How bentonite rotary dryers work
Raw bentonite is mined with 25 to 35% moisture and is often mixed with soda ash and aged to convert calcium bentonite to the sodium form. It is then dried in a co-current rotary dryer to about 8 to 12% moisture, not bone dry, because over-dried bentonite loses swelling and gel strength. Hot gas from a lignite, coal, biomass or oil-fired hot gas generator enters at 450 to 700°C and leaves at a relatively low 80 to 110°C. The dried clay is very fine and hygroscopic, and the humid exhaust carries it to a cyclone and bag filter or wet scrubber, then to the ID fan. The FD fan supplies combustion air to the generator.
Where the ID fan sits
- Hot gas generator
- Rotary dryer
- Cyclone
- Bag filter or wet scrubber
- Exhaust (ID) fan
- Stack
The fan follows the bag filter or wet scrubber at the end of the gas train, with the drum, cyclone and collector upstream, typically 200 to 300 mmWC. It keeps the drum ends under slight suction. Because bentonite is dried only to 8 to 12% moisture, the exhaust is cool, often below 100°C, and it carries a large amount of water. The margin between exhaust temperature and dew point is small.
Gas at the ID fan
| Exhaust temperature, normal | 80 to 110°C after the collector |
| Maximum or upset | Up to 180°C if the drum runs empty |
| Dust at the fan | Very fine, hygroscopic clay; it swells and turns to paste with water |
| Moisture and dew point | Evaporation from 30% feed gives a dew point of about 65 to 75°C, close to the exhaust temperature |
| Corrosive species | SO₂ from lignite or high-sulphur coal in the generator raises the acid dew point |
| Density at the design temperature | About 0.96 kg/m³ at 95°C, 80% of standard air. Pressure developed and power drawn scale with density, so the fan is selected at this condition and the motor is checked at the coldest operating or start-up condition. |
ID Fan selection for bentonite rotary dryers
| Blade form | Backward inclined flat blades that shed deposits, or radial on scrubber duty |
| Material of construction | Mild steel on dry duty; SS 304 wetted parts after a wet scrubber or with high-sulphur fuel |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Pedestal bearings outside the gas stream |
| Flow control | Variable speed drive on drum suction and outlet temperature |
| Also specified | Insulated and cladded casing, drain, wash-down nozzles and a large access door |
What this ID fan has to cope with
Clay paste on the impeller
Bentonite fines that reach a casing near the dew point absorb water, swell and stick to the blades as a paste that hardens as it dries. The build-up throws the wheel out of balance quickly. Insulation, flat blades that shed deposits, wash-down nozzles and a warm purge before shutdown are the standard measures.
Bag filter blinding
A bag filter running on humid bentonite exhaust can blind within hours if the gas cools at start-up or if a door is left open. The fan then cannot hold drum suction. Preheating the filter before feed, insulating it, and choosing a fan with enough pressure margin keep the dryer running.
Over-drying to protect the fan
Some plants raise the outlet temperature to keep the exhaust away from its dew point, but over-dried bentonite loses swelling capacity. It is better to insulate the system and reduce false air, so the exhaust stays warm without spoiling the product. The fan is then selected for the true exhaust temperature, not an artificially raised one.
Energy and control
Bentonite feed moisture varies with the mine face, the ageing time and the season, so the evaporation load and exhaust volume change from day to day. A variable speed drive on the exhaust fan, controlled on drum suction, saves power during drier periods. Reducing air leakage at the drum seals also cuts fan volume and keeps the exhaust warmer, which helps against condensation.
ID fan and FD fan in bentonite rotary dryers
| ID fan | FD fan | |
|---|---|---|
| Position | After the cyclone and bag filter or scrubber | At the hot gas generator |
| What it handles | Very humid exhaust with clay fines | Clean ambient air |
| Temperature at the fan | 80 to 110°C | Ambient |
| Main design concern | Dew point margin and clay paste | Steady firing for product moisture |
| Control | Drum suction | Fuel feed and outlet temperature |
See also the Forced Draft (FD) Fan for bentonite rotary dryers.
Questions buyers ask
Why does bentonite build up in the dryer exhaust fan?
Bentonite is hygroscopic and the dryer exhaust is very humid. When fines reach a fan casing close to the dew point they absorb water, swell and stick to the blades. Insulating the casing, keeping air leaks low and washing the wheel during stops keeps the build-up under control.
Should a bentonite dryer use a bag filter or a wet scrubber?
Both are used. A bag filter recovers dry product but needs the exhaust kept above its dew point. A wet scrubber tolerates the humid gas but wastes the product and turns the fan duty wet. The ID fan materials and drain arrangement follow the choice of collector.
What exhaust temperature is normal for a bentonite rotary dryer?
Usually 80 to 110°C, lower than for most mineral dryers because bentonite is dried only to 8 to 12% moisture. With a dew point of about 65 to 75°C, the margin is small, so the fan and filter must be insulated and the system purged warm before stopping.
Information needed to quote an ID fan for bentonite rotary dryers
- Duty point. Gas volume in m³/h or Am³/h at the fan inlet and the static pressure the fan must develop, with the margin you want on each.
- Gas condition. Normal and maximum gas temperature at the fan, dust loading after the collector, moisture and any acid gases such as SO₂ or HCl.
- System. Boiler or furnace capacity and fuel, collector type and its pressure drop, and the existing fan curve or nameplate if this is a replacement.
- Mechanical. Drive arrangement, flow control (damper, inlet vanes or variable speed drive), rotation and discharge orientation, and a layout drawing or site photograph.
- Feed and product. Raw bentonite moisture, product moisture, throughput and whether soda ash is added.
- Collector. Bag filter or wet scrubber, and the measured exhaust temperature and humidity.
Built and tested in Chennai
Adroit Fantech Engineers has designed and manufactured industrial fans at Ayanambakkam, Chennai since 2004. Each impeller, casing and shaft is built for the gas and temperature it will see, performance tested as per IS 4894 or BS 848 Part I, and balanced to grade G 6.3 or G 2.5 as per ISO 21940-11, with the records supplied with the fan. We also replace rotors and retrofit existing fans of any make.