The Research Of Application In Cement Kilns For Regenerated MgO-CaO Bricks

The Cr3+in magnesite-chrome bricks used in cement kilns is prone to being transformed into Cr6+, hence, it is urgent to explore an environmental substitute. MgO-CaO bricks are kinds of environmental refractories that are considered as the most likely substitutes for magnesite-chrome bricks. However, the hydration of the MgO-CaO bricks has restricted their application in cement kilns. The spent MgO-CaO bricks that contain many impurities such as SiO2, Fe2O3, Al2O3can be made into the regenerated MgO-CaO bricks with a superior hydration resistance due to the aid of these impurities. Therefore, the regenerated MgO-CaO bricks not only solve the problem of the hydration of MgO-CaO bricks, but also reduce the pollution of magnesite-chrome bricks.In this work, the whole spent MgO-CaO bricks were firstly crushed and mixed to obtain spent bricks powder. It was pretreated by calcination and then mixed with fused magnesia powder that was pretreated by drying to form the mixtures of regenerated bricks. Then the green bodies of regenerated MgO-CaO bricks that contained80wt.%,70wt.%,60wt.%MgO were prepared using the mixtures as raw materials and paraffin as a binder. Subseqently the regenerated MgO-CaO bricks were obtained by sintering the green bodies at1873K for2h. It not only realized the large-scale reuse of spent MgO-CaO bricks, but also prepared the regenerated MgO-CaO bricks with good performace properties such as density structure and superior room temperature mechanical properties.The regenerated MgO-CaO bricks sintered under an air atmosphere and isolating system were compared and investigated. It showed that the isolated sintering was a reducing atmosphere which resulted in the reduction of MgO of regenerated bricks, and then the brick structure was damaged. Therefore, the air atmosphere was more conducive to improving the density and room temperature mechanical properties of regenerated MgO-CaO bricks.The hydration resistance and mechanism of the regenerated MgO-CaO bricks were explored respectively. It showed that the maximum mass gain rate of regenerated bricks after the air static and hot-water experiment were0.60wt.%and0.90wt.%, respectively, which showed an excellent hydration resistance. The hydration of free CaO of regenerated bricks was the main reason that resulted in the hydration. The impurities such as SiO2, Fe2O3and Al2O3could enhance the hydration resistance through decreasing free CaO content and improving the density of regenerated samples. It is found that hydration reaction was controlled by diffusion of H2O, and the diffusion rate constant K was0.70x10-6s-1.Performance properties of regenerated MgO-CaO bricks for cement kilns, including high temperature mechanical properties, thermal shock resistance and adhering coating property were investigated subsequently. It showed that the high temperature flexural strength of the regenerated bricks containing80wt.%MgO was much better than that of the regenerated bricks containing70wt.%and60wt.%MgO. The thermal shock resistance and adhering coating property of the regenerated bricks containing80wt.%and70wt.%MgO were both superior, however, compared with poor of the regenerated bricks containing60wt.%MgO. The crush of4CaO·Al2O3·Fe2O3during the thermal shock experiment resulted in the deterioration of thermal shock resistance. The area and thickness of the reaction between cement clinker and MgO of regenerated bricks determined the strength of adhering coating. Comprehensively, the regenerated MgO-CaO bricks with80wt.%and70wt.%MgO were more suitable for cement kilns applications, and the maximum utilization of spent MgO-CaO bricks was66.97wt.%.The regenerated MgO-CaO bricks containing70wt.%MgO with0,1.5,3.0wt.%SiO2(Fe2O3) addition were prepared, and their properties were investigated respectively. It was indicated that the content of3CaO·SiO2increased along with the increasing of SiO2addition. Element Fe and Al were dissolved in3CaO·SiO2and formed the solid solution. It resulted in the decreasing of4CaO·Al2O3·Fe2O3content, so that the bulk densiy, room temperature flexural strength decreased while the high temperature flexural strength and adhering coating strength increased. The content of2CaO·Fe2O3increased along with the increasing of Fe2O3addition. It resulted in the increasing of the bulk densiy and room temperature flexural strength, on the contrary, the high temperature flexural strength decreased, and the adhering coating strength decreased firstly and increased subsequently. The maximum content of SiO2and Fe2O3in regenerated MgO-CaO bricks should be controlled within4.17wt.%and1.57wt.%.The transformation of impurity element of regenerated MgO-CaO bricks was that the impurity element Si was in form of3CaOSiO2, the impurity element Fe was in form of2CaOFe2O3and4CaOAl2O3·Fe2O3, and the impurity element Al was in form of4CaO·Al2O3·Fe2O3. Besides, parts of element Al and Fe were dissolved in3CaO·SiO2, parts of element Fe were dissolved in MgO.