World Gold 2015

Novel and environmentally benign processing options for low grade and polymetallic base

metals-precious metals ores

Jacques Eksteen Gold Technology Group

Curtin University

Outline • Complexity of hydrothermal gold ore deposits • Conventional approaches • Process challenges with conventional processes • Why use amino acids in alkaline oxidizing environments? • Glycine, the simplest amino acid • Selectivity: Metals that leach and metals that don’t leach in

glycine • Benefits of using glycine in alkaline environments • Focus on copper-gold-silver deposits:

– Leaching of copper minerals – Leaching of gold & silver

• Metal recovery from pregnant leach solutions • Conceptual Integrated flowsheets

Complexity of hydrothermal gold ore deposits • Sedimentary ~ 20%, Hydrothermal ~80% • Table below gives breakdown of hydrothermal gold ores

Deposit Average Size Known

Resources

Estimated Resources Depth

(% of deposits)

Type Moz (%) <1 km <3 km

Epithermal 1.1 18.2 31.8 24.0

Orogenic 4.9 34.1 5.8 2.3

Cu-Porphyry 5.2 29.8 42.8 55.9

Skarn 1.1 3.1 8.7 11.4

VMS 0.7 7.7 7.9 2.6

Carlin Type 1.8 4.2 2.7 3.6

IOCG 3.1 2.8 0.2 0.2

Kesler & Wilkinson, 2010

Conventional processing approaches for polymetallic ores

• Nuisance base metal levels (oxide ores): – High cyanide addition, high pH, significant WAD production.

• Recoverable base metals (oxide ores) – Acid (heap) leach followed by cyanidation

• Recoverable base metals (sulfide ores) – Mill/Grind followed by flotation and cyanidation of flotation

tailings. Flotation concentrate is smelted. – Heap bioleaching of low grade base metal sulfides followed by

Cyanide leaching.

Process challenges with conventional processes (1)

Nuisance base metal levels (oxide ores): • High cyanide addition to complex base metals, • high pH, • significant WAD production, • high detox costs, • base metals compete with gold in adsorption, • challenges in gold electrowinning

Process challenges with conventional processes (2)

Recoverable base metals (oxide ores) by acid (heap) leach followed by cyanidation: • pH-swing & neutralisation costs • Silica gels formed by sheet silicates (chlorites/vermiculite) • High acid consumption with acid consuming gangue • Significant slumping for heap leach processes • Removal of dissolved iron and valueless dissolved metals • High residual base metals • Competitive adsorption • Sulfides from transitional domains in the deposit

Process challenges with conventional processes (3) Recoverable base metals (sulfide ores)

– Mill/Grind followed by flotation and cyanidation of flotation tailings. Flotation concentrate is smelted:

• Energy costs associated with fine grind, low grade • Smelter constraints:

– Deleterious elements (As, Hg, Fluoride, Chloride, Mg) – Grade specifications leading to poorer recoveries – Biased sampling of fine floated free gold in trucks – Transport costs associated with transporting low grade

concentrates – Mass pull constraints leading to lower metal units to

concentrate • High residual base metals in float tails

– Often non-sulfide base metals, highly cyanide soluble, high WAD generation

Process challenges with conventional processes (4) Recoverable base metals (sulfide ores)

– Heap Bioleach of low grade base metal sulfide deposits: • Low base metal extraction • Elemental sulfur formation

– Passivates gold/silver – Consumes cyanide to form thiocyanate

• Silica gels formation • Iron partially dissolves or forms jarosite (which locks up silver). • Neutralisation costs • High residual base metal content impacting cyanide

consumption during subsequent cyanidation • Acid consuming gangue

Why amino acids in alkaline (oxidizing) environment (glycine in particular)?

Non-toxic Ease of reagent recovery and reuse

Operated under dilute and concentrated modes

Environmentally benign Reagent Cost Chemical stability

High affinity for Au, Ag, Cu, Zn, Pb, Pd, Hg, Cd, Ni, Co

Can be synergistic with cyanide: accelerated Au leach rates

High adsorption of Au, Ag onto activated carbon. Ease of AARL-type elution

No interaction with acid consuming gangue

Application to various leaching modes

Cheap materials of construction

Selective over non-sulfide gangue minerals

No transportability & logistics, trade restrictions

Ease of base metals removal / recovery

Thermally stable Non-volatile Simple chemistry

Enzymatically destructible Cu- glycinate good oxidant No pH swings

Insignificant Fe, Mn, Mg, Al dissolution

No elemental Sulfur, no silica gels, no jarosites

Pyrite not dissolved

What is an Amino Acid? • Building block of all proteins

• About 500 different types (including peptides).

• Simpler amino acids are produced in bulk quantities and available at low prices (Glycine at $1,600-$1,700 per metric tonne).

• Glycine is simplest amino acid (amino acetic acid).

• Sweet tasting, non-toxic, non-volatile and occurs in human body.

• Crystalline, non-hygroscopic, free flowing powder.

• Glycine has the following structure:

Black: Carbon White-grey: Hydrogen Blue: Nitrogen Red: Oxygen

Metals and their minerals that leach in glycine under alkaline conditions

Complexing Power of Glycine

Focussing on copper and gold • Soluble oxide minerals:

– Cuprite, tenorite, malachite, azurite, atacamite, antlerite

• Soluble minerals that require oxidation – Native copper, chalcocite, covelite,

bornite, chalcopyrite, enargite, tetrahedrite, tennantite

• Poorly soluble copper minerals (~20% Cu extraction): – Chrysocolla

• Gold, electrum are soluble at temperature above 40 °C

Copper-Gold Oxide Ore from Utah, USA

Other base and precious metal minerals already evaluated successfully in alkaline glycine solutions

• Lead: Galena, anglesite, cerussite • Zinc: Sphalerite, zincite, smithsonite • Nickel: Pentlandite • Palladium: Palladium sulfides, tellurides, selenides • Mercury: Cinnabar • Cadmium: Greenockite • Cobalt: Cobaltite

Materials that have been evaluated: • Ores, concentrates, furnace matte, slag, electronic scrap, tailings Liberation / exposure required, as for all leach reactions

Gold Leaching

Effect of pH

0.5M glycine, 1% H2O2, pH, and 60 °C

Leach Time, hr

Au, 103 x µmol/m2.s pH 5.8 pH 10 pH 11

24 8.11 0.59 352

29 8.75 1.30 367

48 5.13 11.47 322

119 4.19 14.34 174

167 3.02 16.93 142

Amino Acids Type

0.5M amino acid, 1% H2O2, pH 10, at 60 °C

Effect of Silver

1M Glycine, 1% H2O2, pH 10, 60 °C

Au, Ag Source Au, 103 x µmol/m2.s Gold from (pure gold sheet) 31.3 Gold (from 50% Au- 50% Ag) 185 Silver (from 50% Au- 50% Ag) 247

Effect of Temperature

1M Glycine, 1% H2O2, pH 10, 60 °C

Effect of glycine concentration in low concentration range on gold leaching

Glycine, 1% H2O2, pH 10, 60 °C

Effect of H2O2 concentration

0.1M glycine, different percentages of H2O2, pH 10.5, 60 °C

Catalytic effect of cupric ions

0.1M glycine, 0.1% H2O2, pH 11, at 30 °C

0.1M glycine, 0.3% H2O2, 4 mM Cu2+, pH 11.9 and 30 °C

Synergistic effect of amino acid mixtures

0.1M amino acid, 1% H2O2, pH 11, at 60 °C

Metal recovery: Adsorption of metal glycinates onto activated carbon

Plot of Log (∆[Me]c/[Me]s) against Log (t) for 4 hours ( 1 M Glycine; pH 10, T=25 °C; Activated Carbon 1.5 g/L) [13.2 kgAu/ton carbon in 4 hours]; [8.89 kgAg/ton carbon]

Time min [Au] mg/L 0 38.70

30 24.60 96 15.82

180 12.08 240 10.26

Time min [Ag] mg/L 0 56.1

30 48.1 96 42.7

180 39.1 240 37.0

Metal recovery: Adsorption of metal glycinates onto activated carbon

Plot of Log (∆[Me]c/[Me]s) against Log (t) for 4 hours ( 0.5 M Glycine; pH 10, Temp=25 °C; Activated Carbon 1.02 g/L) [6.7 kgAu/ton carbon in 4 hours]

Glycine-cyanide synergies

Synergy of cyanide leaching of gold in the presence of Cu2+ ions and glycine

Gold dissolution versus leaching time in solutions containing: (a) 11 mM free cyanide and (b) 5.5 mM Cu, 11 mM CN-, 1 g/L (13.3 mM) glycine at pH 11. 5

Effect of pH on gold leaching with cyanide in the presence of Cu2+ and glycine

Gold dissolution versus leaching time in solutions containing 5.5 mM Cu and 11 mM CN- in the presence of 1 g/L glycine at different leaching pH values

The role of Cu2+ on the dissolution of Au in alkaline cyanide – glycine mixtures

Gold dissolution rates in solutions containing a CN:Cu:Gly molar ratio of 2:1:2 at initial copper concentrations of 4, 5.5 and 15 mM Cu and 12 mM CN- at pH 11.5

Copper Leaching

Alkaline System For Cu-ores

Why?

Challenges in Acidic leaching • Many copper porphyries, Cu-Au deposits and some Au

deposits have significant amounts of alkaline gangue, e.g. calcite, dolomite, siderite, magnesite, etc.

• For acid leaching, each 1 kg CaCO3 requires ~ 1kg H2SO4. • pH shift to >3 precipitates/removes ferric oxidant. • For economic acid leaching, carbonates (as CO2) should

be <2% • Elemental sulfur produced during the leaching of

copper sulfide can deposit and block pores. • Special materials of construction are required

ACID CONSUMPTION AND ESTIMATED TIME TO COMPLETE DISSOLUTION OF MAJOR MINERAL PARTICLES OF SANDSTONE FOR GRAIN SIZE <0.1 mm AND 10 g/dm3 SULPHURIC ACID (IAEA, 2001)

Alkaline Vs Acidic leaching • Alkaline leaching of copper is: More selective. Less corrosive. Lowering reagent consumption. No special materials of construction are required. No sulfur, gels and jarosite precipitation. Particularly useful when followed by alkaline gold or

silver leaching (cyanide or glycine based). Does not passivate gold in a subsequent leach. Convenience of using NaSH in alkaline domain and

high SX extraction selectivity.

Current alkaline lixiviant systems • Ammonia leaching

• Limited ability to recover efficiently and reuse. • Partial decomposition with cheaper oxidants. • Ammonia is not inherently benign • Lower capacity of leaching of acidic compounds • The use of ammonia is limited due to its high evaporation • May have potential at hydrostatic pressures associated with

ISL. • Difficulty in maintaining high concentrations of dissolved

NH3. • Extractors used for copper recovery from ammonia medium

do not have high efficiency.

Cu-NH3 vs Cu-Glycine

Cu-NH3 Cu- Glycine

Use of glycine for Cu leaching in alkaline region

Copper mineral in the concentrate before and after leaching in glycine solutions

Copper extractions from Cu-Au concentrate after two-stages leaching : Stage 2 starts at zero time after stage 1. (Leaching conditions: two-stages, 0.3 M glycine, 1% peroxide, ambient, pH 11, 48 hours)

Effect of leaching solution pH on copper dissolution: 0.3M glycine, 1% H2O2, pH 11, at ambient temperature.

Copper Oxides

Quantitative XRD of Minerals Used

Phase Weight %

Azurite Chrysocolla Cuprite Malachite

Azurite 65.0 - - -

Chrysocolla - 59.2 - -

Cuprite - - 22.6 -

Dolomite - - 62.1 -

Goethite - - 0.5 1.7

Kaolinite-1A 3.4 6.0 - 3.3

Malachite - - - 66.0

Muscovite-1M - 13.5 - -

Muscovite-2M1 6.2 12.8 - -

Quartz 7.4 6.7 1.0 16.7

Rutile - 1.8 - -

Amorphous Content 18 - 27 12.1

Elemental assay of mineral samples used

Mineral/ element Cu (%) Al (%) Si (%) K (%) Zn

(ppm) Ti (%) Mg (%) Fe (%) Ca (%)

Chrysocolla 24.0 2.96 21.6 1.4 - 0.13 0.41 - -

Malachite 41.55 0.13 8.49 0.1 - - - 3.53 -

Azurite 42.1 1.91 6.41 0.6 50 - - - -

Cuprite 20.09 - 0.52 - - - 9.56 1.91 16

Comparison of Cu oxide mineral leaching

Copper Extraction from copper oxide minerals: Gly: Cu=4:1, pH=11, H2O2=0.5%

Oxide minerals: Cu extraction using alkaline glycine/glycinate

Minerals Copper extracted, %

24 hours 48 hours

Azurite 94.2 96.83

Malachite 91.7 92.30

Cuprite 84.5 90.88

Chrysocolla 18.70 20.3

Copper Sulphides - Chalcopyrite

Chalcopyrite sample mineralogy and assay

Quantitative XRD analysis

Phase Chabazite Chalcopyrite Clinochlore Pyrite Quartz Rutile Amorphous

Weight ,% 0.8 35.0 1.5 29.4 0.9 0.4 32.0

Element As Ca Cu Fe Mg Pb Si S Zn

% 0.71 0.31 14.8 30.9 1.08 0.29 3.64 32.40 4.95

Elemental Analysis

Ambient temperature bottle roll tests

Effect of glycine concentration – Ambient Temperature

Copper Extraction from Chalcopyrite: Gly:Cu, pH=11

Effect of pH - Ambient Temperature

Copper Extraction from Chalcopyrite: Gly:Cu, 4:1

Sulfur speciation during/after chalcopyrite leaching with alkaline glycine

Sample ID mg/L % of total S

Sulfate 1142 89.8

Sulfite 74.0 7.2

Thiosulfate 22.2 3.0

Sulfide < 0.1 -

Chalcopyrite leaching: Effect of dissolved O2

Leaching conditions: 0.1 M Glycine, Ambient Temperature, pH 10.5, controlled DO level.

Chalcopyrite concentrate leaching: Effect of glycine concentration

Leaching conditions: 60°C pH 10.5, DO 25 ppm.

Leaching conditions: 0.2 M glycine, DO 25 ppm, pH 11.5

Chalcopyrite concentrate leaching: Effect of temperature

COPPER RECOVERY FROM SOLUTION

Copper recovery from alkaline glycinate solution: NaSH precipitation

Sulfide ions were added to the pregnant liquor at different Cu:S2- molar ratios in order to recover copper from the glycine solution.

The copper recovery was up to 99.1% as Covelite (CuS) at Cu:S2- molar ratio of 1: 0.70 in 10 minutes contact time.

Sample ID Cu, mg/L

Before precipitation 1243

After precipitation Cu:S2- 1:0.70 11

After precipitation Cu:S2- 1:0.5 226

Particle size distribution of CuS precipitate

CuS as precipitated from alkaline copper glycinate solutions

d(0.1):

8.8 µm

d(0.5):

33.1 µm

P80:

67.0 µm

d(0.9):

89.4 µm

GlyLeach Process with CuS Precipitation

Single Stage Solvent Extraction Solvent extraction (SX) experiments show that copper glycinate can be easily extracted from the alkaline aqueous medium. Extractant: LIX 84-I Stripping with 180 g/L H2SO4 and 30 g/L CuSO4.

Concentration in Aqueous Phase, (mg/L) Extraction,

(%)

Stripping,

(%)

Sample ID Equip. pH Cu Cu Cu

Feed 11.5 3596

Test 1 8.8 43.9 98.8 -

Test 2 9.4 65.6 98.2 100

Test 3 10 22.3 99.4 100

GlyLeach Process with Cu SX-EW

Conclusions

• 2 Patents pending (One PCT another Provisional) • Amino acids, or their salts, with a suitable oxidant (O2, H2O2,

Air) opens up a range of leaching options (ISL VL, HL) feasible at alkaline pH.

• Copper, gold and silver dissolved in same pH range (10-11). • Results show high potential for the use of alkaline amino

acids at either room temperature (oxides) or moderately elevated temperatures (40-60 °C, for sulfides) for copper & gold leaching in:

- Heap/Vat leach operations - Tank leach operations - Possible in-situ leaching

Conclusions (Cont’d) • Glycine shows much promise due to bulk availability

and low cost. • Many Cu or Au-Cu deposits have too much alkaline

gangue to make either acid or bioleaching feasible, but which are amenable to amino-acid based leaching.

• High copper extraction from copper oxides was achieved in 24 hours.

• Higher copper extraction were achieved at 4:1 Gly:Cu Very low Fe co-dissolution

• Carbonate minerals are not decomposed. • Precipitation of elemental sulfur, gypsum, jarosite

eliminated.

Conclusions (Cont’d) • In the presence of an air, oxygen or hydrogen peroxide or

a mixture thereof, glycine can dissolve copper from chalcopyrite at either an elevated (40-70 °C) or ambient temperature.

• Pyrite remained unreacted during leaching of chalcopyrite and iron concentration in the final pregnant solution was less than 20 mg/L.

• Recovery and reuse potential of lixiviants after SX, electrowinning and NaSH precipitation.

• Sulfides are oxidised to sulfates. • Applicable to wide range of Cu-minerals (oxide, sulfide,

native copper).