How tin and solder smelting furnaces work
Tin is produced by reducing cassiterite concentrate or recycled tin dross with coke or anthracite in a reverberatory, short rotary or electric furnace at about 1,200 to 1,300°C, under a deliberately reducing atmosphere. Solder and secondary tin plants melt and refine scrap, dross and solder residues in tilting rotary furnaces or oil- and gas-fired pots at lower temperatures. The off-gas carries very fine tin oxide fume, with lead, zinc and antimony oxides from solder scrap and chlorides from fluxes and coated scrap. It leaves the furnace at 700 to 1,000°C, is cooled in a radiant U-tube cooler or by dilution air, and passes a bag filter before the ID fan and stack. Fume hoods over tapping and charging doors usually share the same filter and fan.
Where the FD fan sits
- Inlet filter
- FD fan
- Air flow measurement
- Control valve
- Burner
- Furnace
The fan is mounted beside the furnace or on the burner skid and delivers air through a measuring section and control valve to the burner. On a tilting rotary furnace the air line has a flexible or swivel connection at the burner end. Pressure at the fan outlet is typically 300 to 700 mmWC, enough for the burner at full firing plus the air line and valve. Where a recuperator preheats the air, the fan still sits on the cold side.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Dust | Shed air with fine oxide and coke dust; an inlet filter is advisable |
| Pressure at the fan outlet | Typically 300 to 700 mmWC |
| Turndown | Full firing for melting and reduction, low firing for holding and refining |
| Air-fuel ratio | Below stoichiometric for reduction, close to stoichiometric for melting |
| 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 tin and solder smelting furnaces
| Blade form | Backward curved, for a stable curve and good efficiency |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Direct coupled, often skid-mounted with the burner |
| Bearings and cooling | Standard bearings; no cooling needed |
| Flow control | Variable speed drive or inlet vane control, linked to fuel flow by ratio control |
| Also specified | Inlet filter, flow measuring section and low air pressure interlock |
What this FD fan has to cope with
Holding a reducing flame
Tin reduction depends on a flame with a controlled shortage of air. If the fan delivery drifts with filter loading or supply voltage, the furnace swings between oxidising and over-reducing, losing tin to slag or fume. Measured air flow and ratio control, rather than a fixed damper, keep the atmosphere where it belongs.
Wide turndown on small burners
Pot and rotary furnaces spend long periods holding at low fire. A fan selected for full firing and throttled heavily runs near its stall region. Speed control, or a fan chosen with its best efficiency near the holding duty, gives stable low-fire air.
Dirty inlet air
Smelting sheds are dusty. Oxide and coke dust drawn into the fan builds up and is blown into the burner, disturbing the flame. An inlet filter with a pressure gauge and a sheltered intake solve most of this.
Energy and control
The combustion air fan on a small smelting furnace is modest in power, but it runs for every hour the furnace is hot, much of it at holding fire. Matching fan speed to the firing rate, instead of throttling, reduces power, and good ratio control saves far more in fuel and in tin lost to over-oxidation than the fan itself consumes.
ID fan and FD fan in tin and solder smelting furnaces
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter, before the stack | On the burner skid, feeding the burner |
| What it handles | Cooled furnace gas and hood air with trace fume | Ambient shed air |
| Temperature at the fan | 110 to 150°C | Ambient |
| Main design concern | Hood suction and chloride corrosion | Steady air for a controlled reducing flame |
| Control | Furnace and hood suction | Air-fuel ratio |
See also the Induced Draft (ID) Fan for tin and solder smelting furnaces.
Questions buyers ask
Is the FD fan on a tin smelting furnace the burner blower?
Yes. On most tin and solder smelting furnaces the forced draft fan is simply the combustion air blower feeding the oil or gas burner, sometimes with a separate fan for secondary air at the furnace mouth.
What pressure does a combustion air fan for a rotary smelting furnace need?
Typically 300 to 700 mmWC at the fan outlet, depending on the burner design, the length of the air line and any recuperator. High-momentum burners on rotary furnaces sit at the upper end of that range.
Why does the air-fuel ratio matter so much in tin smelting?
Tin is reduced from its oxide only under a reducing atmosphere. Too much air reoxidises the metal and pushes tin into the slag and fume; too little wastes fuel. The fan and its control are what hold that balance during each stage of the cycle.
Information needed to quote an FD fan for tin and solder 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 make or type, fuel, maximum firing rate and air pressure required at the burner.
- Furnace cycle. Duration of melting, reduction and holding stages and the firing rate in each.
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.