Nickel-based batteries, particularly nickel-cadmium (Ni-Cd) and nickel-metal hydride (Ni-MH) systems, have been widely used in consumer electronics, industrial applications, and early-generation electric vehicles. As these batteries reach end-of-life, efficient recycling methods are critical to recover valuable metals like nickel, cadmium, and rare-earth elements while minimizing environmental impact. Two dominant industrial-scale recycling approaches exist: pyrometallurgical and hydrometallurgical processes. Each method has distinct advantages, limitations, and economic considerations.

Pyrometallurgical recycling involves high-temperature treatment to extract metals from spent nickel-based batteries. The process typically begins with mechanical pretreatment, including shredding and sorting to separate battery components. The material is then fed into a high-temperature furnace, often a rotary kiln or electric arc furnace, where organic materials burn off, and metals are reduced to their metallic form. Nickel and iron form an alloy, while cadmium volatilizes and is recovered from the off-gas through condensation. Rare-earth elements in Ni-MH batteries report to the slag phase, requiring additional processing for recovery. Pyrometallurgy achieves high metal recovery rates, with nickel recovery exceeding 90% and cadmium recovery rates above 95%. The process is robust against varying feed compositions and can handle mixed battery streams. However, energy consumption is significant, ranging between 3-7 kWh per kilogram of processed batteries, and emissions control systems are necessary to capture hazardous fumes.

Hydrometallurgical recycling employs aqueous chemistry to dissolve and separate battery metals. After mechanical pretreatment, the active materials undergo leaching with acids such as sulfuric or hydrochloric acid. Selective precipitation, solvent extraction, or electrowinning then recover individual metals. For Ni-Cd batteries, cadmium can be selectively precipitated as cadmium hydroxide or recovered via electrowinning. Nickel is typically recovered as nickel sulfate or nickel metal through similar methods. Rare-earth elements from Ni-MH batteries require more complex separation due to their chemical similarity, often involving multi-stage solvent extraction with reagents like PC-88A or Cyanex 272. Hydrometallurgical processes achieve high purity outputs, with nickel and cadmium purity levels reaching 99.5% and 99.9%, respectively. The method offers better rare-earth recovery compared to pyrometallurgy, with reported yields of 80-90% for elements like lanthanum, cerium, and neodymium. However, the process generates acidic wastewater requiring neutralization and produces secondary waste streams like iron-rich residues.

Environmental regulations significantly influence recycling operations. The Basel Convention classifies nickel-cadmium batteries as hazardous waste, mandating controlled transboundary movements. The European Union's Battery Directive sets collection and recycling efficiency targets, requiring at least 75% by weight of Ni-Cd batteries to be recycled, with 95% cadmium recovery. In the United States, the Resource Conservation and Recovery Act regulates cadmium-containing waste, while the Toxic Substances Control Act governs cadmium handling. These regulations increase compliance costs but drive innovation in emission control and waste treatment technologies. Hydrometallurgical plants must implement water treatment systems to remove heavy metals before discharge, while pyrometallurgical facilities require scrubbers and baghouse filters to capture particulate matter and volatile cadmium.

Economic feasibility depends on metal prices and process scale. Nickel price volatility directly impacts recycling profitability, with break-even points typically requiring nickel prices above $15,000 per metric ton. Cadmium's declining market due to environmental concerns reduces its economic contribution, though it remains critical for regulatory compliance. Rare-earth elements from Ni-MH batteries can improve economics when prices are high, but separation costs often outweigh their value. Pyrometallurgical plants require capital investments of $50-100 million for medium-scale operations but benefit from lower operational complexity. Hydrometallurgical facilities have lower initial costs but higher reagent and labor expenses. Transportation costs for spent batteries often determine regional recycling viability, with economic radii typically under 500 kilometers for pyrometallurgy and under 300 kilometers for hydrometallurgy due to differing bulk handling requirements.

Closed-loop supply chains are developing for nickel-based battery recycling. Some manufacturers operate take-back programs where recycled nickel is directly reused in new battery production. Automotive manufacturers have established partnerships with recyclers to process hybrid vehicle Ni-MH batteries, with some achieving over 70% closed-loop nickel utilization. However, rare-earth element recycling remains limited due to technical challenges and insufficient collection volumes. The lower energy requirements of recycling compared to primary production present a compelling case, with recycled nickel requiring 85% less energy than mined nickel.

Comparisons with lithium-ion battery recycling reveal fundamental differences. Lithium-ion systems prioritize cobalt and lithium recovery, while nickel-based systems focus on nickel and cadmium. Pyrometallurgy dominates nickel-based battery recycling but faces competition from hydrometallurgy in lithium-ion systems due to the higher value of cobalt. Lithium-ion recycling typically achieves lower metal recovery rates for non-cobalt elements, whereas nickel-based systems maintain high recovery efficiencies across target metals. Environmental concerns differ as well, with nickel-based battery recycling primarily addressing cadmium emissions, while lithium-ion recycling focuses on organic solvent and fluorine management.

Future developments may improve nickel-based battery recycling economics and efficiency. Enhanced sorting technologies could increase feedstock purity, while novel leachants might reduce hydrometallurgical processing costs. Slag treatment innovations could make rare-earth recovery from pyrometallurgical residues more viable. Regulatory pressures will likely increase recycling targets, particularly for rare-earth elements in Ni-MH batteries. However, the declining market share of nickel-based batteries in favor of lithium-ion systems may reduce investment in recycling infrastructure, making scale and feedstock availability ongoing challenges.

The choice between pyrometallurgical and hydrometallurgical recycling depends on regional factors, including regulatory frameworks, energy costs, and available infrastructure. Pyrometallurgy offers simplicity and high throughput for nickel and cadmium recovery, while hydrometallurgy provides better rare-earth recovery and higher purity products. Both methods will remain essential for managing the legacy stock of nickel-based batteries while supporting circular economy principles in the battery industry.