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Low pH cyanidation of gold
MineralsEngineering,Vol.12, No. 12, pp. 1431-1440,1999 © 1999ElsevierScienceLtd All rightsreserved 0892-6875(99)00132-6 0892-6875/99/$- see frontmatter
Pergamon
LOW pH CYANIDATION OF GOLD R. PERRY §, R.E. B R O W N E R §, R. DUNNE t and N. STOITIS ~ § Department of Minerals Engineering and Extractive Metallurgy, Curtin University, PMB 22, Kalgoorlie, WA 6430, Australia. E-marl: [email protected] t Newcrest Mining Ltd. P.O. Box 6380, East Perth, WA 6890, Australia :~ New Celebration Gold Mine, Newcrest Mining Ltd. P.O. Box 2231, Boulder, WA 6432, Australia.
(Received 16 June 1999; accepted 24 August 1999) ABSTRACT
Lime is used in CIP/CIL slurries to increase the pH, which partially stops the formation of aqueous hydrogen cyanide. Hydrogen cyanide gas can be evolved directly from the pulp surface or purged from the leach tank in oxygen or air that is sparged into the tanks to provide oxygen for the dissolution of gold Savings in lime consumption can be achieved by leaching at a reduced pH, especially in aqueous solutions high in magnesium such as to be found in the Kalgoorlie Goldfields in Western Australia, but a greater percentage of cyanide is then present as aqueous hydrogen cyanide. Another proposed procedure to reduce lime consumption is closed tank leaching where a reduced pH or the natural pH of the ore is used and hydrogen cyanide loss is prevented by the closed vessel. The dissolution rate of pure gold as a function of pH for constant total cyanide concentration has been determined Dissolution was directly proportional to the concentration of the cyanide ion present, with the contribution to dissolution from aqueous hydrogen cyanide too low to be measured Ore leached in high magnesium, hyper-saline, process water showed increased gold dissolution at low pH compared to the pure gold. Reducing pH caused more cyanide to be consumed by the pulp but did not significantly increase the amount lost to the atmosphere. © 1999 Elsevier Science Ltd. All rights reserved
Keywords Gold ores; cyanidation; pH control; reaction kinetics
INTRODUCTION The gold industry over the last century has been mainly based on the treatment of free milling gold ores where the gold can be successfully extracted by gravity separation, amalgamation or cyanidation. Cyanidation of gold depends on the stability of the complex ion Au(CN)2- for the dissolution of gold according to the equation:
(1)
4Au + 8CN- + O2 = 4Au(CN)2- + 4OH-
Cyanide is typically added at a rate of between 0.25 to 1.0 kgt-i of ore, and is usually the most significant
reagent cost. Lime is added as a pH modifier to increase the pH and prevent as much as possible the hydrolysis of the cyanide ion to hydrogen cyanide.
1431
1432
R. Perryet al. (2)
CN- + H20 = HCN + OH-
The position of the above equilibrium is dependent on the pH and the salinity of the solution (Verhoeven, Hefter and May, 1990). When cyanidation is carried out in fresh water a pH of 10.5 is easily achieved and 95 percent of the cyanide is present as the cyanide ion. When hypersaline water is used, in areas such as the Kalgoorlie Goldfields of Western Australia, magnesium in the water forms Mg(OH)2 forcing plants to operate at apparent pH values around 9.0. The apparent pH is that measured in the saline water with a calibrated pH probe ignoring the effect of sodium ions on the pH probe. More of the cyanide is hence present as hydrogen cyanide. Two problems associated with this are that; hydrogen cyanide can be lost to the atmosphere increasing cyanide consumption and presenting a safety problem and that; aqueous hydrogen cyanide may not be as effective in leaching gold as the cyanide ion. Costello and von Michaelis (1989) suggested that closed leach tanks can reduce cyanide consumption by minimising HCN gas loss. They conclude that the leaching of gold in a closed vessel could be conducted at a low pH or even at the natural pH of the pulp. The natural pH of the pulp is dependent on the acidity of the process water and the acid generating or consuming power of the ore. Fickers and Owers (1990) report improvements in gold dissolution rates on decreasing pH from 10.5 to 8.0 in non-reproducible tests. A constant cyanide ion concentration was maintained by increasing the total cyanide concentration as the pH was lowered. Dorin and Woods (1991) and LaBrooy and Muir (1994) who used higher cyanide concentrations, possibly making oxygen diffusion the rate controlling mechanism and masking the effect ofpH, did not confirm this phenomenon. The scope to reduce cyanide consumption by using closed tanks, especially when operating in saline water, as suggested by Costello e t al.. (1991) depends on the ability of aqueous hydrogen cyanide to leach gold. A reduction in pH from 10.5 to 8 will reduce the cyanide ion concentration from 95 to 5.8%. A 16-fold increase in cyanide usage would be required to maintain the leaching rate if aqueous hydrogen cyanide did not contribute. The dissolution of gold is an electrochemical process. Equation 1 can be divided into two half equations: Au(CN)z- + e- = Au + 2CN-
(3)
02 d- 4H ÷ + 4e- = 4H20
(4)
Using selected equilibrium constants for the Au-CN--H20 system (Xue & Osseo-asare, 1985) the Eh-pH diagram for the dissolution of gold in a hydrogen cyanide solution was constructed (Figure 1).
3 2.5-
~
21.5 1
Au(CNh
0.5 0 -0.5
-1
Au
2
4
6
8
10
12
pH
Fig. 1 Eh-pH diagram for the dissolution of gold in hydrogen cyanide.
Low pH cyanidationof gold
1433
This diagram for the Au-HCN-H20 system is exactly the same as that for the Au-CN--HzO system. This phenomenon is due to the hydrolysis reaction of the cyanide ion being independent of Eh:
(5)
H20 + CN- = HCN(~q) + OH-
The thermodynamic equilibria presented in the Eh-pH diagram indicate that hydrogen cyanide will leach gold. Gold, therefore, will leach in a cyanide solution regardless of the pH and whether the cyanide is present as CN- o r HCN(aq). The kinetics or speed of the dissolution reaction with hydrogen cyanide needs to be examined to compare the efficiency of hydrogen cyanide to the cyanide ion. This paper examines the leaching rate of gold in cyanide solutions as a function of pH and hence HCN concentration. If the dissolution of gold by hydrogen cyanide is too slow, regardless of the thermodynamic data presented in the Eh-pH diagram, low pH leaching of gold would not be a viable option. Static gold disc experiments were conducted followed by the evaluation of an ore to determine the leaching rate of gold at various pH values with constant total cyanide concentration.
EXPERIMENTAL Static gold disc dissolution
One hundred and forty millilitres of solution of the desired salinity was prepared in a 250mL "head sample" jar. The solution pH was controlled by the addition of sulphuric acid or sodium hydroxide such that when 10 mL of sodium cyanide was subsequently added the required pH would be obtained. Each jar was sparged with oxygen for two minutes and sealed, with a 10mm diameter gold disk suspended in the solution from the lid. The jars were emersed in a water bath to equilibrate at 25°C and then ten millilitres of a sodium cyanide solution was injected through the septum to give a concentration of 0.02% sodium cyanide. After 24 hours the lids with the attached gold disks were removed, stopping the gold dissolution reaction. The pH was measured immediately, a sample extracted for cyanide titration with silver nitrate and the remainder assayed for gold by AAS. The pH reading of the solutions containing 0.75M sodium chloride were adjusted to compensate for the sodium ion effect on the glass pH electrode. Oxide ore leaching
Two experimental procedures were investigated. A sealed rolling reaction vessel was used to determine the effect o f lowering the pH on the gold leaching of an ore. The 3.5L vessel was charged with 500g of ore, 500mL of process water, 0.25g of NaCN and the pH adjusted with 0.15M sulphuric acid. Activated carbon was added (0.5g) for the carbon in leach tests and the vessel sealed and agitated on bottle rolls for 8 hours. No pH or cyanide adjustment was possible during this period. On the completion of the leach the pH was measured, the solution gold grade determined and the gold grade of the filtered, washed and dried tailings also determined. The second procedure used a closed 5L impellor agitated vessel with oxygen sparging situated under the impellor and a gas outlet at the top. Gas exiting the vessel was bubbled through a 0.1 M sodium carbonate solution to collect hydrogen cyanide. A sampling tube extended from the lid down into the pulp such that when the tube was opened to collect a sample only the small amount of gas contained in the tube was lost. The vessel was charged with 2000g of ore, 2000mL of process water, 1.0g of NaCN and the pH adjusted with 0.15M sulphuric acid. Activated carbon was added (2.0g) for the carbon in leach tests. The vessel was sealed and agitated at 150rpm with oxygen sparged through at a rate of 9L/hr. Samples were taken at 1, 2,
R. Perry et aL
1434
3, 4, 5, 6 and 8hrs for analysis of cyanide and gold. Additional cyanide or acid to maintain constant reagent concentrations was injected through a septum.
RESULTS AND DISCUSSION Static gold disc dissolution
Figure 2 shows the gold leaching rate obtained for the solution without any ionic adjustment. Leaching is most efficient at pH values above 10 and decreases to practically zero at low pH values. This indicates that hydrogen cyanide is not participating in leaching gold or the reaction with hydrogen cyanide is too slow to be detected in these experiments. t'-. m
./f"
0.004
E E 0.003 o
t
0"1
E "0 (1)
0.002
,i
0
ill m
0.001
m
0
0 0
-r-
I
I
I
I
I
2
4
6
8
10
12
14
pH Fig. 2
Gold leaching rate as a function of pH for the static gold disc experiments. 0.0 M NaC1 background salinity, 0.02% sodium cyanide.
By sparging with oxygen prior to sealing the jars and using only 200 ppm sodium cyanide excess oxygen was available (Heath & Rumball, 1998) making the reaction cyanide diffusion controlled. The mole fraction of cyanide present as the cyanide ion in a solution is given by:
[CN-] XcN- [HCN]+[CN-]
1 10 (pK'-pH) +1
(6)
where pK,* is the apparent pK, when activity terms are replaced by concentrations. When equation 6 is multiplied by the maximum gold recovery and pKa* is used as an adjustable parameter, the experimental data in figure 2 were fitted to equation 6 using least squares regression. This curve is shown in figure 2 with the value of pKa* = 9.07. A single adjustable pKa* was fitted although this would be expected to change slightly as the ionic strength of the solution changed with the amount of sulphuric acid or sodium hydroxide required to adjust the pH. The results using a 0.75M sodium chloride solution to simulate saline plant water show the same trend (Figure 3) where the adjustable pKa* is 9.04. The pKa* values are in reasonable agreement with those
LowpH cyanidationof gold
1435
determined by Verhoeven, Hefter and May (1990) who give apparent pKa* values of 9.040, 8.949, and 8.946 at sodium chloride concentration of 0.10, 0.50 and 1.0M respectively.
0.004 C
i m
E E 0.003 U
E 0.002 d= U t~
}
0.001
"O m
O
0
I
0
v
2
I
I
I
I
I
4
6
8
10
12
14
pH Fig. 3
Gold leaching rate as a function of pH for the static gold disc experiments. 0.75 M NaCI background salinity, 0.02% sodium cyanide.
Using the pKa* determined above, the percentage of sodium cyanide present as the cyanide ion was calculated for the two conditions of water quality and the gold leached plotted as a function of cyanide ion concentration. The plots (Figures 4a and 4b) show a linear relationship between gold leached and the cyanide ion concentration.
0.004 t-.m
E E 0.003 u E "o
0.002
/o
0.001 0
(.9
0
0
I
I
I
I
20
40
60
80
1O0
Cyanide as the CN- ion / % Fig. 4a Gold leaching rate as a function of percentage of total cyanide as the cyanide ion. 0.0 M background salinity.
1436
R. Perryet aL
It is apparent from the above results that any attempt to reduce lime consumption by operating gold cyanidation tanks at a lower pH will result in reduced cyanide ion concentrations and hence gold leaching rates.
0.004
0.003
0.002
0.001
0 0
I
I
I
I
20
40
60
80
100
Cyanide as the CN- ion 1% Fig. 4b Gold leaching rate as a function of percentage of total cyanide as the cyanide ion. 0.75 M NaC1 background salinity. Cornejo and Spottiswood (1984) have shown that when cyanide concentration is the rate limiting reagent the rate of gold leaching can be represented by the equation: 1
A~oCN-[CN-]b
2
6
Rate = - .
(7)
where, A is the surface area of gold in contact with the leaching solution,
CN- is the diffusion coefficient
of CN-, [CN-]bis the cyanide ion concentration in the bulk solution and 8 is the boundary layer thickness. The cyanide ion concentration can be replaced by equation 6 with pKa* equal to 9.04, which can be regarded as the cyanide concentration effective in leaching gold. The relative rate of gold dissolution as a function of pH can then be determined:
R e l a t i v e rate = _.1 -
2
) 10• (9°~--pH) ,, +1
(8)
8
Decreasing the pH from 9.0 to 8.5 will result in a 53 percent decrease in gold leaching rate requiring a corresponding increase in residence time to achieve the same gold recovery. Alternatively, the total cyanide concentration in the leach tanks could be increased to offset the additional amount present as HCN and maintain the cyanide ion concentration when the pH is lowered.
Low pH cyanidationof gold
1437
O x i d e ore t e s t w o r k
Figure 5 shows the results from sealed rolling bottle tests containing a gold oxide ore compared with the static gold disc leaching tests. At low pH values there is some leaching of gold from the ore compared to virtually nil for the static gold disc leaching. There are at least three significant differences between the tests that bring about the greater leaching of gold from the ore at low pH values. First, the ore tests were conducted in saline process water which would alter the pKa of hydrogen cyanide and change the HCN/CN- ratio. Second, the process water and ore contain cyanide and acid consumers, hence the pH of the closed system was changing throughout the leach. This is represented by the bars for pH in figure 5 giving the initial and final pH. Third, the gold in the ore was finely disseminated presenting a large surface area for dissolution. Consequently, leaching by hydrogen cyanide may be significantly increased compared to the static disc leaching.
E"
90 80
0.004
70
0.003 "E
c
60
> O O
50
"O O
30
t 0.002 "o ""
.= 40
.c o
0.001 "o*~
O 20
O (.9
10
0 4
I
I
I
6
8
10
0 12
Apparent pH • Rolling leach, LH axis Static disc, RH axis Fig. 5
• Carbon in leach, LH axis
Gold recovery from an ore leached at different apparent pH values in sealed bottle roll tests. Straight leach and carbon in leach results compared with static disc leaching rate results.
An activated carbon in leach system may enhance the gold recovery from the ore by removing gold cyanide and preventing an equilibrium gold cyanide value from being reached. Activated carbon in leach results are also shown in Figure 5. The gold leached using carbon in leach tests was lower than that without the carbon. An explanation for this can be obtained from the cyanide consumption figures. Carbon is known to catalyse the decomposition of cyanide to cyanate (Adams 1990) hence the carbon in leach tests had a significantly lower cyanide tenor producing lower leach recoveries. For example the rolling leach at about pH 5 shown in Figure 5 consumed 210gt-i where as the activated carbon in leach test consumed 410gt-1 out of the initial 500gt-~ of sodium cyanide added. To eliminate the imbalance in cyanide concentration found in the activated carbon in leach tests compared to the standard leach test, a closed vessel was used so that cyanide concentration could be monitored and adjusted without releasing hydrogen cyanide gas to the atmosphere. Figure 6 shows the gold recoveries after 8 hours of leaching for the closed vessel leach tests.
1438
R. Perryet
al.
There is a significant improvement in the activated carbon in leach recovery compared to the leach without carbon when the total (HCN and CN ) cyanide concentration is periodically replenished. The results support a carbon in leach (CIL) process for low pH leaching of gold ores but cyanide usage in leaching gold is still not as efficient as working at higher pH values. The activated carbon in leach results could also have been affected by the periodic addition of extra cyanide, in the form of the cyanide ion, that may have enhanced leaching before being partially converted to hydrogen cyanide gas. Figure 7 shows the cyanide consumption for the activated carbon in leach tests.
80 -
- 0.004
70 - 0.003
60 50
4o
0.002
.~ 3 0
8 20
0.001 _
8
10
0 4
I
I
I
6
8
10
0 12
A p p a r e n t pH • Agitated leach, LH axis Static disc, RH axis Fig. 6.
• Carbon in leach, LH axis
Gold recoveries after 8 hours of leaching for the closed vessel leach tests. Straight leach and carbon in leach results compared with static disc leaching rate results. 0.9 08
"~ 0.7 E 0.6 e-
o
U
0.5 0.4
C
~,0.3 o E 0.2 ~o 0.1
0 4
i
i
6
8
10
Apparent pH
Fig. 7
Cyanide consumption for the closed vessel carbon in leach tests. Amounts lost as hydrogen cyanide gas, consumed by the ore and total cyanide used.
LowpH cyanidationof gold
1439
The amount lost as hydrogen cyanide and measured in the sparge gas collection solution remained low with decreasing pH. Cyanide consumed by the ore increases as the pH is lowered, possibly due to cyanide consumers being leached from the ore at the low pH values.
CONCLUSION Lime is used in CIP/CIL slurries to increase the pH, which partially stops the formation of aqueous hydrogen cyanide. Aqueous hydrogen cyanide does not appear to leach gold at a sufficiently fast rate to compete with the cyanide ion in the cyanidation of gold ores. The rate of leaching of pure gold discs was directly proportional to the concentration of the cyanide ion in solution. Some lime is therefore required to maintain a high enough pH to keep at least some of the cyanide present as the cyanide ion to achieve acceptable leaching rates. Gold from an ore leached better than the static gold discs, possibly due to its finer, non pure nature, but gave reduced recoveries as pH was lowered. Adding activated carbon to the leach alters the reagent concentration by catalysing the oxidation of cyanide to cyanate. A Resin in Pulp (RIP) process would eliminate the catalytic oxidation of cyanide. A weak base resin could be used as the functional group would be protonated at lower pH values and adsorb gold cyanide. When total cyanide levels are maintained, activated carbon in the leach improved gold recoveries, but did not compensate for the reduced percentage of cyanide present as the cyanide ion at lower pH values. Cyanide consumed by the ore increases as pH is lowered while that lost to the atmosphere levels off. Reduced pH leaching of gold ores must take into account the reduced cyanide ion concentration brought about both by a greater percentage being converted to hydrogen cyanide and by greater consumption by the ore.
REFERENCES
Adams, M.D., The chemical behaviour of cyanide in the extraction of gold. 1. Kinetics of cyanide loss in the presence of activated carbon, Journal of the South African Institute of Mining and Metallurgy, 1990, 90(2) 37-44. Cornejo, L.M. and Spottiswood, D.J., Fundamental aspects of the gold cyanidation process: A review, Mineral andEnergy Resources, 1984, Colorado School of Mines, pp. 1-17. Costello, M., Dunne, R., Gelfi, P., Michell, I. and Martins, V., Considerations in cyanide savings--closed tanks, oxygen or peroxide, Randol Gold Conference, Cairns '91, 1991, pp. 307-314. Costello, M. and von Michaelis, H., Save cyanide you did not know you were losing--Don't blow your top. Randol Phase IV Workshop on Innovations in Gold and Silver Recovery. Sacramento, California, 1989, pp. 189-192. Dorin, R. and Woods, R., 1991 Determination of leaching rates of precious metals by electrochemical techniques, Journal of Applied Electrochemistry, 21, 419-424. Ficker, A. and Owers, B., Recovery of cyanide from pregnant pulps, Randol Gold Conference, Squaw Valley, '90, 1990, pp. 319-323. La Brooy, S.R. and Muir, D.M., Gold processing with saline water, Procedings of the Australian Institute of Mining and Metallurgy, (A uslMM), 1994, 2, 81-88. Verhoeven, P., Hefter, G.T. and May, P.M., Dissociation constant of hydrogen cyanide in saline solutions, Minerals and Metallurgical Processing, 1990, 185-188. Xue, T. and Osseo-asare, K., Heterogeneous equilibria in the Au~CN-HzO and Ag~2N-H20 systems, Metallurgical Transactions B, 1985, 16B, 455-463.
1440
R. Perry et al.
Correspondence on papers published in Minerals Engineering is invited, preferably by e-mail to [email protected], or by Fax to +44-(0)1326-318352
Pergamon
LOW pH CYANIDATION OF GOLD R. PERRY §, R.E. B R O W N E R §, R. DUNNE t and N. STOITIS ~ § Department of Minerals Engineering and Extractive Metallurgy, Curtin University, PMB 22, Kalgoorlie, WA 6430, Australia. E-marl: [email protected] t Newcrest Mining Ltd. P.O. Box 6380, East Perth, WA 6890, Australia :~ New Celebration Gold Mine, Newcrest Mining Ltd. P.O. Box 2231, Boulder, WA 6432, Australia.
(Received 16 June 1999; accepted 24 August 1999) ABSTRACT
Lime is used in CIP/CIL slurries to increase the pH, which partially stops the formation of aqueous hydrogen cyanide. Hydrogen cyanide gas can be evolved directly from the pulp surface or purged from the leach tank in oxygen or air that is sparged into the tanks to provide oxygen for the dissolution of gold Savings in lime consumption can be achieved by leaching at a reduced pH, especially in aqueous solutions high in magnesium such as to be found in the Kalgoorlie Goldfields in Western Australia, but a greater percentage of cyanide is then present as aqueous hydrogen cyanide. Another proposed procedure to reduce lime consumption is closed tank leaching where a reduced pH or the natural pH of the ore is used and hydrogen cyanide loss is prevented by the closed vessel. The dissolution rate of pure gold as a function of pH for constant total cyanide concentration has been determined Dissolution was directly proportional to the concentration of the cyanide ion present, with the contribution to dissolution from aqueous hydrogen cyanide too low to be measured Ore leached in high magnesium, hyper-saline, process water showed increased gold dissolution at low pH compared to the pure gold. Reducing pH caused more cyanide to be consumed by the pulp but did not significantly increase the amount lost to the atmosphere. © 1999 Elsevier Science Ltd. All rights reserved
Keywords Gold ores; cyanidation; pH control; reaction kinetics
INTRODUCTION The gold industry over the last century has been mainly based on the treatment of free milling gold ores where the gold can be successfully extracted by gravity separation, amalgamation or cyanidation. Cyanidation of gold depends on the stability of the complex ion Au(CN)2- for the dissolution of gold according to the equation:
(1)
4Au + 8CN- + O2 = 4Au(CN)2- + 4OH-
Cyanide is typically added at a rate of between 0.25 to 1.0 kgt-i of ore, and is usually the most significant
reagent cost. Lime is added as a pH modifier to increase the pH and prevent as much as possible the hydrolysis of the cyanide ion to hydrogen cyanide.
1431
1432
R. Perryet al. (2)
CN- + H20 = HCN + OH-
The position of the above equilibrium is dependent on the pH and the salinity of the solution (Verhoeven, Hefter and May, 1990). When cyanidation is carried out in fresh water a pH of 10.5 is easily achieved and 95 percent of the cyanide is present as the cyanide ion. When hypersaline water is used, in areas such as the Kalgoorlie Goldfields of Western Australia, magnesium in the water forms Mg(OH)2 forcing plants to operate at apparent pH values around 9.0. The apparent pH is that measured in the saline water with a calibrated pH probe ignoring the effect of sodium ions on the pH probe. More of the cyanide is hence present as hydrogen cyanide. Two problems associated with this are that; hydrogen cyanide can be lost to the atmosphere increasing cyanide consumption and presenting a safety problem and that; aqueous hydrogen cyanide may not be as effective in leaching gold as the cyanide ion. Costello and von Michaelis (1989) suggested that closed leach tanks can reduce cyanide consumption by minimising HCN gas loss. They conclude that the leaching of gold in a closed vessel could be conducted at a low pH or even at the natural pH of the pulp. The natural pH of the pulp is dependent on the acidity of the process water and the acid generating or consuming power of the ore. Fickers and Owers (1990) report improvements in gold dissolution rates on decreasing pH from 10.5 to 8.0 in non-reproducible tests. A constant cyanide ion concentration was maintained by increasing the total cyanide concentration as the pH was lowered. Dorin and Woods (1991) and LaBrooy and Muir (1994) who used higher cyanide concentrations, possibly making oxygen diffusion the rate controlling mechanism and masking the effect ofpH, did not confirm this phenomenon. The scope to reduce cyanide consumption by using closed tanks, especially when operating in saline water, as suggested by Costello e t al.. (1991) depends on the ability of aqueous hydrogen cyanide to leach gold. A reduction in pH from 10.5 to 8 will reduce the cyanide ion concentration from 95 to 5.8%. A 16-fold increase in cyanide usage would be required to maintain the leaching rate if aqueous hydrogen cyanide did not contribute. The dissolution of gold is an electrochemical process. Equation 1 can be divided into two half equations: Au(CN)z- + e- = Au + 2CN-
(3)
02 d- 4H ÷ + 4e- = 4H20
(4)
Using selected equilibrium constants for the Au-CN--H20 system (Xue & Osseo-asare, 1985) the Eh-pH diagram for the dissolution of gold in a hydrogen cyanide solution was constructed (Figure 1).
3 2.5-
~
21.5 1
Au(CNh
0.5 0 -0.5
-1
Au
2
4
6
8
10
12
pH
Fig. 1 Eh-pH diagram for the dissolution of gold in hydrogen cyanide.
Low pH cyanidationof gold
1433
This diagram for the Au-HCN-H20 system is exactly the same as that for the Au-CN--HzO system. This phenomenon is due to the hydrolysis reaction of the cyanide ion being independent of Eh:
(5)
H20 + CN- = HCN(~q) + OH-
The thermodynamic equilibria presented in the Eh-pH diagram indicate that hydrogen cyanide will leach gold. Gold, therefore, will leach in a cyanide solution regardless of the pH and whether the cyanide is present as CN- o r HCN(aq). The kinetics or speed of the dissolution reaction with hydrogen cyanide needs to be examined to compare the efficiency of hydrogen cyanide to the cyanide ion. This paper examines the leaching rate of gold in cyanide solutions as a function of pH and hence HCN concentration. If the dissolution of gold by hydrogen cyanide is too slow, regardless of the thermodynamic data presented in the Eh-pH diagram, low pH leaching of gold would not be a viable option. Static gold disc experiments were conducted followed by the evaluation of an ore to determine the leaching rate of gold at various pH values with constant total cyanide concentration.
EXPERIMENTAL Static gold disc dissolution
One hundred and forty millilitres of solution of the desired salinity was prepared in a 250mL "head sample" jar. The solution pH was controlled by the addition of sulphuric acid or sodium hydroxide such that when 10 mL of sodium cyanide was subsequently added the required pH would be obtained. Each jar was sparged with oxygen for two minutes and sealed, with a 10mm diameter gold disk suspended in the solution from the lid. The jars were emersed in a water bath to equilibrate at 25°C and then ten millilitres of a sodium cyanide solution was injected through the septum to give a concentration of 0.02% sodium cyanide. After 24 hours the lids with the attached gold disks were removed, stopping the gold dissolution reaction. The pH was measured immediately, a sample extracted for cyanide titration with silver nitrate and the remainder assayed for gold by AAS. The pH reading of the solutions containing 0.75M sodium chloride were adjusted to compensate for the sodium ion effect on the glass pH electrode. Oxide ore leaching
Two experimental procedures were investigated. A sealed rolling reaction vessel was used to determine the effect o f lowering the pH on the gold leaching of an ore. The 3.5L vessel was charged with 500g of ore, 500mL of process water, 0.25g of NaCN and the pH adjusted with 0.15M sulphuric acid. Activated carbon was added (0.5g) for the carbon in leach tests and the vessel sealed and agitated on bottle rolls for 8 hours. No pH or cyanide adjustment was possible during this period. On the completion of the leach the pH was measured, the solution gold grade determined and the gold grade of the filtered, washed and dried tailings also determined. The second procedure used a closed 5L impellor agitated vessel with oxygen sparging situated under the impellor and a gas outlet at the top. Gas exiting the vessel was bubbled through a 0.1 M sodium carbonate solution to collect hydrogen cyanide. A sampling tube extended from the lid down into the pulp such that when the tube was opened to collect a sample only the small amount of gas contained in the tube was lost. The vessel was charged with 2000g of ore, 2000mL of process water, 1.0g of NaCN and the pH adjusted with 0.15M sulphuric acid. Activated carbon was added (2.0g) for the carbon in leach tests. The vessel was sealed and agitated at 150rpm with oxygen sparged through at a rate of 9L/hr. Samples were taken at 1, 2,
R. Perry et aL
1434
3, 4, 5, 6 and 8hrs for analysis of cyanide and gold. Additional cyanide or acid to maintain constant reagent concentrations was injected through a septum.
RESULTS AND DISCUSSION Static gold disc dissolution
Figure 2 shows the gold leaching rate obtained for the solution without any ionic adjustment. Leaching is most efficient at pH values above 10 and decreases to practically zero at low pH values. This indicates that hydrogen cyanide is not participating in leaching gold or the reaction with hydrogen cyanide is too slow to be detected in these experiments. t'-. m
./f"
0.004
E E 0.003 o
t
0"1
E "0 (1)
0.002
,i
0
ill m
0.001
m
0
0 0
-r-
I
I
I
I
I
2
4
6
8
10
12
14
pH Fig. 2
Gold leaching rate as a function of pH for the static gold disc experiments. 0.0 M NaC1 background salinity, 0.02% sodium cyanide.
By sparging with oxygen prior to sealing the jars and using only 200 ppm sodium cyanide excess oxygen was available (Heath & Rumball, 1998) making the reaction cyanide diffusion controlled. The mole fraction of cyanide present as the cyanide ion in a solution is given by:
[CN-] XcN- [HCN]+[CN-]
1 10 (pK'-pH) +1
(6)
where pK,* is the apparent pK, when activity terms are replaced by concentrations. When equation 6 is multiplied by the maximum gold recovery and pKa* is used as an adjustable parameter, the experimental data in figure 2 were fitted to equation 6 using least squares regression. This curve is shown in figure 2 with the value of pKa* = 9.07. A single adjustable pKa* was fitted although this would be expected to change slightly as the ionic strength of the solution changed with the amount of sulphuric acid or sodium hydroxide required to adjust the pH. The results using a 0.75M sodium chloride solution to simulate saline plant water show the same trend (Figure 3) where the adjustable pKa* is 9.04. The pKa* values are in reasonable agreement with those
LowpH cyanidationof gold
1435
determined by Verhoeven, Hefter and May (1990) who give apparent pKa* values of 9.040, 8.949, and 8.946 at sodium chloride concentration of 0.10, 0.50 and 1.0M respectively.
0.004 C
i m
E E 0.003 U
E 0.002 d= U t~
}
0.001
"O m
O
0
I
0
v
2
I
I
I
I
I
4
6
8
10
12
14
pH Fig. 3
Gold leaching rate as a function of pH for the static gold disc experiments. 0.75 M NaCI background salinity, 0.02% sodium cyanide.
Using the pKa* determined above, the percentage of sodium cyanide present as the cyanide ion was calculated for the two conditions of water quality and the gold leached plotted as a function of cyanide ion concentration. The plots (Figures 4a and 4b) show a linear relationship between gold leached and the cyanide ion concentration.
0.004 t-.m
E E 0.003 u E "o
0.002
/o
0.001 0
(.9
0
0
I
I
I
I
20
40
60
80
1O0
Cyanide as the CN- ion / % Fig. 4a Gold leaching rate as a function of percentage of total cyanide as the cyanide ion. 0.0 M background salinity.
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R. Perryet aL
It is apparent from the above results that any attempt to reduce lime consumption by operating gold cyanidation tanks at a lower pH will result in reduced cyanide ion concentrations and hence gold leaching rates.
0.004
0.003
0.002
0.001
0 0
I
I
I
I
20
40
60
80
100
Cyanide as the CN- ion 1% Fig. 4b Gold leaching rate as a function of percentage of total cyanide as the cyanide ion. 0.75 M NaC1 background salinity. Cornejo and Spottiswood (1984) have shown that when cyanide concentration is the rate limiting reagent the rate of gold leaching can be represented by the equation: 1
A~oCN-[CN-]b
2
6
Rate = - .
(7)
where, A is the surface area of gold in contact with the leaching solution,
CN- is the diffusion coefficient
of CN-, [CN-]bis the cyanide ion concentration in the bulk solution and 8 is the boundary layer thickness. The cyanide ion concentration can be replaced by equation 6 with pKa* equal to 9.04, which can be regarded as the cyanide concentration effective in leaching gold. The relative rate of gold dissolution as a function of pH can then be determined:
R e l a t i v e rate = _.1 -
2
) 10• (9°~--pH) ,, +1
(8)
8
Decreasing the pH from 9.0 to 8.5 will result in a 53 percent decrease in gold leaching rate requiring a corresponding increase in residence time to achieve the same gold recovery. Alternatively, the total cyanide concentration in the leach tanks could be increased to offset the additional amount present as HCN and maintain the cyanide ion concentration when the pH is lowered.
Low pH cyanidationof gold
1437
O x i d e ore t e s t w o r k
Figure 5 shows the results from sealed rolling bottle tests containing a gold oxide ore compared with the static gold disc leaching tests. At low pH values there is some leaching of gold from the ore compared to virtually nil for the static gold disc leaching. There are at least three significant differences between the tests that bring about the greater leaching of gold from the ore at low pH values. First, the ore tests were conducted in saline process water which would alter the pKa of hydrogen cyanide and change the HCN/CN- ratio. Second, the process water and ore contain cyanide and acid consumers, hence the pH of the closed system was changing throughout the leach. This is represented by the bars for pH in figure 5 giving the initial and final pH. Third, the gold in the ore was finely disseminated presenting a large surface area for dissolution. Consequently, leaching by hydrogen cyanide may be significantly increased compared to the static disc leaching.
E"
90 80
0.004
70
0.003 "E
c
60
> O O
50
"O O
30
t 0.002 "o ""
.= 40
.c o
0.001 "o*~
O 20
O (.9
10
0 4
I
I
I
6
8
10
0 12
Apparent pH • Rolling leach, LH axis Static disc, RH axis Fig. 5
• Carbon in leach, LH axis
Gold recovery from an ore leached at different apparent pH values in sealed bottle roll tests. Straight leach and carbon in leach results compared with static disc leaching rate results.
An activated carbon in leach system may enhance the gold recovery from the ore by removing gold cyanide and preventing an equilibrium gold cyanide value from being reached. Activated carbon in leach results are also shown in Figure 5. The gold leached using carbon in leach tests was lower than that without the carbon. An explanation for this can be obtained from the cyanide consumption figures. Carbon is known to catalyse the decomposition of cyanide to cyanate (Adams 1990) hence the carbon in leach tests had a significantly lower cyanide tenor producing lower leach recoveries. For example the rolling leach at about pH 5 shown in Figure 5 consumed 210gt-i where as the activated carbon in leach test consumed 410gt-1 out of the initial 500gt-~ of sodium cyanide added. To eliminate the imbalance in cyanide concentration found in the activated carbon in leach tests compared to the standard leach test, a closed vessel was used so that cyanide concentration could be monitored and adjusted without releasing hydrogen cyanide gas to the atmosphere. Figure 6 shows the gold recoveries after 8 hours of leaching for the closed vessel leach tests.
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R. Perryet
al.
There is a significant improvement in the activated carbon in leach recovery compared to the leach without carbon when the total (HCN and CN ) cyanide concentration is periodically replenished. The results support a carbon in leach (CIL) process for low pH leaching of gold ores but cyanide usage in leaching gold is still not as efficient as working at higher pH values. The activated carbon in leach results could also have been affected by the periodic addition of extra cyanide, in the form of the cyanide ion, that may have enhanced leaching before being partially converted to hydrogen cyanide gas. Figure 7 shows the cyanide consumption for the activated carbon in leach tests.
80 -
- 0.004
70 - 0.003
60 50
4o
0.002
.~ 3 0
8 20
0.001 _
8
10
0 4
I
I
I
6
8
10
0 12
A p p a r e n t pH • Agitated leach, LH axis Static disc, RH axis Fig. 6.
• Carbon in leach, LH axis
Gold recoveries after 8 hours of leaching for the closed vessel leach tests. Straight leach and carbon in leach results compared with static disc leaching rate results. 0.9 08
"~ 0.7 E 0.6 e-
o
U
0.5 0.4
C
~,0.3 o E 0.2 ~o 0.1
0 4
i
i
6
8
10
Apparent pH
Fig. 7
Cyanide consumption for the closed vessel carbon in leach tests. Amounts lost as hydrogen cyanide gas, consumed by the ore and total cyanide used.
LowpH cyanidationof gold
1439
The amount lost as hydrogen cyanide and measured in the sparge gas collection solution remained low with decreasing pH. Cyanide consumed by the ore increases as the pH is lowered, possibly due to cyanide consumers being leached from the ore at the low pH values.
CONCLUSION Lime is used in CIP/CIL slurries to increase the pH, which partially stops the formation of aqueous hydrogen cyanide. Aqueous hydrogen cyanide does not appear to leach gold at a sufficiently fast rate to compete with the cyanide ion in the cyanidation of gold ores. The rate of leaching of pure gold discs was directly proportional to the concentration of the cyanide ion in solution. Some lime is therefore required to maintain a high enough pH to keep at least some of the cyanide present as the cyanide ion to achieve acceptable leaching rates. Gold from an ore leached better than the static gold discs, possibly due to its finer, non pure nature, but gave reduced recoveries as pH was lowered. Adding activated carbon to the leach alters the reagent concentration by catalysing the oxidation of cyanide to cyanate. A Resin in Pulp (RIP) process would eliminate the catalytic oxidation of cyanide. A weak base resin could be used as the functional group would be protonated at lower pH values and adsorb gold cyanide. When total cyanide levels are maintained, activated carbon in the leach improved gold recoveries, but did not compensate for the reduced percentage of cyanide present as the cyanide ion at lower pH values. Cyanide consumed by the ore increases as pH is lowered while that lost to the atmosphere levels off. Reduced pH leaching of gold ores must take into account the reduced cyanide ion concentration brought about both by a greater percentage being converted to hydrogen cyanide and by greater consumption by the ore.
REFERENCES
Adams, M.D., The chemical behaviour of cyanide in the extraction of gold. 1. Kinetics of cyanide loss in the presence of activated carbon, Journal of the South African Institute of Mining and Metallurgy, 1990, 90(2) 37-44. Cornejo, L.M. and Spottiswood, D.J., Fundamental aspects of the gold cyanidation process: A review, Mineral andEnergy Resources, 1984, Colorado School of Mines, pp. 1-17. Costello, M., Dunne, R., Gelfi, P., Michell, I. and Martins, V., Considerations in cyanide savings--closed tanks, oxygen or peroxide, Randol Gold Conference, Cairns '91, 1991, pp. 307-314. Costello, M. and von Michaelis, H., Save cyanide you did not know you were losing--Don't blow your top. Randol Phase IV Workshop on Innovations in Gold and Silver Recovery. Sacramento, California, 1989, pp. 189-192. Dorin, R. and Woods, R., 1991 Determination of leaching rates of precious metals by electrochemical techniques, Journal of Applied Electrochemistry, 21, 419-424. Ficker, A. and Owers, B., Recovery of cyanide from pregnant pulps, Randol Gold Conference, Squaw Valley, '90, 1990, pp. 319-323. La Brooy, S.R. and Muir, D.M., Gold processing with saline water, Procedings of the Australian Institute of Mining and Metallurgy, (A uslMM), 1994, 2, 81-88. Verhoeven, P., Hefter, G.T. and May, P.M., Dissociation constant of hydrogen cyanide in saline solutions, Minerals and Metallurgical Processing, 1990, 185-188. Xue, T. and Osseo-asare, K., Heterogeneous equilibria in the Au~CN-HzO and Ag~2N-H20 systems, Metallurgical Transactions B, 1985, 16B, 455-463.
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