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China boasts abundant salt lake lithium resources. However, the industry currently grapples with significant challenges: extremely high magnesium-lithium ratios, complex impurity profiles, low lithium recovery rates, and high energy consumption costs. Traditional processes such as evaporation-crystallization and adsorption struggle to meet the demands of scalable, efficient, and cost-effective lithium production. The mixer-settler, as a core liquid-liquid extraction equipment, offers high-efficiency mass transfer, stable separation performance, flexible adaptability, and environmental sustainability. It has emerged as a critical technological solution to break the bottlenecks in salt lake lithium extraction, providing essential support for the industrialized scale development of this sector.
A mixer-settler is a classic stage-wise contactor for liquid-liquid extraction, fundamentally composed of a mixing chamber and a settling chamber. It achieves efficient separation of lithium from impurities through a two-step process: mixing for mass transfer followed by gravity settling.
Mass Transfer via Mixing: Lithium-bearing brine (aqueous phase) and a specialized lithium extractant (organic phase) enter the mixing chamber at a predefined flow ratio. Under agitation, the two phases are thoroughly dispersed and mixed. The extractant selectively bonds with lithium ions through complexation reactions, efficiently transferring lithium from the aqueous phase to the organic phase. This step simultaneously achieves the initial separation of lithium from impurities like magnesium, sodium, potassium, and boron.
Phase Separation via Settling: The well-mixed liquid overflows into the settling chamber. Utilizing the density difference between the organic and aqueous phases, the liquids rapidly separate under gravity. The upper layer consists of the lithium-loaded organic phase, while the lower layer is the raffinate containing impurities. These separated phases are independently discharged for subsequent washing and stripping processes, ultimately yielding a high-purity lithium solution.
Within the comprehensive salt lake lithium extraction process —salt solar evaporation-pH adjustment-boron extraction-lithium extraction-stripping-precipitation—the mixer-settler serves as the central hub. It is precisely engineered to handle the complex operating conditions inherent in salt lake brines.
Preliminary Impurity Removal: Salt lake brines typically contain high levels of boron, which severely interferes with lithium extraction efficiency and product purity. The mixer-settler enables sequential extraction of boron and lithium. By first using a dedicated boron extractant, it effectively separates boron, creating a clean liquid phase for the subsequent lithium extraction step and eliminating interference from impurities.
Core Lithium Extraction and Enrichment: As the key equipment in the lithium extraction process, the mixer-settler maximizes the complexation efficiency between lithium ions and the extractant by precisely controlling mixing time, temperature, agitation intensity, and phase flow ratios. The optimized design of the settling chamber ensures rapid and complete phase separation, significantly enhancing the concentration of lithium in the organic phase and laying a solid foundation for producing battery-grade lithium salts.
Multi-Stage Configuration for Complex Brines: Addressing the characteristic high magnesium-lithium ratio (exceeding 1500:1 in some salt lakes) of Chinese salt lakes, mixer-settlers can be configured in multi-stage countercurrent/cocurrent series. This setup achieves deep separation of lithium from magnesium, calcium, and other impurities, solving the intractable problem of processing high-impurity brines that traditional technologies struggle with.
By optimizing the agitation structure in the mixing chamber and the residence time in the settling chamber, mixer-settlers ensure thorough contact and rapid phase separation, delivering lithium extraction efficiencies of over 90%. Implementing a multi-stage series design (e.g., 6-stage countercurrent) can boost the overall lithium recovery rate in salt lake processing from 65%-80% (achieved by conventional processes) to an impressive 96%-98%. This substantially reduces lithium loss in mother liquor, enabling the efficient recovery of "hidden lithium reserves."
Mixer-settlers can be flexibly designed as single-stage or multi-stage systems to accommodate varying production scales, from pilot projects to large-scale industrial operations exceeding tens of thousands of tons. The equipment operates continuously and can automate the entire process—feeding, extraction, washing, and stripping—perfectly matching the high-volume, long-cycle, and continuous production requirements of salt lake lithium extraction, and resolving the inefficiencies associated with traditional batch processes.
Adjustable Parameters: The equipment can be customized for each specific salt lake by flexibly adjusting process parameters such as agitation intensity, phase ratio, number of stages, and temperature, implementing a "one lake, one strategy" approach.
Material Compatibility: Tanks are predominantly constructed from high-performance corrosion-resistant materials like PPH and PVC, enabling them to withstand the highly saline and corrosive environment of salt lake brines. The equipment achieves an operational lifespan of over 8,000 hours, significantly reducing maintenance and replacement costs.
Process Integration: It seamlessly interfaces with extractant regeneration and wastewater treatment stages, facilitating the recycling of organic phases and the 达标 discharge of raffinate, ensuring a closed and efficient operational system.
Compared to traditional evaporation-crystallization (energy consumption > 25 kW·h/m³), mixer-settlers reduce energy consumption to less than 15 kW·h/m³, a decrease of over 40%. Additionally, through an efficient co-extraction system, caustic soda consumption can be lowered from 1.2 tons NaOH/t Li₂CO₃ (industry standard) to 0.7 tons/t Li₂CO₃. Water and acid/alkali consumption are also reduced by more than 30%. The recyclable organic phase and low-impurity raffinate lower environmental treatment costs by over 98%, aligning with the green, low-carbon development goals of the salt lake lithium extraction industry.
Featuring a simple structure and no high-speed rotating components, mixer-settlers operate with high stability, minimizing issues like scaling and clogging. The equipment is easy to maintain, with upkeep costs reduced by 60% compared to older extraction technologies. This ensures long-term, stable operation, mitigating production downtime caused by equipment failures and guaranteeing the continuous and efficient performance of salt lake lithium production lines.
Mixer-settlers have already been successfully deployed in numerous salt lake lithium extraction projects across Qinghai and Tibet. For instance, a project in Qinghai processing lithium mother liquor with a 6-stage series mixer-settler increased lithium recovery from 65% to 96%, reducing the production cost per ton of lithium carbonate by over 30%. As the new energy industry drives sustained growth in lithium demand and China's salt lake lithium extraction industry accelerates toward scalability, efficiency, and green transformation, mixer-settlers, armed with their unparalleled technical advantages, are set to become the dominant core equipment in the field. They will further strengthen China's independent supply capacity and industrial competitiveness in lithium resources.
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