Comparison of separation of Mn(II), Co(II), and Ni(II) by oxidative precipitation between chloride and sulfate solutions

: In hydrometallurgy, precipitation would be easier and simpler than solvent extraction as a separation operation. In this work, the separation performance of Co(II), Mn(II) and Ni(II) by oxidative precipitation was investigated. For this purpose, NaClO was employed as an oxidizing agent and the separation behavior of the three ions was compared between chloride and sulfate solutions by varying some factors such as the dosage of NaClO, solution pH and reaction temperature. By controlling the molar ratio of NaClO to Mn(II), Mn(II) were easily separated as MnO 2 by oxidative precipitation from both chloride and sulfate solutions. At the same experimental conditions, precipitation percentage of Co(II) from chloride solution was higher than that from sulfate solution, which can be ascribed to the stronger tendency of Co(II) to form complexes with chloride ion than with sulfate ions. Addition of NaCl to sulfate solution and oxidative precipitation at high temperature enhanced the precipitation percentage of Co 2 O 3 and thus separation degree between Co(II) and Ni(II) was improved. Under the optimum conditions, MnO 2 and Co 2 O 3 powders with 99.9% purity were completely recovered by oxidative precipitation from chloride solution. By contrast, the purities of the MnO 2 and Co 2 O 3 thus recovered from sulfate solution were only 76 and 91%, respectively. Our results indicated that chloride solution would be more effective than sulfate solution in separating Mn(II) and Co(II) by oxidative precipitation with NaClO. Therefore, the use of chloride-based leaching solutions such as HCl and FeCl 3 might be better for the leaching medium of spent lithium-ion batteries.


Introduction
Spent lithium-ion batteries (LIBs) are considered to be important secondary resources due to their containing valuable metals such as lithium, cobalt, and nickel (Lei et al., 2022;Othman et al., 2020;Liu et al., 2019).The consumption of strategic metals for the manufacture of LIBs has grown annually and this trend is anticipated to continue due to the widespread application of LIBs in portable electronics since the end of the 20 th century and the recent rapid growth of the electric vehicle market (Meshram et al., 2020;Al Sultan and Benli, 2023).Meanwhile, the buildup and improper disposal of enormous quantities of outdated LIBs endanger the environment and public health (Meshram et al., 2020;Kang et al., 2013).Thus, from an economic and environmental perspective, it is necessary to recover metals from spent LIBs, and several technologies for the recycling of spent LIBs have been developed (Jena et al., 2021;Rautela et al., 2023).Hydrometallurgical processes for the recycling of spent LIBs have been implemented on a considerable scale in the industry (Yao et al., 2018;Zhang et al., 1998;Joulié et al., 2014).
In the hydrometallurgy process, acidic leaching solutions such as sulfuric acid, hydrochloric acid, citric acid, oxalic acid, and methanesulfonic acid are commonly employed for the dissolution of metals from the powders of spent LIBs (Golmohammadzadeh et al., 2018;Ku et al., 2016;Tran et al., 2022;Nguyen and Lee, 2020;Lu et al., 2021).In general, H2SO4 and HCl are commonly employed in industry due to their benefits over other leaching agents in terms of the cost and metal dissolution effectiveness (Xu, et al., 2008;Li et al., 2022).After the leaching, their acidic leaching solutions typically contain Co(II), Cu(II), Mn(II), and Ni(II).Among these ions, Cu(II) can be separated by cementation with base metal (Nguyen and Lee, 2023;Koseoglu et al., 2022;Tawonezvi et al., 2023).Then it is necessary to separate Co(II), Mn(II), and Ni(II) in order to recover pure compounds of these metals.Although lots of work have been reported on the separation of these three metal ions by solvent extraction due to its high efficiency and selectivity, precipitation is simpler and easier to operate industrially than solvent extraction and other separation methods such as ion exchange and reduction (Pohl, 2020).The advantages of chemical precipitation are process economics due to use of cheap reagent, extensive application range and the possibility of easy scale-up, while its drawbacks are the presence of a limit in the precipitation degree and in the selectivity.Some studies reported the possibility of separating Mn(II) and Co(II) from acidic solutions containing Ni(II) by oxidative precipitation (Nguyen and Lee, 2023;Joulié et al., 2014;Orue et al., 2021;Zhang et al., 2002).Among them, several kinds of oxidizing agents such as O3, NaClO, and gas mixture of SO2 and O2 were employed to separate Mn(II) and Co(II) in the form of MnO2 and Co2O3 by controlling the molar ratio of oxidizing agent to Mn(II).However, it is not easy to separate Co(II) from Ni(II) by oxidative precipitation owing to co-precipitation of Ni(II) and incomplete precipitation of Co(II).Therefore, it is necessary to find a suitable oxidizing agents and some conditions by which the oxidative precipitation of Co(II) could be enhanced, resulting in complete separation of Co(II) and Ni(II).
The selection of oxidizing agent is critical when utilizing oxidative precipitation to separate target metal ions.Common oxidizing agents include hydrogen peroxide (H2O2), potassium permanganate (KMnO4), sodium hypochlorite (NaClO), and ozone (O3), which can be employed in oxidative precipitation to separate metal ions (Singer and Reckhow, 1999).Considering the cost, appropriate reaction conditions, and reduction potential of oxidizing agents, NaClO is a better option for this study.In addition, the chemical properties of Co(II) and Ni(II) in aqueous solutions are very similar, which makes the separation of these two metal ions difficult.The Co(II) has a stronger tendency to form complexes with chloride ion than with sulfate ion.When a metal ion forms several complexes with a ligand, the mole fraction of the free metal ion would be drastically reduced, resulting a decrease in the reduction potential of the free metal ion.By utilizing this, the oxidative precipitation of Co(II) could be increased by introducing chloride ion into the solutions containing Co(II) and Ni(II).
In this work, separation behavior of Co(II), Mn(II) and Ni(II) by oxidative precipitation was compared between chloride and sulfate solution.In this work NaClO was employed as an oxidizing agent.Oxidative precipitation of Mn(II), Co(II) and Ni(II) from synthetic solutions of weak HCl and H2SO4 was investigated by varying some variables such as molar ratio of oxidizing agent to target metal ion, temperature, and solution pH.Especially the effect of adding NaCl or Na2SO4 to either HCl or H2SO4 solutions was investigated.Our results clearly indicated that Mn(II) and Co(II) can be easily separated from Ni(II) in HCl solution by oxidative precipitation.Namely, HCl solution is better than H2SO4 solution when employing oxidative precipitation to separate Mn(II), Co(II) and Ni(II) under the same experimental conditions, and addition of NaCl to weak acid solutions can enhance separation of Co(II) over Ni(II) by oxidative precipitation.Chloride-based leaching solutions are more favorable for the treatment of some resources containing Mn, Co, and Ni in terms of separation performance by oxidative precipitation.

Reagents and chemicals
The powders obtained from the smelting reduction of spent LIBs at high temperature were dissolved in both kinds of acidic solutions and the leaching solutions contain Fe(III), Cu(II), Co(II), Mn(II), and Ni(II).Then, Fe(III) and Cu(II) were removed by hydrometallurgical methods like cementation and solvent extraction and the concentrations of the metal ions after the separation of Fe(III) and Cu(II) from the two acidic solutions were the same as shown in Table 1 (Wen and Lee, 2022).Synthetic solutions containing Co(II), Ni(II), and Mn(II) were prepared by dissolving their corresponding chloride and sulfate salts such as CoCl2•6H2O (Junsei Chemical Co., >97%, Japan), MnCl2

Procedure and analytical methods
All the oxidation and precipitation experiments in this work were carried out by slowly adding a specific amount of oxidative or precipitating agents to the determined volume (20 mL) of synthetic solutions in a 100-mL glass beaker with a cover to prevent the evaporation loss during the reaction.The stirring speed, reaction temperature, and time were controlled by a magnetic stirrer (Daihan Scientific Co., Korea).The resulting solutions after the experiments were filtered using filter paper (ADVANTEC No. 2, 110 mm, Toyo Roshi Kaisha, Ltd).The concentrations of the metal ions in the filtrate were measured by Inductively coupled plasma-optical emission spectrometry (ICP-OES, Spectro Arcos, Cleve, Germany), and the pH of the solutions was determined using a pH meter (Orion Star thermo scientific pH meter, model A221, USA).
The oxidation or precipitation percentages of a metal ion were calculated as where minitial and mfiltrate are the mass of a metal ion before and after the oxidative or precipitating reactions, respectively.

Separation of Mn(II) over Co(II) and Ni(II) from synthetic solutions of HCl and H2SO4 by oxidative precipitation
The oxidative precipitation reaction of Mn(II) by NaClO can be represented as (Nguyen and Lee, 2023) Mn 2+ (aq) + NaClO(aq) + H2O(l) = MnO2(s) + NaCl(aq Even in weak acidic solutions, the nature of a ligand affects the structure of the complex between the metal ion and the ligand, which might affect the chemical activity of the complexes.Therefore, the oxidative precipitation of Mn(II) by NaClO from H2SO4 and HCl solutions containing Co(II) and Ni(II) was investigated.In the treatment of the metallic alloys obtained by smelting reduction of spent lithium ion batteries at high temperature, the concentrations of Ni(II), Co(II), and Mn(II) in the filtrate after the separation of Cu(II) and Fe(III) were 7756, 1704, and 193 mg/L, respectively and solution pH was 3.0.Therefore, the concentrations of Ni(II), Co(II), and Mn(II) in the synthetic H2SO4 and HCl solutions employed in this work were kept at 7756, 1704, and 193 mg/L.The molar ratio of NaClO to Mn(II) was varied from 1 to 5 in both acidic synthetic solutions and the initial pH of solutions was fixed at 3. The reactions were performed with 400 rpm of stirring speed at 25 o C for 30 mins.Fig. 1 shows that the precipitation percentage of Mn(II) from the HCl solution remarkably increased to 100% as the molar ratio of NaClO to Mn(II) increased to 3. It was noticeable that the decrease in the concentration of Co(II) (approximately 4.7%) was observed when the molar ratio of NaClO to Mn(II) was higher than 4, owing to the oxidative precipitation of Co2O3, while the change in Ni(II) concentration was not observed.These results indicated that NaClO could be a potential agent for the selective separation of Co(II) over Ni(II) by the oxidative precipitation from HCl solution.Meanwhile, the precipitation percentage of Mn(II) from the H2SO4 solution was lower than that from the HCl solution at the same conditions.Namely, the precipitation percentage of Mn(II) was increased from 50 to 90% as the molar ratio of NaClO to Mn(II) was increased from 3 to 5 (see Fig. 2).The selective precipitation of Mn(II) over Co(II) and Ni(II) by NaClO solution was ascribed to the difference in the redox potentials of the metal ions in acidic solutions (See Table 2).The Pourbaix diagram indicates that variations in pH can influence the oxidation state of metal ions, which in turn can affect the redox reactions (Cook and Olive, 2012).For example, Ni(II) may be oxidized to Ni2O3 when the pH value is around 3.  These results indicated that chloride ions have a positive effect on the precipitation of Mn(II) compared to sulfate ions.Although the precipitation percentage of Mn(II) from H2SO4 solution was lower than that from HCl solution, no precipitates of Co(II) and Ni(II) were formed from H2SO4 solution.Therefore, H2SO4 solution is more effective in separating Mn(II) by oxidative precipitation than HCl solution on the basis of separation degree.Therefore, the effect of reaction temperature on the precipitation of Mn(II) from the H2SO4 solution was investigated.The reaction temperature was varied from 25 to 95℃ by fixing the molar ratio of NaClO to Mn(II) at 4. In Fig. 3, the precipitation percentage of Mn(II) reached approximately 100% when the reaction temperature was 60 ℃, while that of Co(II) and Ni(II) was negligible.Thus, the selective precipitation of Mn(II) over Co(II) and Ni(II) from two acidic solutions was achieved using NaClO as an oxidative precipitating agent.The best conditions for the precipitation of Mn(II) were determined to be at the molar ratio of 3 for NaClO to Mn(II), pH 3, 25 o C, and 30 mins for the HCl solution and at the molar ratio of 4 for NaClO to Mn(II), pH 3, 60℃, 30 mins for the H2SO4 solution.Compared to HCl solution, the precipitation percentage of Mn(II) from H2SO4 solution was lower.Among the metal ions, Mn(II) can be completely oxidized at suitable conditions from HCl solution and the separation factor between Mn(II) and the sum of Co(II) and Ni(II) from H2SO4 solution was higher than 60000, indicating that the separation effect is also obvious from H2SO4 solution even though a certain amount of Co(II) and Ni(II) were co-oxidized with Mn(II).

Effect of molar ratio of NaClO to Co(II) on the precipitation of Co(II) from HCl and H2SO4 solutions containing Ni(II)
The concentrations of Ni(II) and Co(II) in the filtrate after the separation of Mn(II) were determined to be 7701 and 1689 mg/L, respectively.The addition of NaClO solution during oxidative precipitation of Mn(II) caused a slight decrease in metal concentrations.Since the chemical properties of Co(II) and Ni(II) are rather similar, the complete separation of these two metals by precipitation might be difficult.The results in the previous section indicate that Co(II) could be selectively oxidized to Co2O3 over Ni(II) by NaClO from HCl solution (See Fig. 1).The oxidative precipitation reaction of Co(II) and Ni(II) with NaClO can be represented by Eqs. ( 3) and (4) (Speight, 2005).
To investigate the effect of the dosage of NaClO on the precipitation of Co(II) from H2SO4 and HCl solutions, the molar ratio of NaClO to Co(II) was varied from 4 to 12 and the initial pH, stirring speed, reaction temperature, and time were fixed at 3.0, 400 rpm, 25℃, and 30 mins, respectively.As represented in Figs. 4 and 5, the precipitation percentage of Co(II) from both HCl and H2SO4 filtrates significantly increased with the dosage of NaClO.In Fig. 4, the precipitation percentage of Co(II) in the HCl solution increased from 90% to 99% as the molar ratio of NaClO to Co(II) increased from 4 to 8 and no Ni(II) was precipitated.With the further increase of the molar ratio NaClO, there was no change in the precipitation percentage of Co(II), whereas 5% Ni(II) was precipitated.These results indicate that selective separation of Co(II) from the HCl solution containing Ni(II) is possible when the molar ratio of NaClO to Co(II) was lower than 8.The precipitation percentage of Co(II) by NaClO from H2SO4 solution was lower than that from HCl solution at the same experimental conditions (see Fig. 5).When the molar ratio of NaClO to Co(II) was 8, the precipitation percentages of Co(II) and Ni(II) from H2SO4 solution were 87% and 3%, respectively.These results agreed well with the reported data about the co-precipitation of Ni(II) and Co(II) when the molar ratio of NaClO was high (Nguyen and Lee, 20232).Therefore, a molar ratio of 6 for NaClO to Co(II) was selected for the oxidative precipitation of Co(II) from H2SO4 solution, at which conditions the precipitation percentages of Co(II) and Ni(II) were 84% and zero.To enhance the precipitation efficiency of Co(II) from the H2SO4 solution and improve its purity, the effect of several factors such as pH, the presence of chloride and sulfate anions, and temperature was investigated, which can significantly affect the redox potential of NaClO and the formation of metal complexes.

Effect of several factors on the precipitation of Co(II) from HCl and H2SO4 solutions containing Ni(II)
Oxidative precipitation reactions of Co(II) and Ni(II) with NaClO, Eqs. ( 3) and (4) show that hydrogen ions would be produced during the reaction and thus higher pH would enhance the precipitation of both ions.The basicity of ClO − ion is 3.3110 −7 and thus solution pH would affect the effective concentration of ClO − which can take part in the oxidative precipitation of Co(II) and Ni(II).To consider the effect of solution pH on the selective precipitation of Co(II), the pH of the H2SO4 solution was varied from 2 to 5. In these experiments, the molar ratio of NaClO to Co(II), stirring speed, reaction temperature, and time were fixed at 6, 400 rpm, 25℃, and 30 min, respectively.Fig. 6 indicates that the precipitation percentages of Co(II) and Ni(II) were constant at 84% and zero in these pH ranges.These results could be attributed to the insignificant changes in the chemical activity of NaClO and the speciation of the predominant species of both ions in the studied pH ranges.Therefore, pH solution of 3 was selected in further experiments and the effect of adding chloride and sulfate anions on the precipitation of Co(II) was studied.Some works have reported that addition of chloride or sulfate ion to H2SO4 solution would affect the oxidative precipitation of Co(II) and Ni(II) (Lin et al., 1998).In order to investigate these effects, varying amounts of NaCl and Na2SO4 were added to the H2SO4 solution containing Co(II) and Ni(II).Experiments were performed by fixing the molar ratio of NaClO to Co(II) at 6 under the following conditions: 400 rpm, initial solution pH 3, 30 mins, and 25 o C. Fig. 7a shows that the precipitation percentage of Co(II) significantly rose from 84 to 98% as NaCl concentration increased from 0 to 50 g/L and was constant with the further increase of NaCl concentration to 100 g/L.In these experimental ranges, the precipitation percentage of Ni(II) was below 2%.When the concentration of Na2SO4 in the solution was changed from zero to 50 g/L, there was no change in the precipitation percentages of both Co(II) and Ni(II) and addition of Na2SO4 did not affect the precipitation of both ions (see Fig. 7b).The enhancement in the oxidative precipitation of Co(II) in presence of chloride ion might be ascribed to the stronger tendency of Co(II) to form complexes with chloride ion compared to Ni(II).In these circumstances, the mole fraction of free Co(II) would be rapidly decreased with the concentration of chloride ion, resulting in an increase in the oxidation potential of Co(II).By Contrast, the complex formation constant of Ni(II) with chloride ion is small and the effect of adding chloride ion was not pronounced for the oxidative precipitation of Ni(II), while sulfate complexes with Co(II) and Ni(II) are insignificant.The complex formation constants of Co(II) and Ni(II) with chloride and sulfate ion are listed in Table 3.The difference in the oxidative precipitation behavior of Co(II) in chloride and sulfate solutions could be related to the formation of stable complexes of Co(II) with chloride ions such as CoCl + , CoCl2, CoCl3 -, and CoCl4 2-, which might enhance the oxidation of Co(II) to Co(III) (Lee and Oh, 2005).Table 3 shows that Co(II) has a stronger tendency to form complexes with chloride ions compared to Ni(II), and the formation of these complexes significantly reduces the concentration of free metal ion in the solution.In this case, the reduction potential of the free Co(II) ion would be significantly decreased, which can make the oxidation of the corresponding metal ion occur easily.
Our results show that separation of Co(II) by oxidative precipitation from Ni(II) in H2SO4 solution was possible but the precipitation percentage of Co(II) was about 84%.When NaCl was added to H2SO4 solution, the oxidative precipitation percentage of Co(II) was significantly improved.Therefore, further experiments were done by adding 50 g/L of NaCl to H2SO4 solution.In addition, further experiments were done by adding various amounts of NaCl and Na2SO4 to the HCl solution.In these experiments, the molar ratio of NaClO to Co(II) was fixed at 6. Fig. 8 shows that it was possible to completely separate Co(II) from Ni(II) in synthetic HCl solution when NaCl concentration was controlled to 6 g/L by fixing the molar ratio of NaClO to Co(II) at 8.However, the oxidation of Co(II) was not significantly affected by the addition of Na2SO4.Moreover, this figure shows that the addition of NaCl to HCl solution increased the oxidation of Co(II) via a change in cobalt speciation rather than a change in ionic strength.Our results clearly showed that addition of chloride ion to either H2SO4 or HCl solution showed a positive effect on the separation of Co(II) from Ni(II) by oxidative precipitation with NaClO.
The effect of reaction temperature on the precipitation of Co(II) with the addition of NaCl was considered.Reactions was carried out from 25 to 95℃ by fixing the molar ratio of NaClO to Co(II) at 6 under the following conditions: initial pH 3, 30 mins reaction time, 400 rpm stirring speed, and 50 g/L of NaCl addition.Fig. 9 shows that the precipitation percentage of Co(II) gradually increased as the reaction temperature increased and reached 99.6% at 95 o C, while 2% of Ni(II) was co-precipitated at 95℃.Ref: (Diebler, 1983;Liu et al., 2012) First, in order to precipitate Ni(OH)2, the pH of the filtrates was adjusted from 7 to 11 using NaOH solution and the experiments were done at 25 o C for 30 mins.According to Fig. 10, there was an insignificant difference in the precipitation percentage of Ni(II) between HCl and H2SO4 solutions and most of Ni(II) was precipitated at pH 11.Therefore, pH 11 was the optimum condition for the precipitation of Ni(OH)2.
Second, the precipitation experiments of NiC2O4 from the HCl and H2SO4 solutions were done by varying the molar ratio of Na2C2O4 to Ni(II) from 0.8 to 1.2.The initial pH of the solutions was kept at 3 and reactions were carried out at 25 o C for 60 mins with a stirring speed of 400 rpm.Fig. 11 clearly shows that there was an optimum molar ratio of Na2C2O4 at which the precipitation percentages of Ni(II) in both acid solutions were maximum.The maximum precipitation percentages of Ni(II) were 98.2% for HCl and 94.2% for H2SO4 solution when the molar ratios of Na2C2O4 to Ni(II) were 1.0 and 0.9, respectively.The decrease in the precipitation percentage of NiC2O4 when the molar ratios of Na2C2O4 to Ni(II) were higher than those values is related to the formation of soluble Ni(II) complexes like Ni(C2O4)2 2− (K = 10 3.05 ) and Ni(C2O4)3 4− (K=10 1.36 ) as represented in Eqs. ( 7) and ( 8) (Watters and DeWitt, 1960).These results indicated that it is difficult to completely precipitate NiC2O4 at 25 o C for 60 mins.Therefore, it might be said that precipitation of Ni(OH)2 is better than that of NiC2O4 on the basis of Ni(II) recovery from the filtrates.In this work, oxidative precipitation experiments were carried out to investigate the separation of Mn(II), Co(II), and Ni(II) from synthetic acidic solutions of H2SO4 and HCl.NaClO was employed as an oxidizing agent and Mn(II) and Co(II) were sequentially separated by precipitation of MnO2 and Co2O3, leaving Ni(II) in the filtrate.Our results indicated that there is a certain difference in the oxidative precipitation behavior of the three metal ions between HCl and H2SO4 solutions.In comparing the separation efficiency of the metal ions by oxidative precipitation, it is also important to consider the recovery and the purity of the precipitates thus obtained.The purity of the precipitates depends on the sensitivity of the oxidative precipitation reaction to the molar ratio of NaClO to metal ions.Table 4 lists the optimum conditions for the selective precipitation of Mn(II) and Co(II) and compares the recovery and purity of the precipitates obtained at the corresponding conditions from HCl and H2SO4 solutions.
The recovery percentage and purity of MnO2 and Co2O3 from HCl solution were higher than 99%.By contrast, the purity of the oxides of Mn(II) and Co(II) obtained from H2SO4 solution was only 77 and 91%, respectively.Therefore, it can be said that HCl solution is better than H2SO4 solution in separation than H2SO4 solution.At the same experimental conditions, the precipitation percentages of MnO2 and Co2O3 from HCl solution was higher than those from H2SO4 solution.Addition of NaCl to either HCl or H2SO4 solution enhanced the oxidative precipitation of Co(II) over Ni(II).Ni(II) in the final filtrate after separation of Mn(II) and Co(II) can be recovered as Ni(II) hydroxides by adjusting solution pH at 11.In terms of the recovery percentage and the purity of the powders recovered by oxidative precipitation, HCl solution is more effective than H2SO4 solution as a medium.Therefore, use of chloride based leaching solutions for the dissolution of some resources containing these three components is favorable on the light of the separation efficiency by oxidative precipitation.

Table 2 .
The reduction potential values of MnO2, Co2O3, and Ni2O3 from acidic solution at 25 o C