Improvement of Silver Extraction by Ultra fine Grinding Prior to Cyanide Leaching of the Plant Tailings of a Refractory Silver Ore

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1 Laval University From the SelectedWorks of Ahmet Deniz Bas 2015 Improvement of Silver Extraction by Ultra fine Grinding Prior to Cyanide Leaching of the Plant Tailings of a Refractory Silver Ore Oktay Celep Ahmet Deniz Bas, Laval University Ersin Y Yazici, Karadeniz Technical University Ibrahim Alp, Karadeniz Technical University Haci Deveci, Karadeniz Technical University Available at:

2 This article was downloaded by: [Universite Laval] On: 10 March 2015, At: 06:03 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: Registered office: Mortimer House, Mortimer Street, London W1T 3JH, UK Mineral Processing and Extractive Metallurgy Review: An International Journal Publication details, including instructions for authors and subscription information: Improvement of Silver Extraction by Ultrafine Grinding Prior to Cyanide Leaching of the Plant Tailings of a Refractory Silver Ore Oktay Celep a, A. Deniz Bas a, Ersin Y. Yazici a, İbrahim Alp a & Haci Deveci a a Hydromet B&PM Group, Division of Mineral & Coal Processing, Department of Mining Engineering, Karadeniz Technical University, Trabzon, Turkey Accepted author version posted online: 18 Jun Click for updates To cite this article: Oktay Celep, A. Deniz Bas, Ersin Y. Yazici, İbrahim Alp & Haci Deveci (2015) Improvement of Silver Extraction by Ultrafine Grinding Prior to Cyanide Leaching of the Plant Tailings of a Refractory Silver Ore, Mineral Processing and Extractive Metallurgy Review: An International Journal, 36:4, , DOI: / To link to this article: PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the Content ) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at

3 Mineral Processing & Extractive Metall. Rev., 36: , 2015 Copyright # Taylor & Francis Group, LLC ISSN: print/ online DOI: / Improvement of Silver Extraction by Ultrafine Grinding Prior to Cyanide Leaching of the Plant Tailings of a Refractory Silver Ore OKTAY CELEP, A. DENIZ BAS, ERSIN Y. YAZICI, İBRAHIM ALP, and HACI DEVECI Hydromet B&PM Group, Division of Mineral & Coal Processing, Department of Mining Engineering, Karadeniz Technical University, Trabzon, Turkey Ultrafine grinding (UFG) was proposed as a pretreatment method prior to cyanide leaching of old plant tailings of a refractory silver ore. Direct cyanidation of the as-received tailings (d 80 : 100 mm) led to a low silver extraction of only 43% over 24 h. A size-based diagnostic study has demonstrated that the refractoriness of the tailings is essentially physical in character. Therefore, effects of particle size=ufg (d 99 : 5 75 mm), concentrations of NaCN ( g=l) and Pb(NO 3 ) 2 (0 500 g=t) on the extraction of silver were investigated using a Box Behnken design. The statistical analysis of the experimental data (%Ag extraction at 1 h) revealed that particle size=ufg was the most significant parameter for the extraction of silver, which was substantially improved after UFG. A positive effect of increasing the concentration of NaCN was also observed particularly at finer particle sizes while the effect of concentration of Pb(NO 3 ) 2 was insignificant. Almost complete extraction for silver (i.e., 99%) was demonstrated to be possible at even higher concentrations of cyanide (>2.25 g=l NaCN) when using the finest material (d 99 :5mm) over only 1 h of leaching. Albeit, the consumption of cyanide tended to increase with UFG or increasing the initial concentration of cyanide. The findings showed that UFG can be used as a suitable and environmentally sound pretreatment method to improve the extraction of silver from the refractory silver ore tailings. The diagnostic approach adopted in the current study has proved to be a useful analytical tool to determine the amenability of the ore to ultrafine-grinding as a pretreatment process. Keywords: Box Behnken design, cyanide leaching, refractory, silver, tailings, ultrafine grinding 1. Introduction Cyanide leaching of gold=silver ores has been an industrially accepted process for the treatment of gold=silver ores over a century (Bhappu 1990; Fleming 1992; Marsden and House 2006). Gold and silver are readily dissolved in dilute solutions of cyanide in the presence of air=oxygen. Dissolution reactions for native silver and argentite=acanthite (Ag 2 S) can be expressed by reactions 1 and 2, respectively, (Luna and Lapidus 2000; Senanayake 2006; Zhang et al. 1997) where a stable silver cyanide complex is formed (i.e., AgðCNÞ 2, logk ¼ 20.5) (Senanayake 2006; Zhang et al. 1997). 4Ag þ 8CN þ O 2 þ 2H 2 O! 4AgðCNÞ 2 þ 4OH Ag 2 S þ 4CN þ O 2 þ 0:5H 2 O! 2AgðCNÞ 2 þ 0:5S 2 O 3 2 þ OH Address correspondence to Oktay Celep, Karadeniz Technical University, Department of Mining Engineering, Division of Mineral & Coal Processing, Kanuni Campus, Trabzon, Turkey. ocelep@ktu.edu.tr Color versions of one or more of the figures in the article can be found online at ð1þ ð2þ If sulfide minerals are present in the ore, the addition of lead nitrate at certain levels can enhance gold=silver extraction and reduce cyanide consumption. Lead nitrate decreases the reactivity of sulfide minerals by passivating their surfaces and prevents the formation of an inhibitory layer on the surface of gold=silver (Deschênes et al. 1998; Deschênes et al. 2000). In recent years, owing to the exhaustion of free milling gold=silver ores worldwide, the treatment of old tailings has become increasingly important for the management of these wastes and exploitation of their resource potential with environmental and economic benefits. Plant tailings containing appreciable quantities of gold and silver (e.g., g=t Ag) have been reported to result from conventional cyanidation circuits (rod=ball milling þ leaching) of high grade or refractory ores (Deschênes et al. 2011; La Brooy et al. 1994; Zhou 2010). Refractoriness in gold=silver ores commonly arises from the encapsulation of fine gold=silver particles within the mineral matrix that avoids their contact with the cyanide solution (Bhappu 1990; Celep et al. 2009; Fraser et al. 1991; Fleming 1992; Komnitsas and Pooley 1989; Marsden and House 2006). The literature appears to focus essentially on refractory ores of gold rather than silver. Several pretreatment processes prior to cyanidation such as roasting

4 228 O. Celep et al. (Dunn and Chamberlain 1997), pressure oxidation (Gunyanga et al. 1999), and biooxidation (Ciftci and Akcil 2010; Iglesias and Carranza 1994) have been commercially applied to enhance gold leaching from refractory ores. These pretreatment processes enable the exposure of gold to the action of cyanide by oxidation of encapsulating sulfide minerals (e.g., pyrite and arsenopyrite). Silver minerals (e.g., native silver, pyrargyrite-ag 3 SbS 3, tetrahedrite-cu 12 Sb 4 S 13, and acanthite-ag 2 S) are generally present in hydrothermal veins as associated with base metals (Cu, Zn) sulfides or in oxide zones of ore deposits. Silver coexists with Pb, Zn, Cu, Au, and it happens mainly as fine particles of metallic silver within galena, sphalerite, and chalcopyrite concentrates and in a variety of mineral forms such as tetrahedrite and tennatite (Gasparrini 1984; Bolorunduro 2002). Silver can occur not only as discrete microscopic (visible) silver minerals, but also as submicroscopic (invisible) silver in other minerals. Silver mineralogy is more complex than that of gold, which complicates the metallurgy of silver (Zhou et al. 2009). In fact, silver ores or silver in gold ores often respond poorly to the chemical pretreatment processes leading to low silver extractions compared with gold (Celep et al. 2009; Zhou 2010). Ag-Au alloys or acanthite readily dissolve in cyanide solution when they can be sufficiently exposed, but silver telluride minerals and some silver sulfosalts such as pyrargyrite (Ag 3 SbS 3 ), proustite (Ag 3 AsS 3 ), and stephanite (Ag 5 SbS 4 ) are very slowly dissolved in cyanide solution and hence, they are often regarded as refractory (Bolorunduro 2002; Zhou 2010) (Table 1). Identification of refractoriness of silver ores and their treatment have received no particular attention with most studies being focused on silver-bearing gold ores. Rajala and Deschênes (2009) have reported the exploitation of a proprietary technology called CELP for improved extraction of gold and silver of the Kupol gold=silver ore where silver is present as electrum, acanthite, stephanite, pyrargyrite, proustite, and native silver. In this study, the treatment of the plant tailings of a refractory silver ore with a rather complex mineralogical composition containing native silver, pyrargyrite, tetrahedrite, argentite, and proustite as associated with sulfides and nonsulfides was studied to recover the contained silver. Previous studies (Dinc er 1997) have indicated that extraction of silver from the ore and tailings is low with no significant improvement even after various chemical pretreatment. In the current study, a size-based diagnostic approach for characterization of refractoriness of the tailings was developed. UFG (down to 5 mm) prior to cyanide leaching was evaluated as a potential Table 1. Silver extractions from pure silver minerals during cyanide leach (Bolorunduro 2002) Mineral Composition Silver extraction, % Chlorargyrite AgCl 97 Iodargyrite Agl 99 Argentite Ag 2 S Proustite Ag 3 AsS Pyrargyrite Ag 3 SbS Tehrahedrite Cu 3 SbS pretreatment method. Response surface methodology (i.e., Box Behnken design) was adopted to investigate the main and interactive effects of particle size=ufg (5 75 mm), concentrations of NaCN ( g=l), and Pb(NO 3 ) 2 (0 500 g=t) on the silver extraction (%). Based on the findings in these statistically designed experiments, the influence of even higher concentrations of cyanide (i.e., 5 8 g=l NaCN) was also assessed in separate tests to further improve= maximize the extraction of silver. 2. Material and Methods 2.1 Tailings Sample A sample of the cyanidation plant tailings of a refractory silver ore was used in this study. The chemical composition of the tailings sample was determined by wet chemical analysis methods using ICP ES (Inductively Coupled Plasma Emission Spectroscopy) and XRF analysis was used to determine the major elements (Acme Analytical Laboratories) (Table 2). Silver content was determined by lead collection fire assay fusion-gravimetric finish method (Sevinc 1997). The sample was determined to be rich in silver containing about 133 g=t Ag. The X-ray diffraction pattern of the sample, which was determined using a Rigaku X-ray diffractometer (D=Max-IIIC), revealed the presence of quartz (SiO 2 ) and barite (BaSO 4 ) as the most abundant phases (Figure 1). Earlier mineralogical studies performed on the tailings (Dinc er 1997) and ore (Vıcıl 1982) showed that silver occurs mainly in the form of native silver, pyrargyrite (Ag 3 SbS 3 ), Table 2. Chemical composition of tailings sample Compound % SiO Al 2 O CaO 4.58 Fe 2 O NaO 0.12 Tot C 1.58 Tot S 2.53 K 2 O 1.71 P 2 O TiO Cr 2 O MnO 1.15 MgO 2.49 LOI 9.9 Element g=t Ag 133 Au Cu Pb 1.34% Ni Sb Ba 10.15% Zn 2.08%

5 Improvement of Silver Extraction by UFG Prior to Cyanide Leaching 229 Fig. 1. X-ray diffraction profile of the tailings sample. tetrahedrite (Cu 12 Sb 4 S 13 ), argentite (Ag 2 S), and proustite (Ag 3 AsS 3 ), which are closely associated with and=or encapsulated in other mineral phases, mainly barite, quartz, dolomite, and feldspar (Figure 2). 2.2 Ultrafine Grinding of the Tailings Ultrafine grinding of the tailings was performed using a laboratory scale pin-type vertical stirred mill designed by the authors (Celep et al. 2011a). The optimization of ultrafine grinding of the tailings used in this paper was previously investigated in detail by Celep and Yazici (2013). In the current study, stirred media grinding was carried out to achieve the required particle size of the tailings (d 99 :5,40and75mm). Alumina-based zirconia toughened microgrinding ceramic beads (DMM AZ ) were used as the grinding media. The Al 2 O 3 content, specific gravity and Vickers hardness of the beads were 80%, and 1314 micro, respectively. Particle sizes of the as-received and ground samples were analyzed using Malvern Mastersizer 2000 MU in four replicates. Particle size distribution of the as-received (d 80 : 100 mm) and ground samples is given in Figure 3. particle sizes was performed by hot acid mixture digestion (HCl, HNO 3, HClO 4, and HF) with an atomic absorption spectrophotometer (AAS, Perkin Elmer AAnalyst 400). 2.3 Diagnostic Characterization of Physical Refractoriness High silver content of the tailings can be attributed to the inefficiency of cyanide leaching associated with the refractory nature of the ore (Celep and Yazici 2013). The ore contains refractory minerals including pyrargyrite, tetrahedrite, and proustite, which dissolve slowly in cyanide leaching conditions. Furthermore, silver phases also occur as encapsulated in nonsulfide phases, barite in particular. Therefore, a simple diagnostic approach was developed to characterize the physical refractoriness of the tailings and, hence, ascertain the potential for ultrafine grinding as a pretreatment process. The approach was based on the progressively decreasing the fineness of the tailings from (as-received, d 80 : 100 mm) down to ultrafine sizes (d 80 : mm) and the determination of silver content of the ground tailings products. This allowed the assessment of the degree of physical refractoriness, i.e., the increased availability of silver for extraction. Analysis of silver from the tailings samples prepared at different Fig. 2. Presence and association of silver minerals in ore under ore microscopy (Ba: Barite; A: Argentite; Td: Tetrahedrite; and Pyr: Pyrargyrite) (Vıcıl 1982).

6 230 O. Celep et al. Table 3. Parameters and their corresponding coded and actual levels Symbol Coded levels Parameters Actual Coded Low ( 1) Center (0) High (þ1) Particle size (d 99 ), mm X 1 x 1 5 a 40 b 75 c NaCN concentration, g=l X 2 x Pb(NO 3 ) 2 concentration, g=t X 3 x a d 80 : 1.2 mm; b d 80 :6mm; c d 80 :20mm. Fig. 3. Particle size distribution of the tailings and samples ground by stirred mill. Analyses were performed in five replicates and the mean values were presented. 2.4 Cyanide Leaching Experiments Leaching tests were performed in 1-L glass reactors equipped with pitched-blade turbine impellers using a multi-stirrer experimental set-up. Rotating speed was kept constant at 600 rpm. Top of the reactors were kept covered during the leaching period. All the experiments were carried out at room temperature (25 3 C). Stock solutions of cyanide (5% NaCN) and lead nitrate (5% Pb(NO 3 ) 2 ) were used to prepare leach solutions at different strengths in a final volume of 200 ml. Distilled and deionized water was used for the preparation of all solutions. A required amount of solid sample (60 g) was added to each reactor (200 ml cyanide solution) to achieve a solids to liquid ratio of 30% w=vol corresponding to 23% solids by weight (w=w). ph was monitored at the sampling intervals and controlled by the addition of 5% NaOH solution at ph over the leaching period. The reactors were aerated at 1.8 L=min using air pumps. Samples were taken at predetermined intervals over the leaching period and then centrifuged at 4100 rpm for 5 min. to obtain clear aliquots for the analysis of silver. Silver analysis was carried out using AAS. Free cyanide (CN ) concentration was determined by titrimetric method using a silver nitrate solution (0.02 mol= L) and p-dimethylamino-benzal-rhodanine (0.02% w=w in acetone) as the indicator (Greenberg et al. 1985). The concentration of free cyanide was maintained at the tested levels by the addition of 5% NaCN solution and the consumption of free cyanide was recorded. On the completion of leaching tests, the leach residues were filtered, dried, and finally ground in a tema mill. Then, the metal content of the residues were determined by AAS after hot acid mixture digestion (HCl, HNO 3, HClO 4, and HF) of the residues. Silver extractions were calculated based on the silver content of the leaching residues. 2.5 Experimental Design Response surface methodology (RSM) involves the utilization of mathematical and statistical techniques and allows the evaluation of contribution of several independent parameters (variables) on the response. A Box-Behnken design (BBD), essentially a type of RSM, has three levels (i.e., low ( 1), center (0) and high (þ1) as coded levels) with equally spaced intervals between these levels (Mathews 2005; Montgomery 2001) was adopted to assess the effect of various parameters (i.e., particle size=ufg, concentrations of NaCN and Pb(NO 3 ) 2 ) on the silver extraction in cyanide leaching. Selected variables and their corresponding levels in the cyanide leaching tests are presented in Table 3. Symbols for coded and actual values are shown as x i and X i (i ¼ 1, 2, 3), respectively (Table 3). Minitab (2004) and Design-Expert (2007) softwares were used to perform statistical analysis of the experimental data and to calculate the second-order model coefficients. For the significance test of parameters, P values were used (Montgomery 2001). Simply, the null hypothesis is rejected when a P value smaller than the selected confidence level (e.g., 95%, a ¼ 0.05). This indicates the statistical significance of the parameter tested (Montgomery 2001). 3. Results and Discussion 3.1 Physical Refractoriness of the Tailings Earlier studies by the authors (unpublished data) as well as others (Dinc er 1997) have confirmed poor extraction of silver from the tailings even after various chemical pretreatments. This suggested the encapsulation of silver phases within nonsulfide minerals in the tailings. The diagnostic characterization approach adopted within this study corroborated the physical refractoriness of the tailings (Figure 4). Figure 4 illustrates that the chemical availability of silver increases substantially with finer grinding of the tailings sample. The chemical analysis of the as-received tailings (d 80 : 100 mm) was determined to be only 66 g=t Ag compared to 125 g=t after UFG down to a d 80 of 1.2 mm corresponding to 91% increase in the extraction of silver. These findings indicate that the refractoriness of the tailings (and hence the ore) is physical in nature and UFG as a suitable pretreatment method prior to cyanide leaching may be well exploited to improve silver extraction from the tailings. The adoption of UFG is also more preferable than chemical pre treatment methods from an environmental viewpoint. Notwithstanding this, power consumption and hence

7 Improvement of Silver Extraction by UFG Prior to Cyanide Leaching 231 sulfides (Celep et al. 2011b; Zhang et al. 1997). In contrast, after these initial periods, the extraction of silver from the as-received tailings progressed, albeit at a remarkably slower rate. This difference in the extraction behavior between the as-received and ground samples could be attributed to the increase in the surface area and liberation=reactivity of sulfides after UFG. Accordingly, to eliminate the adverse effect of sulfides on the extraction of silver, the addition of Pb(NO 3 ) 2 was also tested in some detail (Section 3.3). 3.3 Effect of Parameters on Cyanide Leaching Fig. 4. Relationship between silver grade (g=t) and particle size (d 80, mm) of the tailings. treatment costs would significantly increase with increasing the fineness of grind. 3.2 Extraction Kinetics Figure 5 shows the effect of leaching time on the extraction of silver from the as-received tailings (d 80 : 100 mm) and ultrafinely ground sample (d 80 : 1.2 mm, d 99 :5mm). Extraction of silver was observed to occur, to the greatest extent, during an initial period of 1 2 h. After these initial periods, the extraction of silver was almost flattened off. The final extraction of silver was limited to 43% for the as-received tailings, but, it reached up to 86.3% after ultrafine grinding over the leaching period. These findings showed that ultrafine grinding is an appropriate pretreatment method for the tailings ahead of cyanide leaching. It should be noted that, thereafter the initial periods of 1 4 h., a slight decrease in the silver extraction was observed in the cyanide leaching of the ultrafinely ground sample. Such a decrease is often ascribed to the precipitation of silver as Ag 2 S in the presence of soluble Fig. 5. Effect of leaching time on the extraction of silver (1.5 g=l NaCN, ph 10.5, 25 3 C, flow rate of air: 1.8 L=min., 23%w=w). In Table 4, Box-Behnken experimental design for 15 runs including parameter combinations with coded=actual values for each experiment with observed and predicted results (Ag extraction, %) were shown. The predicted values estimated by the regression model were also included. The relative standard deviation of the experimental data was calculated to be 1.29%. The experimental data presented in Table 4 was analyzed in detail to demonstrate the impact and interaction of leaching parameters, i.e., UFG, concentrations of NaCN and addition=dosage of Pb(NO 3 ) Modeling and Statistical Analysis of Data Using the experimental results, a second-order regression model was established (Eq. 3). By the equation (Eq. 3), an unknown response (i.e., Y ¼ Ag extraction (%)) can be estimated for any coded level of particle size (x 1 ), NaCN concentration (x 2 ), and Pb(NO 3 ) 2 concentration (x 3 ). Y ¼ 73: :693 x 1 þ 6:82 x 2 þ 1:415 x 3 9:8329 x 2 1 5:2479 x 2 2 þ 2:30708 x2 3 4:6875 x 1x 2 þ 3:6425 x 1 x 3 0:0725 x 2 x 3 ð3þ The coefficient of multiple determinations (R 2 ) for the model was found to be 0.99, which indicated high predictability of the response (Ag extraction, %) from the independent variables. Figure 6 shows the high correlation between the observed and predicted silver extraction values. The analysis of variance (ANOVA) for the regression model including linear=square=interaction contributions are shown in Table 5. The P value (<0.001) of the regression model shows that the model is statistically significant even at 99.9% (a ¼ 0.001) confidence level (Table 5). The linear (main) and square (quadratic) contributions were found to be significant even at 99% confidence level (i.e., P < a ¼ 0.01) while contribution of interactions was insignificant at 95% confidence level (i.e., P > a ¼ 0.05). This indicates that these parameters are essentially independent of each other. The percent contributions show the relative impact of linear, square and interaction effects with the linear effects being the most dominant (79.8%) (Table 5). Table 6 shows the statistical significance of model terms with their estimated regression coefficients. The linear and quadratic terms of particle size=ufg and NaCN concentration were found to be significant at 95% confidence interval while Pb(NO 3 ) 2 concentration was insignificant. It can be inferred from the absolute values of the regression coefficients (Table 6) that the order of significance of linear (main) effects

8 232 O. Celep et al. Table 4. Experimental layout with observed and predicted results (Ag extraction, %) Levels of parameters Exp. No. Coded Actual x 1 x 2 x 3 UFG (mm) NaCN (g=l) Pb(NO 3 ) 2 (g=t) Ag extraction, % (observed) Ag extraction, % (predicted) þ þ þ1 þ þ þ þ1 0 þ þ þ þ1 þ was particle size=ufg > NaCN concentration > Pb(NO 3 ) 2 dosage. These findings affirm that particle size is the most influential parameter and UFG of the tailings is required for achieving high silver extractions from the tailings. For the verification of the regression model (Eq. 3) further tests apart from Box-Behnken design runs (Table 4) were performed. Experimental conditions for the tests with the observed and the predicted results were presented in Table 7. The relevance of the observed and predicted results for the verification tests was found to be high (Table 7, Figure 6) Response Surface Plots of the Parameters Response surface plots can be used to observe the mode and magnitude of effects of dual parameters. The regression equation (Eq. 3) was used to derive response surface plots to allow the observation of the interaction and dual effects of parameters on the response (i.e., Ag extraction (%)) while the third parameter was held at the centre level (Figure 7). As shown in Figure 7a, reducing the particle size=ufg exerted a profound effect on the extraction of silver particularly at high levels of NaCN concentration. Almost a linear increase in the silver extraction from 47% (d 99 :75mm, Exp. 4) to 87% (d 99 :5mm, Exp. 3) was observed particularly at the high level of NaCN concentration (2.25 g=l NaCN) (Figure 7a). In contrast, at the lowest level of NaCN concentration (i.e., 0.75 g=l), below a critical particle size (i.e., d 99 : 40 mm) only a slight increase in silver extraction was occurred. Figure 7b also illustrates that the silver extraction improved with decreasing particle size irrespective of Pb(NO 3 ) 2 dosage. These finding are consistent with the statistical analysis of the data (Table 6) for the significance of particle size=ufg. In the literature, the studies appeared to focus mainly on gold metallurgy rather than silver and there are many reports showing the positive effect of fine=ultrafine grinding on gold extraction from refractory ores=concentrates=tailings (Corrans and Angove 1991; Davey 2010; Deschênes et al. 2005; Ellis and Gao 2003; Ellis 2003; González-Anaya et al. 2011; Pantchenko et al. 1995). Cyanide leaching of silver phases including pyrargyrite and proustite is reported to be very slow resulting in low extractions. Recently, Rajala and Deschênes (2009) have reported the enhanced extraction of Table 5. Analysis of variance (ANOVA) of the regression model Source Degree of freedom Sum of squares Adjusted mean square P value Contribution (%) Fig. 6. The correlation between observed and predicted silver recoveries (%) values. Regression model Linear Square Interaction Residual Error Total

9 Improvement of Silver Extraction by UFG Prior to Cyanide Leaching 233 Table 6. Regression coefficients and statistical significance test of the model terms Term Symbol Regression coefficient P value Constant b Particle size b NaCN concentration b Pb(NO 3 ) 2 concentration b Particle size particle size b NaCN NaCN b Pb(NO 3 ) 2 Pb(NO 3 ) 2 b Particle size NaCN b Particle size Pb(NO 3 ) 2 b NaCN Pb(NO 3 ) 2 b gold and silver of the Kupol ore containing refractory silver minerals by using CELP, which produces more oxidized mineral surfaces compared with conventional cyanidation. Dissolution of pyrargyrite was reported to even take 120 h allegedly due to the formation of Ag-Sb or Ag 2 S rims, which retards progress of leaching process (Zhou 2010). In this regard, UFG would improve leaching of refractory silver minerals. In effect, the dissolution of silver from the tailings was even faster after UFG (e.g., Figure 5). This suggests that UFG can also be considered as an alternative pretreatment process for those ores containing refractory silver phases with no liberation problems. Consumption of cyanide was observed to increase significantly at finer particle sizes, e.g., 0.71 kg=t (Exp. 2) for the coarse tailings (d 99 :75mm) and 1.42 kg=t (Exp. 1) for the ultrafinely ground tailings (d 99 :5mm) (Table 4). This could be attributed to the increased availability=surface area and even activation of cyanide consuming phases by fine=ufg grinding (Ellis and Gao 2003; González Anaya et al. 2011). It can be inferred that, at fine particle sizes, the concentration of NaCN should be maintained at high levels to achieve high silver extractions. In fact, Figure 7a shows the positive contribution of NaCN concentration to the silver extraction at fine particle sizes, in particular. Deschênes et al. (2011) investigated the effect of cyanide concentration (0.5 2 g=l NaCN) on the extraction of silver and gold from a tailings sample (359.3 g=t Ag, 22.6 g=t Au) produced by gravity separation. The researchers found that increasing the cyanide concentration enhanced the silver extraction, i.e., the silver content in the tailings were 61.6 and 50.5 g=t at 0.5 and 2 g=l NaCN, respectively. The effect of concentration of Pb(NO 3 ) 2 is depicted in Figures 7b c. The statistical analysis of the data showed that the concentration of Pb(NO 3 ) 2 had no significant impact on the extraction of silver and had the lowest contribution to the extraction process (Tables 5 and 6). Pb(NO 3 ) 2 is known to improve gold=silver extraction by inhibiting the reactivity of sulfide minerals or by scavenging the dissolved sulfide from solution and hence, minimizing the formation of a passive sulfide layer on the surface of gold=silver particles (Deschênes et al. 1998; Deschênes et al. 2000). Deschênes et al. (2011) also studied the effect of lead nitrate (0 350 g=t) in cyanide leaching of gold=silver and reported a beneficial effect on gold and silver extraction. The very limited influence of Pb(NO 3 ) 2 observed in the current study (Tables 5 and 6) may be attributed to the relatively low content of sulfide minerals in the tailings (Table 2) in that sulfide minerals may have been leached to some extent in the first cyanidation stage of the feed material Effect of High Concentration of Cyanide In the Box Behnken design runs (Table 4), a maximum silver extraction of 86.5% was obtained (Exp. 3) under the conditions of 5 mm (d 99 ) particle size, 2.25 g=l NaCN and 250 g=t Pb(NO 3 ) 2. The analysis of the experimental data showed that high NaCN concentrations (i.e., 2.25 g=l) in the absence of Pb(NO 3 ) 2 could allow even higher extractions (i.e., 87%) using the finest particle size (d 99 :5mm). Therefore, separate tests were carried out at excess cyanide concentrations of g=l NaCN using the finest tailings (d 99 :5mm) in the absence of Pb(NO 3 ) 2. It was found that almost complete extraction of silver (i.e., 99%) was achieved at >2.25 g=l NaCN over a leaching period of 1 h (Figure 8). A similar time-dependent trend for the extraction of silver to those presented in Figure 5 was also observed at high concentrations of NaCN tested and hence, the leaching profiles were not shown. The increase in the silver extractions at high cyanide concentrations may be attributed to the fact that dissolution rate of native silver and silver minerals are relatively slow and require high concentrations of cyanide (Fleming 1992; Zhou 2010). Fleming (1992) reported that dissolution of argentite (one of the primary silver mineral in the ore, see Section 2.1) in cyanide solutions is more favorable at cyanide concentrations of >1.96 g=l. The consumption of NaCN also significantly increased (i.e., 14.7-fold) with increasing the concentration of NaCN from 1.5 to 8 g=l. At the maximum level of NaCN concentration (8 g=l) tested, NaCN consumption was recorded to be 32 kg=t (Figure 8). These findings suggested that for high silver extractions, high concentrations of NaCN, i.e., >2.25 g=l should be maintained at the expense of high consumption of cyanide. Table 7. Results for the verification tests Exp. No. Coded and actual values (in brackets) Particle size (mm) NaCN concentration (g=l) Pb(NO 3 ) 2 concentration(g=t) Observed Ag recovery (%) Predicted Ag recovery (%) 16 0 (40) 0 (1.5) 1 (0) (40) þ1 (2.25) 0 (250)

10 234 O. Celep et al. Fig. 7. Response surface plots showing the dual effects of parameters on the extraction of Ag (%) (third variable are held at centre level) (a) particle size and NaCN concentration; (b) particle size and Pb(NO 3 ) 2 dosage; and (c) NaCN concentrations and Pb(NO 3 ) 2 dosage Implications for Process Economics The current findings suggest that UFG is the propitious option for recovery of the contained silver from the tailings. It is obvious that the exploitation of UFG depends ultimately on the process economics since power consumption significantly increases with decreasing the particle size (Jankovic 2003). Therefore, the revenue that would be generated by the silver recovered should justify the treatment costs. In fact, this also determines the extent of UFG that can be profitably applied for the treatment of the tailings. In this regard, based on the data generated in this study (1.5 g=l NaCN, d 80 : 1.2 mm, 5 mm, and 20 mm) power costs (10 per kw-h) and cyanide costs (at $2 per kg NaCN) as well as revenues ($21=oz silver) were estimated and presented in Figure 9. Considering the small scale of the current study with the likely overestimation of costs, the power consumption data presented by Jankovic (2003) were also used to generate the Fig. 8. Effect of concentration of NaCN on its consumption and extraction of silver from the tailings (d 99 : 5mm; ph 10.5, 25 3 C, flow rate of air: 1.8 L=min., 23%w=w). Fig. 9. The estimated revenues and costs for power and cyanide as a function of particle size based the data in this study and Jankovic (2003) (1.5 g=l NaCN; d 80 : 1.2 mm, 5 mm and 20 mm; power cost: 10 per kw-h; cyanide costs: $2 per kg NaCN; silver price: $21=oz).

11 Improvement of Silver Extraction by UFG Prior to Cyanide Leaching 235 cost data for comparison. Accordingly, only very fine grinding would not be economic mainly due to the very high power consumption. 4. Conclusions Tailings of cyanide leaching plants of refractory gold=silver ores could contain appreciably high silver values due to low silver extractions (i.e., 80%) in the leaching stage. In this paper, the application of UFG for the pretreatment of a high-grade refractory silver ore tailings (133 g=t Ag) was demonstrated. A size-based diagnostic test was developed for the characterization of physical refractoriness. The test has revealed that the extraction of silver substantially increases with fine grinding. In the leaching tests, which were based on Box Behnken experimental design, the particle size= UFG was identified to be the most important parameter to render the contained silver amenable to the extraction by cyanide as also confirmed by the statistical analysis of data. The extraction of silver, which occurs rapidly reaching plateau over only an hour of leaching, was observed to increase from 43% (for the as-received tailings) to 86.3% after UFG of the tailings (d 80 :1.2mm). However, the consumption of cyanide tends to increase with UFG. Concentration of cyanide was also statistically confirmed to affect the silver extraction and high levels of cyanide (i.e., >2.25 g=l NaCN) was required to achieve almost complete silver extractions. The consumption of cyanide was also determined to substantially increase particularly at high cyanide levels, e.g., 14.7-fold higher at 8g=L NaCN than at 1.5 g=l. The addition of Pb(NO 3 ) 2 did not improve the leaching of silver as its effect was not statistically significant. This study highlights that ultrafine grinding can be exploited as an effective pretreatment method for the treatment of old tailings and ores containing locked silver in nonsulfides or refractory silver minerals such as pyrargyrite, tetrahedrite, and proustite. The diagnostic approach developed within the current study has proved to be a simple and useful tool to characterize the physical refractoriness of ores, concentrates or tailings. Although UFG of the tailings down to 5 mm(d 80 ) was estimated to be economically justified by increasing the recovery of silver, further detailed analysis of process economics based on pilot scale data is required. Acknowledgments The authors would also like to express sincere thanks and appreciation to Dr. Mithat Vıcıl for mineralogical investigation, to Dakot Milling Media (Pty) Ltd. (South Africa) for kindly providing the ceramic microgrinding beads (DMM AZ 20001), and to Yıldızlar Holding for kindly providing the samples. 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