Zn/Mn spinel-based recovered sorbent for the removal of sulphur in gasification processes

The present invention discloses the use of a spinel-based recovered sorbent of formula Zno.85Mn2.15O4 or Zno.25Mn2.75O4, working at a temperature range between 300 ^e C and 500 ^e C, for the removal of sulphur in gasification processes such as coal, biomass and waste gasification processes. Said sorbent is recovered from recycled alkaline batteries and Zn/C batteries.

Therefore, the present invention could be interested for the industry producing gaseous fuels from biomass and waste and for biotechnology industry which produce energy and added-value chemicals from waste and biomass.

STATE OF ART

Removal of reduced sulfur species in gasification processes has been an issue since the 90s. Then, it was mainly associated to coal gasification and to comply with the environmental emission limits associated to syngas combustion in boilers, gas engines and turbines and to prevent corrosion. More recently, the focus is on comprehensive sulfur removal in biomass and waste gasification processes due to the role of syngas and biogas for its further conversion into added-value products such as biofuels or chemicals. Sulfur removal is now related to preventing poisoning of sulfur sensitive catalysts when upgrading syngas or biogas to biomethane. Particularly for syngas, technologies relying on regenerable sorbents working in the 300-500 ^e C temperature range are desirable for better process integration.

Many different types of materials have been tested for H ₂ S removal such as zeolites, activated carbons, iron-based materials and different oxides. It is widely reported that zinc oxide and its derivatives are highly effective sorbents for hydrogen sulfide removal, with the ability to reduce hydrogen sulfide concentration to ppmv levels in experimental studies. However, it is also known that under a reducing environment, zinc oxide tends to be reduced to elemental zinc, reducing its desulfurization capacity. In an effort to solve this limitation, a positive effect of the presence of manganese was observed by Ko et al. [Ko, T.-H., H. Chu, and Y.-J. Liou, A study of Zn-Mn based sorbent for the high-temperature removal of H ₂ S from coal-derived gas. Journal of Hazardous Materials, 2007. 147(1 ): p. 334-341 ] who pointed out that manganese oxide can inhibit the vaporization of Zn in reduced atmospheres, and also improve the sulfur capacity of the sorbent. Therefore, Mn-based oxide sorbents attracted the attention of different research groups and significative work can be found in literature on the advantages of using Mn oxides for desulfurization processes:

• Herszage et al [Herszage, J and M. dos santos Alfonso; Mechanism of Hydrogen sulphide oxydation by Manganese (IV)Oxide in Aqueous Solutions, Langmuir, 2003, 19(23), p: 9684-9692] refers to the advantages of using Mn oxides for desulfurization processes but said studies focused in aqueous solutions.

• Other works involve the use of supported Mn-based sorbents, for instance using AI2O3 [Chytil, S., et aL, Performance of Mn-based H2S sorbents in dry, reducing atmosphere - Manganese oxide support effects. Fuel, 2017. 196: p. 124-133.], MCM [Jie, M., et aL, Semi-Coke-Supported Mixed Metal Oxides for Hydrogen Sulfide Removal at High Temperatures. Environmental Engineering Science, 2011. 29(7): p. 611 -616.] or SiO ₂ [Ko, T.-H., et aL, Performance of ZnMn2O4/SiO2 sorbent for high temperature H2S removal from hot coal gas. RSC Advances, 2017. 7(57): p. 35795-35804.], however therein activity towards sulphur removal is strongly dependent on the sorbent preparation and the effect of the support has to be taken into account.

Therefore, it is needed to develop new sustainable and more efficient sorbents for the effective removal of sulphur in gasification processes, particularly in coal, biomass and waste gasification processes.

DESCRIPTION OF THE INVENTION

The first aspect of the invention refers to the use of a spinel-based recovered sorbent of formula Zno.s5Mn2.15O4 or Zno.25Mn2.75O4, working at a temperature range between 300 ^e C and 500 ^e C, for the removal of sulphur in gasification processes, preferably in coal, biomass and waste gasification processes. Spinel-based recovered sorbent of formula Zno.85Mn2.15O4 or Zno.25Mn2.75O4 show an excellent behavior and are suitable for almost any gassing gas. A higher sulphur retention capacity and efficiency is observed for said spinel-based sorbent of formula Zno.85Mn2.15O4 or Zno.25Mn2.75O4 in comparison to commercial ZnO-based sorbent (Z- Sorb-lll).

Please note that the concentration of H ₂ S in the gas for which the sorbent of the invention is working efficiently surprisingly ranges up to 9000 ppm. This ensures that, in the presence of lower concentrations, for example in gassing gases of concentration of H ₂ S equal or less than 1000 ppm or less, spinel-based recovered sorbent of formula Zno.85Mn2.15O4 or Zno.25Mn2.75O4 of the present invention behave better.

In a preferred embodiment of the invention, the sorbent is obtained from recycled alkaline batteries and Zn/C batteries. The use of energy accumulators is essential for our daily activity, but from an environmental perspective, it is essential both to properly manage these post-consumption products and to recover and reuse their valuable components, in this case, zinc and manganese which form part of the anode and cathode of batteries and accumulators. Recovering the sorbent from recycled alkaline batteries and Zn/C batteries implies a reduction of the use of raw materials as they are obtained from recycled batteries, contributing also to waste reduction and waste sustainable management.

The sorbent is recovered from recycled alkaline batteries and Zn/C batteries. It is a mixed binary oxide of Zn/Mn of formula Zno.85Mn2.15O4 or Zno.25Mn2.75O4, with tetragonal symmetry with space group 141 /amd corresponding to a spinel type structure.

Crystalline structure and composition play a fundamental role in the sorbents used for the sulfur removal in coal, biomass and waste gasification processes. Working temperatures, in the range of 300-500 ^e C, play a crucial role in the sulphur retention mechanism of the sorbents, which consist in the formation of sulphides and other minoritary sulphured species.

The process for obtaining the spinel-based sorbent of formula Zno.85Mn2.15O4 or Zno.25Mn2.75O4 from recycled alkaline batteries and Zn/C batteries comprises the following steps: a) obtaining the black mass of from a recycled alkaline battery and/or a Zn/C battery; b) acidic leaching of the black mass obtained in step (a) at room temperature, preferably using H ₂ O, H2O2 and HCI in a relation of 1 :1 :2; and c) precipitation of the liquid obtained in step (b) at a pH of between 12 and 14, preferably using NaOH.

The first step, step (a) of the process refers to obtaining the black mass of from a recycled alkaline battery and/or a Zn/C battery. In the present invention, "black mass" is understood as the inorganic solid obtained in the dismantling stages of the batteries and containing the electrolytes, the graphite and the manganese and zinc oxides which constitute the anode and cathode of the batteries. The main components of the batteries are manganese dioxide (positive electrode), zinc (negative electrode), the electrolyte (KOH or ZnCI ₂ + NH ₄ CI) and steel (battery cover). These wastes generate great concern in society because of their negative impact on public health and the environment, due mainly to their high content of heavy metals. Mechanical separation is the starting point for obtaining black masses and aims to separate electrodes, steel, paper, and plastics by means of cutting-shredding stages, magnetic separation, dimensional separation (screening), Eddy Current separation (ECS) and dust fraction grinding.

Step (b) of the process refers to a leaching in constant acid medium of black mass obtained in step (a) at room temperature. The resulting solids are disposed of by filtration. The term “room temperature” refers herein to a temperature range between 17 ^e C and 27 ^e C.

Preferably, the acid medium used in this stage (b) is an acid solution of H ₂ O, H2O2 and HCI in a relation of 1 :1 :2, for instance and acid solution of 25 % v/v water, 25 % v/v H2O2 and 50 % v/v HCI of 35 % purity. A lower percentage of H2O2 does not adequately dissolve Mn and a higher percentage increases foams in the dissolution process and causes the reactor to overflow.

This leaching step in acid medium (step (b)) is followed by a precipitation of the liquid obtained in step (b) at a pH of between 12 and 14. Step (c) refers to a selective alkaline precipitation (step (c)) using an alkaline medium. Preferably the alkaline medium of stage (c) is a solution of NaOH or KOH, preferably NaOH. Concentrations of the alkaline medium higher than 6 M are preferred.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skilled in the art to which this invention belongs. Methods and materials similar or equivalent to those described herein can be used in the practice of the present invention. Throughout the description and claims the word "comprise" and its variations are not intended to exclude other technical features, additives, components, or steps. Additional objects, advantages and features of the invention will become apparent to those skilled in the art upon examination of the description or may be learned by practice of the invention. The following examples and drawings are provided by way of illustration and are not intended to be limiting of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 : Breakthrough curves (10 bar, 400 ^e C and SV = 3500 h ^-1 ) of recovered oxides BMO1 , BMO2, BMO3 and BMO4 and Z-Sorb III

FIG. 2. XRD diffractograms for (a) fresh and (b) used samples of recovered oxides BMO1 , BMO2, BMO3 and BMO4.

FIG. 3. TEM images for (a) BMO1 , (b) BMO2, (c) BMO3 and (d) BMO4 fresh samples.

FIG. 4. TEM-EDS analysis for (a) BMO1 , (b) BMO2, (c) BMO3 and (d) BMO4 fresh samples.

FIG. 5. TEM images for (a) BMO1 , (b) BMO2, (c) BMO3 and (d) BMO4 used samples.

FIG. 6 TEM-EDS analysis for (a) BMO1 , (b) BMO2, (c) BMO3 and (d) BMO4 used samples. EXAMPLES

Materials

Different samples recovered from recycled batteries, have been studied herein to investigate their suitability as sulphur removal sorbents applicable to syngas cleaning for its further conversion into added-value products such as fuels or chemicals.

The samples investigated are binary metal oxides and have been identified with the nomenclature BMOX where X goes from 1 to 4.

The recovered oxides BMOX (X=1 -4) investigated have been obtained following a process which consists of an acidic leaching of the black mass, a common waste generated from spent alkaline batteries, followed by a selective precipitation of the resulting solution. First, different amounts of black mass were leached in 250 mL of milliQ water, 250 mL of H2O2 (Panreac®) and 500 mL of HCI (Panreac®) at room temperature for 1 h. Then, in order to obtain the different binary oxides BMOX (X=1 -4), the mixtures were filtered, and the collected liquids were precipitated with NaOH 6 M until pH 12-14 was achieved. In order to obtain the Zno.25Mn2.75O4 or Zno.85Mn2.15O4 binary oxides, 100 g and 200g of the black mass were leached, respectively.

Characterization of the materials

Physic and chemical characterization of all samples was performed before and after the desulphurization tests. X-ray fluorescence (XRF), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) provided relevant information on the structure and composition of the samples while transmission electron microscopy (TEM) analysis was used to gather relevant information on the distribution of the active phases on the samples surface.

First, the mineralogical composition of the samples was determined by X-ray fluorescence (XRF) using a PANalyticalAxios wavelength dispersive spectrometer (4 kW) and the structural characterization was carried out by X-ray diffraction (XRD) using a Siemens D5000 diffractometer (Cu Ka radiation). The collected data were refined by the Rietveld method using the version 4.2 of the Rietveld analysis program TOPAS (Broker ASX). For analyzing the surface of the obtained samples X-ray photoelectron spectroscopy (XPS) was used. Spectra of all samples 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. Binding energies were calibrated to the C 1 s peak at 285.0 eV. The atomic ratios were computed from the peak intensity ratios and reported atomic sensitivity factors.

Table 1 below details composition of samples investigated in terms of main components, determined by X-ray diffraction (XRF).

Table 1 : Elementary composition of samples studied. a Commercial b Recovered from batteries

The commercial Z-Sorb-111 sample has been added to Table 1 for comparison.

In Table 1 it can be observed that active species in recovered samples BMO1 , BMO2, BMO3 and BMO4 consist of mostly of Mn and Zn with low Ni contents (< 1 %), while in Z-Sorb-111 sample majoritarian component is Zn with Ni contents higher than 7%.

Furthermore, it can be observed that all samples BMO1 , BMO2, BMO3 and BMO4 present spinel structure in Table 1 .

Experimental procedure

All samples were supplied in powder form and prepared by first crushing the powder in an agata mortar and then pelletized at sizes in the range of 0.5-1 mm using a Specat 15T manual hydraulic press. Testing of the samples has been carried out in a microactivity-pro lab-scale system, an automatic and computerized laboratory rig used for the study of catalysts and sorbents, equipped with three independent mass flow controllers to produce desired gas mixtures up to 4.5 NL/min of maximum gas flow and where reactions can be studied at up to 600 ^e C and 30 bar. The reactor, with an internal diameter of 9,2 mm and 300 mm long, as well as all the tubing and fittings of the rig, are manufactured of Hastelloy C, to avoid corrosion problems due to the presence of H ₂ S.

Amounts between 2 and 3 grams of each sample, once pelletized, were used in the experiments and desulphurization tests were performed at 10 bar and 400 ^e C, using a feed gas mixture of 0.9 % H ₂ S in N ₂ with a space velocity of 3500 h ^-1 ; as representative of desulfurization of biomass gasification gas streams processes. Inlet and outlet gas composition were analyzed using a CP4900 Varian gas chromatograph equipped with two columns, a Porapack and a molecular sieve column and with two thermal conductivity detectors.

In order to facilitate the comparison between all samples studied, theoretical values of Sulphur sorption capacity (So) and sorption time (to) have been calculated for each sample studied based on reactions stoichiometry (reactions 1 , 2, 3) and the amount of individual active oxides present in each sample:

ZnO + H ₂ S ZnS + H ₂ O (1 )

NiO + H ₂ S NiS + H ₂ O (2)

MnO + H ₂ S MnS + H ₂ O (3)

For the calculation of theoretical sulphur sorption capacity of each sample, So, addition of the contribution of the three individual active species, ZnO, MnO and NiO was carried out using the following expression [1 ]: NiO (%) MW _S 100 MW _Nio

The actual sulfur captured by the sorbent (St (g)), assuming that all sulfur is retained by the sorbent and no sulfur escapes with the gas leaving the reactor until a given sulfidation time t _s , is calculated by the expression [2]: where:

MW _S is the molecular weight of S,

P is the absolute pressure at sulfidation conditions,

Fgas is the volumetric gas flow rate at process conditions,

YH2S represents the inlet H ₂ S mol fraction,

R _g is the universal gas constant and

T is the absolute temperature at sulfidation conditions.

Theoretical sorption time, to, was calculated as the ratio between the theoretical amounts of Sulphur, S, that each sample can adsorb (g) based on its composition and the Sulphur mass flow rate used in each experiment (g/min) according to the expression [3]: to (min)

As can be seen, the theoretical time depends on initial hydrogen sulfide concentration, sulfidation pressure and temperature, as well as on gas flow rate, and it will vary according to experimental conditions. This parameter is used here to compare the performance of all samples investigated.

Table 2 summarizes theoretical values of So and to calculated for all samples studied.

Table 2: Theoretical Sulphur sorption capacity and time of the samples studied a Commercial b Recovered from batteries The experimental Sulphur retention capacity was obtained for all samples using the same procedure: operating conditions were first reached using N ₂ as inert gas. When stable operating conditions have been reached (400 ^e C and 10 bar) gas mixture, consisting of a mixture of 0.9 % H ₂ S in N ₂ was led to enter the reactor. In all experiments, outlet gas composition was continuously analyzed, including the heating and cooling steps and breakthrough curves were obtained until concentration of H ₂ S reached around 0.1 % v/v in the exit gas stream.

For the evaluation of the performance of the samples in terms of Sulphur removal capacity, the efficiency (%) is used, according to expression [4]:

Efficiency = St /So * 100 [4]

Finally, all samples were characterized after the tests by XRD, XPS and XRF in order to identify main sulphured compounds formed and potential changes in structure of the sorbents during the desulphurization process. Results will be shown hereafter.

Results

Breakthrough curves: Sulphur removal capacity

All experiments were performed under the same operation conditions (Pressure = 10 bar, Temperature = 400 ^e C and Space velocity SV = 3500 h ^-1 ). FIG. 1 shows the breakthrough curves for all recovered samples BMO1 , BMO2, BMO3 and BMO4 investigated, where breakthrough curve for commercial Z-Sorb III sample was included for comparison.

In FIG. 1 , breakthrough curves show great differences among the samples investigated: in one extreme it is sample BM02 which outperforms the rest of them and at the other it is BM04 which underperforms the other samples. In the middle of those two, samples BM01 and BM03 showed a closer performance between them.

When compared with a commercial sorbent for syngas desulphurization, Z-Sorb-111 , two of the samples break before and the other two, break later showing these two ones a better performance in terms of efficiency towards Sulphur removal. Moreover, those ones showing a higher sulphur retention capacity than the individual commercial sorbents showed an almost vertical breakthrough, suggesting a fast H ₂ S capture reaction that stops when sorption centers are no more available.

Sulfur removal capacity of the samples is expressed as grams of sulfur adsorbed divided by the mass (g) of sorbent multiplied by 100, according to expression [5]:

S t = (Sadsorb (g)/M _S orbent (g)) 100 [5]

Table 3 summarizes the S capture capacities calculated until H ₂ S reaches 1% of the inlet gas composition.

Table 3: S removal capacity and efficiency (g/g %) a Commercial b Recovered from batteries

According to the results obtained, the order in terms of sulphur removal capacity is: BMO2 > BMO1 > BMO3 > BMO4 which is fully in line with the efficiency calculated and summarized above in Table 3.

Regarding the composition of the samples (see Table 1 ) it can be highlighted that those samples with lower Zn and higher Mn contents (BMO1 and BMO2) showed a better performance in terms of Sulphur removal and higher efficiency than those with higher Zn contents and lower Mn contents (BMO3 and BMO4). Sulfidation process: influence of samples composition

Characterization of spent sorbents BMO1 , BMO2, BMO3 and BMO4 (herein used samples) was performed and results compared to fresh samples BMO1 , BMO2, BMO3 and BMO4 in order to investigate sulfidation mechanisms and the role of Mn in the process. The term “spent or used” sorbents/samples refers herein to BMO1, BMO2, BMO3 and BMO4 samples after being exposed to sulphur under experimental conditions of 10 bar of pressure, 400 ^e C and a gas stream of 0.9 % H ₂ S/N ₂ .

The morphological characterization of the fresh and used samples was carried out by transmission electron microscopy (TEM), using a JEOL JEM 2100 instrument. The samples were dispersed in butanol and the suspensions were deposited onto carbon- coated copper grids. X-ray energy dispersive spectroscopy (XEDS) measurements were carried out using an OXFORD INCA instrument.

The physic-chemical characterization of fresh and used samples investigated showed significant differences in composition, structure and sulfured species formed. Three different analyses have been carried out: XRD provided information on structure, crystallinity and main crystalline phases formed after sulfidation; XPS allowed the identification of different oxidation phases of the sulfured species and finally TEM confirmed the presence of phases, oxides and sulfides. Based on those characterization results some plausible mechanisms for the removal of H ₂ S are outlined in this section.

XRD analysis

As a first sight XRD analysis showed that, after sulfidation, a loss in crystallinity was observed in all the samples studied, which could be indicative of the formation of amorphous sulfur species. As the sorbents investigated are recovered materials no reference pattern is available for direct comparison and only differences among them and comparison with pure MnO and ZnO oxides can be used.

In the case of the XRD patterns for the fresh samples, the more intense reflexion maxima for all obtained binary oxides samples can be indexed to a tetragonal symmetry, with I4iamd space group (JCPDS 24-1133) which is consistent with a spinel-type structure, with Zn _x Mn _3.x O4 general formula, with the corresponding Miller indices (hkl) showed in FIG. 2.

Additional peaks, marked in the XRD patterns with crosses (x), corresponded to some impurities related to the obtention process.

In the case of the BMO4 sample, reflexion maxima indexed to the ZnO phase were recorded, marked in the XRD patterns with circles (o).

Concerning XRD patterns for the used samples, spinel-type structure, with Zn _x Mn ₃ .xC>4 general formula as well as new additional phases were formed as a consequence of the sulfidation process but clear differences between samples were found (See FIG. 2):

• In the case of the BMO2 sample, diffraction peaks which can be indexed to the a-MnS phase with cubic configuration (JCPDS 06-0518) can be observed.

• On the contrary, for the BMO1 sample, a-MnS and y-MnS (JCPDS 40-1289) phases were found.

• BMO3 and BMO4 samples exhibit diffraction maxima attributable to the y-MnS phase.

• An unexpected formation of elemental S was found in BMO1 and BMO2 samples; formation of elemental S occurs for the samples with higher Mn content.

• XRD pattern for the commercial Z-sorb-ll I used sample exhibits the presence of hexagonal wurtzite ZnS phase (JCPDS 36-1450).

Table 3 shows the results obtained from the Rietveld refinements of the XRD data for fresh and used samples.

Table 3. Rietveld refinement results for fresh and used samples. a Commercial; fresh sample composition obtained by X-Ray Fluorescence b Recovered from batteries

An explanation for the observation of additional phases is that manganese sulfide can crystallize in three different structures a-, p-, and y-MnS and at low temperatures (between of 100-400 °C), -, and y-MnS are formed, and turned into a-MnS.

As expected, main sulphured species formed were Zn and Mn sulphides, in the form of ZnS and MnS. However, analysis shows the unexpected presence of elemental sulphur (S) in the used samples and it is worth highlighting that this sulfur formation was quantitatively (> 8 %) identified in those samples showing the highest sulfur removal capacity which were those with higher manganese contents. This formation of elemental sulphur was initially not expected because all experiments were performed in a reducing media, using gas mixtures consisting of H ₂ S/N ₂ , in absence of oxygen.

XPS analysis

Table 4 summarizes the XPS characterization results obtained for fresh (a) and used (b) BMO1 , BMO2, BMO3 and BMO4 samples.

Table 4: a) XPS results of fresh samples

Table 4: b) XPS results of used samples

In the case of the fresh BMO1 , BMO2, BMO3 and BMO4 samples, the obtained bands can be attributed to the bond energies for MnO, Mn ₂ Os and MnO ₂ showing that Mn(ll), Mn(lll) and Mn(IV) are present in the samples.

The analysis of the used BMO1 , BMO2, BMO3 and BMO4 samples shows the presence of different sulphured species, which includes sulfides (ZnS, MnS, MnS ₂ , NiS) but also sulfates (MnSC , NiSC ). It is to mentioned that samples comprising a higher amount of MnS exhibit a lower sulphur retention capacity (See results for samples BMO3 and BMO4).

When comparing XPS bands of fresh and used samples, a slightly displacement towards lower binding energies is observed (Table 5 below) which would suggest the occurrence of some of the transformations during the sulfidation process in the present work (as a reference: MnO ₂ (641 ,9eV-642,6eV) and MnO (640eV-641 ,7eV)).

Table 5: Binding energies for Mn-0 oxides identified by XPS before and after sulfidation tests

During the oxidation removal of H2S, several bulk manganese oxides were shown to be active adsorbents or catalysts and the O/S exchange reaction to generate water and sulfur containing manganese oxides. According to this approach the formation of MnSs from MnOs could be explained by the exchange between oxygen and sulfur in the samples during the sulfurization tests following the sequence:

MnO ₂ + H ₂ S = MnOS + H ₂ O

MnOS + H ₂ S = MnS ₂ + H ₂ O

The formation of sulfates is evident in all the used BMO1 , BMO2, BMO3 and BMO4 samples and, although this is surprising due to the reducing atmosphere of the reaction media, it is somehow coherent with the formation of elemental sulfur observed by XRD. The coexistence of different Mn oxidation states in the fresh recovered materials, already confirmed by XPS (see Table 4), could facilitate the availability of O2 during the sulfidation process which could then be combined with H2S to form SO2 (reaction 5 mentioned above):

H ₂ S + 3O ₂ = SO ₂ + H ₂ O (5)

The temperature (T = 400 ^e C) and the pressure (P = 10 bar) used herein might facilitate the formation of S-0 bonds which further proceed to SO2 formation.

The presence of sulfates in the used BMO1 , BMO2, BMO3 and BMO4 samples can be only explained if some oxygen and SO2 are available during the sulfidation process, its presence in said samples suggests the intermediate formation of SO2 and reaction with oxidized species of Mn following some of the following reactions:

2Mn20s + 4SO2 + 02= 4MnSO4 MnsO4 + 3SO2 + O2 = 3MnSO4 MnS + 2O2 = MnSO4 MnOs + SO2 = MnSO4

The ratio of sulfides/sulfates/S formation during the desulfurization process for each sample studied seems to be very dependent on its structure (spinel) and composition which will be determining the availability of O2 and formation of SO2 under the operating conditions used (400 ^e C and 10 bar). In line with obtained XRD results, those samples with higher concentration of MnS (BMO3 and BMO4) show a lower formation of sulfates which is coherent with the existence of an equilibrium between MnS and MnSC . Please note that, as MnSC requires at least 600 ^e C, no decomposition is expected under the process temperatures used (400 ^e C).

TEM analysis

TEM micrographs of fresh samples (FIG. 3) confirm that spinel ZnxMn _3.x O4 was the main phase, as previously identified by XRD.

Fresh samples BMO1 and BMO2 with higher Mn contents show particles with a regular shape (FIG. 3(a), 4(a) and 3(b), 4(b)) while a nearly-spherical shape was observed (BMO3 and BMO4 FIG. 3 (c), 4(c) and 3(d), 4(d)) for fresh samples with lower Mn contents. Particularly, in the image which corresponds to the BMO4 sample (FIG. 3(d), 4(d)), some rod-like structures were found, which are associated to the presence of ZnO.

An increase of particles agglomeration can be observed in the used samples (FIG. 5), in addition to the presence of un-shaped particles.

The loss of polyhedral forms of the particles seems to indicate the transformation of ZnxMn _3.x O4 spinel-phases to other less crystalline and/or amorphous phases. These results are in good agreement with the XRD section previously described, where the obtained XRD measurements indicate that the sulfidation process leads to a lower crystallinity degree for all analyzed samples, and this is confirmed by TEM micrographs.

The energy dispersive spectra of the fresh and used samples obtained from the TEM- EDS analysis (FIG. 4, 6) also confirms the changes occurred during the sulfidation process. It can be appreciated that for the fresh samples main phases contain Mn and Zn characteristic peaks. For the used samples instead, the S Ka peak was registered in addition. These results clearly confirmed that the H ₂ S was adsorbed by the samples.

Finally, some additional peaks attributable to the carbon-coated copper grids were found.

The performance and characterization results obtained have demonstrated that the spinel-based structure and composition of the recovered mixed-oxides from the recycled batteries, specially the Manganese species, are playing a crucial role in the desulfurization mechanism.