This invention relates to an improved secondary purification process for phosphoric acid. Specifically, it relates to such a process where a portion of the filtrate from the calcium sulfate dihydrate filter is recycled to the same filter.
Superphosphoric acid (SPA), which is used in the manufacture of fertilizers, is phosphoric acid containing about 68 to about 70% by weight of P.sub.2 O.sub.5. It can be manufactured by dissolving calcium phosphate rock with sulfuric acid, producing a slurry of calcium sulfate hemihydrate in a solution of phosphoric acid. The slurry is filtered and the calcium sulfate hemihydrate filter cake is discarded. The filtered acid is evaporated to about 48% P.sub.2 O.sub.5 for further purification.
Some phosphate rock used in this process contains a significant quantity of magnesium, which is dissolved by the sulfuric acid. If the magnesium is left in solution with the phosphoric acid, magnesium pyrophosphate forms when the solution is evaporated to SPA. When the SPA is ammoniated, an ammonium magnesium pyrophosphate sludge forms which creates a serious pluggage problem for liquid fertilizer manufacturers.
In order to avoid this problem, the magnesium is removed from the phosphoric acid. This can be accomplished by the addition of synspar, essentially a calcium fluoride compound made as described in U.S. Pat. Nos. 4,171,342 and 4,243,643, herein incorporated by reference. The addition of the synspar to the phosphoric acid slurry results in the precipitation of Ralstonite, MgAlF.sub.5 (aluminum is normally present in the rock and is dissolved by the sulfuric acid), and hemihydrate gypsum.
To remove the Ralstonite and hemihydrate gypsum, the slurry is filtered on a rotary drum vacuum filter. The hemihydrate cake containing Ralstonite is discharged to a hydration tank, and is reslurried with pond water. The hemihydrate gypsum dissolves and reforms as dihydrate gypsum in this tank. The slurry is then filtered on a second rotary drum vacuum filter. The filtrate is mixed with the filtrate from the first drum filter for further processing. The dihydrate cake containing Ralstonite is disposed of.
In order to maximize the P.sub.2 O.sub.5 recovery, it was, until now, necessary to maintain a minimum of 2% free sulfate ion in the hydration tank. As long as the free sulfate ion concentration in the hydration tank was maintained at least 2%, recrystallization of the calcium sulfate hemihydrate as calcium sulfate dihydrate occurred even though the residence time of the slurry in the hydration tank often varied considerably. But when the free sulfate ion concentration fell below about 2% more time was required to convert the hemihydrate crystals to dihydrate crystals, and frequently the residence time in the hydration tank was not long enough for a complete conversion to occur. When that happened calcium sulfate hemihydrate entered the calcium sulfate dihydrate filter and formed calcium sulfate dihydrate on the filter, plugging the filter.
In order to maintain a free sulfate ion concentration of at least 2% in the hydration tank, it was necessary to add sulfate ion (as sulfuric acid) in excess of the amount required to precipitate the dissolved calcium as calcium sulfate. However, if calcium phosphate rock having a high concentration of magnesium was used in the process, it was necessary to add extra synspar to remove the extra magnesium, and, since the calcium in the synspar removes sulfate ion as calcium sulfate, the free sulfate ion concentration would sometimes fall to less than 2%, or even less than 1%. When the free sulfate ion concentration was less than 2%, the residence time of the calcium sulfate hemihydrate slurry in the hydration tank was frequently not long enough to convert all of the calcium sulfate hemihydrate to calcium sulfate dihydrate. The unconverted calcium sulfate hemihydrate would then enter the calcium sulfate dihydrate filter where it would form the calcium sulfate dihydrate and clog the filter. If the residence time in the hydration tank was increased by shutting off the flow of the slurry from the hydration tank to the calcium sulfate dihydrate filter, the calcium sulfate dihydrate filter would become "dry" and would have to be shut down and restarted again when the required residence time had passed and slurry was again available for filtering. This resulted in P.sub.2 O.sub.5 losses in the lines and equipment, known as "tramp" losses. There did not appear to be any way of solving this problem without reducing the efficiency of the process and lowering the recovery of P.sub.2 O.sub.5.