Start-up procedure in producing phosphoric acid by the hemihydrate process

An improved startup process in a phosphoric acid production system comprising in combination a dissolver vessel for containing a first slurry comprising crystalline calcium sulfate hemihydrate, monocalcium phosphate and phosphoric acid, a crystallizer vessel for containing a second slurry comprising crystalline calcium sulfate hemihydrate, sulfuric acid and phosphoric acid, means in each of said vessels for maintaining a continuous circulation of the slurry therein, a first conduit means interconnecting said dissolver and crystallizer vessels for conducting said first slurry from said dissolver vessel to said crystallizer vessel, a second conduit means interconnecting said vessels for conducting said second slurry from said crystallizer vessel to said dissolver vessel, means for applying a vacuum to said crystallizer vessel to effect temperature control in said crystallizer vessel and means to form a vacuum seal between said dissolver and crystallizer vessels, means for introducing sulfuric acid to said crystallizer vessel, a reslurry vessel for slurrying phosphate rock and phosphoric acid, means for introducing phosphate rock and phosphoric acid to said reslurry vessel, a third conduit means interconnecting said reslurry vessel with said dissolver vessel, a filter feed vessel, and a fourth conduit means interconnecting said filter feed vessel with said crystallizer vessel, for conducting said second slurry to said filter feed vessel, and an agitator means in said filter feed vessel for maintaining the slurry therein in suspension, a fifth conduit means for conducting the slurry from said filter feed vessel to a separation means, for separating crystalline calcium sulfate hemihydrate from phosphoric acid of said last mentioned slurry, a sixth conduit means connecting said separation means with said reslurry vessel for conducting separated phosphoric acid to said reslurry vessel, and a seventh conduit means connecting said filter feed vessel and said reslurry vessel, said improved start-up process comprising the following steps:

• a. substantially filling at least said dissolver and crystallizer vessels with phosphoric acid at a temperature in the range of 165.degree.-215.degree. F. and having a P.sub.2 O.sub.5 analysis in the range of about 28 to 38%,
• b. conducting a portion of said acid from said dissolver vessel through said first conduit means to said crystallizer vessel,
• c. introducing phosphate rock and phosphoric acid to said reslurry vessel, feeding the resulting slurry through said third conduit means to said dissolver vessel and transferring a portion of the slurry from the dissolver vessel through said first conduit means to the crystallizer vessel,
• d. as the volume of fluid in the vessels increases, cycling said fluid from the dissolver vessel through said first conduit means to the crystallizer vessel and from the crystallizer vessel through said fourth conduit means to said filter feed vessel,
• e. recycling said fluid from said filter feed vessel, through a seventh conduit means, to said reslurry vessel,
• f. adding sulfuric acid to said crystallizer vessel to produce a positive sulfate ion content therein, and causing precipitation of calcium sulfate hemihydrate and the production of phosphoric acid, whereby the phosphoric acid strength is increased to the range of 36-42% P.sub.2 O.sub.5 and the solids content in said slurry becomes in the range of 25-35%,
• g. while the sulfate content in the dissolver vessel is maintained at a, negative, level less than stoichiometric for the formation of calcium sulfate from the soluble calcium present in said dissolver, and
• h. transporting said slurry from step (f) containing in the range of 25-35% solids through the filter feed vessel to said separation means,
• i. separating the phosphoric acid from the calcium sulfate hemihydrate, and
• j. recycling a portion of said separated phosphoric acid to said reslurry vessel.