How zinc Waelz kilns work
A Waelz kiln recovers zinc from electric arc furnace dust and other zinc-bearing residues. Pelletised or moist feed is mixed with coke breeze and fed to a long rotary kiln. As the bed moves to the hot end, at about 1,100 to 1,250°C, carbon reduces zinc oxide to zinc vapour, which leaves the bed and burns with air in the freeboard back to zinc oxide. Air is drawn in at the discharge end, or blown in through nozzles, by the kiln draft. The gas leaves the feed end at 650 to 800°C with a heavy load of crude zinc oxide, lead, chlorides and alkalis. It passes a settling chamber, an evaporative cooler or heat exchanger, the bag filter that collects the Waelz oxide product, and the ID fan.
Where the FD fan sits
- Inlet filter
- FD (process air) fan
- Air header
- Kiln shell nozzles or hood nozzles
- Kiln freeboard
The fan sits at the discharge end and supplies air to nozzles in the kiln hood or on the shell, and to the burner used for start-up or heating support. On many kilns most air enters through the open discharge end under the ID fan suction, and the FD fan supplies the controlled part. The pressure required covers the nozzles and header, typically 300 to 700 mmWC.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Dust | Dusty surroundings at the kiln discharge |
| Pressure at the fan outlet | Typically 300 to 700 mmWC |
| Turndown | Varies with feed rate and the required temperature profile |
| 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 zinc Waelz kilns
| Blade form | Backward curved for efficiency on clean air |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled |
| Bearings and cooling | Standard bearings; no cooling needed |
| Flow control | Variable speed drive on air flow |
| Also specified | Inlet filter against discharge-end dust, and flow measurement in the air header |
What this FD fan has to cope with
Dust at the discharge end
The slag discharge end of a Waelz kiln is dusty and hot. An unfiltered fan inlet takes in fine slag dust, which wears the impeller and blocks nozzles. An inlet filter or a duct from a clean location prevents this. The fan motor also needs protection from the radiant heat of the discharge end.
Air distribution and bed temperature
Air delivered too close to the bed raises its temperature and can cause the bed to soften and form accretions. Measured flow and controlled pressure at the nozzles let the operator keep the right temperature profile. Too little air near the discharge end, on the other hand, leaves carbon unburnt in the slag.
Balance with the ID fan
The FD and ID fans together set the air entering the kiln. If the FD fan is pushed hard while the ID fan is fixed, the discharge end can go to positive pressure and blow dust and gas out. The two must be controlled together.
Energy and control
The process air fan runs continuously at a flow set by feed rate and kiln conditions. A variable speed drive set by flow measurement uses less power than a damper and gives more stable control. The balance between blown air and air drawn in by the ID fan also affects energy use, because blown air is controlled while leakage air is not.
ID fan and FD fan in zinc Waelz kilns
| ID fan | FD fan | |
|---|---|---|
| Position | After the Waelz oxide bag filter | At the kiln discharge end |
| What it handles | Cooled kiln gas with chlorides and zinc oxide traces | Ambient air for kiln nozzles and burner |
| Temperature at the fan | 130 to 180°C | Ambient |
| Main design concern | Chloride corrosion and kiln draft | Air distribution along the kiln |
| Control | Feed end draft | Air flow |
See also the Induced Draft (ID) Fan for zinc Waelz kilns.
Questions buyers ask
Does a Waelz kiln need an FD fan?
Not always. Some kilns rely entirely on the ID fan drawing air through the open discharge end. Many use a process air fan to blow a controlled share of the air through nozzles, which gives better control of the temperature profile and zinc burning along the kiln.
What pressure does a Waelz kiln process air fan need?
Typically 300 to 700 mmWC, depending on the nozzle design and the air header. Shell nozzle systems need more pressure than hood nozzles. We calculate the duty from the nozzle data and the required air flow and confirm the margin. Where nozzles are added to an existing kiln, the fan is checked for both flow and pressure.
Why must the FD fan inlet be filtered at a Waelz kiln?
The discharge end of a Waelz kiln, where the slag leaves, is dusty. Fine slag dust drawn into the fan erodes the impeller and blocks the air nozzles. An inlet filter or a duct to a clean air source avoids both problems at low cost.
Information needed to quote an FD fan for zinc Waelz kilns
- 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.
- Air nozzles. Number, location and design of the air nozzles, and the share of total air they supply.
- Burner. Start-up or support burner air requirement.
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.