Cyanidation of a Copper Gold Ore

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ELSEVIER Int. J. Miner. Process. 50 (1997) 127-141

Cyanidation of a copper-gold ore

G. Desch~nes a,*, P.J.H. Prud'homme a a Energy, Mines and Resources Canada, Canada Ctr. for Mineral and Energy Technology, 555 Booth Street,

Ottawa, Ontario KIA OG1, Canada

Received 10 April 1996; accepted 13 January 1997

Abstract

A free milling gold ore, which contains 0.4% copper as chalcopyrite, was treated with cyanide. In the pre-leaching, the kinetics of oxidation of soluble sulphides are not an accurate indication for the length of the treatment. Efficient gold leaching is achieved only under specific conditions using oxygen, lead nitrate and a high concentration of free cyanide. The addition of lead nitrate results in a higher gold extraction, although cyanide consumption cannot be reduced below 1.85 kg/t without a drop in the extraction. Without lead nitrate, the gold recovery is lower than 90% but with lead nitrate, it reaches 98%. The addition of oxygen increases the gold recovery by 1.5%. The extraction of gold is not sensitive to lead nitrate additions when higher than 300 g/t. The redox potential values could thus be used as control parameters for lead nitrate addition, at the different stages of cyanidation, to indicate the state of the system.

The increase of lead nitrate concentration inhibits the dissolution of chalcopyrite but the approach used was not efficient enough to decrease the cyanide consumption. The high concentration of copper in solution requires a concentration of NaCN in the range of 700 mg/L. When the average NaCN concentration is lower than 640 mg/L, gold recovery drops significantly. It was also found that lead nitrate can be added directly at the start of cyanidation to achieve a performance equivalent to the situation in which it is added during pre-leaching. 1997 Elsevier Science B.V.

Keywords: gold; cyanide; extraction; leaching; lead nitrate; oxygen; chalcopyrite

1. Introduction

There are many gold deposits with a significant amount of copper ( > 0.3%). Despite the selectivity of cyanide for gold, copper minerals represent a real problem because of

* Corresponding author. E-mail: [email protected]

0301-751[6/97/$17.00 1997 Elsevier Science B.V. All rights reserved. Pll S0301-7516(97)00008-2

128 G. Desch~nes, P.J.H. Prud'homme / lnt. J. Miner. Process. 50 (1997) 127-141

their solubility in cyanide solutions. Among the copper minerals, chalcopyrite and chrysocolla show a low solubility (Habashi, 1967). Technologies are currently being developed for the treatment of copper-gold ores for gold recovery. Work on an ammonia/cyanide leach system indicates that leaching a flotation tailing under controlled conditions of pH and ammonia allows effective gold recovery and low cyanide consumption (Muir et al., 1995). The Sceresini process allows the recycling of cyanide consumed by copper minerals by loading the copper cyanide onto activated carbon. The Sceresini process and the ammonia/cyanide leaching process have reached commercial scale in the recent years (Zheng et al., 1995), while efforts are still underway for the development of thiourea and thiosulfate leaching processes. Thiourea leaching of a copper-gold ore showed moderate success (Lacoste et al., 1996).

The addition of oxygen and lead nitrate is known to be beneficial for the kinetics of gold dissolution and/or cyanide consumption during the processing of ores containing sulphide minerals (Weichselbaum et al., 1989; Dufresne et al., 1994; Desch~nes and Wallingford, 1995). It has also been demonstrated that the addition of other oxidizing agents increases the kinetics of gold dissolution (Stoychevski and Williams, 1993).

The addition of lead nitrate and oxygen is a common practice during the cyanidation of sulphide bearing gold ores (Desch~nes and Putz, 1995). From the lack of specific information in the literature and diversity of practices in gold plants, it became apparent that a systematic assessment of the addition of lead nitrate and oxygen is required. An exhaustive bibliographical review indicated that lead nitrate acts as a catalyst at the surface of the gold, preventing passivation (Morrison, 1994). It also inhibits the dissolution of metallic sulphides, thereby reducing cyanide consumption.

The goal of the current work is to quantify the influence of lead nitrate on the leaching kinetics and reagents consumption during the leaching of a gold ore containing chalcopyrite and also pyrrhotite as the reactive sulphide minerals to obtain an efficient extraction of gold, an acceptable consumption of cyanide and a modification of the copper mineral activity.

2. Experimentation

2.1. Material

A sample of 27 kg of -2.54 cm ore from Abitibi, Quebec, was crushed in a jaw crusher and in a rotary crusher and then pulverized. The load was then ground to 89% -74 /~m (wet screening) in a ball mill. It was rolled to homogenize the material and break up aggregates. To obtain representative samples, the material was cut into 5-kg batches and then into 500-g batches, which were used for leaching. A 0.635-cm riffle and a PTZ model Retsch rotary feeder were used.

The mineralogical study was done by scanning electron microscopy, on a JEOL 733 scanning electron microprobe in backscattered electron mode and using energy disper- sive X-ray spectrometry. The mineralogical analysis by X-ray diffraction (Table 1)

G. Desch~nes, P.J.H. Prud'homme / Int. J. Miner Process. 50 (1997) 127-141 129

Table 1 Chemical and mineralogical analyses of the copper-gold ore

Chemical analysis (%) Mineralogical analyses

Au 9.55 (g/t) Major phases: chlorite, quartz and albite Si 22.4 Fe 10.9 Minor phases: chalcopyrite, pyrrhotite, ilmenite and magnetite Al 6.9 Ca 4.8 Mg 3.4 Traces: sphalerite and marcasite Sto t 1.58. Cu 0.40 Zn 0.3 As < 0.2 Pb 0.1 Sb


Fig. 2. Backscattered electron image showing inclusions of Au-Ag alloy in pyrrhotite.

Water from the plant was used in the cyanidation tests. The chemical analysis of the water indicates a small amount of free cyanide (15 rag/L) and some copper (39 mg/L) . It also has a relatively high thiocyanate content (347 mg/L).

2.2. Equipment and experimental procedure

The lime, sodium cyanide, lead nitrate and oxygen are all certified reagent-grade chemicals, except for the test to evaluate the oxidation of sulphides during pre-leaching.

To quantify the kinetics of oxidation of soluble sulphides to sulphate and thiosulphate, a pre-leaching test was done. Pre-leaching takes 4 h, during which time, samples are taken at intervals of 30 min, 1 h, 2 h and 4 h. Distilled water is added to maintain a constant pulp density. Lime is added to maintain pH at 10.5. Where lead nitrate is added during preprocessing, it is introduced immediately after the start of agitation.

The gold leaching vessel is made of glass and has a capacity of one litre. The cover has four openings which allow insertion of the electrodes, the agitator and the tube for oxygen addition. A Chemcadet pH monitor gives pH readings. Mixing is provided by an agitator with 2.5-cm Teflon paddles powered by a variable-speed electric motor.

The tests were performed on pulps (50% weight) for a leaching time of 48 h. For each test, a sample of 500 g ore and 500 mL of leaching solution were used. The ore was introduced into the reactor and pulped for a few minutes. Then the cyanide salt (NaCN) was added. Agitation speed was kept constant at 400 rpm. Cyanide was added during the test to maintain a constant concentration of free cyanide (+5%). A 25-mL sample of pulp was taken with a pipette after 4.0 h, 6.0 h, 24.0 h and at the end. After filtration of the samples, the solids were returned to the reactor. The oxygen content was varied by bubbling a mixture of oxygen and air for control purposes. The oxygen

G. Desch~nes, P.J.H. Prud'homme / Int. J. Miner. Process. 50 (1997) 127-141 131

content was kept constant by using a mass flow monitor and an electrode which measured the dissolved oxygen content (DO).

The redox potential was measured with a combination electrode (Pt, Ag, AgC1/KCI 4 M) connected to a multimeter. At the end of the test, the slurry was filtered and the filter cake was washed with 1000 ml of distilled water. This cake was analyzed by precious metals fire assay after being dried, homogenized and sampled. The gold extraction calculations are based on the gold content values of the processed tailings, compared to the gold content value of the mill head sample. There is a 6% difference in the gold content of the average calculated head (gold in the tailing plus gold in solution) and the head assay.

The leach solution and wash solution were titrated for free cyanide with silver nitrate using rhodamine as an indicator and both solutions were assayed for gold and base metals (Fe, Cu) by atomic absorption spectroscopy. The gold concentrations in the solutions gave the extraction kinetics plot. The presence of copper cyanide caused interfer,ence during titration with silver nitrate. Thus, titration resulted in the loss of CN- fi'om the copper/cyanide complex and an increase in the value obtained for free cyanide. This error increased as the copper concentration in the solution increased. The values were not corrected because the plant that processes this ore uses the same titration method.

3. Results

3.1. Pre-leaching

Fig. 3 illustrates the oxidation of soluble sulphides to sulphate and thiosulphate. For this te,;t, distilled water was used (50% weight, room temperature). The pH was

Effect of the lenght of the pre-leach

800 ~ ~" 700 ~ S042

600 / E ~' 500 .~_ 400

W 300

.~. 200

1 O0 $30~ z" S2032"

O- ~ , I , , 60 120 180 240

Time (rain)

Fig. 3. Kinetics of the oxidation of soluble sulphides in the pre-leaching of a copper-gold ore with air. pH 10.5.


controlled at 10.5 and oxidation occurred in the presence of air only. The results indicate that 75% of the soluble sulphides were oxidized to sulphates after 30 min. This period of 30 min represents the time during which the ore is agitated with air in the mill circuit. Oxidation of metallic sulphides to form thiosulphate and sulphate occurs according to the following reactions:

2S2-+ 20 2 + H20--+ SzO 3- + 2OH- (1)

S2 O2- --[- 2OH-+ 202 ~ 2SO42- + H20 (2)

Thiosulphate ($2 O2- ) could also be oxidized to tetrathionate (Latimer, 1938):

2S2 O2- --+ S4 O2 + 2e (3)

Tetrathionate is not stable in alkaline solutions, being converted to thiosulphate and trithionate according to (Goldhaber, 1983):

4S4 O2- + 5OH---+ 5S2 O2- + 2S3062 + 3H20 (4)

Fig. 3 shows a very small concentration of thiosulphate (10 mg/L) and trithionate (58 mg/L) after 30 min; the corresponding sulphate concentration is 626 mg/L . Soluble sulphides that are dissolved are pyrrhotite, marcosite and chalcopyrite, pyrrhotite being the least stable.

3.2. Effect of pre-leaching and oxygen

Table 2 illustrates the effects of pre-leaching with air and lead nitrate and the effect of injecting oxygen during cyanidation. Treatment without pre-leaching and without the injection of oxygen results in a gold recovery of 89.6% and a cyanide consumption of 1.46 kg/ t (test 1). Pre-leaching for 30 min with air (test 2) changes almost nothing in the system, even if oxygen is added during cyanidation. The gold recovery and cyanide consumption are practically similar. The addition of 200 g / t Pb(NO3) 2 during pre- leaching, without the injection of oxygen during cyanidation, results in an obvious increase in gold recovery to 95.3%. There is an increase of 0.10 kg / t in cyanide consumption, to 1.64 kg/ t (test 3).

Table 2 Effect of pre-leaching on the cyanidation of a copper-gold ore Test Pre-leaching 02 NaCN cons. [Cu] [CNS- ] Au rec.

(ppm) (kg/t) (mg/L) (mg/L) (%)

1 none 7 1.46 514 1045 89.6 2 air 15 1.54 554 1071 89.8 3 Pb(NO3)2 7 1.64 627 1275 95.3 4 Pb(NO3) 2 15 1.85 713 1418 96.8

Pre-leaching: 30 min; cyanidation: pH 10.5, NaCN 700 mg/L, 48 h.

G. Desch~nes, P.J.H. Prud'homme / lnt. J. Miner. Process. 50 (1997) 127-141 133

Effect of oxygen and pre-leach

lOO

80

~ 70

._~ 60 atio 50 /~Y~ 1 1 None " Air

f 2 Air 02 "~ 40 3 Pb(NO3)z Air u.I "~ 4 Pb(NO3)2 02 '~ 30

20

10 0 , ~ I ~ ~ ~ I ~ , ~ L ~ ~ i i ~ ~ ~ I ~ i

8 16 24 32 40 48 Time (11)

Fig. 4. Effect of pre-leaching and oxygen on gold leaching of a copper-gold ore. Pre-leaching: Pb(NO3) 2 200 g/t, 30 rain; cyanidation: pH 10.5, NaCN 700 mg/L, 02 15 ppm, 48 h.

Elsner's equation shows the role of oxygen in the oxidation-reduction reaction:

4Au + 8CN- + 0 2 + 2H20 --* 4Au(CN)2 + 4OH- (5)

The combined use of lead nitrate during pre-leaching (200 g /0 and 0 2 (15 ppm) during cyanidation increases the gold recovery to 96.8%. However, cyanide consumption also increases to 1.85 kg/t . Fig. 4 presents the gold dissolution plots for tests 1 to 4. It appears that the use of lead nitrate and oxygen provides the most rapid dissolution of the gold (test 4) and the highest gold recovery.

3.3. Effect of Pb(NO 3)2 concentration during pre-leaching

An examination of Table 3 indicates that gold recovery is increased proportionally to lead nitrate added during pre-leaching. In fact, it rises from 96.5% Au with 100 g / t

Table 3 Effect of the addition of lead nitrate on the cyanidation of a copper-gold ore

Test Pb(NO3) 2 NaCN cons. [Pb] Au rec. (g/t) (kg/t) (mg/L) (%)

5 100 1.86 < 0.05 96.5 4 200 1.85 < 0.05 96.8 6 300 2.06 < 0.05 97.8 7 400 2.16 < 0.05 97.3 8 200 a 2.41 < 0.05 97.9 9 300 2.24 < 0.05 97.4

a Test with the addition of Pb(NO3) 2 during gold leaching: [NaCN] 750 mg/L. Pre-leaching: 30 min; cyanidation: pH 10.5, NaCN 710 mg/L, 02 15 ppm, 48 h.


Effect of lead nitrate concentration

100

90

8O

A 70

i 50 40 20

10

0 0 8 16 24 32 40 48

Time (h)

Fig. 5. Effect of lead nitrate on gold leaching of a copper-gold ore. Pre-leaching: 30 min; cyanidation: pH 10.5, NaCN 710 mg/L, 02 15 ppm, 48 h.

Pb(NO3) 2 to 97.8% with 300 g / t Pb(NO3) 2 (tests 4 to 6). The gold recovery remains the same for Pb(NO3) 2 concentrat ions greater than 300 g / t (test 7).

A recent review (Morrison, 1995) indicates that lead nitrate can act in different ways. Indeed, it can activate the surface of a passivated particle of gold, prevent the formation of a passivation film on the surface of gold, act as an oxidizing agent, precipitate the soluble sulphides or promote the oxidation of soluble sulphides to sulphate. In this case, lead nitrate favours the dissolution of gold, so it probably acts at the surface of the gold particles to prevent passivation. Fig. 5 illustrates the extent of gold dissolution in presence of various concentrations of lead nitrate. Although the system using 400 g / t Pb(NO3) 2 shows the fastest dissolution kinetics during the first 16 h, the gold recovery is about the same at 300 g / t and 400 g / t Pb(NO3) 2 with 97.8% and 97.3%, respectively.

The addition of Pb(NO3) 2 increases cyanide consumption noticeably. In fact, consumption is 1.86 kg/t with 100 g / t Pb(NO3) 2 and rises to 2.16 kg/t with 400 g / t Pb(NO3) 2. Previous work on a sulphide-beating gold ore indicated a 50% decrease in cyanide consumption using lead nitrate while keeping the same gold recovery, but increasing leaching kinetics by 30% (DeschSnes and Wallingford, 1995). The difference in the response of the ore to the addition of lead nitrate is attributed to the duration of the pre-leaching. In the previous work, there was a 4-h pre-leaching for an ore containing 12% pyrite, 1.4% pyrrhotite and 0.14% chalcopyrite. In the present test work, the pre-leaching is only 0.5 h and the material has a copper content 3 times higher than the previous ore. There is no pyrite, and pyrrhotite is also a minor component.

Fig. 6 shows the copper dissolution curve and thiocyanate formation as a function of the amount of added lead nitrate. Between 0 and 100 g / t lead nitrate, there is selective leaching of copper and sulphur. Consequently, the concentrations for copper and



1800~ 1800

1600 700

,00i/ V S o= 12oo

1000 ~ 500

800 ~ 400 0 1 O0 200 300 400

[Pb(NO3)2] (g/t)

Fig. 6. Copper dissolution and formation of thiocyanate during cyanidation as a function of the concentration of lead nitrate. Pre-leaching: 30 min; cyanidation: pH 10.5, NaCN 710 rag/l, 02 15 ppm, 48 h.

thiocyaJaate in solution increase. Accordingly, this causes an increase in cyanide consumption. Work on a different material, with surface analyses by TOF-LIMS (Time of Flight Laser Ionisation Mass Spectrometry) (Martin and Cabri, 1994) indicates that chalcopyrite shows more pronounced dissolution when treated with lead nitrate.

According to Fig. 6, the copper content reaches a maximum around 120 g / t of Pb(NO:~) 2, then decreases as more lead nitrate is added. At this level, lead nitrate passivates the surface of the chalcopyrite. It seems that there is a direct relationship between dissolved copper, cyanide consumption and gold recovery. The relationship among these three factors ceases to apply when the concentration of lead nitrate exceeds 200 g/t . Moreover, the CNS- content fluctuates without indicating any specific tendenc, y. The copper and CNS- contents thus cannot be used as indicators to guide the addition of lead nitrate in defining the optimum system.

The redox potential shows a specific relationship to the system. Fig. 7 illustrates the change in redox potential with time for different concentrations of lead nitrate added during preprocessing. It is clear that the ore reacts differently depending on the amount of lead nitrate added during the pre-leaching. In fact, an increase in Pb(NO3) 2 concentration increases the redox potential. For 100 g / t Pb(NO3) 2, the redox potential varies between - 160 and - 176 mV, while for 400 g / t Pb(NO3) =, the redox potential varies between - 117 and - 145 mV. These values could thus be used as control parameters at the different stages of cyanidation and could indicate the state of the system at different leaching times. For this ore, copper dissolution is about 35%. This value is much higher than that reported in the literature, which is 5.6% for chalcopyrite (Habashi, 1967). Of the minerals containing copper, chalcopyrite shows the lowest dissolution rate.

136 G. DeschOnes, P.J.H. Prud'homme / lnt. J. Miner. Process. 50 (1997) 127-141

Effect of lead nitrate concentration on the redox value

-100 Test [Pb(N03)2]

-110 (g/t) 5 100

-120 " 4 200 \ 6 300

:~A -130 \ ~ _7 400

x -140 o

= -150

-160 ~ ~

-170

-180 , , , t , , , I , i i I , i i I , , i I , , i

0 8 16 24 32 40 48

Time (h)

Fig. 7. Redox potent ia l for go ld leach ing as a funct ion o f lead nitrate added. Pre- leach ing: 30 min; cyan idat ion:

pH 10.5, NaCN 710 mg/L , 02 15 ppm, 48 h.

Fig. 8 illustrates the relationship between gold recovery and the reagents consumption expressed as the gross profit (extraction income less the cost of reagents: Pb(NO3) 2, $1.20/kg; NaCN, $2.00/kg). The optimum system uses between 100 and 300 g / t Pb(NO3) 2. Moreover, we can see that this graph indicates a small variation in profit


175

150 I- . . . ; - , ~ - - - - ~ .., - - .~. 0 n, o.

0 l= 125

100 , i , , I , , i , I i i , , I , , , h I , , , , 0 1 O0 200 300 400 500

[Pb(N03)2] (g/t)

Fig. 8. Prof i t f rom go ld leach ing as a funct ion o f the amount o f lead nitrate added. Pre- leach ing: 30 min; cyan idat ion: pH 10.5, NaCN 710 mg/L , 02 15 ppm, 48 h.


Effect of the location point of lead nitrate addition

~ 70

50 ~ each Cyan ~/ / I 4 Pb(NO3)z 02 I

20

10

0 0 8 16 24 32 40 48

Time (h)

Fig. 9. El~fect of the method of addition of lead nitrate on gold leaching of copper-gold ore with Pb(NO3) 2 200 g/t. Pre-leaching 30 rnin; cyanidation: NaCN 710 rag/L, pH 10.5, 02 15 ppm, 48 h.

(about $1.00/0 in the range of content values between 100 and 400 g / t of Pb(NO3) 2. As the optimum system is defined by economic criteria, it can be seen that the benefit generatqed by the increase in gold recovery is diminished by the cost of adding more lead nitrate and consuming more cyanide. No test was performed between 0 and 100 g / t lead nitrate but repetition of the test without lead nitrate indicates almost the same results.

A study on the addition of lead nitrate during leaching, as opposed to its addition during pre-leaching, showed that this practice is efficient but the pre-leaching is required to reduce the cyanide consumption (Desch~nes and Wallingford, 1995). Fig. 9 illustrates both cases. Curve 4 represents the addition of lead nitrate (200 g/t) during pre-leaching and curve 8 its addition at the start of cyanidation. Curve 8 indicates better cyanidation kinetics,: and a gold recovery that is 1.1% higher. Cyanide consumption is also greater when lead nitrate is added at the start of cyanidation (~ 0.6 kg/0. However, the two system~; are comparable in economic terms. Lead nitrate can thus be added either during cyanid~ttion or during pre-leacbing.

3.4. Ej)~ct of cyanide concentration

The effect of the concentration of free cyanide was assessed with the system using pre-leaching with lead nitrate and injection of oxygen during the 48-h cyanidation period. The results are given in Table 4. The gold extraction is not influenced by cyanide concentrations between 640 to 840 mg/L NaCN (~ 96.6%). The extraction is very sensitive to the free cyanide concentration when the sodium cyanide concentration falls below 640 mg/L; the yield drops rapidly, to less than 90% with 500 mg/L NaCN, and this corresponds to a loss of 6.7% (tests 10 and 12). There is also a significant increase in the cyanide consumption when the concentration of NaCN is greater than 710 mg/L (from 1.85 kg/t to 2.78 kg/t).


Table 4 Effect of cyanide concentration on gold leaching of a copper-gold ore

Test [NaCN] NaCN cons. [Cu] [Fe] Au rec. (mg/L) (kg/t) (rag/L) (mg/L) (%)

10 500 1.56 675 4 89.4- 12 640 1.76 691 6 96.5 4 710 1.85 713 8 96.8 11 840 2.78 770 24 96.6

Pre-leaching: Pb(NO3) 2 200 g/t, 30 rain; cyanidation: pH 10.5, 02 15 ppm, 48 h.

The additional free cyanide does not promote gold dissolution. Rather, the additional cyanide favours the dissolution of copper, the concentration of which increases to 770 ppm. The increase of free cyanide concentration should increase the concentration of Cu(CN)43- and decrease the concentration of Cu(CN) 2 (Hsu and Tran, 1995). In fact, this increase is responsible for only about 0.24 kg/t of additional cyanide consumed. Note that iron from pyrrhotite or chalcopyrite is only very slightly dissolved. The copper dissolution would be responsible for 90% of the cyanide consumed if it is assumed that the copper cyanide complex exists mostly as Cu(CN) 2- complex.

Fig. 10 shows the gold dissolution kinetics for the four concentrations of free cyanide tested (expressed as NaCN). The increase in cyanide content promotes the dissolution of gold during the first 24 h of leaching, but the optimum leaching kinetics are at an intermediate cyanide concentration, in this case 710 mg/L. Work indicated that the gold dissolution rate is accelerated in oxygen-enriched slurries in presence of chalcopyrite

Effect of the concentration of cyanide

100

80

50 f/' (mg/L) "~ // 1o 500 ,,, 40 12 640 = ',~ 30 4 710

11 840 2O

10

0 . , , I , ~ , I , , , I , r , J , ~ J I , , L

0 8 16 24 32 40 48

Time (h)

Fig. 10. Effect of cyanide concentration on gold leaching of a copper-gold ore. Pre-leaching: Pb(NO3) 2 200 g/t, 30 min; cyanidation: pH 10.5, 02 15 ppm, 48 h.


and pyrrhotite (Liu and Yen, 1995). It is suggested that the Cu(CN)~- leaches gold according to the following equation:

Au + 2Cu(CN)~- ~ Au(CN)2 + 2Cu(CN)2 + e (6)

It w~.s found that Cu(CN) 3- is more effective for leaching than CN- (LaBrooy et al., 1991). i[t is believed that the copper(I) cyanide solutions do not show a limiting rate because of the high affinity of oxygen for copper(I) which facilitates the oxygen transfer in solution.

It was indicated earlier that titration with silver nitrate overestimates the amount of free cyanide. In fact, the Cu(CN) 3- complex loses a molecule of free cyanide during titration, in accordance with the following equation:

Cu(CN)]- --> Cu(CN)23 - + CN- (7)

The dissociation of this complex causes a higher concentration of free cyanide. Since the plant uses the same titration method, the values in this report are comparable with the production data, as the error induced is of the same nature.

4. Discussion

The high copper content of this ore (0.4%) creates a kinetic problem for gold extraction, This explains, partly, why the addition of lead nitrate and oxygen enhances gold dissolution and significantly improves gold recovery. Lead nitrate has no effect on the overall gold extraction when added in amounts higher than 300 g/t . However, lead nitrate is not found to be efficient for reducing the cyanide consumption. This effect is probab]Ly related to the very short pre-leaching period used in the current test. A longer pre-leaching would have a positive impact on the cyanide consumption. Indeed, the results indicate that lead nitrate can be added during pre-leaching or directly during the cyanidation without any observable difference in final performance because the gain created by the increase of the gold extraction is attenuated by the increase of cyanide consumed. It also appears that the kinetics of oxidation of soluble sulphides is not a good indication of the length of the pre-leaching required to have a positive effect.

In gold plants, the strategy of lead nitrate addition is not always well defined or undersl:ood (Desch~nes and Putz, 1995). For this ore, it was found that the concentration of thiocyanate and copper cannot be used as reliable indicators to adjust the addition rate. On the other hand, the redox potential could be used as a guide for the lead nitrate addition. A meticulous approach should be practised to standardize the reading of the electrodes used in each leach tank and to ensure avoiding the disturbance related to contamination of the electrolyte.

An excess of lead nitrate is detrimental because it increases the cyanide consumption, probably by oxidizing cyanide. This hypothesis is difficult to confirm because this excess had no effect on gold recovery. The high copper content of this ore justifies the high requirement of free cyanide for leaching. The high concentration of free cyanide also leads to a high consumption of this reagent. Any concentration of cyanide below a critical level, however, means a significant drop in gold extraction.


5. Conclusions

Efficient leaching of a free milling gold ore, with a high copper content, is achieved only under specific conditions using oxygen, lead nitrate and a high concentration of free cyanide. The kinetics of formation of soluble sulphides cannot be used as a direct indication of the length of the pre-leaching required. The addition of lead nitrate results in a higher gold recovery, although cyanide consumption cannot be reduced below 1.85 kg/t when associated to a gold extraction of 97%. The extraction of gold is not sensitive to lead nitrate addition if an addition higher than 300 g / t is employed. The redox potential values could thus be used as control parameters for lead nitrate addition, at the different stages of cyanidation. Lead nitrate inhibits the dissolution of chalcopyrite but the pre-leaching used was not efficient enough to decrease the cyanide consumption. The high concentration of copper in solution requires a concentration of 700 mg/L NaCN for leaching.

Acknowledgements

The authors wish to thank Denis Couture, Claude Dufresne and Jean Ch~teauneuf of Cambior for their participation and collaboration, the financing provided under the project and the information conveyed. Thanks also to our assay laboratory of Mining and Mineral Sciences, to Jean Cloutier for the fire assay, to Gilles Laflamme and J.T. Szymanski for the mineralogical analysis and to Martin Fortier, a student trainee from Laval University, for his assistance in the laboratory tests.

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Cyanidation of a Copper Gold Ore

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