This article describes the damage mechanism of alumina-magnesia-carbon bricks and the influence of carbon oxidation and pores to explain the damage mechanism of magnesium-carbon bricks through chemical erosion, spalling, and carbon oxidation.
Key words: alumina-magnesia-carbon brick, magnesia-carbon brick, damage mechanism
Although carbon-containing refractories have remarkable resistance to molten steel erosion, slag resistance, good thermal shock resistance and high-temperature flexural strength, the problem of carbon oxidation limits its service life. The damage is mainly due to the oxidation of iron oxides in the slag to carbon, and the reduction of carbon to MgO at high temperature, thereby forming a decarburization layer, which leads to the deterioration of the structure of magnesia carbon bricks, and promotes the erosion of slag to the decarburization layer. Low-melting substances appear in the medium, causing erosion and erosion and the oxidation of carbon by oxygen in the furnace. In aluminum-carbon refractories, in addition to the oxidation of carbon, the additive aluminum powder reacts with carbon (4Al+C= Al4C3) to form aluminum carbide whiskers, but aluminum carbide is prone to hydration reaction when it meets water, resulting in a large amount of volume expansion, so cracking and pulverization often occur, which greatly shortens the service life and frequency of refractory materials.
Damage mechanism of alumina-magnesia-carbon bricks
The damage form of alumina-magnesia-carbon bricks is mainly abnormal damage, chemical erosion of refractory brick components by slag and molten steel; wear, thermal spalling and structural spalling caused by slag and molten steel flow; carbon oxidation; slag and molten steel flow. wear and tear.
a. Chemical attack
The common CaO-SiO2-Al2O3 slag and FeO in the slag will react with the alumina and magnesia in the alumina-magnesia-carbon brick. In order to prevent alumina-magnesia-carbon bricks from generating a solution by themselves due to the inclusions contained in the brick components and the Si compound added to prevent oxidation, which promotes the chemical aggressiveness of the slag, the SiO2 component should be reduced as much as possible, and the fused magnesia should be used at a high temperature. Pure magnesia and high-purity graphite.
b. Peel off
The main damage forms of alumina-magnesia-carbon bricks are spalling and chemical erosion. The formation of spinel and the intrusion of slag have a great influence on the spalling of alumina-magnesia-carbon bricks. After the alumina-magnesia-carbon brick is heated, spinel will be formed between the magnesia and alumina, and the generated spinel will expand the volume of the brick, resulting in micro-cracks in the matrix, and micro-cracks and gaps around the MgO particles. It leads to the intrusion of slag into the interior of the brick and structural peeling of the brick while being subjected to chemical corrosion. In order to suppress this phenomenon, it is necessary to control the amount of spinel in the alumina-magnesia-carbon brick and the composition of the magnesia particle size.
C. Oxidation of carbon
When resin-bonded alumina-magnesia-carbon bricks are dried after masonry, the carbon in the bricks will undergo gas phase oxidation with oxygen in the air after preheating, and the formation of a decarburization layer will promote chemical erosion and wear. Therefore, it is required to form a slag layer on the working surface to isolate the air, and at the same time, in order to improve the oxidation resistance, it is necessary to add anti-oxidation materials appropriately.
For alumina-magnesia-carbon bricks, the mineral change of spinel during use has a great influence on the damage form, so the generation and quantity of spinel should be controlled.
Damage mechanism of magnesia carbon brick
Due to the oxidation of carbon and the formation of a decarburization layer, coupled with the large difference in thermal expansion rates between magnesium oxide and graphite at high temperatures (1.4% and 0.2% at 1000°C), the structure of the material becomes loose, the strength decreases, and after melting The erosion of slag, mechanical erosion, etc., make the magnesia particles in the bricks gradually eroded and fall off layer by layer, resulting in damage to the magnesia carbon bricks. The damage process of magnesia carbon bricks is divided into the following steps: oxidation of carbon – formation of decarburization layer – loose structure – slag erosion – mechanical erosion – structure shedding – damage.
Above 1600°C, magnesia and carbon react to generate a large amount of gas, which is the main cause of damage to magnesia carbon bricks.
MgO(s)+C(s)→Mg(g)+CO(g)
When the carbon in the working lining heat surface of the magnesia carbon brick is oxidized, a thin layer of decarburization layer is formed. The formation of the decarburization layer is mainly due to the oxidation of carbon by iron oxides in the slag and O2 in the air, CO2, SiO2, etc. The result of oxidation of substances, including the gasification of carbon by MgO dissolved in molten steel or in bricks; when high-temperature liquid slag infiltrates into pores in the decarburized layer or cracks generated due to thermal stress , the molten slag will react with the cast oxide in the brick to form compounds with low melting points, which will lead to qualitative changes on the surface of the bricks and fall off and be damaged layer by layer under the stress of strong agitation of steel slag and mechanical scour, and so on. , the furnace lining becomes thinner layer by layer, and finally the furnace is mended, repaired and shut down.
a. Oxidation of carbon
It is precisely because the carbon oxidation destroys the network structure of carbon in the brick, making the brick structure loose, the strength of the product decreases, and the pores increase accordingly. At the same time, the erosion of the brick by the slag is intensified. The oxidation of carbon is mainly through the following reaction ongoing:
Fe2O3+C→2FeO+CO
O2+2C→2CO
CO2+C→2CO
SiO2(s)+C(s)→SiO(g)+CO(g)
MgO(s)+C(s)→Mg(g)+CO(g)
b. Influence of stomata:
Another important factor affecting the damage of magnesia-carbon bricks is the pores in magnesia-carbon bricks, especially the open pores. The pores provide channels for the oxidation of carbon and also intensify the erosion of slag on the brick lining, thus causing damage to magnesia-carbon bricks . On the one hand, when cooling, the open pores in the brick inhale air from the outside, and when heating, the oxygen in the air reacts with the surrounding carbon to form CO, which repeats itself to increase the porosity. On the other hand, the binder in magnesia-carbon bricks is an important factor for the generation of pores. As a binder for magnesia-carbon bricks, phenolic resin is generally added in an amount of 3% to 4%, and the porosity of the molded product is about 3%. During the use of the product, after the phenolic resin is heated and decomposed, a large amount of H2O, H2, CH4, CO, CO2 and other gases are evaporated and discharged, forming a large number of pores, so that oxygen in the air and oxides in the slag will pass through The pores erode bricks, which not only promotes the oxidation damage of carbon, but also intensifies the reaction between slag and MgO in bricks, resulting in the damage of magnesia carbon bricks.