International Journal of Mineral Processing 95 (2010) 68–77Contents lists available at ScienceDirect International Journal of Mineral Processing j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i j m i n p r o Cyanidation of gold ores containing copper, silver, lead, arsenic and antimony P. Karimi a,⁎, H. Abdollahi a, A. Amini b, M. Noaparast a, S.Z. Shafaei a, F. Habashi c a School of Mining Engineering, University of Tehran, Islamic Republic of Iran b Geological Survey and Exploration of Iran, Islamic Republic of Iran c Laval University, Quebec City, Canada G1V 0A6 a r t i c l e i n f o a b s t r a c t Article history: Cyanidation tests on two ore samples from two different gold deposits are reported. The first sample Received 11 December 2009 contained 10.5 ppm gold with high arsenic and antimony. The second sample had a low gold content Received in revised form 27 February 2010 (2.5 ppm) but a high silver content (160 ppm). The first series of test work focused on the determination of Accepted 13 March 2010 conditions for extracting gold from the samples ground to − 75 μm. The optimum parameters were Available online 20 March 2010 4000 mg/L for cyanide concentration, pH = 11.1 and 24-h cyanidation time for the first sample; and Keywords: 2500 mg/L, pH = 10.5 and 24 h for the second sample. Silver and gold recoveries were 94.91% and 28.2% Gold ores respectively for the first sample and 92.5% and 93.5% for the second sample. Cyanidation In the second series of test work, H2O2 (0.015 M), air (0.15 L/min) and a mixture of H2O2 and air were used Reaction kinetics as oxidizing agents to improve the gold extraction kinetics of the first sample. It was found that gold leaching Pretreatment recovery followed first order kinetics and that the injection of air had the maximum beneficial effect on the leaching kinetics. In the third series of test work, acid pretreatment (HNO3 and HCl) and roasting (600– 1000 °C) from 0.5 to 2 h were carried out before cyanidation. Acid pretreatment reduced cyanide consumption by 340 and 210 mg/L respectively and the corresponding gold recoveries increased to 98.87% and 95.11%. Cyanidation results on roasted samples showed that cyanide consumption was drastically reduced by 1150 mg/L and gold recovery increased by 5.2%. Furthermore, arsenic, antimony, cadmium and mercury contents were considerably reduced in the roasted sample (2 h at 1000 °C). © 2010 Elsevier B.V. All rights reserved. 1. Introduction reactions and the preferential formation of Au (CN)− 2 on the surface instead of AuSx (Senanayake, 2008). 1.1. General R1 M2S + 4CN− ⇆ 2M(CN)−2 +S −2 Early studies on the dissolution of gold in cyanide solution in the R2 M2S + 4CN− + H2O ⇆ 2M(CN)− − 2 + OH + HS − presence of sulfide minerals have shown that heavy metal compo- nents, such as Cu, Fe and Zn, significantly increase the consumption of The presence of galena, as well as both sulfide and added lead(II), both cyanide and oxygen (Habashi, 1967; Dai and Jeffrey, 2006). In also improved the rate of gold dissolution. The dissolution rate was addition, the sulfide component has been shown to have a strong found to be affected by ions such as lead(II), sulfide, and iron(III) impact on the gold leaching kinetics (Fink and Putnam, 1950; Hedley released by the sulfide mineral. The significant decrease in the rate of and Tabachnick, 1968; Dai and Jeffrey, 2006). The leaching behavior of electrochemical and chemical dissolution of gold caused by chalco- gold in the presence of sulfide minerals depended strongly on both pyrite, pyrite, and pyrrhotite has been related to the various films the solubility of the sulfides and the oxygen concentration in the formed on the gold surface as well as the galvanic interaction solution (Dai and Jeffrey, 2006). Based on the experiments observa- (Lorenzen and van Deventer, 1992; Aghamirian and Yen, 2005; Dai tion, it is postulated that sulfide ions (formed by the decomposition of and Jeffrey, 2006; Senanayake, 2008). The dissolution of sulfides sulfide minerals) show detrimental effect on the cyanidation kinetics results in high cyanide consumption with the formation of cyano of gold and silver. However, the detrimental effect of dissolved sulfide complexes (e.g., of the ions Fe+ 2, Zn+ 2, Cu+ 2, Ni+ 2, Mn+ 2) and SCN−. on gold cyanidation according to reactions R1 and R2 which When sulfide minerals are present in gold ores, gold dissolution can diminishes at higher cyanide concentrations, and favor the reverse be affected in various ways. In one hypothesis (Weichselbaum et al., 1989; Kondos et al., 1995), soluble sulfide (S2− or HS−) generated from mineral dissolution reacts with gold and forms a passive film, which decreases the rate and extent of leaching. In another theory ⁎ Corresponding author. Tel./fax: +98 21 64592257. (Lorenzen and van Deventer, 1992; Kondos et al., 1995), a dynamic E-mail address:
[email protected] (P. Karimi). coupling of reduction forms at the sulfide mineral surface which 0301-7516/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.minpro.2010.03.002 . modified cyanidation are the pretreatment methods currently In this study. 11. Guzman et al. effect when chalcopyrite was present (Dai and Jeffrey. Pyrite FeS2 0. 2009).. due to their cost and complications involved in Arsenopyrite FeAsS 3. 2009).6 leaching (Brooy et al.. The first sample (high simple stage process using a fluidized bed roaster at around 650 °C. and chlorine have been used with more or less success. 1970.7 Feldspar KAlSi3O8 2. / International Journal of Mineral Processing 95 (2010) 68–77 69 results in oxidation on the gold grains (Kondos et al. solid–liquid 2. cone Lapidus. copper presents difficulties due to the solutions of high concentration of cyanide (Tan et al.5 recoveries (N90%) from free milling ores can be readily achieved. While high gold Chalcopyrite CuFeS2 0. and to the presence of Qingcui et al. XRF and chemical analysis interface. is gold grade) was taken from “Hirad gold deposit”. After optimization of parameters. 1995). ultrafine grinding and Deschenes and Prud home. 1992. 2004). 1995. Iglesias and Carranza. optical mineralogy. 2009). Cyanide also forms complexes with a number of included in the sulfide minerals and pretreatment is necessary to metal irons: Fe(CN)4− 2− 6 .2H2O 9. the kinetics of gold dissolution (Stoychevski and Williams. 2006). but it had no beneficial for the second one. pretreatment processes such as acidic leaching and roasting establishing the distribution of gold within different mineral phases of for the first sample were investigated. The main available oxidizing processes for refractory gold inherent mineralogical features with particular reference to the mode ores are roasting (LaBrooy et al. The presence of other catalytic ions in solution and salinity of water also For ore characterization. The studied samples al. Due to the complexity of an ore (Celep et al. Sample preparation Cyanidation has been used for over 100 years to extract precious metals from sulfide ores. Generally. 2005). pressure oxidation. situated in Southern one of the most common methods for the treatment of gold-bearing Khorasan province. Roasting. when it exceeds 0. Clearly the high concentrations of oxygen and cyanide improve initial gold leaching kinetics but high Gold ore Mineral Common mineral formula Composition type name percentage cyanide concentrations also increase cyanide consumption (Ellis and Senanayake. 1990. sulfide Goethite FeO(OH) 21. XRD. biooxidation. After four crushing stages (two jaw crushers. From the electrochemical studies. Celep et al. These ores are usually pretreated by some oxidizing process leaching(Rees and van Deventer. P. Eh of the slurry... Deng and Liao. 2009). pH. the effects of these minerals on gold dissolution in cyanide In the next step. 2009). Since a large proportion of gold ores contain sulfide crusher and roll crusher). It was reported that pre-leaching mineralogy of first sample.. 2009). Zn(CN)4 2− and Ag(CN)− 2 which decompose the mineral structure to liberate gold for subsequent decrease the free cyanide concentration in slurry and retard gold recovery. primary sample size reached to −2830 μm. a were from two different gold deposits in Iran.. 1994. 1997).2 they are not used now. A 1983. 1997. 2006).. 2005.2 (Rubisov et al. quartering method. 2006). In the effect by presence of preg-robber. In despite of negative . Celep et al. 2002) and pressure oxidation (Mason. located in Kerman province calcine with the increased amenability to cyanidation (Roshan. 1987.H3O)(Al. Celep et al. and the second one (low gold grade) was pyrite/arsenopyrite and pyrrhotite concentrates to produce porous provided from “Latala gold deposit”. and due to the Dunn and Chamberlain. it considerably diminishes in leaching of gold using cyanide. The use of oxygen or an oxidizing reagent is Sample 1 Quartz SiO2 61. 2009).1. main cyanidation parameters for two different gold practiced for refractory gold ores and concentrates (Sinadinovic et samples (grade and mineralogy) were compared. Gypsum CaSO4. Nicol et al.6 their handling (Yannopoulos.2. 1990. The presence of Sample2 Quartz SiO2 73 preg-robber components such as carbonaceous matter. 1996. X-ray diffraction (XRD) studies were Senanayake. particle size and temperature. 2009). 2 kg samples were prepared by riffling and cone and solution have interested many researchers. but Hematite Fe2O3 5. In such lack of selectivity of cyanide for gold over copper (Rees and van refractory gold ores. carbonaceous matter (Rubisov et al. 1994. Celep et al.. Despite this fact. Celep et al.. Celep et al. 2009). biological oxidation (Lawrence and Bruynesteyn. Ore preparation and characterization study 1. gold particles may sometimes be occluded or Deventer. 1994. potassium permanganate. Roasting. alkalies. 1991.. 1993... Qingcui et al. of presence and association of gold. 1999). 1996. carbonaceous matters and clay has been also proved to be very useful as an analytical tool for minerals. antimony.Mg. 2004). using the prepared affect the rate of leaching (Habashi. suitable pretreatment process is often required to overcome the Murthy et al. (Karimpour... 2. Diagnostic leaching presence of arsenic. The initial rate of gold dissolution is largely controlled by the factors such as cyanide and oxygen concentrations. Angelidis and Kydros. Ni(CN)4 .. 1999). Karimi et al.Al)4O10[(OH)2. the rate of gold leaching in air-saturated solutions increases with increasing concen- Table 1 tration of cyanide but becomes independent of cyanide concentration Mineralogical compositions of two gold samples.4 gold and silver dissolution rate and recovery. Marsden and Feldspar KAlSi3O8 3 Calcite CaCO3 3 Lain House.6 minerals and aluminosilicate have more influence in decreasing of Calcite CaCO3 2.1 essential for the dissolution of gold under normal conditions.2 Malachite Cu2(CO3)(OH)2 0. Such (H2O)] oxidizing reagents as sodium peroxide. Adams.2. minerals. 1999... It has also been refractoriness and render the gold accessible to the lixiviant action of demonstrated that the addition of other oxidizing reagents increases cyanide and oxygen (Ubaldini et al.Fe)2(Si. after which gold and silver can be recovered by standard cyanidation The refractoriness of gold ores can result primarily from the process.075% KCN. Ellis and polished and thin sections. 2000). The chemical reactions that take place during sampling procedure and about 400 kg from each sample were used for the cyanidation of concentrate for ores can be very complex (Luna and experiments..5 bromine.6 Illite (K.. the reactions involved are Two representative samples were supplied by using a systematic not fully understood. Factors affecting gold leaching 2. 1999. gold dissolution kinetic studies were also successfully overcame the effect of pyrite and pyrrhotite on cyanide evaluated by using different oxidizing reagents which was not applied consumption and the gold leaching kinetics.2 extractions (50–80%) within a conventional cyanide leaching process Celestite SrSO4 0. 1994.2 refractory gold ores are often characterized by the low gold Azurite Cu3(CO3)2(OH)2 0. gold ores can be classified as “free Galena PbS 1 milling” and “refractory” depending on their response to cyanide Sphalerite ZnS 0. a major part of the particle and Sr element in the particle of Fig. Karimi et al. arsenopyrite was reported in the optical observed in the sample which is shown in Fig. O.25 −2000 + 840. As Cu 3000 56 and Fe were detected as major parts of particles in the first sample. gangue mineral was SiO2 which was in three different forms namely Fig. Also all mineralogy studies for the first sample which due to low amount of the qualitative data of the SEM analysis are presented in Table 4.5 electron microscope (SEM) method was used and its results are Ag 2. Fe2O3 5. The relationships between gold and sulfide minerals e. CaO 4. Interlocking of gold and sulfide minerals. 2 As 16676. sphalerite. arsenopyrite. performed to define the main and the trace minerals and their due to the presence of the high amounts of Si. hematite. pyrite. Microscopic mineralogy Qualitative analysis by XRF method. The iron oxide amount was 5. Components First Second Components First Second Mineralogical studies were carried out by optical microscope in (wt.5 2.47 0. malachite. arsenopyrite. 1. Also CaO content in the first sample was ten times higher According to the mineralogical studies. feldspar and calcite whereas the minor ones were galena.90 L. Al2O3 17. −840 + 590. In some cases. heavy minerals were analyzed by The samples were analyzed qualitatively by using X-ray fluores- optical microscope and as it can be seen in Fig.60 0.56 −74 + 62. chalcopyrite.g.I 6. Elements (g/t) First sample Second sample For further identification of first sample characterization. because the It is seen that in the first sample. . arsenopyrite. arsenic.86 Na2O b0.10 0. By comparison of mineralogical data for both samples. second one. was not detected in XRD analysis. clay minerals. − 297 + 149.05 in which their results are previously presented in Table 1.47 14. The second sample is categorized as oxide gold ore. 1. clay minerals (illite). Ca and K interlocking.44 PbO 0. it can be gypsum.3 Pb 9000 14 series.06 MnO 0.20 8. −590 + 297. sphalerite. −149 + 74. sphalerite and galena are shown in Fig. feldspar and calcite for the first resulted that the ore characteristics of the first sample are more sample and quartz. 2(D).91 MgO 0.1 160 presented as particles photos and its component histogram in Fig. scanning Au 10. gypsum.34 TiO2 0. hematite. it can be concluded that some than the second one that could cause some difficulties in cyanidation parts of gold is disseminated in sulfide minerals such as arsenopyrite.47% and 14. Titanium and manganese are also found in the first sample which is given in Fig. etc. / International Journal of Mineral Processing 95 (2010) 68–77 Table 2 2.91 1. In addition. chalcopyrite.%) sample sample order to determine gold liberation size and evaluation of various SiO2 59.9 b 0. The analysis of gold. 2(F) and (G). silver. goethite. sophisticated than the other one and high amount of sulfide minerals Microscopic analysis confirmed the existence of the phases detected causes more cyanide consumption. galena. Zn 4000 53 As it can be seen in Fig. calcite and feldspar for the second one.70 P.76 4.5 543.12 0.05 SO3 0. Low grade gold ore (sample2) results are given in Table 3. process. hydroxide such as goethite and lepidocrosite which are presented in whereas the second sample contains silver much higher than the first Fig.1 – K2O 3. components such as Pb. as well.05 68.3. the main one (80 times). 3 which liberated gold cence (XRF) technique in which its result is presented in Table 2. The major phases of minerals were quartz. antimony is identified as mineralogy of the primary two gold samples are presented in Table 1. −62 + 53 and −53 μm by using polished and thin sections P2O5 0.36 mineral interlocking in different size fractions including + 2000. 2(B) and (E) respectively.3.12 0. pyrite. In Fig. tite. gold and arsenic content are main part of its pyrite minerals was changed to iron oxide and much higher than the second one (by 4 and 32 times respectively). porous particles are by XRD method.06% respectively. azurite and celes- Table 3 Elemental analysis of the primary ores samples by ICP-OES technique. The results of the XRD analysis and optical elements in this particle.73 0. After separation by heavy liquid. zinc and copper were carried out by using atomic adsorption. Also the presence of other heavy the amount of K2O and Al2O3 was twice as much compared to the minerals such as galena and celestite were proved by this method.%) sample sample (wt.2. Na.O. Zn.3. Al. inductivity coupled plasma and optical emission spectrometry (ICP-OES) techniques in which 2.10 ZnO 0.20 2. consequently distinguished. Aluminosilicate minerals such as illite and also feldspar and calcite can be distinguished in Fig.43 b 0. particle as nugget and associated gold with sulfide minerals were Due to the presence of feldspar and clay minerals in first sample. 2(F) and (G). According to thin and polished sections studies.1. 2(H). 4. High grade gold ore (sample1) The major minerals were quartz. lead. 2(A) and (C). The cyanidation experiments were carried to pulp by sparger and hydrogen peroxide as oxidizing reagents. (Ika-RW20. In cyanidation.01 M) and potassium iodide (10%) as indicator. titration was employed with standard silver nitrate (0.85 cm diameter. Germany) with a manual controller unit and stainless In addition. P. 2. a pH meter (744 Metrohm) was utilized and hydrated lime and hydrochloric acid (5%) were used to set the pulp In order to prepare and grind samples for cyanidation process. Fig. Material and methods To control pH. 4 also shows out in a 5-l plastic container equipped with the Ika mechanical mixer that gold grains are free and interlocked with quartz matrix. Karimi et al. crystalline. the presence of a high amount of silver in free and steel impeller with a 9. Scanning electron microscope (SEM) to characterize the first sample. . sodium cyanide was the lixiviant with injected air Denver ball mill was used. Also for determination of free as base metal sulfides is a remarkable point of these studies. a pH. 3. / International Journal of Mineral Processing 95 (2010) 68–77 71 Fig. A 20-l filter press and vacuum sulfide forms and also the low content of other sulfide minerals such filter were applied for pulp filtration. cyanide in solution. microcrystalline and cryptocrystalline. Samples with 5 gr weight were D Fe. K. Sinadinovic et al. Karimi et al. were performed. Mg. to determine the best cyanide concentration. . It is Fig. Celep et al. Ca analyzed for arsenic.. K. and 2500 mg/L suitable conditions for two samples of different origins. 3. antimony.1. roasting test in various C Au. Au. Au roasting (each test were duplicated). Si. Fe. Si. Fe.24% gold recoveries were achieved.5. The effect of different grinding times. Fe. with 25% as solid percent. Au. B: replace most of pyrite minerals by goethite and lepidocrosite. 5 and 6. Si. grinding times for two samples were found as 35 and 55 min In order to increase the process recovery and reduce cyanide respectively in which 91. which included acidic leaching by hydrochloric. Fig. Al. 1994. Al. Fe. to reach the sought particle size. 1999. Si. Sb 5. consumption. Sr. sodium cyanide concentration. Ca. Al. Al. were implemented to eliminate the detrimental elements such as arsenic. cadmium. Effect of grinding time Solution and solids were analyzed by atomic adsorptions. (B) unliberated gold associated with sulfide minerals. 4. Fe. Al. bismuth and mercury after E Mn. inductivity coupled plasma and optical emission spectrometry (ICP-OES). As. Si. 5 and 6 show that the most suitable pH and leaching time. Sb. other parameters were kept constant at Cyanidation experiments were conducted to determine the most 4000 mg/L of cyanide concentration for the first sample.. Na. Sb.5 and 24 h.. As. Results 5. Ca. identical at 10. S. Figs. 75 μm were studied. antimony and carbonaceous matter for the Different cyanidation tests with various cyanide concentrations first sample (Ubaldini et al. Figure Qualitative data The second pretreatment was carried out by 200 ml of HNO3 (30%) with the same condition as to the first test. K detrimental effect of sulfide and metal ions. The results obtained from these grinding times 4. Ca. K temperatures and times were studied. S. As. Si. (A) Liberated gold nugget. diagnostic leaching techniques as “pretreatment pro- cedure”. Ca.8% and 86.2. Mg. Na G Fe. Ti. In order to decrease of A Ca. Ca. The primary pretreatment was performed on the 1 kg of first The qualitative data of the SEM. whereas pH and leaching time for both samples were parameters included grinding time. sample by using HCl in 4 h and at pH of 2. In these experiments. A: The fine gold grain associated with quartz about 12 μm. Fe. These for the second one. / International Journal of Mineral Processing 95 (2010) 68–77 Table 4 2009). As H Sr. nitric 5. Na. As. Si. Pb. Apparatus and experimental procedures versus gold recoveries and passing percent are presented in Figs. Cyanide concentration acid and roasting. Si. Zn B Au. Al.72 P. As. F Ca. K. K. selected as best values for the two samples. 1991. pH sample and 2500 to 6500 mg/L for second one. According to the obtained results that are demonstrated in Fig. dissolution rate for the first sample (Yannopoulos. by increasing cyanide concentration from 4000 to 8000 mg/L in first 5. for the first and second samples.5 interval step). a 48 h test was conducted for each sample. 4000 and 2500 mg/L for cyanide concentrations were leaching time. the quantity of free cyanide in pulp was increased and recovery continued its upward Cyanidation tests were conducted at different incremental pHs trend.1 and 92.4. The gold recovery and passing percent versus grinding times for sample 1 and second samples. Fig.5 to 12 (with approximately 0. Karimi et al. noteworthy that within these tests the other parameters were cyanide in pulp (500 to 1000 mg/L) and acceptable recoveries (higher constant at 35 and 55 min for grinding time. pH = 10.31% to 89. The gold recovery versus cyanidation times for two samples (grinding time: 35 Fig. whereas recovery values were reached from 92.82% from 9. 2000 mg/L 55 min for the first and second samples. pH: 10. The gold recovery and passing percent versus grinding times for sample 2 and a combination of air and H2O2 were utilized to increase the gold (pH: 10. The results of cyanide concentration for two gold samples (grinding time: 35 and and 55 min for the first and second samples. The trend of recovery versus pH for two samples (grinding time: 35 and 55 min for the first and second samples. cyanide concentration: 4000 mg/L. 5.5. cyanide concentration: 4000. .5. Samples (10 mL) from the clear solution obtained after vacuum filtration were used for free cyanide determination and a 30 mL sample was provided to analyze gold content. Regarding the standard free the results of these experiments.5. 8. hydrogen peroxide Fig.5 and 24 h for than 88%). cyanidation time: 24 h). for the two samples respectively. in which the highest recoveries were 94.3. 7. cyanide concentration: 4000. (pH: 10. 8 presents and 88. 6.5. cyanidation time: 24 h). 7. and there was a significant inclined rate in first 10 h whereas maximum recovery reached 92% and 90% respectively. 9. The leaching time was 24 h. 2000 mg/L for the first Fig. Guzman Fig. The results are presented in Fig. / International Journal of Mineral Processing 95 (2010) 68–77 73 Fig. cyanidation time: 24 h). cyanidation time: 24 h). 5.45% to 96. pH: 11.5 for the first and second samples). 5. Leaching time To find out the best cyanidation time. P. It is seen that the trend of gold dissolution was almost identical for two samples.1 and 10.92% respectively. Kinetics of gold dissolution (sample 1) Various types of oxidizing reagent such as air.5% in pH 10.5. 9. cyanide concentration: 2000 mg/L.91% in pH 11. (grinding time: 35 min.35. The content of arsenic after roasting in various temperatures and times (initial effects on omitting of followed elements and warrants a good value: 16. . Gold recovery versus different leaching time by H2O2 + air. cyanide concentration: 4000 mg/L. (initial value: 2214 ppm). 2009). pH: 11. but this trend for antimony and cadmium has a little difference which may be relevant to the boiling point of these elements in which their trends are illustrated in Figs. For each experiment. k = 1. the first one is related to actual data which are obtained from experiments. k = 1.2% (Roshan.6. 1997. roasting in 1000 °C at 2 h had adequate Fig. The content of antimony after roasting in various temperatures and times time: 35 min.1). 11. Dunn and Chamberlain. cyanide concentration: 4000 mg/L.. Fig. / International Journal of Mineral Processing 95 (2010) 68–77 Fig. 1999. Gold recovery versus different leaching time by air. bismuth and mercury decreased.1). Pre-treatment experiments (of sample 1) According to the results. (grinding time: 35 min. 2006). two curves are presented. condition for cyanidation process.650 ppm). cyanide concentration: 4000 mg/L. 15. (grinding time: 35 min.75. pH: 11. Celep et al.1). 10–13. Gold recovery versus different leaching times without oxidizing reagent. Fig. (grinding Fig. based on the least square criteria.. cyanide concentration: 4000 mg/L.74 P. pH: 11. 1990. and the second curve is the modeled one.1). pH: 11. 10.65. the amount of arsenic. k = 1. et al. 14.06. k = 1. 14–18. Fig. Marsden and Lain House. using first order equation as y = ymax(1 − exp(−kx)) which was generated by solver function in Excel. It can be concluded that by increasing of roasting temperature and time. The results are shown in Figs. Cyanidation test was performed on the roasted sample in this condition and obtained results presented a considerable decrease of cyanide consumption by 1150 mg/L and increasing recovery by 5. Karimi et al. 13. Gold recovery versus different leaching time by H2O2. 12. 5. two different types of gold samples were investigated in various cyanidation experiments.56% for the 48 h respectively which showed the high effect of air as an oxidizing reagent. the suspended particles were deposited immediately. This phenomenon was due to the fact that more liberated minerals in higher grinding times were achieved and also gold grains surface was more covered by carbonaceous matter. The results of which are Fig. P.5 and 11 for both samples so that it decreased outside of this range.07 Air 1.5 4000 3875 96. Karimi et al.56 H2O2 1. Gold dissolution rate followed by 1st order kinetics presented as y = ymax(1 − exp (−kx)). the free cyanide intensively increased which is followed by a recovery decrease. In addition. the gold recoveries reached 99. 16.06 10. the curve slope in Fig. at pH Fig.65 min− 1).5 and reached to 2250 and 1750 mg/L at pH = 12 for the first and second samples respectively. Discussion In this study. The content of bismuth after roasting in various temperatures and times (initial shown in Table 5. because of the forming of Ca(OH)2 and Mg(OH)2 complexes.5 and 16676. The first sample was a high grade By increasing of grinding time. Increase of cyanide concentration in leaching pulp caused high cyanide consumption due to the presence of cyanicides and did not improve gold recovery. 22. Summary and conclusions 6. The content of mercury after roasting in various temperatures and times (initial value: 1 ppm).56% and 93.56 .5 4000 3540 99.4% for the first 8 h using second sample.75 min− 1). For instance. In addition. 210 and 1150 mg/L for acidic pretreatment and roasting. cyanide consumption decreased by 340. antimony. value: 1. whereas using mixture of air and H2O2 yielded similar results (1. This could be attributed to the formation of other complexes at low pH which caused the gold recovery to be decreased. / International Journal of Mineral Processing 95 (2010) 68–77 75 in 50 min for the first sample. The amount of arsenic.87% respectively and 97.35 10. 75 and 82% on the roasted sample (1000 °C in 2 h). in which k (rate constant) values were obtained at 1.5% after 48 h respectively. which shows the recovery rate.8 ppm). whereas the concentration of free cyanide was increased from 120 to 240 mg/L at pH = 9. bismuth and mercury elements decreased considerably as 88. For leaching time. With increasing pH.03.5 g/t both sample whereas 2910 mg/L in 20 min was changed to 3680 mg/L respectively and the second sample was a low grade ore with 2.5 4000 3510 98. stable complexes with cyanide. 98. 17.75 10.11% and 98. gold recoveries were achieved 92% for the first 10 h for the first sample and 89. the majority of components were not able to make (initial value: 1. the optimum recovery was in the pH of 10. Also at high pH of solution. The introduction of air during leaching was more effective in kinetics rate increase (1.03 H2O2 + air 1. By using of HCl and HNO3 as acidic pretreatments. In addition. Fig. Furthermore. 18.07 and 97. Oxidant Rate constant Rate constant Cyanide concentration Cyanide consumption Gold recovery (min− 1) (min− 1) (mg/L) (mg/L) (%) Without oxidizer 1. cadmium. and clay minerals (illite). Meanwhile the recoveries finally reached to 96. 7. was high at the first 8 h and gradually decreased until 24 h (changes was up to 2%). Hence. gold passivation at higher pH has been reported and a dramatic decrease of gold extraction in sample 2 at higher pH may be related to this. 9.06 and 1. the value of free cyanide was definitely increased. The results of gold recovery and cyanide consumption versus variation of time and temperature of roasting process are presented in Table 6. cyanide consumption increased for ore and its gold and arsenic grades were 10.5 4000 3650 97. Despite the difference in rate.16 min− 1 for the first and second samples.65 10. The content of cadmium after roasting in various temperatures and times higher than 11. Therefore. gold recoveries reached to 95.5 g/t of Table 5 Rate constant (k) of different oxidizing reagents for the first sample. as the flocculants matters. 34. the final recovery value by using H2O2 as an oxidizer was higher than other reagents.05% by roasting of sample in 1000 °C in 2 h. calcite.1 ppm). Putnam..74 2550 and Abedian and Movaghar Consultant. (Ed.D. 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