Cyanide may be released from process tanks and storage areas or from tailings waste streams and storage areas. The lower the pH, the greater the rate of HCN evolution. The loss of cyanide is actively minimised through regulation of pH and the monitoring of process streams. Toxicity does not normally begin until an exposure level of 20 to 40 ppm. Studies have indicated that there has been no real discernible increase in concentrations of atmospheric HCN as a result of the recent use of cyanide in metallic ore processing in mining.
Tailings are waste slurries produced from mineral extraction processing.
These may contain residual amounts of reagents, including cyanide, used in the extraction process. Concentrations of cyanide in seepage water are generally low, usually only a few parts per million. Analysis of seepage water compared with input solution typically show far lower concentrations in seepage water reflecting strong retention of cyanide within the tailings. These can often make up only a small percentage of total cyanide. Seepage solutions are normally monitored for various substances, including cyanide, and are often recovered via collection trenches and drains and water extraction bores.