Evolutionary Development for Electrokinetic Dewatering of Phosphate Mine Tailings

The processing and storage of phosphatic clay suspensions (or mine tailings) which result as a waste product from phosphate mining industry represent an important economic and environmental problem. A very long time is required to allow gravity-driven settling of clay suspensions with an initial 2-3 wt% solids content, and decades may be required to reach 25-30 wt% solids content. The associated impoundments occupy over 100 square miles of land in Florida alone.

A combination of experimental and modeling approaches were used to explore the use of imposed electric fields to achieve electrokinetic separation. Batch experiments were used to guide the development of a constitutive relationship describing the changes in solids content with time (t) and applied electric field (E)^1,2. A linear behavior between final solids content and Et was found in at short times, and the upper limit of separation was controlled by the value of electric field only.

Subsequently, a  semi-continuous system was designed for continuous removal of supernatant water. This system allowed measurement of supernatant turbidity, pH, and changes in solids content as functions of applied electric field, flow rate, and settling time. Under pseudo-steady-state operation, low-turbidity water was separated from a clay solids suspension by a combination of electrokinetic dewatering and free settling.

A semi-continuous system, which introduced a rotating belt between electrodes to facilitate removing clay sludge, was designed and tested. The batch experiment set up was used to optimize fabric selection. This equipment was able to continuously producing thickened clay cake with a solids content of up to 33 wt%. Evaluation of parameters including electrode spacing, electric field, and residence time provided an understanding of the path for optimization. A similiar constitutive relationship as in batch experiment was also estanblished which correlates final solids content to electric field and residence time.

All the systems were also evaluated from an econonic perspective. The energy requirement of the semi-continuous operation that facilitated sludge removal had a lower energy requirement value at a given electric field as compared with the result from batch experiments and the previous semi-continuous operation. All systems followed the trend that high electric fields have a high energy requirement.

References

1.  J.P. McKinney, M.E. Orazem, A constitutive relationship for electrokinetic dewatering of phosphatic clay slurries, Miner. Metall. Process. 28 (2011) 49.
2. J.P. McKinney, M.E. Orazem, Electrokinetic dewatering phosphatic clay settling areas: numerical simulation and economic assessment, Miner. Metall. Process. 28 (2011) 71.