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What are the alkaline refractory bricks? How much magnesium content can be achieved?

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In the refractory industry, refractory bricks with MgO as the main component and periclase as the main crystal phase are collectively called alkaline refractory bricks. Alkaline refractory bricks are mainly used in thermal kilns used in cement, glass, ferroalloy and other industries.

Taking the glass melting furnace as an example, alkaline refractory bricks are mainly used as checker bricks in the regenerator. They have high thermal conductivity, long service life and high volume density, which increase the heat storage in the regenerator. The porosity of alkaline refractory bricks is low; the thermal expansion coefficient increases linearly as the temperature rises, and is the largest among various refractory materials. Care should be taken to leave sufficient expansion joints during use.

Alkaline refractory bricks are non-insulators above 900°C and have very good conductivity and cannot be used as high-temperature insulating materials.

Alkaline refractory bricks must not be used in gas regenerators, and hydration will occur when exposed to water vapor for a long time at 40 to 160°C, which will cause the brick structure to become loose and damaged, so they must be kept dry during storage. Before laying the checkered bricks, the regenerator must be dried. The physical and chemical properties of foreign alkaline refractory bricks are listed in Table 1.

Physical and chemical indicators of bricks High-purity magnesia bricks Standard magnesia bricks Directly bonded magnesia bricks Magnesia-chrome bricks Forsterite bricks Forsterite bricks containing chromium oxide  Chrome magnesia bricks
Chemical composition (mass fraction)/% MgO 97~98 90~94 70~73 60~70 50~60 45~60 40~42
CaO 0.5~0.15 0.5~1.5 0.5~1.5 1.0~2.0 1.5 0.2~1.5 0.5~1.5
Al₂O3 0.2~0.5 0.5~1.0 8~10 8~12 1~8 4-10 20~22
Cr2O3 8~12 10~14 1.5 4~10 20~24
Fe₂O₃ 0.1~0.4 0~1.0 4.0~6.0 5~7 ~8 6~12 9~11
SiO₂ 0.1~0.3 4.0~6.0 2~3 3.5~4.5 25~40 35 4.5~5.5
Physical properties Bulk density/(t/m³) 2.9~2.95 2.85~2.90 3.0~3.1 2.9~9.0 2.6~2.8 2.7~2.8 2.9~3.0
Porosity/% 15~18 15~18 15~18 17~20 16~23 16~23 20~22
Compressive strength/MP 60~90 70~90 50~70 30~50 >25 >25 35~50
Refractory degree/℃ >2000 >2000 >2000 >2000 >2000
Load softening point/℃ >1730 1650~1700 >1730 1600~1650 1600~1650
High temperature flexural strength (at 1200℃)/MPa 7.0 13.0
Thermal conductivity/[W/(m·K)]300℃ 5.0~5.9 3.4~4.2 2.9~3.7 1.7~2.5
800℃ 4.2~5.0 2.5~3.4 2.5~3.4 1.3~2.1
1300℃ 3.4~4.2 2.1~2.9 2.1~2.9 1.3~2.1

At present, the main varieties of magnesia refractory materials are

01 Ordinary magnesia brick: Made from sintered magnesia stone as raw material, it contains about 91% MgO. It is a magnesia refractory product directly combined with silicate. It is widely produced and used.

02 Direct bonded magnesia brick: Made of high-purity sintered magnesia as raw material and sintered. Containing more than 95% MgO, it is a magnesia refractory product directly bonded between periclase grains.

03 Magnesium Silica Brick: Made from high-silicon sintered magnesia as raw material, it is made by sintering and contains SiO25%~11%. It is a magnesia refractory product combined with forsterite.

04 Magnesia Chrome Brick: Made from sintered magnesia stone as the main raw material, adding an appropriate amount of chromium ore, and sintering, containing Cr2O38%~20%. The composition and properties of typical magnesia chrome bricks are shown in Table 2.

Composition and performance Fused cast magnesia chrome bricks Direct bonded magnesia chrome bricks  Silicate bonded magnesia chrome bricks
SiO₂ content/% 2.81 2.02 4.33
MgO content/% 52.52 82.61 69.32
Cr₂O₃ content/% 20.06 8.72 10.60
Apparent porosity/% 10 15 19
Bulk density/(g/cm³) 3.38 3.08 2.97
Compressive strength/MPa 92~111 59.8 39.8
Load softening initial temperature/℃ >1700 1690 1610
Thermal shock resistance (1100℃ water cooling)/time >9

05 Forsterite Brick: Forsterite refractory material is a refractory material with forsterite as the main crystal phase. It is mostly made of peridotite and pure peridotite as the main raw materials, and the formed products are called forsterite bricks.

06 Magnesia-alumina brick: Made of sintered magnesia as the main raw material, adding an appropriate amount of Al2O3-rich materials, and sintering.

07 Magnesite-calcium brick: Made from high-calcium sintered magnesia stone as raw material, it is made by sintering and contains CaO 6%~10%.

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.

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