Mineral Processing B21 EFFECT OF LEAD NITRATE ON CYANIDATION OF ANTIMONIAL REFRACTORY GOLD AND SILVER ORES MSc.Oktay CELEP Assoc. Prof. Dr. İbrahim ALP Assoc. Prof. Dr. Hacı DEVECİ Department of Mining Engineering, Karadeniz Technical University, Turkey ABSTRACT This paper deals with the effect of the addition of lead nitrate and ultra-fine grinding on the cyanidation of an antimonial refractory gold and silver ore. The results indicated that the recovery of gold was enhanced by 1%, but, silver extraction did not improve with adding lead nitrate or increasing its concentration. Fine grinding (e.g. down to-5µm) did not produce the desired effect on the extraction of Au and Ag. Keywords: gold; silver; refractory ores; antimony minerals; lead nitrate; cyanidation INTRODUCTION The refractoriness of gold ores is linked with their inherent mineralogical features [1, 2] Low gold or silver recoveries in cyanidation are sometimes caused by the sulphide minerals in ores [3]. Lead nitrate is often added to alleviate the negative effect of sulphides present, enhance gold recovery and lower cyanide consumption, although it could be also detrimental to the process at excessively high levels [4]. It was suggested [5] that in a cyanide solution, lead reacts with gold to form AuPb 2, AuPb 3 and metallic lead and accelerates the gold dissolution [6]. However, it is claimed that, if formed, lead hydroxide film decreases gold extraction rate. The Akoluk ore (Ordu / Turkey) having a complex mineralogical composition is an antimonial sulphide ore. The ore consists of predominantly quartz, illite/kaolinite group clay (52.2% SiO 2 ) and barite (17.1% Ba), and to a less extent, sulphide minerals such as pyrite, sphalerite, zinkenite and stibnite. Pyrite, andorite and zinkenite are the main gold and silver-bearing components in the ore. Gold particles containing silver also occur as associated with framboidal pyrite and as inclusions within Sb-S and (-Pb) minerals. Celep et al., [7] suggested that the refractoriness was induced by the dissemination and encapsulation of the very fine gold and silver particles largely within the carbonates, oxides and sulphides and, to a small extent, within silicates present in the ore matrix. Earlier studies on the ore showed that the extraction of gold and silver was severely limited ( 5% Au and 19 Ag) [8]. Antimony minerals do not form stable complexes with cyanide and consequently the presence of cyanide in solution does not appreciably affect the stability of the metal species formed. Under the conditions applied for gold leaching, antimony sulphide minerals decompose to Sb 2 O 2 - and Sb 2 O 3 - (stibnite and stibnate). Dissolution of these minerals has a detrimental effect on gold and silver extraction. This effect is thought to 639
International Multidisciplinary Scientific GeoConference SGEM 21 be due to the formation of passivating layer of antimony oxide layer on the gold surface. Decomposition of these minerals is strongly dependent on ph, with their solubility increasing with increasing ph [9]. The objective of this study was to investigate the effect of lead nitrate for gold and silver recovery from Akoluk antimonial refractory gold and silver ore. Furthermore, ultra-fine grinding was also examined as a pretreatment method to enhance the god and silver extraction. EXPERIMENTAL Material In this study, the antimonial refractory gold/silver ore sample from Akoluk, Ordu (Turkey) was used. Table 1 shows the chemical composition of the ore, which were determined through digestion in aqua regia and then analysed by ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectroscopy) and NAA (Neutron Activation Analysis). The particle size analysis of ground samples was performed by Malvern Mastersizer laser particle size analyzers. Table 1. Chemical composition of the ore sample Compound Content (%) Element Content (%) Element Content (g/ton) SiO 2 52.15 Ba 17.1 Au 2 Al 2 O 3 4.71 Zn 1.5 Ag 22 Fe 2 O 3 1.28 Pb.43 As 261 CaO.14 Sb 1.64 Cu 472 MgO.7 Sr.31 Hg >1 Na 2 O.4 Tot. S 6.89 Ni 6 K 2 O.38 Tot. C.5 Zr 4 TiO 2.8 LOI * 4.6 Cd 62 P 2 O 5.1 Ga 8 MnO.1 Mo 12 * LOI is an abbreviation for loss on ignition Method The cyanide leaching tests were carried out in a stirred tank reactor equipped with a pitched-blade turbine impeller rotating at 75 rpm. A summary of cyanide leaching conditions are shown in Table 2. Effect of lead nitrate on cyanide leaching was examined at 5-4 g/t Pb(NO 3 ) 2. Also, gold and silver extractions were investigated by cyanidation at different particle size and lead nitrate concentrations. During the leaching tests, the concentration of free cyanide was maintained at 1.5 g/l. Over the leaching period, samples were removed and analysed for Au and Ag using an atomic adsorption spectrometer (AAS-Perkin Elmer). On completion of leaching, residues were also analysed. Metal extractions were determined based on the residue analysis. Free CN - concentration was determined by titration with silver nitrate using p- dimethylaminobenzalrhodanine (.2% w/w in acetone) as the indicator. 64
Mineral Processing Table 2. Experimental conditions for cyanide leaching of the ore. Parameter Cyanidation Sample weight, g 7 Particle size; d 8 µm 5-1-15 Pulp density, w/w, % 25 ph (NaOH) 1.5±3 Agitation, rpm 75 NaCN concentration, g/l 1.5 Leach time, hour 24 Temperature, o C 2±3 Aeration, l/min.3 Lead nitrate concentration, g/t 5, 1, 2, 4 RESULTS AND DISCUSSION Effect of lead nitrate addition (up to 4 g/t) on the gold extractions is illustrated at Fig. 1. There was a slight increase (by 1%) in the gold leaching recovery with the addition of lead nitrate up to 1 g/t, but there is no additional improvement at >1 g/t addition. Dissolution of gold was observed to occur largely over an initial period of 3 h. Following these initial periods, the metal dissolution was insignificantly. 1 8 6 4 (a) g/t Pb(NO3)2 5g/t 1g/t 2g/t 4g/t 1 8 6 4 (b) 2 2 4 8 12 16 2 24 1 2 3 4 Pb(NO 3 ) 2 concentration; g/t Figure 1. The effect of lead nitrate on Au recovery in cyanidation. Lead nitrate addition did not effect silver extraction from the ore. Dissolution of silver was observed to occur largely over an initial period of 1 h. Silver recoveries were 12-14% Ag over 24hours (Fig.2). 641
International Multidisciplinary Scientific GeoConference SGEM 21 1 8 6 4 2 (a) g/t Pb(NO3)2 5g/t 1g/t 2g/t 4g/t 1 8 6 4 2 (b) 4 8 12 16 2 24 1 2 3 4 Pb(NO 3 ) 2 concentration; g/t Figure 2. The effect of lead nitrate on Ag recovery in cyanidation. Effect of particle size on the gold and silver extractions is shown in Figure 3 and 4. Particle size reduction to 1 µm improved gold extraction by 5-1% (Fig.3). In contrast to gold, silver recovery decreased with decreasing the particle size 5 µm (d 8 ) (Fig. 4) 1 8 6 4 2 (a) d8:5micron-2g/tpb(no3)2 d8:5micron-4g/t d8:1micron-2g/t d8:1micron-4g/t d8:15micron-2g/t d8:15micron-4g/t 4 8 12 16 2 24 1 8 6 4 2 (b) g/t Pb(NO3)2 2g/t 4g/t 5 1 15 Particle size (d 8 ); micron Figure 3. The effect of lead nitrate and particle size on the extraction of gold. 642
Mineral Processing 1 8 6 4 2 d8:5micron-2g/t Pb(NO3)2 d8:5micron-4g/t d8:1micron-2g/t d8:1micron-4g/t d8:15micron-2g/t d8:15micron.4g/t 1 8 6 4 2 g/t Pb(NO3)2 2g/t 4g/t 4 8 12 16 2 24 5 1 15 Particle size (d 8 ); micron Figure 4. The effect of lead nitrate and particle size on the extraction of silver. Up to 4g/t the addition of lead nitrate reduces the cyanide consumption by.6 kg/t and, cyanide consumption was average 9.3 kg/t (Fig. 5). Reduction of cyanide consumption could be attributed to removal of sulphide released from sulphide phases. Cyanide consumption; kg/t. 12 1 8 6 4 2 1 2 3 4 Pb(NO 3 ) 2 concentration, g/t Figure 5. The effect of lead nitrate concentrations on cyanide consumption. CONCLUSIONS In this paper, the effect of lead nitrate on cyanidation of an antimonial refractory gold and silver ore was demonstrated. Results showed that lead nitrate addition had a limited effect on gold and silver extractions from antimonial refractory Akoluk ore. The addition of lead nitrate reduces the cyanide consumption. Ultra-fine grinding was shown to effect metal extraction to a limited extent. These findings suggest that application of a 643
International Multidisciplinary Scientific GeoConference SGEM 21 suitable pretreatment method is required to improve Au/Ag extraction. Further studies will be conducted to using suitable pretreatment method prior to cyanidation. ACKNOWLEDGEMENTS The authors would like to express their sincere thanks and appreciation to the Research Foundation of Karadeniz Technical University for the financial support, to Gürçelik Mining Trading Ind. Ltd. and Anatolia Minerals Development Ltd. for kindly providing the ore samples. REFERENCES [1] La Brooy, S.R., Linge, H.G. & Walker, G.S. Review of gold extraction from ores, Minerals Engineering, vol. 7/issue 1, 1213-1241, 1994. [2] Marsden, J.O. & House, C.L. The chemistry of gold extraction, Society for Mining Metalurgy and Exploration, 26. [3] Roshan, B.B. Hydrometallurgical processing of precious metal ores, Mineral Processing and Extractive Metallurgy Review, vol. 6, pp 67-8, 199. [4] Deschenes, G., Rousseau, M., Tardif, J. & Prud homme, P.J.H., Effect of the composition of some sulphide minerals on cyanidation and use of lead nitrate and oxygen to alleviate their impact, Hydrometallurgy, vol. 5, pp.25 221, 1998. [5] Deschenes, G., Lastra, R., Brown, J.R., Jin, S., May, O. & Ghali, E. Effect of lead nitrate on cyanidation of gold ores: progress on the study of the mechanisms, Minerals Engineering, vol. 13/issue 12, pp 1263-1279, 2. [6] Deschenes, G., Lacasse, S. & Fulton, M. Improvement of cyanidation practice at Goldcorp Red Lake Mine, Minerals Engineering, vol. 16, pp 53 59, 23. [7] Celep, O., Alp, İ., Deveci, H. & Yılmaz, T. The investigation of gold and silver recovery from Akoluk (Ordu -Turkey) ore, International Conference of Modern Management of Mine Producing, Geology and Environmental Protection-SGEM, Bulgaria, 26, pp 251-258. [8] Celep, O., Alp, İ., Deveci, H. & Vıcıl M. Characterization of refractory behaviour of a complex gold/silver ore by diagnostic leaching, Transactions of Nonferrous Metals Society of China, vol. 19, pp 77-713, 29. [9] Adams, M.D. Advances in Gold Ore Processing, Developments in Mineral Processing 15, Netherlands, Elsevier, pp 172, 25. 644