Abstract: A system for the preparation of phosphoric acid from phosphate rock and sulfuric acid, includes in combination a first reaction vessel containing a first slurry comprising calcium sulfate hemihydrate, monocalcium phosphate and phosphoric acid, a second reaction vessel containing a second slurry comprising calcium sulfate hemihydrate, circulation means including a draft tube disposed centrally in each of said vessels and an agitator positioned axially in each of said vessels within said draft tube, a first conduit interconnecting said first and second reaction vessels for conducting said first slurry from said first reaction vessel to said second reaction vessel, a second conduit interconnecting said vessels for conducting said second slurry from said second reaction vessel to said first reaction vessel, an inlet pipe for introducing phosphate rock and phosphoric acid to said first reaction vessel, said inlet pipe connected with the interior of said draft tube in said first vessel and wherein a vent is connect