POLLUTION CONTROL IN PHOSPHATE
FERTILIZER INDUSTRY
Industry Description and Practices
Phosphate fertilizers are produced by adding acid to ground /
pulverized phosphate rock. If sulfuric acid is used, then single or
normal, phosphate (SSP) is produced having a phosphorous content of
16-21 percent as phosphorous pentoxide (P2O5). If phosphoric acid is
used to acidulate the phosphate rock, then triple phosphate (TSP) is
the result. TSP has a phosphorous content of 43-48 percent as P2O5.
SSP production involves mixing the sulfuric acid and the rock in a
reactor followed by discharging the reaction mixture on to a slow
moving conveyor in a den. It is cured for 4 to 6 weeks before bagging
and shipping.
Two processes are used to produce TSP fertilizers: run-of-pile and
granular. The run-of-pile process is similar to the SSP process.
Granular TSP uses lower strength phosphoric acid (40 percent compared
to 50 percent for run-of -pile). The reaction mixture as a slurry, is
sprayed on to recycled fertilizer fines in a granulator. Granules grow
and are then discharged to a dryer, screened and sent to storage.
Phosphate fertilizer complexes often have sulfuric and phosphoric acid
production facilitates. Sulfuric acid is produced by burning molten
sulfur in air to produce sulfur dioxide which is then catalytically
converted to sulfur trioxide for absorption in oleum. Sulfur dioxide
can also be produced by roasting pyrites ore. Phosphoric acid is
manufactured by adding sulfuric acid to phosphate rock. The reaction
mixture is filtered to remove phosphogypsum, which is discharged to
settling ponds or waste heaps.
Waste Characteristics
Fluorides and dust are emitted to air from the fertilizer plant. All
aspects of phosphate rock processing and finished product handling
generate dust, such as grinders/ pulverizes, pneumatic conveyors, and
screens. The mixer / reactors and dens produce fumes that contain
silicon tetra fluoride and hydrogen fluoride. Liquid effluents are not
normally expected from the fertilizer plant since it is feasible to
operate the plant with a balanced process water system. The fertilizer
plant should generate minimal solid wastes.
A sulfuric acid plant has two principal air emissions: sulfur dioxide
and acid mist. If pyrites ore is roasted then there will also be
particulates in air emissions which may contain heavy metals such as
Cadmium (Cd), Mercury (Hg), and Lead (Pb). Sulfuric acid plants do not
normally discharge liquid effluents except where appropriate water
management measures are absent. Solid wastes from a sulfuric acid plant
will normally be limited spent vanadium catalyst. Where pyrite ore is
roasted, there will be pyrite residue may contain a wide range of heavy
metals such as Zinc, Copper, Lead, Cadmium, Mercury and Arsenic.
Phosphogypsum generated in the process (at an approximate rate of about
5 t/t of phosphoric acid produced) is most often disposed as a slurry
to a storage / settling pond or waste heap (disposal to a marine
environment is practiced at some existing phosphoric acid plants).
Process water used to transport the waste is returned to the plant
after the solids have settled out. It is preferable to have this as a
closed loop operating system where possible to avoid a liquid effluent.
In many climatic conditions, however, this is not possible and an
effluent is generated. This effluent contains phosphorous (as PO4),
fluorides, and suspended solids. The phosphogypsum contains trace
metals, fluorides, and radio nuclides (especially radon gas) that have
carried through from the phosphate rock.
Pollution prevention and control
In a fertilizer plant the major source of potential pollution is
solids-from spills, operating upsets, and dust emissions. It is
essential that tight operating procedures be in places, and that close
attention be paid to constant cleanup of spills and other housecleaning
measures. Product will be retained, the need for disposal of waste
product will controlled, and potential contamination of storm water
runoff from the property will be minimised.
Minimise the discharge of sulfur dioxide fro sulfuric acid plants by
using the double contact double absorption process with high efficiency
mist eliminators. Prevent spills and accidental discharges through god
housekeeping and maintenance practices.
Residues from the roasting of purites may be used by the cement and
steel manufacturing industries.
In the phosphoric acid plant, minimize emissions of fluorine compounds
from the digester / reactor by scrubbers that are well designed, well
operated, and well maintained. Again, design for spill containment is
essential to avoid inadvertent liquid discharges. Maintain an operating
water balance to avoid an effluent discharge.
The management of phosphogypsum tailings is a major problem because of
the large volumes and large area required, and the potential for
release of dust and radon gases, and fluorides and cadmium in seepage.
The following will help to minimise the impacts:
- Maintain a water cover to reduce radon gas release and dust
emissions.
- Where water cover cannot be maintained, keep the tailings wet or
revegetate to reduce dust. (The revegetation Process,
however, may increase the rate of radon emissions.)
- Line the tailings storage area to prevent contamination of
groundwater by fluoride.
- Where contamination of groundwater is a concern a management and
monitoring plant should be implemented.
Phosphogypsum may find a use in the production of gypsum board for the
construction industry.
Treatment Technologies
Scrubbers are used to remove fluorides and acid from air emissions. The
effluent from the scrubbers is normally recycled to the process. If it
is not possible to maintain an operating water balance in the
phosphoric acid plant, then treatment to precipitate fluorine,
phosphorous, and heavy metals may be necessary. Lime can be used for
treatment. Spent vanadium catalyst is returned to the supplier for
recovery or, if unavailable, then locked in to a solidification matrix
and disposed in a secure landfill.
There may be opportunities to use gypsum wstes as soil conditioner (for
alkali soil and be explored so that the volume of the gypsum stack can
be minimised.
Key Issues
The following box summarises the key production and control practices
that will lead to compliance with emission requirements:
- Achieve the highest possible sulfur conversion rate and use the
double contact double absorption process for sulfuric acid
production.
- Consider the use of phosphogypsum to produce gypsum boards for
the construction industry
- Design and operate phosphogypsum disposal facilities to minimise
impacts.
- Maximise product recovery and minimise air emissions by
appropriate maintenance and operation of scrubbers and baghouses.
<>- Eliminate effluent discharges by operating balanced process
water system.
- Prepare and implement an Emergency Preparedness and Response Plan.
- Consider providing pyrites roasting residues to cement and /or
steel making industries.
Source: http://www.worldbank.org/
Date: August 08, 2002