Anhydrous liquid phosphoric acid

摘要

A substantially anhydrous liquid phosphoric acid derived from "wet process" phosphoric acid comprises an equibrated mixture of orthophosphoric acid and acyclic polyphosphoric acid (calculated mole ratio of water to P2O5 of from 2.0 to 3.5) containing non-volatile metallic impurities and free from metaphosphoric acid and salts, the quantity of acyclic polyphosphoric acid being sufficient to retain the impurities in a dissolved state. The concentration of acyclic polyphosphoric acid should preferably be greater than the minimum necessary to prevent metallic impurities from precipitating. This minimum is given by the following equation where P2O5 poly = gram moles of P2O5 as polyphosphoric acid per 100 grams of acid; P2O5 ortho = gram moles of P2O5 orthophosphoric acid per 100 grams of acid; M = total atoms of non-volatile metal impurities per 100 grams of said phosphoric acid; a = average valance of non-volatile metal impurities = wherein the atomic symbols represent the gram atoms of their respective metal per 100 grams of said phosphoric acid and K is between 0 and 0.5 x 10-2. The anhydrous liquid phosphoric acid may have a corrosion rate of 15 mils. per year at 125 DEG F. and a mol. ratio of water to P2O5 of 2.8-3.5. The acid as above is prepared by heating wet process phosphoric acid to 120-400 DEG C. and continuing heating whilst removing water vapour and volatile impurities, e.g. silica and fluorine. The wet process phosphoric acid contains iron, aluminium and magnesium impurities and may be contacted with a cation ion exchange resin to absorb the magnesium. Other impurities present are Fe, Al, Mg, Cr, V, Zn and Cu in the form of sulphates, fluorides and phosphates. The removal of the water vapour and volatile impurities may be performed at atmospheric or subatmospheric pressure. The acid may be neutralized to form ammonium phosphates useful as fertilizers. The anhydrous phosphoric acid may contain between 1 and 10% by weight of metallic and other non-volatile impurities. Specifications 997,932 and 997,933 are referred to.