Gold has been extracted from carbonaceous ore by using cyanide, a chemical which is highly toxic to organisms, since 1887. Conversely, a less toxic method to extract gold from carbonaceous ores was researched and developed, utilising knowledge from the 1970s and further refining the information to achieve the desired outcome. This new method uses thiosulfate, a substance often used in common fertilisers and is significantly cheaper than cyanide, indicating that it is less toxic and more economical. The development of an alternative technology to cyanidation can allow for an application of a sustainable process of extracting gold from their carbonaceous ores. The benefit of using thiosulfate can increase the fertility of the soil as well as reduce the toxicity of the cyanidation process.
Cyanide – its Uses and Harmful Effects
Cyanide is a reactive and highly toxic chemical which are often used in many manufacturing uses, such as making paper, textiles and plastics and removing gold from its ore. Cyanide has a detrimental impact on humans and the environment. When consumed, cyanide kills the cells by preventing it from using oxygen, which damages the brain and the heart the most. Survivors of cyanide poisoning often develop heart, nerve and/or brain damage. Some severe cyanide spills have occurred in the last couple of decades and which caused the requirement of a more sustainable substitute due to political and environmental factors.
Development of Thiosulfate Leaching
Environmental awareness has been a major focus over the years and serious cyanide accidents is much of a public concern, and its toxicity on the surrounding environment and has even caused some countries to campaign against the use of cyanide in mineral processing applications. Scientists at the CSIRO utilised the scientific knowledge from the 1970s, further researched thiosulfate and how it can be optimised to recover gold, and an effective alternative to cyanidation was developed. Scientific knowledge, understanding, and inquiry enable scientists to develop discoveries, design action for sustainability and environmental impacts. In an article in 2014, Australian Mining was told by Paul Breuer at the CSIRO, “thiosulfate has been investigated as a potential alternative for decades, having first been looked at for gold processing back in the 1970s, it has challenges in terms of thiosulfate consumption and recovery of the gold thiosulfate complex”.
They established an elution method which collects gold thiosulfates from a strong base ion exchange resin. “The breakthrough was the use of sulfite in the solution used to recover remove the gold thiosulfate from the resin”. This allowed for “a new area of recovery potential applications of thiosulfate with ion exchange resin for gold recovery”. This process appealed to a couple of Australian miners, including SGS, who has investigated this method for in-situ treatment of gold ores.
Thiosulfate leaching is an alkaline process which removes gold from carbonaceous ores instead of using cyanide. It is less destructive and toxic, and has a higher efficiency with preg-robbing ores as a leaching agent. The major chemical components of thiosulfate are common fertilisers which include ammonium thiosulphate and ammonium sulphate. A two-part method was developed, resulting in it being an economically viable replacement to cyanidation for various types of ores which contained gold; the process of thiosulfate leaching is followed by resin-in-pulp gold extraction.
Thiosulfate leaching can produce comparable gold recovery to cyanidation and in some cases, such as preg-robbing ores which are naturally occurring carbonaceous substances that absorb more of the gold cyanide than the thiosulfate cyanide, it can produce a better recovery than cyanide leaching. By using thiosulfate instead of cyanide, a highly toxic chemical, the impact on the environment can be reduced. The process of thiosulfate leaching uses non-threatening chemicals which increases the potential to operate the technology in regions of the world where using cyanide is subject to intense negative publicity or is prohibited for environmental reasons. Since the main chemical components are common fertilisers, it allows for the possibility of using the mine tailings in agriculture applications in regions where the residential infrastructure and environmental parameters are advantageous.
Even though the process of thiosulfate leaching is more environmentally friendly, it involves more complex chemistry than cyanidation process, thus it is more difficult to optimize and more complicated to operate. It involves a chemical reaction between the thiosulphate anion, metallic gold, oxygen (oxidant), ammonium and copper ions (unconsumed catalysts) in the reaction:
4Au + 8S2O2-3 + O2 + 2H2O ? 4Au(S2O3)3-2 + 4OH
The process of thiosulphate leaching is complicated since the reaction thermodynamics are less favourable than the cyanidation reaction and it requires the optimization of each of the chemical components as well as physical parameters (i.e. pulp density and temperature) to maximize gold turnover and minimize substance losses. Therefore, more concentrated solutions are required to obtain comparable rates of gold leaching. A standard thiosulphate solution has a concentration of 5 to 20 g/L compared to a cyanide concentration of 0.25 to 1 g/L. This slightly counterbalances the cost of thiosulfate which is one fifth the cost of cyanide; significantly lower cost.
The use of thiosulfate leaching in the future is very favourable, however, the current method requires further enhancement to improve the recovery technique to make it more efficient and economical. Countermeasures can be researched and utilised to optimize the technique. Furthermore, the world’s largest gold producer, Barrick Gold, has started his process of commercialising the thiosulfate leaching process and will commence a full-scale production at the Nevada Goldstrike mine of next year. It’s expected that the thiosulfate leaching method will contribute an average of 10,000kg – 12750kg of gold in the first five years. This not only reduces the risk of the negative environmental impacts, but it allows for new opportunities to open up in regions where cyanidation is banned. Gold’s implication of thiosulfate can demonstrate to other gold miners that cyanidation is not the best method of separating gold from its carbonaceous ores.