Production of Zn powder by alkaline treatment of smithsonite Zn Pb ores

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1 Ž. Hydrometallurgy Production of Zn powder by alkaline treatment of smithsonite Zn Pb ores Youcai Zhao a,b,),1, Robert Stanforth a a Department of EnÕironmental Engineering, National UniÕersity of Singapore, Singapore , Singapore b The State Key Laboratory of Pollution Control and Resource Reuse, Tongji UniÕersity, Shanghai , People s Republic of China Received 25 October 1999; received in revised form 20 February 2000; accepted 25 February 2000 Abstract In this work, a production process for Zn powder by alkaline treatment of an oxidized Zn ore, smithsonite Ž ZnCO. 3, was studied. It was found that over 85% of both Zn and Pb, and less than 10% of Al can be leached from the ore when the leaching operation is conducted at over 958C using 5 M NaOH solution as leaching agent. The dissolution of impurities such as Fe, Ca, etc., was negligible. Leaching of Pb can be improved remarkably with addition of NaCl to the leaching systems. Typical composition of the leach solution is grl Zn, grl Pb and grl Al. Zn and Pb contents in the leaching residue were found to be lower than 2.4% and %, respectively. Pb present in the leach solution is separated with the addition of sodium sulfide. The Pb-free solution is then used for the electrolysis of metallic Zn using stainless steel electrodes. Zn metal powder with purity higher than 99.5% is obtained. The specific energy for the Zn electrolysis in alkaline leach solution is around kwhrkg Zn, which is lower than the energy consumption of 3.3 kwhrkg Zn required in the conventional process of Zn electrolysis in acidic sulfate electrolytes. The process developed is considered to be cost-effective, simple and easy to be operated and managed, and the flowsheet for the production of Zn from ores is presented. It is proposed that ores be broken into particles smaller than mm. Then, leached with 5 M NaOH solution at a C temperature for 1.5 h at a phase ratio wnaoh solution added Ž ml. to the ore leached Ž g.x of 6 7. After filtration, the leach solution is subjected ) Corresponding author. The State Key Laboratory of Pollution Control and Resource Reuse, Tongji University, Shanghai , China. address: zyclmk@online.sh.cn Ž Y. Zhao.. 1 On academic leave from the State Key Laboratory of Pollution Control and Resource Reuse, Tongji University under the approval of China Education Ministry while this work was done Xr00r$ - see front matter q 2000 Published by Elsevier Science B.V. All rights reserved. Ž. PII: S X

2 238 Y. Zhao, R. StanforthrHydrometallurgy to electrolysis for the production of metallic Zn after Pb is separated by sodium sulfide. The NaOH solution is recycled to the next leaching operation after most of the Zn is electrowon. To save energy and keep heat loss minimal, all operations can be conducted in an adiabatic environment, including leaching, filtration and electrolysis. Loss of NaOH was estimated to be less than 50 g for the production of 1 kg of Zn, assuming that most NaOH left in the leaching residues and precipitates of lead sulfide could be recovered by washing with dilute NaOH solution and water. However, loss of NaOH is dependent on the type and composition of the ores. The presence and leaching of silicates, carbonates and phosphates in ores would increase the loss of NaOH and deteriorate filtration of leach residues in the alkaline solutions. Therefore, most of the operational cost for this process comprises the consumption of electricity for electrolysis, NaOH, and energy required for heating. q 2000 Published by Elsevier Science B.V. All rights reserved. 1. Introduction Currently, Zn is produced mostly from zinc sulfide ores because sulfides are easy to separate from gangue and to concentrate by conventional flotation techniques. Oxidized Zn ores, such as smithsonite Ž ZnCO., willemiyte Ž Zn SiO., hydrozincite Ž ZnCO3 3ZnŽ OH.., zincite Ž ZnO. and hemimorphite Ž Zn SiO H O , have also long been an important source of Zn. However, their concentration was difficult and, until relatively recently, only rich ores were exploited, using limited concentration by washing and gravity methods. For the low-grade oxidized Zn ores, their exploitation and metallurgy is very limited. Concentration of oxidized Zn ores is usually accomplished by flotation. Recovery of Zn is not high because of incomplete and ineffective transfer of oxidized Zn to sulfided Zn. On the other hand, the flotation requires only a sulfidization of mineral surfaces to be successful. A proper potential control of the sulfidization process is, however, not easy to accomplish. Oxidized Zn ores can be treated by acidic leaching processes. However, besides leachable Zn and Pb, elements such as Ca, Fe, Mg, etc., may also dissolve at the same time. As a result, a large quantity of extra acid will be consumed and a leach solution with complex components will be obtained. This creates the need of many unit operations to purify the solution, with a concomitant loss of Zn during neutralisation, chemical precipitation and solvent extraction. Therefore, this process would not be cost-effective when a low-grade oxidized Zn ore is used as raw material. For smithsonite ores, the content of Zn and Pb is usually low Ž-20% Zn and 5% Pb, respectively.. High-grade concentrates cannot be obtained without heavy losses, regardless of the techniques used. Hence, in many locations that treat smithsonite ores to produce Zn, high-zn grade concentrates are produced from low-grade ores by volatilization techniques at high temperature in a Waelz-type kiln. The Zn content in the collected dusts can be as high as 65 70%, in comparison with the original Zn content of around 20% in the ore. The dusts are dissolved in a sulfuric acid solution, followed by purification and electrowinning to produce Zn metal powder. The volatilization process is very effective, but heavy pollution may occur. Capital investment is also high, and it seems difficult for small and medium enterprises to use this process to produce Zn metal.

3 Y. Zhao, R. StanforthrHydrometallurgy In our previous experiments wx 1, it was found that Zn and Pb in strong alkaline solutions cannot be precipitated by carbonates, phosphates or other precipitants except by sulfides, i.e., carbonates and phosphates of Zn, and certainly, zinc oxide, dissolve in strong alkaline solutions. Thus, it ought to be possible to use an alkaline leaching process for the extraction of Zn and Pb from the oxidized Zn ores. Considering that hydroxides of Fe, Ca, Cd and Mg cannot be leached in alkaline medium, it seems attractive to develop an innovative and cost-effective alkaline leaching process for production of Zn and Pb from oxidized Zn ores. Little has been published as related to an alkaline treatment of oxidized Zn ores. Parallel research can be found in the recovery of Zn and Pb from dusts generated in electric arc furnaces and industrial wastes w2 13 x. For the EAF dusts with low contents of Zn ferrites containing 21.2% Zn, 21.8% Fe, 3.6% Pb and 2.5% Mn wx 2, around 80 85% of Zn and Pb can be transferred into aqueous solution by direct alkaline leaching at 908C. However, when the content of Zn ferrites in the dusts increases, leaching of Zn and Pb becomes less effective wx 8. In our previous work wx 1, two EAF samples were tested; these contained 25% Zn, 1.8% Pb and 33% Fe Ž as Sample A. and 16.8% Zn, 1.12% Pb and 40% Fe Ž as Sample B., respectively. It was found that, depending on the contents of Fe in the dusts, about 38% Ž Sample B. and 53% ŽSample A. of the Zn and over 70% of the Pb Ž both Samples A and B. could be leached from the dusts, while fractions of Zn and Pb remained in the leaching residues, even when leaching was done at the boiling point of water. Therefore, the higher the contents of Fe, the lower extraction of Zn. Obviously, the leaching efficiencies of Zn and Pb are dependent on the chemical forms of Zn and Pb in the feed materials. In this work, an integrated process for alkaline leaching, purification by sulfide precipitation and electrolysis to extract Zn and Pb from oxidized Zn ores, is reported. The possible industrial application is discussed. 2. Procedure The samples Ž smithsonite. used for study were taken directly in situ from the ores plants in which the volatilization process is being used to concentrate the Zn content as dusts Ž around 70% Zn in the dusts.. The ore samples were dried and ground to a particle size mm. Samples were chemically assayed by ICP. The composition or the ore was Ž %. Zn 20.66, Pb 2.83, Cd 0.013, Cu 0.038, Mg 2.46, Al 1.53, Fe Typically, when the leaching processes were carried out at room temperature, 3 g of ore sample Ž and additives if applicable. were added into a glass bottle containing a given volume of 5 M NaOH solution, then the bottles were rotated with an orbital shaker for 42 h Ž 15 rpm.. The supernatant and the residues were analysed. For the leaching experiments at high temperature, 10 g of ore sample was mixed with 70 ml of 5 M NaOH, and heated at boiling point for 2 h while stirring with a magnetic stirrer. The volume was kept constant by addition of water. After the leaching operation was completed, the slurry was filtered and the filtrate was treated to remove Pb by sulfide precipitation. Crystals of sodium sulfide hydrate

4 240 Y. Zhao, R. StanforthrHydrometallurgy Ž Na SP7 9 H O. 2 2 was added to a given volume of filtrate so that the weight ratio of sulfide to Pb was 1.8. In electrowinning, stainless steel electrodes were used as both anode and cathode. The obtained Zn metal powder could easily be removed from the electrode and was washed thoroughly with water and then with ethanol. Finally, it was dried at 508C, weighed and analysed. 3. Results and discussion 3.1. Effect of NaOH concentration It was observed, when the initial NaOH concentration was lower than 5 M, that extraction of Zn from ores increased as the NaOH concentration increased; however, extraction increased only slightly at higher initial concentration Ž Fig. 1.. In fact, zinc Fig. 1. Variation in the leaching rate of Zn and Pb as a function of the concentration of NaOH and Ž. temperature. Phase ratio ml NaOHrg ore : 7 8.

5 Y. Zhao, R. StanforthrHydrometallurgy oxide, zinc carbonate, zinc phosphate and the corresponding Pb complexes, may be dissolved effectively in NaOH solution as long as NaOH concentration is relatively high Ž over 4 M.. In Fig. 1, it can be seen that at room temperature, the maximum leaching and extraction of Zn and Pb Ž 65 75%. is obtained when the initial concentration of NaOH in the leaching solution is higher than 5 M. Use of higher NaOH concentration in leaching solution may be unfavourable to the subsequent filtration step due to a high viscosity of the resultant solutions and a greater loss of NaOH. When the leaching operations were carried out at higher temperatures Žat boiling point of water., the leaching efficiencies increased considerably from 60 75% at room temperature to 80 90% at the higher temperature Ž Fig. 1.. In Fig. 1, it can also be seen that the leaching behaviour of Zn and Pb is very similar Phase ratios effects Higher phase ratios of NaOH solution to the ores facilitate dissolution of Zn and Pb, and improve surface contact between solution and one particles thus increasing the extraction efficiencies of Zn and Pb Ž Fig. 2.. But higher phase ratios would increase the amount of NaOH required and reduce the Zn and Pb concentrations in the resultant leach solutions. Therefore, a phase ratio of 6 8 was considered suitable. The dependence of Zn and Pb extraction on phase ratios was found to be linked closely to the leaching temperature. At room temperature, the leaching efficiencies always increased with an increase in phase ratios. Nevertheless, for extraction at the boiling point of water, the leaching efficiencies increased sharply when the phase ratio was lower than 7, and only slightly when the phase ratio increased further. Hence, a one-stage leaching might be chosen for sufficient extraction for both Zn and Pb, if the phase ratio of 7 and temperature close to boiling are used Leaching time effect The transfer of Zn and Pb from ores to the leaching solution involves a solid liquid reaction and physical dispersion. Relatively longer contacting time between the ore particles and liquid is required to obtain maximum extraction of Zn and Pb. Moreover, extraction of Zn and Pb is also dependent on leaching temperature Ž Fig. 3.. At room temperature, a leaching time of h was sufficient to reach extraction and leaching equilibrium Ž Fig. 3a.. On the other hand, the leaching equilibrium was established faster when the leaching operation was carried out at the boiling point of water. Around 2 h heating time was enough for the maximum extraction of Zn and Pb from the ore Ž Fig. 3b.. In this case, over 85% of Zn and 84% of Pb could be leached from the ore Effect of additiões Organic and inorganic salts were added into the leaching systems and the experimental results are shown in Table 1. It can be seen that extraction of Pb from the ores is

6 242 Y. Zhao, R. StanforthrHydrometallurgy Fig. 2. Variation in the leaching rate of Zn and Pb as a function of the phase ratio and leaching temperature. enhanced by addition of NaCl, possibly because of the formation of soluble Pb Cl complexes. On the other hand, leaching is depressed by an addition of tri-sodium-citrate. The other additives tested had very little effect in the leaching of both Zn and Pb Typical metal contents in the leach solution and residues Under the optimum conditions, i.e., 5 M NaOH solution, a phase ratio Žvolume of NaOH solution in milliliters to the weight of ore in grams. of 7:1, and leaching at over 958C for 2 h, the typical contents of the leach solution were Ž grl. 24 Zn, 3 Pb and 0.5 Al, while the composition of the leach residue was Ž %. 2.4 Zn, 0.5 Pb, Cd, Cu and 12.9 Fe. When NaCl was added to the leaching system at a NaClrore weight

7 Y. Zhao, R. StanforthrHydrometallurgy Fig. 3. Effect of leaching time on the extraction of Zn and Pb from ore at Ž. a 258C and Ž. b 1008C. Phase ratio Ž NaOHrore, mlrg.: 8. ratio of 1:3, the content of Pb in the leach residue was found to decrease to lower than %. In this case, the residue can be disposed of safely in conventional landfill or dumping sites Separation of Pb from Zn in leaching solutions by sulfide precipitation In basic media, the chemical properties of Zn are similar to those of Pb. Therefore, both Zn and Pb dissolve. Before Zn is electrowon, Pb must be removed, usually by addition of Zn powder in an acidic media. In alkaline media, cementation of Pb by Zn is ineffective. In this work, Pb is precipitated selectively by addition of sodium sulfide to the leaching solution. Results are shown in Table 2. When the weight ratio of sodium sulfide added to the Pb is over 1.8, Pb can be quantitatively precipitated. Residues from the precipitation step were identified as mixtures of PbS and Na PbŽ OH. 2 2S, with PbS being the predominant phase.

8 244 Y. Zhao, R. StanforthrHydrometallurgy Table 1 Effect of the addition of salts on the leaching of Zn and Pb from ores Ž 3 g of ore was used. No. Salts addedr Leaching Zn extraction Pb extraction weight Ž. g temperature Ž 8C. Ž %. Ž %. 1 NaClr NaClr NaClr NaClr NaClr NaClr NaClr NaClr CaCO3 r CaCO r CaCO3 r Tri-Na-Citrater Tri-Na-Citrater Tri-Na-Citrater Na-acetater Na-acetater Na-acetater Electrolysis of Zn from Pb-free leach solution Much work has been done on the electrolysis of Zn from alkaline media. It has been shown that electrolysis of Zn from alkaline media is effective and cost-effective w2 5 x. In this work, the electrolysis process was tested, as the leach solution used was more complex than that prepared by the dissolution of zinc oxide in NaOH solution reported in literature w4,5 x. Experimental results are shown in Table 3. The Zn metal deposited on the cathode was gray with a slightly bright metallic luster. It was easily removed from the cathode Flowsheet for the production of metallic Zn from oxidized Zn ore by alkaline process The integrated process based on the above results is shown in Fig. 4. The ore should be broken into particles smaller than mm before leaching. Stirring and heating Table 2 Effect of weight ratio Ž Na SP7 9 H OrPb. 2 2 on separation of Pb from Zn in leach solution by sulfide precipitation Sodium sulfide used: Na 2SP7 9 H 2O; Leach solution: Zn 36 grl; Pb 3.6 grl. No Weight ratio of Na 2SP7 9 H2OrPb Pb removal Ž % Zn removal Ž %

9 Y. Zhao, R. StanforthrHydrometallurgy Table 3 Typical electrowinning conditions for the recovery of Zn from Pb-free leach solution Zn Ž grl. NaOH Electrode Voltage Current density Specific energy Zn Metal Ž M. Ž V. Ž 2 Arm. Consumption Purity Ž %. Ž kwhrkg Zn Stainless steel are necessary to speed up the leaching process. After 2 h of leaching, the filtration operation should be carried out immediately, followed by addition of sodium sulfide to precipitate Pb in the leach solutions. Precipitation of lead sulfide is fast and can be followed by filtration as soon as all the added sodium sulfide is dissolved. The electrolysis can be carried out from the clear filtrate thus obtained. To save energy, heat loss ought to be decreased by proper engineering measures, e.g., in an adiabatic environment. All operations including leaching, filtration, precipitation and electrolysis improved if carried out at a high temperature, especially close to boiling point Discussion and operational cost balances The loss of NaOH in the precipitation of Pb by sulfide is low because most of the Pb is precipitated as PbS and the concentration of Pb in the leach solutions is always lower than 3 grl, which is much lower than the concentration of Zn. From titration analysis, NaOH losses in the whole process are estimated to be around 50 g per kg of Zn. The loss may be beyond this figure in practical application as the loss of NaOH is dependent on the type and composition of the ores. The presence and leaching of silicates, carbonates and phosphates in the ores would increase the loss of NaOH and deteriorate filtration of leaching residues in the alkaline solutions. Such a loss and deterioration may be reduced when zinc oxide ores such as zincite Ž ZnO. are used. The main cost item of the process may be the energy consumed by electrolysis. Around kwh of electricity is needed to electrowin 1 kg of Zn. Additional energy is needed for heating. It is considered to improve the electrolysis of Zn by increasing the temperature to 758C or even higher wx 5. Hence, operational costs would include electrowining of Zn Ž 2.5 kwhrkg Zn., heating of leaching system Ž estimated as less than 1 kwhrkg Zn on average., loss of NaOH caused by entrainments in the leach residue and in the lead sulfide precipitate as well as by an accumulation of sodium salts in the leach solution, and labor. Table 4 compares different processes for the production of metallic Zn from oxidized Zn ores. It can be seen that the alkaline process developed in this work may be an alternative, especially suited for small- and medium-scale companies. The costs shown in Table 4 exclude the profit of selling the pure lead sulfide, which may offset most of the cost of sulfide consumed for the Pb precipitation. Compared with volatilization acidic leaching processes, the energy for alkaline leaching would be cut down considerably. The alkaline leaching electrowinning process can be considered as a potential cost-ef-

10 246 Y. Zhao, R. StanforthrHydrometallurgy Fig. 4. Proposed flowsheet for production of Zn from oxidized Zn ores by alkaline process. Ore: Zn 20% mainly as zinc carbonate, Pb 2.8%. fective and environmentally friendly technology for production of Zn from oxidized Zn ores. The Zn in the ores studied in this work may exist as zinc oxide, zinc carbonate and zinc sulfide. Only the former two types of Zn can be dissolved into NaOH solution. Moreover, the dissolution rates for zinc carbonate, the major component in the smithsonite ores, may be slow, and the dissolution effectiveness may be increased at relatively

11 Table 4 Comparisons for different processes for the production of metallic Zn from oxidized Zn ores Ž based on the production of 1 kg Zn. Processes selected Alkaline process developed Acidic process Volatilisation acidic process in this work Energy for electrolysis kwh Energy for heating leaching system Estimated as 1 kwh ) 1 kwh ) 1 kwh Energy for volatilisation None None 1 kwh Loss of NaOH Moderate Loss for neutralisation and Loss for neutralisation and purification, perhaps quite large purification, perhaps quite large Loss of Zn powder for Pb removal None Necessary Necessary Loss of sodium sulfide for Pb removal Necessary None None Loss of acids None Large quantity Large quantity Capital costs Small to large Small to large Large Operational costs Small to large Quite large Quite large Feasibility Small- to large-scale company Small- to large-scale company Large-scale company Removal of Pb By sulfide precipitation Zn metal powder addition Zn metal powder addition Total costs Low Low High Y. Zhao, R. StanforthrHydrometallurgy 56 ( 2000 )

12 248 Y. Zhao, R. StanforthrHydrometallurgy high temperature and higher NaOH concentration. Even near or at boiling point, around 10% of Zn cannot be leached Ž the Zn content in leach residue is lower than 2.4%.. This Zn may be present in ores as zinc sulfide. Besides Zn and Pb, the oxides of Al, Cr and Cu may also be dissolved in NaOH solution. In our previous experiments wx 1, it was found that the solubility of oxides of Cu and Cr in NaOH solution is very low Ž - 1grL., when Zn and Pb are present. A proportion of Al oxides may dissolve together with Zn and Pb, but their dissolution is depressed by the presence of Zn and Pb. The solubility of these elements in a NaOH solution can be given as follows: Zn)Pb)Al)Cr;Cu. Therefore, some Al in the ores will be leached in the NaOH solution, but its concentration will always be lower than 1 grl. Since Al cannot be removed by sulfide precipitation, it may accumulate in the solution circulating between Zn leaching and electrowinning. 4. Conclusions The oxides and carbonates of Zn and Pb in smithsonite ores can be leached and transferred into NaOH solution effectively, while sulfides remain in the leach residues. Addition of NaCl can improve leaching and extraction of Pb from ores. Pb in the leach solution can be quantitatively separated from Zn by precipitation with sodium sulfide. Zn in the Pb-free solution can be recovered by electrowinning and the Zn-depleted solution can be recycled into the next leaching step. The alkaline leaching sulfide separation electrowinning process developed in this work may be cost-effective and of industrial significance. References wx 1 R. Stanforth, Y. Zhao, Recovery of zinc and lead from EAF dusts by caustic soda fusion and leaching, unpublished reports, 1998, National University of Singapore, Singapore. wx 2 J. Frenay, S. Ferlay, J. Hissel, Zinc and lead recovery from EAF dusts by caustic soda process, in: Electric Furnace Proc., Treatment Options for Carbon Steel Electric Arc Furnace Dust 43 Iron Steel Soc., 1986, pp wx 3 J. St.-Pierre, M. Sider, D.L. Piron, Zinc electrowinning from alkaline and acidic chloride solutions, in: Zinc 85: Proc. Int. Symp. on Extractive Metallurgy of Zinc, Tokyo, Japan, 1985, pp wx 4 J. St.-Pierre, D.L. Piron, Electrowinning of zinc from alkaline solutions at high current densities, J. Appl. Electrochem. 20 Ž wx 5 J. St.-Pierre, D.L. Piron, Electrowinning of zinc from alkaline solution, J. Appl. Electrochem. 16 Ž wx 6 S.R. Swarnkar, B.L. Gupta, R.D. Sekharan, Iron control in zinc plant residue leach solution, Hydrometallurgy 42 Ž wx 7 E. Peters, Hydrometallurgical process innovation, Hydrometallurgy 29 Ž wx 8 E.C. Barrett, E.H. Nenniger, J. Dziewinski, A hydrometallurgical process to treat carbon steel electric arc furnace dust, Hydrometallurgy 30 Ž wx 9 F. Elgersma, G.F. Kamst, G.J. Witkamp, G.M. Van Rosmalen, Acidic dissolution of zinc ferrite, Hydrometallurgy 29 Ž

13 Y. Zhao, R. StanforthrHydrometallurgy w10x R.B. Tippin, R.L. Tate, Metal reclamation and detoxification of brass foundry waste sand, in: Proc. 93rd Annual Meeting, American Foundrymen s Society Transactions, USA, May 7 11 vol. 97, 1989, pp w11x P. Edger, G. Van Weert, Removal of zinc and lead from iron and steel wastes via chlorine pyrohydrolysis, in: A. Takeshi Ž Ed.., Zinc Lead 95, Proc. Int. Symp. Extr. Appl. Zinc Lead, Tokyo, 1995, pp w12x J.R. Donald, C.A. Pickles, Kinetics of the reduction of the zinc oxide in zinc ferrite with iron, in: P.B. Queneau, R.D. Peterson Ž Eds.., Recycl. Met. Eng. Mater., Int. Symp., 3rd, Minerals, Metals and Materials Society, Warrendale, PA, USA, 1995, pp w13x R. Stanforth, Method for reduction of heavy metal leaching from hazardous waste under acidic and nonacidic conditions, US Patent No. 5,037,479, Aug