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Ligand mediated eco-friendly leaching of zinc from spent catalyst in alkaline media
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JIEC-1588; No. of Pages 7 Journal of Industrial and Engineering Chemistry xxx (2013) xxx–xxx
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Ligand mediated eco-friendly leaching of zinc from spent catalyst in alkaline media M. Mohapatra *, Banaja Nayak, K. Sanjay, T. Subbaiah, B.K. Mishra Hydro & Electrometallurgy Department, CSIR-Institute of Minerals and Materials Technology, Bhubaneswar 751 013, Orissa, India
A R T I C L E I N F O
Article history: Received 5 March 2013 Accepted 28 September 2013 Available online xxx Keywords: Zinc oxide Zinc sulphide Alkaline leaching EDTA SEM
A B S T R A C T
A novel leaching process for recovering Zn from spent catalyst in alkaline solution has been discussed. The catalyst was characterized for physico-chemical properties by chemical, XRD, TG-DTA and SEM. More than 92% Zn could be extracted from spent catalyst under the conditions: pulp density 2.5% (w/v), NaOH 1 M, EDTA 0.025 M, temperature 80 8C and time 3 h. Zn extraction increased with the number of stages. At the 4th stage almost all Zn could be extracted. A tentative process flow-sheet has been proposed based on Zn recovery. Characterization of leach residue by XRD and SEM gave an insight into the mechanism. ß 2013 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction Waste residues generated by industrial processes are the major source of environmental contamination. Some of these are recyclable; others are toxic, constituting hazardous wastes. Recycling the valuable part of these wastes instead of landfill is an important alternative from both environmental and economic perspective. For this reason many industrial processes have focused research on the development of methods for their recovery of valuable products from waste materials [1–9]. Spent catalysts contribute a significant amount of the solid wastes generated worldwide. The spent catalyst wastes generated especially in the petrochemical industry [10] pose serious environmental problems, and it has presented a challenging task to recycle and convert these materials to useful products. The zinc oxide catalyst has been widely used to adsorb H2S evolved in the steam reforming process. After certain time, the catalytic activity of ZnO completely ceases due to transformation of ZnO to ZnS. Hence, recovering zinc from the spent catalyst is important to meet the world zinc consumption which is around 490 Mt in the first two quarters in 2009 [11] while addressing the environmental issues. Usually zinc catalyst contains ZnO, supported on alumina along with some filler materials. Some studies have been reported on leaching of zinc spent catalysts [12–
* Corresponding author. Tel.: +91 9432760688. E-mail addresses: [email protected], [email protected] (M. Mohapatra).
14]. Among the vast variety of the available methods for metal recovery, hydrometallurgical methods provide a viable technique. Generally, during acid leaching other associated impurities also get dissolved which requires further unit operations for the solution purification and recovery of zinc values. However, alkaline leaching of wastes has advantage of rejecting iron and silica to residue. Zhao and Stanforth [15] and Feng and Yang [16] studied the production of zinc powder by alkaline treatment of smithsonite ores. Over 85% of both Zn and Pb, and less than 10% of Al could be leached from the ores when the leaching was carried out at a temperature of 95 8C using 5 mol/L NaOH. Zhao and Stanforth [15] reported that electric arc furnace dust containing zinc could be leached affectively with 5 mol/ L NaOH solution. EDTA is a strong chelating agent that is widely used in the food industry as a stabilizer to prevent food spoilage and preserve food colour and flavour. EDTA is one of the lixiviant used for impurity removal from soils and other materials owing to its highly complexing nature. It is also used in detergents to form soluble complexes with calcium and magnesium ions that are often present in hard water. Using EDTA as one of the lixiviant for metal ion during leaching process offers many advantages such as it enhances the extraction process due to coupling of chemical leaching along with complexing technique and also their recyclability from leach solution and further reuse [17,18]. Heavy metals such as Pb extraction from the metal-containing sulfide have been reported where the advantages of use of EDTA were mentioned on extraction processes [19,20]. Recently, Jiangh et al. reported extraction of metals from a zinc smelting slag using two-step procedure combining acid and EDTA [21] and mainly they enlightened the
1226-086X/$ – see front matter ß 2013 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jiec.2013.09.053
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affect of EDTA on Pb extraction. However the synergistic effect of alkaline and complex leaching behaviour by EDTA for zinc ores/ secondary ores/wastes has still not been exploited in zinc hydrometallurgical processes. In aqueous solutions, EDTA can form a complex with Zn2+ in a 1:1 EDTA-to-Zn ratio by forming two bonds with Zn2+. The process will environment friendly, particularly, in case of alkaline leaching where least amount of gangue metals are dissolved from the ore body so the leach solution is not appreciably contaminated. Ligation will give added advantage of enhancing dissolution of metal values. Again, as reported alkaline electrowinning has many advantages over sulfate electrowinning such as lower electricity consumption and production of highly active Zn powder [15,22,23] zinc content of highly-alkaline filtrate solution, can be recovered in the active powder form through zinc alkaline electrowinning. Therefore, in the present study, alkaline leaching was chosen to recover zinc from spent catalyst using a combination of EDTA and NaOH as the lixiviant. 2. Experimental procedure 2.1. Materials Zinc spent catalyst used in the present study was procured from M/S Peekay Metals & Chemicals, Delhi. NaOH and EDTA were of analytical grade from E-MERCK, India. 2.2. Chemical analysis and characterization The samples were acid digested and zinc analysis was done both volumetrically [24] and by atomic adsorption spectrophotometer (Perkin-Elmer Model AA200). The particle size distribution of the original sample was carried by using Mastersizer-2000 of Malvern. X-ray powder diffraction (XRD) measurement of the samples was carried out using a Goniometer = PW3050/60 (Theta/Theta) Pananlytical powder diffractometer with a Mo/Ka X-ray source at 40 kV and 30 mA. The XRD results were recorded with a scan rate of 10/Min and compared with the PDF-4/Mineral powder diffraction file. Philips X-ray Fluorescence (XRF) analytical equipment was used for analysis of metal component in catalyst. SEM micrographs of original zinc spent catalyst and leach residues were examined by SEM using a HITACHI S 3400 instrument at 20 kV. The grains mounted on a stub were coated with Ag for SEM observation. 2.3. Leaching Leaching experiments were carried out in a 250 mL conical flask, which was placed on a thermostatically controlled magnetic stirrer. Temperature was maintained and controlled by passing water through a condenser. The desired amount of spent catalyst was added to 0.1 L leaching solution containing a known amount of NaOH and EDTA. Parameters such as time, concentration of NaOH and EDTA, temperature and pulp density were varied to study the leaching behaviour. After completion of leaching, the solution was filtered with Whatman 41 filter paper. The residue was washed thoroughly and the wash liquor was collected into leach liquor. Zn content in the solution was determined titrimetrically by adopting standard methods [25]. Other elements were analyzed with an atomic absorption spectrophotometer (AAS). 3. Results and discussion 3.1. Physico-chemical analysis and characterization The chemical analysis of the typical sample showed 65.39% Zn (Table 1). Cu, Fe and Ni are in the PPM level. Sulphate content of the sample was measured gravimetrically [25] and found to be 5.12%.
Table 1 Chemical analysis of the zinc spent catalyst. Element
(%)
Zn Fe Co Ni Cu Mn S pH of 2% solids
65.39 0.066 0.002 0.032 0.012 0.009 5.12 6.75
XRF analysis (Table 2) of the sample showed presence of alumina and silica in the sample. The SEM micrograph of spent catalyst is shown in Fig. 1(a). It was observed that the spent catalyst is mainly composed of three types of particles: small spherical black particles (shown by dashed arrow) of ZnS, white agglomerated particles (shown by double solid arrow) of ZnO and ZnO particles seems to be coated with ZnS (shown by solid single arrow). It could be seen from Fig. 1(a) that some black particles were adsorbed on the surface or pores of zinc oxide resulting in blockage of some pores. The particle size distribution of the spent catalyst is shown in Fig. 1(b). It shows that 10% of particles are below 5.18 mm and 50% of particles are below 52.64 mm and 90% of the particles are below 52.64 mm. The thermal behaviour of the spent catalyst sample is illustrated in Fig. 1(c). As can be seen, the thermo-gravimetric (TG) profile shows multi stages of weight loss. The total weight loss of the sample was found to be around 5% (Table 3). The initial weight loss (1.06%) in the temperature range of 100–230 8C was accompanied by an endothermic event that could be considered to be primarily due to the loss of loosely absorbed water. The second weight loss of 0.63% associated with endothermic differential thermal analysis (DTA) peak at 280 8C, was probably attributed to the intercalated water. It can be seen that the change from ZnS to ZnO is caused steeply between 620 and 700 8C with weight gain. An exothermic DTA peak at about 680–700 8C confirms the presence of ZnS phase in the sample. The weight gain obtained from the TG curves is only 0.36%, which agrees with the change from ZnS to ZnO. The XRD pattern of the original spent catalyst sample is given in Fig. 1(d), showing characteristic strong peaks of ‘d’ values at 2.85, 2.63 2.50, 1.92, 1.63, 1.48, 1.38 and 1.30 A˚ corresponding to the major diffraction peaks indexed as (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0),(1 0 3) and (1 1 2) planes of the Zincite (ZnO) structure with lattice constants of a = 3.249 A˚, b = 3.2498, c = 5.206 A˚ (PDF:00-036-1451). The peaks at ‘d’ values of 3.350, 3.16, 2.85, 1.92, 1.77, 1.63 corresponding to the major diffraction peaks indexed as (1 0 0), (0 0 2), (1 0 1), (1 1 0), (1 0 3) and (1 1 2) planes are of the Wurtzite (ZnS) structure with lattice constants of a = 3.821 A˚, b = 3.8212498, c = 6.257 A˚ (PDF00-036-1450) confirming the presence of two phases zincite and wurtize in the spent
Table 2 XRF data of Zinc spent catalyst sample. Analyte
Compound formula (%)
Concentration
Mg Al Si S Ca Fe Ni Cu Zn Cl
MgO Al2O3 SiO2 SO3 CaO Fe2O3 NiO CuO ZnO Cl
0.011 0.843 0.056 5.832 0.081 0.007 0.002 0.002 93.152 0.013
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Fig. 1. (a) SEM, (b) particle size distribution, (c) TG-DTA and (d) XRD of the zinc spent catalyst.
Table 3 Thermogravimetry and differential thermal analysis profile of Zn catalyst. Weight loss (%)
Temperature range (8C)
Endothermic peak (8C)
Expected compound loss
1.06 0.63 0.21 0.37 (Wt. gain) 3.41 Total weight loss 5.31.
100–230 230–300 300–600 600–650 680–1000
185 280 No peak 680–700 (exo) 850–880 (endo) Formation Of ZnO
Adsorbed water Intercalated water
catalyst. No characteristic peak related to any other phase was observed. 3.2. Leaching studies 3.2.1. Effect of time The effect of time was examined in the range of 0–300 min at 30 8C, stirring speed of 500 rpm, and pulp density of 2.5% (w/v) with the leachant composition as: (i) 1 M NaOH (ii) 0.025 M EDTA and (iii) 1 M NaOH + 0.025 M EDTA. It was observed that the dissolution of Zn increased as the time progressed (Fig. 2). Four
Fig. 2. Effect of leaching time on Zn extraction from zinc spent catalyst (Conditions: temperature: 80 8C, Pulp density: 2.5/100 (w/v), [NaOH]: 1 M, [EDTA]: 0.025 M).
Phase transformation of ZnS to ZnSO4
times of the stoichiometric requirement of NaOH to total zinc content in the material was used in the experiments. The recoveries by the end of 5 h were 39.94%, 31.7% and 52%, respectively for the three leachants mentioned above. While there is a marginal improvement with prolonged leaching with NaOH and EDTA alone, the combination experiment passed through maxima at 60% extraction of Zn in 3 h and thereafter dropped. The experiment was repeated and found that the phenomena occurred
Fig. 3. Effect of [NaOH] on Zn extraction from zinc spent catalyst. (Conditions: temperature: 30 8C, pulp density: 2.5/100 (w/v), [EDTA]: 0.025 M (where added), time: 3 h).
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in all the repeat experiments. Hence in all further studies, the time period was limited to 3 h. 3.2.2. Effect of NaOH concentration Effect of NaOH concentration with and without the presence of EDTA was studied. The conditions maintained were: 30 8C, EDTA concentration of nil or 0.025 M, 3 h leaching time. The results given in Fig. 3 indicates that the dissolution of Zn is strongly depended on alkali concentration. When alkali concentration in the absence of EDTA was increased from nil to 3 M, % Zn extraction increased from nil to 39%. This was because in the highly alkaline media, zinc exists in solution as complex anion designated as ZnðOHÞ4 2 [26]. However, in the presence of 0.025 M EDTA the extraction was higher. The extraction efficiency in these experiments also passed through maxima within the range of 0.5–1 M and further increasing NaOH concentration beyond 1 M has an adverse effect. The maximum percentage extraction of Zn was 60% in the optimal range of NaOH concentration i.e., 0.5–1 M. In a study by Zhang et al. [27], it has been reported that solution pH affects the dissociation equilibrium of a complexing agent due to protonation or hydroxylation of the ionic groups and hence had affected dissolution of ZnS nanoparticles. Dissolution of Zn in the presence of EDTA decreased at higher pH. In the present experiments an increase in pH was observed with increase in NaOH concentration as well as with prolonged experimentation. Again some authors [26,28,29] attributed lower zinc extractions in higher concentration of NaOH to an increase in the solution viscosity. Fig. 4 shows the fractional effect of EDTA on Zn extraction efficiency with varying NaOH concentration. As the concentration of NaOH increased, the fraction of Zn extraction contributed by ETDA decreased and with 3 M NaOH the EDTA effect was nullified. Therefore, the enhancement of extraction of Zn in presence of EDTA is a function of NaOH concentration. 3.2.3. Effect of EDTA concentration The percentage of Zn leached with variation of EDTA concentration is presented in the Fig. 5. 1 M NaOH was maintained and EDTA concentration was varied from nil to 0.1 M. In absence of EDTA, 30% Zn was leached and extraction efficiency of Zn increased with increase in EDTA concentration. Inset of Fig. 5 shows the fractional Zn extraction varying EDTA concentration in presence of NaOH. The effect was more or less uniform which indicates that NaOH and EDTA both have equal role in extraction of Zn. About 93% of Zn could be leached with 0.1 M EDTA in presence of 1 M NaOH.
Fig. 4. Role of EDTA with varying [NaOH] on extraction of Zn from Zinc spent catalyst. (Conditions: temperature: 30 8C, pulp density: 2.5/100 (w/v), [EDTA]: 0.025 M (where added), time: 3 h).
Fig. 5. Effect of [EDTA] on percent Zn extraction from zinc spent catalyst. (Conditions: temperature: 30 8C, pulp density: 2.5/100 (w/v), [NaOH]: 1 M (where added), time: 3 h) (Inset: Role of NaOH with varying [EDTA] on fractional extraction of Zn).
Tendy et al. [28] reported that Zn extraction by different chelants almost never exceeded 60%, even at a very high excess of chelants. Therefore, in this case higher extraction is due to synergistic effect of NaOH as lixiviant along with EDTA. 3.2.4. Effect of temperature Fig. 6 presents the effect of temperature on the dissolution of the zinc spent catalyst in the 30–90 8C range. The conditions maintained were: 1 M NaOH, 0.025 EDTA concentration, 3 h time. The recovery of Zn increased as the temperature increased till 70 8C. By increasing the temperature further, it seems there is no effect on leaching. With elevated temperature 93% of Zn could be leached. The temperature moderately influences the zinc dissolution enhances the dissolution from 60.14% to 92.66%. Our results have similar trend as reported by other researchers for the alkali leaching of electric arc furnace (EAF) dusts [29] and the leaching of a zinc silicate ore in the temperature from 60 8C to 90 8C (zinc extraction from 19% to 90%). 3.2.5. Effect of pulp density The effect of pulp density on Zn dissolution is shown in Fig. 7. The rest of the conditions maintained were: 1 M NaOH, 0.025 EDTA concentration, 80 8C, and 3 h time. The Zn extraction decreased from 92.65 to 42.25 as the pulp density increased from 2.5% to 50%
Fig. 6. Effect of temperature on percent Zn extraction from zinc spent catalyst (Conditions: Pulp density: 2.5/100 (w/v), [EDTA]: 0.025 M, [NaOH]: 1 M, time: 3 h).
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Fig. 8. Effect of stage wise leaching on percent Zn extraction from zinc spent catalyst (Conditions: 1 M NaOH, 0.025 M EDTA, Temperature 80 8C and time 3 h). Fig. 7. Effect of pulp density % Zn extraction from zinc spent catalyst (Conditions: temperature: 80 8C, [EDTA]: 0.025 M, [NaOH]: 1 M, time: 3 h).
(w/v). In order to see the role of EDTA and NaOH on leaching efficiency at higher pulp density, two experiments at 20% pulp density were carried out with doubling NaOH and EDTA concentrations respectively. The results showed that doubling NaOH resulted in increase of Zn extraction from 52% to 72% and
doubling EDTA improved Zn recovery from 52% to 78%. Therefore EDTA has predominant role in leaching of zinc catalyst. 3.2.6. Sequential leaching In order to recover complete Zn, sequential leaching was carried out in four steps. The residue generated in first step was leached
Fig. 9. Flow sheet of process developed for complete leaching Zn from zinc spent catalyst.
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Fig. 11. SEM of leach residue obtained from Zinc spent catalyst under leaching cond. 1 M NaOH, 0.025 M EDTA, temp 80 8C and time 3 h.
3.2.7. Process mechanism In the caustic soda leach process, zinc selectively dissolves in sodium hydroxide rejecting iron in the residue. In the present study pH of the leaching system was varied between 11 and 13. Considering thermodynamics of Zn(II)–NaOH–H2O system [11], mainly species of zinc, ZnðOHÞ4 2 and ZnðOHÞ2 2 are formed in the aqueous alkaline medium. The reaction reported by Liu [11] is as follows: 5ZnOðsÞ þ 10NaOHðaqÞ þ H2 O ! 2ZnO2 2 þ 3ZnðOHÞ4 2 þ 10Naþ Fig. 10. XRD patterns of leach residue obtained from zinc spent catalyst leaching at different conditions.
with fresh reactants in the second step and the residue obtained in second step was leached with fresh solutions in third step and so on. Effect of sequential leaching was studied under following conditions: 1 M NaOH, 0.025 EDTA concentration, 80 8C temperature and 3 h time. The pulp densities in the steps were 20, 10, 5 and 2.5, respectively. Fig. 8 shows the cumulative leaching efficiency in each step. By fourth step, most of the Zn could be recovered. Industrially, it is important to run the process at higher pulp density as it will reduce the capital investment. However, the high consumption of reagents, and low extraction efficiency at higher pulp densities can be addressed by step wise leaching. As the mass of the residue decreases with each step of leaching, one can afford decreasing the pulp density in later stages and hence can improve the overall recovery. The tentative overall process is shown in Fig. 9.
ZnO þ 2NaOH ! Na2 ZnO2 þ H2 O
(1) (2)
However, in the present material, ZnS could have been coated on ZnO. Therefore, leaching of ZnS is necessary prior to ZnO dissolution. Zhang et al. [27] reported the effect of pH on dissolution of ZnS nano particles in EDTA medium. EDTA may play a vital role to dissolve ZnS surface coating of ZnO core by following reaction. Zn2þ þ H2 Y2 ! ZnY2 þ 2Hþ
(3)
where Y is EDTA Characterization of leach residue obtained at different conditions of leaching can give an insight into the mechanism. 3.3. Characterization of leach residue 3.3.1. XRD patterns of feed and leach residues The XRD patterns of the leached residue obtained with leaching by (i) 1 M NaOH, (ii) 0.025 M EDTA and (iii) 1 M NaOH + 0.025 M
Fig. 12. Schematic mechanism of dissolution of Zn spent catalyst assisted by EDTA chelation.
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EDTA are shown in Fig. 10. The patterns indicated the presence of two phases; zincite (ZnO) and wurtize (ZnS). The XRD pattern of leach residue obtained under condition (i) was matching with the original spent catalyst (see Fig. 1(d)). However, in the presence of EDTA, the peaks for both ZnS and ZnO diminished marginally. Under the condition (iii) major peaks for ZnS phases are absent in the XRD pattern. As the overall recovery of Zn is directly related to ZnS leaching, absence of major ZnS supports the argument resulting in higher Zn recovery under this condition. 3.3.2. SEM studies The solid residue obtained after leaching for 3 h, at 80 8C, with 1 M NaOH + 0.025 M EDTA was analyzed by SEM (see Fig. 11). It is observed that the small spherical black particles of ZnS (shown by dashed arrow of Fig. 1(a)) are not visible. ZnO particles seems to be coated with ZnS (shown by solid single arrow of Fig. 1(a)) became brighter. As the recovery of Zn in the present experiment is >90%, diminishing of dark areas which might be ZnS phase was dissolved along with ZnO during leaching with NaOH in presence of EDTA. The mechanism of EDTA assisted dissolution of ZnS from ZnO surface may be represented schematically (see Fig. 12). 4. Conclusions Leaching of Zn spent catalyst was carried out with NaOH in presence of EDTA. The chemical analysis of the spent catalyst showed 65.39% Zn content. Cu, Fe and Ni are in the PPM level. Sulphur content of the sample was found to be 5.12%. XRF analysis showed the presence of alumina and silica in the sample. The median particle size (d50) is around 52.6 mm. When alkaline concentration without EDTA increased from nil to 3 M, the percentage of Zn leached increased from nil to 39%. However by addition of 0.025 M EDTA the leaching behaviour has changed. The Zn recovery increased from 27.63% to 60.14% by increasing the concentration of NaOH from nil to 1 M. Further increasing NaOH concentration has detrimental effect on Zn extraction. The recovery of Zn increased at elevated temperature. However, beyond 80 8C a marginal decrease in recovery was observed. By sequential leaching, in four steps complete recovery of Zn could be achieved. Characterization of leach residue by XRD and SEM gave an insight into the mechanism. The downstream processing of the leach liquor can be production of metallic zinc powder by electrowinning or precipitation of ZnO after chemical separation of impurities.
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Acknowledgements The authors are thankful to Prof. B.K. Mishra, Director, Institute of Minerals and Materials Technology, for his kind permission to publish this paper. The financial support provided by CSIR, India under CSC0101 is thankfully acknowledged.
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JIEC-1588; No. of Pages 7 Journal of Industrial and Engineering Chemistry xxx (2013) xxx–xxx
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Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Ligand mediated eco-friendly leaching of zinc from spent catalyst in alkaline media M. Mohapatra *, Banaja Nayak, K. Sanjay, T. Subbaiah, B.K. Mishra Hydro & Electrometallurgy Department, CSIR-Institute of Minerals and Materials Technology, Bhubaneswar 751 013, Orissa, India
A R T I C L E I N F O
Article history: Received 5 March 2013 Accepted 28 September 2013 Available online xxx Keywords: Zinc oxide Zinc sulphide Alkaline leaching EDTA SEM
A B S T R A C T
A novel leaching process for recovering Zn from spent catalyst in alkaline solution has been discussed. The catalyst was characterized for physico-chemical properties by chemical, XRD, TG-DTA and SEM. More than 92% Zn could be extracted from spent catalyst under the conditions: pulp density 2.5% (w/v), NaOH 1 M, EDTA 0.025 M, temperature 80 8C and time 3 h. Zn extraction increased with the number of stages. At the 4th stage almost all Zn could be extracted. A tentative process flow-sheet has been proposed based on Zn recovery. Characterization of leach residue by XRD and SEM gave an insight into the mechanism. ß 2013 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction Waste residues generated by industrial processes are the major source of environmental contamination. Some of these are recyclable; others are toxic, constituting hazardous wastes. Recycling the valuable part of these wastes instead of landfill is an important alternative from both environmental and economic perspective. For this reason many industrial processes have focused research on the development of methods for their recovery of valuable products from waste materials [1–9]. Spent catalysts contribute a significant amount of the solid wastes generated worldwide. The spent catalyst wastes generated especially in the petrochemical industry [10] pose serious environmental problems, and it has presented a challenging task to recycle and convert these materials to useful products. The zinc oxide catalyst has been widely used to adsorb H2S evolved in the steam reforming process. After certain time, the catalytic activity of ZnO completely ceases due to transformation of ZnO to ZnS. Hence, recovering zinc from the spent catalyst is important to meet the world zinc consumption which is around 490 Mt in the first two quarters in 2009 [11] while addressing the environmental issues. Usually zinc catalyst contains ZnO, supported on alumina along with some filler materials. Some studies have been reported on leaching of zinc spent catalysts [12–
* Corresponding author. Tel.: +91 9432760688. E-mail addresses: [email protected], [email protected] (M. Mohapatra).
14]. Among the vast variety of the available methods for metal recovery, hydrometallurgical methods provide a viable technique. Generally, during acid leaching other associated impurities also get dissolved which requires further unit operations for the solution purification and recovery of zinc values. However, alkaline leaching of wastes has advantage of rejecting iron and silica to residue. Zhao and Stanforth [15] and Feng and Yang [16] studied the production of zinc powder by alkaline treatment of smithsonite ores. Over 85% of both Zn and Pb, and less than 10% of Al could be leached from the ores when the leaching was carried out at a temperature of 95 8C using 5 mol/L NaOH. Zhao and Stanforth [15] reported that electric arc furnace dust containing zinc could be leached affectively with 5 mol/ L NaOH solution. EDTA is a strong chelating agent that is widely used in the food industry as a stabilizer to prevent food spoilage and preserve food colour and flavour. EDTA is one of the lixiviant used for impurity removal from soils and other materials owing to its highly complexing nature. It is also used in detergents to form soluble complexes with calcium and magnesium ions that are often present in hard water. Using EDTA as one of the lixiviant for metal ion during leaching process offers many advantages such as it enhances the extraction process due to coupling of chemical leaching along with complexing technique and also their recyclability from leach solution and further reuse [17,18]. Heavy metals such as Pb extraction from the metal-containing sulfide have been reported where the advantages of use of EDTA were mentioned on extraction processes [19,20]. Recently, Jiangh et al. reported extraction of metals from a zinc smelting slag using two-step procedure combining acid and EDTA [21] and mainly they enlightened the
1226-086X/$ – see front matter ß 2013 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jiec.2013.09.053
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affect of EDTA on Pb extraction. However the synergistic effect of alkaline and complex leaching behaviour by EDTA for zinc ores/ secondary ores/wastes has still not been exploited in zinc hydrometallurgical processes. In aqueous solutions, EDTA can form a complex with Zn2+ in a 1:1 EDTA-to-Zn ratio by forming two bonds with Zn2+. The process will environment friendly, particularly, in case of alkaline leaching where least amount of gangue metals are dissolved from the ore body so the leach solution is not appreciably contaminated. Ligation will give added advantage of enhancing dissolution of metal values. Again, as reported alkaline electrowinning has many advantages over sulfate electrowinning such as lower electricity consumption and production of highly active Zn powder [15,22,23] zinc content of highly-alkaline filtrate solution, can be recovered in the active powder form through zinc alkaline electrowinning. Therefore, in the present study, alkaline leaching was chosen to recover zinc from spent catalyst using a combination of EDTA and NaOH as the lixiviant. 2. Experimental procedure 2.1. Materials Zinc spent catalyst used in the present study was procured from M/S Peekay Metals & Chemicals, Delhi. NaOH and EDTA were of analytical grade from E-MERCK, India. 2.2. Chemical analysis and characterization The samples were acid digested and zinc analysis was done both volumetrically [24] and by atomic adsorption spectrophotometer (Perkin-Elmer Model AA200). The particle size distribution of the original sample was carried by using Mastersizer-2000 of Malvern. X-ray powder diffraction (XRD) measurement of the samples was carried out using a Goniometer = PW3050/60 (Theta/Theta) Pananlytical powder diffractometer with a Mo/Ka X-ray source at 40 kV and 30 mA. The XRD results were recorded with a scan rate of 10/Min and compared with the PDF-4/Mineral powder diffraction file. Philips X-ray Fluorescence (XRF) analytical equipment was used for analysis of metal component in catalyst. SEM micrographs of original zinc spent catalyst and leach residues were examined by SEM using a HITACHI S 3400 instrument at 20 kV. The grains mounted on a stub were coated with Ag for SEM observation. 2.3. Leaching Leaching experiments were carried out in a 250 mL conical flask, which was placed on a thermostatically controlled magnetic stirrer. Temperature was maintained and controlled by passing water through a condenser. The desired amount of spent catalyst was added to 0.1 L leaching solution containing a known amount of NaOH and EDTA. Parameters such as time, concentration of NaOH and EDTA, temperature and pulp density were varied to study the leaching behaviour. After completion of leaching, the solution was filtered with Whatman 41 filter paper. The residue was washed thoroughly and the wash liquor was collected into leach liquor. Zn content in the solution was determined titrimetrically by adopting standard methods [25]. Other elements were analyzed with an atomic absorption spectrophotometer (AAS). 3. Results and discussion 3.1. Physico-chemical analysis and characterization The chemical analysis of the typical sample showed 65.39% Zn (Table 1). Cu, Fe and Ni are in the PPM level. Sulphate content of the sample was measured gravimetrically [25] and found to be 5.12%.
Table 1 Chemical analysis of the zinc spent catalyst. Element
(%)
Zn Fe Co Ni Cu Mn S pH of 2% solids
65.39 0.066 0.002 0.032 0.012 0.009 5.12 6.75
XRF analysis (Table 2) of the sample showed presence of alumina and silica in the sample. The SEM micrograph of spent catalyst is shown in Fig. 1(a). It was observed that the spent catalyst is mainly composed of three types of particles: small spherical black particles (shown by dashed arrow) of ZnS, white agglomerated particles (shown by double solid arrow) of ZnO and ZnO particles seems to be coated with ZnS (shown by solid single arrow). It could be seen from Fig. 1(a) that some black particles were adsorbed on the surface or pores of zinc oxide resulting in blockage of some pores. The particle size distribution of the spent catalyst is shown in Fig. 1(b). It shows that 10% of particles are below 5.18 mm and 50% of particles are below 52.64 mm and 90% of the particles are below 52.64 mm. The thermal behaviour of the spent catalyst sample is illustrated in Fig. 1(c). As can be seen, the thermo-gravimetric (TG) profile shows multi stages of weight loss. The total weight loss of the sample was found to be around 5% (Table 3). The initial weight loss (1.06%) in the temperature range of 100–230 8C was accompanied by an endothermic event that could be considered to be primarily due to the loss of loosely absorbed water. The second weight loss of 0.63% associated with endothermic differential thermal analysis (DTA) peak at 280 8C, was probably attributed to the intercalated water. It can be seen that the change from ZnS to ZnO is caused steeply between 620 and 700 8C with weight gain. An exothermic DTA peak at about 680–700 8C confirms the presence of ZnS phase in the sample. The weight gain obtained from the TG curves is only 0.36%, which agrees with the change from ZnS to ZnO. The XRD pattern of the original spent catalyst sample is given in Fig. 1(d), showing characteristic strong peaks of ‘d’ values at 2.85, 2.63 2.50, 1.92, 1.63, 1.48, 1.38 and 1.30 A˚ corresponding to the major diffraction peaks indexed as (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0),(1 0 3) and (1 1 2) planes of the Zincite (ZnO) structure with lattice constants of a = 3.249 A˚, b = 3.2498, c = 5.206 A˚ (PDF:00-036-1451). The peaks at ‘d’ values of 3.350, 3.16, 2.85, 1.92, 1.77, 1.63 corresponding to the major diffraction peaks indexed as (1 0 0), (0 0 2), (1 0 1), (1 1 0), (1 0 3) and (1 1 2) planes are of the Wurtzite (ZnS) structure with lattice constants of a = 3.821 A˚, b = 3.8212498, c = 6.257 A˚ (PDF00-036-1450) confirming the presence of two phases zincite and wurtize in the spent
Table 2 XRF data of Zinc spent catalyst sample. Analyte
Compound formula (%)
Concentration
Mg Al Si S Ca Fe Ni Cu Zn Cl
MgO Al2O3 SiO2 SO3 CaO Fe2O3 NiO CuO ZnO Cl
0.011 0.843 0.056 5.832 0.081 0.007 0.002 0.002 93.152 0.013
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Fig. 1. (a) SEM, (b) particle size distribution, (c) TG-DTA and (d) XRD of the zinc spent catalyst.
Table 3 Thermogravimetry and differential thermal analysis profile of Zn catalyst. Weight loss (%)
Temperature range (8C)
Endothermic peak (8C)
Expected compound loss
1.06 0.63 0.21 0.37 (Wt. gain) 3.41 Total weight loss 5.31.
100–230 230–300 300–600 600–650 680–1000
185 280 No peak 680–700 (exo) 850–880 (endo) Formation Of ZnO
Adsorbed water Intercalated water
catalyst. No characteristic peak related to any other phase was observed. 3.2. Leaching studies 3.2.1. Effect of time The effect of time was examined in the range of 0–300 min at 30 8C, stirring speed of 500 rpm, and pulp density of 2.5% (w/v) with the leachant composition as: (i) 1 M NaOH (ii) 0.025 M EDTA and (iii) 1 M NaOH + 0.025 M EDTA. It was observed that the dissolution of Zn increased as the time progressed (Fig. 2). Four
Fig. 2. Effect of leaching time on Zn extraction from zinc spent catalyst (Conditions: temperature: 80 8C, Pulp density: 2.5/100 (w/v), [NaOH]: 1 M, [EDTA]: 0.025 M).
Phase transformation of ZnS to ZnSO4
times of the stoichiometric requirement of NaOH to total zinc content in the material was used in the experiments. The recoveries by the end of 5 h were 39.94%, 31.7% and 52%, respectively for the three leachants mentioned above. While there is a marginal improvement with prolonged leaching with NaOH and EDTA alone, the combination experiment passed through maxima at 60% extraction of Zn in 3 h and thereafter dropped. The experiment was repeated and found that the phenomena occurred
Fig. 3. Effect of [NaOH] on Zn extraction from zinc spent catalyst. (Conditions: temperature: 30 8C, pulp density: 2.5/100 (w/v), [EDTA]: 0.025 M (where added), time: 3 h).
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in all the repeat experiments. Hence in all further studies, the time period was limited to 3 h. 3.2.2. Effect of NaOH concentration Effect of NaOH concentration with and without the presence of EDTA was studied. The conditions maintained were: 30 8C, EDTA concentration of nil or 0.025 M, 3 h leaching time. The results given in Fig. 3 indicates that the dissolution of Zn is strongly depended on alkali concentration. When alkali concentration in the absence of EDTA was increased from nil to 3 M, % Zn extraction increased from nil to 39%. This was because in the highly alkaline media, zinc exists in solution as complex anion designated as ZnðOHÞ4 2 [26]. However, in the presence of 0.025 M EDTA the extraction was higher. The extraction efficiency in these experiments also passed through maxima within the range of 0.5–1 M and further increasing NaOH concentration beyond 1 M has an adverse effect. The maximum percentage extraction of Zn was 60% in the optimal range of NaOH concentration i.e., 0.5–1 M. In a study by Zhang et al. [27], it has been reported that solution pH affects the dissociation equilibrium of a complexing agent due to protonation or hydroxylation of the ionic groups and hence had affected dissolution of ZnS nanoparticles. Dissolution of Zn in the presence of EDTA decreased at higher pH. In the present experiments an increase in pH was observed with increase in NaOH concentration as well as with prolonged experimentation. Again some authors [26,28,29] attributed lower zinc extractions in higher concentration of NaOH to an increase in the solution viscosity. Fig. 4 shows the fractional effect of EDTA on Zn extraction efficiency with varying NaOH concentration. As the concentration of NaOH increased, the fraction of Zn extraction contributed by ETDA decreased and with 3 M NaOH the EDTA effect was nullified. Therefore, the enhancement of extraction of Zn in presence of EDTA is a function of NaOH concentration. 3.2.3. Effect of EDTA concentration The percentage of Zn leached with variation of EDTA concentration is presented in the Fig. 5. 1 M NaOH was maintained and EDTA concentration was varied from nil to 0.1 M. In absence of EDTA, 30% Zn was leached and extraction efficiency of Zn increased with increase in EDTA concentration. Inset of Fig. 5 shows the fractional Zn extraction varying EDTA concentration in presence of NaOH. The effect was more or less uniform which indicates that NaOH and EDTA both have equal role in extraction of Zn. About 93% of Zn could be leached with 0.1 M EDTA in presence of 1 M NaOH.
Fig. 4. Role of EDTA with varying [NaOH] on extraction of Zn from Zinc spent catalyst. (Conditions: temperature: 30 8C, pulp density: 2.5/100 (w/v), [EDTA]: 0.025 M (where added), time: 3 h).
Fig. 5. Effect of [EDTA] on percent Zn extraction from zinc spent catalyst. (Conditions: temperature: 30 8C, pulp density: 2.5/100 (w/v), [NaOH]: 1 M (where added), time: 3 h) (Inset: Role of NaOH with varying [EDTA] on fractional extraction of Zn).
Tendy et al. [28] reported that Zn extraction by different chelants almost never exceeded 60%, even at a very high excess of chelants. Therefore, in this case higher extraction is due to synergistic effect of NaOH as lixiviant along with EDTA. 3.2.4. Effect of temperature Fig. 6 presents the effect of temperature on the dissolution of the zinc spent catalyst in the 30–90 8C range. The conditions maintained were: 1 M NaOH, 0.025 EDTA concentration, 3 h time. The recovery of Zn increased as the temperature increased till 70 8C. By increasing the temperature further, it seems there is no effect on leaching. With elevated temperature 93% of Zn could be leached. The temperature moderately influences the zinc dissolution enhances the dissolution from 60.14% to 92.66%. Our results have similar trend as reported by other researchers for the alkali leaching of electric arc furnace (EAF) dusts [29] and the leaching of a zinc silicate ore in the temperature from 60 8C to 90 8C (zinc extraction from 19% to 90%). 3.2.5. Effect of pulp density The effect of pulp density on Zn dissolution is shown in Fig. 7. The rest of the conditions maintained were: 1 M NaOH, 0.025 EDTA concentration, 80 8C, and 3 h time. The Zn extraction decreased from 92.65 to 42.25 as the pulp density increased from 2.5% to 50%
Fig. 6. Effect of temperature on percent Zn extraction from zinc spent catalyst (Conditions: Pulp density: 2.5/100 (w/v), [EDTA]: 0.025 M, [NaOH]: 1 M, time: 3 h).
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Fig. 8. Effect of stage wise leaching on percent Zn extraction from zinc spent catalyst (Conditions: 1 M NaOH, 0.025 M EDTA, Temperature 80 8C and time 3 h). Fig. 7. Effect of pulp density % Zn extraction from zinc spent catalyst (Conditions: temperature: 80 8C, [EDTA]: 0.025 M, [NaOH]: 1 M, time: 3 h).
(w/v). In order to see the role of EDTA and NaOH on leaching efficiency at higher pulp density, two experiments at 20% pulp density were carried out with doubling NaOH and EDTA concentrations respectively. The results showed that doubling NaOH resulted in increase of Zn extraction from 52% to 72% and
doubling EDTA improved Zn recovery from 52% to 78%. Therefore EDTA has predominant role in leaching of zinc catalyst. 3.2.6. Sequential leaching In order to recover complete Zn, sequential leaching was carried out in four steps. The residue generated in first step was leached
Fig. 9. Flow sheet of process developed for complete leaching Zn from zinc spent catalyst.
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Fig. 11. SEM of leach residue obtained from Zinc spent catalyst under leaching cond. 1 M NaOH, 0.025 M EDTA, temp 80 8C and time 3 h.
3.2.7. Process mechanism In the caustic soda leach process, zinc selectively dissolves in sodium hydroxide rejecting iron in the residue. In the present study pH of the leaching system was varied between 11 and 13. Considering thermodynamics of Zn(II)–NaOH–H2O system [11], mainly species of zinc, ZnðOHÞ4 2 and ZnðOHÞ2 2 are formed in the aqueous alkaline medium. The reaction reported by Liu [11] is as follows: 5ZnOðsÞ þ 10NaOHðaqÞ þ H2 O ! 2ZnO2 2 þ 3ZnðOHÞ4 2 þ 10Naþ Fig. 10. XRD patterns of leach residue obtained from zinc spent catalyst leaching at different conditions.
with fresh reactants in the second step and the residue obtained in second step was leached with fresh solutions in third step and so on. Effect of sequential leaching was studied under following conditions: 1 M NaOH, 0.025 EDTA concentration, 80 8C temperature and 3 h time. The pulp densities in the steps were 20, 10, 5 and 2.5, respectively. Fig. 8 shows the cumulative leaching efficiency in each step. By fourth step, most of the Zn could be recovered. Industrially, it is important to run the process at higher pulp density as it will reduce the capital investment. However, the high consumption of reagents, and low extraction efficiency at higher pulp densities can be addressed by step wise leaching. As the mass of the residue decreases with each step of leaching, one can afford decreasing the pulp density in later stages and hence can improve the overall recovery. The tentative overall process is shown in Fig. 9.
ZnO þ 2NaOH ! Na2 ZnO2 þ H2 O
(1) (2)
However, in the present material, ZnS could have been coated on ZnO. Therefore, leaching of ZnS is necessary prior to ZnO dissolution. Zhang et al. [27] reported the effect of pH on dissolution of ZnS nano particles in EDTA medium. EDTA may play a vital role to dissolve ZnS surface coating of ZnO core by following reaction. Zn2þ þ H2 Y2 ! ZnY2 þ 2Hþ
(3)
where Y is EDTA Characterization of leach residue obtained at different conditions of leaching can give an insight into the mechanism. 3.3. Characterization of leach residue 3.3.1. XRD patterns of feed and leach residues The XRD patterns of the leached residue obtained with leaching by (i) 1 M NaOH, (ii) 0.025 M EDTA and (iii) 1 M NaOH + 0.025 M
Fig. 12. Schematic mechanism of dissolution of Zn spent catalyst assisted by EDTA chelation.
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EDTA are shown in Fig. 10. The patterns indicated the presence of two phases; zincite (ZnO) and wurtize (ZnS). The XRD pattern of leach residue obtained under condition (i) was matching with the original spent catalyst (see Fig. 1(d)). However, in the presence of EDTA, the peaks for both ZnS and ZnO diminished marginally. Under the condition (iii) major peaks for ZnS phases are absent in the XRD pattern. As the overall recovery of Zn is directly related to ZnS leaching, absence of major ZnS supports the argument resulting in higher Zn recovery under this condition. 3.3.2. SEM studies The solid residue obtained after leaching for 3 h, at 80 8C, with 1 M NaOH + 0.025 M EDTA was analyzed by SEM (see Fig. 11). It is observed that the small spherical black particles of ZnS (shown by dashed arrow of Fig. 1(a)) are not visible. ZnO particles seems to be coated with ZnS (shown by solid single arrow of Fig. 1(a)) became brighter. As the recovery of Zn in the present experiment is >90%, diminishing of dark areas which might be ZnS phase was dissolved along with ZnO during leaching with NaOH in presence of EDTA. The mechanism of EDTA assisted dissolution of ZnS from ZnO surface may be represented schematically (see Fig. 12). 4. Conclusions Leaching of Zn spent catalyst was carried out with NaOH in presence of EDTA. The chemical analysis of the spent catalyst showed 65.39% Zn content. Cu, Fe and Ni are in the PPM level. Sulphur content of the sample was found to be 5.12%. XRF analysis showed the presence of alumina and silica in the sample. The median particle size (d50) is around 52.6 mm. When alkaline concentration without EDTA increased from nil to 3 M, the percentage of Zn leached increased from nil to 39%. However by addition of 0.025 M EDTA the leaching behaviour has changed. The Zn recovery increased from 27.63% to 60.14% by increasing the concentration of NaOH from nil to 1 M. Further increasing NaOH concentration has detrimental effect on Zn extraction. The recovery of Zn increased at elevated temperature. However, beyond 80 8C a marginal decrease in recovery was observed. By sequential leaching, in four steps complete recovery of Zn could be achieved. Characterization of leach residue by XRD and SEM gave an insight into the mechanism. The downstream processing of the leach liquor can be production of metallic zinc powder by electrowinning or precipitation of ZnO after chemical separation of impurities.
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Acknowledgements The authors are thankful to Prof. B.K. Mishra, Director, Institute of Minerals and Materials Technology, for his kind permission to publish this paper. The financial support provided by CSIR, India under CSC0101 is thankfully acknowledged.
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