Cement Manufacturers Association (CMA)

36 changes in their dimensions. These changes can be attributed to various factors: • alterations in allotropic forms affecting specific gravity • chemical reactions giving rise to new materials with different specific gravities • the presence of a liquid phase • sintering reactions • interactions with dust, slag, or alkalis resulting in the formation of alkali-alumina silicates, leading to expansion and disruption. This phenomenon is commonly seen in blast furnaces. Ideally, these changes should occur during manufacturing to minimize economic implications, but time constraints make it challenging. Permanent Linear Change (PLC) upon reheating and cooling of the bricks serves as an indicator of the product’s volume stability as well as the effectiveness of the manufacturing processes. It is particularly significant as a measure of the degree of conversion achieved in the manufacture of silica bricks. Reversible Thermal Expansion: Each material expands when heated and contracts when cooled. The reversible thermal expansion reflects the phase change that occurs during heating and cooling. PLC and reversible thermal expansion are followed in the design of refractory linings to provide expansion joints. In general, those with low thermal expansion co-efficiency are less likely to experience thermal spalling. Thermal Conductivity: The heat flux is defined as the amount of heat that flows through any one area normally within the surface area during a defined period of time with a known temperature gradient. It shows the overall characteristics of the heat flow at the interior of resistor and depends on the composition of minerals and the temperature used. Cool furnace- walls withindustrial applications prefer refractors with low thermal conductivity, because they help in storing thermal energy. The porosity of the heat flow through the refractor is an important factor. The non-resistive heat transfer rate decreases with increasing porosity. Although this is one of the least important attributes when it comes to service, it obviously determines the weight of the brick. How are refractories classified? Refractories can be classified on the basis of chemical composition and the methods of manufacture or physical form. Classification Based on Chemical Composition Refractories are typically classified on the basis of their chemical behaviour, i.e. their reaction to the type of slags. Accordingly the refractory materials are of three classes - Acid, Basic & Neutral. Acid Refractories: Acid refractories are those which are attacked by alkalis (basic slags). These are used in areas where slag and atmosphere are acidic. Examples of acid refractories are: 1. Silica (SiO 2 ), 2. Zirconia (ZrO 2 ), and Neutral Refractories: Neutral Refractories are chemically stable to both acids and bases and are used in areas where slag and atmosphere are either acidic or basic. The common examples of these materials are: 1. Carbon graphite (most inert) 2. Chromites (Cr 2 O 3 ) 3. Alumina Out of these, graphite is the least reactive and is extensively used in metallurgical furnaces where the process of oxidation can be controlled Basic Refractories: Basic refractories are those which are attacked by acid slags but stable to alkaline slags, dusts and fumes at elevated temperatures. Since they do not react with alkaline slags, these refractories are of considerable importance for furnace linings where the environment is alkaline; for example non-ferrous metallurgical operations. The most important basic raw materials are: 1. Magnesia (MgO) - caustic, sintered and fused magnesia 2. Dolomite (CaO*MgO) - sintered and fused dolomite 3. Chromite - main part of chrome ore Chemical characteristics of the furnace process usually determine the type of refractory required. Theoretically, acid refractories should not be used in contact with basic slags, gases and fumes