What is a rotary kiln?
What is a rotary kiln ? Rotary kilns or rotary furnaces are devices that include a steel or steel tube that is insulated with insulation such as cement or firebrick and rotates horizontally and slowly around its axis and from them for calcination. calcination or baking of all kinds of materials are used at temperatures above 300 degrees Celsius. The history of rotary kilns goes back to the time when, for the first time, in the late 1870s, Thomas Milne and his two sons produced their first Portland cement in a cast iron pipe while trying to produce Portland cement in India.
In 1885, an English engineer named Ransome patented a horizontal furnace with a gentle slope that could rotate and move the material from one end to the other in a gradual movement.
Before the invention of rotary kilns, the firing of materials was done in traditional kilns, which were replaced by these kilns after the invention of rotary kilns due to low firing efficiency, non-uniform firing, and much lower production.
How rotary kilns work
Rotary kilns are tubes that direct materials from the kiln entrance to the kiln exit through gentle rotation and gravity. The tubes of the rotary furnace are placed on steel rollers and rotate around the horizontal axis with the power of electricity and the operation of appropriate gearboxes.
Materials enter the rotary kilns from the upper part of the kiln, and except for rare cases, heat enters the kiln from the lower part of the kiln, where the materials exit. In this way, the material enters the rotary kiln with a low temperature (usually ambient temperature), and with the rotation of the kiln, it approaches the end of the kiln and the burner, and while traveling through the pipe of the rotary kiln, it gets hotter as it approaches the burner. Finally, after reaching the hell zone of the furnace, the desired temperature is reached and the cooking of the ingredients is completed, and then they are taken out of the furnace. In the next step, the materials enter the cooler and after the materials reach lower temperatures, they are used next.
Depending on the operating temperature and the distance from the furnace’s hell zone and the type of materials to be fired, the insulation inside the rotary kilns is done with chamotte, aluminum, magnesite or other types of bricks. The fuel used in rotary kiln burners may be natural gas, diesel, or fuel oil depending on the conditions and location, although natural gas is the best type of fuel if it is available.
In order to create air flow inside the rotary kiln and move the heat and gases inside the rotary kiln, a suction fan is used so that the uniform flow of heat causes it to be drawn to the beginning of the kiln. Also, the dust of the materials inside the furnace is separated from the air by bag filters suitable for the size of the furnace.
Length and diameter of rotary kilns
The length of industrial rotary kilns is from 6 meters to 240 meters and their diameter is from 60 centimeters to 15 meters to 7.5 meters depending on the length of the kiln. Basically, in order to achieve the proper efficiency of fuel and temperature, the ratio of length to diameter of numerical rotary furnaces should be between 30 and 40. For example, for a furnace with a length of 60 meters, and an opening diameter of 2 meters, this number is 30. If this number increases to more than 40, there will be no increase in production and efficiency, and if this number is less than 30, we will have energy loss.
Preheater for rotary kilns
In order to increase the efficiency of rotary kilns, a preheater is used. The preheaters pass the exhaust gases from the furnace and use its thermal energy to preheat the materials and cause the materials to enter the rotary furnaces with a hotter temperature and increase the efficiency and production of the product. do
Rotary furnace components
Pipe, refractory brick, burner, rim, gear and pinion, bag filter, suction fan, chassis, rollers.
