How kaolin and clay calciners work
Kaolin is calcined to metakaolin at about 650 to 850°C, or to fully calcined kaolin at 1,000 to 1,100°C, to raise brightness and opacity for paper coating, paint, plastics and ceramics. The feed is washed, refined and spray-dried or pulverised clay of 1 to 2 µm, with about 13 to 14% combined water released during calcination. Calciners are direct or indirect-fired rotary kilns, multiple hearth furnaces or flash calciners, fired with natural gas, LPG or clean oil to avoid colouring the product. The exhaust carries water vapour, combustion gas and a large share of ultrafine clay dust. It passes cyclones and a bag filter, where the dust is recovered as product, before the ID fan and stack.
Where the ID fan sits
- Calciner (rotary, hearth or flash)
- Cyclones
- Bag filter
- ID fan
- Stack
The fan sits after the bag filter, which recovers fine calcined clay for return to the product. On flash calciners the fan draws all the hot gas and product through the calciner and cyclones, at high suction, typically 400 to 700 mmWC. On rotary and hearth calciners the suction is lower, 200 to 400 mmWC. On indirect-fired rotary calciners the fan on the process side handles only the released water and sweep air, while the furnace side has its own flue gas fan.
Gas at the ID fan
| Gas temperature, normal | 150 to 230°C after the bag filter |
| Maximum or upset | Up to 300°C with feed interruption on flash calciners |
| Dust at the fan | Low after the filter; ultrafine clay if bags fail |
| Moisture and dew point | Moderate to high from combined water; dew point about 55 to 70°C |
| Corrosive species | Traces of fluorine from some clays at full calcination |
| Density at the design temperature | About 0.77 kg/m³ at 190°C, 64% 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 kaolin and clay calciners
| Blade form | Backward curved or aerofoil, for clean gas after the filter |
| Material of construction | Mild steel or boiler quality plate; stainless steel where fluorides or condensation are expected |
| Drive and arrangement | Direct coupled, variable speed drive |
| Bearings and cooling | Shaft cooling disc where gas exceeds about 200°C |
| Flow control | Variable speed drive on calciner outlet pressure |
| Also specified | Insulated casing, drain and inspection door |
What this ID fan has to cope with
Product carried by the gas
With 1 to 2 µm particles, a large part of the product travels with the exhaust. A bag leak sends product to the stack and coats the fan blades with fine clay. Leak detection and a blade form that does not trap dust keep the fan clean and the yield up.
Residence time and brightness
On flash and rotary calciners, the gas flow set by the ID fan affects how long the clay stays at temperature. Swings in fan speed change calcination and brightness. Steady control on calciner pressure, not manual damper moves, keeps the product consistent.
Condensation at start-up and stop
The exhaust carries water released from the clay. Cold starts and stops condense it in the fan and filter, turning dust into a paste. Warm-up with dry hot gas and purging before shutdown prevent it.
Energy and control
Kaolin calciners run continuously at steady output for long campaigns, so the ID fan should be sized to run near its best efficiency. A variable speed drive allows trimming for grade changes and feed rates. Reducing air leakage at the calciner seals cuts gas volume and fuel use, as the leaked air must be heated and drawn through the filter.
ID fan and FD fan in kaolin and clay calciners
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter | At the burners or combustion chamber |
| What it handles | Exhaust with released water and clay fines | Ambient air |
| Temperature at the fan | 150 to 230°C | Ambient |
| Main design concern | Steady draft for consistent calcination | Clean flame for brightness |
| Control | Calciner outlet pressure | Air-fuel ratio |
See also the Forced Draft (FD) Fan for kaolin and clay calciners.
Questions buyers ask
What temperature does a kaolin calciner ID fan handle?
Typically 150 to 230°C after the bag filter, depending on the calciner type and how much heat is recovered. Flash calciners can see higher temperatures when feed is lost.
Why does the ID fan affect kaolin brightness?
The gas flow set by the fan controls how long the clay spends at calcining temperature in flash and rotary calciners. Unsteady draft gives uneven calcination and variable brightness. Stable speed control keeps the product within specification.
Does a kaolin calciner need a stainless steel fan?
Not usually. After a bag filter a mild steel fan is normal. Stainless steel is used where some clays release fluorides at high temperature or where the exhaust often condenses.
Information needed to quote an ID fan for kaolin and clay 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. Rotary, hearth or flash calciner, direct or indirect firing, and output.
- Exhaust. Gas flow, temperature and moisture at the bag filter outlet.
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.