Rotary Kilns are used to heat solids to the
point where a required chemical reaction(s) takes place. The Rotary
Kiln is basically a rotating inclined cylinder, solids retention
time in the kiln is an important factor and is set by proper
selection of diameter, length, speed, slope and the internal design.
Construction Features :
Rotary Kilns are normally direct fired and lined with refractory
material to protect the steel shell, reduce radiation losses and
transfer heat to the material. A typical Rotary kiln is cylindrical
shell, slightly inclined from the horizontal and supported on two or
more tyres, which in turn rotate on rollers. One of the tyre is
fixed axially by a set of thrust rollers to care of the thrust load
during kiln movement. The drum assembly along with the tyres are
rotated by a girth gear/ pinion arrangement. Shell diameter and
length are generally designed to suit specific products.
Heat Transfer in Rotary kiln : All kilns are heat exchangers and
the heat is transferred in a variety of ways by radiation,
convection and conduction.
The most effective way of transferring heat is by gas radiation
from the hot flame and a smaller amount of transfer from refractory.
The hot bricks transfer by radiation to material and also by
conduction to the charge
Sealing arrangement :
A seal is essential at each end to prevent ambient air from
entering the process. Air leakage can have varying effects on
different process. Sometimes it reduces efficiency and sometimes it
can create unsafe or unstable conditions. To maintain proper control
of the volume and temperature of gases flowing through the kiln it
is necessary to limit the leakage of the cold air in the kiln
particularly where the rotating shell enters the feed and the
discharge end hoods. Seal rings are provided for this purpose.
Various types of seal rings are used which include spring loaded
types, where two finished metal surfaces are in contact under spring
pressure. The sealing segments require regular adjustment and
Because of high temperature and abrasive material liner is required
and call it as refractory. Refractory is installed as solid block to
fit in to different areas tightly in round formation from sliding or
falling out. There are many types of bricks in different composition
and have different properties such as insulation value, maximum
temperature and resistance to wear. Areas where brick can not be
used liquid refractor called castables are used with anchor support.
Kiln can also have insulation material under the brick and this
extra layer helps to reduce shell losses.
Bearings & Drive Components :
The most common drive components are gear reducers. Gear reducers
are used when reduced speed or increased torque is required. Gear
reducers can be designed differently for applications. Rotary Kilns
are normally gear driven through girth gear mounted over the shell
on gear support base, assembled on a section of the shell away from
the shell end where high temperatures are encountered. To compensate
the thermal expansion the girth gear is mounted on tangential spring
plates attached to the shell. The driving pinion is mounted on the
output shaft of a worm / helical gear unit.
Bearings are in every moving piece of equipment. Bearings are also
uniquely designed for there application. The most common bearings
are the anti-friction or roller bearings. These bearings have become
standardized and perform excellent in almost any condition.
Anti-friction bearings do not work well with heavy loads. Different
kind of bearing are used on the kiln where high loads are seen.
Firing System :
The burner system is an important and an integral component of a
rotary kiln system to optimize the combustion of fuels to release
heat in the kiln. Safety considerations, ease and flexibility of
operation, product quality, energy efficiency, maintenance costs,
and the environmental impact of the products of combustion are some
of the most critical areas that a kiln operator must evaluate in
selecting a kiln burner. In a rotary kiln application heat is
injected directly in the end of the kiln. Firing systems could use
any kind of fuel and be of any size. The burner is where the fuel is
ignited. The fuel could be mixed with air before or in the burner.
The flame temperature is a factor of fuel type, fuel/air ratio, and
other burning conditions.
Dust Collecting Equipments :
Exhaust gases from an oil fired kiln contain certain proportion of
very fine dust which becomes entrained in the air stream while in
contact with materials being calcined. A Dust collector of high
efficiency cyclone type is used. Even the most efficient cyclone
type dust collector will not collect all the dust. Very fine
particles which escape the dry collector may be recovered by mean of
a wet collector in the form of sludge or alternatively by means of a
Rotary Kilns are used for the calcinations of Titanium Dioxide,
Ilmenite, Kaolin, Barium Carbonate, Magnesium Carbonate, Silica
Gel, Lime, Rotary Carbonizer for Activated Carbon, Vermiculite, Barium Sulfide, Magnesium
Oxide, Petroleum Coke, Clay, Gypsum, Alumina, etc.