Cyanide Facts

Cyanide in Gold and Silver Mining

One of the Institute’s objectives is to make information on the use and hazards of cyanide available to all stakeholders. This area of the site has been developed to assist in achieving that objective.

Cyanide Chemistry

The term cyanide refers to a singularly charged anion consisting of one carbon atom and one nitrogen atom joined with a triple bond, CN. It is this triple bond that gives cyanide many of its characteristics and makes it it highly reactive, and it is this high reactivity that is important in understanding its use and behavior.

The most reactive form of cyanide is free cyanide, which includes the cyanide anion itself and hydrogen cyanide, HCN, either in a gaseous or aqueous state. The term “free” is used because these cyanide molecules are are not bonded with any metals or other compound.  Because they are not bonded with other compounds both of these forms are very reactive.  Free cyanide, HCN and CN, are the most reactive forms of cyanide, and because they are most reactive are also the most toxic forms of cyanide.  Although HCN is highly soluble in water, its solubility decreases with increased temperature and under highly saline conditions. Both HCN gas and liquid are colorless and have the odor of bitter almonds, although not all individuals can detect the odor.  In solution, the cyanide ion reacts with water to form HCN.  The relationship between pH, hydrogen cyanide, and the cyanide anion is very important to understand. This relationship affects the form of the cyanide present, and so affects toxicity, health and safety, gold processing economics, and process management. At low pH and at neutral pH, hydrogen cyanide is the dominant form of cyanide, and as pH increases CN- becomes the dominant form.  At a pH of 11, over 99% of the cyanide remains in solution as CN-, while at pH 7, over 99% of the cyanide will exist as HCN.  At a pH of 9.3 – 9.5, CN and HCN are in equilibrium, with equal amounts of each present.

Cyanide forms simple salts with alkali earth cations and ionic complexes of varying strengths with numerous metal cations; the stability of these salts is dependent on the cation and on pH. The salts of sodium, potassium and calcium cyanide are quite toxic, as they are highly soluble in water, and thus readily dissolve to form free cyanide. Mining operations typically use cyanide as solid or dissolved NaCN. Weak or moderately stable complexes such as those of cadmium, copper and zinc are classified as weak-acid dissociable (WAD). Although metal-cyanide complexes by themselves are much less toxic than free cyanide, their dissociation releases free cyanide as well as the metal cation which can also be toxic. Even in the neutral pH range of most surface water, WAD metal-cyanide complexes can dissociate sufficiently to be environmentally harmful if in high enough concentrations.

Cyanide forms complexes with gold, mercury, cobalt and iron that are very stable even under mildly acidic conditions. However, both ferro- and ferricyanides decompose to release free cyanide when exposed to direct ultraviolet light in aqueous solutions. This decomposition process is reversed in the dark. The stability of cyanide salts and complexes is pH dependent, and therefore, their potential environmental impacts and interactions (i.e. their acute or chronic effects, attenuation and re-release) can vary.

Metal cyanide complexes also form salt – type compounds with alkali or heavy metal cations, such as potassium ferrocyanide (K4Fe(CN)6) or copper ferrocyanide (Cu2[Fe(CN)6]), the solubility of which varies with the metal cyanide and the cation. Nearly all alkali salts of iron cyanides are very soluble, upon dissolution these double salts dissociate and the liberated metal cyanide complex can produce free cyanide. Heavy metal salts of iron cyanides form insoluble precipitates at certain pH levels.

The cyanide ion also combines with sulfur to form thiocyanate, SCN. Thiocyanate dissociates under weak acidic conditions, but is typically not considered to be a WAD species because it has similar complexing properties to cyanide. Thiocyanate is approximately 7 times less toxic than hydrogen cyanide but is very irritating to the lungs, as thiocyanate chemically and biologically oxidizes into carbonate, sulfate and ammonia.

The oxidation of cyanide, either by natural processes or from the treatment of effluents containing cyanide, can produce cyanate, OCN. Cyanate is less toxic than HCN, and readily hydrolyzes to ammonia and carbon dioxide.

A variety of sampling and analytical procedures for have been published for cyanide, and recent years have seen advances in techniques and equipment for analyzing cyanide.  Readers interested in sampling and analytical procedures are encouraged to ensure that they are reviewing the most current literature on these topics and information pertaining to their specific analytical needs.  Following are some current published materials that contain information on sampling and analysis of cyanide.

DIN 38405-13: 1981-02, German Standard Methods for the Analysis of Water, Waste Water and Sludge -Anions (Group D) -Determination of Cyanides (D13), German Standards (DIN Normen, Beuth Verlag GmbH, Burggrafenstr. 6, 10787 Berlin/Germany).

South African Water Quality Guidelines, Volumes 1-7, Department of Water Affairs and Forestry, 1996.

Standard Methods For The Examination Of Waters and Wastewater, APHA-AWWA-WEF, 20th Edition, Washington DC, 1998.

Water Quality – Determination of Cyanide -Part 1: Determination of Total Cyanide ISO/DIS 6703/1, International Organization of Standardization.

Water Quality -Determination of Cyanide -Part 2: Determination of Easy Liberated Cyanide ISO/DIS 6703/2, International Organization of Standardization.

USEPA “Methods and Guidance for Analysis of Water”, United States Environmental Protection Agency (USEPA), June 1999.

Use of Cyanide in Gold and Silver Mining

Cyanide is manufactured and distributed for use in the gold and silver mining industries in a variety of physical and chemical forms, including solid briquettes, flake cyanide and liquid cyanide. Sodium cyanide is supplied as either briquettes or liquid, while calcium cyanide is supplied in flake form and also in liquid form. The strength of bulk cyanide reagents varies from 98% for sodium cyanide briquettes, 44-50% for flake calcium cyanide, 28-33% for liquid sodium cyanide and 15-18% for liquid calcium cyanide.

The product strength is quoted on a molar basis as either sodium or calcium cyanide. The form of cyanide reagent chosen for use typically depends on availability, distance from the source and cost. Where liquid cyanide is used, it is transported to the mine by tanker truck or rail car and is off-loaded into a storage tank. The truck or rail car may have a single or double-walled tank, and the location and design of the discharge equipment vary by vehicle. Solid briquette or flake cyanide is transported to the mine in drums, plastic bags, boxes, returnable bins and ISO-containers. Depending on how the reagent is packaged, the mine will design and construct the necessary equipment to safely dissolve the solid cyanide in a high-pH solution. The pH value of cyanide solutions during dissolution should be maintained above pH 12 to minimize the volatilization of hazardous hydrogen cyanide (HCN) gas. The resulting cyanide solution is then pumped to a storage tank prior to introduction into the process.

The cyanide solution is fed from the storage tank into the metallurgical process stream in proportion to the dry mass of solids in the process stream. The feed rate of cyanide is controlled to maintain an optimum cyanide level as demanded by the metallurgy of the ore being treated.

A mine’s inventory of bulk cyanide reagent is dictated by the requirements to maintain continuous operations and avoid transportation constraints, especially for operations in remote areas.

Although the forms of cyanide vary, once introduced into the process, the technologies used for gold and silver recovery are the same.

The process of extracting gold from ore with cyanide is called cyanidation. The reaction, known as Elsner’s Equation, is:

4 Au + 8 CN- + O2 + 2 H2O = 4 Au(CN)2- + 4 OH-

Although the affinity of cyanide for gold is such that gold is extracted preferentially over other metals, cyanide will also form complexes with other metals from the ore, including copper, iron and zinc. The formation of strongly bound complexes such as those with iron and copper will tie up cyanide that would otherwise be available to dissolve gold.

High copper concentrations in the ore lower gold recovery efficiencies and increase costs by requiring higher cyanide application rates to compensate for cyanide that complexes with copper rather than gold.

Copper cyanides are moderately stable; their formation can cause both operational and environmental concerns, as wastewater or tailings from such operations may have significantly higher cyanide concentrations than would otherwise be present in the absence of copper.

Cyanidation is also adversely affected by the presence of free sulfur or sulfide minerals in the ore. Cyanide will preferentially leach sulfide minerals and will react with sulfur to produce thiocyanate. These reactions also enhance the oxidation of reduced sulfur species, increasing the requirement for lime addition to controlling the pH at a sufficient level to avoid the volatilization of hydrogen cyanide (HCN).

Gold and silver are not soluble in water. A complexant, such as cyanide, which stabilizes the gold and/or silver species in solution, and an oxidant such as oxygen are required to dissolve these metals. When gold or silver is dissolved in an aqueous cyanide solution it forms a metal-cyanide complex by oxidizing with an oxidant such as dissolved oxygen and cyanide complexation.  This complex is very stable and the cyanide required is only slightly in excess of the stoichiometric requirement.

Gold and silver typically occur at very low concentrations in ores — less than 10 g/t or 0.001% (mass basis). At these concentrations, the use of aqueous chemical (hydrometallurgical) extraction processes is the only economically viable method of extracting gold and silver from the ore. Typical hydrometallurgical recovery involves a leaching step during which the metal is dissolved in an aqueous medium, followed by the separation of the metal-bearing solution from the residues, or adsorption of the gold and/or silver onto activated carbon. After elution from the activated carbon, the metal is further concentrated by precipitation or electrodeposition.

Typical cyanide concentrations used in cyanide leaching range from 300 to 500 mg/l (0.03 to 0.05% as NaCN) depending on the mineralogy of the ore. However, in practice, the amount of cyanide used in leach solutions is dictated by the presence of other cyanide consumers, and the need to increase the rate of leaching to acceptable levels.

Alternative complexing agents, such as chloride, bromide, thiourea, and thiosulfate form less stable complexes and thus require more aggressive conditions and oxidants to dissolve the gold and/or silver. These reagents present risks to health and the environment and are more expensive. This explains the dominance of cyanide as the primary reagent for the leaching of gold and silver from ores since its introduction in the latter part of the 19th century.

In heap or dump leaching, the ore or agglomerated fine ore is stacked in heaps on a pad lined with an impermeable membrane. Cyanide solution is introduced to the heap by sprinklers or a drip irrigation system. The solution percolates through the heap leaching the gold and silver from the ore, and the resultant metal-bearing solution is collected on the impermeable membrane and channeled to storage facilities for further processing. Heap leaching is attractive due to the low capital cost involved, but is a slow process and the extraction efficiency is a relatively low 50-75%.

In a conventional milling and agitated leaching circuit, the ore is milled in a semi-autogenous ball or rod mills until it is the consistency of powder. The milled ore (slurry) is conveyed to a series of leach tanks. The slurry is agitated in the leach tanks, either mechanically or by means of air injection, to increase the contact of cyanide and oxygen with the gold and silver and enhance the efficiency of the leaching process. The cyanide then dissolves gold and silver from the ore and forms a stable metal-cyanide complex.

The use of oxygen or peroxygen compounds instead of air as an oxidant increases the leach rate and decreases cyanide consumption, due to the inactivation of some of the cyanide consuming species present in the slurry.

The pH of the slurry is raised to pH 10-11 using lime, at the head of the leach circuit to ensure that when cyanide is added, toxic hydrogen cyanide gas is not generated and the cyanide remains in solution to dissolve the gold and/or silver. The slurry may also be subject to other preconditioning such as pre-oxidation at the head of the circuit before cyanide is added.

Highly activated carbon is used in the dissolved gold and/or silver recovery process, either by introducing it directly into the CIL (carbon-in-leach) tanks or into separate CIP (carbon-in-pulp) tanks after leaching. The activated carbon adsorbs the dissolved metal from the leach slurry thereby concentrating it onto a smaller mass of solids. The carbon is then separated from the slurry by screening and subjected to further treatment to recover the adsorbed metal.

When carbon is not used to adsorb the dissolved gold and/or silver in the above-mentioned leach slurry, the metal-bearing solution must be separated from the solids components utilizing filtration or thickening units. The resultant solution, referred to as pregnant solution, is subjected to further treatment (other than by carbon absorption) to recover the dissolved gold and/or silver.

The waste from which the gold and/or silver was removed by any means is referred to as residue or tailings material. The residue is either dewatered to recover the solution, treated to neutralize or recover cyanide, or is sent directly to the tailings storage facility.

Preparation of the ore is usually necessary so that it can be presented to the aqueous cyanide solution in a form that will ensure the optimal economic recovery of gold and silver. The first step in ore preparation is typically crushing and grinding, which reduces the particle size of the ore and liberates these metals for recovery.

Ore that contains free gold and/or silver may not yield a sufficiently high recovery by the sole use of cyanide leaching, due to a very long dissolution time for large metal particles. Such ore may first be subject to a gravity recovery process to recover the free gold and/or silver before being subjected to cyanide leaching.

Ores that contain gold and/or silver associated with sulfide or carbonaceous minerals require additional treatment, other than size reduction, prior to metal recovery. The recovery from sulfide ore is poor because the cyanide preferentially leaches the sulfide minerals rather than the metals, and cyanide is consumed by the formation of thiocyanate. These ores are subject to concentration processes such as flotation, followed by a secondary process to oxidize the sulfides, thereby limiting their interaction with the cyanide during leaching. Carbonaceous minerals adsorb gold and silver once solubilized; oxidizing the ore prior to leaching prevents this. To counter this effect, the leaching process may also be modified by the addition of activated carbon to preferentially adsorb gold and silver.

Gold and silver are recovered from the solution first using either cementation on zinc powder or concentrating the metal(s) using adsorption on activated carbon, followed by elution and concluding with either cementation with zinc or electrowinning. For efficient cementation, a clear solution prepared by filtration or counter-current decantation is required.

The most cost-effective process is to create adsorption of the dissolved gold and/or silver onto activated carbon, resulting in an easier solid-solid separation based on size. To achieve this the ore particles must typically be smaller than 100 µm while the carbon particles must be larger than 500 µm. Adsorption is achieved by contacting the activated carbon with the agitated pulp. This can be done while the metals are still being leached with the CIL-process, or following leaching with the CIP-process. The CIL-process offers the advantage of countering the adsorption of gold and silver on carbonaceous or shale ore particles, but is more expensive due to less efficient adsorption, increased gold and/or silver inventory and increased fouling and abrasion of the carbon.

Activated carbon in contact with a pulp containing gold and/or silver can typically recover more than 99.5% of the metals in the solution in 8 to 24 hours, depending on the reactivity of the carbon, the amount of carbon used and the mixer’s efficiency. The loaded carbon is then separated from the pulp by screens that are air or hydrodynamically swept, thus preventing blinding by the near sized carbon particles. The pulp residue is then either thickened to separate the solution containing cyanide for recovery/destruction of the cyanide, or sent directly to the tailings storage facility from which solution containing residual cyanide may be recycled to the leach plant or milling circuit.

The gold and/or silver adsorbed on the activated carbon is recovered from the carbon by elution, typically with a hot caustic aqueous cyanide solution. The carbon is then regenerated and returned to the adsorption circuit while the gold and/or silver is recovered from the eluate using either zinc cementation or electrowinning. If it contains significant amounts of base metals, the gold/silver concentrate is then either calcined or directly smelted and refined to bullion that typically contains about 70 – 90% gold and/or silver. The bullion is then further refined to either 99.99% or 99.999% fineness using chlorination, smelting and electro-refining. High purity gold and silver is taken directly from activated carbon eluates, using recently developed processes that utilize solvent extraction to produce intensive leaching of gravity concentrates.

Effects on Human Health and the Environment

The following is a general summary of cyanide’s effects on human health and is not intended to be a complete reference to all the health effects of cyanide.

Relatively low concentrations of cyanide can be highly toxic to humans. Liquid or gaseous hydrogen cyanide and alkali salts of cyanide can enter the body through inhalation, ingestion or absorption through the eyes and skin. The rate of skin absorption is enhanced when the skin is cut, abraded or moist; inhaled salts of cyanide are readily dissolved and absorbed upon contact with moist mucous membranes.

The toxicity of hydrogen cyanide to humans is dependent on the nature of the exposure. Due to the variability of dose-response effects between individuals, the toxicity of a substance is typically expressed as the concentration or dose that is lethal to 50% of the exposed population (LC50 or LD50). The LC50 for gaseous hydrogen cyanide is 100-300 parts per million. Inhalation of cyanide in this range results in death within 10-60 minutes, with death coming more quickly as the concentration increases. Inhalation of 2,000 parts per million hydrogen cyanide causes death within one minute. The LD50 for ingestion is 50-200 milligrams, or 1-3 milligrams per kilogram of body weight, calculated as hydrogen cyanide. For contact with unabraded skin, the LD50 is 100 milligrams (as hydrogen cyanide) per kilogram of body weight.

Although the time, dose and manner of exposure may differ, the biochemical action of cyanide is the same upon entering the body. Once in the bloodstream, cyanide forms a stable complex with a form of cytochrome oxidase, an enzyme that promotes the transfer of electrons in the mitochondria of cells during the synthesis of ATP. Without proper cytochrome oxidase function, cells cannot utilize the oxygen present in the bloodstream, resulting in cytotoxic hypoxia or cellular asphyxiation. The lack of available oxygen causes a shift from aerobic to anaerobic metabolism, leading to the accumulation of lactate in the blood. The combined effect of hypoxia and lactate acidosis is a depression of the central nervous system that can result in respiratory arrest and death. At higher lethal concentrations, cyanide poisoning also affects other organs and systems in the body, including the heart.

Initial symptoms of cyanide poisoning can occur from exposure to 20 to 40 ppm of gaseous hydrogen cyanide, and may include headache, drowsiness, vertigo, weak and rapid pulse, deep and rapid breathing, a bright-red color in the face, nausea and vomiting. Convulsions, dilated pupils, clammy skin, a weaker and more rapid pulse and slower, shallower breathing can follow these symptoms. Finally, the heartbeat becomes slow and irregular, body temperature falls, the lips, face and extremities take on a blue color, the individual falls into a coma, and death occurs. These symptoms can occur from sublethal exposure to cyanide but will diminish as the body detoxifies the poison and excretes it primarily as thiocyanate and 2 amino thiazoline 4 carboxilic acid, with other minor metabolites.

The body has several mechanisms to effectively detoxify cyanide. The majority of cyanide reacts with thiosulfate to produce thiocyanate in reactions catalyzed by sulfur tranferase enzymes such as rhodanese. The thiocyanate is then excreted in the urine over a period of days. Although thiocyanate is approximately seven times less toxic than cyanide, increased thiocyanate concentrations in the body resulting from chronic cyanide exposure can adversely affect the thyroid. Cyanide has a greater affinity for methemoglobin than for cytochrome oxidase, and will preferentially form cyanomethemoglobin. If these and other detoxification mechanisms are not overwhelmed by the concentration and duration of cyanide exposure, they can prevent an acute cyanide-poisoning incident from being fatal.

Some of the available antidotes to cyanide poisoning take advantage of these natural detoxifying mechanisms. Sodium thiosulfate, administered intravenously, provides sulfur to enhance the sulfur transferase-mediated transformation of cyanide to thiocyanate. Amyl nitrite, sodium nitrite and dimethyl aminophenol (DMAP) are used to increase the amount of methemoglobin in the blood, which then binds with cyanide to form non-toxic cyanomethemoglobin. Cobalt compounds are also used to form stable, non-toxic cyanide complexes, but as with nitrite and DMAP, cobalt itself is toxic.

Cyanide does not accumulate or biomagnify, so chronic exposure to sublethal concentrations of cyanide does not appear to result in acute toxicity. However, chronic cyanide poisoning has been observed in individuals whose diet includes significant amounts of cyanogenic plants such as cassava. Chronic cyanide exposure is linked to demyelination, lesions of the optic nerve, ataxia, hypertonia, Leber’s optic atrophy, goiters and depressed thyroid function.

There is no evidence that chronic cyanide exposure has teratogenic, mutagenic or carcinogenic effects.

The following is a general summary of cyanide’s behaviour and effects in the environment and is not intended to be a complete reference to all environmental effects of cyanide.

Cyanide is produced naturally in the environment by various bacteria, algae, fungi and numerous species of plants including beans (chickpeas and lima), fruits (seeds and pits of apple, cherry, pear, apricot, peach and plum), almond and cashew nuts, vegetables of the cabbage family, grains (alfalfa and sorghum), roots (cassava, potato, radish and turnip), white clover and young bamboo shoots. Incomplete combustion during forest fires is believed to be a major environmental source of cyanide, and incomplete combustion of materials containing nylon produces cyanide through depolymerization.

Once released in the environment, the reactivity of cyanide provides numerous pathways for its degradation and attenuation:

Complexation: Cyanide forms ionic complexes of varying stability with many metals. Most cyanide complexes are much less toxic than cyanide, but weak acid dissociable complexes such as those of copper and zinc are relatively unstable and will release cyanide back to the environment. Iron cyanide complexes are of particular importance due to the abundance of iron typically available in soils and the extreme stability of this complex under most environmental conditions. However, iron cyanides are subject to photochemical decomposition and will release cyanide if exposed to ultraviolet light. Metal cyanide complexes are also subject to other reactions that reduce cyanide concentrations in the environment, as described below.

Precipitation: Iron cyanide complexes form insoluble precipitates with iron, copper, nickel, manganese, lead, zinc, cadmium, tin and silver. Iron cyanide forms precipitate with iron, copper, magnesium, cadmium and zinc over a pH range of 2-11.

Adsorption: Cyanide and cyanide-metal complexes are adsorbed on organic and inorganic constituents in soil, including oxides of aluminum, iron and manganese, certain types of clays, feldspars and organic carbon. Although the strength of cyanide retention on inorganic materials is unclear, cyanide is strongly bound to organic matter.

Cyanate: Oxidation of cyanide to less toxic cyanate normally requires a strong oxidizing agent such as ozone, hydrogen peroxide or hypochlorite. However, adsorption of cyanide on both organic and inorganic materials in the soil appears to promote its oxidation under natural conditions.

Thiocyanate: Cyanide reacts with some sulfur species to form less toxic thiocyanate. Potential sulfur sources include free sulfur and sulfide minerals such as chalcopyrite (CuFeS2), chalcocite (Cu2S) and pyrrhotite (FeS), as well as their oxidation products, such as polysulfides and thiosulfate.

Volatilization: At the pH typical of environmental systems, free cyanide will be predominately in the form of hydrogen cyanide, with gaseous hydrogen cyanide evolving slowly over time. The amount of cyanide lost through this pathway increases with decreasing pH, increased aeration of the solution and increasing temperature. Cyanide is also lost through volatilization from soil surfaces.

Biodegradation: Under aerobic conditions, microbial activity can degrade cyanide to ammonia, which then oxidizes to nitrate. This process has been shown effective with cyanide concentrations of up to 200 parts per million. Although biological degradation also occurs under anaerobic conditions, cyanide concentrations greater than 2 parts per million are toxic to these microorganisms.

Hydrolysis: Hydrogen cyanide can be hydrolyzed to formic acid or ammonium formate. Although this reaction is not rapid, it may be of significance in groundwater where anaerobic conditions exist.

Effects on Wildlife: Although cyanide reacts readily in the environment and degrades or forms complexes and salts of varying stabilities, it is toxic to many living organisms at very low concentrations.

Aquatic Organisms: Fish and aquatic invertebrates are particularly sensitive to cyanide exposure. Concentrations of free cyanide in the aquatic environment ranging from 5.0 to 7.2 micrograms per liter reduce swimming performance and inhibit reproduction in many species of fish. Other adverse effects include delayed mortality, pathology, susceptibility to predation, disrupted respiration, osmoregulatory disturbances and altered growth patterns. Concentrations of 20 to 76 micrograms per liter free cyanide cause the death of many species, and concentrations in excess of 200 micrograms per liter are rapidly toxic to most species of fish. Invertebrates experience adverse nonlethal effects at 18 to 43 micrograms per liter free cyanide, and lethal effects at 30 to 100 micrograms per liter (although concentrations in the range of 3 to 7 micrograms per liter have been observed to cause death in the amphipod Gammarus pulex).

Algae and macrophytes can tolerate much higher environmental concentrations of free cyanide than fish and invertebrates, and do not exhibit adverse effects at 160 micrograms per liter or more. Aquatic plants are unaffected by cyanide at concentrations that are lethal to most species of freshwater and marine fish and invertebrates. However, differing sensitivities to cyanide can result in changes to plant community structure, with cyanide exposures leaving a plant community dominated by less sensitive species.

The toxicity of cyanide to aquatic life is probably caused by hydrogen cyanide that has ionized, dissociated or photochemically decomposed from compounds containing cyanide. Toxic effects of the cyanide ion itself on aquatic organisms are not believed to be significant, nor are the effects of photolysis of ferro- and ferricyanides. It is therefore the hydrogen cyanide concentration of water that is of greatest significance in determining toxicity to aquatic life rather than the total cyanide concentration.

The sensitivity of aquatic organisms to cyanide is highly species-specific and is also affected by water pH, temperature and oxygen content, as well as the life stage and condition of the organism.

Birds: Reported oral LD50 for birds range from 0.8 milligrams per kilogram of body weight (American racing pigeon) to 11.1 milligrams per kilogram of body weight (domestic chickens). Symptoms including panting, eye blinking, salivation and lethargy appear within one-half to five minutes after ingestion in more sensitive species, and up to ten minutes after ingestion by more resistant species. Exposures to high doses resulted in deep, labored breathing followed by gasping and shallow intermittent breathing in all species. Mortality typically occurred in 15 to 30 minutes; however, birds that survived for one hour frequently recovered, possibly due to the rapid metabolism of cyanide to thiocyanate and its subsequent excretion.

Ingestion of WAD cyanide solutions by birds may cause delayed mortality. It appears that birds may drink water containing WAD cyanide that is not immediately fatal, but which breaks down in the acidic conditions in the stomach and produces sufficiently high cyanide concentrations to be toxic.

Sublethal effects of cyanide exposure to birds, such as an increase in their susceptibility to predators, have not been fully investigated and reported.

Mammals: Cyanide toxicity to mammals is relatively common due to a large number of cyanogenic forage plants such as sorghum, sudan grasses and corn. Concentrations of cyanide in these plants are typically highest in the spring during blooming. Dry growing conditions enhance the accumulation of cyanogenic glycosides in certain plants as well as increase the use of these plants as forage.

Reported oral LD50 for mammals range from 2.1 milligrams per kilogram of body weight (coyote) to 6.0-10.0 milligrams per kilogram of body weight (laboratory white rats). Symptoms of acute poisoning usually occur within ten minutes of ingestion, including: initial excitability with muscle tremors; salivation; lacrimation; defecation; urination; labored breathing; followed by muscular incoordination, gasping and convulsions. In general, cyanide sensitivity for common livestock decreases from cattle to sheep to horses to pigs; deer and elk appear to be relatively resistant.

Although present in the environment and available in many plant species, cyanide toxicity is not widespread due to cyanide’s low persistence in the environment as well as there being no reported bioaccumulation in animals and no evidence of biomagnification in the food chain.

Although chronic cyanide intoxication exists, cyanide has low chronic toxicity. Repeated sublethal doses of cyanide seldom result in cumulative adverse effects. Many species can tolerate cyanide in substantial yet sublethal intermittent doses for long periods of time.


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