Electrochemical Engineering

Clay settling and dewatering

Background

Clay is part of the matrix mined by phosphate companies. The matrix contains roughly equal parts of sand, clay, and phosphate. The finest particles of clay, sand and phosphate are separated in cyclones during the first stages of beneficiation. They are suspended in a watery slurry, commonly called phosphatic clays, and pumped to large impoundment areas for natural settling. The slurry is from 3% - 5% solids when it is pumped to a pond. As it settles, the clear top water is recycled for use in the beneficiation plant. A top crust forms within 3-5 years after the start of dewatering and reclamation, but beneath the crust, however, the clay is still the consistency of pudding. After several years the clay settling areas are still only about 25% solids. It can take 15 to 20 years for the clay to reach a value of 40% solids. As a result, the clay settling areas take up as much as 40% of the land that has been mined.

The Florida Institute of Phosphate Research (FIPR) supported bench- and pilot-scale studies to determine the technical and economic feasibility of a variety of processes to rapidly dewater the clay associated with phosphate mining: freeze-thaw technique, flocculation with a polymer, in-line sand-clay mixing, dredge-mix process, dewatering with moving screens, sand wick, and sand spray process. ^[i] The value of such a process would be to decrease the amount of land used to store the watery clays and/or thicken the clay enough to use for immediate reclamation of mine cuts, as a lightweight aggregate, or for other purposes yet to be defined.^[ii]

Application of an electric field provides an alternative approach for accelerating dewatering of clay. In this concept, direct electrical current is applied to induce movement of either clay particles or water. In dilute suspensions, the electric field induces the movement of clay particles suspended in water. This process is known as electrophoresis. Upon formation of a soil structure, the electric field further induces the movement of water in the soil skeleton. This process is known as electro-osmosis. As the water content of the clay decreases, electrokinetic dewatering of clay progresses smoothly from electrophoresis to electro-osmosis.

Electrokinetic dewatering takes place through interaction between fluid flow, electrical current, and charge adsorbed on the clay particles.[iii] At the anode, the electrochemical reactions include oxygen evolution

which creates an acidic environment near the electrode. A second possible reaction involves corrosion of the electrode

At the cathode,

creating a local basic environment. The water is driven from the anode to the cathode, creating a tendency to dry out the region close to the anode. The solution pH will increase rapidly to approximately 11 or 12 at the cathode and hydrogen will be generated, as shown by reaction (3). Cations are driven to the cathode by the electric gradient and can be reduced (the inverse of reaction (1)). A pH gradient will be generated across the soil as an overall result of the electrode reactions.

Electrophoresis and electroosmotic methods of dewatering phosphate clays have been explored since 1940s. [iv]^{-[v][vi][vii]}  Although the basic technology was considered to have promise it was not considered commercially feasible. Significant progress has since been made in our understanding of the underlying electrokineric and elecroosmatic processes and in the practical implementation of these techniques. For example, recently, electrokinetic methods have been demonstrated for dewatering of soil,[viii] mine tailings,[ix]^,[x] sludge,[xi] and clay,[xii]^{-[xiii][xiv][xv]} including Florida phosphate clay.[xvi] Electrokinetic methods have been demonstrated to increase solids content of phosphate clay from 15 percent to 30 percent in 24 hours.¹¹ In a separate study, the solids content of clay was increased from 17 percent to 60 percent in 30 hours.⁷ Therefore it appears that electrokinetic methods can decrease significantly the time required to dewater clay. It is certainly worth reexamining its potentials in light of the great advances than have been made in the last two decades. However, large-scale implementation of the technology will require advances in several areas:

1.   The power efficiency of the process depends on several factors, including the applied current potential. The reported efficiencies range from 0.6 to 880 kWhr/dry ton.⁵ Most early studies used voltage drops as large as 70 V. The associated large currents result in drying of the soil around the anode, which causes large parasitic potential drops. The large currents also create significant gradients in pH, which change the clay properties and can increase parasitic losses. These effects can be mitigated by reversing polarity or by pausing the current to allow natural diffusion to relax the concentration profiles. We will use the smallest possible voltage gradient. This may increase the time required to process the clay, but it will reduce the problems associated with corrosion of electrodes (see item 2 below), reduce the problems associated with drying of the clay near the cathode, and reduce the power required per unit of clay processed.

2.   Due in part to the large voltages applied, the metal electrodes used in early studies corroded and needed to be replaced on a regular basis. Alternative electrodes such as the dimensionally stable anode used for cathodic protection may provide a cost-effective alternative.[xvii] Fourie has reported that excellent results were obtained for dewatering of mine tailings by using the new electrokinetic geosynthetic electrodes, which comprise a charged porous felt wrapped around a carbon-doped plastic mesh.^{34- NOTEREF _Ref188416499 \h  \* MERGEFORMAT 5} The electrokinetic geosynthetic electrodes also seem to resolve the problems of electrode stability. We will explore the use of both dimensionally stable and electrokinetic geosynthetic electrodes.

3.   The optimal electrode placement requires consideration of the need to vent gases created by the electrochemical reactions, the need to remove the water drawn to the cathode, and the need to provide an appropriate current and potential distribution in the settling area. One unresolved issue is determining whether the rate of dewatering is driven by the current or by the potential gradient.⁵ This point is critical to modeling the dewatering process. Mathematical models can be used to predict the dewatering performance corresponding to various electrode configurations. The operating cost of the process could be mitigated by collecting the hydrogen gas produced at the cathode for eventual use in hydrogen-powered fuel cells. In year 1, we will develop a constitutive relationship for phosphate clay between fluid flow, applied potential gradient, and clay properties. This relationship will be used in subsequent years to model the dewatering process.

Proposed Work

The proposed work will be conducted in stages. The first year will be dedicated to preparation of a state-of-the-art report which will include an economic analysis to establish appropriate targets for time and power requirement. This report will also examine and address a comparative analysis of the electrokinetic approach against other technologically viable approaches to rapid dewatering. Preliminary experiments will used to determine the minimal electric field required to achieve electro-osmotic dewatering of clay and establish a constitutive equation which can be used to relate the current density or electric field to the fluid velocity. This will be used in subsequent years to develop mathematical models for the process.