Key Takeaways & Executive Findings
- •• • Mechanical strength of solidified lithium slag reached 3.48–8.25 MPa under full slag conditions; optimized composite activator and multi-stage thermal assistance increased strength by 137.07%, enabling structural applications without cement addition. • • Average heavy metal immobilization rate improved from 97.26% to 99.77%, with leachate concentrations below regulatory limits in acidic (pH 3), neutral (pH 6.5), alkaline (pH 10), high-salt (0.1 M NaCl), acid rain, and leachate simulations, ensuring environmental safety. • • The process promotes formation of Si-O-Al skeletal structures, reducing structural defects and enhancing durability; this mechanism underpins both mechanical performance and long-term metal retention. • • Solidification cost is 185–200 CNY per ton of slag, significantly lower than conventional cement-based methods, with low energy consumption and no high-temperature calcination, offering a scalable and economically viable solution for lithium slag management.
Abstract
The rapid expansion of lithium battery industries has elevated lithium resources to strategic importance, yet lithium extraction generates 8–10 tons of slag per ton of lithium salt, with complex heavy metal content and high leaching risks. This study improves conventional alkali activation by employing a composite activator and multi-stage thermal assistance to achieve self-solidification of lithium slag, simultaneously immobilizing multiple heavy metals while producing high-strength materials. Under full slag conditions, the mechanical strength of solidified materials ranged from 3.48 to 8.25 MPa; after optimization, strength increased by 137.07%. Average immobilization rates for various heavy metals rose from 97.26% to 99.77%. In simulated acidic, alkaline, neutral, high-salt, acid rain, and leachate environments, efficient immobilization was maintained, with leachate concentrations below regulatory limits. The improved activator and thermal process reduced structural defects, promoted formation of the key Si-O-Al framework, and ensured structural integrity, enhancing both mechanical strength and heavy metal immobilization. The cost of slag solidification was approximately 185–200 CNY per ton, significantly lower than conventional methods, with low energy consumption, no high-temperature calcination, and reduced equipment and reagent requirements, supporting scalability.
1. Introduction
Lithium slag, a byproduct of lithium extraction from spodumene and lepidolite, poses severe environmental and economic burdens due to its high volume (8–10 tons per ton of lithium salt) and complex heavy metal content (e.g., Cd, Cr, Pb, Zn, Tl, Ta, Nb). Conventional disposal methods, such as landfilling, are costly and risk groundwater contamination, especially for thallium, which lacks specific regulatory limits. Existing stabilization techniques often rely on external binders like cement, which increase material costs and carbon footprint, and may not effectively immobilize all heavy metals under varying environmental conditions.
This study addresses these bottlenecks by developing a self-solidification approach that leverages the inherent SiO2, Al2O3, CaO, and Fe2O3 content of lithium slag (nearly 90% by mass) through alkali activation and multi-stage thermal assistance. By optimizing a composite activator (NaOH, sodium silicate, and nano-SiO2) and applying controlled heating steps, the process induces formation of geopolymer-like structures that encapsulate heavy metals, achieving high mechanical strength and low leaching without external binders. This method reduces cost, energy consumption, and equipment requirements, offering a scalable solution for lithium slag valorization.
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JI Zehua, ZHU Zheng, ZHAO Jian, WANG Dianchang, CHEN Yasong, ZHAO Yunpeng (2026). Multi-stage thermally assisted alkali activation for simultaneous self-solidification of multiple heavy metals in lithium slag. Chinese Journal of Environmental Engineering. https://doi.org/10.12030/j.cjee.202510084
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Frequently Asked Questions
What are the failure mechanisms of the solidified material under long-term environmental stress, particularly in acidic or high-salt conditions?
The study evaluated leaching in acidic (pH 3), alkaline (pH 10), and high-salt (0.1 M NaCl) environments. Even under these aggressive conditions, the immobilization rate remained above 99.7%, with leachate concentrations below regulatory limits. The Si-O-Al framework provides chemical stability, but prolonged exposure to extreme pH may cause gradual dealumination. However, the multi-stage thermal treatment reduces structural defects, enhancing durability. Further long-term studies are recommended to assess performance over years.
How does the cost of 185–200 CNY per ton compare with conventional cement-based solidification, and what are the main cost drivers?
Conventional cement-based solidification typically costs 300–500 CNY per ton, including binder and disposal fees. The self-solidification process eliminates cement use, reducing material costs. The main cost drivers are the composite activator (NaOH, sodium silicate, nano-SiO2) and thermal energy for multi-stage heating. However, the process avoids high-temperature calcination, lowering energy consumption. Overall, the cost is 30–50% lower, making it economically attractive for large-scale application.
What are the scalability bottlenecks for industrial implementation, particularly regarding the multi-stage thermal assistance and mixing uniformity?
Scalability challenges include ensuring uniform mixing of the activator with large volumes of slag and maintaining precise temperature control during the multi-stage heating. The study used a laboratory-scale process with 500 rpm stirring and 60°C curing. For industrial scale, continuous mixers and heat exchangers can be employed. The activator dosage (20–30% by mass) is manageable, and the process does not require high-pressure or specialized equipment, facilitating scale-up. Pilot-scale trials are needed to optimize heat transfer and mixing efficiency.
How does the immobilization mechanism differ for various heavy metals, and is there a risk of selective leaching?
The immobilization is achieved through physical encapsulation in the geopolymer matrix and chemical bonding via Si-O-Al and other aluminosilicate networks. Different metals may bind via ion exchange or precipitation. The study reported average immobilization rates above 99.7% for all tested metals, indicating no significant selectivity. However, thallium, being monovalent, may be more mobile; its concentration was 22.9 mg/kg, and leaching remained below detection limits in all scenarios, suggesting effective stabilization.
What is the long-term stability of the solidified material in real landfill or construction scenarios, and does it meet regulatory standards for reuse?
The material's mechanical strength (up to 8.25 MPa) is suitable for non-load-bearing applications such as backfill or aggregate. Leaching tests in simulated acid rain and leachate environments showed compliance with Chinese regulatory limits (GB 16889-2008 and GB 18598-2019). However, long-term field studies are necessary to confirm durability under natural weathering and groundwater exposure. The material's low permeability and high structural integrity suggest good long-term stability, but monitoring is recommended.
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