There are various ways how materials such as metal or minerals are processed to turn them into certain products. Some of the processes include burning and need a certain type of materials to withstand the burning process to obtain the desired results. For this purpose, refractory materials are often needed, such as refractory kaolin used in ceramic manufacture.
In this article, we are going to discuss what refractory materials mean and what is it used for in various industries.
What does Refractory Mean?
Refractories refer to materials that are resistant to high temperatures. Under thermal pressure and a high temperature up to 3000 degrees Celsius, these types of materials will maintain their chemical and physical properties. These materials are mostly used as furnace lining for increased materials temperature during the heating process. It is also used in other applications where it requires thermomechanical properties.
Refractory materials expand when heated, and the expansion degree depends on the chemical composition of said refractories. They are produced from a mixture of compounds and minerals such as chromite, alumina, bauxite, magnesium and kaolin. However, we are going to focus on discussing refractory kaolin.
Kaolin is used to make all kinds of refractory parts for various industries. For example kiln wash is usually made of a 50:50 mixture of kaolin and silica. While kaolin are most commonly known for ceramics industries, the amount used is actually very little compared to other industries. Kaolin is actually used a lot in everything else, such as paper to cosmetics, paint to agricultural products.
In the industrial standard, clay (including kaolin) is called refractory when it does not become soft below 1,850 degrees Celsius. However, a lot of clays soften at much lower temperatures because the real refractory clay can be hard to get. Or if there are any, it can be too expensive.
Kaolin is the best refractory clay type. One of the reasons being pure kaolin clay will not soften below 1,750 degrees Celsius. They are usually found at the location of their parent rock or primary clay that has not been mixed with impurities. Kaolin with impurities will have reduced refractoriness and changed color.
This type of pure kaolin is often expensive because it is usually much in demand for porcelain manufacturers. However, kaolin firing to a buff color is acceptable for producing refractory items.
Calcined kaolin is usually used in manufacturing refractories. That is because they have high alumina content, high melting point, low water absorption, low expansion upon heating, and high plasticity.
This material is produced by heating raw natural kaolin in high temperatures. This process increases the hardness and whiteness of the kaolin. Additionally, it also improves its electrical properties and changes the kaolin particles’ size and shape.
Kaolin itself is used extensively in the ceramic industry for manufacturing whiteware, porcelain and refractories. It contains neither iron, alkalies, or alkaline earth, making it producing desirable ceramic properties.
In whiteware manufacturing, kaolin is usually mixed with silica, feldspar, and sometimes also mixed with ball clay, which is a plastic light-burning clay. These components are important to obtain certain properties that are important in ceramic manufacturing, such as shrinkage, plasticity, vitrification, and so on.
These properties are usually looked for informing and firing the ware. However, Kaolin that is used for refractories is usually used alone and not mixed with anything else. That is why clay needs to be washed to remove the sand and other particles in it before used for refractories.
Kaolin clays usually contain more than 50% sand, therefore it should be removed at the site to save transport costs. The removal process is done by adding water into the clay in a pond, then stirring it until the clay is suspended in the water. When the sand is settled, water clay mixture is transferred to another pond for the clay to settle.
Calcined kaolin or metakaolin appears powdered, white, and non-plastic. This material is made by firing raw kaolin in a rotary calcining kiln with a high enough temperature that it has lost approximately 12% of its crystal water content. You can also make calcined kaolin by bisque firing powdered kaolin in a small bisque vessel with a slow ramp. This way the heat can penetrate the material well.
This process makes the kaolin whiter and more chemically inert. In this form, calcined kaolin becomes more useful and can be used as the materials of a white variety of products. However, not many people know the true value of calcined kaolin, therefore, it is often under-utilized in traditional ceramics making.
Where do You Get Refractory Clays?
For a potter, looking for good quality refractory clays can be difficult. Here are some methods on how you can obtain high-quality refractory materials.
You can obtain information regarding where to find refractory materials from the existing ceramic industries, cement factories, or glass factories. These three industries all have in common, which they all need high-quality refractory materials. Although firebricks may be available, they are usually too expensive. Additionally, you will still need raw refractory clay to produce kiln shelves or saggars.
2. Geological Institutes
You can also ask for more information from geological institutes and mining corporations. However, note that sometimes they are note well informed about where to get the refractory materials. Oftentimes, they only concerned themselves with big high-grade commercial deposits.
Last but not least is by surveying. Some potters may have to look for the refractory clay themselves. However, starting digging holes everywhere is not an ideal way to look for this material. You can ask local farmers or traditional potters, however, whether they know about white or grey-colored clay.
White clays are often used for other purposes such as house whitewashing. You can also contact local well-sinkers and ask them about the soil around the area. They should have some knowledge about the type of soils available below the surface.
Refractory kaolin is an excellent material for various purposes of industries. Therefore, you should be able to find these materials in major suppliers.