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 FD fan sits
- Inlet filter
- FD fan
- Burner air ducts
- Discharge end burner
- Rotary calciner
- Product cooler air
The fan stands at the discharge end, beside the burner platform. It delivers air to the burner, split into primary air for flame shape and secondary air for completing combustion. On many kilns part of the combustion air is pulled through the product cooler by kiln draft, so the fan supplies only the burner air. Outlet pressure is typically 400 to 800 mmWC for gas or oil burners.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Dust | Ambient air near the kiln can carry pigment dust; inlet filter needed |
| Pressure at the fan outlet | Typically 400 to 800 mmWC, depending on burner type |
| Turndown | Burner turndown of about 3:1 across grades |
| Air split | Primary and secondary air controlled separately |
| 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 titanium dioxide calciners
| Blade form | Backward curved, with a steep curve for stable pressure |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled, arrangement 4 or 8 |
| Bearings and cooling | Standard bearings; no cooling needed |
| Flow control | Variable speed drive on air main pressure, with dampers for primary and secondary air |
| Also specified | Inlet filter and silencer; standby fan if the kiln cannot stand a stop |
What this FD fan has to cope with
Flame instability at low firing
At low throughput an oversized fan throttled hard gives uneven air and an unstable flame, which affects crystal conversion and product colour. A properly sized fan with speed control keeps the burner stable over its range.
Pigment dust at the inlet
Fine TiO₂ around the kiln house enters an unfiltered inlet and coats the impeller and burner swirler. An inlet filter, or a duct to clean air, prevents imbalance and burner fouling.
Loss of air on trip
A combustion air fan trip leaves the burner without air and the kiln shell hot and still. The burner shutdown and the kiln auxiliary drive must be interlocked with the fan to protect the shell.
Energy and control
The combustion air fan runs continuously while the kiln fires. Air and fuel follow throughput and grade. Speed control on the fan, with oxygen trim at the kiln inlet, avoids excess air that would carry heat out of the kiln and increase the gas volume through the scrubbing train and ID fan.
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 Induced Draft (ID) Fan for titanium dioxide calciners.
Questions buyers ask
What does the FD fan do on a TiO₂ rotary kiln?
It is the combustion air fan for the discharge end burner, supplying primary and secondary air. It handles ambient air and does not see the acid kiln off-gas.
Why is primary air controlled separately on a TiO₂ calciner?
Primary air shapes the flame, which sets the temperature profile in the product zone and so the crystal form. Separate control lets the operator adjust flame shape without changing total air.
Does a calciner combustion air fan need a filter?
In a pigment plant, yes. Fine TiO₂ dust in the air around the kiln coats the impeller and burner if not filtered, causing imbalance and poor combustion. The filter loss should be included in the fan pressure.
Information needed to quote an FD fan for titanium dioxide calciners
- 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. Burner type, fuel, and primary and secondary air pressure and flow at full firing.
- Cooler air. Whether part of the combustion air is drawn through the product cooler by kiln draft.
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.