3.1. RSE Analysis of the Aluminum Recovery Processes
presents the RSE Analysis of the Al-recovery process using Aalto PP (Top) and CSU PP (Figure 10
b). Additionally, the process fed by the CSU PP, Figure 10
b, keeps the material in a relative constant RSE value until the sieving step; thus, the initial four stages have a minor impact on concentration. In comparison, the Aalto PP presents an earlier decrease in entropy value, as all pre-concentration stages have the purpose of separating LIB components. In both cases, the sieving system produces a major entropy reduction.
The latter stages of Figure 10
describe the entropy change due to the pyrometallurgical process. Observing the process employing Aalto PP stages (Figure 10
a), it is determined that the Al curve (blue solid line, Figure 10
a) increases due the lower amount of pure Al present in the molten phase rate. For comparison, the creation of Al2
, as a consequence of shredding foil, directly affects the creation of entropy during the final stage. Hence, this system generates a higher amount of statistical entropy since new oxidized surfaces are created with the reduction of particle size as Al is diluted to Al2
. The corresponding dashed line for Al represents an ideal scenario where Al ingots are produced with marginal levels of alumina (Al2
) that are lost into the slag phase. The process employing CSU PP stages (Figure 10
b) presents the same trend of entropy increase of Al (gray solid line, Figure 10
b), accompanied by a decrease in RSE value of Cu (solid line). Since Al is not sorted out from the metallic fraction in the CSU PP, its loss into the slag phase increases the Cu concentration in the product molten phase. The absence of a Cu-Al separation stage, in this case, is effectively favoring the recovery of Cu, instead of Al, with the kiln.
A further implication of the Cu-Al sorting stage in the Aalto process on the pyrometallurgical refining stage can be observed in the interdependency of the final RSE values of Cu and Al. While in the Aalto process, the final values of Cu RSE are not significantly influenced by the presence or absence of Al2
dilution, in the CSU process, Al and Cu appear to be heavily linked. Indeed, in the latter case, the loss of Al in the form of Al2
(dilution) represents a concentration of Cu, seen from the Al and Cu RSE curves trending in opposite directions (Figure 10
b). The dashed lines of Al and Cu in Figure 10
represent the ideal scenario where the Al is processed with marginal levels of Al2
. In the CSU process, the Cu final RSE value is comparatively higher as it is diluted with Al in the molten phase. On the other hand, the additional separation of Al in the metal-rich phase by the Aalto PP dissociates Al from Cu as seen from the horizontal Cu RSE value and positive slope Al RSE value (Figure 10
3.2. RSE Analysis Electrode Treatment Processes
depicts the RSE analyses for the hydrometallurgical processes, fed by Aalto and CSU PP stages, top and bottom, respectively. Similar to the pyrometallurgical process, both of the hydrometallurgical processes presented are fed with material containing the same elements but with slight differences in concentration (Table 1
and Table 2
). As observed in Table 4
, the final recovery and quality in both cases are similar. Thus, the analysis of MFA and RSE is bound to provide more detailed information about the influence of PP stages on the final recyclates.
Initially, both processes present abrupt oscillations in RSE value after the PP stages due to the addition of the chemical reagents, hence, influencing the concentration of components in the streams. In general terms, hydrometallurgical processes are designed to separate, by chemical means, the elements by the addition of reagents to form more stable compounds, marginally affecting the rest of elements in ideal cases. Consequently, this may not represent a concentration of a certain element in some cases, as its extraction may be a result of dilution by a reagent. As an example, high purity Al foil [23
] is diluted to aluminum hydroxide in the presence of sodium hydroxide. Consequently, the RSE curve of Al (light blue line in Figure 11
) reflects the dilution of Al in the “Alkali Leaching” step. On the other hand, Fe is found in its diluted form, present in LIB either as cathode dopant [9
] or as residues of steel casing (Table 1
), and, by the effect of the chemical treatment, its relative concentration increases (Figure 11
). Jarosite is a concentrated form of the Fe ions found in LIB; thus, the RSE curve for this element depicts a decrease in entropy towards the stage of “Jarosite Precipitation”. In both cases, the extraction of jarosite and aluminum hydroxide before the last stage of the process would influence the subsequent stage entropy values. Nevertheless, the abrupt negative slope of the Al curve prior to “Alkali Leaching” stage is due to the physical separation carried out during the PP, whereas, the posterior increase in RSE value in the same curve is due to the dilution of Al to ions of Al(OH).
Co presents a similar behavior in both processes (dark blue solid line in Figure 11
), initially undergoing a dilution step due to the addition of NaOH and an immediate concentration during roasting. The further dilution of Co is due to the addition of chemical reagents in subsequent steps for the extraction of all other elements, marginally affecting its entropy value. During the last steps of Co process, this element is once more diluted due to the addition of oxalate reagent and NaOH before the “CoC2
Precipitation” step where entropy decreases due to the formation of this compound, a concentrated form of Co, and separation into a single stream.
The level of readiness, that is, whether a material is suitable for use in the form they were extracted, is affected by the dilution or concentration of the material. It is clear that this concept is subjective and depends on the intended application of the material. However, the forms of the extracted materials, that is, jarosite, manganese dioxide, copper hydroxide, Li-Ni Solution, and cobalt oxalate, may require further processing to become metallic raw materials or to be re-introduced as raw materials for LIB, which from the circular economy perspective is the ultimate objective.
Since the hydrometallurgical process reported similar recovery rates (Table 4
) between the different systems studied, the difference between final elemental RSE values obtained with the Aalto and CSU PP stages (Figure 11
) can be attributed to the influence of PP stages. For example, the Aalto PP presents a maximum RSE value on the Al curve of 0.5 bits while the CSU PP presents a value closer to 1.0 bits.
Furthermore, the stages of the hydrometallurgical process can be interpreted as extraction stages
for different materials. From this perspective, an abrupt change in entropy should be observed when an extracted material is significantly concentrated into a single stream. In cases where the quality of the feeding material obtained by the CSU PP stages leads to undesired interactions between elements during the hydrometallurgical process, the RSE values would be negatively affected. As seen in Figure 11
, the Aalto PP presents sharp changes in entropy at the extraction stage, which, from the RSE perspective, makes it a more logical choice. Admittedly, in the case of extraction as a solution, that is, change from a solid to ionic state, as it is the case of the “SeX (Li, Ni)” step, the MFA and RSE method present a drawback that requires further development. As seen, the separation of Li and Ni is not reflected in Figure 11
a,b even if they are flowing into an extraction point as these elements have been effectively diluted by the leaching process. This is due to the fact that the final form of Li and Ni is heavily diluted in aqueous solution. To avoid this type of un-accuracies in future analysis, the MFA and RSE analysis can be complemented with other types of analyses, such as those applied by [19
]. Nevertheless, an analysis emulating the extraction of water phase from the Li-Ni Solution (dotted Li and Ni lines in Figure 11
), reflect that, indeed, there is a concentration effect of these elements during its extraction stage. Nevertheless, the final RSE value of Li and Ni suggests an overall dilution compared to its initial status. Thus, the application of MFA and RSE for the analysis of recycling processes, including solubilization, require further mathematical developments.