Magnesia carbon brick is made of high melting point alkaline oxide magnesia (melting point 2800℃) and high melting point carbon material that is difficult to be wetted by slag as raw materials, and various non-oxide additives are added. It is a non-burning carbon composite refractory material combined with a carbon binder. Magnesia carbon brick is mainly used for the lining of converters, AC arc furnaces, DC arc furnaces, slag lines of ladles and other parts.
(I) As a composite refractory material, magnesia carbon brick effectively utilizes the strong slag erosion resistance of magnesia sand and the high thermal conductivity and low expansion of carbon, compensating for the biggest disadvantage of poor spalling resistance of magnesia sand.
Its main features are: 1. Good high temperature resistance 2. Strong slag resistance 3. Good thermal shock resistance 4. Low high temperature creep.
(II) The influence of raw materials on the performance of magnesia carbon brick
As the main raw material for the production of magnesia carbon brick, the quality of magnesia sand has a very important influence on the performance of magnesia carbon brick. How to reasonably select magnesia sand is the key to the production of magnesia carbon brick. Magnesia sand includes fused magnesia sand and sintered magnesia sand, which have different characteristics. Fused magnesia: large grains, few impurities, few silicate phases, high degree of direct bonding of grains, and few grain boundaries.
Sintered magnesia: fine grains, relatively more impurities and silicate phases, and poor direct bonding.
For magnesia raw materials, in addition to chemical composition, high density and large crystals are also required in terms of organizational structure. Therefore, as the quality index of magnesia raw materials for the production of magnesia carbon bricks, the following contents should be examined: 1. Magnesium oxide content (purity) 2. Types of impurities, especially C/S and B2O3 content 3. Density, pore diameter, pore morphology, etc. (sinterability) of magnesia.
The purity of magnesia has a significant impact on the slag resistance of magnesia carbon bricks. The higher the magnesium oxide content, the relatively fewer impurities, the lower the degree of silicate phase segmentation, the higher the degree of direct bonding of periclase, and the ability to resist slag penetration and slag melting loss. The juice in magnesia mainly includes calcium oxide, silicon dioxide, and iron oxide. If the impurity content is high, especially the compounds of boron oxide, it will have an adverse effect on the refractoriness and high temperature performance of magnesia.
Magnesia carbon brick production process: fused magnesia and flake graphite are used as the main raw materials, phenolic resin is used as the binder, and a certain amount of antioxidant is added to make it through hot mixing, high pressure molding and heat treatment.
The impurities in magnesia have the following effects:
1. Reduce the direct bonding degree of periclase
2. Form low melting products with magnesium oxide at high temperature
3. Impurities such as iron oxide and silicon dioxide react with carbon before magnesium oxide at 1500-1800℃, leaving pores and making the slag resistance of the product worse.
For magnesia raw materials, in addition to the total amount of impurities, the type and relative content of impurities also have a significant impact on the performance of magnesia. Among them, the influence of CaO/SiO2 ratio and B2O3 content is the most obvious. Generally, magnesia refractory materials usually require CaO/SiO2≥2 to improve the high temperature stability of magnesia carbon bricks.
The carbon material for preparing magnesia carbon bricks is mainly flake graphite.
The organization density of magnesia carbon bricks
The organization density of magnesia carbon bricks depends on the type and amount of binders and antioxidants, the type of magnesia sand, the particle size and amount of graphite, etc. In addition, the molding equipment, brick pressing technology and heat treatment conditions have a certain influence. In order to achieve an apparent porosity of less than 3.0%, ensure that the molding pressure is 2t/cm2, and strengthen the volume density of the matrix part to improve its anti-corrosion performance, magnesia carbon bricks with a particle size of less than 1mm are used in the wind eye bricks and the steel outlet bricks. Different binders also have a certain influence on the density of magnesia carbon bricks. The binder with a high residual carbon rate has a higher volume density. The addition of different antioxidants has significantly different effects on the density of magnesia carbon bricks. Below 800 degrees, the apparent porosity increases with the oxidation of the antioxidant. After exceeding 800 degrees, the apparent porosity of magnesia carbon bricks without metal does not change, while the apparent porosity of those containing metal decreases significantly, and at 1450 degrees it is only half of that at 800 degrees. Among them, the apparent porosity of magnesia carbon bricks with added metal aluminum is the lowest.
The heating speed of magnesia carbon bricks during use will also affect the change of its apparent porosity. Therefore, when using magnesia carbon bricks for the first time, try to increase the temperature at a low speed so that the binder can be completely decomposed at a lower temperature. During the use of magnesia carbon bricks, the temperature difference also has a significant effect on the porosity. The greater the temperature difference, the faster the porosity increases.
High temperature performance of magnesia carbon bricks
01 High temperature mechanical properties Different additives have different effects on improving the high temperature strength of magnesia carbon bricks. Studies have shown that for high temperature flexural strength above 1200℃, no additives < calcium boride < aluminum < aluminum magnesium < aluminum + calcium boride < aluminum magnesium + calcium boride, among which aluminum magnesium + boron carbide is between aluminum magnesium and aluminum magnesium + calcium boride.
02 Thermal expansion performance The participating expansion value of magnesia carbon bricks without metal addition is much lower than the expansion value of metal addition, and the participating expansion value increases with the increase of metal addition.
03 Anisotropy The thermal expansion and high temperature flexural strength of magnesia carbon bricks in different directions are different, mainly due to the orientation of flake graphite, which determines the principles and methods of working lining brick masonry. Magnesium carbon bricks in the vertical direction have higher high temperature strength and lower thermal expansion.
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