How mineral flash calciners work
Flash or gas suspension calciners treat fine minerals in a hot gas stream for a few seconds instead of hours in a kiln. Kaolin is converted to metakaolin at about 700 to 850°C, alumina hydrate to alumina at 1,000 to 1,100°C, and gypsum to plaster at 150 to 180°C, while magnesite and phosphates have their own windows. Feed passes up through cyclone preheaters, enters the calciner vessel where a burner fires oil or gas, and is separated in a hot cyclone. Product then cools in cyclone coolers or a fluid bed, which preheat the combustion air. The whole system typically runs under suction from a single ID fan after the ESP or bag filter, with a smaller FD fan supplying burner air.
Where the ID fan sits
- Cooler cyclones
- Calciner vessel
- Separation cyclone
- Preheater cyclones
- ESP or bag filter
- ID fan
- Stack
The fan is at the cold end, after the dust collector, and provides suction for the entire system. Air enters through the cooling cyclones, picks up heat from the product, becomes combustion air in the calciner, and leaves through the preheaters. With several cyclone stages, the total resistance is high, often 500 to 800 mmWC. The gas temperature at the fan depends on how many preheater stages are fitted.
Gas at the ID fan
| Gas temperature, normal | 150 to 250°C after the preheaters and filter |
| Maximum or upset | 350°C or more if feed is lost with the burner on |
| Dust at the fan | Very fine mineral; low after a bag filter or ESP, heavy during filter upsets |
| Moisture and dew point | Water from dehydroxylation and feed moisture; dew point typically 50 to 70°C |
| Corrosive species | SO₂ from fuel or sulphate impurities; fluorides on some phosphate and clay feeds |
| Density at the design temperature | About 0.75 kg/m³ at 200°C, 63% 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 mineral flash calciners
| Blade form | Backward curved or aerofoil, with wear protection at the leading edges |
| Material of construction | Mild steel casing; wear-resistant blade tips or liners for abrasive minerals |
| Drive and arrangement | Direct coupled, arrangement 8, with a variable speed drive |
| Bearings and cooling | Pedestal bearings with a shaft cooling disc |
| Flow control | Variable speed drive on calciner outlet pressure or temperature |
| Also specified | Temperature trip, inlet damper, casing drain and inspection door |
What this ID fan has to cope with
Gas velocity and product quality
In a flash calciner, residence time depends on the gas velocity, which is set by the ID fan. A drop in draft as the filter loads or a cyclone partially blocks changes the time at temperature and can leave material under- or over-calcined. Speed control on calciner pressure, with good measurement, holds velocity steady.
High system resistance
Several cyclone stages, a calciner and a filter give a high total pressure, and the fan must deliver it at a high gas volume. A high-efficiency backward curved or aerofoil fan matters here, because fan power is a significant part of the calciner's energy use.
Fine dust during filter upsets
Calcined minerals are very fine and often abrasive. If the ESP trips or bags fail, the fan sees heavy dust and wears quickly. Wear-protected blade tips, an interlock that reduces feed when the filter trips, and regular inspection keep the fan reliable. Kaolin and alumina fines are fine enough to pass a damaged bag easily.
Energy and control
The ID fan is often the largest single power consumer on a flash calciner, because it moves all the gas against the resistance of several cyclone stages. A variable speed drive is normal practice. Keeping cyclone dip legs sealed and reducing false air at flap valves lowers the gas volume, and the fan power, without affecting the calcination.
ID fan and FD fan in mineral flash calciners
| ID fan | FD fan | |
|---|---|---|
| Position | After the ESP or bag filter | At the calciner burner |
| What it handles | System gas with fine mineral dust | Clean ambient air |
| Temperature at the fan | 150 to 250°C | Ambient |
| Main design concern | Steady draft through many cyclone stages | Flame stability |
| Control | Calciner pressure or temperature | Burner firing rate |
See also the Forced Draft (FD) Fan for mineral flash calciners.
Questions buyers ask
Why is the ID fan so important on a flash calciner?
It provides the draft that moves both gas and mineral through the whole system. Gas velocity sets the residence time in the calciner and the separation efficiency in the cyclones, so ID fan speed directly affects product quality, output and energy use. On most flash calciners it is the largest motor in the plant.
How much pressure does a flash calciner ID fan need?
Typically 500 to 800 mmWC, depending on the number of preheater and cooler cyclone stages, the calciner vessel and the dust collector. The exact figure comes from the process design, and the fan is selected with a margin for cyclone build-up and filter loading, which raise resistance between cleanings.
Can one ID fan serve a kaolin flash calciner and its dryer?
Sometimes the feed dryer is integrated with the preheater and shares the same fan, using calciner waste heat to dry the feed. The fan must then handle the added water vapour and a lower gas temperature, so it is sized for the combined duty and checked at both the dryer-on and dryer-off conditions.
Information needed to quote an ID fan for mineral flash 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.
- Mineral and process. Mineral calcined, calcination temperature, throughput and number of cyclone stages.
- Gas cleaning. ESP or bag filter, and gas temperature 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.