Phase composition and microstructure of refractory MgAl2O4 spinel grains

Three series of refractory MgAl2O4 spinel grains were processed from differ ent MgO and Al2O3 precursors by firing at 1700 degrees C using a two stage firing process. The grains of the first series were processed from pure, pr ecipitated brucite-gibbsite batches and batches doped with excess MgO, exce ss Al2O3, 1.0%ZrO2, and 2% natural ilmenite mineral. The second and third s eries were processed from Egyptian natural magnesite in combination with gi bbsite and Chinese bauxite, respectively. XRD was used for qualitative dete rmination of solid phase composition of the fired grains and the results co mpared with those calculated according to the available phase equilibria da ta. The effect of Al2O3/MgO ratio and total oxides impurity content on the lattice parameter a(0) of MgAl2O4 was also studied. The densification param eters of fired grains in terms of bulk density, apparent porosity, and line ar shrinkage were investigated in relation to their phase composition and m icrostructure. The microstructures were characterised by SEM examination of polished samples and microanalysis of the major phases by a computerised E DS attachment. Pure stoichiometric spinel grains, consisting mainly of grea ter than or equal to 98.0% direct bonded MgAl2O4 could be processed from pr ecipitated brucite or natural magnesite-gibbsite batches on firing in two s tages up to 1700 degrees C, The bulk density and total open pore volume of these grains were 3.10-3.20 g cm(-3) and greater than or equal to 1.5 x 10( -2) cm(3) g(-1), respectively. Processing of natural magnesite-bauxite batc hes on firing in two stages up to 1700 degrees C formed dense, impure grain s having a bulk density of 3.35 g cm(-3) and total open pore volume of less than or equal to 0.8 x 10(-2) cm(3) g(-1). These grains were composed of c ongruent to 90.0%MgAl2O4 spinel containing some MgFe2O4 and Mg2TiO4 in soli d solution due to the relatively higher impurity content. The spinel solid solution was bonded by Ca, Mg, Al titanoferrite, and silicate phases. The a verage value of the MgAl2O4 spinel lattice parameter in stoichiometric spin el grains decreased significantly from 0.8060 to 0.7991 nm by doping with e xcess Al2O3, at molar Al2O3/MgO ratios of greater than or equal to 1.03. At ratios of less than or equal to 1.03, by doping with excess MgO, a slight increase in lattice parameter from 0.8060 to 0.8080nm occurred. A gradual i ncrease in the total oxides impurity content up to congruent to 10.0% resul ted in a slight increase in lattice parameter lip to 0.8080 nm. BCT/254. (C ) 1998 The Institute of Materials.