Study On Consolidation Mechanism Of Carbonation Pellets With Ferrous Dust

With the development of Chinese iron and steel industry, the steel output is ramping up continually. As a large amount of solid wastes produced from steel mills, the reasonable, effective resource-oriented utilization of ferrous dust has become the key to the long-term coordinated development of resources, environment and economic. Carbonation consolidation has incomparable advantages among many comprehensive utilization methods of ferrous dust. However, the large-scale industrial application of carbonation pellets has great difficulties, not only because of the slow carbonation reaction rate but also because of its low strength. In this paper, proper amount of active lime as a binder was mixed and balled with ferrous dust, and then carbonation experiments were carried. To determine the better conditions and means of carbonated consolidation process, the quality and carbonation characteristics of pellets were studied; to reveal the consolidation mechanism of carbonation pellets, the microstructure characteristics and reaction kinetics were studied. The results can provide theory basis for improving the strength and production efficiency of carbonation pellets. The main results are as follows:The optimal process parameters for preparing green pellets and carbonation pellets were researched through strength, particle size distribution, carbonation conversion rate, and porosity of pellets with ferrous dust in different conditions. The findings are as following: CaO ratio was 25%, moisture content of green ball was 11.5%, pelletizing time was 20min, ball diameter was 10-12.5mm, carbonation reaction temperature was 800℃, gas flow rate was 2 L·min-1, partial pressure of CO2 was 101kPa, and reaction time was 20min.By means of optical microscopy, field emission scanning electron microscope, X-ray diffraction, mercury analyzer and fractal theory, the microstructure variations of pellets with ferrous dust were:studied. With increasing CaO content and lengthening pelletizing time, gelatination of Ca(OH)2 enhanced led to decrease in porosity, increase in compactness; and critical aperture, the most pore size and pore size distribution moved to smaller pore, the complexity of the pore structure increased, the fractal dimension increased. With increasing reaction temperature and CO2 partial pressure, CaCO3 crystallites increased significantly; the fill effect of CaCO3 microlite caused a series of changes on pore structure, for instance, critical aperture, the most pore size and pore size distribution moved to smaller pores, the porosity decreased, the pore structure tended to be more regular and simple, the fractal dimension reduced, and Knudsen diffusion gradually became the main diffusion way of CO2 in the pellets.The carbonation reaction kinetics was studied through thermogravimetric experiment and unreacted shrinking core model, the results indicated that:with the CaO ratio, porosity of green pellets reduced, and the reaction temperature, partial pressure of CO2 increased, the effective diffusion coefficient and reaction rate constant of carbonation reaction increased, the internal diffusion resistance and chemical reaction resistance reduced; accordingly the rate of diffusion and chemical reaction sped up, the total rate of carbonation reaction increased. The process of carbonation reaction could be divided into two stages:in the first stage carbonation reaction was fast, the diffusion resistance rate was smaller than chemical reaction resistance rate, so the carbonation reaction was controlled by interfacial chemical reaction; in the second stage, because of the further volume expansion associated with the formation of CaCO3, porosity decreased and the diffusion of CO2 was limited, which caused the diffusion resistance rate was more larger than chemical reaction resistance rate, CaO conversion rate increased slightly, and it became diffusion limiting which caused the reaction rate to decrease.