Subcritical hydrothermal oxidation of semi-dry ash from iron ore sintering flue gas desulfurization: Experimental and kinetic studies.

Realization of low temperature and high efficiency oxidation of CaSO3 is the key to solve the issue of ecological hazards caused by semi-dry sintering flue gas desulfurization ash. The subcritical hydrothermal technology was employed for the oxidation of CaSO3, achieving 89.83% of CaSO3 at 180 °C, 2 MPa for 120 min with a solid-to-liquid ratio of 1:20. The macroscopic oxidation kinetics of CaSO3 in the subcritical hydrothermal reaction system was investigated. A mathematical model was established, incorporating the intrinsic reaction, CaSO3 dissolution, oxygen diffusion and CaSO4 precipitation. It was concluded that the macroscopic oxidation of CaSO3 was co-controlled by the oxygen diffusion and CaSO4 precipitation. Subcritical hydrothermal technology promises not only higher efficiency, but more importantly, potentially "one-step" preparation of CaSO4 whiskers, enabling cost-effective and high value-added resource utilization of the semi-dry FGD ash.