How lead rotary smelting furnaces work
A short rotary furnace smelts battery paste, grids, drosses and residues in batches. The charge, with soda ash, iron and coke or anthracite as flux and reductant, is loaded through the door, and a burner at one end, air-fuel or oxy-fuel, heats it to 1,000 to 1,200°C while the drum rotates slowly. A batch typically takes several hours from charging to tapping bullion and slag. Off-gas leaves through the door end into a hood and flue at 900 to 1,100°C, carrying lead oxide and sulphide fume, sodium sulphate and SO₂. It passes a settling chamber or afterburner, an air-cooled hairpin cooler or dilution, a bag filter and the ID fan, then an SO₂ scrubber where fitted.
Where the ID fan sits
- Furnace door and hood
- Settling chamber or afterburner
- Hairpin air cooler or dilution
- Spark arrestor
- Bag filter
- ID fan
- Stack
The fan sits after the bag filter. The hood over the door collects both the furnace off-gas and fume released when the door is open, so the suction needed during charging and tapping is higher than during smelting with the door closed. Total suction at the fan is typically 300 to 450 mmWC, set by the cooler and the bag filter at its dirty condition.
Gas at the ID fan
| Gas temperature, normal | 100 to 140°C after the bag filter |
| Maximum or upset | Up to 180°C at peak firing with a fouled cooler |
| Dust at the fan | Traces of lead oxide, lead sulphide and sodium salts after the filter |
| Moisture and dew point | Combustion moisture and moisture in wet paste; acid dew point raised by SO₃ |
| Corrosive species | SO₂ from sulphate in the paste, chlorides from PVC and fluxes |
| Volume swing | Large; door open for charging and tapping, closed while smelting |
| Density at the design temperature | About 0.88 kg/m³ at 130°C, 73% 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 lead rotary smelting furnaces
| Blade form | Backward curved on the clean side, with a wide stable operating range |
| Material of construction | Mild steel with acid-resistant lining at cold spots, or SS 316 casing where condensation is frequent |
| Drive and arrangement | Direct coupled, arrangement 8, with a sealed shaft passage |
| Bearings and cooling | Pedestal bearings with a shaft cooling disc |
| Flow control | Variable speed drive on hood suction with batch-stage set points |
| Also specified | Leak-tight casing, casing drain to effluent, insulation and a fast-acting inlet damper |
What this ID fan has to cope with
Fume escape when the door opens
When the door is swung open for charging or tapping, the gas stream is suddenly diluted by a large volume of room air and fume rolls out at the top of the opening. A fan sized for the open-door flow, with a drive that ramps up as the door opens, keeps the hood capturing the fume.
Sticky sodium sulphate and lead fume
Soda ash flux produces sodium sulphate and low-melting lead compounds that are sticky at the cooler outlet temperatures. If a bag fails these deposit on the blades as a hard crust. A clean-side fan, broken bag detection and a wheel that can be reached through a large access door are the practical response.
Cooler overload at peak firing
At the start of a batch, burner firing is at its highest and the hairpin cooler or dilution may not bring the gas down enough. The bag filter protection damper then opens to admit cold air, and the fan must handle this extra volume without losing hood suction. Fan sizing includes the dilution air at peak firing.
Energy and control
Within each batch the gas volume changes widely, from high firing and an open door during charging to low firing during the final smelt. A variable speed drive with set points for each stage, linked to door position and hood suction, uses far less power than a damper. Oxy-fuel firing reduces the gas volume substantially, and an existing fan may then be slowed or re-wheeled.
ID fan and FD fan in lead rotary smelting furnaces
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter, before the stack | At the burner end of the drum |
| What it handles | Cooled off-gas with lead and sodium salt traces | Ambient air |
| Temperature at the fan | 100 to 140°C | Ambient |
| Main design concern | Open-door hood capture and acid corrosion | Batch firing range |
| Control | Hood suction with batch set points | Burner ratio |
See also the Forced Draft (FD) Fan for lead rotary smelting furnaces.
Questions buyers ask
What size ID fan does a lead rotary furnace need?
It depends on the burner firing rate, the firing method, the door hood design and the cooling method. Air-fuel firing with dilution cooling gives a much larger volume than oxy-fuel firing with an air cooler. We calculate the duty for the open-door condition, which usually sets the fan size, and check it against site measurements.
Why does fume escape from the rotary furnace door?
During charging and tapping the door is open, and the hood has to capture both the off-gas and fume released from the open drum. If the fan cannot provide enough extra suction at that moment, fume escapes. Hood design and a fan that ramps up when the door opens fix this.
Does oxy-fuel firing change the ID fan on a lead rotary furnace?
Yes. Oxy-fuel firing removes most of the nitrogen from the off-gas, so the gas volume at the fan falls considerably. An existing fan may then run at low speed for most of the batch. The open-door hood flow still needs to be covered, so the fan cannot always be reduced in proportion.
Information needed to quote an ID fan for lead rotary smelting furnaces
- 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.
- Batch cycle. Batch size, charging and tapping times, and burner firing rate at each stage.
- Door hood. Hood dimensions and whether the door is open or closed during charging and tapping.
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.