Keywords: clean phosphoric acid, predigestion, crystallisation, fluoride removal, filtration, cadmium removal.

Phosphoric acid for use in fertilizer applications is mainly produced by a 'wet process', i.e. by digestion of phosphate ore with sulfuric acid. In such wet-processes, however, impurities like Cd and Ra, originating from the phosphate ore, are distributed between the phosphoric acid and the by-product, a calcium sulfate modification. The use or disposal of the by-product and probably the use of the phosphoric acid in future as well is limited by its impurity content. The aim of the currently developed process is the direct production of concentrated phosphoric acid with a low Cd content as well as the production of a major amount of calcium sulfate as hemihydrate (HH), with a low phosphate, Cd and Ra content, in a commercially feasible way. For this purpose the phosphate ore is first completely digested in recycled phosphoric acid, containing about 40 w% P₂O₅ and 1.8 w% H₂SO₄. After filtration of the insoluble ore residue together with a minor amount of calcium sulfate hemihydrate, precipitated in the digester, a clear mono-calcium-di-hydrogen-phosphate (MCDHP) solution is obtained. From this solution the Cd-ions can be removed by e.g. ion-exchange. Thereafter the remaining calcium ions are removed by adding concentrated sulfuric acid to the MCDHP-solution in the crystalliser at 90^oC. In this way a clean calcium sulfate hemihydrate can be obtained. In order to optimize the individual process steps, each step had to be investigated separately.

By performing HH crystallisation experiments, a linear relationship was found between the molar phosphate over sulfate ratio in the crystals and in the solution. The phosphate content of the crystals decreases with increasing sulfate concentration in the crystalliser. It was found that above 3 w% H₂SO₄ the phosphate content of the HH was lower than 0.1 w% P₂O₅. The Cd incorporation was also measured as function of the operating conditions and appears to increase significantly with raising sulfate concentrations above 2 w% H₂SO₄.

Filtration studies showed, that the HH crystals obtained during digestion of the ore, are difficult to filtrate, while the HH crystals developed in the crystalliser, filter quite well. A maximum filtration rate was reached, if a sulfate content of about 1.8 w% H₂SO₄ was maintained in the crystalliser.

A study of the fluor emissions in the process led to the development of an expression for the distribution coefficient as a function of the operating conditions. Preliminary results from a continuously operated lab-scale plant, in which part of the process is studied, were in agreement with the results obtained from the separate studies. A preliminary process flow sheet and a mass balance, in combination with our experimental results, shows that concentrated phosphoric acid (40 w% P₂O₅) with less than 5 ppm Cd as well as clean calcium sulfate hemihydrate with less than 1 ppm Cd and less than 0.5 w% P₂O₅ can be produced.

S van der Sluis, Y Meszaros, J A Wesselingh and G M van Rosmalen, Technical University of De;ft, the Netherlands.

30 pages, 13 figures, 2 tables, 35 references.