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In which parts of AOD furnaces are magnesia chrome bricks mainly used, and what are the causes of their damage?

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In which parts of AOD furnaces are magnesia chrome bricks mainly used, and what are the causes of their damage?

The composition and properties of slag refined outside the furnace such as argon oxygen furnace and vacuum argon oxygen furnace change greatly during the smelting process, and the basicity changes from about 0.6 in the initial slag to 4.0~4.5 or higher. The refractory lining is corroded by highly corrosive acidic slag and alkaline slag at high temperatures. Therefore, it is meaningful to clarify the relationship between the alkalinity of slag and the slag resistance of refractory materials, whether it is for selecting appropriate refractory materials or trying to improve the steelmaking operation to increase the life of the furnace lining. For magnesia-chromium bricks, when the alkalinity is about 1.4, the amount of slag erosion is minimal. In the low alkalinity range, slag erosion decreases with increasing alkalinity, but in the high alkalinity range, the opposite is true. The amount of slag erosion increases rapidly with the increase of alkalinity, and greatly exceeds the erosion amount at low alkalinity. In other words, magnesia-chrome bricks have high slag resistance to acidic slag, but poor resistance to high-alkalinity slag.

PART 01Main reasons for damage to the MgO-Cr2O3 refractory lining used in AOD furnaces

(1) The smelting temperature is high and the acidic slag acts for a long time, which will lead to corrosion and penetration of the furnace lining;

(2) Thermal spalling and structural spalling due to temperature fluctuations;

(3) Violent gas → slag → molten steel eddy erosion, especially erosion caused by violent stirring of slag and molten steel. Therefore, refractory materials for AOD furnaces must have good thermal shock stability (TWB), high slag resistance, and strong resistance to high-temperature molten steel and slag erosion as well as mechanical damage. At the same time, the refractory materials are required to have high refractoriness, low porosity, and sufficient strength.

The properties of PART 02AOD furnace refractory materials are as follows

parts tuyere bricks  hearth bricks
Apparent porosity,% 15.7 12.8
Bulk density, g/cm² 3.29 3.09
High temperature flexural strength, MPa 108 6.4
MgO 39.2 70.4
Cr₂O₃ 35.6
Al₂O₃ 2.0 0.3
Fe₂O₃ 2.6 0.6
SiO₂ 0.4 0.3
CaO 0.2 28.4
>Impurities 5.2

The performance and composition of the AOD furnace lining masonry bricks are shown in Table 1.

Air vent brick Properties of Bricks Composition of Bricks
Resistance to slag penetration High Cr₂O₃, MgO ratio
Corrosion resistance High purity, low flux
Thermal shock stability Reduce porosity by improving particle size
Fire resistance Comprehensive furnace lining
hearth bricks Resistance to slag penetration MgO-CaO quality bricks
Corrosion resistance CaO-rich matrix
Wear resistance Ultra-high temperature firing

PART 03 When the AOD furnace is built with magnesia-chromium bricks, it can be roughly divided into three parts: furnace cap, furnace lining and tuyere bricks.

01 furnace cap

Usually, the damage to this area is relatively minor. Therefore, high-aluminum (80% Al2O3) plastic combined with phosphoric acid is used, and metal or ceramics are used to anchor the furnace cap shell. Or cast with 50%MgO-Cr2O3 waste brick particles/50% castable (90% Al2O3). Ordinary magnesia-chromium bricks can also be used for construction.

02 furnace lining

Due to the high smelting temperature of the AOD method, the long-term erosion of acidic slag and the rapid gas-slag-liquid steel vortex, the refractory material is seriously damaged. When MgO-Cr2O3 bricks are used as linings, direct bonded magnesia chromium bricks, co-sintered magnesia chromium bricks and rebonded magnesia chromium bricks are usually used. The United States proposes that the MgO of various magnesia-chromium bricks is 40% to 60%, Cr2O3>18%, the content of easily reducible oxide Fe2O3 should be as low as possible, Cr2O3/Fe2O3≥2.5, and Cr2O3/Fe2O3>3 for the main load parts.

The AOD furnace lining was originally made of directly bonded MgO-Cr2O3 bricks, which are made from sintered magnesia with low impurity content and chromium concentrate sand after mineral processing, and are fired at temperatures above 1700°C or even higher. After high-temperature firing, the main crystal phases form direct bonds, while the low-melting-point silicate phase exists in the form of islands. The high-temperature strength, slag resistance and high-temperature volume stability of this brick are better than those of traditional Mg-Cr2O3 bricks or Cr2O3-MgO bricks. Among them, the direct bonded magnesia-chromium brick with high secondary spinel content has the best performance. Among brick-making methods that produce high secondary spinel content, Cr2O3 spinel is the most effective. The most effective way is to add Cr2O3 powder to the ingredients.

In order to adapt to the operating conditions of the AOD furnace, D.R.F. Spencer developed co-sintered magnesia-chrome bricks. It pre-synthesizes low-impurity magnesia and chromium ore at 1800°C to fully react to produce synthetic MgO-Cr2O3 sand, which is then crushed, granulated, shaped, and then fired at high temperature to obtain uniform spinel distribution. , with low silicate content, its resistance to slag erosion and penetration are better than directly bonded magnesia-chromium bricks.

Another type of magnesia-chromium brick used in AOD furnaces is a fused-cast MgO-Cr2O3 brick made by melting a mixture of MgO and chromium ore in an electric furnace. Not only is the spinel distribution relatively uniform, but the porosity is also very low. Although its corrosion resistance is high, its thermal shock resistance is not good, as shown in Figure 1.

In which parts of AOD furnaces are magnesia chrome bricks mainly used, and what are the causes of their damage

The fused magnesia chromium sand is crushed, granulated, shaped, and then fired at high temperature. This product is called rebonded magnesia chrome brick. Its resistance to slag erosion is good, and its thermal shock resistance is also higher than that of fused magnesia chromium bricks. Japan’s Xingqi Steel Plant has successfully developed the AOD furnace composite blowing method. The mixed gas 02+Ar (N2) is blown from the side of the furnace, and oxygen is blown from the top. The 20t AOD furnace is lined with magnesia-chromium bricks. Its service life reaches 235 heats, and the unit consumption of furnace lining is only 6.7kg/t steel.

Semi-rebonded magnesia-chromium bricks are made of fused magnesia-chromium sand and magnesia (or co-sintered magnesia-chromium sand). Therefore, it has some excellent properties of co-sintered magnesia-chromium bricks and rebonded magnesia-chromium bricks, but overcomes the shortcomings of both.

By observing the use of the above-mentioned magnesia-chromium bricks in AOD furnaces and analyzing the comparative test results in the laboratory, it is believed that if the service life of the directly combined MgO-Cr2O3 brick is 1;

So:

Partially co-sintered MgO-Cr2O3 bricks are 1.4;

All co-sintered MgO-Cr2O3 bricks are 2.0;

Combined with MgO-Cr2O3 bricks it is 2.1.

American Refractory Company believes that rebonded bricks have a 50% longer life than directly bonded bricks.

The typical properties of MgO-Cr2O3 bricks, their resistance to slag erosion and their use in AOD furnaces are shown in Tables 2 and 3 respectively.

Table 2 Typical properties of MgO-Cr2O3 bricks
Brick kinddirectly bonded bricks chemical composition,% Apparent porosity % Normal temperature compressive strength MPa  High temperature flexural strength (1480℃) MPa
MgO Cr₂O₃ Al₂O₃ Fe₂O₃ SiO₂ CaO
semi rebonded brick 71.358.9  16.918.8   6.213.0  2.86.0   1.32.0 1.00.8 16.113~18 80.3658.8~107.8 8.23849.8~14.2
(1450℃)
Recombine bricks 66.153.4  18.727.2   7.86.8  4.710.5   1.31.3 1.30.8 13.114.3 99.9652.92 15.39
(1400℃)7.45
Brick kind 64.4  20.8   6.5  6.5   1.2 0.6 11.9 105.84 12.74
Table 3 Slag resistance of magnesia chromium bricks and their use in AOD furnaces
Brick kind chemical composition,% Apparent porosity % High temperature flexural strength (1500℃) MPa Slag corrosion amount % Relative use effect ①
MgO Cr₂O₃ Al₂O₃ Fe₂O₃ SiO₂ CaO
Electrofusion rebonded magnesium-chromium brick 54.1 21.2 7.7 13.8 2.4 0.9 14 2.94 4 2.1
Fully synthetic magnesium-chrome brick 60.0 18.7 6.4 11.8 1.4 0.7 14.4 5.98 5 2.0
Generally directly bonded magnesium-chromium bricks 64.0 13.8 11.2 6.5 2.1 1.0 17.6 2.94 22 1.0
Traditional Chrome-Magnesia Brick 41.2 24.0 19.0 12.0 2.4 1.0 18.3 1.47 23
① Compared with general direct bonded magnesium-chromium bricks.

Therefore, according to the different service life of different types of MgO-Cr2O3 bricks on the AOD furnace and the inconsistent lining damage, comprehensive lining measures can be adopted to achieve the purpose of balancing corrosion and improving the life of the furnace. In addition, the upper lining of the severely damaged air outlet is generally constructed with combined MgO-Cr2O3 bricks or special MgO-Cr2O3 bricks.

03 Air vent brick

If the AOD furnace lining is divided into three areas: the wind blowing side, the trunnion side and the tapping side, their corrosion severity decreases in this order, and the tuyere brick is particularly critical. Usually, high-purity direct bonded magnesia chromium bricks and rebonded magnesia chromium bricks are selected as tuyere bricks for AOD furnaces. In terms of service life, there is almost no difference between the two bricks when used as tuyere bricks for AOD furnaces. The use results show that when semi-rebonded magnesia-chrome bricks and special magnesia-chrome bricks are selected as AOD furnace tuyere bricks, a higher service life can be obtained.

However, there are also some disadvantages when MgO-Cr2O3 bricks are used as linings for the tuyere of A0D furnaces. The main disadvantages are:

(1) Discontinuous corrosion and less than ideal spalling resistance;

(2) Magnesia-chrome bricks are very sensitive to the use temperature. When the smelting temperature is above 1700°C, every 40K increase in temperature will increase the corrosion loss of MgO-Cr2O3 bricks by 1 to 2 times;

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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