Refractories

REFRACTORIES

A refractory material is one that retains its strength at high temperatures. Refractory materials are used in linings for furnaces, kilns, incinerators and reactors. They are also used to make crucible sand moulds for casting glass and metals and for surfacing flame deflector systems for rocket launch structures. The oxides of aluminium (alumina), silicon (silica) and magnesium (magnesia) are the most important materials used in the manufacturing of refractories. The various combinations of operating conditions, in which refractories are used, make it necessary to manufacture a range of refractory materials with different properties. Refractory materials are made in varying combinations and shapes and for different applications.

PROPERTIES OF REFRACTORIES

❖ Melting point: Pure substances melt sharply at a definite temperature. Most refractory materials consist of high melting particles bonded together. At high temperature, glass fuses and as the temperature rises, the resulting slag increases in quantity by partial solution of the refractory particles. The temperature at which this action results in failure of a test pyramid (cone) to support its own weight is called, for convenience, the melting point of the refractory.

❖ Size: The size and shape of the refractories is a part of the design feature. It is an important feature in design since it affects the stability of any structure. Accuracy and size is extremely important to enable proper fitting of the refractory shape and to minimize the thickness and joints in construction.

❖ Bulk density: A useful property of refractories is bulk density, which defines the material present in a given volume. An increase in bulk density of a given refractory increases its volume stability, its heat capacity, as well as resistance to slag penetration.

❖ Porosity: The apparent porosity is a measure of the volume of the open pores, into which a liquid can penetrate, as a percentage of the total volume. This is an important property in cases where the refractory is in contact with molten charge and slags. A low apparent porosity is desirable since it would prevent easy penetration of the refractory size and continuity of pores will have important influences on refractory behaviour.

❖ Cold crushing strength: The cold crushing strength is a useful property, since it reveals little more than the ability to withstand the rigors of transport and can be used as a useful indicator to the adequacy of firing and abrasion resistance in consonance with other properties such as bulk density and porosity.

❖ Pyrometric cone equivalent (PCE): Temperature at which a refractory will deform under its own weight is known as its softening temperature which is indicated by PCE. Refractories, due to their chemical complexity, melt progressively over a range of temperature. Hence refractoriness or fusion point is ideally assessed by the cone fusion method.

CLASSIFICATION OF REFRACTORIES

Refractories can be classified on the basis of chemical composition, end use and methods of manufacture as described below:

 ❖ Fireclay refractories: Firebrick is the most common form of refractory material. It is used extensively in the iron and steel industry, nonferrous metallurgy, glass industry, pottery kilns, cement industry, and many others. Fireclay refractories, such as firebricks, siliceous fireclays and aluminous clay refractories consist of aluminum silicates with varying silica (SiO2) content of up to 78% and Al2O3 content of up to 44%. This material is often used in furnaces, kilns and stoves because the materials are widely available and relatively inexpensive.

❖ High alumina refractories: Alumina silicate refractories containing more than 45% alumina are generally termed as high alumina materials. The alumina concentration ranges from 45 to 100%. The refractoriness of high alumina refractories increases with increase in alumina percentage. The applications of high alumina refractories include the hearth and shaft of blast furnaces, ceramic kilns, cement kilns, glass tanks and crucibles for melting a wide range of metals.

❖ Silica brick refractories: It is a refractory that contains at least 93% SiO2. The raw material is quality rocks. Various grades of silica brick have found extensive use in the iron and steel melting furnaces and the glass industry. In addition to high fusion point multi-type refractories, other important properties are their high resistance to thermal shock (spalling) and their high refractoriness. The outstanding property of silica brick is that it does not begin to soften under high loads until its fusion point is approached. Other advantages are flux and stag resistance, volume stability and high spalling resistance.

❖ Magnesite refractories: These are chemically basic materials, containing at least 85% magnesium oxide. They are made from naturally occurring magnesite (MgCO₃). The properties of magnesite refractories depend on the concentration of silicate bond at the operating temperatures. Good quality magnesite usually results from a CaO-SiO₂ ratio of less than two with a minimum ferrite concentration, particularly if the furnaces lined with the refractory operate in oxidizing and reducing conditions. The slag resistance is very high particularly to lime and iron rich slags.

❖ Chromite refractories: Chrome-magnesite refractories, which usually contain 15-35% Cr₂O, and 42-50% MgO. They are made in a wide range of qualities and are used for building the critical parts of high temperature furnaces. These materials can withstand corrosive slags and gases and have high refractoriness. Magnesite-chromite refractories, which contain at least 60% MgO and 8-18% Cr2O₃. They are suitable for service at the highest temperatures and for contact with the most basic slags used in steel melting. Magnesite-chromite usually has a better spalling resistance than chrome-magnesite.

❖Zirconia refractories: Zirconium dioxide (ZrO2) is a polymorphic material. It is essential to stabilize it before application as a refractory, which is achieved by incorporating small quantities of calcium, magnesium and cerium oxide, etc. Its properties depend mainly on the degree of stabilization, quantity of stabilizer and quality of the original raw material. Zirconia refractories have a very high strength at room temperature, which is maintained up to temperatures as high as 1500° C.

❖ Oxide refractories: Alumina refractory materials that consist of aluminium oxide with little traces of impurities are known as pure alumina. Alumina is one of the most chemically stable oxides known. It is mechanically very strong, insoluble in water, super heated steam, and most inorganic acids and alkalies. Its properties make it suitable for the shaping of crucibles for fusing sodium carbonate, sodium hydroxide and sodium peroxide. It has a high resistance in oxidizing and reducing atmosphere. Alumina is extensively used in heat processing industries. Highly porous alumina is - used for lining furnaces operating up to 1850° C.

USES OF REFRACTORIES

❖ Refractory materials are used in linings for furnaces, kilns, incinerators brand reactors.

❖ They are also used to make crucibles and moulds for casting glass and metals and for surfacing flame deflector systems for rocket launch structures.

❖ Iron- and steel industry uses approximately 70% of all refractories produced. They are widely used in foundries as well.

❖ Manufacturing of cement, glass, paper, metals.

❖ They are mainly used in muffles and it is also an ideal choice for recuperaters.