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A process development program was initiated for a new type of granular fertilizer based on the acidulation of phosphate rock with nitric acid followed by the formation of urea adducts. The initial investigation of this urea-nitric phosphate (UNP) process was performed using 17 lb/h bench-scale equipment to test continuous processing and various raw materials. On the basis of data from the bench-scale plant, a 350 lb/h granular UNP pilot plant was built for larger scale testing of the main process steps-acidulation, urea adduct formation, granulation, and drying. The success of these tests proved that a new nitric phosphate fertilizer can be produced without the use of a filtration and crystallization unit, with high P205 efficiency, and without waste phosphogypsum. Standard fertilizer properties of the product, initial storage, and application testing were satisfactory for the UNP products.According to the United Nations Industrial Development Organization (UNIDO) study (Nielson, 1987), world fertilizer production is expected to continue to increase, with a tendency toward higher N:P205 weight ratios. Recent trends in fertilizer technology are aimed at higher material and energy efficiencies, manufacture of multicomponent products, reducing nutrient losses by modifying the fertilizer properties, and pollution abatement. At present, the fertilizer industry is the world's major consumer of sulfur, with nearly 50% of this sulfur being processed to waste phosphogypsum in wet-process phosphoric acid production (Becker, 1983). Nearly 60% of all the P205 contained in fertilizer is derived from wet-process phosphoric acid.

A process for oxidizing aqueous elemental phosphorus containing residues (sludges) to produce orthophosphate containing slurries suitable for subsequent reaction with ammonia to produce nitrogen and phosphate containing fertilizer products. It comprises reacting aqueous elemental phosphorus containing residues with certain special mixtures of concentrated nitric acid and sulfuric acid to effect the conversion of the elemental phosphorus into mostly orthophosphoric acid and very little orthophosphorus acid with the relative ratios of the two acids being dependent upon the mole ratio of H2SO4:HNO3 employed in the processing. The resulting aqueous reaction intermediate is neutralized with ammonia during processing to a fluid or solid fertilizer product. Prior to the conversion to products, the aqueous reaction intermediate may be subjected to a solids separation step to remove insoluble salts of certain environmentally undesirable metals, such as Pb, Cd, Ba, and Cr.