Microwave-assisted NaOH roasting followed by HCl leaching for enhanced rare earth elements recovery from bastnaesite concentrate

Bastnaesite is one of the most important rare earth element (REE)-bearing minerals; however, its high chemical stability limits REE extraction by direct HCl leaching, necessitating the use of conventional, energy-intensive NaOH cracking. To overcome these processing challenges, microwave-assisted NaOH roasting followed by HCl leaching was developed to enhance REE recovery from Dong Pao bastnaesite concentrate and compared with direct HCl leaching and conventional NaOH roasting. Direct HCl leaching achieved 72.4 % REE recovery under harsh conditions (12 M HCl, 90 °C, 8 h, and 10 mL/g). Conventional NaOH roasting required treatment at 600 °C for 160 min to achieve 98.5 % REE extraction following HCl leaching. In contrast, microwave-assisted NaOH roasting for just 90 s significantly improved subsequent leaching by promoting mineral transformation and increasing mineral reactivity. Process optimization identified a NaOH-to-concentrate ratio of 0.2 (w/w), HCl concentration of 3.7 M, temperature of 90 °C, and liquid-to-solid ratio of 10 mL/g as the optimum conditions, achieving 98.3 % REE extraction. Characterization results confirmed bastnaesite decomposition and formation of Na-containing products during roasting, while kinetic analysis indicated ash layer diffusion-controlled LREE leaching and diffusion-dominated mixed-control HREE dissolution, with activation energies of 8.49 and 20.13 kJ/mol, respectively. Compared with direct HCl leaching, the proposed process reduced HCl consumption by 73.3 % and energy requirements by 40.6 %, while increasing REE production from 215.6 to 292.7 g per kg of concentrate. Compared with conventional roasting, microwave-assisted roasting enabled a comparable REE extraction while reducing the pretreatment time from 160 min to 90 s and energy consumption by 91.1 %.