolCu problem to be effectively ignored from a cost perspective and the focus returned to the gold. The reason for this is the recovery of the copper, and its associated cyanide in the ReCYN process balances the cost of the additional cyanide required to dissolve the copper in the ore. Cyanide levels within the leach circuit still require careful control to cater to the variable input, but this can be achieved with an automated system that measures Free and WAD cyanide.
THE LONG ROAD
Many excellent papers have been published detailing attempts to overcome the CNsolCu problem (2,3,4), with each concluding that there is no single answer to this problem, or proposing a possible but non-commercialised technology. The reviews detail various processes for removing the copper up-front, reducing copper solubility, and methods to recover the dissolved copper and associated cyanide. The general conclusion has been that none of the existing processes can be universally applied to solving the CNsolCu dilemma. The Red Dome Gold/Copper project (5) that operated between 1986 to 1997 in Queensland is a classic example of the process complexity that is encountered with CNsolCu and the innovations applied to cope with the problems. The success of the project was not solely due to its size and Au/Cu grades, but very much due to the ingenuity of the operations team. With a more limited resource base, the challenge can be insurmountable. An example is the Ravensthorpe Gold Copper Project in WA where for over 50 years, numerous mining companies have attempted to commercialise the deposits at Kundip. There were many reasons the project did not move ahead, including low metal prices and resource size, but always underlain by the CNsolCu issue. Often the copper problem becomes all-consuming, with resources disproportionately directed to first understanding the extent of the problem, quantifying its impact during processing, and then trying to find a technology to overcome the influence. There is a long history of research (2) directed at finding an alternative solvent to cyanide and adsorbants that are more selective. It is evident that no alternative process has so far provided a satisfactory answer. The answer may now be provided following a recent Alta published paper that promoted the benefit of retaining cyanide as the preferred solvent for gold leaching while addressing the negative aspect of cyanide in tailings dams (6). This 2017 paper promoted the use of a proprietary resin adsorbant
Powered by FlippingBook