Investigation of zinc recovery by hydrogen reduction assisted pyrolysis of alkaline and zinc-carbon battery waste

Waste Management xxx (2017) xxx–xxx

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Investigation of zinc recovery by hydrogen reduction assisted pyrolysis of alkaline and zinc-carbon battery waste Burçak 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 20 January 2017 Revised 17 May 2017 Accepted 11 June 2017 Available online xxxx Keywords: Pyrolysis Hydrogen reduction Recycling Zinc particles Manganese oxide Alkaline battery waste

a b s t r a c t Zinc (Zn) recovery from alkaline and zinc-carbon (Zn-C) battery waste were studied by a laboratory scale pyrolysis process at a reaction temperature of 950 °C for 15–60 min residence time using 5%H2(g)-N2(g) mixture at 1.0 L/min gas flow rate. The effect of different cooling rates on the properties of pyrolysis residue, manganese oxide particles, were also investigated. Morphological and structural characterization of the produced Zn particles were performed. The battery black mass was characterized with respect to the properties and chemical composition of the waste battery particles. The thermodynamics of the pyrolysis process was studied using the HSC Chemistry 5.11 software. A hydrogen reduction reaction of the battery black mass (washed with Milli-Q water) takes place at the chosen temperature and makes it possible to produce fine Zn particles by rapid condensation following the evaporation of Zn from the pyrolysis batch. The amount of Zn that can be separated from the black mass increases by extending the residence time. Recovery of 99.8% of the Zn was achieved at 950 °C for 60 min residence time using 1.0 L/min gas flow rate. The pyrolysis residue contains MnO and Mn2O3 compounds, and the oxidation state of manganese can be controlled by cooling rate and atmosphere. The Zn particles exhibit spherical and hexagonal particle morphology with a particle size varying between 200 nm and 3 mm. However the particles were formed by aggregation of nanoparticles which are primarily nucleated from the gas phase. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction Alkaline and zinc-carbon (Zn-C) batteries have been energizing various frequently used electrical tools since the 1960s and their consumption still dominates the portable battery market (Bernardes et al. 2004; Smith et al., 2014). At the end of their service life, the spent batteries are usually landfilled due to the low economical benefit of recycling them. However developed and developing countries enforce the collection, safe storage and/or recycling of the batteries by local regulations (Espinosa et al., 2004; Guevara-Garcia and Montiel-Corona, 2012; Lindhqvist, 2010; Terazono et al., 2015). Although several hydrometallurgical and pyrometallurgical methods were proposed for the recycling of batteries (Bernardes et al., 2004; Ferella et al., 2008), current processes have not been considered sufficiently economically viable for a widespread recycling of these batteries. The recycling of the alkaline batteries is challenging because the total cost of end-of-life recycling, including collection, sorting, transportation, ⇑ 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).

and recycling of the primary batteries currently exceeds the economic value of recovered materials from their waste (Turner and Nugent, 2016). Thus the industry needs innovative recycling approaches which are profitable as well as environmentally sound. Alkaline battery waste contains high amounts of iron (Fe), manganese (Mn), and zinc (Zn). These metals can be recovered separately, and they can also be used as raw material in some industries. For example, spent alkaline batteries are added to molten metal to reclaim the Fe and Mn in the steel industry (Bernardes et al., 2004; Pareuil et al., 2010). Over the last decade, preparation of advanced materials using the spent alkaline batteries as a raw material has attracted scientific attention. Xi et al. (2006) prepared Mn-Zn ferrites by leaching of alkaline batteries and synthesis in a sol-gel process. However, they needed to adjust the concentration of the solution using commercial metal salts. Peng et al. (2008) used Mn-Zn dry batteries, waste scrap iron, and pyrolusite as raw materials to prepare Mn-Zn soft magnetic ferrite powders by a process that includes leaching, purifying, and co-precipitation steps. Kim et al. (2009) reported the production of Mn-Zn ferrite powder from Zn-C batteries by reductive acid leaching and oxidative alkaline co-precipitation. Nanostructured Mn-Zn ferrite particles were also produced by auto-combustion and microwave digestion methods using spent alkaline battery leaching solution

http://dx.doi.org/10.1016/j.wasman.2017.06.015 0956-053X/Ó 2017 Elsevier Ltd. All rights reserved.

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Table 1 Chemical composition of the alkaline battery black mass; before (Ebin et al., 2016a) and after washing (Ebin et al., 2016b).

1 2

Elements

Alkaline battery waste before washing, % (w/w)1

Alkaline battery waste after washing, % (w/w)2

Mn Zn K Fe Ni Co Cu Cr Pb Cd Hg

28 ± 1 25 ± 1 4 ± 0.6 0.83 ± 0.04 0.1 ± 0.06 0.01 ± 0.004 0.03 ± 0.01 0.02 ± 0.005 0.02 ± 0.002 0.01 ± 0.003 0.00

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 Zn nanostructured particles using vacuum separation at high temperatures (650–800 °C) and inert gas condensation from zinc manganese batteries was reported by Xiang et al. (2015). After magnetic separation of the iron shells from the alkaline battery waste, Zn and Mn are the valuable elements to recover. Although fresh alkaline battery contains Zn and manganese dioxide (MnO2), various oxide compounds such as hetaerolite

Ebin et al. (2016a), Waste Manage. Ebin et al. (2016b), J. Anal. App. Pyrol.

due to their interesting ferromagnetic properties (Hu et al., 2011; Yang et al. 2015). Tu et al. (2013) investigated the nanoabsorbent properties of Mn-Zn ferrite particles prepared from battery waste. Qu et al. (2015) showed that spent alkaline batteries can be used to prepare photocatalytic ZnxMn1-xO nanoparticles, which catalyzes the degradation of bisphenol A under solar light irradiation. ZnO nanoparticles have also been prepared from battery waste by a stepwise process consisting of leaching, liquid-liquid extraction and synthesis (precipitation and heattreatment) steps (Deep et al., 2011; Deep et al., 2016). Preparation

150 100

Δ G (kj)

50 0 ZnO + H2(g) = Zn + H2O

-50

0.5 ZnMn2O4 + H2(g) = 0.5 Zn + MnO + H2O 0.75 ZnMn2O4 + H2(g) = 0.75 Zn + 0.5 Mn3O4 + H2O

-100

ZnMn2O4 + H2(g) = Zn + Mn2O3 + H2O

-150

0

200

400

600

800

1000

1200

Temperature (°C)

(a) Fig. 2. Phase equilibrium amount changes by temperature for (a) hydrogen reduction of ZnMn2O4 and (b) hydrogen reduction of ZnO.

30 ZnO + H2(g) = Zn + H2O

20

0.5 ZnMn2O4 + H2(g) = 0.5 Zn + MnO + H2O

ZnMn2O4 + H2(g) = Zn + Mn2O3 + H2O

90

0

Zn recovery, %

log Kp

100

0.75 ZnMn2O4 + H2(g) = 0.75 Zn + 0.5 Mn3O4 + H2O

10

-10 -20

80 70 60

-30

0

200

400

600

800

1000

1200

Temperature (°C)

(b) Fig. 1. Change of the Gibbs standard free energies and log value of the reaction equilibrium quotients (Keq) for hydrogen reduction of the alkaline battery black mass (a) DG°(kj) and (b) logKeq.

50

0

15

30

45

60

Time (min) Fig. 3. Removal of zinc from alkaline and zinc-carbon battery black mass dependent on residence time at 950 °C under 1.0 L/min flow rate of 5%H2-N2 gas mixture.

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B. Ebin et al. / Waste Management xxx (2017) xxx–xxx Table 2 Chemical composition of the pyrolysis residue for the main elements after 15–60 min residence time. Elements

15 min, wt%

30 min, wt%

45 min, wt%

60 min, wt%

Mn Zn K Fe

63 ± 1 8.8 ± 1.1 0.58 ± 0.09 1.18 ± 0.18

76 ± 1.2 0.68 ± 0.03 0.55 ± 0.05 1.30 ± 0.07

77 ± 1 0.08 ± 0.02 0.44 ± 0.09 1.30 ± 0.04

77 ± 2 0.05 ± 0.01 0.35 ± 0.01 1.33 ± 0.08

Fig. 4. XRD patterns of residue particles obtained by hydrogen reduction assisted pyrolysis of alkaline battery black mass for (a) 15 min, (b) 30 min, (c) 45 min and (d) 60 min durations.

Fig. 5. XRD patterns of the residue obtained at 950 °C for 60 min (a) rapid cooling at room temperature under air, (b) cooling to 300 °C by 10 °C/min cooling rate under air and (c) rapid cooling at room temperature under N2(g).

(ZnMn2O4), hausmannite (Mn3O4), and zinc oxide (ZnO) are the major compounds that exist in the industrial pretreated spent batteries (Ebin et al. 2016a). Formation of the various oxides complicates the recycling process. In our previous studies (Ebin et al.,

2016a, 2016b) the recovery of Zn and manganese oxide in particle form by a simple pyrolysis process was investigated. Zn particles are an important industrial materials due to their physicochemical properties. They are used in marine paints as an anticorrosive pig-

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B. Ebin et al. / Waste Management xxx (2017) xxx–xxx Table 3 Carbon amount of the washed alkaline battery black mass and residues depending on residence time (Residues were cooled at room temperature under N2(g) atmosphere). Residence time, min

Carbon, wt%

0 15 30 45 60

6.0 ± 0.1 5.5 ± 0.1 5.0 ± 0.4 4.8 ± 0.5 4.2 ± 0.4

ment (Søresen et al., 2009) and also in various battery technologies as an electrode material (Kim et al., 2013). In the case of manganese oxides, various oxidation states of manganese result in a complex oxide system with several crystal phases, such as MnO, Mn2O3, Mn3O4, and MnO2. Manganese oxides are significant materials for many advanced technology applications due to their catalytic, magnetic and electrochemical features (Gao et al., 2011; Kim and Shim, 2010; Lima et al., 2006; Salazar-Alvarez et al., 2007; Yang et al., 2006). In this study, the recovery of Zn from the alkaline and Zn-C battery waste was investigated using a hydrogen (H2) reduction

(a)

(d)

(b)

(e)

(c)

(f)

Fig. 6. SEM images of the residue obtained at 950 °C for 60 min (a, b) rapid cooling at room temperature under air, (c, d) cooling to 300 °C by 10 °C/min cooling rate under air and (e, f) rapid cooling at room temperature under N2(g).

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B. Ebin et al. / Waste Management xxx (2017) xxx–xxx

assisted pyrolysis process. Additionally, controlling the oxidation state of the manganese in the process was studied using different cooling rates and atmosphere conditions. Particle properties of both Zn and manganese oxides were characterized to discuss possible applications. 2. Experimental The industrial mechanically pre-treated (shredding of batteries and magnetic separation of iron cover) alkaline and zinc-carbon batteries were supplied by Renova AB, Sweden. After the pretreatment, the remaining part of the waste is called battery black mass. The quartering method (Gerlach et al., 2002) was used to take samples from the industrial battery black mass storage, and then the remaining metallic parts from the physical pretreatment step were separated using a 4 mm sieve. The spent battery black mass with particles smaller than 4 mm was ground using a IKA M20 universal mill, and then washed with Milli-Q water (2 ml/g water/powder) to remove the soluble salts. Experimental pyrolysis set-up was illustrated in the previous paper (Ebin et al., 2016b). The weight of the battery black mass was 3 g in the experiments, which was put into ceramic combustion boats as a thick layer (average 2.4 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 15, 30, 45 and 60 min residence times under 1.0 L/min 5%H2 (g)-N2(g) flow rate. The reaction zone was purged by 1.0 L/min N2 (g) flow for 1 min before and after the reaction due to the safety reasons. The flow rate of the gases was measured at room temperature. All experiments were repeated three times. The Zn particles were obtained by rapid condensation of Zn vapor formed by hydro-

gen and 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 wash bottles were filled with acetone. Acetone was used to collect and store the Zn particles to prevent oxidation, and also it rapidly evaporates during the sample preparation for particle characterization studies, which reduces contact time with air. The collected Zn particle samples were washed by acetone. After the stated residence times, the combustion boats were removed from the furnace in three different ways (i) removed at the reaction temperature and cooled at room temperature under air, (ii) cooled to 300 °C by 10 °C/min cooling rate under air, and (iii) removed at the reaction temperature and rapid cooled at room temperature under N2(g) atmosphere. HSC Chemistry 5.11 software was used for thermodynamic calculations regarding the hydrogen reduction reaction and phase equilibrium amount. X-ray diffraction (XRD, Bruker D8 Advance) using Cu Ka radiation was used to examine the crystalline compounds, crystalline structure and crystallite size of the battery waste, prepared Zn particles and manganese oxide particles (pyrolysis residue). The conditions for data collection were continuous scanning with a detector covering a 2b 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 samples were analyzed by inductive coupled plasma optical emission spectrometry (ICP-OES, ICAP 6500, Thermo Fischer) and energy dispersive spectroscopy (EDS). The elemental analysis limit of the EDS is between lithium (Li) to uranium (U), and the detection limits is approximately 0.1 wt%. The battery black mass and pyrolysis residue, 0.2 g powder, were dissolved in 30 mL aqua regia (HNO3 + 3HCl) and diluted in the desired amounts for the ICP-OES analyses. Carbon analyzer (Leco Carbon/Sulfur Analyzer CS744) was used to determine the carbon

Table 4 BET surface area of residues, calculated particle sizes, and average pore sizes. Sample

BET surface area, m2/g

Calculated particle size, nm

Pore size, nm

Mixture of MnO and Mn3O4;rapid cooling at room temperature under air Mn3O4; Cooling to 300 °C by 10 °C/min cooling rate under air MnO; rapid cooling at room temperature under N2(g)

1.30 ± 0.01 0.80 ± 0.01 1.81 ± 0.01

890 1547 618

1.65 1.71 1.65

Fig. 7. XRD patterns of Zn particles obtained by hydrogen reduction assisted pyrolysis of alkaline battery black mass for (a) 15 min, (b) 30 min, (c) 45 min and (d) 60 min durations.

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amount of the alkaline battery black mass. The particle size and morphology of the samples were investigated by a field emission scanning electron microscope (FEI, Quanta 200 FEG ESEM). The surface area and porosity of the samples were measured by BET analysis (Micromeritics ASAP 2020 Surface Area and Porosity Analyzer). BET surface area is also related to the average particle size and the average particle size were calculated by the following equation.

SBET ¼ 6000=q  dBET

flow to separate the Zn from the battery black mass. In our previous study (Ebin et al., 2016b), Zn recovery amount was 80%, when the washed alkaline battery black mass was pyrolyzed at 950 °C

ð1Þ

where SBET is the BET surface area (m2/g), q is the density of the sample (g/cm3) and dBET is the calculated average particle size (nm) (Weibel et al., 2005). 3. Results and discussion 3.1. Chemical characterization of alkaline battery black mass Chemical composition analysis of the alkaline battery black mass were performed by ICP-OES and the results are 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 low amounts of potassium (K) (1.0 ± 0.5 wt%) and 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 probably originate from the brass current collector, steel external case and other components. The washing of the alkaline battery black mass effectively removed the K resulting with the increased Mn and Zn weight percentage. The carbon (C) amounts 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.

(a)

3.2. Thermodynamic Investigation of the process The thermodynamic analyses of H2 reduction of the oxide compounds in the battery black mass were performed using the HSC software over a temperature range between 0 and 1200 °C (Fig. 1). The possible reactions for the hydrogen reduction of alkaline and zinc-carbon battery black mass depending on H2O formation are shown in Fig. 1a. The logarithmic values of reaction equilibrium constant (Kp; partial pressure of reaction products divided by partial pressure of reactants in equilibrium condition) are also plotted as a function of temperature (Fig. 1b). Fig. 2 shows the equilibrium amounts of Zn and oxide compounds versus temperature. The equilibrium amounts were calculated for 1:2 and 1:3 M ratios of ZnO:H2(g) and ZnMn2O4:H2(g), respectively. At the pyrolysis temperature (950 °C), Zn metal vapor and solid MnO can be formed by H2(g) reduction of ZnO and ZnMn2O4. The calculations indicate that H2(g) reduction of battery black mass can be used for the recovery of Zn under constant gas flow to remove the evaporated Zn from the pyrolysis batch.

(b)

3.3. Recovery of manganese and zinc The recovery of Zn from the alkaline battery black mass is shown in Fig. 3 as a function of the residence time at 950 °C under hydrogen reduction condition. The recovery amount sharply increased from 70% to 98% by extending the residence time from 15 to 30 min. The amount of Zn separation reached to 99.8% after 60 min residence time. The results show that hydrogen reduction of zinc oxide compounds effectively took place and that they were reduced to Zn metal under the experimental conditions. At the reaction temperature, the reduced Zn spontaneously evaporated and metal vapors were carried to the collection bottles by a gas

(c) Fig. 8. SEM images of Zn particles produced by hydrogen reduction assisted pyrolysis of alkaline batteries; (a) 5 K, (b) 20 K and (c) 50 K.

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under 1 L/min N2 (g) flow rate for 60 min residence time. Comparison of the results shows that using 5% H2(g)-N2(g) mixture increased the pyrolysis process yield by approximately 19% for 60 min residence time. The chemical compositions of the pyrolysis residues for the main elements are given in Table 2. The wt% of Fe in the pyrolysis residue increased by residence time because the removing of Zn from the batch leads to increase the general composition of the elements. Contrarily, wt% of K decreased with increasing residence time due to partly evaporation with Zn. Fig. 4 shows the XRD patterns of the pyrolysis residue that was removed at reaction temperature to room temperature under N2(g) atmosphere. All samples contained MnO phase. The MnO peaks were indexed with face centered cubic (FCC) crystal structure (Space group: Fm-3 m). The XRD peak observed at 26.5° 2theta position belongs to carbon (C). The residue still had carbon traces after pyrolysis because cooling the samples under N2(g) atmosphere protected the unused carbon. The sample treated for 15 min also had impurities of ZnO and/or Zn0.25Mn2.75O4 due to the low Zn recovery. Fig. 5 shows the XRD patterns of the pyrolysis residue treated for 60 min and removed at different conditions to control manganese oxide phase formations. After hydrogen reduction assisted pyrolysis treatment of the alkaline battery black mass, rapid cooling (removing from 950 °C to room temperature) of the residue under air gave the formation of tetragonal crystal structured Mn3O4 (hausmannite) and FCC crystal structured MnO. Carbon trace was also observed. When the residue was cooled from 950 °C to 300 °C with 10 °C/min cooling rate under air atmosphere, only Mn3O4 was formed, and all the C was burned. MnO was obtained when the residue was removed at the reaction temperature and rapidly cooled at room temperature under N2(g) atmosphere. Table 3 shows the carbon amount of the washed alkaline battery black mass and residues which were cooled at room temperature under N2(g) atmosphere. The process theoretically needs around 9 wt% carbon in the sample to reduce battery black mass to Zn and MnO on the assumptions that (i) carbon does not react with air during the sample feeding, and (ii) the oxides only react with carbon and the exhaust gas is CO(g) at 950 °C. However, it is

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highly possible that carbon partly burns when samples are loaded into the reaction zone at 950 °C under air atmosphere. On the other hand, experimental results show that carbon amount in the pyrolysis residue slightly decreased by increasing residence time. Despite the decreasing carbon amount, its’ consumptions are not enough to reach the obtained Zn recovery amounts and MnO formation. It is obvious that H2(g) reduction of battery black mass is more dominant than the carbothermic reduction at the experimental condition. In the light of these results, the hydrogen reduction steps of the alkaline and Zn-C battery black mass at 950 °C are assumed to occur according to the following reactions;

ZnMn2 O4 þ 2 H2ðgÞ ¡Zn þ 2 MnO þ 2 H2 O

ð1Þ

3 ZnMn2 O4 þ 4 H2ðgÞ ¡3 Zn þ 2 Mn3 O4 þ 4 H2 O

ð2Þ

Mn3 O4 þ H2ðgÞ ¡3 MnO þ H2 O

ð3Þ

ZnO þ H2ðgÞ ¡Zn þ H2 O

ð4Þ

SEM images of the pyrolysis residue treated for 60 min at different conditions are given in Fig. 6. Manganese oxide particles have octahedron and truncated octahedron shape morphologies in the particle size range between 200 nm and 1 lm, but the particles are agglomerated. SEM images show that Mn3O4 particles obtained by cooling the residue to 300 °C with 10 °C/min cooling rate under air had higher level of aggregation than other two samples. BET surface area of the residue particles, calculated particle sizes using BET data and average pore sizes are given in Table 4. MnO particles which were rapidly cooled to room temperature under N2(g) atmosphere had higher surface area, and Mn3O4 particles had lower surface area due to active aggregation during slow cooling rate. It is clear that the residue particles have a microporous structure. The Zn particles were formed by rapid nucleation of the Zn vapor that separated from the alkaline battery black mass. Fig. 7 shows the XRD pattern of the recovered Zn from the alkaline battery black mass for different residence time. The metallic Zn particles had hexagonal crystal structure. Although (1 0 1) peak is the strongest peak of the Zn metal, (0 0 2) peak has higher intensity

Fig. 9. EDS analysis of Zn particles produced by hydrogen reduction assisted pyrolysis of alkaline batteries.

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Fig. 10. Process flow chart to recovery of Zn and MnO from alkaline battery waste by hydrogen reduction assisted pyrolysis process. (*MnO residue obtained at 950 °C for 60 min residence time under 1 L/min H2-N2 gas mixture flow rate and cooling by quenching to room temperature under N2(g) atmosphere).

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for the recovered particles which means that there is a preferred orientation on (0 0 2) planes. The crystallite sizes of the particles were determined by the Scherrer Equation using the (0 0 2), (1 0 0) and (1 0 1) 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 5%H2(g)-N2(g) gas mixture with 1.0 L/min flow rate were around 100 nm. In the proposed process, the residence time does not have a significant effect on Zn particles as expected. SEM images of the Zn particles obtained by hydrogen assisted pyrolysis of the battery black mass for 60 min are given in Fig. 8. The Zn particles have both spherical and hexagonal shape morphology. The particle sizes range from 200 nm to 3 mm, with an average particle size of around 1 mm. However they are all aggregates of primary nucleated particles from the gas phase which have nearly 100 nm particle size, as can be seen in the highly magnified SEM images (Fig. 8c). Fig. 9 shows the EDS analysis of the Zn particles. Possible impurities such as K, which can come from the waste, were not observed. A weak peak of oxygen was detected probably due to the surface oxidation of the sample during the sample preparation. In our previous study on Zn particle production from unwashed alkaline battery sample (contains 4 ± 0.6 wt % K) by carbothermic reduction based pyrolysis (Ebin et al., 2016a), Zn particles had small amount of K in their structure. Washing of alkaline battery black mass removed the K, and its’ amount reduced to 1.0 ± 0.5 wt%. The slightly evaporated K and its’ compounds from the pyrolysis batch probably was trapped in the washing bottles, and it could partly dissolve in the acetone, where Zn particles were also collected. Additional, K could be possibly removed by washing of Zn particles, and its’ concentration reduced to lower than the detection limit of the EDS. Fig. 10 summarized the process steps in a flowchart for the pyrolysis based recovery of Zn from alkaline and Zn-C batteries, and also shares the elemental amount of the battery black mass and recovered materials obtained at the experiments (hydrogen assisted pyrolysis condition: at 950 °C for 60 min residence time with 1 L/min H2(g)-N2(g) flow rate). Nearly 97 wt% of the industrial pre-treated alkaline battery waste is suitable to feed to the suggested pyrolysis process. 3 wt% of the black mass contains brass current collector, organic parts and also small iron parts covered with battery powder which could not be removed by magnetic separation. According to the results, 99.8% of the Zn was recovered from the 3 g battery black mass in the batch experiment by hydrogen dominated reduction process, while only 1.8 wt% C was consumed in the reduction reactions. In our previous study on carbothermic reduction of battery black mass (Ebin et al., 2016b), thermodynamic calculations showed that CO(g) formation is dominant at 950 °C. Additionally, CO(g) formation is also favorable at 950 °C according to Boudouard Equilibrium (Penchini et al., 2013). However, gas outlet of the furnace in the experimental set-up is lower than 950 °C, which means that CO(g) probably decomposes to CO2(g) and C while it moves away from the hottest reaction zone to the outlet according to the Boudouard reaction. Thus the possible exhaust gases of the suggested process can be H2O vapor, CO(g) and CO2(g), as well as unreacted H2(g) and N2(g).

4. Conclusions Zn was successfully recovered from alkaline and Zn-C battery black mass by H2(g) reduction assisted pyrolysis process at 950 °C. The recovery of Zn reached to 99.8% for 60 min reaction duration using 1.0 L/min gas flow rate. H2(g) reduction of the battery black mass is dominant reaction to recover Zn comparing to carbothermic reduction of the waste. The oxidation state of the manganese can be controlled by cooling rate and atmosphere. Par-

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ticles containing pure MnO, pure Mn3O4 and mixtures of these compounds were obtained. Although the manganese oxide particles had fine particle size, they were agglomerated and formed microporous structures. These particles can be suitable for catalytic and battery applications. The surface area of the residue particles was affected by the cooling rate and atmosphere which indicates that these factors can be used to control the residue particle properties. The recovered Zn particles had hexagonal crystal structure and the crystallite sizes were around 100 nm. The pyrolysis duration did not have a significant influence on their structure. The spherical and hexagonal shaped Zn particles that were formed from the gas phase were aggregates of nanosized primary nucleated particles. Results shows that the hydrogen reduction assisted pyrolysis is a highly efficient process not only to recycle the Zn as a nanocrystalline material, but also to reclaim the manganese oxide residue for advanced material applications. The reduction of CO gas emissions of the recycling of spent alkaline batteries using H2(g) as a reducing agent is an environmental advantage of the developed process compared to carbothermic reduction oriented process.

Acknowledgements This research was supported by Swedish Energy Agency (Grant No: 39063-1) and Vinnova-Sweden Innovation Agency (Grant No: 2016-02621).

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