Fertilizers Derived From Phosphoric Acid

Chapter 12. Fertilizers Derived From Phosphoric Acid... 

The production of fertilizers derived from phosphoric acid has increased significantly in the last 50 years because, among other things, these are high-analysis products, thanks to the removal of calcium as byproduct calcium sulfate in phosphoric acid production. Moreover, technical breakthroughs in the field of phosphoric acid and ammonium phosphate manufacture, plus economies of scale, have resulted in high capacity world size" plants, which produce a limited range of products at very competitive prices. 

The phosphate manufacturing and phosphate fertilizer industry includes the production of elemental phosphorus, various phosphorus-derived chemicals, phosphate fertilizer chemicals, and other nonfertilizer phosphate chemicals [1-30], Chemicals that are derived from phosphorus include phosphoric acid (dry process), phosphorus pentoxide, phosphorus penta-sulfide, phosphoms trichloride, phosphorus oxychloride, sodium tripolyphosphate, and calcium phosphates [8]. The nonfertilizer phosphate production part of the industry includes defluori-nated phosphate rock, defluorinated phosphoric acid, and sodium phosphate salts. The phosphate fertilizer segment of the industry produces the primary phosphorus nutrient source for the agricultural industry and for other applications of chemical fertilization. Many of these fertilizer products are toxic to aquatic life at certain levels of concentration, and many are also hazardous to human life and health when contact is made in a concentrated form. 

The manufacture of phosphorus-derived chemicals is almost entirely based on the production of elemental phosphorus from mined phosphate rock. Ferrophosphorus, widely used in the metallurgical industries, is a direct byproduct of the phosphorus production process. In the United States, over 85% of elemental phosphorus production is used to manufacture high-grade phosphoric acid by the furnace or dry process as opposed to the wet process that converts phosphate rock directly into low-grade phosphoric acid. The remainder of the elemental phosphorus is either marketed directly or converted into phosphoms chemicals. The furnace-grade phosphoric acid is marketed directly, mostly to the food and fertilizer industries. Finally, phosphoric acid is employed to manufacture sodium tripolyphosphate, which is used in detergents and for water treatment, and calcium phosphate, which is used in foods and animal feeds. 

The major use of phosphate is to supply phosphorous, one of the three essential plant foods, nitrogen, phosphorus, and potassium. Phosphate rock extraction from its ore, and its subsequent conversion into fertilizer materials and industrial chemicals, is a relatively mature art. Single superphosphate, a mixture of monocalcium monohydrate and gypsum formed by the reaction of sulfuric acid with phosphate rock, has been used as a fertilizer since the mid-1800s. Phosphoric acid, derived by the treatment of phosphate rock with sulfuric acid so as to produce gypsum in a separable form, was manufactured in many locations by batch and countercurrent decantation methods in the 1920s. 

Various phosphates are produced from phosphoric acid which is made either by adding sulphuric acid to phosphate rock (wet process) or by burning phosphorus in air to give phosphorus pentoxide, which is then hydrated. Major uses of phosphoric acid are the production of phosphate and compound fertilizers, formation of sodium tripolyphosphate (which is used as a builder in detergents where it forms stable water-soluble complexes with calcium and magnesium ions) and the production of organic derivatives like triphenyl and tricresyl phosphate. These are used as plasticizers for synthetic polymers and plastics. 

Hnally, sulfuric acid from pyrites, smelter operations, or other byproduct sources may contain impurities that may or may not be deleterious for phosphoric acid production. In at least one case, zinc in smelter acid proved useful since the fertilizer produced from phosphoric add contained enough zinc, mainly derived from the smelter acid, to improve crop yields in zinc-deficient areas. The same benefit applies to another micronutrient, copper. 

Nearly two-thirds of the P2O5 in fertilizers was derived from phosphoric acid in 1988. In fact this figure is around 80% or more in North and Central America, North Africa, and West and South Asia, and around 50%-60% in other areas except Oceania (14%) and China (1%). Moreover, in 1992, 70% of the P2O5 in fertilizers worldwide came from phosphoric add-based products. 

Clarity - Fluid fertilizers that are produced from wet-process phosphoric acids derived from uncalcined phosphate ores are black in color due to the presence of finely divided carbonaceous material. The black color has no effect on the nutrient value of the fertilizers, but it does restrict the sight identification of solid material in solution fertilizers that could clog the... 

A = Ba, Ca, Ce, K, Na, Pb, Sr, Y, X = As, P, Si, V and Z = F, Cl, O, OH, H2O. Solid-solution between end-members is extensive, but complete only in certain cases. Carbonate ion may partially replace the XO4 group, with appropriate charge compensation [2,3]. Fluorapatite (Ca5(P04)3F) is the most common member of the group and the major constituent of phosphorites, which are the main raw materials for the manufacture of phosphoric acid derivatives including fertilizers, foods, pharmaceuticals and other chemicals. Fluorapatite in phosphorites is usually partially carbonated and hydroxylated. Hydroxyapatite (Ca5(P04)30H) is the primary mineral constituent of bones and teeth and hosts a variety of chemical substituents in its structure. These materials are stable for billions of years, even during tectonic events, and over a wide range of solution pH and geological conditions, as can be inferred from the existence of ancient sedimentary phosphorites [4]. 

If the objective is to prepare the safest mineral fiber that it is theoretically possible to manufacture, no trace of a toxic substance can be tolerated There was never any doubt in the author s mind that only food grade raw material could be used in this process, although the question of using purified wet phosphoric acid, derived from fertilizers, was raised from time to time. In my judgement, the best commercially available purified wet phosphoric acid is inferior to furnace acid and, although it probably could be used, it would be done with some loss of a safety margin that has been paramount throughout the history of this project. Safety is the hallmark of phosphate fibers. When safety is not an issue, some other fibers have superior properties. 

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