Silica fume is a by-product obtained from the fume and dust overflowing from the waste gas in the recovery furnace during the production of metallic silicon or ferrosilicon alloy. Under the high temperature of the electric arc furnace, the quartz in the furnace is melted into siOz liquid, and the liquid SiOz is reduced to Si by C. At the same time, a part of SiO₂ was also reduced to SiO gas. The SiO gas overflows out of the furnace with the flue gas, and is absorbed by the air., oxidized to SiOz, and then, these SiO, condensed into glassy spheres with an average particle size <1um, and collected by the dust collector outside the furnace. In addition, the use of zirconWhen producing desiliconized zirconium raw materials, silica fume can also be captured. This silica fume can also be used to make refractories.
The SiO content of silica fume is 90%~98%, the specific surface area is 15~30m/g, the median particle size is 0.3~0.5μm, and the pH=5.3~7.6. Imported silica fume has higher purity and lower pH value. The purity of domestic silica fume is lower and the pH value is higher. The small amount of MgO and R2O contained in domestic silica fume makes it weakly alkaline. In general, refractory castables prepared with imported silica fume have good fluidity, but the cost of raw materials is also high.
