Flocullation of gypsum in phosphoric acid

Follow a patent application I am studying now...Description: FIELD OF THE INVENTION [0001] The present invention is in the technical field of the wet process production of phosphoric acid and additives therefor, and in particular the present invention is a method to make down additives using crude phosphoric acid filtrate as the diluent. BACKGROUND [0002] Phosphoric acid is generally produced by the digestion of phosphate rock with concentrated sulfuric acid in a slurry of recycled phosphoric acid. This reaction also generates solid gypsum (CaSO4) particles, which are typically removed by filtration. In subsequent steps the phosphoric acid is clarified and concentrated by thermal evaporation to drive off more than half the water from the crude filtrate. [0003] Flocculants are added to the crude phosphoric acid gypsum slurry to increase particle size, which improves settling and filtration rate. Traditional flocculants used for this application are supplied as a liquid concentrate or dry powder. The dissolution kinetics of traditional powders and the emulsion inversion kinetics of liquid concentrates prevent their direct feed to the crude slurry. A crude phosphoric acid gypsum slurry reaches the filtering stage in tens of seconds to a few minutes after the flocculant feed point, while emulsions and powders require tens of minutes to a few hours for dissolution or inversion. Thus, a concentrate or powder flocculant is diluted with water at the site to yield a working solution (typically containing 0.5% to 1% of the concentrate) that is subsequently fed into the process stream. Traditional flocculant make-down systems consist of a mixing tank to which concentrate or powder is added with water. After sufficient mixing the dilute flocculant is transferred to a storage tank from which the dilute solution is fed to the process. Use of traditional dilute flocculant solution accounts for about 0.5% to 1% of the total volume of water present in the crude phosphoric acid slurry. A typical phosphoric acid production facility will use 5-10 million gallons of water per year for flocculant make-down. After addition, this water must be removed by thermal evaporation in subsequent processing stages requiring use of substantial thermal energy. Read more: http://www.faqs.org/patents/app/20120171099#ixzz2GIMqqZrQ