OVERVIEW OF CYANIDE TREATMENT METHODS
Michael M. Botz Elbow Creek Engineering, Inc. [email protected]
ABSTRACT cyanide is also used in low concentrations as a flotation reagent for the recovery of base metals An integral and key component of many water such as copper, lead and zinc. At these management systems at mining sites is the operations, cyanide treatment systems may be approach adopted to manage cyanide-containing required to address potential toxicity issues in solutions and slurries. Excluding the bulk regard to wildlife, waterfowl and/or aquatic life. storage of cyanide reagents such as sodium This may include the removal of cyanide from cyanide, most cyanide present at mining sites one or more of the following: will be present in water solutions. Therefore, to a great extent the management of water and the Slurry tailings from milling operations management of cyanide can be considered as one and the same and should be simultaneously Bleed solution from Merrill-Crowe considered when developing water management operations and cyanide management plans. Excess solution from heap or vat leaching All mining sites that utilize cyanide for metals operations recovery should have a comprehensive and well- maintained cyanide management plan. A good Supernatant solution from tailings cyanide management plan will include impoundments descriptions of how cyanide-containing solutions and slurries are to be handled, stored, contained Seepage collected from ponds or tailings and monitored, and in many cases the plan will impoundments also include a description of treatment plants used to remove cyanide from solutions or The term ‘cyanide’ refers to one of three slurries. This article is included in this special classifications of cyanide, and it is critical to edition of Mining Environmental Management to define the class of cyanide that is to be removed describe cyanide treatment processes that have in a treatment plant. The three classes of cyanide been successfully proven on a full-scale at a are: (1) total cyanide; (2) weak acid dissociable number of mining sites worldwide. (WAD) cyanide; and (3) free cyanide as shown
in Figure 1. BACKGROUND
1 Overview of Cyanide Treatment Methods
Strong Metal-Cyanide Complexes of Fe
Weak and Moderately Strong Total Metal-Cyanide Complexes of Cyanide WAD Ag, Cd, Cu, Hg, Ni and Zn Cyanide Free CN- Cyanide HCN
Figure 1 Classifications of Cyanides
The appropriate selection for most situations is CYANIDE DESTRUCTION WAD cyanide since this includes the toxicologically important forms of cyanide, Most cyanide destruction processes operate on including free cyanide and moderately and the principle of converting cyanide into a less weakly complexed metal-cyanides. Total toxic compound through an oxidation reaction. cyanide includes free cyanide, WAD cyanide There are several destruction processes that are plus the relatively non-toxic iron-cyanide well proven to produce treated solutions or complexes. slurries with low levels of cyanide as well as metals. Cyanide treatment processes are classified as either a destruction-based process or a recovery- INCO Sulfur Dioxide/Air based process. In a destruction process, either The sulfur dioxide (SO2) and air process was chemical or biological reactions are utilized to developed by INCO Limited in the 1980’s and is convert cyanide into another less toxic currently in operation at over thirty sites compound. Recovery processes are a recycling worldwide. The process utilizes SO2 and air in approach in which cyanide is removed from the the presence of a soluble copper catalyst to solution or slurry and then re-used in a oxidize cyanide to the less toxic compound metallurgical circuit. cyanate (OCN-).
- Cu 2 Catalyst Selection of an appropriate cyanide treatment SO2 + O2 + H2O + CN - -2 + process involves the consideration of many OCN + SO4 + 2H factors, but generally the number of candidate processes for a particular application can be The primary application of the sulfur dioxide and narrowed following an inspection of the air process is with slurry tailings, but it is also untreated solution/slurry chemistry and the effective for the treatment of solutions for the desired level of treatment. In the following oxidation of free and WAD cyanides. Iron sections, several destruction and recovery cyanides are removed in the process through processes are discussed along with their typical precipitation of insoluble copper-iron-cyanide areas of application. With all of these processes, complexes. laboratory and/or pilot testing is required to confirm the level of treatment achievable and to evaluate the associated reagent consumptions.
2 M. Botz Overview of Cyanide Treatment Methods
The theoretical usage of SO2 in the process is The primary application of the hydrogen - 2.46 grams SO2 per gram of CN oxidized, but in peroxide process is with solutions rather than practice the actual usage ranges from about 3.5 slurries due to the high consumption of hydrogen - to 4.5 grams SO2 per gram of CN oxidized. The peroxide in slurry applications. The process is SO2 required in the reaction can be supplied typically applied to treat relatively low levels of either as liquid sulfur dioxide or as sodium cyanide to achieve cyanide levels that may be metabisulfite (Na2S2O5). suitable for discharge. The hydrogen peroxide process is effective for the treatment of solutions Oxygen (O2) is also required in the reaction and for the oxidation of free and WAD cyanides, and this is generally supplied by sparging iron cyanides are removed through precipitation atmospheric air into the reaction vessels. The of insoluble copper-iron-cyanide complexes. As reaction is typically carried out at a pH of about indicated in the above reaction, hydrogen 8.0 to 9.0 in one ore more agitated tanks, and peroxide reacts to form cyanate and water, a lime is added to neutralize the acid (H+) formed process which limits the build-up of dissolved in the reaction to maintain the pH in this range. solids in the solution being treated. Lime usage is generally on the order of about 3.0 - to 5.0 grams per gram of CN oxidized. As The theoretical usage of H2O2 in the process is +2 - indicated, copper (Cu ) is required as a catalyst, 1.31 grams H2O2 per gram of CN oxidized, but which is usually added as a solution of copper in practice the actual usage ranges from about - sulfate (CuSO4-5H20) to provide a copper 2.0 to 8.0 grams H2O2 per gram of CN oxidized. concentration in the range of about 10 to 50 The H2O2 used in the process is typically mg/L. Upon completion of the indicated provided as a liquid in 50% strength. reaction, metals previously complexed with cyanide, such as copper, nickel and zinc, are The reaction is typically carried out at a pH of precipitated as metal-hydroxide compounds. about 9.0 to 9.5 for optimal removal of cyanide and metals such as copper, nickel and zinc. This process is capable of achieving low levels However, if iron cyanide must also be removed of both cyanide and metals. Generally, the best to low levels, then the pH is lowered somewhat application of this process is with slurries to increase the precipitation of copper-iron- containing low to moderately high initial levels cyanides at the expense of lowering the removal of cyanide when treated cyanide levels of less efficiencies of copper, nickel and zinc. As than about 1 to 5 mg/L are required. In some indicated, copper (Cu+2) is required as a soluble cases, solutions treated with this process may be catalyst, which is usually added as a solution of of suitable quality to permit their discharge. copper sulfate (CuSO4-5H20) to provide a copper concentration in the range of about 10% to 20% Hydrogen Peroxide of the initial WAD cyanide concentration. Upon The hydrogen peroxide treatment process completion of the indicated reaction, metals chemistry is similar to that described for the previously complexed with cyanide, such as INCO process, but hydrogen peroxide is utilized copper, nickel and zinc, are precipitated as rather than sulfur dioxide and air. With this metal-hydroxide compounds. process, soluble copper is also required as a catalyst and the end product of the reaction is cyanate.
- Cu 2 Catalyst - H2O2 + CN OCN + H2O
3 M. Botz Overview of Cyanide Treatment Methods
This process is capable of achieving low levels cyanide is converted to cyanogen chloride of both cyanide and metals. Generally, the best (CNCl) and the second step in which cyanogen application of this process is with solutions chloride hydrolyzes to yield cyanate. containing relatively low initial levels of cyanide - - when treated cyanide levels of less than about 1 Cl2 + CN CNCl + Cl mg/L are required. Oftentimes, solutions treated - - + with this process may be of suitable quality to CNCl + H2O OCN + Cl + 2H permit their discharge. In the presence of a slight excess of chlorine, Caro’s Acid cyanate is further hydrolyzed to yield ammonia Peroxymonosulfuric acid (H2SO5), also known in a catalytic type of reaction. as Caro’s acid, is a reagent used in a recently developed cyanide treatment process that has - Cl2 Catalyst OCN + 3H2O found application at a few sites. + - - NH4 + HCO3 + OH
- - -2 + H2SO5 + CN OCN + SO4 + 2H If sufficient excess chlorine is available, the reaction continues through ‘breakpoint Caro’s acid used in the process must be produced chlorination’ in which ammonia is fully oxidized on-site using sulfuric acid and hydrogen to nitrogen gas (N2). peroxide since Caro’s acid decomposes rather quickly (Norcross, 1996). Caro’s acid is used in + - + 3Cl2 + 2NH4 N2 + 6Cl + 8H slurry treatment applications where the addition of a copper catalyst is not desirable, which is In addition to reacting with cyanide, cyanate and typically only in situations where the sulfur ammonia, the alkaline chlorination process will dioxide and air process is not suited. In solution oxidize thiocyanate, which in some cases can applications, other destruction processes, such as lead to excessively high consumptions of the hydrogen peroxide process, are preferred to chlorine. the Caro’s acid process. - 4Cl2 + SCN + 5H2O The theoretical usage of H2SO5 in the process is -2 - - + SO4 + OCN + 8Cl + 10H 4.39 grams H2SO5 per gram of cyanide oxidized, but in practice 5.0 to 15.0 grams H2SO5 per gram The primary application of the alkaline of cyanide oxidized is required. Acid produced + chlorination process is with solutions rather than in the reaction (H ) is typically neutralized with slurries due to the high consumption of chlorine lime. in slurry applications. The process is typically applied to treat low solutions flows initially Alkaline Chlorination containing low to high levels of cyanide to Alkaline chlorination at one time was the most achieve cyanide levels that may be suitable for widely applied of the cyanide treatment discharge. The process is effective for the processes, but it has gradually been replaced by treatment of solutions for the oxidation of free other processes and is now only used and WAD cyanides, but a lesser amount of iron occasionally. Alkaline chlorination is effective cyanides are removed depending on the levels of at treating cyanide to low levels, but the process other base metals in the solution being treated. can be relatively expensive to operate due to As can be seen in the above reactions, a high reagent usages. The cyanide destruction significant increase in the treated water dissolved reaction is two-step, the first step in which
4 M. Botz Overview of Cyanide Treatment Methods
solids concentration may result, particularly with cyanide (MXFeY(CN)6) compounds such as those chloride. with copper or zinc (Adams, 1992).
+2 - + The theoretical usage of Cl2 to oxidize cyanide to Fe + 6CN + ¼O2 + H - -3 cyanate is 2.73 grams Cl2 per gram of CN Fe(CN)6 + ½H2O oxidized, but in practice the actual usage ranges - +2 -3 + from about 3.0 to 8.0 grams Cl2 per gram of CN 4Fe + 3Fe(CN)6 + ¼O2 + H oxidized. The Cl2 used in the process can be Fe4[Fe(CN)6]3 + ½H2O provided as a liquid Cl2 or as a 12.5% solution of sodium hypochlorite (NaOCl). Chlorine The iron-cyanide precipitation process is limited consumptions for the oxidation of ammonia and in its suitability to situations where the thiocyanate can be calculated from the above precipitation reactions can be controlled and the reactions, which may significantly add to precipitated solids can be separated and properly chlorine required in the process. In addition, the disposed. In the past, this process was widely above reactions generate varying amounts of used to convert free and WAD cyanides to less + acid (H ) which is typically neutralized by toxic iron-cyanide compounds, but its present adding lime or sodium hydroxide to the reaction utility is primarily as a polishing process to vessels. reduce total cyanide concentrations to less than about 1 to 5 mg/L. The reaction is carried out at a pH of greater than 10.0 to ensure cyanogen chloride is completely The process is optimally carried out at a pH of hydrolyzed to cyanate. An advantage of the about 5.0 to 6.0 and iron is added as ferrous process is that copper is not required as a catalyst sulfate (FeSO4-7H2O). Ferrous sulfate usage as with the sulfur dioxide/air and hydrogen ranges from about 0.5 to 5.0 moles Fe per mole peroxide processes. Upon completion of the of CN- depending on the desired level of cyanide oxidation reaction, metals previously treatment (Adams, 1992 and Dzombak et al., complexed with cyanide, such as copper, nickel 1996). and zinc, are precipitated as metal-hydroxide compounds. Activated Carbon Polishing Activated carbon has a relatively high affinity for This process is capable of achieving low levels many metal-cyanide compounds, including the of both cyanide and metals. Generally, the best soluble cyanide compounds of copper, iron, application of this process is with low flows of nickel and zinc. Activated carbon is suitable for solutions containing high to low initial levels of use as a polishing treatment process to remove cyanide when treated cyanide levels of less than cyanide to relatively low levels. This is a simple about 1 mg/L are required. Oftentimes, solutions and effective process, convenient for installation treated with this process may be of suitable at sites where activated carbon is used in quality to permit their discharge. metallurgical processes for precious metals recovery. Iron-Cyanide Precipitation Free, WAD and total cyanides will all react with Biological Treatment ferrous iron to yield a variety of soluble and Biological treatment processes have recently insoluble compounds, primarily become more widespread in the mining industry -3 hexacyanoferrate (III) (Fe(CN)6 ), Prussian blue due to the success of the plant installed at (Fe4[Fe(CN)6]3) and other insoluble metal-iron- Homestake Lead, USA in the 1980’s (Mudder et al., 1998a). In this plant, an aerobic attached
5 M. Botz Overview of Cyanide Treatment Methods growth biological treatment is used to remove acidification-volatilization-reneutralization cyanide, thiocyanate, cyanate, ammonia and (AVR) and Cyanisorb processes, remove cyanide metals from tailings impoundment decant from solution as hydrogen cyanide gas. At a pH solution prior to discharge into a trout fishery. of less than about 8.0, free cyanide and some The plant has been operating successfully for WAD cyanide compounds are converted to over fifteen years, producing high-quality hydrogen cyanide gas, which can then be air- effluent. stripped from solution. Once removed from solution as hydrogen cyanide gas, the hydrogen A biological treatment plant was installed by cyanide is easily absorbed into an alkaline Homestake Nickel Plate, Canada in the mid- solution of sodium hydroxide. 1990’s to treat tailings impoundment seepage (Given, 2001). This plant is a suspended sludge The three main reactions involved with the system with both aerobic and anaerobic cyanide recovery process are as follows. treatment sections to remove cyanide, - -2 thiocyanate, cyanate, ammonia, nitrate and 2CN + H2SO4 2HCN(aq) + 2SO4 metals. (acidification)
Another passive biological treatment process was HCN(aq) HCN(g) developed for the Homestake Santa Fe, USA (stripping) mine to treat draindown from the decommissioned heap leach operation (Mudder HCN(g) + NaOH NaCN + H2O et al., 1998b). This process, known as the (absorption) Biopass process, is suitable for solution flows of 3 less than about 10 m /hour for the removal of This basic process has been used at about ten cyanide, thiocyanate, cyanate, ammonia, nitrate sites worldwide to affect WAD cyanide and metals. recoveries ranging from about 70% to over 95% with both slurries and solutions (Moura, 2001; The applicability of biological processes for the Goldstone & Mudder, 1998; Botz et al., 1995 treatment of cyanide solutions in the mining and Omofoma and Hampton, 1992). Its industry is somewhat limited, but in certain application is primarily with solutions or slurries situations they are the optimal selection. Their with moderate to high concentrations of cyanide applicability is primarily in with continuous under both high and low-flow conditions. The solution flows with temperatures above about presence of dissolved copper in untreated o 10 C. solution can cause difficulties with conventional cyanide recovery processes, and in some cases pre-treatment of the solution with sodium sulfide CYANIDE RECOVERY (Na2S) may be required to precipitate copper sulfide (Cu2S) prior to cyanide recovery. A number of cyanide recovery processes have been investigated over the previous one hundred Water Recycle years, but only two have found widespread Cyanide can effectively be recovered and re-used application. by recycling cyanide-containing solutions within a metallurgical circuit. This is commonly Stripping and Recovery conducted using tailings thickeners or tailings The stripping and recovery approach to filters to separate solution from tailings solids, recovering cyanide, also known as the with the solution being recycled in the grinding
6 M. Botz Overview of Cyanide Treatment Methods and/or leaching circuits. This approach to are accurate under a wide variety of weather, recovering cyanide should be evaluated for all pond/impoundment geometry and chemistry operations utilizing cyanide, and its performance conditions. can be determined by a simple mass balance calculation. An example of the use of natural attenuation would be with a cyanide recovery facility where solution or slurry with a cyanide concentration of NATURAL CYANIDE ATTENUATION about 20 mg/L is placed into a lined pond or impoundment. The lined pond or impoundment It is well known that cyanide solutions placed in could then be designed to maximize natural ponds or tailings impoundments undergo natural attenuation such that the resultant cyanide attenuation reactions which result in the lowering concentration would be less than about 1 mg/L. of the cyanide concentration. These attenuation reactions are dominated by natural volatilization of hydrogen cyanide, but other reactions such as SUMMARY oxidation, hydrolysis, photolysis and precipitation also occur. Natural cyanide As indicated in the previous discussion, there are attenuation occurs with all cyanide solutions over ten cyanide treatment processes that have exposed to the atmosphere, whether intended or been successfully used worldwide for cyanide not. At several sites, ponds or tailings removal at mining operations. The key to impoundments are intentionally designed to successful implementation of these processes is maximize the rate of cyanide attenuation, and in by considering the following: some cases resultant solutions are suitable for discharge. Site water and cyanide balances under both average and extreme climatic conditions Two approaches have been developed to predict the rate of cyanide attenuation in ponds and The range of cyanide treatment processes tailings impoundments. The first method is available and their ability to be used empirical in nature and uses experimentally individually or in combination to achieve derived rate coefficients to estimate the rate of treatment objectives attenuation using a first order decay equation (Simovic et al., 1985). This approach is Proper testing, design, construction, relatively simple to apply, but its applicability at maintenance and monitoring of both water a given site must be verified by conducting field management and cyanide management testwork and the results may not be accurate facilities under changing weather, pond/impoundment geometry or chemistry conditions. By carefully considering these aspects of water and cyanide management before, during and The second approach to modeling natural after mine operation, operators can reduce the cyanide attenuation was developed by Botz and potential for environmental impacts associated Mudder (2000). This approach utilizes detailed with the use of cyanide. solution chemical equilibria and kinetic calculations to predict the rate of cyanide losses Another aspect of cyanide treatment to be from ponds and impoundments through a variety considered is the potential environmental impact of reactions. This approach can be time- of the cyanide related compounds cyanate, intensive to apply at a given site, but the results thiocyanate, ammonia and nitrate. These compounds may be present in mining solutions
7 M. Botz Overview of Cyanide Treatment Methods to varying extents and may require treatment if Table 1 provides a simplified summary of the water is to be discharged. Each of these cyanide general applications of various treatment related compounds is affected differently in the technologies for the removal of cyanide and its treatment processes discussed and this should be related compounds cyanate, thiocyanate, considered when evaluating cyanide treatment ammonia and nitrate. This table represents a alternatives for a given site. very simplified summary, but can be used as a conceptual screening tool when evaluation cyanide treatment processes.
Table 1 Preliminary Selection Guide for Cyanide Treatment Processes Iron WAD Slurry Solution Treatment Process Cyanide Cyanide Application Application Removal Removal SO2/Air Hydrogen Peroxide Caro’s Acid Alkaline Chlorination Iron Precipitation Activated Carbon Biological Cyanide Recovery Natural Attenuation
8 M. Botz Overview of Cyanide Treatment Methods
REFERENCES Moura, W., AngloGold South America, Private Communication, 2001. Adams, M.D., “The Removal of Cyanide from Aqueous Solution by the Use of Ferrous Mudder, T.I., Fox, F., Whitlock. J., Fero, T., Sulphate”, Journal of the South African Institute Smith, G. Waterland. R. and Vietl, J., Mining & Metallurgy, Vol. 92, No. 1, pp. 17-25, “Biological Treatment of Cyanidation January, 1992. Wastewaters: Design, Startup, and Operation of a Full Scale Facility”, In The Cyanide Botz, M.M. and Mudder, T.I., “Modeling of Monograph, Mining Journal Books Limited, Natural Cyanide Attenuation in Tailings London, 1998a. Impoundments”, Minerals and Metallurgical Processing, Vol. 17, No. 4, pp. 228-233, Mudder, T.I., Miller, S., Cox, A., McWharter, D. November, 2000. and Russell, L., “The Biopass System: Phase I Laboratory Evaluation”, In The Cyanide Botz, M.M. and Mudder, T.I., “Mine Water Monograph, Mining Journal Books Limited, Treatment with Activated Carbon”, Proceedings London, 1998b. Randol Gold Forum, pp. 207-210, 1997. Norcross, R., “New Developments in Caro’s Botz, M.M., Stevenson, J.A., Wilder, A.L., Acid Technology for Cyanide Destruction”, Richins, R.T., Mudder, T.I. and Burdett, B., Proceedings of Randol Gold Forum, pp. 175- “Cyanide Recovery & Destruction”, Engineering 177, 1996. & Mining Journal, pp. 44-47, June, 1995. Omofoma, M.A. and Hampton, A.P., “Cyanide Dzombak, D.A., Dobbs, C.L., Culleiton, C.J., Recovery in a CCD Merrill-Crowe Circuit: Pilot Smith, J.R. and Krause, D., “Removal of Testwork of a Cyanisorb Process at the NERCO Cyanide from Spent Potlining Leachate by Iron DeLamar Silver Mine”, Proceedings Randol Cyanide Precipitation”, Proceedings WEFTEC Gold Forum, pp. 359-365, 1992. 69th Annual Conference & Exposition, Dallas, Texas, 1996. Simovic, L., Snodgrass, W., Murphy, K. and Schmidt, J. "Development of a Model to Given, B., Homestake Nickel Plate Mine, Private Describe the Natural Attenuation of Cyanide in Communication, 2001. Gold Mill Effluents", Cyanide and the Environment Vol. II, Colorado State University, Goldstone, A., and Mudder, T.I., “Cyanisorb pp. 413-432, 1985. Cyanide Recovery Process Design, Commissioning and Early Performance”, In The Smith, A.C.S. and Mudder, T.I., Chemistry and Cyanide Monograph, Mining Journal Books Treatment of Cyanidation Wastes, Mining Limited, London, 1998. Journal Books Limited, London, 1991.
Ingles, J. and Scott, J.S., State-of-the-Art of Processes for the Treatment of Gold Mill Effluents, Environment Canada, March, 1987.
9 M. Botz Overview of Cyanide Treatment Methods
By Mr. Michael M. Botz, M.S., P.E. Senior Process Engineer Elbow Creek Engineering, Inc. [email protected]
References to this article should cite:
Botz, M.M., “Overview of Cyanide Treatment Methods’, Mining Environmental Management, Mining Journal Ltd., London, UK, pp. 28-30, May 2001.
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10 M. Botz