Keynote Address: Lessons learned from the copper industry … · 2018. 2. 20. · KEYNOTE ADDRESS:...

KEYNOTE ADDRESS: LESSONS LEARNED FROM THE COPPER INDUSTRY 231

IntroductionThere have been a large number of significantdevelopments in the copper industry over the past twodecades. To a significant extent, these developments havebeen driven by the cyclical nature of the copper price cyclecharacterized by periods of strong metal price (3 to 5 yrs)followed by periods of depressed prices (3 to 5 yrs), asshown in Figure 1. As for every commodity, copper price isdriven by market forces (i.e. supply and demand). Supplyside growth is driven by incremental increases and majorexpansions at existing operations in the short term (1 to 3yrs typically), and new capacity, through exploration andnew project development in the longer term (5 to 10 yrs).Supply side decreases occur during periods of depressedprices through cut backs made at existing operations andcancellation of new development projects (and exploration).

Demand is driven by a variety of market conditions,including; 1) global and regional economic conditions(often measured in terms of GDP and industrialproduction), and particularly economic conditions in themajor copper consuming regions of the world (i.e. China,US, Europe and Japan), 2) copper scrap availability, and 3)substitution opportunities. When supply exceeds demand,the excess copper produced is placed in warehouses (LME,COMEX, Shanghai) and inventories build. When demandoutpaces supply, copper is pulled from warehouses to meetthe demand and inventories fall.

The ore grade of copper deposits being mined decreasedsignificantly through the 20th century. Between 1988 and2006, the average copper head grade declined from 1.08%to 0.84% which is a 22% drop (Brook Hunt estimates). Thishas driven the need to improve metal extraction efficiencythrough more efficient mining and processing technology,and increasing the scale of economy for operations (i.e.

higher throughput and production rates). This isexacerbated during periods of depressed pricing whenextreme measures must be taken by marginal producers toreduce costs and remain competitive, and as other producersstrive to maintain operating cash flow margins.Improvements in operating efficiency at existing operationsmay be achieved in a number of ways, including thefollowing:

• Operating cost reduction for an existing process or unitoperation

• Improvement in metal(s) recovery• Increase in throughput (and production) by de-

bottlenecking and expansions• Improvement in the quality, usually grade, of ore

delivered to the process (generally a short term benefit)• Implementation of a new technology in place of

existing process • Improvements in safety, human health/hygiene,

environmental, sustainable development and socialresponsibility aspects.

In new development projects, there are opportunities tomake improvements in operating efficiency over incumbenttechnology through the prudent use of new technology andequipment, provided that risk is assessed and managedeffectively.

Over the course of the past two decades, significantoperating efficiency improvements have been made in all ofthe areas listed above during this time. Price cyclicality hasbeen a major driver for many of these improvements, and adeterrent for others.

As readers are well aware, the gold market isfundamentally different from the copper market. Gold is not(yet, at least) traded as a commodity and the price dependson many factors including; global and regional financial

MARSDEN, J.O. Keynote Address: Lessons learned from the copper industry applied to gold extraction. World Gold Conference 2009, The Southern AfricanInstitute of Mining and Metallurgy, 2009.

Keynote Address: Lessons learned from the copper industryapplied to gold extraction

J.O. MARSDENPhoenix, Arizona

The copper industry is cyclical with the price determined by market forces (i.e. the balancebetween supply and demand for the metal.) The period of each cycle typically varies between 6 to9 years. These cycles drive decision making for exploration, new project and mine development,mine closures, process improvement, research and development, and technological innovation.The gold industry is fundamentally different from copper in that supply and demand do not, alone,determine metal price and many other factors play a key role, including global and regionalfinancial conditions, political stability, and the global future economic outlook. However, the goldprice experiences trends and longer term cycles that similarly drive business decision making.This paper examines developments in the copper industry that have occurred over the past fewprice cycles (approximately 20 years) and explores how the lessons learned can be applied to goldextraction. A number of specific examples and opportunities are discussed in detail, including;technology development and innovation, cost control and business improvement, materialcharacterization, comminution equipment, biological heap leaching, and concentrate pressureleaching.

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WORLD GOLD CONFERENCE 2009232

market conditions, worldwide, regional and nationalpolitical stability; macro-scale supply and demand for themetal, and the global future economic outlook. Golddemand is principally driven by jewellery requirements(approximately 70% of total demand), mainly in Asia,which, in itself, is a function of many related and unrelatedfactors and sentiment.

Similarly to the copper industry, gold ore grades havedeclined significantly through the 20th century, providing astrong driver for innovation to reduce costs. However, theprevailing gold price fundamentally drives the need forefficiency improvements in existing operations (to maintainoperating margins) and the development of new projects(and exploration activity). The gold market lacks clear price

cycles which occur with reasonable periodicity in thecopper industry. Furthermore, gold projects are typicallyshorter lived than corresponding copper projects, withhigher value per ton deposits mined and processed at lowerthroughput rates. This means that the gold industry tends tobe driven by near to medium term price projections withoutthe expectations of supply demand driven price cycles tocome.

This is a hugely important difference. As can be seen inFigure 2, the gold industry has been very successful indriving step, change efficiency improvements andinnovation during periods of strong pricing. In recenthistory, an exceptional period of innovation occurred in thegold industry during the period 1979 to 1988 when the gold

Figure 2. Gold Price from 1900 to 2008 showing major step change technology developments2 (Note: Insert boxes are placed on timeline toindicate approximately when the technology was commercialized)

Figure 1. Copper price from 1900 to 2008 showing major step change technology developments1 (Note: Insert boxes are placed on timelineto indicate approximately when the technology was commercialized)

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price reached historic highs in real terms, averaging over$750 per oz during the 10 year period. New technologiesintroduced successfully during or immediately after thattime, included heap leaching of low(er) grade ores, carbonadsorption and carbon in pulp, pressure oxidation andfluidized bed roasting of sulphide ores and concentrates,and biological oxidation3. However, during times whengold price is not booming, technology development andinnovation appear to go dormant and no step changedevelopments are evident. This doesn’t mean that noinnovation is occurring; clearly the gold industry workshard on improving existing processes and technology andstrives for better efficiencies all the time.

Necessity is the mother of invention, but the innovationprocess can (and should) be managed and driven evenwithout necessity or a ‘burning platform’. The copperindustry has a ‘burning platform’ every few years. Just asthe copper industry has learnt from the gold (and other)industries in the past, the gold industry can reap rewardsfrom applying lessons learned from the copper industryover the past twenty years or so. This paper discusses anumber of important developments in the copper industrythat have occurred over the past two decades, and exploresthe relevance, and potential application, of each within thegold industry.

Major examples from the copper industry

Managing the innovation processThe innovation process cannot be turned on at the switch ofa button. It has to be carefully planned and nurtured throughperiods of metal price fluctuation and uncertainty.Unfortunately, the innovation process can be (andsometimes is) turned off at the switch of a button. It needsto be flexible and adaptive, adjusting to the changing needsof the organization, but still advancing the core aspects ofinnovation that the business needs over the long term. Thehallmarks of an effective innovation process are thefollowing:

• Experienced, dedicated and self-motivatedindividual(s) to drive innovation and the innovationprocess

• A centralized applied technology development group todrive innovation

• A carefully developed strategic plan for innovation,whether it be continuous improvement or technologydevelopment

• A well coordinated, operationally focused effort, withoperators involved at every step

• The areas for innovation focus must be evaluated,ranked and prioritized

• The innovation process must be communicated clearlyto key stakeholders and progress updated frequently

• Most of the effort should go into the effectiveapplication of innovation and technology within theoperations; fundamental research is fine (andnecessary) but on its own it will not accomplish verymuch

• Progress on innovation should be reviewed routinely(quarterly or bi annually), and the strategic planupdated regularly (annually) to reflect changes inbusiness need and conditions

• Timely ‘go, no go’ decisions are made on eachinitiative (sometimes using a stage-gate process)

• In my experience, less than 5% of your people come up

with 99% of the innovation – find out who they are andput them in roles that maximize their contributions.

There is a great deal of variation in how the innovationprocess has been managed among major copper producers.The producers with the best track record in innovation(including Freeport-McMoRan, Codelco, Rio Tinto andBHP-Billiton) have been able to; 1) dedicate appropriateresources (i.e. people and money), through metal pricecycles, 2) have demonstrated the desire and drive to adoptinnovation within their operations, and 3) have a seniorexecutive management team and board that understands thecritical importance of innovation to be competitive over thelong term (rather than managing solely for quarter byquarter results at the expense of longer term value creation).The enlightened few have recognized the opportunity forsustainable, competitive advantage through effectiveinnovation4. A similar philosophy must be applied toinnovation that is used for exploration and development togrow a prosperous mining company for the long term.

The gold industry can benefit significantly from needswell planned and well executed innovation and technologydevelopment processes.

Cost control and business improvement processesDuring periods of depressed metal price, producers musttake actions to contain and reduce operating costs, and takeother actions necessary to ensure competitiveness andmaintain operating margins. Sometimes, this comes at theexpense of foregoing maintenance and equipmentreplacements on the planned schedule. Several copperproducers have implemented formal cost improvementinitiatives, such as ‘continuous improvement’, ‘six sigma’and ‘lean production’, in order to institutionalize the costimprovement processes and methods within theorganization. The hallmarks of these cost improvementinitiatives are; 1) establishing leadership and accountabilityfor the cost improvement initiative; 2) determining whatprocess will be used; 3) defining how the initiative will beexecuted and rolled out into operations, 4) establishingmetrics to measure progress, 5) setting a timeline forexecution, and 6) measuring and communicating the resultsof the initiative on a regular basis.

In the copper industry, the use of teams to indentify rankand prioritize opportunities for improvements within thebusiness has proven to be an excellent approach to this.Teams are typically comprised of individuals from acrossthe organization and are set up to focus on a particular issueor set of related issues. It is important to heavily weight theteams towards operational and technical expertise, but otherkey resources should also be included, such as; supplychain procurement representatives, human resources,information technology, environmental, safety and health.The team reviews performance and costs in a given area,brainstorms the opportunities and options for costreduction, researches the opportunities and options in detail,prioritizes which options should be tackled and in whatorder, develops an execution plan and, most importantly,follows up on the execution. Areas of focus are generallydivided up by business area (e.g. mining, milling, leaching,flotation, smelting, etc), or by cost center (e.g. energy, steel,labor, reagents, water, maintenance supplies, operatingsupplies), or by some other division that makes sense forthe business. Alternatively, focus areas can be selected in away that forces people to look at the business from adifferent (and fresh) perspective.



Another important aspect of continuous businessimprovement is the identification and specification of bestpractices. Most producers, whether copper, gold or othermetals or minerals, believe that they are utilizing bestpractices. However, few producers go through a formalprocess to determine what these practices are and whetherthey are actually using the best practice. A best practice canbe something as simple as the procedure for cleaning out acopper electrowinning cell or the procedure for eluting goldoff carbon. In either case, it is possible to have a procedurethat gets the job done, however if the procedure does notrepresent best practice then the operation is incurringadditional costs (or losing revenue). If a non optimalprocedure is used for clean out of the copper EW cell, theresult may be production of off specification coppercathode. For the carbon elution example, lack of a bestpractice procedure could result in a significant loss of goldon carbon fines or incomplete stripping of gold within thetime available.

You don’t need a best practice for everything. But you doneed best practices for all of the critical, operational steps.These need to be formally identified, written down clearlyand concisely for all to read and understand, andimplemented company-wide.

Whenever I visit an operation and we are investigating aparticular problem, I generally ask the following questions;

• What is your procedure for this• How would a new operator know how to do this (i.e. is

it written down)• Is it best practice• How do you know it is best practice?I am often surprised by the answers. Either the operator

knows the procedure, shows me a written version andknows that they have a best practice in this area. In thiscase, they nearly always teach me something I didn’t know.They are on top of the issue and we move on to the nextissue. Alternatively, the operator has a general idea of theprocedure, but cannot find it in writing and has no ideawhether or not it is best practice. Worse still, when youlook to clarify the procedure with other operators andmanagers, there is further lack of agreement and clarity. Irest my case.

Material characterization; a core competency in ourbusinessUntil recently, the need for effective materialcharacterization has been hugely under appreciated in thecopper industry. In the 1980s and 90s, copper porphyrydeposits were characterized by geological logging of coreand drill chips, coupled with chemical analysis to providetotal and acid soluble copper assays, and assays for othermetals (by-products such as Au, Ag and Mo, or bad actorssuch as As, Sb, Se etc.). The use of mineralogicaltechniques, (e.g. optical microscopy, x-ray fluorescence, x-ray diffraction), or other more sophisticated techniques, wasreserved to address special problems or concerns and wasrarely employed. This was the domain of universities andspecialist institutions.

The emergence of new tools that allow rapid and accurateanalysis of representative samples of materials hasrevolutionized material characterization in the copperindustry5,6. These techniques include the following;

• automated scanning electron microscopy (e.g.QEMSCAN and mineral liberation analyzer) formineral identification, grain size, mineral grainintergrowths, liberation and locking characteristics

• automated XRD for quantitative mineralogy• automated XRF for quantitative elemental analysis• near infrared (NIR) for quantitative clay and clay-

forming mineral analysis.These techniques are illustrated schematically in Figure 3.The techniques shown in Figure 3 are now applied

routinely, but with varying frequency and intensity, in threeareas of our business as follows;

• ore deposit characterization and modeling formine/process design

• routine production control• Metallurgical problem solving, troubleshooting and

optimization. For each area of the business, a plan must be developed

for what to measure and how often to measure it. Forexample, for ore deposit characterization metal assays areobtained for each sample interval. Mineralogical attributescan, generally, be measured on a small portion of thesample intervals (e.g. one in 20 or one in 50), depending on

Figure 3. Schematic overview of techniques available for ore characterization7



the distribution and variability of the mineral(s) of interest.Alternative mineralogy can be performed on samplecomposites. The same is true for routine production controland for metallurgical problem solving and optimization.While rigorous statistical approaches can be used todetermine the frequency of mineralogical analysis, generalguidelines and rules of thumb can be developed for eachdeposit/project fairly quickly and easily.

One key learning issue from the copper industry is thatthe new mineralogical techniques provided a huge amountof data. When planning material characterizationrequirements for each segment of the business, it is criticalthat the purpose and use of the information is consideredand agreed to by all stakeholders up front. Just because wecan measure something, does not mean that we shouldmeasure it. A good example of this is the use ofQEMSCAN in a copper production environment. TheQEMSCAN can provide an overwhelming amount of datafrom a single sample. The key information for oneparticular copper porphyry operation is what is the amountof chalcopyrite, chalcocite and pyrite in the material andhow are these minerals related to each other (grain size andlocking). If the focus of the material characterization workis limited to these species, the information can beassimilated and communicated to operating staff in ameaningful and manageable way.

Other techniques such as down hole neutron probeanalysis and cross belt neutron activation analysis are nowbeing tested in the copper industry and showing muchpromise.

In a historical perspective, the mineralogy and metallurgyof porphyry copper deposits is, generally, significantlymore complex than gold deposits, and this is certainly thecase for non refractory gold ores which can be processeddirectly by cyanidation. However, this is changing as theavailability of free milling and oxidized gold ore depositsdecreases over time. Increasingly, the gold industry isturning to the development of more refractory deposits(with gold associated with sulphide and carbonaceousminerals) and to more complex copper gold ores. Thisincreased mineralogical complexity requires a much morerigorous approach to material characterization.

Secondly, emerging mineralogical techniques provideopportunities for better ore type classification and improvedinterfacing between mining and processing operations. Agood example of this is the use of near infraredmeasurement to determine clay and clay forming mineralcontent ahead of heap leaching operations. This allows forimproved blending in the mine and between the mine andthe process to control and manage clay content, ensuringgood permeability and performance within the heap. Insome cases, material with excessively high clay contentmust be rejected to the waste stockpiles rather than bedelivered to the process. This is the case at the Morencioperation in Arizona where secondary copper sulphide(chalcocite) ore is processed by three-stage crushingfollowed by biological heap leaching with sulphuric acid.

Comminution – optimum grind size versus cash flowThere are three main aspects of comminution that warrantdiscussion in this paper. The first, considers the question ofgrind size optimization versus cash flow maximization. Forsome time, it appears that the copper industry focused oncash flow maximization at the cost of efficient use of theoverall mineral resource. Operators of large copperconcentrators ran the plants to maximize cash flow at any

given moment in time. This meant maximizing throughputat the expense of grind size. Like most gold ores, mostcopper ores have a grind size versus recovery relationshipthat is usually well established before the plant is built.Once the plant is up and running at design throughput andrecovery (hopefully), then the following question arises:‘Can we get more tonnage through the plant?’ The answeris often yes, but at the expense of grind size. The additionalcash flow from additional metal production almost alwaysexceeds the value of the recovery loss due to the coarsergrind size. It is the job of the plant metallurgist and plantmanagement to make sure that, if a decision is made tooperate at higher throughput, grinding equipment is rapidlyupgraded or expanded to get the plant back to the designoptimum grind size. Failure to do this in a timely mannerresults in misuse of the resource in the ground and loss ofshareholder value over the life of the resource.

Let us look at the example of the Chino copperconcentrator in New Mexico. The optimum grind size forthe resource was 80% minus 150 microns – this was theoriginal plant design specification. In the 1980’s, as a resultof operational improvements and de bottlenecking, theplant was operating to maximize throughput with an actualgrind size of 80% minus 300 microns. The recoverydifference for these two grind sizes was about 8 to 9%. Thecircuit consisted of primary crushing, SAG milling, two-stage crushing of SAG mill discharge oversize, ball milling,and flotation to produce copper and molybdenumconcentrates. Plant management had the choice of reducingthroughput (and consequently metal production) or addinggrinding equipment to achieve the optimum grind size. Thechoice was made to install a tertiary stage of grinding(following the existing ball mills) using Metso Vertimills.Four Metso VTM 1250 units were installed and grind sizeclose to optimum was achieved8.

Many other copper concentrators, particularly olderfacilities, have upgraded their grinding circuits and madeadjustments to operating strategies in recent years to getback to the optimal grind size for the resource.

Comminution – high pressure grinding rollsThe second aspect of comminution considers the use ofhigh pressure grinding rolls (HPGRs) as an alternative toSAG milling in large copper (and gold) operations. Untilrecently, HPGR technology has been confined toapplication in the cement, diamonds and iron ore industries.In 2004, Freeport McMoRan (previously Phelps Dodge)made the decision to install HPGR’s in place of SAG millsfor a 108,000 mt/d concentrator at Cerro Verde nearArequipa in Peru. The plant uses four 2.4 m diameter by 1.6m long HPGR units supplied by Krupp Polysius and fittedwith two 2 500 kW drives. Each HPGR is configured inseries with a double deck vibrating screen. The screens areoperated wet to ensure break down of any caked materialgenerated by the HPGR’s, and this is a key design featureof the circuit. Screen deck oversize material is recycledback to the HPGR feed bins. The plant was constructed at acost of approximately $850 million (2007 US dollar basis)and was commissioned in late 2007. The flowsheet,testwork, and design basis for this circuit has been reportedelsewhere9. The plant is operating at design throughput andclose to design availability.

The key drivers for the decision to use two-stagecrushing, HPGR’s and ball milling instead of conventionalprimary crushing, SAG and ball milling were; 1) significantoperating cost savings of $0.37/mt based on a reduction in



electrical power of 4.2 kWh/mt and lower steelconsumption, 2) manageable risk associated with thetechnology implementation at a large scale, and 3) anincrease in capital cost for the plant to provide in-surancefor equipment and plant availability9. The overall energyconsumption, including the energy equivalent of the wearsteel (grinding balls and mill liners) for a generic circuitemploying HPGR’s in place of SAG milling, has beenestimated to be 9 kWh/mt10. The technical risk for the‘new’ application of HPGR technology to treat copper orewas managed through a combination of pilot scale testwork(at Krupp Polysius in Germany), effective and thoroughmaterial characterization of the deposit, meticulousmetallurgical oversight throughout the design phase, andexperience gained at other operations around the world whowere willing to share their experiences.

Freeport-McMoRan also installed HPGR’s as a fourthstage of crushing in the C1 and C2 concentrators atGrasberg in Indonesia. The primary purpose of the HPGRinstallation was to reduce the flotation circuit feed particlesize and thereby increase copper and gold recovery. Thisapplication allowed the copper and gold production to beincreased by 5% and 4%, respectively, in the twoconcentrators11. These units have been operatingsuccessfully since 2008.

HPGR’s represent a significant opportunity for goldproducers as alternatives to SAG mills in comminutioncircuits. However, more importantly, HPGR’s provide anopportunity to generate heap leach feed material that hasimproved characteristics for leaching (better liberationalong fractures and zones favoring mineralization, and lessfines generation for better heap permeability. The copperindustry is pursuing this for a variety of sulphide and oxideleaching applications.

Comminution: high pressure grinding rolls and stirredmills in seriesThe prior discussions on HPGR and Vertimill applicationsleads to the following question: why not combine theHPGR and stirred milling in a single grinding circuit to takeadvantage of the higher energy efficiency of both unitscompared with the incumbent conventional technology?Stirred milling is estimated to be 20 to 30% more efficientthan conventional ball milling, particularly at finer sizes(<35 mesh; 500 microns). It is estimated, that this couldreduce the overall equivalent energy consumption forgrinding by between 10 to 12 kWh/mt, compared with aconventional SAG ball mill circuit [10], however, this is oretype dependent. About half of this equivalent energyconsumption is electrical energy and the other half isgrinding steel (balls and liners) expressed as an energyequivalent.

Biological heap and stockpile leachingBiological oxidation processes have been utilized forleaching of secondary sulphide (chalcocite dominant)copper ores at run of mine size for over a century. Prior tothe 1950’s, the role of bacteria in the process was notunderstood or appreciated. By the 1970’s, many operationstreating porphyry copper ores containing appreciableamounts of oxide and secondary sulphide were leachinglow grade materials in run of mine stockpiles.

Tremendous advances were made in biological heapleaching of crushed secondary sulphide copper ores in the1990s and early 2000s. Following the first modern day

crushed ore heap leach at Pudahuel in Chile, a number oflarge scale, but relatively low grade, crushed ore heapleaching operations were established at Cerro Colorado,Quebrada Blanca and Zaldivar in Chile, at Cerro Verde inPeru, Girilambone in Australia, and at Morenci in Arizona,USA. The ability to, successfully, heap leach low gradecopper ores and to achieve acceptable metal extraction wasmade possible by a number of technical advances:

• The development of stacking equipment that couldplace crushed ore onto a heap without creatingexcessive compaction and to maintain effectivepermeability within the heap

• The development of forced air injection techniquesusing blowers and distribution piping beneath andwithin the ore heap

• The use of controlled solution application rate to theheap to promote bacterial activity and growth and tooptimize copper extraction

• The control of solution chemistry (acid andferric/ferrous) in the leach solutions to promotebacterial activity and growth.

These technical advances made it possible to achieve copperextractions of 75 to 80% from chalcocite-dominant ores withinshort time frames (12 to 18 months), significantly higher thanthe typical extractions of 40 to 50% obtained from stockpileleaching of these materials at run-of-mine size over muchlonger time frames (3 to 4 years).

More recently, additional advances are in progress,including: genetic mapping of bacteria that are active incopper sulfide leaching; identification of the role andfunction of different bacterial strains in the leachingprocess; development of methods to enhance the numberand activity of bacteria that perform critical, rate-determining steps in the process; and development ofbacterial inoculation and augmentation techniques.

Finally, significant strides have been made in technologyto process primary copper sulphides containingchalcopyrite, bornite and pyrite. Chalcopyrite is quiterefractory to atmospheric acid leaching processes, even inthe presence of ferric and active iron oxidizing bacteria,because of the formation of a passivating poly sulphidelayer on the surface. Based upon experience at operations inthe USA and Chile, copper extractions achieved from large,low grade run of mine stockpiles in which chalcopyrite isthe major copper mineral are typically about 10 to 15%over a five (5) year period. However, the leaching rate ofchalcopyrite can be significantly enhanced by elevatingtemperature and or by controlling solution potential withinthe stockpile or heap. Elevated temperatures can beachieved within heaps and stockpiles by oxidizing pyritecontained in the ore. The conditions required for effectivepyrite oxidation include; sufficient liberation of pyrite toallow access to aerated solution, adequate iron in solution,solution conditions for bacterial activity (pH, no excessconcentrations of deleterious species in solution), sufficientquantity of material (and pyrite) to generate and maintainheat within the stockpile, well managed irrigation rates, andeffective aeration of the heap or stockpile.

It should be noted that the primary copper mineral borniteis less refractory than chalcopyrite, and can be leachedusing similar techniques to those applied for chalcocite-dominant ores, albeit with lower copper extractionsachieved (50 to 60%) after three-stage crushing.

Another important aspect of the development of effectivebiological heap and stockpile leaching processes is theability to model and predict behaviour within the heaps and



stockpiles. There are two reasons for doing this and, in theauthor’s opinion, these reasons, sometimes, becomeconfused and conflicting. Firstly, all heap and stockpileleaching operations need a production model that can beused to measure leaching performance and production, aswell as to forecast future production. Generally, suchmodels are based on empirical information (actualproduction data, the results of metallurgical tests and otherinformation). Secondly, in order to understand and improvethe leaching process within the heap/stockpile, we need todevelop models that adequately describe the most importantphysical and chemical processes that occur. This lattermodel can be described as a phenomenological model.Tremendous advances have been made inphenomenological modeling of heaps and stockpiles, and insome cases, these models have been calibrated against realoperating heaps and stockpiles. While these models are notperfect, they allow the user to examine the effects of manyvariables and to apply the results to the design andoptimization of heap leaching operations. Excellentexamples of the application of such phenomenologicalmodels include; effects of air injection rate, air injectionpiping spacing and aperture spacing, effects of solutionapplication rate and cycle (including on off pulsing), effectof rate of pyrite oxidation, effect of ferric ion and total ironconcentrations, and many other factors12.

The gold industry has been applying bacterial oxidationto treat refractory and semi refractory gold ores for manyyears. However, the application of bacterial oxidation toheap and stockpile leaching has been limited to a few nicheapplications (e.g. in Nevada and Ghana). There is anopportunity to tap into bio heap and stockpile leachingtechnology and expertise from the copper industry to allowlarge scale processing of low grade refractory and semirefractory ores.

Concentrate leaching of copper-gold concentratesThe copper industry, and specifically Freeport McMoRan,has made great strides in the application of pressureleaching technology to treat copper sulphide (mainlychalcopyrite) concentrates. To a significant extent, thiswork built on developments in the gold industry in the1980’s and 90’s with pressure oxidation circuits installed atMcLaughlin, Mercur, Getchell, Goldstrike, Lone Tree andTwin Creeks (all in the USA), Sao Bento (Brazil), Porgeraand Lihir (Papua New Guinea), and Campbell Red Lake(Canada)1. In 2003, the world’s first high temperature(225°C) pressure leaching circuit was commissioned atBagdad, Arizona, to treat approximately 55 000 metric tonsper year of chalcopyrite concentrate13. The process wasintegrated with the existing low grade oxide stockpileleaching, SX and EW operation at the site, and the highgrade pressure leach solution was blended with lower gradestockpile leach solution. Essentially, all of the sulphidesulphur in the concentrate was converted to sulphuric acidand was utilized beneficially in the stockpile leachingoperation. Copper recovery in excess of 98.5% wasachieved. The Bagdad concentrate contained minorquantities of gold and silver, but not sufficient amounts tojustify the installation of a cyanide leaching facility to treatthe pressure leach residue. However, testwork wasconducted on the pressure leach residue and the recovery ofgold and silver was determined to be technically feasible.

In 2007, a medium temperature (160°C) pressureleaching circuit was commissioned at Morenci in Arizonato treat 200 000 metric tons per year of copper concentrate.

In this case, the advantage of medium temperature was theconversion of a significant portion of the sulfide sulfur inthe concentrate to elemental sulphur. This was done tomatch the amount of sulfuric acid generated by the pressureleaching process with the acid requirements at the Morencisite for heap leaching of secondary copper sulphide ore.The medium temperature pressure leaching processrequired super fine grinding of the concentrate to 80%minus 7 μm and 98% minus 15 μm.

In 2008, changes were made to the Morenci mine planthat resulted in a significant increase in the amount of acidrequired for heap leaching. As a result of the changes, theprocess was converted to operate at high temperature(210°C).

These concentrate leaching developments, presentopportunities for application to sulfide-bearing gold-copperand copper gold deposits worldwide. Several flowsheetshave been developed by Freeport- McMoRan for theseapplications. An example of one of these is shown in Figure4 for high temperature (215 to 225°C) pressure leaching inconjunction with heap, stockpile or agitated tank leachingof oxide or secondary sulphide ore to utilize the acidgenerated by the pressure leaching circuit14.

In the flowsheet in Figure 4, pressure leaching isperformed at 215 to 225°C using an oxygen overpressure of120 to 200 psi. After leaching, the copper-bearing leachsolution is separated from the solids by counter-currentdecantation (CCD). The solution is combined with lowgrade leach solution from the heap stockpile tank leachingoperation which itself processes oxide or secondary sulfidecopper ore (usually lower grade material). The ratio of heapstockpile tank leach solution to pressure leach PLS (thedilution ratio) is 8:1 in the example shown. Copper isrecovered from leach solution by solution extraction (SX)and electrowinning (EW). The copper leach residue isneutralized with lime (and/or limestone), the pH adjusted to10.5 and then the slurry is leached with cyanide to extractgold. If there is sufficient (economic value of) silver presentin the ore, then a hot lime boil needs to be utilized to breakdown the silver jarosite formed during pressure leachingand to render the silver amenable to cyanide leaching. Goldand silver are recovered by carbon adsorption.

It is possible, also, to consider stand alone flowsheets forpressure leaching of copper gold ores without heap orstockpile leaching operations included. Such a flowsheet isshown in Figure 5. However, in such a case all of the acidgenerated by pressure leaching must be neutralized(representing a significant cost) and, essentially, all of thesolution must be recycled to the process and/or retreated formetals removal.

Another alternative for pressure metals recovery,particularly from the elemental sulphur-bearing mediumtemperature pressure leaching residue, is the potential useof thiosulphate leaching technology. There are still anumber of hurdles to be overcome with bringing this toreality, including the effective recovery of gold and silverfrom solution, and management of thiosulphate oxidationrate and consumption, but this is clearly an area for futurestudy and consideration3.

ConclusionsThis paper has examined several examples of efficiencyimprovement and innovation in the copper industry over thepast two decades. All of these have potential application tothe gold industry. Some of these are already in the processof being implemented in gold operations and projects, while



others present opportunities for the future. In most cases,the significant step change developments in the copperindustry have grown out of well planned and well managedtechnology development and innovation initiatives.

AcknowledgementsThe author would like to thank Freeport McMoRan Copper& Gold Inc. for permission to publish Figures 3, 4 and 5 asthey appear in this paper.

References

1. United States Geological Survey. www.minerals.usgs.gov.

2. World Gold Council. www.gold.org

3. MARSDEN, J.O. and HOUSE, C.I. The Chemistry ofGold Extraction. SME, Littleton, CO. 2006. 651 p.

Figure 5. Medium temperature pressure leaching with neutralization, SX and EW Stand alone

Figure 4. High temperature pressure leaching with heap stockpile tank leaching



4. MARSDEN, J.O. Technology Development andCompetitive Advantage: Sustainable or Short Term?Proc. Plant Operators’ Forum – Improving andOptimizing Operations: Things That Actually Work,E.C. Dowling and J.O. Marsden. (eds.). SME,Littleton, CO. 2004. pp. 127–137.

5. ZAHN, R.P., BAUM, W., MAGNUSON, R.,JENSEN, D., and THOMPSON, P. QEMSCANCharacterization of Selected Ore Types for PlantOptimization. Proc. Sixth International Copper-Cobre2007 Conference, vol. II. Riveros, P.A., Dixon, D.G.,Dreisinger, D.B. and Collins M.J. (eds.). CanadianInst. Min., Met. & Pet., Montreal, Quebec, Canada.2007. pp. 147–158.

6. KENDRICK, M., BAUM, W., THOMPSON, P.,WILKIE, G., and GOTTLIEB, P. The Use of theQEMSCAN Automated Mineral Analyzer at theCandelaria Concentrator. Proc. Copper-Cobre 2003Conference. vol. III. C.O. Gomez and C.A. (eds.).Barahona. Can. Inst. Min., Met. And Pet., Montreal,Canada. 2003. pp. 415–430.

7. MARSDEN, J.O. Technology Development in theCopper Industry. Plenary presentation. Proc. SixthInternational Copper-Cobre 2007 Conference, vol. IV(Book 1), Riveros, P.A., Dixon, D.G., Dreisinger,D.B., and Collins M.J. (eds.). Canadian Inst. Min.,Met. & Pet., Montreal, Quebec, Canada. 2007. pp. 31–77.

8. VANDERBEEK, J.L. Grinding Circuit Evolution atChino Mines Company. Proc. InternationalAutogenous and Semi-Autogenous GrindingTechnology 1996. vol. 1. A. L. Mular, D. J. Barratt,and D. A. Knight. (eds.). Min. & Min. Proc. Eng.Dept., University of British Columbia, Vancouver,Canada. 1996. pp. 25–40.

9. VANDERBEEK, J.L., LINDE, T.B., BRACK, W.S.,and MARSDEN, J.O. Proc. International Autogenousand Semi Autogenous Grinding Technology 2006.Vol. IV. M. J. Allan, K. Major, B.C. Flintoff, B.

Klein, and A.L. Mular. (eds.). Dept. Min. & Min.Proc. Eng., University of British Columbia,Vancouver, Canada. 2006. pp. 45–61.

10. MARSDEN, J.O. Energy Efficiency and CopperHydrometallurgy. Proc. Hydrometallurgy 2008Symposium in Honor of Robert S. Shoemaker,Phoenix, Arizona. Society for Mining, Metallurgy &Exploration, Inc., Littleton, Colorado. 2008. pp. 29–42.

11. MULAR, M. and MOSHER, J. A pre-productionreview of PT Freeport Indonesia’s high pressuregrinding roll project. Proc. International Autogenousand Semi-Autogenous Grinding Technology 2006.vol. IV. M. J. Allan, K. Major, B.C. Flintoff, B. Klein,and A.L. Mular. (eds.). Dept. Min. & Min. Proc. Eng.,University of British Columbia, Vancouver, Canada.2006. pp. 62–79.

12. COOPER, G.P., ROBINSON, T., BENNETT, C.R.,CROSS, M., and GEBHARDT, J.E. Evaluatingcopper sulfide leach practices with a heap leachmodel. Proc. Sixth International Copper-Cobre 2007Conference, vol. IV, Book 1. Riveros, P.A., Dixon,D.G., Dreisinger, D.B. and Collins M.J. (eds.).Canadian Inst. Min., Met. & Pet., Montreal, Quebec,Canada. 2007. pp. 245–258.

13. MARSDEN, J.O., BREWER, R.E., and HAZEN,W.W. Copper Concentrate Leaching Developmentsby Phelps Dodge Corporation. Proc. Hydrometallurgy2003 Conference, vol. 2. Young, C., Alfantazi, A.,Anderson, C., James, A., Dreisinger, D. and Harris, B.(eds.). The Minerals, Metals & Materials Society,Warrendale, Pennsylvania. 2003. pp 1429–1446.

14. MARSDEN, J.O. and WILMOT, J.C. Sulphate-basedflowsheet options for hydrometallurgical treatment ofcopper sulphide concentrates. Proc. Sixth InternationalCopper-Cobre 2007 Conference, vol. IV, Book 1.Riveros, P.A., Dixon, D.G., Dreisinger, D.B. andCollins M.J. (eds.). Canadian Inst. Min., Met. & Pet.,Montreal, Quebec, Canada. 2007. pp 77–100.

John O. Marsden President, Metallurgium (John O. Marsden LLC)

Twenty-seven years of broad international experience managing major, innovative copper, gold,molybdenum and cobalt operations and projects in the USA, Chile, Peru, Mexico, South Africa,and the Congo (DRC). Marsden has held various operations and technical management positionsduring nine years with Consolidated Gold Fields plc and eighteen years with Freeport-McMoRanCopper & Gold Inc. (formerly Phelps Dodge). Author of “The Chemistry of Gold Extraction” (1stand 2nd editions).


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