marcello m. veiga, associate professor of university of british columbia, vancouver, canada,...

Marcello M. Veiga

Associate Professor of University of British Columbia

Vancouver, Canada

Reducing/Replacing Hg in ASM Operations

Suriname, 2008

Presented at the 8th CASM – Community and Small-scale Mining Annual Conference

Brasilia October 6-12, 2008

Control Hg Control Hg BioavailabilityBioavailability

Technical Solutions for Hg PollutionTechnical Solutions for Hg Pollution

Alternative Alternative Processes to Processes to Replace HgReplace Hg

Reduce Hg Reduce Hg Use and Use and

EmissionsEmissions

Long-term Short-termMedium-term

Replace amalgamation

with other process

Avoid methylation covering or

dredging Hg-contaminated

tailings

Avoid exposure to Hg and

eliminate bad practices

Technical SolutionsTechnical Solutions

AlternativeProcesses

Cyanidation

Electrolytic

Control Hg Bioavailability

Polluted Sites

Mercury Dispersed

Re-suspensionof SedimentsCovering

Dredging & Treatment

BioaccumulationOccurring ?

Monitoring

NY Y

Change ofFood Habits

Fish

Hair

Process

Reduction ofHg Emissions

Systemic Solutions

Individual Solutions

Processing Centers

Organizationof Associations

Law Enforcement

PermanentBiological Monitoring

Education

Retorts & specialFume hoods

Activated Hg

Use Hg far from people

& Technical Assistance

BioaccumulationOccurring ?

Amalgamateconcentrates

Other Lixiviants

Direct Smelting

Amalgamation of Amalgamation of the Whole Orethe Whole Ore

Huge Hg losses, large Huge Hg losses, large environmental problemenvironmental problem

Burning Burning Amalgams in PansAmalgams in Pans

Health problem for miners, Health problem for miners, family and neighborsfamily and neighbors

HgHgoo CH CH33Hg in fishHg in fishCyanidation of Hg-Cyanidation of Hg-

contaminated tailingscontaminated tailings

HgHgo o vapor lungs vapor lungs

and/or

Reduction of Hg Emissions Depends Reduction of Hg Emissions Depends on the Amalgamation Procedureon the Amalgamation Procedure

Amalgamation of Whole Ore is the Main Cause of Hg Loss

• There is a false perception that the main Hg loss in ASM is when miners burn the amalgam in open air

• The main losses occur when the whole ore is amalgamated:– Hg is spread on the ground or in sluice boxes– Cu-Hg plates – Grinding with mercury

Hg is retained in the rifflesgiving the impression that

amalgamation occurred in the pool

Hg is spread in the pooland pumped to the

sluice box

Venezuela, 1995

Amalgamation of the Whole Ore Amalgamation of the Whole Ore (Mercury Spread on the Ground)(Mercury Spread on the Ground)

Hg goes with tailingsHg goes with tailingsBrazil, 1999

Hg is added here

Amalgamation of the Whole Ore Amalgamation of the Whole Ore (Mercury in the Sluice Box)(Mercury in the Sluice Box)

• Between 1860-1895: 6,350 tonnes Hg lost to Carson River, Nevada

• 12,000 tonnes Hg lost in California and Nevada

• Archives from British Columbia: 13kg of Hg/day/sluice used by miners at Cariboo, Canada (1856)

This is 20,000 x more Hg than used by a Brazilian ASM This is 20,000 x more Hg than used by a Brazilian ASM

Amalgamation of the Whole OreAmalgamation of the Whole Ore(Mercury in the Sluice Box)(Mercury in the Sluice Box)

(it was a common practice in North America during gold rush)(it was a common practice in North America during gold rush)

Canada, 1856

Plates: attrition remove Hg from plates = Hg is lost

Amalgamation of the Whole Ore Amalgamation of the Whole Ore (Cu-Plates with Hg)(Cu-Plates with Hg)

Brazil, 2008

Venezuela, 2003

Venezuela, 2003


Zimbabwe, 2005

Use of Copper Use of Copper Amalgamation Amalgamation

Plates Plates generates generates

tailings highly tailings highly contaminated contaminated

with Hgwith Hg(usually 50-200 (usually 50-200

ppm Hg)ppm Hg)


Amalgamation of the Whole Ore Amalgamation of the Whole Ore (Grinding with Hg)(Grinding with Hg)

Indonesia, Talawaan, 2001

Adding Hg into Adding Hg into the Grinding the Grinding

CircuitCircuit

25 to 30% of the 25 to 30% of the Hg added is lost Hg added is lost

with tailings with tailings (“flouring”)(“flouring”)

• Colombia: Hg in the “cocos” (ball mills)

• About 4 ounces of Hg is added in each “coco” (ball mill)

Colombia, 2007

Grinding with Hg Grinding with Hg Antioquia Province - ColombiaAntioquia Province - Colombia

Additives used in Amalgamation Additives used in Amalgamation by Artisanal Minersby Artisanal Miners

Miners use these methods to reduce Hg flouring and consequently Hg losses with Tailings:

• Clean Hg with boiling water (Indonesia)• Lime juice (Laos, Colombia)• Caustic soda (Colombia)• Cyanide (Zimbabwe)• Brown sugar (Ecuador)• Molasse (Colombia)• Urine (Chile)• Sodium-amalgam (Colombia, Brazil)

Brown Sugar in the Ball Mills with Hg

Ecuador, 2004

Amalgamation in Antioquia, Colombia

““coco””coco””

elutriatorelutriator

““coco”coco”(with balls)(with balls)

elutriatorelutriator

cyanidationcyanidation

70kg ore70kg ore30L water30L water

120g Hg120g Hg50kg balls50kg balls

(1/2 volume)(1/2 volume)

200g Ca(OH)200g Ca(OH)22

4 hours4 hours

Excess Hg + Excess Hg + amalgamamalgam

90g Hg90g Hg

10mL molasse10mL molasse

10L lime 10L lime juicejuice

NaHCONaHCO33

pH 5pH 5

pH 11pH 11

3 hours3 hoursExcess Hg + Excess Hg + amalgamamalgam

• Tailings (up to 5000 ppm Hg) are later leached with cyanide

• 50 - 70% of Au recovered by amalgamation

• 25 - 30% of Hg is lost

• 50 to 100 tonnes Hg/a lost in the Antioquia Province

Colombia, 2007

Grinding with HgGrinding with HgAntioquia Province - ColombiaAntioquia Province - Colombia

Using Urine to Amalgamate Using Urine to Amalgamate

Chile, 2008

Forming Sodium-amalgam (increase coalescence = reduces Hg flouring = less Hg loss with tailing)

sodium-amalgam is more consistent

than pure Hg

Also called “Hg Activation”

Battery12 V

wire

Mercury

Water with NaCl (10%)

+ -

Graphite rod

Dr Pantoja’s Method

Brazil, 2006

Zimbabwe, 2006

3800 miners in the GMP site in the Amazon adopted this technique

• Amalgamate only gravity concentrates

• Promote good contact between Hg and gold (particles must be cleaned, e.g. detergent)

• Avoid severe grinding that causes “flouring”

• Additives to reduce Hg surface tension

• Use activated Hg (Na or K-amalgam). K-amalgam is used in Colombia to capture alluvial Pt (Mineros de Antioquia SA)

• Centrifuge amalgam after amalgamation to remove excess mercury

What Can Be Done to Improve Amalgamation What Can Be Done to Improve Amalgamation and Reduce Hg Emissionsand Reduce Hg Emissions

Tanzania, 2005

Manual Amalgamation Must Be AvoidedManual Amalgamation Must Be Avoided

• One part of Hg to 100 parts of concentrate

Glass Amalgamation Barrel (3 L)

Using an Amalgamating Barrel to Improve Amalgamation

Indonesia, 2006Indonesia, 2006

• Excavated pool lined with a plastic trap

• When the pool is full, cover it

• CONTAMINATED TAILINGS MUST NOT BE RECYCLED Brazil, 2007

After Amalgamation, the Amalgam Must Be Separated from the Heavy Minerals

Reducing %Hg in amalgamfrom 40% (manual squeeze)

to 20% (centrifuge)

filtered mercury

PVC cups

amalgam

clampspiece of

cloth


Manual Centrifuge

Brazil, 2006

Filtering Amalgam

Alternative to Reduce/Replace HgAlternative to Reduce/Replace Hg

• Direct smelting of concentrates• Electrolytic Process• Cyanidation• Other lixiviants

Grinding

Ore

Concentration

TailingConcentrateCyanidation

Other lixiviants

Direct smelting

Gold Not Liberated Liberated Gold

Assuming that the dark particles are gold

0.07 mm

Gold Concentration is FundamentalGold Recovery is Low when Gold is not Liberated

Gravity Separation

In order to concentrate it’s necessary to In order to concentrate it’s necessary to LIBERATE gold particlesLIBERATE gold particles

• Liberation is obtained by comminution (crushing and grinding)

• Gold occurs occluded in other minerals

• Manual crushing is inefficient Tanzania, 2007

Good Grinding Does not Need Sophisticated Equipment

Mozambique, 2005Indonesia, 2004

NO Hg ADDED in the ball millsNO Hg ADDED in the ball mills

10 kg of ore ground with 10 kg of ore ground with 14 steel balls for 45 min. 14 steel balls for 45 min.

Panning in a Panning in a closed pool (away closed pool (away from rivers)from rivers)

Some Hg-free gold Some Hg-free gold can be obtained can be obtained but in most cases but in most cases Hg is introduced Hg is introduced at the end of the at the end of the panning step to panning step to capture fine Aucapture fine Au

Mozambique, 2005

Gold Concentration by Panning

Zig-zag SluiceZig-zag Sluice


Zig-zag sluices increase chances of

capturing gold

Suriname, 2007Suriname, 2007

Concentrating Gold in Sluice Box


Centrifuges

• Good for coarse and fine gold (0.05 mm)

• Much more efficient than sluices

• Used by mining companies• High cost• High maintanance and

control• Most common: Knelson

and Falcon, both from B.C., Canada

• S.G. quartz = 2.7

• S.G. gold = 19=

quartzgold

When the particles are subjected to the gravity force (G =1), one gold particle of 0.02 mm fall in water as

fast as a quartz particle of 0.07 mm

Then there is no way to separate them

Why Are Centrifuges More Efficient?

Why Are Centrifuges More Efficient?When the particles are subjected to G = 60, the

difference in fall velocity is much higher than with G = 1

0

1

2

3

4

5

6

0 20 40 60

Gravity (G)Dif

fere

nc

e i

n F

all

Ve

loc

ity

(m

m\s

ec

)

• Introducing new centrifuges developed with UBC and the company Falcon Concentrators(Langley)

• Price <US$4000• 1-2 tonnes/h• Good for fine gold

Canada, 2006

New Falcon Centrifuge for ASM

Cleangold® Sluice Box

Magnetic Sluice BoxesMagnetic Sluice Boxes

Venezuela, 2003Venezuela, 2003

• Ore must have magnetite

• Iron bits from hammer mills can also be attracted

• Like any other gravity concentrator, uliberated gold is lost with middlings

Sudan, 2004Sudan, 2004

Using Small CleangoldUsing Small Cleangold

Panning Tailings with CleangoldPanning Tailings with Cleangold

Concentrating Gold Concentrating Gold Using Fridge MagnetsUsing Fridge Magnets

Mozambique, 2005Mozambique, 2005

Gravity Concentration of a Gold Gravity Concentration of a Gold Ore from Talaawan, IndonesiaOre from Talaawan, Indonesia

HMMS = Homemade magnetic sluice

P80 = 0.25 mmConclusion: gold is not liberated and it’s very fine

Au (ppm or g/t)

Cleangold® HMMS Knelson®

Feed 12.60 12.30 11.50

Concentrate 64.00 117.00 16.00

Tailings 11.95 11.40 11.31

Au Recovery (%) 7% 8% 5%

ConcentrationConcentration

• Gravity Concentration– Depends on gold liberation (grinding) and how

fine is the gold (size of the particle)– Gold in the middling product (unliberated) in a

problem– For primary ore, gold recovery rarely goes

beyond 30-40% (even in industries)

ConcentrationConcentration

• Flotation– Depends on gold liberation (grinding) – Concentrates are usually poor (200 g Au/t)– Concentrate produced must be submitted to

amalgamation or cyanidation– Control of pH and reagents is needed– Little investment in flotation cell is needed

Flotation (Chile)Flotation (Chile)

Chile, 2008Chile, 2008

• Homemade flotation cells

• Use xanthate and pine oil

• Miners sell the gold and copper concentrate

• Sun-dried

ConcentrationConcentration• Agglomeration

– Depends on gold liberation (grinding) – Agglomerates (5 mm) of coal and oil are formed and put

in contact of a pulp of gravity concentrate in one or more cycles.

– Recoveries of 90% were obtained in one step. – Envi-Tech Inc of Edmonton, Canada has its own

proprietary agglomerate (gold absorbent)– Melted paraffin to collect gold in an acidic medium – Encouraging results but none of these methods provide a

simple, cheap and quick alternative for unskilled artisanal miners.

Direct SmeltingDirect Smelting

• Concentrates must be very rich• Smelting of low grade concentrates implies

in Au losses to slag and high amounts of borax used

• Lab tests show that the threshold is around 5,000 g Au/t in a concentrate

• To increase Au in concentrates, Au recovery decreases, i.e more Au is lost in middling

Electrolytic ProcessElectrolytic Process• SALTEM Process (devised by CETEM, Brazil)

– Gold ore (or concentrate) is mixed with NaCl (1 Mol/l) which is transformed by electrolysis into a mixture of sodium hypochorite-chlorate

– Seawater can also be used– More than 95% of the gold dissolves within 4 hours and is

collected on a graphite cathode – Solution is recycled minimizing effluent discharge – NaCl = 100 kg/tonne of ore – Energy = 170 kwh/kg of Au – Plastic tanks are used, reducing investment cost– Very good potential but a bit complicated for ASM

Electrolytic ProcessElectrolytic Process

-+

Cl2

ClO-AuCl4

-

NaCl solution

Pulp of gold ore

CyanidationCyanidation

• ASM are familiar with the process• Problems with NaCN in ASM:

– Poor control of the pH– Use of Zn precipitation followed by burning– High investment in agitated tanks– No CN destruction is applied– Cyanidation of Hg-contaminated material makes

Hg more bioavailable– Poor management of tailings with CN

4Au + 8CN- + O2 + 2H2O = 4Au(CN)2- + 4OH-

2Au(CN)2- + Zn = Zn(CN)4

2- + 2Au (Merrill-Crowe process)

Ecuador, 2006

• Zinc is used to precipitate gold from the cyanide solution

• Zinc is evaporated contaminating the whole area


Zimbabwe, 2005

• Cyanidation of Hg-contaminated tailings increases the bioavailability of mercury

• Mercury-cyanide species are formed


• Abandoned cyanidation heap in São Chico, Brazil near the water stream

• Tailings are still full of Hg when submitted to cyanidation Brazil, SBrazil, Sãão Chico, 2003o Chico, 2003

Hg-contaminated Tailings are Submitted Hg-contaminated Tailings are Submitted to Cyanidation - São Chico, Brazilto Cyanidation - São Chico, Brazil

• Carnivorous fish, Ave = 4.16 ppm Hg

• Non-carnivorous, Ave = 1.33 ppm Hg

• 60% of fish >0.5 ppm Hg (guideline for fish consumption)

• One fish sample = 22 ppm Hg

Hg-contaminated Tailings are Submitted Hg-contaminated Tailings are Submitted to Cyanidation - São Chico, Brazilto Cyanidation - São Chico, Brazil

Ecuador, 2007Ecuador, 2007

Amalgamation of a Pre-concentrate Amalgamation of a Pre-concentrate Followed by Cyanidation - EcuadorFollowed by Cyanidation - Ecuador

A pre-concentrate is obtained in sluice boxes (discharge every hour)… …and amalgamated in

barrels


Some operations use pH = 7 in Cyanidation

Amalgamation of a Pre-concentrate Amalgamation of a Pre-concentrate Followed by Cyanidation - EcuadorFollowed by Cyanidation - Ecuador

Tailings (with or without Hg) are leached with NaCN

Portovelo, Ecuador, 2007Portovelo, Ecuador, 2007

• Tailings with Hg cyanide are dumped into the Amarillo River, Ecuador

• 92 cyanidation tanks in the town of Portovelo

Amalgamation of a Pre-concentrate Amalgamation of a Pre-concentrate Followed by CyanidationFollowed by Cyanidation

Photo AJ Gunson

China, 2002

Hg is added while grinding the ore

Cu plate

Cyanide

• Brazil• China• Colombia• Ecuador• Indonesia• Peru • Philippines• Venezuela• Zimbabwe

Misuse of Cyanidationwith Amalgamation

Cyanidation

Gold Ore from Talaawan, Indonesia

Cleangold® HMMS Knelson® Cyanidation

Feed 12.60 12.30 11.50 12.20Concentrate 64.00 117.00 16.00 32.00Tailings 11.95 11.40 11.31] 1.96Au Recovery (%) 7% 8% 5% 84%HMMS = Homemade magnetic sluice

Au (ppm or g/t)

P80 = 0.25 mm6 hours of leaching in agitated tank

Improving Cyanidation for ASMImproving Cyanidation for ASM

• Instead of using Hg in ball mills, miners could use NaCN

• No additional investment needed• Training is fundamental• Quick leaching time• High Au recovery (>90%)

Mill-leaching(Cyanide in Ball Mill)

Gold Ore from Talaawan, Indonesia Grinding with NaCN for 2h + Leaching Time

Leaching Time, h

pHInitial Final

Final NaCNg/L

%AuRecovered

Au in the sample, g/tonne

2 12.3 12.2 0.88 77.9 14.74 12.3 12.2 0.88 85.5 16.26 12.1 11.9 0.78 93.1 17.58 11.4 11.1 0.88 93.3 15.024 10.7 10.7 0.86 93.6 14.8

• Ore ground in Chilean mill 80% passing 0.150mm• Pre-concentrate from sluice box (17.3 g Au/t):

– Amalgamation (160 kg)– Cyanidation in agitated tank (695 kg)– Field tests of mill-leaching (80kg)

Field Tests in Ecuador


• Amalgamation of the pre-concentrate was manual

• Use of brown sugar

• 8 hours of amalgamation in a batea

• Amalgam was burned in a retort

• Gold recovery was 26%

Result of Amalgamation


• Miners are familiar but not with Carbon in Leaching• Activated carbon was added after 7 hours of

leaching• pH = 11• 41% solids • Gold dissolution in 7 hours of leaching was 62%• Total gold recovered after 31 hours was 94% • Gold grade of the AC was 1235 mg/kg • NaCN consumption of 4.5 g/kg of ore.

Result of Cyanidation in Agitated Tank

Cyanidation in Agitated Tank

Ecuador, 2007

• 95% of gold extracted in 8 h (2h grinding with CN and 6 hours leaching)

• Use of activated carbon• Residual NaCN=1.7 g/L • Free cyanide was

destroyed with bleach before being discharged

• The NaCN consumption was 0.95g/kg of ore

Ecuador, 2007

Results of Mill-leaching(Cyanide in the Grinding Circuit)

Elution of Activated Carbon (AC) • NaCN = 2g/L, NaOH = 10 g/L• Alcohol = 20%• Temperature: 90 °C, 3-4 hours• 97% Au removed from AC• Gold was precipitated with Zinc• Loaded zinc shavings were

leached with nitric acid and melted with borax to produce the gold bullion

• Zinc in the nitric solution was precipitated with lime

Ecuador, 2007

Results of Mill-leaching(Cyanide in the Grinding Circuit)

• Faster than agitated tank leaching:– 8h of mill-leaching = 31 h of agitated tank

• All gold remaining in the moisture in the tailings is recovered

• This is almost 30% more gold which is usually lost with tailings in agitated tanks

• No need for investment in equipment (in the case of Ecuador, Colombia and Indonesia)

Advantages of Mill-leaching(Cyanide in the Grinding Circuit)

Other ReagentsOther ReagentsName Reagent pH Complex formed with Au

Thiourea NH2CSNH2 1-4 [Au(NH2CSNH2)2]+

Bromine Br- 1-7 AuBr4-

Iodine I- 1-5 AuI2-

Thiocyanate SCN- 1-3 [Au(SCN)4]-

Thiosulfate S2O32- 8-11 [Au(S2O3)2]

3-

ChlorineCl-, OCl-, Cl2

ClO3-

1-4 AuCl4-

Adapted from Trindade & Barbosa Filho. Reagentes Alternativos ao Cianeto. Chapter 9, p. 211-252. In: Extração de Ouro - Princípios, Tecnologia e Meio Ambiente. CETEM/CNPq, Rio de Janeiro, Brazil

Other ReagentsOther Reagents

• iGoli Process (Mintek, South Africa)– Gold from gravity concentrates (>1000 g Au/t) is

leached with hypochlorite and HCl– Gold is precipitated with sodium metabisulfite,

or ferrous sulphate or SO2, etc.

– Solution is filtered– Gold powder is hammered to become yellow– Many field tests in Africa– Great potential and open technology– Hard to find reagents in remote areas

iGoliiGoli

Photo: Mintek, South Africa 2001

• There is no panacea• There are many processes to replace Hg but all

cases are site-specific (level of education, level of investment, labor organization, type of ore, access to reagents, etc.)

• Any method must be transparent and simple• Gravity concentration is key to reduce Hg

emissions and promote leaching of concentrates• Gravity concentration alone is efficient in alluvial

ores or when grinding is efficient (rarely more than 30% gold recovery is obtained)

Conclusion

marcello m. veiga, associate professor of university of british columbia, vancouver, canada,...

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