(71d) Efficient Continuous Electrokinetic Dewatering of Phosphatic Clay Suspensions

Phosphate ores have been mined in Florida since the late 1800s, and Florida generates approximately two thirds of mined phosphate in the United States. The phosphate deposits consist of phosphate ore, clay, and sand in approximately equal proportions. The beneficiation of phosphate ore is achieved by froth flotation, and the phosphate is converted to phosphoric acid for subsequent use as an essential nutrient in fertilizer. The sand component is separated and reinserted into the open mines. The remaining effluent, called phosphatic clay, is a dilute clay-water mixture with a solids content of approximately 3 wt%.

The phosphatic clay is typically stored in large impoundments called clay settling areas (CSA). After a few weeks, the solid-rich phase may have a solids content of 10 wt% achieved by hindered gravity settling. Supernatant water collected from settling is recycled to the beneficiation plant. After a few years, a top crust forms on the CSA, covering high-water-content clay. The pseudoplastic character of clay in the CSA limits the amount of weight the land can support. Clay settling areas cover a land area of about 400 km² in the state of Florida.

The objective of the present work was development of an efficient electrokinetic process for dewatering phosphatic clay suspensions. Previous evaluations of industrial-scale in-situ batch electrokinetic dewatering of phosphatic clay yielded the conclusion that the associated large power requirement prohibited economic viability. The present work explored development of a continuous electrokinetic dewatering operation. The goal was to increase the production rate of plastic solids, thereby reducing the associated capital and operating costs. The sequential development of continuous prototypes is described in reference 1.

Two prototypes were developed for continuous electrokinetic dewatering of phosphatic clay suspensions.^2,3 A two-stage design comprised an electrophoretic thickening unit followed by a unit that used electro-osmosis to complete the dewatering. A single-stage design carried out the dewatering in one operation. Rheological measurements showed that the clay suspensions exhibited a yield strength for solids contents greater than 7.5 wt%. The two-stage prototype had low production rates; whereas, the single-stage design demonstrated efficient production of a dewatered cake with a solids content of 35 wt% at a dry-clay production rate of 4.5 kg/h m² from a feed clay of 10 wt%. The poor performance of the two-stage design was attributed to an inefficient design for the electrokinetic thickener unit, which was designed to enhance electrophoretic separation; whereas, the desired range of solids content was better suited for electro-osmotic separation. An economic analysis for the single-stage design yielded an estimated total cost of $11-12/metric ton of dry clay produced. The improved operation of the single-stage system was attributed to its optimization for electro-osmotic separation, which was found to be appropriate for the operating range of solids content.

References

1. R. Kong, A. Dizon, S. Moghaddam, and M. E. Orazem, “Development of Fully-Continuous Electrokinetic Dewatering of Phosphatic Clay Suspensions,” in Electrochemical Engineering: From Discovery to Product, Volume XVIII of Advances in Electrochemical Science and Engineering, R. Alkire, P. N. Bartlett, and M. Koper, editors, John Wiley & Sons, Hoboken, 2018, 159-192.
2. A. Dizon and M. E. Orazem, “Efficient Continuous Electrokinetic Dewatering of Phosphatic Clay Suspensions,” Electrochimica Acta, 298 (2018), 134-141.
3. A. Dizon and M. E. Orazem, “Mathematical Model and Optimization of Continuous Electro-Osmotic Dewatering,” Electrochimica Acta, 304 (2019) 42-53.

Acknowledgements

This work was supported by Mosaic Fertilizer LLC, Paul Kucera, program monitor. The authors express their gratitude for the support of Dr. Yichang Charles Guan, Mosaic Fertilizer LLC, and Profs. Guerry H. McClellan and David Bloomquist, University of Florida. The authors express appreciation as well to the undergraduate researchers, Jeremy Kleiman and Jonathan Ratcliff, at the University of Florida.