The Use of Microsilica in Refractory Castables - Elkem

influencing color. A closer examination of the microsilica by electron microscopy reveals submicronspheres with a mean particle size of approximately 0.15 micron. The surface area, asdetermined by nitrogen adsorption (BET), typically gives values around 20 m 2 /g which isequivalent to a spherical diameter of 0.14 micron, i.e. close to the results obtained by electronmicroscopy.Figure 2:Micrograph of a microsilica cluster showing spherical appearance of the individualparticles.Chemically, microsilica consists of amorphous SiO 2with variable purity. Without going intodetail, it may be stated that there is a close relation between the furnace operation and thequality of the microsilica. Hence traditionally, the purity of the microsilica tends to be rathervariable, reflecting the furnace operation at the time it was collected. As the metal normally isthe main product and microsilica the by-product, a variation in the performance of themicrosilica should thus be expected. This is true for all grades of microsilica except for ElkemMicrosilica grade 971U, 971D and 983U since these are produced as main products with thesilicon metal as the by-product.A typical analysis of a high quality refractory grade microsilica is given in Table 1.