Preparation And Properties Of Bauxite-SiC Composite Refractories Used In The Transition Zone Of Cement Rotary Kiln

Buaxite-SiC composite refractories have been widely used as the lining of transition zone in the cement rotary kiln,which play a critical role in the development of cement industry and cement manufacturing technology.Nowadays,there is a challenge for the service life of such materials.Firstly,the alternative fuels have been applied in rotary kilns from the last decade.The combustion of such fuels produce corrosive alkali vapors,which become more concentrated and continue to circulate in the kiln as time progressed,leading to the permeate degradation of working lining.Secondly,the heat loss for the entire rotary kiln is extremely large,especially at the transition zone,as a result the energy consumption has become another critical issue which needs to be solved.Thirdly,the rotary kilns are expected to further extend their volume capacity and increase the rotation speed,resulting in the drastic abrasion between the lining and clinker.Based on the above phenomena,the performance of the linings in rotary kiln has to be optimized to decrease their thermal conductivity and improve alkali vapor attack resistance and abrasion resistance.In fact,thermal conductivity,alkali vapor attack resistance and abrasion resistance of refractories largely depend on their chemical compositions and microstructures?pore structures?.In order to meet the demands of new-type cement rotary kiln,particular emphasis of this thesis is placed on the optimization of structure and compositions of bauxite-SiC composite refractories.The structural evolution of bauxite-SiC composite refractories during preparation was firstly investigated,especially for the oxidation behaviours and morphology characteristic of SiC aggregates in different areas of the materials,in order to the establish the relationship between microstructure and mechanical properties.The effect of incorporation of several kinds of powder additives?SiC,silicon,?-Al₂O₃ and andalusite,et al?on the microstructure and alkali vapor attack resistance of bauxite-SiC composite refractories was also investigated.To decrease the thermal conductivity,the feasibility of using lightweight mullite aggregates to substitute the dense ones was studied.Besides,SiC aggregates,which possess higher thermal conductivity and excellent abrasion resistance,were also substituted by zirconia-mullite raw materials.Then the effect of incorporation of lightweight aggregates,zirconia-mullite raw materials on the thermal conductivity,alkali vapor attack resistance and abrasion resistance of bauxite-SiC composite refractories was systematically investigated.The above investigation provides theoretical and optimal direction for researching and developing new generation of lining used at transition zone of rotary kiln.On the basis of the work mentioned above,the main conclusions can be made as follows:1.During the preparation of bauxite-SiC composite refractories,different oxidation behaviours of SiC aggregates occurred according to the local oxygen partial pressure,graphitization occurrence was also found on the surface of some SiC aggregates.From the edge zone to center zone of specimens,passive oxidation,active oxidation and non-oxidation of SiC aggregates occurred,respectively.The active oxidation of SiC aggregates deteriorated their interfacial bonding with the matrix,leading to the inferior mechanical properties.In some area,the silicon component of SiC aggregates was vaporized,resulting in the generation of graphite on their surface.The oxidation of SiC aggregates can be controlled by the incorporation of SiC powder and silicon powder to adjust the oxygen partial pressure,enhancing the interfacial bonding,pore structure and mechanical properties.2.The optimized introduction of silicon powder during the preparation of bauxite-SiC composite refractories blocked up the pores,decreased the mean pore size and led to excellent mechanical properties and alkali vapor attack resistance due to its oxidation process at high temperature.The oxidation of silicon powder at high temperature prior to SiC aggregates,leading to the volume expansion and can block up the pores,resulting in the decrease of permeability.Meanwhile,the strong bonding between SiC aggregates and matrix was achieved.According to analysis of the percolation theory,the penetration of alkali vapor decreased and alkali vapor attack resistance of materials was improved.3.The incorporation of micron sized?-Al2O3 and microsilica during the preparation of bauxite-SiC composite refractories changed the mole ratio of Al2O3 to SiO2?Al2O3/SiO2?in the matrix,thus affecting the microstructure?pore structure?,alkali vapor attack resistance and high temperature performance.The introduction of micron sized?-Al₂O₃ accelerated the in-situ formation of mullite at high temperature,improving the refractoriness under load of materials,leading to the better high temperature performance and mechanical properties.By comparison,The interaction of microsilica and impurities led to the formation of silica-rich liquid,which deteriorated the pore structure and refractoriness under load of materials.During the alkali attack testing,the interaction between alkali vapor and alumina-rich matrix resulted in the formation of K-aluminosilicate phases,leading to the volume expansion and degradation of materials.By comparison,the interaction between alkali vapor and silica-rich matrix resulted in the liquid product,which avoided the generation of K-aluminosilicate phases and volume expansion.The better comprehensive performance of materials can be obtained when the mole ratio of Al₂O₃ to SiO₂ was 0.6,approximately.4.The incorporation of andalusite during the preparation of bauxite-SiC composite refractories led to the decrease of mean pore size and variation of the state of corrosion products,resulting in the improved alkali vapor attack resistance.Compared with the microstructure of bauxite and andalusite aggregates,mullite crystals was surrounded by abundant amount of amorphous silica after mullitization process of andalusite at high temperature,accompanying with the volume expansion.Such unique structure and chemical reaction changed the corrosion product during alkali vapor attack,which avoided the volume expansion caused by the formation of K-aluminosilicate phases.On the other hand,the mean pore size also decreased after mullitization of andalusite,which further improved the alkali vapor attack resistance.5.The introduction of lightweight mullite aggregates to substitute the dense bauxite aggregates during the preparation of bauxite-SiC composite refractories decreased their thermal conductivity,the incorporation of silica-sol coated lightweight mullite aggregates drastically improved the alkali vapor attack resistance of materials.Compared with the pristine lightweight aggregates,the silica-sol coated ones possessed better interfical bonding with the matrix,leading to the superior mechanical properties.During the alkali vapor attack,silica-sol coating could prevent the alkali vapor form penetrating into the lightweight aggregates,avoiding the volume expansion,thus promoting the alkali vapor attack resistance.6.During the development of preparation of new generation of bauxite-based composite refractories for the transition zone of cement kiln,zirconia-mullite raw materials was introduced and the matrix of materials was optimized,Compared with bauxite-SiC composite refractories,the enhanced thermal insulation performance of newly prepared materials was obtained whereas their abrasion resistance is satisfactory.Zirconia-mullite raw materials can not be oxidized at high temperature,which on the other hand could form a strong bond with the material at high temperature and enhance the toughness due to the formation of micro-cracks,resulting in the excellent strength and abrasion resistance.The bauxite-based refractories contained zirconia-mullite is promising materials which can be used as the lining at transition zone.