THE PERSISTENCE OF CYANIDE IN HEAP LEACH WASTES AT GOLD MINES

Much has been learned about the chemistry and engineering of cyanide (the CN^- anion) over the past 25 years, and numerous books and reports have recently appeared on these topics. Rather than give a comprehensive review, this presentation will focus on the long-term persistence of cyanide in heap leach wastes at gold mines: geochemical controls, methods for predicting and minimizing persistence, and long-term environmental risks.

Our own empirical studies suggest that, as heap leach operations advance through remediation toward closure, effluent cyanide is increasingly complexed with cobalt. This is a new discovery; these complexes had gone unnoticed previously because they do not report in the WAD and "total" cyanide analytical methods used for regulatory compliance. Cobalt is not abundant in common gold ores, but because process solutions are recycled, and because cyanide complexing enhances its solubility, the metal can be present at concentrations of a few mg/L. This is sufficient to carry several mg/L cyanide as the Co(CN)₆ complex. Although they do not report in the analyses, cobalt cyanocomplexes can complicate regulatory compliance based on WAD or total cyanide standards. The complication stems from photodissociation of the complexes in sunlight to produce free cyanide, which does report. Cobalt cyanide-bearing effluents can show spikes in free cyanide when samples are collected on sunny days or when bottled effluents are exposed to sunlight before analysis. Iron cyanide (Fe(CN)₆)-bearing effluents show similar photochemical effects; these are better understood because Fe(CN)₆ reports in total cyanide analyses and thus can be monitored using data collected for regulatory purposes. Cobalt cyanocomplexes are fairly benign to aquatic life, but they can pose indirect environmental risks in receiving waters for effluents by photodissociating to more toxic free cyanide. The persistence of cobalt-complexed cyanide in heap leach wastes might be minimized by sunlight-exposing effluents via the same procedures that are used to treat iron-complexed cyanide. In the long run, cyanide remediation might be more effective, and more predictable, if attention were paid to cobalt and cobalt cyanocomplexes, despite the fact that these species are not explicitly regulated.