Rotary Dryers
Rotary Dryers
Large throughputs and high evaporative loads.
Operation:
Rotary dryers have the feed materials pass through a rotating cylinder together with a stream of hot gas. Internal lifters or flights elevate the feed and drop it in a cascading fashion across the cross-section of the cylinder. Flights need to be carefully designed to prevent asymmetry of the cascading. Material moves from one end of the dryer to the other by the motion of the material falling and due to the angle of inclination of the cylinder.
- Rotary Dryers are designed for numerous processing applications and are generally considered the workhorse for mineral concentrate drying.
- Rotary Dryers are highly robust with a low maintenance demand and the ability to process a broad range of materials.
- Capacity ranging from 100 kg/hr to in excess of 500 tph. Moisture contents in excess of 30%. But depends on the material properties.
- Materials of construction can range from painted mild steel, through to various grades of stainless steel to special alloys depending on the processing environments and conditions.
- Heat sources range from steam, thermic oil, electric, gas, liquid fuel burner systems and coal-fired fluid bed combustors.
- All equipment is tailor designed to suit the processing requirements.
- The drive systems can range from girth gear and pinion to chain and sprocket depending on the power demand of the process.
- Materials processed: silica sand, chrome sands, coal, phosphate, potash, bauxite, vermiculite, andalusite, graphite to name a few.
Feeding/discharge:
Automatic and continuous. Vibrators, screw feeders, rotary airlocks, double flap valves, gravity chutes
Feed Characteristics:
Powders, granules, non-friable agglomerates, large solid particles. Low to high feed rates
Heat Source:
Steam, electrical, coal, liquid fuels, or gas
Exposure:
Mostly Direct. Flow is co-current or countercurrent for cascade dryers. Flow is a combination of through the bed, cross, co-current or countercurrent for rotary louver.
Residence time:
Controllable. Residence time can be altered by varying the speed of rotation of the drum and the drum angle. Fill can be varied. Fills are typically between 7% and 25% of the cross-sectional area of the drum.
Extraction:
Normally induced. Systems can have combustion, forced and induced draft fans for operation.
Dust Collection:
Cyclones, baghouses, scrubbers, Electrostatic precipitators. Systems require dust collection.
Control:
PLC, infrequently solid state controllers. System controls on discharge temperature. Can modulate energy and feed.
Benefits:
High throughput
Low capital cost
Robust
Can handle variations in feed although the product may be inconsistent
High-temperature operation - can be refractory lined.
The unit can have an integral cooling section.
Limitations:
Clogging although chains and knockers will reduce this
May cause size reduction due to attrition and impact
Inconsistent dried product characteristics
Poor energy efficiency - high losses in cascade units
Poor heat transfer in cascade units
High maintenance
Large real estate required
Many wear components
No implicit dust separation
