Dissolution kinetics of smithsonite ore in ammonium chloride solution

Transcription

1 Hydrometallurgy 8 (25) Dissolution kinetics of smithsonite ore in ammonium chloride solution Shaohua Ju *, Tang Motang, Yang Shenghai, Li Yingnian The College of Metallurgic Science and Engineering, Central South University, Changsha, 4183, People s Republic of China Received 1 May 24; received in revised form 29 November 24; accepted 4 July 25 Available online 24 August 25 Abstract The results of a dissolution kinetics study of smithsonite ore in ammonium chloride are presented. Effect of stirring speed, ore particle size, reaction temperature, and the concentration of ammonium chloride on zinc dissolution rate are determined. The results obtained show that leaching of about 91.2% of zinc is achieved using Am ore particle size at a reaction temperature of 9 8C for 24 min reaction time with 5 mol/l ammonium chloride. The solid/liquid ratio was maintained constant at 1:1 g/ml. Leaching kinetics indicates that chemical reaction at the new particle surface and the diffusion through the inert particle pores together are the rate control steps. A corresponding mixed control model is found suitable to explain the relationship between the reaction time and the fraction of zinc leached. The apparent activation energy of this process is determined to be 21.3 kj/mol. D 25 Elsevier B.V. All rights reserved. Keywords: Chloride hydrometallurgy; Kinetics; Ammonium chloride; Smithsonite; Leaching 1. Introduction It is well known that sulfuric acid is often used as the leaching reactant in most of the hydrometallurgical processes. However, other reactants, such as ammonia with or without sulfate or carbonate, nitric acid, caustic soda and chlorides, can be considered [1]. Nowadays, hydrometallurgy in ammonium chloride solution has been considered as a prospective medium * Corresponding author. address: shaohua_ju@hotmail.com (S. Ju). for extracting nonferrous metals [2]. The reason probably lies in its advantages of both ammonia metallurgy and chloride metallurgy. Firstly, many minerals such as ZnO, PbO, CuO and Ag, can easily dissolve into the solution, because of the high complexation characteristics of both ammonia and chloride ions. In addition, due to the relatively high ph (6 7) of the medium, impurities such as Fe 2 O 3, SiO 2, CaO and MgO are not soluble in the resulting solution. Thirdly, the purification of the leaching solution becomes very simple, since either metallic powder reduction or solvent extraction can be used [8,9]. Finally, treating the ore with high alkaline gangues in ammonium chloride X/$ - see front matter D 25 Elsevier B.V. All rights reserved. doi:1.116/j.hydromet 转载

2 68 S. Ju et al. / Hydrometallurgy 8 (25) solution will not cause excessive consumption of leaching reagent and so much trouble with filtration, as is the case in sulfuric acid media [3 5]. A typical example of using ammonium chloride as a leaching medium is the CENIM-LNETI process [2,6 8] in which a heated and concentrated ammonium chloride solution is used to extract valuable metal elements from complex zinc sulfide ores. The results show that, at 15 8C, 15 kpa oxygen partial pressure, and two countercurrent stages, the extractions of all important metal values, such as Zn, Pb, Cu and Ag, are above 95%. The advantages of the process also lie in the fact that the resulting solution is totally free of iron, as a consequence of the high ph value at which leaching takes place; other impurities such as As, Sb, Bi and Sn are found only at trace levels. The pregnant solution is cooled to crystallize PbCl 2 ; after separating Cu and Zn with solvent extraction, these metals are selectively stripped into sulfate solution for electrolysis. The CENM-LNETI process is, however, not a fully chloride metallurgy process, because the solution medium is finally changed to sulfate system. As would be reasonably expected, the change of solution medium can bring about a number of complications. To break through this limitation, we have developed a process [9] in which smithsonite is leached with ammonium chloride, and then after purification with zinc powder, the result solution containing about 4 g/l zinc is directly electrolyzed in ammonium chloride media with the addition of ammonia. High purity cathode zinc (N %) can be obtained with a relatively low consumption of electrical energy (about 31 kw h/t zinc). What is more, the gas evolved at the anode is N 2 rather than Cl 2. This means that this process is more environment friendly. In this paper, the results of leaching of smithsonite ore with ammonium chloride are presented. The kinetics characteristics of the leaching process are analyzed. A corresponding mixed control model is found suitable to explain the relationship between the reaction time and the fraction of zinc leached; and the apparent activation energy of the process is determined. 2. Experimental 2.1. Materials and apparatus Smithsonite ore from Guangxi province of China was used. After being ground and dry sieved, the ore was analyzed in the Analysis and Detecting Center of the Central South University, China. Table 1 shows the chemical analysis of the ore, while Table 2 shows the mineralogical composition. The tables show that smithsonite (ZnCO 3 ) was the major mineral. In fact, ammonium chloride could only attack smithsonite (ZnCO 3 ) and part of ZnS and ZnSiO 3. Industry grade ammonium chloride with a content of 99.2% was used in this study. The reaction between zinc ore and ammonium chloride solution was performed in a 2-mL round-bottom flask with 3 holes, placed in a thermostatically controlled water bath. Because, at high temperature, there would be a very high water vapor pressure above the liquid phase with the possibility of water loss, a condenser was placed in one of the holes and the other holes were sealed to prevent water loss Procedure 15 g of the smithsonite ore of required size was added at one time to the agitated 15-mL ammonium chloride solution of the required concentration at the required temperature. At an interval of every 3 min, 5 ml solution sample was taken out using a volumetric pipet. Each sample was then filtered and washed. The solids corresponding to that volume were discarded. Finally, the sample of filtrate was chemically analyzed to determine zinc content at a certain time. Consequently, the solid/liquid ratio was maintained constant at 1:1 g/ml. Table 1 Chemical analysis of the ore Element Zn Cu Pb Cd Co S Al 2 O 3 SiO 2 Fe 2 O 3 CaO Content (wt.%)

3 S. Ju et al. / Hydrometallurgy 8 (25) Table 2 Mineralogical composition of zinc in the ore Substance ZnCO 3 ZnSiO 3 ZnFe 2 O 4 ZnS Zn (total) Zinc content (wt.%) The cumulative volume removed by sampling was not significant compared to the original solution, only about 4%. Thus, sampling would cause little change of the leaching condition. In addition, because the content of other metal elements such as Cu, Pb and Co present in the ore was very low, solution analysis did not include these metals. 3. Results and discussion 3.1. Reaction equation and the kinetics model selection Smithsonite reacts with ammonium chloride according to the following chemical equations [1]: NH 4 ClYNH 3(aq) +Cl +H + (1) ZnCO 3(S) +2H + YZn 2+ +CO 2 z+h 2 O (2) Zn 2+ +i(nh 3 ) (aq) YZn(NH 3 ) i 2+ (3) Zn(NH 3 ) Cl YZn(NH 3 ) 2 Cl 2(ag) (4) where i =1,2,3,4. The comparison of the SEM (scanning electron microscope) photos of the ore and the leach residue showed little difference in their size, but the residue became very porous. This suggests the leaching process proceeded according to the shrinking core model with a constant particle size. The rate of reaction might be controlled by chemical reaction at the particle surface, diffusion through the inert particle pore, diffusion through liquid film on the surface of the particle, or a combination of two or three of these processes. When the surface chemical process alone is the rate limitation step, the relationship between the fractional conversion, a, and time, t, is: 1 ð1 aþ 1=3 ¼ k c t ð5þ Table 3 Symbols used in the reaction rate models Symbol Physical and chemistry signification C Bulk leachant concentration DV Effective diffusivity of leachant through the product layer k Overall reaction rate constant k c Surface chemical rate constant k D Pore diffusion rate constant rv Particle radius at time t r Initial particle radius a Fractional conversion, 1 (rv/r ) 3, or leaching fraction t Reaction time V Molar volume of solid reactant b Coefficient indicating surface chemical contribution to overall rate r Stoichiometry factor, moles solid reactant per mole of diffusing leachant When the rate of diffusion of the reactants through the particle pore alone is rate limiting, then 1 2=3a ð1 aþ 2=3 ¼ k D t: ð6þ However, in many cases the leaching rate depends on both the rate of pore diffusion and of chemical reaction, especially in treating oxide ores [11]. When the effect of the liquid film covering the surface of the ore particle is not the controlling process, and on the assumption that pseudo steady-state conditions prevail, this bmixed controlq behavior can be described by the following equation [11 13]: 1 2=3a ð1 a Zn leaching fraction h i Þ 2=3 þ b 1 ð1 aþ 1=3 ¼ kt r/min ð7þ Fig. 1. Effect of stirring speed on the leaching of smithsonite in ammonium chloride solution. Particle size:.97 mm; temperature: 9 8C; ammonium chloride concentration: 5 mol/l, time: 24 min.

4 7 S. Ju et al. / Hydrometallurgy 8 (25) Zn leaching fraction where b ¼ 2DV= ðk c rr Þ ð8þ k ¼ 2DVC n V= r 2 r : ð9þ The various symbols are defined in Table 3. The main task for describing the model is to deduce the values of b and k. From Eq. (7), at a given condition (temperature, particle size, leachant concentration), a linear relationship of 8 >< y ¼ >: a 1 a 3 ð Þ2=3 t against ( ) 1 1 a x ¼ ð Þ1=3 t 9 >= >; Fig. 2. Effect of particle size on the leaching of smithsonite ore in ammonium chloride solution. Temperature: 9 8C; stirring speed: 6 rpm; ammonium chloride concentration: 5 mol/l. Table 4 Values of constants in Eq. (7) for the extraction of zinc from smithsonite ore of different particle sizes Average particle diameter(mm) b k can be set up. Then b is the slope of the line, while k is its intercept Effect of stirring speed A series of experiments, each lasting for 24 min, was conducted to determine how the stirring speed affects Zn extraction. Within the series of tests, the ammonium chloride concentration and leaching temperature were kept constant at 5 mol/l and 9 8C, respectively. The initial particle size was (average.97) mm. The solid/liquid ratio was 1:1 g/ml. The results are given in Fig. 1. Due to the transparent nature of the glass flask, it could be seen that when the stirring speed reached 5 rpm, all ore particles were fully suspended. The results in Fig. 1 show that the zinc extraction increased when stirring rate increasing up to 5 rpm, after which further increasing the stirring rate had little effect on the leaching fraction. This meant that as long as the particles were fully suspended, zinc extraction would not increase further even if the stirring speed increased a lot. In the following experiment, a stirring speed of 6 rpm was adopted. y y =.246x y =.194x Effect of particle size The effect of particle size on the leaching of smithsonite ore was studied using different size fractions: (average.125) mm, (average.97) mm, and (average.75).4 y =.1499x x -.2 Fig. 3. Plot for calculating the values of b and k for experiments using different particle sizes. Table 5 Testing of Eq. (7) using data from Table 4 Particle diameters used (Am) k (d i /d j ) 2 k j /k i d i d j k i k j

5 S. Ju et al. / Hydrometallurgy 8 (25) /3α-(1-α) 2/3 +β[1-(1-α) 1/3 ] mm.97mm.75mm Fig. 4. Plot of zinc leaching data according to shrinking core model with mixed surface and diffusion kinetics. Temperature: 9 8C; stirring speed: 6 rpm; ammonium chloride concentration: 5 mol/l. mm. Within the series of tests, the initial ammonium chloride concentration and leaching temperature were kept constant at 5 mol/l and 9 8C, respectively. The solid/liquid ratio was 1:1 g/ml. The results are given in Fig. 2. Fig. 2 shows that when the initial particle size of the ore decreases to.75 mm, the zinc extraction fraction reached 91.2% after 15 min leaching, which was the maximum of extraction, and further leaching could not increase the leaching fraction. The values of the parameters b and k, for the three different-sized particles used, are calculated with the method discussed in Section 3.1. The linear relationship of dyt against dxt is shown in Fig. 3 and the values of the parameters are given in Table 4. The results for extraction of zinc shown in Fig. 2 were replotted according to Eq. (7) as indicated in Fig. Table 6 Values of constants in Eq. (7) for the extraction of zinc from smithsonite ore at different NH 4 Cl concentrations Initial concentration of NH 4 Cl (mol/l) b k From the excellent linear relationship between leaching time and the mixed model fraction, 1 2/ 3a (1 a) 2/3 +b[1 (1 a) 1/3 ], the conclusion could be drawn that this leaching process accords with the model strictly. During replotting, the last four points of particle size.75 mm and the last point of.97 mm were omitted, because they had reached the maximal leaching fraction, and their tendency obviously could not accord with any model. From Eqs. (8) and (9), the relationship of b or k with the diameter of the ore can be expressed as follows: 2ckj d i =d j =k i : ð1þ If Eq. (7) describes the leaching process adequately, then the ratio of the values of k for two essentially mono-sized fractions should be equal to the square of the ratio of the radius. The comparison is shown as Table 5. From Table 5, the agreement between the expected results and those actually obtained was reasonable, and hence the suggestion that zinc leaching process is under mixed control of both pore diffusion and chemical reaction is reasonable. Zinc leaching fraction M.4 5M.2 6M /3α (1 α) 2/3 +β[1 (1 α) 1/3 ].4.2 4M 5M 6M Fig. 5. Effect of ammonium chloride concentration on the leaching of smithsonite ore. Particle size:.97 mm; stirring speed: 6 rpm; temperature: 9 8C. Fig. 6. Plot of zinc leaching data according to shrinking core model with mixed surface and diffusion kinetics. Temperature: 9 8C; particle size:.97 mm; stirring speed: 6 rpm.

6 72 S. Ju et al. / Hydrometallurgy 8 (25) ln k ln k = 1.33*ln C ln C Fig. 7. Plots of Eq. (11) for the reaction order. Table 7 Values of constants in Eq. (7) for different temperatures under the above condition Temperature (8C) b k between leaching time and the mixed model fraction. This proves again that the process proceeds in accordance with the model strictly. From Eq. (9) and the Arrhenius expression, 3.4. Effect of ammonium chloride concentration The effect of the concentration of ammonium chloride on the leaching of smithsonite was studied using fine ore of (average.97) mm particle size and ammonium chloride concentrations of 4, 5 and 6 mol/l at a constant temperature of 9 8C. The solid/liquid ratio was kept constant at 1:1 g/ml. The results are shown in Fig. 5. It is obvious that the concentration of ammonium chloride also has a pronounced effect on the dissolution of zinc. About 91.2% of zinc present in the fine fraction of the ore is extracted using 6 mol/l ammonium chloride solution after 18 min. Using the method described in Section 3.1 again, values of b and k for different ammonium chloride concentrations were obtained as summarized in Table 6. The results for extraction fraction of zinc shown in Fig. 5 are replotted according to Eq. (7) as Fig. 6. As Fig. 6 shows, there is also a linear relationship Zn extraction fraction ºC 8 ºC 9 ºC Fig. 8. Effect of temperature on the leaching of smithsonite ore in ammonium chloride solution. Particle size:.97 mm; stirring speed: 6 rpm; leachant concentration: 5 mol/l. k ¼ k d 2 C n expð E a =RTÞ ð11þ Using the k values obtained in the above table, a plot of ln k against ln C can be drawn; the slope of the line is the order of the reaction. The result is shown in Fig. 7. It can be seen that the order of this reaction, n, is Effect of temperature The effect of reaction temperature on the percentage of extracted zinc is plotted in Fig. 8 using ore of (average.97) mm particle size and ammonium chloride concentration of 5 mol/l at 7, 8 and 9 8C. The solid/liquid ratio was kept constant at 1:1 g/ ml. Again, using the method described in Section 3.1, the value of b and k for the three different temperatures were obtained, as shown in Table /3α-(1-α) 2/3 +β[1-(1-α) 1/3 ] ºC 8 ºC 9 ºC Fig. 9. Plot of zinc leaching data according to shrinking core model with mixed surface and diffusional kinetics. Particle size:.97 mm; stirring speed: 6 rpm; ammonium chloride solution concentration: 5 mol/l.

7 S. Ju et al. / Hydrometallurgy 8 (25) ln k k y = 57.92x y = x /T Fig. 1. Arrhenius plots for activation energy. The results for extraction of zinc shown in Fig. 8 are replotted according to Eq. (7) as presented in Fig. 9. The linear relationship between the time and mixed model fraction, 1 2/3a (1 a) 2/3 + b[1 (1 a) 1/3 ], further indicates that this leaching process follows the bmixed shrinking core modelq. The Arrhenius plot of this leaching process is shown in Fig. 1. The activation energy was calculated as E =21.3 kj/mol. The Arrhenius plot is consistent with the rate limitation steps described previously. The overall rate constant, k, is a function of the effective diffusivity through the inert particle pore and the effect of surface chemical reaction; b, in contrast, is a function of the effective diffusivity and also of the reciprocal of the surface chemical rate constant (k c ) and so the value of b should decrease rapidly with an increase of temperature since the activation energy for a surface chemical reaction is large. This tendency can also be found in Table Overall rate equation of the leaching model From Eq. (11), for deducing the constant value of k, a linear relationship of k against the value of d 2 C n exp( E a /RT), designated as dmultiplet,.4 can be set up; the slope of the line is k. There are three groups of data that can be used to calculate k : k of different particle size, k of different NH 4 Cl concentration and k of different temperature against their d 2 C n exp( E a /RT), respectively (shown in Table 8). A line of dkt against dmultiplet can be set up and is shown in Fig. 11. As can be seen, the overall rate equation of this leaching process is as follows: k ¼ 57:92 d 2 C 1:33 expð 21:3=RTÞ ð12þ 4. Conclusion multiple Fig. 11. The linear relationship of dkt against dmultiplet (i.e. d 2 C 1.33 exp( E a /RT)). On the basis of the discussion above, the following conclusions can be made: (1) It is feasible to dissolve zinc from ores that mainly contain smithsonite in ammonium chloride solution. The zinc extraction can be more than 91%. (2) Leaching kinetics indicate that chemical reaction at the new particle surface and the diffusion through the inert particle pores together are the rate controlling steps. Table 8 The values of k and d 2 C n exp( E a /RT) marked as dmultiplet Temperature Particle size NH 4 Cl concentration 8C k Multiple mm k Multiple mol/l k Multiple

8 74 S. Ju et al. / Hydrometallurgy 8 (25) (3) The overall rate equation of the dissolution kinetics of smithsonite in ammonium chloride solution is k ¼ 57:92 d 2 C 1:33 expð 21:344=RTÞ ð12þ References Winand, R., Chloride hydrometallurgy. Hydrometallurgy 27, Limpo, J.L., et al., The CENIM-LNETI process: a new process for the hydrometallurgical treatment of complex sulphides in ammonium chloride solutions. Hydrometallurgy 28, Elgersma, F., et al., Simultaneous dissolution of zinc ferrite and precipitation of ammonium jarosite. Hydrometallurgy 34, Cheng, T.C.-M., Demopoulos, G.P., Analysis of the hematite precipitation process from a crystallization point of view. TMS Annual Meeting, Sylvain, Seyer, et al., 21. Jarofix: addressing iron disposal in the zinc industry. JOM 51, Limpo, J.L., et al., Reactions during the oxygen leaching of metallic sulphides in the CEMIM-LNETI process. Hydrometallurgy 28, Figueiredo, J.M., et al., The CENIM-LNETI process for zinc recovery from complex sulphides. International Symposium World ZincT93. The Australasian Institute of Mining and Metallurgy, Hobart, pp Amer, S., et al., The recovery of zinc from the leach liquors of the CENIM-LNETI process by solvent extraction with di(2-ethylhexyl)phosphoric acid. Hydrometallurgy 37, Yang Shenghai, Tang Motang, 2. Chinese patent, CN A. Limpo, J.L., Luis, A., Solubility of zinc chloride in ammoniacal ammonium chloride solutions. Hydrometallurgy 32, Sohn, H.Y., Wadsworth, M.E., Rate Processes of Extraction Metallurgy. Plenum Press, New York, NY, pp Pohlman, S.L., Olson, F.A., A kinetics study of acid leaching of chrysocolla using a weight loss technique. Solution Mining Symposium, AIME Soc. Min. Eng., pp Sato, T., Lawson, F., Differential leaching of some lead smelter slags with sulfurous acid and oxygen. Hydrometallurgy 11,