How bauxite calcining kilns work
Refractory and abrasive grade bauxite is calcined at about 1,400 to 1,600°C to drive off combined water and convert the alumina to dense corundum. Proppant and cement grade bauxite follow similar routes. Long rotary kilns are common, fired with coal, petcoke, furnace oil or gas from the discharge end, with a rotary or grate cooler supplying hot secondary air. The raw bauxite carries 12 to 30% combined water and free moisture, released in the drying and dehydration zones. The kiln exit gas leaves at 700 to 950°C on a long kiln and carries bauxite dust, part of it already calcined and very hard. It is cooled by dilution air or a heat recovery unit, then passes a cyclone and bag filter before the ID fan.
Where the ID fan sits
- Rotary kiln
- Dust chamber
- Gas cooler or dilution air
- Cyclone
- Bag filter
- ID fan
- Stack
The fan sits after the bag filter where one is fitted, so that it handles only the residual dust. On older plants with only cyclones, the fan sees several grams per cubic metre of hard dust. Upstream, the gas is cooled from 700 to 950°C to suit the collector by dilution air or a cooler. The fan works against the kiln, dust chamber, cooler and collectors, typically 300 to 500 mmWC.
Gas at the ID fan
| Gas temperature, normal | 200 to 250°C after cooling and the bag filter |
| Maximum or upset | 350 to 450°C if gas cooling fails |
| Dust at the fan | Low after a filter; heavy and very abrasive after cyclones only |
| Moisture and dew point | Moderate from combined water in the bauxite |
| Corrosive species | SO₂ from coal or petcoke |
| Density at the design temperature | About 0.71 kg/m³ at 230°C, 59% 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 bauxite calcining kilns
| Blade form | Radial-tipped or backward inclined with thick blades and wear allowance |
| Material of construction | Wear-resistant plate on blades and casing liners; hardfacing on leading edges; boiler quality plate stress-checked at upset temperature |
| Drive and arrangement | Direct coupled with moderate tip speed, variable speed drive |
| Bearings and cooling | Shaft cooling disc and heat shield |
| Flow control | Variable speed drive on kiln hood pressure |
| Also specified | Replaceable liners, inspection door for wear checks, spare rotor for critical kilns |
What this ID fan has to cope with
Erosion by calcined bauxite
Calcined bauxite is mostly corundum, among the hardest minerals. Any dust reaching the fan cuts blade leading edges, shrouds and casing quickly. Placing the fan after a bag filter, keeping tip speed moderate, and wear plate and liners on the parts that see the dust extend life.
Ring formation and draft swings
Rings form in the kiln from sticky low-melting phases, and when they break the draft and dust load change suddenly. The fan must have pressure margin for a ringed kiln and the collector must cope with surges.
Gas cooling failures
If dilution air or the cooler fails, gas above 400°C reaches the collector and fan. Fast emergency dilution and a fan designed for the upset temperature protect both.
Energy and control
Bauxite kilns run continuously and the ID fan is a large drive. Speed control on hood pressure saves power at reduced output and avoids high tip speeds, which reduces wear. Lower tip speed and a fan matched to the real duty cut both power and erosion, so the energy and maintenance benefits come together.
ID fan and FD fan in bauxite calcining kilns
| ID fan | FD fan | |
|---|---|---|
| Position | After gas cooling and the collector | On the firing floor, feeding the burner |
| What it handles | Cooled kiln gas with hard dust | Ambient air |
| Temperature at the fan | 200 to 250°C | Ambient |
| Main design concern | Erosion by calcined bauxite | Flame momentum and burner protection |
| Control | Kiln hood pressure | Burner air pressure |
See also the Forced Draft (FD) Fan for bauxite calcining kilns.
Questions buyers ask
Why do ID fans on bauxite kilns wear so quickly?
Calcined bauxite dust is mostly corundum and is extremely hard. Any dust that passes the collector erodes blades and casings. A bag filter before the fan and wear-resistant construction are the main remedies.
What temperature does a bauxite kiln ID fan handle?
After cooling and a bag filter, usually 200 to 250°C. The kiln exit gas is 700 to 950°C, so cooling is essential, and the fan must survive upsets if the cooling fails.
Can a backward curved fan be used on a bauxite calcining kiln?
After a reliable bag filter, a backward inclined fan with wear allowance works well and is more efficient. After cyclones only, a radial-tipped design with liners is safer.
Information needed to quote an ID fan for bauxite calcining kilns
- 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.
- Collector. Type of collector before the fan and the expected dust load at the fan.
- Gas cooling. Method of cooling the kiln gas and the dilution air quantity.
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.