The main raw materials for producing magnesium carbon bricks are: magnesia, graphite, phenolic resin; in addition, there are some auxiliary additives such as: aluminum powder, silicon powder, silicon carbide, high-temperature asphalt powder, etc.
The production of magnesium carbon bricks is actually the process of combining these raw materials in appropriate proportions. According to different use environments and requirements, various different formulas can be designed at different prices, and choosing the right raw materials is particularly important. The following article takes magnesia as an example to discuss how to choose the most suitable magnesia for magnesium carbon brick production.
Magnesia classification, from the production process of magnesia, is mainly divided into two types: one is fused magnesia; the other is sintered magnesia.
Fused magnesia is made from high-quality magnesite by melting and crystallizing in an electric arc furnace. Sintered magnesia is mainly fueled by coal or natural gas, and is obtained by calcining magnesite at 1550~1800℃.
These two types of magnesia have their own advantages. Fused magnesia has high purity and large grains, small pores, high density and strong slag resistance, but its production cost is high. Compared with fused magnesia, sintered magnesia has small grains, large pores and low density, but its production cost is relatively low.
According to the raw materials actually used by the enterprise, the magnesia sand that is more suitable for the production of magnesia carbon bricks is generally the following:
Large crystal magnesia
Large crystal fused magnesia is mainly made of high-purity light-burned magnesia with a magnesium oxide content of 91%-92%, which is melted and insulated in an electric arc furnace.
The appearance of this product is generally white and transparent. Depending on the impurity content, some are slightly grayish white or light yellow. It has high purity, large crystal particles, high density, good slag resistance, and strong thermal shock resistance. It is an important raw material for making high-temperature electrical insulation materials, and is also an important raw material for making various high-end refractory materials such as high-end magnesia bricks and magnesia carbon bricks. It is also widely used in metallurgical industry, chemical industry, national defense research, aerospace, etc.
Ordinary fused magnesia
Ordinary fused magnesia enterprises are referred to as fused magnesia. It is mainly made of magnesite ore with a content of more than 47%, which is melted and kept warm in an electric arc furnace.
Fused magnesia has large grains, high density, low porosity, and good slag resistance. Compared with large crystal fused magnesia, the grains are much smaller and the slag resistance is also slightly worse.
It is often used as an insulating material for high-temperature electricity, and is also the main raw material for making various high-end refractory materials such as high-end magnesia bricks and magnesia carbon bricks. Its main use is in refractory materials.
High purity magnesia (high purity ball)
High-purity magnesia is made from high-purity magnesite ore, which is first lightly burned, then ground and pressed into balls, and calcined in an ultra-high temperature vertical kiln. It is the main raw material for the production of magnesia bricks, magnesia-carbon bricks, cast products and various amorphous refractory materials.
The magnesia sand suitable for producing magnesia carbon bricks are basically the above types. Among them, the magnesium content of large crystal magnesia sand is generally above 97%, the magnesium content of ordinary fused magnesia sand is generally above 96%, and the magnesium content of high purity magnesia sand is generally above 96.5%. Only magnesia sand with such content can be used to produce magnesia carbon bricks. If the quality of magnesia sand is further reduced, it will easily cause product quality problems. The quality of products obtained by different magnesia sands will also be very different.
Factors affecting magnesia
Density of magnesia
When selecting magnesia, the content of magnesium oxide is only one factor, and the density of magnesia is also an extremely important factor. Under the same production process, the higher the content of magnesium oxide, the smaller the pores and the higher the density of fused magnesia. However, driven by economic interests, this is not the case in some cases. Therefore, when selecting magnesia, we must pay attention to it. In addition to the content of magnesium oxide, we must also look at the density of magnesia, which is especially important for manufacturers who purchase granular materials.
Fuel cost
Fuel cost is the most important cost for producing magnesia, whether it is fused magnesia or sintered magnesia. Therefore, in order to reduce production costs, many manufacturers now greatly reduce the time of fusion and sintering to reduce energy consumption. This brings many negative effects, the most prominent of which is the under-burning of raw materials. For magnesia, under-burning is manifested in small magnesia grains, substandard density, and a lot of fine powder after crushing.
When magnesia carbon bricks first appeared, most foreign magnesia carbon bricks were made of sintered magnesia sand. However, after research, people found that magnesia sand and graphite have the following reaction at high temperature: MgO(s)+C(s)=Mg(g)+CO(g)
Studies have shown that this reaction begins at 1600°C, and the reaction speed increases with increasing temperature. This process is one of the important reasons for graphite oxidation and loss of magnesia carbon bricks. The damage of magnesia carbon bricks is first caused by the oxidation of graphite on the working surface under the action of various oxides in the slag and O2, CO2 in the environment, forming a loose decarburized layer, and then the molten slag penetrates into the loose decarburized layer and reacts with magnesia sand to form low-melting products, which reduces the strength of magnesia carbon bricks and accelerates damage. Impurities contained in magnesia sand, such as SiO2, Fe2O3, etc., can accelerate the entire reaction process, so when we choose magnesia sand, we hope that the impurities are as low as possible.
Comparison between fused magnesia and sintered magnesia
Compared with sintered magnesia, fused magnesia has larger grains, better density, lower porosity, and stronger slag resistance, especially large crystal magnesia. In addition, the higher the magnesium oxide content, the lower the corresponding harmful impurities, especially silica. The lower the silica content, the less silicate low-melting products are generated in the product at high temperature. The less low-melting products, the lower the probability of reaction with graphite, and the better the performance of the product. The larger the grains, the higher the degree of direct bonding of the product, and the less liquid phase at the grain boundary, which prevents slag from corroding the product from the liquid phase at the grain boundary.
Therefore, with the development of magnesia carbon bricks, in order to improve the performance of the product, fused magnesia has begun to be used in the production of magnesia carbon bricks. However, due to the high density, low porosity and poor wettability of fused magnesia, sintered magnesia can make up for this problem, so fused magnesia and sintered magnesia are sometimes used together. Due to the poor market conditions in recent years, sintered magnesia has occupied an increasingly large share in the production of magnesia carbon bricks.
According to the results of the use of magnesia carbon bricks in steel mills, magnesia carbon bricks with relatively good on-site use effects are generally produced using magnesia sand with high magnesium oxide content, high density, large crystal particles and high calcium-silicon ratio.
