DISSOLUTION OF FINE PARTICLE CHALCOPYRITE CONCENTRATE IN ACIDIC POTASSIUM DICHROMATE SOLUTION

it appears that most of the studies on chalcopyrite leaching have been carried out various reagents. The ferric ion is most often applied; in addition to cupric ions and oxygen and also some bacteria were used as oxidative leaching agents of chalcopyrite in sulphated and chlorinated medium under atmospheric or pressure conditions. A sulphuric acid (H2SO4) solution containing dichromate ions (Cr2O7

is a strong oxidizing agent capable of dissolving easily the copper sulphides including chalcopyrite. But few investigators interest in the dissolution of sulphide minerals of this reagent. Shantz and Morris (1974) investigated the reaction of dichromate with copper sulphides, Murr and Hiskey (1981) studied the effect on the kinetics of chalcopyrite oxidation in dichromate medium, Antonijevic et al. (1994) investigated the kinetics of chalcopyrite oxidation by potassium dichromate and Ucar (2005) studied the leaching conditions and reaction kinetics of chalcopyrite concentration by potassium dichromate (K2Cr2O7) in acidic medium. In this study, the dissolution of chalcopyrite was carried out with potassium dichromate in sulphuric acid solution. Dichromate ions (Cr2O7

were chosen because it is known as a strong oxidant agent. The oxidative action of dichromate ion in acidic solutions is based on its reduction according to (Jackson,

+ 14H+ + 6e- → 2Cr3+ + 7H2O E0=1,33V (1) This potential value (1.33 V) is adequate to oxidize almost all the metal sulphurs. Sulphuric acid was chosen, because of the increase of hydrogen ions concentration in leaching medium. It achieves to increase the medium potential value. In this way, chalcopyrite oxidation can be obtained effectively. In the experimental studies, the effects of stirring speed, sulphuric acid and potassium dichromate concentration, temperature, particle size and solid/liquid ratio on dissolution of copper from chalcopyrite were investigated, and also optimum condition of copper extraction rate was determined. A paper contained these experimental conditions and dissolution of copper from chalcopyrite was published in XXIII International Mineral Processing Congress (2006). In this study, kinetics of chalcopyrite oxidation by acidic potassium dichromate solutions investigated and all experiments were carried out minus 38 µm particle size.

MATERIAL AND METHODS

1 Material Chalcopyrite concentrate enriched thorough flotation of CuFeS2 - PbS - ZnS complex ore in Menka flotation plant (Sivas-Turkey) was used in this study. Particle size fractions were 212x106, 106x75, 75x45, 45x38 and minus 38 µm that obtained by wet sieving. The materials were dried after wet sieving, and then dry sieving was performed using the same screens. The chemical analysis of each size fractions are given in Table 1. Table 1.Chemical analysis of each fractions of chalcopyrite concentrate Element (%)Particle Size (µm) Cu Fe Zn Pb 212x106 23.78 27.69 1.17 2.58 106x75 29.23 31.58 2.79 2.45 75x45 27.27 27.24 3.76 2.57 45x38 24.62 23.54 4.14 5.27 -38 21.48 18.68 1.76 0.25

2 Experimental procedure Experiments were carried out by agitation leaching using a pyrex beaker of 1L put in the hot water bath whose temperature can be adjusted in the range of 0-100 °C with ±0.2 °C accuracy. The pyrex beaker was closed by a rubber cover which was used to prevent evaporation of solution. Stirring process was provided via a mechanical stirrer whose stirring speed can be adjustable between the 0-2000 rpm equipped with teflon shaft. The experiments were carried out continuously. 500 mL of acidic potassium dichromate solution was used to investigate parameters and was heated to desired temperatures. When the desired temperature was attained, charge of 5 g of chalcopyrite concentrate was added to acidic potassium dichromate solution. At prior definite time intervals, seven samples of 1 mL each were taken for the determination of copper by Unicam 929 model AAS. Distilled water and reagent-grade chemicals were used to make up all required solutions.

RESULTS AND DISCUSSIONS The shrinking core model considers that the leaching process is controlled either by the diffusion of reactant through the solution boundary layer, or through a solid product layer, or by rate of the surface chemical reaction. The simplified equations of the shrinking core model when either diffusion or the surface chemical reactions are the slowest step can be expressed as follows, respectively (Levenspiel, 1972). tkt ar DCMXX d B AB  2 0

2)1( 3 21  (2) tktar CMkX r B ABc  0 31 )1(1  (3) Where X is the fraction reacted, kc is the kinetic constant, MB is the molecular weight of the solid, CA is the concentration of the dissolved lixiviant A in the bulk of the solution, a is the stoichiometric coefficient of the reagent in the leaching reaction, r0 is the initial radius of the solid particle, t is the reaction time, D is the diffusion coefficient in the porous product layer and, kd and kr are the rate constants, respectively, which are calculated from Eqs. (2) and (3), respectively. Eq. (2) reveals that if the diffusion through the product layer controls the leaching rate, there must be a linear relation between the left side of equation and time. If the surface reaction controls the rate, the relation between the left side of Eq. (3) and time must be linear.

1. Effect of temperature The effect of temperature was examined in the range of 20-90 °C. The result obtained showed that the copper dissolution recovery increased with increasing temperature. The apparent rate constants (kr, kd) obtained for both equations and correlation coefficients were calculated for each temperature values. The obtained results are given in Table 2. As can be seen, the dissolution of chalcopyrite for leaching temperature in the range of 20-90 °C fitted to diffusion model given in Eq. (2). Table 2. The kr, kd values and correlation coefficients for each temperature. (H2SO4, 0.5 M; K2Cr2O7, 0.1 M; stirring speed, 400 rpm; solid/liquid ratio, 10g/L; particle size, -38 µm). Temp., °C kr (x10-2min-1) R2 kd x10-3 (x10-2min-1) R2

0,19 0,931 0,0481 0,999

0,26 0,954 0,0702 0,998

0,33 0,961 0,1071 0,999

0,49 0,976 0,1496 0,999

0,62 0,961 0,2392 0,999

0,74 0,959 0,305 0,998

30 60 90 120 Time, min.

31 - 2 / 3 X - ( 1 - X ) 2 / 3 Temperature, °C 20 30 40 50 80 90 Figure 1. Plot of Eq. (2) versus time for different temperatures.

6 2.8 3.0 3.2 3.4 3.6 1000/T, K-1 -8.0 -7.5 -7.0 -6.5 -6.0 -5.5l n k d , m i n - 1 Ea = 22.44 kJ/mol Figure 2. Arrhenius plot of reaction rate against reciprocal temperature. As it has been known that dissolution rate during the leaching decreases with time and it is directly dependent on the activation energy. The activation energy of the diffusion-controlled process is characterised as being 4-12 kJ mol-1, while it is usually >40 kJ mol-1 for a chemically controlled process (Habashi, 1999). Using the apparent rate constants (kd) obtained by application of Eq. (2), the Arrhenius plot was obtained. The activation energy was calculated as 22.44 kJ/mol which supports the view that leaching reaction was controlled by diffusion.

2. Effect of stirring speed The effect of stirring speed on copper dissolution was carried out various stirring speed and leaching times. Diffusion model given at Eq. (2) was applied to the results of stirring speed tests and the apparent rate constant values kd were calculated. The results obtained indicate that the copper dissolution from chalcopyrite increased with increased stirring speed in the range of 100-400 rpm (Fig. 4). Fig. 5 shows at linear equation between ln kd - ln stirrer speed. The order of reaction with respect to moderate stirrer speed of 100-400 rpm was determined 2.24 power of stirrer speed ((n)2.24) with a correlation coefficient of 0.986.

40 80 120 Time, min.

251 - 2 / 3 X - ( 1 - X ) 2 / 3 Stirring Speed, prm 100 200 300 400 Figure 4. Plot of Eq. (2) versus time for different stirrer speed.

4 4.8 5.2 5.6 6.0 ln (s. speed) -10 -9 -8 -7 -6l n k d , m i n - 1 (s.speed)2.24 Figure 5. Plot of ln kd- ln stirrer speed.

3. Effect of sulphuric acid concentration The effects of sulphuric acid concentration on the dissolution of chalcopyrite were investigated in the range of 0.1-0.5 M H2SO4 concentrations. The results of these experiments were shown that the copper dissolution from chalcopyrite concentration increased with increasing H2SO4 concentration. Fig. 6 shows the linear kinetics plot of 1- 2/3X-(1-X)2/3 versus time at various H2SO4 concentrations. From the slopes of the plot apparent rate constant values were determined.

30 60 90 120 Time, min.

251 - 2 / 3 X - ( 1 - X ) 2 / 3 H2SO4Concentration, M

5 Figure 6. Plot of Eq. (2) versus time for different H2SO4. -2.4 -2.0 -1.6 -1.2 -0.8 -0.4 ln [H2SO4] -9 -8 -7 -6l n k d , m i n - 1 [H2SO4]1.42 Figure 7. Plot of ln kd - ln H2SO4. The natural logarithm of kd versus [H2SO4] plot (see in Fig. 7) is constructed to determine the order of dependency with respect to H2SO4 concentration. Reaction order with respect to sulphuric acid concentration is 1.42.

4. Effect of potassium dichromate concentration The effects of potassium dichromate concentration were investigated in the range of

02-0.1 M. Diffusion model was applied to the experiment results of K2Cr2O7 concentration. Fig. 8 was drawning after this application. As seen from Fig. 8, the obtained results are shown that the copper dissolution increased with increasing K2Cr2O7 concentration.

30 60 90 120 Time, min

251 - 2 / 3 X - ( 1 - X ) 2 / 3 K2Cr2O7 Concentration, M

1 Figure 8. Plot of Eq. (2) versus time for different K2Cr2O7. -4.0 -3.6 -3.2 -2.8 -2.4 -2.0 ln [K2Cr2O7] -10 -9 -8 -7 -6l n k d , m i n - 1 [K2Cr2O7]2.16 Figure 9. Plot of ln kd - ln K2Cr2O7. The apparent rate constant values were calculated for K2Cr2O7 concentration. Values of ln kd, the rate constants, were plotted versus ln K2Cr2O7. As seen in these figures, the order of reaction with respect to for H2SO4 concentration was found 2.16 power (K2Cr2O72.16) with a correlation coefficient of 0.998.

5. Effect of particle size The experiments for determination of the effect of particle size on the dissolution of copper were performed various particle size fractions. As seen in Figure 10, the copper dissolution recovery increases with decreasing particle size. The apparent rate constants were also determined and these showed an increasing trend with the decreasing particle sizes. Thus kd were found to be 0.00219, 0,001083, 0,00051, 0,00028 and 0,00016 min-1 for the particle sizes minus 38, 45x38, 74x45, 106x75 and 212x106 µm, respectively. The order of reaction with respect to mean particle radius was determined to be inversely proportional to 1.54 power of particle sizes (r0 -1.54) with correlation coefficient of 0.99.

2 3.6 4.0 4.4 4.8 5.2 ln r0 -9 -8 -7 -6l n k d (r0)-1.54 Figure 10. Plot of ln kd - ln r0.

0000 0.0004 0.0008 0.0012 1/r02

0025k d Figure 11. Dependence of kd on 1/r0

Eq. (2) reveals that if the diffusion through the product layer controls the leaching rate, there must be a linear relation between the left side of equation and time. The slop of the line is the rate constant kd, it must be directly proportional to 1/r0

Linear relation between the kd values and the reciprocal of mean particle radius 1/r0

This reaction is predicted by the shrinking core model for a diffusion controlled process.

6. Effect of solid/liquid ratio The effect of solid/liquid ratio was investigated at solid/liquid ratio values in the range of 10-200 g/L. The results obtained showed that the copper dissolution from chalcopyrite increased with decreasing solid/liquid ratio. And the apparent rate constant values were also calculated for last parameter solid/liquid ratio. Values of the rate constants were plotted versus ln solid/liquid ratio and ln kd, yield a perfect linear relationship with a high correlation coefficient (Fig. 12). The order of reaction with respect to solid/liquid ratio was determined inversely proportional to 1.46 power of solid/liquid ratio (S/L -1.46) with correlation coefficient of 0.99.

2 3 4 5 ln S/L, g/L -11 -10 -9 -8 -7 -6l n k d , m i n - 1 [S/L]-1.46 Figure 12. Plot of ln kd - ln S/L.

CONCLUSIONS On the basis of the results of this study, the following conclusions can be drawn: ∙The dissolution kinetics of copper from chalcopyrite study showed that activation energy and the order of reaction values with respect to stirrer speed, sulphuric acid and potassium dichromate concentration, particle size and solid/liquid ratio was also correspond to the shrinking core model for a diffusion-controlled process, 1-2/3X-(1- X)2/3. The activation energy obtained was 22.44 kJ/mol for the temperature range 20-90 °C. ∙The rate of chalcopyrite oxidation by potassium dichromate in sulphuric acid was directly proportional to stirring speed, sulphuric acid and potassium dichromate concentrations and leaching temperature. On the other hand, it increased with decreasing particle size and solid/liquid ratio.

ACKNOWLEDGEMENTS This study was supported by The Research Foundation of Selcuk University under Project No. BAP-07701067.

Antonijevic, M.M., Jankovic, Z.D., Dimitrijevic, M.D., 1994. Investigation of the kinetics of chalcopyrite oxidation by potassium dichromate. Hydrometallurgy, 35, 187-

Antonijevic, M.M., Jankovic, Z.D., Dimitrijevic, M.D., 2004. Kinetics of chalcopyrite dissolution by hydrogen peroxide in sulphuric acid. Hydrometallurgy, Issues 3-4, 329-334.

Habashi, F., 1999, Kinetics of Metallurgical Processes, 2nd ed. Metallurgie Extractive Quebec, Quebec, Canada. Jackson E., Hydrometallurgical Extraction and Reclamation, 1986, Ellis Harwood Ltd., London.

Levenspiel, O., 1972. Chemical Reaction Engineering, 2nd ed., Wiley, New York, NY.

Murr, L.E., Hiskey, J.B., 1981. Kinetic effect of particle size and crystal dislocation density on the dichromate leaching of chalcopyrite. Metallurgical Iron Saction B, Volume 12 B, p. 255-267.

Shantz, R. and Morris, T.M., Engr. Mining J., 175 no. 5, 71-72 (1974).

Ucar, G., Aydogan, S., Canbazoglu , M., 2006. Dissolution of chalcopyrite concentrate by dichromate ions in sulphuric acid medium. XXIII International Mineral Processing

Congress, XXIII International Mineral Processing Congress Volume 2, p. 1410-1414.

Ucar, G., 2005. Determination of leaching conditions of chalcopyrite concentrate in sulphuric acid medium under effect of potassium dichromate. Selcuk University, Master thesis.