cip - who needs it(a combination circuit of gravity)

7/29/2019 CIP - Who Needs It(a Combination Circuit of Gravity)

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Canadian Mineral Processors Conference - Vancouver, J anuary 2003

Sandy Gray, J ennifer Abols, Andrew McCallum, Gary Patrick, Garry Johansen.

CIP - who needs it? A combination circuit of Gravity, Flotation and Intensive Leach may provide the OptimalEnvironmental and Cost Outcome for Gold Plants.

CIP - WHO NEEDS IT? A COMBINATION CIRCUIT OF GRAVITY,FLOTATION AND INTENSIVE LEACH MAY PROVIDE THE OPTIMAL

ENVIRONMENTAL ANDCOST OUTCOME FOR GOLD PLANTS.

Authors: Sandy Gray, Technical Director, Gekko Systems AustraliaJennifer Abols, VP Canadian Operations, Gekko Systems Inc CanadaAndrew McCallum, Process Metallurgist, Gekko Systems AustraliaGary Patrick, Consultant Metallurgist, Celtic ResourcesGarry Johansen, Exploration Manager, Bendigo Mining

Key words: intensive cyanidation, leaching, gravity concentration, flotation, gold

ABSTRACT

The authors challenge the traditional thinking that whole ore cyanidation followed by Carbon inPulp (CIP) or Carbon in Leach (CIL) should be applied as a standard process in the majority ofgold processing flowsheets. The concept of whole ore leaching by using cyanide can result inunnecessary environmental and capital risk. Alternative, currently available technologies can beused to provide the desired level of gold recovery at a much lower installed cost and less impacton the environment. Gravity concentration, alone or in conjunction with flotation, can produce agold bearing concentrate that can then be fed directly to a batch or continuous intensive

cyanidation unit.

The InLine Leach Reactor (ILR) is an intensive cyanidation unit developed by Gekko Systems ofAustralia. Advantages of this technology include significantly reduced capital outlays forgreenfield plants or plant upgrades and increased investor returns. Reduction of onsite cyanideand carbon inventories improves plant safety, materials handling and site security. By usingdirect electrowinning to treat the gold bearing solutions produced by the ILR, the need for aconventional carbon stripping circuit is removed. Potential also exists to increase grind size saving energy, further reducing capital and operating costs and decreasing the environmentalimpact of a mill. Detoxification of cyanide before the release of solution to the tailings dam canminimise tailings disposal and containment requirements.

When used in the right application the proposed process will have little or no impact on goldrecovery and may actually increase recovery for complex ores. Areas applicable to this type oftreatment include: plants in environmentally sensitive regions, lower tonnage higher gradedeposits, coarse gold deposits, and preg robbing ores - some of which may not currently beeconomically feasible using standard processes and flowsheets. We have provided threeexamples of such projects, where an alternative to whole ore leach and CIP or CIL has deliveredthese benefits.

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INTRODUCTION

The vast majority of recently commissioned gold processing plants utilise whole ore cyanideleaching followed by CIP or CIL. These processes can be very effective and high goldrecoveries can be achieved when used in the proper circumstances. Whole ore leaching followedby CIP or CIL has been applied so widely however, that project owners may have sufferedfinancially because the processes selected did not fully meet the specific needs of the deposit.We also believe that some potentially lucrative projects may not have gone ahead due toinsufficient ore resources required for and/or the high capital and environmental costs associatedwith whole ore cyanidation.

Both the gravity concentration and flotation of gold and gold bearing sulphides have beensuccessfully incorporated into many gold circuits throughout the world, although the treatmentoptions of such concentrates have often created further difficulties. With the recent development

and commercialisation of a continuous intensive cyanidation process capable of the successfultreatment of such concentrates the need for whole ore cyanidation and CIP or CIL can now bereconsidered.

Gravity recovery is a concentration process based on differences in specific gravity betweenoreand gangue minerals. Separation is performed by shaking tables, Humphreys spirals, jigs,hydrocylones, dense media, etc. (Dictionary of Earth Sciences, Oxford University Press 1999).The use of gravity to recover gold is the oldest, most cost effective and environmentally friendlymethod available. With the advent of froth flotation and later whole ore leach in conjunctionwith the Merrill Crowe Process, CIP or CIL the use and further development of gravityconcentration devices for the recovery of gold in modern gold plants went into decline.

Limitations of the CIP/CIL process recently saw the re-emergence of gravity concentrators eitherin the grinding circuit or in treating the CIP/CIL tailings in many operations. Batch centrifugalconcentrators (BCCs) such as the Knelson Concentrator, Falcon Concentrator and InLineSpinner in grinding circuits and Kelsey jigs on tailings streams have been implemented torecover coarse gold and gold bearing sulphides that would otherwise require much greaterresidence times than are afforded by whole ore leach circuits.

Maximising Gravity Recovery through Continuous Concentrate Technology

The development and application of a continuous concentrate unit, such as the InLine Pressure

J ig (IPJ), is at the centre of the innovative new flowsheet, designed to produce high gold gravityrecoveries at a coarse grind with a manageable mass. With a combination of gravityconcentrators and or flotation a combined concentrate containing a significant portion of the totalgold in the feed can now easily be treated with leach recoveries expected in the order of 95percent and above.

Continuous concentrators, like the IPJ , allow gravity recovery to be optimised for a particular oreby "climbing the recovery curve". This involves the production of a continuous stream of high

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grade heavies, resulting in mass yields to concentrate of between one percent to a maximum of30 percent, with 10 percent being typical (Longley et al., 2002).

To illustrate this point, figures 1 and 2 show the possible recovery and grade as a function ofyield for a typical high grade gold ore sample tested at Gekko Systems Pty Ltd. As the massyield is increased the gold recovery increases. Batch units such as BCCs are limited to massflow to concentrates of less than 0.05 percent and accordingly cannot generally achieve the highrecovery levels required for a gravity only circuit. For the material tested below, batch units at ayield of 0.1 percent would be limited to recoveries of around 50 percent, whereas a continuousunit has the potential to recover over 90 percent at a yield above 10 percent.

Gold Grade Yield Curve

0

100

200

300

400

500

600

700

800

0% 20% 40% 60% 80% 100%

Yield

Grade(ppm)

Gold Recovery Yield Curve

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0% 20% 40% 60% 80% 100%

Yield

Recovery

Fig 1: Gold Ore Recovery Yield Curve Fig 2: Gold Ore Grade Yield Curve

This extra recovery as yield increases is due to either poorly liberated gold or gold associatedwith heavy minerals, typically sulphides. This association with sulphides often leads todifficulties processing such ores through conventional CIP/CIL circuits, however the concentrateusually responds very well to intensive cyanidation. The ability of the IPJ to produce a highyield of sulphide rich concentrate can result in the economic treatment of an extremely complex

ore that may otherwise require a treatment route similar to that of a truly refractory ore.

The gravity concentration flowsheet detailed in figure 3 incorporates both IPJs and BCC's. TheIPJ 's treat the entire underflow stream from the hydrocyclones. Since the IPJ tailing is recycledback to the mill the gravity concentrators have multiple opportunities to recover the goldminerals before they report to cyclone overflow.

On the overflow stream from the hydrocyclones, IPJ s/BCC's are used to scavenge any fine goldminerals; again they treat the entire stream. In this case it is a single pass opportunity to recover

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the gold minerals. Further recovery on the overflow stream can be achieved by addingconventional or flash flotation.

The mass recovery to concentrate is determined from grade recovery testwork done in thelaboratory.

ConcU/F

Tail

Conc

O/F

Tail

Milling Hydrocyclones IPJ s/BCCs

IPJ

Fig 3: Gravity Concentration Flowsheet.

The storage requirements for the gravity and or flotation tailing stream may differ significantlyfrom conventional CIP/CIL tailings or intensive cyanidation tailings streams. If the ore has ahigh sulphide content, the gravity concentrators and or flotation will produce a sulphide richconcentrate and hence a sulphide depleted tailing, significantly reducing the opportunity for acidgeneration and heavy metal mobilisation in the tailings impoundment area. This means there areoften significant advantages if the two tails streams are stored separately.

INTENSIVE CYANIDATION

Until recently gravity concentrates from BCCs were generally treated by tabling, with the tableconcentrates being smelted while table middlings and tailings were recycled to the milling circuitfor further grinding and leached in the CIP/CIL circuit. Gold recovery on production tables isrelatively low, often only 30 percent, and rarely above 60 percent. Significant factors in tablingperformance are mineralogy and the operators experience and ability. Complex, sulphideassociated gold particles are generally lost to table tails and, through attrition, will eventuallyreport to the cyclone overflow.

For the gravity recovery process to be viable, in the absence of whole ore cyanidation, it isnecessary to reliably obtain high recoveries from the gravity concentrate - in excess of 97percent. The only proven technology giving high gold recoveries from gravity concentrates isintensive cyanidation coupled with either direct electrowinning or carbon absorption.

The chemistry of cyanidation is proven and well known with the majority of gold operations inthe world using the process:

4 Au +8 CN- +2 H2O =4 Au(CN)2- +4 OH-

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Intensive cyanidation typically uses cyanide levels of about 2 percent and oxygen or peroxide areadded, at a concentration of approximately 20 ppm. The ILR process, patented by GekkoSystems, does not require any exotic reagents or oxidants and equipment is modularized and skidmounted, which simplifies shipping and installation.

There are over fifteen mines worldwide using InLine Leach Reactors for intensive cyanidation ofgravity gold concentrates. These include both batch and continuous units. While batch ILR's arecost effective for small concentrate masses, higher feed rates require a continuous reactor.

CONTINUOUS INL INE LEACH REACTOR

The only commercially manufactured continuous intensive cyanidation reactor is the InLine

Leach Reactor (ILR), produced by Gekko Systems Pty Ltd. Continuous ILRs have beensuccessfully operated in Australia, Malaysia, Ghana, Tanzania, Mali and Argentina atthroughputs from 50kg/h up to 10t/h.

The continuous ILR is based on the same principle as the laboratory bottle roll. It consists of ahorizontal drum, with internal baffles and lifters, rotating slowly on support rollers to ensureeffective contact of the solids with reagents. Solids and solution are fed continuously andoverflow continuously. The solution flow rate through the ILR is largely independent of thesolids flow rate allowing solution grade to be controlled within fairly wide limits. Gray andKatsikaros (1999) have described the ILR in detail.

The ILR operates continuously in conjunction with an existing or dedicated electrowinning cell(or carbon column) with the barren solution recycled back to the ILR to minimise reagent use.For mass balance reasons a small bleed solution is also produced carrying trace amounts of goldthat can be recovered via a carbon column. The continuous intensive cyanidation flowsheet,shown in figure 4, includes an ILR and a dedicated electrowinning cell.

The leached solids are continuously dewatered, washed if necessary, and then transferred to thetailings storage or detoxification facility. Dewatering is carried out using a combination of cones,screens and cyclones to take full advantage of the coarse nature of the concentrate. Flocculantscan be used if required to settle slimes.

Intensive cyanidation of a gravity concentrate typically gives leach recoveries of over 95 percent.The aggressive leach conditions with high cyanide and oxidant levels are easily able to treatcoarse particles and complex ores that under standard CIP/CIL conditions would give extremelypoor leach performance. These conditions give high gold dissolution rates even when there isonly minor exposure of the gold surface and in the presence of passivating sulphides.

ILRs are successfully recovering gold from gravity concentrates containing free gold, pyrite,arsenopyrite, mixed sulphides and preg robbing carbon.

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SOLIDS

STORAGE

SOLUTION

STORAGE

REAGENT

ADDITION

REACTOR

DRUM

MOTOR

2.2KW

MOTOR

4KW

WATER

ADD

MOTOR

4KW

THICKNER CLARIFYER

WATERADD

RAKE

ELECTROWIN

SOLID TAIL

DEWATERING

SCREEN

FEED FROM

CONCENTRATOR

MOTOR

4KW

MOTOR

.55KW

Fig 4: Continuous Intensive Cyanidation Flowsheet

An intensive cyanidation leach profile is provided in figure 5. This leach profile is typical, with

very rapid initial leach rates. The size of the required ILR for a given application is determinedfrom the leach rates and the required throughput. Laboratory tests have proven to be veryreliable in predicting full scale performance.

The use of an ILR increases security by removing the need to manually handle gold concentrates.The design means high value concentrates are stored in a single closed vessel with any accessand sample points padlocked for security if desired. Once the solids are fed into the reactor theyare inaccessible until after leaching.

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Fig 5: Typical Intensive Cyanidation Kinetics

SAFETY

Cyanide is a highly toxic chemical that has a high recognition factor in the general community.Yet cyanide has been extensively used in the mining and numerous other industries for manyyears, generally in a safe and responsible manner.

In contrast, the use of intensive cyanidation reduces the overall usage and inventory of cyanide inthe plant compared to a conventional CIP/CIL plant treating the same ore. This is due to thereduced mass of material treated and the correspondingly small size of the leach circuit.

The high concentration of cyanide requires similar management practices as are used currently inthe elution and electrowinning areas of a CIP/CIL plant. Where the ILR is installed indoors,good ventilation, preferably with forced extraction, is recommended as a precaution.

The potential evolution of high levels of hydrogen cyanide is small due to cyanide's high pKavalue and at the high concentration used; the natural solution pH is above 11. With the drumfully enclosed and the small area of cyanide solution exposed during operation forced extractionin an indoor installation is straightforward. Lastly the ILR continually monitors pH to prevent theevolution of hydrogen cyanide.

Au Leaching

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0 6 12 18 24

time (hours)

%l

eached

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TAILINGSDISPOSAL

Of increasing concern to both mining companies and society is the long term liability and riskassociated with large tailings storage facilities containing cyanide residues. The gravityconcentration, flotation and intensive cyanidation process that weve presented can result intypically 80 percent to 90 percent of tailings being relatively coarse and devoid of cyanideresidue.

With a coarse grind and the removal of sulphides from the bulk of the tailings stream, thepotential mobilisation of heavy metals and acid generation is considerably reduced.

Due to the small volume of gravity concentrate treated by the ILR it is economic to treat the ILRtails via a cyanide destruction and detoxification facility. In this case the site would not have anycyanide in the tailings pond(s). The ILR tails, being a much smaller volume, could also be stored

in a facility that allows for encapsulation of the material to a higher standard than wouldnormally be economic for the complete tailings stream.

These features are extremely important in areas with fragile environments e.g. Alaska, Greenlandand adjacent to National Parks. The benefits of restricting cyanide tailings to small volumes,which can be easily detoxified or encapsulated, should also allow faster approvals bygovernment and environmental agencies, increasing the financial return and reducing risk toinvestors.

CAPITAL

The gold mining industry is compelled to provide a high return to investors; otherwise they willinvest elsewhere. The need is to build plants with reduced capital requirements and to provide ashorter lead time for approval and construction ensuring a faster return on capital. Both thesefactors also make it more attractive to build in countries of high political risk, further increasingexploration targets.

The capital cost savings of the proposed flowsheet are considerable. For example the cost of acomminution circuit grinding to a P80 of 500mm compared to a P80 of 75mm will be 50 percentless (Lanz and Noakes, 1993). Capital costs of an IPJ , BCC, ILR and electrowinning flowsheetare 40 percent lower compared to the traditional leach, CIP and carbon strip circuit. Further

savings in tailings containment can be expected.

Additionally, environmental approvals can be expected to be faster due to the reduced cyanideinventory and increased feasibility of completely detoxifying all residues. Environmental bondsmay also be lower leading to the greater availability of operating capital.

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SITE EXAMPLES

NEW BENDIGO GOLD PROJ ECT

The Bendigo Goldfield has historically produced over 22 million ounces of gold with 82 percentof this coming from quartz reef mining carried out from surface to an average depth of 500 mand the deepest mine going to 1,400m. Between 1854 and 1954 quartz reef mining yielded about18 million ounces of gold from 40 million tonnes of ore, the recovered grade being 0.5 oz/t(14.3g/t).

No processing plant presently exists although a bulk sampling plant has been modified to trialprocess the 65,000 tonnes of development ore that will be produced whilst exploring theSheepshead and Deborah lines of the reef during 2003. This trial processing will assist withfurther metallurgical testing. Initial test work indicates greater than 90 percent recovery using

gravity, with up to 99 percent recovery by flotation of the gravity tail without the use of cyanidein the main circuit.

Metallurgical testwork performed on bulk samples taken from the Bendigo Mine Developmenthave shown that excellent gravity gold recoveries are achievable. Due to the large amount ofcoarse gold present, gravity concentration recoveries of up to 98 percent have been achieved.Even at low head grades (1.38g/t) recoveries of greater than 80 percent have been reported.

Combined with flotation, the recovery at low head grades exceeded on average 91.4 percent andup to 99.6 percent at higher head grades (+8 g/t). These results are shown in the figure 6, below.

Bendigo New Moon Metallurgical Recoveries Vs Head Grade

1.6611.4

9892

80

99.69891.4

0

20

40

60

80

100

7781.38

Head Grade (g /t)

Recovery(%) Flotation Recovery

Gravity Recovery

Total Recovery

Fig 6: Metallurgical Recoveries vs. Head Grade for Bendigo

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The total recovery by gravity/flotation has also been found to be grind size independent belowapproximately 500 microns. The possibility of processing at 1 mm is being considered, as testwork has indicated that recoveries drop off only marginally, even at this grind size.

Bendigo anticipates approximately 90 percent recovery by simple gravity processing. Flotationor extra gravity concentration aimed at sulphide recovery (1 to 2 percent of ore) will increasetotal recovery to the high 90s.

The original bulk sampling plant incorporated two of Gekko Systems Inline Spinners. Theupgraded bulk sampling plant, recently commissioned, treats up to 15 tph and includes a coarsegold trap developed by Gekko Systems, two Inline Pressure J igs and a shaking table toconcentrate and recover gold.

A proposed flow sheet for the operating plant at Bendigo, incorporating intensive cyanidation is

shown in figure 7, below.

FLOTATION PLANT

FINAL TAIL

CONTINUOUS

ILR FLOWSHEET

MILL FEED

TO WATER SUPPLY

FEED FROM

CONCENTRATOR

CLASIFICATION

TO BE CONFIRMED

POSSIBLE

SCAV CIRCUIT

ROUGHER GOLD

IPJ2400

CLEANER GOLD

IPJ1500

Fig 7: Proposed Flowsheet for New Bendigo Gold Plant

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NEZHDANINSKOYE PROJ ECT

The Nezhdaninskoye project is located in North Eastern Yakutia, Russia and is 50 percent ownedand managed by Celtic Resources. Historically the mine was in production from 1975 and closedin 1992, Celtic Resources reopened the mine in 2001.

Past metallurgical test work had shown that gravity concentration could yield both a gold productsuitable for smelting and gravity gold sulphide concentrate that contained up to 80 percent of thegold. Flotation of the gravity tailings recovered a further 18 percent of the gold. Combined goldrecoveries were reported to be up to 97 percent.

However final recoveries were ultimately lower. An investigation of fine grinding followed byresin in leach reported recoveries of less than 90 percent gold recovery from the gold sulphideconcentrate and 70 percent gold recovery from the flotation concentrates.

Further test work involving gravity concentration, followed by CIL leaching conducted inAustralia in 1997 showed recoveries of 91 percent from the Quartz Veins while recoveries ofonly 75 percent were achieved from the Breccia Zones. A preg-robbing effect was preventinghigh leach recoveries, ruling out RIL and CIL circuits.

Recent test work conducted by Ammtec in Australia investigated gravity recovery and directcyanidation of the gravity sulphide concentrate. The test work indicated that recovery could beimproved through the use of high intensity gravity separation units, coarser grinds to minimisecarbon liberation and the use of the Gekko Systems ILR to enable gold recovery as soon as it isliberated (Armitage et al., 2002).

Tabling Recovery Yield Curve

0%

10%

20%

30%

40%

50%

60%

70%

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0% 20% 40% 60% 80% 100%

Yield

Recovery

Au Leachin g

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%l

eached

Fig 8: Gravity recovery mass yield curve Fig 9: Concentrate leach recoveries forfor Nezhdaninskoye Nezhdaninskoye

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The recent test work confirmed that very high gravity recovery is achievable. The grade/recoveryrelationship for the quartz vein ore indicated that over 90 percent of the gold can be concentratedinto a yield of less that 16 percent of the ore. The recovery / yield curve is pictured above infigure 8.

The concentrate leach results indicate that over 90 percent recovery is achievable using intensivecyanidation conditions. Leach recoveries have since been optimised with grind size reductionand kerosene addition to combat the preg-robbing issue. Results are shown in figure 9, above.

From the test work results a simplified flow sheet for Nezhdaninskoye was developed and isshown in figure 10, below. This flow sheet incorporates IPJ s, BCCs and tabling to produce ahigh yield gravity concentrate. Gekko Systems continuous ILR10000 is to be used to leach theconcentrate. The ILR plant is currently under construction.

O/F

U F

Dor

Cathod

Conc

U/F

Conc

Classifying Cyclone O/F

Conc

Tail

Conc

O/F Process Water

TailingsDam

Tail

Crushing and Screening

Primary Grinding Circuit andInLine Pressure Jigs

Dewatering Cyclones

Secondary Grinding Circuitand InLine Pressure Jigs

Concentrating Tables

ROM

Mid

BCCs

Tailings Thickener

Conc

Cleaner Table

Regrind

Gekko ILREW

Calcine Oven &Furnace

Fig 10: Simplified flowsheet for Nezhdaninskoye

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NOVODNEPROVKA

Novodneprovka Gold Mine in Kazakhstan had been traditionally run using gravity only. Lowerthan expected gravity gold recoveries have resulted in the plant being upgraded to a totalgravity/flotation, continuous ILR and detoxification circuit. From a feed of 20 tph thegravity/flotation circuit can recover approximately95 percent of the gold into approximately400kg/hr of concentrate.

The concentrate is then leached in a continuous ILR1000, providing up to 98 percent leachrecovery. Gold is recovered from solution using a combination of electrowinning and carboncolumns. Tailings from the ILR are treated by a detoxification system to ensure a benign tail isproduced from the plant. The plant upgrade is currently in progress with the ILR achieving theexpected 98 percent recovery during recent commissioning.

Figure 10 below shows the continuous ILR 1000 prior to shipping to Novodneprovka Gold Minein Kazakhstan.

Fig 11: Continuous ILR 1000, destined for Novodneprovka Gold Mine in Kazakhstan

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SUMMARY

The intensive leaching of gravity and/or flotation concentrates using high levels of cyanide andoxygen, when used in the proper application, can result in overall gold recoveries comparable toor greater to those seen with traditional whole ore leach circuits. Advantages of our proposedflowsheet include:

Maximised return to shareholders with a combined reduction in capital outlay and risk Reduced table losses to cyclone overflow High security and less gold room labour Easy installation modular design A reduction in equipment lead time, Smaller plant footprint Reduced cyanide and carbon consumption

Lower reagent handling Reduced tailings disposal and environmental costs Lower energy costs

A flowsheet comprising of the production of a gold bearing concentrate, followed by intensivecyanidation and tailings detoxification mark a significant changing of thinking as to the mostappropriate gold processing route in the future. Environmental, political and investor factors aswell as and project size and location may demand an alternative to the traditional whole oreleach, CIP/CIL gold circuit. We believe we have offered a possible viable replacement withproven, currently available technology.

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REFERENCES

Armitage, Mick et al., 2002, Celtic Resources AIM Prospectus, Section Five CompetentPersons Report, Celtic Resources Holdings PLC, 2002.

Gray, A H and Katsikaros, N, 1999, The InLine Leach Reactor The New Art in IntensiveCyanidation of High Grade Centrifugal Gold Concentrates Proceedings, Randol Goldand Silver Forum, Denver, May 1999.

Lanz, T and Noakes, M. 1993. Cost Estimation Handbook for the Australian Mining Industry -AusIMM 1993.

Longley, R J, Katsikaros, N, Hillman, C, 2002, A New Age Gold Plant Flowsheet for theTreatment of High Grade Ores, Paper presented at AusIMM Metallurgical Plant

Design and Operating Strategies Conference, Sydney.

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cip - who needs it(a combination circuit of gravity)

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