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Descriptions of Refractory brick lining

Views: 20     Author: Site Editor     Publish Time: 2023-05-31      Origin: Site

In materials science, a refractory (or refractory) is a material that resists decomposition by heat, pressure or chemical attack and retains its strength and shape at high temperatures.Refractory materials are polycrystalline, heterogeneous, inorganic, non-metallic, porous and heterogeneous. They usually consist of oxides or carbides, nitrides, etc. of the following elements: silicon, aluminum, magnesium, calcium, boron, chromium and zirconium.ASTM C71 defines refractories as "nonmetallic materials having chemical and physical properties making them suitable for use in components of structures or systems exposed to environments above 1,000 °F (811 K; 538 °C)". Refractories are used in furnaces, kilns, incinerators and reactors. Refractories are also used in the manufacture of crucibles and molds for casting glass and metals, and in the surface treatment of flame deflection systems for rocket launch structures.Today, the steel industry and the metal foundry industry use approximately 70% of all refractory materials.

Refractory materials:Refractory brick lining

Refractories must be chemically and physically stable at high temperatures.Depending on the operating environment, they must be resistant to thermal shock, chemically inert, and/or have a specific range of thermal conductivity and coefficient of thermal expansion.

Oxides of aluminum (alumina), silicon (silica) and magnesium (magnesia) are the most important materials used in the manufacture of refractories. Another oxide commonly found in refractories is that of calcium (lime).Refractory clay is also widely used in the manufacture of refractory materials.Refractory materials must be selected according to the conditions they face.Some applications require special refractory materials.Zirconia is used when the material must withstand extremely high temperatures.Silicon carbide and carbon (graphite) are two other refractory materials used in some very severe temperature conditions, but they cannot be used in contact with oxygen because they will oxidize and burn.Binary compounds such as tungsten carbide or boron nitride can be very refractory.Hafnium carbide is the most refractory binary compound known, with a melting point of 3890 °C.The ternary compound tantalum hafnium carbide has one of the highest melting points of all known compounds (4215 °C).The melting point of molybdenum disilicide is as high as 2030°C, and it is often used as a heating element.

Uses:

Refractories can be used for the following functions:

1.As an insulation layer between the heat medium and the container wall

2.Withstand physical stress and prevent the erosion of the container wall by the heat medium

3.Anti-corrosion

4.Provide insulation

Refractory materials have a variety of useful applications.In the metallurgical industry, refractory materials are used for the lining of furnaces, kilns, reactors and other containers that hold and transport heat media such as metals and slag.Refractories are also used in other high temperature applications such as fired heaters, hydrogen reformers, ammonia primary and secondary reformers, cracking furnaces, utility boilers, catalytic cracking units, air heaters and sulfur furnaces.

Classification of refractory materials:

Refractories are classified in multiple ways, based on:

1.Chemical composition

2.Method of manufacture

3.Fusion temperature

4.Refractoriness

5.Thermal conductivity

Based on chemical composition

Acidic refractories

Acidic refractories are generally not affected by acidic substances, but are easily corroded by alkaline substances, so they are used together with acidic slag in acidic environments.They include substances such as silica, alumina and fireclay bricks.Well-known  reagents that can etch alumina and silica are hydrofluoric acid,phosphoric acid and fluorinated gases (e.g. HF, F2). At high temperatures, acidic refractories may also react with lime and basic oxides.Siliceous refractories are refractories that contain more than 93% silicon oxide (SiO2).They are acidic and have high thermal shock, flux and slag resistance as well as high spalling  resistance.Silica bricks are commonly used as furnace charge in the steel industry. An important property of silica brick is its ability to maintain hardness up to its melting point under high loads.Silica refractories are generally cheaper and therefore easier to handle. When mixed with organic resins, the new technology can provide higher strength and longer casting duration with less silica (90%).Zirconia refractories are refractories mainly composed of zirconia (ZrO2).They are often used in glass furnaces because they have low thermal conductivity, are not easily wetted by molten glass, and have low reactivity with molten glass.These refractory materials can also be used in high temperature building materials.Aluminosilicate refractories are mainly composed of alumina (Al2O3) and silica (SiO2). Aluminosilicate refractories can be semi-acidic, refractory clay composites or high alumina content composites.

Alkaline refractories

Alkaline refractories are used in areas where the slag and the atmosphere are alkaline.They are stable to alkaline substances but can react with acids which is important e. g. When removing phosphorus from pig iron.The main raw material belongs to the reverse osmosis group, a common example of which is magnesium oxide (MgO).Other examples include dolomite and magnesite. In the first half of the 20th century, the steelmaking process used artificial periclase (roasted magnesite) as furnace lining material.Magnesite refractories contain ≥ 85% magnesium oxide (MgO).They have high slag resistance to lime and iron-rich slags, are highly wear and corrosion resistant, and have high refractoriness under load and are commonly used in metallurgical furnaces.Dolomite refractories are mainly composed of calcium magnesium carbonate.Generally, dolomite refractories are used in converters and refining furnaces.Magnesia-chromium refractories are mainly composed of magnesium oxide (MgO) and chromium oxide (Cr2O3).These refractory materials have high refractoriness and high resistance to corrosive environments.

Neutral refractories

These are used in areas where the slag and atmosphere are acidic or basic and are chemically stable to both acids and bases.The main raw material belongs to but not limited to the R2O3 family.Common examples of these materials are aluminum oxide (Al2O3), chromium oxide (Cr2O3), and carbon.Carbonaceous graphite refractories are mainly composed of carbon.These refractories are often used in highly reducing environments, and their high refractoriness gives them excellent thermal stability and slag resistance.Chromite refractories consist of sintered magnesia and chromium oxide.They have constant volume at high temperatures, high refractoriness and high slag resistance.

  1. Alumina refractories contain ≥ 50% alumina (Al2O3).

Based on manufacturing method:

  1. Dry pressing process

  2. Casting

  3. Manual molding

  4. Forming (plain, fired or chemically bonded)

  5. Unformed (integral plastics, ramming and gunning materials, castables, mortars, dry vibration cement.)

  6. Unformed dry-process refractories.

Shaped:

These come in standard sizes and shapes.These can be further divided into standard shapes and special shapes.Dimensions for standard shapes are followed by most refractory manufacturers and are usually suitable for the same type of kiln or furnace.The standard shape is usually a brick with the standard dimensions of 9 inches by 4.5 inches by 2.5 inches (229 mm by 114 mm by 64 mm), known as a "one brick equivalent". "Brick Equivalent" is used to estimate how many refractory bricks are needed to install into an industrial furnace.A range of standard shapes in different sizes are manufactured to produce walls, roofs, arches, pipes and circular holes etc.Special shapes are made specifically for specific locations within the furnace and for specific kilns or furnaces. Special shapes are usually less dense and therefore less wear-resistant than standard shapes.

Unshaped (unshaped refractories)

These have no definite form and only take shape when applied.These types are known as monolithic refractories.Common examples are plastic blocks, ramming materials, castables, injection molding materials, filling materials, mortars, etc.Dry vibratory linings, commonly used for induction furnace linings, are also monolithic, sold and shipped as a dry powder, often containing magnesia/alumina components, with other chemicals added to alter specific properties.They also find more applications in blast furnace linings, although this use is still rare.

Based on fusion temperature

  • Refractory materials are divided into three types according to the melting temperature (melting point).

  • Common refractory materials have a melting temperature of 1580–1780 °C (eg refractory clay)

  • Advanced refractories have a melting temperature of 1780–2000 °C (eg chromite)

  • Melting temperature of super refractories >  2000 °C (eg zirconia)

  • Based on refractoriness

  • Refractoriness is the property of multiple phases of a refractory material to achieve a specified degree of softening at high temperatures without load, as measured by the high temperature cone equivalent (PCE) test.Refractory materials are classified as follows:

  • Supertax: PCE value 33–38

  • High load: PCE value 30–33

  • Intermediate duty: PCE value 28–30

  • Light load: PCE value 19–28

Based on thermal conductivity

Refractory materials can be classified by thermal conductivity as conductive, non-conductive or insulating.Examples of conductive refractories are silicon carbide (SiC) and zirconium carbide (ZrC), while examples of non-conductive refractories are silica and alumina.Insulating refractory materials include calcium silicate materials, kaolin and zirconia.Insulating refractories are used to reduce the rate of heat loss through the furnace walls.These refractory materials have low thermal conductivity due to their high porosity, ideally porous structure with small and uniform pores evenly distributed throughout the refractory brick to minimize thermal conductivity. Insulating refractory materials can be further divided into four types:

1. Heat-resistant insulating materials with a service temperature of ≤1100°C

2. Refractory insulation materials with a service temperature of ≤1400°C

3. High refractory insulating materials with operating temperature ≤ 1700°C

4. Ultra-high refractory and heat-insulating materials with a service temperature of ≤2000°C.


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