Preparation And Properties Of SiC Based Refractories For Plasma Gasifier

The disposal of highly toxic and polluting special hazardous waste can realize harmlessness through plasma gasification process,by which instant high temperature of 3000?~10000? occurs during the air ionization in the plasma gasifier.Refractories containing Cr2O3 are widely used as lining materials in the plasma gasifier at present,chromium pollution can be caused during production and application.Si C is an important refractory for the lining materials of the plasma gasifier due to its good mechanical performance and slag resistance at high temperature.However,the operating temperature reaching up to 1700? and constant feeding air around the plasma torch will cause serious oxidation degradation of Si C materials.Meanwhile,the water vapour formed by waste burning corrodes the upper Si C materials.So the investigation of Si C refractory with good anti-oxidation,excellent wet air corrosion resistance and mechanical behavior has important significance for the development of environmental protection.Therefore,in this thesis,the damage mechanism of Al2O3–Cr2O3 brick(original lining materials)was firstly investigated.Silica sol bonded Si C castable and Mg Al2O4-Cr2O3 castable were prepared,the slag resistance of the two castables and the Al2O3–Cr2O3 brick was investigated for the choice of lining materials used in the plasma gasifier.Secondly,?-Al2O3 micropowder and Si O2 micropowder were added into the silica sol bonded Si C castable to improve the bonding strength after during.In order to improve the water vapor corrosion resistance,Si C particles were replaced by compact calcium hexaluminate(CA6)aggregates to prepare CA6-Si C castable.And water vapor corrosion mechanism of Si C castable,Mg Al2O4-Cr2O3 castable and CA6-Si C castable were investigated.Finally,the microtructures and mechanical properties of Si C castables with different contents of metal Zr/Ti were investigated.The physical parameters of Si C castables with different metals after SPS treatment(simulating actual working condition)were researched to improve the oxidationresistance and verify the microstructure evolution law under the process of SPS,which is used to reveal the oxidation kinetic characteristics and anti-oxidation mechanism of Si C castable.On the basis of the work above,the main conclusions can be made as follows:1.Although Fe O in the slag enters into the refractory and reacts with Al2O3 and Cr2O3,forming the composite spinel Fe O·Cr(Al)2O3 in hang slag layer.The slag reacts with the Al2O3–Cr2O3–Zr O2–Si O2 particles and corundum particles to forming CAS2 and glass phase,which causes damage of Al2O3–Cr2O3 brick.2.SiC castable and Mg Al2O4-Cr2O3 castable perform better slag resistance compared with the Al2O3–Cr2O3 brick.The mechanical properties of Si C castable and Mg Al2O4-Cr2O3 castable depend on temperature obviously.The strength of Si C castable after 110?×24 h heat treatment is small due to the bonding agent of silica sol and performs high mechanical properties after medium-high temperature heat treatment.In-situ Si C whisker is formed by the reaction between Si O(g)and CO(g),which significantly enhances the fracture energy and performs better thermal stress resistance compared with Mg Al2O4-Cr2O3 castable.The formation of amount of Si C whisker significantly improves the strength after 1700? heat treatment,which improve the strength obviously.3.The ?-Al2O3 micropowder contributes to accelerating the network formation and hardening of the silica sol at lower temperature and shorter time,which benefits for the improvement of strength after 110?×24 h during.However,excessive ?-Al2O3 micropowder negatively affects the workability and increases the apparent porosity of the castables due to the rapid condensation rate.The specimen with 2 wt% ?-Al2O3 micropowder and 3 wt% Si O2 micropowder occupies big strength after 110?×24 h during.The CA6 aggregates consist of a small quantity of ?-Al2O3,which can reach reinforcement function.4.The castable that Si C aggregates replaced by CA6 aggregates performs high strength after during.The microstructure of CA6 aggregates remain stable after1000?×100 h wet air corrosion.However,Si C and Cr2O3 can react with the air containing water vapour.Firstly,Si C reacts with O2 to forming Si O2,then the Si O2 will be corroded by water vapour with the continuous oxidation of wet air,which differs from the oxidation condition of dry air.The siloxane network(Si-O-Si)can be corroded to become silanol group(Si-OH),which destroys the Si O2 protective layer to become irregular porous layer and can not keep the inner Si C from being oxidized.The Mg Al2O4-Cr2O3 castable performs weight loss and the formation of big pores after1000?×100 h wet air corrosion since that Cr2O3 can react with H2O(g)and O2(g)to forming Cr O2(OH)2(g).The CA6-Si C castable occupies better wet air corrosion than Si C castable and Mg Al2O4-Cr2O3 castable.5.The addition of metallic Zr and Ti can improve the oxidation resistance of a Si C castable.As Zr/Ti is preferentially oxidized following volume expansion jamming,the porosity of the castable decreased and the declining partial pressure of O2 prevented a depletion in the amounts of Si C and C by forming Si C whisker and graphite,which gives the silica sol bonded Si C castable the ability to repair and enhance itself after 1500? heat treatment.While too much Zr/Ti is harmful to the properties because of the big apparent porosity caused by excess volume expansion when being oxidized at high temperature.Si C castable with 0.6 wt% Zr/Ti can improve the mechanical performance and oxidation resistance.At 1700?,adding Zr reduced the partial pressure of the oxidizing gases(O2 and CO2)in the matrix.Further,Zr O2 product reduces the equilibrium partial pressure of Si O2(g)to a certain degree,which is beneficial for the generation of a dense Zr O2-Si O2 protective layer on the surface,improves the anti-oxidation and mechanical properties and prevents the degradation of the Si C castable to make the silica sol bonded Si C castable be a self-protecting refractory material.Ti O2 is not only more difficult to volatilize compared with Si O2,but also can decrease the viscosity of Si O2 glass to improve the mobility of liquid,which is good for sealing the holes and protecting the inner Si C from being oxidized to improve the mechanical property and oxidation resistance.6.The SiC particles bonded by Si O2 glass shows continuous network structure in the Si C castable after SPS treatment for 1600?×15 min.The Si C castable obtains high densification level and good mechanical performance after SPS compared with hot pressing sintering.A double electrode layer is formed at the interface between melting Ti and Si O2 glass(introduced by Si O2 micropowder and silica sol)when adding Ti into the Si C castable.And the surface tension gradient caused by high frequency pulse electric field during SPS processing occurs because of the electrocapillary principle results in the melting Ti and Si O2 glass move in an opposite direction to separate,which forms the structure that Si C particles is surrounded by melting Ti.Meanwhile,the Ti drops react with Si and carbon black to forming Ti3 Si C2(MAX phase).The structure that Si C particles with sheet Ti3 Si C2 coating significantly improves the strength of Si C castable.At 1500?,the Si C castable performs good oxidation resistance after SPS due to its high densification level,and Si C coated by sheet Ti3 Si C2forms the structure that solid Ti O2 nailed in the Si O2 glass and a dense protective layer on the surface of castable,which gives the Si C castable good ability to resist oxidation at 1700?.