Rotary Kilns

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.

Standard 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 replacement.

Refractory :
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 Bag Filter

Application :
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.

Rotary Dryers are used to remove moisture from material using hot gases to heat the feed material and to evaporate the water. Rotary Dryers are suitable for drying wide range of materials continuously because of its ability to process material having considerable variation in size and composition.

Construction :
Rotary Dryers are slightly inclined cylinder shell supported by two nos. riding rings(tyres) running on a set of support Rollers. Fabricated in carbon, stainless steel and carbon+Ebonite lined with flights or lifters are welded or bolted internally to produce cascade of particles falling through hot gas stream. The mechanical lifting of the material allows to dry materials from filter cake to coarse minerals also helps in breaking-up lumps for uniform drying. The length of the drum may range from 4 to 10 times its diameter. Normal dryers the length will be 10 times its diameter and usually operate with 12 to 15% of their volume filled with the material.

End closer seals :
Both the ends of the drum are closed by steel fabricated hoods with air inlet/outlet chute with suitable seals in between the hood and the dryer drum. If slight ingress of air is acceptable simple felt seals can be used, bolted to a stationary end plate and positioned in between to seal rings. If air is unacceptable spring loaded friction seals are provided, where two finished metal surfaces are in contact under spring pressure. The sealing segments require regular adjustment and replacement.

Operation :
Dying is achieved by passing the material being processed through the horizontal rotating drum which also passes a current of hot air. Lifters or flights fitted internal surface of the drum lift and cascade the material through the hot air stream. The hot air steam can be directed through the cylinder either in the same direction as material (co-current) or in the opposite direction (counter current). Counter current applications are more efficient in terms of energy input but give rise to high product temperature, with the associated handling problem.

Air Heater :
The air may be heated indirectly by means of thermal oil or electricity-heated or steam heat exchanger.

Air Handling System :
In Rotary Dryer the flow of air is induced by a centrifugal fan located up stream of the air heater with filter at fan inlet. The exhaust air is removed either by a second centrifugal fan at the outlet or direct at outlet end of the dryer and through dust collector.

The exhaust gas stream from any drying operation inevitable include a significant amount of fine particles and these fine particles may be collected by passing the gases through cyclones or bag filter or wet scrubber or any combination on any or all of these, depending upon the nature and quantity of dust involved.

Process Control :
For any system there is relationship between the product moisture content and exhaust air temperature. Therefore product moisture can be controlled through the control of exhaust air temperature by regulating the flow of heat by means of temperature controller with a thermocouple located in the exhaust air duct. The optimum performance can be achieved if the dryer operates as per design conditions and with constant evaporative load. Any variation in evaporation load due to variation in feed rate or moisture content results in a corresponding variation in exhaust temperature which in turn increases or decreases air heater output and it raises or lowers the inlet air temperature. Thus evaporative load may be controlled by measurement of inlet air temperature using controller with temperature probe located in the inlet air duct and output signal to a variable rate feeder, which generally provides control of the drying system.

Dust Collecting Equipments :
Exhaust gases from Rotary Dryer contain certain proportion of very fine dust which becomes entrained in the air stream while in contact with materials being dryed. 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 Bag Filter

Application :
Rotary Dryers are used to dry Potassium Chlorate, Barium Nitrate, Sodium Chloride, Sodium Nitrate, Ilmenite, Urea Crystals, etc.