The effects of dissolved oxygen and cyanide dosage on gold extraction from a pyrrhotite-rich ore

The effects of dissolved oxygen and cyanide dosage on gold extraction from a pyrrhotite-rich ore

Hydrometallurgy 72 (2004) 39 – 50 www.elsevier.com/locate/hydromet The effects of dissolved oxygen and cyanide dosage on gold extraction from a pyrrh...

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Hydrometallurgy 72 (2004) 39 – 50 www.elsevier.com/locate/hydromet

The effects of dissolved oxygen and cyanide dosage on gold extraction from a pyrrhotite-rich ore Sandra Ellis a, Gamini Senanayake b,* b

a Newmont Jundee Operations, PO Box 1652, Subiacao WA 6904, Australia Extractive Metallurgy and Mineral Science, A.J. Parker Cooperative Research Center for Hydrometallurgy, Murdoch University, Perth, WA 6150, Australia

Received 16 November 2001; received in revised form 17 January 2003; accepted 7 June 2003

Abstract The results of cyanidation of a pre-oxidized pyrrhotite-rich gold ore in laboratory leaching experiments and in plant trials are presented to highlight the effects of particle size, oxygen and cyanide concentrations during pre-oxidation and cyanidation on the overall gold extraction. The Multi Mix Systems (MMS) unit for oxygen injection significantly enhances the mass transfer of reagents and allows the achievement of high gold extraction. Pre-oxidation of ore of P80 = 63 Am for 12 h followed by cyanidation at a molar ratio of [CN]/[O2] c 12 provided the best gold extraction in the first cyanidation tank. The %Au extracted in the first cyanidation tank reached a maximum of 83% when the product [CN]  [O2] reached c 5.6 mmol2 L 2. D 2003 Elsevier B.V. All rights reserved. Keywords: Gold; Pyrrhotite; Cyanide; Oxygen; Pre-oxidation; Leaching, sulphide

1. Introduction The Bounty Gold Mine located within the Forrestania Greenstone Belt of the Southern Cross Province, Western Australia has recovered over a million ounces of gold from 6.1 megatonnes of ore at an average grade of 5.5 Au g/t. The underground orebody has estimated in-situ reserves (excluding resources) of over 325,000 ounces of gold and a pre-mining reserve of greater than 1.5 million ounces of gold. The economic mineralisation within the Bounty Horizon is dominated by pyrrhotite with minor pyrite,

* Corresponding author. Fax: +61-8-93606343. E-mail address: [email protected] (G. Senanayake). 0304-386X/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0304-386X(03)00131-2

marcasite and trace chalcopyrite. Table 1 lists the mineral composition of Bounty ore and Table 2 lists the composition of important elements. The gold rich lodes within the orebody can be as high as 25 g/t. Gold distribution in gold-rich pyrrhotite breccia matrix (5– 25 Au g/t) is either as clouds of very fine gold ( < 5 Am) associated with pyribole in the breccia matrix or much coarser gold (up to 30 Am) associated with quartzite/chert clasts. The gold-poor breccia ( < 1 Au g/t) can be visually distinguishable from the gold-rich breccia by a subtle colour change and a decrease in grain size. Typical gold grades vary considerably, ranging from 1 to 15 Au g/t and the economic cut-off grade for the underground mining strategy is 3.5 Au g/t. The presence of iron sulphide minerals, especially pyrrhotite, has influenced the optimisation of the

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Table 1 Mineral composition of Bounty ore Mineral name

Common mineral formula

Composition percentagea

Quartz Pyrrhotite Clinopyroxene

SiO2 Fe7S8 Family of (metal)AlSi2O6 and (metal)FeSi2O6 NaAlSi3O8 – CaAl2Si2O8 (Ca,Na)2 – 3(Mg,Fe,Al)5(Si,Al)8O22 Fe3O4 K(Mg,Fe)3AlSi3O10(OH,F)2 FeS2 (Mg,Fe)7Si8O22(OH)2

30 (22 – 37) 16 (14 – 20) 14 (12 – 15)

Plagioclase Hornblende Magnetite Biotite Pyrite Mg Fe amphibole Garnet Carbonate Epidote Chalcopyrite Galena Arsenopyrite a

(metal)3(metal)2Si3O12 (metal)CO3 Ca2Al3Si3O12(OH) CuFeS2 PbS FeAsS

13 9 7 4 2 2

(10 – 18) (5 – 10) (4 – 11) (2 – 9) (1 – 5) (1 – 3)

1 (0.5 – 2) 1 (0 – 1) 1 (0 – 1) 0.5 (0 – 1) 0.5 (0 – 1) 0.5 (0 – 1)

Values in parentheses show the range.

processing plant over the years. Without pre-oxidation, the gold cannot be amenable to cyanidation at a cost-effective rate and thus pre-oxidation of the ore is a crucial factor in the leaching circuit. This paper presents a summary of the initial test work carried out in the laboratory and some of the plant trials with particular attention to examining the extent of gold leaching with respect to particle size, preoxidation, cyanide dosage and dissolved oxygen concentrations.

The concentration of cyanide determines the rate of anodic dissolution of gold whilst the concentration of oxygen determines the cathodic reduction of oxygen. Thus the initial rate of gold dissolution is largely controlled by the factors such as cyanide and oxygen concentrations, pH and temperature. The presence of other catalytic ions in solution and salinity of water also affect the rate of leaching (Habashi, 1970; Nicol et al., 1987). Although the carbonaceous (preg-robbing) material has a negative effect on gold leaching, the Bounty ore is very low in organic matter (Table 2). The optimum concentration ratio of [CN]/[O2] has been evaluated on the basis of the fact that the anodic oxidation of gold and the cathodic reduction of oxygen should take place at the same rate, according to the mixed potential theory. This model estimates an optimum concentration ratio of [CN]/[O2] = 6 for Eq. (1) and [CN]/[O2] = 12 for Eq. (2), where the H2O2 formed is an intermediate product (Habashi, 1970). It is widely accepted that the overall reaction for the dissolution of gold in alkaline cyanide solution is best described using the Elsner’s equation (Eq. (2)) (Elsner, 1846). From the electrochemical studies reported by Kudryk and Kellogg (1954), the rate of gold leaching in air-saturated solutions increases with increasing concentration of cyanide but becomes independent of cyanide concentration when it exceeds 0.075% KCN.

Table 2 Elemental analysis of typical Bounty ore in two assays Element

2. Chemical background The gold dissolution described by Eqs. (1) and (2) is an electrochemical process (Kudryk and Kellogg, 1954; Nicol et al., 1987; Yannopoulos, 1991; Li et al., 1992; Xue and Osseo-Asare, 2001). 2Au þ 4NaCN þ O2 þ 2H2 O ¼ 2NaAuðCNÞ2 þ 2NaOH þ H2 O2

ð1Þ

4Au þ 8NaCN þ O2 þ 2H2 O ¼ 4NaAuðCNÞ2 þ 4NaOH

ð2Þ

Au Ag As Al Ba Ctotal COrganic CCarbonate Ca Cu Fe K Mg Na Pb STotal Sr

Assay1, g/t

Assay2, g/t

5 1.1 13

5.5 1 13

75

76

81

79

3900

3720

3910 51

3860 53

32

32

Assay1, %

Assay2, %

1.41

1.44

0.13 < 0.03 0.13 3.43

0.17 < 0.03 0.17 3.45

18.1

18.3

1.32

1.32

5.05

5.11

S. Ellis, G. Senanayake / Hydrometallurgy 72 (2004) 39–50

This occurs when the diffusion of O2 to the gold surface becomes the rate controlling step. This has been confirmed by showing that the calculated values of rate of gold dissolution based on the mixed potential theory (Li et al., 1992) were in reasonable agreement with the measured values (Kudryk and Kellogg, 1954). Therefore the agitation should be sufficient to suspend all the particles in the slurry and to ensure that the mass transfer of both the dissolved O2 and CN ions is fast enough for the surface electrochemical reaction to be rate controlling. Crundwell and Godorr (1997) used an electrochemical reaction mechanism to describe the initial rate of gold leaching according to the equation: d½AuðIÞ=dt ¼ k½CN 0:5 ½O2 0:5

ð3Þ

As the reaction proceeds the surface area of exposed gold becomes less and hence the heterogeneous kinetic models become important (Crundwell and Godorr, 1997). Oxygen injection, instead of air, gives a high concentration of O2 and hence high overall gold extraction. Clearly the high concentrations of oxygen and cyanide improve initial gold leaching kinetics and thus reduce the residence time and increase throughput; but high cyanide concentrations also increase cyanide consumption. The reported experimental optimum ratio of [CN]/[O2] is in the range 4.6 – 7.4 (Lorenzen and van Deventer, 1992). However, the consumption of oxygen from the solutions by ions such as Fe2 +, S2 , HS, etc., which are very common in cyanide pulps, is a widespread problem. The depletion of oxygen from the pump retards cyanidation and extends cyanidation time, which in turn consumes more cyanide. Pyrrhotite is the most reactive and the highest cyanide and oxygen consuming iron sulphide mineral due to the formation of Fe(OH)3 and SCN (Hedley and Tabachnick, 1968; Marsden and House, 1992). Gold(I) forms a stable complex with thiocyanate. However, at the pH employed for cyanidation Au(CN)2 is the predominant species due to the higher stability constant of 1038 for Au(CN)2 compared to 1024 for Au(SCN)2 (Osseo-Asare et al., 1984). Pre-oxidation with air or pure oxygen prior to cyanidation causes degradation of oxygen and cya-

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nide consuming minerals and hence improves the gold oxidation kinetics and gold recovery (Hedley and Tabachnick, 1968; Osseo-Asare et al., 1984; Kondos et al., 1995; Descheˆnes and Prud’homme, 1997; Descheˆ nes et al., 1998; Descheˆ nes and Fulton, 1998). The high oxygen dissolution rate yields effective oxygen uptake. The oxygen dissolution rate depends on many factors, such as pressure, temperature, ionic strength, slurry agitation as well as oxygen injection system. The creation of a large number of small oxygen bubbles and dispersing them in the slurry long enough and deep enough provides adequate oxygen concentration for gold dissolution (Stephens, 1988). The injection of oxygen into an existing slurry pipeline takes advantage of high pressure and turbulence to increase the oxygen solubility (McMullen and Thompson, 1989).

3. Plant description 3.1. Leaching and adsorption circuit The plant processes a blend ratio of up to 1:1 of underground sulphide ore and open pit ore at a rate of 115 tph. The gravity circuit recovers the fine free gold ( P80 c 75 Am), 10 – 20% of the total amount of gold inputted into the plant, to produce a gravity concentrate with a fine gold purity of between 75% and 80%. Fig. 1 illustrates the leaching and adsorption circuit schematically, outlining the cyanide and oxygen addition points. The residence time in tanks 1and 2 is 4.5 h at the rate of 115 tph and 50% solids w/w and for other tanks 3.5 h. The last six tanks in Fig. 1 are the gold cyanide adsorption stages. The activated carbon is regenerated in a horizontal gas-fired kiln before being returned to the circuit. The loaded carbon pulled from the circuit each day is consequently stripped using a split Anglo-American system prior to electrowinning. 3.2. Influence of water quality The water utilised in the process plant is hypersaline and is made up from the tails return water, thickener overflow water, underground de-watering activities, production bores and any fresh rain-water that is collected in catchment dams. The quality of

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Fig. 1. Bounty gold plant leaching and adsorption circuit.

the process water varies, as shown in Table 3, with the seasons and the amount of available rain water. The addition of lime is commonly used to maintain a high pH in the gold leaching circuit and thus to minimise the formation of hydrogen cyanide causing

Table 3 Elemental analysis of process water used in testworka Analyte

T.D.S. pH Au Ag Cu Ni Pb FeTotal Fe2 + Fe3 + Cl SO24  SCN Free CN Na+ K+ Mg2 + Ca2 + a b

cyanide loss (as HCN gas) due to the following reactions: CN þ H2 O ¼ HCN þ OH

ð4Þ

CN þ H2 O þ CO2 ¼ HCN þ HCO 3

ð5Þ

Testwork water

Plant water concentration range Minimum

Maximum

95,000 6.6 0.03 0.66 1.8

80,000 6.1 0.01 0.01 0.4 0.4

120,000 7.5 0.05 0.16 1.7 2.5

b

b

b

The pH is modified to 9.3 by the addition of slaked lime to the slurry as it flows from the thickener into the first pre-oxidation tank. The presence of clay material in open pit ore can increase the lime consumption rate by 50 – 100%. The high salinity of the Bounty process water, as shown in Table 2, limits flexibility of pH due to the buffering effect of the high concentration of MgSO4. This also leads to the precipitation of gypsum causing a high lime consumption of 5 kg/t.

6.2 6.2

2 2

13 13

MgSO4 ðaqÞ þ CaOðsÞ þ H2 O

b

b

b

50,000 4000 260 0.63 20,000 240 1800 2500

45,000 2400 150 0.63 20,000 200 1500 385

75,500 6000 350 2.6 40,000 550 5550 4450

b

Units: mg/L. Too low to be measured.

¼ MgðOHÞ2 ðsÞ þ CaSO4 ðsÞ

ð6Þ

3.3. Multi Mix System The first four tanks in the circuit (Fig. 1) represent the pre-oxidation and cyanidation stages. The Multi Mix Systems (MMS) oxygen blasters (Sceresini, 1997) are capable of producing very small gas bubbles to the slurry in tanks 1 and 2, which are in closed

S. Ellis, G. Senanayake / Hydrometallurgy 72 (2004) 39–50

circuit with Warman pumps that recycle the slurry. The intense mixing significantly enhances mass transfer rate of the reactants and allows possible saturation of slurry with oxygen to achieve high dissolved oxygen (DO) levels. During oxidation in tank 3, with no further oxygen or air added to this tank, the DO level depletes to less than 0.1 mg/L. The high level of DO contained in the discharge pulp from tank 2 is available for oxidation of slower reacting oxygen consuming species within tank 3, prior to the cyanidation. The liquid cyanide, 28 – 32% NaCN by weight, is diluted to half strength with barren eluate from the electrowinning circuit and fresh rain water to contain between 75 and 85 g/L CN. The third MMS oxygen blaster is in tank 4, which is also the first cyanide addition point as shown in Fig. 2. The liquid cyanide is supplied by a positive displacement pump to overcome the line pressure associated with injecting it into the suction side of the Warman slurry recycle pump. This injection method operates very effectively and has lowered the cyanide consumption rate by 36% initially. The processing plant consumes a total of 5.5 –6 t of oxygen per day, which is equivalent to 1.8 kg of oxygen per tonne of dry ore. A secondary cyanide dosing point is positioned in the preceding launder leading to tank 6. As long as the dissolved oxygen levels are maintained above 20 mg/L in tanks 1 and 2 and there are no disruptions to the cyanide dosing system into tank 4, then it is normally not necessary for any more cyanide dosing into tank 6 or further downstream. It is preferred that a CN level of

Fig. 2. First cyanide addition point.

43

at least 53 mg/L is discharged with the slurry to the tails dam to ensure that any leachable gold is dissolved and returned to the plant in the tails return water. Typically, the solution losses to the tails dam are 0.01 Au g/t and the tails return water contains 0.03 Au g/t.

4. Experimental Drill core material was crushed and rod milled to the desired grind size for subsequent leaching. The ground ore was thoroughly cleaned from the rod mill and transferred to a 20 cm diameter laboratory scale leach tank at ambient temperature fitted with baffles and an overhead stirrer to achieve the target density of 50% solids (w/w) in the leaching tank using the wash water. The oxidation was carried out by delivering compressed oxygen gas (Industrial Grade) via a small sparger to the slurry to maintain the desired DO level. To enhance the oxygen transfer rate, the speed of the overhead stirrer was increased to disperse the bubbles into the slurry. The DO level of each slurry tank was measured using a Syland Dissolved Oxygen Meter and Probe with a salinity compensation up to 3.5% or 35 g/L NaCl. The pH was measured using a standard pH probe with a TPS 900 series meter calibrated with standard pH buffers. A temperature probe was attached to both meters to compensate for the temperature of the slurry. The solid sodium cyanide used was of A.R. grade and the free cyanide was measured by performing a colorimetric titration with silver nitrate using rhodanine solution as the indicator. Additional experiments carried out using potentiometric titrations showed that the presence of background chloride has no significant influence on the cyanide concentration determined using colorimetry and potentiometry. The pH in the testwork was maintained above 9.0 during cyanidation by adding lime. The slurry samples removed using a small scoop were vacuum filtered and the solids were washed with distilled water before being dried in an oven set at 40 jC to avoid oxidation prior to further analytical testwork. Standard techniques were used to analyse iron (acid digestion followed by titration or atomic absorption spectroscopy), sulfide, sulfate (gravimetry), and thiocyante (titration). The collection of the circuit profile data was undertaken when the plant was in a steady state. All of the

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samples associated with the laboratory testwork and plant trials were analysed for gold using standard techniques: atomic absorption spectroscopy for gold in solution and fire assay for gold in solids.

5. Results and discussion 5.1. Laboratory experimental data Fig. 3 shows the relationship between the grind size of the ore and gold recovery at different time intervals from the pre-oxidized ore for 8 h. A moderate particle size ( P80 = 45 Am) favors the initial leaching kinetics and hence gives the highest %Au leached in 2, 4 and 8 h; but small particle size ( P80 = 38 Am) gives a better recovery after 32 h of leaching. However, as shown in Fig. 4, for particles of P80 smaller than 75 Am there is a dramatic increase in NaCN consumption with the increase in gold recov-

Fig. 4. Relationship between particle size ( P80), gold recovery and NaCN consumption (conditions same as in Fig. 3).

Fig. 3. Effect of initial particle size ( P80) on gold extraction at different time intervals (pre-oxidation time: 8 h).

ery. Moreover, the grinding cost also increases with the decrease in particle size and thus the optimum operating particle size has been established as P80 = 63 Am for the underground ore. This particle size gave a reasonable %Au extraction of c 90% (Fig. 3). A high concentration of O2 during pre-oxidation increases the %Au recovery and decreases the cyanide consumption (Haque, 1992). This was observed in the laboratory tests LT7 –LT10 (Table 4). The gold recovery with no pre-oxidation was as low as 79.1%. It increased to 90.2% due to pre-oxidation whilst the cyanide consumption dropped by 15.8%. The increase in DO level during pre-oxidation from 10 to 20 mg/L (LT8 to LT9) caused a further decrease (36.8%) in cyanide consumption during cyanidation. Although a further increase in DO to 30 mg/L did not have any influence on the cyanide consumption (LT10), it increased the gold extraction to 91.7%. In contrast, the increase in cyanide from 400 to 500 mg/L during cyanidation caused an increase in both gold extraction and cyanide consumption (LT11 to LT12). The process water contained a large concentration of sulfate

S. Ellis, G. Senanayake / Hydrometallurgy 72 (2004) 39–50 Table 4 Impact of concentration of O2 and cyanide on gold extraction and cyanide consumption in leaching experiments with process water Test

LT7 LT8 LT9 LT10 LT11 LT12

[O2]a mg/L

CNb mg/L

Au%c

10e 10 20 30f 20f 20f

400 400 400 400 400 500

79.1 90.2 90.9 91.7 89.5 91.7

CNd kg/t 1.9 1.6 1.2 1.2 1.2 1.7

Percentage change in Au

CN d

+ 11.2 + 11.8 + 12.6

 15.8  36.8  36.8

+ 2.2

+ 41.7g

a

Pre-oxidation time 4 h, pulp density 50%, P80 63 Am, pH 9.3. b Added as NaCN, leaching time 32 h. c Based on solid analysis. d Consumption. e No pre-oxidation. f Pre-oxidation time 12 h. g With respect to LT11.

as well as other ions (Table 3). Therefore a laboratory test was carried out with deionized water to monitor the change in the ratio of S2 (wt.%)/Fe (wt.%) in solids and the concentration of sulfate, thiocyanate and total iron in solution during pre-oxidation and cyanidation (LT13 in Table 5). The results provide evidence for the decrease in S2 /Fe ratio during preoxidation and the formation of sulfate and thiocyanate ions during cyanidation. The ore without pre-oxidation consumes cyanide and oxygen according to the following equation: 2FeSðsÞ þ 2NaCN þ 3H2 O þ 1:5O2 ¼ 2FeðOHÞ3ðsÞ þ 2NaSCN

(Table 4). This is partly prevented by the pre-oxidation of pyrrhotite, marcasite and pyrite according to the following chemical reactions (Habashi, 1970; Nicol et al., 1987; Descheˆnes et al., 1998): 4FeSðsÞ þ 5O2 þ 4H2 O þ 4OH ¼ 4FeðOHÞ3ðsÞ þ 2S2 O2 3

ð8Þ

  2 S2 O2 3 þ CN þ 0:5O2 ¼ SCN þ SO4

ð9Þ

 2 S2 O2 3 þ 2O2 þ 2OH ¼ 2SO4 þ H2 O

ð10Þ

4FeS2ðsÞ þ 15O2 þ 16OH ¼ 4FeðOHÞ3ðsÞ þ 8SO2 4 þ 2H2 O

ð11Þ

The Bounty process water contains approximately 260 mg/L SCN (Table 3) which drops to about 200 mg/L during pre-oxidation and then increases to about 400 mg/L during cyanidation. These observations indicate the reactions of sulfide minerals and intermediate sulphur compounds formed during pre-oxidation with cyanide (Hedley and Tabachnick, 1968) (Eqs. (8) –(11)). The results summarized in Table 6 show how the increase in NaCN from 400 to 500 mg/L during cyanidation improves the overall gold extraction due to the enhanced rate, particularly during the first 2 – 4 h of cyanidation of a pre-oxidized ore sample.

ð7Þ

and this explains the low gold extraction and high cyanide consumption in Test LT7 compared to LT8 Table 5 Change in S2 /Fe ratio, sulfate, thiocyanate and total iron concentration during pre-oxidation and cyanidation with 30 mg/L dissolved oxygen (Ellis, 2001) Testa

Time, h

S2 /Feb

[SCN]c mg/L

[SO24 ]c mg/L

c [Fe]total mg/L

LT13

0 12 44

0.4545 0.4285 0.4849

0 0 300

0 0 400

0 0 19

a

45

Used deionized water, pre-oxidation 12 h, cyanidation with 500 mg/L CN for 32 h. b Weight percent ratio based on solid analysis. c In solution.

5.2. Plant trials Table 7 lists the details of the selected plant trials since the commissioning, to summarise the various stages of development of the oxygen and cyanide addition strategies. The shifting of the first cyanidation point from tank 2 to tank 3 and then to tank 4 (Fig. 1) increased the pre-oxidation time. This, as well

Table 6 Effect of cyanide on %Au extraction in laboratory testsa Time, h

0

2

4

24

32

CN 400 mg/L CN 500 mg/L

1.6 2.0

65.9 66.6

68.4 74.0

82.0 82.1

89.5 91.7

a

Tests LT11 and LT12 in Table 4.

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Table 7 Effect of concentrations of NaCN and dissolved O2 and multi mix systems on gold leaching in selected plant trials (pH c 9.2 unless stated otherwise) Trial

Pre-oxidation tank Tank no.

Oxygen lances used A 1 B 1 C 1 D 1 E 1+2 F 1+2 One Multi Mix System used G 1+2 Two Multi Mix Systems used H 1+2+3 I 1+2+3 Three Multi Mix Systems used J 1+2+3 K 1 + 2 + 3d L 1+2+3 M 1+2+3 N 1+2+3

1st cyanidation tank Tank no.

[CN] mg/L

[O2] mg/L

[CN]/[O2]b

mg/L 3 12 10.4 7.6 32.8 4.2

2 2 2 2 3 3

350 350 350 300 350 350

20 20 20 20 20 2

21.5 21.5 21.5 18.5 21.5 215

8.41 8.41 8.41 7.21 8.41 0.84

61.2 71.5 55.4 48.9 58.1 19.9

37

3

400

6

82.1

2.88

68.9

46.2 47.8

4 4

400 450

8 8

61.5 69.2

3.85 4.33

79.6 70.6

47.8 20 29.5 54.2 59

4 4 4 4 4

450 223 212 265 255

20 20 22 21 26

10.8 5.36 5.61 6.69 7.97

83.4 80.5 83.2 80.7 83.4

a [O2]total

27.6 13.7 (pH 9.1) 11.8 (pH 10.3) 15.5 (pH 11.4) 12.1 (pH 9.0)

[CN]  [O2] (mmol/L)2

%Auc

a

Sum of [O2] in all pre-oxidation tanks. Molar ratio. c First cyanidation tank. d Tank 3 off line. b

as the change in oxygen lances to MMS led to the changes in DO levels in the respective tanks listed in Table 7. The effect of these changes on the total DO during pre-oxidation stages and the concentration ratio [CN]/[DO] are also listed in Table 7. According to Eq. (3), the initial rate of oxidation of gold is directly proportional to the square root of the concentration product [CN]  [DO]. Therefore this quantity along with the %Au leached in the first cyanidation tank are also listed in Table 7. The effect of different cyanide and DO levels obtained using oxygen lances on the %Au extraction in the first cyanidation tank are compared in trials A – D. Since the [CN] and [O2] were kept the same, the increase in gold extraction (A < B) is a result of the increase in [O2] in the pre-oxidation tank. The comparison between trials E and F shows the effect of failure in oxygen lances in trial F which led to low [O2] of 4.2 mg/L during pre-oxidation and 2 mg/L in the first cyanidation tank and hence a very low gold extraction of 20%. The increase in MMS from 1 to 2

improves the gold extraction by 10% (trials G and H) and the use of 3 MMS systems increased the gold extraction to over 80% (trials J – N). Fig. 5 examines the effect of extent of pre-oxidation on gold leaching immediately after the preoxidation by plotting %Au leached in the first cyanidation tank against the total dissolved oxygen in all the pre-oxidation tanks. According to Table 7 and Fig. 5 the general trend is that the %Au leached in the first cyanidation tank increases with the shift in the first cyanide dosing point from tank 2 to 3 to 4 and with the increase in dissolved oxygen concentration in the pre-oxidation tanks. It is of interest to note that when three MMS were in operation the %Au leached in the first cyanidation tank reached 80% at [O2]total as low as 20 mg/L in pre-oxidation tanks (trial K). This means that a higher residence time of sulphide ore in pre-oxidation stages with high dissolved oxygen levels favor the %Au extraction in the first cyanidation tank. The MMS system provides high concentration of DO in the pre-oxidation tanks, leading to

S. Ellis, G. Senanayake / Hydrometallurgy 72 (2004) 39–50

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overall gold extraction by 10% prior to adsorption onto carbon. Fig. 7 shows the %Au extraction in the overall circuit to show the positive impact on the gold extraction achieved by increasing the number of MMS to 3 and moving the first cyanide dosing point from tank 3 to tank 4 to lengthen the pre-oxidation stage by 4 h. This has improved the plant performance considerably to an overall gold recovery of 95%. The increase in gold extraction from trial I to J is due to the increased number of MMS. As shown in Table 7, subsequent trials K – N gave high gold extraction in the first cyanidation tank at lower cyanide dosages. 5.2.1. Effect of [CN]/[O2] on gold leaching Although the optimum ratio of [CN]/[O2] for gold leaching has been reported as 6 – 12 (Habashi, 1970), a higher ratio is generally maintained in gold plants to ensure that the gold recovery is not at risk due to insufficient reagent levels caused by side reactions

Fig. 5. Effect of total dissolved oxygen during pre-oxidation on gold extraction in first cyanidation tank (data from Table 7).

successful oxidation reactions and thus most of the CN and DO in the first cyanidation tank are available for gold leaching. Prior to upgrading the oxygen addition regime using MMS, the DO levels remained low. Fig. 6 summarizes the impact of the different low oxygen levels on gold extraction in plant trials and compares the historical performance of the plant with the various stages of development. Only the gold extracted in the first half of the circuit (up to tank 6 in Fig. 1) is shown in Fig. 6 to emphasize the impact the dissolved oxygen and cyanide levels have on the cyanidation. At the time of these preliminary plant trials the oxygen injection was only occurring in the first two tanks and the cyanide was being added to tank 3 and 6. The implementation of a MMS oxygen blaster in tank 1 and high cyanide in tank 3 in the preliminary trials have improved the

Fig. 6. Impact of different dissolved oxygen levels on overall gold extraction based on plant-solid analysis (pre-oxidation time: 8 h).

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For example, in trial K the [O2]total was 20 mg/L during pre-oxidation (with tank 3 off line) and was lower than [O2]total of 32.8 mg/L in trial E. The product [CN]  [O2] in the first cyanidation tank in trial K was also lower at 5.36 compared to 8.41 in trial E. Despite these lower figures the gold extraction in the first cyanidation tank was as high as 80.5% in trial K compared to 58.1% in trial E (Table 7). This is also evident from Fig. 8 which shows that %Au extraction in the first cyanidation tank is higher with MMS than that with lances, but becomes relatively independent of the product [CN]  [O2] above c 5.6. As noted previously the electrochemical studies have indicated that the rate of gold leaching in air saturated solutions (0.25 mmol/L O2) become independent of cyanide concentration when it exceeds

Fig. 7. Impact of implementation of MMS oxygen blasters on overall gold extraction based on plant solid analysis. The first cyanide dosing point is after 12 h in trial G (Tank 3), and after 16 h (Tank 4) in other trials.

that consume CN and O2. However, as noted in the lab tests LT11 and LT12 in Table 4, the use of excess cyanide may improve the gold extraction as well as the cyanide consumption. This was the case in plant trials listed in Table 7 where the [CN]/[O2] ratio in the first cyanidation tank changed in the range 12 – 215. The general trend is that the %Au extracted in the first cyanidation tank decreases with the increase in [CN]/[O2] ratio. In plant trials J– N which made use of three MMS, much higher gold extraction was observed in the first cyanidation tank, even at low [CN]/[O2] ratios close to 12. 5.2.2. Effect of [CN][O2] on gold leaching When oxygen lances were used in the plant, %Au extracted increased with the increase in [CN]  [O2] in the first cyanidation tank (F < D < E in Table 7). However, in the case of MMS %Au extracted seems fairly independent and is as high as 80% even at low values of [CN]  [O2] in the first cyanidation tank.

Fig. 8. Effect of [CN]  [O2] mmol/L 2 in first cyanidation tank on gold extraction in the same tank (data from Table 7, Lances: open triangles, MMS: closed triangles).

S. Ellis, G. Senanayake / Hydrometallurgy 72 (2004) 39–50

0.075% KCN (Kudryk and Kellogg, 1954), i.e. 11.5 mmol/L CN . This corresponds to the product [CN]  [DO] c 3 (mmol/L)2 in a clear electrolyte solution with no solids. For plant trials G –N which made use of the MMS at a pulp density of 50% and [O2] c 20 mg/L the optimum [CN]  [DO] product appears to be c 5.6 (mmol/L) 2 which extracts c 83% Au in the first cyanidation tank (Fig. 8). The lower gold extraction of 80.7% in trial M is a result of the high pH of 11.4. (Table 7).

6. Conclusions n Pre-oxidation and appropriate [CN]/[O2] during cyanidation is critical to achieving good gold leaching kinetics in the first cyanidation tank. n The use of Multi Mix Systems for oxygen addition provides high dissolved oxygen concentrations in the two pre-oxidation tanks, 1 and 2, allowing a total of 12 h of pre-oxidation without any oxygen injection to tank 3 prior to cyanidation in tank 4. n The use of the third Multi Mix Systems unit in tank 4 for oxygen and cyanide addition in tank 4 provides good mixing and a high dissolved oxygen of z 20 mg/L for cyanidation. This lowers the concentration of cyanide required in tank 4 to achieve good leaching kinetics and avoids the need for further addition of cyanide or oxygen in tanks 5, allowing 8 h of cyanidation prior to the adsorption onto activated carbon. n A particle size of P80 = 63 Am and a pH of 9 – 10 gave best gold leaching. Material with a lower particle size consumed more cyanide and a higher pH of 11 lowered the gold leaching kinetics. n The lowest [CN]/[O2] molar ratio tested was c 12 and this gave the best gold extraction. With the use of MMS oxygen blasters the optimum product of [CN]  [O2] in the first cyanidation tank appears to be c 5.6 mmol2 L 2.

Acknowledgements The authors wish to acknowledge Viceroy Australia for financial assistance and permission to publish this technical information. The contribution of the staff at AMMTEC and MPL Laboratories in

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Perth in analytical aspects of this project is gratefully appreciated.

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