M. D. Turan 1, H. S. Altundoğan 2

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1 J. Cent. South Univ. (013) 0: 6 68 DOI: /s Leaching of copper from chalcopyrite concentrate by using ammonium persulphate in an autoclave: Determination of most suitable impeller type by using response surface methodology M. D. Turan 1, H. S. Altundoğan 1. Department of Metallurgical and Materials Engineering, Fırat University, Elazığ 379, Turkey;. Department of Bioengineering, Fırat University, Elazığ 379, Turkey Central South University Press and Springer Verlag Berlin Heidelberg 013 Abstract: Some effective parameters on the copper extraction from Küre chalcopyrite concentrate were optimized by using response surface methodology (RSM). Experiments designed by RSM were carried out in the presence of ammonium persulfate (APS) and different types of impeller in an autoclave system. Ammonium persulfate concentration and leaching temperature were defined numerically and three types of impellers were defined categorically as independent variables using experimental design software. The optimum condition for copper extraction from the chalcopyrite concentrate is found to be ammonium persulfate concentration of kg/m 3, leaching temperature of K and wheel type of impeller. The proposed model equation using RSM has shown good agreement with the experimental data, with correlation coefficients R and R adj for the model as 0.89 and 0.84, respectively. Key words: copper; chalcopyrite; leaching; response surface methodology; ammonium persulfate; optimization 1 Introduction The most widespread method of producing copper is the smelting of copper sulfide concentrates that have been obtained from the flotation plant. This pyrometallurgical route causes some environmental and economical problems. So, during the pyrometallurgical operation of sulfide concentrates SO x, emission toward the atmosphere causes acid rains. Pyrometallurgical processes also include expensive initial investment cost. Currently, hydrometallurgical techniques have been studying as alternative of pyrometallurgical processes to exceed the topics by researchers. Hydrometallurgical processes offer great potential for treating sulphide concentrates such as chalcopyrite concentrate. On the other hand, leaching of chalcopyrite is difficult and strong oxidant or pressure conditions must be used. These effective conditions are usually provided by using oxygen gas in the autoclave system [1 6], but feeding gas into a pressure autoclave system may be difficult and costly. Persulfate compounds have strong oxidizing effect and been reduced to sulfates after performing the process of oxidation in solution. Alternatively, using persulfate salts may be thought as oxidant in the autoclave system. Another important issue in the leaching studies is the stirring profile. In the leaching experiments, mixture of the concentrate and reagent solution must be stirred effectively to extract the metals from solid materials, for instance, concentrate ores. Especially, chalcopyrite has a high density (~4 000 kg/m 3 ) and it sinks at the bottom of the leaching solution and effective stirring can mostly not be provided. There are a lot studies about the importance of the stirring speed on the hydrometallurgical metal extraction [7 10]. Unfortunately, there is not any study about the effect of impeller type on the metal extraction in agitation leaching systems. In fact, this work is a part of an investigation on copper extraction from chalcopyrite concentrate by using ammonium persulfate in an autoclave. Present work may shed light on the impeller type for metal extraction in the autoclave system. Response surface methods (RSM), known as factorial design, have been frequently used in the hydrometallurgical studies recently. This method has some advantages such as easy optimization, need of less experimental data, to reach results quickly and availability of evaluation statistically. Because of these advantages, the experiments were designed by using RSM. In the present work, during leaching of chalcopyrite concentrate, different types of stirring impeller were studied in the presence of ammonium persulfate. Received date: ; Accepted date: Corresponding author: M. D. Turan; Tel: ; E mail: mdturan@firat.edu.tr

2 J. Cent. South Univ. (013) 0: Experimental design and evaluation of the results were performed via face centered design (FCD). Materials and methods.1 Materials Chalcopyrite concentrate namely Küre concentrate was obtained from Karadeniz Copper Plant, Samsun, Turkey. This concentrate was classified by sieving through 00 mesh. The fraction passed (90 % of total mass) through this sieve was used in all experiments. This fraction was dried in a furnace at K for 1 h and sample was stored in a closed vessel for later use. For the chemical analyses of the chalcopyrite concentrate, sample was dissolved by using microwave assisted acid dissolution technique. After the dissolution, chemical analyses of clear supernatant were carried out by ICP OES (Perkin Elmer Optima 000 DV). Sulphur content of the concentrate was determined gravimetrically [11]. Mineralogical analyses of the concentrate were made by using X ray diffraction system (Shimadzu XRD 6000) and powder diffraction technique. Particle size distribution and surface area measurements of the concentrate were carried out by using laser scattering technique (Malvern Instruments MasterSizer X) and N BET method (Micromeritics ASAP 00), respectively. The chemical used was 98% ammonium persulfate salt (Merck ) without any further purification. Double distilled water was used in all experiments. Extraction of the copper from Küre chalcopyrite concentrate was investigated in the presence of ammonium persulfate by using an autoclave system. Leaching tests in the autoclave system were conducted in a 0.3 L Parr autoclave (Parr Model No. 4561). This autoclave was equipped with a heating mantle, a PID temperature controller, a variable speed stirrer with one impeller mounted on shaft, an internally mounted cooling coil and pressure monitoring system. System parameters such as temperature, stirring speed and pressure were controlled and monitored by CALGrafix software (Parr Inst). Three different types of impellers were used in the experiments. Three impellers have same height and width but different shapes of turbine, anchor and wheel. Impellers are shown in Fig. 1.. Experimental methods In this work, extraction of copper from the chalcopyrite concentrate was investigated to identify the effect of different types of stirring impeller and a few leaching parameters in the pressure autoclave system by using the FCD experimental design layout. For the Fig. 1 Impeller types used in leaching experiments: (a) Anchor; (b) Wheel: (c) Turbine response analyses and optimization of the results, State Ease Ver trial was used with three factors defined as independent variables. Totally 33 experiments were designed with three center points for each impeller. All experiments were accomplished discontinuously. The factors examined and inspection intervals are ammonium persulfate concentration ( kg/m 3 ), leaching temperature ( K) and impeller type (anchor, wheel and turbine). Ammonium persulfate concentration and leaching temperature were defined numerically and the impeller types were defined categorically as variables. Other leaching parameters were kept constant with fullness ratio of the vessel of 50%, leaching time of 90 min, liquid/solid ratio of 10 L/kg and stirring speed of 300 r/min. FCD experimental design array of the investigating range of parameters is given in Table 1. Mixtures of the lixiviant solution and chalcopyrite sample were placed in the autoclave vessel and assembled into investigating impeller. After the temperature reached to desired value, stirring was

3 64 Table 1 Independent variables and their investigation ranges for FCD design Variable APS concentration/ (kg m 3 ) Symbol Value or description A (Numerical) Leaching temperature/k B (Numerical) Impeller type C (Categorical) Anchor, Wheel, Turbine commenced and kept constant at the temperature in the range of ±75.15 K until end of that experiment. At the end of preferred leaching time, the autoclave was rapidly cooled and slurry was filtered through analytical filter paper. Leaching solutions obtained were diluted with double distilled water and analyzed. 3 Results and discussion Ammonium persulfate has active oxygen of 7% when it decomposes in solution to produce active oxygen [1]: (NH 4 ) S O 8 + H O NH 4 (HSO 4 ) + H O (1) Oxidizing leaching reaction of metal sulfurs by the ammonium persulfate may be represented by following reaction: Me x S y + xs O 8 xme + + ys 0 + xso 4 As can be seen from the above reactions, usage of ammonium persulfate salt in the autoclave system seems probable for extraction of copper from the chalcopyrite concentrate instead of oxygen gas. Firstly, some pre experiments were performed to see the availability of using ammonium persulfate instead of oxygen gas into autoclave system. For this purpose, ammonium persulfate concentration and leaching temperature were investigated in the range of kg/m 3 and K, respectively. All of the pre experiments were repeated with three types of impellers. Obtained results show that copper does not pass into the leaching solution at the leaching temperature of K. Previous researchers have also indicated that elemental sulfur layer exists in the course of oxidizing leaching of the sulphide ores and concentrate result of oxidation of the sulfide sulfur shows that elemental sulphur layer covers on the surface of the particulate. Afterwards, the elemental sulfur layer melts over K of leaching temperature and this melted passive layer covers all particulate surfaces [13 15]. On the other hand, results obtained indicate that small quantity of copper ions pass to leaching solution used in low concentration of the ammonium persulfate, () J. Cent. South Univ. (013) 0: 6 68 and when it is used over 300 kg/m 3 of concentration, ammonium persulfate is inefficient in the copper extraction. In the light of data obtained from pre experiments, study range of examining parameters were narrowed and utilized from response surface methods to optimize the effect of impeller type on the result. 3.1 Characterization of chalcopyrite concentrate A series of studies were examined to characterize the Küre chalcopyrite concentrate. Firstly, chemical composition of the chalcopyrite concentrate was determined, as given in Table. For the chemical analyses of the chalcopyrite concentrate, samples were dissolved by using microwave assisted acid dissolution technique. Table Chemical compositions of Küre chalcopyrite concentrate Element w/% Cu.0 Fe 8.85 Al 0.5 Mn 0.44 K 1.4 Pb 1.44 S 8.01 Ni Co LOI* 4.88 * Loss of ignition for K. According to results of the X ray analysis, chalcopyrite (CuFeS ), pyrite (FeS ) and α S of mineralogical phases were identified in the Küre chalcopyrite concentrate (Fig. ). Fig. XRD patterns of Küre chalcopyrite concentrate

4 J. Cent. South Univ. (013) 0: Particle size distribution and specific surface area were determined for physicochemical characterization of the chalcopyrite concentrate. The particle size distribution of concentrate is presented in Fig. 3 and the obtained data of particle distribution with surface area result are presented in Table 3. The measurements show that average particle size of chalcopyrite concentrate is µm and 90% of the concentrate particle is smaller than 3.7 µm and 10% of it is smaller than 3.68 µm. Particle size distribution data indicate that chalcopyrite concentrate, supplied from flotation plant, does not need to grind for use and has also small enough particle size for leaching studies. expert software. ANOVA results are presented in Table 4. ANOVA results show that R and R adj correlation coefficients for quadratic model are evaluated quite satisfactorily as 0.89 and 0.84, respectively. The Prob>F values less than 0.05 indicate that the model terms are significant, whereas bigger ones are usually considered as insignificant. Table 4 shows that all terms are significant on the response. According these results, all terms affect linearly on the response. In addition to these, ammonium persulfate concentration and leaching temperature have quadratic effect on the result (A, B ). This means that variations of the leaching temperature and APS concentration are reflected to square degree on the result. According to the experimental results, the approximating functions of copper extraction for each impeller are presented as following equations. Anchor: η(cu)= A+3.780B A 0.017B AB (4) Wheel: η(cu)= A+3.877B A 0.017B AB (5) Turbine: η(cu)= A+4.093B A 0.017B AB (6) Fig. 3 Particle size distributions of chalcopyrite concentrate Table 3 Particle size distribution and N BET surface area of chalcopyrite concentrate d(0.10)/µm d(0.50)/µm d(0.90)/µm Surface area/ (m kg 1 ) Response analysis RSM helps to optimize the process, influenced by number of operating parameters with a minimum number of experiments as well as to analyze the interaction between the parameters. In the RSM studies, the responses can be simply related to chosen factors by linear or quadratic models. A quadratic model, which also includes the linear model, is given as k η = β 0 + j = j β x j + 1 j = k k β jj x j + 1 i j = β x i x j + e i ij (3) where η is the response, x i and x j are variables, β 0 is the constant coefficient, β j β jj and β ij are interaction coefficients of linear, quadratic and the second order terms, respectively, and e i is the error [16]. The runs were designed in accordance with face centered design. Results were evaluated by aid of design Table 4 Analysis of variance (ANOVA) results of quadratic model for copper extraction Source Sum of squares Degree of freedom Mean square F value Prob>F Model < A < B C < A B AB AC < BC < Residual R = 0.89; R = 0.84; Std. Dev. = 7.04; Adeq. Precision = 15.50; A: APS adj concentration; B: Leaching temperature; C: Impeller type. The effect of experimental factors can be visualized on the copper extraction in terms of the model adequacy by using Eq. (3). Some response analysis data and the model adequacy check are important parts of response analysis procedure. Figure 4 shows the studentized residuals and normal percentage probability plot of the copper extraction from the chalcopyrite concentrate. In Fig. 4, residuals exhibit normal distribution.

5 66 J. Cent. South Univ. (013) 0: Optimization The effects of the different parameters on the copper extraction in the autoclave system were examined and the results were optimized by using design expert software. Maximum amount of copper in solution was aimed to determine the optimum conditions. The response surface graphs of the copper extraction are shown in Fig. 6. These graphs demonstrate interaction of the parameters and effect on the response. It is shown that the extraction rates of copper for all types of impellers decrease with the increase in the leaching Fig. 4 Normal probability versus studentized residual On the other hand, Box Cox plot indicates that studies have been performed whether in coefficient interval or not. The Box Cox plot is a tool to help analyzer determine the most appropriate power transformation to apply to response data. Most data transformations can be described by the power function: σ=f(µα) (7) where σ is the standard deviation, µ is the mean and α is the power. Also, one of the axis in the Box Cox graph, λ, is equal to 1 α in all cases. Figure 5 presented Box Cox plot where the dashed line shows the current transformation. In this case, it points to a value of 1 for λ, which symbolizes the power applied to response values. The value of λ of 1 indicates no transformation. The axial bold line indicates the best λ value, while the double lines demonstrate the 95% of confidence interval surrounding it. This plot represents that experimental studies have been performed 95% of coefficient interval and not necessary to transformation. Fig. 5 Box Cox plot for leaching experiments (SS: Sum of square, C.I: Confidence interval) Fig. 6 Effect of impeller type on copper extraction yields: (a) Anchor (b) Wheel (c) Turbine (Leaching time: 90 min stirring speed: 300 r/min, liquid to solid ratio: 10 L/kg fullness ratio of vessel: 50%)

6 J. Cent. South Univ. (013) 0: temperature. This evident dropping of the copper extraction, especially beyond K, is due to the formation of a liquid sulfur layer which covers the surface of particle based on reaction (Eq. ()) and inhibits further copper extraction [13]. The maximum predicted value is indicated by the surface confined at the smallest ellipse in the contour diagram. Elliptical contours are obtained when there is a perfect interaction between the independent variables [17]. In Fig. 6, confined elliptic contours are shown, where APS concentration and leaching temperature reach optimum regions against copper extraction for all impeller types. Moreover, as can be seen from response surface graphs, wheel impeller is more appropriate in the autoclave system for copper extraction from the chalcopyrite concentrate. The copper extraction from the chalcopyrite concentrate is around 35% 40% under the condition used in anchor and turbine, whereas copper extraction is possible to reach around 65% by using wheel type impeller. This is probably related to the shape of impeller so that wheel impeller may be stirred more effectively for the mixture of the leaching solution chalcopyrite in the autoclave system via operative wings of the wheel impeller. All of the experimental studies were performed in 50% fullness ratio of the total vessel volume. This fullness ratio is necessary to stir effectively 0.15 L of the mixture of leaching solution in the autoclave system so that operative wings are considered as wheel type impeller, which ensures important advantage for copper extraction. The two factor interaction plot of copper extraction is shown in Fig. 7. This figure represents the effect on the response by using impeller types and leaching temperature under condition of 60.0 kg/m 3 of APS concentration. Figure 7 indicates that wheel impeller is more effective on the result in these experimental conditions. Optimum leaching conditions with model validation to maximize the copper extraction were identified by using Design Expert software. Table 5 indicates the optimum conditions to be ammonium persulfate concentration of kg/m 3, leaching temperature of K and wheel type of impeller, which results in the copper extraction of 63.86%. This also reports that in copper extraction, anchor and turbine type impellers are less used. Table 5 Optimum leaching conditions with model validation APS concentration/ (kg m 3 ) Leaching temperature/ K Impeller type Cu extraction/% Predicted Experimental Wheel Anchor Turbine Conclusions 1) The proposed quadratic model agrees well with experimental data, with correlation coefficients R and R adj of 0.89 and 0.84, respectively. ) Ammonium persulfate concentration, leaching temperature and all of the stirrer impeller types are found to have significant effects on the copper extraction. 3) Optimum conditions for maximum copper extraction in the autoclave system are identified to be an ammonium persulfate concentration of kg/m 3, a leaching temperature of K, and wheel type impeller with the copper extraction of 63.86%. 4) The response surface methodology can be used to optimize non numeric categorical parameters besides the other numerical parameters that have significant effects on the results. Acknowledgements This work was supported by the TUBITAK (Scientific and Technological Research Council of Turkey) under the Project No: 106M177. References Fig. 7 Two factor interaction plot for copper extraction by using various impeller types [1] PADILLA R, PAVEZ P, RUIZ M C. Kinetics of copper dissolution from sulfidized chalcopyrite at high pressure in H SO 4 O [J]. Hydrometallurgy, 008, 91: [] McDONALD R G, MUIR D M. Pressure oxidation leaching of chalcopyrite. Part I: Comparison of high and low temperature reaction kinetics and products [J]. Hydrometallurgy, 007, 86: [3] McDONALD R G, MUIR D M. Pressure oxidation leaching of chalcopyrite. Part II: Comparison of medium temperature kinetics and products and effect of chloride ion [J]. Hydrometallurgy, 007,

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