GENERIC ENVIRONMENTAL RISK ASSESSMENT: Summary Of Key Findings For Dry Tails Stacking

Assessment of environment risk plays a vital part in process decision making to reduce adverse effects of commercial and industrial operations on the environment. The purpose of this assessment was to examine the environmental risks posed by the general applications of Clean Mining’s (CM) new gold leaching and extraction technology combined with dry stackable tailings.

Commonwealth Scientific and Industrial Research Organisation (CSIRO) is an Australian Governmental agency responsible for scientific research who has recently examined the environmental risks posed by CM’s new thiosulphate gold leaching and extraction technology. Thiosulphate on its own is intrinsically less toxic to living organisms than cyanide and poses minute risks to the environment, during operations, transport of reagents and disposal of dry tailings.

CM’s blended reagents can be classified as “non-toxic” by the Globally Harmonized System of Classification and Labelling of Chemicals (GHS) but take note that any substance can be “toxic” at the right concentration, e.g., oxygen. Transportation of CM’s reagent can be labelled as “Not Dangerous Goods” by GHS standards.

The components of the chemical formulation used in the leaching and extraction processes were screened using standard hazard assessment protocols which utilise information on toxicity, persistence, and bioaccumulation. Thiourea was the only reagent flagged as being of potential concern due to its potential toxicity. The scientific literature and assessment results indicated this reagent readily degrades in both soil and water environments to produce non-toxic products over relatively short timescales (half-lives of several weeks) in both soils and waters. However, with the thiourea component of the CM’s reagent comprising at a rate of <10% of the formula, concentration levels are below detections limits that are considered potentially harmful. We can therefore conclude with confidence that residue from CM’s processed tailingsresults in no harmful effects to the environment.

Application of the extraction process to ores results in the production of waste liquors that contain a range of dissolved metals which are mobilised into solution through their affinity for thiosulphate. These include lead, copper, cobalt, and zinc. The risks posed by toxicity of the mobilised metals such as copper and gold, in a leachate is greater than from the reagents themselves.

Thiosulphate has a relatively low toxicity level, but literature has pointed out the potential to oxidize through bacterial oxidation of thiosulphate and other sulphur compounds (thiosalts), which can result in the formation of sulphuric acid. However, it is important to note the production of dry tailings drastically reducesthe risk of any sulphidic waste. The fate of thiosulphate and its potential to produce acid was independently investigated by CSIRO’s Land and Water department, in a series of experiments involving soils from Western Australia and tailings from Menzies demonstration plant. Added leachates and thiosulphate were incubated under conditions representative of those found on Western Australia mine sites. The experiments did not indicate any acidification of soils and showed progressive decreases in extractable thiosulphate concentrations over a 6-week time period. This finding was supported by CSIRO field work conducted at the Menzies demonstration site which indicated no signs of soil acidification both within stored tailings piles and the sub-surface soils, two and a half years after the cessation of operations.

The information obtained was used in a generic risk assessment of the following scenarios:

  • Dry tailings with no washing to recover reagents (typically 8-15% leachate content)
  • Dry tailings with washing to recover reagents (1.6-3% leachate content)

Overall, it is concluded that CM’s thiosulphate leaching, and extraction processes combined with dry stackable tailings poses minute environmental risks. The risks identified are manageable through the application of appropriate monitoring and controls.

Apte, S.C., Kirby, J.K., Jarolimek, C.V., Fiebiger, C. and Johnston, C. (2021). An assessment of the environmental risks posed by the thiosulfate gold leaching process. Land and Water Technical Report EP211345 CSIRO, Australia.