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 FD fan sits
- Inlet filter
- Burner air FD fan
- Burner air channels
- Burner
- Kiln
The fan stands on the firing floor beside the kiln hood. It supplies primary air through the burner channels, at typically 800 to 2,000 mmWC, while most of the combustion air enters hot from the cooler, drawn in by the ID fan or blown by grate cooler fans. On oil-fired kilns the primary air also helps atomisation and flame shaping.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Dust | Bauxite dust near the kiln; filtered intake needed |
| Pressure at the fan outlet | Typically 800 to 2,000 mmWC |
| Flow | A small share of combustion air, typically 8 to 15% |
| Variation | Adjusted for flame length and kiln conditions |
| Density at the design temperature | About 1.13 kg/m³ at 40°C, 94% 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. |
FD Fan selection for bauxite calcining kilns
| Blade form | Backward curved or radial at high tip speed |
| Material of construction | High-tensile steel impeller |
| Drive and arrangement | Direct coupled, with standby on continuous kilns |
| Bearings and cooling | Standard bearings with vibration monitoring |
| Flow control | Variable speed drive with channel dampers at the burner |
| Also specified | Inlet silencer and emergency air for burner protection |
What this FD fan has to cope with
Flame shape and ring formation
A flame that is too long or impinges on the lining encourages rings in the kiln. Steady primary air pressure with axial and swirl control lets the operator keep the flame compact and off the refractory.
Burner protection
The burner sits in a kiln at over 1,400°C. Loss of primary air distorts the burner pipe quickly, so a standby or emergency fan is advisable.
Dirty intake air
Bauxite dust around the kiln is abrasive and wears the impeller and inlet cone if drawn in, while deposits upset the balance. A filtered intake from a clean area protects it.
Energy and control
The burner air fan runs continuously at high pressure. Speed control saves power over throttling, but the larger gain is in fuel: correct primary air lets more of the combustion air come hot from the cooler, lowering heat consumption on a kiln that burns at very high temperature. A standby fan avoids the cost of a burner damaged during a trip.
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 Induced Draft (ID) Fan for bauxite calcining kilns.
Questions buyers ask
What is the FD fan on a bauxite kiln?
On a bauxite calcining kiln the forced draft fan is normally the burner primary air fan. Most combustion air comes hot from the cooler, so the primary air is a small share at high pressure.
How does the burner fan affect bauxite kiln rings?
Primary air controls flame shape. A long or impinging flame overheats the lining and encourages rings. Steady pressure and good burner control help keep the kiln clear.
What pressure does a bauxite kiln burner fan need?
Typically 800 to 2,000 mmWC, depending on the burner design and fuel. Oil and petcoke burners for high temperature kilns are usually at the upper end of the range.
Information needed to quote an FD fan for bauxite calcining kilns
- Duty point. Combustion air quantity in m³/h or kg/h and the static pressure at the fan outlet, with the margins you want.
- Air condition. Site ambient temperature range and altitude, and whether the fan takes ambient air or air already heated.
- System. Boiler or furnace capacity and fuel, resistance of the air heater, windbox, grate or burner, and the existing fan curve or nameplate if this is a replacement.
- Mechanical. Drive arrangement, flow control (damper, inlet vanes or variable speed drive), noise limit, rotation and discharge orientation.
- Burner and fuel. Burner type, fuel and primary air flow and pressure required.
- Cooler. Cooler type and how secondary air reaches the kiln.
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.