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 ID fan sits
- Furnace and tapping hoods
- Radiant cooler or dilution air
- Spark arrestor
- Bag filter
- ID fan
- Stack
The fan sits after the bag filter, where the tin oxide fume has been collected for return to the furnace and the gas carries only a trace of dust. Upstream it pulls through the furnace mouth or flue, the U-tube cooler, the hood branches and the filter, typically 200 to 350 mmWC in total. Placing it on the clean side protects the impeller from the sticky, fine fume and keeps the whole collection system under suction, so a leaking joint draws air in rather than letting lead-bearing dust out.
Gas at the ID fan
| Gas temperature, normal | 110 to 150°C after the bag filter |
| Maximum or upset | Up to 200°C if the cooler or dilution damper fails, limited by the filter bag rating |
| Dust at the fan | Low after the filter; sub-micron tin, lead and zinc oxide fume if a bag fails |
| Corrosive species | Chlorides and HCl from fluxes and coated scrap; some SO₂ from sulphidic feed |
| Moisture and dew point | Low to moderate; chloride-bearing gas becomes corrosive if it cools near its dew point at shutdown |
| 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 tin and solder smelting furnaces
| Blade form | Backward curved, on clean gas after the filter; radial-tipped where the filter is weak or bypassed |
| Material of construction | Mild steel with corrosion allowance; SS 316 or coated casing where chloride loading is high |
| Drive and arrangement | Belt or direct drive with variable speed; arrangement suited to outdoor mounting beside the filter |
| Bearings and cooling | Standard bearings outside the gas stream; no cooling needed at these temperatures |
| Flow control | Variable speed drive on furnace or hood suction, with motorised branch dampers |
| Also specified | Shaft seal, casing drain, inspection door and access for cleaning fume deposits |
What this ID fan has to cope with
Fume escaping at doors and taps
Tin furnaces charge, rabble and tap through open doors, and the hood air needed during these steps is several times the furnace off-gas. A fan sized for the furnace alone leaves the shed full of fume. The duty is set for the furnace plus the active hoods, with branch dampers so the suction goes where the work is.
Chloride corrosion at shutdown
Fluxes and plastic-coated solder scrap release chlorides. When the fan and ducts cool below the dew point after a shift, the condensate attacks mild steel quickly. A corrosion allowance, stainless or coated wetted parts where chlorides are high, and purging with warm air before stopping all extend life.
Sticky fume deposits
Fine oxide fume that passes a damaged bag sticks to blade faces in a thin, uneven layer and causes vibration. Bag leak detection, a fan blade form that sheds deposits and an inspection door for periodic cleaning keep the rotor in balance.
Cooler upset and hot gas
If the U-tube cooler is fouled or the dilution damper sticks, gas reaches the filter above its rating. The fan control and a temperature trip on the dilution damper protect the bags first; the fan itself is comfortable at these temperatures.
Energy and control
Smelting and refining run in campaigns of charging, reduction, tapping and holding, and the hood demand changes with each step. A variable speed drive tied to furnace suction, with dampers that close the idle hoods, lets the fan follow the cycle instead of running at full flow through every holding period. On small solder plants that work one or two shifts, the saving from turning the fan down during holding is a large part of the fan's running cost.
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 Forced Draft (FD) Fan for tin and solder smelting furnaces.
Questions buyers ask
What size ID fan does a tin smelting furnace need?
The fan is sized on the furnace off-gas after cooling plus the air drawn through every hood that can be open at once, at the bag filter outlet temperature. On a tin smelting furnace the hood air is often the larger part, so a list of hoods and their face velocities is needed before a selection is made.
Why do solder recycling fans corrode so quickly?
Solder scrap and dross carry fluxes and coatings that release chlorides. These form acidic condensate whenever the fan and duct cool below the gas dew point, usually after shutdown. Purging, insulation and corrosion-resistant wetted parts slow the attack; a plain mild steel fan without allowance can fail in a few years.
Should the fan be before or after the bag filter on a smelting furnace?
After. The fume is fine, sticky and contains lead and tin worth recovering. A fan on the clean side runs with little dust, and the collection system stays under suction so leaks draw air inward instead of releasing fume.
Information needed to quote an ID fan for tin and solder 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.
- Hoods and capture points. Number and size of hoods over charging, tapping and casting, and which can be open together.
- Feed and fluxes. Type of feed, such as concentrate, dross or solder scrap, and the fluxes used.
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.