The Leaching of Valuable Metals (Li, Co, Ni, Mn, Cu) from Black Mass from Spent Lithium-Ion Batteries

Abstract

Near-complete (>99%) dissolution of lithium and cobalt was achieved by the leaching of black mass from spent (end-of-life) lithium-ion batteries (LiBs) using 4 M H₂SO₄ or HCl at 60 °C. Raising the temperature to 90 °C did not increase the overall extraction of lithium or cobalt, but it increased the rate of extraction. At 60 °C, 2 M H₂SO₄ or 2 M HCl performed similarly to the 4 M H₂SO₄/HCl solution, although extractions were lower using 1 M H₂SO₄ or HCl (~95% and 98%, respectively). High extractions were also observed by leaching in low pulp density (15 g/L) at 60 °C with 2 M CH₂ClCOOH. Leaching was much slower with hydrogen peroxide reductant concentrations below 0.5 mol/L, with cobalt extractions of 90–95% after 3 h. Pulp densities of up to 250 g/L were tested when leaching with 4 M H₂SO₄ or HCl, with the stoichiometric limit estimated for each test based on the metal content of the black mass. Extractions were consistently high, above 95% for Li/Ni/Mn/Cu with a pulp density of 150 g/L, dropping sharply above this point because of insufficient remaining acid in the solution in the later stages of leaching. The final component of the test work used leaching parameters identified in the previous experiments as producing the largest extractions, and just sulphuric acid. A seven-stage semi-continuous sulphuric acid leach at 60 °C of black mass from LiBs that had undergone an oxidising roast (2h in a tube furnace at 500 °C under flowing air) to remove binder material resulted in high (93%) extraction of cobalt and near total (98–100%) extractions of lithium, nickel, manganese, and copper. Higher cobalt extraction (>98%) was expected, but a refractory spinel-type cobalt oxide, Co₃O₄, was generated during the oxidising roast as a result of inefficient aeration, which reduced the extraction efficiency.

1. Introduction

The increasingly widespread adoption of electric vehicles and distributed battery storage is increasing the demand for critical metals such as lithium, cobalt, nickel, manganese, and copper for use in lithium-ion batteries (LiBs). Long term, as batteries reach the end of life, this will increase the amount of waste material containing these metals, which can be harmful if discarded improperly [1,2]. The development of effective metallurgical processes to recycle spent batteries addresses both problems, particularly as the concentrations of these elements in battery cathode material are significantly higher than in the original ores from which they were produced. The term black mass is widely used to describe the powdered mix of cathode materials and graphite from crushed batteries [3,4,5,6,7]. Various thermal pre-treatments of spent LiBs may be necessary to remove the organic materials from the cathode material or to improve the solubility of metals from the mixed oxide phase [4]. Several thermal pre-treatments using conventional and microwave-based heating under oxidising and reducing conditions were tested in a previous study and shown to improve the extraction of the major elements using a variety of lixiviants [8]. The use of thermal pre-treatments has the disadvantage of consuming a lot of energy and of releasing hazardous fumes [9]. It may be possible to avoid the need for pre-treatments through the optimisation of the leaching process parameters, which was the aim of this study.

A comparison of different lixiviants in the early stages of this work [8] showed that sulphuric acid or hydrochloric acid was effective when combined with hydrogen peroxide. The use of sulphuric acid together with hydrogen peroxide has been widely studied for the extraction of lithium, cobalt, nickel, and manganese from NMC cathode material, Li(Ni,Mn,Co)O₂ in spent lithium-ion batteries [7,10]. Strangely, hydrogen peroxide acts as a reductant in this process [11,12]. Nickel, manganese, and cobalt are reduced to the divalent state and dissolve together with lithium by the following reaction (1) [7]

$$\begin{gathered} 6 {\mathrm{L}\mathrm{i}\mathrm{N}\mathrm{i}}_{1/3}{\mathrm{M}\mathrm{n}}_{1/3}{\mathrm{C}\mathrm{o}}_{1/3}{\mathrm{O}}_2+18 {\mathrm{H}}_{\left(\mathrm{a}\mathrm{q}\right)}^{+}+3 {\mathrm{H}}_2{\mathrm{O}}_{2\left(\mathrm{a}\mathrm{q}\right)} \\ \to 6 {\mathrm{L}\mathrm{i}}_{\left(\mathrm{a}\mathrm{q}\right)}^{+}+2 {\mathrm{N}\mathrm{i}}_{\left(\mathrm{a}\mathrm{q}\right)}^{2+}+2 {\mathrm{M}\mathrm{n}}_{\left(\mathrm{a}\mathrm{q}\right)}^{2+}+2 {\mathrm{C}\mathrm{o}}_{\left(\mathrm{a}\mathrm{q}\right)}^{2+}+12 {\mathrm{H}}_2\mathrm{O}+3 {\mathrm{O}}_{2\left(\mathrm{g}\right)} \end{gathered}$$

(1)

Other reductants such as sulphites have been investigated as well [13,14,15], but are less effective than hydrogen peroxide at the same dose [12]. The reductive dissolution of cathode materials with sodium metabisulphite proceeds via the following reaction (2) [14], where M is Co, Ni, or Mn:

$$4 \mathrm{L}\mathrm{i}\mathrm{M}{\mathrm{O}}_2+{\mathrm{N}\mathrm{a}}_2{\mathrm{S}}_2{\mathrm{O}}_5+6 {\mathrm{H}}_2{\mathrm{S}\mathrm{O}}_4\to 2 {\mathrm{L}\mathrm{i}}_2{\mathrm{S}\mathrm{O}}_4+4 {\mathrm{M}\mathrm{S}\mathrm{O}}_4+2 {\mathrm{N}\mathrm{a}\mathrm{H}\mathrm{S}\mathrm{O}}_4+5 {\mathrm{H}}_2\mathrm{O}$$

(2)

Metallic copper, aluminium, and iron can act as reductants, but are oxidised by hydrogen peroxide. This, in turn, diminishes the capacity of hydrogen peroxide to reductively dissolve the NMC material [16].

Ammonia and ammonium salts have been tested for the recovery of cobalt and nickel from spent lithium-ion batteries, but higher temperatures (>80 °C) are needed for effective dissolution [13,17]. Wang et al. (2020) extracted 95–100% of the cobalt together with 74–98% of the Ni and 85–98% of the Li from spent LiBs with reductive ammoniacal leaching in an autoclave for 30 min at 150 °C [18].

At the time this study was conducted, there were no battery recycling plants domestically within Australia. The purpose of this work was to develop the capability to recycle black mass from spent lithium-ion batteries locally. The aim was to identify the most effective lixiviant for the extraction of lithium, cobalt, nickel, manganese, and copper. Both sulphuric and hydrochloric acids were selected. Following this, the aim was to identify the most effective leaching conditions with several series of leach tests at varied acid concentration, temperature, hydrogen peroxide addition, and pulp density. Finally, the aim was to adapt this to a semi-continuous countercurrent process. Much of the published research on battery recycling papers only focuses on batch processes, and there is not much published information available for continuous processes apart from a few recent studies [19,20,21].

This semi-continuous countercurrent leaching process was conducted on black mass that had been roasted for 2 h at 500 °C. The study on pre-treatments is the subject of another paper [8]. This produced a highly concentrated solution suitable for the recovery of lithium, cobalt, nickel, manganese, and copper, discussed in a separate paper [22]. Recovery of these elements as a mixed hydroxide, (Ni,Mn,Co)(OH)₂ simplifies the overall recycling process, and the mixed hydroxide is suitable as a precursor to re-synthesise Li(Ni,Mn,Co)O₂ [23].

2. Materials and Methods

2.1. Raw Material

Two batches of black mass from spent LiBs were used in this study. Batch 1 was used in the leaching kinetics tests, and batch 2 was used in the controlled potential and seven-stage semi-continuous leach test after roasting for 2 h at 500 °C. This was due to limitations in the amount of sample available. X-ray diffraction analyses (XRD) showed Li(Co,Ni,Mn)O₂ and graphite as the main phases present with minor amounts of metallic iron, copper, and aluminium in both batches.

2.1.1. Feed Batch 1

Eight major elements each made up more than 1% of the feed material by mass (

Table 1. Major elements exceeding 1% of the mass of batch 1.

Element	Mass %	Element	Mass %	Element	Mass %	Element	Mass %
Co	30.7%	Al	2.01%	Cu	1.54%	Mn	1.23%
Li	3.78%	P	1.90%	Ni	1.24%	Fe	1.09%

). The sample contained carbon and fluorine, but the sample was not quantitatively analysed for carbon or fluorine, though they were identified in qualitative analyses. Carbon (as graphite) was identified in XRD analyses, while fluorine was identified in some energy dispersive X-ray analyses discussed in a separate paper [8].

Of the major elements, five were monitored in the leaching tests: cobalt, lithium, nickel, manganese, and copper. The first four of these occurred in the cathode materials as mixed oxides (Li(Ni,Mn,Co)O₂), while copper was present as shreds of metallic foil.

Another 11 elements were analysed to track hazardous elements in the battery material (lead, arsenic, and cadmium), and each of these was below 1% or 10,000 ppm (

Table 2. Minor elements, <1% of batch 1 by mass.

Element	ppm	Element	ppm	Element	ppm	Element	ppm
Zn	5137	Na	530	Cr	184	Nd	28
Pb	4219	Cd	499	La	122	Pr	14
As	815	Ca	271	Ce	82

). The concentrations of these minor elements have been expressed in ppm.

2.1.2. Feed Batch 2

Batch 2 contained less cobalt compared to batch 1, but greater amounts of copper, nickel, and manganese, with comparable amounts of lithium (

Table 3. Major elements exceeding 1% of the mass of batch 2.

Element	Mass %	Element	Mass %	Element	Mass %	Element	Mass %
Co	24.0%	Ni	3.81%	Mn	2.80%	Na	1.77%
Cu	4.49%	Li	3.54%	Al	2.65%

).

Iron was the main element under 0.5% by mass in batch 2. Of the hazardous elements, lead and cadmium were the highest, while arsenic levels were low compared with batch 1 (

Table 4. Minor elements, <1% of batch 2 by mass.

Element	ppm	Element	ppm	Element	ppm	Element	ppm
Fe	5217	Cd	1240	Cr	294	Nd	68
Pb	3829	Ca	1038	P	226	As	55
Zn	3433	La	323	Ce	193	Pr	29

).

2.2. Solution Assays

All feed solutions and filtrate samples were diluted and analysed for Co, Li, Cu, Ni, Mn, Cr, Al, Fe, Ca, Na, P, Zn, As, Cd, Pb, La, Ce, Nd, and Pr using a Thermo Scientific ICP-MS instrument, Waltham, MA, USA, model iCAP-Q ICP-MS at Murdoch University. For the initial test work with different lixiviants, only Co, Li, Cu, Ni, Mn, Cr, Al, Fe, Ca, and Na were analysed.

2.3. Determination of Acid Consumption During Leaching

Leach solution samples (0.3 mL) were mixed with 5 mL 30 wt. % potassium oxalate and made up to 20 mL with deionised water and stirred with a magnetic stirrer. The pH was monitored, and the solution was titrated with 0.1 M AR-grade NaOH solution to an end point of 6.57. Comparisons of the inflection points on the titration curves were used to determine the acid consumption during leaching.

The potassium oxalate solution was prepared from AR potassium oxalate (Rowe Scientific, Minto, NSW, Australia). 0.01 mol/L H₂SO₄ (AR grade, Merck, Darmstadt, Germany) was added to bring the pH of the potassium oxalate solution down to 6.57.

2.4. Solid Analyses

2.4.1. Solid Assays

Several digestion methods were tested in the early stages of the study to determine their suitability to analyse the composition of the raw black mass, roasted black mass, and leach residues. The method that gave the most reliable agreement with assays of the same material conducted at external commercial mineral processing laboratories by four-acid digest (concentrated HCl, HNO₃, HF, and HClO₄) was a modified aqua regia digest method, with the addition of hydrogen peroxide and vanadium (V) to oxidise the carbon or graphite within the solids. Vanadium (V) was added as sodium metavanadate, NaVO₃ (99.9%, Sigma-Aldrich, St. Louis, MO, USA). This gave reliable assays without the use of the highly dangerous reagent hydrofluoric acid. Vanadium (V) compounds such as NaVO₃ or NH₄VO₃ have long been used as oxidising agents in the decomposition of inorganic samples for analysis [24], and vanadium (V) catalyses the oxidation of organic material by hydrogen peroxide [25]. All black mass samples and leach residues were assayed by dissolving ~0.5–1 g (weighed to the nearest 0.0001 g) in 50 mL aqua regia (2:1 mix by volume of hydrochloric acid (HCl) (32%, AR grade, Rowe Scientific) and nitric acid (HNO₃) (70%, AR grade, Ajax)) where 1 mL of a NaVO₃ solution containing 10 g/L V (10 mg V) was added to catalyse the oxidation of carbon. A volume of 3 mL of hydrogen peroxide was slowly added before boiling for 30 min. All solutions were filtered, diluted appropriately, and analysed by the methods described in Section 2.2.

2.4.2. X-Ray Diffraction

Selected solid samples were analysed using X-ray diffraction (XRD) to determine their mineralogical composition and mineral associations. All XRD analyses were performed in-house with a GBC Enhanced Multi-material Analyser (EMMA). The X-ray tube was operated at a voltage of 35.0 kV and a current of 28.0 mA with copper Kα X-rays (λ = 0.1540562 nm). Diffraction patterns were collected over a range of 10° ≤ 2θ ≤ 70° using a 2° diverging slit, a 0.2° receiving slit, and a 2° scattering slit. A step size of 0.02° was used at a rate of 2°/min (0.6 s per step). All diffraction analyses were compared against references from the ICDD powder diffraction file (PDF), and the relevant references are included in the plots with their PDF identification numbers.

2.5. Leaching Kinetics Tests

Five separate flasks for a given lixiviant containing the same masses of lixiviant and raw Batch 1 black mass material were placed in a water bath and agitated for up to three hours. Individual flasks were removed after 15 min, 30 min, 1 h, 2 h, and 3 h.

When each flask was removed, the slurry was filtered, and the filtrate retained. The pH and E_h of the filtrate were measured. The wet solids were then re-pulped with deionised water and filtered again. These second filtrates are described throughout the report as wash liquors. Solids were dried overnight at 60 °C and digested by the methods described in Section 2.4.1. Selected solids were analysed by X-ray diffraction or SEM-EDX techniques.

While the extraction kinetics of ten different elements were monitored (Co, Li, Cu, Ni, Mn, Cr, Al, Fe, Ca, and Na), most of the discussion will focus on just cobalt and lithium in the interest of brevity. Final extractions of copper, nickel, and manganese are shown where relevant. Eight different lixiviants were tested and compared: sulphuric acid (H₂SO₄) (AR grade, Merck), hydrochloric acid (HCl) (32%, AR grade, Rowe Scientific), nitric acid (HNO₃) (70%, AR grade, Ajax), chloroacetic acid (CH₂ClCOOH) (≥99.0%, Sigma-Aldrich), citric acid (C₆H₈O₇) (BDH Chemicals, Dubai, United Arab Emirates), oxalic acid (C₂H₂O₄) (98%, Thermo Fisher Scientific), glycine (NH₂CH₂COOH at pH 10), and a mixture of ammonia and ammonium carbonate (NH₃/(NH₄)₂CO₃) (both sourced from Ajax).

Following the initial comparison of lixiviants, most of the remaining leaching experiments took place with either sulphuric acid or hydrochloric acid solutions. The effects of temperature (30–90 °C), acid concentration (1–4 M), reductant addition (0.2–1.0 M H₂O₂), and pulp density (15–250 g/L solids) were tested. The temperature that was used for most of the leaching experiments to assess the effects of acid concentration, reductants, and pulp density was 60 °C.

2.6. Leaching Tests on Roasted Batch 2 Material

These tests were all conducted on samples of Batch 2 black mass material that had been roasted in air for 2 h at 500 °C.

2.6.1. Controlled Potential Leach Tests

Controlled potential tests were carried out with black mass batch 2 material that had been roasted for 2 h in air at 500 °C with 262 g solids per 1000 mL of 4 M sulphuric acid lixiviant. The solids concentration was set so that the amount of acid would be 110% of the stoichiometric requirement for full dissolution based on the concentration of each acid-consuming element present in the feed and their individual acid requirements to form the soluble species dominant under the given conditions based on thermochemical data. The concentrations of the major elements in the roasted material used in the controlled potential leaches are shown in

Table 5. Assays of major elements in the feed for the controlled potential leach tests.

Sample	Co	Li	Cu	Ni	Mn	Al	Fe
Batch 2 Roast 2	34.0%	5.07%	3.66%	5.26%	4.09%	3.65%	0.354%

.

These tests were run for 3 h at 60 °C, with either hydrogen peroxide or iron to maintain a constant redox potential. The redox potential was checked and adjusted every 15 min. Leaching experiments were conducted under flowing nitrogen to exclude atmospheric oxygen, which could potentially have interfered with the results.

2.6.2. Seven-Stage Semi-Continuous Test

Two separate oxidative roast treatments of black mass (BM) batch 2 materials were carried out in a tube furnace for 2 h at 500 °C under flowing air.

The two roast products (295 g in total) were combined, homogenised, and split into 5 × 50 g charges for semi-continuous leaching, with the remainder of the material prepared for characterisation. The homogenised roasted material was assayed in duplicate (

Table 6. Assays of major elements in the seven-stage leach feed.

Sample	Co	Li	Cu	Ni	Mn	Al	Fe
Batch 2 Roasts 3+4 digest 1	36.3%	5.09%	2.88%	5.40%	4.21%	3.79%	0.268%
Batch 2 Roasts 3+4 digest 2	36.7%	5.17%	4.02%	5.69%	4.36%	3.80%	0.303%
Batch 2 Roasts 3+4 avg.	36.5%	5.13%	3.45%	5.54%	4.28%	3.79%	0.285%

), and the results showed good reproducibility. The average result was used to calculate the amount of leach liquor required to be added to give 110% stoichiometric acid addition using 200 mL of 4 M H₂SO₄ solution at 60 °C.

A seven-stage semi-continuous leach was then performed using this material in locked-cycle mode over two weeks to simulate a two-stage counter-current leaching process. The flows of the solid materials and recycled filtrates are summarised in Figure 1.

Based on the assays, 48.67 g of roasted black mass material was used in each of the leaching stages that involved fresh roasted black mass feed. The temperature and acid concentration were selected based on what had been found to be effective in the controlled potential leach tests. Leaching was conducted under the same conditions as the controlled potential tests: 4 M H₂SO₄, 60 °C, 243 g/L solids, with acid addition being 110% of the amount stoichiometrically required for complete dissolution.

All these parameters were selected based on the results of the leaching kinetics study. Fresh samples of roasted black mass were used in the odd-numbered stages, and the residues were re-leached in the even-numbered stages with fresh acid. Filtrates from stages 2, 4, and 6 were used to leach fresh roasted black mass material in stages 3, 5, and 7, respectively.

3. Results and Discussion

3.1. Leaching Kinetics Study

Leaching tests were performed for 3 h to determine the most effective lixiviant, lixiviant concentration, hydrogen peroxide concentration, temperature, and pulp density.

When an individual parameter was not being studied, the values used for that parameter were pulp density of 15 g/L solids, temperature of 60 °C, lixiviant concentration of 4 M, and hydrogen peroxide concentration of 1 M. The value of a parameter used when studying that parameter is specified for the relevant tests.

3.1.1. Comparisons of Different Lixiviants

Of the eight lixiviants studied, sulphuric acid was the most effective for cobalt extraction, while chloroacetic acid was the most effective for lithium extraction (Figure 2). Almost no cobalt was extracted when leached with oxalic acid or ammonia. Given the results in Figure 3 for all elements, subsequent studies on the effect of temperature, acid concentration, reductant concentration, and pulp density were only carried out with sulphuric and hydrochloric acid. These two acids, along with chloroacetic acid, were the only lixiviants tested that gave consistently good results for all five of the target elements (Figure 3).

Chloroacetic acid did not have any clear advantages over sulphuric or hydrochloric acid, so the use of this reagent was not investigated further. Chloroacetic acid is toxic in addition to its corrosive effects [26]. Any waste streams containing chloroacetic acid or chloroacetate would have to be processed to remove them prior to disposal, complicating the process.

The use of ammonia can be effective for extracting cobalt from spent lithium-ion batteries, though significantly better extraction is achievable at 80 °C compared to 60 °C [13,17]. The use of ammonium carbonate is ineffective for the extraction of manganese, even at 150 °C, due to the formation of insoluble MnCO₃ [18].

3.1.2. Effect of Varied Temperature, 4 M Sulphuric or Hydrochloric Acid

Leaching at 60 °C was sufficient for effective lithium or cobalt extraction in 4 M H₂SO₄ (Figure 4). A similar trend was apparent when leaching in hydrochloric acid, although temperatures of 90 °C were necessary for extractions above 95% in hydrochloric acid.

3.1.3. Effect of Varied Concentrations of Sulphuric and Hydrochloric Acid

There was nearly complete leaching extraction of Co and Li at 2 M H₂SO₄ or at 1 M HCl (Figure 5), though there was a noticeable drop in cobalt extraction with 4 M HCl.

3.1.4. Effect of Varied Reductant Concentration

The initial rate of cobalt dissolution was faster the higher the hydrogen peroxide concentration, although the final cobalt extractions were similar for the 0.5 and 1.0 M H₂O₂ tests (Figure 6). Overall, the final extractions did not vary significantly between 0.5 and 1 M H₂O₂. In sulphuric acid, 0.5 M H₂O₂ is sufficient for the rapid extraction of cobalt from the black mass. Variations in hydrogen peroxide concentration did not have a significant effect on the final metal extraction values once a certain threshold was exceeded. This is consistent with what is expected from the literature. Leaching black mass from mobile phone batteries in 2 mol/L H₂SO₄, Çuhadar et al. (2023) observed a significant increase in Co/Ni/Mn/Li recovery when increasing the hydrogen peroxide concentration from 0 to 10 g/L (0.29 mol/L) with minimal improvement on further increase to 50 g/L (1.47 mol/L) [27]. Similarly, Zhang et al. (2015) found the limiting dose to be 4 vol. % of 30 wt. % H₂O₂ (0–12 vol. %, 2 vol. % increments) at 70 °C in 3 mol/L CCl₃COOH [12]. Zhang et al. (2015) noted the spontaneous decomposition of H₂O₂ at high concentrations above 60 °C as a reason for the rate of dissolution not increasing past a particular H₂O₂ concentration [12].

3.1.5. Effect of Varied Pulp Densities

Increasing the pulp density decreased the rates of cobalt and lithium extraction for both H₂SO₄ and HCl. The effect of solids density on extraction was weaker in sulphuric acid compared with hydrochloric acid for all the major elements other than cobalt (Figure 7). Higher pulp densities have the advantage of producing more concentrated leach liquors as well as decreasing the size of the leach tanks that must be used. At a pulp density of 250 g/L, the amount of sulphuric acid present in a 4 M solution is not far above 100% of the stoichiometric requirement, and the amount of acid remaining in solution was not sufficiently high for the reaction to continue. This would explain the sharp drop in the final extractions of metals other than cobalt above 150 g/L pulp density in 4 M H₂SO₄.

Higher pulp densities, up to 262 g/L, proved to be effective when leaching roasted black mass material in the controlled potential (Figure 8) and semi-continuous leach tests (

Table 7. Extraction of key elements based on the masses in the feed and residue.

Leaching	Co	Li	Cu	Ni	Mn
Stage 1	86.0%	99.1%	97.2%	97.6%	99.2%
Stage 1 + 2	93.0%	99.8%	99.8%	98.9%	99.8%
Stage 3	84.6%	98.9%	98.6%	97.2%	98.8%
Stage 3 + 4	92.5%	99.7%	99.8%	98.6%	99.8%
Stage 5	85.7%	99.0%	98.8%	97.2%	98.8%
Stage 5 + 6	93.0%	99.7%	99.8%	98.9%	99.8%
Stage 7	84.8%	98.6%	98.6%	96.4%	98.7%

).

3.2. Controlled Potential Leaches

Roasted material was leached in 4 M H₂SO₄, with the solid addition adjusted so that the amount of acid present was 110% of the amount needed to dissolve all the roasted black mass added to the leach. Iron and hydrogen peroxide were tested as reductants; each was added gradually over the 3 h leach period to maintain a target E_h potential of 50 mV for iron and 600 mV for hydrogen peroxide. The extractions, which are summarised in Figure 8, were high for all the major elements present except for Cu. Copper extraction was much lower when the reductant was iron, likely because of cementation. Maintaining the target E_h proved difficult with Fe.

The E_h values and cumulative additions of each reductant over time are summarised in Figure 9. It was observed that the E_h was slower to stabilise when Fe was used, likely because of slow reaction kinetics.

Fe consumption was 119 kg/t, and H₂O₂ was 844 kg/t (as a 30% solution). There were some difficulties with temperature control in the peroxide leaches, due to exothermic reactions between the reagent and the slurry, resulting in periodic temperature rises of up to 10 °C above the set point.

3.3. Semi-Continuous Leach Test

3.3.1. Leach Results

The extraction of the main elements from the seven-stage leach test based on the assays of the solids is summarised in Table 7. In stages 2, 4, and 6, the residues from stages 1, 3, and 5, respectively, were re-leached with fresh acid, and extractions from these stages have been grouped with the previous stage so that the table reports the overall extractions. For example, the re-leaching of the Stage 1 residue in Stage 2 extracted approximately half of the remaining un-leached cobalt, raising the overall cobalt extraction from 86% to 93%.

The combined extractions of all the key elements were very high, and after a single re-leach of residue, the average extractions were 92.8% Co, 99.7% Li, 98.8% Ni, and 99.8% Mn. However, 99.8% of the Cu was also dissolved, and this would need to be dealt with by neutralisation/impurity separation.

The final pH and redox potential (E) data are summarised in

Table 8. Solution and slurry properties.

Stage	Filtrate pH	Filtrate E, mV vs. Ag/AgCl
1	−0.16	725
2	−0.29	611
3	−0.21	701
4	−0.48	609
5	−0.22	718
6	−0.29	612
7	−0.02	735

. The pH was too low to be reliably measured, but the redox data indicate the potential during the leaching of fresh feed ranged between 600 and 1020 mV vs. Ag/AgCl, but was stable at approximately 610 mV when residue from a previous leach stage was re-leached with fresh acid. The re-leach stages, therefore, had a correspondingly lower requirement for a reductant. The E vs. time profiles are illustrated in Figure 10.

At the completion of each leaching test, the free acid concentration was determined by titration. Sulphuric acid (95%) was added to increase the concentration to the starting value of 4 mol/L before the next stage. The acid consumption was calculated from the difference in the free acid concentration before and after each leaching stage, and is reported in

Table 9. Acid consumption observed in each stage.

Stage	Acid Consumption, g/100 mL	Acid Consumption, kg/t
1	28.84	1353
2	4.54	962
3	31.53	1405
4	5.34	1074
5	32.32	1430
6	5.28	1111
7	33.04	1536

. The results show that the average acid consumption was 1431 kg/t when leaching fresh feed, decreasing to an average of 1049 kg/t when re-leaching the residues. While the acid consumption was quite high, the cost could potentially be justified based on the high value of cobalt and other metals extracted.

3.3.2. Leach Residue Characterisation

The average total cobalt extractions for the seven-stage semi-continuous leaching tests were in the range 92–93%, while for other elements (Li, Ni, Mn, and Cu), the extractions were 98–100%. This suggests that some of the cobalt was present in a refractory phase. The leach residues were analysed by XRD, which confirmed the presence of a spinel-type cobalt oxide, Co₃O₄. This was the only cobalt-containing solid identified in the residues. This was likely formed during the oxidative roast prior to leaching. The characterisation results are summarised in

Table 10. Grades (%) of the leach residues and the phases identified by XRD.

Residue	Co	Li	Cu	Ni	Mn	Al	Fe	Phases Identified
Stage 1	21.6	0.2	0.4	0.6	0.2	0.5	0.4	Not analysed
Stage 2	13.4	0.0	0.0	0.3	0.0	0.3	0.2	Graphite, Co3O4
Stage 3	22.8	0.2	0.2	0.6	0.2	0.6	0.4	Not analysed
Stage 4	14.2	0.1	0.0	0.4	0.0	0.5	0.2	Graphite, Co3O4
Stage 5	22.0	0.2	0.2	0.7	0.2	0.5	0.3	Not analysed
Stage 6	13.8	0.1	0.0	0.3	0.0	0.5	0.1	Graphite, Co3O4
Stage 7	22.4	0.3	0.2	0.8	0.2	0.8	0.5	Graphite, Co3O4

and Figure 11. Further optimisation of the roasting process is needed to prevent the formation of spinel-type cobalt oxide.

The residues from leach stages 1, 3, and 5 were not analysed by XRD because most of those residues were used as feed for the leach stages 2, 4, and 6, respectively, and there was too little remaining residue material.

4. Conclusions

Leaching raw black mass material with 4 M H₂SO₄ or 4 M HCl at 60 °C together with 1 M H₂O₂ resulted in nearly complete dissolution of the five elements studied. Increasing the temperature to 90 °C with the same reagents resulted in faster dissolution but did not significantly improve the final amounts extracted after 3 h of leaching. A hydrogen peroxide concentration of at least 0.5 M was needed for effective extraction of the major elements. Increasing the solids density above 150 g/L resulted in decreased extractions of all major elements from raw black mass.

Higher extractions were achieved at a solids density of 250 g/L with 4 M H₂SO₄ at 60 °C in two controlled potential leaches. Hydrogen peroxide proved to be an effective reductant for all elements apart from lithium, while iron filings were effective for all elements apart from copper.

To simulate a continuous two-stage counter-current leaching process, a seven-stage semi-continuous leach was conducted on material processed under the optimal conditions determined in the batch leaches. An oxidising roast pre-treatment for 2 h at 500 °C for the removal of binder, followed by leaching in 4 M sulphuric acid at 60 °C resulted in high extractions of cobalt (92.8%), Li (99.7%), Ni (98.8%), Cu (99.8%), and Mn (99.8%) from black mass. The lower cobalt extraction was due to a refractory spinel-type cobalt oxide, Co₃O₄, being generated during the oxidising roast because of inefficient aeration. Further optimisation is needed to suppress the formation of this material during the roast.