Title：Introduction to the performance and production process of refractory magnesia-carbon bricks
Keyword：Magnesia, Carbon, Graphite, Powder, Magnesia carbon bricks
Magnesia-carbon bricks are refractory materials made of high melting point basic magnesium oxide (melting point 2800°C) and high melting point carbon materials that are difficult to be infiltrated by slag, adding various non-oxide additives and combining with carbonaceous binders. Magnesia-carbon bricks are mainly used for the lining of converters, AC electric arc furnaces, DC electric arc furnaces, and slag lines of ladle.
Usually, the melting loss of magnesia-carbon bricks is carried out by the reaction between magnesia and slag on the working surface. The size of the melting loss rate is not only related to the properties of magnesia itself, but also depends on the size of magnesia particles. Larger particles have higher corrosion resistance, but they are more likely to float away from the working face of magnesia-carbon bricks and float into the slag. Once this happens, the damage speed of magnesia-carbon bricks will be accelerated.
The absolute expansion of large magnesia particles is larger than that of small particles, and the expansion coefficient of magnesia is much larger than that of graphite, so in MgO-C bricks, the interface between large particles of magnesia/graphite is larger than that of small particles of magnesia/graphite. The stress is large, and the resulting cracks are also large, which indicates that the critical particle size of magnesia in the MgO-C brick is small, which will have the effect of relieving thermal stress.
From the perspective of product performance, the critical particle size becomes smaller, the open pores of the product decrease, and the pore diameter becomes smaller, which is beneficial to the improvement of the oxidation resistance of the product; at the same time, the internal friction between the materials increases, forming difficult, resulting in a decrease in density. Therefore, in the production of MgO-C bricks, it is very difficult to generally determine the critical particle size of magnesia magnesia. It is usually necessary to determine the critical particle size of magnesia according to the specific use conditions of MgO-C bricks. Generally speaking, MgO-C bricks used in parts with large temperature gradient and intense thermal shock need to choose a smaller critical particle size; and parts that require high corrosion resistance, the required critical particle size needs to be larger. In order to improve the bulk density of the product, for manufacturers with small tonnage of molding equipment, the critical particle size can be appropriately larger.
Magnesia fine powder
In order to maintain the overall uniformity of the thermal expansion of the particles and the matrix in the MgO-C brick, a certain amount of magnesia fine powder should be added to the matrix, which is also conducive to maintaining a certain integrity of the structure after the matrix is partially oxidized.
However, if the added magnesia fine powder is too fine, the reduction speed of MgO will be accelerated, thereby accelerating the damage of MgO-C bricks. Magnesia less than 0.01mm is easy to react with graphite, so it is best not to mix such too fine magnesia in the production of MgO-C bricks. In order to obtain MgO-C bricks with excellent performance, the ratio of magnesia to graphite less than 0.074mm in MgO-C bricks should be less than 0.5. If it exceeds 1, the porosity of the matrix will increase sharply.
Graphite addition amount
The amount of graphite added should be considered in combination with different brick types and different use parts of the brick. Under normal circumstances, if the amount of graphite added is less than 10%, it is difficult to form a continuous carbon network in the product, and the carbon cannot be effectively used. Therefore, the amount of graphite added is generally between 10% and 20%. According to different parts, different amounts of graphite are selected. The melting loss of MgO-C bricks is dominated by the oxidation of graphite ink and the dissolution of MgO into slag. Increasing graphite can reduce the erosion rate of slag, but it increases the damage caused by gas phase and liquid phase oxidation. damaged.
The density of graphite is light, and it is easy to float on the top of the mixture during mixing, so that it is not completely in contact with other components in the batch. Generally, a high-speed mixer or a planetary mixer is used. When producing MgO-C bricks, if you do not pay attention to the feeding sequence during mixing, the plasticity and formability of the mud will be affected, thereby affecting the yield and performance of the product.
The correct feeding sequence is: magnesia (coarse, medium) → binder → graphite → mixed powder of magnesia fine powder and additives. The mixing time varies slightly depending on the different mixing equipment. If the mixing time is too long, the graphite and fine powder around the magnesia will easily fall off, and the mud will dry out due to a large amount of volatilization of the solvent in the binder; if it is too short, the mixture will be uneven and the plasticity will be poor, which is not conducive to molding .
Forming is an important way to increase the filling density and densify the structure of the product. Therefore, high-pressure forming is required, and at the same time, it is pressed in strict accordance with the operating procedures of light first, then heavy, and multiple pressures. When producing MgO-C bricks, the density of bricks is commonly used. Control the molding process. Generally, the higher the tonnage of the press, the higher the density of the brick, and the less binder required for the mixture (otherwise, due to the shortening of the distance between particles and the thinning of the liquid film, there are several local binders, resulting in The structure of the product is not uniform, which affects the performance of the product and also produces elastic after-effects that cause brick cracking).