Experimental Study on Recycling of Spent Lithium-Ion Battery Cathode Materials

The standard cathode materials (> 99.99% pure) for validation the methods were obtained as battery-grade reagents from MTI Corp. All chemicals (H₂SO₄, NaOH, Na₂CO₃, NaHS, Na₂S, LiOH, (NH₄)₂CO₃) were analytical grade, and all solutions were prepared with deionized water.

Leaching tests were conducted in a 1000 ml baffled reactor equipped with a condenser and overhead agitator. The reactor was placed in a heating mantle for temperature control. The cathode material was mixed with sulfuric acid solution and SO₂ was sparged into the resulting slurry, targeting a pre-determined ORP (Oxidation-Reduction-Potential) set point. Parameters including acid concentration, leaching time, pH and ORP were monitored and controlled as necessary. At the end of the leach stage, the products (leachate and reaction residue) were filtered and the leach residue was thoroughly washed with DI water. Subsequent precipitation tests were done using the same type of reactor. At the beginning of a precipitation test, the solution was sparged with nitrogen gas to remove the dissolved oxygen. Typically, 200 ml of the leachate was used for batch tests studying different reagents and conditions. At the end of each test, the products were filtered, and the precipitate was washed thoroughly and dried at 70 °C. A sketch of the reactor is given in Fig. 1.

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The quantitative metal contents for Al, Mn, Ni, Co and Li were analyzed by ICP-OES. The concentration of sulphate was determined using the method, sulphate by turbidimetric (EPA 9038), where sulphate ion was converted to a barium sulphate suspension under controlled conditions. The resulting turbidity was measured using a HACH TU5200 laser turbidimeter. The concentration of dithionate was directly measured by HPLC using sodium perchlorate eluent along with an AS17 ion exchange column.

The locked-cycle campaign used 100 grams of cathode material per cycle at a pulp density of 8%, with 1.2M H₂SO₄ to maintain a pH close to 1.5 and an ORP target of 350 mV. The leach residence time was 2 h per cycle. The temperature was maintained at 50 °C after the initial exothermic phase to help ensure that the leach reaction went to completion.

The first campaign used sodium hydroxide addition under nitrogen sparging to precipitate cobalt as cobalt hydroxide. The residence time was 2 h. The filtrate from the cobalt hydroxide precipitation stage was heated to 95 °C and mixed with 1.2 times the stoichiometric requirement of sodium carbonate for 0.5 h to precipitate lithium carbonate. The precipitated lithium carbonate was filtered and washed with the saturated Li₂CO₃ at 95 °C. Except for Cycle 1, all the saturated Li₂CO₃ wash solution was prepared using Li₂CO₃ solids generated from previous cycles. The filtrate from the lithium precipitation stage was cooled to 5 °C to initiate Na₂SO₄ crystal formation. The crystallization stage was run for 2 h before the resulting crystals were filtered. The crystallization filtrate and leach residue wash solution were then combined with evaporated wash solution from the cobalt precipitation stage and the resulting solution was acidified for use as the leach solution for the next cycle.

In the second locked-cycle campaign, the Co and Li were precipitated together by adding a 1.2 times stoichiometric quantity of sodium carbonate at 95 °C to the leach solution after first adjusting the pH to 11 with sodium hydroxide to form cobalt hydroxide. The combined precipitate was filtered and washed with a saturated Li₂CO₃ solution (using reagent Li₂CO₃). As with the first campaign the filtrate was then chilled to 5 °C for 2 h before filtering the resulting sodium sulphate crystals. For this campaign, the filtered crystals were also subjected to a re-pulp wash in the saturated Na₂SO₄ solution, also at 5 °C. The crystallization filtrate was then combined with the crystal wash solution, Co-Li precipitate wash solution and the leach residue wash solution. This combined solution was evaporated to the target leach volume and acidified for use as the leach solution for the next cycle.

Three different cathode materials, NMC, NCA and LMO, were leached using sulphuric acid with the objective of extracting valuable metals. Typical leach conditions were 10% solids, 1.2 M H₂SO₄ and SO₂ sparging targeting an ORP of 350 mV. To determine the optimal reaction time, kinetic samples were taken at time intervals. Figures 2a–2c presents examples of the leaching kinetics of NMC, NCA, and LMO material.

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Figure 2a shows that 80% of metals (Li, Co, Ni, Mn) from NMC are extracted within 30 min, and that almost a complete dissolution of cathode material can be seen within 2 h of the reaction time. A repetitive test shows 97.8% reproductivity. Further leaching tests with NMC indicated that the acid concentration could be reduced to 1.0M H₂SO₄ without sacrificing metal extraction. The same was true for leaching tests with NCA shown in Fig. 2b and LMO shown in Fig. 2c. The total metal extractions were over 99% in all tests with NMC and NCA materials. The extractions were similar for LMO material, and it was found that less acid (∼0.8M) was required to achieve the same overall metal extraction. The selected test results are given in Table I.

Along with metal extraction values, Table I presents the sulphate and dithionate concentrations in leach solutions. The dithionate concentration varies between 21–30 g l⁻¹ and sulphate concentrations ranges from 150 to 198 g l⁻¹. These values are important to monitor to avoid exceeding the saturation levels and generating undesirable precipitates, particularly in the closed systems with the circulating solution streams. Examination of the complete test results revealed that higher acid concentrations and less SO₂ addition generated less dithionate. Although, the SO₂ addition was not crucial for lithium dissolution, this reagent was necessary to dissolve other metals. The SO₂ addition impacts the oxidation state of the transition metals, maintaining them in their more soluble forms.

Three different precipitating agents were used to recover the dissolved metals from leach solutions. Sodium hydroxide was used to recover metals as hydroxides. Sodium carbonate produced carbonate compounds and was mainly used for Lithium recovery. Sodium sulphide formed insoluble metal sulphides and was tested as an option to minimize the Mn in Ni/Co mixed metal precipitates. Some tests explored the option of recovering all metals simultaneously and some used a sequential precipitation approach and various conditions were tested for each approach. Metal recovery results from NMC leach solutions are given in Table II.

The results shown in Table II indicate that increasing the temperature (50 °C in NMC-2-PT3 vs ∼20 °C in NMC-2-PT2) at pH ∼8 can only increase Co and Ni recoveries slightly whereas Mn recovery to the Ni-Co product is increased by 15%. Complete removal of all three metals (Co, Ni and Mn) was achieved when the pH was adjusted to 10 in test NMC-2-PT7. In this test the consumption of caustic increased but the overall reagent requirement was reduced. A small amount of caustic was still needed in test NMC-2-PT5, using sodium sulphide precipitation at pH 5. Co and Ni recoveries for this test were over 99.5%. A small amount of Mn was co-precipitated with the Co and Ni sulphides, but overall, this test showed the most effective separation of Mn from the Co-Ni product. The remaining Mn and Li could then be precipitated as two separate products using the controlled sodium carbonate addition and temperature.

The NCA leach solution required an initial treatment stage to remove Al separately from other metals. The Al was removed through neutralization with caustic at pH 4.5 to 5.5. Within this range, higher pH gave better Al removal. After Al removal, the pH was further increased to above 10 with caustic to collect the Co and Ni as the mixed metal hydroxides. Following that, sodium carbonate was added to recover the lithium. The test results are given in Table III.

Table III shows that close to 99% of Al can be precipitated from solution at pH 5.5 (NCA-PT4), while just over half of Al (56%) is removed at pH 4.5. Just over 90% of Co and Ni are recovered at both pH 10.5 and 11.5, with the balance lost to entrainment in the voluminous AlOOH precipitate. In test NCA-PT 6, a simultaneous co-precipitation of Al, Co and Ni as hydroxides can be successfully demonstrated at pH 10.5. Sulphide addition in test NCA-PT 8 shows similar levels of Co and Ni recovery to test NCA-PT 4, but the precipitate is significantly larger.

A similar approach was taken in treating LMO leach solution, although the composition was much simpler. Both caustic and carbonate precipitation methods were used to recover Li and Mn from solution. Test results are shown in Table IV.

From Table IV, it is clear that Mn recovery is completed at pH 10 using either caustic or carbonate. The lower concentrations can make the recovery of lithium particularly challenging with LMO, and only about 20% of the initial Li feed is recovered as carbonate, whether the reagent was added as sodium carbonate powder or as CO₂ gas sparges in alkaline solution.

Figure 3 summarises the metal recovery from the leach solutions produced from all three cathode material types (NMC, NCA and LMO) as a function of pH. Good recovery values for Co, Ni and Mn can be achieved at pH 10 using the hydroxide system. Similar recovery of Co and Ni can be seen at a lower pH (5.0) with aqueous sulphide addition (not included in Fig. 3). The lithium recovery is low, but the expectation is that the recovery would be increased by recycling lithium-rich process solutions back to the leaching stage, therefore no alternative lithium recovery methods were included in this part of the investigation.

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Following the positive results from bench scale tests, a locked-cycle campaign was initiated to evaluate the effects of the recycled process solutions on overall efficiency. Two different campaigns were run, with each campaign consisting of 10 cycles. Both campaigns were based on the same conditions for leaching an LCO active cathode material, using sulphuric acid and sulphur dioxide under the same target conditions. The main difference between these campaigns was in the subsequent metal recovery flowsheet. The first locked cycle campaign recovered the leached metals sequentially using sodium hydroxide to remove Co followed by sodium carbonate for Li recovery. The second locked cycle campaign used a combination of sodium hydroxide and sodium carbonate to recover both valuable metals simultaneously. After metal recovery, the remaining metal-deficient solutions were treated by chilling for sodium sulphate removal before being recycled back to the next cycle's leaching stage. The chiller was added to lower the equilibrium sulphate concentration and prevent uncontrolled precipitation of sodium sulphate in the system. ¹³ Evaporation of the recycled solutions was incorporated as needed to maintain the overall water balance.

Figure 4 shows the schematic of the first locked-cycle campaign. The optional of nanofilter showed in Fig. 4 was not executed during the locked-cycle campaign. The extraction values for the first locked-cycle campaign are shown in Fig. 5. Near 100% extractions can be achieved for both metals for the initial 8 cycles.

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The recovery values are declined in Cycles 9 and 10 to some extent (82%–88% for both Li and Co). This can be attributed to the changing composition of the recycle streams and will be discussed further. The recoveries of leached metals from solution are shown in Fig. 6. Cobalt is precipitated with sodium hydroxide and lithium is recovered with sodium carbonate. Results indicate consistent cobalt recovery through all cycles of 90% or greater, and a steady rise in lithium recovery with each cycle, as is predicted.

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To crystalize the Na₂SO₄ and remove it from the recycle stream, the filtrate from lithium precipitation was cooled to 5 °C. Generally, the sodium removal was approximately 30% of the total sodium in the system in each cycle.

Overall, the first locked-cycle test showed high extraction of metals from LCO cathode material and 93%–98% cobalt recovery from acid leach solution. Lithium recovery from solution was around 55% initially and exceeded 85% once the locked cycle circulating load was established in later cycles. The material fed and products recovered in each cycle and their compositions are given in Table V.

The sulphate and dithionate concentrations at different stages in the process over the 10 cycles are shown in Figs. 7 and 8, respectively. It is clear that the sulphate level can quickly stabilize around 300 g l⁻¹ in the leach solution, which is reduced to approximately 50 g l⁻¹ after removing sodium sulphate crystals in the final crystallization stage. In Cycle 6 crystallization treatment leaves higher sulphate (100 g l⁻¹) but this does not significantly affect the sulphate level in the subsequent leach, which remains near 300 g l⁻¹ like most other cycles. In contrast, the dithionate level is consistently higher after sulphate crystallization (dithionate does not reach saturation to crystalize) and reaches a level of 14–16 g l⁻¹ in Cycles 8 to 10, which coincides with the reduced leaching efficiency in those cycles as noted above. The presence of dithionate affects the leach solution oxidation-reductive potential and can contribute to some variations in cobalt extraction. The reactions responsible for dithionate formation and the important reactions for metal dissolution and recovery are as follows ¹⁴ :

$$\begin{eqnarray}&&2{\mathrm{LiCoO}}_{2}+{\mathrm{SO}}_{2}+2{{\rm{H}}}_{2}{\mathrm{SO}}_{4}\to {\mathrm{Li}}_{2}{\mathrm{SO}}_{4}+2{\mathrm{CoSO}}_{4}+2{{\rm{H}}}_{2}{\rm{O}}\end{eqnarray} \tag{ 1 }$$

$$\begin{eqnarray}&&2{\mathrm{LiCoO}}_{2}+4{\mathrm{SO}}_{2}+2{{\rm{H}}}_{2}{\mathrm{SO}}_{4}\to {\mathrm{Li}}_{2}{{\rm{S}}}_{2}{{\rm{O}}}_{6}+2{\mathrm{CoS}}_{2}{{\rm{O}}}_{6}+2{{\rm{H}}}_{2}{\rm{O}}\end{eqnarray} \tag{ 2 }$$

$$\begin{eqnarray}&&{\mathrm{CoSO}}_{4}+2\mathrm{NaOH}\to \mathrm{Co}{\left(\mathrm{OH}\right)}_{2}+{\mathrm{Na}}_{2}{\mathrm{SO}}_{4}\end{eqnarray} \tag{ 3 }$$

$$\begin{eqnarray}&&{\mathrm{CoS}}_{2}{{\rm{O}}}_{6}+2\mathrm{NaOH}\to \mathrm{Co}{\left(\mathrm{OH}\right)}_{2}+{\mathrm{Na}}_{2}{{\rm{S}}}_{2}{{\rm{O}}}_{6}\end{eqnarray} \tag{ 4 }$$

$$\begin{eqnarray}&&{\mathrm{Li}}_{2}{\mathrm{SO}}_{4}+{\mathrm{Na}}_{2}{\mathrm{CO}}_{3}\to {\mathrm{Li}}_{2}{\mathrm{CO}}_{3}+{\mathrm{Na}}_{2}{\mathrm{SO}}_{4}\end{eqnarray} \tag{ 5 }$$

$$\begin{eqnarray}&&{\mathrm{Li}}_{2}{{\rm{S}}}_{2}{{\rm{O}}}_{6}+{\mathrm{Na}}_{2}{\mathrm{CO}}_{3}\to {\mathrm{Li}}_{2}{\mathrm{CO}}_{3}+{\mathrm{Na}}_{2}{{\rm{S}}}_{2}{{\rm{O}}}_{6}\end{eqnarray} \tag{ 6 }$$

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Table VI shows the concentrations of sodium sulphate through the process for each cycle, calculated based on the sulphate and sodium concentrations measured in each solution. By comparing the sodium sulphate concentrations in the recycled solution with those in the leach filtrate, it can be seen that they are very close, suggesting that there is little or no formation of sulphate crystals in the leach during this locked-cycle campaign.

The second locked-cycled campaign repeated the leach conditions of the first (also using LCO cathode material as feed) followed by combined precipitation of Co and Li as hydroxides and carbonates. As with the first campaign, the final filtrate was treated by chilling for sodium sulphate removal prior to being recycled back to the next cycle's leach. For the second campaign, a limited re-pulp wash was added for the recovered sodium sulphate crystals and this wash solution was added to the recycled solution, along with other wash streams, which were also recycled in the first campaign. As there were higher volumes of wash water in the second campaign, to control the water balance the combined recycle stream was evaporated to the target volume before the next leach cycle, whereas in the first campaign only the dilute cobalt precipitate wash solution was evaporated to control the solution volume. Figure 9 shows the schematic of the second locked cycle campaign. The optional of nanofilter showed in Fig. 9 was not executed during the locked-cycle campaign.

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Since the feed material and leaching conditions were similar in both campaigns, it was expected that the leaching performance would also be similar. Initially, Co extraction was near 100% and Li was above 90%, which was similar to the first campaign, but after Cycle 2 the cobalt extraction dropped significantly, as shown in Fig. 10 . The lost cobalt could be recovered from the leach residue by washing with large amounts of water, which is shown in Fig. 10 as the "Leach Wash".

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Following leaching, the solubilised metals were recovered with sodium hydroxide and carbonate as shown in Fig. 11. Overall, the recovery of lithium rises rapidly and is generally good through all cycles except the first one. Lithium recoveries were likely slightly overestimated in these tests due to the requirement for addition of a fresh saturated Li₂CO₃ solution to the second wash of the co-precipitated filter cake from each cycle. A theoretical saturation of lithium was used in accounting for this addition, but temperature variations likely resulted in higher concentrations. Despite this uncertainty, the overall lithium recovery appeared to be comparable to that achieved in the first campaign. Cobalt recovery was affected in early cycles by losses to crystal formation in the leach but increased in later cycles as this material was recycled back into the process. The excess recoveries for both metals in Cycle 9 reflect recovery of losses from previous cycles after decreasing the pulp density in the later cycles.

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As in the first locked-cycle campaign, the second campaign showed consistent extraction of the metals from LCO material. The main issue was the entrapment of leached metal into sulphate crystals formed during leaching. Another indication of crystal formation was the higher mass of leached residues for Cycles #3 through #7 shown in Table VII.

Figures 12 and 13 show sulphate and dithionate levels in the leach, in precipitation filtrates and after sulphate crystal removal. The sulphate concentrations in leach filtrates are consistently higher (>300 g l⁻¹ SO₄) when compared to the first campaign (Fig. 7). In addition, it is noteworthy that high levels of dithionate in Cycles 3 and 4 coincide with the lowest extractions of Co in the leach stage.

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Table VIII presents the calculated sodium sulphate levels in the principal process streams for each cycle. The sodium sulphate in the recycled solution is over 220 g l⁻¹ while the leach filtrate is significantly lower, with concentrations between 145 and 164 g l⁻¹. This confirms the formation of sodium sulphate crystals during leaching. The high sulphate concentrations in the recycle stream are due to the evaporation of the combined process solutions to control the water balance.

As simple as sodium sulphate chemistry might seem, its crystallization behaviour can be a complex phenomenon. As many as three phases may form under certain condition: stable thenardite, (Na₂SO₄) and mirabilite, (Na₂SO₄ × 10H₂O) as well as a metastable heptahydrate, Na₂SO₄ × 7H₂O. In a system involving more than a single solid phase, metastable Na₂SO₄ × 7H₂O may be expected to crystallise and it is not always reproducible. ¹³ The residence time is also an important factor in the formation of precipitates. Considerable care must therefore be taken to ensure solution concentrations are well below the saturation levels. High levels of hydration with sodium sulphate crystals can also increase crystallization by removing water from the system. Figure 14 shows the solubility curves for all three phases of sodium sulphate and it shows that sodium sulphate levels over 200 g l⁻¹, as observed in the second locked-cycle campaign, could have generated crystals in the leach solution.

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The experimental evidence collected in both campaigns showed that both simultaneous and sequential precipitation methods were successful, recovering over 90% of cobalt and lithium from LCO active cathode material, although the grades of cobalt and lithium in the products were higher (60% for Co and 18% for Li, Table V) using sequential precipitation with hydroxide for Co and carbonate for Li compared with the single combined product (about 40% Co and 6% Li average for 10 cycles, Table VII). The control of sulphate levels in the recycle stream with crystallization should be an important addition to the earlier bench optimization study and it could prove to be a necessary step to prevent losses associated with sulphate crystal formation due to circulating sulphate loads in recycle streams. In addition to sulphate, the dithionate generated in the leach can increase with recycle but does not reach saturation levels. When dithionate concentration reaches saturation, it could also be controlled by crystallization along with sulphate. ¹⁴

The principal difference between the first locked-cycle campaign and the second locked-cycle campaign as shown in Figs. 4 and 9 lies in the metal precipitation stage. The flowsheet in Fig. 4 recovers cobalt separately from lithium, while the flowsheet in Fig. 9 uses a single stage to produce a combined Co/Li product. Both flowsheets include the crystallization stage and recycled streams (remaining lithium and sodium) to minimize water consumption and maximize lithium recovery in the system. On a larger scale, dilute wash recycle streams can be treated by nanofiltration to produce clean water for washing products and to control the overall water balance rather than using evaporation. The concentration from nanofiltration may be recycled back to the crystallizer to maximize sodium sulphate removal.