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Effects of gas flow rate on zinc recovery rate and particle properties by pyrolysis of alkaline and zinc-carbon battery waste
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Effects of gas flow rate on zinc recovery rate and particle properties by pyrolysis of alkaline and zinc-carbon battery waste Burc¸ak Ebin ∗ , Martina Petranikova, Britt-Marie Steenari, Christian Ekberg Nuclear Chemistry and Industrial Material Recycling, Department of Chemistry and Chemical Engineering, Chalmers University of Technology, S-412 96 Gothenburg, Sweden
a r t i c l e
i n f o
Article history: Received 27 March 2016 Received in revised form 9 August 2016 Accepted 10 August 2016 Available online xxx Keywords: Pyrolysis Recycling Zinc particles Manganese oxide Alkaline and zinc-carbon batteries Battery waste
a b s t r a c t Zinc (Zn) recovery rate and the properties of Zn particles obtained by pyrolysis of alkaline and Zn-C battery waste were studied at a reaction temperature of 950 ◦ C for 60 min residence time using various N2(g) flow rate (0.5–3.0 L/min) without using any additive. The battery black mass was characterized with respect to the properties of waste battery particles, and chemical content. The thermodynamics of the pyrolysis process was studied using the HSC Chemistry 5.11 software. A carbothermic reduction reaction of the washed battery black mass by Milli-Q water takes place at choosen temperature and makes it possible to produce fine Zn particles by a rapid condensation following the evaporation of zinc from the pyrolysis batch. The amount of Zn that can be separated from the black mass slightly increases at higher N2(g) flow rates than 0.5 L/min and stabilizes by controlling the gas flow. Zn recovery of 80% was achieved at 950 ◦ C and 60 min residence time using 1.0 L/min and higher flow rates by pyrolysis of the washed battery black mass. The pyrolysis residue was shown to be mainly composes of MnO and Mn2 O3 with traces of impurities. The particle size of the produced Zn particle decreased from 874 nm to 534 nm with increasing flow rate and those particles are formed by the aggregation of primary condensed particles with nano-range sizes. The morphology of the zinc particles also changes from hexagonal shape to spherical morphology by increasing gas flow rate. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Zinc (Zn) is one of the most economically important industrial metal after iron, aluminum and copper. It is widely used in several applications such as in metallurgy, construction, electrical and chemical sectors, as well as defense and oil industries due to its anticorrosion characteristic and mechanical properties [1]. Especially the particle form of Zn have been attracting a growing attention to be applied as an anti-corrosive pigment in dyes, cathode materials for battery technologies and catalyzer for chemical reactions [2–8]. The research showed that anti-corrosive properties of Zn rich coating/paint were improved by reduced particle size to the submicron range [3,4]. Moreover, the Zn powders have been a key component of the primary alkaline batteries, and its usage are spreading to the secondary (rechargeable) battery technology like Zn-Ni and Zn-air batteries. The particle size and morphology significantly influence
∗ Corresponding author at: Nuclear Chemistry and Industrial Material Recycling, Department of Chemistry and Chemical Engineering, Chalmers University of Technology, Kemivägen 4, S 412 96 Gothenburg, Sweden. E-mail addresses: [email protected], [email protected] (B. Ebin).
the battery performance. For example, Zn electrodes with smaller particle size and larger surface area have been preferred to improve discharge performance at high currents [5–7]. The Zn particles have been also utilized as catalyst [8] and advanced technological applications have been developed [9]. As result of the recent industrial development and incoming technologies, the world consumption of Zn has shown a long-term upward trend since the middle of the 20th century. From then on, roughly half of all the zinc used during the human history was mobilized to supply the modern life demands [10]. However, Zn is currently accepted as one of the endangered elements that may have potentially restricted reserve in the next century. Therefore, the reclaiming of Zn from various waste streams including batteries, electronic devices, metallurgical waste or municipal solid waste is becoming an extremely important subject for the sustainable Zn supply to industry [11,12]. Furthermore, the recycling of Zn reduces the energy requirement to nearly a quarter of the energy consumed for the primary production, as well as carbon footprint to one out of five, and also minimizes the environmental impact of Zn mining [13,14].
http://dx.doi.org/10.1016/j.jaap.2016.08.014 0165-2370/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: B. Ebin, et al., Effects of gas flow rate on zinc recovery rate and particle properties by pyrolysis of alkaline and zinc-carbon battery waste, J. Anal. Appl. Pyrol. (2016), http://dx.doi.org/10.1016/j.jaap.2016.08.014
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Fig. 1. (a) Process flowchart and (b) illustration of experimental set-up.
1.1. Alkaline and zinc-carbon battery waste as a secondary Zn source The recycling of Zn is challenging because of not only due to the difficulty to find suitable waste sources, but also due to technological limitations. Large amount of Zn is used for galvanizing and brass production, i.e. materials which have long service life. The difficulties of removing Zn from steel before re-melting and the loss of Zn from electrical arc steel making furnaces as zinc rich oxide dust, which now is disposed of in landfills by steelmakers, are the crucial obstacles on its recycling [1,10,12]. Alternatively, alkaline and zinc-carbon (Zn-C) battery black mass can be a significant secondary source for Zn, 20,000 t/year, and also almost the same amount of manganese (Mn) can potentially be recovered [15]. The current technology for alkaline battery recycling is based on hydrometallurgical and pyrometallurgical processes. Hydrometallurgical recycling methods includes acidic or alkaline leaching of battery waste followed by precipitation or electrolysis. The metal cations can then be separated from the leachate by the solvent extraction method using an organic solvent, followed by recovery Table 1 Elemental analysis of battery waste. Elements
Battery Waste% (w/w)
Mn Zn K Fe Ni Co Cu Cr Pb Cd Hg
32.8 ± 2.6 29.1 ± 2.8 1.0 ± 0.5 0.8 ± 0.1 0.14 ± 0.03 0.01 ± 0.003 0.03 ± 0.01 0.01 ± 0.003 0.02 ± 0.004 0.01 ± 0.004 0.00
of the metals by electrolysis [16–18]. On the other hand, alkaline battery wastes are smelted at 1500 ◦ C to produce steel and ferromanganese alloys in pyrometallurgical treatments. During this process, Zn can be recovered by vaporization and condensation steps in the form of a dust. However, it usually ends up in steel making dust as a waste [19–21]. According to the European Union battery directive, 50% by average weight of the alkaline, Li-ion and NiMH batteries should be recycled [22]. The recycling processes for alkaline battery waste have been focused on the reclaiming of heavier metallic components like the iron shell and brass current collector. On the other hand, the recovery of battery black mass containing Zn and Mn is still not effective due to the low economic benefit with current recycling technology, and the lack of regulations. Thus, Zn and Mn recycling from alkaline battery waste is not widespread, and still needs innovative approaches. To further emphasize the importance of functioning recycling methods for Zn it should be considered that landfilling of the alkaline battery black mass can cause environmental problems in the long run due to its fine particle morphology, which can be in submicron scale [23,24], as well as high content of Zn and Zn compounds [24,25]. Although, Zn is necessary for normal growth, reproduction and other processes, it may become toxic to aquatic organisms at high concentrations and should not be spread to aquatic systems [26]. In addition, it is reported that if zinc oxide (ZnO) nanoparticles reach the sufficiently high level in the environment, which is as low as 1 mg/L, this can lead to a significant risk for the environmental biota [27]. Moreover, landfilled alkaline battery black mass is a potential contamination risk to nature due to their fine particle size and their nanotoxicology aspects. Airborne ultrafine particles and nanoparticles can have series of impacts on both environment and human [28], which should be taken into account for the battery waste disposal. The economic and green recovery of the Zn and Mn in the spent alkaline and Zn-C batteries is a challenge, and innovative processes
Please cite this article in press as: B. Ebin, et al., Effects of gas flow rate on zinc recovery rate and particle properties by pyrolysis of alkaline and zinc-carbon battery waste, J. Anal. Appl. Pyrol. (2016), http://dx.doi.org/10.1016/j.jaap.2016.08.014
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Fig. 2. XRD pattern of washed and unwashed alkaline and zinc-carbon battery black mass.
should be developed for sustainable future. In this study, the possibility of Zn recycling and controlled production of submicron metallic Zn particles from alkaline and zinc-carbon battery waste were investigated using a single step pyrolysis process without any additive at various gas flow rates.
2. Experimental The industrial mechanically pre-treated (shredding of batteries and magnetic separation of iron cover) alkaline and zinc-carbon battery black mass was supplied by Renova, Sweden. The quartering method [29] was used to take samples from the industrial battery black mass storage, and then the remaining metallic parts from the physical pre-treatment step were separated using a 4 mm sieve. The spent battery black mass smaller than 4 mm was ground using a IKA M20 universal mill, and then was washed with MilliQ water (2 mL/g water/powder) to remove the soluble salts. The weight of the battery black mass was 3 g in the experiments, which was put into ceramic combustion boats as a thick layer (nearly 3 mm). The combustion boat was placed inside the horizontal quartz tube located in an electrically heated tube furnace with a temperature control of ±1 ◦ C at the reaction temperature. The pyrolysis processes were performed at 950 ◦ C for 60 min residence time under 0.5, 1.0, 2.0 and 3.0 L/min N2(g) flow rates. All experiments were repeated three times. The process flowchart and the experimental set-up are given in Fig. 1. The Zn particles were obtained by rapid condensation of Zn vapor formed by carbothermic reduction of zinc oxide compounds in the black mass. The particles were collected in the washing bottles connected to the outlet of the quartz tube. The washing bottles were filled with acetone. The collected Zn particle samples were washed by acetone. After the stated residence times, the combustion boats were removed from the furnace at the reaction temperatures and cooled at room temperature in an air atmosphere. HSC Chemistry 5.11 software was used for thermodynamic calculations regarding the carbothermic reaction and phase equilibrium amount. X-ray diffraction (XRD, Bruker D8 Advance) using Cu K␣ radiation was used to examine the phase content, crystalline structure and size of the battery waste, prepared Zn particles and manganese oxide particles (pyrolysis residue). The conditions for data collection were continuous scanning of a detector covering a 2; angular range from 10◦ to 90◦ with a step size of 0.04 and a wavelength of 1.541874 Å. The chemical compositions of the sam-
ples were analyzed by inductive coupled plasma optical emission spectrometry (ICP-OES, ICAP 6500, Thermo Fischer) and energy dispersive spectroscopy (EDS). The battery black mass and pyrolysis residue, 0.2 g powder, were dissolved in 30 mL aqua regia (HNO3 + 3 HCl) and diluted in the desired amounts for the ICP-OES analyses. Carbon analyzer (Eltra Carbon/Sulfur Analyzer CS-800) was used to determine the carbon amount of alkaline battery black mass. The particle size and morphology of the samples were investigated by field emission scanning electron microscopy (FEI, Quanta 200 FEG ESEM). The particle size and size distribution analysis of the battery waste and the pyrolysis products were performed using, BI-90 particle sizer instrument by Brookhaven Instruments Corporation. The surface area of the samples were measured by BET analysis (Micromeritics ASAP 2020 Surface Area and Porosity Analyzer).
3. Results and discussion 3.1. Characterization of spent alkaline and Zn-C battery black mass Chemical composition analysis of the sieved and washed alkaline battery black mass performed by ICP-OES is shown in Table 1. Mn (32.8 ± 2.6 wt%) and Zn (29.1 ± 2.8 wt%) were the main elements in the powder mixture together with potassium (K) (1.0 ± 0.5 wt%), low amounts of iron (Fe) (0.8 ± 0.1 wt%) and minor quantities of nickel (Ni), cobalt (Co), copper (Cu), chromium (Cr), lead (Pb) and cadmium (Cd). Fe and other impurities that probably originate from the brass current collector, steel external case and other components. K was mainly removed by washing comparing to the unwashed sample, which was reported in the previous study [24]. XRD patterns for the washed and unwashed alkaline battery black mass are shown in Fig. 2. Washed battery powder contains mainly hetaerolite (ZnMn2 O4 ), zincite (ZnO), hausmannite (Mn3 O4 ) and graphite (C) phases. On the other hand, unwashed battery powder additionally contains KCl salt, which indicates that this salt is effectively removed from the sample by the washing. Carbon amount of the washed and unwashed sample are 6 wt% and 8 wt%, respectively. Some part of the fine carbon powders were stuck into the filter papers during filtration of washed samples. Fig. 3 shows the SEM images of the alkaline and zinc-carbon battery black mass. The majority of the powders are fine particles with a particle size of smaller than 20 m and have irregular shape morphology including spherical, rode-type and polygonal shapes.
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Fig. 3. SEM images of alkaline and zinc-carbon battery black mass (a) ×1000, (b) ×5000, (c) ×10,000 and (d) ×30,000.
However, magnified images, as seen in Fig. 3b, show that these particles are in fact aggregates of submicron spherical particles. The BET surface area of the particles is 6.12 ± 0.03 m2 /g which conform to the determined fine particle size. A comparison with the results obtained for the unwashed sample (3.67 ± 0.03 m2 /g [24]), the surface area was enlarged by washing due to the removing of KCl which can act as a binder of fine particles. In the case of the contamination of the battery black mass to the nature, it can show a harmful environmental effect due to the toxicity of the fine particles.
3.2. Pyrolysis of spent alkaline and Zn-C battery black mass The separation efficiencies for Zn production from the alkaline battery black mass as a function of the N2 gas flow rate at 950 ◦ C pyrolysis temperature for 60 min residence time are shown in Fig. 4. The percentages of separated Zn from the battery black mass obtained under 0.5, 1.0, 2.0 and 3.0 L/min N2(g) flow rates were 75%, 80%, 80% and 79% at 950 ◦ C for 60 min residence time, respectively. The graph details are given in the Supplementary material.
Fig. 4. Removal of zinc from the alkaline and zinc-carbon battery black mass depending on gas flow rate.
Please cite this article in press as: B. Ebin, et al., Effects of gas flow rate on zinc recovery rate and particle properties by pyrolysis of alkaline and zinc-carbon battery waste, J. Anal. Appl. Pyrol. (2016), http://dx.doi.org/10.1016/j.jaap.2016.08.014
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Fig. 5. Change of the Gibbs standard free energies by temperature for the decomposition reaction under N2 atmosphere and carbothermic reduction of ZnMn2 O4 .
All flow rates show laminar flow in the quartz reactor at 950 ◦ C according to the fluid dynamics. The calculated Reynolds number and friction factors are shown in the Supplementary material. The friction factor for the gas flow decreases with increasing flow rate which leads to better sweeping of the air and reaction products from the reaction zone. Thus, the recovery of Zn slightly increased by changing the flow rate from 0.5 to 1.0 L/min and then stabilized. On the other hand, the Zn recovery rate was 97%, when unwashed battery black mass was treated at 950 ◦ C for 60 min residence time and 1.0 L/min N2 gas flow rate conditions [24]. It is clear that the decreasing amount of carbon in the battery black mass due to loss of carbon particles in the filtering step caused the insufficient reduction of zinc oxide compounds to Zn metal, and decreased the efficiency of the process. According to the results from ICP analysis of the samples pyrolysed at 950 ◦ C for 60 min with various flow rates, all pyrolysis residue powders contained Zn between 7.4–8.8 wt%, and nearly 1.3 wt% Fe and 0.6 wt% K impurities. The thermodynamics of the high temperature reactions and equilibrium amounts of compounds were investigated using HSC software to determine reaction steps. Fig. 5 shows the Gibbs standard free energy change of decomposition and carbothermic reduction of ZnMn2 O4 , which is the main component of the battery black mass. The Gibbs standard free energies for the decomposition of ZnMn2 O4 to ZnO and manganese oxides (Mn2 O3 , Mn3 O4 and MnO) gets negative value at temperatures above 400 ◦ C in N2 atmosphere. In the case of C reducing environment, ZnMn2 O4 compound spontaneously decomposes to ZnO and Mn3 O4 at lower temperatures than when only N2(g) environment. In Fig. 6, diagrams show the Zn and oxide compounds equilibrium amount versus temperature. The equilibrium amounts were calculated using 1 mol oxide compound for N2(g) atmosphere and 1:1 mol ratio for oxide compound per reducing agent, C, amounts. At the pyrolysis temperature (950 ◦ C), ZnMn2 O4 decomposes to ZnO, Mn2 O3 , Mn3 O4 , MnO and MnO2 , when C reducing agent is not used. In carbon reducing environment, Zn and MnO are the stable phases due to the reduction of the ZnMn2 O4 and ZnO by C. So zinc compounds effectively reduced to Zn and instantly evaporates at 950 ◦ C. Another important issue is the exhaust product of the reaction. According the thermodynamic approach, CO (g) dominates at the pyrolysis temperature. XRD patterns of the pyrolysis residues shown in Fig. 7 were obtained to observe the reaction products and to identify the reactions. In all samples MnO is the main compound in the residue. However depending on the gas flow rate, Mn3 O4 and minor ZnO and C phases were also determined. Although most of the XRD peaks of Mn3 O4 and ZnMn2 O4 are overlapped, the ZnMn2 O4 compound should not be in the pyrolysis residue treated at 950 ◦ C for 60 min according to the thermodynamic investigation. However
Fig. 6. Phase equilibrium amount changes by temperature for (a) the decomposition of ZnMn2 O4 reaction under N2 atmosphere, (b) carbothermic reduction of ZnMn2 O4 and (c) carbothermic reduction of ZnO.
samples were removed from the furnace at pyrolysis temperature and rapidly cooled at room temperature under air atmosphere. Therefore, trace amount of ZnMn2 O4 may be formed due to the reaction between manganese oxide and unseparated zinc in the residue during the rapid cooling under air. Although MnO was the stable phase in the pyrolysis condition, Mn3 O4 was also partly formed in the cooling step. A carbon peak with low intensity also appeared for samples pyrolysed under 2.0 and 3.0 L/min N2(g) flow rates. The reduction of alkaline battery black mass using carbon
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Table 2 Size distrubition of Zn particles produced by pylorysis of spent batteries at 950 ◦ C and various N2 flow rates. Sample
Average particle size (nm)
0.5 L/min 1.0 L/min 2.0 L/min 3.0 L/min
874 ± 11 nm 765 ± 8 nm 669 ± 9 nm 534 ± 13 nm
Cumulative Undersize (nm) 10% below
25% below
50% below
75% below
90% below
304 nm 301 nm 341 nm 285 nm
453 nm 421 nm 448 nm 368 nm
704 nm 613 nm 605 nm 489 nm
1096 nm 892 nm 818 nm 649 nm
1634 nm 1250 nm 1074 nm 837 nm
Fig. 8. XRD pattern of Zn particles.
Fig. 7. XRD pattern of pyrolysis residue.
occurs through solid-state reactions, and the reactants should have physically contact with each other for the reactions. Probably the carbon traces were observed due to the heterogonous structure of battery black mass. In the light of our previous study [24] and the current results, the carbothermic reduction steps of the alkaline and Zn-C battery black mass are assumed to occur according to the following reactions; ZnMn2 O4 + C → 3 ZnO + 2 Mn3 O4 + CO (g)
(1)
Mn3 O4 + C → 3 MnO + CO (g)
(2)
ZnO + C → Zn + CO (g)
(3)
3.3. Properties of Zn particles The metallic Zn was recovered from alkaline and zinc-carbon battery black mass by a carbothermic reduction of zinc oxide based compounds at 950 ◦ C. Following to the reduction of oxides, Zn metal spontaneously evaporated at reaction temperature and was separated from battery black mass. Then the Zn vapor was transferred from the hot zone by N2(g) flow. When it reached the out of the furnace, metal vapor rapidly condensed and was collected in the washing bottles filled with acetone which was placed outlet of the heated zone. The obtained fine Zn particles by pyrolysis of battery black mass is one of the valuable product of the proposed pyrolysis process. The XRD patterns of the Zn particles prepared at 950 ◦ C under 0.5–3.0 L/min N2(g) flow rate conditions from alkaline and zinc-carbon battery black mass are shown in Fig. 8. The diffraction peaks of the samples are indexed to hexagonal crystal structure with a P63/mmc space group. The (101) peak has generally the highest intensity, and also (002) and (100) peaks follow
as second and third intense peaks for hexagonal crystal structured Zn. However the (002) peak became the highest one by increasing flow rate, which indicates the changing of orientation. Zn particles shows partly single crystal growth on the (002) plane. The crystallite sizes of the particles were determined by the Scherrer Equation [30] using the (002), (100) and (101) peaks in the XRD patterns. Instrumental broadening was taken into account to obtain the accurate crystallite size in the calculation. The crystallite sizes of the Zn particles produced at 950 ◦ C under 0.5, 1.0, 2.0 and 3.0 L/min N2(g) flow rates were calculated to be 120, 115, 106 and 98 nm, respectively. These results indicate that controlling the crystallite properties of Zn is possible by adjusting the gas flow rate is possible in the proposed experimental process. The change in gas flow rate probably changes the cooling gradient. The SEM images of Zn particles produced by the pyrolysis of alkaline and zinc-carbon battery black mass at 950 ◦ C with various N2(g) flow rates are shown in Fig. 9. The Zn particle morphology changes from a hexagonal to a spherical morphology by increasing gas flow rate. The average particle size and particle size distribution measured by laser particle sizer are given in Table 2. The average particles sizes of the samples produced at 950 ◦ C using 0.5, 1.0, 2.0 and 3.0 L/min N2(g) flow rates were measured around 874, 765, 669 and 534 nm, respectively. However, smaller particles varying in size 100–200 nm size can be easily observed for all Zn samples especially at magnified SEM images. It can be obviously seen from the SEM images that firstly Zn vapor condensed as a spherical dense particle around 100–200 nm sizes, and then these primary crystals/particles formed coarser particles by aggregation. The sintering and aggregation of the primary particles are lower for higher gas flow rate due to increasing cooling rate leading to finer crystallite and particle sizes. The BET surface areas and calculated particle sizes from these results of the Zn particles are shown in Table 3. It is assumed that
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Fig. 9. SEM images of Zn particles produced at 950 ◦ C and various N2(g) flow rates (a) 0.5 L/min, ×5 K, (b) 0.5 L/min, ×20 K, (c) 0.5 L/min, ×50 K, (d) 1.0 L/min, ×5 K, (e) 1.0 L/min, ×20 K, (f) 1.0 L/min, ×50 K, (g) 2.0 L/min, ×5 K, (h) 2.0 L/min, ×20 K, (i) 2.0 L/min, ×50 K, (j) 3.0 L/min, ×5 K, (k) 3.0 L/min, ×20 K, and (l) 3.0 L/min, ×50 K. Table 3 BET surface area of Zn particles produced by pylorysis of spent batteries at 950 ◦ C and various N2 flow rates. Sample
BET Surface Area
Calculated Particle Size (nm)
0.5 L/min 1.0 L/min 2.0 L/min 3.0 L/min
3.71 ± 0.18 m2 /g 8.02 ± 0.13 m2 /g 13.26 ± 0.14 m2 /g 16.14 ± 0.19 m2 /g
227 nm 105 nm 64 nm 52 nm
particles are spherical and dense for the particle size calculation using surface area. The BET surface area of Zn particles dramatically increased with increasing N2(g) flow rate from 0.5 to 3.0 L/min. According the surface area results, the calculated particle sizes are 227, 105, 64 and 52 nm for 0.5, 1.0, 2.0 and 3.0 L/min gas flow rates, respectively. The differences between the measured size and calculated size from the surface area results are probably due to the agglomeration of small particles to larger clusters. While the mea-
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Fig. 10. EDS analyses of Zn particles produced at 950 ◦ C (a) 0.5 L/min, (b) 1.0 L/min, (c) 2.0 L/min, and (d) 3.0 L/min N2 (g) flow rates.
sured particle size represents a small portion of the Zn sample, all produced Zn particles were used for BET surface area analysis. EDS analyses (Fig. 10) were performed on 10 different points of the images both on coarser and finer particles zones. Small amounts of oxygen were detected in all samples due to the rapid oxidation of fine Zn particles during SEM specimen preparation. The carbon peak on the EDS results comes from the adhesive carbon tape used for sample fixation. On the other hand, traces of K and Cl peaks were also observed for all samples, and these impurities are present in negligible amounts (<1 at.%). The results show that high purity Zn particles with controlled size in nano-range can be produced from alkaline and Zn-C battery waste by the suggested pyrolysis process.
and further the zinc oxide is reduced by carbothermic reduction at 950 ◦ C pyrolysis temperature. The recovered Zn was collected as submicron metallic particles in the pyrolysis process. The particle size of the products were varying around 534 nm–874 nm depending on gas flow rate. The crystallite sizes of the Zn particles were reduced from 120 nm to 98 nm by increasing N2(g) flow rate. Results clearly shows that Zn particle and crystallite size can be easily controlled by adjusting the gas flow rate, and also it is possible to produce the Zn particles with sizes in nano-range. The present results indicate that the suggested pyrolysis system is a promising alternative process for not only efficient Zn recovery, but also the industrial recycling of alkaline and Zn-C battery waste.
4. Conclusion
Acknowledgement
Zn was recovered from washed alkaline and Zn-C battery black mass by a pyrolysis process without using additional chemicals at 950 ◦ C. The remaining KCl salt from the battery electrolyte was removed by Milli-Q water washing of the battery black mass to reduce the K and Cl contamination to high purity metallic Zn product. The battery black mass consists of hetaerolite (ZnMn2 O4 ), zincite (ZnO), hausmannite (Mn3 O4 ), graphite (C), and traces of Fe and K with nearly 32.8 ± 2.6 wt% Mn and 29.1 ± 2.8 wt% Zn. The surface area of the washed alkaline battery black mass particles increased, which is the sign of the decreased particle size, due to de-agglomeration by removing soluble salts. The N2(g) flow rate variation between 0.5–3.0 L/min did not have a dominant effect on Zn recovery at 950 ◦ C for 60 min residence time. However, the Zn recovery rate slightly increased from 75 to 80% by increasing the flow rate from 0.5 to 1.0 L/min, and then stabilized. The sweeping of the air from the experimental set-up is more efficient at higher flow rates than 0.5 L/min due to the decreasing friction factor. According to XRD results and thermodynamic calculations, at high temperature reaction pathway for the Zn production was also suggested. At 950 ◦ C, the stability of CO2(g) is lower than that of CO (g) , and thus the main exhaust gas should be CO (g) . It is assumed that hetaerolite decomposes to zinc oxide and manganese oxides in the first step,
This research was supported by Swedish Energy Agency (Grant No: 39063-1). The authors would also like to thank Renova AB Gothenburg to provide the pre-treated battery waste. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jaap.2016.08.014. References [1] G. Xueyi, Z. Juya, S. Yu, T. Qinghua, Substance flow analysis of zinc in China, Resources, Conversation Recycl. 54 (2010) 171–177. [2] T.H. Yun, J.H. Park, J.-S. Kim, J.M. Park, Effect of the surface modification of zinc powders with organosilanes on the corrosion resistance of a zinc pigmented organic coating, Prog. Org. Coat. 77 (2014) 1780–1788. [3] K. Schaefer, A. Miszczyk, Improvement of electrochemical action of zinc-rich paints by addition of nanoparticulate zinc, Corros. Sci. 66 (2013) 380–391. [4] A. Olad, M. Barati, H. Shirmohammadi, Conductivity and anticorrosion performance of polyaniline/zinc composites: investigation of zinc particle size and distribution effect, Prog. Org. Coat. 72 (2011) 599–604. [5] Y. Ito, M. Nyce, R. Plivelich, M. Klein, D. Steingart, S. Banerjee, Zinc morphology in zinc-nickel flow assisted batteries and impact on performance, J. Power Sources 196 (2011) 2340–2345.
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Effects of gas flow rate on zinc recovery rate and particle properties by pyrolysis of alkaline and zinc-carbon battery waste Burc¸ak Ebin ∗ , Martina Petranikova, Britt-Marie Steenari, Christian Ekberg Nuclear Chemistry and Industrial Material Recycling, Department of Chemistry and Chemical Engineering, Chalmers University of Technology, S-412 96 Gothenburg, Sweden
a r t i c l e
i n f o
Article history: Received 27 March 2016 Received in revised form 9 August 2016 Accepted 10 August 2016 Available online xxx Keywords: Pyrolysis Recycling Zinc particles Manganese oxide Alkaline and zinc-carbon batteries Battery waste
a b s t r a c t Zinc (Zn) recovery rate and the properties of Zn particles obtained by pyrolysis of alkaline and Zn-C battery waste were studied at a reaction temperature of 950 ◦ C for 60 min residence time using various N2(g) flow rate (0.5–3.0 L/min) without using any additive. The battery black mass was characterized with respect to the properties of waste battery particles, and chemical content. The thermodynamics of the pyrolysis process was studied using the HSC Chemistry 5.11 software. A carbothermic reduction reaction of the washed battery black mass by Milli-Q water takes place at choosen temperature and makes it possible to produce fine Zn particles by a rapid condensation following the evaporation of zinc from the pyrolysis batch. The amount of Zn that can be separated from the black mass slightly increases at higher N2(g) flow rates than 0.5 L/min and stabilizes by controlling the gas flow. Zn recovery of 80% was achieved at 950 ◦ C and 60 min residence time using 1.0 L/min and higher flow rates by pyrolysis of the washed battery black mass. The pyrolysis residue was shown to be mainly composes of MnO and Mn2 O3 with traces of impurities. The particle size of the produced Zn particle decreased from 874 nm to 534 nm with increasing flow rate and those particles are formed by the aggregation of primary condensed particles with nano-range sizes. The morphology of the zinc particles also changes from hexagonal shape to spherical morphology by increasing gas flow rate. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Zinc (Zn) is one of the most economically important industrial metal after iron, aluminum and copper. It is widely used in several applications such as in metallurgy, construction, electrical and chemical sectors, as well as defense and oil industries due to its anticorrosion characteristic and mechanical properties [1]. Especially the particle form of Zn have been attracting a growing attention to be applied as an anti-corrosive pigment in dyes, cathode materials for battery technologies and catalyzer for chemical reactions [2–8]. The research showed that anti-corrosive properties of Zn rich coating/paint were improved by reduced particle size to the submicron range [3,4]. Moreover, the Zn powders have been a key component of the primary alkaline batteries, and its usage are spreading to the secondary (rechargeable) battery technology like Zn-Ni and Zn-air batteries. The particle size and morphology significantly influence
∗ Corresponding author at: Nuclear Chemistry and Industrial Material Recycling, Department of Chemistry and Chemical Engineering, Chalmers University of Technology, Kemivägen 4, S 412 96 Gothenburg, Sweden. E-mail addresses: [email protected], [email protected] (B. Ebin).
the battery performance. For example, Zn electrodes with smaller particle size and larger surface area have been preferred to improve discharge performance at high currents [5–7]. The Zn particles have been also utilized as catalyst [8] and advanced technological applications have been developed [9]. As result of the recent industrial development and incoming technologies, the world consumption of Zn has shown a long-term upward trend since the middle of the 20th century. From then on, roughly half of all the zinc used during the human history was mobilized to supply the modern life demands [10]. However, Zn is currently accepted as one of the endangered elements that may have potentially restricted reserve in the next century. Therefore, the reclaiming of Zn from various waste streams including batteries, electronic devices, metallurgical waste or municipal solid waste is becoming an extremely important subject for the sustainable Zn supply to industry [11,12]. Furthermore, the recycling of Zn reduces the energy requirement to nearly a quarter of the energy consumed for the primary production, as well as carbon footprint to one out of five, and also minimizes the environmental impact of Zn mining [13,14].
http://dx.doi.org/10.1016/j.jaap.2016.08.014 0165-2370/© 2016 Elsevier B.V. All rights reserved.
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Fig. 1. (a) Process flowchart and (b) illustration of experimental set-up.
1.1. Alkaline and zinc-carbon battery waste as a secondary Zn source The recycling of Zn is challenging because of not only due to the difficulty to find suitable waste sources, but also due to technological limitations. Large amount of Zn is used for galvanizing and brass production, i.e. materials which have long service life. The difficulties of removing Zn from steel before re-melting and the loss of Zn from electrical arc steel making furnaces as zinc rich oxide dust, which now is disposed of in landfills by steelmakers, are the crucial obstacles on its recycling [1,10,12]. Alternatively, alkaline and zinc-carbon (Zn-C) battery black mass can be a significant secondary source for Zn, 20,000 t/year, and also almost the same amount of manganese (Mn) can potentially be recovered [15]. The current technology for alkaline battery recycling is based on hydrometallurgical and pyrometallurgical processes. Hydrometallurgical recycling methods includes acidic or alkaline leaching of battery waste followed by precipitation or electrolysis. The metal cations can then be separated from the leachate by the solvent extraction method using an organic solvent, followed by recovery Table 1 Elemental analysis of battery waste. Elements
Battery Waste% (w/w)
Mn Zn K Fe Ni Co Cu Cr Pb Cd Hg
32.8 ± 2.6 29.1 ± 2.8 1.0 ± 0.5 0.8 ± 0.1 0.14 ± 0.03 0.01 ± 0.003 0.03 ± 0.01 0.01 ± 0.003 0.02 ± 0.004 0.01 ± 0.004 0.00
of the metals by electrolysis [16–18]. On the other hand, alkaline battery wastes are smelted at 1500 ◦ C to produce steel and ferromanganese alloys in pyrometallurgical treatments. During this process, Zn can be recovered by vaporization and condensation steps in the form of a dust. However, it usually ends up in steel making dust as a waste [19–21]. According to the European Union battery directive, 50% by average weight of the alkaline, Li-ion and NiMH batteries should be recycled [22]. The recycling processes for alkaline battery waste have been focused on the reclaiming of heavier metallic components like the iron shell and brass current collector. On the other hand, the recovery of battery black mass containing Zn and Mn is still not effective due to the low economic benefit with current recycling technology, and the lack of regulations. Thus, Zn and Mn recycling from alkaline battery waste is not widespread, and still needs innovative approaches. To further emphasize the importance of functioning recycling methods for Zn it should be considered that landfilling of the alkaline battery black mass can cause environmental problems in the long run due to its fine particle morphology, which can be in submicron scale [23,24], as well as high content of Zn and Zn compounds [24,25]. Although, Zn is necessary for normal growth, reproduction and other processes, it may become toxic to aquatic organisms at high concentrations and should not be spread to aquatic systems [26]. In addition, it is reported that if zinc oxide (ZnO) nanoparticles reach the sufficiently high level in the environment, which is as low as 1 mg/L, this can lead to a significant risk for the environmental biota [27]. Moreover, landfilled alkaline battery black mass is a potential contamination risk to nature due to their fine particle size and their nanotoxicology aspects. Airborne ultrafine particles and nanoparticles can have series of impacts on both environment and human [28], which should be taken into account for the battery waste disposal. The economic and green recovery of the Zn and Mn in the spent alkaline and Zn-C batteries is a challenge, and innovative processes
Please cite this article in press as: B. Ebin, et al., Effects of gas flow rate on zinc recovery rate and particle properties by pyrolysis of alkaline and zinc-carbon battery waste, J. Anal. Appl. Pyrol. (2016), http://dx.doi.org/10.1016/j.jaap.2016.08.014
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Fig. 2. XRD pattern of washed and unwashed alkaline and zinc-carbon battery black mass.
should be developed for sustainable future. In this study, the possibility of Zn recycling and controlled production of submicron metallic Zn particles from alkaline and zinc-carbon battery waste were investigated using a single step pyrolysis process without any additive at various gas flow rates.
2. Experimental The industrial mechanically pre-treated (shredding of batteries and magnetic separation of iron cover) alkaline and zinc-carbon battery black mass was supplied by Renova, Sweden. The quartering method [29] was used to take samples from the industrial battery black mass storage, and then the remaining metallic parts from the physical pre-treatment step were separated using a 4 mm sieve. The spent battery black mass smaller than 4 mm was ground using a IKA M20 universal mill, and then was washed with MilliQ water (2 mL/g water/powder) to remove the soluble salts. The weight of the battery black mass was 3 g in the experiments, which was put into ceramic combustion boats as a thick layer (nearly 3 mm). The combustion boat was placed inside the horizontal quartz tube located in an electrically heated tube furnace with a temperature control of ±1 ◦ C at the reaction temperature. The pyrolysis processes were performed at 950 ◦ C for 60 min residence time under 0.5, 1.0, 2.0 and 3.0 L/min N2(g) flow rates. All experiments were repeated three times. The process flowchart and the experimental set-up are given in Fig. 1. The Zn particles were obtained by rapid condensation of Zn vapor formed by carbothermic reduction of zinc oxide compounds in the black mass. The particles were collected in the washing bottles connected to the outlet of the quartz tube. The washing bottles were filled with acetone. The collected Zn particle samples were washed by acetone. After the stated residence times, the combustion boats were removed from the furnace at the reaction temperatures and cooled at room temperature in an air atmosphere. HSC Chemistry 5.11 software was used for thermodynamic calculations regarding the carbothermic reaction and phase equilibrium amount. X-ray diffraction (XRD, Bruker D8 Advance) using Cu K␣ radiation was used to examine the phase content, crystalline structure and size of the battery waste, prepared Zn particles and manganese oxide particles (pyrolysis residue). The conditions for data collection were continuous scanning of a detector covering a 2; angular range from 10◦ to 90◦ with a step size of 0.04 and a wavelength of 1.541874 Å. The chemical compositions of the sam-
ples were analyzed by inductive coupled plasma optical emission spectrometry (ICP-OES, ICAP 6500, Thermo Fischer) and energy dispersive spectroscopy (EDS). The battery black mass and pyrolysis residue, 0.2 g powder, were dissolved in 30 mL aqua regia (HNO3 + 3 HCl) and diluted in the desired amounts for the ICP-OES analyses. Carbon analyzer (Eltra Carbon/Sulfur Analyzer CS-800) was used to determine the carbon amount of alkaline battery black mass. The particle size and morphology of the samples were investigated by field emission scanning electron microscopy (FEI, Quanta 200 FEG ESEM). The particle size and size distribution analysis of the battery waste and the pyrolysis products were performed using, BI-90 particle sizer instrument by Brookhaven Instruments Corporation. The surface area of the samples were measured by BET analysis (Micromeritics ASAP 2020 Surface Area and Porosity Analyzer).
3. Results and discussion 3.1. Characterization of spent alkaline and Zn-C battery black mass Chemical composition analysis of the sieved and washed alkaline battery black mass performed by ICP-OES is shown in Table 1. Mn (32.8 ± 2.6 wt%) and Zn (29.1 ± 2.8 wt%) were the main elements in the powder mixture together with potassium (K) (1.0 ± 0.5 wt%), low amounts of iron (Fe) (0.8 ± 0.1 wt%) and minor quantities of nickel (Ni), cobalt (Co), copper (Cu), chromium (Cr), lead (Pb) and cadmium (Cd). Fe and other impurities that probably originate from the brass current collector, steel external case and other components. K was mainly removed by washing comparing to the unwashed sample, which was reported in the previous study [24]. XRD patterns for the washed and unwashed alkaline battery black mass are shown in Fig. 2. Washed battery powder contains mainly hetaerolite (ZnMn2 O4 ), zincite (ZnO), hausmannite (Mn3 O4 ) and graphite (C) phases. On the other hand, unwashed battery powder additionally contains KCl salt, which indicates that this salt is effectively removed from the sample by the washing. Carbon amount of the washed and unwashed sample are 6 wt% and 8 wt%, respectively. Some part of the fine carbon powders were stuck into the filter papers during filtration of washed samples. Fig. 3 shows the SEM images of the alkaline and zinc-carbon battery black mass. The majority of the powders are fine particles with a particle size of smaller than 20 m and have irregular shape morphology including spherical, rode-type and polygonal shapes.
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Fig. 3. SEM images of alkaline and zinc-carbon battery black mass (a) ×1000, (b) ×5000, (c) ×10,000 and (d) ×30,000.
However, magnified images, as seen in Fig. 3b, show that these particles are in fact aggregates of submicron spherical particles. The BET surface area of the particles is 6.12 ± 0.03 m2 /g which conform to the determined fine particle size. A comparison with the results obtained for the unwashed sample (3.67 ± 0.03 m2 /g [24]), the surface area was enlarged by washing due to the removing of KCl which can act as a binder of fine particles. In the case of the contamination of the battery black mass to the nature, it can show a harmful environmental effect due to the toxicity of the fine particles.
3.2. Pyrolysis of spent alkaline and Zn-C battery black mass The separation efficiencies for Zn production from the alkaline battery black mass as a function of the N2 gas flow rate at 950 ◦ C pyrolysis temperature for 60 min residence time are shown in Fig. 4. The percentages of separated Zn from the battery black mass obtained under 0.5, 1.0, 2.0 and 3.0 L/min N2(g) flow rates were 75%, 80%, 80% and 79% at 950 ◦ C for 60 min residence time, respectively. The graph details are given in the Supplementary material.
Fig. 4. Removal of zinc from the alkaline and zinc-carbon battery black mass depending on gas flow rate.
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Fig. 5. Change of the Gibbs standard free energies by temperature for the decomposition reaction under N2 atmosphere and carbothermic reduction of ZnMn2 O4 .
All flow rates show laminar flow in the quartz reactor at 950 ◦ C according to the fluid dynamics. The calculated Reynolds number and friction factors are shown in the Supplementary material. The friction factor for the gas flow decreases with increasing flow rate which leads to better sweeping of the air and reaction products from the reaction zone. Thus, the recovery of Zn slightly increased by changing the flow rate from 0.5 to 1.0 L/min and then stabilized. On the other hand, the Zn recovery rate was 97%, when unwashed battery black mass was treated at 950 ◦ C for 60 min residence time and 1.0 L/min N2 gas flow rate conditions [24]. It is clear that the decreasing amount of carbon in the battery black mass due to loss of carbon particles in the filtering step caused the insufficient reduction of zinc oxide compounds to Zn metal, and decreased the efficiency of the process. According to the results from ICP analysis of the samples pyrolysed at 950 ◦ C for 60 min with various flow rates, all pyrolysis residue powders contained Zn between 7.4–8.8 wt%, and nearly 1.3 wt% Fe and 0.6 wt% K impurities. The thermodynamics of the high temperature reactions and equilibrium amounts of compounds were investigated using HSC software to determine reaction steps. Fig. 5 shows the Gibbs standard free energy change of decomposition and carbothermic reduction of ZnMn2 O4 , which is the main component of the battery black mass. The Gibbs standard free energies for the decomposition of ZnMn2 O4 to ZnO and manganese oxides (Mn2 O3 , Mn3 O4 and MnO) gets negative value at temperatures above 400 ◦ C in N2 atmosphere. In the case of C reducing environment, ZnMn2 O4 compound spontaneously decomposes to ZnO and Mn3 O4 at lower temperatures than when only N2(g) environment. In Fig. 6, diagrams show the Zn and oxide compounds equilibrium amount versus temperature. The equilibrium amounts were calculated using 1 mol oxide compound for N2(g) atmosphere and 1:1 mol ratio for oxide compound per reducing agent, C, amounts. At the pyrolysis temperature (950 ◦ C), ZnMn2 O4 decomposes to ZnO, Mn2 O3 , Mn3 O4 , MnO and MnO2 , when C reducing agent is not used. In carbon reducing environment, Zn and MnO are the stable phases due to the reduction of the ZnMn2 O4 and ZnO by C. So zinc compounds effectively reduced to Zn and instantly evaporates at 950 ◦ C. Another important issue is the exhaust product of the reaction. According the thermodynamic approach, CO (g) dominates at the pyrolysis temperature. XRD patterns of the pyrolysis residues shown in Fig. 7 were obtained to observe the reaction products and to identify the reactions. In all samples MnO is the main compound in the residue. However depending on the gas flow rate, Mn3 O4 and minor ZnO and C phases were also determined. Although most of the XRD peaks of Mn3 O4 and ZnMn2 O4 are overlapped, the ZnMn2 O4 compound should not be in the pyrolysis residue treated at 950 ◦ C for 60 min according to the thermodynamic investigation. However
Fig. 6. Phase equilibrium amount changes by temperature for (a) the decomposition of ZnMn2 O4 reaction under N2 atmosphere, (b) carbothermic reduction of ZnMn2 O4 and (c) carbothermic reduction of ZnO.
samples were removed from the furnace at pyrolysis temperature and rapidly cooled at room temperature under air atmosphere. Therefore, trace amount of ZnMn2 O4 may be formed due to the reaction between manganese oxide and unseparated zinc in the residue during the rapid cooling under air. Although MnO was the stable phase in the pyrolysis condition, Mn3 O4 was also partly formed in the cooling step. A carbon peak with low intensity also appeared for samples pyrolysed under 2.0 and 3.0 L/min N2(g) flow rates. The reduction of alkaline battery black mass using carbon
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Table 2 Size distrubition of Zn particles produced by pylorysis of spent batteries at 950 ◦ C and various N2 flow rates. Sample
Average particle size (nm)
0.5 L/min 1.0 L/min 2.0 L/min 3.0 L/min
874 ± 11 nm 765 ± 8 nm 669 ± 9 nm 534 ± 13 nm
Cumulative Undersize (nm) 10% below
25% below
50% below
75% below
90% below
304 nm 301 nm 341 nm 285 nm
453 nm 421 nm 448 nm 368 nm
704 nm 613 nm 605 nm 489 nm
1096 nm 892 nm 818 nm 649 nm
1634 nm 1250 nm 1074 nm 837 nm
Fig. 8. XRD pattern of Zn particles.
Fig. 7. XRD pattern of pyrolysis residue.
occurs through solid-state reactions, and the reactants should have physically contact with each other for the reactions. Probably the carbon traces were observed due to the heterogonous structure of battery black mass. In the light of our previous study [24] and the current results, the carbothermic reduction steps of the alkaline and Zn-C battery black mass are assumed to occur according to the following reactions; ZnMn2 O4 + C → 3 ZnO + 2 Mn3 O4 + CO (g)
(1)
Mn3 O4 + C → 3 MnO + CO (g)
(2)
ZnO + C → Zn + CO (g)
(3)
3.3. Properties of Zn particles The metallic Zn was recovered from alkaline and zinc-carbon battery black mass by a carbothermic reduction of zinc oxide based compounds at 950 ◦ C. Following to the reduction of oxides, Zn metal spontaneously evaporated at reaction temperature and was separated from battery black mass. Then the Zn vapor was transferred from the hot zone by N2(g) flow. When it reached the out of the furnace, metal vapor rapidly condensed and was collected in the washing bottles filled with acetone which was placed outlet of the heated zone. The obtained fine Zn particles by pyrolysis of battery black mass is one of the valuable product of the proposed pyrolysis process. The XRD patterns of the Zn particles prepared at 950 ◦ C under 0.5–3.0 L/min N2(g) flow rate conditions from alkaline and zinc-carbon battery black mass are shown in Fig. 8. The diffraction peaks of the samples are indexed to hexagonal crystal structure with a P63/mmc space group. The (101) peak has generally the highest intensity, and also (002) and (100) peaks follow
as second and third intense peaks for hexagonal crystal structured Zn. However the (002) peak became the highest one by increasing flow rate, which indicates the changing of orientation. Zn particles shows partly single crystal growth on the (002) plane. The crystallite sizes of the particles were determined by the Scherrer Equation [30] using the (002), (100) and (101) peaks in the XRD patterns. Instrumental broadening was taken into account to obtain the accurate crystallite size in the calculation. The crystallite sizes of the Zn particles produced at 950 ◦ C under 0.5, 1.0, 2.0 and 3.0 L/min N2(g) flow rates were calculated to be 120, 115, 106 and 98 nm, respectively. These results indicate that controlling the crystallite properties of Zn is possible by adjusting the gas flow rate is possible in the proposed experimental process. The change in gas flow rate probably changes the cooling gradient. The SEM images of Zn particles produced by the pyrolysis of alkaline and zinc-carbon battery black mass at 950 ◦ C with various N2(g) flow rates are shown in Fig. 9. The Zn particle morphology changes from a hexagonal to a spherical morphology by increasing gas flow rate. The average particle size and particle size distribution measured by laser particle sizer are given in Table 2. The average particles sizes of the samples produced at 950 ◦ C using 0.5, 1.0, 2.0 and 3.0 L/min N2(g) flow rates were measured around 874, 765, 669 and 534 nm, respectively. However, smaller particles varying in size 100–200 nm size can be easily observed for all Zn samples especially at magnified SEM images. It can be obviously seen from the SEM images that firstly Zn vapor condensed as a spherical dense particle around 100–200 nm sizes, and then these primary crystals/particles formed coarser particles by aggregation. The sintering and aggregation of the primary particles are lower for higher gas flow rate due to increasing cooling rate leading to finer crystallite and particle sizes. The BET surface areas and calculated particle sizes from these results of the Zn particles are shown in Table 3. It is assumed that
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Fig. 9. SEM images of Zn particles produced at 950 ◦ C and various N2(g) flow rates (a) 0.5 L/min, ×5 K, (b) 0.5 L/min, ×20 K, (c) 0.5 L/min, ×50 K, (d) 1.0 L/min, ×5 K, (e) 1.0 L/min, ×20 K, (f) 1.0 L/min, ×50 K, (g) 2.0 L/min, ×5 K, (h) 2.0 L/min, ×20 K, (i) 2.0 L/min, ×50 K, (j) 3.0 L/min, ×5 K, (k) 3.0 L/min, ×20 K, and (l) 3.0 L/min, ×50 K. Table 3 BET surface area of Zn particles produced by pylorysis of spent batteries at 950 ◦ C and various N2 flow rates. Sample
BET Surface Area
Calculated Particle Size (nm)
0.5 L/min 1.0 L/min 2.0 L/min 3.0 L/min
3.71 ± 0.18 m2 /g 8.02 ± 0.13 m2 /g 13.26 ± 0.14 m2 /g 16.14 ± 0.19 m2 /g
227 nm 105 nm 64 nm 52 nm
particles are spherical and dense for the particle size calculation using surface area. The BET surface area of Zn particles dramatically increased with increasing N2(g) flow rate from 0.5 to 3.0 L/min. According the surface area results, the calculated particle sizes are 227, 105, 64 and 52 nm for 0.5, 1.0, 2.0 and 3.0 L/min gas flow rates, respectively. The differences between the measured size and calculated size from the surface area results are probably due to the agglomeration of small particles to larger clusters. While the mea-
Please cite this article in press as: B. Ebin, et al., Effects of gas flow rate on zinc recovery rate and particle properties by pyrolysis of alkaline and zinc-carbon battery waste, J. Anal. Appl. Pyrol. (2016), http://dx.doi.org/10.1016/j.jaap.2016.08.014
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Fig. 10. EDS analyses of Zn particles produced at 950 ◦ C (a) 0.5 L/min, (b) 1.0 L/min, (c) 2.0 L/min, and (d) 3.0 L/min N2 (g) flow rates.
sured particle size represents a small portion of the Zn sample, all produced Zn particles were used for BET surface area analysis. EDS analyses (Fig. 10) were performed on 10 different points of the images both on coarser and finer particles zones. Small amounts of oxygen were detected in all samples due to the rapid oxidation of fine Zn particles during SEM specimen preparation. The carbon peak on the EDS results comes from the adhesive carbon tape used for sample fixation. On the other hand, traces of K and Cl peaks were also observed for all samples, and these impurities are present in negligible amounts (<1 at.%). The results show that high purity Zn particles with controlled size in nano-range can be produced from alkaline and Zn-C battery waste by the suggested pyrolysis process.
and further the zinc oxide is reduced by carbothermic reduction at 950 ◦ C pyrolysis temperature. The recovered Zn was collected as submicron metallic particles in the pyrolysis process. The particle size of the products were varying around 534 nm–874 nm depending on gas flow rate. The crystallite sizes of the Zn particles were reduced from 120 nm to 98 nm by increasing N2(g) flow rate. Results clearly shows that Zn particle and crystallite size can be easily controlled by adjusting the gas flow rate, and also it is possible to produce the Zn particles with sizes in nano-range. The present results indicate that the suggested pyrolysis system is a promising alternative process for not only efficient Zn recovery, but also the industrial recycling of alkaline and Zn-C battery waste.
4. Conclusion
Acknowledgement
Zn was recovered from washed alkaline and Zn-C battery black mass by a pyrolysis process without using additional chemicals at 950 ◦ C. The remaining KCl salt from the battery electrolyte was removed by Milli-Q water washing of the battery black mass to reduce the K and Cl contamination to high purity metallic Zn product. The battery black mass consists of hetaerolite (ZnMn2 O4 ), zincite (ZnO), hausmannite (Mn3 O4 ), graphite (C), and traces of Fe and K with nearly 32.8 ± 2.6 wt% Mn and 29.1 ± 2.8 wt% Zn. The surface area of the washed alkaline battery black mass particles increased, which is the sign of the decreased particle size, due to de-agglomeration by removing soluble salts. The N2(g) flow rate variation between 0.5–3.0 L/min did not have a dominant effect on Zn recovery at 950 ◦ C for 60 min residence time. However, the Zn recovery rate slightly increased from 75 to 80% by increasing the flow rate from 0.5 to 1.0 L/min, and then stabilized. The sweeping of the air from the experimental set-up is more efficient at higher flow rates than 0.5 L/min due to the decreasing friction factor. According to XRD results and thermodynamic calculations, at high temperature reaction pathway for the Zn production was also suggested. At 950 ◦ C, the stability of CO2(g) is lower than that of CO (g) , and thus the main exhaust gas should be CO (g) . It is assumed that hetaerolite decomposes to zinc oxide and manganese oxides in the first step,
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Please cite this article in press as: B. Ebin, et al., Effects of gas flow rate on zinc recovery rate and particle properties by pyrolysis of alkaline and zinc-carbon battery waste, J. Anal. Appl. Pyrol. (2016), http://dx.doi.org/10.1016/j.jaap.2016.08.014