Study On Coal Slag Corrosion Resistance Of SiC-CA₆ Composites

The key technology for clean and efficient utilization of coal is modern coal gasification technologies,among which coal water slurry gasification technology accounts for more than 50%.The most widely used lining materials in coal water slurry gasifiers are high chrome?Cr₂O₃-Al₂O₃-ZrO₂?refractories.However,although the Cr³⁺is non-poisonous,the Cr₂O₃ could change to Cr⁶⁺valence during the manufacture and/or servicing process,which is harmful for water,soil and human health.Therefore,it is of great significance to study chromium-free alternatives and develop chromium-free refractory lining materials for coal-water slurry gasifiers.SiC materials are characterized by high melting point?2827°C?,high hardness,excellent abrasion and chemical corrosion resistance.CA₆ materials have excellent mechanical properties at high temperature,high stability in reducing atmosphere and alkaline environment.The combination of SiC materials and CA₆ materials may be one of the promising materials used in coal-water slurry gasifiers as chromium-free refractories.However,few studies were carried out about the preparation and performance improvement of SiC-CA₆ composites,especially the resistance to penetration and corrosion of the coal slag.Therefore,SiC-CA₆ composites were investigated in this paper.Effects of firing temperatures on the phase composition,mechanical properties and microstructures of SiC-CA₆ composites were systematically studied.On this basis,the samples with better comprehensive properties were selected to take the coal slag corrosion experiments by the static crucible method.The coal slag corrosion mechanism of SiC-CA₆ composites was analyzed.Furthermore,the comparison of the slag penetration and corrosion resistance among SiC-CA₆ composites with two chrome-containing materials?high chrome materials and chrome corundum materials?and another chrome-free materials?SiC-MgAl₂O₄ composites?was investigated.The main experiment results are as the followings:?1?In buried carbon bed powder atmosphere,the sintering of SiC-CA₆ composites was affected by the oxidation behavior of SiC under special temperature?1300-1650°C?.When firing temperature was lower than 1500°C,the oxidation products of SiC were mainly SiO₂,which could partly react with CA₆ in the matrix to form CAS₂,causing expansion and weight gain.Since SiC-CA₆ composites couldn't be sintered,there were few effects of firing temperatures on the density and strength.When firing temperature was higher than 1500°C,there was slowly active oxidation of SiC in the SiC-CA₆composites causing the weight loss,meanwhile,CAS₂ melted to form liquid phase which could promote the sintering of SiC-CA₆ composites,thus resulting in the shrinking of the sintered materials,the strength of SiC-CA₆ composites increased due to the decrease of apparent porosity and improvement of the bulk density.However,when firing temperature was 1650°C,the active oxidation of SiC accelerated and the new formed CA₆ was precipitated through the remained liquid,causing the improvement of apparent porosity and decrease of the bulk density.Comprehensively,the optimizing temperature of SiC-CA₆ composites is at 1600°C,with little oxidation of SiC,the composites had low porosity?16.1%?,high bulk density(2.75 g·cm^-3)and bending strength?7.6 MPa?.?2?The slag corrosion test of SiC-CA₆ composites was carried on by using static crucible method in buried carbon bed powder atmosphere at 1500°C for 3 h.It could find that the corrosion and penetration wasn't obvious in the tested samples.The slag corrosion mechanism was investigated:The most of FeO in the slag could be reduced by CO in the atmosphere and SiC in the samples and Fe-Si metal formed at the interface between the slag and composites.The penetrated slag could react with the SiO₂produced by SiC oxidation and the CA₆ matrix to form CAS₂ and fill into the porosity in the sample,which causing the compacting of the reaction layer.Because of the reaction between the infiltrated slag and the components in the brick,the composition and properties of the local slag changed and the penetration of the coal slag was inhibited.?3?The slag corrosion resistance of SiC-CA₆ composites was compared with high chrome materials,chrome corundum materials and SiC-MgAl₂O₄ composites by static crucible method and rotary slag resistance method respectively.In the case of static crucible test at 1500°C,these materials had better corrosion resistance,these materials had little corrosion except chrome corundum materials;as for penetration resistance,SiC-CA₆ composites were better than the other three materials.For the rotary slag test at1600°C,the erosion of SiC-CA₆ composites was serious,the reason might be the relatively low mechanical strength of SiC-CA₆ composites at room temperature and high temperature.Therefore,the mechanical erosion of slag and air flow has a great impact on their failure.The microstructure analysis of the residual bricks showed that SiC-CA₆ composites still had better slag penetration resistance than chromium-containing materials.