METS has designed and operated over 12 DETOX facilities. The following paper highlights some of the issues with DETOX or recovering cyanide.

Full paper including photos and tables available from: [email protected]

ABSTRACT

More and more gold plants around the world are being required by law to destroy cyanide in tailings. This paper evaluates the options and what influences the process selection for cyanide destruction and recovery. The cost effectiveness, comparison of common detoxification methods with advantages and disadvantages are explored. This paper also looks at the safety and hazards of cyanide and explores various cyanide degradation methods so as to make it safe to people and the environment upon disposal. 

Fundamental aspects of the International Code for Cyanide Management are covered, and highlighted are references to all major Australian bodies who are consulting on the management of cyanide. Case studies are examined so as to illustrate major hazards and mitigations that may occur and be implemented in gold processing operations. 

The various environmental issues pertaining to the usage of cyanide, including the effects on wildlife, monitoring procedures and strategies undertaken to prevent and control any risks due to exposure to the environment will be discussed. Treatment procedures, and methods, for cyanide poisoning are also briefly covered as well as standard transport and storage procedures for the handling of cyanide.

Introduction

More and more gold plants around the world are being required by law to destroy cyanide in their tailings. This is to minimize the exposure to wildlife particularly birds who have a toxicity limit of 50 ppm cyanide in water. In some states of South America and North America the use of cyanide is forbidden. Cyanide is a very reactive toxic compound which forms lethal hydrogen cyanide gas (HCN) in humid and acidic conditions. HCN gas affects respiration and incurs cell suffocation. Even so, a large amount of cyanide is used in mining industries.  Since cyanide is very reactive, it readily binds metals as a strong ligand to form complexes of variable stability and toxicity. Because these compounds induce large damage to humans and organisms if they get into soil and groundwater, wastes containing cyanide must be treated very carefully with suitable methods. There are many possible methods for treating wastes containing cyanide. Destructive methods include natural attenuation, alkaline chlorination, the Inco process, hydrogen peroxide, ferrous sulfsulfate, Caro’s acid, ozonation, electrolytic oxidation and biodegradation. Newer cyanide recovery methods such as acidification volatilisation reneutralisation (AVR) and for high copper ores using sulfsulfidisation acidification recycle and thickening (SART) and resins can be used to recover cyanide. This is cost effective with the current high cyanide cost.

The International Cyanide Management Code has resulted in much higher standards of shipping, handling and emergency response protocols for cyanide. Cyanide impacts on the environment at Baia Mare, Guyana, Rapu Rapu and Parkes have resulted in risk assessments for new projects and far fewer incidents in recent times. Increased attention has been focused on the occupational health and safety and exposure to hydrogen cyanide gas and long term health effects on the workforce.

HISTORICAL INCIDENTS

There have been a number of significant incidents around the world with cyanide spills and hereafter a number of major ones are referred to. The Baia Mare spill was a very significant event and created a bad image around the world relating to cyanide use and motivated the European Commission to regulate the use of cyanide more strictly.

 Baia Mare Spill, Europe

The full implementation of the International Cyanide Management Code could have contributed to the prevention of the Baia Mare incident which resulted from the collapse of a tailings dam from the Aurul mine in Romania, in January 2000.  The reports, established by the governments of Romania and Hungary, evaluated the causes, impacts of the spill and measures needed to reduce those risks in the future.  There were six main causes for the spill:

1)         Large rainfall and snowmelt event

2)         Hydrocyclones weren’t operational and prevented building up the dam wall

3)         No provision for discharge of excess water in tailing

4)         Inefficient monitoring of pond level

5)         Impoundment construction did not meet specifications, and

6)         There was not enough water balance oversight performed by the government

 Guyana, South America

During the night of 19-20 August, 1995, an estimated 4.5 million m3 of cyanide-bearing processed solution was dumped into Omai river after a tailings dam failed.  Three days after the accident, the president declared 80km of the Essequibo (a river into which the Omai connected to) an environmental disaster zone.  The Omai Gold Mines Ltd disaster had an impact on the following areas:

• Loss of animals and aquatic life in the river system
• Children suffered nausea, lesions and skin rashes
• Over 50% of the local residents reported some type of health effect
• Severe effect on potable water supply
• Affected food supplies of 33% of households
• Three cases of suspected cyanide poisoning reported
• Possibly affected the life of nearly 50,000 Amerindians
• Cleanup costs = G$426 million (government share = G$314 million)

 

Process SELECTION CRITERIA

 When selecting suitable methods for cyanide detoxification, many factors affecting treatment times and costs must be considered. The factors include reagent, capital equipment, operating and engineering costs as well as licensing fees and initial cyanide concentration.

There are a number of technologies available for destroying cyanide and its metal complexes, and the choice of the best method for a particular application is not simple.

Complexity arises because:

• Recovery is favoured over destruction when high cyanide consuming copper gold ores are processed.
• Each project is unique with respect to cyanide species in the tailings
• Environmental regulations and legislation vary with respect to allowable concentration limits in the treated tailings
• The choice of cyanide destruction methods is large
• The cost and efficiency of each method varies

 COST EFFECTIVENESS

The cost of treating tailing can be a significant percentage of total operating costs, and unlike other costs it generates no income. It is therefore important, for both regulatory and economic reasons, to select the correct process, and then optimize the operating conditions to minimize reagent dosages. This requires a good knowledge of local regulations as well as laboratory test work and piloting.

 The cost for cyanide destruction, particularly for sodium metabisulfsulfite, e.g. in the Inco process, can be a large cost impost on a project as high as $A0.50 to $2.00/tonne of tailings treated plus the new cyanide cost of approximately $1.50/tonne. On the other hand the cost of recovering cyanide can be as low as $0.50/tonne of tailings when copper credits are added back.

 The CAPEX cost is a serious consideration when cyanide recovery is involved compared to cyanide destruction. The AVR or SART processes treat solution tailings rather than pulp and therefore the CAPEX cost of solid liquid separation has to be added.

Recycled cyanide is much cheaper than new cyanide and also has the advantage of less environmental risk. Less new cyanide is required so there are less transport, storage and handling issues.

DETOXIFICATION ALTERNATIVES

 Natural Attenuation

 The natural degradation of cyanide is well established and has historically been the preferred method. In climates like Australia where cyanide degradation is enhanced by high temperature this is an appropriate strategy.

The downside is the negative impact on the environment particularly birds and fish or native animals such as kangaroos and emus. Seepage of cyanide water into groundwater is now causing authorities to insist on lined tailings dams (i.e. Boddington which also has DETOX).

Going forward, this is less likely to be the preferred method due to local environmental issues.

 Alkaline Chlorination

 Alkaline chlorination is the oldest chemical process for the treatment of cyanide wastes. Nowadays it is used primarily in the plating industry because it has recently fallen out of favour due to the environmental complications of the process. In alkaline chlorination, hypochlorite ions oxidize cyanide to form CO2 and N2 gases according to the following reactions:

 CN- + H+ + ClO- → CNCl + OH-                                                                      (1)

CNCl + 2OH- → CNO- + Cl- + H2O                                                                      (2)

2CNO- + 3ClO- + H2O → 2CO2(g) + N2(g) + 3Cl- + 2OH-            (3)

 All these reactions are dependent on pH, which must be over 10 in reactions (1) and (2). The optimal pH for reaction (3) is 8.5. Oxidation destroys free cyanide and cyanide complexes, except for iron cyanide complexes. One of the major disadvantages of this method is the potential for forming chlorinated organics. Some of the products (e.g. cyanogen (CN)2) are even more harmful and toxic than cyanide itself. This was an issue at the Henty gold mine causing them to switch to the INCO process.

 INCO SO2/Air

 Inco of Canada developed a specific cyanide destruction process which is used around the world to detoxify cyanide plant tailings. 

The SO2/Air process is no longer proprietary technology and no longer requires a license prior to its use.  It uses an agitated tank into which SO2 (in liquid, as metabisulfsulfite, or in gas form) is added with air to work on the cyanide remaining in any type of total tailings, and water recycled from an impoundment area.  A very small amount of soluble copper must also be present to serve as a catalyst.  The process offers continuous single-stage operation.  Despite being a simple process, optimisation is different for each cyanide tailings tested. 

As a fail safe operation the tailings are often processed in batches using two storage tanks with the tailings from a tank only being pumped to tails after measurements have determined that no cyanide is present.

The oxidising agent consists of air or liquid oxygen for the O plus SO either in gaseous or liquid form.  Other reagents normally include CuSO, and lime or NaOH for pH control.  The oxygen via air sparging and the reagents are dispersed in the effluent using a well-agitated mixer.  Retention times are variable, depending on the composition of the solution to be treated, but generally range from 20 minutes to 2 hours.

Stoichiometrically, the reactions, as described in Table 1, require about 2.46g of SO per gram of WAD cyanide.  After treatment, cyanide levels are reduced to between 0.1ppm (0.00001%) and 0.5ppm.  The process does not produce any toxic intermediaries.

A WAD laboratory cyanide analysis unit is used to monitor the process.

 The INCO process is robust and widely used throughout the gold industry.

 Hydrogen Peroxide

 The hydrogen peroxide treatment process chemistry is similar to that described for the INCO process, but hydrogen peroxide is utilized rather than sulfur dioxide and air.

With this process, soluble copper is also required as a catalyst and the end product of the reaction is cyanate.

 HO + CN- → OCN- + HO

 The primary application of the hydrogen peroxide process is with solutions rather than slurries due to the high consumption of hydrogen peroxide in slurry applications. The process is typically applied to treat relatively low levels of cyanide to achieve cyanide levels that may be suitable for discharge. The hydrogen peroxide process is effective for the treatment of solutions for the oxidation of free and WAD cyanides, and iron cyanides are removed through precipitation of insoluble copper-iron-cyanide complexes.  

 The theoretical usage of HO in the process is 1.31 grams HO per gram of CN- oxidized, but in practice the actual usage ranges from about 2.0 to 8.0 grams HO per gram of CN- oxidized.  The HO used in the process is typically provided as a liquid in 50% strength. The reaction is typically carried out at a pH of about 9.0 to 9.5 for optimal removal of cyanide and metals such as copper, nickel and zinc.  However, if iron cyanide must also be removed to low levels, then the pH is lowered somewhat to increase the precipitation of copper-iron-cyanides at the expense of lowering the removal efficiencies of copper, nickel and zinc.  As indicated, copper (Cu2+) is required as a soluble catalyst, which is usually added as a solution of copper sulfate (CuSO4-5H2O) to provide a copper concentration  in  the  range  of  about  10%  to  20%  of  the  initial  WAD  cyanide concentration.  

This process is capable of achieving low levels of both cyanide and metals. Solutions treated with this process may be of suitable quality to permit their discharge. The cost of hydrogen peroxide is high.

 Ferrous Sulfate

 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)3-), Prussian blue (Fe[Fe(CN)]) and other insoluble metal-iron-cyanide (MFe(CN)) compounds such as those with copper or zinc (Adams, 1992).

 Fe2+ + 6CN- + ?O + H+                   →         Fe(CN)3- + ?HO

 4Fe2+ + 3Fe(CN)3- + ?O + H+  → Fe[Fe(CN)] + ?HO

 The iron-cyanide precipitation process is limited in its suitability to situations where the precipitation reactions can be controlled and the precipitated solids can be separated and properly disposed. The process has fallen out of favour. The process is optimally carried out at a pH of about 5.0 to 6.0 and iron is added as ferrous sulfate (FeSO-7HO).Ferrous sulfate usage ranges from about 0.5 to 5.0 moles Fe per mole of CN- depending on the desired level of treatment.

 Caro’s Acid

 Peroxymonosulfuric acid (HSO), also known as Caro’s acid, is a reagent used in a recently developed cyanide treatment process that has found application at a few sites.

 HSO + CN-       →  OCN- + SO2- + 2H+

 Caro’s acid used in the process must be produced on-site using sulfuric acid and hydrogen  peroxide  since  Caro’s  acid  decomposes  rather  quickly. Caro’s acid is used in slurry treatment applications where the addition of a copper catalyst is not desirable, which is typically only in situations where the sulfur dioxide and air process is not suited.  In solution applications, other destruction processes, such as the hydrogen peroxide process, are preferred to the Caro’s acid process.

The theoretical usage of HSO in the process is 4.39 grams HSO per gram of cyanide oxidized, but in practice 5.0 to 15.0 grams HSO per gram of cyanide oxidized is required. Acid produced in the reaction is typically neutralized with lime.

 Ozonation

 Treatment of cyanide bearing wastewater has been carried out with ozone.

Advantages of ozone oxidation include:

• Extremely effective against all free and complexed cyanides either alone or in combination with UV light
• Does not form any undesirable by products such a chlorinated organics or ammonia
• Does not require the purchase, storage or handling of dangerous chemicals on site
• Ozone is produced on site from air using an ozone generator

 The reaction with ozone does not require high temperatures or pressures

Ozone, with an electrode potential of +1.24 V in alkaline solutions, is one of the most powerful oxidizing agents known.
Cyanide oxidation with ozone is a two-step reaction similar to alkaline chlorination. Cyanide is oxidized to cyanate, with ozone reduced to oxygen per the following equation:

CN– + O → CNO– + O

 Then cyanate is hydrolyzed, in the presence of excess ozone, to bicarbonate and nitrogen and oxidized per the following reaction:

2 CNO- + 3O + HO → 2 HCO3- + N + 3O

The reaction time for complete cyanide oxidation is rapid in a reactor system with 10 to 30 minute retention times being typical. The second-stage reaction is much slower than the first-stage reaction. The reaction is typically carried out in the pH range of 10-12 where the reaction rate is relatively constant. To complete the first reaction requires 1.8 – 2.0 gram of ozone per gram of CN-.

The metal cyanide complexes of cadmium, copper, nickel, zinc and silver are readily destroyed with ozone. The presence of copper and nickel provide a significant catalytic effect in the stage one reaction but can reduce the rate of the stage two reaction (oxidation of cyanate).

 Electrolytic Oxidation

 Electrochemical oxidation is an alternative process for destroying cyanide ions at the anode and collecting heavy metals from the cathode. Free cyanide, cyanide complexes and concentrated cyanide solution can be handled with the electrochemical oxidation method.

First cyanide and cyanocomplexes become oxidized to form cyanate ions at the anode. The ions are then decomposed into carbon dioxide and nitrogen gas. Dissociated metal cations are reduced at the cathode.

 Biodegradation

 Cyanide is produced in the environment by plants, fungi and bacteria. Therefore, a number of micro-organisms and their enzymes have the ability to degrade cyanide and metal cyanide complexes (also stable iron cyanide complexes) into less toxic compound like ammonia, formic acid and formamide. Microbes can utilise these compounds as a source of nitrogen and carbon for their own growth. Reed beds are particularly effective in using cyanide and taking up heavy metals. In Finland cyanide solution was used to irrigate pine trees which flourished on the cyanide.

 CYANIDE RECOVERY ALTERNATIVES

 Cyanide recovery has gained in importance since the price of cyanide increased from $1.60/kg to over $3.20/kg. This has the advantage of satisfying environmental aspects and economic benefits.

 Water Recovery

 Cyanide can effectively be recovered and re-used by recycling cyanide-containing solutions within a metallurgical circuit. This is commonly conducted using tailings thickeners or tailings filters to separate solution from tailings solids, with the solution being recycled in the grinding and/or leaching circuits. This approach to recovering cyanide should be evaluated for all operations utilizing cyanide, and its performance can be determined by a simple mass balance calculation including the improved economics. In a dry Australian climate water recovery is favoured because water is scarce.

 Acidification Volatilization Reneutralisation (AVR)

 The process consists of adding sulfuric acid followed by air sparging to strip the hydrogen cyanide from the slurry. The strip gas is then pumped to a stripping tower with caustic soda to form sodium cyanide for recycle to the process. The stripped tailings are neutralized with lime before being pumped to the tailings dam.  The first operation was at Golden Cross in NZ followed by DeLamar silver mine in Idaho followed by Cerro Vangaurdia in Argentina.  The process works technically but has a high CAPEX and the engineering design to minimize HCN exposure is strictly adhered to.

 Sulfidisation Acidification Recycling Thickening (SART)

 SART was developed to treat gold plant tailings solutions that contain high concentrations of copper cyanide. Cyanide recovery has a significant impact on the economics of processing high cyanide-consuming copper-gold ore bodies since less cyanide is required overall in the plant.

When cyanide is destroyed in a gold recovery operation, the costs and hazards associated with the purchase, delivery and destruction of the reagent represents a substantial portion of total operating costs. This cost can be converted into a new stream of revenue when cyanide and copper are recycled from the tailings and the recovered CuS is sold as a by-product. At the same time, savings in the purchase and delivery of new cyanide increase your bottom line. As well, the requirement for less cyanide means less is transported to your site. Overall, SART helps ensure compliance with environmental regulations concerning cyanide.

There are four steps in the SART process:

1. Sulfidization – Sulfur ions are added to the gold tailings.
2. Acidification – The pH of the solution is lowered to around 4.5. Under these conditions, the copper cyanide complex breaks down completely, releasing the cyanide as HCN gas and converting the copper to the mineral chalcocite (CuS).
3. Thickening – The chalcocite is thickened and filtered. The mineral is now a saleable by-product.
4. Recycling – The recycled cyanide is returned to the gold recovery segment of the operation.

The SART process has been used successfully at several full-scale mining operations and more plants currently are under construction throughout the world.  Telfer was one of the first operations to employ SART technology.

Adsorption Using Resins

Fiscal concerns and new environmental regulations regarding the transportation and destruction of cyanide have led to a renewed interested in recovering cyanide and copper from gold processing operations. The Hannah Process is particularly well suited to the recovery of copper cyanide, and can be applied to cyanide in solutions or pulp. The process uses a strong base resin technology to extract free cyanide radicals as well as metal-cyanide complexes from gold tailings. The process offers a number of important benefits including:

• Efficient extraction of free cyanide and metal cyanide complexes in 2 or 3 adsorption stages
• Rapid elution of cyanide and base metals under ambient conditions
• Separation and recovery of valuable by-products in the eluate, including copper compounds
• Significant cost savings. The Hannah Process allows the efficient recovery of cyanide from solutions or pulp at one quarter to one sixth of the average cost of traditional recovery techniques
• Increased plant safety. Cyanide is recovered for direct recycle-to-leach, without volatilizing toxic hydrogen cyanide gas (HCN)
• Reduced transportation costs and lower risk of spills during transport-to-site. Cyanide recovery saves you money on transportation and decreases your environmental risks

• Regulatory compliance. Cyanide recovery enables you to comply with strict new environmental regulations concerning cyanide in gold tailings 

 

SUMMARY

Table 2 provides a simplified summary of the general applications of various treatment technologies for the removal of cyanide and its related compounds; cyanate, thiocyanate, ammonia and nitrate.  This table represents a very simplified summary, but can be used as a conceptual screening tool when evaluation cyanide treatment processes.

 The INCO process is well proven and widely used. The AVR process is attractive for cyanide recovery or the SART process for high copper ores.

 

ASSESSING RISK

 Risk is defined in the Australian Standard on Risk Management (AS/NZS 4360:1995), as “the chance of something happening that will have an impact on objectives”.

Risk has two characteristics that need to be understood to be managed. They are likelihood and consequences.

Risk management process can be applied to resource projects as an essential part of good business management practice.

This involves hazard identification to determine the adverse effects or potential to cause harm and rank the risk with possible mitigation.

Environmental

The main risks are birds, native animals and fish species. Contamination of groundwater is also an issue.

Occupational Health & Risk

There has been increased emphasis on minimizing exposure to hydrogen cyanide gas levels. The limit of 10 ppm for an 8 hour day has not changed in 50 years.

 THE INTERNATIONAL CYANIDE MANAGEMENT CODE

 The International Cyanide Management Code (hereinafter “the Cyanide Code”) and other documents or information sources referenced at www.cyanidecode.org.

The Cyanide Code is a voluntary initiative for the gold mining industry and the producers and transporters of the cyanide used in gold mining. It is intended to complement an operation’s existing regulatory requirements. Compliance with the rules, regulations and laws of the applicable political jurisdiction is necessary; this Code is not intended to contravene such laws.

 The Cyanide Code focuses exclusively on the safe management of cyanide that is produced, transported and used for the recovery of gold, and on mill tailings and leach solutions. The Cyanide Code originally was developed for gold mining operations, and addresses production, transport, storage, and use of cyanide and the decommissioning of cyanide facilities. It also includes requirements related to financial assurance, accident prevention, emergency response, training, public reporting, stakeholder involvement and verification procedures. Cyanide producers and transporters are subject to the applicable portions of the Cyanide Code identified in their respective Verification Protocols.

CASE STUDIES

1) INCO process tested in the laboratory and found to be suitable. The process worked extremely well on oxide ore for many years with zero free and WAD cyanide. Near the end of project life sulphide ore was processed and the tailings resulted in zero free cyanide but high levels of thiocyanate. The legislation did not mention cyanate and the operation closed due to a lack of ore before a solution could be found.

2) INCO process tested in the laboratory and found to be suitable. The process worked extremely well on oxide ore for many years with zero free and WAD cyanide. The project then went underground and processed fresh sulphide ore the tailings contained zero cyanide and no thiocyanate. The project continues to operate today.

3) INCO process tested on solution for a Merrill Crowe project. High cyanide levels resulted in longer residence times required in practice and high cost for reagents. The requirement to DETOX complicated by the water balance. AVR may have been a better solution in hindsight but would have incurred a higher CAPEX cost.

 

CONCLUSIONS

 The historical spill incidents such as Baia Mare and Guyana have created an environment where regulators are wary of gold cyanide projects. Either DETOX or cyanide recovery is seen as being responsible and minimising the potential environmental risk.

As indicated in the previous discussion, there are over nine cyanide treatment processes that have been successfully used worldwide for cyanide removal at mining operations.  The key to successful implementation of these processes is by considering site water and cyanide balances under both average and extreme climatic conditions and the range of cyanide treatment processes available and their ability to be used individually or in combination to achieve treatment objectives. Proper testing, design, construction, maintenance and monitoring of both water management and cyanide management facilities are required.

Process selection is not straight forward and each project is unique in finding the best solution. With regards to cost effectiveness the economics have moved to cyanide recovery in preference to destruction.

By carefully considering these aspects of water and cyanide management before, during and after mine operation, operators can reduce the potential for environmental impacts associated with the use of cyanide. Each of these  cyanide related compounds is affected differently in the treatment processes discussed and this should be considered when evaluating cyanide treatment alternatives for a given site.

 Assessing the hazards of using cyanide should be based on the Australian Risk Assessment standard. It is in the interests of all Gold mining companies adopting the International cyanide Management Code.