Removal of magnesium in zinc hydrometallurgy system via freezing crystallization: zinc sulfate solution property and impurities (Mg 2+ , Mn 2+ , K + ) behaviour

Magnesium is a major, difficult-to-remove impurity in zinc hydrometallurgy. This study applies a green and efficient freezing crystallization method to systematically investigate the behaviour of impurity ions (Mg2+, Mn2+, K+) and the effects on zinc sulfate solution properties and magnesium sulfate removal efficiency. Results show that Mg2+ concentration below 15 g/L enables magnesium removal exceeding 90% with zinc loss kept below 10%. At 4.5 g/L Mn2+, density reaches 1.302 g/cm3 and viscosity drops to 4.580 mPa·s, promoting crystal settling and low zinc loss. At 15 g/L K+, density peaks at 1.312 g/cm3 with 40% magnesium removal efficiency. SEM-EDS analysis shows that K+ induces the formation of a cubic crystal structure and Mg2+, Mn2+, K+ promote co-crystallization of zinc and magnesium sulfates. This study first clarifies the influence mechanisms of Mg2+, Mn2+ and K+ on crystallization behaviour during freezing crystallization for magnesium removal and defines their concentration ranges. This research deepens the understanding of crystallization in complex solutions and achieves high-efficiency magnesium removal with minimal zinc loss without chemical additives, providing a theoretical foundation and technical pathway for green magnesium removal in zinc hydrometallurgy.