Leaching - Gold and Silver Recovery

Quadra's extensive product offering covers the complete range of reagents needed to operate and facilitate the leaching/absorption/desorption/recovery/treatment of your precious metals.

GOLD AND SILVER RECOVERY – LEACHING

Since the late 19th century, cyanide has been the industry standard used for recovery of gold and silver from multitude of ore types. The cyanide leaching process can be described by the contacting of a cyanide solution, in presence of oxygen, with gold and silver particles contained in the ore processed and their resulting transition from the solid to the liquid state as per the Elsner’s Equation below:

This contact can take place in multiple forms such as laboratory bottle roll, large agitated tanks as well as large crushed rock emplacements (Heap Leach) or batch & semi-batch application such high intensity and/or vat leaching.

During this contact period, some type of oxygen addition to the system will be necessary to provide the amount required by the reaction as well as replace the amount that will be consumed by other reducing minerals. Multiple oxygen sources have been used throughout history for this purpose such as compressed air, high purity oxygen, and multiple oxidizing agents such as hydrogen peroxide, lead nitrate and 3-nitrobenzene sulphonate to name a few.

Gold ore processing jets water

For safety reasons, pH typically needs to be controlled in the 9.5-12.0 range depending on applications. While it feasible to achieve this using caustic, lime is usually the economical choice for large volumes. Depending on available supply sources, location and overall project economics, this lime can be delivered to site pre-hydrated or not.

Leaching Circuit

Molten gold process in gold bar casts

Quadra can supply the following product to meet the needs of your leaching circuit:

  • Sodium Cyanide
  • Hydrogen Peroxide
  • Lead Nitrate
  • Sodium Nitrate
  • 3-nitrobenzene Sulphonate
  • Glycine
  • Lime

ADR CIRCUIT

The most common way to recover gold and silver dissolved from the ore in the leaching process is the use of an Adsorption/Desorption/Recovery circuit commonly referred to as ADR circuit.

In this circuit, activated carbon, typically made from coconut shell, is used to recover the gold and silver ions from the slurry. Due to their coarser nature, activated carbon particles can then be separated from the slurry using screenings methods.

The activated carbon containing the gold and silver ions (aka loaded carbon) is subsequently treated with a caustic solution at high temperature do desorb the gold and silver from the activated carbon.  This high-grade desorption solution (aka pregnant) is either accumulated (preg tank) for later treatment in the refinery or sent directly to the refinery for recovery by electrowinning. The electrowinning sludges are then dried, with or without additional filtration, for final treatment in a melting furnace to pour gold and silver dore bars

Quadra can supply the following products to meet the needs of your ADR circuit:

  • Activated Carbon
  • Sodium Hydroxide
  • Hydrochloric Acid
  • Nitric Acid
  • Refinery Fluxes, such as:
    • Borax
    • Fluorspar
    • Silica
    • Sodium Nitrate
    • Sodium Carbonate
Raw carbon ready for processing

MERRILL-CROWE

In certain occasions, such as when the ore silver content is higher than the gold content, the ADR circuit will be formed by a combination of filtration and Merrill-Crowe circuit. In this circuit, the gold and silver in solution can be separated from the slurry using a variety of filtration equipment such as drum, disk, linear and press filters.

Following the filtration step, the solution is then de-aerated and gold and silver in solution is precipitated by reaction with zinc sulphate powder. The precipitated particles are then recovered through a second filtration stage. At this stage, the recovered filtered material will be dried and fed to a melting furnace to pour gold and silver dore bars.

Gold and Silver Ore Processing Water Jets

Quadra can supply the following product to meet the needs of your Merrill-Crowe circuit:

  • Zinc Sulfate
  • Hydrochloric Acid
  • Nitric Acid
  • Filtration Aid
  • Refinery Fluxes such as:
    • Borax
    • Fluorspar
    • Silica
    • Sodium Nitrate
    • Sodium Carbonate

Slurry Treatment/Cyanide Detoxification

Following gold and silver recovery by either ADR or Merrill-Crowe processes, it will typically be necessary to treat the slurry prior to pumping it to the tailing impoundment area or for preparation as underground backfill for the mining operation.

The most common treatment as this step will by the detoxification step for the free and wad cyanides, commonly referred to as cyanide destruction. Multiple treatment processes such as INCO SO2/Air, Hydrogen Peroxide as well as Caro’s Acid are feasible. Selection of the optimal treatment will depend on the solution/slurry characteristics to be treated, the feed and required discharge contaminant level as well as regional pricing for the necessary chemicals.

Quadra can supply the following product to meet the needs of your cyanide detoxification circuit:

  • Copper Sulfate
  • Sodium Metabisulfite (SMBS)
  • Hydrogen Peroxide
  • Ferric Sulphate

LEACHING PROCESS PRODUCTS & SERVICES

Quadra can support you in the operation of these circuits with:

  • Activated Carbon Circuit Evaluation
  • Activated Carbon Improvement Guidelines
  • Activated Carbon External Testing

 

Quadra is proud to support our client base with geographically distributed warehousing locations, a large dedicated logistical team and technical know-how to help you achieve your goals.

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