GRAVITY SEPARATION EQUIPMENTFOR GOLD MINING

IHe Holland have been involved in the design and manufacture ofmining equipment since the end of the last century Most of this equipmentis used in alluvial mining projects all over the world.

In the early days, most of the dredgers built for mining were of thebucket type, with sluice boxes, and these were used for gold mining oper­ations. But gradually the emphasis changed to alluvial tin mining. With thefixed, low gold price between 1945 and 1973, hardly any equipment wassold for gold mining projects.

Alluvial tin mining, however, continued to develop and bucket dredg­ers with integrated treatment installations were built in large numbers.

especially in the small particle sizeranges, this concept improvesfeed distribution, requires less con­struction space, water and power,and reduces operational costs.The present range is based on amodular design consisting of sepa­rated cells as shown in Figs. 1 and2.

More and more attention wasgiven to the efficiency of the treat­ment installations, and this ulti­mately led to the development ofthe IHC jig.These jigs were first used on themodern bucket dredgers built foruse in alluvial tin projects, and after1973 again for alluvial gold oper­ations.

In the early Eighties it was realizedthat the highly efficient IHC treat­ment installations on board bucketdredgers, could be used to advan­tage in other mining applications.IHC developed skid-mountedplants for onshore operations anda pontoon-mounted version to workin conjunction with backhoe, grab,cutter or wheel dredgers.With the decline of the tin marketafter 1985, these new conceptshave come to be used primarily ingold mining projects. These not

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only include alluvial mining, butalso other applications like hard­rock gold mining and the recoveryof gold as a by-product of sandand gravel winning.

In this article we shall examinethese various applications in goldmining using IHC gravity separa­tion equipment, and highlight someof the specific operational parame­ters for these applications.

IHe HOLLAND JIGS

The basic design of IHCHolland jigs was developed in thelate Sixties and combines the tra­pezoidal shape with the IHC saw­tooth drive characteristic, i.e. arapid upward stroke followed by aslow downward movement of thepulsator.Besides higher recovery rates,

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For gold processing, each cell ormodule will as a rule be able toprocess 32 ton/hr of dry solids.Depending on the required capaci­ty, modules can be combined, themaximum number, twelve, forminga circular jig.If additional capacity is required,further jigs, each consisting of anumber of modules, can be in­stalled.For smaller capacities, a mini-mod­ule jig and a number of small rec­tangular jigs are available.

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THE USE OF JIGS INGOLD SEPARATION

In general, jigs are well suit­ed to use in gold separation pro­cesses. The high specific densityof gold is an important parameterin assuring high efficiency in gravi­ty separation, even for smaller par­ticle size ranges.The thick bed characteristics of thejig, as compared to thin flowing filmconcentrators such as spirals andshaking tables, guarantees a sta­ble operation and makes the jigless sensitive to fluctuations in feedcapacity and material parameters.

Furthermore, due to its ductilecharacter, gold tends to form longflat particles, giving problems in

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the separation process when usingthin flowing film concentrators.

This makes the jig superior to othergravity methods, particularly forrougher duty/primary concentra­tion, or as pre-concentrator wherefeed fluctuations cannot be avoid­ed.

ze is discharged by belt conveyorsand the undersize pumped to thejig installation.The jig feed material is processedthrough two cyclones to get rid ofthe slimy material. This is necessa-

ry as clay contents of over 15% re­duce the efficiency of the jigs.

For this kind of alluvial gold oper­ations, three jig stages are normal­ly used, with the tailings of the pri-

In the following sections, varioususes of IHe jigs in goldminingoperations will be examined. Everyoperation depends on the operat­ing parameters encountered, notonly in terms of capacity, but alsoof flowsheet layout, efficiency andoperational control.

IHe JIGS IN ALLUVIALGOLD MINING

The treatment installationshown in Fig. 3 was designed andbuilt for an alluvial gold miningoperation in West Africa and con­sists of a scrubber/screening unitand a separate jig plant with sixmodules as the primary stage.

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The feed material is fed into thescrubber/screen drum assisted bymonitors, thoroughly washed and '" 4

screened off at 9.5 mm. The oversi-

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a sand and gravel operation in theU.S.A. The deposit contains alluvialgold, which is recovered as a by­product.The gold used to be recovered bytwo sluice boxes, but analysisshowed that an important part ofthe gold was lost by using thismethod.A three-stage IHC jig system wasinstalled. consisting of a 12-mod­ule jig as the primary stage, a 2­module jig as the secondary stageand a mini-module as the tertiarystage.Fig. 7 shows the installation of theprimary jig.

mary stage being discharged, ascan be seen in the flowsheetshown in Fig. 4.The primary stage concentrate isfed into the secondary jig stage. ofwhich the tailings are recirculatedvia the primary stage.The secondary stage concentrateis further upgraded in the tertiaryjig stage, producing the final con­centrate. Tailings from the tertiarystage are also recirculated via thesecondary stage.FIg 6

The purpose of the primary jig is toget the highest possible recovery,while the secondary and tertiaryjigs are used as clean-up stages toincrease the quality of the concen­trate.

Fig. 5 gives a detailed view of thesix-module jig plant with cyclones.Even when treating material withhigh clay content, recovery rates ofover 96% have been measured ongold particles down to 50 11m.Using IHC jigs in three stagesenables ratios of concentrationbetween 2000 and 4000 to beachieved.

IHC JIGS FOR GOLDRECOVERY IN SAND AND

GRAVEL OPERATIONS

The next example of the useof IHC jigs is shown in Fig. 6. Thispicture shows the process tower of

Important factors in the choice ofthe IHC jig system were:

- The system had to have a higherrecovery rate than the sluiceboxes.

- The jig operation might not inter­fere with the sand and gravelprocess, and thus possibly delaythe main product operation.

- The system had to be able to co­pe with surges of sand and grav­el which exceeded the averagedesign capacity.

- As the system had to be builtwithin the existing plant, a mini­mum of construction space wasdemanded.

The IHe jig system proved itselffrom the outset, giving very highgold recovery rates at high capaci­ties and with a minimum of mainte­nance.

Fig. 8 shows the primary jig inoperation.

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IHC JIGS IN HARD-ROCKGOLD OPERATION

Fig. 9 shows two 12-moduleIHC jigs installed as primary gravi­ty separators in a hard-rock goldprocessing plant.The flowsheet of this operation isreproduced in Fig. 10.

Using a gravity stop between theballmills and the chemical recoverysystems will result in recovery ofthe free gold at an earlier stageand more effectively in terms ofcost.

Furthermore, the cyclone classi­fies, which means that it separatesnot only by particle size (for whichit is installed), but also by specificgravity.Gold, with its very high specificgravity, will report earlier to theapex discharge of the cyclone thansimilar sized rock-matrix material.

This results in a higher percentageof gold than average material recir­culating in the ballmill circuit, andis known as overmilling of the gold.The installation of a gravity step willreduce this effect, as the gold willbe recovered before it can be re­circulated by the cyclone. More­over, gold particles with an oxidiz­ed coating will be concentrated bythe gravity step, whereas certainchemical processes will have prob­lems.

High slime contents, very smallballmill discharge and feed surgescause the jigs to operate near thelimit and demand much more at­tention, compared with the othertwo examples described above.However, operating under properprocess conditions, the IHC jigs

have proved able to maintain ahigh performance in terms of bothrecovery and ratio of concentra­tion, while requiring only minimummaintenance.FI(} 10

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