Preg-Robbing Potential of Sulphide-Bearing Gold Ores

Transcription

1 Laval University From the SelectedWorks of Ahmet Deniz Bas 2 Preg-Robbing Potential of Sulphide-Bearing Gold Ores Ahmet Deniz Bas, Laval University Pelin Altinkaya, Karadeniz Technical University Ersin Y Yazici, Karadeniz Technical University Haci Deveci, Karadeniz Technical University Available at:

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3 Preg-Robbing Potential of Sulphide-Bearing Gold Ores Bas, A. D. *, Altinkaya, P., Yazici, E. Y. and Deveci, H. Div. of Mineral & Coal Processing, Dept. of Mining Engineering. Karadeniz Technical University, 18, Trabzon, Turkey. ABSTRACT: In this study, the preg-robbing characteristics of a copper-rich gold ore and a pyritic gold ore were evaluated based on the adsorption of gold by ore from the spiked solutions (-5 mg/l Au). A free milling gold ore and a synthetic preg-robbing gold ore, which was prepared by adding activated carbon to the free milling gold ore, were also tested to confirm the validity of the method adopted. Cyanide leaching of the copper-rich ore and the pyritic ore resulted in low gold extractions of only ~.7% and 21.5%, respectively, indicating that they are refractory in nature. The gold extraction from the free milling ore sample was almost complete over 24 h of cyanide leaching while the addition of activated carbon (1% w/w) severely suppressed the extraction of gold (13.3%) into solution apparently due to the adsorption of dissolved gold from the leach solution by the activated carbon added. The preg-robbing tests revealed that the (natural) gold ores exhibited a varying degree of preg-robbing behaviour. In this regard, the pyritic ore had the highest preg-robbing potential while the free milling ore showed minimal preg-robbing activity. These findings are consistent with the level of gold extractions occurred in cyanide leaching suggesting that the preg-robbing peculiarities of the copper-bearing ore and the pyritic ore contribute to the refractoriness of these ores. INTRODUCTION The presence of naturally occurring constituents (e.g. carbon, silicates, sulphides and clays) in gold ores could adversely affect the extraction of gold in cyanide leaching since the solubilised gold can be adsorbed by these constituents (Marsden and House, 2; Helm et al. 29). Such ores are often known as preg-robbing or carbonaceous ores (Miller et al., 25). This phenomenon may well impart refractoriness to a gold ore. Natural carbon (e.g. heavy hydrocarbons, organic acids), sulphide (e.g. chalcopyrite, pyrrhotite and pyrite) and silicate minerals (particularly clays) present in the ore exhibit a varying extent of preg-robbing properties (Rees and van Devanter, 2; Miller et al., 25). Gold losses by sulphides and carbonaceous matters may be reduced and controlled by maintaining high concentrations of free cyanide (Celep et al., 21). Since activated carbon adsorbs gold faster than the naturally occurring constituents, CIL process could be alternative for the treatment of refractory gold ores. Several pretreatment methods such as roasting, aqueous oxidation and blinding can be also used to minimize or eliminate the preg-robbing behaviour (La Brooy et al., 1994; Goodall et al., 25). The determination of preg-robbing potential of an ore is of practical importance for the selection of a suitable treatment process (La Brooy et al., 1994). A variety of procedures for the assessment of pregrobbing behaviour of an ore were proposed (Miller et al., 25; Goodall et al., 25). These procedures essentially involve the addition of spiked gold solution to a certain amount of ore and the determination of gold loss from solution over a certain period of time. The procedure developed by Barrick Gold Mines Incorporate (BGMI) for the Carlin-type ores in which only a small amount of sample (5 g in 1 ml solution) is contacted with gold-cyanide solution (3 ppm Au) for only 15 min is extensively used due to its simplicity. Since adsorption of gold-cyanide is often reversible and dissolution of gold is likely to occur during the test, such a short contact time as well as small sample size can lead to misleading results. Considering these detractions to such procedures, Goodall et al. (25) developed a new procedure for the evaluation of the preg-robbing potential, which was based on the utilization of multi spike solutions, a large sample size (1 g per 2 ml) and a long equilibrium time of 24 h. Earlier studies have shown that Mastra copper-rich ore (CRO) and pyritic gold ore (PO) were found to be refractory in character due to the poor gold extractions by conventional cyanidation (Bas et al. 211; Bas et al., 2; Altinkaya, 2). In this study, the contribution of preg-robbing phenomenon to the 13

4 refractoriness of CRO and PO was investigated. In addition, a synthetic preg-robbing gold ore (SO) which was prepared by adding activated carbon to a free milling gold ore were also used to test the validity of the procedure for the determination of preg-robbing potential. MATERIAL AND METHOD Ore Samples Three different ore samples (Table 1 ) were used in this study. A synthetic gold ore which was prepared by adding activated carbon (AC) to the free milling gold ore was also used. Free milling (FMO) and copper-rich gold ore (CRO) samples were obtained from Mastra Gold Mine (Gümüşhane) and pyritic gold ore from Hazinemağara (Gümüşhane) deposit. Ore samples (-4 mm) were ground to 8% passing - 75 µm (d 8 ) by a rod mil. The ground ore was packed as 1 g portions to use in the experiments. FMO and CRO are composed predominantly of quartz while PO contains pyrite as the predominant phase (Celep et al., 21; Bas et al., 2). Copper sulphides including chalcopyrite and covelline were also identified in CRO. Table 1. Ore samples used in the preg-robbing tests Ore Name Code Origin Grade Mineralogy Free Milling Gold Ore FMO 2 g/ton Au 1 g/ton Ag Mainly quartz Synthetic Gold Ore 85% SiO Mastra- 2 SO %1 Activated Carbon (AC) Plus AC Gümüşhane CRO 5 g/ton Au Mainly quartz, Copper-Rich Gold Ore 1 g/ton Ag Cu-sulphides 1.1% Cu (CuS and CuFeS 2 ) PO Hazinemağara- 42.4% S Predominantly Pyritic Gold Ore Gümüşhane g/ton Au 2 g/ton Ag pyrite Preg-robbing Tests Reagent grade chemicals and deionised-distilled water were used to prepare all the solutions. Goldcyanide solutions were prepared from 1 ppm calibration standard (AuCl - 4 ). Preg-robbing tests were performed in 25-ml polypropoline bottles. A required aliquot of standard - solution of AuCl 4 was spiked into a cyanide solution (2 g/l NaCN at ph ) to produce test solutions with Au concentrations of 1, 3 and 5 ppm. These solutions were thoroughly stirred for 3 min to - ensure the conversion of AuCl 4 to Au(CN) - 2. The test solutions (2 ml) and ore samples (1 g) were added into the bottles. The top of the bottles was closed tightly to prevent diffusion of air during the experiment. An agitation apparatus rotating in an end-over-end fashion at 3 rpm was used to mix the bottle contents. The bottles were sampled at.25,.5, 1, 2 and 24 h and these samples were then centrifuged at 41 rpm for 5 min to obtain clear supernatants from which the residual gold concentrations were determined. Atomic absorption spectrophotometer (Perkin Elmer AAnalyst 4) was used for analysis of gold. All tests were performed in duplicate and mean values were presented in the results. Preg-robbing potential of the ore samples were determined based on the procedure proposed by Goodall et al. (25). Accordingly, standard addition of gold (ppm Au) versus final gold concentration (ppm Au) was plotted for actual and expected (i.e. no preg-robbing behaviour) results and the deviation from the expected trendline (i.e. linearity) indicating the preg-robbing potential was determined as: 14

5 PRP(%) (1 m)1 (1) where PRP (%) and m are the preg-robbing potential and slope, respectively. Ergun s test, which is a One-way Analysis of Variance (ANOVA) was also adopted for the assessment of statistical significance of the difference between the trendlines of actual and expected values (Celep et al., 21; Deveci et al., 28; Yazıcı et al., 27). RESULTS AND DISCUSSION Fig. 1demonstrates the extraction of gold from the ore samples by conventional cyanidation and, hence, their refractory characteristics. These data were in fact derived from the preg-robbing tests performed as blanks i.e. no spiked gold addition. Gold extraction from FMO was almost complete over 24 h consistent with the earlier findings (Celep et al., 21). Poor extractions of gold from CRO and PO indicated the refractory behaviour of these ore samples. Bas et al. (211 and 2) also reported low gold extractions from CRO. It was interesting to note that the extraction of gold peaked at 5% over the initial period of an hour with a sharp decline to 41% prior to a trend of steady decrease after 2 h. A similar trend for Au extraction was also observed for SO sample, which contained 1% activated carbon, vindicating the loss of gold dissolved apparently via adsorption by activated carbon. 1 Extraction of Au (%) Copper-Rich Gold Ore Free Milling Gold Ore Pyritic Gold Ore Synthetic Gold Ore Time in hours Figure 1. Gold recovery from the ore samples by direct cyanidation Preg-robbing potentials of these ores were calculated using Eq. 1 (Goodall et al.,25) and presented in Table 2. The findings illustrate that CRO and PO, in particular, have a high preg-robbing potential compared with FMO. Considering the difference in the peak and final extractions observed for PO, pregrobbing phenomenon may significantly contribute to the refractoriness of pyritic gold ores. It is also pertinent to note that the addition of activated carbon (1% by weight of the ore) resulted in a preg-robbing potential of 97%. Table 2. Preg-robbing potential of the ore samples used in the current study Time in hours Preg-robbing Potential, %PRP FMO CRO PO SO

6 Fig. 2 illustrates the results of preg-robbing tests as the plots of initial standard addition of Au (ppm) vs Au (ppm) for the ore samples. No significant deviation of the expected trendline from the actual was evident for FMO (Table 2; Fig. 2c,d), which was consistent with high gold extractions obtained (Fig. 1). However, the actual trendline was observed to significantly deviate from the expected for CRO and PO, indicating the preg-robbing characteristics of these ores (.25 h) (a) (24 h.) (b) (.25 h) (c) (.25 h) (e) Figure 2. Plots of initial standard addition of Au (ppm) vs Au (ppm) for CRO ((a) and (b)); FMO ((c) and (d)); PO ((e) and (f)) and (24 h.) (d) (24 h.) (f) 1

7 (.25 h) (g) Figure 2. (continued) for SO ((g) and (h)) (24 h.) (h) Table 3. Summary of the statistical analysis of the experimental results using Ergun s test Statistical Test P value Alpha (α) Significance CRO.25 h.47 1 h.58.5 (5%) Significant 24 h.13 FMO.25 h h (5%) Not significant 24 h.9799 PO.25 h.931 Not significant 1 h (5%) Not significant 24 h <.1 Significan SO.25 h.22 1 h (5%) Significant 24 h <.1 The statistical analysis of the experimental results using Ergun s test are presented in Table 3. Ergus s test, which tested the statistical significance of the differences between the actual and expected trendlines, confirmed the preg-robbing potential of the ore samples used in this study. It is of practical importance to note that no significant difference between the actual and expected trendlines was detected for the data obtained over a short contact time of 15 min and 1 h probably due to the insufficient time to reach equilibrium conditions. This also confirms the shortcomings of the procedures such as BGMI that are based on the short contact time. CONCLUSIONS This study has shown that the procedure proposed by Goodall et al. (25) could be suitably used for the evaluation of pregrobbing potential of gold ores. It can be inferred that the refractory gold ores containing sulphides are susceptible to pregrobbing phenomenon and pyritic ores, in particular, have high pregrobbing potential. Short contact times for preg-robbing tests may lead to erroneous results. The Ergun s 17

8 test for the statistical significance of the results is a useful tool for accurate evaluation of pregrobbing potential of an ore and analysis of data. ACKNOWLEDGEMENT The authors would like to express their sincere thanks to Assoc. Prof. Dr. I. ALP and Assist. Prof. Dr. O. CELEP for their help in preg-robbing tests and to Mastra Gold Mine-Koza Gold Operations for kindly providing ore samples. REFERENCES Altinkaya, P., 2. cevherlerinin preg-robbing özelliklerinin araştırılması, BSc. Thesis, Karadeniz Technical University, Dept. of Mining Engineering, Karadeniz Technical University, Trabzon, Turkey. Bas, A. D., Ozdemir, E., Yazici, E. Y., Celep, O. and Deveci, H., 211. Ammoniacal thiosulphate leaching of a copper-rich gold ore, Proceedings of Int. Conference on Environmental and Mineral Processing EaMP 211, Ostrava, Czech Republic, pp Bas, A.D., Yazici E.Y. and Deveci, H., 2. Treatment of A Copper-Rich Gold Ore by Ammonia Assisted Cyanide Leaching, Proceedings of XXVI International Mineral Processing Congress (IMPC 2), September 24-28, New Delhi, India. Celep, O., Yazici, E.Y., Bölük, D., Deveci, H. and Alp, İ., 21. Evaluation of preg-robbing potential of refractory gold ores, Proceedings of XII th International Mineral Processing Symposium (IMPS 21), - 8 October, Cappadocia, Nevşehir, Turkey, pp Deveci, H., Jordan, M., Powell, N., and Alp, I., 28. Effect of salinity and acidity on bioleaching activity of mesophilic and extremely thermophilic bacteria. Transactions of Nonferrous Metals Society of China, Vol. 18, No. 3, pp Ergun, S., 195, Application of Principle of Least Squares to Families of Straight Lines, Industrial & Engineering Chemistry, Vol. 48, No. 11, pp Goodall, W.R., Leatham, J.D., and Scales, P.J., 25. A new method for determination of preg-robbing in gold ores, Minerals Engineering, Vol. 18, pp Helm, M., Vaughan, J., Staunton, W.P. and Avraamides, J. 29. An investigation of the carbonaceous component of preg-robbing gold ores, Proceedings of World Gold Conference 29, pp La Brooy, S.R.; Linge, H.G. and Walker, G.S., Review of Gold Extraction from Ores, Minerals Engineering, Vol. 7, No. 1, pp Marsden, J.O. and House, C.L., 2. The Chemistry of Gold Extraction, Society for Mining Metallurgy and Exploration. Miller, J.D., Wan, R.-Y., and Diaz X., 25, Chapter 38: Preg-robbing gold ores, Advances in Gold Ore Processing, M.D. Adams, ed., 25, Developments in Mineral Processing Elsevier, Vol. 15 Amsterdam, pp Ozdemir, E., 211. Thiosulphate leaching of gold from copper-rich Mastra gold ore, BSc. Thesis, Karadeniz Technical University, Dept. of Mining Engineering, Trabzon, Turkey. (In Turkish). Rees, K.L. and van Deventer, J.S.J., 2. Preg-robbing phenomena in the cyanidation of sulphide gold ores, Hydrometallurgy, Vol. 58, pp Yazıcı, E.Y., Deveci, H., Alp, I. and Uslu, T., 27. Generation of hydrogen peroxide and removal of cyanide from solution using ultrasonic waves, Desalination, Vol. 21, pp