The Synthesis Of Light Weight Forsterite And Its Application In Light Weight MgO-SiO₂Castable

There are rich and price competitive magnesia and silica raw material resourcesin China. It is meaningful to synthesize light weight （LW） forsterite aggregates usingsuch resources and develop LW MgO-SiO₂castables containing such aggregates tomeet specific applications, as compared with dense basic castables, they can helpreduce thermal capacity and conductivity and improve thermal shock resistance; whilecompared with aluminosilicate refractories for the same purposes, they can enablehigher service temperature and better adaptability with basic working linings whenused as back linings.By using naturally occurred magnesite powders with MgO≥46%, silica powderswith SiO₂≥98%and caustic magnesia powders with MgO≥90%as starting materials,properly batching semi-dry pressing and calcinating, forsterite aggregates with bulkdensity of1.4g/cm³and1.8g/cm³were synthesized. The influences of molding pressure,the ration of the caustic magnesia and firing temperature on forsterite conversion, bulkdensity and cold crushing strength of the LW aggregates were investigated. By addingthe prepared two types of LW forsterite aggregates respectively to replace magnesiaaggregates in a magnesia based castable with MgO-SiO₂-H2O binding, at additions of10%,20%,30%and40%respectively for the1.4g/cm³type, and15%,30%and45%respectively for the1.8g/cm³type, properties of the castables were investigated incorrelation with the LW forsterite aggregate addition. The effect of silica fumeaddition ranging from3%,5%to7%on cold strengths and permanent linear change ofthe castable added with40%of the1.4g/cm³type LW aggregate was investigated and3%of its addition was found proper and adopted in the later work. To tackle with theproblem of big shrinkage of the castables added with the LW aggregates when heatedat and above1300℃, white fused alumina and special grade bauxite grog powders,both at additions of8%,16%and24%, were respectively incorporated in the castables added respectively with30%of the two types of LW aggregates, making use of theexpansion effect from the in situ spinel formation to compensate the shrinkage.Microstructure analysis on related samples was also carried out by means of SEM.For the synthesis of the LW forsterite, higher molding pressure is beneficial to theforsterite conversion and strength. Increase of caustic magnesia addition in the batchand raising calcination temperature lead to increased bulk density and strength ofsynthesized materials. Addition of the LW forsterite aggregates into the dense MgObased castables is a feasible way to prepare lightweight or semi-lightweight castablesin MgO-SiO₂system. The appropriate silica fume addition in this work is3%. Bulkdensity of the castables heated at different temperatures decreases gradually with moreadditions of the two types of LW forsterite, implying lightening dense castables can beachieved in this way. Volumetric stability of the MgO-SiO₂castables with synthesizedLW forsterite at high temperature can be improved by incorporating corundum or highalumina bauxite fines, which can react with MgO in the castable to form in-situ spinel,accompanying volumetric expansion. Castables after high temperature heating changesfrom shrinkage to slight expansion with increased incorporation of the fused aluminaor bauxite fines.According to this research, synthesis of LW forsterite aggregates with differentbulk density from natural MgO and SiO₂resources and using them to prepare MgO-SiO₂castables are both feasible and meaningful. The bulk density and addition of suchLW forsterite can be adjusted and chosen according to specific service conditions inpractical use. In the developed MgO-SiO₂castables, Al₂O₃component as from aluminasource raw material, can react with MgO to form in-situ spinel. The accompaniedvolume expansion can compensate the shrinkage when subjecting to high temperatureheating so as to obtain a satisfactory volumetri stability at high temperature.