How alumina calciners work
An alumina calciner converts aluminium hydroxide from the Bayer process into smelter grade or specialty alumina by driving off free moisture and chemically bound water at about 950 to 1,100°C. Most plants use gas suspension or circulating fluidised bed calciners; older and specialty plants use rotary kilns. Wet hydrate enters a flash dryer or venturi drier, is preheated in cyclone stages by hot gas from the calciner, calcined in a furnace fired with natural gas or fuel oil, and cooled in cyclones and a fluid bed cooler that preheat the combustion air. The exhaust gas is very humid, with 35 to 50% water vapour, and leaves the drying stage at 140 to 200°C carrying fine alumina to the ESP and ID fan.
Where the ID fan sits
- Fluid bed cooler and cooling cyclones
- Calcining furnace
- Preheater cyclones
- Flash or venturi dryer
- ESP
- ID fan
- Stack
The fan sits after the ESP, so it sees only the alumina that the ESP does not catch. It draws air in through the product coolers, where the air is preheated before combustion, and pulls the whole system under suction. Total suction is high, typically 600 to 900 mmWC, because the gas passes several cyclone stages, the dryer and the ESP.
Gas at the ID fan
| Gas temperature, normal | 140 to 200°C after the ESP |
| Maximum or upset | Up to 250°C when wet feed is interrupted |
| Dust at the fan | Fine alumina, extremely hard; 30 to 100 mg/Nm³ after the ESP, much more on ESP field trips |
| Moisture and dew point | 35 to 50% water vapour, giving a water dew point of about 75 to 85°C |
| Corrosive species | Low; sulphur only from fuel oil firing |
| Density at the design temperature | About 0.80 kg/m³ at 170°C, 67% 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 alumina calciners
| Blade form | Backward curved with wear protection, at moderate tip speed |
| Material of construction | Mild steel casing with wear liners at the scroll; impeller in high strength or wear-resistant plate with leading edge protection |
| Drive and arrangement | Direct coupled, double inlet or single inlet arrangement 8 for large flows |
| Bearings and cooling | Pedestal bearings with cooling disc and vibration monitoring |
| Flow control | Variable speed drive on calciner draft and production rate |
| Also specified | Insulated casing, casing drain, and bolted wear liners at the cut-off |
What this ID fan has to cope with
Erosion by alumina dust
Alumina is one of the hardest common industrial dusts. Even at the low concentrations after an ESP, it wears blade leading edges and the scroll cut-off over months, and an ESP field trip can cause rapid damage. Lower tip speed, wear-resistant plate and replaceable liners extend impeller life.
Condensation at start-up and shutdown
The exhaust carries so much water vapour that its dew point is around 80°C. During start-up, shutdown or low feed, the casing can fall below it, and wet alumina forms hard scale. Insulating the casing, warming the system before feeding and keeping the fan running until the gas is dry avoid this.
Unbalance from uneven wear and scale
Uneven erosion and patches of scale both unbalance the impeller. On a large calciner fan this means high vibration and the risk of an unplanned stop. Vibration monitoring and in-situ balancing at planned shutdowns keep the fan running between maintenance periods. Keeping a spare rotor ready for exchange shortens the stop when wear does reach its limit.
Energy and control
The calciner ID fan is one of the largest motors in the alumina refinery's calcination area and runs continuously. A variable speed drive matched to production rate and draft saves considerable power compared with damper control. Reducing air ingress at cyclone flaps and the ESP, and recovering impeller efficiency lost to erosion, save energy directly.
ID fan and FD fan in alumina calciners
| ID fan | FD fan | |
|---|---|---|
| Position | After the ESP, before the stack | At the fluid bed cooler and burners |
| What it handles | Humid exhaust with fine alumina | Clean ambient air |
| Temperature at the fan | 140 to 200°C | Ambient |
| Main design concern | Erosion and dew point | Steady fluidising pressure |
| Control | Calciner draft and production rate | Fluidising air flow |
See also the Forced Draft (FD) Fan for alumina calciners.
Questions buyers ask
Why does the alumina calciner ID fan wear out?
Alumina dust is extremely hard, and even the small amount that passes the ESP erodes the blades and casing over time. Wear increases sharply when an ESP field trips. Moderate tip speed, wear-resistant plate and liners, and quick restoration of ESP fields protect the fan.
What is the dew point of alumina calciner exhaust gas?
Typically about 75 to 85°C, because the gas carries free moisture from the hydrate and chemically bound water released during calcination, often 35 to 50% water vapour by volume. The fan and ESP must stay well above this to avoid wet scale. Insulated casings help at start-up.
How much suction does an alumina calciner ID fan need?
Typically 600 to 900 mmWC, depending on the number of cyclone stages, the dryer type and the ESP. The fan must also cope with higher resistance from cyclone build-up. We size the fan from the plant's heat and mass balance and measured site data. Margin for build-up matters.
Information needed to quote an ID fan for alumina calciners
- 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.
- Calciner type and capacity. Gas suspension, circulating fluidised bed or rotary kiln, and the output in t/day.
- Gas data. Gas flow, temperature, water vapour content and dust at the fan inlet.
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.