Hemihydrate type process for phosphoric acid manufacture using additive to improve filterability

A process for the preparation of phosphoric acid from phosphate rock and sulfuric acid comprising the steps of:

• (a) adding phosphate rock and phosphoric acid to a first reaction vessel, the dissolver, which contains a first slurry comprising calcium sulphate hemihydrate, monocalcium phosphate, and phosphoric acid, including dissolved calcium, whereby the added phosphate rock is substantially converted to monocalcium phosphate, calcium sulfate hemihydrate, and phosphoric acid while maintaining the first slurry at a soluble or excess sulfate concentration at less than about 0.0%, that is, the soluble sulfate concentration is less than the stoichiometric amount required for the formation of calcium sulfate, with the dissolved calcium, and at a temperature and P.sub.2 O.sub.5 concentration such that the calcium sulfate by product comprises the hemihydrate;
• (b) adding sulfuric acid to a second reaction vessel, the crystallizer, which contains a second slurry comprising calcium sulfate hemihydrate, monocalcium phosphate, sulfuric acid, whereby the sulfuric acid reacts with the monocalcium phosphate and the phosphate rock to form calcium sulfate hemihydrate and phosphoric acid, said second slurry being maintained at a soluble sulfate concentration of about 0.7% to +4.5% and at an effective concentration of an organic sulfonic acid or a derivative thereof in the range of about 1 to about 1000 ppm by weight based on the total weight of the slurry transferred to the separation section of step(e) to improve the filterability of the calcium sulfate hemihydrate;
• (c) circulating a first portion of the first slurry from the dissolver through a first conduit into the crystallizer and circulating a first portion of the second slurry from the crystallizer through a second conduit into the dissolver, the circulation being continuous;
• (d) circulating a second portion of the first slurry within the dissolver and circulating a second portion of the second slurry within the crystallizer, and
• (e) separating, by filtration, in a separation section a third portion of the second slurry into a liquid comprising phosphoric acid and a solid comprising calcium sulfate hemihydrate.