Ceramic fiber modules and Flat-topped tunnel kilns

A brick kiln is a type of tunnel kiln where wet bricks made of gangue and shale are fired into finished bricks. It is usually more than 140m long and 2.8m, 3.3m, 4.6m, 6.9m or 9.2m wide, according to the size of the kiln.

With the development of the brick and tile industry, tunnel kilns are widely used. Issues such as the ends, firing quality and energy consumption of the kiln are gradually emerging. The dome of the traditional dome tunnel kiln is built with T-shaped refractory bricks. Since refractory bricks have poor thermal shock resistance, there are always slitting and spalling problems. In addition, the horizontal thrust of the dome kiln is large, so the wall requires special reinforcement. This consumes a lot of materials and time.

Because the upper space is large and the temperature difference is great, this greatly affects the quality of products and increases coal consumption. In the recent decade, the tunnel kiln begins to be flat-topped. Initially, the flat top is built with refractory cement blocks and refractory concrete. But since these materials have poor thermal shock resistance, the service life is just 2 years, so it greatly increases the cost.

Ceramic fiber is made of alumina powder. The powder is melt at 2000℃ and then spun and blown into fiber. It has high purity, good thermal shock resistance and good acid and alkali resistance. Its service life can reach up to 10 years. It is easy and safe to install. It is only 1/20 of refractory bricks and has good energy saving performance.

Corundum refractory bricks used in RH furnaces

A RH vacuum degasser was initially used as a degasser and the refractory lining was mainly built with high alumina brick and fire clay brick. Now, the functi refractories are used.
With the increase in productionon of RH furnaces has been extended to the oxygen blowing and dusting. Since the conditions become more demanding, advanced of advanced special steel, methods that can increase the circulation and the amount of gas blown are promoted for stable production and high-speed processing of ultra-low carbon steel. The increase of circulation increases the wear of the lining; the cold air causes the high temperature spalling; the intake volume of ladle slags can exacerbate the structural spalling and erosion. All these intensify the damage of lining materials. Now, the lining of RH/RH-OB furnaces is mainly built with direct bonded corundum bricks and the oxygen blowing mouth is semi-bonded or rebounded corundum brick and magnesia carbon brick.
The refractory materials used in the top and the upper slot of RH devices are no in direct contact with molten steel and slags, so they are not damaged co severely compared to the lower parts. The middle parts are easy to be damaged since they are in direct contact with molten steel and slags. The lower slot including the immersion tubes are the high corrosive areas and the weak point of RH furnaces, so it often determines the service life of RH furnaces. The lining of the lower slot is always built with direct bonded corundum bricks. The circulation pipe is the most seriously damaged part in the lower parts of RH furnaces, because the lining structure limits its thickness and bricks with complex shapes should be heated twice. In addition, in RH-OB furnaces, OB also has great effect on the refractory materials. When the spray gun is set on the upper parts, refractory materials are subjected to the corrosion by oxygen, iron oxides in molten steel and high temperature gases, so, MgO-Cr2O3 bricks with a high content of Cr2O3 should be used.
Corundum bricks including direct bonded, rebounded and semi-rebounded corundum bricks have good resistance to low basicity slags, so they are widely used in refining furnace linings.

Refractory Materials Used In Rotary Kilns

The high temperature in a rotary kiln can severely exacerbate the damage to refractory materials. C3A and C4AF in the melt of cement clinkers are corrosive. The overheating in the kiln can severely exacerbate the damage of thermal stress.
The higher the kiln speed is, the higher the output is, the bigger the damage of mechanical stress and fatigue.
Components such as alkali, chlorine and sulfur are corrosive. Sulfates and chlorides are volatile chemicals. They volatize and are enriched via repeated cycle. This exacerbates the potential of structural spalling.
Since the diameter of the kiln is large, the skin of the kiln is not stable.
The structure of the kiln is complex. The mechanical and electrical failures increase. The frequent turning on and shutdown of the kiln exacerbates the damage.
Refractory bricks used in the rotary kiln include high alumina bricks, alkali resistant bricks and basic refractory bricks.
(1)High alumina bricks
Non-basic bricks are silicate alumina refractory products contain more than 48% Al2O3. It is composed ofα-Al2O3, mullite and glass phase. Its content is up to the ratio of Al2O3/SiO2 and the types and amount of impurities.
According to the content of Al2O3, non-basic bricks can be divided into Grade Ⅰ (Al2O365%-70%), Grade Ⅱ (Al2O355%-65%) and Grade Ⅲ (Al2O348%-55%).
Wear resistant bricks, mullite red bricks, silica mullite bricks and spalling resistant bricks used in the rotary kiln belong to high alumina bricks.
(2) Basic bricks
Basic bricks used in the rotary kiln include magnesia alumina spinel bricks, magnesia iron spinel bricks and direct bonded magnesia chrome bricks.
In order to improve the thermal stability of magnesia bricks, aluminum oxide or iron oxide is added to the raw materials to produce magnesia bricks with spinel as the main structure. A thin protective layer of calcium aluminate is generated when the oxides react with the clinker. Liquid phase cannot penetrate into the bricks with the protection of the layer.
The direct bonded magnesia chrome brick is fired at 1750-1850℃ in a tunnel kiln. It has good chemical corrosion resistance. Compared to spinel bricks, it has good thermal stability and spalling resistance, but poorer chemical corrosion resistance than spinel bricks.
(3) Alkali resistant bricks
They are used in tertiary air pipes and whirlwind tubes. They are composed of 25%-30% Al2O3 and more than 65% SiO2. When alkalis in the air are agglutinated on the surface of bricks, they can form a viscous glaze layer. This layer can enclose pores through which alkalis can penetrate into the bricks.
Refractory castables belongs to monolithic materials. They are a mixture of refractory aggregates and binders. They are dry material before use. During use, water or other liquid is added. The binders are hydraulically settable binding agents or chemical binding agents. The common construction methods are casting, vibration or tamping method. they can be cured without heating.

Features of high quality ceramic fiber products

The quality of ceramic fiber products is up to the quality of raw materials, so improving the purity of raw materials is an effective way to improve the quality of ceramic fiber products.
High quality ceramic fiber products are made of high purity synthesized powder as the raw material. The content of harmful impurities (Fe2O3, Na2O, CaO) is less than 1%. This improves the quality and thermal properties of ceramic fiber boards.
High quality ceramic fiber products have the following advantages:
(1) Their continuous working temperature can reach up to 1000℃, while its short-term working temperature can reach up to 1260℃.
(2) Have good resistance to acid, alkalis and molten metal.
(3) Have good strength at high temperature and good insulation performance.
(4) Cloth, ropes, tapes and yarns made of ceramic fiber and reinforced with E-glass fiber have better electrical insulation than glass fiber.

What Is Ceramic Fiber?

Ceramic fiber is a type of fibrous lightweight refractory material. Due to its lightweight, high temperature resistance, good thermal stability, low thermal conductivity, low specific heat and good thermal shock resistance, it is widely used in the machinery, metallurgy, chemical, petroleum, ceramics, glass, electronics and other industries.

Its manufacturing process, applications and features are similar to glass fiber, but it has better high temperature resistance and corrosion resistance. it is made of high quality ceramic fiber cotton, mainly by the wet forming method.

Ceramic fiber is characterized by non-asbestos, uniform fiber distribution, white color, non-hierarchical and low shot content (four times centrifugal cleaning). Its bulk density can be adjusted according to the application. It contains reinforced fiber which greatly increases its strength. It has good elasticity and is easy to process.

Ceramic fiber products are mainly used for insulation, sealing, electrical insulation, sound absorption and filtration in high temperature equipment and furnaces.

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