How to Prolong the Service Life of Magnesia Carbon Bricks

The damage process of magnesia carbon bricks can be divided into 7 steps: oxidation, decarbonization, loosing, corrosion, erosion, spalling and damage. 

First, due to the oxidation of carbon, a decarburized layer is formed. Due to the big differences between the thermal expansion coefficients of MgO and graphite, the structure of bricks becomes loose. This reduces the strength of bricks. Coupled with the slag erosion and mechanical erosion, the MgO particles are gradually corroded and fall off layer by layer. All these result in the damage of the bricks. 

 

A lot of researches have proved that, above 1600℃, the following reaction is the main reason for the damage of the bricks: MgO(s)+C(s)→Mg(g)+CO(g)

 

Some ways to expand the service life of magnesia carbon bricks are as follows:

(1) Use high quality fused magnesia as the main raw material. High quality fused magnesia has well developed and large crystals, high bulk density, low chemical activity and good corrosion resistance. It can also resist the reducing effect of carbon and prevent corrosion to MgO particles. Fused magnesia with high content of magnesia and low content of impurities, especially SiO2, contains a low content of silicate phase. This helps prevent the reaction between fused magnesia and graphite and the oxidation of carbon. 

Besides, the direct bonding of crystals can prevent the corrosion by liquid phase in the grain boundaries and can prevent the penetration of slags. 

Since high quality fused magnesia is expensive, for seriously corroded parts, high quality fused magnesia is used, while for other parts, sintered magnesia can be used. Magnesia carbon bricks are used in different parts of converters.

(2) Improve the purity of graphite. Graphite is the simple substance with the highest melting point currently, as high as 3500℃. It has low thermal expansion coefficient and good chemical stability. Its purity has significant effect on the properties of the bricks. 

With the improvement of the purity of graphite, the corrosion resistance of the bricks increases. In general, graphite with 98% carbon is used. Since purer graphite contains less ash, the content of SiO2 is lower. Since SiO2 can oxidize the carbon in bricks at high temperature. This results in the oxidation of carbon and the increasing of porosity and makes the structure loose. SiO2 can also react with low-melting-point substances such as MgO and Fe2O3. Besides, since carbon has good thermal conductivity and low expansion coefficient, the bricks are not easy to be wetted by slags. So, improving the purity of graphite can improve the thermal shock of products and prevent the penetration of slags into the bricks through pores. 

(3) Add metal antioxidants. Magnesia carbon bricks have poor oxidation resistance and low strength at high temperature. To overcome the two weak points, metal antioxidants are added. This can improve the oxidation resistance and high temperature strength of the bricks. 

Common antioxidants include alumina powder and silica powder. The melting point of alumina powder is 659℃. It has better reducing property than carbon, so it can prevent the oxidation of carbon. So does silica powder. Si can generate SiC with C and form high melting point substance 2MgO·SiO2 with MgO. This is companied with volume expansion. These reaction products can block the passages or fill the pores, which prevent the transfer of CO and CO2 into the brick and protect carbon from being oxidized.