S. Al-Hassan & R. K. Amankwah & R. S. Nartey, University of Mines and Technology, Ghana,...

Direct Smelting of Gold Concentrates as an Alternative to Amalgamation in Small-

scale Gold Mining Operations

S. Al-Hassan & R. K. Amankwah & R. S. NarteyUNIVERSITY OF MINES AND TECHNOLOGY, GHANA

presented at the workshop at

CASM CONFERENCE, BRASILIA, BRAZILOctober 6 – 12, 2008

Today’ s Presentation

• Objective• ASSM Mining methods in Ghana

• Available alternatives

• Direct smelting – the technique

• Laboratory and field trials

• Challenges

• Conclusions

Objective

Identify and introduce techniques or processes to eliminate the use of mercury by artisanal and small-scale gold miners (ASM) in Ghana

Map of Africa showing position of Ghana

•Area is about 230 020 km2

•Population is about 21 million

•Climate is tropical

Geological map of Ghana showing rock formations that contain gold

Location of Small-scale Location of Small-scale Mining District Centres, Mining District Centres,

GhanaGhana

District Centres District Centres LocationLocation

RegionRegion

Tarkwa, Bibiani, & Tarkwa, Bibiani, & AsankraguaAsankragua

WesternWestern

Assin Fosu, & Dunkwa-Assin Fosu, & Dunkwa-on-Offin on-Offin

CentralCentral

Akim OdaAkim Oda EasternEastern

Bolgatanga Bolgatanga Upper EastUpper East

ASSM ASSM Gold & Diamond Gold & Diamond ProductionProduction

YearYear Gold Prodn (oz)Gold Prodn (oz)Diamond Prodn Diamond Prodn

(carats)(carats)

20002000 2,457,152 2,457,152 878,011 878,011

20012001 2,381,345 2,381,345 1,090,072 1,090,072

20022002 2,236,833 2,236,833 963,493 963,493

20032003 2,274,627 2,274,627 904,089 904,089

20042004 2,031,971 2,031,971 905,344 905,344

20052005 2,138,944 2,138,944 1,065,923 1,065,923

20062006 2,342,722 2,342,722 972,991 972,991

20072007 2,628,290 2,628,290 839,235 839,235

19891989Gold 2%Gold 2%Diamonds 40%Diamonds 40%

20012001Gold 6%Gold 6%

Diamonds 95%Diamonds 95%

20072007Gold 12%Gold 12%

Diamonds 98%Diamonds 98%

Total Ghana ProductionTotal Ghana Production

Source: Minerals Commission, Ghana

ASSM Mining Methods in ASSM Mining Methods in GhanaGhana

Washing and Panning (alluvials Washing and Panning (alluvials along rivers)along rivers)

Shallow pitting (near surface Shallow pitting (near surface placers)placers)

Underground mining (reefs)Underground mining (reefs) Tailings and waste dumps (of old Tailings and waste dumps (of old

closed mines)closed mines)

Mining in Dry Area Mining in Dry Area (Eluvial)(Eluvial)

90% greater than 1 mm

Concentration (Dry Area)Concentration (Dry Area)

Winnowing Washing

Mining of Alluvial DepositsMining of Alluvial Deposits

Mining Hard Rock (Reefs)Mining Hard Rock (Reefs)

SSM Dredging in Ghana SSM Dredging in Ghana (Suction)(Suction)

Milling (Hard Rock)Milling (Hard Rock)

Flow sheet for operations

Ore (hard rock)

crushing/grinding

gravity concentration

mercury amalgamation

filtration of excess mercury liquid mercury

burning of amalgam mercury vapor

sponge gold

smelting

bullion

Sluicing

Panning

Gold concentrate in a pan

Amalgamation

Squeezing excess mercury from amalgam

Fireplace for burning amalgam

Spongy gold

Gather and sell

Open hearth for smelting gold

Mercury Loss

• Spillage during amalgamation

• Poor amalgam distillation

• Disposal of amalgamation tailings

• Further processing of sponge gold

• it is estimated that for every gram of gold recovered at least 0.43g of mercury is escapes

Alternatives

Physical methodsShaking tables , special sluicesCentrifugal concentrators

Chemical methodsCoal gold agglomerationIGoliHaber processCyanide leachingSmelting

Alternative suitable for galamseyers

Effective

Easy - requires no special equipment

Quick

Cheap

Suitable for processing small batches of concentrate

Visible - the miners can see their products throughout the whole process

Locally available

Choice of direct smelting

Fulfils all main criteria

Uses non-toxic, cheap chemicals, borax, sodium carbonate, silica sand

Technology not developed for artisanal use

Smelting

Smelting is a high-temperature melting process used to recover metals from ores and concentrates

Fluxes are added to the concentrate to assist melting and react with impurities so that the metal separates out

The final products are pure metal and a glassy slag containing the unwanted components

Smelting vs amalgamation

Smelting gave 99.9% recovery

In the lab amalgamation gave around 97% recovery

In the field amalgamation gave around 88% recovery

Furnace (sika bukyia)

Removable chimney

Steel

shell

Refractory bricks

Stabilizer bar

Combustion chamber

Pipe for exhaust

fan

Direct Smelting

Mixing charge and feeding crucible

Feeding the furnace

Heating crucibles in furnace

Temperature control

Removing crucible after smelting

Pouring of molten material

Cooling of melt in a mould

Separation of gold from slag

Gold

Gold beads

Field trials

Demonstration of MethodDemonstration of Method

Cost of Direct SmeltingCost of Direct Smelting

ItemItem Weight/Weight/numbernumber

Cost Cost (GH¢)(GH¢)

FluxFlux 200 g200 g 0.40.4

CruciblesCrucibles 22 0.40.4

CharcoalCharcoal 12 kg12 kg 0.80.8

TotalTotal 1.61.6

Cost comparism

Amalgamation

Cost, GHC

Direct smelting

Cost, GHC

Half teaspoonful of mercury (27 g)

4.00 4 crucibles (2 operations)

3.20

Heating of amalgam

0.50

Smelting 1.50

Total 5.50 3.20

Challenges (alternative fuels)PALM KENNEL

Challenges (concentrate pretreatment)

Sulphidic concentrates

Concentrates with high metal content

Challenges (alternative fuels)GAS

Crucibles

Gas cylinder

Furnace

Challenges (alternative fuels)GAS

Challenges (integrity of crucibles)

Conclusions

Direct smelting can be used to replace amalgamation

It is cheaper on consumables

It takes a similar length of time

It uses no hazardous reagents

It is suitable for small batches

It is visible

Cost estimates for kitCHARCOAL

Item Number Unit cost, ¢ Total Cost, ¢

Total cost, €

Total Cost, $

Furnace 1 4,000,000 4,000,000 355 444.44

Tongs 2 60,000 120,000 10 13.33

Cupels 2 100,000 200,000 16.7 22.22

Hammer 1 25,000 25,000 2 2.78

Gloves 2 pairs 20,000 40,000 3.33 4.44

Goggles 1 40,000 40,000 3.33 4.44

Spoons 2 1,000 2,000 .16 0.22

Weighing balance

1 800,000 800,000 66.7 88.90

Generator 1 2,500,000 2,500,000 208 277.78

Crucibles 100 2,000 200,000 1.67 222.22

Flux reagents 1 2,000,000 2,000,000 167

Mixing bowls 2 2,000 4,000 3.3 0.44

Total 9,931,000 827.2

1083.45

Cost estimates for kitGAS

Item Number Unit cost, GH¢ Total cost GH¢ Total cost $ Furnace & accessories 1 640.00 640.00 640.00

Tongs 2 6.00 12.00 12.00 Cupels 2 10.00 20.00 20.00

Hammer 1 2.50 2.50 2.50 Gloves 2 pairs 2.00 4.00 4.00

Goggles 1 4.00 4.00 4.00 Spoons 2 0.10 0.20 0.20

Weighing balance 1 80.00 80.00 80.00 Crucibles 50 0.20 10.00 10.00

Flux reagents 1 200.00 200.00 200.00 Mixing bowls 2 0.20 0.40 0.40

TOTAL 973.10 973.10

Acknowledgements

The on-going direct smelting studies is being sponsored by the EU. The great support given by GBS and Wardell Armstrong is worth commending. I am grateful to the organizers of this CASM Conference for sponsoring me for this presentation.

RIDE A CROCODILE RIDE A CROCODILE AT PAGA GHANAAT PAGA GHANA