Microfluidic strategy for rapid and efficient extraction of scandium ions from red mud

Abstract

Red mud is an alkaline solid waste generated during alumina production and harbors significant scandium (Sc) resources. Sc holds substantial scientific and economic value due to its critical applications in metallurgy, chemistry, and electronics. However, traditional methods for extracting Sc from red mud are hindered by challenges such as prolonged mixing times, high energy consumption, and susceptibility to emulsification, which severely limit industrial application efficiency. To address these issues, this study designed and developed a microfluidic extraction platform utilizing laminar flow, slug flow, and microdroplet systems. It systematically investigated the separation process of Sc³⁺ using di-(2-ethylhexyl) phosphoric acid (P204) as the extractant. The microfluidic chip significantly enhanced the rapid mass transfer of Sc³⁺ between phases by providing a larger interface area, thereby accelerating the extraction kinetics. This characteristic enabled the extraction process to reach equilibrium more quickly while ensuring operational stability and reproducibility. Experimental results demonstrated that, compared to solvent extraction (with an extraction efficiency of 74.6 %), the laminar flow achieved the selectivity separation efficiency for Sc³⁺ ranging from 94.1 % to 96.4 %, with the reaction time reduced to 7.14–10 min. The slug flow improved mass transfer efficiency by promoting internal circulation within the droplets, achieving an extraction efficiency of 86.9 % to 94.8 %. Meanwhile, the microdroplet attained the highest mass transfer efficiency of 92.9 % to 97.4 % by leveraging the large specific surface area and internal flow characteristics of the droplets. Therefore, the laminar flow is highly suitable for continuous operation and the rapid and efficient separation of rare earth elements from bulk solid waste. Additionally, the performance of the slug flow and micro-droplet is significantly superior to traditional solvent extraction methods, making them particularly suitable for high-selectivity separation of rare earth ions and small-scale processing applications. This study not only provides a novel strategy for the efficient recovery of Sc resources from red mud but also promotes the application and development of microfluidic technology in hydrometallurgy engineering.