Killer beans! 

Killer beans! 


For years the gold mining industry has been criticized for its use of the chemical cyanide. But this may be more about a means of attacking the mining industry than the practice. Many disingenuous arguments are made against the mining industry, very often where cyanide use is concerned. We hope that this article will open your eyes to cyanide being more than a toxic pollutant used only by mining companies. For starters lets review these simple facts, generally never mentioned by those who attack mining usage of cyanide:

  • Cyanide  is a very important naturally produced molecule in the plant and animal world and has been around long before humans. It even occurs in outer space!     
  • Mining accounts for less than 15% of industrial usage.
  • Many products  you likely use every day like nylon (over 40% of industrial cyanide usage!) and blue jeans are dependent on cyanide for their production.
  • Many plants and organisms naturally produce and consume cyanide.
  • Plant produced cyanide can occur in the soil around plant root systems. If gold also occurs in the soil it can be dissolved and concentrated by this natural cyanide. This is the same methodology used in human mining operations at a larger scale.
  • Cyanide introduced to the environment does not accumulate: it is rapidly broken down and dissipated as part of the natural cyanide cycle.
  • In some plants, cyanide occurs in concentrations that would be judged “hazardous” if it were associated with manufactured sources.
  • Many fruits and vegetables  you may consume every day contain elevated levels of naturally produced cyanide.
  • Mining is  one of the most government-regulated industries in existence, especially where cyanide usage is concerned. The industry has also developed a rigorous code of conduct concerning cyanide use.
  • For many mines cyanide is the most expensive consumable used and out of economic interest alone it is recycled and consumed as fully as possible. 
  • The largest source of human produced cyanide in the environment is likely from road salt, fire retardants and the waste from thousands of former Manufactured Gas Plants across the US and Europe.
  • Millions of liters of chemical retardants containing about 400 tonnes of iron cyanide derivatives are being applied in an effort to fight forest fires in the United States each year.
  • Biomass burning, specifically forest fires, are the largest source of hydrogen cyanide gas in our atmosphere. 
  • The most significant human exposure risk for cyanide is from the consumption of vegetables and fruits and the inhalation of cigarette smoke and vehicle exhaust.


Perhaps the biggest hindrance to understanding cyanide and the risks of using this chemical is that "cyanide" is a general term. The media and even scientific writings routinely make this mistake by treating all the forms of cyanide as if they were the same. Indeed government environmental regulations have failed to take this into account in some cases. In fact there are many forms of cyanide of varying toxicity and chemical reactivity. Cyanide is the term used to designate chemical with a carbon atom triple bonded to a nitrogen molecule (CN). Cyanide mostly occurs regardless of origin in non-organic form. The main forms of cyanide can be organised as follows:

  • Free Cyanide and Simple Cyanides
  • Metal cyanides
  • Cyanate and Thiocyanate
  • Organocyanides
  • Simple and Free Cyanides

The free form of cyanide is hydrogen cyanide (HCN) and the cyanide ion CN- that it dissociates to in water.  HCN is soluble in water and, above 26 degrees Celsius, is a colourless gas that smells of bitter almonds. At a pH of less than 8 (almost all surface water on earth) HCN volatises, meaning it naturally escapes from the water into air where it is dispersed and rendered harmless. This process is natural and detoxifies HCN solutions. Simple metal cyanide solids include the cyanide salts sodium cyanide (NaCN) and potassium cyanide (KCN). These solid salts readily dissolve in water to produce free cyanide. Cyanogen halides are cyanogen chloride (CNCl) and cyanogen bromide (CNBr) which are colorless gases and commonly found in Municipal sewage treatment facilities as a result of processing the waste. The simple and free forms of cyanide are the most toxic of the cyanide species. The gaseous escape (volatility) of dissolved HCN from water and subsequent transformation to benign compounds in air, is an important process. It is a natural mechanism for that removes free cyanide from natural and industrial waste waters. This is why in the plants at gold mines the pH of cyanide solutions at maintained at values near 10.5 in order to prevent volatilization. This preserves cyanide in solution so that it can be used to recover gold and at the same time limits the risk of worker exposure to HCN gas in a confined space.

Metal Cyanides

The cyanide molecule can associate with heavy metals such as copper, nickel, zinc, cadmium, iron, silver, gold to form complexes of variable stability and toxicity. Metal complexed cyanides can be separated into two groups, strong and weak metal cyanide complexes. Both are water soluble.

Strong metal cyanide complexes are CN- associated with metals such as iron, gold and cobalt. An example is the iron cyanide dye "Prussian blue". These complexes are chemically very stable, difficult to oxidize and therefore are not toxic or low toxic forms of cyanide. The weak or weak-acid dissociable (WAD) metal cyanides, CN- associated with copper, nickel, silver, cadmium and zinc, are ‘‘toxicologically significant’’ forms of cyanide because they far more easily breakdown into free cyanide.<p>

Cyanate and Thiocyanate

Free cyanide can be oxidized to form cyanate. The oxidiation of CN is hard to acheve so cyanates are rare. CNO-. Cyanate is much less toxic than free cyanide. Thiocyanates are CN combined with sulfur, S, a relatively non-toxic form of cyanide. Both cyanates and thiocyanates can combine with metals to form metal-cyanates and metal-thiocyanates respectively.


The organocyanide complexes are cyanide associated with organic molecule, such as nitriles, cyanobenzene, cyanogenic glycosides, where the cyanide is associated with the organic molecule. Organic cyanide is produced in nature and in manufacturing.


HCN is comprised of hydrogen, carbon and nitrogen, three of the four most common atoms that make up life forms on earth. Cyanide compounds and HCN have been around long before humans came on the scene. At the start of earth history cyanide is thought to have played a pivotal role in the development of life on earth by helping form amino acids, peptides, lipids and membranes. More than four billion years later cyanide continues to be an integral part of nature on earth and in the universe. Cyanide gas clouds occur in interstellar space and HCN gas has been found in the atmospheres of other planets included Titan, a moon of Saturn.

Naturally produced cyanide occurs throughout our biosphere. HCN is a naturally produced part of the earth’s atmosphere in concentrations up to 900 parts per trillion (by volume). Research shows that cyanide gets into the atmosphere mostly through forest fires, as the burning of biomass produces HCN gas. Many plants, animals, microbes and fungi organically produce and consume cyanide.

Don't eat the pits! 

Don't eat the pits! 

Over 2,600 plants (130 families) are known to produce cyanide. Most cyanide produced by plants is for use as a defense mechanism but almost all fruit-bearing plants release HCN as part of the fruit ripening process. This is why fruit seeds contain such high concentrations of cyanide. Various microorganisms can produce and/or destroy cyanide. They also degrade this same cyanide either to detoxify it, or to use it as a source of nitrogen for growth. Significant amounts of cyanide are formed by a wide range of fungi and a few bacteria. Many forms of bacteria have been identified that use cyanide as a source of nitrogen for growth. Recently it has been discovered that vascular plants like Willow trees, fungi and bacteria can be produced and used to detoxify cyanide. Experiments are underway to remove cyanide from industrial waste using such organisms which convert the cyanide into nitrogen food.

If you listen to your mother and eat vegetables you consume cyanide every day. For example sweet potatoes, corn, lima beans, almonds, radishes, cabbage, kale, brussel sprouts, cauliflower, broccoli, turnips, lettuce and kidney beans and the seeds of cherries, plums, apricots, pears and apples all contain cyanide. The cyanide is produced by the plant to aid in the fruit ripening process. Many fruit pits contain small amounts of cyanohydrins, which slowly release HCN; one hundred grams of crushed apple seeds can yield roughly about 10 mg of HCN. Up to one gram of hydrogen cyanide per kilogram may be obtained from the "bitter" roots of the cassava plant. Cassava is part of the diet of hundreds of millions of people living in equatorial countries of the world. If improperly prepared, cyanide can be released at toxic levels resulting in a chronic paralysis of the arms and legs particularly amongst young people. Several thousand cases of have been documented in African countries.

Due to organic metabolic processes, cyanide occurs naturally in the soil around the root systems of many plants. Since gold and silver naturally dissolve in a weak cyanide solution, these metals have been found concentrated around plant roots where they have been dissolved out of gold rich soil and re-deposited. Typically natural CN in water is in concentrations <0.01 mg/L as it decays rapidly when exposed to UV radiation from sunlight. In tropical soils natural cyanide values up to 0.1 mg/L have been reported.


Cyanide has been known for over 2,000 years. The word cyanide is derived from the Greek cyanine meaning blue. It has been used as a blue dye in the form of the strong metal cyanide iron ferrocyanide "Prussian Blue" since its discovery in 1704. This cyanide die is still used today make jeans blue. Many other industries use cyanide, and without it our daily lives would not be the same.  Over 2 million tonnes of cyanide are produced a year for industrial use. Cyanide is produced industrially in one of two ways: as a by-product of the manufacture of acrylic fibres and certain plastics, or by combining natural gas and ammonia at high temperatures and pressures to produce hydrogen cyanide (HCN) gas. There are three primary producers of solid, liquid and gaseous cyanide in the world: Dupont, in the United States, ICI, in England, and Degussa Corporation, in Germany.

Cyan Jeans

Cyan Jeans

Mining use only accounts for less than 15% of cyanide usage, but mining companies bear the brunt of criticism regarding the use of cyanide. About 85% of cyanide used today is in the manufacture of a wide range of industrial and medical products. Useful items containing cyanide are nylon, plastic, glass, perfume, soaps, fertilizers and animal food supplements, paints and adhesives. Cyanide forms a very stable complex with several metals including gold and silver, copper and zinc. This property is why it is used in the gold and silver mining industry. It also makes cyanide very useful in metal electroplating, to make items such as silver plated dinnerware and brass plated door handles that you likely use every day. Other uses of cyanide include pest control (as a poison), an anti-clumping agent in road salt and fire retardants.  HCN vapour has been used to fumigate buildings and ships, to exterminate rabbits, rodents, and large predators, and in horticultural practice to control insect pests that have developed resistance to other pesticides. Sodium cyanide was used for about 50 years by the US Fish and Wildlife Service to control coyote numbers and still used by the New Zealand government to control possums where cyanide-laced pellets are dropped from the air.

NZ Government declares war on Possums! 

NZ Government declares war on Possums! 

While these industries use cyanide as a part of the manufacturing or mining process, other human activities produce cyanide as a byproduct. Everyday airborne sources of cyanide arise from fires and cigarette smoke. Coffee and table salt also contain cyanide. Manufactured gas plants (MGP) produced cyanide wastes and there are thousands of past MGP sites in the US and Europe. Chemical manufacturing, iron and steel production, petroleum refining and aluminum smelting all produce cyanide waste materials. Some Municipal sewage treatment plants can produce toxic levels of HCN.

Medical Uses

The cyanide compound sodium nitroprusside is occasionally used in emergency medical situations to produce a rapid decrease in blood pressure in humans; it is also used as a vasodilator in vascular research. The cobalt in artificial Vitamin B12 contains a cyanide ligand as an artifact of the purification process. During World War I, a copper cyanide compound was briefly used by Japanese physicians for the treatment of tuberculosis and leprosy. Cyanide is used in pharmaceuticals such as the anti cancer drug laetrile and in specialised surgical dressings that foster healing and reduce scarring. Laetrile (an anti-cancer preparation made from apricot kernels) and sodium nitroprusside (a drug used to reduce high blood pressure), release cyanide upon metabolism.

Mining Use

Over 90% of the gold mined in the world is extracted using cyanide. This process is similar to what plants do in tropical rain forests. The cyanide plants produce can dissolve, concentrate and redeposit gold in soil. It has been used in mining operations since 1887 to extract gold, silver, copper, zinc and molybdenum from mined rock. Today its main use is in gold and silver mining because of how readily these metals dissolve in a weak cyanide solution. Cyanide used in mining is in a liquid form as water based solution. Gold mining
operations use very dilute solutions of sodium cyanide, typically in the range of 0.01% and 0.05% cyanide (100 to 500 parts per million). The process of dissolving metal is called leaching. A weak cyanide solution dissolves the gold contained in the rock. Once dissolved the gold can be recovered from solution allowing the cyanide solution to be used again.

There are two types of mining operations that use cyanide gold leaching to extract the gold:

  • Tank Leaching: Leaching in a tank of gold from rock that is ground to a fine powder;
  • Heap Leaching: outdoor leaching of rock crushed to gravel size put in piles called pads.
Tank Leaching

Tank Leaching

Tank Leaching

Tank leaching is the conventional method, in which the rock is crushed and then ground to a fine powder. The powder is then mixed a weak cyanide solution and the gold is dissolved. The gold is then removed from the cyanide solution which can be recycled to be used again. The finely ground rock now stripped of gold is mixed with water and pumped into a storage area. This material is called tailing discussed in greater detail below and here.

 Heap Leaching

Heap Leaching Schematic

Heap Leaching Schematic

With this method, the rock is only crushed to pea size and placed in large rock piles known as heaps. The cyanide solution is trickled through these heaps to dissolve the gold in the rock. Underneath the heap there is a special rubber mat called a liner. The gold-cyanide solution is very valuable and it is collected in a pond where. From there it is pumped into a mill building where the gold is separated from the solution and the cyanide solution is recycled to leach gold once again.


Other chemicals used regularly are as toxic as cyanide. But cyanide has a special place in our lore. Its toxicity may have been exceeded by the infamy it has gained from bad espionage movies depicting dying spies foaming at the mouth. But apart from drama, given a past history of purposeful misuse, it is no wonder that cyanide can be perceived with alarm by the public. As a deadly poison, cyanide has been associated with events such as World War Two genocide gas chambers, Middle Eastern wars, and the Jonestown suicides. It seems from history that the bad reputation of cyanide comes from misuse by governments not mining companies.

Cyanide in the movies

Cyanide in the movies

In over 100 years of mining use of cyanide in Canada, the US, New Zealand and Australia there have only been three documented human deaths resulting from accidental cyanide poisoning.

Nevertheless there is justifiable public concern about the use of any toxin, cyanide included. Cyanide is a toxic substance and can be lethal if ingested or inhaled in sufficient amounts and in certain forms. This is also true for many other chemicals such as gasoline and common household cleaning supplies. As is the case for the thousands of other chemicals used in our modern industrial processes, knowledge, proper handling procedures and a responsible attitude are critical to the safe and beneficial use of cyanide.

Free forms of cyanide, HCN and CN are the most toxic cyanides. Free cyanides nevertheless are the most readily detoxified by natural and human treatment. HCN gas outdoors is rapidly dispersed. If you are exposed to cyanide gas or ingest cyanide, and you are able to talk about it later, you will have no long term or unknown health problems.  Cyanide is not toxic in all forms or all concentrations. The toxic forms of cyanide, HCN and free cyanide do not persist in the environment and do not accumulate in organisms or the environment.

The significant major health and environmental concern with some synthetic chemicals is that they do not decompose readily and can thereby accumulate in the food chain. Cyanide, however, is transformed by natural physical, chemical and biological processes into other, less toxic chemicals. 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. Cyanide is attenuated through natural processes. Over time, natural processes such as exposure to sunlight can reduce the concentration of toxic forms of cyanide in solutions to very low values.

"Workers in the following industries may be exposed to higher concentrations of thiocyanate than the general population: manufacture of electronic computing equipment, research and development laboratories, newspaper and other commercial printing, general medical or surgical hospitals, production of adhesives and sealants, pesticide application, building and furniture construction, and handling,treatment, or disposal of thiocyanate-containing wastes from industrial processes." - US Department of Heath Human Services

In terms of toxicity it is worth making some comparisons:

  • A lethal dose of cyanide is contained in 3.7 pounds of lima beans.
  • A lethal dose of pure water is 17 litres consumed very quickly.
  • A lethal dose of gasoline is the same as cyanide.

Cyanide can be manufactured, stored, transported, used and disposed of in a safe manner.  There is no evidence that cyanide is teratogenic, mutagenic, carcinogenic, or bioaccumulative in humans, animals, or aquatic life (Smith & Mudder 1994).

There are no reports of cyanide biomagnification or cycling in living organisms, probably owing to its rapid detoxification (Hagelstein 1997). The majority of the absorbed cyanide reacts with thiosulphate in the presence of enzymes to produce thiocyanate, which is excreted in the urine over several days. Owing to this rapid detoxification, animals can ingest high sub lethal doses of cyanide over extended periods without harm (Eisler 1991).

"Among the general population, sub-populations with the most likely potential for exposure to cyanide and thiocyanate include active and passive smokers (EPA 1981e) and people who are exposed to house or other building fires" - US Department of Heath Human Services

For example, a New Zealand case history documents the non-fatal incidence of a three and a half year old girl who ingested an unspecified number of apricot kernels. The child rapidly developed signs of acute poisoning, and her breath smelled strongly of bitter almonds. When tested, apricot kernels from the same batch contained 0.33% of CN, an average of 2.15 mg/kernel.  

Is Cyanide dangerous to Use in Mining Operations?

To begin we need to dispel some misunderstandings. Cyanide is often confused with acid drainage. This is something quite different and discussed in detail here. Furthermore, cyanide is not a heavy metal and it is not radioactive. Organic plant produced cyanide dissolve gold in soil and concentrate it. This natural process is exactly what human mining operation do.

Leach Pad Being Constructed

Leach Pad Being Constructed

Cyanide is one of only a few chemical reagents that will dissolve gold in water. It is a common industrial chemical that is readily available at a reasonably low cost. Cyanide has been used in metal extraction since 1887 and is now safely used and managed in gold recovery around the world. Gold mining operations use very dilute solutions of sodium cyanide, typically in the range of 0.01% and 0.05% cyanide (100 to 500 parts per million). Cyanide compounds are not persistent in water or soil. This means that they are rendered non-toxic rapidly as natural processes break down the compound itself. If exposed to air, the majority of the cyanide is volatilized to the atmosphere where it is quickly diluted and destroyed by ultra violet light. Cyanide is also broken down by biological oxidation and sunlight. There is no evidence of bioaccumulation in the food chain, and hence, secondary poisoning does not occur from the introduction of cyanide into the environment.

Swimming in a former cyanide tailings pond! 

Swimming in a former cyanide tailings pond! 

In 19th century New Zealand cyanide was first used to produce gold at a time of no safeguards in mining operations. During this time mine cyanide was disposed directly into the Ohinemuri River from various mining operations. In Bulgaria where cyanide was used at a number of operations in the first half of last century - the water downstream from these is drinkable - today. Despite this practice, there have been no long term adverse environmental effects from cyanide use in the mining industry there or anywhere else. Similarly in early 20th Century Bulgaria cyanide was used at a number of operations and not carefully stored and monitored but today the water downstream from these is drinkable. One of the earliest countries to use cyanide extensively was Mexico. Amongst the first cyanide plants working in North America were the mines of Pachuca, Guanajuato and Fresnillo all of which were installed in the first decade of the 19th century. All three still have mining operations and are vibrant and large cities today. Cyanide use has continues today unabated since 1906 at both Pachuca and Guanajuato. At Fresnillo the public park, Parque los Jales, was built on the tailings facility used by the cyanide plant and is perfectly safe today and is used for swimming.

Cyanide in Tailings

Tailings are rock ground to a powder and water. This material is what is left over after gold and silver has been removed from the mined rock at the mine’s mill or processing plant and are a mixture or “slurry” of powdered rock and water. Tailings are put into a lined hollow area that looks like a small pond. Tailings ponds are designed so that they cannot mix with ground waters until they have been removed of cyanide. Generally only low residual concentrations of cyanide are found in tailings storage areas, with concentrations of cyanide averaging around 15 grams per cubic metre. This is well within the level of 50 grams per cubic metre which is the upper level considered safe for migratory birds and water fowl (see Mudder 1994). But because of the many reactions and transformations that it undergoes naturally, cyanide does not remain at these concentrations and is broken down rapidly in the environment.

Mining companies monitor the decay of cyanide beneath tailings ponds by drilling water monitoring wells. These typically show that the concentration of cyanide decreases with depth, due to decay and transformation processes. In the upper few metres of an active tailings pond, free cyanide rapidly decays. However, below that depth, the remaining cyanide is present mainly as copper cyanide complexes. Transformation processes convert the copper cyanide to the non-toxic iron cyanide form and insoluble CuCN. Iron cyanide is a very stable complex. These processes have been observed at many mine sites, and prevent long term leaching of cyanide from the tailings.

Mining companies install drains at the bottom of tailings ponds and monitor for any remaining cyanide in water discharge. Typically as tailings build up over the course of a mine’s life they become compacted and after a period of time the flow of water and concentrations of cyanide in the drains begins to decrease. Water treatment plants are installed that destroy any remaining cyanide in the drain water. For example water from the Waihi gold mine (located in New Zealand) tailings pond following treatment for the removal of residual cyanide is allowed by government consent to be discharged to the Ohinemuri River.

At the same Waihi mine, it is expected that the water quality in the tailings pond in current use will require treatment for approximately three years once the mine ceases. Studies of plant growth trials undertaken on mine tailings of both grass and native species in both fertilized tailings, and tailings with compost added have been undertaken. Pasture yields from plots without compost were 11,000 kilograms per hectare, compared to 14,000 kilograms per hectare from compost added plots. The survival rate of native plants in tailings was 100%. This is in no small part because of the Nitrogen fertilizer provided by the breakdown of cyanide in the tailings.


 Government Reports and Policy Manuals

Australian Government's Cyanide Management Handbook

YUKON MERG Report 2001-2

UK Health Protection Agency Discussion of Cyanide


Swedish Environmental Protection Agency HCN Discussion

Dutch Government's Cyanide Risk Assessment Report

Cyanide in Nature

Cyanide in plants 

Metabolism of Hydrogen Cyanide by Higher Plants

Cyanide Compounds in Biology By CIBA Foundation Symposium

Mechanism, regulation, and ecological role of bacterial cyanide biosynthesis

Cyanide-bearing Elderberries?

How to get cyanide out of Cassava! 

Recycling Cyanide in the environment 

 Plants Eliminate Anthropogenic Cyanide!

Cyanide Degredation by Microorganisms 

Forest Fires are the major source of HCN in the atmosphere!

Plant produced cyanide concentrates gold in soil! 

Microbial Destruction of Cyanide in Mine Tailings 

Recommended Mining Industry Documents and Management Codes

The Management of Cyanide in Gold Extraction

International Cyanide Management Code for the Gold Industry  

Minerals Council of Australia Cyanide Factsheet 

Corporate article with scientific references: Newmont's Waihi Gold Mine

Dr. Muddler's Letter to the Prime Minister of Romania: A Technical Statement on Cyanide

 Frederick W. De Vries: Cyanide Myths and Facts

Cyanide Process Discussed in the 1902 US Census Office Mines and Quarries Special Report


The Organic Codes and Introduction to Semantic Biology (Tells the story of HCN and the origin of life)

Cyanide in Water and Soil: Chemistry, Risk, and Management

The Management of Cyanide in Gold Extraction 

The Chemistry and Treatment of Cyanidation Wastes 


Formation of Free Cyanide and Cyanogen Chloride from Chloramination of Publicly Owned Treatment Works Secondary Effluent

Living with Cyanide - A Great Introduction to the Subject

Cyanide and Society: A Must to Understand Mining and Cyanide Usage 

Cyanide in table salt! 

US Fish and Wildlife Uses Cyanide Traps to Kill Coyotes 

 List of Scientific References by Topic