Views: 6 Author: Site Editor Publish Time: 2022-08-29 Origin: Site
Before the 1980s, various refractory materials were used as AOD furnace tuyere bricks without satisfactory results, so magnesia-calcium refractory materials were used. Magnesium-calcium refractories, especially dolomite bricks, contain a large amount of CaO, and have gradually achieved good results. At that time, when magnesia-calcium-based refractory materials were applied to AOD furnaces, calcined magnesia-calcium bricks were mostly used considering that dolomite was easy to hydrate and the corrosion resistance of slag was not as good as that of MgO-dolomite bricks.
In the early days of using magnesium-calcium-based refractories on AOD furnaces, it was noticed that there was a very dense "marbling" layer between the hot surface (working surface) and the original brick layer, and its thickness varied depending on the type of use, usually in the Between 25/100mm. At the same time, the fact that the cracks behind the working surface partially healed was also observed, such as the cracks formed after the working surface were healed due to the recrystallization of CaO in the cracks as the working surface went inward.
The importance of this finding is that the recrystallization of f-CaO in Mg-Ca-based refractories enables cracks to heal in service and promotes the formation of a very dense working surface that limits slag penetration and erosion.
This shows that the magnesium-calcium-based refractory used in the AOD furnace can only have a better use effect when it contains a large amount of f-CaO. While a material with a low CaO/MgO ratio tends to improve slag resistance, it also tends to cause a change in the physical properties of the material during use, reducing the material's resistance to spalling.
In the early 1980s, Japan developed a CaO-magnesium dolomite brick with fused CaO added to the matrix.
Compared with the corresponding dead-fired dolomite bricks, the bricks made of fused dolomite have lower heating shrinkage and room temperature strength, which is because the firing temperature is not enough.
The microstructure of fused dolomite bricks shows low ceramic bonding, and only during the firing process, little contact is formed between fine particles and coarse particles; while the ceramic bonding dominates in dead-fired dolomite bricks. This is the internal reason for the different physical properties of the two.
Magnesia-calcium bricks for AOD furnaces are usually made of high-purity raw materials and fired at temperatures above 1750 °C, so their porosity is low and high-temperature shrinkage is small.
The main disadvantages of fired dolomite bricks used on AOD furnaces are:
1. Discontinuous erosion occurs during use;
2. The peeling resistance is relatively poor.
When fired dolomite bricks are used in AOD furnaces, the above two typical disadvantages cannot be overcome by asphalt impregnation. This is because the matrix of the asphalt-impregnated fired dolomite bricks used in the AOD furnace is selectively eroded and the apparent porosity increases.
During the operation of the AOD furnace, the selective loss of the matrix components of the asphalt-impregnated and fired dolomite bricks leads to an increase in porosity and a loose structure, which are the main reasons for the rapid erosion of this type of dolomite bricks. However, the submerged slag composition is small, and it stops at a relatively shallow part (although the residual C is slightly less on the working face side, the total residual amount is 1.4% to 2.3%, which is still good). On the other hand, the slag in the fired dolomite brick not impregnated with asphalt is greatly impregnated and densified.