Effect of ore pulp on the adsorption rate of gold cyanide on activated carbon

Hydrometallurgy, 22 (1989) 231-238

231

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Technical Note

Effect of Ore P u l p on the A d s o r p t i o n R a t e of Gold C y a n i d e on A c t i v a t e d Carbon* W.G. JONES and H.G. LINGE

CSIRO, Division of Mineral Products, c/o Curtin University of Technology, Bentley, W.A. 6102 (Australia) (Received June 6, 1987; revised and accepted August 18, 1988)

ABSTRACT Jones, W.G. and Linge, H.G., 1989. Effect of ore pulp on the adsorption rate of gold cyanide on activated carbon. HydrometaUurgy, 22: 231-238. The effect of the presence of a clay-laterite ore pulp (20-40 wt.% ) on the adsorption rate of gold on activated carbon has been studied. A substantial reduction in the rate (e.g., by up to an order of magnitude) occurs when the pulp is present. The reduction in rate appears to be caused by two effects: permanent carbon pore blocking by ore particles and temporary shielding of the carbon particle surface, slowing gold access to the carbon as a result of the physical presence of the fine pulp. However, the pulp had no systematic effect on the equilibrium adsorption of gold on the carbon.

INTRODUCTION

Granular activated carbon is now used widely in gold processing by the Carbon-in-pulp (CIP) process [ 1 ]. In this process, the essential reaction involving the carbon is adsorption of the gold cyanide (aurocyanide) ion Au ( CN ) 2 from leached ore pulp. In practice, high pulp densities are used and viscosity modifiers are usually added to lower the pulp viscosity, enabling mixing and transfer of the pulp. It has been observed [2,3 ] that the presence of pulp depresses the adsorption rate of A u ( C N ) 2 onto the carbon but the detailed operational mechanism of this interference is obscure. It is not clear whether the effect is a result of: (1) a decreased mixing efficiency in the presence of pulp; (2) a chemical poisoning of the carbon surface by the pulp or by the viscosity modifier; or (3) a physical blinding of the carbon surface by ore particles. These separate aspects of the influence of ore-pulp are studied here using a coconut CIP carbon which is widely used in Australia, in contact with an in*Presented at Regional Conference of Aust. I.M.M., Kalgoorlie, October 1987.

0304-386X/89/$03.50

© 1989 Elsevier Science Publishers B.V.

232 dustrial clay-laterite gold ore pulp. The effect of a commercial viscosity modifier, which satisfactorily lowers pulp viscosity at economic addition rates, is also investigated. EXPERIMENTAL CIP coconut carbon Calgon GRC22, obtained from the Australian distributor Wright and Co. Pty. Ltd., was cleaned by acid-washing ( 1 M HCh 1 M HF ), rinsed (distilled water), stored in vacuo and used as coarse particles ( - 8 + 16 mesh). The gold loading on GRC22 was found to be dependent on pH, ionic medium and ionic strength. Therefore, a standard cyanide-free borate buffer (pH = 10.0, ionic strength ~ 0.11 ), similar to that recommended by Davidson et al. [4], was used throughout this work. Gold loading on GRC22 approached equilibrium within 24 h, compare the long-term change of loading with time (each quoted as fraction of 24-h value): 40 min/0.38, 13 h/0.96, 5 day/1.02, 1 month/1.05, 2 month/1.08. In this work, gold loading on carbon was obtained in stoppered flasks and was considered to be at equilibrium after 24 h shaking at constant temperature using a shaking table (Heto Lab Equipment, Denmark). A temperature of 30 ° C was chosen as representative of conditions in Australian gold plants. Gold loading rates were measured at 30 °C in a thermostatted glass cell (0.21 1) stirred by an efficient multi-blade Teflon stirrer. During the experiment, the carbon particles (dosage 1.2 g 1-1) were held in a 52 mesh Pt cage, which prevented carbon attrition. Small volumes of solution were assayed periodically for dissolved gold by flame atomic absorption spectrophotometry (Varian Instrument AA-375). Pulp ore was obtained from Worsley Alumina Pty. Ltd. and consisted of a 70:30 wt.% clay-laterite blend originating from the company's Boddington mine. The as-received blend was sieved to pass - 2 3 0 mesh to ensure that the ore particles flowed freely through the Pt cage, and this size fraction accounted for 72 wt.% of the original blend. The major constituents of the blend were: A1 (18.0%), Fe (10.1%), Si (15.4%), K (6800 ppm), Mg (1500 ppm), Na (960 ppm ) and Ca (480 ppm ). The pulp density used in the experiments was usually 40 wt.% solids, which matches the industrial usage level (B. Sceresini, pers. commun., 1985). Pulp sampling involved withdrawal of ~ 5 ml of the pulp using a syringe. The solution was separated from the pulp by centrifuging and filtering through a Millipore 0.2-zm filter, followed by AA analysis of the filtrate. The sampling was proven to be representative of the content, and an analysis of the constituent weights of a typical sample is shown in Table 1. The viscosity modifier used was Freevis 528 (ICI Australia Operations Pty. Ltd., St. Marys, N.S.W., Australia), which is a sodium poly-carboxylate. The recommended (B. Sceresini, pers. commun., 1985) concentration of this reagent to achieve a manageable pulp at 40 wt.% is 150 ppm. This reagent con-

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TABLE 1 Test results of sampling 40.0 wt.% pulp Mass filtrate (g) Mass wet filtercake (g) Mass dry filtercake a (g) Mass solution in pulp (g) Total mass solution (g) % water % solids

1.761 4.674 2.582 2.092 3.853 59.9 40.1

"Dried at 60°C for 24 h.

centration was used in this work and the apparent pulp viscosity was reduced from 0.25 to 0.11 Pa s at a shear rate of 100 s -1. R E S U L T S AND D I S C U S S I O N

The amount of gold adsorption on carbon was calculated from the measured gold solution concentration and the gold adsorption rates obtained by numerical differentiation of the digitized form of the gold loading-time variation followed by data-smoothing [5]. The gold adsorption rate on fresh carbon in a clear borate buffer solution (pH--10.0, 30°C) containing ~ 5 ppm Au (as KAu (CN)2 ) varies with the gold solution concentration, as shown in Fig. 1 for two solution stirring rates. In each case, the reaction rate is proportional to the gold solution concentration. Thus, the reaction is characterized by the rate constant (k) given by the slope of this line. The constant k was determined at stirring speeds ranging from 250 to 5000 r.p.m. This work [6] shows that the rate constant is dependent on stirring speed below 1500 r.p.m., indicative of a solution mass-transport controlled gold adsorption reaction below this stirring speed. By increasing the stirring speed beyond 1500 r.p.m., the adsorption rate enters a region where mass transport control is no longer important and the rate constant is then a true indicator of the surface reaction. To achieve this condition, a stirring speed of 2000 r.p.m, was used to test surface conditioning effects, and for this condition the standard rate constant was 3.5 X 10 -4 1 g-1 s - 1. This value is equivalent to an apparent first-order rate constant of 1.5 h - 1 at a carbon dosage of 1.2 g l - 1. Before determining the effect of pulp on the gold adsorption on carbon, it was demonstrated that the pulp had no direct effect on the gold solution concentration. A standard pulp was prepared in pH 10 borate buffer containing 9.1 ppm of dissolved gold and sampled by the standard technique. The determined concentration of gold in the sample filtrate after 24 h contact was 9.6 ppm, which is in reasonable accord with the analytical result. As the gold concentration of the pulp in the absence of carbon was constant it is clear that

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Fig. 1. Gold adsorption rate on Calgon GRC22 vs. gold solution concentration at two solution stirring speeds (carbon dosage 1.2 g 1-1, pH = 10 borate buffer, ionic strength 0.11, 30°C). there was no true adsorption (i.e., phase separation) of water on the ore. Similarly, the result shows that there is no gold leaching from the ore, as would be expected for a cyanide-free electrolyte. Figure 2 shows the effect of pulp on gold adsorption from 5 p p m aurocyanide solution onto carbon compared with the behaviour of carbon in clear electrolyte (in Fig. 2, [Au] is the concentration of gold at time t, [Au]o is the concentration at time t = 0). The stirring speed chosen for these tests was 1000 r.p.m., to highlight any influence of pulp viscosity on the gold mass-transport rate, which determines the loading rate under these conditions. The results show that in the presence of pulp, the data scatter more widely than in the reference experiments, even though care was taken during sampling. Within this scatter, there is no pronounced effect from the presence of Freevis 528, even though the apparent pulp viscosity was reduced from 0.25 to 0.11 Pa s (measured at a shear rate of 100 s - 1). Similarly, there was no effect from changing the pulp density from 40 to 20 wt.%. However, it is clear that the loading rate is very much reduced whenever pulp is present. Furthermore, the reaction kinetics changed whenever pulp is present. This is demonstrated by the data points in Fig. 3, in which the rate of reaction was obtained from the line drawn through the data in Fig. 2 pertaining to gold adsorption in a pulp. In Fig. 3, the dashed curve represents the adsorption rate in clear buffer solution, and it is seen that

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Fig. 2. Effect of pulp with and without Freevis 528 on gold adsorption on carbon in ~ 5 ppm KAu(CN)2/borate buffer solution at 1000 r.p.m, and 30°C. (A = 40 wt.% pulp; O = 40 wt.% pulp and 150ppm Freevis528; + = 20 wt.%pulp and 150 ppm Freevis528; • -- clearelectrolyte). the percentage rate reduction in the presence of pulp increased during the reaction. Initial values are 82% of the unhindered rate at 5 ppm to values as low as 16% at 3.8 ppm (2 p p m = 10 -s M). This large decrease in the rate could not have resulted from hydrodynamic factors such as viscosity or impeller stirring efficiency, as the presence of viscosity modifier had no systematic effect on the loading rates, suggesting t h a t the diffusion of A u ( C N ) ~ is unaffected by the change in the pulp viscosity. Similarly, the rate data in Fig. 3 imply t h a t Freevis 528 does not poison the carbon surface as is the case with other organic surfactants, such as flotation reagents [3]. This latter point was demonstrated separately in tests of gold loading of carbon at 2000 r.p.m, in solutions to which only the viscosity modifier had been added, at a concentration of 150 ppm. For these conditions, the rate still varied linearly with gold solution concentration and the value of the rate constant was 3.2 X 10 -41 g - 1 s - 1. This value is 91% of the standard value. Thus, Freevis 528 does not significantly foul the carbon surface. The presence of ore pulp could affect the gold loading rate temporarily by directly blinding the carbon surface or permanently by contaminating the carbon surface. Such contamination could arise chemically by a dissolution-reprecipitation mechanism involving ore components or by a physical mechanism such as carbon-pore blocking with an ore particle. To distinguish between

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Fig. 3. Rate of gold adsorption vs. gold solution concentration at 30°C in borate buffer at 1000 r.p.m, obtained from the data in Fig. 2. ([] = average rate for gold adsorption in pulps, as given in Fig. 2; dashed curve represents gold adsorption in clear borate buffer). these mechanisms, samples of carbon were preconditioned separately in pulp for 1 h and 3 days and the gold loading examined subsequently in clear solution at 2000 r.p.m, after the pulp had been carefully separated from the carbon. There was no significant difference between preconditioning for either 1 h or 3 days but there was a significant reduction in the gold adsorption rate as compared with clear solution, as shown in Fig. 4. The plot shows that the rate of loading is still proportional to the gold solution concentration but the rate is reduced to 66% of that of untreated carbon. The level of this reduction is less pronounced than that observed when ore particles are present, which lowered the rate to as little as 16% of that of untreated carbon; this suggests that there are at least two distinct mechanisms by which ore pulp hinders the reaction. Further insight into these mechanisms was obtained by examining the effect of pulp ore on the equilibrium loading of gold on carbon. The equilibrium loading in clear aurocyanide solution of carbon, which had been precontacted with pulp, was 83% of the value of fresh carbon. The effect of the presence of pulp on the equilibrium capacity was determined similarly. In this test, the duration of the experiment was extended from 24 h, anticipating that it would take longer to reach equilibrium than in the standard test. Table 2 shows the result

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carbon for the same adsorption conditions). TABLE 2

The proportion of gold loading on carbon in the presence of pulp as compared with the 24-h isotherm of carbon in a clear solution Time (days)

Ratio of gold loading in pulp to gold loading in solution

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0.76 0.90 1.16 1.16

as a proportion of gold loaded on fresh carbon in clear solution in the standard test as a function of contact time. According to these data, there is no systematic effect from pulp on the equilibrium loading of gold on carbon, in contrast to the effects observed on the kinetics. As it would be expected that a chemical modification of the carbon surface arising from pulp contact should also affect the equilibrium gold loading, our results suggest that pulp influences the gold loading process by both: (1) temporary general blinding of the exposed carbon

238 surface, a n effect w h i c h d i s a p p e a r s w h e n t h e p u l p p a r t i c l e s are r e m o v e d f r o m the system; and (2) a p a r t i a l c a r b o n p o r e b l o c k i n g w i t h ore particles, w h i c h slows t h e r a t e of r e a c t i o n b u t does n o t h i n d e r e v e n t u a l access into t h e s e p o r e s a f t e r a n e x t e n d e d p e r i o d of solution c o n t a c t . ACKNOWLEDGEMENT We t h a n k L. M e a g h e r for m a k i n g t h e p u l p v i s c o s i t y m e a s u r e m e n t s .

REFERENCES 1 Woodcock, J.T., 1982. Carbon-in-pulp technology in gold extraction. In: Aust. I.M.M. Symp. Carbon-in-Pulp Technology for the Extraction of Gold, Australasian Inst. Min. Metall., Melbourne, Vic., pp. 1-6. 2 Potter, G.M., 1981. Effects of clay content on carbon-in-pulp process. World Mining, July, p. 33. 3 Fleming, C.A. and Nicol, M.J., 1984. The adsorption of gold cyanide onto activated carbon. III. Factors influencing the rate of loading and the equilibrium capacity. J. S. Afr. Inst. Min. Metall., 84(4): 85-93. 4 Davidson, R.J., Douglas, W.D. and Tumitty, J.A., 1982. The selection of granular activated carbon for use in carbon-in-pulp operation. In: Aust. I.M.M. Symp. Carbon-in-Pulp Technology for the Extraction of Gold. Australasian Inst. Min. Metall., Melbourne, Vic., pp. 199-218. 5 Savitzky, A. and Golay, M.J.E., 1964. Smoothing and differentiation of data by simplified least squares procedures. Anal. Chem., 36 (8): 1627-1639. 6 Jones, W.G., Klauber, C. and Linge, H.G., 1988. The adsorption of Au{CN)~ onto activated carbon. In: Perth Int. Gold Conf., Randol Int. Ltd., Golden, Colo., pp. 243-248.