| Since chrome-containing refractory materials have the potential polluting water and soil,it is very urgent and important to develop chrome-free materials to support the green development of coal gasifier industry.It was reported that SiC-MgAl2O4composite has good coal slag corrosion resistance.However,the low mechanical strength of SiC-MgAl2O4composite at high temperature impedes its application as working lining materials..In this study,new design conception of SiC-MgAl2O4 composites was developed to improve the comprehensive properties of SiC-MgAl2O4 composites by adding silicon or aluminum powder,then the designed composite was heat-treated in N2 atmosphere,a series of nitride/carbide compounds could form according to thermodynamic prediction,which were considered as reinforcing phases in SiC-MgAl2O4composites.The effects of the amount of silicon and aluminum powder and nitriding sintering temperature on the properties of SiC-MgAl2O4 composites were systematically investigated.The results are the followings.（1）Much more nitride phases with fibrous or platelet morphologies formed in the SiC-MgAl2O4 composites with addition of silicon powder after nitriding at high temperature,improving the densification.Compared with the sample without silicon powder addition,the CMOR and HMOR of the designed composites are significantly improved by 930%,With the increase of the amount of silicon powder,the weight gain and the mechanical strength of the sample increased with the increased of the content of nitride fibers,but the other properties such as linear change and density didn’t change obviously.In the nitriding temperature range of 1350～1600℃,with the increase of nitriding temperature,the amount of the in-situ formed nitrides fibers increased firstly and then decreased.When the sintering temperature was higher than 1500℃,β-Sialon phase with fiber morphology decreased and become to granulous shape.The mass change rate and the linear change rate with the nitriding temperature rising showed the phenomenon of increase firstly and then decrease,while the bulk density appeared a trend of decrease generally,although the density of the sample nitrided at 1550℃was slightly higher than that of the sample sintered at 1500℃.In this study,it was found that SiC-MgAl2O4 composite with 8wt%silicon addition presents the optimal comprehensive properties when nitride at 1400℃for 4 h.（2）Many kinds of metal carbonitrides,such as MgAl ON,Al-O-C and Al-O-C-N,formed in the SiC-MgAl2O4 composites with addition of aluminum powder after nitriding at high temperature,and the densification and mechanical properties of the SiC-MgAl2O4composites were greatly improved.Compared with the samples without aluminum powder,the densification,mechanical strength at room temperature and high temperature were significantly improved and the increase rate of the HMOR was about 4000%.The results showed that the content of fibrous reinforced phase increased first and then decreased with the increase of aluminum powder at the range of 0～8%,the bulk density and mechanical strength also increased first and then decreased.With the increase of nitriding sintering temperature from 1400℃to 1600℃,the content of fibrous reinforcement phase increased first and then decreased while the content of lamellar reinforcement phase increased gradually.However,the mass change rate and linear change rate of the samples after sintering did not change obviously;The BD,EM,CMOR and HMOR showed the trend of first increase and then decrease.In this study,it was found that SiC-MgAl2O4 composite with 4wt%aluminum addition presents the optimal comprehensive properties when nitride at 1450℃for 4 h.（3）The corrosion resistance to coal molten slag of SiC-MgAl2O4 composite without any addition（A0）,with 4wt%aluminum（A4）and 8wt%silicon（S8）,respectively,was investigated by using static crucible method at 1500℃for 3h in CO atmosphere.The results showed that all the three samples had scarcely any corrosion,but there was big difference in coal slag penetration.The thickness of penetration layer of A0,S8 and A4samples was 12mm,2mm and<1mm,respectively.The results showed that the addition of silicon and aluminum powder to SiC-MgAl2O4 composite can improve the slag penetration resistance of materials.It was considered thatβ-Sialon,O’-Sialon,and MgAl ON in-situ formed in the composite with Si or Al addition could increase the contact angle between the material and the molten coal slag,thus inhibiting the slag penetrating into the material,and consequently reducing the dissolution of magnesium aluminum spinel matrix,Especially due to no reaction between MgAl ON with Ca O,Mg O,Al2O3 and Si O2 in the slag the molten coal slag infiltrating along the microcracks between the aggregate and the matrix could be prevented when meeting the matrix containing MgAl ON.（4）Rotary slag resistance test was used to simulate the application environment of coal gasification.The slag resistance of the SiC-MgAl2O4 composite with without aluminum powder addition was compared with that of high chromium material and chromium corundum material.The results showed that the two kinds of SiC-MgAl2O4 materials both had good slag permeability resistance and thermal shock resistance,and the SiC-MgAl2O4material with Al addition had a better slag resistance.However,the chromium containing materials（high chromium materials and chromium corundum materials）had serious penetration,transverse cracks and cross cracks.The above experimental results show that the composite addition of aluminum powder can not only improve the high temperature mechanical properties and the thermal shock resistance of SiC-MgAl2O4 material,but also the prepared SiC-MgAl2O4 composite has excellent coal slag permeability and corrosion resistance.This proving that the SiC-MgAl2O4 composite was a promising chrome free green refractory for coal gasification. |
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