Synthesis Of Periclase-forsterite Lightweight Heat-insulating Refractory By Molten Salt Method

In this thesis,the feasibility of synthesizing periclase forsterite lightweight heat-insulating refractory with sodium chloride molten salt as sintering medium and low-grade magnesite and tertiary talc in Haicheng,Liaoning Province as raw materials was explored.The effects of factors such as the type of molten salt,the amount of molten salt added,the reaction temperature and the holding time on the synthesis of forsterite by molten salt method were studied.The synthesis mechanism of forsterite with sodium chloride molten salt as the medium was discussed in detail,Finally,a lightweight heat-insulating refractory of periclase-forsterite with good physical properties and high temperature service performance was prepared.The conclusions are as follows:1.Under the test conditions,forsterite powder M1 synthesized with Na Cl as the molten salt medium has very high purity,and there are almost no impurities in the XRD spectrum;The purity of forsterite in forsterite powder M2 and M3 synthesized with Na₂CO₃or Na Cl+Na₂CO₃as molten salt medium is very low,because Na₂CO₃reacts with raw material analytical pure Mg O,Si O₂and the generated forsterite to generate a large amount of Na₂Mg Si O₄,and the higher the content of Na₂CO₃,the less the remaining forsterite in the products M2 and M3;The purity of forsterite sample M4 synthesized with Na₂SO₄as molten salt medium is high,but the viscosity of molten Na₂SO₄at 1100℃is high and the mass transfer rate is slow,so there is a small amount of Mg Si O₃left,which does not react with Mg O to generate forsterite.2.In the sodium chloride molten salt system,the optimal reaction conditions are as follows:when the Na Cl molten salt content accounts for 20%of the total mass of the mixture,the sintering temperature is 1200℃,and the holding time is 6 h.Under this process parameter,forsterite can be synthesized from four kinds of raw materials with no enstatite residue found in the raw materials.Na Cl molten salt can not only be used as a liquid phase to promote the sintering of magnesium olivine phase,but also reduce the sintering temperature.It can also be used as a pore forming agent of materials after evaporation at high temperatures.The self decomposition of Mg CO₃and Mg₃[Si₄O₁₀]（OH）₂in the tertiary talc ore,evaporation after Na Cl melting at high temperature,forsterite formation and grain growth during the reaction are all reasons for the formation of pores.3.When the mass ratio of light burned magnesia powder to talc powder is 5:5,the lightweight thermal insulation refractory of periclase-forsterite for ladle can be prepared with a bulk density of 1.46 g/cm³,an apparent porosity of 55%,a compressive strength of 31 MPa,a residual strength retention of 93%,and a thermal conductivity（test temperature 1000℃）of 0.58 W·m^-1·K^-1.In addition to serving as the silicon source for synthesizing the target phase（magnesium olivine phase）,talc is decomposed into pores when heated,which also reduces the thermal conductivity of the sample.From M1 to M4,the increase of talc content will promote the formation of intergranular pores.Using Na Cl molten salt as reaction medium,the mechanism of synthesizing forsterite by molten salt method is mainly"template growth".The forsterite particles take silica particles as templates,and periclase dissociates to the surface of silica particles through the diffusion of molten Na Cl,and reacts with it to form a"core shell"structure with silica and forsterite as the"core"and"shell"respectively,until the synthesis reaction of forsterite is completely completed.