Recovery Of Pure Zno Nanoparticles From Spent Zn

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Recovery of Pure ZnO Nanoparticles from Spent Zn-MnO2 Alkaline Batteries Akash Deep1*, Kamal Kumar2, Parveen Kumar1, Pawan Kumar1, Amit L Sharma1, Bina Gupta3, Lalit M Bharadwaj1 1 Biomolecular Electronics and Nanotechnology Division, Central Scientific Instruments Organization, Sector 30C, Chandigarh 160030, India 2 3 Department of Physics, Gurukula Kangri University, Haridwar 249404, India Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247667, India *Corresponding author email: [email protected] Abstract Present work reports the recovery of pure zinc nanoparticles from a leaching solution of spent Zn-Mn dry batteries. Black powder from the dismantled spent batteries was reacted with 5 mol L-1 HCl to produce a leaching solution containing 2.90 g L-1 Zn and 2.02 g L-1 Mn. Selective extraction of Zn(II) was carried out with a 0.10 mol L-1 solution of Cyanex 923 in n-hexane. An A/O (Aqueous/Organic) phase ratio of 1 was employed for the quantitative transfer of zinc. The loaded organic phase was subjected to combustion at high temperature to yield ZnO nanoparticles. The important characteristics of the prepared nanoparticles was assessed by Field Emission Scanning Electron Microscopy (FESEM), Energy Dispersive Xray Spectroscopy (EDX), X-ray Diffraction Spectroscopy (XRD) and Atomic Force Microscopy (AFM). Key Words: ZnO nanoparticles, spent batteries, solvent extraction, Cyanex 923 1. Introduction Zinc – manganese oxide alkaline batteries are frequently used to operate many electronic and electrical appliances. Recycling of spent Zn-MnO2 alkaline batteries is very important to reduce possible environmental damage due this ever-accumulating waste. However, recycling is still not a favourable business particular in developing countries where economic interests supersede environmental obligations. In this scenario, the recovery of high cost products from the spent waste may be useful to attract more industry to explore the toxic discards as possible cheap raw material. Several pyrometallurgical and hydrometallurgical processes have been suggested to recover metal components from different types of spent batteries [1-10] including Zn-MnO2 alkaline batteries [11-21]. A recent review article [22] summarizes the status of technological advancements in the recovery of metals from spent batteries. However, most of the suggested processes are not so user friendly due to high energy requirements and low value of the recovered product. Liquid-liquid extraction of pure metals from the spent batteries [7-10] is a viable method both in terms of simplicity and achievable purity of the recovered product. Using this technique, some researchers have also proposed the separation of metals from spent Zn-MnO2 alkaline batteries [19-21]. These reports highlight the use of phosphonic and phosphinic acid derivatives, such as Cyanex 272 and Cyanex 301. In an earlier published study, we identified a phosphine oxide reagent, Cyanex 923 (a mixture of four tri alkyl phosphine oxides), to be very useful for the extraction of transitions metals from chloride media [23-25]. This extractant offers fast extraction kinetics, better loading capacity, and improved phase separations over Cyanex 272 and 301. Moreover, the phosphine oxide extractant has a much better loading capacity than Cyanex 272 and 301 and is also free from foul smell. Apparently, the recovery of zinc in the form of ZnO nanoparticles is highly beneficial due to their specific electrical, catalytic, photochemical and optoelectronic properties. These nanoparticles find important applications in piezoelectric transducers, gas sensors, photonic crystals, light-emitting devices, photodetectors, photodiodes, optical waveguides, transparent conductive films, varistors and solar cells. Different techniques have been developed for the preparation of ZnO nanoparticles, which include sol–gel method, evaporative decomposition of solution, template-assisted growth, wet chemical synthesis, and gas-phase reaction [26]. Using these techniques, ZnO have been transformed to different morphologies including nanoparticles [26], nanorods [27, 28], nanowires [28], nanotubes [29] and nanorings [30]. The present work demonstrates the use of Cyanex 923 for the extraction of zinc from the leaching solution of the black powder of spent Zn-MnO2 alkaline batteries. The organic phase loaded with pure zinc has been evaporated at high temperature to produce ZnO nanoparticles, which have been characterized by Field Emission Scanning Electron Microscopy (FESEM), Energy Dispersive X-ray Spectroscopy (EDX), X-ray Diffraction Spectroscopy (XRD), Atomic Force Microscopy (AFM) and Ultra Violet-Visible (UV-Vis) Spectrophotometry. The adopted approach is novel in reporting the synthesis of pure ZnO nanoparticles from spent alkaline battery waste. 2. Experimental 2.1. Materials and equipment Metal standards for atomic absorption spectroscopy were purchased from Merck Chemicals, India. Other reagents, solvents, and indicators were A.R. grade materials from Fisher Scientific or Sigma Aldrich. Cyanex 923 was received as a gift sample from Cytec Canada Inc. This product was a 93% pure mixture of four trialkylphosphine oxides: R 3P=O, R′R2P=O, R2R′P=O, and R3′P=O, where R and R′ represented n-octyl and n-hexyl hydrocarbon chains, respectively. Concentration of metals in aqueous solutions was determined by Atomic Absorption Spectrometer (AAS, Perkin Elmer – AAnalyst 200). Nanoparticles were characterized on UV –Vis spectrophotometer (Varian – Cary 5000), Field Emission Scanning Electron Microscope – Energy Dispersive X-ray Spectroscope (FESEM-EDX, Hitachi – 4800 SE). X-ray diffraction analysis was carried out on SHIMADZU-6000 X-ray diffractometer using Cu Kα radiation (λ = 1.54056 Å). Atomic Force Microscopy of the sample was carried out by XENSOM AFM system from Park Systems, Korea. 2.2. Dismantling of spent Zn-MnO2 alkaline batteries and their elemental composition Spent Zn-MnO2 dry alkaline cells (AA size) of a particular brand were collected from the local sources. A set of 20 spent batteries was manually dismantled. Scrap papers, plastic films and outer metallic body were carefully removed. Black powder from the cathodic and anodic materials was separated and dried for 24 h at 1200C. The resulting powder was manually ground to make a homogenous matrix. 1 g of the sample was subsequently treated with 20 ml of aqua regia. The contents were heated to boil for 1 h. After cooling, the solution was appropriately diluted with double distilled water to determine the concentration of contained metals by Atomic Absorption Spectroscopy. 2.3. Leaching of zinc from the black powder 10 g sample of the black powder, obtained by the dismantling of spent batteries, was treated with 100 ml of 5 mol L-1 HCl at 700C for 2 h. After the reaction, the contents were allowed to cool and then filtered to obtain leaching solution. Residual mass was washed in four steps with a total of 100 mL of double distilled water. Final volume of the leaching solution was made up to 500 mL. 2.4. Liquid-liquid extraction studies Required volume of 0.15 mol L-1 of Cyanex 923 solution was prepared in n-hexane. Equal volumes of the leaching solution of spent Zn-MnO2 alkaline batteries and the extractant solution were shaken with an orbital shaker (250C) for 5 min to ensure complete equilibrium. The phase separation was achieved within 30 sec. The two phases were then separated and the concentration of metals in the aqueous phase was determined by AAS. The concentration of metals in the organic phase was calculated by mass balance. Stripping of the extracted zinc was carried out by contacting the organic phase with an aqueous acidic solution (pH 3) at 250C for 2 min. The recovery was then determined by AAS. 2.5. Synthesis of ZnO nanoparticles After the extraction of zinc, 10 mL volume of the organic phase in a platinum crucible was heated at elevated temperatures for 1 h. The particles thus formed were characterized by FESEM-EDX, XRD, AFM and UV-Vis spectrophotometry. 3. Results and Discussion 3.1. Composition of spent Zn-MnO2 alkaline batteries and leaching tests AAS analysis (Based on 3 measurements) of the digested black powder (1 g solid in 50 mL of final volume) from spent batteries revealed the following concentration of metals: Zn – 3.05±0.05 g L-1, Mn – 7.05±0.10 g L-1, Ni – (3.1±0.05)×10-3 g L-1, Fe – (51±1.0)×10-3 g L-1, Cd – (2.5±0.02)×10-3 g L-1. Metallic composition of different sets of batteries was also evaluated and the overall study highlighted average composition of the black powder from spent alkaline batteries to be as follows: 15 ± 0.3% Zn, 35 ± 0.5% Mn, 0.015 ± 0.005% Ni, 0.25 ± 0.03% Fe, 0.012 ± 0.005% Cd. Results of the leaching of the black powder with 5 mol L-1 HCl solution are given in Table 1. It is apparent that 2 h of leaching solubilized almost 96% of zinc. Recovery of manganese was lower (30%). Nickel, iron and cadmium were almost completely dissolved. Free acid content in the final leaching solution was estimated by acid-base titration using bromophenol blue indicator. It is pertinent to mention that the dissolved metals did not precipitate out during the titration reaction. The experiment revealed the final free HCl concentration in the leaching solution to be 0.82 ± 0.05 mol L-1. 3.2. Extraction of metals from leaching solution of spent batteries by Cyanex 923 The leaching solution from spent batteries was subjected to extraction with varying concentrations (0.01, 0.05, 0.075, 0.10, 0.125, 0.15 mol L-1) of Cyanex 923 in n-hexane. As shown in Figure 1, the extraction of Zn in Cyanex 923 is higher than that of Fe and Cd. Mn and Ni are negligibly (< 1%) extracted in Cyanex 923 and their extraction data has not been plotted. About 87% extraction of Zn is possible in a single stage operation with 0.10 mol L-1 Cyanex 923. About 22% of iron and 25% of cadmium are co-extracted. In all further studies, the use of 0.10 mol L -1 Cyanex 923 was preferred as it was possible to extract sufficiently high percentage of Zn with minimum possible interference by Fe and Cd. HCl content in leaching solution influences the metal extraction in Cyanex 923. Therefore, the extraction pattern of Zn, Mn, Ni, Fe and Cd was further investigated as a function of initial aqueous phase acidity keeping the extractant concentration constant at 0.10 mol L-1 (Figure 2). For this, synthetic solutions were prepared to match the composition of leaching solution as depicted in Table 1, but with different final HCl concentrations of 0.50, 0.60, 0.70, 0.80, 0.90, 1.0, 1.1 and 1.2 mol L-1. The extraction of Zn, Fe and Cd is directly proportional to the HCl concentration in the initial aqueous phase. The extraction of zinc becomes more than 90% in aqueous solutions with HCl acidity of 1.0 mol L-1 or higher. Lower values of percentage extractions in relatively dilute HCl solutions may be attributed to the formation of non-neutral anionic ZnCl- species, whose co-presence with extractable ZnCl2 species reduces overall extraction. Presence of high concentration of HCl in the leaching solution tends to increase interference from Fe and Cd as the presence of their extractable neutral species also starts to dominate. 3.3 Extraction equilibrium The extraction of zinc with solvating extractant, Cyanex 923 (R), can be proposed to proceed according to the following reaction (1) The stoichiometric extraction constant of above reaction (1) is (2) Since is the distribution ratio D, logarithmic form of equation (2) can be written as (3) Apparently, at constant Cl- concentration, the slop of the plot (Figure 1) between the varying extractant concentration and log D gives the value of n = 2. Therefore, the complex formation in the organic phase can be concluded to take place as neutral ZnCl2.2R. Solving equation 3 for different extractant concentrations at fixed [Cl-] of 0.82 mol L-1 gives a conditional average extraction constant Kex value of (1.92±0.04) × 104. Figure 1 shows the results of the study of the effect of extractant concentration keeping a fixed acid concentration. Investigation of the same parameter with respect to the different HCl concentrations is important to verify the repeatability of results under varying leaching conditions. The extraction of Zn2+ has therefore been monitored in varying HCl and extractant concentrations. Figure 3 shows the results. This study was carried out with synthetic metal salt solutions and the results have revealed no change in the slope value of log [Cyanex 923] vs. log D. The extraction constants were again calculated for every set of the experiment. The estimated values of Kex are almost similar under different conditions of the aqueous phase acidity, e.g. Kex,0.6 M HCl = (1.93±0.02) × 104; Kex,0.8 M HCl = (1.97±0.02) × 104; Kex,1.0 M HCl = (1.97±0.03) × 104; and Kex,1.2 M HCl = (1.96±0.03) × 104. Cyanex 923 can thus be utilized for the recovery of Zn from the leachates of the spent batteries irrespective of varying leaching conditions as stated above. Also, the adopted method of calculating the conditional extraction constants gives fairly precise values without complex speciation modelling. 3.4 Extraction isotherm Figure 4 shows the loading of the organic phase with increasing amounts of zinc. This experiment was carried out by maintaining a particular volume of aqueous phase (leaching solution) and then investigating the distribution of Zn2+ in different volumes of organic phase (0.10 mol L-1 Cyanex 923) such as to collect data for A/O ratios of 4, 3, 2, 1, 0.5, 0.33 and 0.25. The plot is useful to estimate the number of extraction stages required for the complete distribution of Zn2+ in the organic phase at above A/O ratios. Under selected leaching conditions and by taking an A/O ratio of 1, the complete extraction of Zn2+ can be obtained in three counter-current steps. 3.5 Extraction of Zn2+ from the leaching solution After optimizing the extraction parameters, 200 mL aliquot of the spent battery leaching solution containing 2.93 g L-1 Zn, 2.03 g L-1 Mn, 3.07 × 10-3 g L-1 Ni, 50.5 × 10-3 g L-1 Fe and 2.47 × 10-3 g L-1 Cd was equilibrated in three counter-current stages with 0.10 mol L-1 Cyanex 923 keeping an A/O ratio of 1. The concentration of the leaching solution before and after the extraction is shown in Table 2. The estimated composition of the organic phase is also given. Apparently, Zn was almost completely extracted. Fractions of Fe and Cd were co-extracted as a result of three counter-current extraction steps. Mn and Ni remained in the aqueous raffinate. Subsequently, the loaded organic phase was washed four times with 50 mL of a scrubbing solution (2.0 mol L -1 HCl), which selectively removed the co-extracted impurities and left pure Zn in the organic phase (Table 2). This was confirmed by taking a small (1.0 mL) sample of the above organic phase and transferring the metal contents to an aqueous phase (1.0 mL of 2.0 mol L-1 HNO3), which was then analysed on AAS. It was previously checked that the taken concentration of HNO3 could remove all the possibly entrained metals from Cyanex 923. 3.6 Treatment of loaded organic phase to yield concentrated zinc plant electrolyte or nanoparticles Pure zinc, remained in the organic phase, was further processed by two approaches in order to obtain a usable product. In the first approach, the stripping of pure zinc from the loaded organic phase was carried out with a simulated spent plant electrolyte (90 g L -1 ZnSO4 in 1.1 mol L-1 H2SO4). Required stripping isotherms (Figure 5) were studied at different A/O ratios (0.1, 0.25, 0.5, 0.75, 1, 1.5). The numbers of stages required for the complete stripping have been estimated. Evidently, an A/O ratio of 0.25 should provide almost complete stripping in two counter-current steps. Based on the above data, the recovery of zinc from 80 mL aliquot of the loaded organic phase was carried out in two counter-current stripping stages with 20 mL (in each stage) of 90 g L -1 ZnSO4 in 1.1 mol L-1 H2SO4. Finally recovered zinc solution had composition: Zn – 101 g L-1 ; Mn, Fe, Ni, Cd – not detected. The first approach of recovering usable zinc from the organic phase has yielded concentrated zinc plant electrolyte solution which is suitable for electrowinning cells. Alternatively, the zinc loaded organic phase has been subjected to combustion at high temperature yielding ZnO nanostructure. For comparative purpose, the surface characteristics and the elemental composition of the black powder of spent batteries were investigated (Figure 6). Macroscopic structure is observed with no particular shape of particles. EDX pattern suggests the presence of Zn, Mn, Fe, Cd and Ni. Expectedly, Zn and Mn are major fractions. Figure 7-10 depict the surface pattern and the elemental composition of ZnO particles after the combustion of zinc loaded organic phase at different temperatures. At 300 and 4000C, the size of particles is more than 100 nm with significant containment of phosphorus. Cyanex 923 is the source of phosphorus impurity during combustion at lower temperatures. Solution combustion at temperatures (500 and 6000C) much higher than the boiling point of Cyanex 923 results in the formation of ZnO nanoparticles free from any metallic or non-metallic impurity. In overall, the formation of ZnO nanoparticles from the leaching solution can be represented by the following reactions. (4) (5) It can be concluded that the combustion of the zinc loaded Cyanex 923 phase at 6000C produces ZnO nanoparticles with size ranging around 35 – 50 nm. AFM data of the synthesized ZnO nanoparticles are shown in Figures 11 (a) and 11 (b). True Non-Contact AFM surface topography image at the 2 µm x 2 µm scale reveals their maximum particle size to be of 40-50 nm range. Figure 3 (a) further displays a three dimensional (3D) view of the scan area supporting the FESEM investigations about the possible particles agglomeration in dried form. Figure 12 highlights the UV-Vis spectrophotometric data of ZnO6000C nanoparticles. An absorption peak at 351 nm is observed suggesting a crystalline structure and the semiconductor nature of the formed nanoparticles arisen due to electronic transitions between valence and conduction bands. A well-defined exciton peak at 351 nm and a sharp absorbance onset are blue-shifted compared to the value for bulk ZnO at about 385 nm. An XRD spectrum (Figure 13) of these nanoparticles is characterized with well defined peaks for crystalline structure. All the observed diffraction peaks can be correlated with hexagonal phase of zinc oxide. There were no diffraction peaks from other impurities. 4.0 Conclusions Recovery of pure ZnO nanoparticles from spent rechargeable battery has been achieved. The problem of huge waste generation by an ever-increasing use of rechargeable batteries is on continuous increase especially in developing countries like India. Using such materials for the production of pure ZnO nanoparticles can invoke economic interest. Proposed hydrometallurgical method is quite simple and can be carried out in typically used industrial mixer-settlers with no particular modification. The extraction is fast enough to enable the use of column extractors as well. The extraction of Zn in Cyanex 923 is fairly selective and any co-extracted impurity can be easily washed out with dilute HCl solution. If required, the zinc can be recovered from the organic phase with typical electrowinning electrolyte. Pure ZnO nanoparticles have been synthesized by combustion of the loaded organic phase. Apart from offering high yield, the adopted cost effective and facile combustion process does not require complex equipment and complicated operation. Various data reveal the important physical characteristics of the product. The prepared ZnO nanoparticles would be usable in sensing applications as well in other areas of electronics, pigments etc. Acknowledgements Authors are thankful to Dr Pawan Kapur, Director, CSIO, Chandigarh, India for providing necessary infrastructure facilities. We thank Ms. Manpreet for helping in part of the experimental work. References [1] K. Huang, J. Li, Z. 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Composition of leaching liquor obtained after treating 10 g of the black powder of spent batteries with 100 mL of 5 mol L -1 HCl at 700C (final volume = 500 mL) Metal Concentration in digested sample g L-1 Concentration in leaching sample g L-1 % Leaching Zn 3.05±0.05 2.93±0.04 96.1 Mn 7.05±0.10 2.03±0.04 28.8 Ni (3.1±0.05) × 10-3 (3.07±0.05) × 10-3 ≥ 99 Fe (51±1) × 10-3 (50.5±1) × 10-3 ≥ 99 Cd (2.5±0.02) × 10-3 (2.47±0.02) × 10-3 ≥ 99 Table 2. Extraction and recovery of zinc from the leaching liquor in three step countercurrent extraction with 0.10 mol L -1 Cyanex 923 (A/O =1) followed by the scrubbing of zinc loaded organic phase with 2.0 mol L -1 HCl (4 times, A/O = 0.25) Metal Concentration in leaching sample (g L-1) Concentration in aqueous phase after extraction (g L-1) Estimated concentration in organic phase after extraction (g L-1) Concentration in organic phase after scrubbing with 2.0 mol L-1 HCl (g L-1) % Recovery with purity Zn 2.93±0.04 0.04±0.01 2.89 2.88±0.02 ≥ 98% (recovery) Mn 2.03±0.04 2.03±0.05 < 0.01 not detected Ni (3.07±0.05) × 10-3 (3.07±0.05) × 10-3 < 0.01 not detected Fe (50.5±1) × 10-3 (20.0±0.50) × 10-3 30.5 × 10-3 < 0.01 Cd (2.47±0.02) × 10-3 (1.25±0.02) × 10-3 1.22 × 10-3 < 0.01 ≥ 99 (relative purity) List of Figures Fig 1. Extraction of metals from leaching solution of spent batteries with varying concentrations of Cyanex 923 in n-hexane {[Cyanex 923] = 0.01, 0.05, 0.075, 0.10, 0.125, 0.15 mol L-1; A/O ratio = 1; t = 5 min; T = 250C} Fig 2. Extraction of metals from synthetic solution as a function of initial aqueous phase acidity {[Cyanex 923] = 0.10 mol L-1 ; [HCl] = 0.50, 0.60, 0.70, 0.80, 0.90, 1.0, 1.1, 1.2 mol L-1; A/O ratio = 1; t = 5 min; T = 250C} Fig 3. Extraction of Zn with respect to varying HCl and extractant concentrations {[Cyanex 923] = 0.01, 0.05, 0.075, 0.10, 0.125, 0.15 mol L-1 ; [HCl] = 0.60, 0.80, 1.0, 1.2 mol L-1; A/O ratio = 1; t = 5 min; T = 250C} Fig 4. Extraction isotherm of Zn from leaching solution {[Cyanex 923] = 0.10 mol L-1 ; A/O ratio = 4, 3, 2, 1, 0.5, 0.33, 0.25; t = 5 min; T = 250C} Fig 5. Stripping isotherms of Zn {[Cyanex 923] = 0.10 mol L-1 ; A/O ratio = 0.1, 0.25, 0.5, 0.75, 1, 1.5; t = 5 min; T = 250C} Fig 6. FESEM and EDX data of black powder obtained by the dismantling of spent batteries Fig 7. FESEM and EDX data of ZnO nanoparticles synthesized by the combustion of Zn loaded Cyanex 923 phase at 3000C Fig 8. FESEM and EDX data of ZnO nanoparticles synthesized by the combustion of Zn loaded Cyanex 923 phase at 4000C Fig 9. FESEM and EDX data of ZnO nanoparticles synthesized by the combustion of Zn loaded Cyanex 923 phase at 5000C Fig 10. FESEM and EDX data of ZnO nanoparticles synthesized by the combustion of Zn loaded Cyanex 923 phase at 6000C Fig 11 (a & b) Non-contact AFM topography & 3-dimensional NCM phase of ZnO6000C nanoparticles Fig 12. UV-Vis spectrophotometric data of ZnO6000C nanoparticles Fig 13. XRD spectrum of ZnO6000C nanoparticles