WHAT ARE REFRACTORIES?

Refractories are essentially non-metallic construction materials which are capable of retaining their form and strength at extremely high temperatures, under varying conditions of mechanical stress and chemical attack by hot gases, liquids, or molten and semi-molten materials such as glass, metals or slags.

In processes which involve high temperatures, a refractory is almost certainly required to contain and control the heat, as well as protect other materials involved in the process.

There is no single refractory material that is capable of withstanding all the possible conditions presented by a wide variety of uses to which refractories are put. A choice must be made in order to meet the main requirements of a particular application.

Refractories belong to a family of industrial products known as "ceramics".

The need to raise the efficiency and productivity of basic industrial processes is the spur to improvements in the performance of refractories. Refractories have to resist melting, erosion, creep, deformation, corrosion and thermal stresses. Their role is an essential one and their failure can result in very expensive down-time. Refractories also influence the quality of the materials being produced.

HISTORY OF REFRACTORIES

It is not known who made the first refractory, but it is thought to have been either the Phoenicians or the Chinese. The earliest types were made from minerals directly dug from the earth's crust. It is almost certain that refractory bricks were used to build furnaces as long ago as 1500BC to extract iron and copper from natural minerals.

As a result of the industrial revolution, greater demands were placed on refractories, and as the range of heater processes widened, so the range of refractory materials increased. This trend has continued to the present day, and during the last fifty years has increased rapidly to the point where a remarkably wide range of refractory materials are now available.

IN WHICH INDUSTRIES ARE REFACTORIES USED?

Refractories are key components in a wide variety of industries. Virtually all high temperature processes depend upon the reliable performance of refractory materials such as the production of metals, cement, glass, ceramics, chemicals, petrochemicals, energy and many other commodities worldwide. They are critical materials that anonymously support the production of products that allow us to maintain and continually improve our way of life. But despite their tremendous importance, refractories are virtually unknown to the general public.

The biggest user is the iron and steel industry, which accounts for around 60% of total refractories production worldwide. Traditionally, since it is the major consumer and also the one which presents the widest variety of problems, the iron and steel industry has always exerted a considerable influence on the pace and pattern of development in the refractories industry.

WHAT DO REFRACTORY MATERIALS CONSIST OF?

Refractories generally consist of oxides or mixtures of oxides, notable exceptions being carbon, carbides and nitrides. The main refractory compounds are based on the oxides or mixtures of oxides of magnesium, aluminium, silicon, calcium, chromium and zirconium. Some are natural, others are man-made.

In common with all other technologies, ceramics has its own particular jargon and this is often a source of confusion to people entering the industry, even those who enter with a background in Chemistry. The art and science of ceramics are closely associated with the earth and other earth sciences such as geology, mineralogy and petrography. Much of the terminology used within ceramics has been borrowed from these disciplines.

WHAT TYPES OF REFRACTORIES ARE THERE?

Refractories are supplied in two main forms - shaped or unshaped.

Shaped products are supplied in the form of bricks, blocks or special shapes, and can be either fired or unfired. Bricks are the most common form of shaped refractory, and the range of sizes and shapes that can be produced is almost infinite, however, there is some degree of standardisation within the refractories industry. Furnaces and other equipment requiring a refractory lining come in all shapes and sizes, therefore many variations of a brick have to be produced in order that curves and arches can be built. Various shaped bricks have been given names by the industry, including squares, splits, side arch, end arch, wedges and keys. Special shapes products include,crucibles, nozzles and sliding gate plates.

Unshaped products are applied at the point of use and are formed into single structural pieces know as monolithics. A monolithic structure is defined as a structure which has no joints. Monolithics are essentially two phase composites consisting of an aggregate phase and a binder phase, and are initially rendered mouldable by the addition of liquids and/or the use of energy to make them flow. There are many different types of monolithics all having a wide range of compositions. They can generally be divided into the following groups.

• Castables - which are essentially high temperature concretes, generally consisting of an aggregate and a hydraulically setting bonding system, which includes high alumina cement.
• Gunning Mixes - products which are sprayed onto the surface of a structure.
• Mouldables, Plastics, Ramming Mixes - stiff plastic materials that are rammed into place by hand or pneumatic rammer.
• Dry Vibratables - supplied in dry form and vibration compacted using some type of former.
• Mortars - used to bond individual bricks together.

Additionally, there is a special range of refractory products which do not fit into the shaped/unshaped classification, and these are refractory ceramic fibres. Ceramic fibres are man-made fibrous refractory materials used for high temperature insulation purposes. Products based on ceramic fibres are supplied in shaped form such as blanket, modules, vacuum formed shapes, rope, tape or paper, and in unshaped form as loose fibre.