Study On The Expansion Character Of Perlite Covering Flux

With the output of foundry goods getting higher and higher by years, the demand of the casting covering flux becomes more and more. At the same time, the perlite covering flux is used widely in the industry field as a high efficiency covering flux. The expansion character of perlite is its leading nature in its use characteristics, Based on this, the study on the expansion character of perlite has great significance.The inherence impetus of expansion is the water existed in the inner of perlite, but just with the proper quality, much more or less is disadvantage for the expansion. So the pretreatment technology is necessary in order to remove the excess water in the raw ore.Firstly, the paper discussed the degree of the influence on the last expanding rate through orthogonal test generated by the three main factors which are preheat temperature, preheat time and particle size. The result said that the particle size and preheat temperature was the most important factor, following them was the preheat time, and the affect of their interaction was not important. Through the single factor test in this experiment, the best pretreatment technology parameter which were 750℃for preheat temperature ,10s for preheat time and 0.9mm～1.7mm for particle size was found out.Secondly, the perlite expansion character was discussed through three aspects that the mechanism of action of water, micropore and viscosity With the mechanism of action of water, in raw perlite the form of water that the fracture water, structure water encapsulated in the glassiness, exist as water molecule and hydroxy, and the crystal water which can be ignored because of its microcontent by the DTA differential thermal analysis and FTIR infra-red spectrum was found out. The change of viscosity and desiccation of Si-OH bond was the most important factor in the expanding of perlite. With the mechanism of action of micropore, it made for the max last expanding rate because of the water vapor's quickly gathering in the micropore forming in the high temperature instant-preheat process. With the mechanism of action of viscosity, the high temperature-instant-preheat process could increase the polymerization degree of the silicon-oxy tetrahedron so as to increase the viscosity of the perlite, which was advantage for the max expanding rate.Finally, a new covering flux was discussed. As a result, the product dried after being immersed in the Na3AlF6 is preferable, and it gained good effect in industry application.