For the recycling of ferrous sludge from steel industry,the carbonation pellet process should be considered as a "green" process,since no impurities are added as well as CO2 can be sequestrated and consumed.Through the thermodynamic calculation,the carbonation reaction can occur spontaneously and is an exothermic reaction.Based on the kinetic analysis through unreacted core model,the interfacial chemical reaction was the rate controlling step in the initial fast stage of carbonation,and the CO2 diffusion through the CaCO3 product layer was the rate controlling step in the following extremely slow stage.For the carbonation bonded mechanism,the pellet strength was gained by the formation and growing of CaCO3 product layer.Since the interfacial chemical reaction was the critical stage of the entire carbonation process,the emphasizes should be focused on the improvement of sorbent activity and the optimization of process parameters,such as pore structure,pore surface area,and total pressure,CO2 partial pressure,reaction temperature,etc to accelerate the reaction rate and to improve the quality of carbonation pellets.
The carbonation and microstructure characteristics of pellets containing ferrous dusts were investigated during carbonation consolidation at different reaction temperatures and CO2 partial pressures.The results indicated that green pellets had loose and network supporting structure with initial strength,and large cracks and pores existed in the pellets.The carbonation reaction was controlled by interfacial chemical reaction at the initial fast stage,which limited diffusion and thus caused the reaction rate to decrease.With increasing reaction temperature and CO2 partial pressure,the conversion rates of CaO and the number of microcrystalline CaCO3 particles increased,and the volume expansion of CaCO3 led to a decrease in the open porosity,average pore size and specific surface area of the pellets.Micro-pores were occluded,and the number of smaller pores(diameter less than 50nm)increased,thereby resulting in the more compact and uniform structure of carbonated pellets.Simultaneously,the dense structure prevented CO2 diffusion into the product layer,affecting the increase in carbonation conversion rate.
