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Selection of a cyanide treatment process requires consideration of three primary criteria:
Characteristics of the Stream to be Treated
w Solution or slurry
w Flow rate (maximum, minimum, average)
w Intermittent versus continuous treatment
w Untreated solution chemistry (pH, temperature, cyanide and metals levels, etc.)
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Treatment Goals
w Discharge location (surface water, groundwater, impoundment, land application, process recycle)
w Hydrograph controlled releases for surface water discharges
w Free versus WAD versus total cyanide removal
w Metals and cyanide related compounds removals
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Treatment Process Performance
w Capability of the process to consistently and reliably meet treatment goals
w Treatment process construction and operating costs
w Availability of required reagents, fuels and power
w Availability of sufficiently skilled treatment plant operators
w Ability to further treat or dispose treatment by-products
The following chart can be used as an initial guide in selecting candidate cyanide treatment processes. Click on the Treatment Process links for further information about the processes.
* Most often iron-cyanide removal occurs through metal-cyanide complex precipitation. With natural attenuation, iron-cyanide is photolytically degraded.
Further information regarding many of these cyanide treatment processes can be found in the following free publication.
Overview of Cyanide Treatment Methods
Download a recently published technical paper that discusses the most widely used methods to treat and recover cyanide in the mining industry. Download Now (155 kB)
Brief Cyanide Treatment Process Descriptions
SO2/Air
The SO2/Air process utilizes sulfur dioxide and air in the presence of a soluble copper catalyst to oxidize cyanide to the less toxic compound cyanate (OCN-).
SO2 + O2 + H2O + CN- → OCN- + SO4-2 + 2H+
Metals previously complexed with cyanide, such as copper, nickel and zinc, are precipitated as metal-hydroxide compounds. Iron cyanide removal is affected through precipitation as a copper-iron-cyanide complex.
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Hydrogen Peroxide
The hydrogen peroxide process utilizes hydrogen peroxide in the presence of a soluble copper catalyst to oxidize cyanide to the less toxic compound cyanate (OCN-).
H2O2 + CN- → OCN- + H2O
Metals previously complexed with cyanide, such as copper, nickel and zinc, are precipitated as metal-hydroxide compounds. Iron cyanide removal is affected through precipitation as a copper-iron-cyanide complex.
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Caro's Acid
The Caro's acid process utilizes peroxymonosulphuric acid (H2SO5), also known as Caro's acid, to oxidize cyanide to the less toxic compound cyanate (OCN-).
H2SO5 + CN- → OCN- + SO4-2 + 2H+
This process may require the addition of a soluble copper catalyst to improve the reaction efficiency and rate.
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Alkaline Chlorination
The alkaline chlorination process consists of two-steps, the first in which cyanide is converted to cyanogen chloride (CNCl) and the second in which cyanogen chloride hydrolyzes to yield cyanate.
Cl2 + CN- → CNCl + Cl-
CNCl + H2O → OCN- + Cl- + 2H+
In the presence of a slight excess of chlorine at alkaline pH, cyanate is further hydrolyzed to yield ammonia in a catalytic reaction.
OCN- + 3H2O → NH4+ + HCO3- + OH-
If sufficient excess chlorine is available, the reaction continues through "breakpoint chlorination" in which ammonia is oxidized to nitrogen gas (N2).
3Cl2 + 2NH4+ → N2 + 6Cl- + 8H+
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Iron Precipitation
Free, WAD and total cyanides will all react with ferrous iron to yield a variety of soluble and insoluble compounds, primarily hexacyanoferrate (III) (Fe(CN)6-3), Prussian blue (Fe4[Fe(CN)6]3) and other insoluble metal-iron-cyanide (MXFeY(CN)6) compounds such as those of copper or zinc (Adams 1992).
Fe+2 + 6CN- + ĵO2 + H+ → Fe(CN)6-3 + ½H2O
4Fe+2 + 3Fe(CN)6-3 + ĵO2 + H+ → Fe4[Fe(CN)6]3 + ½H2O
These reactions act to lower the free and WAD cyanide concentrations by converting them to stable iron cyanide compounds (soluble and insoluble), while the iron-cyanide concentration is lowered as a result of precipitation reactions.
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Activated Carbon
Activated carbon has an affinity for many metal-cyanide compounds, including the soluble cyanide compounds of copper, iron, nickel and zinc. Activated carbon is suitable for use as a polishing process to remove cyanide to low levels when the initial cyanide concentration is already low.
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Biological
Cyanide can be aerobically treated in a biological system, using either suspended or attached growth process configurations. The biologically mediated cyanide oxidation reaction is:
CN- + ½O2 + 3H2O → HCO3-
+ NH4+ + OH-
Other cyanide related compounds can also be biologically treated in either aerobic or anaerobic systems. The stable iron-cyanide compounds are not biologically oxidized in this process, though a small portion may be sorbed into biomass.
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Cyanide Recovery
The stripping and absorption approach to recovering cyanide, also known as the acidification-volatilisation-reabsorption (AVR) and Cyanisorb processes, remove cyanide from solution as hydrogen cyanide gas. The three main reactions involved with the process are:
2CN- + H2SO4 → 2HCN(aq) + 2SO4-2 (acidification)
HCN(aq) → HCN(g) (volatilisation)
HCN(g) + NaOH → NaCN + H2O (absorption)
This process can affect WAD cyanide recoveries ranging from about 70% to over 95% with both slurries and solutions.
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Reverse Osmosis
Reverse osmosis is a membrane process, primarily used for the production of high purity water, such as for boiler feedwater preparation. The process has found application in treating cyanide solutions, where very low concentrations of cyanide and other constituents are required. The primary issue of concern is handling, treatment and disposal of the waste brine generated in the process.
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Natural Attenuation (Natural Degradation)
Cyanide solutions placed in open ponds or impoundments undergo natural attenuation reactions which result in the lowering of the cyanide concentration. These attenuation reactions are dominated by natural volatilization of hydrogen cyanide, but other reactions such as biological degradation, oxidation, hydrolysis, photolysis and precipitation also occur.
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