Development of Electrokinetic Dewatering for Phosphate Mine Tailings

Solid-liquid separation represents a significant problem for the phosphate mining industry. A dilute suspension of phosphatic clays is a waste product of the phosphate ore beneficiation (mineral processing). The suspensions, containing initially 3-5 wt% solids, are pumped to large impoundment areas termed clay settling areas. Addition of flocculating agents causes a rapid but partial separation, resulting in a suspension containing roughly 10 wt% solids. Further increase in solids content, however, proceeds very slowly. Hindered settling and self-consolidation of clay requires as much as 25 years to reach a solids content of 40 wt%. In central Florida, clay settling areas cover over 150 square miles, representing 30% of the mined land.

Discovery of new exploitable phosphate ore deposits in Florida, coupled with increased environmental awareness as reflected by regulatory agencies, has motivated a search for a technology that can achieve rapid and economic separation of solids from water. The use of an applied electric field for dewatering was explored in the late 1990s. Electrokinetic separation was found to be promising, but the technology was not considered economical.

Under funding from Mosaic, a major phosphate mining company, our group produced bench-top results similar to those reported in the literature. Using a large number of experimental results, we then identified constitutive relations among the increase in solids content, the electric field, and the elapsed time. The analysis showed that for short times, an increase in solids content could be achieved at any electric field, but the maximum increase depended on the strength of the electric field. The constitutive relation was used in boundary element simulations for application of electrokinetic separation to an existing clay settling area. The energy costs were found to be acceptable, but the power requirement was prohibitive.

We turned then to development of a fully continuous electrokinetic separation device that could be used as a process unit for a beneficiation plant. This development was stepwise. A semi-continuous system was designed for continuous removal of supernatant water. Under pseudo-steady-state operation, low-turbidity water (<10 NTU) was separated from a clay solids suspension by a combination of electrokinetic dewatering and gravity settling.

A second semi-continuous system, which introduced a rotating belt between electrodes to facilitate removal of clay sludge, was designed and tested. A series of small-scale batch experiments were used to identify the optimal belt properties. A constitutive relation similar to that developed for batch operations showed that this system had greater efficiency. This equipment produced thickened clay cake with a solids content greater than 33 wt%. The information obtained from the semi-continuous prototypes was used to design a fully continuous system which provides both clear water and thickened clay.

The academic team working on the development includes chemical, mechanical, and civil engineers. The approach taken can be characterized as application of fundamental engineering science to a practical problem of great societal importance.