Ceramic Kilns

Industries kilns are huge ovens, which carry the work of thermal processing at very high temperatures mostly above 2500°F. They are used on a wide scale to make ceramic materials and for drying wood and wood products. They are also used to break down waste materials like paper pulp and used tires.

There are a variety of industrial kilns:

1 Brick Kilns
2 Electric Kilns
3 Rotary Kilns
4 Ceramic Kilns

These kilns are of two basic types i.e. Continuous and Intermittent.
A continuous kiln is also known as a tunnel because of the long tunnel like shape. In a continuous kiln there is a cool end from where the material is inserted and only the central portion is heated. As the material moves (on rollers or other types of conveyers) from the insertion point, the temperature gradually increases till it reaches the center, which is the hottest part of the kiln. Then the material is carried towards the exit, where it returns back to room temperature.
An intermittent kiln is one in which the firing process starts once the material is inserted into it and the kiln is sealed. The temperature of the kiln is steadily increased to the required temperature. The kiln and the material are cooled after the firing process is completed.

Ceramic is a refractory material used in kilns. The crucibles and linings used in kilns are made of this ceramic refractory. Ceramic refractors have highly specialized characteristics. Refractors are insulators that can resist heat of high temperatures. This material prevents fusion, softening, the action of fluxes and slag, controls temperatures, etc. These are heavy insulators that provide solutions to challenges of thermal management. These are used in kilns as they utilize heat and encounter high temperature. They are powered by gas, propane or electricity. Most of the modern day kilns use electricity and natural gas. Most of the large pottery industries are using natural gas now days as it is efficient, easy to use and is clean too.

There are various types of ceramics insulations used in kilns like:

1 Ceramic fiber insulations
2 Ceramic textiles
3 Ceramic kiln furniture

These insulators mostly consist of fibers like cloth, rope, batting, paper and many other such products. Braiding, extrusion, knitting and weaving are different ways ceramic fibers are made. There are layers of such materials in the kilns known as blankets. These blankets are of two types:

Woven: These are used to form multi-dimensional structures and composite tooling.

Non-woven: These materials are in the form of mats made of randomly selected insulating fibers.

Features of ceramic kilns:

1 Provides high resistance to abrasion, chemical erosion
2 High resistance to thermal shock
3 Low thermal conductivity
4 Highly resistant to pressure and provides structural integrity

Potters and glassmakers use ceramic kilns on a wide scale. They are also used to make moulds, tools, glazes, etc.

Refractory Materials

Refractory materials are those, which stubbornly resists softening, fusion, the action of fluxes and slag’s, and other effects, physical and chemical, of high temperatures. Usually, a material that can withstand a temperature of 1100° C without softening is termed as refractory material. The ability to withstand exposure to elevated temperatures without undergoing appreciable deformation is measured in terms of refractoriness. When heat is utilized and high temperatures are encountered and needs to be contained, refractory materials are used. Furnace linings and thermal barriers are typical applications. These heat treatment and containment products are used extensively for high technology applications in aerospace, automotive and industrial environments.

Refractory materials usually consist oxides of elements such as silicon, aluminum, magnesium, calcium and zirconium of single or mixed high melting point. There are also some non-oxide refractories like carbides, nitrides, borides and graphite. Atmosphere, temperature, and the materials that it will be in contact with are some of the operating factors that determine the composition of refractory materials.

Refractory materials are available in a variety of different densities and porosities. Usually, thermal conductivities are higher for refractories with low porosity. High porosity materials tend to be highly insulating as a result of high volume of air they trap, because air is a very poor thermal conductor. Such materials, however, do not work with higher temperatures and direct flame impingement, and are likely to shrink when subjected to such conditions. As a result, low porosity materials are generally used in hotter zones, while the more porous materials are usually used for thermal backup.

The key properties of refractory materials are:
1 The ability to withstand high temperatures and trap heat within a limited area, like a furnace.
2 The ability to maintain sufficient dimensional stability at high temperatures and after/during repeated thermal cycling.
3 To maintain sufficient mechanical properties (e.g. compressive strength) at high temperatures.

Some refractory materials commonly used are:
Magnesia: It is a white solid mineral formed by an ionic bond between one magnesium and one oxygen atom. It is used as a basic refractory material for lining crucibles.
Lime: It is a naturally occurring mineral and the materials derived from it are used in various industrial applications.
Fire Clay: This is a specific kind of clay used in manufacturing firebricks. It has a high percentage of silicon and aluminum oxides, which enables it to withstand temperatures upto 1600° C. Hence it is usually used for lining furnaces.
Silicon Carbide: It is a ceramic compound made of silicon and carbon. With a melting point of nearly 2700 ° C, and being highly chemically inert, it is widely used to make furnace parts for heating and casting metals.
Zirconia: This is a white crystalline oxide of zirconium, with a melting point of 2715 ° C. It is used as a refractory material in insulation, abrasives, and ceramic glazes.
Thus, as can be seen, refractories are usually used to line furnaces in order to keep the heat within the hearth of the furnace. As a result of using refractories as insulation, furnace efficiency is increased.

Industrial Ceramics

Ceramics are inorganic, non-metallic materials formed due to heat. The American Society for Testing and Materials (ASTM) defines a ceramic article as, “an article having a glazed or unglazed body of crystalline or partly crystalline structure, or of glass, which body is produced from essentially inorganic, non-metallic substances and either is formed from a molten mass which solidifies on cooling, or is formed and simultaneously or subsequently matures by the action of the heat.” Most ceramic materials are porous, hard, and brittle. Ceramics has many advantages over metal, alloys, and compounded materials.

Industrial Ceramics have very good thermal, wear, and structural properties. It provides high performance in harsh environments where there are high temperatures, and in erosive, corrosive and abrasive conditions. Due to such qualities, ceramics are widely used in industries.

Refractory materials are those, which can retain their strength even at high temperatures. Such materials are used to make crucibles and linings for kilns, furnaces, and incinerators. Usually, a material that can withstand a temperature of up to 1100° C without softening is termed as refractory material.

Some of the refractory materials commonly used in industries are:

1   Magnesia – It is a white solid mineral formed by an ionic bond between one magnesium and one oxygen atom. It is used as a basic refractory material for lining crucibles.
2   Lime – It is a naturally occurring mineral and the materials derived from it are used in various industrial applications.
3   Fire Clay – It is a specific kind of clay used in manufacturing firebricks. It has a high percentage of silicon and aluminum oxides, which makes it able to withstand temperatures of up to 1600° C. Hence it is suitable for lining furnaces.
4   Silicon Carbide – It is a ceramic compound of silicon and carbon. With a melting point of nearly 2700 ° C, and being highly chemically inert, it is widely used to make furnace parts for heating and casting metals.
5   Zirconia – It is a white crystalline oxide of zirconium, with a melting point of 2715 ° C. It is used as a refractory material, in insulation, abrasives, and ceramic glazes.

Kilns are thermally insulated chambers resembling ovens, in which controlled temperature conditions are created. They are often used in industries to burn, harden or dry materials. Kilns are an vital part of the ceramic manufacturing process, which usually requires heat treatment at high temperature. During this process, various physical and chemical reactions occur, as a result of which the materials properties are permanently changed. The composition and preparation of the clay item will determine the final characteristics, so will the temperature at which it is fired, and the glazes that are used. Part of the firing process includes sintering. This process heats the clay until the particles partially melt and bond, which creates a strong, composite mass, which has a glassy phase, and is interspersed with pores and crystalline material. Through firing, due to reduction in pore size, the material shrinks. The crystalline material formed is a matrix of mostly silicon and aluminium oxides, and is very hard and strong. However, these materials tend to be brittle.

The two broad types of kilns are:    Intermittent and Continuous.
In the intermittent kiln, the ware to be fired is loaded, and taken out after the firing and cooling of both, the ware as well as the kiln.
The continuous or tunnel kiln has a centrally heated portion where the ware to be fired is continuously fed, somewhat like a car wash system.