Cyanide is a toxic substance that is commonly found in compounds such as sodium cyanide, potassium cyanide, and hydrogen cyanide. Hydrogen cyanide, a colorless gas or liquid with a faint, bitter almond odor, is found at low levels in air from car exhausts. Chronic exposure through inhalation to this substance is highly toxic to humans. It can result in effects on the central nervous system such as headaches, dizziness, loss of visual acuity, and tremors [1].
The air from car exhausts is a significant source of cyanide, but it can also be found in other airborne sources such as gas emissions from municipal waste incinerators, chemical processing, and other industries [1].
At Anderson, we do a lot of environmental characterization and assessment at previous gold mines were cyanide was used as a leaching agent.
Cyanide Remediation
Cyanide treatment consists of the following[2]:
Physical Methods
Dilution – Dilution is the only method that does not involve the destruction or separation of cyanide. It is a cheap and straightforward method but is not usually recommended because the total amount of cyanide that is discharged does not change.
Membranes – A membrane is utilized to separate cyanide from a solution using electrodialysis or reverse osmosis.
Electrowinning – This method is primarily used in gold processing but has found its way into cyanide remediation. This method performs well in concentrated solutions.
Hydrolysis/Distillation – Free cyanide is hydrolyzed in water producing aqueous hydrogen cyanide, which, in turn, will volatilize as hydrocyanic gas. It is then distilled and captured for recycling or release into the atmosphere.
Rio Tinto completing the mining ‘circle of life’
Complexation Methods
Acidification/volatilization – Cyanide volatilizes at low pH. The lower the pH, the higher the rate of volatilization. After solutions are acidified and liberated from cyanide, they are re-neutralized for recycling or disposal.
Metal Addition – The addition of metals and cations form metal-cyanide complexes and precipitates, rendering the cyanide inactive.
Flotation – Flotation is the process of adding a heteropolar surfactant, usually a cationic amine, to react with the anionic complex to precipitate an organic double salt. This method works well for separating strong acid dissociable.
Solvent Extraction – This method involves the use of an organic solvent to solubilize the organic extractant and successfully remove the extractant, in this case, cyanide, from the sample.
Adsorption Methods
Minerals – Cyanide absorption is commonly a combination of two ways: precipitation interaction or ion exchange. Some examples of minerals that are effective in adsorbing cyanide are ilmenite, bauxite, zeolites, and feldspars.
Activated Carbon – Activated carbons have a high adsorptive ability due to its high surface area and high porosity. The only downside to the use of activated carbon is its nonselective nature.
Resins – Resins are polymers that have functional groups with either ion exchange or chelation capabilities. Metal-cyanide complexes are said to adsorb more strongly, but this is still dependent on what resin is used, and the pretreatments are done to both the resin and the solution.
Oxidation Methods
The oxidation method is the subsequent method used to destroy cyanide after the aforementioned process has been performed. Cyanide oxidation can be accomplished through the following:
Bio-oxidation – The chief mechanism occurs through the transformation of cyanide to a cyanate, which will further be converted to ammonium and bicarbonate ions through hydrolysis.
Catalysis – Adsorption of cyanide to activated charcoal in the presence of oxygen triggers the catalysis of cyanide to cyanate. This reaction is increased in the presence of copper. With excess copper, the hydrolysis of cyanate to ammonium and bicarbonate ions is also enhanced.
Electrolysis – This process has the same operating principles as electrowinning.
Chemical addition – The most popular method used for the destruction of cyanide is the addition of oxidants. Oxidants have high electron affinity and therefore strip off the electron from cyanide forming cyanate. The most common oxidants include ozone, oxygen, hydrogen peroxide, and sulfur dioxide.
Photolysis – This method makes use of the electromagnetic radiation obtained from the ultraviolet rays to catalyze electron transfer processes which lead to the destruction of cyanide.
