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Heap Leach Pads 

 

Summary

This review looks at the basic concepts of heap leaching and describes the various types of heap leach pads. It also explains the design considerations for heap leach pads. Performance and stability of the pads are also discussed, and links to several software are given.

INTRODUCTION

Heap leaching is the process to extract precious metals like gold, silver, copper and uranium from their ore by placing them on a pad (a base) in a heap and sprinkling a leaching solvent, such as cyanide or acids, over the heap. This process dissolves the metals and they collect at the bottom of the pad. The metal is then further processed. This methodology is mostly used for low-grade ores, and the basic processing steps involve crushing and sometime grinding.

The stages for heap leaching can be described as:

  1. Ground Preparation and pad construction: Here the soil on a slightly sloping ground is compacted and covered by an impermeable pad (can be made of plastic).
  2. Ore stacking: Then the crushed ore is stacked in the form of big heaps. Amount of fines is decreases as low as possible to improve permeability.
  3. Then the leaching agent such as cyanide or acid is sprayed over the heap.
  4. As, the reagent passes through the heap; the valuable metals get dissolved in it.
  5. The solution containing metal is drained from the heap and collected in a pond and the solution is sent for subsequent process for metal recovery.

Here is an illustration of the process:

Source: http://wiki.biomine.skelleftea.se/wiki/images/a/a2/HeapLeaching.png

This diagram is from http://wiki.biomine.skelleftea.se/wiki/


DESIGN


Let us now take a look at what the general guidelines say about the construction of heap leach facilities. "This Guideline provides the minimum Environmental Health and Safety (EHS) requirements for the design, construction, operation, management and closure of tailings, heap leach and water storage facilities. This Guideline does not replace any regulatory or authoritative industry Guideline or Standard."

Conceptually there are a number of designs of heap leach pads to consider. The design will be determined by the geography, amount of space available or climate. The principal types available are:

  • Reusable Pad
  • Expanding Pad
  • Valley Method

For further detail on these I would recommend studying the Leach Pad & Facilities section on the EduMine course on Heap Leach Pads. A very brief overview of these three pad designs can be found on this Golder Associates Publication.

Source: Design and Operation of Heap Leach Pads - by John F. Lupo (Golder Associates)

Source: Design and Operation of Heap Leach Pads - by John F. Lupo (Golder Associates)

In terms of the current methodologies in today's Heap Leach designs, Richard Thiel's publication on the current State of Practice, offers much insight into the thinking behind their design.

"The critical aspects of heap leach design, from a geotechnical and containment perspective, can be summarized as depth of the ore (or equipment loads for on/off pads), presence of water and local terrain."

The main geotechnical concerns of the design include:

Slope Stability
  • Global and deep-seated failures due to extreme heights and base pressures
  • Sliding block stability along geomembrane interfaces
  • Effects of active leaching, with elevated degrees of saturation
  • Long-term chemical and biological degradation of ore
  • First-lift stability affected by lift thickness (5m to 50m) and stacking direction

Seismic Stability
  • Liquefaction
  • Earthquake-induced failures
  • Possible static liquefaction flowslides

Water Management
  • Tropical installations can have large surplus water balances
  • Designs include interim catch benches and temporary caps
  • Phreatic levels range from 1 to 60m over the base liner

Liner Durability & Leakage
  • Coarse rock "overliner" systems
  • Extreme pressures caused by weight of heap and equipment
  • Durability against chemical attack - especially for 96% H2SO4
  • Valley fill systems create very high solution levels

Slope Stability

When heaping ore for leaching, the stacking heights can in some cases reach 100m in height, so an analysis of slope stability is of high importance. In general space is also limited in mining operations so there is an economic incentive maximise the volume of the ore that can be placed on a lined facility. Breitenback's paper on slope stability in heap leach he analyses the factors in heap leach construction that slope stability is calculated upon, and concludes that:

"analyses for construction of loosely placed ore heap fills is typically conservative for safe operations and do not consider the increase in fill and liner strength with time during operations"

In order to analyse the slope stability of the heap leach pad, a slope stability modelling software such as SLOPE/W could be used. 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.

Seismic Stability

Thiel remarks that while standard circular and block type failure analysis is important and can be used, but attention should also be paid to liquefaction due to areas in the pile which have become locally saturated due to leach solution irrigation. 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. The paper "Simplified Liquefaction Analysis for Leach Heaps and Dumps" gives a basic overview of the factors that affect liquefaction and how to militate against this. The seismic stability of the cover is something that would also have to be considered after heap leaching has concluded. Methods used in the landfill industry to evaluate the seismic response of landfills are also to quantify the potential for liner sliding and mass deformation. This is documented by Kavazanjian & Matasovic in their publication on hazardous wastes.

Erosion

Control of erosion during operation of the HLP is generally best affected 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.


TYPES OF HEAP LEACH PADS


Different researchers have classified leach pads into different categories.

Dirk Van Zyl (2012) has classified heap leach pads into three categories: Reusable/On-Off pads, Valley fills and expanding pads.

John F. Lupo divided leach pads into four categories: Dedicated (single use pads), Reusable/On-Off pads, Valley fills and Hybrid pads.

Richard Thiel and Mark E. Smith divided them into four categories: Conventional or flat pads, dump leach pads, valley fills and on/off pads.

Richard Thiel and Mark E. Smith also explained the key geotechnical concerns of leach pads as:

Table 1: Summary of Key Geotechnical Concerns of Leach Pads " Heap. Source: http://www.rthiel.com/Heap_Leaching-GRI_Dec03.pdf

Source: State of the Practice Review of Heap Leach Pad Design Issues. Richard Thiel and Mark E. Smith. 2003.


COMPONENTS OF A LEACH PAD


Pad Liner system: Liner system selection depends mostly on the type of material and the thickness of the layer.
Dirk Van Zyl described the liner for a heap leach pad system as:

For more information on liner systems, please click here.

Pond Liner system:

Leach Solution collection system: It's a combination of pipes and filter material which is installed directly on top of the liner.

Ore stacking system: It can be dozer and dumpers, conveyor and radial arm stackers or combination of these units.


GEOSYNTHETICS IN HEAP LEACH


Liner

The Liner system employed is one of the most crucial aspects associated with the construction of a heap leach facility. As noted in the above section, there are many factors that are considered before laying down any synthetic material at the base of the heap leach pad. According to Lupo, the geomembrane which is tasked with containing the lixiviant is only one part of the general liner configuration, which depends on the type of leach facility (single use, on-off, or valley pad). Components include:

i. Foundation
ii. Underliner
iii. Geomembrane
iv. Overliner materials

Each of these components are discussed in detail in Lupo's paper.

Foundation: The ideal foundation is one that consists of homogeneous, firm materials. A firm foundation is desirable to minimize settlements under loads which would translate to strain on the geomembrane liner and the piping networks in the overliner. However, ideal foundation conditions are seldom encountered in mine sites that may be located anywhere from the high Andean mountains of South America to the marshlands in central Asia.

Underliner: An integral component of any liner system is the underliner material. The purpose of the underliner is to provide a low-permeability layer beneath the geomembrane liner to minimize leakage of leach solutions from the facility. Ideally, the underliner should consist of fine-grained soils with the following characteristics:

Maximum particle size - 38 mm
Non-gap graded particle size distribution -- moderate to high fines (minus 200 mesh) content
Moderate plasticity
Saturated hydraulic conductivity of 1x10-6cm/sec or less

Since the underliner is in direct contact with the geomembrane, the internal and interface shear strength of the underliner are very important. The design of the underliner material is a balance between permeability and shear strength.

Geomembrane Liner: The geomembrane liner is the primary component in a liner system. Common geomembrane liner materials used in the design of heap leach facilities include Linear Low Density Polyethylene (LLDPE), High Density Polyethylene (HDPE), and Polyvinyl Chloride (PVC), with the majority of facilities being constructed using LLDPE and HDPE. When selecting a geomembrane type, it is important to consider all of the properties of the liner with respect to the anticipated loading conditions, local climatic conditions, experience of installation crew, and local construction conditions. In addition, the thickness and type of geomembrane liner should consider the following factors:

o Foundation settlement and maximum strain;
o Anticipated ore loads;
o Underliner material characteristics (maximum particle size, internal, and interface friction);
o Overliner material characteristics (maximum particle size, internal, and interface friction); and
o Slope stability requirements for the facility.

Overliner: Depending on the type of ore and leaching process, the overliner layer may consist of the following configurations:

o Single, permeable drainage layer with or without solution collection piping;
o Compacted, low permeability protection layer overlain by permeable drainage layer with Solution collection piping; and
o Single, permeable drainage layer with solution collection piping, overlain by a permeable protection layer with or without air injection piping.

Testing: Prior to the placement of any geosynthetic barrier in a heap leach pad, extensive testing must be carried out to determine what the effect of normal operation will be on the liner system. The Use of Geosynthetics in Mining Works includes a detailed study of geosynthetic components and their selection. They acknowledge that because of the harsh conditions in the area of heap leach pads, the correct choice of the synthetic liner is crucial. The specified geomembrane has to survive:

  • High chemical attack
  • Point loads from mineral drainage layer on top of the geomembrane (requirement of a geotextile)
  • Heap loading conditions
  • Site specific topography
  • Site specific climate conditions
  • Site specific construction conditions (quality control during liner installation and installation of initial mineral layers)

The thickness and the type of geomembrane (e. g. raw material, surface structure) have to be determined. Geomembrane thicknesses between 1 mm and 3 mm have been used (5 mm in tanks) (Defilippis M.O.). In the area of heap leach pads typically 1.5 mm thick geomembranes are common.

The selection of the geomembrane thickness can be derived using the "Liner-Load Test" used by Giroud (1995), reference in paper. For liner-load tests, rock particles are manually placed on the underliner surface and directly on the geomembrane to simulate field conditions. The paper gives an important key to choose the geomembrane thickness. Nevertheless it is a laboratory approach which cannot cover all possible risks. Long term creep of the geomembrane at point loads and stress cracking cannot be proven with this laboratory approach.

Many of the leaks that develop in heap leach facilities are related to rock particles left on the underliner surface or that have collected at the bottom of the mineral drainage layer. As the leach pad is loaded with ore, point loads (from the rock particles) develop on the geomembrane surface, resulting in puncture. Details of the experiment types carried out are also included in this paper.

Interestingly the manufacture's point of view can also be understood from SL Limitada's publication on the use of geosynthetics in heap leach. Testing is carried out with respect to density, thickness, stress crack resistance, oxidative induction time (OIT) and delamination to get a better idea of the durability of a geosynthetic system.

Leak Detection: Often a leak detection layer can be employed in the design of a heap leach pad. Their use in double liner systems is used to control the head on the lower liner system. According to Lupo the performance of this leak detection layer is measured through its ability to achieve these goals:

o Prevent puncture of the upper and lower geomembranes. This can be evaluated by conducting a series of liner-load tests;
o Provide sufficient permeability under load to allow collection of leakage solution, if it occurs;
o Sufficient internal shear strength to maintain heap stability under anticipated loads; and
o Sufficient interface shear strength to maintain heap stability under anticipated loads.

Thiel, Beck & Smith have run a cost-benefit analysis on the installation of leak detection system on both copper and gold mining operations. They conclude that the probability of the system's installation being economical is 96 & 97% respectively.

Drainage Collection: It is also important to remember that the collection system you use at the base of your heap leach system will also be subject to the load of the ore placed on top of the rest of the pad. Thiel & Smith bring forth the idea of soil compressibility in terms of stress distribution on the pipe. They remark "The more compacted the soil, the less deformation and stress the pipe will receive. Terzaghi's famous arching equation relates the stress on the pipe to the friction angle of the adjacent soil, presuming a granular soil. Most overliner material indeed is granular, and internal shear strengths of well over 35 degrees would be expected for the crushed stone usually employed at mine sites". Details of testing can be found in this paper and also in Lupo's paper.


ORE PREPARATION


Agglomeration

"The formation of a lump by the coalescence of smaller globules; refers to briquetting, nodulizing, sintering, etc".

A persistent cause of failure of heap leach operations is the presence of excess fines in the materials placed on the pad. Excess fines results in a low permeability material and thus the seepage rate of the lixiviant is too slow for economic pad operations.

A similar situation occurs if you intend to leach tailings. Generally tailings are primarily clay and siltthe seepage rate of most lixiviants through these fine-grained, low permeability materials is generally too slow for cost-effective heap leaching.

It may be possible to agglomerate the fines in the ore or the tailings to be leached. This normally involves adding a binder to the fines so that the resulting material consists of individual particles that are larger than the original material and hence of higher permeability.

Additives for effective agglomeration vary greatly and depend on the ore type and gradation, the chemistry of the ore and the lixiviant, and the physical characteristics of the heap leach pad, including the height and the stresses imposed on the agglomerated fine grained materials.

Typical agglomerating materials include Portland cement, lime, or ash for gold ores. Polymers have been used for coppers ores. In some cases, after crushing, sulfide ores may be treated by roasting, autoclaving, bio-oxidations, or chlorination prior to heap leaching. The web notes these recent patent applications for agglomeration additives:

No 6428597: Methods and compositions for increasing the recovery of precious metals from ore during heap leaching operations are disclosed. The methods add polypropylene glycol and alkylphenol ethoxylate in a paraffin oil solvent with the cyanide lixiviant to the ore heap.

No 5186915: Agglomerating agent and method for use in heap leaching of mineral bearing ores. A moderate to high molecular weight anionic polymer in combination with lime provides a highly effective agglomerating agent. The anionic polymer is preferably a copolymer of acrylamide and acrylic acid. The polymer preferably has a molecular weight of about 1 to 8 million or higher.

Crushing & Preconditioning

In addition to agglomeration, increasing the leachability of the ore may be carried out through further crushing, and preconditioning. The economic return associated with these increased process costs should be considered, through column testing etc., before implementation on the heap leach pad. This paper analyses the increased return of gold ores with crushing with regards to heap leaching.

In some cases the economics can be quite favorable. This link for a low grade gold ore body suggests that "leach operation requiring only a minor crush or perhaps even no crushing at all are feasible".


OPERATION


Application of Lixiviant

The materials in a heap leach pad constitute a heterogeneous, anisotropic mass. Material hydraulic conductivities vary greatly from point to point. This random variation from point to point of hydraulic conductivity is the result of the inherent in-situ variability in the ore being mined, variations in the comminution of the ore as a result of blasting, loading, and dumping, and segregation and blinding that occur during placement

While it is tempting to think of seepage of leach solution and lixiviants as a uniform vertically downward flow regime, this is not the case. The paths that the solution will take as it flows down through the mass of heap leach material will depend on these, and probably many other factors:

  • The heterogeneity of the mass, and hence the presence and pattern of channels or paths of greater permeability.
  • The moisture content of the ore which depend on the moisture content as mines, as placed, and as resulting from ambient conditions including antecedent rainfall percolation
  • The rate and pattern of application of the solution and lixiviant.

O'Kane Consultants have carried out much research into the area and they note:

Layers of coarse and fine textured ore inevitably develop within heap and dump leach piles as natural processes segregate coarse and fine material during material placement. Segregation of heap leach material will occur regardless of whether the material is agglomerated or non-agglomerated... Under such conditions leaching solution flows preferentially in the more conductive layer, potentially leaving areas within the heap unleached. The preferred flow path is not dependent entirely on the physical properties of each layer, but also on the stress state and resulting degree of saturation, and therefore the solution application rate. For this reason either the coarse or the finer material can be the preferred flow path.

Thus it is not quite as simple as multiplying the area of the pad by the saturated hydraulic conductivity if you want to establish the maximum possible application rate.

If you do succeed in applying enough solution to the top of the pad to create fully saturated flow through the heap leach materials, you will certainly be getting lots of solution through the materials, but you may not be getting the metal recovery you seek or could achieve by less aggressive solution application.

To quote O'Kane again:

Column testing revealed that solution application rates greater than the saturated hydraulic conductivity of the finer material resulted in preferential flow in the coarser layer. The preferred flow path became the finer textured material when application rates were less than the saturated hydraulic conductivity of the fine material.

In other words, to get the metal out of the finer materials and into the solution, you need to get the solution to seep through these finer materials. And that happens best when the material is partially saturated, and the seepage retreats, as it were, into the finer channels.

This leads to the counterintuitive conclusion: to increase recovery, it may be better to reduce solution application rates, rather than increase them.

Keep in mind also if you increase application rates too much you may create a saturated zone near the base of the pad, and that could induce slope failure.

Here is a random result from the web that supports the greater recovery from finer material concept:

Samples were subjected to leach ranging from 6", 4", 2" and 1" top crush size. The samples were subjected to various amounts and rates of acid addition to the column leaches. The following results have been reported by Plenge Labs for some selected samples. 1" top size intrusive mineralization containing 0.33% total copper of which 53.5% was cyanide soluble copper was subjected to an acid cure and leached in a ferric sulfate environment for 295 days. The intrusive sample leached well and 72.9% of the total copper was leached. The sample consumed 25.7 kg/t sulfuric acid. Non-cyanide soluble appeared to leach well and the mineralization was still producing copper at the time the leach was terminated. 4" top size intrusive mineralization containing 0.33% total copper, of which 53.5% was cyanide soluble copper, was subjected to a leach using an application rate of leach solution of 15 L/hr-m2 of 5 gpL H2SO4. The rock was leached for 247 days and yielded 53.5% total copper recovery. An identical sample crushed to 6" top size is still running and the total copper recovery has exceeded 60%. The copper is still leaching in the 6" column. Total acid consumption of the 4" material was 21.9 kg/t.

Maybe it is time to get a consultant to help. Here is the description of services from one:

WMC developed its proprietary Heap Leach Dynamic Technology (HLDT) to assist mines in designing, operating and closing heap leach facilities. HLDT uses a combination of geophysical, laboratory, field and modeling techniques to fully characterize and optimize the heap leaching process. With its wide understanding and experience in water flow and solute transport, WMC is able to assist mine metallurgists in optimizing key design and operational conditions that affect the hydrodynamics of the leaching process. This gives the operator control of solution-to-rock interaction, thereby greatly increasing the potential for higher yields during the leaching operation. WMC has successfully improved leaching efficiency, increased recovery and lowered costs through application of this technology at mines worldwide. Results include operational costs reduced by up to 30%, expansion of economically leachable reserves, ability to leach ores with very high fine contents and significantly increase recoveries have been demonstrated. Services offered for HLDT include:

  • Electrical resistivity tomography (ERT) surveys of heap leach facilities
  • Laboratory testing of unsaturated flow parameters affecting heap leaching in our Tucson laboratory facility
  • Field testing and instrumentation of heap leach facilities to monitor flow and leach efficiency
  • Integration of hydrodynamics with standard column tests
  • Numerical modeling of heap leach hydrodynamics
  • Evaluation of existing heap facilities for overall efficiency
  • Determination of optimum leaching application rates and dripper spacing
  • Design of agglomerates to improve hydrodynamics and recovery
  • Development of material handling and placement criteria to improve recovery
  • Studies of optimum leach cycles including variable application rates
  • Heap leach closure

Bio heap leaching

Microbes are also used to extract metals from ores in heap leach pads. This is known as bioleaching and can be used to enhance leaching efficiency. The microbes, as set out in this paper, change the redox state of the metal being harvested, or change the redox state of other metal cations which also render the desired metal more soluble and easier to harvest. Bio heap leaching is cost effective (U.S. $ 0.34-$0.60 per pound of cathode copper, depending on ore grade) and used to recover other metals such as zinc, nickel, uranium, gold and silver ores. This article by Pradhan gives us a review of the current bio-leaching techniques that are currently being employed in the copper industry.


COLD CLIMATE HEAP LEACHING OPERATIONS


The operation of a Heap leach facility in a cold climate offers a number of challenges that milder regions don't encounter, and so consideration must be given to:
  • Heap, recovery plant, ponds and fluid handling facilities must be carefully engineered to minimise heat loss.
  • Construction on permafrost should be avoided, but this may not be possible. Thawing of this would result in significant settlement and could destroy the liner.
  • Cold sensitive liners should be avoided.
  • Mining ore in winter months could contribute to the depression of the heap temperature and should be considered in planning.
  • Heap leaching above the continuous permafrost line may not be practical.
  • Insulation of pipes etc. will be necessary.
  • Low evaporative rates will cause a net increase of moisture in the system and toxic chemicals destruction will have to be considered in the design process.
  • Due to the heat sink that develops in the heap, the valley method will probably prove to be the best design option.
  • Careful management of the fluids in irrigation should prevent freezing for most of the heap so consideration should be giving to the stability issues that may arise from this.

Kenneth E. Smith's Publication on cold weather gold heap leaching is worth a read and sheds some light on the operational methods used.


CLOSURE


Principles of Closure

For a full overview on closure there are few more comprehensive resources available than the workshop on Closure, Remediation & Management of Precious Metals Heap Leach Facilities 99, which if you have enough time, you should definitely have a look at.

If are you are unable to commit yourself wholly to the concept of closure, then I would suggest you have a look at these 10 questions on the decommissioning stage of heap leach facilities. They are from the Yukon Environmental Assessment Act, under Additional Information Requirements proposed. It recommended that the proponent prepare the supporting documentation on the proposed conceptual heap decommissioning plan. This work should include, but not be limited to, the following considerations. Here are a number of questions that the Act laid out.

"10 good questions"

1. Discussion of the methods of testing done on the spent ore;

2. Presentation of all data generated from the tests;

3. Interpretation of test results, geochemical modelling, and discussion of the feasibility of detoxification of the spent ore, time estimates for how long it will take to detoxify the heap and implications of precipitate formation;

4. Details of leach rinsing and detoxification procedures including volumes, scheduling, duration, and factors that indicate when rinsing and detoxification will cease;

5. Design criteria and preliminary design for the closure treatment system;

6. Discharge effluent quality that protects the aquatic resources in the surrounding environment, and sludge disposal;

7. A discussion of the risks and uncertainties associated with the conceptual plan;

8. A evaluation of possible failure modes, contingencies in place, and an evaluation of any risks or uncertainties;

9. Description of ongoing or planned studies, objectives, and scheduling; and how study results will be incorporated into the heap decommissioning plan; and

10. Activities to be implemented in the case of a temporary closure including criteria to define when temporary closure constitutes permanent closure and final reclamation measures are to be

This paper on closure plans reports on practices gathered by one company for a number of their heap leach facilities and how they were implemented. It includes their approaches to closure and some of the tools they use in good closure practice. It is short, non-technical, but gives a good deal of useful information on the challenges a planner would have to overcome to facilitate a successful closure program.

A specific closure case study that I will also be include is on the Tonopah Mine which was closed in 2003 and became the world's first closed and reclaimed heap leach facility.

Covers

In this review we try to keep strictly to the technology associated with heap leach systems. However, there are some supporting areas that while they are not specific to heap leach, they do play an important role in the life cycle of the technology. It is for this reason I include this piece on the role of covers in Heap Leach systems.

When one considers using a cover over a heap leach pad, a permanent end of operational life cover comes to mind. However the use of temporary covers also known as Raincoat Liners (RCLs) are now used in areas where a significant amount of rainfall can occur in a relatively short amount of time or extended throughout the month-to-month wet season. There are a number of advantages to using these systems, namely, the avoidance of a surplus water balance which requires recycled storage within the ore heap, treatment and discharge to the environment, or both. Details of this can be found in this article. Thiel & Smith also include their temporary cover in their "State of Practice" paper on heap leaching.

The other type of cover is permanent and placed after the mine has ceased producing. The style of cover mentioned in this piece is designed to limit the infiltration of water into the heap leach pile where it could pick up cyanide solution. The original concern was that this affected water would find its way to Badger Creek where and affect the Red Band Trout. This was remedied by placing a geosynthetics liner over the pad, followed by waste rock and topsoil which was then reseeded. While it is common practice to line heap leach pads with geosynthetics, capping is usually done with soil mixtures only. Wood Gulch was the first case of a heap leach pad being capped using a geosynthetic liner as part of the cover solution.

I also include in this closure section, a PowerPoint on covers by Maritz Rykaart, worthwhile, as it takes a numerical modelling approach to determine the success of a number of cover designs.


HEAP LEACH TECHNOLOGY


Modelling Software:

Heap leach pads are complex problems to analyse. As we have already seen in this review, issues with application rate, collection, and partially saturated flow all serve to complicate this issue further. Computer codes to simulate and model the performance of leach pads aboundany 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. Some are more specialist than others. This paper by Jensen & Taylor takes a detailed look at the various heap leach models available, their roles and their limitations.

GoldSim is a simulation software that is used in engineering that uses Monte Carlo methods to simulate complex dynamic systems. One major application is the its use in water balance, and hence in heap leach systems. This software is used in heap leach for probabilistic simulation of rainfall inputs, and controlling parameters, processes and events that are uncertain or stochastic. SRKs website briefly discusses this topic of GoldSim in heap leach.

SoilVision Systems sells us SVFlux, which is a powerful tool that can be used in the design of a heap leach pad to analysis unsaturated flow. This can also be combined with add-ons like CHEMFLUX and SVAirFlow to further augment the analysis. The following link uses SVAirFlow to simulate leaching with air injection. . This link also gives a good run through of the different disciplines SoilVision packages that are covered in the area of heap leach simulation.

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.

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 byTerraplus.


QUALITY CONTROL


Risk Management

Risk management enables you to identify risks and take action to avoid them before you get hurt. At least that is the theory. In practice, it may scare you so much that you decide instead to do nothing.

The problem lies in the definition of risk. Try making a list of the things that could go wrong as you put your pants on each morning - enough to make you Scottish and a kilt devotee!

Heap leach pad operation can be equally as hazardous if you list the risks. At the link is a list of risks associated with operating a heap leach pad. The list includes suggestions as to how you might go about avoiding them, or at least mitigating their consequences.

This site gives you all you need to know about a risk based probabilistic assessment of a long-term cover systems for waste isolation. Generally this is relevant to reclamation of a heap leach pad but the technique can also be applied to many other processes.


PERMITTING


It is in the interest of mining to keep the environmental impact of heap leaching as low as reasonably possible. It is in many cases the only economically viable method available, so maintaining a good relationship with those who live around the mine is crucial. There have however been some disastrous public relations affairs that have blackened its name and even caused moratoria to be placed on the use of certain lixiviants, which can render viable ore bodies uneconomical. The lixiviant that is most controversial in heap leaching is cyanide.

Montana

One example where the misuse of cyanide has resulted in serious environmental damage is in that of the state of Montana. In this state, the practices of two mines in particular lead to phase out open pit and cyanide leach mining. This was set out in state law in the 1998 citizens initiativeI-137.

This citizens initiative has since halted any development of other gold projects in the state. The Seven-Up Pete Joint Venture is one such project that has been rendered not economically viable. Metallurgical testing has been carried using alternative lixiviants for extraction, but have all been proven that cyanide heap leaching is the only real option. For more on this, sign in or register for OneMine and read the article by Richard H. De Voto on banning cyanide at McDonald mine.

Romania

Cyanide also has a bad name outside of North America, in Europe this is due largely to the environmental incident that occurred on 30th January 2000. Baia Mare is a small town in northwestern Romania with a history of mining especially in gold and silver. Waste from seven mining sites was stored in over 200 different ponds and improvised tailings facilities in the area. The mine planned to reprocess these waste sites and extract the remaining gold and dispose of the tailings in a location which didnt hinder the citys development and was safe.

The process used high concentrations of cyanide, much of which was contained in a dam encircling a tailings facility in Baia Mare. The cause of the spill was deemed to be a combination of design defects, unexpected conditions and bad weather, which resulted in the spilling of 10,000m3 of liquid from the facility much of which made its way into the local river.

The result of this spill was what became known as the largest environmental disaster in Europe since Chernobyl. For more information on this please see this spill please see this report (note: Click on it first and then open a duplicate tab to view the publication).

Other Cyanide Restrictions

A similar bill was also introduced in Wisconsin after the spill in Romania and I-137 was introduced in Montana. To be added to that list, cyanide has also been banned in Czech Republic and Hungry. The importance of proper management of cyanide can be seen from these case studies as once a precedent has been set, countries and states will follow, and it will be to the determent of the industry as a whole.

"Because of the dangers of cyanide, and the disasters seen across the world, I have introduced a bill in the legislature to ban the use of cyanide in mining operations in Wisconsin. My bill would prevent a disaster such as the recent disaster in Romania and others that have been seen across the country"

-Representative Spencer Black

Failure

One of the most prominent heap leach failures in recent years was the Bellavista heap leach pad in October 2007, located in Costa Rica. The failure of this heap leach pad however can be traced long before any heavy rains encouraged any moved of the pile. From all the information that is available, this tropical region was clearly unsuitable for a heap leach pad placed on a slope. Here are the opinions of Anna Cederstav included from a newspaper article:

"Putting an open-pit gold mine in a mountainous, tropical region prone to landslides and torrential rainfall is a disaster waiting to happen"

Her concerns were documented in a 21 page review that was conducted before the mines opening. The consequences named in the report included:

"Potential landslides", "erosion and sedimentation of rivers," and "the creation of a boom-and-bust economy"

Even though these risks were highlighted in her report, it was not the report that mattered as the consultants who carried out the reviews of the mine were ultimately answering to the mining company and not the government and where hence reluctant to offer their true analysis of the situation which would have led to difficulties in procuring future work.


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-, andin situleaching, 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.


COURSES


It is also worth keeping in mind the large number of courses available on EduMine (a division of InfoMine) that deal with heap leaching. These courses are diverse and deal with them main principles of heap leaching from cyanide management to closure and everything in between.

"Heap Leach Pads": This Course is includes design, mechanics of prepared ore, construction, operation, physics of liberation of ore, and ultimately closure of the heap leach pad.

"Hydrometallurgy 1 - Introduction to Leaching": Covers the theoretical basis of solution chemistry, leaching mechanisms & processes amongst other things.

"Hydrometallurgy 2 - Leaching Processes" : Takes us through the leaching processes used for hydrometallurgical treatment of mineral ores, concentrates and metals.

"Hydrometallurgy 3 - Concentration and Purification of Leach Solutions" : Takes us through the leaching processes used for hydrometallurgical treatment of mineral ores, concentrates and metals

"Hydrometallurgy 4 - Precipitation from Leach Solutions": The final instalment of this series covers the recovery from the leached solutions.

"Cyanide Management in Mining - 3: Geochemical Properties and Environmental Fate of Cyanide": This course attempts to provide the user with the necessary background for development of a cyanide management plan that meets the unique requirements of each operating mine.

"Mine Water and Chemical Balance Analysis" : This courses could prove to be valuable in the context of a heap leach pad.

"Groundwater in Mining" : This course includes a section on heap leach pads and how they relate to groundwater

"Geotechnical Engineering for Mine GeoWaste Facilities": As is in the description of this course, it is intended for anybody involved in mining that has to manage, review, pay for, design, construct, operate, or close a geotechnical structure at a mine.

"Mine Closure: The Basics of Success" : Is the final course I include and could also be looked at last final stage in a heap leach life cycle so it is still of the utmost importance.


BOOKS


The most important book for understanding the basic of the heap leaching is The Chemistry of Gold Extraction by John Marsden and Iain House. Other important books are:

Improvements in heap leaching to recover silver and gold from low-grade resources by G E McClelland

Enhancing percolation rates in heap leaching of gold-silver ores by H J Heinen

Gold and Silver Leaching, Recovery and Economics by W J Schlitt


SOFTWARE


For heap leach pad design and analyzing the performance there are some softwares available in the market that are listed as:
  • SVFlux
  • METSIM
  • Leach, Inc.

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
A typical heap leach pad (Photo: SoilVision Systems Ltd.) 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.


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