The Basic Features of Mullite Refractories

Mullite is a general name of a series of minerals composed of aluminum silicate. It is rare. Mullite is the mineral generated by aluminum silicate at high temperature. Natural mullite crystals are needle-like and tufted. It can be used to produce refractory materials.

Mullite is the only binary compound under atmospheric pressure among Al2O3 -SiO2 binary system, expressed as 3Al2O3-2SiO2. Natural mullite is rare, so it is generally synthesized by the sintering method or fusing method. Its Mohs hardness is 6-7. Its refractoriness is 1800℃. At 1800℃, it is stable, while it is decomposed into corundum and liquid at 1810℃.

Mullite brick is a high quality refractory material. It has a uniform expansion, excellent thermal shock resistance, high softening point under load, small high temperature creep, high hardness and good chemical corrosion resistance. Currently, mullite bricks mainly include high purity fused mullite brick, common fused mullite brick, sintered mullite brick made of natural bauxite ore and light burned mullite brick.

High temperature mullite insulation brick is a new energy-saving refractory material. It has high temperature resistance, high strength, small thermal conductivity and good energy-saving effects. It is widely used in petroleum cracking furnaces, metallurgy hot stoves, ceramics roller kilns, tunnel kilns, porcelain drawer kilns, crucible furnaces and the lining of a variety of electric furnaces. It can contact with flame directly.

How to Control the Casting Temperature of Fused Cast Refractories

The casting process of most fused cast refractories, especially those with multi-component, is very complex, since every component has its melting point and crystallization temperature and they can be quite different.
Before discharged, the melt of fused cast refractories should be heated. For steel, before casting, it should be heated to 100℃ higher than its melting point, so as to completely fill the model and obtain castings with exact dimensions. For fused cast AZS, the temperature should be 200℃ higher. When melting and casting multi-component materials, each component can only crystallize simultaneously at their eutectic point or invariant point.
If the temperature of the melt of multi-component fused cast refractories does not reach the temperature of the component most difficult to be melt, it relies on the excessive heating of other components to discharge the melt. But in this case, there will be unmelted components. In general, each component of refractory material has no big difference, but in the ternary system of fused cast AZS and chrome spinel, the biggest difference is about 1000℃.

The Development of Fused Cast refractories

In 1927, Corhart was founded. It is a refractory company producing fused cast refractories. Raw materials are melted in an electric arc furnace and then casted in a mold, just like metal casting. The lining of the electric arc furnace is made of the same materials. The temperature is up to the requirements of refractory products, generally 1800-2500℃.

The first fused cast refractory is fused cast mullite refractory. It has been widely used in the sidewall of glass furnaces. Later zircon is added to the raw materials. The melting temperature is reduced and the slag resistance of the products is improved.

In 1952, fused cast magnesia chrome brick, a new fused cast product, was developed by Corhart. It is mainly used in the steel industry.

When producing fused cast refractories, the melt is casted into a mold with a large cap. The wall of the mold is made of refractory materials. When casting is finished, the mold wall is removed. Then the mold is put into insulation materials to reduce thermal shock to obtain crystals with uniform sizes and minimum gaps.

Fused cast refractory is larger than the standard bricks, so it is difficult to cast. Currently, fused cast products for sale are always cut from big blocks.

Article Source: http://refractorybrick.over-blog.com/2016/07/the-development-of-fused-cast-refractories.html

Abvantages Of Corundum Brick

Corundum brick is made of corundum as the main raw material. It contains more than 90% Al2O3. Due to its good stability, good corrosion resistance and excellent wear resistance, it is widely used in the petrochemical industry, metallurgical industry and the lining of coal gasifier, residual oil gasifier, pulp waste gasifier, the carbon black reactor and other industrial furnaces.

The bulk density is related to the particle size, particle distribution and particle shape. Generally, bricks with coarse particles have lower bulk density than those with fine particles. The smaller the gap between the particles, the greater the bulk density of the bricks. Bricks with mixed particles have higher bulk density than those with single particles. The bulk density of corundum brick increases with the density of particles.

Corundum brick has high cold compressive strength , which will directly affect the quality of the bricks. It has high Al2O3 content. The higher the Al2O3 content, the better the refractory performance of the bricks. It has high softening temperature and good high temperature performance. It has good chemical properties and excellent resistance to acid and alkali. It has good thermal shock resistance and high strength.It has wide applications.

Advantages Of Diatomite Insulation Brick

Diatomite is a great non-metallic raw materials for heat insulation, sound insulation, and thermal insulation. It can withstand high temperature up to 1380℃.

Diatomite insulation brick is made of high quality diatomite earth. It have a vast number of pores. Compared to other insulation products made of fire clay, high alumina, cenospheres and fires, it have light weight, high strength, low bulk density, low thermal conductivity, insulation performance, high quality and low price.
Diatomite brick is not deformed, non-toxic, odorless, safe and convenient to construct. It is an ideal material for insulation and sound insulation. It can increase the temperature, shorten the heating time and reduce energy consumption.
It is widely used in the electrolyzer, pipe insulation, furnace wall, high hot air channel, high-temperature equipment in the electric power industry, metallurgy industry, chemical industry, petroleum industry, cement industry, textile industry, nonferrous furnaces and other industries as insulation materials.
The advantages of diatomite insulation brick include:
(1) Cost
It can reduce the cost of the base, reduce the cross section of the frame, save reinforced concrete and reduce the overall cost of the buildings. Compared to fireclay brick, it can reduce the overall cost by 5%.
(2) Practicability
Light weight brick can increase the usable area.
(3) Construction
It is easy to process and construct. Since it is large and light, it can reduce the labor intensity, improve construction efficiency and shorten the construction period.

The Difference Between Heat Insulating Material And Refractory Material

Heay insulating material is a composite material used in the building envelope or thermal equipment to resist the heat transfer. It includes insulation material and cold preservation material.

Heat insulating material can not only maintain the thermal environment of buildings and thermal equipment, but also save energy. Therefore, it is considered as the the fifth largest energy source following the coal, oil, natural gas and nuclear energy. It can efficiently resist the heat transfer. It is mainly used in the insulation of the walls, roof or tubes of buildings and furnaces.

According to the principl, it can be divided into

(1) Porous materials

Its insulation properties rely on the gases with low thermal conductivity in the gaps. Generally air is used as the insulation medium. It always has fibrous tissue accumulation and porous structure. Foam plastic has good insulation properties, followed by mineral fibers (eg asbestos), expanded perlite, porous concrete and foam glass.

(2) Reflective materials

For example, aluminum foil can reduce the thermal radiation heat transfer by reducing reflection. Several layers of aluminum foil or paper can form a omposite structure with a thin layer of air.

The heat insulating material is usually used in the insulation of walls, roof and floor of buildings in the form of loose material, sheet, plate and prefabricated blocks, etc. it can be directly used (such as aerated concrete) or as the core material in the roof and envelope structures or paved into the floor insulation layer. Fibers or granulated insulating material can be filled in walls or sprayed on the walls, for insulation, sound absorption, decoration and fire-resistance.

There are many different types of insulation products, including foam plastic, mineral wool products, foam glass, expanded perlite insulation products, EPS powder particle insulation slurry, spray mineral cotton and foam cement insulation products.

In addition to the thermal conductivity, the water absorption, combustion performance, strength and other indicators of the material should taken into consideration.

Refractory material is a type of inorganic non-metallic material that can withstand high temperature above 1580 ℃. It is widely used in the metallurgy industry, chemical industry, petroleum industry, machinery industry silicates industry, power industry, etc.. It has the largest consumption amount in the metallurgical industry, accounting for 50% to 60% of total output.

The commonly used refractory materials inlcude AZS refractory, corundum brick, directly bonded magnesia-chrome brick, silicon carbide bricks, silicon nitride bonded silicon carbide bricks, nitrides, silicides, sulfides, borides, carbides, calcium oxide, chromium oxide, aluminum oxide, magnesium oxide, and beryllium oxide refractories. Insulation materials incldue diatomite products, asbestos products and heat insulation panels. Monolithic refractories frequently used include fettling material, ramming refractory materials, refractory castable, fire-resistant plastic, refractory clay, gunning refractory, refractory castables, refractory coatings, lightweight refractory castable, stemming etc.

5 Features Of Magnesia Chrome Bricks

The chemical composition of magnesia chrome brick varies with the compostion of raw materials. Generally, it contains 55%-60% MgO, 10% Cr2O3 and a small amount of Fe2O3, Al2O3, SiO2 and CaO. Magnesia chrome brick mainly consists of magnesia-chrome spinel and magnesium ferrite.

Since it has a low content of periclase and high content of magnesia-chrome spinel and magnesium ferrite, and its eutectic compounds have a low melting point, it has a lower refractoriness than magnesia brick, generally only 1850 -1960℃.

The periclase and magnesia-chrome spinel in the magnesia chrome brick can form a network sketeton. Since there are only a few low-melting substances filled in the network gaps, when subjected to high temperature loads, although the low-melting material is softened, the network skeleton can still support the load. Therefore, its softening temperature under load is higher, generally above 1550℃. The refractoriness is lower than that of magnesia brick.

It has a high content of magnesia-chrome spinel and magnesium ferrite and a lower linear expansion coefficient. Meawhile, the porosity of magnesia chrome brick is higher than that of magnesia brick, so it has good thermal stability.

It has a low content of calcium forsterite, so it is not easy to generate anisotropy expansion stress in it. Magnesium ferrite spinel is dissolved in the magnesia-chrome spinel and has a high solubility and wide dissolution temperature range, so the precipitation and dissolution of magnesium ferrite will not produce a large stress. Therefore, it has good thermal shock resistance and its water-cooled frequency is up to about 25 times.

Magnesia chrome spinel is the compound of magnesium oxide and chromium oxide and chromium oxide is typical of neutral oxide, so it has a strong corrosion resistance to both alkaline and acidic slag.