precious metals heap leach facilities design, closure and reclamation

7/23/2019 Precious Metals Heap Leach Facilities Design, Closure and Reclamation

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Closure, Remediation & Management of Precious Metals Heap Leach Facilitiesedited by Dorothy Kosich and Glenn Miller. January 14-15. 1999

PRECIOUS METALS HEAP LEACH FACILITIESDESIGN, CLOSURE AND RECLAMATIONbyCarl J. Burkhalter, P E., Richard H. Allison, P E., andBrett F. Flint, P E.ofKnight Piesold LLC, Denver Colorado and Elko Nevada

ABSTRACTModem mining and heap eaching practices ncorporatestate of the art design o provide functionality, economy,expansion apability,and environmental rotection. Optimizing he functionalityand economyof heap each acilitieswith closure and reclamationplans s key to a successful roject. Whenclosureand reclamation eedsare dentifiedat he beginning of the designprocess, balancebetween peration, xpansion, nd closureand reclamation eedscanbe achieved. Currentpractice in the designof heap each acilities and currentclosureand reclamation re examinedand suggestions or an integratedapproach re developed.

1.0 INTRODUCTIONChemical solutions have been used to extract metals for hundreds of years in mining history. Modem heap leachingstarted in the copper industry in the early part of this centtJry mainly in the form of , Dump Leaching." Heap leachingto recover precious metals became wide spread in the 1960's with Nevada gold producers leading the way. In recentyears several of these pioneer facilities have reached the end of their useful life and have been successfully closed andreclaimed. As higher grade ore bodies are depleted, and heap leach technology improves, the use of heap leaching torecover precious metals will become more prevalent. As with other aspectsof mining, heap eaching inevitably involvessome necessary and unavoidable environmental impacts. Responsible mining practices and environmental legislationdictate that mining operations be planned, and completed in a manner to minimize impacts to the local environment.With proper planning and design, many of the environmental impacts due to mining, and heap leach facilities can bemitigated and reclaimed, while still allowing the development of an economic resource. Modem mining design mustconsider existing environmental conditions and long term closure and reclamation needs from the beginning of theplanning process.

1.1 Definition of Heap LeachingHeapLeaching is the tenD used or describing he hydro metallurgicalprocess f removingmetals rom lower gradeores by advancing solution at a relatively slow rate through a pile or "heap" of ore. The solution is chemicallyformulated to react with the ore at a microscopic evel o dissolveandmobilize economicmetals, hereby leaching"the desiredmetal rom the ore. The heap s placedon a gentlysloping"pad" or within a naturalvalley o allow gravitydrainageat the baseof the facility and he collection of the resulting pregnant"solution. Pregnant olution may bestored within the heapbehind a containmentberm or in an outsideholding pond from which it is pumped o a metalrecoveryplant. In the plant, he metalsare chemically emoved rom the solution, eavinga "barren" solution,whichis usually stored in an outside holding pond or tank where it is chemically econstituted or re-useon the heap. Thisprocess llows the recirculation of the solutionand maximizesefficiency. Often he pregnantsolution s recirculatedthrough he heap o "build up" it's assayand maximize he recoveryof metal.

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1.2 Definition of Closure and ReclamationAfter economical leaching of the ore is completed, the heap leach facility must be closed and reclaimed to either anexternal (regulatory) or internal (corporate) standard. Closure of the facility is defined as the chemical and physicalstabilization of the material within the heap, such that the fate and transport of solutions from the heap are conn-oIledor mitigated to prevent the contamination of the local water resources or other environmental degradation. The chemicalstabilization usually involves rinsing the heap with fresh water and/or detoxifying agents o remove any residual metalsand/or chemicals to environmentally safe standards. This process usually involves the replacement of several porevolumes of solution. The physical stabilization of the heap may involve regrading the side slopes for seismic stabilityand redirecting the outflow from the base o an infiltration or evaporation facility. Closure may also involve placingsome type of cover over the heap to minimize infiln-ation of precipitation, reduce erosion, and prepare the facility forreclamation.Reclamation of a heap leach facility is generally defined as those processes aken to rettlm the area to a post mining landuse that is considered to be equivalent to, or more productive than, the pre-mining conditions. This process addresseslong term physical stability of the facility, surface water drainage and re-vegetation. Reclamation of a heap is similarto that used for other mine facilities such as uneconomical mine rock storage areas (waste dumps). The goal of closureand reclamation is to address the visual, physical, and hydrological impacts of the leach process, and to provide apermanent system that will remove, minimize or mitigate long term impacts.

1.3 ScopeThis paper s intended o present n overview of heap each designand closure/reclamation ractice in Nevada. Itpresents discussionsand practical solutions based on the authors' experienceand points out the advantagesofincorporatingclosureand reclamation onsiderationsnto the initial stagesof planning and design. Detailed designmethods,conclusions,and/or ecommendationsre beyond he intendedscope.2.0 CURRENT HEAP LEACH DESIGN PRACTICE

Current heap leach design practice, which has developed in the past few decades, nvolves several key factors including:Site selection.Selectionof the heap ype and configuration.Metallurgical ssues.Hydrological ssues.Geotechnicalssues.Detenninationof designparameters.

Minimum design riteria for facilities o be built in Nevadaare prescribed n the NevadaAdministrative Code (NAC)Section 45A. These egulations rebasedon the performanceof the facility, so use of minimum designcriteria doesnot release he operator rom responsibility or protecting he environmentaland water esources n the area.2.1 Site SelectionThe selection of the site will depend on several actors and will influence other key components of the design process.

In turn. the key components will aid the designer in evaluating each potential site and in developing an economicalfacility that will meet the needs of the mining operation and have desirable features from an environmental point ofview. Information obtained during pre-feasibility, and feasibility level, studies is used to evaluate alternatives anddevelop a system to select the optimal site. Basic information needed will be site topography, ownership/accessboundaries, deposit location, mining plan and adjacent and use, along with available geotechnical, geological and hydrogeological information. Using this information a desk study can be completed and several site alternatives evaluated.Factors o be considered include haul distance from the pit, location of the process plant, regrading required to obtainfavorable topography, depth to groundwater, presenceof surface water (springs) and potential environmental impacts.Once an optimal site (or sites) is located, the other components discussed below are examined and the site eitheraccepted or rejected. The process is an iterative one, taking into account the variables discussed below and therelationship with other mine components. It is always good practice to have at least one alternative site in case ofunforseen events (such as discovery of economical ore beneath the facility during condemnation drilling.)


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2.2 Heap Type/ConfigurationAlthough each heap leach project is as unique as the ore it is designed to leach, there are three basic types of facilitiescurrently used:.Reusable (On/Off) Pads -A relatively small pad area is loaded with one lift of ore which is leached, rinsed andremoved. The pad is then reused again for another lift of ore. The spent ore is usually placed in a waste pile similar

to uneconomical mine rock. The pad area is usually flat and graded to drain to the opposite side from which theore is loaded and unloaded. Pad lining materials must be durable and able to withstand the constant traffic ofloading equipment. It is common to incorporate a substantial "cover" or "overliner" in any lining system thatincludes geomembranes. Pregnant solution is generally stored in a holding pond outside of the heap to avoidunloading problems. Figure I depicts a typical schematic of a reusable pad layout and basic sequence of activities.

.Permanent Ex~anding Pad -A relatively large area is graded and loaded with several lifts of ore which remainspermanently on the pad. To save on initial capital costs, the pad is usually constt1lcted in stages, hat may expandboth horizontally and vertically. Pad lining materials can be a number of options, with geomembranes being themost common. Depending on the loading method for the first lift, the lining systems may include an overliner orprotective cover. Pregnant solution is generally stored outside of the heap in a holding pond. Figure 2 illustratesa typical schematic layout for an expandable pad and the basic sequence of activities..Vallej: Fill Pad -A natural valley is used to form the leach pad. A retaining stt1lcture, usually an earthenembankment, is used to contain the ore and solution at the down gradient toe. Pad lining options can vary withgeomembranes he most common. In the area with permanent solution storage, regulations may require a doubleliner with leak detection capability. Figure 3 shows a typical schematic layout for a valley-fill leach pad and basicsequence of activities.

2.3 Metallurgical IssuesSeveralmetallurgicalssues re critical o the pad designand ayout. Most of theseare dictated by the type of ore andthe economicsof metal recovery. Key issues or pad layout nclude:.Ore PreI2aration nd Pad Loading -A successfuleachingprocess equires hat solution be in contactwith largeareasof the ore. Dependingon the host rock porosity and run-of-mine size, crushing may be needed o increasethe surfaceareaavailable or solutioncontact. For ores hat contain ine soil or rock particles agglomerationwitha binding agentcould be required o allow adequate olution low. -.Lift Height Lift height s a function of the pad areaandproposedoading ate. An iterativeprocesss usuallyusedto optimizeeconomicsof the liner areaversusexpected ecoveryand pumping requirements. Typical lift heightsrange rom 20 to 50 feet..Leach Cycle and ApplicationRate These are detennined rom laboratorycolumn leach esting. The leachcycleis basedon the time and amountof solution equired o optimize he metal ecovery-.Leach Cells -Sometimes he leacharea s physicallydivided nto cells to provide the heapoperatorwith a methodof accounting or which areasand methods re the mostsuccessful.The use of cells can help optimize solutionrecovery nd recycle. The size of the cell will be basedupon he leachcycle, solution applicationand ore oadingrates.

2.4 Hydrological IssuesA water balance for the facility is developed using the natural water and leach cycles. The natural water cycle includesprecipitation and evaporation for the site, and is detennined from meteorological data collected at the site or at regionalweather stations. Data for the natural water cycle are used to evaluate operational conditions during nonnal, wet, anddry cycles. This infonnation is used with the elements of the leach cycle (application rate, cycle time and ore adsorption)to create a water balance summary that indicates solution storage, make-up water and stonn water containment needs.The key to a successful water balance is the inclusion of all possible variables and sufficient time related meteorologicaldata to be "statistically significant" (at least 3 to 5 times the expected heap life).Infonnation from the water balance is used to design the collection system beneath the heap, size the solution holdingponds. and to predict process water supply needs. The heap collection system usually consists of a network ofperforated piping (corrugated polyethylene tubing) placed within a drainage layer at the base of the ore heap. Thesepipes direct solution flow to larger pipes or open channels, which carry process solutions to the holding (pregnant) pondlocated either inside or outside the heap. The ponds are sized to provide operating solution volume, emergency heapdraindown and a design stonn event runoff. In areas such as Nevada, where evaporation exceeds precipitation, the goal

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is to have a "zero discharge facility" that maintains process solutions in a closed circuit. In areas of high precipitation,covers or skirts may be used on the heap leach facility to limit the amount of precipitation that enters he process circuit.It is also common to use a series of ponds to improve solution management. It is often economical to size operationalponds for "normal' operating conditions and to provide an "events' pond to handle solutions from storm events or otherabnonnal operating conditions. This events pond may have a single lining on the premise that solutions entering thepond will be removed in a short period of time, once operations have returned to nonnal.

2.5 Geotechnical IssuesGeotechnical issues or leach pad design involve the stability of embankmentsand the ore heap, seepage,and settlementwithin the facility foundation. It is beyond the scope of this paper to provide a detailed discussion of slope stability,and foundation issues. Briefmention is made of key issuesbased on heap each design experience and analysis of heapleach facilities which have experienced slope or foundation movement:

Foundation StTeng!h lnfonnation on the stTength of foundation materials, especially at the "downhill" toe of thefacility, is critical to a successfuldesign. Soft foundation material may present an unsuitable condition leading toslope and liner failures. A well planned, and adequately detailed geotechnical investigation is a must.Liner Interface Shear StTeng!h Most heaps are designed with geomembrane and/or clay lining systems. The heapgeometry almost always leads o conditions conducive to block failures with principle sliding planes along the linersurface. When analyzing these failure planes it is important that good shear strength data, from the materials to beused in the facility, is evaluated and used in the design.First Lift Anal~ses -In addition to analyzing the overall slope of the heap in a block failure mode, it is importantto analyze the first lift at the down gradient toe of the facility. StTict ecommendations on lift height and loadingdirection may be required (downhill loading of the first lift should be avoided).

Seepageand settlement ssuesneed to be considered when selecting a lining system. If the system includes a syntheticgeomembrane, a liner integrity test (puncture test), using the proposed materials, liner section and anticipated loads isrecommended. Settlement analyses should focus on the evaluation of differential settlements which may lead to linerfailure.

2.6 Design ParametersBased on the above analyses, several key design parametersshould be developed for the leach pad site. These includebut are not limited to:.Liner Cross-section -Typical liners sections for heap leach facilities include single, composite and double liner

systems (Figure 4). Most leach pads use the composite system that consists of a geomembrane overlaying acompacted, fme-grained soil liner. Solution ponds are generally designed with double liners with a leak detectionsystem incorporated between he primary and secondary iners. In valley fill leach pads the in-heap solution storagearea will typically require a double lined system as well.

.Loading Plan -Using the proposed mining rates and the metallurgical information discussed in Section 2.3, aloading plan for the facility is developed. The loading plan is based on the available facility area and the leachcycle to allow sufficient time for each lift of ore to be leached and rinsed if needed, before the next lift of ore isplaced, or the ore is removed. Preliminary loading plans are prepared early in the design process to help size theleach pad area.

.Slopes. Setbacks and Containment Berms -The geotechnical study will provide recommendations on the maximumoverall slope and first lift setback so that heap material and solutions are contained during the design seismic event,or other emergency. The containment berms also prevent surface runoff from outside the pad from entering thesystem. Depending on the topographic layout and design storm event, engineered channels may also be requiredto divert surface flows away from the facility.

3.0 CURRENT HEAP LEACH RECLAMA TION/CLOSURE PRACTICE IN NEVADA3.1 General GuidelinesThe Nevada regulations for heap leach pads require a final closure plan to be prepared and submitted two years prior

to the estimated time of permanent facility closure (NAC445A.447). Regulations for heap leach pad reclamation arefound in NAC519A.345. The regulations for reclamation address requirements for stable slopes land use and re-vegetation. while the closure regulations are written to protect ground water. The final closure and reclamation plans


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must be written to specifically addresses he requirements of the regulations. These plans need to contain sufficienttechnical data to demonstrate that the requirements of the regulations will be met. As has been noted, the Nevadaregulations are performance based; f the work an operator performs for a closure plan will not meet he criteria for bondrelease, he operator must continue to revise the plan and perform modifications on the heap until these criteria are met.Since the burden of proof is on the operator, technical data must be provided in the [mal closure and reclamation plansto substantiate that methods proposed will meet the requirements of the regulations for stability, re-vegetation andprotection of water resources. The final closure plan should include data on rinsing and/or detoxification process forthe spent ore. Such infonnation can be obtained from column testing. A 4 to 12 inch diameter column is loaded withsamples of ore taken from the pit and rinsed with freshwater, or subjected to a proposed chemical or biologicaldetoxification process. The column test will provide information on the number of pore volumes of rinse water requiredto meet effluent standards and on the effectiveness of any detoxification procedures. Often a series of tests arecompleted to evaluate the effectiveness of various rinse/detoxification processes. If effluent is to be disposed of in anattenuation/infiltration site, laboratory analysis of the attenuation capacity of the soils and the percolation rate at theproposed site will be required for the final closure plan.The final reclamation lan will need o incorporate tability analyses, overdesignand re-vegetationssues. Field andgeotechnicalesting of foundationmaterialsand spentore may be required o provide input nto slope stability modelssuchas XSTABL and SLOPE/Wwhich can predict he long termstaticand pseudo-static tability of fmal slopes. Soilcover designmay nvolve laboratory esting of the proposedmaterial o provide informationon permeability,porosity,wilting point, field capacity,and soil particle distribution of the proposed overmaterials. This informationcan thenbe used in computermodeling programssuchas SoilCover,HELP, OPUS,and UNSATII to predict how runoff andinfiltration processwill function in the reclaimed acility and how it may effect he proposed egetation.

3.2 Reclamation and Closure PlansClosure and ReclamationPlansare two distinct documents n the Stateof Nevadaand are reviewed by two separategroups. The closure plan addresseshe fate and transportof the effluent rom the heapand how it will affect groundand surfacewater. Whereas he reclamationplan addressesonverting he land o anapprovedpost-mining anduse.Closure regulations,as stated n NAC 445A43, require rinsing of the heapuntil the WAD cyanide evel s reduced o0.2 mg/l or less, pH is in the range of 6 to 9, and othercontaminatesre at evelswhich will not degradeWatersof theState. Paragraph of this regulationallows alternatemethodsof chemicalstabilization o be used, f the operator andemonstratehat he resulting effluent will not degradeWatersof the State. Therefore,detoxificationwith chemicalssuchas hypochlorite or hydrogenperoxide may be approved. Biological detoxification s also being approvedwiththe use of cyanideconsumingbacteria. In addition, echnologies reavailablewhich provide a bio-reductionof metalsin the heap. Thereare currentlya numberof provenand developingechnologieshatareavailable o provide adequatestabilization of spentheap ore. As some of these echnologies an be costly, with the tradeoff of reducing he timerequired to achieve closure,a detailed laboratory esting program o evaluate ariousalternatives f recommended.In order to provide a productive post-mining and use, he reclamation lan mustaddressong term stability and re-vegetation of the heap eachpad. Generallyspeaking, mostheap eachgrading plansare successfulwith slopes latenough o preventerosion. The state may require a stability analysison slopesdependingon the site conditions. Allore must remain on he pad unless he operator an demonstratehat contaminants ill not be eleasedo Watersof theState.The MeteoricWaterMobility Testprocedure s often usedas a basis or this determination.Ideally. the final grading plan for the heap eachpad will produceoverland low. Any ditches or channels hat arerequired or long termsurfacewater control mustbe designedo be non-erosive.Generally. he industrystrives o re-vegetate o a level of productivity which existed prior to mining. The reclamationvegetation an be compared o areferencearea at the mine. It is advisable o conductsite specific re-vegetation tudies o evaluatedplant species ndplanting/seeding techniques or inclusion in the final reclamation lan. Erosion which occurson the site must berepaired and areas with high plant mortality will require replantingbefore he reclamationbond will be released.Therefore. a well engineered eclamationplan is generallya good nvestment.


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3.3 Heap Leach Pad Reclamation and Closure Design FeaturesTypical design feanlres of a modern closure design may include one or more of the following: clay or geomembranecaps. in-heap passive treatment, out of heap reatments, nfiltration/soil attenuation fields, grading which maximizes sunexposure and groWth media covers. Clay or geomembranecaps reduce infiltration, capillary breaks and growth mediaare used to stimulate vegetation. Typical clay caps have an infiltration rate of IxIO-6cm/sec or less. In-heap passivetreatments include bio-oxidation and bio-reduction treatments. The bio-oxidation treatments cultivate bacteria whichwill consume cyanide. However, these ypes of treatmentshave the potential to produce acid rock drainage if there aresignificant amounts of sulfides present in the ore. A bio-reduction treatment fills the heap with carbon dioxide andproduces bacteria which reduce the metals and cyanide in the heap by bio-reduction without the potential for creatingacid rock drainage. The effluent in the heap may be conveyed to an infiltration field where the metals in the effluentmay be attenuated by the soils.

4.0 PRACTICAL SUGGESTIONS FOR AN INTEGRATED APPROACHExamination of the designand reclamation uidelines ndicates hat good, optimal designand efficient closure andreclamation re often not completely ompatible. As mine operationsare typically sensitive o up-front capital costs,the tendency s to design acilities hatminimize up-frontcapital nvestment. These acilities may incur higher closureand reclamation ostsat he end of the facility life. By identifying potentialclosure and reclamationneedsduring theinitial planningand design process, n ntegrated esign an bedeveloped hat will meet he needsof project fmancing,operation,and final closureand reclamation. 4.1 Initial PlanningIn the initial planningstages,epresentativesrom all phases f the leach acility life shouldbe included. Typically, theprojectowner,operators, ivil andprocess ngineers, nvironmental oordinator, closure and reclamationsspecialists,and national or state regulatorsshould all be involved in the initial planning stagesof heap leach acility. Earlyinvolvement of all thesegroups will help o identify relevant ssues hat will need o be considered hroughout hedesign,operationand decommissioning f the facility.The planning process should include an identification of post-mining land use goals. Identification of post-mining landuse can then be used to develop feasible closure and reclamation alternatives that can be considered during the designprocess. It should be noted that reclamation does not imply restoration. Generally, there is no requirement to restorethe area to the same configuration and land use as existed prior to mining activities. In some cases, he resulting landforms from mining may be a beneficial addition to the area. For example, the surface of a reclaimed valley fill leachfacility may provide a relatively level area conducive to agriculture in an area of high relief. In other cases, it may bedesirable to have the reclaimed facility blend into the surrounding terrain. In either case, identification of the post-mining needs will allow the designer o incorporate elements nto the initial layout and operation of the facility that mayreduce the time and cost requirements for closure and reclamation. Involving appropriate regulatory agencies in thisprocess, or at a minimum, a thorough investigation of the regulatory requirements for closure and reclamation can alsoallow initial designs to address future needs.

4.2 Design ConsiderationsDesign issues that may impact final closure and reclamation include site selection and facility configuration. Placinga square leach pad in a flat plain may prove to be economical for design and operation, but will be difficult to blend intothe surrounding topography at the end of mine life. It may be possible to site the leach facility close to existing hills,or other mine facilities such as uneconomical mine rock storage areas so that the rmal, reclaimed configuration has anatural appearance. The use of a round pad may be as economical from an operational standpoint as a square pad andwill blend with natUral fonDs. Undulating side slopes may also blend in better.The metallurgical process used should be evaluated from the stand point of detoxification of the leached ore, and finalfate of the leach solutions. Surface water design elements should consider the needs of reclamation, including theserviceability and durability of any diversion structures that will be required as part of the fmal reclaimed configuration.Mined ore may be stable at steepslopes for the operational life of the facility, but require extensive regrading to achieveslopes that are stable for the long term and amenable to re-vegetation. Perhaps a flatter heap slope can be used.Consideration of these requirementsduring the initial design phase may allow for practical, cost effective solutions thatwill allow for economical construction, operation. closure and reclamation.


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FIGURE 1REUSABLE lON/Om PAD SCHEMATIC

,.--// OREMINE PIT

/\ , /EXPANDING LEACH PAC

,FUTURE'SEGMENT IIIIIJo~ A=vmI1SORE AND STACK ON PAD

If" NECESSARYSPENT ORE PILE IN PlACE qj:-~-"~ WATER/RfAGENT pH

~~=::::Y=~~=~~=~REGNANT RECOVERY BARRENSownoN POND SOlunDN POND

FIGURE 2PERMANENT EXPANDING PAD SCHEMATIC

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WATER/REAGENT/pH_EN SOLUTION ,lINE PIT ..LEACHING SOLUTION A

I~L- ~BARRENSOLUTION PONOI UE:TAlRECOVERY

ORE UFTS

r RETAINING STRUCTURE

PREGNANT SOLUTIONCONTAINMENT(SATURATED ZONE)

~O~CE OF l=v1Tll!S-ulNE ORE-PREPARE AND STACK IN VALLEY-LEACH-COVER WITH NEW ORE-L.EACH.REPEAT PROCESs.RINSE/TREAT If' NECESSARY-L.EAVE IN PLACE.RECLAIM

FIGURE 3VALLEY FILL PAD SCHEMA11C

A) SINGLE UNERS

ClAY

! /," " " " " , , ...",{--: ;-: ;-: ;-: ;-:;~ CONCRETE~,' ,",' ,",' ,"," ,"," ,","'-",

precious metals heap leach facilities design, closure and reclamation

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