Recovery efficiency is based on economic considerations, and don’t usually target very low free cyanide levels. Recoveries of about 90% to 95% are considered sufficient. The following are typical solution values obtained in the Mirah recovery circuit, which targets a CN WAD of <50ppm. CN Free CN WAD For detox applications the chemistry becomes important and depends on whether the target level is to satisfy TSF limits (typically <50ppm CN WAD), or more stringent levels for river discharge. For TSF levels often the removal of CN Free is sufficient, unless soluble base metal levels are high. If excessive CN WAD is caused by base metals such as copper, a small metal recovery circuit is also added. When the detox process is required to achieve river discharge quality, more attention is paid to the number of adsorption stages and the metal recovery circuit. The same mechanical detail applies. For river quality, it is more economical to simply treat the water to be discharged with the RECYN Process as this entails a lower water volume and treating a solution. Cyanide and metal levels will already be low in the tailings dam, because they’ve already been treated in the plant. Designs must take into account the changing properties of ore bodies as they get deeper, and where varying oxidation and mineralogy properties are encountered. This applies to detox as much as primary metal recovery. ECONOMIC IMPLICATIONS The economic benefits of the RECYN process are wide ranging and substantial. It can provide a step change in project feasibility, making marginal projects economic and economic projects even better. ● Firstly, the ability to recycle 50% of the cyanide added to the leach circuit at a much reduced cost is easily measured. Capital cost is generally equivalent to, or less than, a conventional detox circuit, the difference being that there is usually a payback period of less than 12 months for the recovery circuit. ● Secondly, the use of a recycle process eliminates expensive conventional detox circuits. Detox costs can be up to $3/t ore treated. Other benefits more difficult to measure are: ● The cyanide recycle eliminates the need for tailings thickeners, which can have a negative effect on the metal circuit due to viscosity problems when flocculent is recycled to the milling and CIL circuits. ● The lower cost of cyanide makes it more economical to increase the cyanide levels in leaching to improve gold and silver recovery. ● Recovered base metals can provide a significant income stream, especially at high solution levels. CIL Tailings 350 360 RECYN Tailings 20 30
CONCLUSION
The benefits of the RECYN process are significant: ● Cyanide is economically recycled. ● CIL plant tailings are economically detoxified.
ALTA 2017 Gold-PM Proceedings
7
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