Superphosphoric acid

Phosphoric acid when concentrated above 54% P2OS forms superphosphoric acid (SPA). This name is given to phosphoric acid in which an appreciable proportion, usually 30% or more, of the P2O5 is in the form of condensed or polyphosphoric acids [general formula H +2 

For concentration from 54% to 70% P2OS, about 1 tonne of high-pressure steam (about 2.7 MPa pressure and 230 C temperature) is required per tonne of P2O5. Power requirements are about 24 kWh/tonne of P2O5. When a Dowtherm heat transfer medium is used, the fuel requirement is about 2.7 GJ/tonne of P2O5. 

The compositions of superphosphoric acids produced from several types of phosphate rock are shown in Table 11.25. 

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A breakthrough came in 1957 with the introduction by TVA of superphosphoric acid and of base solutions made by ammoniation of that acid (90,91). This superphosphoric acid, which at first was made by the electric-furnace process, contained 76% P2 5 compared to 54% P2 s commercial... 

Fig. 23. TVA pipe reactor system for production of 10—34—0 or 11-37-0 base solution from low conversion superphosphoric acid.

At equihbrium, the specific composition of a concentrated phosphoric acid is a function of its P2 s content. Phosphoric acid solutions up to a concentration equivalent of about 94% H PO (68% P2O5) contain H PO as the only phosphoric acid species present. At higher concentrations, the orthophosphoric acid undergoes condensation (polymerization by dehydration) to yield a mixture of phosphoric acid species (Table 5), often referred to genericaHy as polyphosphoric or superphosphoric acid, H20/P20 = - 3, or ultraphosphoric acid, H20/P20 = - 1. At the theoretical P2O5 concentration for orthophosphoric acid of 72.4%, the solution is actually a mixture containing 13% pyrophosphoric acid and about 1% free water. Because the pyrophosphoric acid present is the result of an equihbrium state dependent on the P2 5 content of the solution, pure orthophosphoric acid can be obtained because of a shift in equihbrium back to H PO upon crystallization. 

Condensed Phosphoric Acid. The largest use of polyphosphoric (superphosphoric) acid is as an intermediate in the production of high quahty Hquid fertilizers. The TVA pioneered the development of electric-furnace superphosphoric acid for this appHcation. However, wet-process superphosphoric acid prepared by evaporation of water from wet-process phosphoric acid has almost completely replaced furnace-grade acid in fertilizer manufacture. 

Phosphate Fertilizer Industry Superphosphoric Acid Plants... 

Superphosphoric acid, diammonium phosphate, triple superphosphate and granular triple superphosphate. 

Crystalline polyammonium catena-polyphosphate, (NH4) H2P 03 + 1, has been prepared by heating urea and monoammonium orthophosphate under ammonia vapor for 16 hours,1 by ammoniation of superphosphoric acid,2 by thermal condensation of urea phosphate,3 and by heating various ammonium phosphates in a current of ammonia.4,5 The procedure given below, in which crude ammonium tetrametaphosphate is reorganized and condensed to a long-chain polymer in a stream of ammonia, is straightforward and permits the use of common laboratory equipment and supplies. 

At the U.S. Forest Products Laboratory, many of the research programs involve fire-retardant-treated wood. This has included extensive basic study of pyrolysis and combustion reactions of wood and its components and the effects of chemical additives on these reactions (15,24-26,28,29,66). A cooperative study (9) with the Division of Chemical Development of the Tennessee Valley Authority, showed the effectiveness of liquid ammonium polyphosphate fertilizers as fire retardants for wood. The commercial use of these products, made from electric furnace superphosphoric acid, has been shown to be economically feasible. Work has been completed by Schaffer (33) on the rate of fire penetration in wood treated with different types of chemicals. Some results of this study are reported elsewhere in this paper. 

If even less water is used for hydration, a product known as polyphosphoric acid results. Ordinary phosphoric acid is a solution of the monomer, H3P04, in water, and is called orthophosphoric acid. If a molecule of water is removed between two orthophosphate molecules, the dimer, pyrophosphoric acid, H4P207, is formed. Similarly, the trimerand higher polymers can be made. Superphosphoric acid is a mixture of orthophosphoric acid and polyphosphoric acid and is now made from wet process acid as described later in this chapter. 

Solution fertilizers have become very popular in the United States. The principal source of P205 for these fertilizers is wet process Superphosphoric acid containing about 70 percent P205, where 35 percent or more of the P205 is present in the polyphosphoric form. When this acid is ammoniated and diluted, the iron, aluminum, and magnesium... 

In addition, a potassium polyphosphate solution of 0-26-27 grade has been produced from superphosphoric acid and potassium hydroxide it contains a mixture of ortho, pyro, and higher polyphosphates. 

The problem of limited plant-food solubility was much alleviated by TVA s introduction of superphosphoric acid in 1957.37 Superphosphoric acids are acids of high P205 content (up to 80% P205) that contain signif-... 

The most popular scheme by which solution-type fertilizers are made from low-conversion, wet-process superphosphoric acid is outlined in Fig. 24.20. The first step is ammoniation of the acid to produce ammonium polyphosphate base solution. By carrying out this ammoniation in a pipe reactor, a temperature of over 200°C is developed, which promotes an increase in polyphosphate content to 65-70 percent of total P2Os in the ammoni-ated acid.39,40 The grade of the resultant base is usually 10-34-0 an 11-37-0 grade can be made, but its storage properties are not as... 

Phillips, A. B., Concentration Wet-Processes Acid to Superphosphoric Acid, Farm Chem., 126, 36, 38, 62 (June 1963). 

Standards of Performance for the Phosphate Fertilizer Industry Wet-Process Phosphoric Acid Plants Standards of Performance for the Phosphate Fertilizer Industry Superphosphoric Acid Plants Standards of Performance for the Phosphate Fertilizer Industry Diammonium Phosphate Plants Standards of Performance for the Phosphate Fertilizer Industry Triple Superphosphate Plants Standards of Performance for the Phosphate Fertilizer Industry Granular Triple Superphosphate Storage Facilities... 

E.G. Sherif and A.G. Ciamei, East-setting cements from superphosphoric acid, US Patent 4,734,133, 1988. E.G. Sherif and E.A. Via, Production of solid phosphorous pentoxide containing materials for fast-setting... 

Subpart U Standards of Performance for the Phosphate Fertilizer Industry Superphosphoric Acid Plants... 

Properties Viscous, water-white liquid. Water soluble. Does not crystallize on standing hygroscopic. The commercial acid is a mixture of orthophospho-ric acid with pyrophosphoric, triphosphoric, and higher acids and is sold on the basis of its calculated content of H3P04, e.g., 115%. Superphosphoric acid is a similar mixture sold at 105% H3P04. These acids revert slowly to orthophosphoric acid on dilution with water. 

Hydration, too, releases some additional heat. The properties of phosphorus pentoxide and the absorption process inevitably leave as much as 25 % of the oxide plus a phosphoric acid mist in the exit gases from the absorption tower. These are captured on passage through an electrostatic precipitator. By variations of the process details and equipment, grades (concentrations) of phosphoric acid from 75-105% (ortho, or superphosphoric acid) H3PO4 may be made in this manner [28]. 

Sulfide Precipitants. Sulfide precipitation of heavy metals from 10-34-0 prepared from Western U.S. superphosphoric acid was also investigated. Portions of the 10-34-0 were then treated with varying amounts of Na2S and Na2S2.32 solutions. The results of these tests are shown in Table VI. 

Superphosphoric acid Tetraphosphoric acid. Acid used in the manufacture of phosphates, phosphate esters, catalysts, fuel cell electrolytes, metal cleaning and brightening, organic rextions. Viscous liquid d = 2.1000. Albright Wilson Americas Inc. 

By far the most important use of phosphoric acid is in the production of fertilizers. At least four major types of fertilizers are made from phosphoric acid diammonium phosphate ((NH4)2HP04 DAP), monoammonium phosphate (NH4H2P04 MAP), granulated triple superphosphate (GTSP), and superphosphoric acid, the only liquid among the group. An additional 5 percent of the phosphoric acid produced is used as an animal feed supplement. 

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