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2.6 Refractory

2.6.5 Insulating refractories

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Porosity is a significant factor in heat flow through refractories. The thermal conductivity of a refractory decreases on increasing its porosity. Although it is one of the least important properties as far as service performance is concerned, it evidently determines the thickness of brick work. Others include : cold crushing strength, Pyrometric Cone Equivalent (PCE), refractoriness under load, creep at high temperature, volume stability, expansion and shrinkage at high temperature and reversible thermal expansion:

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2.6.5.1 Characteristics of good insulating refractory

According to Fragoulis et al. (2004), the desirable feature of insulating bricks is the low thermal conductivity, which usually results from a high degree of porosity. Structure of air insulating material consists of minute pores filled with air which have in them very low thermal conductivity. The air spaces inside the brick prevent the heat from being conducted but the solid particles of which the brick is made conduct the heat. So, in order to have required insulation property in a brick a balance has to be struck between the proportion of its solid particles and air spaces. The thermal conductivity is lower if the volume of air space is larger. Importantly, the thermal conductivity of a brick does not so much depend on the size of pores as on the uniformity of size and even distribution of these pores. Hence, uniformly small sized pores distributed evenly in the whole body of the insulating brick are preferred.

The high porosity of the brick is created during manufacturing by adding a fine organic material to the mix, such as sawdust. During firing, the organic addition burns out, creating internal pores (Kogel et al., 1996). Other ways to accomplish high porosity involves: a) by using materials which expand and open up on heating; b) by using volatile compounds like naphthalene; c) using aluminium (Al) powder in combination with NaOH solution (called chemical bloating);

d) by using substances which by themselves have open texture e.g. insulating brick grog, vermiculite, ex-foliated mica, raw diatomite etc; e) using foaming agents to slip; f) aeration etc Because of their high porosity, insulating bricks inherently have lower thermal conductivity and lower heat capacity than other refractory materials (Ghanbarnezhad et al., 2014).

2.6.5.2 Types of insulating refractories

Insulating materials can be classified with respect to application temperature: i) heat resistant insulating materials for application temperatures up to 2000 °F: calcium silicate materials;

products from siliceous earth, perlite or vermiculite; silica based micro porous heat insulators;

alumino-silicate fibres; ii) refractory insulating materials for application temperatures up to 2500°F: lightweight chamotte and kaolin bricks; lightweight castables; mixed fibres and aluminium oxide fibres; iii) high refractory insulating materials for application temperatures up to 3100°F: lightweight mullite and alumina bricks; lightweight hollow sphere corundum

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castables and bricks; special high refractory fibres; iv)Ultra-high refractory insulating materials for application temperatures up to 3600 °F: zirconia lightweight bricks and fibers; non-oxide compounds. Other types of insulating refractories include; castables, granular insulation, and ceramic fibre insulation, which is light weight. Extremely lightweight materials have a porosity of 75 to 85% and ultra-lightweight, high-temperature insulating materials have a total porosity greater than 85% (Ghanbarnezhad et al., 2014).

2.6.5.3 Applications and advantages

These are widely used in the crowns of glass furnaces and tunnel kilns. They can also be used as linings of furnaces where abrasion and wear by aggressive slag and molten metal are not a concern. These offer several distinct advantages; decreased heat losses through the furnace lining and less heat loss to the refractory leads to savings in fuel cost, the insulating effect causes a more rapid heat-up of the lining and lowers heat capacity of the insulating refractory, thinner furnace wall construction to obtain a desired thermal profile and less furnace mass due to the lower mass of the insulating refractory (Subramanian et al., 1996; Ghanbarnezhad et al., 2014) 2.6.5.4 Some drawbacks of insulating bricks

The porosity in insulating refractories creates a large amount of free surface area. Although porosity decreases thermal conductivity and density of the brick, it also degrades the mechanical strength of the brick as compared to a dense refractory firebrick. The poor strength of insulating bricks due to their high porosity can pose structural design problems.

Porosity in insulating refractory leads to poor chemical resistance as compared to dense refractories of similar compositions. Gases, fumes, liquids such as slags, molten glass etc. at high temperatures can penetrate porous bricks easily, making insulating fire bricks unsuitable for direct contact with such liquids or gases.

Insulating fire bricks often suffer from thermal spalling problems, particularly in an environment of rapidly changing temperature. Since these bricks are good insulators, a substantial temperature gradient will occur between the hot and the cold face of each brick. The hot face will expand more than the cold face. The thermal gradient thus, gives rise to a mechanical stress in the body of the brick (Bhatia, 2011).

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2.6.5.5 Ceramic fibres

Ceramic fibres are a family of insulating refractory products that is fluffy, white cotton like fiber and can be spun and fabricated into textiles, blankets, felts, boards, blocks, etc. These products have low thermal conductivity, very low heat storage, extremely light weight, immunity to thermal shocks and are chemically stable. The lightweight construction ensures that the required temperature in high-temperature plants is reached more rapidly, as only a small proportion of the temperature released into the processing vessel/furnace is used for the heating of the walls.

Ceramic fibres composed of 52 Al

2O

3 - 48 SiO

2 combinations can be applied as a hot face insulation material up to ~2600°F, whereas a 62 Al

2O

3 - 38 SiO

2 combinations impart greater refractoriness to fibre. 42% Al

2O

3, 52% SiO

2 and 6% ZrO

2 produces extra long staple fibre of 10 inches and are used for manufacturing ceramic fibre textiles and ropes ( Bhatia, 2011).

The advantages of light weight ceramic fibre are; better fuel economy (savings as high as 60%

are feasible in the case of certain intermittent furnaces), higher productivity capacity of furnaces, due to reduced heat storage capacity, higher service life of the furnace and reduced maintenance costs due to longer refractory life and ease of installation.

They are used with great success in metal treating furnaces, ceramic kilns, and numerous other periodic operations whose atmosphere do not negate their revolutionary thermal and lightweight qualities. Fibre mats also continue to be used in expansion joints and door seals, and in tunnel kilns and other exposed - brick structures as either original or retrofit layers on the outside or cold-face surface. (Bhatia, 2011)

2.6.5.6 Limitations

The chief limitation of ceramic fibre is shrinkage at high temperatures. A high quality ceramic fibre blanket rated for continuous use at 2400 °F will have 5% shrinkage after 24-hr exposure at 2400 °F. Shrinkage will not continue past this level in normal operating conditions, but this shrinkage must be carefully considered in designing a furnace lining. Others include ;

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Mechanical strength of ceramic fibres is poor. These are not really structural materials. Proper support must be given to all refractory fibre products, fibre tend to sag at high temperature due to softening of fibres if improperly supported.

They are not suitable in severe hostile environments. These are handy repositories of dusts, fogs, and combustible fumes; not to mention for process liquids like slags and metals.

They are more expensive than conventional refractories however; installation labour savings and energy savings offset the high initial costs (Bhatia, 2011)