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...

Fireclay Refractory Bricks...

Fireclay Refractory Bricks  Fireclay refractory bricks are manufactured from unfired refractory bond clay and fireclays (chamotte), fired refractory clay or similar grog materials . Fireclay refractory bricks have two main components namely 18 % to 44 % of alumina (Al2O3) and  50 % to 80 % of silica (SiO2). The variety of clays and manufacturing techniques allows the production of numerous brick types appropriate to particular applications. The usefulness of fireclay refractory bricks are largely due to the presence of mineral mullite, which forms during firing and is characterized by high refractoriness and low thermal expansion. Raw materials for fireclay refractory bricks Refractory fireclay essentially consists of hydrated aluminum silicates with minor proportion of other minerals. The general formula for these aluminum silicates is Al2O3.2SiO2.2H2O, corresponding to 39.5 % alumina, 46.5 % silica, and 14 % water (H2O). Kaolinite is the most common member of this group. At high temperature, the combined water is driven off, and the residue theoretically consists of 45.9 % alumina and 54.1 % silica. However even the purest clays contain small amounts of other constituents , such as compounds of iron, calcium, magnesium, titanium, sodium, potassium, lithium, and usually some free silica. The total quantity of these fluxing agents, which lower the melting point, should be at a level of 5 % to 6 % maximum. TiO2 is not regarded as fluxing agent and was previously counted together with alumina. The name fireclay is given to a group of refractory clays which can generally withstand temperatures above pyrometric cone equivalent (PCE) value of 19. Refractoriness and plasticity are the two main properties needed in fireclay for its suitability in the manufacture of refractory bricks. A good fireclay should have a high fusion point (greater than 1580 deg C) and...

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...

Introduction to Refractories...

Introduction to Refractories Refractories are defined in ASTM C71 as non metallic materials having those chemical and physical properties that make them applicable for structures or as components of systems that are exposed to environments above 538 deg C. Refractories are inorganic, nonmetallic, porous and heterogeneous materials composed of thermally stable mineral aggregates, a binder phase and additives. These materials have ability to retain its physical shape and chemical identity when subjected to high temperatures. Refractories perform four basic functions namely (i) act as a thermal barrier between a hot medium and the wall of the containing vessel, (ii) represent a chemical protective barrier against corrosion, (iii) ensure a physical protection, preventing the erosion of walls by the circulating hot medium and (iv) act as thermal insulation for heat retention. Refractories are classified in the different following ways. Classification based on chemical composition –  Refractories are classified on the basis of their chemical behaviour into following three classes. Acid refractories – These are those refractories which are attacked by alkalis or basic slags. These are used in acidic atmosphere or where slags are acidic. Example of these refractories are silica and zirconia. Basic refractories – These refractories are attacked by acid slags but stable to alkaline slag, dust and fumes at the elevated temperatures. These refractories are used in alkaline atmospheres. Example of these refractories are magnesia, dolomite and chromite. Neutral refractories – These refractories are chemically stable to both acids and bases and used in the areas where slag and environment are either acidic or basic. Examples are carbon graphite, chromites and alumina. Grphite is the least reactive and is extensively used in the furnaces where the process of oxidation can be controlled. Classification based on physical form – Refractories are classified according...