s. al-hassan & r. k. amankwah & r. s. nartey, university of mines and technology, ghana,...
TRANSCRIPT
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