Cyanide Treatment: Physical, Chemical and ...

Chapter 36 Cyanide treatment: Physical, chemical and biological processes

M.M. Botza, T.I. Mudderb, and A. Akcilc a

Elbow Creek Engineering, Billings, Montana, USA

b

c

Sheridan, Wyoming, USA

Suleyman Demirel University, Isparta, Turkey

Abstract

Several proven and effective chemical, physical and biological treatment processes have been developed for the removal and recovery of cyanide from mill tailings and process solutions. These treatment processes are well understood and have been utilized for years at mine sites worldwide. The purpose of this chapter is to provide background information regarding these cyanide treatment processes, including basic chemistry and reagent usages, common areas of application and treatment performance that can be expected at full-scale. Emphasis is placed upon those treatment processes with proven full-scale success, as well as those processes exhibiting significant potential for specific application at mine sites. Coupled with the description of treatment processes are discussions of water management and effluent discharge strategies.

Key Words: cyanide, treatment processes, natural attenuation, analysis, discharge strategies, water management

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Mr. Mike BOTZ Mr. Mike Botz is president and owner of the consultancy Elbow Creek Engineering, Inc. and is a registered professional chemical engineer. Mr. Botz holds a BS degree in Chemical Engineering from Montana State University and an MS degree in Chemical Engineering from Purdue University. As a Senior Process Engineer, Mr. Botz has over twenty years of experience related to the evaluation, testing, modeling, design, cost estimating, construction, commissioning and troubleshooting of cyanide treatment plants. Projects completed by Mr. Botz have spanned North America, Latin America, Europe, Australia, Africa and Asia and have ranged in scope from conceptual evaluations through pilot testing, full-scale facility construction and plant commissioning. In addition to industrial project work, Mr. Botz has provided course instruction to academic, government and industry groups, and has presented research findings at numerous industry professional societies. Mr. Botz is active in the Society for Mining, Metallurgy and Exploration (SME) through presenting papers and chairing technical sessions. Mr. Botz has authored and co-authored numerous publications in regard to the management and treatment of cyanide and related compounds. Mr. Botz was also co-author the books The Cyanide Monograph and the Chemistry and Treatment of Cyanidation Wastes, both published through Mining Journal Books Limited in London.

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Dr. Terry. I. MUDDER Dr. Terry I. Mudder was co-owner of Times Limited a consulting firm located in Sheridan, Wyoming (USA) for the past two decades. He was previously employed as Chief Environmental Engineer AND Research Chemist at the Homestake Gold Mine in the U.S. and later a partner and branch manager with SRK Consulting. Dr. Mudder holds a B.S. and M.S. degree in analytical and organic Chemistry, and a Ph.D. in Environmental Science and Engineering. He has thirty-five years experience in investigation of various aspects of cyanide wastes and environmental issues in the mining industry. He served as an adjunct professor and thesis advisor in multiple departments at universities worldwide. He has worked on scores of mining projects, has over one hundred publications, and has been involved with numerous short courses on cyanide, acid mine drainage, and closure. He has co-authored many manuals, pamphlets, and books, including the Chemistry and Treatment of Cyanidation Wastes and the Cyanide Monograph and Compendium. Dr. Mudder has developed several novel chemical, physical, and biological treatment processes for which he has received international awards and patents. He has been the member of many national and international scientific organizations and associated professional committees, as well as a manuscript reviewer and technical advisor for several international journals. He co-created the cyanide information website located at www.cyantists.com. Dr. Mudder co-sponsored the Dr. Adrian Smith International Award presented to outstanding individuals for lifetime achievement in environmental stewardship in mining. He has served as technical advisor to multiple industries, all levels of governments, regulatory agencies, the general public, native peoples and NGO’s including the United Nations Environmental Program (UNEP), the European Bank for Reconstruction and Development and International Finance Corporation. He was recently inducted into the International Mining Hall of Fame.

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Professor Ata AKCIL, PhD-Eng.

Professor Ata Akcil (PhD-Eng.) is a professionally registered engineer with 25 years of experience in process, hydrometallurgical engineering, engineering services, teaching, research, professional service, consulting, management and industrial plant operations. His global assignments as coordinator, researcher, consultant, expert, reviewer, examiner have included projects in Europe (Germany, Italy and Belgium), RSA, Montenegro, Canada, Kazakhstan, Turkmenisthan, Hungary, Jordan, Turkey and Romania. Experienced and trained in mineral processing and hydrometallurgy, he also has extensive academic and industrial experiences in hydrometallurgy, environmental metallurgy, environmental mining management, cyanide management, recycling and mineral processing. He has worked and developed hydrometallurgical and biohydrometallurgical technologies for leaching, agitated leaching, precious metal recovery, base metal recovery, REEs, process control, process design and separations. He has been responsible for lab work, pilot plant work, research, process development, engineering design, start-up, operations, management, corporate budgeting, contracting and environmental affairs for mineral processing and hydrometallurgical plants producing gold, quartz, nickel, copper, cobalt, molibdenium, vanadium, zinc, gallium, alluminium, REEs. Professor Ata Akcil is the Head of the Mineral-Metal Recovery and Recycling (MMR&R) Research Group, SDU, Isparta, Turkey. He has participated in 50 R&D funded projects, awarded 25 scientific awards, is now active reviewer in 20 peerreviewed scientific journals, and is author&co-author in 100 research papers in national&int. journals, 1 book, 10 book chapters, 50 International Conference proceedings and over 1500 citations. He has been the member of many national and international scientific organizations and associated professional committees. He has been a board member of the Hydrometallurgy (published by the Elsevier) and the Associate Editor of the Waste Manegment (published by the Elsevier). He has been a technical expert of the International Cyanide Management Institute (ICMI) since 2009. Knowledgeable of English (Good Level) and Greek (Fair Level).

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Chapter 36 Cyanide Treatment: Physical, Chemical and Biological Processes M.M. Botza, T.I. Mudderb, and A. Akcilc a

Elbow Creek Engineering, Billings, Montana, USA

b

c

Sheridan, Wyoming, USA

Suleyman Demirel University, Isparta, Turkey

2.6.1.1 Introduction In the mining industry, cyanide is primarily used for leaching gold and silver from ores, but it is also used in low concentrations as a flotation reagent for the recovery of base metals such as copper, lead and zinc. At many of these operations, cyanide treatment systems may be required to address potential toxicity issues in regard to the health of humans, wildlife, waterfowl or aquatic life. This may include the removal of cyanide from one or more of the following:     

Slurry tailings from cyanidation operations; Excess solution from Merrill-Crowe operations; Excess solution from heap or tank leaching operations; Supernatant solution from tailings storage facilities, and Seepage collected from ponds or tailings storage facilities.

Cyanide treatment is generally classified as either a destruction-based process or a recovery-based process. In a cyanide-destruction process, either chemical or biological reactions are utilized to convert cyanide into another less toxic compound, usually cyanate. Cyanide recovery processes, described in more detail in Chapter 2.6.2, are a recycling approach in which cyanide is removed from the solution or slurry and then re-used in a metallurgical circuit. Selection of an appropriate cyanide-treatment process for a particular site involves the consideration of many factors, but normally the number of candidate processes for a particular application can be narrowed following review of the untreated solution or slurry chemistry, the desired effluent quality and the availability of reagents or suitable process waters. Common applications for cyanide treatment in the mining industry are described below: 

Tailings slurry treatment is employed when the cyanide level must be lowered prior to being discharged into a tailings storage facility. In this application, the initial tailings slurry weak acid dissociable (WAD) cyanide level typically ranges from about 100 to 500 mg/L and treatment to less than 50 mg/L cyanide is commonly established as the goal for wildlife and waterfowl protection (Hagelstein and Mudder, 2001; ICMI, 2002).



Solution treatment is employed when the cyanide level in decant or process solution must be lowered prior to being discharged into the environment. Treatment of WAD cyanide to low levels is normally required to ensure the protection of human health or aquatic ecosystems. Treatment technologies for solutions commonly employ chemical oxidation and polishing processes, which are applicable to relatively low concentrations of cyanide and generate high-quality effluent.

2.6.1.2 Cyanide Management Plan A key component in relation to treating cyanide is development of a site-wide cyanide-management plan. The importance of properly developing and administering such a plan has been highlighted by incidents at mine sites

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involving the inadvertent release of cyanide to the environment (Mudder and Botz, 2001a). Aside from the potential for environmental impact, such incidents broadly and negatively affect the image of the mining industry and have led to emotional and damaging political responses, such as the banning of cyanide in some regions. Numerous guidance documents have been developed in regard to cyanide management, and these documents should serve as a template for developing site-specific cyanide management plans at mine sites (CMSA, 2001; ICMI, 2002; ECIC, 2003; Cyantists, 2004; Mudder and Botz, 2004). Implementation and adherence to a cyanide-management plan, augmented by experienced scientific and engineering judgement, will help reduce both the number and severity of environmental incidents involving cyanide. The International Cyanide Code (ICMI, 2002) is one such plan that is attracting significant commitment from the international gold mining community and is reviewed in Chapter 1.3.1. The management of water and the management of cyanide are intimately related, and development of a cyanidemanagement plan should proceed in concert with development of a water-management plan. A good cyanidemanagement plan will include descriptions of how cyanide-containing solutions and slurries will be handled, stored, contained and monitored, and in many cases the plan will also include a description of treatment plants used to remove cyanide from solutions or slurries. At sites where natural cyanide attenuation is important, the cyanide-management plan should address the specifics of predicting and monitoring the effectiveness of the attenuation processes. Decommissioning and closure are important phases in the life cycle of cyanide management and should be addressed in the cyanide-management plan (refer to the chapters in Section 1.4). 2.6.1.3 Analysis of Cyanide The term cyanide generally refers to one of three classifications of cyanide, and it is critical to define the class of cyanide that is to be treated. The three classes of cyanide are: (1) total cyanide; (2) WAD cyanide; and (3) free cyanide, as shown in Figure 2.6.1.1. Each of these forms of cyanide has specific analytical methodologies for its measurement, and it is important that the relationship between these forms be understood when analysing cyanide-containing solutions. As indicated in Figure 2.6.1.1, for a given solution the total cyanide level is always greater than or equal to the WAD cyanide level, and likewise, the WAD cyanide level is always greater than or equal to the free cyanide concentration.

Figure 2.6.1.1 General classifications of cyanide compounds The appropriate approach to assessing the quality of water samples in most situations is to analyse for WAD cyanide since this includes the toxicologically or environmentally important forms of cyanide, including free cyanide and moderately and weakly-complexed metal-cyanides. Total cyanide includes free cyanide, WAD cyanide plus the relatively non-toxic iron-cyanide complexes. Complete characterization of a cyanide solution generally includes analyses for pH, total cyanide, WAD cyanide, thiocyanate, cyanate, ammonia, nitrate, nitrite and base metals such as copper, iron, nickel and zinc. Total dissolved solids (TDS) and oxidation-reduction potential (ORP) measurements may also be useful. Shown in Table 2.6.1.1 are the free cyanide and metalcyanide complexes often encountered in cyanidation solutions. Other metals, such as cadmium and mercury, also form complexes with cyanide, but these are normally present at low levels.

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Table 2.6.1.1 Free Cyanide and Metal-Cyanide Complexes Free Cyanide HCN CN–

Copper Cyanides Cu(CN)2– Cu(CN)32– Cu(CN)43–

Iron Cyanides Fe(CN)63– Fe(CN)64–

Nickel Cyanides NiCN+ Ni(CN)42– Ni(CN)53–

Zinc Cyanides ZnCN+ Zn(CN)2o Zn(CN)3– Zn(CN)42– Zn(CN)53–

2.6.1.4 Biological Cyanide Destruction Processes A wide variety of microorganisms are naturally present in water, including domestic and industrial wastewaters. Biological treatment processes promote the growth and development of large populations of bacteria, which are essential for treatment. Bacteria convert soluble organic contaminants into energy, cell mass and other less toxic by-products. Many microbial species (bacteria, fungi and algae) and plants can detoxify cyanide to environmentally acceptable levels and into less harmful by-products (Akcil, 2003; Akcil and Mudder, 2003; Akcil, et al, 2003; Trapp, et al, 2003; Gurbuz, et al, 2004; Kuyucak and Akcil, 2013). Over the past decade, biological cyanide treatment processes have become increasingly widespread in the mining industry due to their ability to simultaneously remove multiple contaminants, their relatively low operating cost and ability to produce high-quality effluent. Biological treatment processes are used to treat decant solution, or in some cases process solutions, but are normally not suitable as a direct tailings-treatment process. There are several possible configurations of biological treatment processes, with the general divisions being aerobic versus anaerobic and attached growth versus suspended growth. Shown in Table 2.6.1.2 are constituents normally removed in aerobic and anaerobic biological treatment processes. In the aerobic process, cyanide, thiocyanate, nitrite and ammonia are oxidized to nitrate, while in the anaerobic (or anoxic) process, nitrate and nitrite are removed as nitrogen gas. With both processes, incidental metals removal may occur through biomass sorption or precipitation as metal carbonates, hydroxides or sulphides. Table 2.6.1.2 Constituent Removals in Aerobic and Anaerobic Biological Processes Aerobic Biological Treatment Cyanide Ammonia Thiocyanate Nitrite Metals

Anaerobic Biological Treatment Nitrate Nitrite Metals

With attached-growth biological processes, biogrowth occurs on a fixed solid media, for example in a rotating biological contactor (RBC) or trickling filter. Periodically, biomass slough from the media and are carried away with the effluent. Attached growth systems are generally used with low influent constituent levels to avoid overloading the media with biomass. With suspended-growth biological processes, biogrowth occurs in a suspended sludge system, similar to a slurry suspension. Waste biomass is removed as underflow from a clarifier, with most biomass recycled from clarifier underflow to the feed water stream. Suspended growth systems are generally used with higher influent constituent levels due to the higher rate of biomass growth. The biologically-mediated cyanide oxidation reaction is: CN– + ½O2 + 3H2O  HCO3– + NH4+ + OH–

(2.6.1.1)

The stable iron-cyanide compounds are not biologically oxidized in this process, although a small portion may be sorbed into the biomass. In the above reaction, cyanide is oxidized to ammonia, with about 0.54 grams of ammonia (as N) being formed per gram of cyanide oxidized.

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The oxygen demand for this reaction is about 0.62 grams per gram of cyanide oxidized, while biomass production is about 0.05 to 0.10 grams per gram of cyanide oxidized. The biologically-mediated thiocyanate oxidation reaction is: SCN– + 3H2O + 2O2  HCO3– + NH4+ + SO42– + H+

(2.6.1.2)

In this reaction, thiocyanate is oxidized to ammonia and sulphate, with about 0.24 grams of ammonia (as N) being formed per gram of thiocyanate oxidized. The oxygen demand for this reaction is about 1.10 grams per gram of thiocyanate oxidized, while biomass production is about 0.08 grams per gram of thiocyanate oxidized. Ammonia generated by cyanide and thiocyanate oxidation is oxidized in the aerobic biological process. The end product of this nitrification reaction is nitrate according to the following reaction: NH4+ + 2O2  NO3– + 2H+ + H2O

(2.6.1.3)

In this reaction, one gram of nitrate (as N) is formed for each gram of ammonia (as N) oxidized. The oxygen demand for this reaction is about 4.57 grams per gram of ammonia oxidized (as N), while biomass production is about 0.17 grams per gram of ammonia oxidized. Nitrite is also aerobically oxidized according to the following reaction: NO2– + ½O2  NO3–

(2.6.1.4)

The oxygen demand for this reaction is about 1.14 grams per gram of nitrite (as N) oxidized, with minimal biomass production. The biological oxidation of cyanide, thiocyanate, ammonia and nitrite occurs simultaneously in aerobic systems, with the thiocyanate reaction being somewhat more rapid and less sensitive to temperature. Often it is necessary to heat water to maintain a treatment temperature of approximately 10 oC to 15 oC, since lower water temperatures result in slower reaction kinetics and proportionally larger treatment equipment. Following aerobic treatment, anaerobic (or anoxic) denitrification is often used to remove nitrate and residual nitrite as nitrogen gas: 6NO3– + 5CH3OH  3N2 + 5HCO3– + 7H2O + OH–

(2.6.1.5)

Nitrate is converted to inert nitrogen gas in this process (nitrogen gas is the main component of atmospheric air). Total sludge production in this reaction is about 0.55 grams per gram of nitrate removed. Methanol (CH3OH) is commonly used as an organic carbon source in this process, although other sources of organic carbon can be used, such as ethanol or molasses. Active and passive biological cyanide-treatment processes have become relatively widespread in the mining industry due to the success of the first plant installed at the Homestake Lead mine in the USA in the 1980s (Mudder, et al, 2001a). In this plant, an aerobic attached-growth biological treatment is used to remove cyanide, thiocyanate, cyanate, ammonia and metals from tailings-impoundment decant solution prior to discharge to surface water. The plant has been operating successfully for over fifteen years, producing high-quality effluent as summarized in Table 2.6.1.3 (Mudder, Botz and Smith, 2001). Table 2.6.1.3 Homestake Lead Biological Treatment Performance Decant Solution Constituent Total Cyanide WAD Cyanide Thiocyanate Ammonia Nitrate Copper Iron Nickel Zinc

Untreated (mg/L) 3.39 2.34 -5.31 -0.49 0.1 to 5.0 0.01 to 0.04 0.01 to 0.1

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Treated (mg/L) 0.37 0.03 -0.27 21.9 0.04 0.27 0.03 0.01

A two-stage suspended-growth biological-treatment plant was installed by Homestake Nickel Plate, Canada in the mid-1990s to treat tailings-impoundment seepage. This plant is a suspended-sludge system with both aerobic and anaerobic treatment sections to remove cyanide, thiocyanate, cyanate, ammonia, nitrate and metals. Representative treatment results for this plant are shown in Table 2.6.1.4 (Given, et al, 2001). Table 2.6.1.4 Homestake Nickel Plate Biological Treatment Performance Seepage Water Constituent Total Cyanide WAD Cyanide Thiocyanate Ammonia Nitrate Copper Iron Nickel Zinc

Untreated (mg/L) 1.04 0.33 379 25.3 2.8 0.02 0.06 ---

Treated (mg/L) 0.44 0.04 0.08 0.15 0.13 0.005 0.02 ---

A passive biological-treatment process was installed at the Homestake Santa Fe (USA) mine to treat draindown from a decommissioned heap-leach pad. This process, known as the passive Biopass process, is suitable for solution flows of less than about 10 m3/h for the removal of cyanide, thiocyanate, cyanate, ammonia, nitrate and metals. Representative treatment results for this plant are shown in Table 2.6.1.5. Table 2.6.1.5 Homestake Santa Fe Biological Treatment Performance (Mudder, et al, 2001b)

Constituent Total Cyanide WAD Cyanide Thiocyanate Ammonia Nitrate Copper Iron Nickel Zinc

Heap Leach Pad Draindown Untreated Treated (mg/L) (mg/L) --14