Heap leaching discussed - Knight Piésold - Heap... · Heap leaching discussed at roundtable Bryan Ulrich, member SME, is vice president, Nevada Operations, ... During the early conceptual

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On Feb. 11, 2005, more than 20 mining profession-als gathered in Elko, NV for a day-long round-table discussion on heap leaching. Attendees in-

cluded project managers, operators and environmentalprofessionals. They represented 10 mining organizations.Knight Piésold sponsored the special event. Its sevenrepresentatives included three staff members from thecompany’s Elko Operation, and one each from its Den-ver, Cajamarca (Peru) and Vancouver offices.

The roundtable format was chosen to allow peoplefrom the mining industry to share ideas and experiencesin an open forum. Rather than presenting prepared talks,spontaneity was encouraged. This resulted in a lively andenjoyable discussion. Agenda items included:

• Facility siting and layout considerations.• Role of geotechnical site investigations.• Geotechnical design.• Liner selection.• Drainage and solution conveyance/collection.• Solution and storm water ponds.• Water balances.• Contractor selection.• Role of quality assurance in construction.• Permitting issues and environmental considerations.

• Heap loading, cycle times and lift thickness.• Problems.

Any one of these topics could have led to a daylongdiscussion. As would be human nature, discussions alsoled to related tangents, such as tailings storage facilities.In addition, lines were blurred between the topics, anddiscussions with related areas often filtered into the vari-ous subjects. Feedback forms filled out by the partici-pants indicated that the event was beneficial and thatadditional roundtable discussions should be held in thefuture.

Knight Piésold personnel initiated each topic with a

Heap leaching discussedat roundtable

Bryan Ulrich, member SME, is vice president, Nevada Operations, with Knight Piésoldand Co., 460 Silver St., Suite 106, Elko, NV 89801, e-mail [email protected] Lahti is senior associate with Knight Piésold and Co., Jr. Etan 124, Cajamarca,

Peru, e-mail [email protected].

Bryan Ulrich and Kerry Lahti

Placement and compaction of a soil liner and the placement of a geomembrane liner at a heap leach operation.

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brief introductory comment. They either opened the topicfor discussion or called on specific individuals to sharetheir experiences with the group. This article provides abrief summary of the discussions that took place duringthe meeting. This is not meant to be an authoritative andcomplete compilation of information pertaining to heapleaching, but rather a summary of the discussions thattook place.

Facility siting and layout considerationsDesigning for closure is vital to the long-term suc-

cess of any mining operation. So it is essential that min-ing facilities be situated as optimally as possible. Landownership issues often dictate facility siting. Cognizanceof property boundaries, ownership of adjacent lands andpit location play a central role in siting a mine’s variousfacilities. This facility siting process often involves a de-sign team that would include mine personnel and engi-neering consultants. A properly organized team can beequipped with the foresight to arrange all the facilitiesadequately.

During the early conceptual period of a project, thevarious facilities related to a mining project are oftenroughly sited, with predictions of future gold prices be-ing used as a guideline for the placement of facilities. Thegroup discussed the difficulty of accurately predictinggold prices. This factor can lead to facilities that becomepoorly situated in the future. Also, early in the life of aproject, funds can be limited. This restricts the amountof work that is completed for optimizing the siting of thefacilities.

Designing to allow some flexibility to changes in themetals market would have obvious advantages.

Role of geotechnical site investigationsThe importance of a well-conceived geotechnical site

investigation was discussed. Several operators spoke ofpoorly functioning facilities (not limited to the heap leachfacilities) that could have been better designed and builtif a proper geotechnical site investigation and engineer-

ing design had been completed at the inception of thework. An old adage was shared with the group: you payfor a site investigation whether you have one or not.Several anecdotes of poorly completed site investigationswere shared with the group. Each offered a lesson onreasons to carry out an intelligently formulated site in-vestigation.

The amount of time, energy and money spent ongeotechnical investigations by the mining companies de-pends on the amount of information already available.When little information is available, the mines often relyon input from engineering consulting firms to providereasonable guidelines for establishing the level of worknecessary for the site investigation. The geotechnical en-gineer should establish a justifiable site investigation.And he or she should be able to offer an explanation ofeach feature of the investigation. Many mines completerisk analyses and audits for guidance in establishing theamount of investigation that should be required. The par-ticipation of a qualified heap leach facility designer wouldimprove the results of such risk analyses and audits.

It was noted that the results of geotechnical site in-vestigations infrequently alter the general layout of thefacilities. Rather, the geotechnical design often must becompleted to take unanticipated conditions into consid-eration. For example, soft soils may require removal fromwithin the footprint area of a heap leach facility.

The early identification of material to use in thedrainage layer should be a priority of the site investiga-tion. This material can represent a major variable in thecapital cost of the heap leach facility. The selection of apoorly functioning material can become an economiccatastrophe to the long-term operation of a heap leachfacility.

The materials in immediate contact with thegeomembrane liner are critical to the proper operationof a heap leach facility, affecting solution recovery andslope stability. As such, it is imperative that these mate-rials be identified early in the design process. Redesignsteps may be necessary depending on the materials that

are identified for use as soil liner andoverliner (or protective layer).

Recontouring of the pad topog-raphy, for example, can be under-taken to overcome some of thedifficulties associated with low inter-face shear strengths. Geosyntheticclay liners (GCLs) have been usedin certain situations if insufficientlow-permeability soil is available onthe site. Designs using GCLs shouldbe developed with considerations ofits possible effects on slope stability.Facilities can also be designed usingbentonite-amended soil. Decisionsto use GCLs or bentonite amend-ment should be made recognizingpossible physical and chemicalchanges to the clay due to the chemi-cal constituents of the leachate solu-tion.

There is little information toguide a mine owner in the properway to conduct geotechnical site in-

An example of field reconnaissance activities related to heap leach facilitysiting studies. This one included a global positioning system survey.

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vestigation. However, it is evident from the discussionthat a well-developed investigation is imperative to theproper operation of a heap leach facility.

Geotechnical designThe quantification of geotechnical parameters and

properties can greatly improve the operation of a heapleach facility. The permeability of the various materialsinvolved plays a key role in the recovery of fluids. Op-erators typically apply leachate solutions at rates justbelow ponding thresholds of the ore. The heap and itsunderlying drainage system should be permeable enoughto percolate the fluids through the heap. And the net-work of drainage pipes at the base of the heap should bedesigned to rapidly convey the fluids to the perimetertrenches/pond system.

Pore pressure buildup within the heap is counterpro-ductive to metals recovery and it can jeopardize slopestability. It is, therefore, important that the solutions beallowed to exit the heap as efficiently as practicable. Onlya small percentage of heap leach facilities have instru-mentation such as piezometers installed. As such, it isnot practical to make adjustments to the heap afterwardsto correct for pore pressure buildup. And the geotech-nical characteristics of the materials should be well un-derstood beforehand to avoid such an occurrence.

Permeability testing of the ore can be beneficial inunderstanding the expected behavior of a heap leach fa-cility. The permeability tests should be carried out to rep-resent the range of conditions expected in the field. Thiswould include tests on agglomerated material, aged ma-terial and materials confined under the pressures antici-pated to be imposed by the heap.

Side slope leaching is carried out by a few of the op-erations present for the roundtable discussion. Such a

practice can lead to a buildup of solutions near the toe ofthe heap — an area where slope stability can be at itslowest. Operations where such a practice is carried outshould be developed with consideration of its impactson slope stability.

The resistance to sliding along a soil-to-geomem-brane interface is often the controlling factor that dic-tates the stability of a heap leach facility. Propercharacterization of this strength using the materials iden-tified for use in the construction of the facility is criticalto the geotechnical design of a heap leach facility. Testsshould be carried out using the range of loads anticipatedin the field, with materials compacted considering thedensity and moisture content specified for the project.Great care is necessary to correctly interpret the resultsof interface shear tests.

A sound geotechnical design can be an irreplaceabletool in establishing a priori the range of expected behav-iors that a heap may exhibit during its life. This designwould couple laboratory testing with modeling to assessslope stability as well as fluid movement through the heap.

A well-conceived geotechnical design can improvethe long-term success of a heap leach facility. It can alsohelp guide the way toward necessary alterations to thedesign.

Liner selectionThe selection of an appropriate geosynthetic liner is

crucial to the success of a heap leach facility. With heapheights continually on the rise, liner selection becomesmore critical. Thicker high-density polyethylene (HDPE)geomembranes are often selected because they offergreater puncture resistance. However, alternatives toHDPE can also be considered. These include very-low-density polyethylene (VLDPE) and linear low-density

Heap leach material being placed on a pad using a radial stacker.

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polyethylene (LLDPE). Load testing of the geomem-brane with the site materials that will be placed on eitherside of the geomembrane can be beneficial in selectingan appropriate geomembrane to match the project re-quirements.

Textured geomembranes are used more often todaybecause they provide a greater interface shear resistancewith the adjacent soils when compared to smoothgeomembranes. Welding techniques for texturedgeomembranes have improved, making their installationand quality assurance a more streamlined process. Thecost of placing texturing the geomembrane depends onthe process used to apply the texture. The performanceof the textured geomembrane also depends on the manu-facturing process. It was noted that texturing can resultin a liner with reduced elongation characteristics.

Interpretation of laboratory interface shear tests us-

ing textured geomembrane must becompleted with consideration of theintegrity of the texturing as very littleshearing may be required to erodethe texturing.

The purpose of texturing inslope stability is to increase the fric-tional resistance at that soil-to-geomembrane interface to a pointwhere a failure surface would beforced to pass through the soil ratherthan along the interface. If the tex-turing is eroded or sheared off atvery low normal loads, the “useful”interface shear strength may be es-sentially the same as that determinedfor a smooth geomembrane.

Several precautions were statedfor mining companies that are con-sidering purchasing geomembranesfrom foreign manufacturers or ThirdWorld countries. Third party con-formance testing before shipment isencouraged. An example of a heapleaching operation in Nevada thatused such a troublesome geomem-

brane was shared with the attendees.It was mentioned that geomembranes from differ-

ent suppliers might be incompatible for welding. Differ-ent resins may not match sufficiently to allow an efficientweld to be made. Foreign manufacturers may use a widervariety of resins in making their geomembranes com-pared with those manufactured in the United States.

The selection of a geomembrane is vital to the suc-cess of a heap leach facility. The final decision in geo-membrane selection should be made with adequateflexibility to allow for changes in the operation, such asincreased ultimate heap height.

Drainage and solution conveyance/collectionThe drainage layer is one of the most important fea-

tures of a successful heap leach facility. One participantnoted that a major heap leach facility had accumulated a

significant inventory buildup, not be-cause of a clayey ore, but due to poordrainage at the base of the heap.

On occasion, a cushion layermay be required between the drain-age layer and the geomembrane.This would be especially common ifthe only appropriate drainage mediaincludes large or angular particles.When such a cushion layer is not nec-essary, the terms “overliner” and“drainage layer” are often used in-terchangeably. Several commentswere made regarding the character-istics of a desirable drainage layer.

There is considerable value inobserving the placement of theoverliner material. Observationsduring this process should includeperformance of the drainage pipes,especially at their couplings. This en-

Solution delivery pipes and mounded heap material created by “fluffing”to reduce the effects of material compaction due to equipment traffic.

Placement of drainage material on a heap leach pad.

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sures no oversize or otherwise inap-propriate material is placed on theliner.

Other observations includedcarefully checking for tears in theliner and monitoring the thickness ofthe layer. Wrinkles in the geomem-brane can often be avoided by in-stalling the liner during coolerweather, including the earlier partsof the day.

Attendees reported thicknessesof overliner ranging from 460 mm to0.9 m (18 in. to 3 ft). Placement isdone using scrapers, 777 Caterpillarhaul trucks and low-ground-pressuredozers, and belly dumping trucks.

Compaction of the drainagelayer should generally be avoidedbecause the permeability of the layercan be reduced. This would reduceits ability to drain the leachate fromthe heap. It was noted that there is acurrent trend to compact materialadjacent to drainage pipes. Engi-neers and owners need to considerthis practice carefully because suchan operation could decrease the abil-ity of materials surrounding the pipesto allow fluids to flow into the drain-pipes.

Some of the operators presentprefer to have internal separationsdividing the pads into smaller cells.These separations can be achieved with berms or trenches.Without such separations, it is difficult to make adjust-ments to account for operational experiences gained asthe heap develops.

Solution and storm water pondsThere are some strides being made to avoid tradi-

tional freestanding solution ponds. Sumps and in-heapponds are two of the options being used as alternatives.Cross-valley heaps can be used for in-heap solution stor-age, to avoid traditional ponds. For facilities using sumps,there are some difficulties that designers must overcome,including concrete-to-geomembrane tie-ins.

Special considerations are often required for designsthat include concrete sumps. Concrete is often not ac-cepted as being impermeable, necessitating an additionalfeature to achieve acceptance. For sites that are topo-graphically favorable, tanks can be an acceptable alter-native to sumps.

For facilities that include traditional ponds, design-ers and owners should consider operational aspects dur-ing the design process. For example, sedimentationbuildup can create dead space in the pond. This decreasesits capacity and necessitates a cleaning program. Pondcleaning operations may necessitate additional ponds.

In addition to environmental issues, sizing of theponds should be made with consideration of a numberof factors. These include storm events, the possibility ofdowntime or electrical outages, draindown time, pump-ing capacity and backup pumps.

Water balancesThe primary purpose of a water balance is to size

ponds to accommodate operational volumes and storm-event volumes under a variety of operating and climato-logical scenarios. High-quality input is needed so that areliable model is built. This may include ore propertiesas well as daily precipitation, temperature and evapora-tion data. Intense storm events may be the driving fac-tor in the required sizing of a pond system. Heap loadingplans should be as detailed as possible, as the phasing ofheap construction can have a major impact on the pre-dictions of the water balance. Modelers should coordi-nate with facility operators to ensure the modelsufficiently reflects that way the owners will be operat-ing the facility.

The use of a Monte Carlo type simulation for cli-matic forecasting may not be appropriate. Monte Carlosimulation uses a stochastic technique. This means that ituses random numbers and probability statistics to obtainforecasts. Since precipitation and evaporation influenceeach other, independent random generation of these twofactors may be invalid.

In addition, a simple Monte Carlo simulation hasbeen observed to be a poor predictor of the cyclical na-ture of these climatic factors. Instead, the use of histori-cal data to create a statistical assessment of futureconditions provides a better sense of reliability for suchforecasts.

The impact of an extreme storm event should be as-sessed for any given month in the model. The consider-

Geotechnical laboratory testing of soils recovered during a site investiga-tion showing flex wall permeability testing.

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ation of storm events occurring in quick succession shouldalso be made as part of the water balance modeling ef-forts. Several stories of back-to-back storm events wereshared among the participants, including such occurrencesearly in the operation of a facility.

Prolonged periods of drought should also be consid-ered because the extreme lack of available solution mayaffect the project economics drastically. Through goodprediction, it may be possible to store sufficient water tokeep ahead of demand during periods of drought.

Water balances can range in complexity from simpleto sophisticated. Mine owners should select the type ofwater balance model that best fits their needs based onthe complexity of their operation as well as the level ofrisk they are willing to accept with the pond sizing.

Contractor selectionWhen selecting a contractor for the construction of

a heap leach facility, it was noted thatprice should not be the primary driv-ing factor. You are essentially select-ing individuals. Staffing ofconstruction jobs is of primary im-portance to the successful outcomeof a construction project. People ad-judicating the cost proposals of con-tractors should assess the résumés ofthe proposed superintendents andforemen. Having a trustful relation-ship with a contractor is beneficialto the construction process.

In times of economic upswingswith metals prices also on the rise,contractors can become busy, mak-ing contractor availability a criticalissue. Personnel selection can be-come an even more vital matter.

A cradle-to-grave approach withcontractors and engineering consult-ants can be beneficial to a project.Mining companies often move their

staff from project to project. In such cases, the institu-tional knowledge sometimes resides only with the con-tractors and design engineers.

Role of quality assurance in constructionThe role of construction quality control and assur-

ance (CQA) is to ensure that the client’s needs are metand that the intent of the engineering design and techni-cal specifications are met. The CQA activities are the“eyes and ears” of the designer. Complete CQA cover-age typically results in good construction documentation.It can also offer solutions for difficulties that may arisein the future, such as locating buried drainage pipes. Highquality surveying is important for solving future prob-lems. It also expedites the completion of the construc-tion report.

Mention was also made of selecting qualified per-sonnel to carry out the CQA work and that continuity of

service providers can be beneficial.Some mines have general policies toaward the CQA work to the design-ing firms, while others generally putthis work out to bid.

Permitting issues andenvironmental considerations

Although the topic was not spe-cific to closure, much of the discus-sion focused on closure issues.Attendees expressed the sentimentthat heap leach facilities are gener-ally less problematic to close thantailings facilities. However, severalchallenges still exist.

It was noted that the best engi-neering decisions for heap leach fa-cilities are often also the bestenvironmental decisions. Designingfor closure is now commonplace.Most companies throughout theworld work at or above the level of

Drainage pipes at the base of a heap leach will assist in the recovery ofleachate solutions.

Example of dust suppression and moisture conditioning of soil liner mate-rial at a heap leach operation.

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the applicable existing environmen-tal regulations. Wherever the facil-ity is, guidelines such as those of theWorld Bank should be considered.

A few of the attendees had com-pleted the closure and reclamationof a heap leach facility. In general,there is an effort toward construct-ing evapotranspiration (E/T) covers.This application has worked well inarid climates, especially once thedraindown is managed.

Depending on the characteris-tics of the ore, the draindown periodcan be lengthy, or it can be quiteshort. Various models can be usedto successfully guide closure issuesregarding draindown. With good in-put parameters, a knowledgeablepractitioner can make useful predictions that can helpguide closure initiatives.

A study is being undertaken on behalf of the U.S.Environmental Protection Agency to assess the perfor-mance of various models. The results of this study maybe useful in establishing realistic closure bonds for heapleach facilities.

Column leach tests are used to generate data on theleaching characteristics of an ore, and to allow predic-tion of the recovery/time curves. Parameters used tomodel the draindown of a heap are often based on theresults of column tests. Although these tests are typi-cally carried out under low stress conditions, compactingthe ore into the test columns at various densities to rep-resent various burial depths can approximate the effectsof deep burial. This practice can improve the outcomeof the modeling efforts.

Heap leach facilities are continually designed withgreater ultimate heights. Designers and operators mustbecome more aware of the several affects that such apractice entails. Among these considerations would bethe increased draindown time required before closing andreclaiming the facility, including the associated financialimplications.

Heap loading, cycle times and lift thicknessTruck loading is the most common method of deliv-

ering ore to a heap leach facility. Compaction issues area primary concern with the use of large mine haul trucks.Ripping and “fluffing” using backhoes or dozers, espe-cially in haul roads, is typical. One operator stated thathis mine has instituted a program to isolate finer materi-als in the central areas of a heap. This would improveslope stability conditions, as elevated phreatic surfacesand materials of lower shear strength would be isolatedin areas away from the outer slopes.

The issue of optimal lift height was raised. This di-mension depends on the particular properties of the oreand on the operating considerations. At times, the liftheight is also selected to minimize compaction of the oredue to trafficking by the loading equipment. When fea-sible (economically or for construction purposes), the useof a conveyor system is one way to avoid excessive com-paction.

Pad designs that are developed based just on the to-

tal tonnage of the material to be stacked may be inad-equate. Pads should be designed considering areas spe-cifically available for leaching, as opposed to stacking,aging, pre-wetting or other such activities. This is espe-cially important for the upper lifts where the top surfacearea decreases.

ProblemsThe attendees discussed several types of problems

in heap leach design. Insufficient overliner placementwas noted to cause damage to drainage pipes. Soil linermaterial that is too wet in the borrow area is a typicaldifficulty during construction, a condition that should beforeseeable.

There was some discussion on the use of interlift lin-ers. When ore is especially clayey, or when the drainagelayer at the base of the heap leach facility is insufficientto drain solutions freely, the use of liners and/or drainsbetween lifts can be considered. These operations aresometimes useful, but their success depends on the na-ture of the materials involved. Engineering analyses usu-ally indicate that a well-constructed liner and completedrainage system is needed to ensure the success of interliftliners.

Despite these predictions, there have been somenoted successes where facilities have installed only drain-age piping between lifts. Installing an interlift liner anddrain may preclude the lower lifts from future leachingor rinsing. Alternatively, several companies are investi-gating the feasibility of installing an irrigation systembeneath the interlift liner.

ClosingThe roundtable discussion was a considerable suc-

cess. It offered heap leach operators, project managersand environmental professionals an open forum in whichto express their thoughts and gain from the experiencesof others.

As the sponsor of the event, Knight Piésold has madethe decision to make this a recurring event. The nextroundtable discussion will be held in Elko early in 2006.It will focus primarily on tailings storage facilities. ■ (Thephotos for this article are not a product of the roundtablediscussion. They are Knight Piésold’s file photos cap-tured during the past several years.)

Example of a poorly performing heap leach facility. Photo shows sloughingof heaped materials due to elevated fluid levels within the heap.

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