Mathematical Model and Optimization of Continuous Electro-Osmotic Dewatering
Authors: Dizon Arthur , Orazem Mark E 
Published: February 2020

A dilute 2-3 wt.% suspension of phosphatic clay is produced as a waste stream in the beneficiation of phosphate ore. 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. 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.1

The objective of the present work was to develop a mathematical model based on soil mechanics, electrokinetic phenomena, and transport phenomena that describes a fully continuous electrokinetic dewatering apparatus for processing a dilute phosphatic clay to a dewatered plastic cake. The physical system studied is based on a prototype developed by Dizon and Orazem.2 A constrained optimization strategy was developed that yielded the process parameters of a cost-optimized continuous electrokinetic dewatering process and was used to simulate an industrial-scale operation.3

The model developed in this work adequately represented experimental data collected for the operation of the lab-scale continuous electrokinetic dewatering prototype. The optimization strategy consisted of a constrained maximum electric field and final solids content of 35 wt% for a continuous electrokinetic dewatering process starting with a feed clay with a solids content of 10 w%. Optimal operating parameters were identified for an industrial-scale continuous electrokinetic dewatering process capable of processing the effluent production rate of the Mosaic Co. Four Corners Mine. The simulation results suggest that lower total costs may be achieved with small electrode gaps. The cost-optimized process parameters were a feed-side gap of 8.5 cm, a residence time of 1.3 hours, and a dry-clay production rate of 7.58 kg/h m2. An estimated total electrode area of 0.17 km2 was required. The total cost of an optimized industrial-scale continuous electrokinetic dewatering process was estimated to be $9.12 per metric ton of dry clay.

References

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.