Table of Contents
1.1 Heap leach fundamentals
The purpose of mining is to provide beneficial minerals demanded by modern society. For doing so, mining companies secure resources from mineral deposits worldwide and use different techniques to recover the valuable materials from the ore.
The selection of a suitable technique, which is both economically viable and environmentally safe, to process mineral resources largely depends on the type of ore that is mined and the physical conditions associated with the mine site’s location.
Heap leaching is a well-established mining technique enabling the processing of different ores that could not otherwise be exploited under viable economic conditions. Modern-day heap leaching, which has a comparatively low level of energy consumption, is, for example, successfully utilised for the beneficiation of certain types of gold ores. As a result, it contributes to the substantial development of a sustainable gold mining sector in many countries.
In other words, heap leaching is among the best available techniques (BAT) for suitable ores for the reason that it allows the cost-effective processing of ore that would otherwise be uneconomic under circumstances that can technically attain acceptable regulatory levels of environmental risk mitigation.
Cyanidation entails both heap and tank leaching. Wastes include spent ore and tailings with residual cyanide levels. Before final closure, these excess materials are usually processed to neutralise or destroy cyanide.
1.2 Mining waste
As a result of the essence of mining and mineral processing, the size of mining wastes are unquestionably larger than those of both domestic and industrial wastes. The waste’s chemical characteristics — particularly the mobility of metal constituents — are often of concern. For instance, the volumes of mine waste greatly exceed the total volumes of materials handled by civil engineering activities worldwide. The crushed rock circulated into the processing plant to extract the desired product is off-loaded from the plant’s tail end, and in many parts of the world, composes the greatest volume of mine waste. However, at open-pit mining enterprises, the volume of waste rock may exceed the volume of tailings.
Tailings dams store fine particulate matter in the form of impoundments. The material is implanted hydraulically and so is loose and nearly saturated. Consequently, any significant movement of the retaining boundaries can prompt shearing strains that derange the structure of the tailings mass, inducing a soaring upsurge of pore water pressures and liquefaction of a section of the impoundment, causing even greater forces to be applied to the remaining boundaries.
Collapse of the retaining dam can release liquefied tailings that can travel for long distances, and because of its greater weight, wiping out everything in its path. Instead of water, which will flow through and around buildings, liquefied tailings can destroy the structures. Thus, the tendency is for tailing dams to become ever higher and impoundments ever larger.
Similarities between embankment dams designed to collect water and tailings dams have enabled many of the design techniques used with the former to be applied to produce safe tailings dams. However, despite significant improvements, a tailings dam has reported a failure almost every year for the past 40 years.
The damage caused by these failures in the form of casualties, pollution of the environment, destruction of property, disruption of communications, and economic loss to the mining industry is enormous.
1.3 Why are standards useful?
Heap leach management and tailings dam standards mainly emanate from a standard known as civil society sector standards. These reflect a confluence of the growth of higher incomes in major consuming markets and the globalisation of value chains. As prosperous economies grew in wealth, so civil society organisations grew in importance. They began to pinpoint the ethical and environmental character of the products which they were consuming. Under what conditions were labour employed in meeting the products which they utilised? What was the impact on the environment of the products which they consumed? Were these products safe to use?
Given the flourishing role of standards in global value chains, governments and international agencies are providing support for dedicated producers — these are progressively seeking to enter the global economy to provide for sustainable income growth. However, promoting standards is a complex challenge, and a suitably multi-pronged and nuanced response is required.
Consequently, there are five main reasons why standards have become necessary for producers participating in global commodity markets:
- Standards have become an influential determinant of market access, particularly in high-income spots;
- Many high-margin market segments are specified by product and process standards (for example, organic foods);
- While developing the capacity to achieve standards, many producers expand capabilities that enhance their efficiency and systematically increase productivity;
- However, meeting standards is generally a costly process, and this can act as a barrier to entry for small scale and informal producers;
- Standards require coordinated actions along the value chain, and this systemic performance may be hard to achieve.
Indeed, history is full of examples that show the profound importance of standards in shaping the natural resources marketplace and the technologies that come to distinguish it. But at an ultimate level, standards are essential because they state the rules and provide benchmarks and best practices. Moreover, standards are necessary to guarantee that such rules of the game are recognised, adhered to, and respected by all stakeholders.
2 TAILINGS DAMS AND HEAP LEACH
2.1 Tailings dams: What for?
Tailings dams are built to preserve impoundments of tailings, and when possible, the elements extracted from the tailings themselves are used in their construction. Thus, they have many features in common with embankment dams built to maintain water reservoirs. Indeed, in many cases, they are built as water-retaining dams, mainly where there is a necessity for water storage over the tailings. These have to be protected by a water top to prevent aerial pollution.
While identical methods used to design and construct embankment dams can be applied to tailings dams, there are significant differences between the two types. Embankment dams are venerated structures used to profitably store water, whereas tailings dams are required to store unwanted waste desirably at minimum cost. The former are usually built to full height during one construction period, designed and their construction supervised by competent engineers (regulated by law in many countries). Modern tailings dams are also often planned by qualified consulting engineers., but because they are built gradually in stages over the years, and conditions may also change with time, supervision of their construction may become faulty.
Knowledge about the factors that command the behaviour of tailings dams has improved dramatically during the past twenty years. Also, the consequences and public perception of failures have increased considerably, causing owners and managers to become more aware of the risks involved in constructing impoundments. Every site and dam is unique, so a linear relationship from one to another is seldom practicable. However, some common principles and the lessons acquired from incidents can be applied somehow to other situations.
2.1 Origins and evolution of heap leaching
Since the early 1980s, heap leaching has evolved into an effective method of beneficiating a wide range of low-grade, oxidised gold ores. It has many benefits over tank leaching, including ease of design, lower capital and operational costs, and faster startup times.
In many cases, heaps are built on lined pads using ore that has been shipped straight from the mine (run-of-mine ore) with little to no preparation. However, in approximately half of heap leaching operations, the ore is crushed and agglomerated before placing it on the heap — this is done to improve permeability and preserve the high pH needed for leaching to occur. (Bureau of Mines 1986).
Agglomeration requires mixing the crushed ore with lime, portland cement, ash, or other materials. In some cases, sulfide ores may be treated by bio-oxidation, chlorination, roasting or autoclaving after crushing and before heap leaching.
Two common types of pads used in gold heap leaching include permanent heap structure on a buffer from which the leached ore is not removed and on-off pads, allowing the consumed ore to be pulled out following the leach cycle and fresh ore be placed on the pad. Continual heaps are typically built in lifts, each composed of a 1.5- to a 9-meter layer of ore. On-off pads are not regularly used in the industry and are constructed to admit spent ore to be evacuated after the leaching cycle and reuse of the pad.
Leaching occurs according to the following reactions, with most of the gold dissolving in the second reaction (van Zyl et al. 1988):
- 4Au + 8NaCN + O2 + 2H2O 4NaAu(CN)2 + 4NaOH
- 2Au + 4NaCN + O2 + 2H2O 2NaAu(CN)2 + H2O2 + 2NaOH
Once the leaching process is complete and no further gold can be recovered, the spent ore and residual cyanide solution become wastes. There are several modus operandi to the decommissioning of cyanide-contaminated ore piles and neutralising cyanide solutions. Typically, the waste load is rinsed with water until the cyanide concentration in the effluent is below a specific standard set by the local environmental regulatory agency. Occasionally, analysis of the heap solids is required. The heap may then be recycled with wastes in place.
The heap is an on/off pad; the depleted ore will have been periodically removed to a permanent disposal area. Solution ponds and other areas are also neutralised and closed, sometimes with residue or wastes in place, before reclamation.
Accidents at tailings management facilities (TMFs) may lead to accidental water pollution. TMFs store vast amounts of solid waste, which are generated as a by-product when extracting minerals. By its very nature, they can pose severe threats to individuals and the environment, especially in their inaccurate design, handling or management. Accordingly, a failure may result in uncontrolled spills of tailings, dangerous flow slides or the release of hazardous substances, leading to lamentable environmental catastrophes. The devastating effects on human health and the environment of such incidents and their far-reaching and acute transboundary consequences have been demonstrated by major past accidents, such as those listed in Section 3.1.
Many publications have issued guidelines for designing, constructing, and closing safe tailings dams, including ICOLD Bulletins, ANCOLD Guidelines, etc. If the instructions given in these guidelines were to be closely observed, the risk of a failure or unsafe event with a tailings dam and impoundment would be significantly reduced. Unfortunately, the number of dangerous incidents continues at an average of more than once a year.
In most countries, water reservoirs are controlled by legislation, and in some, the legislation applying to these are equally applied to tailings dams. However, there continues to be a need for a more comprehensive implementation of regulations to the non-revenue-generating practice of storing waste tailings to reduce the occurrences of failures and dangerous behaviour.
The International Commission on Large Dams (ICOLD)
The ICOLD Committee on Tailings Dams and Waste Lagoons provides a forum for technical interaction amongst designers and constructors and has spotted the importance of learning from failures and accidents with dams. ICOLD has numerous technical teams that publish bulletins giving guidance to various aspects of dam design, construction and monitoring.
In the past, the body has attempted for tailings dams a similar approach to the one developed for freshwater dams but has encountered averseness amongst the owners of tailings dams to expose incidents or failures unless they came into the public domain through the media or published papers.
Improvements are made by learning the lessons of experience and using them to avoid repeating past mistakes. This Bulletin is intended to help all those connected in any way with tailings storage facilities, including owners, managers, contractors, engineers, personnel responsible for day-to-day construction, and government officials concerned with regulation. In highlighting accidents, the aim is to learn from them, not to condemn them. UNEP commends ICOLD for its thorough work in compiling and analysing this extensive database of accidents, incidents and remedial action.
The ICOLD Tailings Dams Committee has reckoned that a vow can only effectively reduce the cost of risk and failure from owners to the adequate and enforced application of state-of-the-art engineering technology to the design, construction and closure of tailings dams and reservoirs over the entire period of their working life.
As an example, Bulletin 121 can be referred to. The report found that the main reasons for failure could be attributed to a lack of attention to detail. The slow construction of tailings dams can span many staff changes and sometimes changes of ownership. As a result, original design heights are often exceeded, and the properties of the tailings can change. Lack of water balance can lead to “overtopping”: so-called because that is observed but may be due to rising phreatic levels causing local failures that produce crest settlements.
Accordingly, the ICOLD Tailings Dams Committee recommended owners and operators that the design, construction, operation and closure of dams and impoundments with risk potential to downstream shall include the following requirements:
- Detailed site investigation by experienced geologists and geotechnical engineers to determine the possible potential for failure, with in situ and laboratory verification to determine the properties of the foundation materials.
- Application of state of the art procedures for design.
- Expert construction supervision and inspection.
- Laboratory testing for “as-built” conditions.
- Routine monitoring.
- Safety evaluation for experimental conditions including “as-built” geometry, materials and shearing resistance. Observations and effects of piezometric conditions.
- Dam break studies.
- Contingency plans.
- Periodic safety audits.
The Australian National Committee on Large Dams Incorporated (ANCOLD Inc.) is an association of organisations and individual professionals interested in dams in one of the world’s mining superpowers.
Whilst in Australia there is no federal legislation covering the safety of dams, their development and surveillance are controlled by legislation and regulations in Queensland, NSW, Victoria and Tasmania. The other states and territories do not currently have specific dam safety legislation. Regulated items vary but cover safety, environmental, cultural heritage, and other value issues to the community. In most cases, they require compliance with the ANCOLD guidelines.
ANCOLD members have been involved in the drafting and operation of dam legislation and regulations. It encourages and supports the state regulators to meet and identify improvements to achieving consistent dam safety outcomes.
Some of the most prominent guidelines are: Guidelines For Geotechnical Investigations of Dams, Their Foundations And Appurtenant Structures (May 2020); Guidelines for Design of Dams and Appurtenant Structures for Earthquake (July 2019); Regulation and Practice for the Environmental Management of Dams in Australia (June 2014); Guidelines on Design Criteria for Concrete Gravity Dams (September 2013); Guidelines for Dam Instrumentation and Monitoring Systems (1983); Guidelines on Risk Assessment (2003); Guidelines on Dam Safety Management (2003); Guidelines on the Environmental Management of Dams (2001); Guidelines on Selection of Acceptable Flood Capacity for Dams (2000); Guidelines on Strengthening and Raising Concrete Gravity Dams (1992).
UNECE safety guidelines and good practices for tailings management facilities
Since the early 1990s, the United Nations Economic Commission for Europe (UNECE) has committed itself to the anticipation of preparedness for and response to industrial accidents, specifically those with transboundary effects in its region.
These guidelines establish a minimum set of requirements to ensure a basic level of safety for TMFs. In addition, they highlight features to be considered to achieve a satisfactory level of security through applying different policies, measures and methodologies.
Below is an extract of policy, administrative and legal recommendations to the ECE member countries, competent authorities and TMF operators:
- ECE member countries should ensure the development and upkeep of national inventories of operating, closed, abandoned, or orphaned TMFs that may pose a danger to human health or the environment. National inventories of TMFs that have been closed, lost, or orphaned should address all existing impacts and the risks of potentially harmful consequences (accidents, spills and leaks).
- For TMFs with significant risks to outer communities, external emergency plans should be developed in collaboration with operators, community organisations, municipal councils, and rescue services. These plans should be implemented off-site in the case of an accident.
- The competent authorities should put on risk identification and assessment of closed or abandoned TMFs using a step-by-step approach, beginning with a simple screening of sites. Resources are gradually directed towards the sites with the highest risk.
- TMF operators should educate their employees and strengthen and update their safety skills, especially how to recognise potentially hazardous accidents or circumstances.
- When a tangible danger of a major accident has been detected, an uncontrolled incident that could lead to a significant accident happens, or a significant accident has happened, TMF operators should develop and execute internal emergency procedures and enforce them on-site. In addition, TMF operators can revisit, test, revise, and upgrade internal emergency plans regularly and if there is a change in mine operation or management.
The South African Bureau of Standards code of practice for mine residue deposits
The South African Government appointed the Council for Scientific and Industrial Research (CSIR) to investigate the matter and essentially confirmed the conclusions reached. These investigations highlighted the inherent deficiencies in the operation of this facility and others of its kind. This resulted in a 1995 Draft Code of Practice for the ‘Design, Operation and Closure of Tailings Dams’ in South Africa.
The Code of Practice was intended to address the life cycle of residue deposits, including tailings, regarding their safety, environmental considerations, construction and management. The code contains fundamental objectives, principles and minimum requirements for good practice, all aimed at ensuring that no unavoidable risks, problems or legacies are left to future generations.
- Objectives: safety to life, limb and property; environmental responsibility; effectiveness; efficiency
- Principles: continual management throughout the life cycle; minimisation of impact and risk; precautionary approach to promote prevention; internalised costs; cradle-to-grave control
Mining personnel were also then more aware of their legal responsibilities when they make appointments. They would make sure that staff have the necessary skills and appropriate experience to perform their functions. Unions started being more active in assisting mining personnel to become trained in different skills. The South African Government promulgated several new laws to ensure that companies train staff sufficiently and incorporate environmental issues in tailings-dam management.
Heap leaching is more efficiently accomplished when air is introduced to the ore pile. A new ASTM International standard calls how corrugated polyethene (PE) pipes and fittings can be more effectively used to assist the aeration process.
F2987-Specification for Corrugated Polyethylene Pipe and Fittings for Mine Heap Aeration Applications are intended for deep underground applications such as dump leaching process or heap leach pile aeration pipe under a mine. Corrugated PE pipes and fittings in this setting are subject to harsh chemical exposure from corrosive effluents.
According to specialists, corrugated PE pipe has been used in heap leach applications for more than 30 years. In the last ten, there have been extra efforts to increase the effectiveness of the leachate in extracting the sought-after metal from the ore by adding air into the heap leach pile. This requires some exceptional perforation designs and tight joints. It is detrimental for the leachate to enter these pipes; it is also necessary to protect the gaps from clogging or plugging.
Hence, F2987 sets a quality level that offers some assurance that the pipe manufactured to it will perform as planned by design. Pipe and resin quality, perforation scheme, and placement are critical in the mine heap aeration application.
3 LESSONS LEARNT FROM PRACTICAL EXPERIENCES
3.1 Examples of tailings dam accidents
Kachin State, Myanmar
First, a couple of waste heap failures killed at least 130 people in two different jade mines by the end of 2015. More recently, a waste heap collapsed into a lake after heavy rain, triggering a wave of mud and liquid that buried many workers; at least 126 people were killed.
Mishor Rotem, Israel
A phosphogypsum dam failure caused 100,000 m3 of acidic wastewater to surge through a dry riverbed and left a trail of ecological destruction more than 20 km long.
In 2018, a dam failure resulted in about 250,000 m3 of released tailings travelling 29 km downstream, most of which were deposited along the course of a surrounding river. The Federal Attorney’s Office for Environmental Protection said that the tailings don’t contain cyanide or any heavy metals. Three workers were killed, two wounded, and four were reported missing.
Minas Gerais, Brazil
One of the most terrible accidents occurred not long ago in a Vale’s iron mine near Brumadinho. On January 25, 2019, tailings dam No. 1 suddenly failed, releasing almost its complete load of 12 million cubic metres of wastes in a big burst.
The tailings wave devastated the mine’s administrative area, loading station, and two smaller sediment retention basins; it then travelled approx. 7 km downhill, thereby spreading to parts of the local community Vila Ferteco. Two hundred fifty-nine people were killed, and 11 were reported missing.
Free State, South Africa
The 31 -meter high wall of the number four tailings dam of the Harmony Gold mine collapsed. The tailings dam is situated up-slope of Virginia in the Free State Goldfields. At least 2.5 million tonnes of liquefied tailings ripped through the mining village. Eighty houses were swept away, and 200 others were severely damaged. Seventeen people were killed.
3.2 Examples of heap-related mine accidents
San Juan, Argentina
In September 2015, the Veladero mine was hit by a million-litre spill of cyanide solution. Barrick appears to have missed deadlines on several orders from local authorities, including replacing pipes, before subsequent minor spills in 2017.
Colorado, United States
Spills of cyanide and other contaminants from the Summitville gold mine produced severe environmental problems on a 27-kilometre stretch of the Alamosa River. The operation was inaugurated in 1986 and abandoned in 1992.
Montana, United States
The Zortman-Landusky gold mine opened in 1979; it was the first large-scale cyanide heap-leach mine in the U.S. The mine underwent repeated leaks and discharges of cyanide solution throughout its working life, resulting in wildlife mortality and severe contamination of streams and groundwater.
Nevada, United States
Following the failure of a leach room structure in 1997, the Gold Quarry mine near Carlin released almost a thousand cubic meters of cyanide-laden waste into two local creeks. Prior, a series of eight cyanide leaks occurred at McCoy/Cove gold mine, releasing a total of about 400 kilograms of cyanide into the environment.
In April 1998, a dam at the Los Frailes zinc mine in southern Spain ruptured, discharging an estimated 4.9 billion litres of acid, metal-laden tailings into a significant river and over adjacent farmlands. While news reports of the related massive fish kill did not allude to cyanide or associated compounds, their presence seems likely given the nature of the metals extracted at this site.
3.3 Practical lessons learnt
Development of any heap leaching project
Every operator should finish the exploration program first, ensuring that he has well-characterised mineral resources for the first ten years of operation, considering the range of production rates that he is targeting. In other words, you ought to wait until the geology and mining options are well understood.
Heap leaching cannot be fast-tracked. It takes time to safeguard sufficient orebody knowledge is available, complete the test work, and safeguard land access to do the geotechnical and hydrodynamic characterisation of the project site takes time. But redoing the engineering design every time new information is available will always take longer and will be significantly more expensive.
Do not underestimate the hydrodynamic aspect of the test work program. Inferring the permeability characteristics of the ore is fundamental to designing a safe heap leach operation. Nobody should use results from tests done on fresh ore samples only; instead, it is wise to complete as many experiments as possible using fully leached ore samples and conduct a statistical analysis to establish the operating window. Also, numerical models cannot replace the metallurgical test work program. Models are great tools to simulate process scenarios (for example, changes in operation parameters, different ore blends, changes in external conditions, etc.). However, they cannot substitute test work results in defining process design criteria and inputs for financial evaluation.
Key Geotechnical Concerns of Leach Pads & Heaps
Pile tips and natural springs
On occasion, Accident Investigation Boards have attributed disasters to ‘natural unknown springs’ beneath works. For example, ore discard was deposited on hillsides above the miners’ villages in some mining assets. Over the years, waste deposited on top of sandstone — a highly porous rock layer that contains many springs — might become a significant risk. Consequently, heavy rainfall resulted in increased spring discharge and runoff, which partly liquefied. It loosened the discard, resulting in subsidence of a portion on the upper flank of the tailings pile. This phenomenon was followed by the liquefaction of thousands of cubic meters of waste that flowed downhill at high speed, ripping through everything in its path.
Tragic accidents like the one described above have led to regulations that charge quarry owners with securing the safety of solid and liquid tips and provide for design, supervision, inspection, notification, records and the making of tipping rules.
Piping through dam walls
Otherwise, experts have was concluded that the main reason for mismanagement was piping through dam walls. Piping is an erosional process at the interface of course and fine layered material; it is consistent with the jet of water observed shooting out of broken dam walls during the initial stages of a failure event.
The main issue revealed by pipe-related events was that the factors contributing to the formation of pipes should be controlled. These are:
- Thin layers (1–20mm thick) of very fine, saturated tailings should not be sandwiched between thick layers (200–500mm thick) of coarser, partly saturated tailings.
- Fine tailings should not be deposited in small pools of a limited extent.
- Coarser, partially saturated material will always be liable to collapse.
Wall angles and siting
Tailings embankments have also been wrongfully constructed on ridges. The arched surface under these dams causes the water table to mound, resulting in poor drainage and seepage at the sides and the base. In addition, the perimeter walls of the tailings dam can be excessively steep (e.g. more than 35 degrees).
When heavy rain happens, a saturation of the internal tailings occur, and because penstock is not in proper working order, the pool can move away from the penstock to one of the walls of the tailings dam. Thus, the freeboard capacity of the dam is exceeded, and the surface water level overtops the bunds. If breaches develop, they will release several tonnes of liquefied metal tailings.
The primary outcomes are:
- The walls of tailings dams should not be steeper than 35° — although recent regulations in places like Zimbabwe state that angles should not exceed 26°;
- Plenty of freeboard should always be available in tailings dams to allow for periods of excessive rainfall;
- The siting of the tailings dam and good drainage are fundamentally crucial for good tailings-dam construction and management.
Monitoring of tailings dams
Digital satellite image processing is increasingly used to monitor and manage tailings dams. It has proved to be a cheap, easy and effective management tool. Monitoring of tailings dams on the ground now includes an inspection by a competent person with observations recorded in terms of:
- Toe seepage
- Drying off of vegetation planted on the dam walls
- Slope erosion
- Tailings build up in the erosion paddocks at the base of the tailings dam
- Any signs of wall bulging
- Cracking on top of the walls
- The amount of freeboard available
- Sinkhole occurrences
- Condition of the penstock
This article explored the geological and management causes of heap-leach and tailings-dam disasters and provided guidelines for the future management and monitoring of these facilities.
The seeming lack of application of the lessons learnt from historical events in mining operations becomes apparent when relatively new infrastructure fail similarly. This demonstrates the need to study and learn something from each failure event.
Following some tragedies and stringent environmental and social governance, a fundamental reassessment of the philosophies for the design, management and operation of tailings worldwide was initiated by states and the mining industry. As a result, operators and regulators now view tailing dams and their associated problems far more seriously than in the past and is now required to carry out regular, independent self-audits.
 ICOLD, U. (2001). Tailings dams–risk of dangerous occurrences, lessons learnt from practical experiences (bulletin 121). Commission Internationale des Grands Barrages, Paris, 155.
 Kaplinsky, Raphael (2010). The role of standards in global value chains and their impact on economic and social upgrading. World Bank.
 Thiel, R., & Smith, M. E. (2004). State of the practice review of heap leach pad design issues. Geotextiles and Geomembranes, 22(6), 555-568.
 Proceedings of the Heap Leach Solutions Conference. September 22−25, 2013, Vancouver, Canada. Editors: Dirk van Zyl, University of British Columbia, Canada Jack Caldwell, Robertson GeoConsultants, Canada.
 Marsden, J.O. (2009) Keynote address: Lessons learned from the copper industry applied to gold extraction. In World Gold Conference 2009 (pp. 231−239), The Southern African Institute of Mining and Metallurgy.
 Zanbak, C. (2012). Heap leaching technique in mining within the context of best available techniques (BAT). Euromines–The European association of mining industries, metal ores & industrial minerals.