Properties of Refractories...

Properties of Refractories Refractories are those materials which have high melting points and have properties which make them suitable to act as heat resisting barriers between high and low temperature zones. Refractories are inorganic, nonmetallic, porous and heterogeneous materials composed of thermally stable mineral aggregates, a binder phase and additives. The general requirements of refractories include (i) ability to withstand high temperatures and trap heat within a limited area such as a furnace, (ii) ability to withstand action of liquid metal, hot gasses and liquid slag by resisting erosion and corrosion etc. (iii) ability to withstand load at service environment, (iv) ability to resist contamination of the material with which it comes into contact, (v) ability to maintain necessary dimensional stability at high temperatures and after/during repeated thermal cycling, and (vi) ability to conserve heat. Important properties of refractories include chemical composition, bulk density, apparent porosity, apparent specific gravity and strength at atmospheric temperatures. These properties are frequently among those which are used as ‘control points’ in the manufacturing and quality control process. The chemical composition serves as a basic for classification of refractories and the density, porosity and strength are influenced by many other factors. Among these are type and quality of the raw materials, the size and fit of the particles, moisture content at the time of pressing, pressure at mould, firing temperature, duration of firing, and the rate of cooling. Properties of the refractories can be classified to resist four types of service stresses namely (i) chemical, (ii) mechanical, (iii) thermal, and (iv) thermo-technical. A suitable selection of the refractories for the furnace lining can only be made with an accurate knowledge of the refractory properties and the stresses on the refractories during service. The relationship between service stresses and important...

Insulation Refractory Bricks...

Insulation Refractory Bricks  Insulating refractory brick (IRB) is the term used for heat insulating bricks and  covers those heat insulating materials which are applied up to 1000 deg C. IRBs are often mistakenly referred to as rear insulation materials. These bricks are assigned to the group of lightweight refractory bricks and are manufactured on the basis of naturally occurring lightweight raw materials. IRB is a class of brick, which consists of highly porous fireclay or kaolin. IRBs are lightweight, low in thermal conductivity, and yet sufficiently resistant to temperature to be used successfully on the hot side of the furnace wall, thus permitting thin walls of low thermal conductivity and low heat content. The low heat content is particularly important in saving fuel and time on heating up, allows rapid changes in temperature to be made, and permits rapid cooling. IRB is characterized by the presence of large amount of porosity in it. The pores are mostly closed pores. The presence of porosity decreases the thermal conductivity of the insulating bricks. IRBs  were developed in the 1930s, and they were the predominant form of insulation until the development of insulating castable and fiber refractories. There are two types of bricks namely (i) bricks based on clay and gypsum using the burnout of sawdust to create high porosity (and thereby provide better insulating value), and (ii) bricks based on lightweight aggregate and clays. Like all alumina-silica brick, IRBs have a duty rating (service limit). Over the years, IRBs have been made in a variety of ways, such as mixing of organic matter with clay and later burning it out to form pores; or a bubble structure incorporated in the clay-water mixture which is later preserved in the fired brick. IRBs are characterized by the presence...

Introduction to the Characteristics of Refractories and Refractory materials...

Introduction to the Characteristics of Refractories and Refractory materials A suitable selection of the refractory lining material for a furnace can only be made with an accurate knowledge of the chemical and physical properties of the refractories and refractory materials, and of the stresses of the materials during service. There are four types of stresses which refractories face during their period of service. These are given below: Thermal – The important properties for thermal stresses are pyrometric cone equivalent (PCE), refractoriness under load (RUL), Thermal expansion under load (creep), hot modulus of rupture, thermal expansion, reheat change (after-shrinkage and after-expansion) and thermal shock resistance. Thermo-technical – The important properties for thermo-technical stresses are thermal conductivity, specific heat, bulk density, melting point, thermal capacity and temperature conductivity. Mechanical – The important properties for mechanical stresses are cold modulus of rupture and deformation modulus, crushing strength, abrasion resistance, porosity and density. Chemical – The important properties for chemical stresses are chemical composition, mineralogical composition and crystal formation, pore size distribution and types of pores, gas permeability and resistance to slag, glass melts, gases and vapours. Some of the important physical and chemical properties are given below: Melting point – Melting point (melting temperatures) specify the ability of materials to withstand high temperatures without chemical change and physical destruction. The melting points of major elements that constitute refractory composition in pure state vary from 1700 deg C to 3480 deg C. The melting point serves as a sufficient basis for considering the thermal stability of refractory mixtures and is an important characteristic indicating the maximum temperature of use. Size and dimensional stability – The size and shape of the refractories is an important feature in design since it affects the stability of any structure. Dimensional accuracy and...