How single super phosphate plants work
Single super phosphate is made by mixing finely ground rock phosphate with sulphuric acid, typically 60 to 70% strength, in a cone or paddle mixer. The slurry drops into a den, where it sets over 30 to 60 minutes and is then cut out by a rotary cutter and sent to storage for curing. The reaction releases steam, CO₂ and fluorine compounds, mainly silicon tetrafluoride and hydrogen fluoride, from the 2 to 4% fluorine in the rock. Den and mixer gases are drawn by an exhaust fan through a series of cyclonic or spray scrubbers, where the fluorides are absorbed in water to form fluosilicic acid, and then discharged through a stack. Granulated SSP adds a granulator and rotary dryer with their own fans.
Where the ID fan sits
- Acid and rock mixer
- Den
- Den off-gas duct
- Fluorine scrubbers
- Mist eliminator
- Den exhaust (ID) fan
- Stack
The fan usually sits after the scrubbers, so it handles cooled, nearly saturated gas with residual fluorides and mist. The suction at the fan covers the den, the long off-gas duct, typically two to four scrubber stages and the mist eliminator, and is usually 150 to 300 mmWC. Placing it after the scrubbers keeps the hot, concentrated SiF₄ away from the fan, which would otherwise deposit hard silica on the impeller. The fan also sets the den suction that protects operators at the cutter and discharge.
Gas at the ID fan
| Gas temperature, normal | 40 to 60°C after the scrubbers |
| Maximum or upset | About 80 to 100°C if scrubber water fails |
| Dust at the fan | Little dust; fine silica gel and fluosilicate solids carried over with mist |
| Moisture and dew point | Saturated, with droplets carried past the mist eliminator |
| Corrosive species | HF, SiF₄ and fluosilicic acid mist; aggressive to steel, stainless and glass fibre |
| Density at the design temperature | About 1.08 kg/m³ at 55°C, 90% 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 single super phosphate plants
| Blade form | Backward curved or radial with open passages, so silica deposits can be washed off |
| Material of construction | Rubber-lined mild steel, polypropylene or FRP with a fluoride-resistant synthetic veil in place of glass on the wetted surface |
| Drive and arrangement | Belt driven, arrangement 9, with the motor outside the gas path |
| Bearings and cooling | Pedestal bearings outside the casing with a lip or labyrinth shaft seal |
| Flow control | Inlet damper or variable speed drive on den suction |
| Also specified | Water wash nozzles, a casing drain with seal pot and a large inspection door |
What this ID fan has to cope with
Silica scaling
SiF₄ hydrolyses in contact with water to form gelatinous silica, which deposits in ducts, scrubbers and on the fan. Scale grows unevenly on the impeller and causes vibration. Regular water washing, open blade passages and efficient scrubbing upstream keep the deposit thin. Spray nozzles at the fan inlet are sometimes used for continuous washing on plants with heavy scaling.
Fluoride attack on materials
Hydrofluoric acid attacks glass fibre and the passive layer on stainless steel. Rubber lining, polypropylene or FRP built with a synthetic, non-glass veil in the wetted layer resists it. Standard glass-reinforced FRP fails early on this duty. Shaft seals and fasteners in the gas path need the same attention, since a stainless bolt can fail long before the lining does.
Loss of den suction
When scaling raises the system resistance, the den loses suction and fluorine fumes escape at the cutter, which is both a health hazard and an emission problem. The fan is sized with margin for a scaled system, and duct and scrubber cleaning are scheduled with fan washing.
Energy and control
SSP dens operate in batches or continuously depending on design, and the fume load peaks just after mixing. A variable speed drive holding den suction saves power between peaks and responds better than a fixed damper as scale builds up. Keeping the scrubbers and mist eliminator clean is the main energy measure, since every millimetre of extra resistance is paid for at the fan.
Questions buyers ask
What material is used for the den exhaust fan in an SSP plant?
Rubber-lined mild steel, polypropylene, or FRP made with a synthetic veil resistant to hydrofluoric acid on the wetted surface. Ordinary glass-reinforced FRP and standard stainless steel are attacked by fluorides on this duty. The choice also depends on the gas temperature during upsets, because rubber linings and polypropylene have upper temperature limits that must not be exceeded if scrubber water fails.
Why does the SSP den fan vibrate?
Silica from the hydrolysis of silicon tetrafluoride deposits on the impeller and grows unevenly. Washing the fan regularly with water through nozzles in the casing, and keeping the scrubbers efficient, controls the build-up. An open blade form makes washing more effective. A vibration trend shows when the next wash is due, before the scale grows enough to damage bearings.
Where should the ID fan be in an SSP fluorine scrubbing system?
After the scrubbers and mist eliminator, so the fan handles cooled, mostly cleaned gas rather than the hot, fluoride-rich den gas. This reduces both scaling and corrosion on the fan. The fan also benefits from the cooler, denser gas, which reduces its size and power. A mist eliminator just before the fan stops droplets carrying acid to the impeller.
Information needed to quote an ID fan for single super phosphate 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.
- Scrubbing system. Number and type of scrubber stages, mist eliminator, and the pressure drop across them.
- Present fan condition. Material of the existing fan and photographs of scale and corrosion.
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.