Hydrometallurgical refining plays a critical role in recovering valuable metals from black mass, the shredded material derived from spent lithium-ion batteries. This process involves several stages, including leaching, precipitation, solvent extraction, electrowinning, and crystallization, to isolate and purify metals such as lithium, cobalt, nickel, and manganese. Each step is optimized to achieve high purity while minimizing environmental impact and operational costs. Commercial recycling operations employ these techniques to meet the growing demand for battery materials in a sustainable manner.
The first stage in hydrometallurgical refining is leaching, where black mass is dissolved in acidic or alkaline solutions to extract metals into a liquid phase. Sulfuric acid is commonly used due to its effectiveness and cost efficiency, often combined with hydrogen peroxide as a reducing agent to enhance metal dissolution. The resulting leachate contains a mixture of metal ions, including lithium, cobalt, nickel, and manganese, alongside impurities such as aluminum, copper, and iron.
Following leaching, precipitation is employed to remove impurities and selectively recover certain metals. By adjusting pH levels, impurities like iron and aluminum can be precipitated as hydroxides and filtered out. For instance, raising the pH to around 3-4 with sodium hydroxide or lime causes iron and aluminum to form insoluble compounds, leaving cobalt, nickel, and manganese in solution. Further pH adjustments can selectively precipitate manganese as manganese hydroxide, separating it from cobalt and nickel. Precipitation is a cost-effective method but may require additional steps to achieve high purity.
Solvent extraction is then used to separate cobalt, nickel, and other valuable metals from the leachate. This technique relies on organic extractants that selectively bind to specific metal ions. For cobalt recovery, phosphinic acid extractants like Cyanex 272 are highly effective, allowing cobalt to be separated from nickel in multiple extraction stages. The loaded organic phase is then stripped using hydrochloric or sulfuric acid, yielding a concentrated cobalt solution. Similarly, nickel can be extracted using oxime-based reagents such as LIX 84, followed by stripping to produce a pure nickel solution. Solvent extraction offers high selectivity and purity but involves complex reagent management and organic waste disposal.
Electrowinning is the next step for producing high-purity metal deposits from purified solutions. In this electrochemical process, metal ions are reduced onto cathodes, forming pure metal sheets or powders. Cobalt and nickel are commonly recovered via electrowinning, with cobalt typically deposited from sulfate solutions at optimized current densities and pH levels. Nickel electrowinning follows a similar approach, often requiring chloride-based electrolytes for efficient deposition. The resulting metals can achieve purity levels exceeding 99.9%, suitable for battery-grade applications. Electrowinning is energy-intensive but provides a direct route to high-purity metals.
Lithium recovery requires different approaches due to its high solubility in aqueous solutions. After removing other metals, lithium remains in the solution and is concentrated through evaporation or membrane filtration. Crystallization is then employed to precipitate lithium as lithium carbonate or lithium hydroxide. Adding sodium carbonate to the lithium-rich solution forms lithium carbonate, which is filtered and dried. Alternatively, lime can be used to produce lithium hydroxide. Both compounds must meet strict purity benchmarks, with battery-grade lithium carbonate requiring less than 0.2% impurities. Crystallization is a straightforward method but demands careful control of temperature and concentration to avoid co-precipitation of impurities.
Process flowsheets vary depending on the target metals and desired purity levels. A typical flowsheet for black mass recycling might include leaching with sulfuric acid, impurity removal via precipitation, cobalt and nickel separation by solvent extraction, electrowinning for metal recovery, and lithium crystallization. Each step is interconnected, with intermediate filtrations and solution treatments to ensure optimal performance. Reagent choices are critical, balancing cost, efficiency, and environmental considerations. For example, using lime for pH adjustment is economical but generates large volumes of gypsum waste, whereas sodium hydroxide offers cleaner operation at higher costs.
Environmental and economic trade-offs are inherent in hydrometallurgical refining. The process generates acidic waste streams, spent organic solvents, and solid residues that require careful management. Neutralization and wastewater treatment are necessary to comply with environmental regulations. Recycling reagents and recovering by-products can improve sustainability but add complexity to operations. Economically, the process is sensitive to metal prices and reagent costs, with cobalt and nickel recovery being more profitable than lithium due to their higher market values. However, advances in process efficiency and scale are reducing costs over time.
Commercial recycling operations demonstrate the practical application of these techniques. Companies like Umicore and Li-Cycle employ hydrometallurgical refining to recover metals from black mass at industrial scales. Umicore’s integrated process combines pyrometallurgical pretreatment with hydrometallurgical refining to achieve high recovery rates for cobalt, nickel, and lithium. Li-Cycle uses a purely hydrometallurgical approach, focusing on maximizing lithium yield through advanced solvent extraction and crystallization. These operations highlight the importance of tailored flowsheets to accommodate varying feedstock compositions and market demands.
Purity benchmarks are stringent for battery-grade materials. Cobalt and nickel must exceed 99.5% purity, with strict limits on impurities like sulfur and iron. Lithium carbonate and hydroxide require even higher purity, often above 99.9%, to ensure optimal battery performance. Achieving these benchmarks demands precise control over each refining stage, from leaching to final crystallization. Analytical techniques such as inductively coupled plasma spectroscopy are used to verify purity levels throughout the process.
In conclusion, hydrometallurgical refining of black mass is a multifaceted process that enables the recovery of high-purity lithium, cobalt, nickel, and manganese for reuse in battery production. By leveraging precipitation, solvent extraction, electrowinning, and crystallization, recyclers can transform spent batteries into valuable raw materials. While environmental and economic challenges persist, ongoing advancements in process optimization and waste management are enhancing the sustainability and viability of metal recovery. Commercial operations continue to refine these techniques, ensuring a steady supply of critical materials for the growing energy storage industry.