How mineral rotary dryers work
Mineral rotary dryers dry silica sand, quartz, feldspar, limestone, dolomite, bauxite, kaolin, gypsum and heavy mineral sands such as ilmenite, rutile, zircon and garnet. Feed moisture is usually 5 to 15%, and the product leaves at below 0.5% for glass, foundry and abrasive grades. Most are co-current drums fired by oil, gas or a coal hot air generator, with inlet gas at 600 to 900°C because minerals tolerate high temperatures. Some sand dryers are counter-current to discharge a hot, dry product for screening. The exhaust, at about 90 to 150°C, carries hard, fine mineral dust through cyclones and a pulse-jet bag filter. The collected fines are often valuable and are returned to the product.
Where the ID fan sits
- Hot air generator
- Rotary drum
- Discharge hood
- Cyclones
- Pulse-jet bag filter
- ID fan
- Stack
The fan follows the bag filter. Under normal conditions it sees little dust, but the filter is bypassed during start-up, on high temperature or after bag failure, and the fan then takes the full cyclone outlet load of hard fines. Its suction covers the furnace, drum, discharge hood, cyclones and filter, typically 250 to 400 mmWC. The hood is held at slight suction so dust does not escape at the drum seals.
Gas at the ID fan
| Gas temperature, normal | 90 to 150°C at the fan inlet |
| Maximum or upset | Up to 200°C or more if feed is lost |
| Dust at the fan | Hard mineral fines; low after the bag filter, heavy if bypassed |
| Moisture and dew point | Moderate; dew point usually 40 to 55°C |
| Other | Dense heavy mineral dust, such as ilmenite and zircon, erodes faster than its size suggests |
| Density at the design temperature | About 0.90 kg/m³ at 120°C, 75% 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 mineral rotary dryers
| Blade form | Backward curved with wear allowance, or radial-tipped for dusty duty |
| Material of construction | Wear-resistant plate on blades and scroll, with replaceable liners |
| Drive and arrangement | Belt or coupled, variable speed |
| Bearings and cooling | Pedestal bearings with cooling disc |
| Flow control | Variable speed drive on drum suction |
| Also specified | Replaceable liners, inspection doors, vibration monitoring |
What this ID fan has to cope with
Erosion during filter bypass
Quartz and feldspar dust wears blade leading edges and the scroll quickly, and even a few hours of bypass at start-up does measurable damage. Wear-resistant plate on the blades, bolted replaceable liners in the scroll and a conservative tip speed keep the rotor in service between planned overhauls.
Dense heavy mineral dust
Ilmenite, rutile, zircon and garnet dust is roughly one and a half to two times as dense as silica, so each particle strikes the blades with more energy. Fans in beach sand separation plants need thicker wear protection and closer inspection than fans of the same size on silica sand.
Hotter exhaust from counter-current sand dryers
Counter-current dryers discharge a hot product and exhaust at a higher temperature, sometimes above 150°C. This lowers gas density, raises the volume the fan must move and may need higher temperature bags. The fan is rated and its density corrected for the highest exhaust temperature.
Energy and control
Mineral dryers run continuously, with throughput set by screening, grinding or bagging downstream, and feed moisture that rises sharply after rain on open stockpiles. A variable speed drive on the ID fan follows these swings without damper losses. Tight drum seals and an insulated drum and hood cut both the gas volume the fan handles and the fuel burnt per tonne.
ID fan and FD fan in mineral rotary dryers
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter | At the burner or hot air generator |
| What it handles | Exhaust with hard mineral fines | Ambient combustion air |
| Temperature at the fan | 90 to 150°C | Ambient |
| Main design concern | Abrasion during filter bypass | Burner turndown |
| Control | Drum suction | Firing rate |
See also the Forced Draft (FD) Fan for mineral rotary dryers.
Questions buyers ask
Why does a sand dryer ID fan wear out so quickly?
Quartz and heavy mineral dust are harder than ordinary steel. Even after a bag filter, bypass at start-up and leaking bags send dust to the fan, where it erodes blade leading edges and the scroll. Wear-resistant plate, replaceable liners and a lower tip speed extend the life considerably.
What temperature does a mineral rotary dryer exhaust run at?
Usually about 90 to 150°C at the fan for co-current dryers, and higher for counter-current sand dryers that discharge hot product. The upper limit is often set by the bag filter fabric, and the fan is selected at the density of the hottest normal condition.
Can a mineral dryer ID fan be placed before the bag filter?
It can, and some older plants do this, but the fan then handles the full dust load after the cyclones and wears in months. Placing it after the bag filter is normal practice and keeps erosion to the periods when the filter is bypassed.
Information needed to quote an ID fan for mineral rotary dryers
- 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.
- Mineral. Mineral type, density and particle size of the dust.
- Dryer flow. Co-current or counter-current and the exhaust temperature range.
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.