In Part 1 of our series on thiocyanate, we looked at why this compound is now becoming an issue for the gold industry when this precious metal has been mined for centuries. We identified the mining of complexed gold ore deposits, recycling of process water, tightening regulations and the ineffectiveness of conventional cyanide detox to address thiocyanate as some of the reasons for this new concern.
Thiocyanate generation during the metallurgical extraction of gold presents challenges beyond economics and environmental compliance, as mines today must also operate in a socially acceptable manner. With knowledge about why thiocyanate is a problem, we now turn to explore different solutions for the management of thiocyanate in Part 2 of this series.
Biological Removal for Environmental Compliance
Implemented at several gold processing facilities to remove thiocyanate, this process consists of sequential treatment involving biological oxidation, nitrification, and denitrification to convert thiocyanate into a non-toxic nitrogen gas and relatively benign sulphate to produce treated effluent compliant with discharge requirements. This is achieved with a series of three biological reactors. In the first, sulphur in the thiocyanate is oxidized to sulphate and ammonia. In the second, ammonia is oxidized to nitrate. And in the third, nitrate is reduced to nitrogen gas.
Biological treatment may also be suitable when nitrogen species such as ammonia, nitrite, and nitrate require removal. These species are introduced into mine water from ammonium nitrate explosives used in blasting or from cyanide destruction. When simultaneous removal of these nitrogen species is also required, the same sequence of treatment for thiocyanate will achieve this need.
Limitations and disadvantages of the biological approach include:
- Not suitable for sites with seasonal or intermittent discharge owing to the slow start-up and acclimation of microorganisms after a prolonged shut-down.
- Not suitable for highly variable hydraulic flow and/or mass load of nitrogen species as biological systems are difficult to ramp up and down compared to chemical processes.
- Costly to construct and operate due to the need to heat cold water and/or permanently accommodate the periodic maximum inventory of biomass at peak mass loading.
- Incomplete oxidation of ammonia or reduction of nitrate produces nitrite which can trigger effluent toxicity.
- Denitrification increases the risk of effluent toxicity caused by residual organics, ammonia, and organo-selenium species when selenium is present in the feed water.
- Residual biomass finely suspended in plant effluent is generally difficult to remove and can increase total suspended solids levels above the 15 mg/L limit adopted by regulators worldwide.
Chemical Oxidation for Environmental Compliance
Facilities with seasonal or intermittent flows, variable wastewater flows and compositions, or where the treatment facility does not operate full-time may find chemical oxidation to be more suitable for removing thiocyanate to comply with environmental requirements. While the commonly used SO2/air cyanide destruction process that oxidizes cyanide to cyanate performs poorly for oxidizing thiocyanate and cannot be relied upon for environmental compliance, other stronger chemical oxidants such as ozone and chlorine can remove thiocyanate effectively by oxidizing it to cyanate and sulphate.
Cyanate generated from thiocyanate oxidation hydrolyzes to form ammonia and bicarbonate. While bicarbonate is not harmful and may even be slightly beneficial as a pH buffer, ammonia is subject to environmental regulations. Thiocyanate treatment utilizing chemical oxidation would need to account for ammonia management to meet applicable discharge limits.
The main limitations and disadvantages of chemical oxidation are:
- Strong chemical oxidants are very expensive and with high thiocyanate loads the operating cost could be higher than that of biological systems.
- Additional ammonia or nitrate treatment may be necessary to remove these species that are either generated as by-products of thiocyanate oxidation or are present in the plant feed.
Electro-Oxidation for Environmental Compliance and Cyanide Recovery
Two main features distinguish this treatment method for thiocyanate compared to biological removal and chemical oxidation. First, the process can be applied as treatment within the battery limits of the mine and metallurgical process to facilitate process water recycle or for environmental discharge. And second, the process recovers cyanide while removing thiocyanate to enable value recovery from waste.
Process Water Recycle
New mine and metal extraction plants are often constructed with a closed water balance characterized by a high degree of water recycle and no environmental discharge during operations. In these scenarios, thiocyanate concentrations and the overall thiocyanate inventory within the project battery limits can build up to high levels. While this build-up does not necessarily create interference with the metal extraction process during active mine operations, this large inventory of thiocyanate in heap leach solutions or tailings dams can become a significant liability during closure and post-closure. To eliminate or reduce the build-up of thiocyanate during the project operations phase and mitigate the liability once mine operations ceases, direct partial electro-oxidation of thiocyanate can be utilized.
How this works is that process water laden with thiocyanate is passed through electrocells composed of anodes and cathodes. As an electric current is passed through the cell, the main anodic reaction is the partial oxidation of thiocyanate to sulphate and cyanide while the main cathodic reaction is the evolution of hydrogen. Discharge from the electrocell is recycled back to the metallurgical process rather than released into the environment. This allows residual thiocyanate levels to remain elevated and requires a current efficiency that is typically quite high in a process where electricity is the only consumable. If low electricity costs from renewable or hydroelectric sources are available, the cost of thiocyanate removal can be defrayed in part or in whole by the value of cyanide recovered.
In this application, thiocyanate concentrations in the plant feed may already be quite low with the treated effluent needing to contain even lower levels to avoid effluent toxicity and comply with permitted limits. An electro-oxidation step combined with ion exchange can achieve this strict discharge requirement. The process works by having the ion exchange remove thiocyanate to low levels suitable for discharge while producing a small volume of concentrated thiocyanate solution from the ion exchange regeneration process that is directed to the electro-oxidation step for cyanide recovery.
Although there are currently no full-scale facilities that recover cyanide from thiocyanate, both concepts – recycle to metallurgical process and environmental discharge – have been successfully tested on process waters from several different mining operations in Canada and Latin America.
Thiocyanate removal by partial electro-oxidation offers the same advantages as chemical oxidation compared to biological systems. However, electro-oxidation also offers reduction in the overall carbon footprint associated with the manufacturing, purchase and transport of fresh sodium cyanide and cyanide destruction reagents.
Adopting sound thiocyanate management strategies at gold mining and extraction facilities can help organizations adhere to increasingly stringent environmental regulations and achieve corporate and societal mandates to minimize the environmental impacts and decrease the carbon footprint of gold mining and processing activities. Part of a holistic thiocyanate management strategy could include cyanide recovery from thiocyanate in addition to thiocyanate removal to offer both environmental compliance and operational improvements.
H.C. Liang, PhD, PChem