How titanium dioxide calciners work
In the sulphate route to titanium dioxide, washed metatitanic acid pulp from the leaf filters, still holding water and 5 to 8% sulphuric acid, is fed to a long direct-fired rotary kiln. Gas or oil burners at the discharge end heat the product to 850 to 1,000°C, where it dries, loses its acid and converts to anatase or rutile pigment. The gas flows counter-current and leaves the feed end at 300 to 450°C carrying water vapour, SO₂, sulphuric acid vapour and SO₃, and fine pigment. A dust chamber or cyclone returns the coarse fines; the gas is then quenched, scrubbed in a venturi or packed tower and passed through a wet ESP to remove acid mist before the ID fan and stack. The chloride route uses an oxidiser instead of a calciner and is not covered here.
Where the ID fan sits
- Rotary calciner
- Dust chamber or cyclone
- Quench
- Venturi or packed scrubber
- Wet ESP
- ID fan
- Stack
Placing the fan after the scrubber and wet ESP keeps it clear of the hot acid vapour and the bulk of the pigment, which is recovered upstream. The gas is saturated at 60 to 80°C. The fan must overcome the kiln, dust chamber, quench, scrubber and ESP, typically 400 to 700 mmWC, and holds the kiln feed end at a slight suction so that acid fume does not escape at the feed chute and seals.
Gas at the ID fan
| Gas temperature, normal | 60 to 80°C, saturated after the scrubber |
| Maximum or upset | Up to 200 to 300°C if quench water fails; a temperature trip protects the fan |
| Dust at the fan | Low; fine TiO₂ pigment, sticky when wet |
| Moisture and dew point | Saturated, with free droplets carried from the wet ESP |
| Corrosive species | Dilute sulphuric acid mist, residual SO₂ and SO₃ |
| Density at the design temperature | About 1.02 kg/m³ at 75°C, 85% 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 titanium dioxide calciners
| Blade form | Backward curved or backward inclined, with open passages that drain |
| Material of construction | Rubber-lined or FRP lined mild steel, or high alloy stainless such as 904L for the impeller |
| Drive and arrangement | Belt or direct drive, arrangement 9 or 8, bearings outside the gas stream |
| Bearings and cooling | Standard bearings; a cooling disc only where the upset temperature requires it |
| Flow control | Variable speed drive on kiln feed end draft |
| Also specified | Casing drain, wash nozzles, acid-resistant shaft seal and a quench failure trip |
What this ID fan has to cope with
Dilute acid corrosion
Acid mist that passes the wet ESP collects on the impeller and casing and concentrates as water evaporates, attacking ordinary stainless steel at welds and crevices. Lined construction or high alloy stainless, with smooth, fully welded surfaces, gives a long life.
Pigment and gypsum deposits
Fine pigment, and gypsum from any lime used in scrubbing, settle as a wet paste on the blades. As it dries and flakes unevenly the fan vibrates. Wash nozzles at the inlet and a regular wash routine keep the rotor clean.
Quench failure
If quench water fails, hot acid gas reaches lined ducts and the fan, damaging rubber or FRP within minutes. A temperature trip, emergency water and a design that tolerates the short upset protect the fan.
Energy and control
TiO₂ calciners run continuously, with throughput varied by grade. The ID fan works against a high, fairly steady resistance from the scrubbing train. A variable speed drive on kiln draft lets the fan follow grade changes and avoids pulling excess air through the kiln seals, which would cool the product zone and raise burner fuel.
ID fan and FD fan in titanium dioxide calciners
| ID fan | FD fan | |
|---|---|---|
| Position | After scrubber and wet ESP, before the stack | At the kiln discharge end, feeding the burner |
| What it handles | Saturated acidic kiln off-gas | Ambient air |
| Temperature at the fan | 60 to 80°C | Ambient |
| Main design concern | Dilute sulphuric acid corrosion | Stable burner air |
| Control | Kiln feed end draft | Air main pressure and air split |
See also the Forced Draft (FD) Fan for titanium dioxide calciners.
Questions buyers ask
Where is the ID fan placed on a titanium dioxide kiln?
Usually after the quench, scrubber and wet ESP, where the gas is saturated at 60 to 80°C. This keeps the fan out of the hot acid gas and the main pigment dust load, but requires corrosion-resistant construction.
What material is used for a TiO₂ calciner ID fan?
Rubber-lined or FRP lined mild steel, or high alloy stainless steel for the impeller, because the gas carries dilute sulphuric acid mist. Plain mild steel and standard grades corrode quickly in this duty.
Why does a titanium dioxide calciner need draft control?
The kiln must stay at slight suction at the feed end so that acid fume and pigment do not escape at seals and chutes. Too much suction draws in cold air, which cools the kiln and increases fuel use.
Information needed to quote an ID fan for titanium dioxide 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.
- Off-gas cleaning train. Equipment between kiln and fan, with the pressure drop and outlet temperature of each.
- Acid content. Expected acid mist and SO₂ at the fan inlet, and quench failure temperature and duration.
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.