Quoting from a 1999 paper The Management of Cyanide in Gold Extraction by Logsdon, Hagelstein, and Mudder and released by the International Council on Metals and the Environment, “Cyanide is a general term for a group of chemicals containing carbon and nitrogen. Cyanide compounds include both naturally occurring and human-made (anthropogenic) chemicals….” Some naturally occurring sources of cyanide compounds are found in apricots, bean sprouts, cashews, cherries, peanuts, potatoes, lima beans, sorghum, and many, many more common items we see, often touch, and occasionally eat, including common table salt. This brief report will focus not on the natural sources of cyanide but on human-made cyanide and its uses, particularly in mining and the extraction of precious metals.
In their 1999 paper, Logsdon, et.al., reported the 1996 annual production of HCN (hydrogen cyanide) gas was approximately 1.4 million tonnes, with the bulk of production coming from 3 companies, one of which is Dupont in the U.S. A tonne is 1000 kilograms or approximately 2204.6 pounds, which means the 1996 production was around 3,086,440,000 pounds of HCN. Of this production, it is estimated somewhere between 13% to 20% is used to produce sodium cyanide (NaCN), usually shipped to users in the form of solid briquettes in sealed containers, although sometimes it is shipped in liquid form in “specially designed tanker trucks.” (p.11, Logsdon, et.al.) Included in any shipment of NaCN is a Material Safety Data Sheet listing the chemistry, toxicity, accident instructions, emergency contact information, and other information deemed appropriate by the manufacturer. In addition to mining companies using NaCN in the extraction of precious metals, numerous other industrial uses for cyanide compounds abound: nylon and acrylic plastic production, electroplating, steel hardening, synthetic rubber production, anti-caking additives (such as in road salt), rodent control (with HCN), and at least one anti-cancer drug.
Even though twenty-first century humanity evidently needs products produced by using cyanide compounds, the word itself causes fear. This is because cyanide can be lethal for humans – and deer, and fish and …. In humans, ingestion, absorption, or inhalation can be deadly. Logsdon, et.al., list the 1998 “Threshold Limit Values for Chemical Substances and Physical Agents” (published by the American Conference of Governmental Industrial Hygienists). The lethal inhalation dose for humans is 250 parts per million of HCN gas in air. The lethal skin absorption and mouth ingestion values are somewhat dependent on body weight.
Our concern as residents of Michigan and Wisconsin is the use of cyanide by Aquila in its Back 40 Project along the Menominee River.
In its November 12, 2015, application (page 5) Aquila states: “During full production combined annual production of gold and silver at the oxide plant will be approximately 1.4 million ounce ( oz. ). The precious metals are recovered from the ore through use of a cyanide leach process to produce a precious metal bearing solution (pregnant liquor). The cyanide leach process occurs within contained vessels located inside the mill buildings. Further processing occurs to recover gold and silver from the pregnant liquor using a zinc dust precipitation process to produce a dore product for refining off-site.”
How much NaCN will be used? Table 5-6 in Aquila’s application lists an estimate of 166 tonnes per year for the oxide plant, for the 7 years ( from page 8 of the application) the mine is expected to be in production. That’s slightly under 366,000 pounds of NaCN per year that will have to be brought to the mine site, unloaded, stored, used, and then eventually “destroyed” – for each of the years the mine is in operation.
In Section 8 of the Logsdon, et.al. paper, evaluating and managing the risks of cyanide include: (1) Hazard Indentification (What physical, health, and ecological areas could be adversely affected by cyanide exposure); (2 ) Dose – Response (How much? What will happen now?); (3) Exposure Assessment (How did this happen?); and (4) Risk Characterization (How can we prevent this from happening again?). According to the paper Risk Assessment must include not only possible impacts on the general population and the environment, but especially for the workers who are “most likely to be exposed to the hazard.” Transporting, receiving, unloading, handling, storing, and using NaCN are each possible exposure points. A quick survey of the table of contents of the application and an admittedly quick reading of the 45 pages of Volume 1 of the application did not reveal any mention of worker training. The only mention is a short paragraph on page 44 stating: “Possible HSE (health, safety, environment) risks to on-site workers will be addressed by Aquila through a health and safety plan which is (sic.) complies with Occupational Safety and Health Administration (OSHA) and Mine Safety and Health Administration (MSHA) requirements.” Also, there is a bullet point on page 20 stating that MSHA safety standards during mine operations will be met.
Let us all hope that the systems engineering is accurate, that the systems function correctly, and that all stated requirements and standards for safety are followed and will indeed protect the workers, the environment, and all of us from contamination and worse.
Mark J. Lagsdon, MSc; Karen Halelstein, PhD, CIH; Terry I. Mudder, PhD, The Management of Cyanide in Gold Extraction, International Council on Metals and the Environment, 294 Albert Street, Suite 506, Ottawa, Ontario, Canada K1P 6E6., 1999. It should be noted that a search for ICME will be redirected to the International Council on Mining and Metals (ICMM) and that the web site for ICME will display the word “Obsolete.”