Progress in the resource utilization of calcium-based semi-dry desulfurized ash in the iron and steel industry

With the introduction of an ultralow emission policy for the steel industry, nearly 70% of the steel enterprises in China have integrated desulfurization, denitrification, dust removal, and collaborative treatment transformation. The semi-dry desulfurization process, which mostly uses CaO or Ca(OH)₂ as the desulfurization absorber, has produced large amounts of desulfurization byproducts—desulfurized ash—which is difficult to resource. Relevant data show that China annually produces approximately 20 million tons of semi-dry desulfurized ash, which is the third largest solid waste in China. At present, the treatment of semidry desulfurized ash is mainly based on stockpiling, leading to increased land occupation. Semidry desulfurized ash has the characteristics of slow hydration reaction, strong alkalinity, and high content of metal impurities, and long-term stockpiling also causes environmental pollution; therefore, the resource utilization of semidry desulfurization byproducts has become a hot research and practical issue in the environmental field. This research team conducted in-depth research on the oxidative modification of flue gas desulfurized ash from Shougang and Tanggang circular fluidized bed (CFB) and dense phase tower (DFA). The mineral phase composition of the desulfurized ash mainly includes CaSO₃·0.5H₂O, CaSO₄, CaCO₃, and Ca(OH)₂, which are slightly different from those of the wet desulfurized ash (the main composition is CaSO₄·2H₂O). The average particle size of the desulfurized ash is 7.252 μm, with a median particle size (D₅₀) of 4.521 μm, and 80% of the particles are in the range of 1.039–16.162 μm. The specific surface area is 2.25 m²·g⁻¹, the specific surface area is relatively large, and the particle size of desulfurized ash by the spray drying absorption method is significantly larger than that of the CFB and DFA methods. The main physical characteristics of the desulfurized ash are as follows: water content, 0.02%–0.36%; bulk density, 0.85–1.0 t·m⁻³; bulk density, 0.55–1.0 t·m⁻³; true density, 2.25–2.69 t·m⁻³; and compacted density, 1.4 t·m⁻³. The desulfurized ash particles are of various sizes and mostly irregularly shaped small spheroids with many pores and pore diameters in the region of 0.1–0.2 μm. This article presents the results of systematic characterization of the physical and chemical properties of semidry desulfurized ash, modification, and industrial application; systematically analyses the main directions and progress of research on the resource utilization of semidry desulfurized ash in iron and steel enterprises and other semidry desulfurized ashes; and proposes that the modification of desulfurized ash, such as the oxidation of CaSO₃·1/2H₂O to CaSO₄, is one of the important means to achieve resource utilization. In addition, the use of semi-dry desulfurized ash to prepare various new materials can facilitate resource utilization.