| POLLUTION
CONTROL IN PHOSPHATE FERTILIZER INDUSTRY
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.
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.
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. 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:
Phosphogypsum may find a use in the production of gypsum board for the construction industry.
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.
The following box summarises the key production and control practices that will lead to compliance with emission requirements:
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