The recovery of lithium from pyrometallurgical slag byproducts has emerged as a critical process in battery recycling, particularly as demand for lithium continues to rise. Smelting processes for battery recycling often generate slag rich in lithium, but extracting it efficiently requires careful consideration of slag chemistry, leaching methods, and purification steps. This article examines the technical and economic aspects of lithium recovery from these slags, focusing on slag modification, leaching optimization, and silicon removal challenges.

Pyrometallurgical recycling of lithium-ion batteries typically involves high-temperature smelting to recover metals such as cobalt, nickel, and copper. Lithium, however, tends to report to the slag phase due to its high oxygen affinity. The composition of this slag varies depending on the feed material and processing conditions but often includes oxides of lithium, silicon, aluminum, and calcium. To enhance lithium recovery, slag chemistry must be modified to increase lithium solubility in subsequent leaching steps.

The addition of fluxing agents such as calcium oxide or sodium carbonate during smelting can alter slag properties, reducing viscosity and promoting lithium partitioning into more leachable phases. Research indicates that slags with higher basicity, achieved through controlled flux additions, improve lithium extraction rates. For example, slags with a basicity index (CaO/SiO₂ ratio) between 0.8 and 1.2 have shown higher lithium recoveries in leaching experiments. The formation of lithium aluminosilicates, which are resistant to acid leaching, can be minimized by optimizing the smelting atmosphere and cooling rate.

Once the slag is properly conditioned, hydrometallurgical methods are employed to extract lithium. Acid leaching, typically using sulfuric or hydrochloric acid, is the most common approach. The efficiency of this process depends on acid concentration, temperature, and leaching duration. Studies demonstrate that sulfuric acid concentrations between 1.5 and 2.5 M at temperatures of 60 to 80°C yield lithium recoveries exceeding 80%. Alkali leaching, using sodium carbonate or sodium hydroxide, is an alternative, particularly for slags with high silica content. However, alkali methods often require higher temperatures and longer processing times, increasing operational costs.

A major challenge in lithium recovery from slag is the presence of silicon, which complicates downstream processing. During acid leaching, silicon dissolves and can reprecipitate as gelatinous silica, impeding filtration and reducing lithium purity. Several strategies have been developed to mitigate this issue. Pre-treatment steps, such as roasting the slag with sodium salts, can convert silicon into insoluble silicates, reducing its interference during leaching. Alternatively, controlled pH adjustment during leaching can minimize silica dissolution. Another approach involves adding flocculants or filtration aids to manage silica gel formation in leach solutions.

Purification of the lithium-rich leach solution is necessary to isolate lithium in a usable form. Solvent extraction, precipitation, and ion exchange are common methods. Precipitation using sodium carbonate is widely used due to its simplicity and effectiveness, producing lithium carbonate of battery-grade purity. However, residual impurities such as calcium and magnesium must be carefully controlled through pH adjustment and multiple washing steps.

The economic viability of lithium recovery from slag depends on several factors, including slag lithium content, processing costs, and market prices. Slags with lithium concentrations above 2% Li₂O are generally considered economically feasible for extraction. Integration with existing smelter operations can reduce costs by utilizing existing infrastructure for slag handling and leaching. However, additional investments in purification and waste management may be required.

A key advantage of this approach is its compatibility with established pyrometallurgical recycling processes. By recovering lithium from slag, recyclers can achieve a more complete material recovery loop, improving overall sustainability. However, the process must compete with direct recycling methods that recover lithium without smelting. The choice between pyrometallurgical and hydrometallurgical routes depends on feedstock composition, scale, and regional regulatory frameworks.

In conclusion, lithium extraction from pyrometallurgical slag is a technically feasible but complex process requiring optimization at multiple stages. Slag chemistry modification, efficient leaching, and effective silicon management are critical to achieving high recovery rates. While economic viability depends on market conditions and process integration, this method offers a promising pathway to enhance lithium supply from recycled batteries. Continued research into slag conditioning and purification techniques will further improve the efficiency and scalability of this approach.