Engineering Chemistry S Datta

SILICATE TECHNOLOGY 431
(vi) They must be able to withstand abrasion or erosion of the furnace charge and also
the pressure of the load.
(vii) Porosity of a refractory is a deciding factor of the degree of penetration by molten
fluxes and gases and thus the refractory material easily disintegrates, i.e., the greater
the porosity, the greater is the susceptibility of the refractory to chemical attack by
molten fluxes and gases. Decrease in porosity increases the refractory’s strength and
also increases its heat capacity, thermal conductivity and chemical strength.
(viii) Least porous bricks have highest thermal conductivity. The reason for such incidence
is due to the absence of air in the void space.
Manufacturing of refractories
The steps can be displayed schematically as follows:
Crushing in jaw
crushersto25mm
size of raw materials
Grinding in a
grinding machine
to 200 mesh
Screening to remove
unwanted materials
by settling or
magnetic separation
or chemical method
Moulding
1. Manual
2. Mechanical
Mixing, uniform
mixing causes
moulding easy
Storage in
storage bins
Deairing by
applying vacuum
Drying carried
out very slowly
in tunnel driers
Firing in tunnel
kilns or rotary
kilns. Temp.–avg.
1480–1870°C
Common Refractory Bricks
1. Silica bricks: Silica bricks contain (92-95% silica, 2% lime), the main raw materials
are—quartz, quartzite, sandstone etc. During manufacture, silica bricks are heated to about
1500°C within a span of 24 hours. The cooling of the bricks takes about 1-2 weeks. During
heating, the quartzite converts to crystobalite. During slow cooling, crystobalite is converted
to tridymite. So silica brick contains a mixture of tridymite and crystobalite. If during heating
of silica bricks, quartzite is not converted to tridymite and crystobalite the bricks will expand
during use in the furnace and the refractory structure will break and fall. To attain the right
composition 12 hours heating at 1500°C of the bricks is very much essential.
Uses
Main applications of silica bricks are:
• In open hearth furnaces
• In coke-oven walls
• Glass-furnaces
2. Fire clay-bricks: Raw material is fire clay (clay–Al 2
O 3
.2SiO 2
.2H 2
O)
Generally fire clay bricks contain 55% silica, 35% alumina. This is acidic fire clay brick.
Basic fire clay bricks contain 55% alumina and 40% silica, rest consists of K 2
O, FeO, CaO,
MgO.