Gold Cyanidation - Cyanide Remediation Processes

Cyanide Remediation Processes

The various species of cyanide that remain in tails streams from gold plants are potentially toxic, and on some operations the waste streams are processed through a detoxification process prior to tails deposition. This reduces the concentrations of these cyanide compounds, but does not completely eliminate them from the stream. The two major processes utilised are the INCO licenced process or the Caro’s acid process. Both processes utilise oxidants to oxidise cyanide to cyanate, which is not as toxic as the cyanide ion, and which can then react to form carbonates and ammonia. The Inco process can typically reduce cyanide concentrations to below 50 mg/L, while the Caro’s acid process can reduce cyanide levels to between 10 and 50 mg/L, with the lower concentrations achievable in solution streams rather than slurries. Hydrogen peroxide and alkaline chlorination can also be used, although these are typically less common.

One of the alternative oxidants for the degradation of cyanides that has been attracting industrial interest is Caro’s acid – peroxomonosulphuric acid (H₂SO₅). Caro’s acid converts cyanide to cyanate. Cyanate then hydrolyses in the water to ammonium and carbonate ions. The Caro's acid process is able to achieve discharge levels of WAD below 50 mg/L, which is generally suitable for discharge to tailings. Generally, the best application of this process is with tailings slurries containing low to moderate initial levels of cyanide and when treated cyanide levels of less than about 10 to 50 mg/L are required.

Over 90 mines worldwide now use an Inco SO₂/air detoxification circuit to convert cyanide to the much less toxic cyanate before waste is discharged to a tailings pond. Typically, this process blows compressed air through the tailings while adding sodium metabisulfite which releases SO₂, lime to maintain the pH at around 8.5, and copper sulfate as a catalyst if there is insufficient copper in the ore extract. This procedure can reduce concentrations of "Weak Acid Dissociable" (WAD) cyanide to below the 10 ppm mandated by the EU's Mining Waste Directive. This level compares to the 66-81 ppm free cyanide and 500-1000 ppm total cyanide in the pond at Baia Mare. Remaining free cyanide degrades in the pond, while cyanate ions hydrolyse to ammonium. Recent studies show that residual cyanide trapped in the gold-mine tailings causes persistent release of toxic metals (e.g. mercury ) into the groundwater and surface water systems.