Hydrometallurgical recycling of nickel-metal hydride (NiMH) batteries presents a critical pathway for recovering valuable rare earth elements (REEs), nickel, and cobalt. Unlike lithium-ion batteries, NiMH batteries contain significant quantities of lanthanides, making their recycling economically and environmentally significant. The process typically involves leaching, purification, and separation stages, each optimized for maximum recovery of target metals while minimizing waste generation.

Leaching serves as the primary step for dissolving metals from spent NiMH batteries. Sulfuric acid is the most common leaching agent, often combined with hydrogen peroxide as an oxidizing agent to enhance dissolution efficiency. Typical leaching conditions involve concentrations of 1-3 M sulfuric acid at temperatures between 60-90°C, achieving over 90% recovery rates for nickel, cobalt, and REEs. Hydrochloric acid is also effective, particularly for REE extraction, but poses greater challenges in handling and downstream processing. Organic acids such as citric or acetic acid offer milder, more environmentally friendly alternatives, though with slightly lower leaching efficiencies.

Selective leaching can be employed to separate nickel and cobalt from REEs. Adjusting pH and redox potential allows preferential dissolution of nickel and cobalt in the initial stage, leaving REEs in the solid residue for subsequent processing. This approach reduces the complexity of later separation steps. For instance, maintaining a pH below 2 with sulfuric acid dissolves nickel and cobalt effectively, while REEs remain insoluble until subjected to stronger acidic conditions or alternative reagents.

Following leaching, solution purification removes impurities such as iron, aluminum, and residual organic compounds. Precipitation is a common method, where pH adjustment causes impurities to form insoluble hydroxides. For example, raising the pH to 3-4 precipitates iron and aluminum while keeping nickel, cobalt, and REEs in solution. Solvent extraction is another powerful technique, particularly for separating nickel and cobalt. Extractants like di-(2-ethylhexyl) phosphoric acid (D2EHPA) selectively bind cobalt over nickel, allowing their separation through multiple extraction stages.

REE recovery requires specialized approaches due to their chemical similarities. Solvent extraction remains the dominant method, with extractants such as PC-88A or Cyanex 272 showing high selectivity for lanthanides. A typical flowsheet involves multiple extraction and stripping stages to isolate individual REEs. For instance, neodymium and praseodymium can be separated from other lanthanides using selective stripping with hydrochloric acid at varying concentrations. Alternatively, precipitation with oxalic acid produces high-purity REE oxalates, which are subsequently calcined to oxides.

Comparison with lithium-ion battery recycling reveals key differences in hydrometallurgical approaches. Lithium-ion batteries prioritize lithium, cobalt, and nickel recovery, often using similar acids but with different separation sequences. Cobalt is typically the highest-value target in lithium-ion systems, whereas REEs drive NiMH recycling economics. Additionally, lithium-ion recycling must address organic electrolytes and graphite, which are less prominent in NiMH systems. Both processes share challenges in managing fluorine and phosphorus compounds from electrode materials.

Market drivers for NiMH battery recycling stem from supply security and environmental regulations. REEs, particularly neodymium and dysprosium, are critical for permanent magnets in electric vehicles and wind turbines. China dominates global REE production, creating incentives for alternative sources through recycling. The European Union and United States have classified certain REEs as critical materials, further promoting recovery initiatives. Nickel and cobalt prices also influence recycling viability, with market volatility driving interest in secondary sources.

Economic feasibility depends on process efficiency and metal prices. Hydrometallurgical methods generally offer lower energy consumption compared to pyrometallurgical routes but require careful management of reagent costs and waste streams. Optimizing leaching conditions and minimizing chemical usage improve profitability. For example, recycling one ton of NiMH batteries can yield approximately 100-150 kg of REE oxides, alongside significant nickel and cobalt quantities, depending on battery composition.

Environmental considerations include wastewater treatment and emissions control. Neutralization of acidic effluents generates sulfate or chloride salts, requiring proper disposal or further treatment. Closed-loop systems that regenerate leaching agents are under development to reduce chemical consumption. Life cycle assessments indicate hydrometallurgical recycling can lower the carbon footprint of battery production by reducing primary mining demands.

Future advancements may focus on improving selectivity and reducing process steps. Ionic liquids and deep eutectic solvents are emerging as alternative leaching agents with potential for higher metal specificity. Membrane-based separation technologies could replace solvent extraction in some applications, simplifying flowsheets. Automation and real-time monitoring may enhance process control, ensuring consistent recovery rates.

In summary, hydrometallurgical recycling of NiMH batteries provides a technically viable route for recovering rare earth elements, nickel, and cobalt. The process leverages well-established leaching and separation techniques while adapting to the unique composition of NiMH systems. Market and regulatory pressures continue to drive innovation in this field, positioning battery recycling as a key component of sustainable material supply chains.