Research On Synthesizing Sialon Multiphase Materials Using Coal Gangue By Secondary Carbon Thermal Reduction Nitridation Reaction(CRN)

Coal gangue is a kind of solid waste from coal industry,and,the annual emission and accumulated stock pile of coal gangue are huge.Because coal gangue is non renewable mineral resources whose price is very cheap,the researchs on comprehensive and recycle utilization of coal gangue have very important realistic significance and commercial value.Using Inner Mongolia coal gangue as main material,?-Sialon based material was synthesized by carbothermal reduction and nitridation method in N₂ atmosphere with active carbon as reductant.Adding?-Al₂O₃?SiO₂ powder to adjust Al₂O₃/SiO₂ mass ratio,using corundum as aggregate and coal gangue base?-Sialon fine powder from primary reaction as matrix,?-Sialon mutiphase material was produced by secondary carbon thermal reduction nitridation reaction.Using electronic universal testing machine and thermal shock resistance testing machine,the strength,thermal shock resistance and oxidation resistance were carried out;XRD diffraction pattern and scanning electron microscope?SEM?were used to analysis mineral composition and microstructure.The research results showed as follow:Because of complex raw material composition,multiple reactions occurring at the same time and multi-reaction product reciprocal transforming,and the carbon thermal reduction nitridation reaction of coal gangue is a very complex physiochemical process so,it is diffcult to control the reactive system toward target product and get single phase?-Sialon material.Under different dynamic and thermal conditions,these phases such as?-Sialon,X,O?-Sialon,SiC and mullite can produce during the carbon thermal reduction nitridation reaction.In reacted products,X phase is relatively less,for example,X phase is only accounts for 15.29%under the reactive condition whose reaction temperature is above 1420?with carbon content is over 10%.The carbon content and reaction temperature significantly affect on the preparation of?-Sialon phase and O?-Sialon phase.The experimental result showed that it is benefit to produce?-Sialon phase,O?-Sialon phase when the sintering temperature is above 1420?and the carbon content is 10%¹6%.At 1420?,when the carbon content is 14%,the content of?-Sialon is 57.21%;at 1500?,Z=0.4,the content of O?-Sialon phase is23.80%when the carbon content is 16%.During the carbon thermal reduction nitridation reaction of coal gangue,mullite phase first appears,?-Sialon phase is synthesized by nitride reductive decomposition of mullite later.When the mass ratio in the coal gangue is adjusted from 0.85 to2.60,namely the mass ratio of Al₂O₃/SiO₂ slightly more than theory constitute of mullite,mullite content maximize during high-temperature calcinations of coal gangue which lay the foundation of maximize producing?-Sialon phase by carbon thermal reduction nitridation reaction subsequently.Because of large volume change,high porosity and low strength of?-Sialon material synthesized by primary reaction.It is difficult to preparing?-Sialon multiphase material by primary carbon thermal reduction nitridation reaction of coal gangue which limit the?-Sialon directly applied as high temperature material.Using primary synthesized?-Sialon fine powder as matrix,and corundum as aggregate,adding SiO₂ or SiC fine powder and metal Si powder,high-performance Sialon-Al₂O₃and Sialon-Al₂O₃–SiC mutiphase material can be successfully prepared by secondary carbon thermal reduction nitridation reaction.Adding 43wt%corundum,10wt.%SiO₂ fine powder to?-Sialon fine powder synthesized during primary reaction,by secondary carbon thermal reduction nitridation reaction,Sialon-Al₂O₃ multiphase material involving 93.6%Sialon+Al₂O₃ is produced under the reaction condition of over 20wt%carbon contene,N₂ atmosphere and fired temperature 1420?for 3h.Synthesized Sialon-Al₂O₃ material is dense and has the highest compressive and flexural strength.During secondary carbon thermal reduction nitridation reaction,under the action of reducing agent of carbon,the mullite synthesized in the primary carbon thermal reduction nitridation reaction decomposes and results in?-sialon by reacting with SiO₂ fine powder.Corundum phase and?-Sialon phase generated in matrix are intertwined and overlapping growth,which guarantees excellent property of mutiphase material.Adding 45-50wt%corundum,10wt%SiO₂ fine powder and 5wt%metal Si powder to?-Sialon fine powder synthesized during primary reaction,Sialon-Al₂O₃-SiC multiphase material involving 90%Sialon+Al₂O₃+SiC is produced under the reaction condition of over 50wt%of carbon,N₂ atmosphere and fired temperature 1450?for3h by secondary carbon thermal reduction nitridation reaction.Synthesized Sialon-Al₂O₃–SiC material is dense and has advantages of high compressive and flexural strength with excellent oxidation resistance and thermal shock resistance.During secondary carbon thermal reduction nitridation reaction,besides the decomposition reaction of mullite and synthetic reaction of?-Sialon phase,also includes the following in-situ reaction that excessive carbon powde reacts with metal Si powder SiC.The microstructure analysis shows that corundum aggregates are surrounded by fine crystalline grain of?-Sialon which present a granular pile up mixed with fibrous SiC phase in the middle.Due to self reinforcing toughening and compound dispersoid toughening of?-Sialon and SiC crystals of in-situ generation,the mutiphase material has good mechanical properties and thermal shock resistance.After water quenching twice at1100?,the Sialon-Al₂O₃–SiC multiphase material has advantage of high retention rate of residual strength.