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Causes of damage and improvement methods of magnesia-chrome bricks and magnesia-calcium bricks used in VOD furnaces

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Regarding the configuration of refractory materials for the VOD furnace lining, direct bonded magnesia-chromium bricks are mostly used at the bottom and molten steel parts; semi-rebonded magnesia-chromium bricks and electrofused rebonded alumina-magnesia-chrome bricks are used in the slag line. Furnace lining repair material uses MgO-MgO·Al2O3 castable.

Causes of damage to magnesia-chrome bricks used in VOD furnaces

Mainly due to the infiltration of molten slag, the magnesia becomes loose, chemically corroded, structurally peeled off, and pulverized; at the same time, the infiltration of molten slag causes high-temperature peeling, the molten steel is strongly stirred or the slag is worn, etc. After magnesia-chromium bricks are heated and cooled, the strength and toughness of the material decrease, and the structure cracks, which promotes the penetration of molten slag into the bricks, making the particles easy to lose and accelerating the corrosion of the bricks. In order to improve the resistance of magnesia-chrome bricks to the erosion of VOD slag at high temperatures greater than 1700°C and improve their performance, the manufacturing process of magnesia-chrome bricks should be improved.

Improvements

(1) Strictly control the chemical composition of bricks.

Cr2O3 content.

For magnesia-chromium bricks used in the VOD slag line, generally the corrosion rate decreases as the Cr2O3 content in the brick increases. Therefore, increasing the Cr2O3 content can increase the corrosion resistance of magnesia-chromium bricks.

w(Cr2O3)/w(MgO) ratio. w(Cr2O3)/w(MgO)=2/3 is optimal. When the Cr2O3 content exceeds 25%, the secondary magnesia-chromium spinel in the material reaches saturation, which can prevent the penetration of low-alkalinity slag and improve the corrosion resistance of the brick.

SiO2 content.

SiO2 is the main factor that reduces the corrosion resistance of magnesia-chromium bricks fired at high temperatures. Its content should be as low as possible, so its content should be strictly controlled when making bricks, preferably less than 1%.

Fe2O3 and Al2O3 content.

The spinel in magnesia chrome bricks increases as the Fe2O3 content decreases and the Al2O3 content increases. Therefore, adjusting the Fe2O3 and Al2O3 content in the bricks can promote the spinel content and improve the corrosion resistance of the bricks.

Strictly control the content of impurity CaO. CaO can form a low melting point silicate phase with SiO2 in the slag and MgO in the bricks, reducing the corrosion resistance of the material, so its content should be strictly controlled.

(2) Brick making process.

Particle size and distribution. Multi-stage ingredients of coarse, medium and fine discontinuous particles are used to fill the gaps between coarse particles to obtain products with low porosity.

High pressure molding. The density of the semi-finished product is the key to obtaining low-porosity products, so high-pressure molding is also beneficial to increasing the density of the material, reducing the porosity of the material, reducing the erosion channels of slag, and improving the anti-erosion effect of the material.

Firing at ultra-high temperature. Controlling the cooling rate and increasing the oxygen partial pressure in the firing atmosphere are important aspects to improve the performance of magnesia-chrome bricks.

Damage mechanism of magnesia-calcium bricks for VOD furnace

The VOD slag line uses asphalt or tar combined with dense dolomite products or magnesia carbon bricks (w(C)=10%); the bottom and molten steel parts use asphalt or tar combined with dolomite products.

The damage mechanism of magnesia-calcium bricks for VOD furnaces is similar to that of magnesia-chrome bricks, which are still chemical erosion, structural peeling, high temperature peeling, wear, etc. The difference is that when magnesium dolomite bricks or dolomite bricks are used, the SiO2 in the low-alkalinity slag and the CaO in the bricks generate 2CaO·SiO2 or 3CaO·SiO2, which can turn the slag into a high melting point and high viscosity slag. However, when the solubility of FeO or Al2O3 in the slag is high, it will form low melting point substances with CaO in the brick, aggravating its damage.

ways to improve

(1) Improve corrosion resistance and inhibit cracking.

The main method is to increase the MgO content in dolomite or increase the CaO content in magnesium dolomite; in order to improve the thermal shock resistance of magnesium dolomite bricks, zirconia can be added to the material.

(2) Enhance the hydration resistance of raw materials.

1) Additives. Various additives (Fe2O3, CuO, CaCl2, 2CaO·Fe2O3 or CaO·TiO2) are added to the raw materials to promote the crystallization of CaO particles, and CaO is completely coated, improving the microstructure of magnesia-calcium sand.

2) Special treatment of raw materials. In the production process of raw materials, special additions such as rare earths (La2O3, CeO2, Y2O3) can be added to the raw materials to improve the hydration resistance of magnesia-calcium sand; magnesia-calcium sand or calcium sand is carbonated and passivated. Soaking treatment (soak the raw materials in an aqueous solution such as phosphoric acid or boric acid to generate a layer of inorganic calcium salt on the surface of the particles to coat the surface of the material). Spraying treatment (spray organic solvents such as styrene, butadiene, and 2% borax solution on the surface to isolate the air).

(3) Product waterproofing measures:

1) Improve the brick-making process. Oil immersion or paraffin impregnation can block the pores in the bricks and prevent air from entering and hydrating the free CaO in the product. When packaging, use vacuum and aluminum foil packaging to isolate air and prevent hydration of the product.

2) Use anhydrous resin. Use anhydrous resin to bind magnesium calcium carbon to reduce hydration during the mixing process.

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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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