Hydrogen cyanide (HCN)

• Hydrogen cyanide (HCN) is widely used as an industrial chemical in the production of synthetic fibres, plastics and nitrites and as fumigant and rodenticide. It is released by the combustion of nitrogen containing plastics. It is an extremely toxic compound, rapidly producing symptoms, including headache, loss of consciousness, cardiac arrest and death.
• Exposure is typically by inhalation of HCN vapour. Ingestion and absorption do not occur unless individuals are in contact with HCN liquid or contaminated water. Liquid is also rapidly absorbed across the skin.
• Although cyanide salts are toxic, most inhalation poisoning results from exposure to HCN. Other cyanide compounds may also produce cyanide poisoning and some may be intensely irrigating.

WHAT IS CYANIDE?

• Cyanide (inorganic) compounds refer to a group of substances containing the cyanide ion (CN^-). CN^- forms strong complexes with metals such as cobalt, gold, iron and silver and weaker complexes with metals such as zinc. It is because of these properties that cyanide is used extensively within precious and base metal extraction and concentration.
• CN^- complexes are found in nature in some plants, blue green algae, bacteria and fungi.
• Cyanide salts are odorless when dry, however, when damp they may have a slight odor of hydrogen cyanide. Cyanide gas is highly flammable and in liquid form is both very volatile and flammable. Exposure of cyanides to strong oxidizers such as nitrates and chlorates may cause fires and explosions.
•  Cyanide compounds include cyanide salts, such as sodium cyanide (NaCN), potassium cyanide (KCN) and calcium cyanide (CaCN) and the gas hydrogen cyanide (HCN) also known as hydrocyanic gas or prussic acid.

WHAT IS HYDROGEN CYANIDE

• Hydrogen cyanide (CAS# 74-90-8) is a colourless or pale blue liquid or gas. 
• Hydrogen cyanide may be synthesized directly from ammonia and carbon monoxide or from ammonia, oxygen (or air) and natural gas. 
• It may also be prepared by reacting a cyanide salt with a strong acid, e.g. sulfuric acid or by thermal decomposition of formamide. Because impure hydrogen cyanide can undergo spontaneous explosive polymerization and decomposition, a small amount of stabilizer (usually phosphoric acid) is added to it.

CHEMICAL AND PHYSICAL PROPERTIES OF HCN

At normal temperature and pressure, HCN is a volatile, colourless gas or a yellow brown liquid. It has a boiling point of 26⁰ C and its melting point is –13.4 ⁰C. It is less dense than air and therefore, dispersed rapidly.

• It is a liquid at below 2 degrees C.
• Some people can detect a distinct odour of bitter almonds at concentrations between 0.6 to 4.4 ppm. The ability to detect the odour, however, is genetically determined and many people are unable to detect its presence.
• Potassium, sodium and calcium cyanides are white, deliquescent, non-combustible solids with faint bitter almond odour.
• Water solutions of hydrogen cyanide containing sulfuric acid as a stabilizer severely corrode steel above 40 ⁰C.
• Liquid hydrogen cyanide will attack some forms of plastics, rubber and coatings.
• Hydrogen cyanide is miscible with water and alcohol and slightly soluble in ether. It is incompatible or reactive with amines, acids, sodium hydroxide, calcium hydroxide, sodium carbonate, caustics and ammonia. It is intensely poisonous even when mixed with air and is highly toxic to all species in water.
• Synonyms are hydrocyanic acid, prussic acid, aero liquid HCN, carbon hydride nitride, formic anammonide and formonitrile.

 WHAT HAPPENS TO CYANIDE WHEN IT ENTERS THE ENVIRONMENT?

           

Cyanides are not persistent in water or soil. Cyanides may accumulate in bottom sediments, but residues are generally as low as <1mg/kg even near polluting sources. Majority of an accidental release of cyanide is volatilized to the atmosphere where it is quickly diluted and degraded by ultra violet. Other factors, such as biological oxidation, precipitation and the effects of sunlight also contribute to cyanide degradation. There is no evidence of bioaccumulation in the food chain and hence, secondary poisoning does not occur.

            Cyanides enter air, water and soil as a result of both natural processes and human industrial activities. In air, cyanide is present mainly as gaseous hydrogen cyanide. A small amount of cyanide in air is present as fine dust particles. This dust eventually settles over land and water. Rain helps remove cyanide particles from air. The gaseous hydrogen cyanide is not easily removed from the air by settling or rain. The half-life of hydrogen cyanide in the atmosphere is about 1 to 3 years. Most cyanide in surface water will form hydrogen cyanide and evaporate. Some cyanide in water will be transformed into less harmful chemicals by microorganisms or will form a complex with metals, such as iron. The half-life of cyanide in water is not known. Cyanide in water does not build up in the bodies of fish.

            Cyanide in soil can form hydrogen cyanide and evaporate. Some of the cyanide will be transformed into other chemical forms by microorganisms in soil. Some forms of cyanide remain in soil, but cyanide usually does not seep into underground water. However, cyanide has been detected in underground waters of a few landfills. At the high concentrations found in some landfill leachates (water that seeps through landfill soil), cyanide becomes toxic to soil microorganisms. Since these microorganisms can no longer change cyanide to other chemical forms, cyanide is able to pass through soil to underground water. Less is known about what happens to thiocyanates when they enter the environment. In soil and water, thiocyanates are changed into other chemical forms by microorganisms. At near normal temperatures (30⁰C), evaporation or sorption (binding to soil) does not seem to be important for thiocyanates in soil. 

CYANIDE USE

            

The predominant users of cyanide are the steel, electroplating, mining and chemical industries. The principal cyanide compounds used in industrial operations are potassium and sodium cyanide and calcium cyanide, particularly in metal leaching operations. Cyanides have well established uses as insecticides and fumigants; in the extraction of gold and silver ores; in metal cleaning; in the manufacture of synthetic fibers, various plastics, dyes, pigments and nylon; and as reagents in analytical chemistry. Cyanogen has been used as a high-energy fuel in the chemical industry and as a rocket or missile propellant; cyanogens and its halides are used in organic synthesis, as pesticides and fumigants, and in gold-extraction processes.

            As a commercially available product, hydrogen cyanide is sold as a gas and is also available as a technical grade liquid in concentrations of 5, 10 and 96-99.5%. Almost all grades of hydrogen cyanide contain a stabilizer such as phosphoric acid to prevent decomposition and explosion.

            Cyanide salts have various uses. The most significant applications of compounds included in this profile are uses in electroplating and metal treatment, as an anti-caking agent in road salts and in gold and silver extraction from ores. Minor applications include use as insecticides and rodenticides, as chelating agents and in the manufacture of dyes and pigments. Calcium cyanide is used as a cement stabilizer and has had limited use in rodent control and as a beehive fumigant. Formerly used as a polymerization catalyst and as an antifouling agent in marine paints, copper (1) cyanide continues to be used in plating baths for silver, brass and copper-tin alloy plating. Many metal polishes contain potassium or sodium cyanide. Potassium cyanide has a primary use in silver plating and is also used as a reagent in analytical chemistry. Potassium and sodium cyanide are used in combination for nitriding steel. One method of achieving hardened, weather-resistant metal surfaces uses a process known as cyaniding which involves heating the metal in a liquid solution of sodium cyanide, sodium chloride and sodium carbonate in the presence of atmospheric oxygen. Fumigation of fruit trees, railway cars and warehouses and treatment of rabbit and rat burrows and termite nests are included among the former uses for sodium cyanide.

           

HOW CYANIDE IS USED?

            

Cyanide combines with many organic and inorganic compounds. Because of its unique properties, cyanide is used in the manufacture of metal parts and numerous common organic products. About 1.4 million tonnes of HCN are produced annually worldwide, of which only about 20% is converted into sodium cyanide and mainly used in the extraction of precious metals such as gold, silver and others. The remaining 80% of the HCN is used in electroplating, metallurgy and in the production of a wide range of chemicals such as plastics, cosmetics, paints, pharmaceuticals, rocket propellant and others.   

HOW IS CYANIDE USED IN MINING?

            Cyanide is one of only a few chemical reagents that dissolve gold in water. It is a common industrial chemical that is readily available at a reasonably low cost. For both technical and economic reason, cyanide is the chemical of choice for the recovery of gold from ores. In gold mining, a dilute cyanide solution is sprayed on crushed ore that is placed in piles, commonly called heaps or mixed with ore in enclosed vats. The cyanide attaches to minute particles of gold to form a water-soluble, gold-cyanide compound from which the gold can later be recovered.

            Cyanide is used in a similar manner to extract silver from ores. In the extraction of non-precious metals such as copper, nickel, cobalt and molybdenum, cyanide is used in the milling and concentration processes to separate the desirable metals from the wastes. Consequently, cyanide and related compounds often are contained in mine tailings.  

CYANIDE EMISSIONS FROM MINING

1)    Hydrogen cyanide emissions to air

Cyanide emissions to air in the mining industry mainly result from the use of sodium cyanide in precious and base metals extraction. Cyanide may be released from process tanks and storage areas or from tailings waste streams and storage areas. Emitted cyanide is almost exclusively in the form of HCN.

            The rate of HCN release (volatilization) from cyanide solutions is largely governed by the pH of the solution. The lower the pH, the greater the rate of HCN evolution.

            The environmental hazards posed by emissions of HCN to air from mineral extraction are considered to be low for the following reasons:

·         Cyanide is not persistent in the environment but is degraded via oxidation to elemental nitrogen, carbon monoxide and oxides of nitrogen.

·         Ambient HCN levels around tailings storage facilities and above leach and adsorption tanks are monitored regularly and are typically less than 1 ppm at distances of 1 m or more from the source.

·         HCN is lighter than air so it tends to disperse rapidly into the upper atmosphere.

  2)    Cyanide in seepage

Seepage of tailings solutions from active tailings storage areas represents the most common form of release of cyanide (inorganic) compounds to land from mining facilities. Tailings are waste slurries produced from mineral extraction processing.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 shows far lower concentrations in seepage water reflecting strong retention of cyanide within the tailings.Emissions of cyanide to land in seepage water are reported as total cyanide but in reality it is only the weak acid dissociable (WAD) and free forms of cyanide that have any real environmental consequence. These can often make up only a small percentage of total cyanide.Cyanide recycling, destruction and neutralization are employed to ensure that cyanide levels in tailings are minimized.

CYANIDE REMEDIATION

           

Various procedures exist for treating cyanide and are comprised of several physical, adsorption, complexation and/or oxidation methods. These methods predominantly involve separation or destruction processes and may occur naturally. Separation processes include the physical, adsorption and complexation methods that are used to concentrate and thereby recover cyanide for recycling. On the other hand, destruction processes are used to severe the carbon-nitrogen triple bond thereby destroying the cyanide and producing non-toxic or less-toxic species.

1)    Physical methods

Physical methods for cyanide treatment can be accomplished using dilution, membranes, electrowinning and hydrolysis/distillation.

·    Dilution

Dilution is the only treatment method, which does not separate or destroy cyanide. This method involves combining a toxic cyanide waste with an effluent that is low in or free of cyanide to yield a wastewater below discharge limits. Consequently, dilution is simple and cheap and is often used as a stand-alone or back-up method to insure that discharge limits are satisfied. Dilution is usually considered to be unacceptable since the total amount of cyanide discharge is not altered and since naturally occurring processes such as adsorption and precipitation can attenuate and thereby concentrate the cyanide in ground and surface waters. 

·    Membranes

Cyanide can be separated from water using membranes with either electrodialysis or reverse osmosis. In electrodialysis, a potential is applied across two electrodes separated by a membrane permeable to cyanide. The cyanide solution requiring purification is placed in the half-cell containing the cathode or negative electrode. Cyanide, because it is negatively charged, will diffuse through the membrane and concentrate in the half-cell containing the anode or positive electrode. In reverse osmosis, pressure is applied to a cyanide solution needing treatment but, in this case, water is forced through a membrane impermeable to cyanide. Both of these methods have been shown to be applicable to solutions containing free and metal complexed cyanide. 

·    Electrowinning

Strong acid dissociable (SADs) and WADs can be reduced to corresponding metals by applying a potential across two electrodes immersed in the same solution:

M(CN)_x^y-x + ye^- à M + xCN^-                                       (1)

Thiocyanate does not respond. Free cyanide is liberated which makes solutions more amenable to other recovery and remediation processes. Because this electrowinning reaction involves the reduction of an anion at the cathode, a negatively charged electrode, metal recoveries and current efficiencies are low. This is compensated for by using steel wools as high surface area cathodes, increasing agitation to further increase mass transport, increasing solution temperatures, using appropriate solution pHs and conductivities, increasing metal concentrations if possible and redesigning the electrowinning cell of which four designs have been developed for gold processing. Furthermore, reaction (1) must be accompanied by a corresponding oxidation reaction, usually oxygen evolution, at the anode with an equivalent number of electrons, e^-, being donated:

Y/4.[4OH^- à 2H₂O + O₂ + 4e^- ]                              (2a)

Y/4.[2H₂O à 4H⁺ + O₂ + 4e^- ]                                (2b)

Electrowinning is predominantly used for gold processing. Electrowinning performs well in concentrated solutions; at dilute concentrations, hydrogen evolution predominates, possibly masking Reaction 1 completely:

                         2H⁺ + 2e^- à H_2(g)                                          (3)

Progress is continuing to make electrowinning technology economically viable to dilute solutions. Direct applications to cyanide remediation may then be possible. 

·     Hydrolysis/distillation

Free cyanide naturally hydrolyzes in water to produce aqueous hydrogen cyanide:

                      CN^- + H⁺ à HCN _(aq)                                       (4)

The aqueous hydrogen cyanide can then volatilize as hydrocyanic gas:

                           HCN _(aq) à HCN_(g)                        (5)

Because hydrocyanic gas has a vapor pressure of 100kPa at 26^oC, which is above that of water (34kPa at 26^oC), and a boiling point 79^oC, which is below that of water (100^oC), cyanide separation can be enhanced at elevated temperatures and/or reduced pressures. Distillation rates can also be increased by increasing the agitation rate, the air/solution ratio and the surface area at the air/solution interface. Hydrocyanic gas can be captured and concentrated for recycling in conventional absorption-scrubbing towers. It can also be vented to the open atmosphere and has been noted to occur naturally in tailings ponds, especially in warm and arid environments.

2)    Adsorption methods 

Minerals, activated carbons and resins adsorb cyanide from solution. Several types of contact vessels can be used for this purpose and include elutriation columns, agitation cells, packed-bed columns and loops. Once the cyanide is adsorbed, the material is separated from the solution by, for example, screening, gravity separation or flotation. The material is then placed into another vessel where the cyanide is desorbed into a low volume solution and thus concentrated. Finally, the material is separated again and in most cases, reactivated and recycled for further use. 

 Activated carbon

Active or activated carbons are typically prepared by partial thermochemical decomposition of carbonaceous materials- predominantly wood, peat, coal and coconut shells. Adsorption characteristics change with preparation method and material. Because activated carbons have high porosity and high surface area, adsorption capacities and rates are high. Adsorption, however, is not very selective; cations, anions and neutral species can be adsorbed simultaneously at various sites via ion exchange, solvation, chelation and coulombic interactions. Activated carbons are commonly used in packed-bed systems for treating wastewaters and gases. Applications to cyanide wastewaters have been reported with packed-bed systems and shown to be applicable at dilute cyanide concentrations, with increased adsorption of WADs and SADs with copper or silver pretreatment. In gold leaching with cyanide, packed-bed systems are usually avoided and have given rise to the use of continuous operations exemplified by carbon-in-column (CIC), carbon-in-leach (CIL) and carbon-in-pulp (CIP) processes. Although these processes have been designed for gold-cyanide adsorption, it does not preclude applying them for the remediation of free or other metal-cyanide complexes. High adsorption capacities and rates as well as strong ability for reactivation are important parameters for all activated carbons. 

Resins 

Resins are usually polymeric beads containing a variety of surface functional groups with either chelation or ion-exchange capabilities, somewhat similar to solvent extraction processes. They can be selective and have high adsorption capacities depending on whether the chelating or ion exchange properties are strong or weak. Resins that are deposited on substrates as thin films are predominantly used in packed-bed systems, whereas resins that do not require a substrate are mostly used in continuous processes like those used with activated carbons for gold. Gold-blatt developed the first resin column for cyanide recovery. Metal-cyanide complexes have since been found to adsorb more strongly but this adsorption is dependent on which resin is being used and how the solution and/or resin are pretreated. Thiocyanate appears to adsorb weakly. In a comparison of resins to activated carbons, resins can be more cost effective since they resist organic fouling, regenerate more efficiently, have longer life and desorbs faster. At present, the only resin being used in industry for cyanide recovery is the chelation resin. 

3)    Treatment with hydrogen peroxide. 

While hydrogen peroxide, H₂O₂ will oxidize free cyanide; it is common to catalyze the reaction with a transition metal such as soluble copper, vanadium, tungsten or silver in concentrations of 5 to 50 mg/l. The oxidation requires 1.26 lbs H₂O₂ per lb cyanide and is described as follows: 

CN^- + H₂O₂ à CNO^- + H₂O (pH 9-10/ catalyst)

This simple system is adequate for treating both free cyanide and some weak acid dissociable cyanides such those complexed with zinc, copper or cadmium. Less reactive cyanides such as those complexed with nickel or silver may require addition of a chelating agent to encourage dissociation. Inert cyanides such as ferricyanides can only be destroyed by photoactivation (using UV- H₂O₂).

With any peroxygen system, a pH of 9-10 should be maintained if cyanide is present to avoid release of hydrogen cyanide gas. Reaction rates can be increased by several means: raising the temperature, increasing catalyst dose and/or using excess H₂O₂. For example, 25 degrees C and without catalysis, the conversion of free cyanide to cyanate takes two to three hours; and at 50 degrees C, one hour or less. The inclusion of 10 mg/l Cu will increase the rate 2-3 fold, while a 20% excess of H₂O₂ will increase the rate by about 30%.

As with alkaline chlorination, the product of the H₂O₂ reaction is cyanate (CNO^-), which is 1000 times less toxic than cyanide and is often acceptable for discharge. Alternatively, cyanate can be destroyed through acid hydrolysis, forming carbon carbon dioxide and ammonia. The equation is: 

CNO^- + 2H₂O à CO₂ + NH₃ + OH^- (acid)

           

The lower the pH, the faster the hydrolysis. At pH 2, CNO^- is hydrolyzed in 5 minutes, at pH 5, 60 minutes and at pH 7, 22 hours. 

4) Treatment with peroxymonosulfuric acid

If the rate of cyanide oxidation is important, peroxymonosulfuric acid (Caro’s acid) is recommended. Caro’s acid is an equilibrium product formed from H₂O₂ and sulfuric acid and is typically produced onsite using a compact, modular generator:

                                  

H₂O₂ + H₂SO₄ <==> H₂SO₅ + H₂O

This process is used at some of the world’s largest gold refining operations. For smaller scale operations, Caro’s acid can be prepared through hydrolysis of ammonium persulfate:

(NH₄)₂S₂O₆ + H₂O à NH₄HSO₄ + NH₄HSO₅

(H₂SO₄/ steam)

With Caro’s acid, the conversion of cyanide to cyanate is complete in a few minutes, according to the following equation:

CN^-  + H₂SO₅ à CNO^- + H₂SO₄

(pH 10)

The additions of excess Caro’s acid will hydolize the cyanate to carbonate and nitrogen in the same step:

2OH^- + CNO^- + 3SO₅^2- à 2CO₃^2- + N₂ + 3SO₄^2- + H₂O

Under acidic conditions, a smaller amount of Caro’s acid will be needed since cyanate hydrolysis (second reaction below) is greatly accelerated. The equations are as follow:

2H⁺ + 2CNO^- + 3H₂SO₅ à 2CO₂ + N₂ + 3H₂SO₄ + H₂O

CNO^- + 2H₂O à CO₂ + NH₃ + OH^-

      5)   Rhodacs Process

Crude gas produces in the manufacturer of coal gas (COG) contains hydrogen cyanide. If it is released directly, it would contaminate air or cause corrosion on industrial facilities. Rhodacs process can remove hydrogen cyanide in the crude gas efficiently and at low cost.

By scrubbing crude gas containing hydrogen cyanide with sulfur suspended alkalic solution, hydrogen cyanide is removed in the form of thiocyanate, which is non-toxic. It can be applied on purification for coal gas or oil gas. The advantages of this process are:

·         Decyanization efficiency can be adjusted in the design up to 100% according to the customer’s requirement.

·         When H₂S, NH₃ and HCN are contained simultaneously in the process, gas, toxic substances can be removed through chemical reactions of these compounds, thus improving the process economy.

·         Compact system.

·         Easy operation and maintenance.

SUMMARY

            Hydrogen cyanide is released by the combustion of nitrogen containing plastics and is an extremely toxic compound. Cyanide compounds refer to a group of substances containing the cyanide ion that found in nature in some plants, blue green algae, bacteria and fungi. Cyanide gas is highly flammable and in liquid form is both very volatile and flammable. Hydrogen cyanide is a colourless or pale blue liquid or gas. It may be synthesized directly from ammonia and carbon monoxide or from ammonia, oxygen and natural gas. At normal temperature and pressure, hydrogen cyanide is a volatile, colourless gas or a yellow brown liquid. Its boiling point is at 26⁰C and its melting point is at –13.4⁰C.  The ability to detect the odour, however, is genetically determined and many people are unable to detect its presence. Hydrogen cyanide is miscible with water and alcohol and slightly soluble in ether. It is intensely poisonous even when mixed with air and is highly toxic to all species in water. Synonyms are hydrocyanic acid, prussic acid, carbon hydride nitride, formic anammonide and formonitrile. Cyanides are not persistent in water or soil and may accumulate in bottom sediments. In air, cyanide is present mainly as gaseous hydrogen cyanide. Most cyanide in surface water will form hydrogen cyanide and evaporate. Cyanide in soil can form hydrogen cyanide and evaporate too. The predominant users of cyanide are the steel, electroplating, mining and chemical industries. In gold mining, a dilute cyanide solution is sprayed on crushed ore that is placed in piles. Cyanide and related compounds often are contained in mine tailings. 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 hydrogen cyanide evolution. Various procedures exist for treating cyanide and are comprised of several physical, adsorption, complexation and/or oxidation methods. 

REFERENCES:

1.    Cyanide Remediation. C.A Young and T.S Jordan. Department of Metallurgical Engineering, Montana Tech.

2.    http://www.mineralswa.asn.au/

3.    http://www.nsc.org/library/chemical/Hydrocya.htm.

4.    http://www.infoplease.com

5.    http://www.bartleby.com

6.    http://nett21.unep.or.jp

7.    http://www.h2o2.com