How slag drying plants work
Steel plants dry slag for sale or reuse: granulated blast furnace slag before grinding or dispatch, and crushed steelmaking slag fines before magnetic separation or as aggregate. Wet granulated slag carries about 8 to 15% moisture; steelmaking slag fines carry less water but contain free lime and metallic iron. Drying is done in rotary or fluid bed dryers heated by a hot air generator, which in a steel plant often burns blast furnace gas, coke oven gas or mixed gas, sometimes with oil or coal. Hot gas at 400 to 700°C enters the dryer and leaves at 90 to 130°C with water vapour and a heavy load of abrasive, glassy dust, which is cleaned in a cyclone and bag filter before the ID fan.
Where the ID fan sits
- Hot air generator
- Rotary or fluid bed dryer
- Cyclone
- Bag filter
- ID fan
- Stack
The fan sits after the bag filter, so it normally sees residual dust only. On older plants with just a cyclone or a wet scrubber, the fan handles the heavy dust load and wears fast. The gas at the fan is 90 to 130°C, carrying the water driven off the slag, so its dew point is high. The fan holds a slight suction at the dryer feed end and works against the dryer, cyclone and filter, typically 250 to 450 mmWC.
Gas at the ID fan
| Gas temperature, normal | 90 to 130°C after the bag filter |
| Maximum or upset | 180 to 250°C with wet feed interrupted or HAG overfiring |
| Dust at the fan | Below 50 mg/Nm³ after a bag filter; several g/Nm³ after a cyclone alone |
| Dust character | Glassy, angular slag particles; free lime and metallic iron in steelmaking slag |
| Moisture and dew point | High water vapour; dew point typically 50 to 65°C |
| Density at the design temperature | About 0.93 kg/m³ at 110°C, 77% 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 slag drying plants
| Blade form | Backward curved with wear liners after a bag filter; radial with replaceable liners after a cyclone |
| Material of construction | Wear-resistant plate impeller or hard-faced blades; mild steel casing with liners |
| Drive and arrangement | Direct or belt drive, at moderate tip speed to limit wear |
| Bearings and cooling | Pedestal bearings outside the gas stream |
| Flow control | Variable speed drive on dryer suction |
| Also specified | Casing drain, insulated casing, and bolted wear liners at the scroll cut-off |
What this ID fan has to cope with
Abrasive wear
Granulated slag is glassy and hard, and any dust that passes the filter erodes blade leading edges and the casing scroll. Wear liners, lower tip speed and a well-maintained bag filter with broken bag detection extend fan life.
Condensation at start-up and stop
The exhaust is close to its dew point, and on start-up or with wet feed the fan casing and ducts can condense water. Slag dust then forms a cement-like deposit. Insulation, a warm-up sequence before feed and casing drains limit this.
Free lime deposits
Steelmaking slag contains free lime that hydrates in the moist exhaust and then carbonates, forming a hard scale on the impeller and in the inlet cone. The scale grows unevenly and is difficult to remove once set. Keeping the gas above dew point, avoiding long idle periods with wet gas in the casing, and inspection at each stop control the build-up.
Energy and control
Slag dryers run to dispatch or grinding demand, often at varying throughput and feed moisture. The ID fan should follow evaporation load on speed control rather than run at full speed with a throttled damper. Lower exhaust volume also means less heat lost from the dryer, so keeping false air out at the seals saves both fan power and fuel.
ID fan and FD fan in slag drying plants
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter | Feeding the HAG burner |
| What it handles | Moist exhaust with slag dust | Ambient air |
| Temperature at the fan | 90 to 130°C | Ambient |
| Main design concern | Abrasion and condensation | Fuel gas variability and turndown |
| Control | Dryer suction | Burner firing and dryer inlet temperature |
See also the Forced Draft (FD) Fan for slag drying plants.
Questions buyers ask
Why does a slag dryer ID fan wear so quickly?
Granulated slag is glassy and very hard, and fine slag particles behave like abrasive grit. Any dust that passes the filter erodes the fan. Wear liners, hard-facing and keeping the bag filter in good condition are the main ways to extend fan life.
What temperature does a slag dryer ID fan handle?
Usually 90 to 130°C after the bag filter, with a high moisture content from the water driven off the slag. The fan should be rated for 180 to 250°C to cover feed interruptions, when the hot air generator keeps firing into an empty dryer and the exhaust temperature rises quickly.
How is a slag dryer for a steel plant different from one for a cement plant?
In a steel plant the dryer is often fired with blast furnace or mixed gas, and may handle steelmaking slag with free lime and metallic iron. In a cement plant the slag is usually granulated blast furnace slag fed to a mill or dryer heated by coal or kiln gas.
Information needed to quote an ID fan for slag drying 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.
- Slag type and moisture. Granulated BF slag or steelmaking slag, feed moisture and output moisture.
- Dust collection. Cyclone, bag filter or both, and the dust load at the fan.
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.