A Rotary kiln is a pyroprocessing device used to raise materials to a high
temperature in a continuous process. Materials produced using rotary kilns
include: Cement
Lime Refractories
Metakaolin Titanium dioxide
Alumina Vermiculite
Iron ore pellets They are also used for roasting a wide
variety of sulfide ores prior to metal extraction.
Principle of Operation 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, but usually in the
opposite direction. 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 which acts like a large bunsen burner. The fuel for this may be
gas, oil, pulverized petroleum coke or pulverized coal.
Construction The basic components of a rotary kiln
are the shell, the refractory lining, support tyres and rollers, drive gear
and internal heat exchangers. History
The rotary kiln was invented in 1873 by the Englishman Frederick Ransome.
= Kiln Shell = This is made from rolled mild steel
plate, usually between 15 and 30 mm thick, welded to form a cylinder which
may be up to 230 m in length and up to 6 m in diameter. This will be usually
situated on an east/west axis to prevent eddy currents.
Upper limits on diameter are set by the tendency of the shell to deform under
its own weight to an oval cross section, with consequent flexure during rotation.
Length is not necessarily limited, but it becomes difficult to cope with
changes in length on heating and cooling if the kiln is very long.
= Refractory Lining = 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, or may be absent in
zones of the kiln that are below around 250°C. The refractory selected depends
upon the temperature inside the kiln and the chemical nature of the material
being processed. In some processes, such as cement, the refractory life is
prolonged by maintaining a coating of the processed material on the refractory
surface. The thickness of the lining is generally in the range 80 to 300 mm. 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 failure. = Tyres and 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 a variety of "chair" 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 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. A well-engineered kiln, when
the power is cut off, will swing pendulum-like many times before coming
to rest. The mass of a typical 6 x 60 m kiln, including refractories and feed,
is around 1100 tonnes, and would be carried on three tyres and sets of
rollers, spaced along the length of the kiln. The longest kilns may have 8 sets
of rollers, while very short kilns may have only two. Kilns usually rotate at
0.5 to 2 rpm, but sometimes as fast as 5 rpm. The Kilns of most modern cement
plants are running at 4 to 5 rpm. The bearings of the 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
rollers" bearing against the side of tyres, that prevent the kiln from
slipping off the support rollers. = Drive Gear =
The kiln is usually turned by means of a single Girth Gear surrounding a cooler
part of the kiln tube, but sometimes it is turned by driven rollers. The gear is
connected through a gear train to a variable-speed electric motor. This must
have high starting torque in order to start the kiln with a large eccentric
load. A 6 x 60 m kiln requires around 800 kW to turn at 3 rpm. The speed of
material flow through the kiln is proportional to rotation speed, and so a
variable speed drive is needed in order to control this. When driving through
rollers, hydraulic drives may be used. These have the advantage of developing
extremely high torque. In many processes, it is dangerous to allow a
hot kiln to stand still if the drive power fails. Temperature differences
between the top and bottom of the kiln may cause the kiln to warp, and
refractory is damaged. It is therefore normal to provide an auxiliary drive for
use during power cuts. This may be a small electric motor with an independent
power supply, or a diesel engine. This turns the kiln very slowly, but enough
to prevent damage. = Internal heat exchangers =
Heat exchange in a rotary kiln may be by conduction, convection and radiation, in
descending order of efficiency. In low-temperature processes, and in the
cooler parts of long kilns lacking preheaters, the kiln is often furnished
with internal heat exchangers to encourage heat exchange between the gas
and the feed. These may consist of scoops or "lifters" that cascade the
feed through the gas stream, or may be metallic inserts that heat up in the
upper part of the kiln, and impart the heat to the feed as they dip below the
feed surface as the kiln rotates. The latter are favoured where lifters would
cause excessive dust pick-up. The most common heat exchanger consists of chains
hanging in curtains across the gas stream.
Other equipment The kiln connects with a material exit
hood at the lower end and to ducts for waste gases. This requires gas-tight
seals at either end of the kiln. The exhaust gas may go to waste, or may
enter a preheater which further exchanges heat with the entering feed.
The gases must be drawn through the kiln, and the preheater if fitted, by a
fan situated at the exhaust end. In preheater installations which may have a
high pressure-drop, a lot of fan power may be needed, and the fan is often then
largest drive in the kiln system. Exhaust gases contain dust 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. Thermal efficiency
The thermal efficiency of the rotary kiln is about 50-65%.
See also List of ovens
Sources and further reading R H Perry, C H Chilton, C W Green,
Perry's Chemical Engineers' Handbook, McGraw-Hill, sections 12.56-12.60,
23.60, ISBN 978-0-07-049841-9 K E Peray, The Rotary Cement Kiln, CHS
Press, ISBN 978-0-8206-0367-4 Boateng, Akwasi, Rotary kilns :
transport phenomena and transport processes. Amsterdam ; Boston :
Elsevier/Butterworth-Heinemann, ISBN 978-0-7506-7877-3
References