(Invited) Process for Continuous Electro-Osmotic Dewatering of Phosphatic Clay Suspensions

Abstract

A recent spill of wastewater at a former phosphate mining facility in Piney Point Florida, reported in March 2021, highlights the need for alternatives to creating large impoundments of the dilute 2-3 wt.% suspension of phosphatic clay produced as a waste stream in the beneficiation of phosphate ore. A 1997 report highlights releases in Florida.1 In 1994, the earthen berm of a clay settling area operated by Hopewell Mine failed along a section of approximately 100 feet. Approximately 482 million gallons of water were lost. A series of releases of large volumes of turbid water from clay settling areas into nearby surface waters occurred between 1989 and 1993 at the MMM Nichols Phosphate Mine. In 1994, the dike of an inactive settling area was breached over a section of approximately 150 linear feet, causing very rapid dewatering of the impoundment.A typical Florida phosphate mining operation produces more than 6,300 L/s of phosphatic clay. The clay-water suspension is pumped into large impoundments called clay settling areas in which separation is achieved by hindered settling and self-consolidation. As it settles, the supernatant water is recycled for use in the beneficiation plant. A top crust is formed after a few years, but the clay beneath the crust has a large water content and a pseudo-plastic character that limits the amount of weight the settling area can support. Clay settling areas cover roughly 390 square kilometers of land in Florida. Uses for the land are limited by the properties of the clay that leave the settling areas unstable, even after 50 years of elapsed time.This presentation will describe the results of a research project, funded by Mosaic, to create an electrokinetic alternative to creation of clay settling areas. Research in use of electrokinetic phenomena to accelerate the dewatering process has been active for more than 50 years.2 The use of an electric field to separate the water from the solids is attractive because the inherent stability of the clay suspension is due, in part, to the surface charges residing on the platelets. A single-stage design was developed for continuous electro-osmotic dewatering of phosphatic clay suspensions that 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.%.3,4 The results were supported by measurements of yield stress and a mathematical model that accounted for compressibility of the cake and for both electro-osmotic and hydraulic permeability.5 References Damage Cases and Environmental Releases from Mines and Mineral Processing Sites, U.S. Environmental Protection Agency, Washington, DC, 1997.Dizon and M. E. Orazem, "Advances and Challenges of Electrokinetic Dewatering of Clays and Soils," Current Opinion in Electrochemistry, 22 (2020), 17-24.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.Dizon and M. E. Orazem, “Efficient Continuous Electrokinetic Dewatering of Phosphatic Clay Suspensions,” Electrochimica Acta, 298 (2018), 134-141.Dizon and M. E. Orazem, “Mathematical Model and Optimization of Continuous Electro-Osmotic Dewatering,” Electrochimica Acta, 304 (2019) 42-53.