Hydrothermal Oxidation Of Iron Ore Sintering Semi Dry Desulfurization Ash And Simultaneous Preparation Of Calcium Sulfate Whiskers

The iron ore sintering process is the main source of SO₂ emission in iron and steel companies.The existing desulphurization technology for flue gas has achieved significant social benefits,but at the same time results in a large number of desulfurization byproducts.Particularly,semi-dry flue gas desulphurized ash is difficult to be effectively utilized,due to its main component unstable CaSO₃.So it is urgent to find a new way to utilize the desulfurization ash with high efficiency,low cost and high added value.In this paper,semi-dry sintering flue gas desulphurized ash from one steel company was studied.The calcium sulfate whiskers were prepared by hydrothermal oxidation method.The effects of reaction temperature,slurry concentration and initial pressure on the hydrothermal oxidation of desulfurization ash and the microstructure of CaSO₄ whiskers were investigated.The macro-reaction mechanism of CaSO₃heterogeneous oxidation was explored.The relationship between CaSO₃ oxidation rate and CaSO₄ whisker solid content was revealed.With the increase of reaction temperature,the content of CaSO₃ in desulfurization ash is significantly reduced,and the oxidation efficiency is accelerated.When the solid-liquid ratio is 1:20-1:5,the slurry concentration in the hydrothermal reaction system has no obvious effect on the oxidation efficiency of CaSO₃ in the desulfurization ash.The initial pressure of the system has a great influence on the oxidation of CaSO₃ in the desulfurization ash.At 0-1MPa,the effect is more obvious.At 1-2 MPa,the pressure response is not obvious.When the system temperature is 180℃,solid-liquid ratio is1:20 and pressure is 2MPa,the content of CaSO₃ in desulfurization ash is reduced to7.32%and the oxidation rate is 89.74%.The total reaction rate of thermal oxidation of desulfurization ash is controlled only by the oxygen diffusion rate and the crystallization rate of CaSO₄.Oxidation mechanism is:temperature increases,reducing the oxygen concentration in the solution,increasing the supersaturation of CaSO₄,so that the oxygen transfer rate and the rate of crystallization faster.The change of the solid-liquid ratio（1:20-1:5）has no effect on the steps in the reaction,and the oxidation rate remains basically unchanged.Due to the influence of saturated vapor pressure,the increase in the mass transfer rate of oxygen decreases,and the effect at 1-2 MPa is not significant,limiting the overall reaction rate.The oxidation rate of CaSO₃ in desulfurized ash is positively correlated with the solid content of calcium sulfate whiskers in the product,and it is not directly related to the orientation growth behavior of calcium sulfate whiskers.The effect of temperature on the growth of calcium sulfate whiskers is manifested as the first to promote the inhibition.At the same time,the aspect ratio increases first and then decreases with the increase of the solid-liquid ratio,and also increases with the increase of the pressure.Combined with the oxidation results,the optimum process conditions for synergistic preparation of calcium sulfate whiskers by semi-dry calcined flue gas desulphurization ash were:reaction temperature 140°C,solid-liquid ratio 1:10,initial pressure 2 MPa.