PECO Process for Producing Phosphoric Acid from Flotation Feed
For a great many years the potential advantages of the mono calcium process for producing phosphoric acid have been debated. In this process phosphate rock is dissolved by phosphoric acid to form a mono calcium phosphate solution that is reacted with sulfuric acid in a subsequent processing step to generate phosphoric acid and phosphogypsum.
It has been claimed that the mono-calcium process will produce better filtering phosphogypsum resulting in higher yields and production rates. A second claimed benefit is that less of the iron, aluminum and magnesium in the phosphate rock will be solubilized by the phosphoric acid and if the residue is removed before the sulfuric acid is added, the resulting phosphoric acid product will be purer and it will be easier to make guaranteed analysis (18-46-0) DAP (diammonium phosphate). It is also thought that less of the fluorine in the phosphate rock would appear in the phosphoric acid and/or be evolved. It has been proposed that the phosphate rock fluorine would react with the phosphoric acid and then immediately react with the calcium in solution to form insoluble calcium fluoride. Some test work has indicated that with a step-wise addition of sulfuric acid to the monocalcium phosphate solution, the first small portion of the phosphogypsum formed would contain all of the radium from the phosphate rock and the major portion of the phosphogypsum would be radium free. Accomplishing any of these claims would also reduce the sulfuric acid consumed per ton of phosphoric acid produced.
It is apparent that the realization of any or all of these claims could significantly impact the phosphate industry from both an environmental and economic standpoint. It could even positively affect mining practices by allowing the use of higher impurity phosphate rock that is not now mined or is discarded after mining.
While there were some positive aspects of this research the results were not sufficiently encouraging to expect that there will be any interest in pursuing this processing scheme on a commercial scale.
The PECO process uses phosphoric acid to leach the phosphate from the ore, to form a monocalcium phosphate (MCP) solution, which is then reacted with sulfuric acid to form phosphoric acid and gypsum. An initial sludge is removed from the MCP solution consisting of silica, clays, and an initial gypsum precipitate. The MCP solution is then reacted with sulfuric acid to produce phosphoric acid and the final precipitate of gypsum. Part of the phosphoric acid is recycled to dissolve more phosphate from the phosphate ore.
Radium contained in the phosphate ore is removed with the gypsum from the phosphoric acid process and is the source of the radiation levels of the gypsum. Since calcium has an affinity for radium, tests were conducted to determine if the major amount of radium could be removed by the initial precipitate of gypsum in the sludge, thus allowing the production of the majority of the gypsum at low, acceptable levels of radium. These tests did not confirm this hypothesis. The radium did track the calcium in the system, however it tracked not only the calcium in the gypsum but it also tracked the calcium in the monocalcium phosphate. It was therefore not possible to remove most of the radium in the initial precipitate.
The PECO process can utilize flotation feed as a source of phosphate for phosphoric acid. This would eliminate the need for a flotation operation and its capital and operating costs. In addition to saving $24.00 per ton P2O5 it would also extend the reserves of Florida by 20%, reduce the area required for slime disposal and decrease the use of fresh water by 20%. These tests showed while these objectives could be achieved, the phosphate content of the flotation feed was so low (in some cases as low as 7.4% BPL) that 3.76 times as many tons of feed would have to be transported to the phosphoric acid plant to produce the same amount of phosphate product. The transportation costs may be larger than the benefits. When this large amount of material is fed to the phosphate acid plant as a wet feed, the amount of water contained with this feed would significantly reduce the wash water on the gypsum filter resulting in unacceptable low phosphate recoveries.
The PECO process did indicate a reduction of about 30% in the iron content of the phosphoric acid.