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 FD fan sits
- Inlet filter
- FD (combustion air) fan
- Air control valve
- Burner on the end wall
- Rotary drum
The fan is installed on the burner platform at the closed end of the drum, away from the door and tapping area. Air passes a control valve and enters the burner, which is mounted on a fixed end wall with a seal against the rotating shell. The pressure required is typically 300 to 600 mmWC, depending on the burner. Keeping the fan at this end also keeps it away from the fume around the door.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Dust | Fume and dust from charging if the inlet is near the door end |
| Pressure at the fan outlet | Typically 300 to 600 mmWC |
| Turndown | High fire after charging, reduced during smelting and holding |
| 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 lead rotary smelting furnaces
| 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 with burner ratio control |
| Also specified | Inlet filter, inlet taken from outside the building, and low pressure interlock to the burner |
What this FD fan has to cope with
Firing rate changes through the batch
The burner fires hard after charging to melt the cold charge, then much less while the slag and bullion separate. A fan with a damper wastes power for most of the batch and gives coarse ratio control. A variable speed drive tied to the burner ratio controller handles the full range smoothly.
Burner end wall seal leakage
The burner sits on a fixed end wall sealed against the rotating shell. When that seal wears, air leaks in or gas leaks out, and the burner flame changes. The combustion air fan cannot correct for this; it is a maintenance item, but the fan duty includes some margin for normal seal wear.
Oversized fan after oxy-fuel retrofit
After conversion to oxy-fuel, the original air fan runs nearly closed and noisy, or is left idle. A small cooling and start-up air fan sized for the new duty is more efficient and quieter than keeping the old fan throttled at a fraction of its capacity.
Energy and control
The burner firing rate varies through each batch, and furnaces often hold between batches waiting for charge. Speed control on the combustion air fan, linked to burner ratio, avoids wasting power at low fire. Where the plant uses oxy-fuel or oxygen enrichment, the air fan duty and power fall in proportion to the oxygen share.
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 Induced Draft (ID) Fan for lead rotary smelting furnaces.
Questions buyers ask
What does the FD fan do on a lead rotary furnace?
It is the combustion air fan for the burner that heats the drum. It supplies ambient air through a control valve to the burner on the fixed end wall. It does not handle off-gas or fume, so mild steel construction and standard bearings are used, and its main requirement is good control across the batch.
Can I convert my lead rotary furnace burner from air to oxy-fuel?
Many plants have done so to reduce fuel use and gas volume. The burner is changed and the combustion air fan becomes much smaller or redundant. The ID fan and bag filter then handle less gas, which usually leaves margin for higher throughput or lower fan speed.
Why is the combustion air inlet ducted from outside?
The air near the door end of a lead rotary furnace carries lead fume and dust from charging and tapping. Ducting the inlet from outside keeps the fan and burner clean, protects the people who maintain them, and gives a steadier, cooler air supply with more consistent density.
Information needed to quote an FD fan for lead rotary smelting furnaces
- 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, firing range, and whether air-fuel, oxy-fuel or oxygen-enriched.
- Batch firing profile. Firing rate during charging, smelting and holding.
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.