High purity refractories are generally sintered at higher temperatures, and additives can give the materials the desired initial strength and high-temperature performance. Additives shall not significantly reduce the fire resistance of furnace lining materials; Sintering has no effect on the integrity of the material. The commonly used additives are resin powder, linnate, sulfate, borate, silicate and oxide ultrafine powder, etc. Sintering aids include boric acid, brine, water glass, titanium dioxide, etc., which can improve the sintering properties of the materials. At present, boric acid is the most widely used additive.
Boric acid can reduce the sintering temperature of ramming material. Generally, the sintering temperature of corundum and magnesia is higher than 1600 ℃ and that of quartz sand is about 1450 ℃. Boric acid exists in the form of B2O3 after heating and dehydration. It forms low melting point compounds with Cao, MgO and SiO2 at 1000 and 1300 ℃ to reduce the sintering temperature.
However, excessive addition of boric acid will reduce the fire resistance of the material, so the amount of boric acid must be controlled. For the lining of vacuum induction furnace, boric acid and its reaction products will increase boron in liquid steel, so the amount of boric acid must be controlled to the minimum, and other measures must be taken if necessary.
