Phosphorous acid solns. of low iron and arsenic content - prepd. by converting sec. calcium phosphite to prim. calcium phosphite solns., removing iron and arsenic by pptn. etc

摘要

The prepn. of phosphoric acid solns. with low iron and arsenic levels comprises (a) converting sparingly water-soluble sec. calcium phosphite into a soluble prim. calcium phosphite; (b) removing iron and arsenic from this soln. as much as is possible; (c) liberating the phosphorous acid using sulphuric acid, and pptg. out the calcium sulphate. Step (a) is pref. carried out using stoichiometric amts. of phosphorous acid to form prim. calcium phosphite, or with stoichiometric amts. of sulphuric acid, in which case the calcium sulphate pptd. is removed. Pref. the iron ions are removed in step (b) by adding solns. of alkali hexacyanoferrates, e.g., potassium hexacyanoferrate-(II) and ferrate-(III), and the arsenic impurities are removed by treating the soln. with sodium thiosulphate soln. USE/ADVANTAGE - Phosphorous acid is used in the prepn. of complexing phosphonates. The process of the invention uses sec. calcium phosphite obtd. as a by-prod. in hypophosphate prepn. The process gives phosphorous acid solns. which are low in iron and arsenic and therefore suitable for phosphonic acid prepn. The solns. obtd. can be used to prepare phosphonic acid without further concn.