Summary
This review describes heap leach pads. It lists several consulting companies specializing in heap leach pad design, construction and operation. Performance and stability of the pads is also discussed, and links to several software are given.
PAD DESCRIPTION
The basic components of heap leach pads are described by Hutchinson and Ellison (1992). A recent review of the
state of practice of heap leach pad layouts is that by Thiel and Smith (2003). Wikipedia has a brief
overview. See also the many papers in the
InfoMine library.
CONSTRUCTION & OPERATION
General
Operation of the HLP is done in accordance with
industry-, company-, and site-specific procedures. Generally this involves compliance with formal and/or informal
Standard Operating Procedures,
Health and Safety Plans,
Emergency Response Plans, permits, and relevant and applicable
laws and regulations.
Ore Placement Systems
Suppliers of systems to apply solution to the HLP are listed in the
Infomine supplier's catalogue.
Solution Application Systems
Suppliers of systems to
apply solution to the HLP are listed in the
Infomine supplier's catalogue. For example, see
Ore Max and
Black-Line International.
Contract Mining
Your heap leach pad can be managed by contract. Contact the
Washington Group International for example.
AMES lists four projects where they have been involved in
heap leach pad construction.
PERFORMANCE EVALUATION
Software
Computer codes to simulate and model the performance of leach pads abound-any code that calculates a factor of safety against instability will work on a heap leach pad problem, and the same is true for most aspects of heap leach pad performance.
Jensen and Taylor compare some codes. Here are others:
SOILVISION SYSTEMS sells SVFlux to analyze unsaturated flow in a pad-when coupled with ChemFlux you can study the effect of the input chemistry on the output chemistry.
METSIM has a heap leach model that is part of their comprehensive suite of computer codes to address complex chemical, metallurgical, and environmental processes. The heap leach module performs mass balances around the heap leach process including chemical reactions, precipitation and evaporation, solids and water inventories, heap drainage and control logic. The model is non-steady state and generates time dependent plots.
Leach, Inc. has a software package to scale up and simulate a heap leaching operation based on engineering and kinetic fundamentals of the leaching process. Using results from a column leach test or a commercial heap operation LEACH will allow you to quickly predict heap performance under different operating conditions such as particle size distributions, heap heights, solution flow rates and solution chemistry.
Stability - Static
Industry standard
methods and/or computer programs may be used to evaluate the factor of safety of the slopes of Heap Leach Pads. Key to the use of any of these programs is the geometry of the Heap Leach Pad, the strength of the ore as placed in the pad, and the angle of friction between the liner beneath the pad and the materials placed directly above the liner.
Stability - Seismic
The response of a Heap Leach Pad to an earthquake depends on the magnitude of the event, the properties of the materials in the pad, and the liner interface strength. Potential responses include:
Methods used in the landfill industry to evaluate the seismic response of landfills may be used to quantify the potential for liner sliding and mass deformation - see Kavazanjian and Matasovic.
Erosion
Control of erosion during operation of the HLP is generally best effected by appropriate surface contouring as part of the placement of the ore on the HLP, by limiting runon, and by directing runoff to sediment control dams and basins. See the TechnoMine
review on this topic.
Performance Monitoring
HeritageGeophysics.com describes installation of an electronic leak detection system under a heap leach pad to monitor the liner performance. See a similar program by
Terraplus.
LEACH PAD COVERS
Follow this link to a course on EduMine where I have written extensively about the design, construction, operation, and closure of heap leach pads.
But I say nothing in the course about placing a “synthetic liner” over the pile at closure and reclamation. Yet here is an excerpt from a story in the Washington Examiner about a story on the old Wood Gulch mine in Nevada where Barrick has done just that:
Barrick returned to the site this summer to install the liner to prevent melting snow from traveling through the leach pad and picking up old cyanide solution at the small mine near Mountain City that hasn’t produced gold since the end of 1990. “We were still seeing a large volume of drain-down solution from the snow so we’re trying to cut off any more water from entering the pad,” she said.
Rory Lamp, a supervisory wildlife biologist for the Nevada Department of Wildlife, said the original concern at Wood Gulch was that contaminated water would reach a nearby stream, which could threaten the red band trout in Badger Creek. “They are trying to resolve that risk,” Lamp said.
Sierra Geosynthetic Services Inc. installed the special liner supplied by Agru and designed for better drainage. Logan Jensen of the engineering firm Knight Piesold said roughly 430,000 square feet of liner went on the pad. High Mark Construction worked to finish covering the liner with waste rock that will then be recovered with topsoil and reseeded. The company earlier removed topsoil on the pad, regraded and compacted the material so the liner could be laid and welded to an original liner under the pile of rock and ore.
Wood Gulch was a small mine, so the leach pad covers only 12 acres. The mine was the first and only one Homestake Mining Co. developed under a small mines division, Brito said. Barrick acquired Wood Gulch when it acquired Homestake in 2001. The mine started in mid-1988, closed for the 1988-89 winter, and went into continuous operation from April 1989 to November 1990. During operation, the mine produced 34,852 ounces of gold and 66,955 ounces of silver, according to Barrick. Reclamation began in 1991. The heap was rinsed in 1991 and 1992, and horizontal drains were drilled in 1993 before the pit highwall resloped, and the heap was covered with growth soil and seeded.
The interesting part of this story is how long it took after mine closure to decide they needed to reduce infiltration to the old, so-called, closed pile. You can almost hear the panic in the voice of the person talking to the reporter–also obviously a young and impressionable person who wrote the following:
She said the decision to install the liner over the Wood Gulch leach pad made sense for the location. “This is very site specific,” she said, adding that others in the mining industry were concerned the Barrick work would set a costly precedent. Modern mines place synthetic liner down before starting a heap leach pad or expanding an existing pad, but they cap the heaps with soil mixtures. Lamp said Nevada mines continue to learn from leach-pad closures, but Wood Gulch was reclaimed years ago. “We have 20 years of information now. It’s been a learning process as we go along,” he said. Brito said that while the synthetic liner topping a leach pad is a first for mining in Nevada, the liner has been used for projects elsewhere and is commonly used at landfills. “We borrowed the idea,” she said.
I would love to get details of how they intend in the long-term to keep the soil in place on the slopes over the “liner.” Sadly this report gives no indication of the type of geosynthetic used as a “liner,” so can make no inferences. I once put a layer of Claymax down as part of the top cover of a uranium mill tailings pile, but have never been able to persuade myself or any client to do likewise on the sideslopes of a mine waste facility. Maybe I will phone Logan Jensen of Knight Piesold, a consulting company that started life in South Africa as F.E. Kanthack, changed its name to Watermeyer, Legge, Piesold, and Uhlman, and then again to Knight Piesold.
CONSULTANTS
I suspect that any reasonable consulting group servicing the mining industry will tell you they have and can design a heap leach pad. All that is needed is the ability to prepare a grading plan, select a liner, specify drains, and establish the slope stability of a heap, write a plausible closure plan, and leave it to the metallurgists and chemist to work out the right solutions to apply when. Maybe, but as always past success is the best indicator of future performance. So here are a few consultants whose heap leach experience is documented in their websites. If I omit your company, be assured this is my oversight, not conscious decision-for I have sought to include all companies I am aware of.
Golder Associates provides a comprehensive suite of services related to heap leach pad design, construction, operation, and closure.
Kappes, Cassiday & Associates in Reno, Nevada provides consulting, testing, and design services for metal heap leaching.
Knight Piesold has undertaken extensive research, testing and analysis on the deformation and collapse capacities of drain pipes, liner systems and their compatibilities with foundation and overliner materials as well as with hydraulic and geotechnical stability of ore piles.
Leach, Inc. will design and supervise laboratory and field test programs to evaluate the heap leaching of copper and gold ores. They have an extensive library of publications that are available on request.
LinRae Consulting Services, Inc offers to undertake design and construction support of heap leach facilities from their Denver office.
O'Kane Consultants have done the work and have written about it. I much admire their innovative approaches.
W. Joseph Schlitt is a consulting metallurgist-hydrometallurgist. He writes the best papers I have read-see the list of publications on his website and the proceedings of the 2006 SME Conference. Here is a quote from his website: W. Joseph Schlitt has 40 years of worldwide hydrometallurgical experience. His experience includes engineering, process assessments, research and development, and plant-oriented studies in the nonferrous industry. Primary emphasis is in hydrometallurgy and chemical processing, particularly copper heap, dump, bio-, and in situ leaching, plus metal value recovery. Additional experience covers other leaching and recovery systems including molybdenum, zinc, vanadium, uranium, nickel and precious metals.
SRK has the specialists, including for example William Gibson in Perth, Australia. And they have worked in the most amazing places which they have written about-see the book Copper Leaching, Solvent Extraction, and Electrowinning Technology by Gerald Jergensen.
Sinclair Knight Merz is a leading water and environmental consulting company that has applied their skills to heap leach pad water management.
Water Management Consultants describes their proprietary Heap Leach Dynamic Technology as being able to "improve leaching efficiency, increase recovery and lower costs."
Vector Engineering and Mark Smith is the original heap leach pad consultant in my mind. They dominate the South American market for the design, construction oversight, operation, and closure of big heap leach pads. Mark Smith et al have written most of the definitive technical papers on the topic.
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