An Article To Read The Performance And Application Of Various Refractory Materials

Introduction to the types of refractory materials and performance analysis

Refractory materials - clay bricks

Refractory clay bricks are a kind of refractory material with hydrated aluminum silicate as the main component and a small amount of other minerals, and their production accounts for about 75% of all refractory materials. They are characterized by low cost and versatility, and are widely used in iron and steel, non-ferrous metals, glass, ceramics, cement and other industries.

The properties of refractory clay bricks vary depending on their composition and process, and there is a wide variation in general. According to the standards of the American Society for Testing and Materials (ASTM), refractory clay bricks can be categorized into four grades: ultra-high-temperature-resistant clay bricks, high-temperature-resistant clay bricks, intermediate-grade refractory clay bricks, low-grade refractory clay bricks, and semi-silica bricks. The alumina content of these grades is about 18% to 44%, and the silica content is about 50% to 80%. Within certain limits, these grades depend primarily on the degree of refractoriness (PCE), which in turn is related to the alumina-silica ratio. For example, super refractory clay bricks have a PCE of about Cone 33, which is equivalent to 3175° F. This does not mean that the bricks can be used at temperatures of 3175° F. In fact, if the bricks are used at this temperature, they can be used at temperatures of up to 3,000° F. In fact, at this temperature, the brick will not be able to support its own weight if the environment is clean, free of slag, neutral or slightly oxidized. The following table shows the trend of decreasing refractoriness of refractory clay bricks as the impurity content increases and the alumina content decreases.

Refractory clay bricks begin to soften well below their melting point and deform permanently under load. The amount of deformation is related to the magnitude of the load, and once started, deformation is a slow and continuous process unless the load or temperature is reduced. Because of this, refractory clay bricks are not suitable for use in vaults in high temperature furnaces.

High Aluminum Refractories

Alumina refractory is an engineered ceramic with alumina as the main component, mixed with small amounts of other materials. Alumina is one of the most stable oxides with high hardness, strength and resistance to spalling. It is insoluble in water and superheated steam, as well as in most inorganic acids and bases. Alumina refractory not only has the properties of refractory clay brick, but remains stable over a much higher temperature range and is suitable for furnace linings at temperatures up to 3,350°F. It has high resistance to erosion in both oxidizing and reducing atmospheres and is widely used in the thermal processing industry.

The refractoriness of high alumina refractories increases with the increase of alumina content. 50%, 60%, 70% and 80% alumina grades contain the corresponding proportion of alumina with an allowable error of plus or minus 2.5%.

According to the standards of the American Society for Testing and Materials (ASTM), high alumina bricks are classified into the following categories according to their alumina content:

- Mullite Refractories: Mullite refractory bricks contain approximately 72% alumina and 28% silica. They have excellent volume stability and strength at high temperatures. They are well suited for use in the roofs of electric furnaces, blast furnaces and blast hearths, as well as in the superstructures of glass tank furnaces.

- Corundum Refractories: Corundum refractories are refractories that contain more than 99% alumina. This type of refractory includes monocrystalline, polycrystalline and alpha-alumina. High alumina bricks are commonly used in cement, lime and ceramic kilns, glass storage tanks, crucibles for melting various metals, blast furnace hearths and shafts, and lead slag furnaces.

These materials are very economical for use in the lower portion of dip tanks in the iron and steel industry, primarily because of their resistance to erosion by ferrous slag. The cost and price of production of high-alumina refractories increases with the alumina content, and therefore experiments or tests are required to determine the most suitable alumina content for each application.

Silica Brick

Silicon bricks are a refractory material containing more than 93% SiO2 and are second only to clay bricks in terms of production. The outstanding feature of silica bricks is their high mechanical strength even at temperatures close to the melting point. This is in contrast to many other refractory materials, such as aluminum silicate, which begin to soften and deform at temperatures well below the melting point. However, silica bricks also have one major disadvantage. They tend to spall at temperatures below 1200°F. If the temperature fluctuates above 1200°F, silica bricks have good resistance to spalling. However, many furnaces need to be cooled to near room temperature on a regular basis, so silica bricks are not very practical in this situation. As might be expected, silica bricks give good service in contact with silica-rich slag and have been used successfully in converter tops, glass storage tanks, and reflector furnaces for melting and refining copper and many other metals.

Magnesium Brick

Magnesia refractories are alkaline refractories containing more than 85% magnesium oxide as the main component. They are made from natural magnesite (MgCO3) and silica (SiO2). Good quality magnesia bricks usually consist of ferrite with a low CaO-SiO2 ratio and a minimum amount of ferrite, especially when the furnace lining is operated under oxidizing and reducing conditions.

The physical properties of these bricks are generally poor, but they are highly resistant to alkaline slag, especially to lime slag and iron-rich slag. This type of refractory brick is the most important refractory material in the steelmaking process. In addition to metallurgical furnaces, magnesite bricks are now successfully used in glass tank inspectors, lime kilns and cement kilns. Raw magnesite (MgCO3) is usually used in the form of light or heavy fired. Lightly fired magnesite is used in wastewater treatment, composite fertilizers and as a chemical raw material. Heavy-fired magnesite is a widely used raw material in the refractory industry. Recently, in order to further improve corrosion resistance, electrofused magnesia sand, magnesia sintered ore with higher crystallinity and magnesia sintered ore with very high purity have been used. Electrofused magnesia is produced by electrofusing refractory grade magnesia or other magnesia precursors in an electric arc furnace. The electrofused material is removed from the electric arc furnace, cooled and pulverized. The material is used in the production of refractory materials.

Refractory Products-Dolomite Brick

Dolomite refractories are a kind of alkaline refractories with calcium oxide and magnesium oxide as main components. They are transformed from natural dolomite (CaCO3-MgCO3) by high temperature calcination. The CaO+MgO content of high purity dolomite is greater than 97%. Dolomite refractories are considered to be the most compatible material with cement kiln clinker because of its excellent coating stability, very good resistance to thermal shock and resistance to alkali attack under different kiln operating conditions. These zirconia-rich refractories can be used for anti-cracking.

Chromite Refractories

Chromium-magnesium refractories and magnesium-chromium refractories are two different kinds of refractories.

Chromium-magnesium refractories usually contain 15-35% Cr2O3 and 42-50% MgO; Chromium-magnesium refractories are used in the construction of critical parts of high-temperature furnaces. These materials are resistant to corrosive slags and gases and have a high degree of refractoriness.

Magnesium-chromium refractories contain at least 60% MgO and 8-18% Cr2O3; magnesium-chromium refractories are suitable for use at the highest temperatures and are in contact with the most alkaline slags used in steel melting.

Magnesium-chromium refractories are generally superior to chromium-magnesium refractories in terms of spalling resistance.

Refractory Products-Zirconia Refractories

Zirconia refractory is a polycrystalline material. There are some difficulties in its use as a refractory material and in the manufacturing process. Before it can be used as a refractory material, it must be stabilized. This can be achieved by adding small amounts of stabilizers such as calcium, magnesium and cerium oxide. The properties depend largely on the degree of stabilization, the amount of stabilizer and the quality of the original raw material. Zirconia refractories have very high refractory properties. They maintain their strength at room temperature up to 2700°F. Therefore, they can be used as high-temperature building materials for furnaces and kilns. Zirconia refractories have much lower thermal conductivity than most other refractories and are therefore used as high temperature insulating refractories. Zirconia is particularly suitable for the manufacture of refractory crucibles and other containers for metallurgical purposes because of its extremely low heat loss and the fact that it does not react easily with liquid metals. Zirconia is a useful refractory for glass furnaces, mainly because it is less likely to be wetted by molten glass and has a lower reaction with molten glass.

An amorphous refractory is a general term for an amorphous refractory product that is constructed as a material in some sort of suspended form that eventually solidifies into a solid block. Most amorphous refractory formulations consist of a large-grained refractory (aggregate), a fine-grained refractory (to fill the voids between the particles), and a binder (to allow the particles to solidify together in the unsintered state). Unshaped refractories are replacing traditional sintered refractories at a faster rate in many applications, including industrial furnaces.

There are various methods of construction of indefinite refractories, such as pounding, spraying, painting, slurrying, and so on. Pounding is mainly used in cold applications where proper solidification of the material is important. The same method can be used for both air-solidified and heat-solidified materials. Proper selection of tamping tools is required.

Types of Unshaped Refractories

There are many types of unshaped refractories, which can be categorized according to the raw material material, type of bonding agent and construction method, etc. The following are some common types of unshaped refractories. Some common types of indefinite refractories are introduced below:

Potting compound: A potting compound is a water-hardened material containing a cementitious binding agent, usually an aluminate cement.

Thermal Insulating Castables: Thermal insulating castables are a specialized type of unshaped refractory material, mainly used for thermal insulation of cold surfaces. They are made of lightweight aggregates such as vermiculite, perlite, expanded balls, bubbling alumina and expanded clay. They have lower density and thermal conductivity, but also lower mechanical strength.

Plasticizable: Plasticizable is a mixture prepared in a hard plastic state and delivered in blocks wrapped in polyethylene. For use, the block of coroplast is cut into pieces without further processing and tamped into place with a pneumatic tamper or wind pick. The plastisol has good plasticity and adaptability and can be tamped into any shape or profile.

Tamped material: Tamped material is similar to plastisols, but harder. The particle size is carefully graded and the final product is usually dry, mixed with a small amount of water before use. Pounding compounds are also delivered wet and are ready to use immediately after opening. Tamping compounds require pneumatic tampers or manual tamping for construction.

Patch: Patch is an amorphous refractory material used to patch the lining of a kiln or furnace. It is similar to coroplast but is very soft and malleable and can be hand tamped into place.

Coating: A coating is an unshaped refractory material used to protect a refractory lining from chemical attack or to improve surface quality. Coatings are usually used only to cover the working surface of the lining. Coatings are usually thin and can be applied by hand or machine or sprayed.

Mortars: Mortars are fine powdered refractory materials that are malleable when mixed with water. They are primarily used to bond refractory bricks or prefabricated parts into a strong monolithic unit, providing cushioning and sealing at joints. Mortars can be masoned with a masonry knife or similar tool.

SPRAY MATERIALS: Spray materials are granular refractory materials that are sprayed onto the application area using a variety of air lances. They are thermosetting and are used for repair and maintenance work in kilns and furnaces.

PRESSURIZED MATERIALS: Pressurized materials are also granular refractory materials that function similarly to sprayed materials, but pressurized materials are pressed into the kiln to be repaired using a slurry pump or other pressurized equipment.

Heat-insulating refractories are refractories with high porosity, low bulk density, low thermal conductivity and low heat capacity, which can effectively reduce the heat loss of the kiln, save energy and reduce the heat storage capacity and weight of the kiln. Heat-insulating refractories are mainly used as heat-insulating and thermal-retaining layers in industrial kilns, and some of them can also be used as working layers. Compared with general dense refractories, heat-insulating refractories have poorer slag erosion resistance, mechanical strength and abrasion resistance, and should not be used in load-bearing structures and direct contact with slag, charge, molten metal and other parts. There are many kinds of heat-insulating refractories, which can be classified according to the raw material material, use temperature, use mode, bulk density, material form, organizational structure and so on.

Characteristics of heat-insulating refractory materials

Powdered or flaked materials: This type of insulating refractory consists of powdered or granular inorganic materials bonded in the desired form, such as diatomaceous earth panels, perlite panels, and alumina hollow sphere panels. They have low density and low thermal conductivity and can be used at temperatures up to 2000°F.

The main characteristics of insulating refractories are high porosity, low bulk density, low thermal conductivity, and low heat capacity, which make them effective in reducing heat loss from the kiln, saving energy, and reducing the heat storage capacity and weight of the kiln. The porosity of insulation refractories is generally between 40% and 90%, the bulk density is generally between 0.4 and 1.2 g/cm3, and the thermal conductivity is generally between 0.1 and 0.6 W/(m-K).

Characteristics of heat-insulating refractory materials

The high porosity of insulating refractories is mainly achieved by adding different types of porogenic agents during the manufacturing process. Porogenic agents can be categorized as follows:

(1) Combustion porous agent: This kind of porous agent refers to the organic or inorganic materials that will be completely burned out during the firing process, such as sawdust, wood chips, rice hulls, naphthalene, aluminum sulfide and so on. They can produce uniformly distributed round or oval pores.

(2) Expansion Porosifiers: These are materials that expand and open when heated, such as vermiculite, perlite, expansion spheres, and alumina hollow spheres. They can produce porous or hollow aggregates.

(3) Volatile porous agents: Such porous agents are compounds that volatilize or decompose into gases when heated, such as aluminum powder, silica powder, ammonium sulfate, etc. They can produce irregularly shaped gases. They can produce irregularly shaped pores.

(4) Foaming porous agent: this porous agent refers to the mixture will produce a large number of bubbles of substances, such as foaming agent, aeration agent, etc.. They can produce small and connected pores.

The low thermal conductivity of insulating refractories is mainly determined by the large amount of still air contained within them. Air is an excellent insulator and has a very low thermal conductivity. The solid particles in an insulating refractory, however, conduct heat, so in order for the material to have the desired insulating properties, a balance must be struck between the ratio of solid particles to air space. In general, the larger the volume of air space, the lower the thermal conductivity. However, if the air space is too large or too small, the thermal conductivity will be affected. Therefore, the optimum thermal conductivity varies with the size of the air space within a certain range.

The thermal conductivity of insulating refractories is also affected by factors such as their internal structure and service temperature. In general, the shape, size, distribution and connectivity of the air pockets affect the thermal conductivity. The ideal pore structure is a uniform distribution of small, uniformly sized pores throughout the volume of the insulating refractory. In addition, the thermal conductivity of an insulating refractory increases significantly with increasing temperature, due to the increase in thermal convection and radiative heat transfer from the air.

The small heat capacity of insulating refractories is mainly determined by their low bulk density. Heat capacity is the amount of heat absorbed or released per unit mass or per unit volume of the material when the temperature changes. The heat capacity per unit volume of insulating refractories is generally between 0.2 and 0.8 MJ/(m3-K), which is much lower than that of general dense refractories.

Types of insulating refractories

(1) Heat-resistant insulation for applications up to 2000°F: calcium silicate materials; silica clay, perlite, or vermiculite products; silica-based microporous insulation; and aluminum silicate fibers;

(2) Refractory insulation materials for applications up to 2500°F: including lightweight kaolin and kaolin bricks; lightweight castables; hybrid fibers and alumina fibers;

(3) High refractory insulation for applications up to 3100°F: lightweight mullite and alumina bricks; lightweight hollow-ball corundum castables and bricks; and special high refractory fibers;

(4) Ultra-high refractory insulation for applications up to 3600°F: zirconia lightweight bricks and fibers; non-oxide compounds;

Several other types of insulating refractories include castables, granular insulations, and ceramic fiber insulations, which are lightweight. Very lightweight materials have a porosity of 75 to 85 percent, and ultra-lightweight high-temperature insulation materials have a total porosity greater than 85 percent.

Applications and advantages

Widely used as crowns for glass kilns and tunnel kilns. They are also used as kiln linings, where wear caused by aggressive slag and molten metal is not a problem. They offer several distinct advantages:

- Heat loss from the lining is reduced and heat loss from the refractory is minimized, thus saving fuel costs;

- The insulation effect causes the lining to heat up more quickly and reduces the heat capacity of the insulating refractory;

- A thinner furnace wall construction results in an ideal thermal profile;

- The kiln has a smaller mass due to the smaller mass of the insulating refractory.

Some disadvantages of insulating bricks

Insulation brick is an insulating refractory material that has a high porosity resulting in a large specific surface area. While the porosity reduces the thermal conductivity and bulk density of the insulating brick, it also makes the mechanical strength of the insulating brick much lower than that of dense refractories. As a result, insulating bricks can be problematic in terms of structural design. Another disadvantage of insulating bricks is their poor resistance to chemical attack. Gases, fumes, and liquids (e.g., slag, molten glass, etc.) at high temperatures can easily penetrate into the porous structure of insulation bricks and cause damage to them. Therefore, insulation bricks are not suitable for direct contact with these liquids or gases. Thermal insulation brick is also prone to thermal spalling, especially in the case of rapid temperature changes. Due to the good thermal insulation properties of insulation bricks, a large temperature difference is generated between the hot and cold sides of each insulation brick. Thus, the expansion of the hot side will be greater than that of the cold side. As a result, the temperature difference creates mechanical stresses inside the insulation brick.

Ceramic Fiber

Ceramic fiber is a lightweight heat-insulating refractory material, which is in the form of white cotton wool and can be made into textiles, blankets, felts, boards, blocks and other forms. Ceramic fiber is characterized by low thermal conductivity, small specific heat capacity, light weight, good thermal shock resistance and chemical stability. Due to their lightweight construction, ceramic fibers allow high-temperature equipment to reach the desired temperature more quickly, as only a small portion of the heat released into the process vessel/kiln is used to heat the walls. Ceramic fibers are used at different temperatures depending on their composition. Ceramic fibers composed of 52% Al O23 and 48% SiO2 can be used as high temperature insulation up to ~2600°F, while ceramic fibers composed of 62% AlO23 and 38% SiO2 have higher refractoriness. Ceramic fibers containing 42% Al O23 , 52% SiO2 and 6% ZrO2 can be made into ultra-long fibers for the manufacture of ceramic fiber textiles and ropes. Zirconium fibers (usually glass fibers bonded to zirconium) can also be used to make ceramic fiber textiles and ropes. Lightweight ceramic fibers have the following advantages:

- Higher fuel economy (some batch kilns can save up to 60% of energy consumption);

- Increased kiln capacity due to the low specific heat capacity;

- Longer kiln life and lower maintenance costs due to longer refractory life;

- Easy installation.

Ceramic fibers have been used with great success in metal treatment kilns, ceramic kilns, and many other periodic operations where atmospheric conditions do not affect their excellent insulating and lightweight properties. Ceramic fiber mats can also be used for expansion joints and door seals, as well as in tunnel kilns and other exposed brick structures, as an original or retrofit layer for exterior or cold surfaces.

Limitations

The main limitation of ceramic fiber is high temperature shrinkage. High quality ceramic fiber blankets rated for continuous use at 2400°F will shrink by 5% after 24 hours of exposure at 2400°F. Under normal operating conditions, shrinkage will not exceed this level, but this shrinkage must be carefully considered when designing kiln liners.

Ceramic fibers have low mechanical strength. They are not a true structural material. Proper support must be provided for all refractory fiber products.

If not properly supported, they will sag at high temperatures due to fiber softening.

Not suitable for use in harsh environments. They are prone to the accumulation of dust, smoke and combustible fumes, not to mention erosion by process fluids such as slag and metals.

Prices tend to be higher than traditional refractories; however, the labor and energy savings during installation can offset the high initial cost.

Temperature-resistant ceramic materials for applications up to 1,475°F are sometimes considered insulation rather than refractory products.

In reality, no single refractory material can be adapted to all operating conditions. Often it is desirable to have a refractory with a low thermal conductivity so that the heat can be contained within the kiln more efficiently. However, there are times when a refractory with a high thermal conductivity is required; for example, the protective muffle in some ceramic kilns is designed to prevent combustion gases from entering the ceramic ware. It must transfer as much heat as possible to the ceramic ware, so electrically conductive ceramic materials such as silicon carbide are often used to make muffle furnaces.

You must take into account the specific metal you will be working with, the temperature you will be reaching, the processing time, the amount of time you will be holding the metal in the kiln, how much inductive stirring will take place, the additives or alloying agents you will be using, and your method of relining the kiln. Refractory selection should therefore be based on the most critical factors, such as wear patterns and operating parameters, in order to achieve the best possible technical economy. The right choice not only extends the service life of the refractory, but also helps to minimize downtime, thus increasing productivity.

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