How aluminium dross processing plants work
Dross skimmed from melting and holding furnaces contains 30 to 80% metallic aluminium mixed with oxide, nitride, carbide and salt. Hot dross is cooled quickly in a rotary dross cooler, a press or under inert gas to stop the metal burning. The cooled dross is then crushed, screened and milled to separate metallic concentrate, which goes back to a rotary furnace, from fine non-metallic product. Every step makes dust: fume from hot dross, and fine oxide, metallic aluminium and salt dust from crushing and screening. A dust extraction system with hoods at each point, ducts, a spark arrestor, a bag filter and an ID fan serves the plant. There is usually no forced draft duty.
Where the ID fan sits
- Dross cooler and crusher hoods
- Screen and mill hoods
- Ducting
- Spark arrestor
- Bag filter
- ID fan
- Stack
The fan sits after the bag filter, on the clean side, so it handles air with only traces of dust. Upstream, the hoods take air from the dross cooler, where it is warm and carries fume, and from crushing and screening points at near ambient temperature. Suction at the fan is typically 200 to 350 mmWC. Keeping the fan on the clean side keeps combustible metallic dust out of the rotating equipment.
Gas at the ID fan
| Air temperature, normal | 40 to 80°C at the fan |
| Maximum or upset | Up to 150°C if hot dross is crushed before cooling is complete |
| Dust at the fan | Traces of alumina, salt and fine metallic aluminium after the filter |
| Moisture and dew point | Low; moisture must be kept away from dross dust |
| Reactive gases | Ammonia from aluminium nitride, and traces of phosphine and methane, when dross dust meets moisture |
| Density at the design temperature | About 1.06 kg/m³ at 60°C, 88% 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 aluminium dross processing plants
| Blade form | Backward curved on the clean side, with wear allowance on the leading edge |
| Material of construction | Mild steel with earthing; non-sparking inlet ring or rubbing strip where required by the explosion risk assessment |
| Drive and arrangement | Direct coupled, arrangement 8 or 4 |
| Bearings and cooling | Standard pedestal bearings outside the air stream, with temperature monitoring |
| Flow control | Variable speed drive on duct suction, with branch dampers |
| Also specified | Earthing and bonding, vibration monitoring, and inclusion in the filter explosion protection design |
What this ID fan has to cope with
Metallic aluminium dust explosion
Fine aluminium dust is a serious explosion hazard. The bag filter carries explosion venting or suppression, and the fan, though on the clean side, must not be an ignition source. Earthing, close control of impeller clearance to avoid rubbing, bearing temperature monitoring and vibration trips address this.
Ammonia and other gases from wet dross
Aluminium nitride in dross reacts with moisture to release ammonia, and carbides and phosphides release methane and phosphine. If the plant is washed down or rain enters the ducts, these gases form in hoppers and ducts. Keeping the system dry and extraction running prevents gas build-up.
Abrasive dust in the ducts and fan
Alumina and corundum in dross are very hard. In the ducts, bends and the fan inlet erode quickly. Low duct velocities consistent with conveying, wear-resistant bends, and a clean-side fan with leading edge protection limit wear. Bends and the inlet cone are the first parts to check at each shutdown.
Hood capture at the dross cooler
Hot dross emits fume and burning particles. The cooler hood must capture these without drawing so much air that the cooler oxidises the metal. A dampered branch sized for this point balances capture against metal recovery. The branch damper is set at commissioning and checked whenever the cooler is modified.
Energy and control
Dross plants run in batches as dross arrives from the melting shop, and not all points are active at once. Branch dampers that close idle points, with a variable speed drive holding duct suction, cut fan power substantially. Correct duct velocity also saves power, because oversized velocity adds pressure loss and wear without improving capture.
Questions buyers ask
Is aluminium dross dust explosive?
Yes. Dross dust contains fine metallic aluminium, which is combustible and can explode as a dust cloud. Dust collection systems in dross plants need explosion protection on the filter, earthing throughout, and fans that are not ignition sources. The fan is normally placed on the clean side of the filter.
Why does a dross processing plant smell of ammonia?
Aluminium nitride formed when molten aluminium reacts with nitrogen is present in dross. When the dross or its dust becomes wet, the nitride reacts with water to release ammonia. Keeping dust and dross dry and the extraction running prevents the gas building up. Good ventilation of storage areas also helps.
Does an aluminium dross plant need an FD fan?
Usually not. The extraction system is a single induced draft system pulling air through hoods at each process point. Rotary dross coolers are normally cooled externally with water or air, and any inert gas is supplied separately. The ID fan is the main fan duty.
Information needed to quote an ID fan for aluminium dross processing plants
- 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.
- Process points. Dross cooler, crushers, screens and mills on the system, and the air flow at each hood.
- Dust properties. Metallic aluminium content of the dust and any explosion test data for it.
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.