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Analysis of the causes of damage to alumina-magnesia carbon bricks and magnesia-carbon bricks during use of carbon-containing refractory materials

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Analysis of the causes of damage to alumina-magnesia carbon bricks and magnesia-carbon bricks during use of carbon-containing refractory materials

Carbon-containing refractory materials

Although carbon-containing refractory materials have significant resistance to molten steel erosion, slag resistance, good thermal shock resistance and high temperature flexural strength, the oxidation problem of carbon limits its service life. During use, magnesia carbon bricks The damage is mainly due to the oxidation of carbon by iron oxides in the slag and the reduction of MgO by carbon at high temperatures, which in turn forms a decarburization layer, leading to the deterioration of the organizational structure of the magnesia carbon bricks, prompting the slag to erode towards the decarburization layer, and during the reaction Low-melting substances appear in the furnace, causing corrosion and erosion as well as the oxidation of carbon by oxygen in the furnace body. In aluminum carbonaceous refractory materials, in addition to the oxidation of carbon, the additive aluminum powder reacts with carbon (4Al+C= Al4C3) generates aluminum carbide whiskers, but aluminum carbide is prone to hydration reactions when exposed to water, resulting in a large amount of volume expansion. Therefore, cracking and powdering often occur, which greatly shortens the service life and frequency of refractory materials.

magnesia carbon brick
magnesia carbon brick

01Damage mechanism of alumina-magnesia carbon bricks

The damage forms of alumina-magnesia carbon bricks are mainly abnormal damage, chemical erosion of the refractory brick components caused by molten slag and molten steel; wear, thermal spalling and structural spalling caused by the flow of slag and molten steel. Oxidation of carbon; wear caused by the flow of slag and molten steel.

a.Chemical attack

The common CaO-SiO2-Al2O3 series slag and FeO in the slag will react with the alumina and magnesia in the alumina-magnesia carbon bricks. In order to prevent alumina-magnesia carbon bricks from forming a solution on their own due to inclusions contained in the brick composition and Si compounds added to prevent oxidation, which promotes chemical corrosion with slag. The SiO2 component should be reduced as much as possible, and fused magnesia, high-purity magnesia and high-purity graphite should be used.

b.Peeling off

The main damage forms of alumina-magnesia-carbon bricks are spalling and chemical erosion. The generation of spinel and the invasion of slag have a great impact on the spalling of alumina-magnesia-carbon bricks. After the alumina-magnesia carbon brick is heated, spinel will be generated between the magnesia and alumina. The generated spinel will expand the volume of the brick, causing micro-cracks in the matrix part. The micro-cracks and gaps around the MgO particles are connected, causing the slag to invade into the interior of the brick, causing the brick to be chemically attacked and structurally peeled off. In order to suppress this phenomenon, it is necessary to control the amount of spinel generated 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 construction, the carbon in the preheated bricks will undergo gas-phase oxidation with oxygen in the air, forming a decarburized layer that will promote chemical erosion and wear. This requires the formation of a slag layer on the working surface to isolate the air. At the same time, in order to improve the oxidation resistance, appropriate addition of antioxidant materials is required.

For alumina-magnesia carbon bricks, the mineral change of spinel generated during use has a great impact on the damage form, so the generation and quantity of spinel should be controlled.

02Damage mechanism of magnesia carbon bricks

Due to the oxidation of carbon and the formation of a decarburization layer, coupled with the huge difference in thermal expansion rates of magnesium oxide and graphite at high temperatures (1.4% and 0.2% respectively at 1000°C), the structure of the material becomes loose, the strength is reduced, and after melting Due to the erosion of slag and mechanical erosion, the magnesium oxide particles in the bricks are gradually eroded and fall off layer by layer, causing 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 falling off – damage.

Above 1600°C, magnesium oxide reacts with carbon 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 hot surface of magnesia carbon bricks is oxidized, a thin layer of decarburization is formed. The formation of the decarburization layer is mainly the result of the oxidation of carbon by iron oxides in the slag and O2 in the air as well as CO2, SiO2 and other oxides. It also includes the gasification of carbon by MgO dissolved in the molten steel or bricks; When high-temperature liquid slag penetrates into the pores in the decarburized layer or cracks caused by thermal stress, the slag will react with the oxidation cast in the brick to form a low melting point compound. These low-melting-point compounds will cause qualitative changes on the surface of the bricks and cause them to fall off and be damaged layer by layer under the stress of strong agitation of steel slag and mechanical erosion. Repeatedly, the furnace lining becomes thinner layer by layer, and eventually the furnace is repaired, repaired, and shut down.

a.Oxidation of carbon

It is precisely because of carbon oxidation that the carbon network structure in the brick is destroyed, making the brick’s organizational structure loose. The strength of the product decreases and the pores increase accordingly. At the same time, the erosion of the bricks by the slag is intensified. The oxidation of carbon is mainly carried out through the following reactions:

Fe2O3+C→2FeO+CO

O2+2C→2CO

CO2+C→2CO

SiO2(s)+C(s)→SiO(s)+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 the magnesia carbon bricks, especially the open pores. The pores provide a channel for the oxidation of carbon, and also intensify the erosion of the slag on the brick lining, thus causing damage to the magnesia carbon bricks. . On the one hand, when cooling, the open pores in the brick suck in air from the outside. When heating, the oxygen in the air reacts with the surrounding carbon to generate CO samples, and the cycle repeats, increasing the porosity. On the other hand, the binding agent in magnesia carbon bricks is an important factor in the generation of pores. As the binding agent of magnesia carbon bricks, phenolic resin is generally added in an amount of 3% to 4%. The porosity of the molded product is about 3%. During the use of the product, the phenolic resin is heated and decomposed to produce a large amount of H2O, H2, CH4, CO, CO2 and other gases that evaporate and are emitted, forming a large number of pores. The oxygen in the air and the oxides in the slag will corrode the bricks through the pores, which not only promotes the oxidation damage of carbon, but also intensifies the reaction between the slag and the MgO in the bricks, causing damage to the magnesia carbon bricks.

 

 

LMM YOTAI established in 2007. Our production technology comes from Japanese Yotai. As an experienced and international player in the refractories industry. We have succeeded in expanding both the breadth of its product range and the depth of its services. From raw material selection, refractory portofio & optimization, installation & services & recycle of used refractories on site to further reduce client’s Opex & Capex in refractory consumption per ton steel output, meanwhile improve product quality of client.

Our Product have been supplied to world’s top steel manufacturer Arcelormittal, TATA Steel, EZZ steel etc. We do OEM for Concast and Danieli for a long time

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