Spent Nickel Metal Hydride Research Articles

All-metal recovery from spent Ni-MH batteries based on electrolysis of sodium sulfate solution

A sustainable strategy for all-metal recycling from spent nickel metal hydride (Ni-MH) batteries was proposed, where H2SO4 and NaOH solutions by the electrolysis of Na2SO4 solution were used as the leaching agent and precipitation agent, respectively. The leaching of electrode materials was performed in mild condition, and more than 99% of the rare earths were firstly precipitated at pH 1.0 in the form of double sulphates, which were converted to hydroxides by reacting with NaOH. In the following purification steps, Al3+ and Fe3+ were deposited at pH 5.5, and Zn2+ and Mn2+ were extracted by saponified D2EHPA–kerosene from the leaching solution, and two hydroxide byproducts with industrial application were obtained. Ni2+ and Co2+ were precipitated at pH 9.5 with a total recovery rate of 97.5%. The recovery efficiency of Na2SO4 was up to 97% in total. This metal recovery approach produces considerable economic profit, without solid or liquid waste in the closed-loop process.

Selective Precipitation of Rare Earth Double Sulfate Salts from Industrial Ni–MH Battery Leachates: Impact of Downstream Processing on Product Quality

This work focuses on the recovery of rare earth elements (REEs = La, Ce, Nd, Pr) from spent nickel–metal hydride batteries by hydrometallurgical processing. The REEs were precipitated in the form of sodium-lanthanide double sulfate salts by adding Na2SO4 to a leach liquor prepared from industrially processed spent batteries. The objectives were to better understand the parameters driving the purity of the product and to identify the phases involved, as well as their crystallographic structure. The methodology included experiments performed in a 2 L reactor, thermodynamic calculations and product characterization. We confirmed that high REE precipitation yields (>95%) can be achieved under a wide range of hydrodynamic conditions. Furthermore, we demonstrated and quantified how appropriately washing the product allows for a significant reduction in nickel losses while maintaining control over REE product purity. Finally, using X-ray Diffraction analyses, it was established that REEs form a solid solution with a chemical formula (Na0.9K0.1)(La0.65Ce0.24Pr0.04Nd0.07)(SO4)2·H2O, which has not been reported so far.

Upcycling of nickel oxide from spent Ni-MH batteries as ultra-high capacity and stable Li-based energy storage devices

Due to the rapid expansion of portable electronics and electric vehicles market, the projected demand of rechargeable batteries is huge and may lead to shortage of critical minerals, especially nickel (Ni) metal. In the present study, we upcycled NiO material from spent Nickel–Metal hydride batteries (Ni-MH) as electrodes in Li-ion battery (LIB) and supercapacitor. Intriguingly, recycled NiO was applied successfully to develop sustainable LiNi0.5Mn1.5O4 (LNMO) cathode, delivering the promising capacity. As the recovered NiO-anode in LIBs, the device exhibited ultrahigh capacity of 1248 mAh g−1 at 0.1C, exhibiting excellent rate capability and cycle life in traditional carbonate-based electrolyte. The full cell with NMC cathode gave a high discharged capacity of 137.4 mAh/g. Addition to conventional electrolyte, a safe and non-flammable diglyme electrolyte using recovered NiO was investigated under identical condition. The maximum reversible capacity of 642 mAh g−1 at 0.1C was achieved along with good rate performance. Likewise, the NiO-based supercapacitor electrode delivered a maximum capacitance of 106C g−1 at 0.5 A g−1. Hence, upcycling of abundant NiO waste as low-cost electrode materials from end-of-life batteries paves the way for future generation of Li-based energy storage devices and sustainable supply of critical minerals and clean environment.

Selective separation of lanthanide group in spent NiMH battery acidic leaching solutions

A novel selective separation of lanthanide group (Ln) known as rare earth elements (REEs) from acidic leaching solutions of spent nickel metal hydride (NiMH) batteries was examined using precipitation at low pH with disodium phosphate, Na2HPO4, as precipitation agent. The acidic leaching solutions containing lanthanides (La, Ce, and Nd) and base metals (Ni, Cd, Mn, Fe, Al, and Zn) were from spent NiMH batteries subject to subcritical water extraction (SWE) process using 0.5 mol/L of HCl, HNO3, and H2SO4 solution, respectively. The effects of important parameters such as the molar ratio of lanthanides to phosphate ions, Ln/P, to form precipitates of lanthanide phosphates (Ln(PO4)), pH and temperature were investigated. The recovery efficiency of lanthanides increased when decreasing Ln/P, while increased with increasing pH and temperature. The decrease in the concentration of Ln remaining in HCl and HNO3 leaching solutions was proportional to the decrease in the P concentration in leaching solutions, while different results were found in the leaching solution of H2SO4 due to the formation of another insoluble lanthanide mineral, double sulfate (NaLn(SO4)2·H2O). This separation process demonstrated that the high-purity lanthanide group including La, Ce, and Nd from spent NiMH batteries could be obtained simply by adding phosphate ions at low pH. It is efficient, effective, and selective for critical rare metals recovery from spent NiMH batteries.

A systematic analysis of the costs and environmental impacts of critical materials recovery from hybrid electric vehicle batteries in the U.S.

SummaryCritical materials such as rare earth underpin technologies needed for a decarbonized global economy. Recycling can mitigate the supply risks created by the increasing demand and net import dependence, and enable a circular economy for critical materials. In this study, we analyze the feasibility and life-cycle impacts of recovering critical materials from spent nickel metal hydride batteries from hybrid electric vehicles in the U.S., accounting for stocks, battery scrappage, and end-of-life reverse logistics, given uncertain future availability scenarios. Our results show that the total collection and recycling costs depend strongly on future battery availability, with marginal costs exceeding marginal revenues when the availability of spent batteries declines. We quantify the potential of recycling to reduce primary imports, as well as the accompanying climate change and resource impacts. We explore the underlying reverse logistics infrastructure required for battery recycling and evaluate strategies for reducing associated capital investment risk.

Efficient Recovery of Rare Earth Elements and Zinc from Spent Ni-Metal Hydride Batteries: Statistical Studies.

Considering how important rare earth elements (REEs) are for many different industries, it is important to separate them from other elements. An extractant that binds to REEs inexpensively and selectively even in the presence of interfering ions can be used to develop a useful separation method. This work was designed to recover REEs from spent nickel–metal hydride batteries using ammonium sulfate. The chemical composition of the Ni–MH batteries was examined. The operating leaching conditions of REE extraction from black powder were experimentally optimized. The optimal conditions for the dissolution of approximately 99.98% of REEs and almost all zinc were attained through use of a 300 g/L (NH4)2SO4 concentration after 180 min of leaching time and a 1:3 solid/liquid phase ratio at 120 °C. The kinetic data fit the chemical control model. The separation of total REEs and zinc was conducted under traditional conditions to produce both metal values in marketable forms. The work then shifted to separate cerium as an individual REE through acid baking with HCl, thus leaving pure cerium behind.

Dry chlorination of spent nickel metal hydride battery waste for water leaching of battery metals and rare earth elements

An efficient leaching process was developed for nickel, cobalt, and the rare earth elements (REEs) from spent nickel metal hydride (NiMH) battery waste. The process involves dry chlorination with ammonium chloride in low temperature to produce water-soluble chlorinated compounds, followed by simple water leaching. The factors affecting the conversion and solubilization were studied, including the amount of ammonium chloride, residence time and temperature in dry chlorination, and solid to liquid ratio, time and temperature in water leaching. As a result, the dry chlorination process was found to produce ammonium and chloride containing products, depending on the temperature of the process: ammonium metal chlorides were produced in temperatures of 250–300 ℃, while increasing the temperature to 350 ℃ resulted in formation of metal chlorides. Overall, highest metal recoveries were achieved during 60 min residence time at a temperature of 350 °C, where ammonium is no longer present and ammonium metal chlorides and metal chlorides have formed. Water leaching was found to proceed rapidly, especially for REEs, and yields of 87% for Ni, 98% for Co, 94% for Ce, and 96% for La were attained during 60 min of leaching in room temperature. This study introduces a process, which is considered as an environmentally more benign alternative to traditional mineral acid leaching, resulting in high metal leaching efficiencies with neutral leachates, requiring no chemical-intensive neutralization steps in the following processing.

Recovery of rare earth elements from spent NiMH batteries using subcritical water extraction with citric acid

Spent nickel-metal hydride (NiMH) batteries contain high concentrations of rare earth elements (REEs), nickel (Ni), and cobalt (Co). Subcritical water extraction (SWE) process with citric acid as the leaching reagent was applied to spent NiMH batteries. The effects of acid concentration, solid to liquid ratio (S/L), reaction time, and reaction temperature were examined. Leaching efficiency increased with increasing acid concentration, reaction time and temperature, and decreased as S/L increased. Total of 90.85% of lanthanum (La), 88.84% of cerium (Ce), 90.85% of neodymium (Nd), 90.70% of Ni, and 91.62% of Co were leached out using 2.0 N of citric acid, S/L of 10 g/L, at 150 °C in 5 min. Selective precipitation of REEs was induced by the addition of diluted waste phosphoric acid (H3PO4) to the leaching solutions. At [H3PO4]:[REEs] of 10:1, pH 2.0, and 65 °C, 99.3% of La, 98.8% of Ce, and 99.0% of Nd were precipitated, which correspond to overall recovery of 90.2% of La, 87.8% of Ce, and 89.8% of Nd. The results showed that SWE using citric acid followed by selective precipitation with H3PO4 could effectively recover REEs from spent NiMH batteries.

Recovery of nickel, cobalt and rare-earth elements from spent nickel–metal-hydride battery: Laboratory tests and pilot trials

The recycling of nickel, cobalt and rare earths from spent nickel–metal-hydride batteries was investigated. Nickel and cobalt were recovered as a nickel–cobalt mixed sulfide, which can be used as an intermediate raw material in existing nickel refineries. Rare earths were recovered as double sulfates, which can be recycled into industrial materials. In the recovery of rare earths, yttrium, a heavy rare earth, could be precipitated and recovered by co-precipitation with a light rare-earth double sulfate, which was considered impossible by the double-sulfate precipitation method because of high solubility of yttrium double sulfate. On the other hand, the coprecipitation behavior of yttrium was found to be unstable at the pilot scale because the stirring force was weaker than that in the laboratory experiments. In this study, the optimum conditions for leaching, double-sulfate precipitation, and sulfide precipitation reactions were determined in a laboratory test and the good performance was demonstrated in a pilot test. Based on the results, the estimated metal yields were 87% nickel, 90% cobalt, 54% yttrium, 99% lanthanum and 99% cerium. The nickel and cobalt impurities in the double sulfate that were recovered by this process were 0.04% and 0.005%, respectively, and the rare-earth impurities in the nickel–cobalt mixed sulfide were low at 0.049% yttrium, 0.007% lanthanum and 0.003% cerium. This process combined with a precipitation method showed a high separation performance.

Pilot-Scale Lanthanide Precipitation from Sulfate-Based Spent Ni-MH Battery Leachates: Thermodynamic-Based Choice of Operating Conditions

Access to critical metals required for high-performance technologies, particularly, the light rare earth elements (REEs = La, Ce, Nd, Pr), has become a major challenge for import-dependent economies such as the European Union. In this regard, the recycling of spent nickel metal hydride (Ni-MH) batteries by hydrometallurgical processes can serve as an attractive secondary source of REEs. In such processes, precipitation of REEs from pregnant leach solutions (PLS) in sulfate media using Na2SO4 is often reported. However, little consideration is given as to whether and how sodium ions influence the precipitation efficiency and selectivity. This work focuses on a better understanding of the precipitation process by coupling pilot-scale (2 L) experiments on industrially sourced PLS containing 50 g/L of Ni and 17 g/L of REEs, with thermodynamic modeling, to assess the influence of temperature (25 °C < T < 60 °C) and the Na/REEs molar ratio (0.8:1 < Na/REEs < 3.2:1). Equilibria calculations were performed using OLI Systems Inc. software whose database covers rare earth sulfate compound properties and an accurate description of the aqueous electrolytes. Highly selective precipitation was obtained at 60 °C and for a Na/REEs molar ratio of 4:1. A lanthanide-alkali solid solution was identified by multianalytical characterization.

Selective recovery of rare earth elements (REEs) from spent NiMH batteries by two-stage acid leaching

The current study investigated selective recovery of rare earth elements (REEs) from spent nickel metal hydride (NiMH) batteries using two-stage acid leaching followed by precipitation at acidic pH. The first-stage conventional leaching using 2.5 M of waste phosphoric acid (H3PO4) at 25 °C for 60 min removed 60–100% heavy metals, such as Ni, Co, Zn and Cd, and converted more than 90% REEs into insoluble phosphate precipitates, REE(PO4). Subcritical water extraction (SWE) using mild concentration of sulfuric acid (H2SO4) in the second stage leached out all REEs. The leaching efficiency of REEs increased by increasing acid concentration, and 100% leaching efficiency of REEs, Ni, Co and Mn were achieved using 1 M of H2SO4, solid concentration of 20 g/L, at 125 °C for 30 min. As predicted by species distribution of metals at equilibrium, REEs in the leaching solution were selectively precipitated as NaREE(SO4)2·H2O at low pH (0.5–2) by the addition of NaOH solution, and separated from other metals (Ni, Co, Mn). Precipitation at pH 1 resulted in 82.59% of La, 90.75% of Ce, and 85.97% of Nd recovered as hexagonal rod crystals. This study provides an alternative process to selectively recover REEs from electronic waste (e-waste).

Process intensification for valuable metals leaching from spent NiMH batteries

Acid leaching is an essential unit in hydrometallurgy, yet it is time consuming and requires concentrated acids. Process intensification is examined for the leaching of valuable and rare earth elements (REEs), such as Ni, Co, La, Nd, and Ce, from spent nickel metal hydride (NiMH) batteries using subcritical water extraction (SWE) and microwave-assisted extraction (MAE). Results of SWE revealed that HCl performed better than H2SO4 and HNO3. Leaching efficiency increased as reaction temperature increased from 100 to 150°C, and it increased with HCl concentration. SWE was very effective using 0.5 N HCl, solid concentration of 20 g/L, initial pressure of 9.8 bar, 100°C for 5 min, and leaching efficiency of 99.85% for Ni, 91.72% for Co, 96.26% for La, 85.75% for Nd, and 84.37% for Ce was found, and the total energy consumption was 2,640 kJ. Leaching efficiency of MAE using 0.5 N HCl decreased slightly when heating rate increased from 30 to 70°C/min, and it increased as reaction time increased from 5 to 30 min. Compared with SWE and conventional leaching, MAE was the most energy-efficient, and total of 874.1 kJ was required to reach 85.93% for Ni, 77.60% for Co, 64.04% for La, 75.01% for Nd, and 57.17% for Ce, respectively under experimental conditions of 1 N of HCl, solid concentration of 20 g/L, 100°C for 5 min. Novel acid leaching methods showed promising potential for metals recovery from e-waste.

A review on the recovery of metal values from spent nickel metal hydride and lithium-ion batteries

A review on the recovery of metal values from spent nickel metal hydride and lithium-ion batteries

Nickel recovery from spent nickel-metal hydride batteries using LIX-84I-impregnated activated charcoal

Nickel (Ni) is a ferromagnetic and corrosion-resistant metal, whose demand has risen remarkably in recent years. Depletion of its sulphide and laterite ores has necessitated the need to find secondary sources of the metal. Spent nickel metal hydride (Ni-MH) batteries are considered to be possible sources of Ni recovery and recycling, and have been the object of some scientific investigations. This recovery is also important from environmental perspective, as disposal of the waste batteries constitute a significant release of Ni to the environment. However, previous studies have focused only on solvent extraction processes for Ni uptake from the battery leachates, neglecting the possibility of adsorption-based methods. Here, we optimized and achieved 99.6 % Ni uptake from the leachate of Ni-MH battery cathode, via the use of an adsorbing composite generated from 2-hydroxy-5-nonylacetophenone oxime (LIX-84I) and activated charcoal. Desorption with HCl led to the production of NiCl2.6H2O of 99.9 % purity, useful as a versatile industrial catalyst and for electroplating Ni onto other metals. The adsorption was endothermic and spontaneous, following a pseudo-second order kinetics while fitting best to Temkin isotherm model (R2 = 0.9913). This is the first report of an adsorption-based method optimizing Ni recovery from Ni-MH battery wastes.

Waste of batteries management: Synthesis of magnetocaloric manganite compound from the REEs mixture generated during hydrometallurgical processing of NiMH batteries

In the present study, rare earth elements (REEs, i.e., La, Ce, Nd, and Pr) were hydrometallurgically recovered in oxalate form with presence of very low concentration of Co, Al, Zn and Ni from solution after processing of spent Nickel metal hydride (Ni-MH) batteries. The recovered mixture was used as alternative source in the synthesis of magnetocaloric materials. In this study, a manganite sample with general formula ABO3 was selected to be prepared since it is relatively easy to synthesize and is tuneable by adjustment of the doping concentration. The conventional solid-state reaction method was used to prepare an orthorhombic structure of manganite with presence of REE2O3 and MnO2 as secondary phases reported from x-ray pattern at room temperature. The thermomagnetic measurements showed a PM to FM transition at 184 K in a 0.01 T magnetic field that shifts to 194 K by increasing the magnetic field to 1.8 T. The magnetocaloric properties were determined by calculating the isothermal entropy change and directly measuring the adiabatic temperature change. A reversible magnetocaloric effect was observed.

A multi-analytical methodology for the characterization of industrial samples of spent Ni-MH battery powders

A multi-analytical methodology is implemented to characterize several sieving fractions of industrial samples of Black Mass (BM) powders originating from the thermo-mechanical treatment of cylindrical and prismatic-type spent nickel metal-hydride (Ni-MH) batteries. Elemental analyses of 17 elements (including C and O) indicate that the elemental composition of the powders (greater than93 %wt) does not depend on the battery type nor on the sieving fraction. XRD analyses evidence several phases (including Ni, NiO, CeO2 and C) but their quantification is not possible. Beyond these standard characterisations, magnetic susceptibility measurements demonstrate that the amount of metallic nickel versus nickel oxide increases with the sieving fraction, and that powders from prismatic-type batteries contain twice as much metallic nickel than cylindrical ones. Thanks to statistical analysis (based on clustering algorithms) of an electron probe µ-analysis (EPMA) compositional map, the complete methodology allows us to propose a full phase distribution for the BM particles. Three types of particles are identified and quantified. They originate from the partial oxidation of the battery components (anode active mass, anode current collector, cathode active mass and cathode current collector). The whole picture highlights the joint importance of battery ageing mechanisms, thermal deactivation and BM sieving steps on powder composition.

An alternative approach to reclaim spent nickel–metal hydride batteries

AbstractSpent nickel‐metal hydride (NiMH) batteries contained some valuable metals which are worthy of recovery. This study focuses on the reclamation of spent NiMH batteries using an alternative approach, thermal separation process (TSP). Spent NiMH batteries were mixed with cullet, limestone, and dolomite. The mixture was heated to 1,450°C and cooled to room temperature without forced convection. During the TSP, the output‐materials were divided into slag, ingot, and flue gas according to the boiling points and densities of metals. Results show that metals having a relatively low boiling points, such as Cd, Hg, Pb, and Zn, were mostly vaporized into flue gas. Among the metals retained in molten materials, Co, Cu, Fe, and Ni predominately went into the bottom layer and formed the ingot; on the contrary, Al, Ca, Mg, and Si stayed in the upper layer and formed slag. Therefore, the main components of slag were Si (316,000 mg/kg), Ca (289,000 mg/kg), and Mg (32,400 mg/kg) and the major crystalline phases were CaSiO3, CaMgSi2O6, and Ca2MgSi2O7. The slag was pended as non‐hazardous materials and could be reused as road pavement or building materials. The ingot had high levels of Ni (408,000 mg/kg), Fe (349,000 mg/kg), and Co (55,400 mg/kg) and could act as an additive in stainless steel manufacturing process.

Leaching Mechanisms of Industrial Powders of Spent Nickel Metal Hydride Batteries in a Pilot-Scale Reactor.

In view of a sustainable recycling process, the leaching mechanisms of nickel and rare-earth elements (REEs) contained within industrial samples of spent nickel metal hydride battery powders were investigated in HCl and H2 SO4 , under mild temperature (25-60 °C) and pH (3-5.5). First, in-depth characterization of the heterogeneous battery powder was carried out with powder XRD, SEM, electron probe microanalyzer wavelength-dispersive spectroscopy (EPMA-WDS) quantitative analyses of individual particles, and inductively coupled plasma optical emission spectrometry (ICP-OES) elemental analysis. An unusual result is the identification of particles that exhibit a core-shell structure, which is related to anode active mass aging mechanisms. Then, a leaching study in a 10 L pilot-scale reactor demonstrated the selective dissolution of REEs, with respect to nickel, at pH 3, which is attributed to 1) the kinetic inhibition of nickel metal dissolution, and 2) the specific core-shell structure of aged mischmetal particles. Furthermore, the use of H2 SO4 led to coprecipitation of lanthanide-alkali double sulfates and nickel salts.

Symmetric All-Quinone Aqueous Battery

Lithium-ion batteries continue to dominate portable electronics and electric vehicles markets for its high energy density/specific energy.1However, combustible electrolytes and expensive cathodes pose safety and cost concerns to consumers.2 Further cost reduction is expected to be limited after decades of development.3Next generation energy storage devices call for safe, cheap, resource-abundant and flexible batteries.2, 4 Rechargeable batteries with cost-effective redox active materials and aqueous electrolytes can potentially meet these requirements.5 Aqueous battery chemistries include lead-acid batteries, nickel-metal hydride aqueous batteries, and emerging aqueous lithium/sodium-ion batteries, but they all have their Achilles’ heels.6 - 12 Here we report a symmetric all-quinone aqueous battery based entirely on Earth-abundant elements that uses a naturally-occurring dye as the redox-active material in both positive and negative electrodes. We demonstrate a symmetric all-quinone cell with 1.04 V of open circuit voltage, 163 mAh/g of capacity, and 100 cycles at 10C with 100% of depth of discharge. The use of the same quinone in a symmetric setup expands the repertoire of inexpensive redox active materials for aqueous rechargeable batteries, and the simple cell design will enable optimizations toward safe, cheap, lightweight, and flexible electronics in the future. Natural abundance and cheap commercial source promise its low cost when produced at large scale. In addition, we demonstrate that other fused quinone derivatives can also be used for symmetric quinone-acid batteries. Further optimization of fused quinones with improved reduction potential and stability, and cell engineering of electrode composition and morphology can further improve the performance of the battery. References Zubi, G.; Dufo-López, R.; Carvalho, M.; Pasaoglu, G., The lithium-ion battery: State of the art and future perspectives. Renewable and Sustainable Energy Reviews 2018, 89, 292-308. Armand, M.; Tarascon, J. M., Building better batteries. Nature 2008, 451, 6. Hsieh, I.-Y. L.; Pan, M. S.; Chiang,Y.-M.; and Green, W. H. Learning only buys you so much: practical limits on battery price reduction Applied Energy 2019, 239, 218. Larcher, D.; Tarascon, J. M., Towards greener and more sustainable batteries for electrical energy storage. Nat. Chem. 2015, 7(1), 19-29. Beck, F.; Rüetschi, P., Rechargeable batteries with aqueous electrolytes. Electrochimica Acta 2000, 45, 16. Chen, H. Y.; Li, A. J.; Finlow, D. E., The lead and lead-acid battery industries during 2002 and 2007 in China. Journal of Power Sources 2009, 191(1), 22-27. van der Kuijp, T.; Huang, L.; Cherry, C. R., Health hazards of China's lead-acid battery industry: a review of its market drivers, production processes, and health impacts. Environmental Health 2013, 12(61), 10. Ruetschi, P., Aging mechanisms and service life of lead–acid batteries. Journal of Power Sources 2004, 127(1-2), 33-44. Zou, X., Kang, Zongxuan, Shu, Dong, Liao, Yuqing, Gong, Yibin, He, Chun, Hao, Junnan, Zhong, Yayun, Effects of carbon additives on the performance of negative electrode of lead-carbon battery. Electrochimica Acta 2015, 151, 89-98. Rodrigues, L. E. O. C.; Mansur, M. B., Hydrometallurgical separation of rare earth elements, cobalt and nickel from spent nickel–metal–hydride batteries. Journal of Power Sources 2010, 195(11), 3735-3741. Ying, T. K.; Gao, X. P.; Hu, W. K.; Wu, F.; Noréus, D., Studies on rechargeable NiMH batteries. Int. J. Hydrogen Energy 2006, 31(4), 525-530. Kim, H.; Hong, J.; Park, K. Y.; Kim, H.; Kim, S. W.; Kang, K., Aqueous rechargeable Li and Na ion batteries. Chem. Rev. 2014, 114(23), 11788-827. Figure 1

Two stage leaching process for selective metal extraction from spent nickel metal hydride batteries

In this study, a closed loop two-stage leaching process has been experimentally and theoretically established for the selective dissolution of metals from electrode material of spent Ni-MH batteries. In the first step, baking parameters, such as temperature, acid concentration, and duration were optimized as 300 °C, 2 mL H2SO4 for 1.5 g of cathode powder, 90 min. The thermodynamic feasibility of the process ascertained the spontaneous formation of water-soluble sulfates of nickel, zinc, and rare earth elements during the acid baking. Post-baking, a two-stage leaching was performed for leaching of different metals present in the baked material. In first stage leaching with water, 91.73% Ni, 94% Zn, 91% rare earth elements were extracted with a low recovery of 36.5% Co, 22.1% Fe, 23.5% Mn at 75 °C in 1 h. Owing to the low recovery of Co and Mn, 2nd stage leaching was performed using NaHSO3 in H2SO4 at 95 °C for 1 h, improving extraction to Co (55%) and Mn (74%). The overall recovery of the metals was found to be 98.2% Ni, 91.4% Co, 98% Zn, 97.8% Mn and 96% rare earth elements. The two-stage process (baking and leaching) outperforms direct sulfuric acid leaching of cathode powder due to lower acid consumption and its selectivity of metals.