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 ID fan sits
- Kiln freeboard
- Feed end housing
- Settling chamber
- Evaporative cooler
- Waelz oxide bag filter
- ID fan
- Scrubber or stack
The fan sits after the bag filter that collects the product, so the dust that reaches it is low. Its suction covers the kiln, the settling chamber, the cooler and the filter, typically 350 to 550 mmWC. Because the kiln is open to atmosphere at the discharge end, the fan speed sets how much air enters the kiln, and so controls the burning of zinc vapour and the bed temperature.
Gas at the ID fan
| Gas temperature, normal | 130 to 180°C after the bag filter |
| Maximum or upset | Up to 250°C on cooler failure, before the filter protection operates |
| Dust at the fan | Fine zinc oxide with lead and chlorides; low after the filter, but very fine |
| Moisture and dew point | High from evaporative cooling and wet feed; dew point of hygroscopic chloride deposits close to fan temperature |
| Corrosive species | Chlorides, fluorides, alkalis and SO₂ |
| Density at the design temperature | About 0.82 kg/m³ at 160°C, 68% 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 zinc Waelz kilns
| Blade form | Backward curved on the clean side, with smooth blade surfaces |
| Material of construction | SS 316 or higher alloy wetted parts where chlorides condense; mild steel with insulation where gas stays well above dew point |
| Drive and arrangement | Direct coupled, arrangement 8, casing on sliding supports |
| Bearings and cooling | Pedestal bearings with shaft cooling disc |
| Flow control | Variable speed drive on kiln inlet draft or feed end pressure |
| Also specified | Insulated casing, casing drain, large access door and provision for water washing of the impeller |
What this ID fan has to cope with
Chloride deposits and corrosion
EAF dust carries chlorides that volatilise in the kiln and condense as fine salts after the cooler. On a cold fan casing they absorb moisture and form a corrosive paste. Insulation, keeping the gas well above dew point, SS 316 where needed and washing the fan during stops keep the casing intact.
Kiln draft and zinc recovery
Too little draft leaves zinc vapour unburned and the kiln runs reducing at the feed end; too much brings in excess air, cools the gas and increases volume. Fine speed control on the ID fan, tied to feed end pressure and oxygen, holds the kiln in balance and helps product quality.
Rings and kiln upsets
Accretion rings in the kiln change the resistance and the air distribution, and the fan draft has to be raised to compensate. The fan needs pressure margin above the clean kiln condition to keep the kiln running while rings are managed. Rings also break away suddenly, giving a short surge of dust and gas that the filter and fan must ride through.
Product loss through a failed bag
The Waelz oxide is the product, so bag failure loses product and coats the fan. Broken bag detection and a fan wheel that sheds deposits limit both the loss and the vibration that follows. Because the oxide is valuable, many plants also return any material washed from the fan casing to the product stream rather than to waste.
Energy and control
Waelz kilns run continuously, and the ID fan is the largest single power user in the kiln line. A variable speed drive controlling feed end pressure saves power compared with damper control, and keeps air ingress steady. Reducing false air at the kiln seals and feed end housing reduces gas volume and fan power directly.
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 Forced Draft (FD) Fan for zinc Waelz kilns.
Questions buyers ask
What controls the air flow in a Waelz kiln?
Mainly the ID fan. The kiln is open at the discharge end, and the fan suction draws air through it to burn the zinc vapour and coke. Some kilns add air through nozzles with a process air fan, but the ID fan speed still sets the total gas flow and the draft at the feed end.
Why does the Waelz kiln ID fan corrode?
EAF dust contains chlorides and alkalis that pass the kiln as vapour and condense as fine, hygroscopic salts after the cooler. Deposits on a cool casing absorb moisture and attack mild steel. Keeping the casing hot, insulating it and using stainless steel at the cold spots reduce corrosion.
Where is the ID fan placed on a Waelz kiln?
After the bag filter that collects the Waelz oxide product. Placing it on the clean side protects the impeller from the very fine, sticky oxide and lets the fan run between filter maintenance stops. The fan suction then covers the kiln, cooler and filter resistance.
Information needed to quote an ID fan for zinc Waelz kilns
- 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.
- Kiln and feed. Kiln dimensions, feed rate and zinc, lead and chloride content of the feed.
- Gas cooling. Type of cooler, outlet temperature and the filter design temperature.
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.