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Production Facts Waste Disposal
- Waste Disposal
There are several ways that gold mines deal with waste once gold is extracted: the most prevalent term for such waste is "tailings". This section explains the different forms of "tailings" and how the gold industry manages this issue.
Background
Tailings are the by-product of minerals processing or a milling operation. They are finely crushed and ground rock particles from which the valuable mineral portion has been extracted. For the purposes of this discussion, tailings do not include pit-stripping waste, or underground muck, or even fly ash handled in a dry form. They are a simple waste product on the one hand, but also possess distinct physical and chemical characteristics. As such, they are treated in the current regulatory arena on a case-by-case basis, due to a growing public and technical awareness of mining-related environmental impacts.
From a hard-rock mining standpoint, which is the focus of this overview, recent regulatory developments in the United States have addressed tailings management in areas characterized by net precipitation conditions. This is in contrast to major gold-producing states like Nevada and Arizona where tailings are managed under a net evaporation climate with no discharge from tailings storage facilities (TSF) to “waters of the U.S.”. In a net precipitation area like Washington and Alaska, the U.S. EPA (EPA) and U.S. Army Corps of Engineers (Corps) working closely with the Alaska Department of Environmental Conservation have even developed a management policy whereby tailings may be placed as fill in a lake or wetlands after meeting strict performance standards under a Corps 404 permit. The discharge of treated effluent from the lake would also have to meet stringent effluent guidelines and water quality standards. Waters of the U.S. are defined as creeks, streams, mudflats, potholes, ponds and wetlands. “Waters of the U.S.” specifically excludes waste treatment systems, disposal sites, and treatment ponds or lagoons designed to meet the requirements of the Clean Water Act.
In other countries, like Chile, environmental regulations may not be as specific. Discharges of waste water and industrial or mining wastes into rivers or lakes are prohibited “without first providing such discharges with necessary treatment”.
A common attribute of mining operations is that they all generate mine waste in the form of waste rock and tailings. Management of these sources must consider sound science and best engineering judgment. Three methods of tailings management are described in this document. They include:
• conventional “wet” tailings management behind a dam-like facility;
• submarine (sea) tailing placement;
• deposition underwater in a lake; or
• dry-stack tailings management.
Conventional Wet Tailings Management
Traditionally, tailings management has been in surface impoundments that retain tailings and mill effluent. This practice in the U.S. is supported by the 1982 EPA research report: Development Document for Effluent Limitations Guidelines and Standards for Ore Mining and Dressing – Point Source Category. In this document EPA profiles many mining sources (gold, silver, copper and molybdenum mines, etc) regulated under the Clean Water Act, discusses New Source Performance Standards, and outlines Best Available Treatment and effluent limitation guidelines. This document is the basis for designating tailings ponds and settling/treatment as best available treatment technology, and defining the requirements for zero discharge with the net precipitation and ground water infiltration exceptions. These typically apply in certain areas of Idaho and Alaska. The exceptions effectively “overlay” the requirements for issuance of NPDES permits for these discharges.
Water-retention type dams and raised embankments have been used for decades to store and treat tailings. They are carefully designed and constructed so as to discharge only treated effluent from the tailings ponds into waters of the U.S. These discharges must meet all federal and state water quality standards in the receiving waters. An NPDES Permit is the basis for discharge of treated effluent into a receiving stream. Only that volume of water that equals the net precipitation minus evaporation can be discharged after meeting water quality standards. This discharge cannot exceed 20 mg/l total suspended solids (TSS), along with other limits for pH, turbidity, metals, etc. The discharge of tailings into the pond is governed by the Corps’ “fill rule”, which was established in 2002.
Tailings ponds and/or dams are used throughout the western U.S. and elsewhere to store and treat mine tailings. They must be reclaimed and revegetated at cessation of mining. Typically, a reclamation bond (real cost) is required by state and/or federal authorities. Environmental requirements are: 1) maximize the integrity of the impoundment to ensure against the release of tailings, 2) minimize seepage to protect the ground water, 3) mitigate seismic risk, 4) return the impacted area to pre-mining productive use via reclamation/restoration, and 5) treat discharges from the impoundment such that they meet all applicable Water Quality Standards. The basic objectives of tailings impoundment reclamation are to achieve long-term stability, prevent environmental contamination, and return the “wet impoundment area” to a productive use after closure.
Specific design and operational requirements regarding the location, water management, maintenance, and control and monitoring are fairly universal outside the U.S. For example in Bolivia an Environmental Impact Assessment (EIA) involves a sworn statement regarding waste generation and handling. A risk analysis and contingency plan is usually required if the project is in close proximity to a population centre. The Environmental Suitability Statement is issued by a competent environmental authority before construction can begin. Specific licenses for tailings dams are also required.
Submarine Tailing Placement
Submarine Tailing Placement (STP) is an engineered technology developed as a disposal alternative for mines in close proximity to coastlines. Application of STP is evaluated on a case-by-case basis, and is an option where land-based disposal would have more substantial environmental impacts. A detailed set of screening criteria has been developed to evaluate a site's suitability for STP. The screening criteria include proximity to deep-water coastlines, severe precipitation or flooding potential, seismic loading, and land use considerations.
STP has been employed at more than twenty sites, in the following countries: Canada, Denmark (Greenland), Norway, Papua New Guinea, Peru, and Turkey. The Island Copper Mine in British Columbia, Canada operated its STP system very successfully for more than twenty years. STP has not been employed in the United States, as few U.S. mines are located close enough to the coastline to consider the feasibility of using STP.
The Government of Indonesia selected subsea tailing placement after an extensive environmental review based on cumulative impacts to land, ecology, and the local population. Sometimes subsea tailings placement is a more expensive approach to tailings management than land based technologies.
Underwater Tailings Placement in a Lake
Typically, the storage of mine waste (tailings) in an on-land disposal facility is required or preferred. If the tailings are acid-generating, then the waste needs to be placed in a protected environment. One option for doing this is to place tailings underwater in a lake ecosystem. Following completion of mining, the aquatic ecosystem within the lake can be re-established by raising the bottom of the lake with tailings fill, and raising the water cover over the top of the tailings with fresh or treated water. This effectively creates a “natural design” at closure, which allows sunlight to penetrate into the lake where previously it did not. The enhanced natural design further allows plants in the lake to produce oxygen, improving the biology and fishery of the lake.
In Alaska, the Corps recently issued a Section 404 Dredge and Fill Permit for placing benign tailings fill from a gold milling process into a small, biologically unproductive lake. The EPA has also issued a Section 402 NPDES Permit for discharge of treated tailings decant water to the receiving stream. The term “section” above refers to sections of the Clean Water Act, which along with implementing regulations, provide legal basis for this authority. To issue the permits, the Corps and EPA participated in an environmental impact statement, Section 404(b)(1) site-specific environmental analysis, an EPA New Source evaluation, and a Risk Assessment. Subaqueous disposal was considered the environmentally preferred alternative in the EIS, and the risk assessment determined “upon closure of the tailings storage facility, water depths and substrate composition would stabilize improving conditions for plants and macroinvertebrates”.
The application of subaqueous underwater tailings placement is very common in Canada. In the past decade, six mines have been approved and fisheries compensation measures implemented at Canadian mine sites, which haven been successful in restoring fisheries and wetlands habitat. Examples include: the Kemess Mine and Benson Lake in British Columbia, Buttle Lake on Vancouver Island, and Mandy and Anderson Lakes in Manitoba.
Canada has also successfully employed this technology to mitigate the potential impacts of acid-rock drainage (ARD) from reactive waste rock by placing the material under water. The basic science is that in this protected environment of low oxygen, ARD and fisheries impacts will be mitigated through sound science and engineering. All these examples in the U.S. and Canada were preceded by exhaustive environmental baseline studies, EIAs, and EISs. Water cover of tailings was selected over dry covers, thus reducing oxidation of any remaining sulphide minerals. Corps 404(b)(1) guidelines and Provincial requirements in Canada make approval of this method of tailings management a case-by-case, project-by-project process.
In Chile, mining development projects including mineral processing plants, tailings, and waste disposal facilities that have the potential to cause environmental harm are subject to the environmental impact evaluation system (Base Law of the Environment). This would include underwater placement, dry stack tailings, conventional wet tailings placement, and submarine or “offshore” disposal.
Dry Stack Tailings Management
Hardrock mining projects also “dewater” tailings to essentially a moist solid form (typically less than 18% moisture) and stack the material in engineered lifts. Belt or vacuum filters may be used to accomplish the dewatering. The “dry cake” may also be placed underground as backfill in the case of an underground mine to maximize ore recovery.
Tailings are stacked using conveyors, radial stackers or other means. Individual sandwich-like layers of clean crush rock may be required to facilitate internal drainage. These stacks may be 200 to 300 ft. high, depending on the design. In locations of high precipitation, some sort of capping with till-like material and possibly retention berms are usually required, or the pile is subject to re-saturation and movement.
The advantage of this type of design is the area required for storage may be smaller with less infiltration. Sometimes depending on siting constraints, these facilities may also be located outside “waters of the U.S.”, sometimes eliminating the need for any impoundment. Environmental issues may include: visual impacts, energy requirements for dewatering, long-term stability and reclamation maintenance, and erosional water quality impacts. The capital and long-term operating costs for dewatering, stacking and capping are typically significantly more than for conventional wet or underwater subaqueous storage.
There remains considerable controversy over the economics, feasibility, practicability and environmental trade-offs with dry tailings disposal especially in areas of high precipitation and runoff. However, in dry climates where residual water can be filtered and recycled, this option may be employed on an individual basis. Typically, this is on very flat topography at sites close to the mill where runoff is limited, or in areas of low seismic risk.
Conclusion
All mining operations that produce tailings fill require careful planning and design, and first and foremost application of an appropriate site-specific tailings management approach for that particular project. On a case-by-case basis, natural water bodies, conventional tailings impoundments, dry stack tailings, or outside of the U.S. submarine tailings placement may be permitted. The potential environmental hazards caused by specific contaminants must be evaluated.
Understanding of tailings management begins with a knowledge of the processes by which they were produced. Tailings characteristics, physical and biological conditions at a site, logistics of transport, technology, and economic practicability (costs) should and do determine final tailings management needs. This is particularly the case in the U.S. No “one size fits all” management approach is applicable in the rapidly changing world of current day mining.
Tailings are the by-product of minerals processing or a milling operation. They are finely crushed and ground rock particles from which the valuable mineral portion has been extracted. For the purposes of this discussion, tailings do not include pit-stripping waste, or underground muck, or even fly ash handled in a dry form. They are a simple waste product on the one hand, but also possess distinct physical and chemical characteristics. As such, they are treated in the current regulatory arena on a case-by-case basis, due to a growing public and technical awareness of mining-related environmental impacts.
From a hard-rock mining standpoint, which is the focus of this overview, recent regulatory developments in the United States have addressed tailings management in areas characterized by net precipitation conditions. This is in contrast to major gold-producing states like Nevada and Arizona where tailings are managed under a net evaporation climate with no discharge from tailings storage facilities (TSF) to “waters of the U.S.”. In a net precipitation area like Washington and Alaska, the U.S. EPA (EPA) and U.S. Army Corps of Engineers (Corps) working closely with the Alaska Department of Environmental Conservation have even developed a management policy whereby tailings may be placed as fill in a lake or wetlands after meeting strict performance standards under a Corps 404 permit. The discharge of treated effluent from the lake would also have to meet stringent effluent guidelines and water quality standards. Waters of the U.S. are defined as creeks, streams, mudflats, potholes, ponds and wetlands. “Waters of the U.S.” specifically excludes waste treatment systems, disposal sites, and treatment ponds or lagoons designed to meet the requirements of the Clean Water Act.
In other countries, like Chile, environmental regulations may not be as specific. Discharges of waste water and industrial or mining wastes into rivers or lakes are prohibited “without first providing such discharges with necessary treatment”.
A common attribute of mining operations is that they all generate mine waste in the form of waste rock and tailings. Management of these sources must consider sound science and best engineering judgment. Three methods of tailings management are described in this document. They include:
• conventional “wet” tailings management behind a dam-like facility;
• submarine (sea) tailing placement;
• deposition underwater in a lake; or
• dry-stack tailings management.
Conventional Wet Tailings Management
Traditionally, tailings management has been in surface impoundments that retain tailings and mill effluent. This practice in the U.S. is supported by the 1982 EPA research report: Development Document for Effluent Limitations Guidelines and Standards for Ore Mining and Dressing – Point Source Category. In this document EPA profiles many mining sources (gold, silver, copper and molybdenum mines, etc) regulated under the Clean Water Act, discusses New Source Performance Standards, and outlines Best Available Treatment and effluent limitation guidelines. This document is the basis for designating tailings ponds and settling/treatment as best available treatment technology, and defining the requirements for zero discharge with the net precipitation and ground water infiltration exceptions. These typically apply in certain areas of Idaho and Alaska. The exceptions effectively “overlay” the requirements for issuance of NPDES permits for these discharges.
Water-retention type dams and raised embankments have been used for decades to store and treat tailings. They are carefully designed and constructed so as to discharge only treated effluent from the tailings ponds into waters of the U.S. These discharges must meet all federal and state water quality standards in the receiving waters. An NPDES Permit is the basis for discharge of treated effluent into a receiving stream. Only that volume of water that equals the net precipitation minus evaporation can be discharged after meeting water quality standards. This discharge cannot exceed 20 mg/l total suspended solids (TSS), along with other limits for pH, turbidity, metals, etc. The discharge of tailings into the pond is governed by the Corps’ “fill rule”, which was established in 2002.
Tailings ponds and/or dams are used throughout the western U.S. and elsewhere to store and treat mine tailings. They must be reclaimed and revegetated at cessation of mining. Typically, a reclamation bond (real cost) is required by state and/or federal authorities. Environmental requirements are: 1) maximize the integrity of the impoundment to ensure against the release of tailings, 2) minimize seepage to protect the ground water, 3) mitigate seismic risk, 4) return the impacted area to pre-mining productive use via reclamation/restoration, and 5) treat discharges from the impoundment such that they meet all applicable Water Quality Standards. The basic objectives of tailings impoundment reclamation are to achieve long-term stability, prevent environmental contamination, and return the “wet impoundment area” to a productive use after closure.
Specific design and operational requirements regarding the location, water management, maintenance, and control and monitoring are fairly universal outside the U.S. For example in Bolivia an Environmental Impact Assessment (EIA) involves a sworn statement regarding waste generation and handling. A risk analysis and contingency plan is usually required if the project is in close proximity to a population centre. The Environmental Suitability Statement is issued by a competent environmental authority before construction can begin. Specific licenses for tailings dams are also required.
Submarine Tailing Placement
Submarine Tailing Placement (STP) is an engineered technology developed as a disposal alternative for mines in close proximity to coastlines. Application of STP is evaluated on a case-by-case basis, and is an option where land-based disposal would have more substantial environmental impacts. A detailed set of screening criteria has been developed to evaluate a site's suitability for STP. The screening criteria include proximity to deep-water coastlines, severe precipitation or flooding potential, seismic loading, and land use considerations.
STP has been employed at more than twenty sites, in the following countries: Canada, Denmark (Greenland), Norway, Papua New Guinea, Peru, and Turkey. The Island Copper Mine in British Columbia, Canada operated its STP system very successfully for more than twenty years. STP has not been employed in the United States, as few U.S. mines are located close enough to the coastline to consider the feasibility of using STP.
The Government of Indonesia selected subsea tailing placement after an extensive environmental review based on cumulative impacts to land, ecology, and the local population. Sometimes subsea tailings placement is a more expensive approach to tailings management than land based technologies.
Underwater Tailings Placement in a Lake
Typically, the storage of mine waste (tailings) in an on-land disposal facility is required or preferred. If the tailings are acid-generating, then the waste needs to be placed in a protected environment. One option for doing this is to place tailings underwater in a lake ecosystem. Following completion of mining, the aquatic ecosystem within the lake can be re-established by raising the bottom of the lake with tailings fill, and raising the water cover over the top of the tailings with fresh or treated water. This effectively creates a “natural design” at closure, which allows sunlight to penetrate into the lake where previously it did not. The enhanced natural design further allows plants in the lake to produce oxygen, improving the biology and fishery of the lake.
In Alaska, the Corps recently issued a Section 404 Dredge and Fill Permit for placing benign tailings fill from a gold milling process into a small, biologically unproductive lake. The EPA has also issued a Section 402 NPDES Permit for discharge of treated tailings decant water to the receiving stream. The term “section” above refers to sections of the Clean Water Act, which along with implementing regulations, provide legal basis for this authority. To issue the permits, the Corps and EPA participated in an environmental impact statement, Section 404(b)(1) site-specific environmental analysis, an EPA New Source evaluation, and a Risk Assessment. Subaqueous disposal was considered the environmentally preferred alternative in the EIS, and the risk assessment determined “upon closure of the tailings storage facility, water depths and substrate composition would stabilize improving conditions for plants and macroinvertebrates”.
The application of subaqueous underwater tailings placement is very common in Canada. In the past decade, six mines have been approved and fisheries compensation measures implemented at Canadian mine sites, which haven been successful in restoring fisheries and wetlands habitat. Examples include: the Kemess Mine and Benson Lake in British Columbia, Buttle Lake on Vancouver Island, and Mandy and Anderson Lakes in Manitoba.
Canada has also successfully employed this technology to mitigate the potential impacts of acid-rock drainage (ARD) from reactive waste rock by placing the material under water. The basic science is that in this protected environment of low oxygen, ARD and fisheries impacts will be mitigated through sound science and engineering. All these examples in the U.S. and Canada were preceded by exhaustive environmental baseline studies, EIAs, and EISs. Water cover of tailings was selected over dry covers, thus reducing oxidation of any remaining sulphide minerals. Corps 404(b)(1) guidelines and Provincial requirements in Canada make approval of this method of tailings management a case-by-case, project-by-project process.
In Chile, mining development projects including mineral processing plants, tailings, and waste disposal facilities that have the potential to cause environmental harm are subject to the environmental impact evaluation system (Base Law of the Environment). This would include underwater placement, dry stack tailings, conventional wet tailings placement, and submarine or “offshore” disposal.
Dry Stack Tailings Management
Hardrock mining projects also “dewater” tailings to essentially a moist solid form (typically less than 18% moisture) and stack the material in engineered lifts. Belt or vacuum filters may be used to accomplish the dewatering. The “dry cake” may also be placed underground as backfill in the case of an underground mine to maximize ore recovery.
Tailings are stacked using conveyors, radial stackers or other means. Individual sandwich-like layers of clean crush rock may be required to facilitate internal drainage. These stacks may be 200 to 300 ft. high, depending on the design. In locations of high precipitation, some sort of capping with till-like material and possibly retention berms are usually required, or the pile is subject to re-saturation and movement.
The advantage of this type of design is the area required for storage may be smaller with less infiltration. Sometimes depending on siting constraints, these facilities may also be located outside “waters of the U.S.”, sometimes eliminating the need for any impoundment. Environmental issues may include: visual impacts, energy requirements for dewatering, long-term stability and reclamation maintenance, and erosional water quality impacts. The capital and long-term operating costs for dewatering, stacking and capping are typically significantly more than for conventional wet or underwater subaqueous storage.
There remains considerable controversy over the economics, feasibility, practicability and environmental trade-offs with dry tailings disposal especially in areas of high precipitation and runoff. However, in dry climates where residual water can be filtered and recycled, this option may be employed on an individual basis. Typically, this is on very flat topography at sites close to the mill where runoff is limited, or in areas of low seismic risk.
Conclusion
All mining operations that produce tailings fill require careful planning and design, and first and foremost application of an appropriate site-specific tailings management approach for that particular project. On a case-by-case basis, natural water bodies, conventional tailings impoundments, dry stack tailings, or outside of the U.S. submarine tailings placement may be permitted. The potential environmental hazards caused by specific contaminants must be evaluated.
Understanding of tailings management begins with a knowledge of the processes by which they were produced. Tailings characteristics, physical and biological conditions at a site, logistics of transport, technology, and economic practicability (costs) should and do determine final tailings management needs. This is particularly the case in the U.S. No “one size fits all” management approach is applicable in the rapidly changing world of current day mining.
