Precious Metal Recovery from Low Grade Gold and Silver Tailings Samples

Precious Metal Recovery from Low Grade Gold and Silver Tailings Samples

A client approached hrltesting to investigate options for precious metal recovery from a gold tailings dam project. The project examined options for re-processing the tailings material in the existing operational site plant.

The goals of the project were threefold:

  • Investigate and optimise gold and silver recoveries by cyanide leaching
  • Evaluate any beneficiation options available to the project
  • Identify a robust cyanide detoxification process for cyanide solutions

The project faced some specific challenges, including:

  • Strict Environmental Protocols: Cyanide discharges were to be limited to an extremely low <0.1 ppm CNWAD, substantially below typical requirements of 2 ppm CNWAD. Similar low levels were required for other solutes including calcium and sulphate.
  • Climactic Variability: The operating temperature at the plant could vary seasonally from 5°C to 45°C.
  • Capex/Opex Constraints: Any new process needed to be added to the existing plant operation with minimal Capex/Opex.
  • Integration with Existing Plant: the testwork scope was limited to investigating options that were closely aligned with existing plant operating conditions.

The project was successful in identifying a flowsheet capable of being added to the existing plant, recovering precious metals, and treating cyanide slurries to comply with local discharge requirements.

hrltesting worked collaboratively with the client, who was present at the laboratory for much of the testwork. hrltesting allowed rapid access to data and tests, and provided consultation throughout the project in order to tackle the difficult nature of the project.

Beneficiation

hrltesting received a set of tailings samples for testing from the recovered stock of 430 kg. Size by size assays on the head test sample indicated that precious and base metals were generally evenly distributed across the size range tested and on this basis the tailings were not amenable to any processing or beneficiation based on size distribution. Therefore, it was determined that the tailings should be assessed as a whole.

  • Several other options for optimising the recoveries were identified. These included:
  • Flotation of the master composite, with reasonable sulphide recovery but lower gold and silver recovery.
  • Cyanide leaching of the concentrates and tails, with very good recoveries.

However, the flotation and other options trialled were deemed slightly too expensive for commercial application or involved too many changes to the current plant design. The project instead moved on to focus on cyanide leaching of whole tailings.

Cyanide Leaching

A series of cyanide leach tests were carried out under specific conditions identified to model viable processing options for integration into the existing plant. Recovery data for key variables were obtained, including temperature, grind size, oxygenation, cyanidation and the effect of pre-conditioning. Gold and silver recoveries were optimised for each of these variables. Key conditions were repeated to demonstrate repeatability. These tests also identified leaching kinetic rates for both gold and silver and allowed for optimisation of the leaching.

Once the optimal conditions were identified, these were further tested for the extreme weather conditions on both the small (2 L) and large scale (60 L) leach tanks. Gold and silver recoveries under these extreme conditions allowed for a better understanding of the expected recoveries from the processing plant during different times of the year.

Improvements in recovery of up to 22%, compared to baseline leaching, were obtained in the optimisation testwork.

Cyanide Detox and Removal of other Environmental Penalties

Cyanide leach slurries were then sent for cyanide destruction testing via a 3-stage destruction process. The cyanide leach filtrates were first subjected to sulphur dioxide (SO2) based cyanide detoxification, with varied results. Weak Acid Dissociable cyanide (CNWAD) was destroyed at various doses, temperatures and residence times. Outstanding results were achieved (<10 mg/L CNWAD and CNtotal) under specific conditions prior to further downstream destruction tests. Again, these specific operating conditions were further tested using the same extreme weather conditions as in the cyanide leach tests. Rapid turnaround of solution assays by hrltesting’s internal laboratory sped up the execution of this programme.

In order to meet the local water discharge requirements, other components in solution needed to be kept below regulated targets. Precipitation tests were trialled to reduce calcium and sulphate concentrations and TDS in the post SO2 based cyanide detoxification liquors.

This testing incorporated the following methods:

  • Ettringite precipitation (Sodium aluminate) based tests
  • Calcium aluminate (Istra 50) based tests
  • Sodium hypochlorite cyanide detoxification tests (optimised for best economics)
  • Hydrogen peroxide final stage destruction
  • Conditions were identified that gave soluble components below that of the local discharge requirements.

Further cyanide detoxification tests were conducted to lower the thiocyanate (SCN) to below the local restrictions. Cyanide detoxification tests on the final liquor oxidised the SCN in the final filtrate to < 0.5 mg/L. The CNtotal was also lowered to < 0.07 mg/L at this dose rate which met the 0.1 ppm target given by the client.

In summary, despite the wide array of challenging factors, hrltesting successfully assisted our client to identify critical process and environmental data needed to establish a business case for the client to progress the project further. Key to this success were:

  • Equipment on site at hrltesting for a wide range of specialist tests
  • hrltesting’s open-door and collaborative approach to executing testwork with the client
  • hrltesting’s technical expertise to identify and evaluate processing options in a disciplined manner, coupled with access to rapid assay turnaround.