How phosphoric acid plants work
Wet process phosphoric acid is made by digesting ground phosphate rock in sulphuric acid in a series of agitated reactor compartments. The rock carries 2 to 4% fluorine, and the reaction releases HF and silicon tetrafluoride along with water vapour and acid mist. Reactors are cooled either by a vacuum flash cooler or, in older designs, by drawing a large flow of air across the slurry surface. The gypsum is filtered off on tilting pan or belt filters, and the acid is concentrated in vacuum evaporators. Fumes from the reactor, filter hoods and tanks are drawn through a fluorine scrubber, usually a venturi or cross-flow spray tower using water or dilute fluosilicic acid, by an exhaust fan before the stack.
Where the ID fan sits
- Reactor and filter hoods
- Fume ducts
- Venturi or cross-flow scrubber
- Mist eliminator
- Exhaust (ID) fan
- Stack
The fan sits after the fluorine scrubber and its mist eliminator, so it handles saturated air at about 45 to 65°C. Placing it after the scrubber keeps the fluoride load low and the reactor and filters under suction. On air-cooled reactors the same fan, or a separate one, also draws the large cooling air flow across the slurry. Suction covers the hoods, ducts and scrubber, typically 200 to 400 mmWC.
Gas at the ID fan
| Air temperature, normal | 45 to 65°C, saturated after the scrubber |
| Maximum or upset | 70 to 80°C if scrubber water is lost |
| Dust at the fan | Little dust; fluosilicate and gypsum carry-over |
| Moisture and dew point | Saturated with free droplets |
| Corrosive species | HF, SiF₄ and fluosilicic acid mist, with traces of sulphuric and phosphoric acid |
| 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 phosphoric acid plants
| Blade form | Radial tipped or backward inclined, open enough to shed deposits |
| Material of construction | Rubber-lined mild steel, or FRP with a synthetic veil rather than glass on the wetted surface, or high alloy stainless |
| Drive and arrangement | Belt driven, arrangement 9, bearings outside the gas stream |
| Bearings and cooling | Standard bearings; no cooling needed |
| Flow control | Variable speed drive on reactor or hood suction |
| Also specified | Wash water nozzles, casing drain and inspection door |
What this ID fan has to cope with
Fluoride attack on glass and stainless
HF attacks glass fibre, so ordinary FRP with a glass surface veil fails; fluosilicic acid also pits ordinary stainless steel. Rubber lining, FRP with a synthetic veil or high alloy steel is used for parts in contact with the gas.
Silica scale
SiF₄ hydrolyses in water to fluosilicic acid and hydrated silica, which deposits as a hard scale on the impeller and casing. Uneven scale causes vibration. Washing on a routine, and a good mist eliminator before the fan, keep it under control.
Loss of suction at the reactor
If the fan loses suction from scaling or a fouled scrubber, fluoride fumes escape at the reactor and filter hoods. A fan with pressure margin and suction monitoring at the hoods protects people working nearby.
Energy and control
Phosphoric acid plants run continuously, with output varied by rock supply and filter capacity. On flash-cooled reactors the fume fan duty is fairly steady; on air-cooled reactors it changes with ambient temperature. Speed control on hood suction reduces power and limits the air drawn through hoods, which also reduces the water and energy used in the scrubber.
Questions buyers ask
What fan is used for a phosphoric acid plant fluorine scrubber?
A centrifugal exhaust fan after the scrubber, usually rubber-lined steel, FRP with a synthetic veil, or high alloy stainless. It must resist HF and fluosilicic acid mist. Radial tipped or open blade forms are preferred because silica scale builds up quickly.
Why can't standard FRP be used for phosphoric acid fume fans?
Hydrogen fluoride attacks the glass fibre in standard FRP. A synthetic surface veil and a suitable resin protect the laminate, or rubber-lined steel is used instead. The resin itself must also be chosen for fluoride service.
How do you stop scale on a phosphoric acid plant exhaust fan?
The scale is mainly silica from SiF₄ hydrolysis. Good mist elimination before the fan, wash water at the inlet and a blade form without pockets keep it manageable.
Information needed to quote an ID fan for phosphoric acid 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.
- Fume sources. Reactor, filter and tank vents connected, with the flow from each, and whether the reactor is air-cooled.
- Scrubber. Scrubber type, scrubbing liquor and pressure drop.
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.