A ROTARY KILN is a Direct Fired, Refractory
Lined equipment for high temperature applications where it is necessary to
change the 'state' of the material in a continuous process or in batch type
process. Since this process usually requires a long residence time, the
length to diameter ratio is often in excess of 10:1 in a continuous kiln. In
most cases an oil or gas burner fires directly into the discharge end of the
unit. The material is heated in three ways - by radiation from the burner
flame, conduction from the refractory lining and convection by contact with
the hot gases. They are also used for roasting a wide variety of ores.
The kiln is a cylindrical vessel, inclined slightly
to the horizontal, which is rotated slowly about its axis. The material to
be processed is fed into the upper end of the cylinder. As the kiln rotates,
material gradually moves down towards the lower end, and may undergo a
certain amount of stirring and mixing. Hot gases pass along the kiln,
sometimes in the same direction as the process material (co-current), but
usually in the opposite direction (counter-current). The hot gases may be
generated in an external furnace, or may be generated by a flame inside the
kiln. Such a flame is projected from a burner-pipe.
The fuel for this may be gas, oil or pulverized coal. 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.
The basic components of a rotary kiln are the
refractory lined shell, support tyres and rollers, drive gear and burner
system. A typical Rotary Kiln is cylindrical shell, slightly inclined from
the horizontal and supported on two or more tyres, which in rotate on
rollers. One of the tyre is fixed by a set of thrust rollers to take care of
the thrust load during kiln movement. The drum assembly along with tyres is
rotated by a girth gear/pinion arrangement.
To maintain proper control of the volume and the
temperature of the gases flowing through the kiln, it is necessary to limit
the leakage of the external cold air in to the kiln where the rotating shell
enter at the feed and discharge hood. Seal rings are provided at both ends
between the hood and shell for this purpose. Various types of seal rings are
used which include spring loaded rubbing type where two finished metal
surfaces are in contact under spring pressure and spring plate type.
TYRES & SUPPORT ROLLERS
Tyres, sometimes called riding rings, usually
consist of a single annular steel casting, machined to a smooth cylindrical
surface, which attach loosely to the kiln shell through wedge arrangements.
These require some ingenuity of design, since the tyre must fit the shell
snugly, but also allow thermal movement. The tyre rides on pairs of steel
support rollers, also machined to a smooth cylindrical surface and set about
half a kiln-diameter apart. The rollers must support the kiln and allow
rotation that is as nearly frictionless as possible. Depend on size and
capacity an additional tyre can be provided with a set of support rollers.
Kilns usually rotate at 0.5 to 2 rpm. The bearings of the support rollers
must be capable of withstanding the large static and live loads involved,
and must be carefully protected from the heat of the kiln and the ingress of
dust. In addition to support rollers, there are usually upper and lower "retaining
(or thrust) rollers" bearing against the side of tyres, that prevent
the kiln from slipping off the support rollers.
BEARINGS & DRIVE COMPONENTS
Bearings are uniquely designed for this
application. The most common are the anti-friction or spherical roller
bearings. These bearings have become standardized and perform excellently in
almost any condition.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 temperature is encountered. To
compensate the thermal expansion, the girth gear is mounted on a tangential
spring plates attached to the shell.
The purpose of the refractory lining is to insulate
the steel shell from the high temperatures inside the kiln and to protect it
from the corrosive properties of the process material. It may consist of
refractory bricks or cast refractory concrete. The refractory selected
depends upon the temperature inside the kiln and the chemical nature of the
material being processed. Mostly 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 cannot be used liquid refractor called
castables are used with anchor support. Also provided insulation material
under the brick and this extra layer helps to reduce the heat transfer to
the shell. A typical refractory will be capable of maintaining a temperature
drop of 1000°C or more between its hot and cold faces. The shell
temperature needs to be maintained below around 350°C in
order to protect the steel from damage and continuous infrared scanners are
used to give early warning of "hot-spots" indicative of refractory
The hot gases may be generated in an external
furnace, Hot Air Generator (HAG), or may be generated by a flame inside the
kiln. Such a flame is projected from a burner-pipe. The fuel for this may be
gas, oil or pulverized coal. 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.
SCRUBBER & DUST COLLECTION
Exhaust gases from an oil fired kiln contain
certain proportion of very fine dust, entrained in the air stream while in
contact with materials being calcined and there may be undesirable
constituents such as sulfur dioxide or hydrogen chloride. Equipment is
installed to scrub these out before the exhaust gases pass to atmosphere.
A Dust Collector of high efficiency cyclone type is used to trap the fine
dust. Very fine particles which escape the dry collector may be recovered by
means of a wet collector in the form of a sludge or alternatively by means
of Bag Filter.
ROTARY COOLER is a part of Rotary kiln system used
to lower the temperature of the hot products discharged from the kiln to a
temperature approaching ambient. The Rotary Cooler is installed beneath the
Rotary Kiln and interconnected with the kiln discharge hood. The
construction is similar to Rotary kiln with a cylindrical shell inclined to
the horizontal with tyres, support rollers, feed hood and driven by girth
gear & pinion. A part of the cooler shell is castable lined to take care
of the shell from the hot discharge.
The Rotary Cooler operates in counter-flow, the hot gases being substituted
by ambient or chilled air, which is in direct contact with the hot material.
For high temperature applications the material is cooled to a temperature
approaching ambient by continuous water spray over the shell and operates on
the principle of heat transfer through the rotating cylinder wall which is
continuously irrigated by a series of external water sprays mounted over the
unit. The water is partially evaporated as steam while the excess is
collected from the bottom of the unit and re-circulated. The material thus
cooled is discharged from the other end of the shell.
ROTARY CARBONISER (KILN) FOR ACTIVATED CARBON UNITS
Rotary carbonisers (kilns) are used in Activated
carbon units for the steam activation of carbon granules (charcoal) at a
controlled temperature. The kiln is unique in design with a spiral wound
cooling coil arrangement fixed over the discharge end of the shell to lower
the product temperature during the discharge
through cooling coil. The system is complete with
moveable type feed hood for easy maintenance, rubbing type, spring loaded
sealing arrangement at both ends of the carboniser to seal and to control
the atmospheric air entry in to the burning zone through adjustable damper
valve provided on the feed hood, Steel casted girth gear with back-up plate
to take-up the expansion during operation, cone at discharge end for free
flow of flue gas through flue gas pipe etc.
Rotary kilns are used for the calcinations of
Titanium dioxide, Illeminate, kaolin, Barium Carbonate, Magnesium Carbonate,
Silica Gel, Lime, Activated Carbon, Vermiculite, Barium Sulphide, Magnesium
Oxide, Petroleum Coke, Clay, Gypsum, Alumina etc.
Drum Dryer is a drying system in which
water / solvent removed from a solution / suspension by the heating source.
Drum Dryer is a slow speed rotating drum, a part of the drum is submerged in
an attached trough contains wet feed. Heat is applied to the inside of the
rotating drum through drum hollow shaft and thorough rotary joint and the
wet feed clings to the outside of the drum.
The material is dryed during a part of revolution and is removed by a blade
or scraper. The condensate inside the drum will come out through other port
of rotary joint The operation of the drum dryer is controlled by the
adjustment of the drum speed and the heat applied.
The material of construction and the size of the drum depend upon the
character of the product and its capacity