Phosphoric acid Phosphoric acid, more accurately referred to as orthophosphoric acid with chemical formula H3PO4, represents in its pure form colorless hygroscopic diamond-shaped crystals with melting point of 42,3°С. However this kind is rarely met, therefore 75% or 85% phosphorous water solution is usually referred to as phosphoric acid. Phosphoric acid can be mixed with water in any proportions which results in producing slightly acidulous solutions. Hence we are accustomed to regard the phosphoric acid as colorless thick odourless liquid which being cooled off precipitates hemihydrate Н3РО4 * 0.5 Н2О. Under normal conditions phosphoric acid is underactive and reacts only with hydroxides, carbonates and some metals. When being heated to more than 80°С, phosphoric acid reacts with inactive oxides, silicates and silicas. Besides when heating the phosphoric acid, the water is evaporated which leads at first to the formation of pyrophosphoric acids and then to metaphosphoric acids.
Phosphorus is an indispensable element for all beings on Earth from microbes and primitive plants to humans. It is essential for growing bones, teeth, shells, claws and etc. Living creatures contain phosphorus only in the form of ortho-phosphate-anions or organic phosphoric esters. Application All kinds of phosphorus compounds are applied in a wide range of fields: • Production of fertilizers which consumes a main part of worldwide produced phosphoric acid. More than 100 million tons of phosphorus ore (90%) are processed every year to produce fertilizers. The main manufacturers of these fertilizers are Russia, USA and Morocco, while almost all countries of Western Europe, Asia and Africa are among their main consumers. Plants absorb this element from phosphates by way of anions (H2PO4- or HPO42-) and from the polyphosphates after hydrolysis. They use phosphorus to grow seeds and fruits which actually are their most important parts. Furthermore, phosphoric acid increases their ability to withstand low temperatures, drought and other unfavorable conditions. The use of phosphoric fertilizers is especially important in northern regions with short vegetation period. They have a positive effect on the soil and promote the growth of soil bacteria. • Cattle breeding. Phosphoric acid is indispensable for prophylaxis of excessive stomach acidity and urolithiasis. • Food industry. Diluted phosphoric acid is a flavour intensifier for syrups, marmalades, effervescent beverages. It is registered as a food additive Е338. Backing powder also includes phosphates which improve the flavour of baked goods. • Woodwork industry. Phosphoric acid solution and phosphates are used in impregnation of wood which makes the material incombustible. • Production of household chemicals and building materials. Phosphoric acid is involved in production of flame retardant paint and lacquer materials: enamels, lacquers, impregnation and others, flame retardant phosphate styrofoam, wood based foams and other building materials. Phosphates are used for softening water; they make part of detergents and antiscale agents. Production methods A small amount of phosphoric acid is easily produced under laboratory conditions by oxidizing phosphorus by 32% nitric acid solution. Extraction and thermal methods are applied in the industry to obtain phosphoric acid. Extraction method is more economical. It lies in decomposing natural phosphates by different acids, mainly sulphuric one, less nitric and hydrochloric acids, and aims at extracting P2O5 in the form of H3PO4. For that to happen, phosphates are dissolved by H2SO4, calcium sulphate is filtered out of received slurry thus producing pure phosphoric acid. Strict requirements are imposed on the feedstock used for phosphoric acid production: natural phosphates with high content of carbonates, Al, Mg, Fe compounds and other organic substances are not usable! Russia and CIS generally prefer to use Khibiny apatite concentrate and Karatau phosphorite as feedstock for phosphoric acid production. Thermal method which produces the purest phosphoric acid consists of the following stages: combustion of elemental phosphorus, P4O10 hydration and its absorption by water or acid, condensation and mist entrapment from gas. There are three thermal methods of phosphoric acid production depending on applied gas cooling method: evaporation, heat exchange & evaporation and circulation & evaporation. Generally, Russian plants apply a technology with circulation & evaporation method of cooling.