This invention relates to an improved process for the reduction of air and water pollution resulting from the manufacture of fertilizer.
In the manufacture of fertilizer, aqueous phosphoric acids or acid salts thereof are produced and reacted with ammonia to produce various nitrogen grades of fertilizer such as mono ammonium or diammonium phosphate. In one common process, phosphate rock containing fluorapatite [Ca.sub.5 F(PO.sub.4).sub.3 ] is ground and reacted with aqueous sulfuric acid. After filtering to separate gypsum and other insoluble by-products, the crude aqueous phosphoric acid is concentrated by evaporation. Substantial quantities of fluoride gases are produced which are scrubbed from the effluent gases and substantial cooling is required. Large ponds are employed as repositories for insoluble by-products and as a source of recycle water for washing the filters and for scrubbing and cooling. Such ponds are costly to build and operate and are of increasing concern as a source of water and air pollution.
When aqueous phosphoric acid is reacted with ammonia, the resulting effluent gases contain substantial quantities of ammonia, fluoride, and other particulates. Conventionally, these gases have been washed with aqueous phosphoric acid to recover ammonia and a portion of the particulates, and thereafter scrubbed with pond water in a packed enclosure before exhausting to atmosphere. Washing with phosphoric acid releases fluoride gases such as HF and SiF.sub.4 and copious quantities of pond water are subsequently required for cooling to more effectively remove the fluoride gases and submicron particulates.
It has been estimated that the cooling ponds employed cost per acre between $20,000 and $30,000 to build and between $2,000-$5,000 per year to maintain. Expensive piping is also required and all overflows must be carefully neutralized. A typical integrated site will require a cooling pond area of one hundred acres or more, about one-half of which may be required to scrub the effluent gases from the ammonia-phosphoric acid reactor.
The phosphoric acid used to wash the gases has been restricted to aqueous concentrations between about 20-40% P.sub.2 O.sub.5 by weight because, at higher concentrations, increasing quantities of submicron fluoride particulates such as NH.sub.4 F, NH.sub.4 F.HF and (NH.sub.4).sub.2 SiF.sub.6 are produced which are very difficult to remove from the gases. As a result, substantial water is present in the ammoniated acid when it is returned to the process for drying and conversion to product. About 85-99% of the ammonia in the effluent gases has been thus recovered.
Since pond water typically is highly acidic, scrubbing with phosphoric acid is sometimes omitted and pond water is employed throughout.