Properties and Testing of Refractory materials

Refractory materials are basically inorganic non-metallic substances which are composed of thermally stable mineral aggregates, a binder phase, and additives. Refractory substances are supposed to be able to withstand the varying degrees of severity in terms of exposure to abrasive and corrosive solids, liquids, or gases at extremely high temperatures. The principal raw materials used in the production of refractories are: Oxides of silicon, aluminium, magnesium, calcium and zirconium, and some non-oxide compounds of carbides, nitrides, borides, silicates and graphite. There are a number of different types of refractory materials, in terms of physico-chemical composition and designs, which are manufactured depending upon the operating service conditions.

The general desirable characteristics of refractory materials are as follows:

• Able to withstand high & sudden change of temperatures.
• Able to withstand action of molten metal slag, glass, and hot gases.
• Able to withstand mechanical stress/load and abrasive environments at high temperatures.
• Able to prevent or minimize loss of heat from furnaces or reaction chambers.
• Should have low coefficient of thermal expansion.
• Should be able to maintain chemical integrity and not contaminate substances.

Uses/Applications of Refractories

Refractories are mainly used in the metallurgical industries for the purposes of providing internal linings of furnaces, kilns, reactors, boilers, and other vessels for holding and transporting metal and slag. They are even used in non-metallurgical industries, where they are mainly used in fired heaters, hydrogen reformers, ammonia primary and secondary reformers, cracking furnaces, incinerators, utility boilers, catalytic cracking units, coke calciner, ducting, stacks, etc.

Insulating materials

Insulating materials are primarily used with the purpose of preventing heat loss or gain in the application areas, like manufacturing structures and heating chambers. They should possess low thermal conductivity and their heat capacities depend on their bulk densities and specific heats. They are generally porous, containing a large number of dormant air cells. Insulating materials can be classified into two broad categories: Organic and Inorganic materials. The organic foams include polystyrene, polyurethane, phenol foam, polyethylene foam etc. The inorganic ones include mineral wool, calcium silicate, cellular glass, micro porous silica, magnesia, ceramic fibre, vermiculite and pearlite.

Types of refractory

Classification based on Chemical Composition This classification is based on the behaviour, i.e. chemical reaction of refractory materials towards slag. Three types are recognized:

Acidic refractories

These types of refractories are attacked by basic slags (alkalies), and are stable to acidic ones. Hence, these are used in those places where the atmosphere and slag composition are acidic. Examples include Silica (SiO2), Zirconia (ZrO2), etc.

Basic refractories

These refractories are attacked by acidic slags, and are stable to alkaline slags, dusts and fumes at higher temperatures. Thus, they find use in places, like furnace linings, where the slags and atmospheric conditions are basic. In terms of chemical composition, they mainly include Magnesia (MgO), Dolomite (CaO.MgO), and Chromite (main part of chrome ore).

Neutral refractories

These are chemically stable to both acidic and basic slags, hence are used in areas where the slag and atmospheric conditions are either acidic or basic. Some of the common neutral refractories are Carbon graphite (most inert), Chromites (Cr2O3), & Alumina (Al2O3). Graphite is extensively used in metallurgical furnaces for controlling the process of oxidation, since it is extremely stable and inert.

Classification Based on Physical Form

Based on their physical forms, refractories can be classified into 'Shaped' and 'Unshaped' refractories. The former is commonly referred to as refractory bricks and the latter as "monolithic" refractories