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Refractories

Any material which can withstand high temperature Importance in construction of

**1. Metallurgy

2. Engineering
3. Chemical industries**

KHINYARAM KAKAR

MNIT Jaipur

Introduction

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.

2. To resist abrasive and corrosion action of molten metals, slags** and gases at high temperatures, without undergoing softening **or distortion in shape. Uses
3. Construction of the linings of the furnaces, tanks, converters,** **Kilns, crucibles, ladles etc.
4. Manufacture of metals (Ferrous or non-ferrous), cement, glass,** ceramics, paper, steel etc.

Classification of Refractories

1. Acid Refractories: important members of this group is Alumina, **Silica gel fireclay refractories.

2. Basic Refractories: Cao, MgO
3. Neutral Refractories: made from weakly acid/basic materials** like Chromite(FeO. CrO 2 ), Zirconia (ZrO 2 ) Important members of this group are Graphite, Chromite, Zirconia and carborundum (SiC) refractories.

Properties of Refractories

**1. Refractoriness

2. Strength or Refractoriness-under load
3. Dimensional Stability
4. Chemical Inertness
5. Thermal Expansion
6. Thermal Conductivity
7. Porosity
8. Thermal Spalling
9. Resistance to abrasion or corrosion
10. Electrical conductivity
11. Heat capacity
12. Texture
13. Permeability**

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 Cone

Pyramid Shaped having triangular base

38 mm high and 19 mm long sides

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.

Segar Cones Number Fusion temperature

1 1110 2 1120 3 1140 4 1160 5 1180 6 1200 7 1230 8 1250 9 1280

R.U.L. Test

Refractories-under load Test

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.)

3. Dimensional Stability

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.)

5. Thermal Expansion

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.

6. Thermal Conductivity

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.)

6. Thermal Conductivity

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.)

7. Porosity

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.

A good refractory, in general , should have low porosity.

17 Properties of Refractories (contd.)

8. Thermal Spalling

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

• Using high porosity, low coefficient of expansion and good thermal conductivity refractory bricks.
• (^) Avoiding sudden temp. changes.
• By overfiring the refractories at high temp. for a sufficiently long time, whereby mineral inversion et. takes place making the material less susceptible to uneven expansion or contraction, when heated.

Properties of Refractories (contd.)

12. Texture

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.)

13. Permeability

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