An important way to improve the quality of refractory materials is to improve the purity of refractory bricks, reduce the number of low-melt substances, and increase the density and high-temperature strength of refractory bricks. High-density magnesia refractories and high-purity direct-bonded magnesia refractories are an important trend in the development of magnesia refractories at home and abroad in recent years.
(1) High-purity raw materials, high-pressure molding, high-temperature firing and their significance
A basic lesson in refractory production is based on concentrate, i.e. high purity. The impurity content of the flux has a great influence on the slag resistance and high temperature strength of the lining brick. And directly affect the life of the furnace lining.
The purpose of high pressure forming is to improve the strength and density of refractory bricks. The greater the density of the brick, the more difficult it is for the slag to penetrate inside the brick.
High-temperature sintering is a measure taken to achieve full sintering on the basis of high-purity raw materials. The so-called direct bonding structure can be formed in the brick under the condition of little liquid phase, and the density of the refractory brick can be increased at the same time, and the high temperature strength of the brick can be improved. It should be said that the improvement of high temperature strength of refractory bricks is the comprehensive result of factors such as reduction of impurity content, high pressure forming and high temperature firing. Tests and experience have shown that the high temperature strength of bricks can predict the ability of bricks to resist scrap wear.
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(2) Composition of brick making
①Control the ratio of c/s. The smaller the ratio of CaO/SiO2 is, the secondary phases coexisting with the main crystal phase periclase in magnesia refractories are mainly minerals with low melting point such as forsterite and iron rose pyroxene. When the CaO/ratio is increased, there are dicalcium silicate and tricalcium silicate high melting point minerals. Therefore, the secondary relative bricks other than periclase have a great influence on the high temperature strength of the brick. The CaO/SiO2 ratio of magnesia brick should be controlled within the optimum range to obtain the maximum strength. Otherwise, low-melting silicates will be formed, resulting in a significant decrease in strength. Considering the slag resistance, a high CaO/SiO2 value is also necessary, because magnesia bricks with a high CaO/SiO2 ratio have better corrosion resistance to the initial slag when used in a converter.
②Reducing the flux effect of B2O3 This is a problem brought about by using seawater magnesia as raw material.
③Adjust the mineral composition of refractory bricks, increase the initial temperature of liquid phase and reduce the amount of liquid phase at high temperature.
The use of high-purity raw materials, high-pressure molding, and high-temperature firing can significantly improve the quality of refractory materials, but this is often constrained by cost. In improving quality, a practical approach is to understand the role of low melting materials, so as to control it and reduce its harmful effects. This can be done from two aspects: avoiding the production of low melts, that is, increasing the initial temperature of liquid phase generation and reducing the amount of liquid phase at high temperature; controlling the distribution of liquid phase so that the liquid phase does not surround refractory solid particles at high temperature, and are repelled in the interparticle gaps, so that the liquid phase does not penetrate the particles, thereby increasing the solid-phase interparticle contact and achieving direct bonding.
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(3) Control the structure of refractory materials
The so-called structural control is both the phase distribution in the refractory material. The practical significance of this research work was developed from the emergence of so-called direct-bonded magnesia-chrome bricks. At high temperatures, the liquid phase tends to move towards the inter-particle gaps, rather than surrounding the particles, forming a direct bond between the solid-phase particles.
① For magnesia bricks containing MgO and liquid phase at high temperature, in order to improve the degree of direct bonding between periclase, or to prevent the liquid phase from penetrating the periclase particles. Then adding Cr2O3 is very beneficial.
② For materials with two solid phases and liquid phases at high temperature (such as dolomite bricks, magnesia-chrome bricks or chrome-magnesium bricks), the combination of different particles is easier than the combination of the same particles. Therefore, it is very advantageous to use spinel or M2S, C2S refractory minerals as secondary relatively direct bonds.
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