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Refractory lecture notes for short course that describes the type, process and use of them
Typology: Lecture notes
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Any material which can withstand high temperature Importance in construction of
**1. Metallurgy
Refractory is any material which can withstand high temperature, without softening or suffering a deformation in shape. Main Objective
**1. To confine heat e.g. to resist loss of heat.
Classification of Refractories
1. Acid Refractories: important members of this group is Alumina, **Silica gel fireclay refractories.
Properties of Refractories
**1. Refractoriness
Pyrometric Cones Test (Segar Cones Test) The refractoriness is, usually, determined by comparing the behaviour of heat on cone of material to be tested with that of a series of Segar cones of standard dimensions.
Segar cones melt or fuse at definite temperature when heated under standard conditions of 10°C / min. So the temp. at which the fusion or softening of the test cones occurs is indicated by its apex touching the base. The PCE value of the given refractory is taken as the no. of the standard cone, which fuses along with the test cone.
1 1110 2 1120 3 1140 4 1160 5 1180 6 1200 7 1230 8 1250 9 1280
R.U.L. test is performed by applying a constant load of 3.5 or 1.75 kg/cm^2 to the refractory specimen (of size 5 cm^2 and 75 cm high) and heating in a carbon-resistance furnaces at a standard rate of 10°C / min. The record of the height of the specimen vs. temperature is made by a plot, until the test-piece deforms or collapses by 10%. The R.U.L. is expressed as the temperature at which 10% deformation takes place.
Properties of Refractories (contd.)
Resistance of a material to any volume changes, which may occur on its exposure to high temperature, over a prolonged time. These dimensional changes may be permanent (irreversible) or reversible. Irreversible changes may result either in the contraction or expansion of a refractory. The permanent contraction is due to the formation of increasing amounts of liquid from the low fusible constituents of the refractory brick, when it is subjected to a long period of soaking at the high temperature. The liquid gradually fills the pores of the refractory body, causing a high degree of vitrification and shrinkage.
Properties of Refractories (contd.)
Solid materials, on heating, expands and on cooling it contracts. So in the designing of the practical furnaces, a refractory material should have least possible thermal expansion as the expansion affects all dimensions (e.g. length, area, volume) of the body.
In industrial operations, refractory materials of both high thermal conductivity and low thermal conductivity are required, depending upon the type of the furnaces. In most cases, furnaces is lined with refractories of low heat conductivities to reduce the heat losses to the outside by radiation; otherwise maintenance of high temp. inside furnaces will become difficult.
Properties of Refractories (contd.)
A good heat conductivity of the refractory material is desirable for effective heat transmission in furnace construction. The densest and least porous brick have the highest thermal conductivity, owing to the absence of air-voids. On the other hand, in porous bricks, the entrapped air in the pores, acts as a non-heat conducting material. For making porous refractory bricks, the refractory material is mixed with a liberal amount of carbonaceous material, then mould into bricks and burnt. The carbonaceous material burns off; leaving behind minute voids, which enhances the insulating quality.
Properties of Refractories (contd.)
Porosity decreases Strength resistance to abrasion resistance to corrosion/ penetration by slags, gases ec. Porosity increases resistance to thermal spalling ( i.e. thermal shock-resistance The densest and least porous brick have the highest thermal conductivity, owing to the absence of air-voids. In porous bricks, the entrapped air in the pores, acts as a non-heat conducting material.
17 Properties of Refractories (contd.)
Breaking, cracking, peeling off or fracturing of a refractory brick or block, under high temperature. So good refractory must show a good resistance to thermal spalling. Spalling is caused by rapid changes in temperature, which causes uneven expansion and contraction within the mass of refractory, thereby leading to development of internal stresses and strains. Spalling may also be due to slag penetration into the refractory brick, thereby causing variation in the coefficient of expansion. Spalling can be decreased by
Properties of Refractories (contd.)
Course or light –textured bricks, because of their large porosity, are light in weight and hence, they are more resistant to sudden changes in temperature. However, their crushing strength is low. Such bricks are more susceptible to the action of abrasion and corrosion. on the other hand, fine or dense-textured bricks possess low porosity and hence are light in weight. These are not so resistant to sudden changes in temp. However, such bricks are less susceptible to action and corrosion.
Properties of Refractories (contd.)
Measure of rate of diffusion of gases, liquids and molten solids through a refractory. Permeability depends upon the size and number of connected pores. Permeability α temperature α 1
- - - - - - - - - - - - - - - - - - - - - - - - - - - Viscosity of molten material