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With the maturity of magnesia-carbon brick production technology, the application range of magnesia-carbon brick is more and more wide. In 1994, hu chaoqun et al. used magnesia-carbon brick as the lining of electric arc furnace, and greatly improved the service life of the lining. Shihezi today, magnesia carbon brick has become the main refractory of most domestic iron and steel enterprises. Although magnesia-carbon brick is widely used in the metallurgical process, its service life is still a big problem because of its harsh working conditions, especially the ladle slag line, the damage of magnesia-carbon brick is especially serious.
In ladle, the chemical composition of slag is complex and changeable, and the temperature changes violently and frequently, especially in ladle slag line, so magnesium-carbon brick with excellent performance is often used in ladle line. The erosion mechanism of magnesia-carbon brick in ladle in slag has been studied deeply at home and abroad.
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[/vc_column_text][vc_column_text]In ladle, because of the complex physical and chemical environment of the slag line, the furnace lining is most easily damaged. The chemical erosion of magnesia-carbon brick by slag is mainly caused by the dissolution of magnesium oxide and the oxidation of carbon in the matrix of magnesia-carbon brick, and the damage of magnesia-carbon brick is caused by the following factors:
- influence of alkalinity: the lower the alkalinity of slag, the more favorable the erosion of magnesiac-carbon brick. If the alkalinity of slag increases, the lower the activity of SiO2 in slag can reduce the oxidation of carbon.
- Influence of MgO: Osbom et al. found that the content of MgO in slag-hanging layer was as high as 30% when analyzing the composition of LF slag line, and believed that the higher the content of MgO in slag, the slower the erosion of magnesia-carbon brick, the higher the alkalinity, the slower the erosion of magnesia-carbon brick by slag.
- The influence of Al2O3: the Al2O3 in slag will reduce the melting point and viscosity of slag, increase the wettability of slag and refractory, make slag more easy to permeate from the grain boundary of magnesia, make the magnesite from the matrix of magnesia carbon brick.
- the influence of FeO: first of all, FeO in slag is easy to oxidize graphite in magnesia-carbon brick at high temperature, and produce bright white iron beads, forming decarbonization layer, as shown in figure 1. Secondly, the magnesia-carbon brick in the cube magnesium stone also reacts with FeO in slag to produce low melting point products.
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Figure 1 metal iron around decarburization layer
[/vc_column_text][vc_column_text]In the process of ladle repeatedly heating and cooling, the magnesium oxide on the surface of refractory is broken due to the inconsistence of the thermal expansion rate between the magnesium iron composite low melting point product and the magnesium iron ore, which leads to the dissolution of brick. Foreign scholars also believe that the increase of iron content in steel slag is unfavorable to the life of magnesia-carbon brick. Firstly, iron FeO accelerates the oxidation of carbon on the surface of magnesia-carbon brick. Secondly, FeO reacts with MgO to loosen the structure of magnesia-carbon brick.[/vc_column_text][vc_text_separator title=”Oxidation of carbon in carbon – magnesite brick” border_width=”3″][vc_column_text]When the magnesia-carbon brick is in contact with the slag, the carbon will decarbonize with the oxides such as FeO in the slag, forming the decarbonization layer under certain conditions, resulting in the loose structure of the working face of the magnesia-carbon brick, which is the main reason for the damage of the magnesia-carbon brick. Carbon reacts with oxides such as CO2, O2 and SiO2 and is continuously oxidized by iron oxides in slag. Secondly the loose structure of the formation of decarburization in the thermal expansion and the slag flushing under the action of creating greater crack and pore, make convenient slag penetration, and with MgO style formation of low melting point, at the same time in molten pool severe mechanical agitation and the steel slag flushing the surface layer of magnesia carbon brick structure under the action of organization change, eventually outside-in gradually damaged, cause serious destruction of magnesia carbon brick. When the temperature exceeds a certain value, the brick structure will be damaged and corroded rapidly. This is because at high temperature, MgO and graphite begin to react with each other.[/vc_column_text][vc_text_separator title=”Stomatal effect” border_width=”3″][vc_column_text]The erosion of magnesia-carbon brick is more likely to occur due to the existence of pores in and on the surface of magnesia-carbon brick. In the process of using magnesia-carbon brick, the porosity accelerates the formation of the decarburization layer, and the slag corrodes the magnesia-carbon brick refractory more seriously. When the external air enters the pores in the magnesia-carbon brick for cooling, the oxygen in the air reacts with the surrounding carbon to produce CO gas, which is discharged through the micro pores. The continuous occurrence of the two processes makes the porosity and pore size gradually increase. The most important factor to produce porosity is the choice of binder in magnesia-carbon brick. Generally, the binder is phenolic resin. If a small amount of phenolic resin is added to the magnesia-carbon brick, the porosity will not be too high under the cold state of about 3%, but the phenolic resin after heating, will decompose to produce water, hydrogen, methane, carbon monoxide (2) and other gases, and in the flow of these gases to form pores, increased porosity. Therefore, magnesium-carbon bricks are eroded by slag passing through the pores, which makes the oxidation of carbon and the dissolution of MgO more intense, thus causing damage to magnesium-carbon bricks. Due to the repeatability of the process of producing gas, the damage of magnesia-carbon brick is increasing.[/vc_column_text][/vc_column][/vc_row][vc_row][vc_column][ult_buttons btn_title=”Conclusion” btn_align=”ubtn-center” btn_size=”ubtn-large” btn_title_color=”#ffffff” btn_bg_color=”#6b96bf” icon_size=”32″ btn_icon_pos=”ubtn-sep-icon-at-left” btn_font_style=”font-weight:bold;” btn_font_size=”desktop:20px;”][vc_column_text]
About magnesia carbon brick damage mainly from two aspects of slag chemical attack and physical penetration to consider, and in different metallurgical reactors, because of the different operating process and slag composition, the erosion mechanism of magnesia carbon brick is not the same, because the steelmaking converter of furnace protection by slag splashing technology on the basic, so on the furnace life of converter are above all furnace, steel-making furnace and refining ladle lining erosion is in serious condition. The oxidation of carbon and the dissolution of magnesium oxide are the main factors for the corrosion behavior of magnesia-carbon brick in slag.
The damage process of magnesia-carbon brick can be summarized as: oxidation, decarburization, loosening, erosion, erosion, fall off and damage. First working face of magnesia carbon brick form graphite oxidation decarburization layer, the decarburized layer of magnesia in thermal stress (graphite and sintered at 1000 ℃, the thermal expansion rates were 1.4% and 0.2% respectively), chemical erosion and mechanical erosion conditions gradually eroded off, fall off suddenly and violently leak after graphite, continue to be oxidation decarburization layer formation, and then to the magnesia dissolution process, under the effect of repeated, cause the damage of magnesia carbon brick.
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