Cyanide Usage in the Gold Mining Industry Under Wet Tropical Conditions
Bryne Ansin Morna, SMAusIMM
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The extraction of gold (from both open cut and underground mines) involves land usage and a considerable amount of chemicals. One of the most predominant chemicals in the industry is cyanide, which has the potential to impact heavily on the environment, affecting land, water and air, and thus the biosphere. With many major mining companies currently operating in the developing third world countries which are often located in wet tropical regions and coupled with the inadequacy of standards and regulations governing the usage of cyanide, the propensity for damage in wet tropical environments is great.
Cyanide has been used to extract precious metals from crushed rock for more than 100 years. Modern recovery methods that utilise cyanide in water-based solution can recover nearly 100% of the contained precious metals (eg. gold and silver), making it profitable for mining companies to process low-grade ores. Despite its toxicity to the environment, the mining industry will continue to use cyanide, since no practical alternatives are available. Given the relatively low gold price, cost efficient processing is all the more important, if the gold mining industry is to continue.
Several cyanide accidents have recently occurred. Spills and leaks from processing operations together with accidental drops from helicopter shave been noted in the press. These accidents have caused significant environmental damage and have raised public concern over cyanide. Avoiding such accidents and incidents, and providing a base for lessening their impact, when and where they occur is what this paper seeks to address.
It is necessary to have a better understanding of the impacts of cyanide on the environment in the wet tropics. It is also most appropriate to develop a baseline for its usage under these conditions to avoid recurrence of catastrophes. To this end, Best Environmental Management Practices (BEMP), have to be developed to provide the tools that incorporate Risk Management (RM) and Environmental Management System (EMS).
Keywords: cyanide, gold, tropical
The aim of this paper is to, examine cyanide usage in the gold mining industry under wet tropical conditions, promote an understanding of the problems involved, and develop a BEMP for cyanide usage under such conditions. The overall aim is to improve management of the chemical to protect the environment and local communities.
Mining is an industrial activity that takes place in the natural environment, disturbing areas around where it occurs. These mining operations come with several direct and indirect environmental impacts which include waste-water spills and water pollution (eg. cyanide spills), visual changes, solid waste generation (containing waste cyanide solutions), ground vibration, noise pollution and air pollution.
Cyanide is used in mining to extract gold (and silver) from ores, particularly in low-grade ores or ores that can not be readily treated through simple physical processes such as gravity. While cyanide-bearing solutions are used in mining because they react with gold, they also react with other metals such as Cu, Zn, Co and Hg. These reactions form weak cyanide complexes, often referred to as 'weak acid dissociable' (WAD) which can dissociate in solution to produce environmentally significant concentrations of free cyanide .
Cyanide is a general term for a group of chemicals containing carbon and nitrogen. Over one million tonnes of cyanide, representing about 80% of annual production, are used in the production of organic chemicals and other industrial applications. It is also used in minute quantities in pharmaceutical applications. The remaining 20% of cyanide production are used to manufacture sodium cyanide. Of this, 90% (i.e. 18% of global production) is used in mining around the world, mostly for gold recovery .
Cyanide has been used to extract precious metals from crushed rock for more than 100 years. Modern recovery methods that utilise cyanide in water-based solution can recover nearly 100% of the contained precious metals, making it profitable for mining companies to process low-grade ores. However, cyanide is toxic to humans and to animal species because it binds to key iron containing enzymes required for cells to use oxygen . The major problems originate from the transportation, handling, usage and disposal of this chemical.
Cyanide ingestion can result in either acute poisoning (including death) or chronic poisoning to humans and animals. Cyanide can remove trace elements from the environment. It can also disturb the balance of trace elements that are available in a natural situation.
The treatment of cyanide effluent is also one of the main problems the mining industry faces . The cost of clean-ups and compensation for both government and companies is prohibitive. Despite these toxicity and environmental problems, the mining industry continues to use cyanide, since no practical alternatives are available. With the relatively low gold price, cost efficient processing is all the more important, if the gold mining industry is to continue producing gold. The gold being derived from resources for which there is no reasonable medium term (foreseeable with accuracy) depletion date. The last inclusion will be a controversial inclusion to some people.
Several cyanide incidents and accidents have occurred the world over with disastrous consequences and huge environmental damage. The most recent big accident was the spill of cyanide-rich tailing waste from the Aurul Sa Baia Mare Company into the river system near Baia Mare in North West Romania, which occurred on the 30th of January 2000  causing a major fish kill. Again on the 21st of March 2000 in Papua New Guinea, while flying from the capital Port Moresby to the Tolukuma mine, a Dome helicopter accidentally dropped a crate containing one tonne of sodium cyanide into the rainforest . Others include:
- Remin mine in Romania in March 2000 which contaminated a stream,
- Placer (Surigao) in Philippines, April 1999 which buried 17 homes,
- Kumtor in Kyrgystan, May 1998, caused 2 deaths,
- Omai in Guyana, August 1995, causing minor fish kill and
- North Parkes, NSW, 1995, duck kill.
These accidents have caused significant environmental damage. Avoiding such accidents and incidents, and providing a base for lessening their impacts on the environment, when and where they occur will further justify the continuing usage of this chemical and reduce company liabilities.
The avoidance of risk will also lessen the chance of public outrage, with outrage being a major hazard of the mining industry.
It is therefore necessary to have a better understanding of the impacts of cyanide on the environment. We are concentrating on the wet tropics, since that is of greatest interest to Queensland, much of the rest of Australia and to many of our S. E. Asian neighbours.
It is also most appropriate to develop a baseline for its usage under these conditions to avoid recurrence of catastrophes. To this end, Best Environmental Management Practices (BEMP), have to developed to provide the tools that incorporate Risk Management (RM) and Environmental Management System (EMS).
Risk Management (RM) would be applied to assess the level of the risk companies, workers and communities are expose to as a result of transportation, usage and disposal of cyanide in gold mining operations. Important aspects of Environmental Management System (EMS) will be highlighted to serve as a prompt prior to and during mining activities. The EMS will and justify the need for continuous improvement in the environmental performance of the industry.
3.0 IMPACTS OF CYANIDE
Besides being a fast acting acute poison, cyanide can cause chronic poisoning in humans and animal species . Other problems with cyanide include :
- cyanide and cyanide heavy metal contamination in catchments and loss of potable water,
- extinction of some fish species in the neighbourhood of the contaminated rivers ,
- water birds and carnivorous animals dying after eating contaminated fish,
- surface and groundwater resources of communities along rivers becoming contaminated with heavy metals in the medium term, and
- socio-economic problems for communities along rivers due to temporary or longer-term loss of livelihood.
Cyanide consumption is one of the major components of the total operating costs of a typical gold-producing plant. Only 0.3 to 0.4 grams tonne of typical ore should be required to dissolve and extract the gold. However, in practice consumption ranges from 300 grams per tonne to more than 2000 grams per tonne . The "excess" cyanide consumption is partly accounted for by complexation with copper, iron and zinc or though reaction with sulphur species to form thiocyanate and loss through volatilisation as HCN gas. Cyanide complexes in particular eventually find their way to tailing dams and then, potentially, into the wider environment.
Biological oxidation decomposes free cyanide into cyanate (CNO-) and thence to HCO3- and NH3 with further biological nitrification producing NO2- and NO3-. Other degradation products such as SCN- are likewise subject to biological degradation and produce HCO3-, HSO4- and HN3.
Apart from the direct environmental and health impacts of cyanide accidents, the cost involved in the clean-ups and compensation from both government and companies are prohibitive. For example the total clean-up cost after cyanide and heavy metal leaks from the Sumitville  gold mine in the USA in December 1992 was US$150 million. As a result of the organisational health safety and environmental impacts and costs of these accidents, there is predetermine the risk as each accident is unique and location specific.
There are generic sets of principles that apply to all cyanide-using industries. At the same time, in adopting these principles, mining operations must take account of site specific requirements and stages of the operating life cycle .
3.1 Potential cause of Accidents under Wet Tropical Conditions
Generally, there are many causes of cyanide accidents but most likely identified to occur under wet tropical conditions will include the following :
- Design deficiencies in dams and heap leach pads
- Operating deficiencies
- Process and people changes
- Operations staff lack awareness of time
- Lack of fund to adequately maintain dams and heap leach pads
- Inadequate assessment of climatic conditions during design (eg. high rainfall in wet tropics)
- Inadequate risk assessment
These events are exacerbated by lack of emergency preparation by some companies, thus the inadequate response to problems when they occur.
3.2 Cyanide in the Environment
Cyanide may occur in the following forms:
1. Free form, as hydrocyanic acid (HCN) or as cyanide ion (CN-);
2. Simple or ionising form, as compounds of the type of A(CN)x , where A may be alkaline earth or heavy metal: mostly present as HCN at pH below 6.0;
3. The complex form, such as a,
4. the organic cyanide (nitrile) form, as R-CN where R may be either aliphatic aromatic .
Cyanide forms stable bonds with metals such as iron and nickel. These compounds show little or no indication of free cyanide. Cyanate and thiocyanate also exist in the environment but are not considered as 'cyanides' since they do not produce a reaction for cyanide, and are in fact degraded forms of cyanide.
3.3 Human and Animal Toxicity.
Cyanide enters the body upon inhalation of HCN and ingestion or injection of hydrocyanic acid or cyanide ions(CN-). It can also be absorbed by the skin, this being most common when working directly with the chemical as in the mining industry (e.g. gold extraction plant). When cyanide enters the body, it quickly enters the blood stream. Part of the cyanide is converted to thiocyanate, which is less harmful, and leaves the body in urine. Some of the cyanide in the body combines with hydroxocobalamin to form vitamin B12. Most of the cyanide and its products will normally leaves the body within 24 hours of exposure .
From studies on Cassava Toxicity , cyanide is a pernicious and serious acute toxin for those that have a regular sub-acute ingestion. From the reference the following may be gleaned:
1. Chronic Cassava Toxicity occurs amongst the poor and poorly fed of Africa and other regions, where cassava is the food of last resort,
2. The toxicity turns up as Ataxic Neuropathy (nerve damage) and goitre. The people showed a marked loss of sulphur amino acids,
3. Thiocyanate was found in blood plasma, and
3.4 Chronic cyanide poisoning in mining populations. Does it exist? Probably in some form, in some individuals.
In a mining scenario chronic toxicity could occur as follows:
- People working with cyanide ingest or absorb small (non-acutely toxic) amounts,
- Those that have high levels of sulphur amino acids show and have no problems,
- Those with LOW levels of sulphur amino acids, show mild symptoms of nerve disruption as indicated by a general lack of good health as may be indicated by dermatitis or mild goitre.
Those with low levels of sulphur amino acids could be:
° heavy drinkers,
° poor eaters (or vegetarians),
° people with eating disorders or bowl problems (eg Irritable Bowl Syndrome) and
° possibly smokers.
The means of detecting cyanide from chronic poisoning instances would be in urine tests for thiocyanate. The level of damage caused by chronic cyanide poisoning may be indicated in a thorough medical examination. In summary chronic poisoning can lead to loss of body sulphur, bringing on ataxic neuropathy.
The problem would show in communities where minute quantities of cyanide remove sulphur that is already in short-supply from dietary deficiencies.
Management of furtive emissions, and the monitoring of workers health is essential.
(The monitoring of thiocyanate in 'cyanide' workers urine is planned for a WA study, that should soon be commenced.)
4.0 CYANIDE UNDER WET TROPICAL ENVIRONMENT
In the wet tropics the following climatic conditions may be experienced:
· A severe monsoon season,
· Distinct wet and dry seasons,
· Unpredictable storm events including cyclones.
Elevated temperatures (to that of temperate climates) will be experienced throughout most of the year.
The effect of elevated temperatures will principally effect the solubility of the following gases in aqueous solution; HCN, O2, Cl2, SO2. The lower solubility of HCN in solution will decrease the efficiency of the cyanidation process, and cause more loss of cyanide to the atmosphere than would be experienced in cooler climates. The lower solubility of oxygen will decrease the efficiency of gold dissolution.
(Note. The solubility of oxygen in water at Std. Press. is 10mg/L at 15°C. At 30°C it is 7.5 mg/L. . )
Chlorine and sulphur dioxide are two reagents that are used to destroy waste cyanide. Their lower solubilities will decrease the efficiency of their respective destruction processes.
Processing gold ores using cyanidation in a tropical environment will thus be less efficient than when a similar operation is conducted in a cold or temperate climate. The result of this loss of efficiency is that more cyanide will be wasted to the environment.
In the tropics climatic conditions may increase difficulties in tailings management, hence the need to control cyanide in heap-leach circuits and tailings dams to a greater extent. Monsoonal rains will make heap-leaching more difficult to control, tailing dams harder to maintain, and cause process solution dilution problems during the 'wet season'. Cyclones and lesser storm events will be unpredictable factors that will make management that much more harder.
Cyanide management practices will need to be tighter during periods of heavy precipitation or when major storms are likely. Also cyanide degradation technologies should be selected to suit the climatic conditions (eg. rainfall and temperature variations)
4.1 The Fate of Cyanide
In active heap leach circuit and tailings dumps, cyanide can be lost by the following means:
1. off-gassing of HCN to the atmosphere,
2. microbial or abiotic oxidation to form dissolved inorganic carbon and nitrogen species, and
3. retention in ore heaps as precipitated, co-precipitated , or absorbed species .
Cyanide as the free cyanide ion may oxidise to cyanate, and thence further oxidise to ammonia, carbon dioxide and water. A good fate, and one that will be enhanced at elevated temperatures. This 'fate' should be encouraged, by BEMP, for cyanide that is liable to be emitted to the environment.
Cyanide may also combine with some heavy metals (eg copper and zinc) and form semi-stable complexes. This cyanide may remain active in the environment, and will only have a marginal increase in degradation at tropical temperatures. A bad fate.
Cyanide may also combine with iron, and form very stable complexes, that will not be readily available to the environment. A reasonable fate, and probably not influenced by the climate.
5.0 BEST ENVIRONMENTAL MANAGEMENT PRACTICES (BEMPs)
The 'ten commandments' which must be followed to achieve best practice for cyanide as proposed by Environment Australia, 1998  with comments from the authors in italics, are as follows:
1. Implement an overall planning procedure, from conception to closure and rehabilitation, for all mine operations that use cyanide, based on an assessment of risks that maximises the benefits and minimises liabilities and environmental impacts (in the light of the climatic situation of themining operation).
2. Establish a cyanide management strategy as part of the mine's environmental management plan for implementing best practice.
3. Implement initial and ongoing cyanide management training for managers, workers and contractors, including maintenance contractors handling or exposed to cyanide - this training should cover both the everyday roles of personnel and how they respond to cyanide-related emergencies (including those caused by storm events).
4. Establish well-defined responsibilities for individuals with clear chains of command and effective lines of communication within the workforce.
5. Institute safe procedures for cyanide handling governing transport, storage, containment, use and disposal (including during periods of monsoonal rain).
6. Integrate the mine's cyanide and water management plans (taking into account seasonal and storm events).
7. Identify and implement appropriate options for reusing, recycling and disposing of residual cyanide from plant operations.
8. Conduct regular cyanide audits and revise cyanide management procedures where appropriate (and use the audit to more tightly manage cyanide during the wet and cyclone season).
9. Develop a cyanide occupational and natural environment monitoring program, supported through a sampling, sample preservation, analysis and reporting protocol.
10. Establish a carefully considered and regularly practiced emergency response procedure (that may include the shutting-down of the operation during major storm or other deluge events).
These rules can only achieve their desired aims when incorporate in the risk management plan (RMP) and environmental management system of the company, with commitment from management and with adequate resources being provided.
5.1 Risk Assessment and Management
Risk assessment is an analytical tool used widely to evaluate the potential risk of hazardous chemicals like cyanide to human health and or the environment. There four major steps in risk assessment. These include:
1. Identifying the chemical,
2. Evaluating its characteristics,
3. Classifying the risk under significance impacts, and
4. Assessing the exposure and chemical pathways for the chemical.
For cyanide, monitoring of what complexes are in the mine process solutions is essential. The management of the cyanide in terms of the alkalinity of the leach and make-up solutions will also need to be monitored, and fail-safe systems for alkalinity maintenance installed.
Under wet tropical conditions a large rainstorm may cause water containing cyanide to drain from heap leach pads and dams and mix with surface water or ground water. Through modelling of water releases, the assessment of the concentration of cyanide that may be expected in rivers and groundwater after a rainstorm may be estimated and the degree of damage (contamination) predicted. There will however be a potential source of error from inadequate or poor rainfall data collected for the model.
It is standard to use a 100-year, 24-hour rain is event is for this evaluation to ensure that the design standards are environmentally near fool-proof. However, the possibility of these computer models failing is dependent on the data used.
5.2 Environmental Management Systems
In Table 1 is the proposed environmental management controls system for cyanide usage.
6.0 Application - Environmental Good Practice
A comprehensive monitoring plan to monitor the significant impacts should be established. The monitoring plan should provide information to manage the operation's impacts on the environment. Remedial action should form part of the plan. Adequate financial provision should be made for, maintenance, routine checks and reviews, recording procedures and reporting obligations to fulfil this function.
The review should be conducted by a management committee regularly and be documented. The environmental coordinator of the company should set the programs for reviews and schedule the reviews with the assistance external experts. Risk management principles should be incorporated in the day to day operational activities.
6.3 Prior Assessment
Having processing activities in harmonisation with the climate. No new operation or extension to an existing operation or process or modification to an existing activity should be undertaken unless proper consideration is given to the legal health, safety and environmental risk, and impacts of the proposed activity. Prior assessment provides the information to manage the impacts of the proposed activity.
6.4 Openness and the need for dialogue
The company should recognise the importance of ongoing communication with employees and interested and affected parties. Their concerns about the impact of the Company's operations on the health safety and environment with the use cyanide usage and storage should be appropriately addressed.
A creative approach to minimising the impacts of the chemical on occupational health and safety, and the environment should be encouraged at all levels in the operation. The understanding of the climate in which the mine operates is crucial. In a mine situated in the wet tropics full account must be taken of seasonal variations, and the propensity for storm events.
6.6 Communication and Consulting
A formal procedure for the maintenance of central records of communication with interested parties, issues raised by interested parties and response should be kept by the company. Concerns should be addressed and best as possible. Fears should be attended to and cleared. Upfront due diligence process very import to foresee any possible future impact and communicate to all stakeholders. A proactive approach will reduce a companies exposure to risk and build good corporate image.
To avoid and lessen hazard from cyanide use, the cyanidation of ore should be employed in the context of the climatic situation in which a mine/mill exists. The wet tropics will create specific challenges for mining and milling operations that will require careful management of cyanided tailings, barren solutions and all other occurrences of concentrated and dilute cyanide. The creation of Environmental Management Systems that take into account the climatic variations will help achieve safe milling practice.
1. Update on the European assessment of the Danube, Cyanide Use in the Mining Industry World Health Organisation. Report 2000
2. Best Management Practice. Environment Australia, 1998
3. Carrillo-Pedroza F. R. et al. Cyanide Oxidation by Ozone in Cyanidation Tailings. Minerals Engineering, Vol. 13, pp 541-548, 2000)
4. Cyanide Spill at Baia Baia - Romania, Report of Assessment Mission, UNEP/OCHA, 2000).
5. Mineral Policy Center, A history of Accidents, URL: www.alphacdc.com/treaty/cyanide2.html,Dec.2000
6. Environmental poisoning. Clean Water Action: www.cwac.net/news2001june1.html#cyanidedisasters
7. SRK, Managing Risk in the Mining Industry, URL: http://mineralresourceforum.unepmanagement%/20-%20final/sld001.htm, May 2001
8. Padiyar V. et al., 1995, Cyanide Detoxification - A Review, Trans IChemE, Vol 73, Part B, pp. 1-4
9. Chronic Cassava Toxicity; Proceedings of an interdisciplinary workshop, London, Jan 1973. Centre De Researches, Pour L'Development International, Ottawa, Canada.
10. Cyanidation of Gold and Silver Ores, Dorr & Bosqui, McGraw-Hill, p 216, 1950
11. Johnson C. A. et al Fate of Cyanide used in the Processing of Gold Ores, New Evidence fro Isotopic Measurements. U.S. Geological Survey. 1998
Table: Proposed Environmental Management Controls System for Cyanide Usage in Wet Tropical Conditions
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SOURCE: M.E.T.T.S. Pty. Ltd. Website http://www.metts.com.au
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