Taking magnesia refractory as an example, the damage mechanism of magnesia refractory is described as follows:
The damage of magnesia materials is mainly manifested in thermal erosion caused by flowing molten steel and chemical erosion caused by penetration of slag components into materials. Manufacturer of fused magnesia
During smelting, the solution will penetrate into the refractory matrix through the capillary channels in the refractory matrix to erode the furnace lining. The components penetrating into the refractory matrix include; SiO2 and FeO in slag; Fe, Si, Ai, Mn, C in molten steel, even metal vapor, CO gas, etc. These infiltrated components are deposited in the refractory capillary channels, resulting in the discontinuity between the physical and chemical properties of the refractory working face and the original refractory matrix. Cracks, flaking and structural looseness will occur under the sudden change of operating temperature. Strictly speaking, this damage process is much more serious than the dissolution damage process.
The metal materials added into the furnace will bring in various oxides, and the slag composition of different materials and different heats will be different. Most of the oxides, carbides, sulfides and various forms of compound compounds in the slag will react with the furnace lining to generate new compounds with different melting points. Some low melting point oxides generated in the reaction, such as iron olivine (FeOSiO2) and manganese olivine (MnOSiO2), generally have a melting point of about 1200 ℃. Low melting point slag has good fluidity, which may form the role of flux and cause severe chemical erosion to the furnace lining, thus reducing the service life of the furnace lining.
The high melting point slags generated in the reaction, such as mullite (3Al2O3 · 2SiO2), forsterite (2MgO · SiO2), and some high melting point metal elements can have a melting point of more than 1800 ℃. The high melting point slags and low melting point slags suspended in the metal liquid also have a relatively complex interpenetrating and mutual dissolution effect. These slags are easy to adhere to the furnace wall and form a accumulation, causing serious slag sticking, affecting the power Melting rate and capacity until affecting the service life of furnace lining.
With the increase of furnace capacity, the proportion of heat lost on the surface of molten steel decreases, the temperature of slag is higher than that of small capacity furnace, and the fluidity of slag is also better than that of small capacity furnace, so the erosion of furnace lining is intensified. Large induction furnaces usually adopt the method of mixing steel and slag for tapping, which requires good fluidity of slag to adapt to tapping conditions. Therefore, the slag line is severely eroded, which is another reason for the decline of lining service life. Because of the above reasons, the service life of large induction furnace lining is lower than that of medium and small induction furnaces. In order to improve the service life of furnace lining, the thickness of furnace lining should be appropriately increased. However, with the increase of lining wall thickness, the resistance value increases, the reactive power loss increases, and the electrical efficiency decreases. Therefore, the thickness of furnace lining wall is limited to the range. Therefore, reasonable wall thickness must be selected to ensure high electrical efficiency and service life of furnace lining.
Solution design
The above erosion causes the so-called structure peeling under the cyclic fluctuation of temperature. In the production process, the slag penetrates into the pores of the refractory matrix, forming a large thickened refractory layer. Some physical and chemical properties of refractories soaked by slag will change. Due to the different thermal expansion coefficient between the permeable layer and the residual inconvenient layer, when the temperature changes, a large stress will appear at the junction of the two layers, which will lead to cracks parallel to the working face and eventually cause the furnace lining to peel off. The slag infiltrated into the refractory matrix will dissolve the refractory particles and weaken the combination between particles, thus leading to the decline of the refractoriness and high temperature resistance of the material. As a result, the refractory of slag penetration layer is damaged faster under the erosion of flowing molten steel.
The basicity of slag shall be compatible with the lining material. Magnesium lining materials can be eroded by high slag and SiO2 slag. The amount of CaF in the slag should be controlled. Excess CaF will erode the basic furnace lining, causing premature melting in the slag line area. When the fluoride and manganese ions in the slag are high or the bath temperature is above 1700 ℃, the viscosity of the solution will also drop sharply, the lining will be damaged faster, and the lining life will be greatly reduced. When slag free smelting is conducted under vacuum, the service life of furnace lining is longer than that of non vacuum smelting.
The high iron oxide content infiltrated into the furnace lining will damage the microstructure of the original furnace lining, reduce the refractoriness, and reduce the viscosity of the - Ai2O3-SiO2 system slag, so that the slag can penetrate into the deeper layer of the material. However, the amount of iron oxide contained in the original furnace lining is conducive to the rapid sintering of the furnace lining, reducing the opening pores and permeability of materials. Especially the amount of iron oxide contained in molding materials, the rapid sintering, sand washing and sand inclusion of materials are very prominent.
Increasing the magnesium oxide content and slag viscosity is not only beneficial to reducing the erosion of slag on the furnace lining, but also beneficial to improving the slag collection effect. When the basicity of slag is low, the erosion of magnesia lining is more serious, and the life of lining decreases accordingly; On the contrary, when the slag alkalinity is high, the erosion of the furnace lining is slight, and the service life of the furnace lining is relatively improved. Increase slag alkalinity and MgO content in slag, and reduce FeO content in slag, which is beneficial to reduce the corrosion of slag on refractory. Therefore, when using slagging agent, attention should be paid to the selection of high magnesium oxide materials. Reasonably configure slag material structure, accelerate slag forming speed, shorten smelting time and reduce iron oxide content in slag.
Proper slag shall be selected according to the material of furnace lining. Alkaline slag is suitable for magnesium lining, but it can be eroded by high slag and SiO2 slag. Excess CaF2 will also erode the basic lining, causing premature melting in slag line area. Acid slag is suitable for quartz lining, while magnesium aluminum lining can only be used for weak alkaline or neutral slag. Under high temperature, the alumina lining will show typical amphotericity in different acid-base degrees. It can adapt to the slag with different acid-base degrees, but it is slightly worse than the acid lining and the alkaline lining. For this reason, some use high-purity magnesia and add spinel to change the matrix properties of pure magnesia furnace lining materials when selecting materials, but the corrosion resistance of high-purity corundum materials is also obviously inferior to that of sintered magnesia with low purity. Acid slag is suitable for quartz lining, while magnesium aluminum lining can only be used for weak alkaline or neutral slag. Under high temperature, the alumina lining will show typical amphotericity in different acid-base degrees. It can adapt to the slag with different acid-base degrees, but it is slightly worse than the acid lining and the alkaline lining.
In a word, considering the main damage mechanism of magnesia furnace lining, through continuous summary and exploration, the resistance to slag penetration of materials can be improved by limiting the opening pores and permeability, and the high temperature corrosion resistance and anti stripping of furnace lining matrix can be improved by increasing the high temperature bending strength and severe softening temperature.
The performance of furnace lining depends on many factors, such as particle size distribution, physical and chemical properties of materials and sintering temperature of furnace lining.
epilogue
1) The furnace lining material shall have the characteristics of high thermal strength, low air permeability and low porosity.
2) The initial destruction of the furnace lining is due to the front end erosion caused by the dissolution of the sintering layer of the furnace lining refractory in the slag, and the cracks caused by the cyclic change of temperature. Another factor causing crack growth in the integral lining is the stress caused by the difference of expansion coefficient between the "three belts" caused by heating to too high temperature.
3) The service life of furnace lining depends on correct operation, including the detection of lining body, and the timely repair of cracks with "furnace mending material" to prevent molten steel and slag from infiltrating into the furnace lining matrix.
