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

Abstract

The beneficiation plant associated with a typical Florida phosphate mining operation produces more than 6,300 L/s of a dilute 2-3 wt.% suspension of phosphatic clay. The clay-water suspension is pumped into large impoundments called clay settling areas in which partial separation is achieved by hindered settling and self-consolidation. As clay 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. In addition, failure of the impoundments has caused major releases into local rivers.1 Research in use of electrokinetic phenomena to accelerate the dewatering process of phosphatic clay has been active for more than 50 years.2,3 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. This presentation will describe the results of a 7-year research project, funded by Mosaic, to create an electrokinetic alternative to the creation of clay settling areas. Through a systematic development of prototypes, from batch4,5 to semi-continuous with emphasis on water clarification6 to semi-continuous with emphasis on solids extraction, our team has developed a fully continuous prototype using electrokinetic dewatering. The fully continuous prototype itself underwent several prototype designs, culminating in a single-stage design 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.%.7,8 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.9 Patents were published that correspond to the above stages of development.10-12 References Damage Cases and Environmental Releases from Mines and Mineral Processing Sites, U.S. Environmental Protection Agency, Washington, DC, 1997.A. Dizon and M. E. Orazem, “Advances and Challenges of Electrokinetic Dewatering of Clays and Soils,” Current Opinion in Electrochemistry, 22 (2020), 17-24.J. Q. Shang and K. Y. Lo, “Electrokinetic Dewatering of a Phosphate Clay,” Journal of Hazardous Materials, 55 (1997), 117-133J. P. McKinney and M. E. Orazem, “A Constitutive Relationship for Electrokinetic Dewatering of Phosphatic Clay Slurries,” Minerals & Metallurgical Processing, 28 (2011), 49-54.J. P. McKinney and M. E. Orazem, “Electrokinetic Dewatering Phosphatic Clay Settling Areas: Numerical Simulation and Economic Assessment,” Minerals & Metallurgical Processing, 28 (2011), 71-76.R. Kong and M. E. Orazem, “Semicontinuous Electrokinetic Dewatering of Phosphatic Clay Suspensions,” Electrochimica Acta, 140 (2014), 438-446.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.A. Dizon and M. E. Orazem, “Efficient Continuous Electrokinetic Dewatering of Phosphatic Clay Suspensions,” Electrochimica Acta, 298 (2018), 134-141.A. Dizon and M. E. Orazem, “Mathematical Model and Optimization of Continuous Electro-Osmotic Dewatering,” Electrochimica Acta, 304 (2019) 42-53.M. E. Orazem and R. Kong, “Electrokinetic Dewatering of Phosphatic Clay Suspensions,” U.S. Patent No. 10,486,108 B2, Issued: November 22, 2019.M. E. Orazem, R. Kong, S. Moghaddam, H. Lai, D. Yu, Y. Huang, and D. Bloomquist, “Continuous Electrokinetic Dewatering of Phosphatic Clay Suspensions,” U.S. Patent No. 10,315,165 B2, Issued: June 11, 2019.M. E. Orazem and A. R. Dizon, “Device for Efficient Continuous Electrokinetic Dewatering of Phosphatic Clay Suspensions,” U.S. Patent No. 11,208,342 B2, Issued: Dec. 28, 2021.