By: Engr. Rizwan Nasir B.Sc. Chemical Engineering September 29, 2009 Refractories By: Engr. Rizwan Nasir B.Sc. Chemical Engineering September 29, 2009

Definition In general usage, products applied at temperatures > 600 °C are referred to as refractories. A refractory material is one that retains its strength at high temperatures. ASTM C71 defines refractories as: “ Non-metallic materials having those chemical and physical properties that made them applicable for structures, or as components of systems, that are exposed to environments above 1000 °F (800K, 500 °C)”

Refractory Materials Refractory materials must be chemically and physically stable at high temperatures. Depending on the operating environment, they need to be resistant to thermal shock, be chemically inert, and/or have specific ranges of thermal conductivity and of the coefficient of thermal expansion. The oxides of aluminum (alumina), silicon (silica) and magnesium (magnesia) are the most important materials used in the manufacturing of refractories

Types of Refractories Refractories can be classified on the basis of chemical composition, method of manufacture, physical form or according to their applications. There are four basic types of refractories: Acidic Refractories Basic Refractories Neutral Refractories Super Refractories

Acidic Refractory A refractory that is composed principally of silica and reacts at high temperatures with bases such as lime, alkalies, and basic oxides. These are used in areas where slag and atmosphere are acidic. They are stable to acids but attacked by alkalis. The main components of these refractories are silica along with alumina (Al2O3). The steel industries are the largest consumer of acidic refractories.

Basic Refractories These are used on areas where slags and atmosphere are basic, stable to alkaline materials but reacts with acids. The main raw materials is magnesia (MgO) is a very common example. Other examples includes dolomite (MgCO3 + CaCO3) and chrome-magnesia (Cr2O3 + MgO).

Neutral Refractories These are used in areas where slags and atmosphere are either acidic or basic and are chemically stable to both acids and bases. The common examples of these materials are alumina (Al2O3), chrome ( Cr2O3) and carbon. Normally we have to use acidic and basic refractories combined but we use neutral bricks to avoid the reaction. The neutral bricks are made of graphite and chromites.

Super Refractories These refractories are manufactured for exceptional high temperature application higher than 1800oC. For example zirconia (ZrO2) its melting point 2340oC – 2550oC, Thoria (ThO2) its melting point is 3200oC.