Kinetics of Oxidative Roasting of Complex Copper Concentrate

Transcription

1 Materials Transactions, Vol. 49, No. 5 (28) pp to 1198 #28 The Japan Institute of Metals XPRSS RGULAR AICL Kinetics of Oxidative Roasting of Complex Copper Concentrate Byung-Su Kim 1; *, un-young Kim 2, Chi-Kwon Kim 1, Hoo-In Lee 1 and Jeong-Soo Sohn 1 1 Minerals & Materials Processing Division, Korea Institute of Geoscience & Mineral Resources, Daejeon, Korea 2 Department of Resources Recycling, University of Science and Technology, Daejeon, Korea As copper concentrates are progressively becoming more complex and low in grades, it is meaningful to remove sulfur from complex copper concentrates for smelting them by a carbon reduction process. In the present work, a kinetic study on the oxidative roasting of complex copper concentrate was experimentally investigated under nonisothermal condition in air using TGA equipment. Nonisothermal experiments were carried out at various linear heating rates up to 1123 K. Intermediates formed in each stage of the oxidative roasting of the complex copper concentrate were identified. After the first weight loss step, sulfate compounds were mainly formed in the second stage, and about 55% of sulfur contained in the concentrate was removed. In the third stage, the sulfur removal reaction was carried out, the rest of sulfur was nearly removed in this stage. Kinetics of the third stage were analyzed from the dynamic TGA data by means of Coats and Redfern equation. The nucleation and growth model yielded a satisfactory fit to these experimental data. [doi:1.232/matertrans.mr27311] (Received December 5, 27; Accepted February 27, 28; Published April 25, 28) Keywords: complex copper concentrate, oxidative roasting, copper, nucleation and growth model 1. Introduction Copper concentrates involving copper as well as small amounts of precious metals such as gold, silver and so on are becoming more and more complex and low grade since the sources of clean deposits are progressively depleted. Thus, in the recent, a carbon reduction process for smelting valuable metals like copper and precious metals from such complex and low grade copper concentrates becomes to be worthy of consideration. 1) The carbon reduction process contains the removal of sulfur by oxidative roasting, followed by smelting of the dead-roasted concentrate in a furnace, i.e. the process treats the dead-roasted copper concentrates to produce blister copper without producing and converting matte. Thus, the process allows great flexibility with respect to the selection of copper ore types, and do not impose limits on the minimum quantities of contained base copper metal or sulfur. In addition, the process can be applied for treating several copper containing secondary sources like waste printed circuit boards, byproducts from smelters and refiners, and industrial and consumer recyclable products. Figure 1 shows the flow sheet of the carbon reduction process that largely consists of oxidative roasting and reduction processes. Many researches for the oxidative roasting of copper concentrates were performed. 2 5) But, in spite of many researches related to the oxidative roasting of copper concentrates, sufficient fundamental kinetic data on the oxidative roasting of complex and low grade copper concentrates are not available. Therefore, in the present work, a kinetic study on the oxidative roasting of complex copper concentrate was experimentally investigated under nonisothermal condition in air using TGA equipment. 2. xperimental 2.1 Material A natural complex copper concentrate was obtained from Mankayan mine in Benguet, Philippine, which mainly *Corresponding author, -mail: bskim@kigam.re.kr contains about 13.4% Cu, 28.3% Zn, 11.9% Fe and 3.7% S. Table 1 shows chemical composition of the complex copper concentrate used in the study. The sample was grinded under 2 mesh by ball mill. Figure 2 presents the XRD pattern of the complex copper concentrate. The XRD analysis found metals like copper, zinc, lead, and iron in the complex copper concentrate are present as sulfides such as CuFeS 2, ZnS, PbS, and FeS, respectively. 2.2 Thermogravimetric analysis xperiments on the oxidative roasting of complex copper concentrate were carried out in a typical thermogravimetic analysis (TGA) unit. Figure 3 shows a schematic diagram of the apparatus. The apparatus consisted of a recording microbalance from one arm of which a shallow silica tray for holding complex copper concentrate was suspended by a platinum chain into a reactor tube located within vertical tubular furnace. The microbalance continuously recorded the weight changes taking place during the reaction. The reactor tube was an Inconel tube of 5. cm i.d. and 65 cm length. By means of a gas delivery system, mass flow controllers of air and nitrogen were also used. The exit gas was cleaned by bubbling it through a 1% hydrogen peroxide solution in water to remove produced sulfur dioxide gas before being discharged. The gas temperature in the reactor was measured by chromel-alumel thermocouples. A uniform temperature profile of 4 K was achieved over 2 cm length of the reaction tube. The experiments were conducted at four heating rates of 2, 4, 6, 8 K/min from room temperature to 1123 K. A sample weight of about 6 mg (5 mg) of complex copper concentrate was used for each run. During heating, a steady flow of 1 L/min of dry air was maintained through the reactor tube, the microbalance protected from sulfur dioxide and hot gases by flushing it with a dry nitrogen gas of 2 L/min. The morphological characterization of the samples was performed using a scanning electron microscope (JSM- 638LV, JOL Ltd, Tokyo, Japan) equipped with an energy dispersive X-ray spectrometer (Link Isis 3., Oxford Instrument plc, Oxon, U.K). The XRD patterns were obtained

2 Kinetics of Oxidative Roasting of Complex Copper Concentrate 1193 Fig. 1 Flow sheet of the carbon reduction process. Table 1 Chemical composition of the natural complex copper concentrate used in the study. (mass%) Cu Zn Fe Pb Au Ag SiO 2 S ppm 145 ppm Fig. 2 XRD pattern of the complex copper concentrate obtained from Mankayan mine in Benguet, Philippine. using a X-ray diffractometer (Rigaku D-max-25PC, Rigaku/MSC, Inc., TX, U.S.A) with Cu K radiation ( ¼ :154 nm) operated at 4 kv and 3 ma. Samples before and after the oxidative roasting were analyzed for Fe, Si, and S by wet chemistry and for Cu, Zn, and Pb by the inductively coupled plasma (ICP) method (JY-38 plus, Horiba Ltd, Kyoto, Japan). 3. xperimental Results Figure 4 shows the TGA curve of the oxidative roasting of complex copper concentrate obtained at a heating rate of 4 K/min under dry air atmosphere. The oxidative roasting pattern obtained under other heating rates was similar except for the usual increase in the observed reaction temperatures with increase in heating rate. As shown in Fig. 4, the first weight loss of about 2% occurred below about 68 K. The reason was not detail investigated because of the complication of the natural complex copper concentrate used in the study. It was only considered that one of the reason might be the formation of CuO and CuSO 4 CuO compounds prior to that of CuSO 4 from CuS contained in the complex copper concentrate. 6) After the first weight loss step, the weight increase of about 2.2% occurred below about 988 K. During the weight increase step, the sulfur removal ratio was approximately 55%. In this step, the weight increase might be due to sulfate compounds such as ZnSO 4, CuSO 4, PbSO 4, Fe 2 (SO 4 ) 3, and FeSO 4 formed prior to the formation of metal oxides like ZnO, CuO, PbO, and Fe 2 O 3, even though lots of sulfur is removed. The phenomenon was investigated by XRD analysis of samples before and after heating up to 988 K and calculation of the equilibrium composition of solid phase for the step at the temperature range. Figure 5 presents the XRD pattern of a sample after heating up to 988 K. As shown in Fig. 2, sulfide compounds such as CuFeS 2, ZnS, PbS, and FeS were detected at the complex copper concentrate before heating up to 988 K, while as shown in Fig. 5, sulfate compounds such as ZnSO 4, CuSO 4, and PbSO 4 and Fe 2 O 3 were mainly detected at a sample after heating up 988 K. However, other metal oxides like Zn, Cu, and Pb except for iron oxide were not detected at

3 1194 B.-S. Kim,. Kim, C.-K. Kim, H.-I. Lee and J.-S. Sohn Fig. 3 Schematic diagram of the experimental apparatus for TGA thermal analysis. 1. Micro balance, 2. Data collector, 3. Pressure display, 4. xhaust gases, 5. Mass flow controller, 6. Thermocouple, 7. Sample tray, 8. Temperature controller, 9. Reactor tube, 1. Furnace, 11. Ceramic ball Weight (mg) Temperature (K) Fig. 4 TGA curve of oxidative roasting of complex copper concentrate at a heating rate of 4 K/min in air atmosphere. the sample after heating up 988 K as shown in Fig. 5. This might be the reason that the amounts of the metal oxides produced at the temperature range are relatively smaller than those of their sulfate compounds. Also, iron sulfates like Fe 2 (SO 4 ) 3 and FeSO 4 were not detected because those are decomposed to iron oxide easily, as shown in Fig. 6 which will be explained. Figure 6 presents the equilibrium composition of solid phase calculated for the reaction of complex copper concentrate and air at the temperature range of 723 to 123 K. The equilibrium composition at each temperature was calculated using HSC Chemistry 5.1(A. Roine, Outokumpu, 22) version. The input data are given in Table 2, among which the input amount is based on the chemical composition shown in Table 1. The activities of Fig. 5 XRD pattern of the product formed after heating up to 988 K at a heating rate of 4 K/min in air atmosphere. all species considered were assumed to be unity because it is almost impossible to know the accuracy activity of each species present in the complex copper concentrate. Figure 6 indicates that ZnSO 4, CuSO 4, and PbSO 4 and Fe 2 O 3 are mainly present in the solid phase below about 9 K and as the temperature increases over 9 K, the degree of decomposition of the sulfate compounds progressively become large. Specially, it was shown in the figure that iron sulfates like Fe 2 (SO 4 ) 3 and FeSO 4 are easily decomposed at even low temperature. Figure 7 was also obtained from Fig. 6, which presents the weight change of solid phase calculated from the weight change of sulfur and oxygen occurred from the reaction of complex copper concentrate and air at the temperature range of 723 to

4 Kinetics of Oxidative Roasting of Complex Copper Concentrate ZnSO 4 ZnO Log X (kmol) -1-2 CuSO 4 Fe 2 O 3 PbSO 4 CuO*CuSO 4 Fe 2 (SO 4 ) 3 CuFeO 2 CuO ZnFe 2 O 4 Fe 2 ZnO 4 CuO*Fe 2 O 3 Weight change (%) Decrease effect Increase effect 975 K FeSO 4 Cu 2 O Temprature (K) Fig. 6 quilibrium composition of solid phase calculated from the reaction of complex copper concentrate and air using data in Table Temperature (K) Fig. 7 Weight change of solid phase calculated from the reaction of complex copper concentrate and air using data in Table 2. Table 2 Input data and chemical species considered in the calculation of equilibrium composition. Species Input (kmol) Species Input (kmol) N 2 (g) Fe 2 O 3 CuO(g) Fe 3 O 4 CuS(g) FeSO 4 Cu 2 S(g) Fe 2 (SO 4 ) 3 FeO(g) Fe 2 ZnO 4 FeS(g) PbO O 2 (g) PbO 2 Pb(g) Pb 2 O 3 PbO(g) Pb 3 O 4 Pb 2 O 3 (g) PbO PbSO 4 PbS(g) 2 PbO PbSO 4 PbS 2 (g) PbSO 4 Pb 2 S 2 (g) PbS 2 O 3 S(g) PbS 3 O 6 S 2 (g) ZnFe 2 O 4 SO 2 (g) ZnO SO 3 (g) ZnO 2ZnSO 4 Zn(g) ZnSO 4 ZnO(g) CuFeS 2 ZnS(g) Cu 5 FeS 4 CuFeO 2 CuS.24 CuFe 2 O 4 Cu 2 S CuO FeS.24 Cu 2 O FeS 2 CuO CuSO 4 PbS.3 CuO Fe 2 O 3 ZnS.49 Cu 2 O Fe 2 O 3 Cu CuSO 4 Fe Cu 2 SO 4 Pb FeO Zn 123 K. Here, solid line indicates the weight increase effect caused by the formation reactions of sulfate compounds like ZnSO 4, CuSO 4, and PbSO 4, FeSO 4, and Fe 2 (SO 4 ) 3 and dot line does the weight loss effect caused by those of metal oxides like ZnO, CuO, PbO, and Fe 2 O 3. It is shown in Fig. 7 that the weight increase effect caused by the formation reactions of sulfates equals to the weight loss effect caused by those of metal oxides around 975 K. Thus, based on these analyses, it was indirectly verified that the phenomenon of weight increase for the reaction of complex copper concentrate and air happens at the temperature range of 68 to 988 K. On the other hand, it was investigated that the rate of weight increase is relatively slow at the temperature between 798 K and 928 K in the weight increase step. This might be explained by the fact that sulfate compounds like ZnSO 4, CuSO 4, and PbSO 4 is produced from the reaction of complex copper concentrate and air, which has lager volume than that of sulfide compounds like CuFeS 2, ZnS, and PbS contained in the natural complex copper concentrate. The phenomenon was indirectly verified by scanning electron microscopy (SM) analysis of samples before and after heating up to 928 K. Figure 8 presents the SM pictures. Figure 8(a) showing the complex copper concentrate particles before reacting indicates that the surface of the particles is impervious. After reacting at 928 K, the product particles are shown on the surface of the complex copper concentrate particles as shown in Fig. 8(b). This might be the reason that sulfate compounds were formed on the surface of the natural complex copper concentrate particles. After the second weight increase step, the weight loss occurred with increase in the oxidative reaction temperature in the third step, as shown in Fig. 4. This is because the third step is mainly sulfur removal process. Thus, XRD pattern of metal oxides was only detected at samples after heating up to 1123 K. For an example, Fig. 9 shows the XRD pattern of a sample after heating up to 1123 K at a heating rate of 2 K/min. Therefore, it was investigated in the oxidative roasting of the natural complex copper concentrate under air condition that sulfate compounds like ZnSO 4, CuSO 4, and PbSO 4 and Fe 2 O 3 as intermediate compounds were mainly formed up to 988 K, and after the step the weight loss occurred is mainly caused by the removal of sulfur.

5 1196 B.-S. Kim,. Kim, C.-K. Kim, H.-I. Lee and J.-S. Sohn (a) (b) Fig. 8 SM images of samples before (a) and after (b) heating up to 928 K. 1. In the third stage Removal ratio of sulfur, α K/min 4 K/min 6 K/min 8 K/min Temperature (K) Fig. 9 XRD pattern of the product formed after heating up to 1123 K at a heating rate of 2 K/min. Fig. 1 Changes in the removal ratio of sulfur from the complex copper concentrate with four different heating rates as a function of temperature. 4. Kinetic Analysis of Thermal Data Kinetic analysis of the thermal data obtained from the reaction of oxidative roasting of the natural complex copper concentrate at the temperature range of 988 to 1123 K were done in order to interpret the mechanism of the reaction and to deduce the kinetic parameters. Figure 1 shows changes in the removal ratio of sulfur from the complex copper concentrate with four different heating rates as a function of temperature. The removal ratio of sulfur from the complex copper concentrate at a particular temperature during the oxidative roasting was determined by multiplying the weight change ratio of the solid sample up to that temperature and the total removal ratio of sulfur at finishing the oxidative roasting. In the present study, the removal ratio of sulfur between.1 and.9 was investigated for the kinetic analysis of the reaction. In general, kinetic parameters can be obtained employing either a model fitting method or an isoconversional (modelfree) method. But, the model fitting method is found to be better one compared to that of model-free method because the model-free method employs much generalized rate expression and cannot predict the mechanism of the reaction. Thus, in the present study, kinetic parameters were evaluated employing model fitting method. 7,8) The rate of the reaction under nonisothermal condition can be expressed by the following relation: 7 9) d dt ¼ kðtþ f ðþ ð1þ where is the removal ratio of sulfur at temperature T, f ðþ the conversion (the removal ratio of sulfur) function which is dependent on the mechanism of the reaction, the rate of heating employed in the experiment, and kðtþ is the rate constant as a function of temperature. quation (1) can be represented by its integral form as follows: 7 9) Z Z d T gðþ ¼ f ðþ ¼ A exp If ¼ x ) gðþ ¼A R Z e X x x 2 dx ¼ A R dt; Z 1 x pðxþdx ð2þ where A is the pre-exponential factor (frequency factor), the activation energy, R the gas constant, and pðxþ is the temperature integral. The algebraic expression of the integral gðþ functions that are tested in this work are listed in Table 3. These expressions are generally applied for the

6 Kinetics of Oxidative Roasting of Complex Copper Concentrate 1197 Table 3 List of the rate expressions of reaction models used in the study. Model Integral form gðþ ¼kt Nucleation and growth models Power law (P2) 1=2 Power law (P3) 1=3 Avarami-rofe ev eq. 1 (A2) ½ lnð1 ÞŠ 1=2 Avarami-rofe ev eq. 2 (A3) ½ lnð1 ÞŠ 1=3 Avarami-rofe ev eq. 3 (A4) ½ lnð1 ÞŠ 1=4 Geometrical contraction models Contracting area (R2) ½1 ð1 ÞŠ 1=2 Contracting area (R3) ½1 ð1 ÞŠ 1=3 Diffusion models 1D Diffusion (D1) 2 2D Diffusion (D2) ½ð1 Þ lnð1 ÞŠ þ 3D Diffusion Jander eq. (D3) ½1 ð1 Þ 1=3 Š 2 Ginstling-Brounshtein (D4) ð1 2=3Þ ð1 Þ 2=3 Reaction-order models Zero-order (F/R1) First-order (F1) lnð1 Þ Second-order (F2) ð1 Þ 1 1 Third-order (F3) :5½ð1 Þ 2 1Š kinetic analysis of solid state reactions and encompass most common mechanism. However, quation (2) has no analytic solution, and thus, several approximations were made. One of the most widely used approximations has been given by Coats and Redfern approximation which gives the expression: 7 9) ln gðþ ¼ ln AR T 2 2R T If : 1 ) ln gðþ T 2 1 2R T ¼ ln AR where T is the mean experimental temperature. Thus, quation (3) was used to analyze the thermal data shown in Fig. 1. It is clear from eq. (3) that a plot of lnðgðþ=t 2 Þ versus 1/T gives a straight line when the correct gðþ function is chosen. As explained previously, the gðþ function describes the mechanism of the reaction. The best fit for the thermal data obtained from the reaction of oxidative roasting of the natural complex copper concentrate at the temperature range of 988 to 1123 K is obtained using nucleation and growth model (A3) with considering all models listed in Table 3, which is shown in Fig. 11. Figure 11 shows plots of lnf½ lnð1 ÞŠ 1=3 =T 2 g versus 1/T using the thermal data of Fig. 1 according to eq. (3). It was thus verified that the nucleation and growth model was applicable as shown in Fig. 11. From Fig. 11, straight lines with high-correlation coefficient (r > :98) were selected to represent the possible controlling mechanism. Here, the activation energy can be calculated from the slope, the frequency factor from the intercept. The corresponding kinetic parameters are shown in Table 4. Theoretically, the activation energy of a reaction is expected not to change with change in heating rate. But, the activation energy was found to decrease with increase in heating rate, as shown in Table 4. Similarly the frequency factor was also found to be change with heating rate. Similar trends at nonisotermal ð3þ ln {[-ln(1-α)] 1/3 /T 2 } Heating rate, 4 K/min =61.9 kj/mol T -1 / 1-4 K -1 condition have been reported in literatures. 8,1) As shown in Table 4, the activation energy of the reaction of the removal of sulfur was calculated to be kj/mol, which is higher than 38 kj/mol obtained under isothermal condition. 11) The reason for the somewhat higher activation energy of the reaction was not known at this stage of our research. However, it was considered that the higher activation might be the characterization difference of natural complex copper concentrate used. 5. Conclusion Heating rate, 2 K/min = 68.6 kj/mol Heating rate, 6 K/min =59.4 kj/mol Heating rate, 8K/min = 55.8 kj/mol Fig. 11 Plots of lnf½ lnð1 ÞŠ 1=3 =T 2 g versus 1/T using the thermal data of Fig. 1 according to eq. (3). Table 4 Kinetic parameters of the reaction for the oxidative roasting of natural complex copper concentrate in air under nonisothermal condition. Heating rate (K/min) (kj/mol) log A/min A kinetic study on the reaction of the oxidative roasting of natural complex copper concentrate was experimentally investigated under nonisothermal condition in air atmosphere using TGA equipment. The experiments were carried out at four heating rates of 2, 4, 6, 8 K/min from room temperature to 1123 K. It was found that the reaction of the oxidative roasting of natural complex copper concentrate was largely divided by three stages. In the first stage, the weight loss of about 2% occurred below about 68 K. In the second stage, sulfate compounds like ZnSO 4, CuSO 4, and PbSO 4 and Fe 2 O 3 as intermediate compounds were mainly formed, and about 55% of sulfur contained in the complex copper concentrate was removed. And, in the third stage, the removal reaction of sulfur occurred, the rest of sulfur almost removed in the stage. In the third stage, the reaction of the removal of sulfur follows nucleation and growth model, and the activation energy of the reaction was calculated to be kj/mol with heating rate.

7 1198 B.-S. Kim,. Kim, C.-K. Kim, H.-I. Lee and J.-S. Sohn RFRNCS 1) R. Prajsnar, Z. Smieszek, J. Czernecki, S. Plucilski, M. Warmuz and Leszek Garycki: Conroast: Sulfide Smelting 22, ed., by R. L. Stephens, H. Y. Sohn (Seattle, Washington, U.S.A, 22) pp ) G. Lindkvist and A. Holmstrom: Advances in Sulfide Smelting, ed., by H. Y. Sohn, D. B. George, A. D. Zunkel, (San Francisco, California, U.S.A, 1983) pp ) A. R. Udupa, K. A. Smith and J. J. Moore: Advances in Sulfide Smelting, ed., by H. Y. Sohn, D. B. George, A. D. Zunkel, (San Francisco, California, U.S.A, 1983) pp ) S.. Khalafalla and I. D. Shah: Met. Trans. 1 (197) ) I. D. Shah and S.. Khalafalla: Met. Trans. 2 (1971) ) H. Y. Sohn and M.. Wadsworth: Rate Process of xtractive Metallurgy, (Plenum Press Inc., 1979) pp ) S. Vyazovkin and C. A. Wight: Thermochimica Acta (1999) ) A. Khawam and D. R. Flanagan: Thermochimica Acta 436 (25) ) S. Vyazovkin: J. of Thermal Analysis 49 (1997) ) P. K. Heda, D. Dollimore, K. S. Alexander, D. Chen,. Law and P. Bicknell: Thermochimica Acta 255 (1995) ) I. Mihajlovic, N. Strbac, Z. Zivkovic, R. Kovacevic and M. Stehernik: Minerals ngineering 2 (27)