Refractory
Refractory materials are specialized inorganic substances capable of withstanding high temperatures and corrosive environments without losing their structural integrity. Typically, these materials can endure temperatures surpassing 1000 degrees Fahrenheit (538 degrees Celsius) and are essential in various industrial applications, particularly in furnaces and storage vessels for hot materials. The evolution of refractory materials dates back to ancient times, with early examples seen in Bronze Age ceramic firing pits. Over the centuries, advancements in metallurgy led to the development of more sophisticated refractories, each tailored to specific manufacturing needs.
There are multiple classifications of refractory materials, including basic, acid, and neutral refractories, each responding differently to chemical interactions. Special refractories and insulating refractories serve targeted functions, with some designed to retain heat, thereby enhancing manufacturing efficiency. Cermets, a composite of ceramic and metal, are another notable class, utilized in extreme environments such as aerospace and nuclear applications. The continual innovation in refractory technology has significantly enhanced metal and material production processes, underscoring their critical role in modern manufacturing.
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Refractory
Refractory refers to a material that can withstand the action of corrosive or abrasive substances at high temperatures. These materials retain their strength and form even when exposed to temperatures in excess of 1000 degrees Fahrenheit (538 degrees Celsius). There are many different types of refractory materials, and they can be used for many different purposes. The extremes to which they are subjected mean that refractory materials often have a short useful life span; however, many of them can be recycled and reused.
Background
Ancient people understood the concept of a refractory, even if they did not recognize the material as such. As far back as 3000 BCE, Bronze Age workers dug pits in the ground and built the fires they used to harden earthenware ceramics. In this firing process, the earth surrounding the pit was the refractory material, containing the heat and flames without changing shape or consistency. This met all the requirements of an early refractory, although ancient people were probably most concerned with its ability to contain and concentrate the fire.
The earliest interest in a material that would not only retain and contain heat but also resist any effects of heat and other substances likely occurred in the Iron Age. Iron is formed by melting the iron ore out of the rock where it is found. Melting iron required even higher temperatures than firing ceramics. Humankind was now interested in melting iron ore in standing furnaces that burned charcoal instead of a wood-burning pit. Ways were discovered and invented to combine naturally occurring rocks and minerals to create materials that could resist heat and corrosion during the iron-making process. This included fireclay, a material that could be shaped into bricks but included alumina and silica, making the bricks ceramic-like once they were hardened.
Around 300 BCE, people in China discovered how to make steel. This was done by adding carbon to the iron as it was being melted and formed. Steelmaking also required furnaces lined with a refractory material that could withstand heat and the effects of carbon and any other substances released during the manufacturing process. While the sophistication of the furnaces changed, the basic process of using a refractory-lined furnace to melt the ore and extract the iron remained unchanged.
By the fourteenth century CE, the technology for making iron and steel had spread around the world. This meant an increased demand for more furnaces and the need for greater quantities of refractories. In place of charcoal fuel, many of these larger facilities used coke (the residue of coal), especially as the Industrial Age of the eighteenth and nineteenth centuries arrived. This also necessitated a change in the type of refractory used. Instead of the same standard fireclay bricks, new clays were developed that included more silica as well as magnesium compounds and tar-bonded dolomite, elements that increased the heat resistance and the resistance to acids, alkalis, and bases that could be released during the metal-making process. The furnaces themselves were also redesigned to include blowers and features to control the flow of melted metal such as nozzles and stoppers; therefore, it became necessary to find refractories that could be formed into different shapes and still maintain their strength and integrity.
In the early twentieth century, increased demand for steel and the development of the steel industry in the United States led to further enhancements to the types of refractories available. Silicon carbide, originally discovered as an abrasive, was found to be a very effective component of refractory materials, and new sources were found for traditional refractories. More and more attention was paid to refractory engineering, or the process of developing new refractories and finding new ways to use them.
Overview
Refractory materials are inorganic (not from living material), non-metal substances that can withstand extremely high temperatures without any loss of strength or shape. They are used in devices, such as furnaces, that heat substances and in tanks and other storage devices that hold hot materials. There are many different types of refractories, based on the material they are made of and the substances that affect them. They include basic refractories, acid refractories, and neutral refractories. There are also some special purpose substances known as special refractories, insulating refractories, and cermets.
Basic refractories are affected by acids but not by bases. Bases are chemical substances that, when liquid, are slippery in feeling, bitter in taste, react with litmus paper, and have some other specific properties. These types of refractories are often made with dolomite, magnesia-chromite, forsterite, and magnesite. They are not suitable for iron making, but they are used in many other types of furnaces.
Acid refractories can be affected by bases but are not affected by acids. They include fireclay, aluminosilicate, silica, and kyanite. These are often used in the iron-making and steelmaking industries.
Neutral refractories are those that are not affected by either bases or acids. They are often made of graphite, chromite, silicon carbide, and zirconium. Neutral refractories are often found in furnaces that process metals.
Special refractories are manufactured using more expensive materials that are free of impurities and produce a refractory with special properties. They are therefore costlier and are used when no other refractory can be used or when cost is no object. These refractories are often made of alumina, beryllia, boron nitride, and zirconia.
Insulating refractories help the furnace or other equipment that use them to retain heat. They help control costs in the manufacturing process. Insulating refractories are made of china clay, glass wool, mica, and ceramic, among other materials. Another type of insulating refractory is asbestos. This was used more extensively prior to the discovery that it causes lung cancer when small amounts of it become airborne and are inhaled. Asbestos is still used in some applications, but with much greater care.
Another class of refractories are cermets. A cermet is a composite substance usually made of both ceramic and metal. The metal is sintered, or processed with heat and pressure to make it more compact. Cermets are made of oxides, carbides, borides, and metals or metal alloys. They are often used in situations resulting in extreme heat, such as space vehicles and nuclear power plants.
Refractories have made it possible to produce substances such as refined metal and glass that require high heat for processing. Improvements in the types of refractories available have in turn improved the manufacturing processes of these materials. New types of refractories have also made it possible to make better, stronger metals and materials and to expand their uses.
Bibliography
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