Cyanide Detoxification
As a result of a general regulatory shift to tighter environmental controls over mining project proposals, and a number of adverse incidents in recent years, the environmental implications of the use of sodium cyanide in the extraction of gold has become a significant issue now being considered during the early stages of gold project feasibility studies.
At AMMTEC we have experienced a significant increase in the amount of this type of work being conducted recently. In addition, ores containing significant levels of copper are becoming more commonplace, and means of detoxifying and perhaps recovering the cyanide as part of a cost reduction strategy is becoming significantly more important in these days of low gold prices.
Over the years, a number of detoxification processes have been developed, tested and elaborated. These include:
- Natural degradation by Ultraviolet radiation (e.g. in the tailings dam),
- Oxidation by Ozone,
- Oxidation by hydrogen peroxide,
- Acidification and recovery by volatilisation (AVR),
- Conversion into stable iron cyanide precipitates,
- Oxidation by chlorination.
- Sulphidisation, Acidification, Recycling and Thickening (SART)
- INCO process (oxidation by SO2). This technology is robust and has been successfully implemented into a significant number of operations worldwide. The principle behind the INCO process is the oxidation of cyanide and thiocyanate anions into cyanate, as shown below (the thiocyanate reaction is somewhat slower and may not proceed to completion in the same timeframe required for the first reaction):
Metal cyanide complexes are also broken down resulting in precipitation of both the metal hydroxides (e.g. those of Cu, Zn, Ni) and retro cyanide solid precipitates (e.g. Cu2Fe(CN)6))
S02 can be introduced in the gaseous/liquid form or as sodium metabisulphite. The reaction is catalysed by the presence of copper, which may have to be added (as copper sulphate). Air is also required, as is lime to maintain the pH at optimum levels (8-10) as the reaction proceeds.
The entire INCO process can take place within simple stirred tank reactor circuits and is quite simple to operate and monitor its performance. One drawback of the INCO process is that the aim is to achieve cyanide destruction. In the presence of significant levels of copper, the cyanide associated with the copper can be considerable and its destruction is therefore an unrecoverable operating cost.
Recently AMMTEC has tested a relatively new process called SART for several clients. SART is an acronym for Sulphidisation, Acidification, Recycling and Thickening.
Sulphidisation refers to controlled addition of stoichiometric additions of soluble sulphide salts (e.g. sodium hydrosulphide) to the waste cyanide solution. Acidification results in the dissociation of metal cyanide complexes (e.g. Cu, Zn) and the formation of HCN. The free metal cations thus created combine with the sulphide ions and precipitate out as insoluble metal sulphides. These precipitates are Thickened and separated for separate processing. After removal of solids, the liquors can be neutralised with Sodium hydroxide or lime and then Recycled back to the leaching process, thereby recovering and recycling the cyanide.
The overall process chemistry can be summarised by the following equation, which assumes the presence of copper (as the tetracyanide complex), however zinc behaves similarly.
The high solubility of HCN under ambient conditions results in a clean solution after removal of the precipitate, which if treated with sodium hydroxide or lime, can be recycled to the leaching process. The technology is particularly appropriate when high levels of copper are present in solution allowing the processing, by intensive cyanidation, of copper rich gold concentrates.
If sufficiently free of residual cyanide the sulphide precipitates can be sold or otherwise disposed of via further metal processing facilities (e.g. copper smelters). The presence of iron can lead to losses as ferrocyanide salts to the precipitate. This contributes to the cyanide content of the precipitate (which cannot be reduced by washing), and also results in a loss of cyanide, which cannot be recycled in the neutralised solution. Other losses of cyanide can occur, such as the formation of thiocyanate in the leach circuit through the presence of excess sulphide ions in the recycled solution. Therefore, the addition rate of sodium hydrosulphide needs to be carefully monitored.
SART is a simple, efficient process and does not require complex process equipment. Typical test programs explore the sulphidisation stoichiometry, the optimum pH control point and an optimised run inclusive of an overall cyanide balance.
AMMTEC has developed the Cynatec® process in which copper and cyanide are simultaneously recovered by solvent extraction, the cyanide being recycled, and copper being recovered in a saleable form. Thiocyanate is converted into various compounds for disposal or sale. Subject to testwork being carried out at AMMTEC and completion of non-disclosure documentation, a zero cost license fee will be negotiated for the use of this technology.