Design of electrolytic dewatering systems for phosphatic clay suspensions

Phosphatic clays arise as a waste product of the Florida phosphate mining industry. The clays exist initially as a 2-6 solids weight percent slurry which is pumped to large impoundment areas for natural settling. The clay settling process takes as long as 25 years to reach a value of 40% solids. The clay settling areas currently cover an area of over 100 square miles in Central Florida, representing 40 percent of the land that has been mined.;The Florida Institute of Phosphate Research (FIPR) has supported bench- and pilot-scale studies to determine the technical and economic feasibility of a variety of processes to speed the dewatering of clays associated with phosphate mining. The approaches considered included using a freeze-thaw cycle to remove water, adding sand layers to enhance drainage, and adding flocculants to enhance settling. While flocculants are used commercially, the other techniques have not been permanently implemented on a large scale, and settling to an acceptable solids content still requires as much as 25 years.;Application of an electric field provides an alternative approach for accelerating the removal of water from 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 which is known as electrophoresis. Upon formation of a solid matrix, the electric field further induces the movement of water in a process known as electro-osmosis.;The objective of this work was to use small-scale electrokinetic experiments to develop parameters that can be used for large-scale design. A bench-top Plexiglas cell was built to perform the experiments. Clay slurry samples were obtained from a phosphate mine located in Central Florida. A set of experimental results were used to calculate scaling parameters to aid in predicting the solids content as a function of operating time and the electric field applied. This was done by scaling the change in solids content by the applied electric field. A linear relationship was found at short times while at longer times a maximum solids weight percent was reached as a function of the electric field. A constitutive relationship was established which relates the increase in solids content to time and the applied electric field.;A mathematical model previously developed at the University of Florida was used to model and evaluate varying electrode configurations in a one-square-mile clay settling area. The electrical current generated from the applied voltage was calculated to project electrical power and energy requirements for such a process. For a given electrode configuration, the associated electric field can also be calculated. The experimental work suggests a relationship between the solids content of the clays with the electric field. The model results can then be used to identify regions where the electric field is nonuniform, which indicates regions where the clays would have a nonuniform solids weight percent. Therefore, the model allows selection of an optimal electrode configuration based on electrical power requirements as well as the most uniform drying of the clays.