Perspective
The manufacture of wet process phosphoric acid has often been described as the art of producing an acceptable by-product, phosphogypsum. Both the size and the shape of the phosphogypsum crystals are critical to achieving optimum filtration rates and filtration rates dictate both production rates and yields.
It has long been recognized that the size and/or shape of the phosphogypsum crystals can be modified by the addition of trace quantities of certain chemicals and much time and effort has been expended to find an economical crystal modifier that would guarantee optimum filtration rates under any and all operating conditions. Improved production rates and yields would offer financial benefits to the industry and the improved yields would help reduce the potential for ground water contamination by lowering the acid content of the pond water.
This project looked at two methods of modifying the phosphogypsum crystals. The first procedure is perhaps the most traditional where a chemical agent is added after the crystals are formed to flocculate the smaller sized crystals that impede the filtration separation of the phosphoric acid from the phosphogypsum.
The second crystal modification method is perhaps more interesting in that a chemical agent is added to the phosphoric acid reactor to modify the phosphogypsum crystals as they are formed from the reaction of phosphate rock and sulfuric acid. Modifying the crystals during the formation stage has the potential to produce decidedly superior crystals that can be readily separated from the phosphoric acid at high production rates with superior yields.
While the flocculation method improved the crystals the second method, changing the way the crystals grow, was decided superior. If this second technique can be demonstrated successfully enough on a plant scale that it is widely adopted, it could make a significant environmental and financial contribution to the phosphate industry.
SUMMARY
This study has focussed on improving phosphogypsum filtration through the addition of additives, as well as gaining a more clear understanding the role of additives in enhanced filtration. The results from this study show that several additives are effective in enhancing filtration. The most effective additives are polyethyleneoxide, alkylated sulfonates, and polyacrylamides. The addition of the additives has been shown to increase the hydrodynamic size of the phosphogypsum particles when added during the digestion stage of phosphoric acid production. Experimental results indicate that the phosphogypsum particle size is altered during the nucleation stage of crystal growth. Additional results from NMR spectroscopy indicate that the additives alter the fluorine-19 and phosphorous-31 levels in the phosphogypsum. It is hoped that the findings in this study will lead to increased productivity in the Florida phosphate industry.
Brij M. Moudgil, University of Florida. December 1995.