- Email: [email protected]
Pelletizing and alkaline leaching of powdery low grade zinc oxide ores
Available online at www.sciencedirect.com
Hydrometallurgy 89 (2007) 305 – 310 www.elsevier.com/locate/hydromet
Pelletizing and alkaline leaching of powdery low grade zinc oxide ores Linyong Feng ⁎, Xianwan Yang, Qingfeng Shen, Mingli Xu, Bingjie Jin Faculty of Materials and Metallurgical Engineering, Kunming University of Science and Technology, People's Republic of China Received 6 January 2007; received in revised form 15 July 2007; accepted 4 August 2007 Available online 16 August 2007
Abstract Low grade powdery zinc oxide ores (5.2% Zn, b2 mm) are mixed with cement 5 wt.%, pelletized and solidified. The diameters of the pellets obtained are between 5 mm to 8 mm. When the pellet solidification periods are 3 days, 10 days and 45 days respectively, the alkaline leaching rates of zinc in the pellets are up to 92.2%, 87.3% and 72.9% respectively. Decreasing the solidification time can reduce reaction time, increase dissolution of zinc in pellets and lower the effect of initial zinc concentration on leaching rate. The experiment results show that the minimum solidification time is three days, and the kinetic study indicates that alkaline leaching of the low grade zinc oxide pellets is controlled by the diffusion of the leach liquor through the gangue layer in the whole leach process, and the apparent rate constants are 3.51 × 10− 2 day− 1, 8.09 × 10− 3 day− 1, 4.74 × 10− 3 day− 1 respectively. © 2007 Elsevier B.V. All rights reserved. Keywords: Pelletizing; Ammonia leaching; Low grade powdery ores; Zinc oxide ores; Column reactor
1. Introduction The processing of zinc oxide ores is becoming more attractive due to the depletion of zinc sulfide ores as well as the restriction on sulfur emissions during their processing. There is very abundant zinc oxide ores in Yunnan province (China). The zinc oxide ores usually contains a low grade zinc, high grade calcium magnesium carbonate and silica. Leaching of zinc can be performed either by hydrometallurgical or pyrometallurgical routes. The low grade zinc of the ores results in high con⁎ Corresponding author. 76# mail box, Faculty of Materials and Metallurgical Engineering, Kunming University of Science and Technology, Kunming 650093, People's Republic of China. Tel.: +86 871 5182554. E-mail address: [email protected] (L. Feng). 0304-386X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2007.08.002
sumption of energy in pyrometallurgical processing due to the necessity of heating high contents of gangue materials. The high content of silica usually enters solution as silica gel with zinc when acid leaching method is used. The formation of silica gel makes filtration difficult. To overcome the filtration difficulty, a new flocculating agent (Magnafloc 156) was used by Bodas (1996) and the quantity required was very small compared to the other flocculating agents used. A method proposed by Ikenobu (2000) made it possible to precipitate silica in a form having excellent solid–liquid separation characteristics by feeding a compound containing pre-adjusted silica contents. Perry (1966) used Al2(SO4)3 as the flocculating agent to prevent clogging of the filters with silicic acid. Matthew and Elsner (1977) adjusted the pH of the leaching solution to 4.0–5.5, using a neutralizing agent, to precipitate and coagulate the
306
L. Feng et al. / Hydrometallurgy 89 (2007) 305–310
colloidal SiO2. In those processes the operating parameters must be properly controlled, otherwise colloidal silica will not be effectively precipitated from the solution. For the effective control of silica gel, Dufresne (1976) presented a method of treating zinc silicate ores, called the quick leaching. It was also applied to an Egyptian zinc silicate ore by Abdel Aal and Shukry (1997). Microwave irradiation by Hua et al. (2002) was applied to the quick leaching of zinc silicate ore. The leaching percentage of zinc was 99% and the dissolution of silica and iron was as low as 0.30% and 0.10%. Those investigations show that hydrometallurgical methods are more attractive than pyrometallurgical ones for the treatment of zinc oxide ores, especially for the low grade ores. Mineralogical studies showed that smithsonite can be completely leached but hemimorphite is relatively difficult to leaching by alkaline liquor. To this day, little has been published as related to an alkaline treatment of low grade powdery ores. Zhao and Stanforth (2000) studied the production of zinc powder by alkaline treatment of smithsonite ores. The best leaching results were obtained in presence of 5 M NaOH, at 90–95 °C and a reaction time of 90 min. Frenay (1985) studied leaching of zinc oxide ores in various solution media and obtained the best leaching results with alkaline media. In previous paper, directly leaching the ores was discussed whose diameter is between 2.0 mm–10.0 mm. But when the ores size is −2.0 mm, they could not be directly leached in a column reactor because they are too fine and the leaching liquor is difficult to go through the ores layer. In order to overcome the effect by silica gel, reduce the acid consumption, shorten technical process and economically utilize the ores, in this paper, as a fundamental study, powdery zinc oxide ores are mixed with cement, pelletized by disc balling machine, solidified and leached by ammonium sulphate solutions in a column reactor.
Fig. 1. XRD pattern of raw ores.
8.5% Fe, 25.3% CaO, 1.1% MgO, 15.1% SiO2, 0.7% Al2O3 and 0.02% Cu. X-ray diffraction (XRD) analysis shows smithsonite(ZnCO3) (8.4%), calcite (CaCO3) (42.6%) and quartz (SiO2) (15.1%) as the major components, Franklinite {(Zn, Fe, Mn)(Fe,Mn)2O4} (0.1%) as the minor ones and marmatite (nZnS·mFeS) (0.08%) is present in trace amounts in raw ores. Zinc is mainly in the form of smithsonite (86.0%) whose distribution shows in Table 2. Low grade powdery zinc oxide ores (5.2% Zn, b 2 mm) are separated from the raw ores, mixed with cement 5 wt.%, pelletized and solidified. The diameters of the pellets obtained are between 5 mm to 8 mm and the pellet solidification periods are 3 days, 10 days and 45 days respectively in order to get enough compressive strength. According to the result of previous paper, nine experiments are designed. In all experiments, the liquor and solid ratio is 4:1.Other different experiment parameters such as solidification time, concentration of (NH4)2SO4, pH and initial zinc concentration are showed in Table 3. Those chemical reagents such as (NH4)2SO4 and NH3·H2O are of analytical purity. 2.2. Procedure
The zinc oxide ores used in the present study was from Lanping town in Yunnan Province of China. The raw ores ground was analyzed by chemical method and examined by Xray powder diffraction (XRD). The chemical composition is given in Table 1 and the XRD pattern is shown in Fig. 1. The chemical method shows that the sample contains 9.6% Zn,
The experiments are carried out in a column reactor that is fabricated from 5 mm thick glass. The column showed in Fig. 2 is 650 mm high with an internal diameter of 45 mm, which is filled with pellets and fed with the leaching liquor at the rate of 95 L/(m2 h); The leaching liquor immerged all pellets in column passes through the pellets sample slowly by gravity and then re-circulates through a side loop with a pump. A container with a capacity of 10.0 L collects the solution draining from the column. When the zinc concentration in solution doesn't increase, the leaching is the end. In the leaching process, the column system is comprised of 850 g pellets and the liquid and solid ratio is always 3400: 850 cm3/
Table 1 Chemical composition of raw ores (wt.%)
Table 2 Mineral components of raw ore (wt.%)
2. Experimental 2.1. Materials
Zn
Fe
CaO
MgO
SiO2
Al2O3
Cu
Smithsonite
Willemite
Blende
Franklinite
9.6
8.5
25.3
1.1
15.1
0.7
0.02
86.0
12.6
1.1
0.4
L. Feng et al. / Hydrometallurgy 89 (2007) 305–310
307
Table 3 Experiment parameter Experiment Solidification time Concentration pH Initial zinc of (NH4)2SO4 concentration Number
Day
g/dm3
1 2 3 4 5 6 7 8 9
45 45 10 10 10 10 10 10 3
2.0 2.0 2.0 2.0 2.0 1.5 1.0 1.0 2.0
g/dm3 9.0 9.0 9.0 9.0 9.0 9.0 9.0 7.0 9.0
4.13 2.59 2.59 2.0 0 0 0 0 2.59
g, namely 4:1 cm3/g. The cumulative volume removed by sampling is not significant compared to the original leaching liquor. The samples are chemically analyzed for the determination of zinc content every five days. After that, the leaching percentage of zinc is calculated. 3. Results and discussion 3.1. Effect of concentrations of ammonia and ammonium sulfate Liquor : solid ratio ¼ 4 : 1; initial zinc concentration ¼ 0g=dm3
Fig. 3. Leaching percentage of zinc as affected by concentration.
of ammonia and ammonium sulfate. When the concentration of ammonium sulfate decreases from 2.0 mol/dm3 to 1.0 mol/ dm3 and pH is at 9.0 by adding ammonia to leaching liquor, the percentage of leached zinc decreases from 85.0 to 21.2. Without addition of ammonia to (NH4)2SO4 solution, pH is at 7.0 and the percentage of leached zinc is only 21.2. The solubility value of zinc increases with increase in the concentration of ammonia and ammonium sulfate, which are 2.8 g/dm3, 5.3 g/dm3, 9.7 g/dm3 and 11.0 g/dm3 respectively from curve 1 to curve 4 in Fig. 3. 3.2. Effect of solidification time
A plot of the percentage of leached zinc against time is presented in Fig. 3. The results indicate that the dissolution of zinc is strongly dependent on the increase in the concentrations
Fig. 4 shows the variation in recovery of zinc as a function of the solidification time for the same initial zinc concentration. As expected, it is obvious that the solidification time has a pronounced effect on the dissolution of zinc. The percentage of leaching zinc rapidly increases with in the decrease of solidification time. When the pellets are solidified for three days (curve1 in Fig. 4), the chemical reaction velocity is very faster and maximum leaching percentage amounts to 92.2 in 10 days. For the pellets solidified for 45 days (curve3 in
Fig. 2. Schematic and size of the column reactor.
Fig. 4. Leaching percentage of zinc as affected by solidification time.
308
L. Feng et al. / Hydrometallurgy 89 (2007) 305–310
leaching the pellets solidified for 10 days, because there is some zinc in the water phase unavoidably that needs to be recycled to leach other pellets. C ðNH4 Þ2 SO4 ¼ 2:0 mol=dm3 ; pH ¼ 9:0; 3.4. Kinetic aspects Pellets are reacted with ammonium sulphate solution according to the following reaction equations (Abdel Basir and Rabah, 1999): ZnCO3 þ iNH3 →½ZnðNH3 Þi2þ þ CO3 2
Fig. 5. Leaching percentage of zinc as affected by initial zinc concentration.
Fig. 4), the percentage of leached zinc only amounts to 72.9 in 25 days. The optimum leaching result can be obtained with the decrease of solidification time. But when the solidification time is less than three days, the pellets have not enough intensity, break up too much and the leaching liquor is difficult to go through the material layer. liquor : solid ratio ¼ 4 : 1; initial zinc concentration ¼ 2:59g=dm3 ; CðNH4 Þ2 SO4 ¼ 2:0mol=dm3 ; pH ¼ 9:0; 3.3. Effect of initial zinc concentration When the solidification period is 45 days, the effect of initial zinc concentration in leaching liquor on the percentage of leached zinc is shown in Fig. 5. It can be seen that the initial zinc concentration has a noticeable effect on the recovery of zinc. For a given reaction time, zinc recovery in pellets increases significantly with the decrease of initial zinc concentration in leaching liquor. From Fig. 5 it can also be seen that when the initial zinc concentration is 4.13 g/dm3, the recovery of zinc increases very slightly and remains almost constant after thirty days leaching; when the initial zinc concentration is 2.59 g/dm3, the recovery percentage of zinc still shows a tendency to increase after thirty days leaching. So, it can deduce that 0 g/dm3 of initial zinc concentration is found to be optimum.
ð1Þ
The reaction of pellets can proceed in a topochemical manner, whereby the thickness of an outer shell of insoluble gangue layer is progressively increased while the inner core of unreacted particle is decreased. It is clear that the rate of reaction decreases with time in Figs. 4, 5 and 6. This is due to the reduction of the reactant surface and the increase in path length for the diffusion of ions (Xuin et al., 1986; Pohlman and Olson, 1974). The rate of reaction is given for the models based on control by (a) chemical reaction at the pellets surface, (b) diffusion through the gangue layer, and (c) a combination of both. In this paper, the kinetics is discussed for the same initial zinc concentration but different solidification time. 3.4.1. Rate control by chemical reaction (Habashi 1969; Amer 1994) KC t ¼ 1 ð1 xÞ1=3
ð2Þ
where KC = reaction rate constant(day− 1 ); t = time(day); x = fraction reacted of zinc. x ¼ kZn leaching=100
ð3Þ
Based on the experimental data in Fig. 4, a plot of the righthand side of Eq. (2) vs time is given in Fig. 7. A linear fit is
C ðNH4 Þ2 SO4 ¼ 2:0 mol=dm3 ; pH ¼ 9:0; When the solidification period is 10 days, the effect of initial zinc concentration in leaching liquor on the percentage of leached zinc is shown in Fig. 6. It is obvious that the initial zinc concentration has an inconspicuous effect on the dissolution of zinc. From Fig. 6 it can be seen that the maximum percentage margin of leached zinc is only 5.1. That is to say, decreasing the solidification time can lower the effect of initial zinc concentration on leaching rate. Namely, after extract zinc from solution, the water phase is more suitable for
Fig. 6. Leaching percentage of zinc as affected by initial zinc concentration.
L. Feng et al. / Hydrometallurgy 89 (2007) 305–310
309
operated through zero respectively and the correlation coefficients R1, R2, and R3 are obtained which showed in the top left corner of Fig. 7. In the case of straight lines, the slope equals the rate constant KC: KC ¼ MbKCC C=qr day1
ð4Þ
where KCC chemical rate constant (cm/day); b = stoichiometric coefficient (dimensionless); M = molecular weight of the major zinc mineral; C = concentration of ammonium sulphate (mol/m3); r = radius of unreacted pellets (m);ρ = density of zinc ore (kg/m3). 3.4.2. Rate control by diffusion through gangue layer When the diffusion through the gangue layer is rate controlling, the kinetics may be correlated graphically using the Valensi equation: Kp t ¼ 1 2=3x ð1 xÞ2=3
Fig. 8. 1 − 2 / 3x − (1 − x)2 / 3 vs. time.
ð5Þ −1
where KP = chemical rate constant (cm day ). Again, based on the experimental data in Fig. 4, a plot of the right-hand side of Eq. (5) against time is given in Fig. 8 The slope of these straight lines is KP: KP ¼ 2bMDC=qr2 day1
ð6Þ
where D = diffusion coefficient of zinc ions in porous medium (m2/day). A linear fit is also operated through zero respectively and the correlation coefficients R4, R5, and R6 are obtained which showed in the top left corner of Fig. 8. When the correlation coefficients R1, R2 and R3 are compared with R4, R5 and R6, it is clear that the correlation coefficients R4, R5 and R6 all are bigger and closer to one than R1, R2 and R3. That is to say, the distribution of dots in Fig. 8 is more close to a linear, and it is indicated that the rate of reaction is controlled by diffusion through the gangue layer during the whole reaction time. The leaching rate equations. of R4, R5 and R6 are 1 − 2 / 3x − (1 − x)2 / 3 = 3.51 × 10 − 2 t; 1 − 2 / 3x − (1 − x)2 /
Fig. 7. 1 − (1 − x)1 / 3 vs. time.
3 = 8.09 × 10 − 3 t, 1 − 2 / 3x − (1 − x)2 / 3 = 4.74 × 10 − 3 t respectively.
4. Conclusions (1) Zinc is successfully recovered by pelletizing and alkali leaching in a column reactor. The percentage of leached zinc increases with in the concentration of ammonia and ammonium sulfate, so does the solubility values of zinc in leached saturation which are 14.48 g/dm3, 9.71 g/dm3, 5.29 g/dm3 and 2.75 g/dm3 respectively in No.5, No.6, No.7 and No. 8 experiments in Table 3. When the pellets solidification time are 3 days, 10 days and 45 days, the maximum leaching rates of zinc in the pellets are 92.2%, 87.3% and 72.9% respectively. (2) Decreasing the solidification time can reduce reaction time, increase dissolution of zinc in pellets and lower the effect of initial zinc concentration on leaching rate. The experiment results show that the minimum solidification time is three days, or else the pellets have not enough intensity, break up too much and the leaching liquor is difficult to go through the material layer. (3) When the solidification period is 45 days, zinc recovery in pellets increases significantly with the decrease of initial zinc concentration in leaching liquor. When the solidification period is 10 days, the initial zinc concentration has an inconspicuous effect on the zinc recovery in pellets. Decreasing the solidification time can lower the effect of initial zinc concentration on leaching rate, which is very useful in practice when the water phase is
310
L. Feng et al. / Hydrometallurgy 89 (2007) 305–310
recycled to leach other pellets after extracting zinc from solution. (4) The kinetic study indicates that alkaline leaching of the low grade zinc oxide pellets is controlled by the diffusion of the leach liquor through the gangue layer in the whole leach process, and the leaching rate equations of R4, R5 and R6 are 1 − 2 / 3x − (1 −x)2 / 3 = 3.51 × 10 − 2 t; 1 − 2 / 3x − (1 − x)2 / 3 = 8.09 × 10 − 3 t, 1 − 2 / 3x − (1 − x)2 / 3 = 4.74 × 10 − 3 t respectively. Acknowledgements This research is supported by Green Metallurgy Research Institute in Kunming University of Science and Technology, China. The author gratefully acknowledges Prof. Fei, Prof. Ma and Prof. Shenli for help. References Abdel Aal, E.A., Shukry, Z.E., 1997. Application of quick leaching method to an Egyptian zinc silicate ore. Transactions of the Institution of Mining and Metallurgy. Section C: Mineral Processing and Extractive Metallurgy, 106, pp. 89–90. Abdel Basir, S.M., Rabah, M.A., 1999. Hydrometallurgical recovery of metal values from brass melting slag. Hydrometallurgy 53 (1), 31–44.
Amer, A.M., 1994. Hydrometallurgical processing of Egyptian black shale of the Quseir-Safaga region. Hydrometallurgy 36 (1), 95–107. Bodas, M.G., 1996. Hydrometallurgical treatment of zinc silicate ore from Thailand. Hydrometallurgy 40 (1–2), 37–49. Dufresne, R.E., 1976. Quick leaching of siliceous zinc ore. Journal of Metals 28, 8–12. Frenay, J., 1985. Leaching of oxidized zinc ore in various media. Hydrometallurgy 15 (2), 243–253. Habashi, F., 1969. Principles of Extractive Metallurgy, vol. 1. Gordon & Breach, New York, pp. 153–163. Hua, Y., Lin, Z., Yan, Z., 2002. Application of microwave irradiation to quick leaching of zinc silicate ore. Minerals Engineering 15 (6), 451–456. Ikenobu, S., 2000. Method for processing siliceous zinc ores. In: Dutrizac, J.E. (Ed.), Lead-Zinc 2000. Minerals, Metals and Materials Society, Warrendale, pp. 427–435. Matthew, I.G., Elsner, D., 1977. The hydrometallurgical treatment of zinc silicate ores. Metallurgical transactions. B, Process Metallurgy 8 (1), 73–83. Perry, W., 1966. Refining zinc silicate ore by special leaching technique. Chemical Engineering 73 (21), 182–184. Pohlman, S.L., Olson, F.A., 1974. A kinetic study of acid leaching of Chrysocolla using a weight loss technique. Solution Mining Symposium. AIME, New York, pp. 447–460. Xuin, G.H., Yu, D.Y., Su, Y.F., 1986. Leaching of Scheelite by hydrochloric acid in the presence of phosphate. Hydrometallurgy 16 (1), 27–40. Zhao, Y., Stanforth, R., 2000. Production of Zn powder by alkaline treatment of smithsonite Zn–Pb ores. Hydrometallurgy 56 (2), 237–249.
Hydrometallurgy 89 (2007) 305 – 310 www.elsevier.com/locate/hydromet
Pelletizing and alkaline leaching of powdery low grade zinc oxide ores Linyong Feng ⁎, Xianwan Yang, Qingfeng Shen, Mingli Xu, Bingjie Jin Faculty of Materials and Metallurgical Engineering, Kunming University of Science and Technology, People's Republic of China Received 6 January 2007; received in revised form 15 July 2007; accepted 4 August 2007 Available online 16 August 2007
Abstract Low grade powdery zinc oxide ores (5.2% Zn, b2 mm) are mixed with cement 5 wt.%, pelletized and solidified. The diameters of the pellets obtained are between 5 mm to 8 mm. When the pellet solidification periods are 3 days, 10 days and 45 days respectively, the alkaline leaching rates of zinc in the pellets are up to 92.2%, 87.3% and 72.9% respectively. Decreasing the solidification time can reduce reaction time, increase dissolution of zinc in pellets and lower the effect of initial zinc concentration on leaching rate. The experiment results show that the minimum solidification time is three days, and the kinetic study indicates that alkaline leaching of the low grade zinc oxide pellets is controlled by the diffusion of the leach liquor through the gangue layer in the whole leach process, and the apparent rate constants are 3.51 × 10− 2 day− 1, 8.09 × 10− 3 day− 1, 4.74 × 10− 3 day− 1 respectively. © 2007 Elsevier B.V. All rights reserved. Keywords: Pelletizing; Ammonia leaching; Low grade powdery ores; Zinc oxide ores; Column reactor
1. Introduction The processing of zinc oxide ores is becoming more attractive due to the depletion of zinc sulfide ores as well as the restriction on sulfur emissions during their processing. There is very abundant zinc oxide ores in Yunnan province (China). The zinc oxide ores usually contains a low grade zinc, high grade calcium magnesium carbonate and silica. Leaching of zinc can be performed either by hydrometallurgical or pyrometallurgical routes. The low grade zinc of the ores results in high con⁎ Corresponding author. 76# mail box, Faculty of Materials and Metallurgical Engineering, Kunming University of Science and Technology, Kunming 650093, People's Republic of China. Tel.: +86 871 5182554. E-mail address: [email protected] (L. Feng). 0304-386X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2007.08.002
sumption of energy in pyrometallurgical processing due to the necessity of heating high contents of gangue materials. The high content of silica usually enters solution as silica gel with zinc when acid leaching method is used. The formation of silica gel makes filtration difficult. To overcome the filtration difficulty, a new flocculating agent (Magnafloc 156) was used by Bodas (1996) and the quantity required was very small compared to the other flocculating agents used. A method proposed by Ikenobu (2000) made it possible to precipitate silica in a form having excellent solid–liquid separation characteristics by feeding a compound containing pre-adjusted silica contents. Perry (1966) used Al2(SO4)3 as the flocculating agent to prevent clogging of the filters with silicic acid. Matthew and Elsner (1977) adjusted the pH of the leaching solution to 4.0–5.5, using a neutralizing agent, to precipitate and coagulate the
306
L. Feng et al. / Hydrometallurgy 89 (2007) 305–310
colloidal SiO2. In those processes the operating parameters must be properly controlled, otherwise colloidal silica will not be effectively precipitated from the solution. For the effective control of silica gel, Dufresne (1976) presented a method of treating zinc silicate ores, called the quick leaching. It was also applied to an Egyptian zinc silicate ore by Abdel Aal and Shukry (1997). Microwave irradiation by Hua et al. (2002) was applied to the quick leaching of zinc silicate ore. The leaching percentage of zinc was 99% and the dissolution of silica and iron was as low as 0.30% and 0.10%. Those investigations show that hydrometallurgical methods are more attractive than pyrometallurgical ones for the treatment of zinc oxide ores, especially for the low grade ores. Mineralogical studies showed that smithsonite can be completely leached but hemimorphite is relatively difficult to leaching by alkaline liquor. To this day, little has been published as related to an alkaline treatment of low grade powdery ores. Zhao and Stanforth (2000) studied the production of zinc powder by alkaline treatment of smithsonite ores. The best leaching results were obtained in presence of 5 M NaOH, at 90–95 °C and a reaction time of 90 min. Frenay (1985) studied leaching of zinc oxide ores in various solution media and obtained the best leaching results with alkaline media. In previous paper, directly leaching the ores was discussed whose diameter is between 2.0 mm–10.0 mm. But when the ores size is −2.0 mm, they could not be directly leached in a column reactor because they are too fine and the leaching liquor is difficult to go through the ores layer. In order to overcome the effect by silica gel, reduce the acid consumption, shorten technical process and economically utilize the ores, in this paper, as a fundamental study, powdery zinc oxide ores are mixed with cement, pelletized by disc balling machine, solidified and leached by ammonium sulphate solutions in a column reactor.
Fig. 1. XRD pattern of raw ores.
8.5% Fe, 25.3% CaO, 1.1% MgO, 15.1% SiO2, 0.7% Al2O3 and 0.02% Cu. X-ray diffraction (XRD) analysis shows smithsonite(ZnCO3) (8.4%), calcite (CaCO3) (42.6%) and quartz (SiO2) (15.1%) as the major components, Franklinite {(Zn, Fe, Mn)(Fe,Mn)2O4} (0.1%) as the minor ones and marmatite (nZnS·mFeS) (0.08%) is present in trace amounts in raw ores. Zinc is mainly in the form of smithsonite (86.0%) whose distribution shows in Table 2. Low grade powdery zinc oxide ores (5.2% Zn, b 2 mm) are separated from the raw ores, mixed with cement 5 wt.%, pelletized and solidified. The diameters of the pellets obtained are between 5 mm to 8 mm and the pellet solidification periods are 3 days, 10 days and 45 days respectively in order to get enough compressive strength. According to the result of previous paper, nine experiments are designed. In all experiments, the liquor and solid ratio is 4:1.Other different experiment parameters such as solidification time, concentration of (NH4)2SO4, pH and initial zinc concentration are showed in Table 3. Those chemical reagents such as (NH4)2SO4 and NH3·H2O are of analytical purity. 2.2. Procedure
The zinc oxide ores used in the present study was from Lanping town in Yunnan Province of China. The raw ores ground was analyzed by chemical method and examined by Xray powder diffraction (XRD). The chemical composition is given in Table 1 and the XRD pattern is shown in Fig. 1. The chemical method shows that the sample contains 9.6% Zn,
The experiments are carried out in a column reactor that is fabricated from 5 mm thick glass. The column showed in Fig. 2 is 650 mm high with an internal diameter of 45 mm, which is filled with pellets and fed with the leaching liquor at the rate of 95 L/(m2 h); The leaching liquor immerged all pellets in column passes through the pellets sample slowly by gravity and then re-circulates through a side loop with a pump. A container with a capacity of 10.0 L collects the solution draining from the column. When the zinc concentration in solution doesn't increase, the leaching is the end. In the leaching process, the column system is comprised of 850 g pellets and the liquid and solid ratio is always 3400: 850 cm3/
Table 1 Chemical composition of raw ores (wt.%)
Table 2 Mineral components of raw ore (wt.%)
2. Experimental 2.1. Materials
Zn
Fe
CaO
MgO
SiO2
Al2O3
Cu
Smithsonite
Willemite
Blende
Franklinite
9.6
8.5
25.3
1.1
15.1
0.7
0.02
86.0
12.6
1.1
0.4
L. Feng et al. / Hydrometallurgy 89 (2007) 305–310
307
Table 3 Experiment parameter Experiment Solidification time Concentration pH Initial zinc of (NH4)2SO4 concentration Number
Day
g/dm3
1 2 3 4 5 6 7 8 9
45 45 10 10 10 10 10 10 3
2.0 2.0 2.0 2.0 2.0 1.5 1.0 1.0 2.0
g/dm3 9.0 9.0 9.0 9.0 9.0 9.0 9.0 7.0 9.0
4.13 2.59 2.59 2.0 0 0 0 0 2.59
g, namely 4:1 cm3/g. The cumulative volume removed by sampling is not significant compared to the original leaching liquor. The samples are chemically analyzed for the determination of zinc content every five days. After that, the leaching percentage of zinc is calculated. 3. Results and discussion 3.1. Effect of concentrations of ammonia and ammonium sulfate Liquor : solid ratio ¼ 4 : 1; initial zinc concentration ¼ 0g=dm3
Fig. 3. Leaching percentage of zinc as affected by concentration.
of ammonia and ammonium sulfate. When the concentration of ammonium sulfate decreases from 2.0 mol/dm3 to 1.0 mol/ dm3 and pH is at 9.0 by adding ammonia to leaching liquor, the percentage of leached zinc decreases from 85.0 to 21.2. Without addition of ammonia to (NH4)2SO4 solution, pH is at 7.0 and the percentage of leached zinc is only 21.2. The solubility value of zinc increases with increase in the concentration of ammonia and ammonium sulfate, which are 2.8 g/dm3, 5.3 g/dm3, 9.7 g/dm3 and 11.0 g/dm3 respectively from curve 1 to curve 4 in Fig. 3. 3.2. Effect of solidification time
A plot of the percentage of leached zinc against time is presented in Fig. 3. The results indicate that the dissolution of zinc is strongly dependent on the increase in the concentrations
Fig. 4 shows the variation in recovery of zinc as a function of the solidification time for the same initial zinc concentration. As expected, it is obvious that the solidification time has a pronounced effect on the dissolution of zinc. The percentage of leaching zinc rapidly increases with in the decrease of solidification time. When the pellets are solidified for three days (curve1 in Fig. 4), the chemical reaction velocity is very faster and maximum leaching percentage amounts to 92.2 in 10 days. For the pellets solidified for 45 days (curve3 in
Fig. 2. Schematic and size of the column reactor.
Fig. 4. Leaching percentage of zinc as affected by solidification time.
308
L. Feng et al. / Hydrometallurgy 89 (2007) 305–310
leaching the pellets solidified for 10 days, because there is some zinc in the water phase unavoidably that needs to be recycled to leach other pellets. C ðNH4 Þ2 SO4 ¼ 2:0 mol=dm3 ; pH ¼ 9:0; 3.4. Kinetic aspects Pellets are reacted with ammonium sulphate solution according to the following reaction equations (Abdel Basir and Rabah, 1999): ZnCO3 þ iNH3 →½ZnðNH3 Þi2þ þ CO3 2
Fig. 5. Leaching percentage of zinc as affected by initial zinc concentration.
Fig. 4), the percentage of leached zinc only amounts to 72.9 in 25 days. The optimum leaching result can be obtained with the decrease of solidification time. But when the solidification time is less than three days, the pellets have not enough intensity, break up too much and the leaching liquor is difficult to go through the material layer. liquor : solid ratio ¼ 4 : 1; initial zinc concentration ¼ 2:59g=dm3 ; CðNH4 Þ2 SO4 ¼ 2:0mol=dm3 ; pH ¼ 9:0; 3.3. Effect of initial zinc concentration When the solidification period is 45 days, the effect of initial zinc concentration in leaching liquor on the percentage of leached zinc is shown in Fig. 5. It can be seen that the initial zinc concentration has a noticeable effect on the recovery of zinc. For a given reaction time, zinc recovery in pellets increases significantly with the decrease of initial zinc concentration in leaching liquor. From Fig. 5 it can also be seen that when the initial zinc concentration is 4.13 g/dm3, the recovery of zinc increases very slightly and remains almost constant after thirty days leaching; when the initial zinc concentration is 2.59 g/dm3, the recovery percentage of zinc still shows a tendency to increase after thirty days leaching. So, it can deduce that 0 g/dm3 of initial zinc concentration is found to be optimum.
ð1Þ
The reaction of pellets can proceed in a topochemical manner, whereby the thickness of an outer shell of insoluble gangue layer is progressively increased while the inner core of unreacted particle is decreased. It is clear that the rate of reaction decreases with time in Figs. 4, 5 and 6. This is due to the reduction of the reactant surface and the increase in path length for the diffusion of ions (Xuin et al., 1986; Pohlman and Olson, 1974). The rate of reaction is given for the models based on control by (a) chemical reaction at the pellets surface, (b) diffusion through the gangue layer, and (c) a combination of both. In this paper, the kinetics is discussed for the same initial zinc concentration but different solidification time. 3.4.1. Rate control by chemical reaction (Habashi 1969; Amer 1994) KC t ¼ 1 ð1 xÞ1=3
ð2Þ
where KC = reaction rate constant(day− 1 ); t = time(day); x = fraction reacted of zinc. x ¼ kZn leaching=100
ð3Þ
Based on the experimental data in Fig. 4, a plot of the righthand side of Eq. (2) vs time is given in Fig. 7. A linear fit is
C ðNH4 Þ2 SO4 ¼ 2:0 mol=dm3 ; pH ¼ 9:0; When the solidification period is 10 days, the effect of initial zinc concentration in leaching liquor on the percentage of leached zinc is shown in Fig. 6. It is obvious that the initial zinc concentration has an inconspicuous effect on the dissolution of zinc. From Fig. 6 it can be seen that the maximum percentage margin of leached zinc is only 5.1. That is to say, decreasing the solidification time can lower the effect of initial zinc concentration on leaching rate. Namely, after extract zinc from solution, the water phase is more suitable for
Fig. 6. Leaching percentage of zinc as affected by initial zinc concentration.
L. Feng et al. / Hydrometallurgy 89 (2007) 305–310
309
operated through zero respectively and the correlation coefficients R1, R2, and R3 are obtained which showed in the top left corner of Fig. 7. In the case of straight lines, the slope equals the rate constant KC: KC ¼ MbKCC C=qr day1
ð4Þ
where KCC chemical rate constant (cm/day); b = stoichiometric coefficient (dimensionless); M = molecular weight of the major zinc mineral; C = concentration of ammonium sulphate (mol/m3); r = radius of unreacted pellets (m);ρ = density of zinc ore (kg/m3). 3.4.2. Rate control by diffusion through gangue layer When the diffusion through the gangue layer is rate controlling, the kinetics may be correlated graphically using the Valensi equation: Kp t ¼ 1 2=3x ð1 xÞ2=3
Fig. 8. 1 − 2 / 3x − (1 − x)2 / 3 vs. time.
ð5Þ −1
where KP = chemical rate constant (cm day ). Again, based on the experimental data in Fig. 4, a plot of the right-hand side of Eq. (5) against time is given in Fig. 8 The slope of these straight lines is KP: KP ¼ 2bMDC=qr2 day1
ð6Þ
where D = diffusion coefficient of zinc ions in porous medium (m2/day). A linear fit is also operated through zero respectively and the correlation coefficients R4, R5, and R6 are obtained which showed in the top left corner of Fig. 8. When the correlation coefficients R1, R2 and R3 are compared with R4, R5 and R6, it is clear that the correlation coefficients R4, R5 and R6 all are bigger and closer to one than R1, R2 and R3. That is to say, the distribution of dots in Fig. 8 is more close to a linear, and it is indicated that the rate of reaction is controlled by diffusion through the gangue layer during the whole reaction time. The leaching rate equations. of R4, R5 and R6 are 1 − 2 / 3x − (1 − x)2 / 3 = 3.51 × 10 − 2 t; 1 − 2 / 3x − (1 − x)2 /
Fig. 7. 1 − (1 − x)1 / 3 vs. time.
3 = 8.09 × 10 − 3 t, 1 − 2 / 3x − (1 − x)2 / 3 = 4.74 × 10 − 3 t respectively.
4. Conclusions (1) Zinc is successfully recovered by pelletizing and alkali leaching in a column reactor. The percentage of leached zinc increases with in the concentration of ammonia and ammonium sulfate, so does the solubility values of zinc in leached saturation which are 14.48 g/dm3, 9.71 g/dm3, 5.29 g/dm3 and 2.75 g/dm3 respectively in No.5, No.6, No.7 and No. 8 experiments in Table 3. When the pellets solidification time are 3 days, 10 days and 45 days, the maximum leaching rates of zinc in the pellets are 92.2%, 87.3% and 72.9% respectively. (2) Decreasing the solidification time can reduce reaction time, increase dissolution of zinc in pellets and lower the effect of initial zinc concentration on leaching rate. The experiment results show that the minimum solidification time is three days, or else the pellets have not enough intensity, break up too much and the leaching liquor is difficult to go through the material layer. (3) When the solidification period is 45 days, zinc recovery in pellets increases significantly with the decrease of initial zinc concentration in leaching liquor. When the solidification period is 10 days, the initial zinc concentration has an inconspicuous effect on the zinc recovery in pellets. Decreasing the solidification time can lower the effect of initial zinc concentration on leaching rate, which is very useful in practice when the water phase is
310
L. Feng et al. / Hydrometallurgy 89 (2007) 305–310
recycled to leach other pellets after extracting zinc from solution. (4) The kinetic study indicates that alkaline leaching of the low grade zinc oxide pellets is controlled by the diffusion of the leach liquor through the gangue layer in the whole leach process, and the leaching rate equations of R4, R5 and R6 are 1 − 2 / 3x − (1 −x)2 / 3 = 3.51 × 10 − 2 t; 1 − 2 / 3x − (1 − x)2 / 3 = 8.09 × 10 − 3 t, 1 − 2 / 3x − (1 − x)2 / 3 = 4.74 × 10 − 3 t respectively. Acknowledgements This research is supported by Green Metallurgy Research Institute in Kunming University of Science and Technology, China. The author gratefully acknowledges Prof. Fei, Prof. Ma and Prof. Shenli for help. References Abdel Aal, E.A., Shukry, Z.E., 1997. Application of quick leaching method to an Egyptian zinc silicate ore. Transactions of the Institution of Mining and Metallurgy. Section C: Mineral Processing and Extractive Metallurgy, 106, pp. 89–90. Abdel Basir, S.M., Rabah, M.A., 1999. Hydrometallurgical recovery of metal values from brass melting slag. Hydrometallurgy 53 (1), 31–44.
Amer, A.M., 1994. Hydrometallurgical processing of Egyptian black shale of the Quseir-Safaga region. Hydrometallurgy 36 (1), 95–107. Bodas, M.G., 1996. Hydrometallurgical treatment of zinc silicate ore from Thailand. Hydrometallurgy 40 (1–2), 37–49. Dufresne, R.E., 1976. Quick leaching of siliceous zinc ore. Journal of Metals 28, 8–12. Frenay, J., 1985. Leaching of oxidized zinc ore in various media. Hydrometallurgy 15 (2), 243–253. Habashi, F., 1969. Principles of Extractive Metallurgy, vol. 1. Gordon & Breach, New York, pp. 153–163. Hua, Y., Lin, Z., Yan, Z., 2002. Application of microwave irradiation to quick leaching of zinc silicate ore. Minerals Engineering 15 (6), 451–456. Ikenobu, S., 2000. Method for processing siliceous zinc ores. In: Dutrizac, J.E. (Ed.), Lead-Zinc 2000. Minerals, Metals and Materials Society, Warrendale, pp. 427–435. Matthew, I.G., Elsner, D., 1977. The hydrometallurgical treatment of zinc silicate ores. Metallurgical transactions. B, Process Metallurgy 8 (1), 73–83. Perry, W., 1966. Refining zinc silicate ore by special leaching technique. Chemical Engineering 73 (21), 182–184. Pohlman, S.L., Olson, F.A., 1974. A kinetic study of acid leaching of Chrysocolla using a weight loss technique. Solution Mining Symposium. AIME, New York, pp. 447–460. Xuin, G.H., Yu, D.Y., Su, Y.F., 1986. Leaching of Scheelite by hydrochloric acid in the presence of phosphate. Hydrometallurgy 16 (1), 27–40. Zhao, Y., Stanforth, R., 2000. Production of Zn powder by alkaline treatment of smithsonite Zn–Pb ores. Hydrometallurgy 56 (2), 237–249.