Cyanide in Mining: Some Observations on the Chemistry, Toxicity and Analysis of Mining-related Waters

The use of cyanide compounds in mining is frequently a controversial issue. Environmental groups often focus on the acutely toxic properties of many cyanide compounds to humans. The mining industry has argued that the dilute cyanide concentrations employed, the methods of use, and the rapid decomposition of these compounds make cyanide extraction a very safe alternative. Clearly the spill of almost two tons of sodium cyanide while being transported to the Kumtor mine, and the associated medical complaints and deaths have caused the public, especially in Kyrgyzstan, to reexamine these claims. While it is reasonable to be concerned about the acute poisoning of humans and other organisms from miningrelated accidents, the more common environmental problems are likely to result from the chronic contamination of surface and ground waters by lower concentrations of cyanides and related breakdown compounds. Such chronic releases are much more difficult to notice and evaluate than are acute, high concentration spills that are often associated with rapid, observable deaths of aquatic organisms. Also, because mining-related waste waters are usually complex mixes of cyanides, metals, organic reagents and other anions, it is difficult to determine which chemical constituents are causing the toxicity problems. Contrary to much of the literature published in mining and regulatory documents, not all of the cyanide used in mineral processing breaks down quickly into largely harmless substances. Many of the breakdown compounds, while generally less toxic than the original cyanide, are known to be toxic to aquatic organisms, and may persist in the environment for significant periods of time. Some of these toxic breakdown forms include the free cyanides, metal-cyanide complexes, organic-cyanide compounds, cyanogen chloride, cyanates, thiocyanates, chloramines, and ammonia. Unfortunately, many of these chemical species are not detected in the routine laboratory analyses normally performed on mining-related waters. Thus, it is often assumed that they do not exist. For many reasons, national and international regulatory (and lending) agencies do not require monitoring for many of these chemical species. Adequate sampling and analysis of waters for cyanide and related breakdown compounds is complex and subject to considerable uncertainty. Because of these technical and regulatory limitations, a great deal is not known about the actual presence, persistence, and toxicity of these compounds in natural waters. As a result, it is likely that the negative impacts to aquatic organisms, especially sensitive fish populations, from releases of cyanide and related breakdown compounds at mineral processing sites is underestimated and undetected—in both developed and less developed countries. NOTE: The following paper discusses only selected aspects of the chemistry and toxicity of cyanide (CN) in waters associated with hardrock mining and is not intended to be a comprehensive review. It was written for a general audience with limited knowledge of chemistry. In this paper, mining-related waters refers to water associated with mining wastes-such as heap leach process solutions, tailings, heap leach piles, waste rock---and water such wastes may impact. The latter may include rivers, lakes, ground water, pit lakes, oceans. Introduction. The use of cyanide (CN) compounds by the mining industry raises many conflicting and confusing issues. Mining and regulatory documents often state that cyanide in water rapidly breaks down, in the presence of sunlight, into largely harmless substances such as carbon dioxide and nitrate. A recent report sponsored by the mining and cyanide manufacturing industries (Logsdon, M.J., et. al., 1999) states: “ Since cyanide oxidizes when exposed to air or other oxidants, it decomposes and does not persist. While it is a deadly poison when ingested in a sufficiently high dose, it does not give rise to chronic health or environmental problems when present in low concentrations.” However, cyanide also tends to react readily with many other chemical elements and molecules to form, as a minimum, hundreds of different compounds (Flynn and Haslem, 1995). Many of these breakdown compounds, while generally less toxic than the original cyanide, are known to be toxic to aquatic organisms, and persist in the environment for significant periods of time. In addition, there is evidence that some forms of these compounds can be accumulated in plant tissues (Eisler, 1991) and may be chronically toxic to fishes (Heming, 1989; and