Investigation On The Relation Of Composition, Microstructure And Properties Of MgO Based Castables Added With Synthesized Forsterite

The vigorous development of monolithic refractories has also been evidenced byincreased MgO based castables since recent years. For magnesia based refractories,there remain some technical barriers to be surmounted such as poor resistances tothermal shock resistance and slag penetration and high thermal conductivity,nevertheless they have advantages in raw material resource, high refractoriness, goodresistance to basic slag, etc. Taking into the consideration of forsterite having theadvances of high melting point, good crystal stability, no hydration problem, goodresistance to molten metals, good compatibility with magnesia at high temperatures andhigh impurity level of natural occurred olive ore, this work synthesized high purityforsterite, using magnesite, caustic magnesia and quartz as starting materials and addedthe synthesized forsterite as aggregates into MgO based castables. The influence offorsterite addition on properties of the castables was investigated. On the other hand, theshrinkage problem of the castables heated at high temperature was tackled with byadding high alumina bauxite to make use of its reaction with magnesia to form spineland the accompanied volumetric expansion to compensate the shrinkage.Based on a scrutiny into the MgO-SiO₂phase diagram, a magnesia richcomposition was chosen for the synthesized forsterite with a mass ratio at60:40of MgOover SiO₂. By comparing the effect on properties of the forsterite of grinding by dryprocess vs. wet process, wet co-grinding was adopted in the synthesizing process,consisting of wet co-grinding, semi-dry compacting and firing at1550℃. The so madeforsterite is intact, with well developed short columnar and granular forsterite crystals,and its purity is appreciably high. Using the castable with dead sintered magnesia as rawmaterial and MgO-SiO₂-H₂O binding system as a reference,15%,30%,45%and60%of the synthesized forsterite as aggregates were respectively added to replace the sintered magnesia, and the influence on properties of the castables was investigated. Inthe castable with60%forsterite, the influence of respective addition of3%,6%,9%and12%special grade bauxite powders on properties of the castables was also investigated.Microstructure analysis of related samples was carried out by means of SEM, EDAXand XRD.Adding forsterite to the castables leads to reduced density, favorable to achievinga lighter magnesia based dense castable; HMOR at1400℃can be improved by theforsterite incorporation, at60%forsterite addition, HMOR is enhanced by45%compared with the reference; thermal sock resistance, by1100℃-air quenching andtesting the residual CMOR, of the castables pre-heated at1500℃for3h can also beimproved. Cracks on the specimens tend to reduce along with increased addition of theforsterite; slag penetration resistance is meanwhile improved. At60%of the forsteriteaddition, the corroded depth is only56%of the reference. Incorporating bauxite in the60%forsterite added castable can obviously reduce the negative value of permanentlinear change, when the bauxite addition reaches12%, there is almost no residualshrinkage. Some micro-cracks as a result of the expansion effect by the in-situ formedspinel, help improve thermal shock resistance, evidenced by higher residual CMORratio when subjected to1100℃-air quenching.This work has confirmed a synthesizing technology for high purity denseforsterite. The target of property improvement by adding forsterite to the MgO basedcastable has been met and the relationship of composition, microstructure and propertyhas been revealed by this work, which is guidable to the production and application ofsynthesized forsterite and property optimization and application of such MgO-SiO₂castables.