The recycling of heavy metals contained in alkaline batteries allows minimizing the environmental impact and gives an alternative use to this waste, which can be used in the pyrometallurgical industry. In the present research work, we evaluated the possibility of reducing the manganese oxide black mass from discarded alkaline batteries to produce metallic manganese, using charcoal as a reducing agent. The procedure begins with the characterization of the raw materials, the stoichiometric calculations and the preparation of a practical method to produce self-reducing pellets, composed of manganiferous material, charcoal and bentonite as agglomerant. Computer simulations were performed, to establish the appropriate thermodynamic conditions for reduction. The tests were carried out in a tubular-type furnace, and the results obtained were evaluated using optical microscopy, scanning electron microscopy coupled with microchemical analyses and X-ray diffraction techniques. It was verified that the agglomerates showed a self-reducing behavior, so an increase of the %Mn in the samples due to increasing the temperature of the reduction treatment was found, as well as the presence of a metallic manganese phase that was identified by X-ray diffraction analysis.
Graphical Abstract
Flow sheet for the production of self-reducing pellets containing eucalyptus charcoal for the recycling of manganese contained in spent alkaline batteries
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Hinweise
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Introduction
Currently, we live in a time in which we constantly interact with technological devices, which work thanks to the energy produced by batteries. This situation has triggered a series of environmental problems, due to the inadequate handling given to the waste generated by the batteries, when they have completed their useful life. In the same way, the increase in the consumption of batteries causes a greater need for landfills for the elimination of these residues and a more intensive exploitation of mineral resources (Belardi et al. 2014; Melchor et al. 2021).
To overcome this problem, recycling methods have been implemented to give an alternative use to this waste and minimize its environmental impact on the ecosystems. Alkaline batteries are generally recycled using hydrometallurgical or pyrometallurgical techniques (Bernardes et al. 2004); however, the processes to a greater or lesser degree alter the environment. The pyrometallurgical industry is one of the areas interested in the use and processing of the metals contained in the alkaline batteries that are discarded (Toro et al. 2023). One of the elements contained in these batteries is manganese, in the form of oxide within the cathode black mass. Manganese is an industrial metal with very high demand nowadays (Eghbali et al. 2023) that can be recovered from its oxides through a reduction process (Singh et al. 2021). So, the manganese material contained in discarded batteries can be treated through pyrometallurgical reduction.
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Several pyrometallurgical reduction procedures have been developed for recycling spent alkaline batteries. For instance, proprietary processes like Sumitomo, Batrec and Citron have been extensively documented in the literature (Espinosa and Mansur 2019; Sayilgan et al. 2009; Bernardes et al. 2004), along with various experimental systems (Belardi et al. 2012; Hu et al. 2021; Yeşiltepe et al. 2019). In general, the pyrometallurgical recycling of spent alkaline batteries involves two primary stages: a mechanical treatment, such as shredding, followed by a thermal process in a suitable reactor or furnace. During the thermal step, carbon-based reductors are added to facilitate the metallization of manganese oxides within the batteries (Espinosa and Mansur 2019). The metal manganese then combines with the steel casing of the batteries to form a Fe–Mn alloy (Hu et al. 2021) aided by fluxing agents like lime, which create a slag (Toro et al. 2023). Due to the volatility of zinc in the batteries, this element is recovered from the gas cleaning units of the furnace, typically as ZnO (Belardi et al. 2012). A process flow diagram summarizing this generic operation is shown in Fig. 1.
Fig. 1
Generic process flow diagram for the pyrometallurgical recycling of spent alkaline batteries
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In the pyrometallurgical unit, reduction of oxides takes place commonly using coal or metallurgical coke as a reducing agent. This approach has the drawback that being a carbonaceous material of mineral origin, it generates significant amounts of uncompensated carbon monoxide and dioxide in the atmosphere, which makes it polluting, since they are emitted directly into the atmosphere during the reactions of the process (Sommerfeld and Friedrich 2021; Harvey et al. 2022). Consequently, the application of biomass in metallurgy has been considered, proposing the replacement of fossil reductants (coal and coke) by charcoal, whose CO and CO2 emissions can be compensated. For example, in typical combustion procedures applied in ferrous pyrometallurgy, such as coal injection, the substitution of coal by charcoal promoted a 33% reduction in CO2 emissions (Harvey et al. 2022).
Additionally, there are studies in which the use in pyrometallurgy of this type of charcoal from lignocellulosic origin presents higher yields when it is used to reduce oxide-type residues, which contain heavy metals (Griessacher et al. 2012). Furthermore, Yeşiltepe and Şeşen (2020) reported the use of ground coffee waste as a reducing agent, which was pelletized alongside black mass from spent alkaline batteries to produce ferromanganese alloys.
In this study, we investigate the reduction of manganese oxide in cathode black mass material extracted from spent alkaline batteries. We use eucalyptus charcoal as a reducing agent, offering an eco-friendly alternative to traditional pyrometallurgical waste recovery methods. Our approach aligns with the principles of a circular economy, aiming to reintegrate manganese into the production chain while minimizing environmental impact.
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Materials and methods
As shown in Fig. 2, each of the stages of the experimental methodology developed in the present investigation will be enunciated and explained, starting from obtaining the raw materials used in the manufacture of the agglomerates, up to the evaluation of the results obtained after the reduction tests performed in the tubular furnace.
Fig. 2
Flowsheet of the experimental methodology
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Characterization of raw materials
Spent batteries pretreatment.
Spent alkaline C-size batteries from various brands were collected from recycling containers at a local recycling facility in Bucaramanga, Colombia. These batteries were disassembled using manual tools, with great care taken to extract the electrode materials without cross-contamination between the anode and cathode fractions. The zinc-rich anode of the batteries was not used in this study, while the cathode (referred to as the ‘black mass’), was extracted, milled and sieved (100% passing 100 Tyler mesh sieve). According to the literature (Molina-Silva et al. 2019; Gallaway et al. 2015), manganese in the cathode black mass can be found in several chemical forms, including MnOOH, Mn(OH)2, and some spinels such as Mn3O4 or ZnMn2O4 depending on the discharge degree.
The black mass was digested using a 10% HNO3 solution in a microwave oven, and the metals in the resulting supernatant were chemically analyzed using atomic absorption spectroscopy (AAS) with a GBC SavantAA spectrometer. Carbon and sulfur content in the black mass were determined using a Leco CS 600 analyzer. The elemental chemical analysis of the black mass is presented in Table 1.
Table 1
Elemental chemical analysis of the black mass
Chemical element
Weight (%)
Mn
51 ± 2
Zn
1.38 ± 0.05
K
9.4 ± 0.3
Fe
0.041 ± 0.005
Ni
0.013 ± 0002
C*
6.5 ± 0.1
S*
0.9 ± 0.1
*Determined using a Leco CS 600 equipment
Charcoal and agglomerating agents
To create a self-reducing spheroidal pellet from the manganese-bearing cathode black mass, a combination of a reducing agent and an agglomerating agent is required. This pellet should possess inherent self-reducing capacity, eliminating the need for an external reducing gas to facilitate the metallization of manganese oxide. In our study, we employed eucalyptus charcoal as the reducing agent, a carbonaceous biomass-derived product known for enhancing the sustainability of pyrometallurgical processes (Sommerfeld and Friedrich 2021; Harvey et al. 2022). Table 2 summarizes the key characteristics of the charcoal utilized in this research.
Table 2
Characteristics of the eucalyptus charcoal powder used
Parameter
Weight (%)
Fixed carbon
87.6
Volatile matter
4.1
Ash
2.7
Moisture
5.6
Sulfur
1.2
Calorific value (Btu/Lb)
13,457
Granulometry
100% passing Tyler mesh #200
The bentonite used as a binder to obtain cohesion in the agglomerated charcoal pellets and cathode black mass was purchased from the company Bentonitas Colombianas Ltda-Bentocol, with the composition and main characteristics shown in Table 3.
Table 3
Chemical composition and physical characteristics of the bentonite used as agglomerant
Component
Weight (%)
SiO2
56.1
Al2 O3
22.3
Fe2O3
3.8
Na2O
6.2
CaO
2.4
Moisture
12.0
Granulometry
80% passing Tyler mesh #200
Color
Light yellow
Determination of agglomeration conditions
To produce the self-reducing pellets, a mixture comprising cathode black mass, eucalyptus charcoal, bentonite and distilled water was prepared. Initially, the cathode black mass was dried for 2 h at 100 °C in an oven. Subsequently, it was dry mixed with eucalyptus charcoal powder and bentonite. The resulting mixture was homogenized using a Retsch RM 200 mortar grinder for 20 min, ensuring that 100% of the mixture passed through a 200 Tyler mesh sieve. To determine the mass of both reagents in the mixture, we considered the fixed carbon content in the charcoal (refer to Table 3) and the manganese content in the black mass (refer to Table 1). We employed a stoichiometric ratio of 1 mol of Mn3O4 per 4 mol of carbon. The amount of bentonite added to the mixture was set at 1.5% based on references in the literature, which suggested that the binder quantity for fine mineral granules in pelletization typically falls within the range of 1–2% (Man et al. 2014; Man and Feng 2016; Guo et al. 2015). A critical step in the process was the controlled addition of water to ensure the formation of high-quality pellets. Water was added drop by drop to the mixture, with every 10 drops followed by homogenization using a glass bar to develop plasticity while preventing slurry formation. The optimal water proportion was determined to be 11 ml per 10 g of the dry mixture. The final mixture was manually molded into spheroids (green pellets) weighing 240 ± 20 mg. These green pellets underwent a drying process at 100 °C for 24 h before being subjected to the reduction experiment in a tubular furnace. This meticulous procedure ensured the production of mechanically stable pellets capable of withstanding a vertical drop test from a height of 50 cm. The passing criterion was achieving 25 drops without any visible cracks.
Simulation of thermodynamics reduction conditions
Using the information available in the literature (Zhang et al. 2013), the possible chemical species that could be formed during the reaction of cathode black mass and eucalyptus charcoal were selected. The HSC chemistry software version 6.0 was used to simulate the reaction of Mn3O4 and C, where the conditions used in the first simulation were: 1:1 molar ratio of the reagents, temperature range from 0 to 2500 °C and constant pressure of 1 bar. This simulation was intended to identify, in a precise way, the relevant chemical species in the system. Afterward, a second simulation was carried out, considering the main reactions identified in the system, as follows:
In consequence, to determine the metallization temperature of the manganese within the system, a second simulation was performed varying the initial quantities of the reagents to 1 mol of Mn3O4 and 4 mol of carbon, to satisfy the stoichiometric demand of both reactions.
Design of reduction tests
For the reduction experiments, a horizontal tubular furnace with an alumina tube of 90 mm internal diameter was used. The design of the reduction treatment took into account relevant data from the bibliography (Guo et al. 2016; Man et al. 2014; Abd Rashid et al. 2014; Zhang et al. 2012). The chosen heating time was 180 min to reach the desired holding temperature and the holding time at the desired temperature was 120 min. The heating rate used was set to 5.4 °C/min. The holding temperature was determined considering the simulation carried out in the HSC Chemistry software; based on this, the reduction temperatures for the tests 1 and 2 were selected: 1000 °C and 1200 °C, respectively.
Mineralogical, microchemical and microstructural analysis of the agglomerates
The agglomerate self-reducing pellets were photographed using a digital camera. For the petrographic preparation, the samples were subjected again to a crushing and grinding process in the mortar mill; in this way, fine particles were extracted, which were later deposited in a mold with cold resin. The preparation of the samples was carried out under the ASTM E3-01 standards. Subsequently, the microstructure of the pellets was revealed and photographed, through the optical microscopy technique, using an OLIMPUS GX 41 inverted metallurgical microscope, with a maximum magnification of 1500×, in which micrographs were taken at different magnifications.
The microstructural and microchemical analysis of the samples was carried out using a field emission scanning electron microscope Quanta 650 FEG (Field Emission Gun) coupled to an energy dispersive X-ray spectroscopy (EDS) detector EDAX Apolo X, with a 126.1 eV resolution, at an accelerating voltage of 25 kV. The EDS microchemical analysis were semi-quantitative, and detection limits are typically considered to be ≥ 1% for low atomic number elements (C, O) and ≥ 0.1% (1000 ppm) for elements with higher atomic number. The standard deviation of the semi-quantitative analysis for major components ranges from 2 to 5%, depending on surface roughness. Two types of EDS analysis were performed: area analysis (entire image) and point analysis (selected spots in the images).
The mineralogical characterization was carried out by means of X-ray diffraction-XRD, using a BRUKER model D8 ADVANCE powder diffractometer with DaVinci geometry, under the following conditions: voltage 40 kV, current 40 mA, slit divergence 0.6 mm, primary Soller slit 2.5°, secondary Soller slits 2.5°, 0.02035° 2θ step size, 0.6 s time per step, 2.0° 2θ to 70.0° 2θ recording range, CuKα1 radiation, nickel filter, antiaircraft scattering, LynxEye linear detector and type step sweep.
Results and discussion
Thermodynamics simulation of reduction conditions
Figure 3 shows the speciation diagram obtained by free energy minimization, using the HSC chemistry software, and considering the components of the self-reducing pellets at a molar relationship oxide/carbon 1:1. The presence of MnO, Mn and CO are evident as the temperature increases and the consumption of C and Mn3O4 advances, up to 2000 °C where the reaction ends. It should be noted that CO2 is completely consumed and gives way to the production of CO, while MnO2 is not generated during the entire simulation; therefore, these last two species are discarded from further simulation analysis.
Fig. 3
Simulated speciation diagram, obtained by free energy minimization, considering the components of the self-reducing pellets at a molar relationship oxide/carbon 1:1
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Figure 4 shows a second simulation, based on the stoichiometric calculations at a molar relationship oxide/carbon 1:4 and considering the species chosen in the first simulation, for the Mn3O4 reduction process. Graphically, the metallic manganese (Mn) appears at a temperature of approximately 1153 °C and is produced until the end of the reaction, just like CO. The other species in the simulation are completely consumed at a temperature of 2000 °C. The information provided in the speciation diagram of the second simulation graphically showed the temperature at which the metallic manganese began to form and helped to establish the ideal temperature ranges for the design of the carbothermal reduction treatment applied in the tubular furnace to the self-reducing pellets made of cathode black mass.
Fig. 4
Simulated speciation diagram, obtained by free energy minimization, considering the components of the self-reducing pellets at a molar relationship oxide/carbon 1:4
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Visual appearance of the agglomerates in reduction experiments
After applying the agglomeration conditions and the reduction tests to the samples, physical changes were observed in the pellets. In each of the stages of the process, the agglomerates underwent morphological changes. Figure 5A shows the uniform size of the pellets. In Fig. 5B, smaller agglomerates can be observed due to moisture loss. In Fig. 5C, there are evident changes in the size of the agglomerates and the appearance of shiny particles not only on the surface of the pellets, but also on the walls of the crucible in which they are deposited.
Fig. 5
Physical changes of the pellets presented in the stages of A mixing in green, B drying at 100 °C and C reduction at 1200 °C
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Optical microscopy was performed to analyze the microstructure of the samples under study. Figure 6A corresponds to the microstructure of sample 2 reduced at 1000 °C, areas like those obtained in the microstructure of sample 1 can be observed, but with a finer texture and the appearance of a bright iridescent colored particle, characteristic feature of a metal. The possible metallization after the reduction process can be seen, as well as traces of carbon, metallic oxides, and some discolored and rounded areas, corresponding to porosities of the liquid resin in the casting stage. Figure 6B corresponds to the microstructure of sample 3 after being exposed to a reduction at 1200 °C, allows to show areas of manganiferous material with uniformly dispersed shiny particles, and empty areas, which correspond to porosities generated by the liquid resin. Areas are observed where some oxides were possibly reduced, passing to a stable metallic phase, at room temperature, taking on an iridescent hue, showing areas of metallic oxides that were not reduced.
Fig. 6
Optical micrographs of A sample 2 reduced at 1000 °C and B sample 3 reduced at 1200 °C, both at 500× (original magnification)
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Microchemical study of agglomerates by SEM–EDS
Figure 7 corresponds to sample 1, this was examined in pellet form. Optimal agglomeration conditions are observed due to the absence of pores and angular grains with different characteristics indicated in points A, B, C. Point A corresponds to charcoal, whose laminar shape is characteristic of this type of substance. Point B and C show material with greater granulometry and less dark compared to point A, characteristics similar to the cathode black mass extracted from the alkaline batteries, and light-colored particles can also be observed around them and with finer granulometry, similar to bentonite. The percentage by weight of the different points of microchemical composition taken are presented in Table 4.
Fig. 7
Backscattered electron mode SEM micrographs of sample 1 at 100 °C at 100× (A) and (B) at 400×
Table 4
Microchemical composition of sample 1
Sample 1
Point
Percentage point by weight of the elements (%Wt)
C%
O%
Al%
Si%
S%
K%
Mn%
Zn%
Ca%
N%
100 °C
General
41
18
0.5
0.7
0.4
2.6
28.2
8.2
N/A
N/A
A
88
7
0.3
0.3
0.2
0.7
2.8
0.9
N/A
N/A
B
12
19
0.5
0.4
0.4
2.2
36.6
29.4
N/A
N/A
C
10
22
0.5
0.6
0.7
4.1
59.2
2.4
N/A
N/A
×
Figure 8 corresponding to sample 2 is chemically analyzed in powder form. It shows morphological changes and variation of chemical composition in the selected points A, B, C and D. In Fig. 8A and B, fines of different sizes are presented; these correspond to the manganiferous material in the form of oxide, and to the charcoal that it was not consumed in the treatment, during the reduction stage. In Fig. 8C, the formation of dendritic ramifications is evidenced, whose morphology presents rounded drop-shaped endings, in addition, irregular grains of different sizes and non-uniform distribution. The percentages by weight of the different points of microchemical composition taken are presented in Table 5.
Fig. 8
Backscattered electron micrographs SEM of sample 2 reduced at 1000 °C at 100× (A), 400× (B) and 6000× (C)
Table 5
Microchemical composition of sample 2
Sample 2
Point
Percentage point by weight of the elements (%Wt)
C%
O%
Al%
Si%
S%
K%
Mn%
Zn%
Ca%
N%
1000 °C
General
39
12
0.7
1.0
0.4
0.7
45.2
N/A
N/A
N/A
A
37
4
1.7
3.0
0.8
2.4
49.8
N/A
1.3
N/A
B
11
2
N/A
N/A
N/A
N/A
86.9
N/A
N/A
N/A
C
45
17
N/A
0.9
0.3
0.5
36.1
N/A
N/A
N/A
D
18
9
N/A
0.9
0.8
N/A
70.9
N/A
N/A
N/A
×
Figure 9 corresponds to sample 3, this was analyzed in pellet form. Significant morphological changes and variation of chemical composition are evident in the selected points A, B and C. Figure 9 (point A) shows greater dendritic growth compared to sample 2, and the appearance of small porosities due to the consumption of reagents over time. In Fig. 9 (point B), irregular grains of rounded morphology (drop-shaped) can be observed; these refer to dendritic growth terminations, which are characteristic of metals, in Fig. 8C of sample 2 something similar is observed. The percentage by weight of the different points of microchemical composition taken is shown in Table 6.
Fig. 9
Backscattered electron micrographs SEM of sample 3 reduced at 1200 °C
Table 6
Microchemical composition of sample 3
Sample 3
Point
Percentage point by weight of the elements (%Wt)
C%
O%
Al%
Si%
S%
K%
Mn%
Zn%
Ca%
N%
1200 °C
General
9
10
2.2
2.8
2.0
N/A
70.5
N/A
0.8
2.7
A
11
10
0.5
0.4
0.3
N/A
76.8
N/A
0.1
0.8
B
8
6
0.7
0.7
0.5
N/A
83.4
N/A
0.3
0.6
C
3
2
0.4
0.7
0.3
N/A
93.8
N/A
0.3
N/A
×
It is noteworthy that the manganese-rich cathode black mass used in this work contains zinc (see Table 1). The presence of this element is confirmed in the pellets by EDS analysis, as shown in Table 4. This element can be incorporated in the material as hetarolite, a spinel isostructural to Mn3O4 (Gallaway et al. 2015). Zinc oxides can undergo carbothermic reduction, and due to the volatile nature of zinc (boiling point 907 °C), it can be recorded that no Zn was found in the reduced pellets at 1000° (Table 5) and 1200 °C (Table 6).
Mineralogical analysis of agglomerates by XRD
The mineralogical analysis by XRD was carried out to verify and compare the crystalline phases that were formed in the samples of each test carried out as shown in Fig. 10.
Fig. 10
X-ray diffractograms of charcoal/manganese oxide pellets treated at 100 °C (sample 1), 1000 °C (sample 2) and 1200 °C (sample 3)
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Figure 10 shows the diffractogram of sample 1, in which manganese oxide (Mn3O4) from alkaline batteries and carbon (C), either from eucalyptus charcoal or from graphite in the batteries are identified, as the components used to manufacture the pellets. No peaks related to bentonite were detected, due to its low presence in the mixture. In the qualitative analysis of sample 2 (treated 1000 °C), the presence of MnO is observed; this means that during the holding temperature, the Mn3O4 reduction reaction took place. Moreover, in sample 3, metallic Mn was identified, allowing us to affirm that at 1200 °C, part of the Mn3O4 content was reduced to metallic manganese, demonstrating why the percentage by weight of manganese increased notoriously, as previously shown in Table 6.
Discussion
The tests shown above were carried out in order to characterize and compare the results in each of the samples. The EDS and XRD tests showed how manganese metallization was favored during the Mn3O4 reduction reaction, in the tests conducted at 1000 °C and 1200 °C. These results allowed us to observe elements and chemical species, previously seen in the simulation of the thermodynamic speciation diagrams, obtained through the free energy minimization method, presented in the stability diagram of Fig. 4.
As previously said, the carbon contained in the eucalyptus charcoal promotes the reduction of the Mn3O4 contained in the cathode black mass of the alkaline batteries. This occurs in sample 2 at 1000 °C, where reaction (3) occurs, in which Mn3O4 reacts with carbon (C) to produce a reduced manganese oxide (MnO) and carbon monoxide (CO), as observed in Fig. 4.
In the test carried out on sample 3, at 1200 °C, the increase in the self-reducing condition due to temperature increase in the pellets is evident. At this temperature, reaction [Eq. (4)] generates what would be a secondary reduction, which promotes the reduction of the previously reduced oxide obtained in reaction (3), when it contacts the carbon in the charcoal.
After performing the microchemical analysis (EDS) of the selected general areas in sample 1 (Table 4), sample 2 (Table 5) and sample 3 (Table 6), the results show a successive increase in the manganese content as the temperature of the reduction process increases. Reducing the pellets at 1000 °C (sample 2) increased the manganese 60.3%, whereas reduction at 1200 °C (sample 3) increased 150% the manganese content, as compared to the original pellets (sample 1). The increase in the manganese content in the material is consistent with the elimination of oxygen from the sample, and thus, indicates that the reduction is taking place. An inverse trend in the carbon content is also detected in the EDS analysis as the temperature of the reduction treatment increases, indicating the consumption of the reducing agent.
On the other hand, the XRD (Fig. 10) corroborated that during the treatment applied to samples 2 and 3, there was formation of reduced manganese species such as MnO, making effective reduction reaction [Eq. (3)]. Nevertheless, only in sample 3, manganese oxides were reduced in a significant amount to their metallic form, as indicated by reaction (4). Since we did not employ specific manganese speciation techniques, such as XPS spectroscopy, in this study, we can estimate the reduction rate of manganese roughly based on the EDS analysis data presented in Tables 5 and 6. We assume that stable oxides, such as Al2O3, SiO2 and K2O, are formed in the reduced pellets. Therefore, any remaining oxygen in the analysis can be attributed to its bonding with manganese, resulting in the formation of MnO in sample 2 (reduced at 1000 °C) and Mn2.03O4 in sample 3 (reduced at 1200 °C). By performing the appropriate stoichiometric calculations and mass balances, we estimate the reduction rate, or metallization, of manganese to be 9.2% at 1000 °C and 96.1% at 1200 °C. However, it is important to exercise caution when interpreting these numbers, as oxygen measurements using EDS can be subject to higher inaccuracies.
Furthermore, XRD analysis and thermodynamic simulations did not detect the formation of manganese carbides in the self-reducing pellets. However, it is worth noting that literature sources (Surup et al. 2020) suggest that these phases can develop at temperatures exceeding 1400 °C.
The metallographic characterization carried out on the samples by optical microscopy and SEM allowed us to observe the dendritic growth and the brilliant iridescent colored particles (see Fig. 6) in the agglomerates to which the reduction treatment was applied. These characteristics were more clearly observed in the granules of sample 3, whose reduction temperature was the highest. The iridescent color is a characteristic of metallic manganese precipitates, at room temperature, as it is evidenced in a previous work (Elliot and Barati 2018). As for dendrite-like growth detected in Fig. 9, this is a structure with repetitive ramifications, which occurs in molten metals that have been subcooled below their solidification point (Askeland 2011); in the case of manganese, its melting point is 1246 °C, which is close to the reduction temperature used (1200 °C), considering the fact that impurity elements in the system (Zn, S) or even carbon may dissolve in metal manganese, lowering its liquidus temperature. In the XRD analysis of Fig. 9, it was possible to observe the presence of metallic manganese beta (Mn-β), a chemical species identified in the diffractogram of sample 3. This high temperature allotrope of manganese could probably be metastable at room temperatures in the samples by cooling conditions or by the action of zinc, an element initially present in the pellets. Beta manganese can dissolve up to 52.4% Zn (ASM International 2016). In addition, sample 3 exhibits a higher valence oxide, Mn2.03 O4, suggesting the possibility of re-oxidation of the reduced species during the process. Literature sources (Stobbe et al. 1999) suggest that re-oxidation can occur within a temperature range of 727–927 °C. It is important to note that this secondary re-oxidation does not significantly affect the overall metallization efficiency of the process.
Conclusions
Through a systematic process of experimentation and refinement, we successfully developed a practical formulation for self-reducing pellets using cathode black mass extracted from spent alkaline batteries, eucalyptus charcoal and bentonite. To ensure the effectiveness of this formulation, we utilized thermodynamic simulations to validate temperature conditions and predict the formation of various chemical species.
Our reduction experiments, conducted at both 1000 °C and 1200 °C, along with the microstructural analysis, unequivocally demonstrate the self-reducing behavior of these agglomerates. Notably, the emergence of a manganese metallic phase at 1200 °C confirms this behavior, a finding further substantiated by the mineralogical characterization obtained through XRD analysis.
In conclusion, our findings strongly suggest the viability of using eucalyptus charcoal as a reductant for manganese metallization in the recycling of spent alkaline batteries. This approach holds great promise for reducing CO2 emissions in pyrometallurgical recycling processes, and it aligns with the promotion of innovative circular economy strategies within this sector.
Declarations
Conflict of interest
The authors have no competing interests to declare that are relevant to the content of this article.
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