SGS MIN 2012 04 Recovering Refractory Resources en 13 09

$: SGS MIN 2012 04 Recovering Refractory Resources en 13 09$
7/18/2019 SGS MIN 2012 04 Recovering Refractory Resources en 13 09

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148 PRECIOUS METALS PROCESSING

Recovering

refractoryresourcesRefractory gold ore needs pre-treatmentfor cyanidation to be effective in goldrecovery. Ailbhe Goodbody looks at theadvantages and disadvantages of themost common pre-treatment options,and speaks to some companies thatoffer or use the processes

Arefractory gold ore is gold-containing ore that is resistant torecovery by direct cyanidation

and carbon adsorption processes. Morespecifically, it is an ore that has a goldrecovery rate of less than 80% when directcyanidation is applied to it.

Dr Chris Fleming, senior metallurgicalconsultant at SGS Minerals, says: “In thepast two to three decades, gold recoveryfrom refractory ores has received anincreased amount of attention due to ahigher number of orebodies failing torespond adequately.”

Refractoriness (the degree of resistanceto standard recovery methods) isgenerally due to total encapsulation ofextremely fine gold particles by a hostmineral that is impervious to the cyanideleach solution. As a result, refractory oresrequire physical or chemical pre-treatmentfor adequate gold recovery to beachieved through traditional cyanidationand carbon adsorption processes. Pyriteand arsenopyrite are the most commonhost minerals in refractory gold deposits.

“Recent introduction of hydrometallurgi-cal pre-treatment processes, such as

pressure oxidation and bacterial leaching(bio-oxidation), has given miningcompanies more options for treatingrefractory ores,” says Dr Fleming. “In manycases, these new technologies have foundfavour over traditional roasting practices.”

There are four common pre-treatmentoptions for refractory gold ores: roasting;pressure oxidation; bio-oxidation; andultra-fine grinding.

In roasting, pressure oxidation andbio-oxidation, the iron sulphide mineralsare oxidised to create sulphur dioxide(SO2) gas in the case of roasting, orsulphate ions in pressure oxidation andbio-oxidation. The iron component isoxidised to the trivalent state and formssolid compounds such as haematite(roasting), basic iron sulphate or jarosite(pressure oxidation and bio-oxidation) and

soluble compounds such as ferric sulphate(pressure oxidation and bio-oxidation).Ultra-fine grinding is a strictly physicalprocess and there is no chemical changeto minerals in the feed.

Mineral concentration steps, such assulphide flotation, often precede thepre-treatment processes. By reducing thevolume of ore going in to the pre-treat-ment process, the size of the equipmentneeded is also decreased.

“The infrastructure needed forrefractory gold processing variesdepending on the method chosen,” says

Rachel Bridge, metallurgist at Hatch. “Theprocess steps common to each methodare crushing, grinding, gravity concentra-tion, intensive cyanidation of the gravityconcentrate and all refinery processesdownstream of carbon-in-leach (CIL).”

Dr Fleming comments: “It is importantto note that no single pre-treatmentprocess provides the best economic andenvironmental outcome for all cases. Eachprocess has strengths and weaknesses.Often at SGS we evaluate all fourpre-treatment options during feasibilitystudies to ensure the customer gets an

economically viable solution.”The decision on which method of

refractory gold recovery to choose isusually based on the economics of

investment and operating costs (OPEX).These processes are all expensive andeconomic considerations have traditionallyweighed heavily on the final processdecision. Marcus Runkel, senior processengineer at Outotec, says: “The economicsare driven by the gold recovery rate. Thepre-treatment of refractory gold ore shouldincrease the gold recovery rate in thecyanide leach process by 30-40% in orderto be economical”

“Large operations tend to chooseroasting or pressure oxidation, whilesmaller operations tend to select bacterial

leaching,” says Dr Fleming. “Otherfactors, such as gold and silver recoveryrates and environmental regulations, alsodetermine which process will be used.”

However, environmental regulationscan sometimes outweigh cost and goldrecovery. “It is very difficult or evenimpossible to get a permit to build aroaster in some countries,” adds DrFleming. “Other countries will not allowarsenic to be handled in any way otherthan pressure oxidation. While there isn’tone set procedure for refractory gold oreprocessing, SGS looks at every situation

separately, to evaluate which option is thebest fit.”

ROASTINGRoasting is generally applied to sulphideminerals and refractory ores. It involves athermal gas-solid reaction during whichsulphides and sometimes organic carbonare oxidised to SO2 and carbon dioxide(CO2) at temperatures between 500ºCand 700°C. Mr Runkel of Outotec says:“Roasting plants can be designed with athroughput up to 5,000t/d or higher perline, depending on the ore composition.”

Dr Fleming says: “Prior to the 1970s,roasting was the only process available tometallurgists for treatment of refractorygold ores and it was applied in most of

The advantagesthat Gold Fields’BIOX offers are

very project-specific, but the

company says

that it has seenimproved rates

of gold recovery

A sample of veingold from

Timmins Ontariomining camp

“In the past

two to three

decades,

gold

recovery

from

refractory

ores has

received an

increased

amount of

attention”


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the gold-producing regions of the world.”The reactor types are: bubbling fluidised

bed (FB) or circulating fluidised bed (CFB).A FB roaster operates at relatively low gasvelocities, with the particles kept inbalance against their own gravity. Ms

Bridge says: “A CFB roaster operates athigher gas velocities, and is used toevaporate residual moisture and roast theflotation concentrate, oxidising 98% of thesulphide and 65% of the carbon.”

Gold ores are typically pre-concen-trated through a flotation process prior toroasting. For ores that do not respondwell to flotation, recent technicalinnovations in roasting have made itpossible to treat whole ores. This has alsoallowed for the re-evaluation of depositspreviously thought uneconomical forexploitation, such as those found in the

southwestern US.Mr Runkel adds: “The removal of

impurities such as arsenic and mercury inparticular can be done very effectively in aroasting step, which is always designed incompliance with the environmentalregulations. Another factor is theproduction of sulphuric acid and heatrecovery in the form of high-pressuresteam, which in special applications canimprove the economic model.”

The advantages of roasting include

high gold recovery and the potential for a

slight capital cost (CAPEX) advantageover alternative methods of treatingrefractory gold ore. “The roasting oxidantcan be air or pure oxygen, so there mayno need for a dedicated oxygen plant,which will lower the overall CAPEX,”explains Dr Fleming. “If air is used, theSO2 that is generated is generally toodilute to make acid, so it is scrubbed fromthe gas phase and neutralised withlimestone. If the plant is going to producesulphuric acid as a saleable by-product,

then it will generally have to use oxygenas the oxidant and the CAPEX will behigher. This can be offset by acid sales.”

However, roasting will not provide largesavings if an oxygen plant, in combinationwith comprehensive scrubbing of the

gases, is required to meet stringentmodern standards for discharge of toxicelements in the gas phase. Roasters alsohave to recover arsenic trioxide (As2O3)and SO2 from the gas very efficiently tomeet environmental standards and theCAPEX of gas capture can be expensive.

Ms Bridge says: “While the goldrecovery by roasting is high, the higherCAPEX and OPEX requirements neededto comply with environmental standardshave caused roasting to be lesseconomically attractive in some regions.”Concerns over the potential for

environmental damage have made itdifficult to obtain an operating permit fora roaster in a number of countries.

Another product of the roasting processis haematite (Fe2O3), which is made byconverting the iron in pyrite andarsenopyrite. This is a desirable product forthickening, filtering and storing in tailingsas it is dense and compact. It is alsochemically inert, eliminating the possibilityof release of acid or heavy metals into theenvironment. However, when the feed

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SGS autoclaveset-up for gold

processing

“While gold

recovery by

roasting is

high, the

higher

CAPEX and

OPEX have

caused

roasting tobe less

attractive in

some

regions”



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153

September 2012

PRECIOUS METALS PROCESSING

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contains arsenopyrite, the process canbecome quite complex, as the roastershave to be operated in two stages.

When implementing a roasting plant,the longest period is typically in theconcept stage. Mr Runkel explains:

“Roasting test work and finalising thedesign basis should be done within a year.For a lump sum turnkey roasting projectincluding the roasting, gas cleaning andsulphuric acid unit, the timeframe fromstart of the contract to mechanicalcompletion and start-up of the plant cantake 2-3 years.”

PRESSURE OXIDATION

Pressure oxidation, or autoclaving, wasoriginally developed for processing basemetal concentrates. In the last 30 years, ithas been adapted for processing

refractory gold ores and concentrateswhere gold is trapped in sulphideminerals, such as pyrite or arsenopyrite.

During this process, the sulphides areoxidised by pure oxygen at an elevatedtemperature and pressure in an aqueousslurry. This breaks the sulphides down to asolution phase consisting of metal sulphatecompounds and sulphuric acid. The goldlocked in the original sulphide mineral iscompletely liberated, allowing very highgold recovery to be achieved when theproduct is treated with cyanidation.

The use of pressure oxidation has

become common in the past 20 years as aresult of better gold recovery efficiencywhen compared to roasting, and aninability of first-generation roasting plantsto meet increasingly strict environmentalcontrols on sulphur and arsenic discharge.

Advantages of pressure oxidationinclude: high gold recovery; and sulphideminerals break down completely intosolution, allowing the gold locked in thesulphides to be fully liberated. Pressureoxidation is also capable of handling arange of feed rates. Dr Fleming says:“Pressure oxidation gold recovery rates

are a minimum of 10% better than thoseof roasting.”

Toxic elements such as arsenic andsulphuric acid are produced in solution,rather than in the gas phase as in roasting.These by-products are much easier tocontain and stabilise as environmentallybenign products when in solution. Arsenicis converted to ferric arsenate (scorodite)in the autoclave, which is as stable as theoriginal arsenic sulphide mineral and canbe disposed of to the process tailingswithout further treatment. The acid isconverted to gypsum, a stable and

environmentally benign compound.However, the technology has some

disadvantages as well: it has a higherthroughput and requires larger equip-

ment, such as an oxygen plant, makingthe overall CAPEX typically higher thanbio-oxidation or roasting. Also, aworkforce with technical expertise isrequired to operate high-pressure vessels.

Pressure oxidation is not well suited for

feed materials with high levels of silver, asthe silver often reacts with iron in theautoclave to form a silver jarositecompound that is resistant to cyanideleaching. Expensive measures have to beadopted to liberate and recover the silver.

There are some environmental concernsabout the technology. Traditionally, only asmall percentage of the iron in pyrite andarsenopyrite is converted to haematiteduring pressure oxidation, and the majorityends up as jarosite or basic iron sulphate.These compounds can cause metallurgicalchallenges in downstream precious metal

recovery and both have the potential torelease acid and heavy metals to theenvironment.

To meet environmental regulations inmost gold-producing nations, the tailingsfrom a pressure oxidation plant may haveto be deposited in a lined tailings pond.Creating a tailings pond will result in anincreased CAPEX for the project.

BIO-OXIDATION

In bio-oxidation, certain strains of bacteriaare used to accelerate the natural processof sulphide oxidation. These bacteria

metabolise energy from the oxidation offerrous ions and sulphides. The by-prod-ucts then report to the aqueous phase asmetal sulphate compounds and sulphuricacid (similar to what happens in thepressure oxidation process). Goldrecovery is very high and is similar to whatis achievable using pressure oxidation.Bio-oxidation is a reasonably well-estab-lished technology with several plantsoperating globally.

The largest interest in bio-oxidation isfocused on high-grade concentrates bytreatment of finely milled solids in stirred

tank reactors. Dr Fleming adds: “Animportant characteristic of bacterialleaching reactions is that the bacteria areautocatalytic; ie, the bacteria, which arethe catalyst for the reaction, are also

beneficiaries of the reaction, allowingthem to multiply.”

Bio-oxidation enjoys lower CAPEX than

roasting and pressure oxidation, and has aslightly lower net present value (NPV) thanpressure oxidation, so it often producesthe best overall economics for smaller-scale operations. The process uses air tooxidise the sulphides, so no oxygen plantis needed. The process is simple tooperate with limited expertise required,so operators can be sourced from localcommunities and trained.

“For the bio-oxidation option,alternatives in various design details canbe investigated to reduce CAPEX andOPEX,” says Ms Bridge. “Also, bio-oxida-

tion designs are exceptionally well suitedfor incremental expansion to accommo-date a mine plan calling for a productionramp-up over time.”

Bio-oxidation has a lower environmentalimpact than some other technologies.Toxic elements such as arsenic and acid aregenerated in solution and are easier tocontain and stabilise as environmentallybenign by-products. The sulphide mineralsbreak down completely into solution.However, arsenic is converted to anamorphous ferric arsenate compound inthe tailings, which is not as stable as

scorodite. Amorphous ferric arsenate couldcreate issues when attempting to meetenvironmental regulations in some regions.

Bio-oxidation is best suited forsmaller-scale operations due to the longresidence time required for oxidation ofthe sulphides. Dr Fleming explains:“Residence time for bio-oxidationprocesses is typically 4-5 days, comparedwith roughly 1h for pressure oxidation andless than 30 minutes for roasting. The plantfootprint will, therefore, be much larger forbio-oxidation than the other alternativesfor a given ore throughput capacity. This

becomes an issue if space is limited.”Cyanide consumption can also be a

problem. Cyanide consumption is typically10-20kg/t, versus a maximum of 1-2 kg/t

Gold Fields’BIOX uses amixed population

of bacteria tobreak down thesulphide mineralmatrix

Gold Fields’BIOX technologyis in use at

Eldorado Gold’sJinfeng plant inChina

“Bio-

oxidation

designs areexception-

ally well

suited for

incremental

expansion

to accom-

modate a

mine plan

calling for a

productionramp-up

over time”



155PRECIOUS METALS PROCESSING

September 2012 www. .com

after pressure oxidation or roasting. Thehigh cost of new cyanide purchases, aswell as the cost of cyanide detoxificationin the tailings, can negate any benefitsgained due to the lower cost of oxidationby air. As a result, OPEX in bio-oxidation

can be the highest of the three processes.Minor metal impurities may poison the

bio-organisms. The main consumer ofcyanide is the reaction between cyanideions and sulphur, which is a by-product ofthe bacterial oxidation of sulphides. Thisreaction produces thiocyanate in solution.Thiocyanate is very toxic to the bacteriaused in the bio-oxidation processes, sogreat care must be taken to avoidrecycling tailings water containingthiocyanate back to the bio-oxidation partof the process.

Gold FieldsGold Fields owns the BIOX biologicalprocess, which uses a mixed population ofbacterial cultures to break down thesulphide mineral matrix and liberate theoccluded gold for subsequent cyanidation.

BIOX development started in the late1970s and early 1980s at Gencor’s ProcessResearch Facility (now Billiton ProcessResearch). At that stage, Gencor ownedthe Fairview gold mine in the Barbertonarea of South Africa and it needed alow-cost technology to treat a concen-trate stock pile in parallel with the

Edwards roasters it was operating. Thefirst 10t/d plant was commissioned in1986 to treat the refractory concentrate.

The performance of the process wassuch that Gencor continued to operateBIOX in parallel with the roasters, feedingfresh concentrate after depletion of thestockpile. The roasters were nearing theend of their life at that stage, and in 1991the decision was taken to switch to BIOXtreating 100% of the feed concentrate.The decision was based on the improvedprocess performance and lower OPEX ofBIOX over the roasters. Importantly, BIOX

was more environmentally friendly thanthe roasters, as it was able to fix thearsenic in the ore in a stable precipitatethat could be discharged to the tailingsdam.

In 1997, the gold assets from Gencormerged with those of Gold Fields SouthAfrica to form Gold Fields. The Fairviewmine and the ownership of the BIOXtechnology transferred to Gold Fields atthe same time. Gold Fields sold theFairview mine shortly after but retainedthe BIOX technology.

The advantages that BIOX offers over

alternative processes for refractory goldrecovery are very project-specific, but thecompany says that, in general, it has seenimproved rates of gold recovery,

significantly lower CAPEX, lower running

cost, more robust technology that issuited to remote locations, lower levels ofskills required for operation, low impacton the environment and on-going processdevelopment and improvement.

The technology is also backed by 25years of operating experience, and GoldFields says the fact that it is owned by amajor gold mining company ensurestechnical support into the future. Thecompany says that approximately 1.5Mozof gold was produced from BIOX in 2011,with the cumulative gold production fromBIOX estimated at around 16Moz to date.

“Although CAPEX and OPEX structuresare project-specific, we have seen anincreased focus on the ‘non-financial’advantages,” says Jan van Niekerk, seniormanager for BIOX at Gold Fields. “Theavailability of skilled people and theability to use local semi- or unskilledoperators on the plant become importantconsiderations when operating a mine inremote locations. With increasingvariability in ore grade and quality in mostnew projects, we are seeing factors suchas the flexibility of the technology,turndown capacity and ease of expansion

also becoming important factors duringthe selection of the technology. This isagain where BIOX offers significantadvantages over processes such asroasting and pressure oxidation.”

Another advantage of BIOX is thetreatment of arsenic-containing concen-trates. Arsenic is dissolved in the BIOXprocess, and afterwards is fixed as a ferricarsenate during neutralisation. BIOX putsthe arsenic into a stable form that can besafely deposited onto a tailings dam. Testwork on the old tailings dams at PanAfrican Resources’ Fairview mine in South

Africa confirmed that, after 25 years, thearsenic precipitates are still stable.

Gold Fields is working on the fourthgeneration (Generation IV) of BIOX, and

hopes to have it available by the end of

2013.For more information on the BIOX and

ASTER processes, see the May 2012 issueof MM for an exclusive interview with Janvan Niekerk.

REBgoldREBgold owns a bacterial oxidationtechnology called BACOX, which usesnaturally occurring bacteria to separate outthe gold. The technology provides asuitable environment for the bacteriawithin tanks, creating what the companycalls a ‘Garden of Eden’ for them to live in.

Originally, a group from King’s CollegeLondon, UK, isolated the bacteria thatcould make the iron, arsenic and sulphideminerals associated with refractory goldores soluble and cultivated them in thelaboratory. Studies were then conducted toevaluate the bacterial requirements, suchas nutrients, temperature and acidity, tocreate agitated aerated reactors thatprovide the ideal environment for bacterialgrowth and division, as well as for thesolubilisation of refractory gold ores.

BacTech was formed to commercialisethis technology in the early 1990s, and it

discovered how to scale up the processand make it work on a continuous basis totreat refractory gold ores as part of anormal gold processing operation. Thetechnology was also developed further fortreating complex base metal concen-trates, allowing the production of on-sitemetals from polymetallic ores that aredifficult to treat.

The first application of the technologywas in Western Australia in 1994, wherethe company was originally based beforeobtaining a listing in Canada. Thetechnology was then applied in Tasmania

and in China. The plant in China treats avariety of different concentrate types, andwas so successful that it was doubled incapacity a few years ago.

In addition to BIOX, Gold Fieldsalso offers its Activated SludgeTailing Effluent Remediation

(ASTER) technology. This is a fullycommercial process that focuseson the destruction of biologicalcyanide and thiocyanate, whichare by-products of BIOX. The firstoperation has been running forover 18 months and the companysays it has been giving consistentlygood results, so it would be ofinterest to any mine evaluatingoptions for cyanide and/orthiocyanate destruction.

ASTER

Gold Fields’ ASTER technology focuses on the destruction ofbiological cyanide and thiocyanate

“BIOX was

moreenvironmen-

tally friendly

than the

roasters, as

it was able

to fix the

arsenic in

the ore in a

stable

precipitate”




REBgold says that the separationtechnology achieves in six days whatwould take 20 years to occur in nature. Italso says that the tailings created arebenign and there is no risk of environmen-tal damage. The typical gold recovery rateis 90-95%.

The BACOX process can work at a

variety of scales. Concentrate treatmentrates vary from 30t/d to 1,000t/d ofconcentrate, which is generally equivalentto between 200t/d to 5,000t/d of ore. Thecompany’s largest installation processes200t/d of concentrate, which is roughlyequivalent to 1,000t/d of ore. The processis modular, making expansion at a laterdate very easy.

The process can treat a variety of oretypes through the life of a project, fromtransition ores to fully refractory ores.

REBgold says that for many operations,bioleaching technologies are now thetreatment method of choice, due to thelower costs and environmentallyresponsible practices associated with theprocess. BACOX is well suited to a variety

of locations, even those in remote areas.A reasonable quality of water is

required together with a good source oflocal limestone, which the company saysare usually readily available. REBgold hasbuilt a plant in a desert region where thewater was mildly saline, and says that itworks well. The process maximises waterre-use to minimise discharge require-ments.

The timeframe for implementing theBACOX process is project-specific.However, REBgold says that gettingpermits for projects is often easier with

BACOX processing due to the productsbeing environmentally benign, and thatlead times can be shorter than with othertechnologies, as no specialised equipmentis required.

Test work protocols are well establishedfor taking projects from a conceptual basisthrough to commercialisation, so it cantake as little as one year from test work toengineering, design, construction andcommissioning, depending on permitting.

The capital investment is also very

project-specific, but the company saysthat it is generally half of that required forpressure oxidation.

REBgold says that the technology isvery good at managing toxic elements.The company says that it handles arsenic

and iron as part of the ore composition inmany projects; the bacteria make thearsenic and iron soluble, and a benignstable waste is produced by neutralisationof this solution with limestone. Theprecipitate produced as part of theprocess binds the arsenic very stronglyinto a matrix, meeting US EnvironmentalProtection Agency (EPA) or equivalentregulations for safe disposal.

REBgold is currently examining anumber of projects for the possibleapplication of BACOX in a variety of coun-tries. It is active in Finland, where there is

a mixture of prospects from conventionalnon-refractory gold resources through tosulphides where use of the technologymay become appropriate.

The company believes that there aremore projects in Europe and SouthAmerica where the technology couldprovide an advantage, as well as an entryto projects by economic participation, asREBgold not only provides the technologybut also help to secure funding for theright project.

A REBgoldBACOX plant in

operation

Thermophilicbacteria used in

REBgold’sBACOX process



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ULTRA-FINE GRINDINGUltra-fine grinding is when grinding is usedto reduce the particle size of the hostmineral so that part of the gold surface isexposed, allowing contact with cyanidesolution. A benefit of this technique is that

the host mineral is not destroyed in anoxidative chemical reaction, which avoidsthe resulting problems of how to treat thereaction products.

However, such fine grinding isincreasingly energy-intensive with each sizereduction step. Dr Fleming explains: “Goldparticles in refractory deposits aregenerally less than 0.1micron in size and itis impractical, if not impossible, to grindsulphides to a fine enough state to liberatethe gold particles.”

He continues: “Ultra-fine grinding, ifsuccessful at liberating gold, provides the

lowest CAPEX and OPEX of the fourprocesses. Unfortunately, it is very seldomeffective as a technique. However, becauseof the favourable economics, ultra-finegrinding should always be tested andevaluated during project feasibility studiesfor the rare occasion that it proves to bebeneficial.”

Maelgwyn Mineral ServicesMaelgwyn owns the Leachox process,which was developed by combining itsexperience in gold processing with its flota-tion technologies to modify the Imhoflot

pneumatic flotation aerator. This allowed itto be used as a highly efficient mass-trans-fer device to introduce oxygen into slurry,which is combined with ultra-fine grindingof a flotation concentrate.

The concentrate is then fed toMaelgwyn’s Aachen reactors to providelow-pressure partial oxidation of sulphidescombined with high shear to removepassivation of gold surfaces oftenassociated with other oxidative leachingprocesses. The shear action facilitates thethinning of the diffusion or boundary layeraround the mineral particles, while also

enhancing kinetics and reduced cyanideconsumption. The typical recovery rate is afunction of the ore mineralogy, but canoften be in excess of 90% compared with30-60% without Leachox.

“The Leachox process is very flexibleand so the circuit can be modified tohandle any required tonnage,” says SteveFlatman, general manager at Maelgwyn.“While Leachox and other competitorprocesses are generally associated withrefractory gold treatment, the Aachenreactors themselves, which are the corepart of the Leachox process, are

increasingly being used on run-of-mine(ROM) circuits to accelerate leach kinetics,or in a pre-oxidation mode where a lightpre-oxidation is required.”

MINERAÇÃOAngloGold Ashanti has a number of mines thatfeature refractory gold. For example, the

Mineração mining complex in Minas Gerais,southeastern Brazil, uses the roasting process totreat its refractory ore.

The company says that the roasting process waschosen because it is the best match to the type ofore at the mine; the choice was made based on theprocess that would provide the best metallurgicalefficiency. The roasting process promotes therelease of the gold contained in the ore concen-trate, and thus increases the metallurgical recoveryof the gold treatment process. It also allows theproduction of another value-added product – sul-phuric acid. This way, AngloGold Ashanti canmaximise the mineral resource mined at its mines.

The roasting process used at AngloGold Ashantiis the single-stage FB type. In this process, the pulpconcentrate at a typical particle size (80% passingthrough 400-mesh, or 38 microns) is fed by 12compressed air ‘feed guns’. The slurry inside theroaster reacts with the air blown from the bottomof the furnace, which has a dual function: it fluidisessolids and provides the oxygen required for thevarious sulphide oxidation reactions.

The ore is processed at the Queiroz and Cuiabáplants. The Queiroz plant roasting process can treat848t/d of concentrate, which is equivalent to5,000t/d of ore, bearing an average gold content of7.5g/t. Therefore, the Cuiabá metallurgical plant

has the capacity to treat 37.5kg/d of gold (13.7t/yor 440,000oz/y).

Infrastructure required included a power supply,water and a site to build the processing plant andtailings dams. It took 3-4 years to proceed frominitial test works, studies, and assessment ofmetallurgical processes alternatives to implementa-tion and plant start-up. The capital invested in theroasting plant and sulphuric acid plant was US$35million.

CÓRREGO DO SÍTIOThe ore from the company’s Córrego do Sítio mine,also in the Minas Gerais region of Brazil, is highly

refractory, and therefore requires a more intenseoxidation process to achieve high recovery. In thiscase, after metallurgical tests were carried out,pressure oxidation was chosen. It works at 230°Cand 36bar pressure by injecting oxygen to attainoxidation. This plant began operation early thisyear, and is currently in the stabilisation phase. Noby-product is produced in this particular process.The investment made in the pressure oxidationsystem was US$25 million.

The process for treating SO2 uses doubleoxidation and double absorption. The processcomprises three steps:

• gas drying – removal of all moisture (water

vapour) from the gas. It is accomplished byrecirculating the 96% sulphuric acid stream incounter-current in the drying tower;

• conversion of SO2 to sulphur trioxide (SO3)– once dry, the SO2 contained in the gas needsto be converted to SO3 with the aid of a catalyst

to enable the absorption of the reaction tobecome thermodynamically feasible in suchoperating conditions; and

• absorption of SO3 – the SO3 produced in theprevious step is absorbed by dilution watercontained in the sulphuric acid circulating in theinterpass and final absorption towers, whichleads to the production of additional sulphuricacid (H2SO4) molecules.

OBUASIGold Fields’ BIOX process is used at the Obuasiplant in Ghana. Different treatment routes wereevaluated in 1991, and the choices were narroweddown to pressure oxidation and bio-oxidation. Thecompany says that it chose bio-oxidation for thefollowing reasons:

• there was the need to move away from roasting,

which had caused environmental pollutionthrough emission of arsenic trioxide;

• bio-oxidation was perceived to provide a cleaneroperating environment, as arsenic compoundsare safely neutralised and impounded toconform to EPA requirements;

• the relative simplicity of operation and controlfor bio-oxidation when compared with pressureoxidation; and

• the low CAPEX and OPEX.

The BIOX plant at Obuasi is made up of fourmodules; each contains six stainless steel reactorsthat are each 895m3 in volume. There is also a 25th

reactor that is common to all of the modules. Theplant also has four counter current decantation(CCD) thickeners, a neutralisation circuit made upof six tanks, and ancillaries such as a cooling tower,blowers and a nutrient mixing dosing facility.

It took two years to conduct the pilot test work,design and construct the plant, and it wascommissioned in 1994 with a capital investment ofUS$25 million.

The BIOX plant was initially designed to treat720t/d of concentrate, and the capacity wasincreased to 1,050t/d with the addition of extrareactors in 1998.

The waste stream from the process, which

contains soluble arsenic, is precipitated as insolubleferric arsenate, which is considered stable, and thendisposed of on the tailings storage facility.

Case study: AngloGold Ashanti

AnglogoldAshantiuses bio- oxidation

at Obuasi



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The efficiency of Leachox is not afunction of grade, so it is not limited to justhigh-grade flotation concentrates. This canenable mines to increase their flotationmass pull, albeit at a lower grade toenhance flotation metal recovery.

Maelgwyn says there are a number ofadvantages to using Leachox. Mr Flatmanexplains: “Firstly, Leachox is generallysignificantly more cost-effective thanother refractory processes from both aCAPEX and OPEX viewpoint. This isessentially because the process is flexibleand is able to combine a number ofMaelgwyn’s core technologies toformulate a bespoke solution for aspecific orebody, rather than trying tomake the orebody fit the process.

“Secondly, the flexibility also extends torecovery, where the number of passes

through the Aachen reactor can be tailoredto achieve an economic recovery, asopposed to maximum recovery.”

The only ‘chemical’ that Leachox uses isoxygen, so the process is very environ-mentally friendly. Leachox also reducescyanide consumption, particularly whenultra-fine grinding is required. While notstrictly part of Leachox, Maelgwyn canalso incorporate its cyanide destructionprocess, which reduces residual cyanidelevels to below international cyanide

guidelines, again through using its Aachenreactors.

An oxygen supply is the majorrequirement for Leachox. For remoteplants this would typically be a pressureswing adsorption or vacuum swingadsorption (PSA/VSA) plant. Imhoflotpneumatic flotation cells would thennormally be used to produce a flotationconcentrate. Depending on the mineral-ogy, ultra-fine grinding may be necessary,requiring an ultra-fine grinding mill.Historically, Maelgwyn has used vendormills for this, but has also developed its

own in-house ultra-fine grinding mill.Thereafter, the Aachen reactor contains

no moving parts, with the flow through thereactor provided by a feed/recirculationpump. Mr Flatman concludes: “Due to itsinherent simplicity, the Leachox process is

ideally suited to remote locations.”Maelgwyn has extensive laboratory

facilities in South Africa, through itssubsidiary Maelgwyn Mineral ServicesAfrica. “The amount of test work and thetimeframe required is a function of howmuch test work the client has alreadydone,” says Mr Flatman. “Normally,scouting work would be needed to narrowdown the process options, followed bymore detailed confirmatory work. Atimeframe of six months is typical.Thereafter, another six months formanufacture and installation/commission-

ing should be allowed.”The Leachox process has been used

commercially at both the Galaxy GoldMining’s Agnes mine near Barberton andTransvaal Gold Mining Estates’ operationnear Pilgrim’s Rest, both in South Africa.A Leachox circuit is also being installed ata west African gold producer’s mine.Additionally, several mining companieshave committed to incorporating theprocess into their planned operationspending funding finalisation.

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SGS columnleach pilot set-up

in Chile

“Leachox is

generally

significantly

more cost-

effective

than other

refractory

processes

from both a

CAPEX and

OPEX

viewpoint”

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