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Gold cyanidation with potassium persulfate in the presence of a thallium (I) salt
Hydrometallurgy 54 Ž2000. 185–193 www.elsevier.nlrlocaterhydromet
Gold cyanidation with potassium persulfate in the presence of a thallium žI / salt L. Guzman a , J.M. Chimenos b, M.A. Fernandez b, M. Segarra b, F. Espiell b,) a
b
Department of Physical Chemistry, UniÕersidad Mayor de San Andres, ´ La Paz, BoliÕia Department of Chemical Engineering and Metallurgy, UniÕersity of Barcelona, Martı´ i Franques, ` 1, 08028 Barcelona, Spain Received 11 June 1999; received in revised form 31 July 1999; accepted 10 October 1999
Abstract The use of potassium persulfate as oxidant in gold cyanidation is proposed at pH values between 10 and 11. The addition of thallium ŽI. ions enables high cyanidation rates to be maintained at pH values higher than 11, as it acts as a catalyst of the process. When gold cyanidation is performed in the simultaneous presence of 10 mM persulfate and 0.5 mM thallium ŽI. the dissolution rate is eight times higher than that obtained by conventional cyanidation, and the working pH can achieve values near 13. The kinetics of the process have been studied, and the results compared with those obtained by conventional cyanidation. The process is shown to be diffusion-controlled with an activation energy of 19.2 kJ moly1. Finally, an equation of the reaction rate that describes the kinetics of the overall process for gold cyanidation with potassium persulfate and thallium ŽI. is presented, and the wCNyxrwS 2 O 82y x ratio at which the dissolution rate reaches its limiting value is shown to be 2.5. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Gold cyanidation; Potassium persulfate; Thallium
1. Introduction When gold cyanidation is performed at minimum conditions of temperature and atmospheric pressure, conventional cyanidation gives low reaction rates that make the operating times long compared to those obtained under the best operating conditions of temperature and oxygen pressure. The recovery of gold from new gold sources such as )
Corresponding author. Tel.: q34-3-4021316; fax: q34-3-4021291; e-mail: [email protected]
0304-386Xr00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 8 6 X Ž 9 9 . 0 0 0 6 7 - 5
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L. Guzman et al.r Hydrometallurgy 54 (2000) 185–193
electronic scrap, in which gold is present as thick gold, also needs other gold leaching processes with higher cyanidation rates in order to diminish the operating times. In conventional gold cyanidation, generally performed at high cyanide concentrations Žbetween 0.04 and 0.1 M., cyanidation rate is determined by the dissolved oxygen concentration, slowing down the processes due to its limited solubility. Therefore, the use of alternative oxidants, such as permanganates, peroxides, ozone, dichromates, bromides and hydrogen peroxide, has been studied for many years although gold cyanidation with atmospheric oxygen is still the most-used process. An alternative oxidant must comply with certain requirements: it must not react with cyanide and thus must be stable in solution with respect to cyanide concentration. It must also be active as an oxidant in the potential range of gold anodic dissolution; and as far as possible, it must be compatible with the cyanide products. Furthermore, it must be soluble, stable and must not be adsorbed on gold surfaces in order to prevent possible passivation. An example of an oxidant that complies with all these requirements is the persulfate anion, which has been proved to increase the gold recovery from pyrite concentrates w1x. An increase of gold cyanidation rate can be achieved by the addition of thallium ŽI. ions in the leaching media w2,3x. This element is able to act on the passivant species w4–7x and catalyze both the gold anodic dissolution and the cathodic reduction of the oxidant. The simultaneous use of persulfate and thallium ŽI. has been studied in order to increase the gold cyanidation rate by combining both effects: an alternative oxidant and a catalyst of the reaction. The use of these reagents could be very important in the manufacture of electronic devices for etching electroplated gold. However, the interest in using persulfate and thallium for conventional cyanidation of ores depends on the cost and the benefit of gold extraction and exhaustive studies may be carried out with different gold ores.
2. Experimental 2.1. Materials and equipment Gold of electrolytic purity Ž999.5r1000. was used for all the experiments. It was melted in a magnesite cupel at 10508C for 30 min and the resulting button was cold-rolled to a thickness of less than 50 mm. Plates were cut with a surface area ca. 1 cm2 . To obtain a good reproducibility of the reaction rate measurements, it was necessary to heat the plate in a bunsen flame just before the experiment in order to remove hydrocarbons adsorbed on the gold surface w7x. Sodium cyanide, potassium persulfate and thallium nitrate were of analytical grade and distilled water, filtered through a Millipore Milli-Q system prior to the solutions preparation, were used in all the experiments. The pH was corrected by the addition of drops of 0.1 M sodium hydroxide solution. The dissolution rate measurements of the gold plates were carried out in a 500 ml cylindrical reactor, magnetically stirred, with two holes in the cover for inserting a thermo-regulator and a holder for the metal plate, so that it could be removed from the
L. Guzman et al.r Hydrometallurgy 54 (2000) 185–193
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reactor after a given reaction time. These plates were immediately washed with distilled water, dried at 1108C and weighed. During the experiments, samples of 2 ml were also taken at various time intervals and their gold concentration was analyzed by atomic absorption spectrophotometry. In all cases, the weight loss of the metallic plate was proved to be equal to the amount of gold found in solution. The plots of gold concentration vs. reaction time were always linear, thus confirming that the metal dissolution took place under steady-state conditions. 2.2. Results 2.2.1. Effect of pH The influence of pH on the kinetics of gold cyanidation with potassium persulfate at atmospheric pressure was determined under the following conditions: 0.02 M NaCN, 0.005 M K 2 S 2 O 8 , 400 miny1 and 258C. The corresponding results are shown in Fig. 1. At low pH values, cyanidation rate increases until pH 10.8, from which it begins to decrease with increasing pH. In this case, cyanidation rates are significantly higher than those obtained by conventional cyanidation under the same experimental conditions, which are also plotted in Fig. 1. This fact allows cyanidation to be performed at pH values higher than the cyanide hydrolysis pH, thus preventing the loss of cyanide by formation of HCN. In the presence of persulfate, at pH higher than 11, the gold cyanidation rate decreases, thus indicating the inhibition of the persulfate reduction. In order to improve cyanidation rates at these pH values it is necessary to eliminate this inhibition. This objective can be achieved by adding small amounts of thallium ŽI. ions to the leaching solution, as it acts as a catalyst both on the gold anodic dissolution and the persulfate reduction w8x. The use of thallium ions to improve leaching rates in conventional cyanidation has been previously published w7x. The results obtained when adding 0.5 mM TlNO 3 to the conventional cyanidation and cyanidation with persulfate are also shown in Fig. 1. It can be observed that the high values for the cyanidation rate obtained
Fig. 1. Effect of pH on gold cyanidation rate in: conventional cyanidation Ža., with thallium ŽI. Žb., with persulfate Žc. and with persulfate and thallium ŽI. Žd., at 0.02 M NaCN, 0.21 atm pO 2 , 400 miny1 and 258C. Experimental conditions: Žb. 0.85 mM thallium ŽI.; Žc. 5 mM K 2 S 2 O 8 ; Žd. 5 mM K 2 S 2 O 8 and 0.5 mM thallium ŽI..
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Fig. 2. Effect of persulfate concentration on gold cyanidation with persulfate and thallium ŽI.. Experimental conditions: 0.02 M NaCN, 0.5 mM Tl ŽI., pH 11.5, 0.21 atm pO 2 , 400 miny1 and 258C.
with persulfate as well as for conventional cyanidation are maintained over a wide range of pH when thallium is present. 2.2.2. Effect of persulfate concentration The effect of persulfate concentration on the cyanidation rate was determined by measuring the gold dissolution rate at 0.02 M NaCN, 0.5 mM TlNO 3 , 400 miny1 , pH 11.5, 258C and atmospheric pressure ŽFig. 2.. Under these conditions, gold cyanidation increases with the persulfate concentration, and levels off reaching a nearly constant value for a concentration of 15 mM persulfate. Therefore, for persulfate concentrations of 15 mM or higher, the gold cyanidation rate is controlled by the cyanide concentration. Previously, the effect of the persulfate on the stability of the cyanide solutions has been studied, under the cyanide concentrations described bellow, and no variation in the cyanide concentration has been observed with persulfate addition. 2.2.3. Effect of cyanide concentration The effect of cyanide concentration on the cyanidation rate was determined by measuring the gold dissolution rate at 0.5 mM TlNO 3 , 400 miny1 , pH 11.5, 258C and atmospheric pressure at different persulfate concentrations ŽFig. 3.. At low cyanide
Fig. 3. Effect of cyanide concentration on gold cyanidation with persulfate and thallium ŽI.. Experimental conditions: 0.5 mM Tl ŽI., pH 11.5, 0.21 atm pO 2 , 400 miny1 and 258C.
L. Guzman et al.r Hydrometallurgy 54 (2000) 185–193
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Fig. 4. Arrhenius plot for gold cyanidation with persulfate and thallium ŽI.. Experimental conditions: 0.02 M NaCN, 5 mM K 2 S 2 O 8 , 0.5 mM Tl ŽI., pH 11.5, 0.21 atm pO 2 and 400 miny1 .
concentrations, the gold dissolution rate is proportional to the cyanide concentration, reaching a maximum value that depends on the persulfate concentration and remains nearly constant for higher cyanide concentrations. The wCNyx rwS 2 O 82y x ratio at which the dissolution rate reaches its limiting value can also be determined from Fig. 3. The average value obtained for this ratio is 2.6. The maximum value for the cyanidation rate in each case is directly proportional to the persulfate concentration. 2.2.4. Effect of the temperature The Arrhenius plot for the gold cyanidation with persulfate and thallium ŽI. is shown in Fig. 4. The experiments were carried out in air atmosphere, with 0.02 M NaCN, 0.005 M K 2 S 2 O 8 , 0.5 mM TlNO 3 , 400 miny1 , pH 11.5 and 258C. The value of the activation energy found was 19.2 kJ moly1 , which is typical for diffusion-controlled reactions. 2.2.5. Effect of the stirring speed The effect of the stirring speed on the dissolution rate is plotted in Fig. 5. The stirring speed was varied while maintaining the rest of experimental conditions as in the
Fig. 5. Napierian logarithm of the reaction rate against Napierian logarithm of the stirring speed for gold cyanidation with persulfate and thallium ŽI.. Experimental conditions: 0.02 M NaCN, 0.005 M K 2 S 2 O 8 , 0.5 mM Tl ŽI., pH 11.5, 0.21 atm pO 2 and 258C.
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previous series ŽFig. 4.. The slope of the straight line obtained when representing the Napierian logarithm of the reaction rate vs. the logarithm of the stirring speed is 0.5, which is also typical of diffusion-controlled systems.
3. Discussion 3.1. Mechanism of reaction A mechanism and kinetic equation for gold cyanidation in the presence of thallium ŽI. has been previously reported by Chimenos et al. w7x, by considering that gold cyanidation is a process-controlled by oxygen and cyanide diffusion towards the gold surface. Based on this model, a kinetic equation has been established for gold cyanidation with persulfate in the presence of thallium ŽI., taking into account that persulfate exchanges two electrons and thallium ŽI. ion concentration practically does not affect the rate. 3.2. Determination of the rate equation for gold cyanidation with persulfate in the presence of thallium (I) Considering the following anodic and cathodic reactions: y
Au q 2CNy™ Au Ž CN . 2 q ey
Ž 1.
S 2 O 82y q 2ey™ 2SO42y
Ž 2.
and the global reaction: y
2Au q 4CNyq S 2 O 82y ™ 2Au Ž CN . 2 q 2SO42y
Ž 3.
The gold dissolution rate equation can be expressed by: y
rd s
1 d Au Ž CN . 2 2
s
dt
1 d SO42y 2
dt
1 d w CNy x sy
sy 4
dt
d S 2 O 82y dt
Ž 4.
As has been stated before, the process is controlled by diffusion and therefore, gold cyanidation under these conditions will be controlled by cyanide and persulfate diffusion towards the gold surface. Thus, according to the Fick’s law: d w CNy x sy
DCN y
dt d S 2 O 82y dt
sy
d
Sa Ž w CNy x y w CNy x i .
D S 2 O 82y
d
Sc Ž S 2 O 82y y S 2 O 82y
Ž 5. i
.
Ž 6.
where D S 2 O 82y and DCN y are the diffusion coefficients for the persulfate and the cyanide ions, respectively, wS 2 O 82y x and wCNyx the concentrations in the bulk solution, wS 2 O 82y x i and wCNyx i the concentrations in the interface, d the thickness of the Nernst boundary layer and Sa and Sc the surfaces where anodic and cathodic reactions take place, respectively.
L. Guzman et al.r Hydrometallurgy 54 (2000) 185–193
191
The concentrations at the interface in a diffusion-controlled reaction become nil and, as a consequence, Eqs. Ž5. and Ž6. can be written as: d w CNy x DCN y sy Sa Ž w CNy x . Ž 7. dt d D S O 2y d S 2 O 82y s y 2 8 Sc Ž S 2 O 82y . Ž 8. dt d The sum of the anodic and cathodic surface fractions is equal to the total surface area. Thus, considering these later equations and replacing them in Eq. Ž4. we obtain: y
1 d Au Ž CN . 2 S
s
dt
2 DCN y D S 2 O 82y w CNy x S 2 O 82y
d Ž DCN y w CNy x q 4 D S 2 O 82y S 2 O 82y
Ž 9.
.
This equation represents the rate equation for gold cyanidation with persulfate in the presence of thallium ŽI.. It was previously shown w7x that the thallium ŽI. ion concentration practically does not affect rate, thus it was neglected in this work, too. As the experiments have been performed under oxygen at atmospheric pressure, and oxygen is an active oxidant also in this case, Eq. Ž9. must be corrected taking into account the effect of oxygen. Thus, considering the rate at which gold is dissolved by cyanide under atmospheric oxygen in the presence of thallium ŽI. w7x, the complete expression for the equation rate becomes: y
1 d Au Ž CN . 2 S s
dt 2 DCN y D S 2 O 82y w CNy x S 2 O 82y
d Ž DCN y w CNy x q 4 D S 2 O 82y S 2 O 82y
q
.
4 DCN y DO 2 w CNy xw O 2 x
d Ž DCN y w CNy x q 8 DO 2 w O 2 x .
Ž 10 . y9
2
y1
y9
2
y1
where w8,9x: DCN ys 1.83 = 10 m s , D S 2 O 82y s 1.15 = 10 m s , DO 2 s 2.76 = 10y9 m2 sy1 , and wS 2 O 82y x and wCNyx are expressed in mmol my3 , and d in m. When gold cyanidation is performed with thallium ŽI. in the absence of persulfate, a limiting value of 12 has been found for the wCNyx rwO 2 x concentration ratio at which the dissolution rate becomes constant w7x. Therefore, for cyanide concentrations higher than 12 times the oxygen concentration at atmospheric pressure, i.e., 3.24 mM, the second term in Eq. Ž10. becomes constant. As all the experiments have been performed at cyanide concentrations higher than 0.01 M, Eq. Ž10. can be expressed as: y
1 d Au Ž CN . 2 S
s
dt
2 DCN y D S 2 O 82y w CNy x S 2 O 82y
d Ž DCN y w CNy x q 4 D S 2 O 82y S 2 O 82y
.
q rO 2
Ž 11 .
where rO 2 is the cyanidation rate at atmospheric pressure without persulfate. At low cyanide concentrations, the first term in the denominator may be neglected compared with the second one, so that the simplified equation becomes: y
1 d Au Ž CN . 2 S
dt
s
DCN y 2d
w CNy x q rO 2
Ž 12 .
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192
Under these conditions, the experimental results verify the dependence of the dissolution rate on the cyanide concentration only. At high cyanide concentrations, the second term in the denominator of Eq. Ž11. may be neglected compared with the first term, and the equation becomes: y
1 d Au Ž CN . 2 S
s
dt
2 D S 2 O 82y
d
S 2 O 82y q rO 2
Ž 13 .
Therefore, at high cyanide concentrations, the cyanidation rate only depends on the persulfate concentration. The wCNyx rwS 2 O 82y x concentration ratio at which the dissolution rate reaches its limiting value can be calculated when Eqs. Ž12. and Ž13. give the same value:
w CNy x D S O 2y s4 2 8 wS 2 O8 x DCN y
Ž 14 .
Taking into account the previous values for the diffusion coefficients of persulfate and cyanide, the concentration ratio is now:
w CNy x s 2.5 wS 2 O8 x
Ž 15 .
which is very close to the limiting ratio found experimentally: 2.6. The correlation between Eq. Ž11. and the experimental results is shown in Fig. 6. A value of 1.55 = 10y5 m has been calculated for the thickness of the Nernst boundary layer, which is in the range of values reported by other authors under different conditions w10x. In general, a good correlation can be observed. The cyanidation rate is limited by the persulfate solubility in the media Ž1.75 gr100 cm3 in water at room temperature. and, at high persulfate concentrations, by the stability of thallium ŽI., as it can be oxidized to Tl 3q and precipitate as TlŽOH. 3 in the bulk leaching solution.
Fig. 6. Comparison of experimental reaction rates and those calculated with the deduced kinetic model.
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4. Conclusions Potassium persulfate can be used to improve gold cyanidation rates at pH between 10 and 11. The dissolution rates are nearly eight times higher than those obtained by conventional cyanidation. However, at pH higher than 11, cyanidation rates dramatically decrease. This can be avoided by adding a small amount of a thallium ŽI. salt, which acts as a catalyst. Thus, when gold cyanidation is performed in the simultaneous presence of persulfate and thallium ŽI., the higher dissolution rates can be maintained until a pH value of 13. Gold cyanidation with persulfate in the presence of thallium ŽI. is a diffusion-controlled process, with an activation energy of 19.2 kJ moly1 . Gold cyanidation rate depends only on cyanide concentration at low cyanide concentrations and both on persulfate and oxygen concentrations at high cyanide concentrations. If the cyanidation is performed at atmospheric pressure, the gold dissolution rate mainly depends on the persulfate concentration in the range 2–10 mM and then reaches a limiting value. At high persulfate concentrations, Tl ŽI. can be oxidized to Tl 3q and precipitate as TlŽOH. 3 in the bulk solution.
References w1x V. Sacelean, G. Sigartau, L. Crivei, Effect of autoclaving on extraction of gold and silver from pyrite concentrates, Rev. Chim. ŽBucharest. 47 Ž11. Ž1996. 1046–1048. w2x M. Kameda, Effect of alkalies, lead acetate and some impurities contained in foul cyanide solutions, Sci. Rep. Res. Inst. Tohoku Univ., Ser. A 1 Ž1949. 435–444. w3x C.G. Fink, G.L. Putman, The action of sulfide ion and metal salts on the dissolution of gold in cyanide solutions, Am. Inst. Min. Metall. Eng. Trans. 187 Ž1950. 952–955. w4x R. Amadelli, N.M. Markovic, R.R. Adzic, E. Yeager, Oxygen reduction on electrode surfaces modified by underpotential deposited species: thallium on gold, J. Electroanal. Chem. 159 Ž1983. 391–412. w5x K.J. Cathro, F.A. Koch, The dissolution of gold in cyanide solutions — an electrochemical study, Proc. Ans. Inst. Min. Metall. 207 Ž1963. 181–205. w6x M.J. Nicol, The anodic behaviour of gold: Part II. Oxidation in alkaline solutions, Gold Bulletin 13 Ž2. Ž1980. 105–111. w7x J.M. Chimenos, M. Segarra, L. Guzman, A. Karagueorguieva, F. Espiell, Kinetics of the reaction of gold cyanidation in the presence of a thallium ŽI. salt, Hydrometallurgy 44 Ž1997. 269–286. w8x F. Habashi, Kinetics and mechanism of gold and silver dissolution in cyanide solutions, Mont., Bur. Mines Geol., Bull. 59 Ž1967. . w9x R. Mills, V.M.M. Lobo, Self-diffusion in lectrolyte solutions. A critical examination of data compiled from the literature. Elsevier, Netherland, 1989. w10x L. Guzman, Doctoral Thesis, University of Barcelona, Barcelona, Spain, 1997.
Gold cyanidation with potassium persulfate in the presence of a thallium žI / salt L. Guzman a , J.M. Chimenos b, M.A. Fernandez b, M. Segarra b, F. Espiell b,) a
b
Department of Physical Chemistry, UniÕersidad Mayor de San Andres, ´ La Paz, BoliÕia Department of Chemical Engineering and Metallurgy, UniÕersity of Barcelona, Martı´ i Franques, ` 1, 08028 Barcelona, Spain Received 11 June 1999; received in revised form 31 July 1999; accepted 10 October 1999
Abstract The use of potassium persulfate as oxidant in gold cyanidation is proposed at pH values between 10 and 11. The addition of thallium ŽI. ions enables high cyanidation rates to be maintained at pH values higher than 11, as it acts as a catalyst of the process. When gold cyanidation is performed in the simultaneous presence of 10 mM persulfate and 0.5 mM thallium ŽI. the dissolution rate is eight times higher than that obtained by conventional cyanidation, and the working pH can achieve values near 13. The kinetics of the process have been studied, and the results compared with those obtained by conventional cyanidation. The process is shown to be diffusion-controlled with an activation energy of 19.2 kJ moly1. Finally, an equation of the reaction rate that describes the kinetics of the overall process for gold cyanidation with potassium persulfate and thallium ŽI. is presented, and the wCNyxrwS 2 O 82y x ratio at which the dissolution rate reaches its limiting value is shown to be 2.5. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Gold cyanidation; Potassium persulfate; Thallium
1. Introduction When gold cyanidation is performed at minimum conditions of temperature and atmospheric pressure, conventional cyanidation gives low reaction rates that make the operating times long compared to those obtained under the best operating conditions of temperature and oxygen pressure. The recovery of gold from new gold sources such as )
Corresponding author. Tel.: q34-3-4021316; fax: q34-3-4021291; e-mail: [email protected]
0304-386Xr00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 8 6 X Ž 9 9 . 0 0 0 6 7 - 5
186
L. Guzman et al.r Hydrometallurgy 54 (2000) 185–193
electronic scrap, in which gold is present as thick gold, also needs other gold leaching processes with higher cyanidation rates in order to diminish the operating times. In conventional gold cyanidation, generally performed at high cyanide concentrations Žbetween 0.04 and 0.1 M., cyanidation rate is determined by the dissolved oxygen concentration, slowing down the processes due to its limited solubility. Therefore, the use of alternative oxidants, such as permanganates, peroxides, ozone, dichromates, bromides and hydrogen peroxide, has been studied for many years although gold cyanidation with atmospheric oxygen is still the most-used process. An alternative oxidant must comply with certain requirements: it must not react with cyanide and thus must be stable in solution with respect to cyanide concentration. It must also be active as an oxidant in the potential range of gold anodic dissolution; and as far as possible, it must be compatible with the cyanide products. Furthermore, it must be soluble, stable and must not be adsorbed on gold surfaces in order to prevent possible passivation. An example of an oxidant that complies with all these requirements is the persulfate anion, which has been proved to increase the gold recovery from pyrite concentrates w1x. An increase of gold cyanidation rate can be achieved by the addition of thallium ŽI. ions in the leaching media w2,3x. This element is able to act on the passivant species w4–7x and catalyze both the gold anodic dissolution and the cathodic reduction of the oxidant. The simultaneous use of persulfate and thallium ŽI. has been studied in order to increase the gold cyanidation rate by combining both effects: an alternative oxidant and a catalyst of the reaction. The use of these reagents could be very important in the manufacture of electronic devices for etching electroplated gold. However, the interest in using persulfate and thallium for conventional cyanidation of ores depends on the cost and the benefit of gold extraction and exhaustive studies may be carried out with different gold ores.
2. Experimental 2.1. Materials and equipment Gold of electrolytic purity Ž999.5r1000. was used for all the experiments. It was melted in a magnesite cupel at 10508C for 30 min and the resulting button was cold-rolled to a thickness of less than 50 mm. Plates were cut with a surface area ca. 1 cm2 . To obtain a good reproducibility of the reaction rate measurements, it was necessary to heat the plate in a bunsen flame just before the experiment in order to remove hydrocarbons adsorbed on the gold surface w7x. Sodium cyanide, potassium persulfate and thallium nitrate were of analytical grade and distilled water, filtered through a Millipore Milli-Q system prior to the solutions preparation, were used in all the experiments. The pH was corrected by the addition of drops of 0.1 M sodium hydroxide solution. The dissolution rate measurements of the gold plates were carried out in a 500 ml cylindrical reactor, magnetically stirred, with two holes in the cover for inserting a thermo-regulator and a holder for the metal plate, so that it could be removed from the
L. Guzman et al.r Hydrometallurgy 54 (2000) 185–193
187
reactor after a given reaction time. These plates were immediately washed with distilled water, dried at 1108C and weighed. During the experiments, samples of 2 ml were also taken at various time intervals and their gold concentration was analyzed by atomic absorption spectrophotometry. In all cases, the weight loss of the metallic plate was proved to be equal to the amount of gold found in solution. The plots of gold concentration vs. reaction time were always linear, thus confirming that the metal dissolution took place under steady-state conditions. 2.2. Results 2.2.1. Effect of pH The influence of pH on the kinetics of gold cyanidation with potassium persulfate at atmospheric pressure was determined under the following conditions: 0.02 M NaCN, 0.005 M K 2 S 2 O 8 , 400 miny1 and 258C. The corresponding results are shown in Fig. 1. At low pH values, cyanidation rate increases until pH 10.8, from which it begins to decrease with increasing pH. In this case, cyanidation rates are significantly higher than those obtained by conventional cyanidation under the same experimental conditions, which are also plotted in Fig. 1. This fact allows cyanidation to be performed at pH values higher than the cyanide hydrolysis pH, thus preventing the loss of cyanide by formation of HCN. In the presence of persulfate, at pH higher than 11, the gold cyanidation rate decreases, thus indicating the inhibition of the persulfate reduction. In order to improve cyanidation rates at these pH values it is necessary to eliminate this inhibition. This objective can be achieved by adding small amounts of thallium ŽI. ions to the leaching solution, as it acts as a catalyst both on the gold anodic dissolution and the persulfate reduction w8x. The use of thallium ions to improve leaching rates in conventional cyanidation has been previously published w7x. The results obtained when adding 0.5 mM TlNO 3 to the conventional cyanidation and cyanidation with persulfate are also shown in Fig. 1. It can be observed that the high values for the cyanidation rate obtained
Fig. 1. Effect of pH on gold cyanidation rate in: conventional cyanidation Ža., with thallium ŽI. Žb., with persulfate Žc. and with persulfate and thallium ŽI. Žd., at 0.02 M NaCN, 0.21 atm pO 2 , 400 miny1 and 258C. Experimental conditions: Žb. 0.85 mM thallium ŽI.; Žc. 5 mM K 2 S 2 O 8 ; Žd. 5 mM K 2 S 2 O 8 and 0.5 mM thallium ŽI..
188
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Fig. 2. Effect of persulfate concentration on gold cyanidation with persulfate and thallium ŽI.. Experimental conditions: 0.02 M NaCN, 0.5 mM Tl ŽI., pH 11.5, 0.21 atm pO 2 , 400 miny1 and 258C.
with persulfate as well as for conventional cyanidation are maintained over a wide range of pH when thallium is present. 2.2.2. Effect of persulfate concentration The effect of persulfate concentration on the cyanidation rate was determined by measuring the gold dissolution rate at 0.02 M NaCN, 0.5 mM TlNO 3 , 400 miny1 , pH 11.5, 258C and atmospheric pressure ŽFig. 2.. Under these conditions, gold cyanidation increases with the persulfate concentration, and levels off reaching a nearly constant value for a concentration of 15 mM persulfate. Therefore, for persulfate concentrations of 15 mM or higher, the gold cyanidation rate is controlled by the cyanide concentration. Previously, the effect of the persulfate on the stability of the cyanide solutions has been studied, under the cyanide concentrations described bellow, and no variation in the cyanide concentration has been observed with persulfate addition. 2.2.3. Effect of cyanide concentration The effect of cyanide concentration on the cyanidation rate was determined by measuring the gold dissolution rate at 0.5 mM TlNO 3 , 400 miny1 , pH 11.5, 258C and atmospheric pressure at different persulfate concentrations ŽFig. 3.. At low cyanide
Fig. 3. Effect of cyanide concentration on gold cyanidation with persulfate and thallium ŽI.. Experimental conditions: 0.5 mM Tl ŽI., pH 11.5, 0.21 atm pO 2 , 400 miny1 and 258C.
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Fig. 4. Arrhenius plot for gold cyanidation with persulfate and thallium ŽI.. Experimental conditions: 0.02 M NaCN, 5 mM K 2 S 2 O 8 , 0.5 mM Tl ŽI., pH 11.5, 0.21 atm pO 2 and 400 miny1 .
concentrations, the gold dissolution rate is proportional to the cyanide concentration, reaching a maximum value that depends on the persulfate concentration and remains nearly constant for higher cyanide concentrations. The wCNyx rwS 2 O 82y x ratio at which the dissolution rate reaches its limiting value can also be determined from Fig. 3. The average value obtained for this ratio is 2.6. The maximum value for the cyanidation rate in each case is directly proportional to the persulfate concentration. 2.2.4. Effect of the temperature The Arrhenius plot for the gold cyanidation with persulfate and thallium ŽI. is shown in Fig. 4. The experiments were carried out in air atmosphere, with 0.02 M NaCN, 0.005 M K 2 S 2 O 8 , 0.5 mM TlNO 3 , 400 miny1 , pH 11.5 and 258C. The value of the activation energy found was 19.2 kJ moly1 , which is typical for diffusion-controlled reactions. 2.2.5. Effect of the stirring speed The effect of the stirring speed on the dissolution rate is plotted in Fig. 5. The stirring speed was varied while maintaining the rest of experimental conditions as in the
Fig. 5. Napierian logarithm of the reaction rate against Napierian logarithm of the stirring speed for gold cyanidation with persulfate and thallium ŽI.. Experimental conditions: 0.02 M NaCN, 0.005 M K 2 S 2 O 8 , 0.5 mM Tl ŽI., pH 11.5, 0.21 atm pO 2 and 258C.
L. Guzman et al.r Hydrometallurgy 54 (2000) 185–193
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previous series ŽFig. 4.. The slope of the straight line obtained when representing the Napierian logarithm of the reaction rate vs. the logarithm of the stirring speed is 0.5, which is also typical of diffusion-controlled systems.
3. Discussion 3.1. Mechanism of reaction A mechanism and kinetic equation for gold cyanidation in the presence of thallium ŽI. has been previously reported by Chimenos et al. w7x, by considering that gold cyanidation is a process-controlled by oxygen and cyanide diffusion towards the gold surface. Based on this model, a kinetic equation has been established for gold cyanidation with persulfate in the presence of thallium ŽI., taking into account that persulfate exchanges two electrons and thallium ŽI. ion concentration practically does not affect the rate. 3.2. Determination of the rate equation for gold cyanidation with persulfate in the presence of thallium (I) Considering the following anodic and cathodic reactions: y
Au q 2CNy™ Au Ž CN . 2 q ey
Ž 1.
S 2 O 82y q 2ey™ 2SO42y
Ž 2.
and the global reaction: y
2Au q 4CNyq S 2 O 82y ™ 2Au Ž CN . 2 q 2SO42y
Ž 3.
The gold dissolution rate equation can be expressed by: y
rd s
1 d Au Ž CN . 2 2
s
dt
1 d SO42y 2
dt
1 d w CNy x sy
sy 4
dt
d S 2 O 82y dt
Ž 4.
As has been stated before, the process is controlled by diffusion and therefore, gold cyanidation under these conditions will be controlled by cyanide and persulfate diffusion towards the gold surface. Thus, according to the Fick’s law: d w CNy x sy
DCN y
dt d S 2 O 82y dt
sy
d
Sa Ž w CNy x y w CNy x i .
D S 2 O 82y
d
Sc Ž S 2 O 82y y S 2 O 82y
Ž 5. i
.
Ž 6.
where D S 2 O 82y and DCN y are the diffusion coefficients for the persulfate and the cyanide ions, respectively, wS 2 O 82y x and wCNyx the concentrations in the bulk solution, wS 2 O 82y x i and wCNyx i the concentrations in the interface, d the thickness of the Nernst boundary layer and Sa and Sc the surfaces where anodic and cathodic reactions take place, respectively.
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The concentrations at the interface in a diffusion-controlled reaction become nil and, as a consequence, Eqs. Ž5. and Ž6. can be written as: d w CNy x DCN y sy Sa Ž w CNy x . Ž 7. dt d D S O 2y d S 2 O 82y s y 2 8 Sc Ž S 2 O 82y . Ž 8. dt d The sum of the anodic and cathodic surface fractions is equal to the total surface area. Thus, considering these later equations and replacing them in Eq. Ž4. we obtain: y
1 d Au Ž CN . 2 S
s
dt
2 DCN y D S 2 O 82y w CNy x S 2 O 82y
d Ž DCN y w CNy x q 4 D S 2 O 82y S 2 O 82y
Ž 9.
.
This equation represents the rate equation for gold cyanidation with persulfate in the presence of thallium ŽI.. It was previously shown w7x that the thallium ŽI. ion concentration practically does not affect rate, thus it was neglected in this work, too. As the experiments have been performed under oxygen at atmospheric pressure, and oxygen is an active oxidant also in this case, Eq. Ž9. must be corrected taking into account the effect of oxygen. Thus, considering the rate at which gold is dissolved by cyanide under atmospheric oxygen in the presence of thallium ŽI. w7x, the complete expression for the equation rate becomes: y
1 d Au Ž CN . 2 S s
dt 2 DCN y D S 2 O 82y w CNy x S 2 O 82y
d Ž DCN y w CNy x q 4 D S 2 O 82y S 2 O 82y
q
.
4 DCN y DO 2 w CNy xw O 2 x
d Ž DCN y w CNy x q 8 DO 2 w O 2 x .
Ž 10 . y9
2
y1
y9
2
y1
where w8,9x: DCN ys 1.83 = 10 m s , D S 2 O 82y s 1.15 = 10 m s , DO 2 s 2.76 = 10y9 m2 sy1 , and wS 2 O 82y x and wCNyx are expressed in mmol my3 , and d in m. When gold cyanidation is performed with thallium ŽI. in the absence of persulfate, a limiting value of 12 has been found for the wCNyx rwO 2 x concentration ratio at which the dissolution rate becomes constant w7x. Therefore, for cyanide concentrations higher than 12 times the oxygen concentration at atmospheric pressure, i.e., 3.24 mM, the second term in Eq. Ž10. becomes constant. As all the experiments have been performed at cyanide concentrations higher than 0.01 M, Eq. Ž10. can be expressed as: y
1 d Au Ž CN . 2 S
s
dt
2 DCN y D S 2 O 82y w CNy x S 2 O 82y
d Ž DCN y w CNy x q 4 D S 2 O 82y S 2 O 82y
.
q rO 2
Ž 11 .
where rO 2 is the cyanidation rate at atmospheric pressure without persulfate. At low cyanide concentrations, the first term in the denominator may be neglected compared with the second one, so that the simplified equation becomes: y
1 d Au Ž CN . 2 S
dt
s
DCN y 2d
w CNy x q rO 2
Ž 12 .
L. Guzman et al.r Hydrometallurgy 54 (2000) 185–193
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Under these conditions, the experimental results verify the dependence of the dissolution rate on the cyanide concentration only. At high cyanide concentrations, the second term in the denominator of Eq. Ž11. may be neglected compared with the first term, and the equation becomes: y
1 d Au Ž CN . 2 S
s
dt
2 D S 2 O 82y
d
S 2 O 82y q rO 2
Ž 13 .
Therefore, at high cyanide concentrations, the cyanidation rate only depends on the persulfate concentration. The wCNyx rwS 2 O 82y x concentration ratio at which the dissolution rate reaches its limiting value can be calculated when Eqs. Ž12. and Ž13. give the same value:
w CNy x D S O 2y s4 2 8 wS 2 O8 x DCN y
Ž 14 .
Taking into account the previous values for the diffusion coefficients of persulfate and cyanide, the concentration ratio is now:
w CNy x s 2.5 wS 2 O8 x
Ž 15 .
which is very close to the limiting ratio found experimentally: 2.6. The correlation between Eq. Ž11. and the experimental results is shown in Fig. 6. A value of 1.55 = 10y5 m has been calculated for the thickness of the Nernst boundary layer, which is in the range of values reported by other authors under different conditions w10x. In general, a good correlation can be observed. The cyanidation rate is limited by the persulfate solubility in the media Ž1.75 gr100 cm3 in water at room temperature. and, at high persulfate concentrations, by the stability of thallium ŽI., as it can be oxidized to Tl 3q and precipitate as TlŽOH. 3 in the bulk leaching solution.
Fig. 6. Comparison of experimental reaction rates and those calculated with the deduced kinetic model.
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4. Conclusions Potassium persulfate can be used to improve gold cyanidation rates at pH between 10 and 11. The dissolution rates are nearly eight times higher than those obtained by conventional cyanidation. However, at pH higher than 11, cyanidation rates dramatically decrease. This can be avoided by adding a small amount of a thallium ŽI. salt, which acts as a catalyst. Thus, when gold cyanidation is performed in the simultaneous presence of persulfate and thallium ŽI., the higher dissolution rates can be maintained until a pH value of 13. Gold cyanidation with persulfate in the presence of thallium ŽI. is a diffusion-controlled process, with an activation energy of 19.2 kJ moly1 . Gold cyanidation rate depends only on cyanide concentration at low cyanide concentrations and both on persulfate and oxygen concentrations at high cyanide concentrations. If the cyanidation is performed at atmospheric pressure, the gold dissolution rate mainly depends on the persulfate concentration in the range 2–10 mM and then reaches a limiting value. At high persulfate concentrations, Tl ŽI. can be oxidized to Tl 3q and precipitate as TlŽOH. 3 in the bulk solution.
References w1x V. Sacelean, G. Sigartau, L. Crivei, Effect of autoclaving on extraction of gold and silver from pyrite concentrates, Rev. Chim. ŽBucharest. 47 Ž11. Ž1996. 1046–1048. w2x M. Kameda, Effect of alkalies, lead acetate and some impurities contained in foul cyanide solutions, Sci. Rep. Res. Inst. Tohoku Univ., Ser. A 1 Ž1949. 435–444. w3x C.G. Fink, G.L. Putman, The action of sulfide ion and metal salts on the dissolution of gold in cyanide solutions, Am. Inst. Min. Metall. Eng. Trans. 187 Ž1950. 952–955. w4x R. Amadelli, N.M. Markovic, R.R. Adzic, E. Yeager, Oxygen reduction on electrode surfaces modified by underpotential deposited species: thallium on gold, J. Electroanal. Chem. 159 Ž1983. 391–412. w5x K.J. Cathro, F.A. Koch, The dissolution of gold in cyanide solutions — an electrochemical study, Proc. Ans. Inst. Min. Metall. 207 Ž1963. 181–205. w6x M.J. Nicol, The anodic behaviour of gold: Part II. Oxidation in alkaline solutions, Gold Bulletin 13 Ž2. Ž1980. 105–111. w7x J.M. Chimenos, M. Segarra, L. Guzman, A. Karagueorguieva, F. Espiell, Kinetics of the reaction of gold cyanidation in the presence of a thallium ŽI. salt, Hydrometallurgy 44 Ž1997. 269–286. w8x F. Habashi, Kinetics and mechanism of gold and silver dissolution in cyanide solutions, Mont., Bur. Mines Geol., Bull. 59 Ž1967. . w9x R. Mills, V.M.M. Lobo, Self-diffusion in lectrolyte solutions. A critical examination of data compiled from the literature. Elsevier, Netherland, 1989. w10x L. Guzman, Doctoral Thesis, University of Barcelona, Barcelona, Spain, 1997.