Acid leaching of mixed spent Li-ion batteries

2.1 Material

Different types of spent LIBs, (BL-5B, BP-4L, BL-5CB, BL-5CA, BL-6F, BL-5C, BL-4C, BL-4U, and BR-5C) were dismantled using a manual procedure described elsewhere (Dorella and Mansur, 2007; Mantuano et al., 2006) to remove both plastic and steel cases that cover the batteries. Once dismantled, anode and cathode were crushed carefully and sieved to separate the scrap paper, plastic film, outer metallic body, and membrane. Then, the black powder obtained was collected and washed with water to remove entrained electrolyte. The resulted powder was dried for 24 h at 60 °C, sieved with screens of 0.5, 2.0, and 5.0 mm.

In order to characterize the metal content in the powder of spent LIBs, Table 1, a sample of powdered materials that cover anode and cathode foils (Carbon corresponds to 16–18 wt.% of the overall sample) are dried and analyzed by X-ray fluorescence (XRF spectrometer, Asios, Sequential WD-XRF Spectrometer, PANalytical 2005) at National Research Centre, Egypt. Lithium concentration is analyzed by ICP-OES (Perkin Elmer Optima 2000 DV) at the Central Metallurgy R&D Institute, Helwan, Egypt.

The quantitative metal contents for Al, Mn, Cu, Ni, Co, and Li were assessed by atomic absorption spectrometry (AA spectrometer, GBC 932 plus model) after dissolving samples (anode and cathode, including metallic foils) in the leaching solutions.

2.2 Experimental procedure

All the experiments were conducted in batches, and the dissolution experiments were performed in 500-ml conical flasks. After adding known amounts of the mixed battery powder sample with NH₄OH solution, the mixture was heated under specific conditions. The decomposition conditions were fixed at 4.0 M NH₄OH solution at 60 °C for 60 min with a liquid/solid mass ratio of 15/1 except otherwise cited. At the end of each decomposition process, the slurry (paste) was filtered, washed with distilled water, and was taken for further acid leaching investigations.

The effect of agitation on the decomposition and leaching processes was studied using different stirring rates varying from 125 to 425 rpm. The results obtained shows that the dissolution rate increases with increasing stirring rate, but becomes almost independent of stirring above 250 rpm. Therefore, all experiments were performed at a stirring rate of 250 rpm to ensure good mass transfer.

3.1 Decomposition with NH₄OH

In this section, decomposition processes are carried out for the raw material by using different concentrations of NH₄OH solution. The particles finer than 0.5 mm (gives better recovery efficiency) was treated by digesting with NH₄OH and the alkali paste was dried at 80 °C for 120 min. The main parameters that influence this process are decomposition time, NH₄OH concentration, NH₄OH-to-raw material (L/S) mass ratio and temperatures.

Dissolution of Al, Mn, Cu, Ni, Co, and Li was studied in the decomposition process. Recovery percentage (%E) of Al was increased very fast to reach about 97% after 50 min, while the recovery percentage (%E) of Cu, Mn, Ni, Co, and Li was slightly increased with time, reaching a plateau after about 60 min by using 4.0 M NH₄OH with L/S mass ratio of 15/1 at 60 °C (97.8% Al, 64.7% Cu). Therefore, all remaining tests were carried out at a constant time of 60 min.

The effect of NH₄OH concentration on dissolution of Al, Cu, Mn, Ni, Co, and Li from powder materials of LIBs was studied for 60 min with L/S mass ratio of 15/1 at 60 °C as shown in Fig. 1a. (%E) of Al increased from 20.5% to 99%, and (%E) of Cu also increased from 6.5% to 61.2% when the concentration of NH₄OH was increased from 0.5 M to 6.0 M. (%E) of Li and Ni increased from 4.6% to 14.8%, and from 1.6% to 4.7%, respectively, as NH₄OH concentration was increased from 0.5 M to 6.0 M. (%E) of Mn and Co (%E <1.0%) by NH₄OH solutions is slightly affected, and their behaviors are nearly similar under these conditions. At 4.0 M NH₄OH, a plateau is formed with high (%E) values for Al and Cu (97.8% Al and 64.7% Cu). Therefore, all remaining experiments were carried out by 4.0 M NH₄OH solution.

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Figure 1a Effect of NH₄OH concentration on the decomposition rate of spent LiBs (contact time = 60 min, L/S mass ratio = 15/1, T, °C = 60 °C).

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As shown in Fig. 1b, the effect of temperature on the recovery percentage (%E) of the investigated elements was studied by using 4.0 M NH₄OH with L/S mass ratio of 15/1 for 60 min in the range of 25–80 °C. It can be observed that the dissolution efficiency of Al and Cu increases as the temperature increases until 60 °C. Moreover, as the temperature increases, small amounts (%E <1.0%) of Mn and Co (their behaviors are nearly similar), about 11% Li and 4.5% Ni are recovered in the solution.

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Figure 1b Effect of temperature on the decomposition rate of spent LiBs by 4.0 M NH₄OH (contact time = 60 min, L/S mass ratio = 15/1).

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Generally, the decomposition of spent LIBs by NH₄OH was found as a very important and interesting step to be included in a treatment route for spent LIBs because it is very selective for Al and Cu over the other metal content. The alkali paste obtained is washed with water in order to remove the impurities and then dried at 80 °C for 120 min. The resulted paste contains the main bulk of Mn, Co, Ni, and Li. This paste was subjected to the following acid leaching investigations.

3.2 Leaching study

3.2.1

3.2.1 Effect of leaching time

To study the effect of leaching time on leaching process of the alkali paste of spent LIBs by 2.0 M H₂SO₄, several experiments were carried out at a time from 15 to 180 min. The other dissolution parameters were fixed with an L/S mass ratio of 10/1, 4.0% H₂O₂ at 70 °C. The results obtained, Fig. 1c, shows that the leaching efficiency (%E) of Co reached 92.8% at first 45 min and a plateau is obtained after 60 min with (%E) of 99.1%. Leaching percentage was increased to 66.5% for Mn, 57.7% for Ni, and to 59.7 for Li, respectively, when the time was increased to 60 min. Furthermore, as the time increases to 120 °C, (%E) of Mn, Ni, and Li increases to the maximum values. Further increase in leaching time from 120 to 180 min has a slight effect on the dissolution rate of Mn, Ni, and Li and adverse effect on the dissolution rate of Co. Therefore, all remaining tests were carried out at a constant leaching time of 120 min.

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Figure 1c Effect of time on the leaching of alkali residue of spent LiBs by 2.0 M H₂SO₄ (H₂O₂ = 4.0%, L/S mass ratio = 10/1, T, °C = 70 °C).

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3.2.2

3.2.2 Effect of a liquid/solid mass ratio

Effect of L/S mass ratio on leaching efficiency (%E) of Mn, Ni, Co, and Li from the alkali residue of spent LIBs was studied using 2.0 M H₂SO₄ acid solution with reaction time of 120 min and 4.0% H₂O₂ at 70 °C, Fig. 2a. The results obtained indicate that (%E) was increased to 97.8% for Mn, 99.4% for Ni, 99.6% for Co and 98.8% for Li, respectively, as the liquid/alkali paste mass ratio increases from 1/1 to 10/1. When the L/S mass ratio increases from 10/1 to 15/1, (%E) of Mn slightly increased, while no significant change in the leaching percent of Ni, Co, and Li was observed. The leaching of Mn, Ni, Co, and Li decreased when the L/S mass ratio was increased from 15/1 to 20/1.

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Figure 2a Effect of L/S mass ratio on the leaching of alkali residue of spent LiBs by 2.0 M H₂SO₄ (H₂O₂ = 4.0%, time = 120 min, T, °C = 70 °C).

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The increase in leaching efficiency of the investigated elements when the liquid/alkali paste mass ratio increased can be attributed to the decrease in the viscosity and consequently, decreases the mass transfer resistance in the acid–alkali paste of the spent LIB interface. Further, at a low L/S mass ratio there is insufficient acid to react with the alkali paste whereby at a certain higher L/S mass ratio more acid is available to react with it to give maximum recovery of Mn, Ni, Co, and Li. Thus, the best condition of L/S ratio was 10/1.

3.2.3

3.2.3 Effect of H₂ SO₄ concentration

Several leaching experiments were performed using H₂SO₄ solutions with concentrations varying from 0.5 to 4.0 M for 120 min with 4.0% H₂O₂ and L/S mass ratio of 10/1 at 70 °C as shown in Fig. 2b. The results obtained show that (%E) of Ni increases rapidly to 87.8% as the H₂SO₄ concentration increases to 1.0 M while (%E) of Mn, Co, and Li slightly increased in this range of concentration. Using higher concentration of H₂SO₄ up to 2.0 M, (%E) of the investigated metals rapidly increased up to 97.8% for Mn, 99.4% for Ni, 99.6% for Co and 98.8% for Li. With further increase in the H₂SO₄ concentration from 2.0 to 4.0 M, (%E) of the investigated metals increased slowly in comparison to lower H₂SO₄ concentration solutions in the presence and absence of H₂O₂.

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Figure 2b Effect of H₂SO₄ concentration on the leaching of alkali residue of spent LiBs. (H₂O₂ = 4.0%, time = 120 min, L/S mass ratio = 10/1, T, °C = 70 °C).

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This behavior can be explained by the following chemical equations of dissolution of alkali paste of spent LIBs (such as LiCoO₂ and LiMnO₂ types) in the H₂SO₄solution in the absence and in the presence of H₂O₂ solution (Nan et al., 2005; Zhu et al., 2012).

(1a)

$$4{\text{LiCoO}}_{2(\text{s})}+6{\text{H}}_2{\text{SO}}_{4(\text{aq})}\to 4{\text{CoSO}}_{4(\text{aq})}+2{\text{Li}}_2{\text{SO}}_{4(\text{aq})}+6{\text{H}}_2{\text{O}}_{(\text{g})}+{\text{O}}_{2(\text{g})}$$

(1b)

$$2{\text{LiCoO}}_{2(\text{s})}+3{\text{H}}_2{\text{SO}}_{4(\text{aq})}+{\text{H}}_2{\text{O}}_{2(\text{aq})}\to 2{\text{CoSO}}_{4(\text{aq})}+{\text{Li}}_2{\text{SO}}_{4(\text{aq})}+4{\text{H}}_2{\text{O}}_{(\text{g})}+{\text{O}}_{2(\text{g})}$$

(2a)

$$4{\text{LiMnO}}_{2(\text{s})}+6{\text{H}}_2{\text{SO}}_{4(\text{aq})}\to 4{\text{MnSO}}_{4(\text{aq})}+2{\text{Li}}_2\text{SO}4(\text{aq})+6{\text{H}}_2{\text{O}}_{(\text{g})}+{\text{O}}_{2(\text{g})}$$

(2b)

$$2{\text{LiMnO}}_{2(\text{s})}+3{\text{H}}_2{\text{SO}}_{4(\text{aq})}+{\text{H}}_2{\text{O}}_{2(\text{aq})}\to 2{\text{MnSO}}_{4(\text{aq})}+{\text{Li}}_2{\text{SO}}_{4(\text{aq})}+4{\text{H}}_2{\text{O}}_{(\text{g})}+{\text{O}}_{2(\text{g})}$$

These equations show that the addition of the reacting substances can facilitate the forward reaction resulting in an increase in (%E) of the investigated metals.

3.2.4

3.2.4 Effect of H₂O₂ concentration

Effect of concentration of H₂O₂ on the leaching of alkali paste of spent LIBs by 2.0 M H₂SO₄ was studied in the range from 0.0 to 6.0%, Fig. 2c. The results obtained show only 51.6% of Mn and about 42.7% of Co were leached in the absence of H₂O₂ solution, whereas more than 82.2% of Ni and about 74.5% of Li were leached at the same conditions. (%E) of Mn and Co was increased significantly with the increase in concentration of H₂O₂ from 0.0% to 4.0% to reach about 97.8% of Mn and 99.6% of Co and (%E) of Ni and Li increased from 82.2% to 99.4% and 74.5% to 98.8%, respectively, under the investigated conditions.

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Figure 2c Effect of H₂O₂ concentration on the leaching of alkali residue of spent LiBs by 2.0 M H₂SO₄ (time = 120 min, L/S mass ratio = 10/1, T, °C = 70 °C).

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Fig. 2c shows that (%E) did not increase significantly when more than 4.0% H₂O₂ was used. This can be due to the fact that is instability of H₂O₂ solutions. When heated, H₂O₂ solution can be decomposed according to the following equation (Eq. (3));

(3)

$${\text{H}}_2{\text{O}}_{2(\text{aq})}\to {\text{H}}_2{\text{O}}_{(\text{aq})}+1/2{\text{O}}_{2(\text{g})}$$

Therefore, an increase in H₂O₂ concentration could accelerate its decomposition, resulting in no significant increase in the leaching efficiency of alkali paste of spent LIBs by 2.0 H₂SO₄ solution at high concentration of H₂O₂ (above 4.0%).

3.2.5

3.2.5 Effect of temperature

Effect of leaching temperature on the dissolution of alkali paste of LIBs by 2.0 M H₂SO₄ was studied in the range of 20–100 °C, while the other leaching parameters remained constant. The results obtained are shown in Fig. 2d. It can be observed that (%E) of Mn, Ni, Co, and Li increases with the increase in temperature. The data obtained illustrate that leaching efficiency is significantly affected by temperature. The results indicate that only 52.7% Mn, 23.8% Ni, 29.7% Co, and 18.6% Li are leached at 20 °C. At 70 °C, about 97.8% Mn, 99.4% Ni, 99.6% Co, and 98.8% Li was recovered. A further increase in the temperature does not show any significant increase in the recovery of metals. Thus, the best condition of temperature is 70 °C. This indicates that the leaching process of metals from spent LIBs is an endothermic reaction (Sakuitung et al., 2007).

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Figure 2d Effect of temperature on the leaching of alkali residue of spent LiBs by 2.0 M H₂SO₄ (H₂O₂ = 4.0%, time = 120 min, L/S mass ratio = 10/1).

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3.3 Kinetic analysis

In order to determine the kinetic parameters and rate controlling step in the leaching of the investigated elements (Mn, Ni, Co, and Li) from the alkali residue of spent LIBs by 2.0 M H₂SO₄ in the presence of 4.0% H₂O₂, the experimental data in Fig. 2d, are processed and correlated to various kinetic models for solid–liquid reactions. The equations of the shrinking core model when either the surface chemical reactions or diffusion reactions are the slowest step can be expressed as follows, respectively, (Levenspiel, 1998).

The results obtained for Mn, Ni, Co, and Li, presented in Fig. 3a–d as a relation between 1 − (1 − X)^1/3 against time (t), gave straight lines at different temperatures and pass through the origin. This verifies Eq. (4). Under these conditions, the reactions are controlled by surface chemical reactions. The correlation coefficients of these straight lines are about 0.99.

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Figure 3a Plots of 1 − (1 − X)^1/3 vs. time for Mn leaching at various temperatures by 2.0 M H₂SO₄ solution.

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Figure 3b Plots of 1 − (1 − X)^1/3 vs. time for Ni leaching at various temperatures by 2.0 M H₂SO₄ solution.

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Figure 3c Plots of 1 − (1 − X)^1/3 vs. time for Co leaching at various temperatures by 2.0 M H₂SO₄ solution.

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Figure 3d Plots of 1 − (1 − X)^1/3 vs. time for Li leaching at various temperatures by 2.0 M H₂SO₄ solution.

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For estimation of the activation energy, the reaction between the overall rate constant and temperature from the Eq. (4) can be expressed by the Arrhenius equation, as follows;

The rate constants for different temperatures at different times were calculated from Fig. 3a–d for the investigated elements. A plot of ln k vs. 1/T for this model is linear (Fig. 4) and the values of the apparent activation energy (E_a) for the leaching elements were determined from the line slopes. The calculated values are 30.1 kJ/mol for Mn, 36.7 kJ/mol for Ni, 41.4 kJ/mol for Co, and 37.4 kJ/mol for Li.

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Figure 4 Arrhenius plot for Mn, Ni, Co, and Li leaching in 2.0 M H₂SO₄.

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The relatively high values of activation energies obtained for leaching of these elements by H₂SO₄ in the presence of H₂O₂ is clearly indicative of a chemical controlled reaction in this leaching process.

3.4 Recovery of leached metals from leaching liquor of spent LIBs

Based on the results obtained, a flow sheet for the separation of Mn, Ni, Co, and Li is developed and tested. After the decomposition process of LIBs by 4.0 M NH₄OH, the resulted alkali paste has been leached by 2.0 M H₂SO₄ and 4.0% H₂O₂. The resulted leach liquor (1000 ml) was found to contain 32.73 g Co, 18.7 g Mn, 2.265 g Li, 0.075 g Ni, and other impurities, Fig. 5.

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Figure 5 Flow sheet for the recovery of Mn(II), Ni(II), Co(II), and Li(II) from spent LIBs.

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Leach liquor was, first, purified by filtration to remove any suspended material. By adjusting the pH value of the leach liquor to 7.5 by adding 2.0 M NaOH solution into the filtration, Mn was precipitated with saturated solution of Na₂CO₃. White to faint pink precipitate of MnCO₃ is obtained and washed using hot water to remove the soluble salts. The resulting MnCO₃ is dried at 100 °C for 1.0 h. However, the analytical results show that about 94% of Mn is recovered as a precipitate.

The pH value of leach liquor was adjusted by 2.0 M NaOH to 9.0. With adding a solution of Na₂CO₃ and stirring for 1.0 h at room temperature, nickel is precipitated as light green precipitate (NiCO₃). Solid precipitate was washed with water to remove the soluble salt and then dried at 60 °C for about 4.0 h. At these conditions, about 91% of Ni is recovered from the leaching liquor.

A saturated solution of NaOH was added to the leach liquor under stirring to reach a pH of 11–12. After 2.0 h under stirring at room temperature, the rose red precipitate Co(OH)₂ is separated by filtration. Then, solid precipitates were washed with warm water to remove different soluble salts and then dried at 100 °C for about 4.0 h. About 95% of Co is precipitated from the leaching solution under these optimum conditions.

By adding the Na₂CO₃ solution to the filtrate, the solution was agitated at a speed of 250 rpm for 1.0 h to precipitate Li₂CO₃. The white precipitate obtained is separated from the leach liquor by filtration. The precipitate obtained is washed using hot water to eliminate the entrained sodium and then dried at 100 °C for 1.0 h. The data obtained show that about 90% of Li is recovered as a precipitate.