Minerals recovery from a rare earth extraction wastewater by a combined chemical precipitation and membrane distillation process

Rare earth extraction wastewater (REEW) derived from rare earth (RE) extraction contains high value-added resources, such as magnesium. Therefore, it is highly desirable to recover the minerals from REEW to reuse in the RE extraction process. This study investigated the combined chemical precipitation and direct contact membrane distillation (DCMD) process to treat REEW for minerals recovery. Results showed that 99.7% Mg²⁺ in REEW was precipitated as Mg(OH)₂ by adjusting pH of the solution. The MgO was obtained through the calcination of Mg(OH)₂. The specific surface area (SSA) and reactivity of the calcined MgO decreased with the increase of calcination duration. MgO calcined at 500 °C for 2 h exhibited higher reactivity and SSA of 188.2 m²/g. DCMD performance decline was mainly caused by gypsum during treatment of the Mg-separated stream and membrane fouling was effectively mitigated by almost complete removal of Ca²⁺, resulting in the concentrated Na₂SO₄ solution. Subsequently, Na₂SO₄ was successfully recovered via freeze crystallization by controlling its solubility. The recovered Na₂SO₄ exhibited narrow crystal size distribution with mean size of 43.3 μm. With this combined process, it was possible to successfully recover 20.1 g/L MgO, 49.7 g/L Na₂SO₄ along with 72.6% of the water from REEW. The recovered MgO and Na₂SO₄ with purity of 98.5% and 99.2%, respectively, achieving industrial grade. The results suggest that the combined technologies, chemical precipitation and DCMD process, can be used as a viable engineering solution for minerals recovery from REEW.