From spent alkaline batteries to ZnxMn3−xO4 by a hydrometallurgical route: synthesis and characterization

A series of Zn/Mn binary oxides with different molar ratios (1.4–11) were synthesized via co-precipitation from a solution obtained through the acidic (HCl) leaching of a black mass originating from the mechanical recycling of spent alkaline and Zn–C batteries. The oxides obtained were characterized by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Magnetic properties of the samples were also investigated. The Raman spectroscopy results showed all the binary metallic oxides belong to the ZnxMn3−xO4 (0.25 ≤ x ≥ 1.75) type. All showed a spinel crystalline structure. The saturation magnetization decreases with the Zn/Mn molar ratio; a maximum of 13.19 emu g−1 was found for the molar ratio of 11 at the Curie temperature (25.5 K). XPS showed that all the synthesized compounds contained Mn2+, Mn3+ and Mn4+. Mn2+ was the most prominent at a molar ratio of 11, Mn3+ was most common at a molar ratio of 2, and Mn4+ at 1.4.


Introduction
In recent decades, technological advances have allowed batteries to be used as energy sources in many electronic devices (toys, computers, cell phones, watches, remote controls, cameras, etc.). 1 Unfortunately this has led to the need to dispose of increased quantities of spent batteries. 2 The yearly global demand for batteries is currently growing at 7.7%by 2019, battery consumption will reach 80 000 million units per year, representing a market worth US$120 billion. 3 In Spain, only about 24% of spent batteries (representing some 3031 tonnes) are currently collected for proper disposal. By 2019, however, it is expected that 65% will be recovered for recycling. 4 The main components of batteries are manganese dioxide (positive electrode), zinc (negative electrode), an electrolyte (KOH or ZnCl 2 + NH 4 Cl), and the steel casing. These compounds and any other heavy metals they may contain (e.g., cadmium, mercury, lead, lithium) may seep out, negatively affecting the environment and human health. [4][5][6] These dangers, plus the elevated costs associated with the adequate management of these spent products, make the recycling of batteries an attractive option. 7 The techniques developed for processing spent batteries fall into three groups: mechanical separation, pyrometallurgical treatment, and hydrometallurgical treatment. 8 Mechanical separation is commonly required before any further processing, especially hydrometallurgical processing. The aim is to separate the electrodes, steel casing and any plastic or paper components. 9 Mechanical separation usually involves cutting/milling, magnetic separation, dimensional separation (screening), eddy current separation (ECS) employing Foucault currents, and the nal milling of the particulate fraction. 7 The result is a so-called 'black mass' (consisting largely of electrolytes, graphite, and oxides of zinc and manganese). Although pyrometallurgical treatment is the most commonly used method 10,11 hydrometallurgical treatment is gaining attention given its low cost and more environmentally friendly nature. 12 The hydrometallurgical treatment of spent battery black mass takes place in several stages: pre-treatment, acidic or alkaline leaching, and the recovery of the Zn and Mn via electrolysis, liquid-liquid extraction, or selective precipitation. 13 The use of biological processes for this is also of interest since they are likely to be less environmentally harmful. 14,15 Several hydrometallurgical processes have been patented for the industrial scale recovery of Mn and Zn from spent batteries, including the Bateaux, 16 Recupyl 17 and Revabat 18 processes, among others.
Previous work performed by the present authors showed it is possible to obtain highly pure ZnO from black mass via leaching with ammoniacal ammonium carbonate. 19,20 This ZnO has good luminescent properties and can be used in gas sensors. 21 Ternary compounds of metallic oxides that contain transition metals of the AB 2 O 4 type are currently a focus of research. In this context ZnO, doped with a certain amount of Mn, forms interesting spinel structures of potential use in shortwave magneto-optical 22 and spintronic devices. 23 In addition, semiconductors doped with small quantities of transition metals (TM), known as diluted magnetic semiconductors (DMS), are of great technological interest given their capacity to control spin as well as electric charge, a property that makes them potentially useful in the manufacture of new generation spintronic products. 24 In addition, ZnO and TM doped ZnO compounds (such as Cr, Mn, V) exhibit photocatalytic applications under UV and solar radiation. 25,26 ZnO : Mn also processes the capacity to capture CO 2 and SO 2 . 27 All of this makes these compounds potentially useful in several technological applications.
A number of studies have reported various ways of synthesizing the spinel ZnMn 2 O 4 . Courtel et al. synthesized ZnMn 2 O 4 nanoparticles from their corresponding acetates via a hydrothermal process 28 and via coprecipitation. 29 The production of ZnMn 2 O 4 microspheres via solvothermal synthesis from the corresponding acetates has been also reported. 30 Recent studies have also shown that ZnMn 2 O 4 spinel can be obtained via autocombustion processes 31,32 and sol-gel methods, 33,34 among others. However, while several authors have described the recovery of different valuable metals from spent lithium ion batteries, [35][36][37][38] few of them describe in detail a comparative study of the structural characteristics and magnetic properties of these type of compounds.
The present work describes the synthesis of Zn x Mn 3Àx O 4 (0.25 # x $ 1.25) compounds with molar ratios ranging between 1.4 and 11, and with a spinel crystalline structure, via coprecipitation from the solution obtained following the acidic leaching of spent battery black mass. [35][36][37][38] Experimental procedure

Preparation of black mass
Black mass produced from spent alkaline and Zn-C batteries was provided by Envirobat España, S.A (Guadalajara, Spain). Table 1 shows its mean chemical composition, as revealed by Xray uorescence (XRF) analysis using a PANalytical Axios wavelength dispersive spectrometer (4 kW). The mineralogical composition was determined by X-ray diffraction (XRD) using a Siemens D5000 diffractometer equipped with a Cu anode (Cu Ka radiation) and a LiF monochromator. Morphological analysis was performed by scanning electron microscopy (SEM) using a FEI Inspect microscope equipped with an X-Ray energy dispersive spectrometer (EDS).

Synthesis of zinc and manganese oxides
Different binary metallic oxides (BMO) with different proportions of Zn/Mn were produced from the black mass via (1) the acidic (HCl) leaching of Zn and Mn, followed by (2) the precipitation of Zn 2+ and Mn 2+ cations in an alkaline medium.

Acidic leaching
100, 200, 300 or 400 g of black mass were dispersed in a 1 L suspension in 250 mL milliQ water plus 500 mL of 6 M HCl (Panreac®) and 250 mL of H 2 O 2 (Panreac®). Aer mixing (500 rpm) for 1 h at room temperature, the suspension was ltered through a Millipore Holder lter at a pressure of 7 bars. The solid phase was discarded and the resulting ltered solutions termed L iq , where q refers to the 100, 200, 300 or 400 g used. The Zn and Mn contents of these solutions were determined by atomic absorption spectroscopy (AAS) using a Varian Spectra AA 200 spectrometer. The pH of the solutions was, in all cases, approximately 0. The solutions were then subjected to precipitation in an alkaline medium.

Precipitation in an alkaline medium
The ltered solutions (L iq ) were subjected to a precipitation procedure involving 6 M NaOH. The pH was monitored using a pH meter at room temperature until a value between 12-14 was obtainedallowing the precipitation of zinc and manganese oxides. The solutions were then ltered, producing a solid phase of different hues of brown containing the Zn/Mn binary oxides (BMOs) and a nal liquid phase (L j ) that was discarded. The solids obtained were termed BMO1, BMO2, BMO3 and BMO4 with reference to their precipitation from L1 to L4 respectively. Fig. 1 summarizes the procedure for the production of these BMOs.

Characterization of the Zn/Mn binary metallic oxides
The chemical composition of the Zn/Mn BMOs was determined by XRF using the above-mentioned PANalytical Axios wavelength dispersive spectrometer. Their mineralogical composition was determined by XRD using the above-mentioned Siemens D5000 diffractometer. The Rietveld method was used to calculate structural parameters from the XRD patterns, using TOPAS v4.2 soware (Bruker ASX) and taking into account the crystallographic information for the different phases from Pearson's crystal structure database for inorganic compounds. 39 The morphology of all the BMO samples was studied by scanning electron microscopy (FE-SEM) using a JEOL JSM 7600 apparatus, as well as by transmission electron microscopy (TEM) using a JEM 2100 HT device. Fourier-transformed infrared spectroscopy (FTIR) using a Varian 670 FTIR spectrometer (spectral range 1600-400 cm À1 , spectral resolution of 4 cm À1 ) in transmittance mode was also performed. In addition, micro-Raman spectra were obtained using a confocal Horiba Jovin-Yvon LabRAM HR800 system. The samples were excited by a 633 nm He-Ne laser on an Olympus BX 41 confocal microscope with a 10Â objective.
The temperature dependence of the magnetic susceptibility was measured in the 2-300 K range in a magnetic eld of 1000 Oe using a Quantum Design XL-SQUID magnetometer. Hysteresis measurements were taken at 10 K with a maximum eld of 5 T.
The surface chemistry of the BMO samples was examined by XPS. Spectra were recorded using a Fisons MT500 spectrometer equipped with a hemispherical electron analyzer (CLAM2) and a non-monochromatic Mg Ka X-Ray source operated at 300 W. Spectra were collected at a pass energy of 20 eV (typical for highresolution conditions). The area under each peak was calculated aer subtraction of the S-shaped background and tting the experimental curve to a combination of Lorentzian and Gaussian lines of variable proportions. Binding energies were calibrated to the C 1s peak at 285.0 eV. The atomic ratios were computed from the peak intensity ratios and reported atomic sensitivity factors. 40

Results and discussion
Characterization of the black mass Table 1 shows the composition of the black mass, which consisted mainly of Mn (36.8 wt%) and Zn (23.7 wt%). The major crystalline phases were zincite (ZnO), hetaerolite (ZnMn 2 O 4 ) and sylvite (KCl). Fig. 2 shows SEM images and the corresponding EDS microanalysis which reveal that the hetaerolite (Fig. 2a) and zincite (Fig. 2b) phases are present. Table 2 shows the composition of the acidic leachates. These solutions were pinkish in colour, characteristic of solutions containing Mn 2+ ions, the intensity of the colour depending on the Mn 2+ concentration. Eqn (1) and (2) show the dissolution reactions: 12

Synthesis of binary metallic oxides
Table 2 reveals how the Mn content decreases as the amount of dissolved black mass increases. The highest Mn content was obtained for L i200 sample. The concentration of Zn increases with the amount of black mass dissolved.
During the alkaline precipitation process, the precipitation of Zn(OH) 2 (eqn (3)) starts at pH 7.5 and increases with the pH. The precipitate was white.
Zn(OH) 2 (K sp at 25 C ¼ 3 Â 10 À16 ) continues to form until a pH of 11.5 is reached, aer which (bearing in mind the amphoteric nature of this compound) it dissolves to form zincate ions (eqn (4)): The Mn 2+ is stable up to pH 8.5, aer which manganese hydroxide (K ps ¼ 2.5 Â 10 À13 ) is formed, along with mixed Zn/ Mn hydrates (eqn (5)): These hydrated oxides are rapidly oxidized to form anhydrous oxides with a spinel structure (eqn (6)): The reactions shown above explain the formation of the Zn/ Mn BMOs. In the case of BMO4 large amounts of ZnO were detected. This is a consequence of the high concentration of Zn 2+ in the acidic solution (63.0 g L À1 ); during the alkaline precipitation process some Zn 2+ did not combine with the Mn 2+ . This excess Zn 2+ precipitated out as Zn(OH) 2 , and later became ZnO through dehydration.
Regarding the composition, with the exception of the odd minority elements, all are composed of Zn and Mn, the exact content of each depends on the amount of solution subjected to alkaline precipitation The Zn and Mn contents and stoichiometry as determined by XRF (Table 3) and Rietveld rened XRD (    (Table 3). Nevertheless, if we take into account the data from Table 4, in this sample, an equivalent amount of ZnO (12% wt) is detected, then accounting for the 10.7 wt% excess of Zn observed.

Characterization of the binary metallic oxides
The composition of the crystalline phases was examined by XRD. Fig. 3 shows the diffractograms recorded. In most cases, a tetragonal symmetry with a I4 1 /amd spatial group are obtained, which, according to JCPDS Card no. 24-1133, ts well with an spinel-type structure. However, the diffractogram for BMO4 also shows a diffraction maximum corresponding to ZnO (JCPDS Card no. . Table 4 shows the composition of the crystalline phases of the BMOs as determined by Rietveld renement; where a very clear variation in the proportions of Zn and Mn is observed. The different Zn/Mn BMOs obtained also show a very high purity (95-96%).
The measured value for the a lattice parameter of the ZnMn 2 O 4 phase is 5.758Å, i.e., slightly higher than the values reported in the literature (5.709-5.722Å). The value for b parameter (9.240Å), however, is much closer to previously reported values (9.222-9.238Å). 37,41,42 The value of a parameter for the Zn/Mn BMO stoichiometries decreases with increasing Zn content. This agrees with previous results reported for ZnMn 2 O 4 phases obtained by the ceramic method. 43 The intensities of the diffraction maxima decrease as the Mn content increases. An increasing Mn content is therefore associated with a reduction in the crystallinity of the BMOs. Other authors have reported similar results for ZnO doped with Mn obtained via hydrothermal and sol-gel synthesis. 44,45 Table 3). The mean diameter of the particles is 40, 50 and 35 nm for BMO1, BMO2 and BMO3 respectively. An increase in the Mn content leads to an increase in particle size, as previously reported. 43 Fig . 6 shows the FTIR spectra for the BMOs. All show two main absorption bands at approximately 527 and 634 cm À1 . These may be respectively attributed to distortion vibration of Mn 3+ cations in an octahedral environment, and stretching vibration at Mn-O tetrahedral sites. A weaker band is also observed at around 420 cm À1 , due to stretching vibration of Mn 3+ at octahedral sites. [44][45][46] Fig . 7a shows the normalized Raman spectra for the different Zn x Mn 3Àx O 4 samples. In all the spectra the bands are attributable to ZnMn 2 O 4 . These results are consistent with those obtained in the XRD analysis. According to the group theory, oxides of the stoichiometry XMn 2 O 4 (where X ¼ Zn, Mn) with a spinel-type crystalline structure and I4 1 /amd space group have 10 active Raman modes: G ¼ 2A 1g + 3B 1g + B 2g + 4E g . 47 However, the present spectra do not show all these modes; indeed, the maximum number ever observed has been seven. 48,49 The vibration modes above 600 cm À1 , correspond to the movement of oxygen atoms in the tetrahedral AO 4 groups, and reect an A 1g symmetry.
The low frequency modes are characteristic of octahedral sites (BO 6 ). Other authors report a high variation in the position of the Raman peaks for ZnMn 2 O 4 . Malavasi et al. 48 reported peaks at 300.2, 320.5, 381.9, 475.5 and 677.6 cm À1 , Tortosa   Fig. 7b shows a zoom of this Raman peak. Note that as the Mn content increases, the height of the peak decreases. According to XPS, in the sample with the higher Mn content, the predominant oxidation state is Mn 2+ , hence the differences observed in the Raman peak could be explained by the substitution of Zn 2+ by Mn 2+ (which is larger) in the BMOs, 51 which would lead to a reduction in the bond distances. In addition, a broadening of the Raman bands can be appreciated related to the purity and crystallinity of the samples. These results are in good agreement with those obtained from the Rietveld renement where highest percentage of impurities leading to a broader Raman bands. Fig. 8 shows the variation in magnetic susceptibility per unit mass (c g ) vs. temperature over the range 2-300 K in a 1000 Oe eld. All the BMOs behave in a similar way, the magnetic susceptibility rises to a peak value and then decreases. These inection points allow us to determine the Curie temperature (T c ) for each BMO ( Table 5). As the value of x increases in the nominal composition of the Zn x Mn 3Àx O 4 phases, the T c reduces, as reported by Nádherný et al. 52 BMO2 shows the greatest magnetic susceptibility, in agreement with the XPS results (Fig. 9). For this sample, the majority charge state for Mn ions is +2, hence, since Mn 2+ has the largest magnetic dipole moment (5.9mB), a largest magnetic moment and saturation magnetization are expected.

Magnetic properties
Different theories exist regarding the magnetic properties of (Zn, TM) O-type (with transition metals) oxides. Sharma et al. 53    Table 6 shows the elemental atomic percentages for the different Zn/Mn ratios, and the percentages according to the Mn oxidation state. In all the examined BMOs, mixtures of Mn 2+ , Mn 3+ and Mn 4+ are obtained. The XPS spectra for the O 1s are also shown in Fig. 9. In all cases, the obtained spectra exhibit asymmetric peaks. In this respect, there is a controversy in the interpretation of O 1s XPS spectra for different oxides. The band peaked at around 534 eV in some cases is attributable to hydroxyl groups OH, or other radicals on the sample surface 58,59 or oxygen vacancies. 60,61 For the present BMOs, none of these possibilities can be ruled out. Either, the existence of oxygen vacancies created to compensate the electric charge or the different Mn ions could be equally responsible for the magnetic behavior. Fig. 10 shows the magnetization curves versus applied eld. All are representative of a ferromagnetic material with magnetic domains. As expected, the magnetic saturation (M s ) increased with the Mn content, reaching a value of 13.2 emu g À1 for BMO2.    and Mn 4+ . The synthesis described provides a simple method to obtain Zn x Mn 3Àx O 4 compounds useful potential for several applications. This process would appear to be a good way to obtain spinel from spent batteriesan useful product from a dangerous waste.

Conflicts of interest
The authors declare no conict of interest.