GB689642A - Improvements relating to the manufacture of phosphoric acid - Google Patents

Abstract

In the production of phosphoric acid by digesting crude phosphate with sulphuric acid and precipitating calcium sulphate as anhydrite the phosphate is mixed with an amount of phosphoric acid circulating in the process and containing at least 40 per cent, say 45 per cent by weight P2O5, the ratio between the amount of P2O5 circulating during unit time and the amount subsequently formed during unit time being at least 2:1, say 3:1, and the resulting mixture is then introduced into sulphuric acid, or a sulphuric-phosphoric acid mixture, the temperature of the reaction mixture being maintained at at least 80 DEG C, say 100 DEG C. The initial mixing may be effected at a lower temperature, the phosphoric acid being used cold. Heat for the second stage is supplied by pre-heating the sulphuric acid to 120 DEG C, or the reaction mixture. Precipitation may be effected in say three vessels, the phosphoric acid-phosphate mixture being added to the sulphuric acid passing through the vessels. Part of the slurry from the last vessel may be passed to the first. In an alternative procedure, half of the crude phosphate may be treated with the circulating acid, the balance being added after mixing with sulphuric acid. The anhydrite-phosphoric acid slurry is filtered through a cloth of nitrated cotton, silk, nylon or metal.