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Application of high-quality magnesia-calcium bricks in AOD refining furnace

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Table of Contents

At present, refractory materials for AOD furnaces are roughly divided into three types: magnesia-chrome bricks, magnesia-dolomite bricks (magnesia-calcium bricks) and dolomite bricks. European AOD furnace linings generally use calcined dolomite bricks, while most Japanese AOD furnaces still use MgO-Cr2O3 bricks, and some use composite masonry. MgO-Cr2O3 bricks are used below the 10th floor of the wind eye area, and MgO-CaO bricks are used on the front wall, furnace bottom and other parts. In the initial stage of the completion of the TISCO 18t AOD furnace, all furnace linings were made of magnesia-chromium bricks. With the development of magnesia-calcium materials, the use of magnesia-calcium bricks has been gradually promoted in areas outside the wind eye area. Electro-fused semi-rebonded magnesia-chrome bricks have been used in the wind eye area. This choice of materials and integrated masonry methods continued until 2000. With the expansion and transformation of AOD and the implementation of the single-slag refining process, the erosion rate of the MgO-Cr2O3 brick area is significantly higher than that of the MgO-CaO brick area. To this end, a full calcium magnesia furnace lining test was conducted, and a high and stable service life was obtained.

Overview of TISCO AOD furnace

Taiyuan Steel currently has three exchangeable AOD furnaces with a capacity of 40 t. Each AOD furnace has three tuyere openings of <19 mm outside and 15 mm inside. The air supply method is bottom side blowing. The maximum blowing temperature is 1730°C, the blowing time of each furnace is 70 to 80 minutes, and the slag basicity is ≥1.8. The smelting steel types include 304, 304L, 321, 316, 305, 316L, 409, and 430.

The technical conditions and inspection results of magnesia-calcium bricks for Taigang AOD furnace are shown in Table 1.

The bottom of the furnace is laid flat with 5 layers of TZ-3 standard bricks, and then one layer is laid vertically with T-8 bricks. The furnace wall masonry is shown in Figure 1. The brick joints of the furnace lining are required to be ≤2 mm, and the ramming layer should be compacted.

Table 1 Technical conditions and inspection results of magnesia-calcium bricks for AOD furnaces

indexTaibiao 350. 25 -98 actual measurement
w(CaO)/%≥2020. 24
w (MgO + CaO) / %≥9089. 77
Total impurity content/%≤33. 12
Load softening temperature/℃≥1700≥1700
Apparent porosity/  %≤8≤8
Bulk density/(g·cm – 3)≥3. 03. 11
Normal temperature compressive strength/MPa≥7080

Figure 1 Expanded view of AOD furnace wall masonry

The built refining lining needs to be baked for 20 hours before it can be put into use. The lining temperature is required to be ≥1000°C during steel mixing. Due to production rhythm or other reasons, when the furnace shutdown time exceeds 2 hours, the furnace needs to be re-baked to 1000°C before it can be used with steel.

Furnace lining usage

The two sets of MgO-CaO brick furnace linings tested both achieved high service life, 90 and 94 times respectively. The erosion conditions of the two sets of residual linings were basically the same: both caused the furnace to shut down due to erosion and falling off of the slag line on one side of the trunnion. The thickness of the remaining lining around the fallen part is about 50 to 100 mm, and the thickness of the remaining bricks in other parts is 100 to 200 mm; the furnace bottom is corroded into a pot shape, with the maximum erosion depth of 350 mm; the erosion in the eye area is also relatively serious. However, after the furnace structure was modified, the eye areas of the two sets of furnace linings were thickened by 300 mm and 200 mm respectively, making the slag line area the weak link of the current furnace lining. The erosion speed of each layer in the two sets of furnace linings is shown in Figure 2.

Figure 2 Erosion speed of each layer of two sets of furnace linings

Analysis of MgO-CaO bricks after use

The post-consumer MgO-CaO bricks in the slag line area were sampled and analyzed. Judging from the appearance and cross-section of the MgO-CaO bricks after use, most of the remaining bricks have micro cracks, but no large cracks that can accelerate peeling have yet been observed. The change in the residual thickness of the furnace lining is proportional to the number of heats. The corrosion rate of the entire furnace is almost the same, and the brick joints are damaged faster, so the remaining bricks are all pencil-shaped. The reaction layer on the hot surface of the MgO-CaO brick is relatively thick, about 180 mm. Figures 3 and 4 show the changes in the physical properties of the residual bricks, and Table 2 shows the changes in the chemical composition of the residual bricks.

Figure 3 Changes in apparent porosity of residual bricks in the slag line area of AOD furnace lining

Figure 4 Changes in volume density of residual bricks in the slag line area of AOD furnace

Table 2 Changes in chemical composition of residual bricks in the slag line area of AOD furnace (w) %

Distance from hot surface/mmMgOCaOSiO2Al2O3Fe2O3
1062 . 525. 32. 850. 331. 4
306424. 61. 950. 341. 75
11067 . 223. 21. 150. 421
19068210. 80. 310. 8
2006920. 50. 80. 320. 9

The residual bricks can be roughly divided into 4 zones: 0 to 5 mm is the slag layer, which decomposes and peels off due to β2C2S < 400 ℃ γ2C2 S during the cooling process. 5 to 30 mm is the reaction layer; 30 to 180 mm is the C removal layer. In the operating atmosphere of the AOD furnace, in the matrix of the brick layer 5 to 180 mm away from the hot surface, MgO and CaO react with residual C:

MgO(s)+ C(s)= Mg (g)+ CO(g) (1)

CaO (s) + C(s) = Ca(g)+ CO ( g) (2)

Especially in the high temperature and low pCO atmosphere during the oxidation and reduction periods, the generated gas is released from the bricks and the reaction will continue, so the apparent porosity in this area increases and the volume density decreases. CaO(s) is relatively stable than MgO(s), reaction (1) is more violent than reaction (2), and the MgO content in the brick gradually decreases. However, at a distance of about 30 mm from the hot surface, the cracks heal during use due to the recrystallization of free CaO in the bricks. It also promotes the formation of a very dense working layer surface, which limits the penetration and erosion of slag, increases the volume density of the area, and reduces the porosity. ≥180 mm is the original brick layer, and the physical and chemical properties are basically unchanged. Taking the two together, the changes in volume density and apparent porosity at different levels of the residual bricks are shown in Figures 3 and 4. Due to the influence of the intrusion of CaO and other components with high content in the slag, the chemical composition of CaO, SiO2, Al2O3, and FeO at different levels of the residual lining changes to varying degrees. The details are shown in Table 2.

Conclusion

(1) The two sets of all-calcium magnesia furnace linings in this test both achieved a higher service life, exceeding the comprehensive masonry furnace linings.

(2) Both sets of furnace linings were scrapped due to damage to the slag line on the side of the trunnion. Although the wind eye area was seriously eroded, the slag line area became the weak link of the current furnace lining due to the 300 mm thickening.

(3) Through the analysis of the residual bricks, it was found that the MgO-CaO bricks contain highly reactive free CaO during use. It can repair material cracks and form a very dense and impermeable working surface with strong corrosion resistance.

(4) After passing the test, the full MgO-CaO furnace lining has been fully promoted in Taiyuan Iron and Steel Co., Ltd.

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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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