Commercial Superphosphoric Acid

Superphosphoric acid is the term used industrially for impure concentrated acid containing about 76% P2O5, which has a third or more of this P2O5 in the form of polyphosphoric acids. Superphosphoric acid is more viscous but less corrosive than the wet process ortho acid from which it is derived by evaporation. The metallic impurities are held in solution by the sequestering action of the polyphosphoric acids (see Section 5.4). 

The solidification temperature of commerdal superphosphoric acid is lower than that of 100% H3PO4 and about the same as that of 85% H3PO4. It can usually be transported and handled as a liquid above about 18°C. Superphosphoric and commercial polyphosphoric acids (see below) are more economic to handle than the less concentrated varieties. The main use of superphosphoric acid is in the production of fertilisers (Chapter 12.2). 

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Liquid micronutrient compositions are made by dissolving metal salts such as CUSO4 and MnCl2 in phosphoric acid, and then neutralising with anunonia. A small amount of a phosphonate of type (12.13a) or (12.13b) may be added to complex and prevent any precipitation of the metallic salts. Commercial superphosphoric acid, when used for superphosphate manufacture, functions as a source of micronutrient elements. The add itself can be used for liquid fertilisers, since the small quantities of polyphosphates which are present will sequester the impurity metal ions present. 

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... 

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. 

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. 

Field-operating experience has shown that alloy 625 exhibits excellent resistance to phosphoric acid solutions, including commercial grades that contain fluorides, sulfates, and chlorides that are used in the production of superphosphoric acid (72% P2O5). 

The initial phosphoric acid solution may contain 23-33% P2O5, but can be concentrated by the evaporation of water to produce commercial- or merchant-grade phosphoric acid, which contains about 54% P2O5. Further evaporation of water yields superphosphoric acid with a P2O5 concentration above 70%. 

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