In Direct Rotary Calciners the heat transfer occurs mainly by radiation thanks to the flame which envelopes the radiating heat to the refractory wall and to the material. Only a small portion of heat is given by conduction and convection.
Due to flame in direct contact to material, the direct calciner can work up to 1000 – 1200 °C.
The calcining process normally involves the following stages that can be carried out with rotary kiln
- Drying stage to draw out the superficial water that take place in the first section, feeding side, of the rotary kiln.
- Calcining section
The rotary unit consists mainly of a revolving cylinder, slightly inclined to the horizontal, supported in two riding rings resting on two trunnion rolls each, with the trunnion roll bearing mounted on structural steel bases.
Based on the initial moisture of material the rotary kiln can include an initial portion provided with spiral and lifting flights to dry the material before it enters in the calcining zone.
On either side of one of the riding rings is an adjustable thrust roll for holding the cylinder in its longitudinal position.
The cylinder is rotated by means of a girt gear meshing with a spur pinion mounted on the low speed shaft of a speed reducer.
The high speed shaft of the reducer is directly coupled to an electric motor.
The interior of the cylinder is lined with two layers of bricks.
At the feed end the cylinder is enclosed in a refractory lined head provided with flanged connection for gas exit and feed chute.
At discharge end the cylinder is enclosed in a refractory lined breeching with a central opening for the burner installation.
Seals are provided at feed and discharge end to limit air leakage between rotary cylinder and stationary heads.
The kiln burner is of our construction operating with natural gas or other fuel.
The flue gases at the rotary kiln exit are conditioned with external air to reduce the temperature to about 180°C at the bag filter entrance for dedusting before the discharge into the atmosphere.
A damper mounted on the blower suction governs the exhaust flow and the necessary negative pressure inside the rotary kiln.
The calcined material at the rotary kiln exit can be cooled preheating the combustion air supplied under the cooling grate with blower.
Products that can be processed by direct calciners:
- Regeneration of sands
- Mercury extraction
- Ferrite and magnetite
- Welding dust