Phosphate mining operations create a waste product, consisting of a dilute clay/sand/phosphate suspension that, due to the properties of the suspended solids (primarily clay), requires 25 to 50 years to densify via hindered settling and self-weight consolidation. The objective of this research was to develop and apply an effective electrokinetic separation process to phosphatic clay suspensions, generated during the beneficiation of phosphate ore, a major component of fertilizer. The use of an electric field to separate the water from the solids is attractive because the inherent stability of the clay suspension is due, in part, to the surface charges residing on the platelets. Previous work by others has demonstrated the feasibility of batch-wise electrokinetic separation of phosphatic clay suspensions, but the process is widely regarded as uneconomical. The present research is predicated on the assumption that an economically feasible separation requires a continuous, electrokinetic separation procedure. Solid-liquid separation represents a significant problem for the phosphate mining industry The waste suspensions, initially containing 3-5 wt% solids, are pumped to large impoundment areas termed clay settling ponds. When incorporated, the addition of flocculating agents produce a rapid, but partial, separation, to approximately 10 wt% solids. A further increase in solids content proceeds very slowly. Hindered settling, followed by self-weight consolidation, requires as much as 25 years to reach a solids content of 40 wt%. This is an important target value, because its shear strength will then allow the addition of a surcharge (typically a sand cap) to increase the consolidation rate and magnitude. Through a systematic development of prototypes, from batch to semi-continuous with emphasis on water clarification to semi-continuous with emphasis on solids extraction, our team has developed a fully continuous electrokinetic dewatering prototype. The system takes advantage of the charged nature of the suspended solids. When subjected to an applied electric field, the solids are attracted to the anode and clarified water is collected at the cathode. The process is capable of generating a 40 wt% clay cake and clarified water with turbidity levels below 1 NTU within hours rather than years. Optimization of the design was possible by developing a constitutive relationship between the final solids content, the applied electric field, and the residence time. Successful implementation of a continuous electrokinetic separation process will reduce the vast amount of water consumed by industry. While the process was developed for phosphatic clay suspensions, it is anticipated that electrokinetic dewatering may be applicable to other mining operations such as those associated with the oil sand tar tailings in Canada. This presentation will describe the current state of development, including energy and power requirements.

The by-products of phosphatic clay suspensions are a major waste product of the phosphate mining industry. The phosphate clay suspensions produced from beneficiation are about 130,000 gallons per minute (GPM) at about a solids content of 2 wt%. After flocculation the settled clay suspensions at 10% solids content are pumped into the clay settling areas (CSAs) for natural settling. This natural clay settling process takes as much as 25 years to reach the demanded value of 40 wt% solids content. Therefore, the CSAs are frequently constructed on the unmined land and currently cover an area of over 100,000 acres in Florida. Disposal of the phosphatic clay suspensions has become the phosphate mining industry’s major challenge. Thus, it is necessary to develop a technique to increase the solids content of the effluent of phosphate clay suspensions from a phosphate mining beneficiation plant and thereby can save the land used for CSAs and recycling water. A semi-continuous electrokinetic dewatering apparatus for phosphatic clay suspensions has been designed and its performance has been evaluated. A 23 factorial design was applied to determine the optimum process parameters. The results show that the electric field was found to have the greatest positive influence on the solids content. The maximum change in solids content reached a plateau as a function of operating time at a given electric field. Moreover, the plateau occurred earlier at larger electric fields, indicating that for the maximum solids content the optimum parameters of the electric field and the operating time can be determined. The CEDA had lower energy consumption and continuously produced the thickened clay which had the highest solid content of about 33 wt%. The CEDA can be operated to cope with dilute initial solids content (3-10%) from a phosphate mining beneficiation plant. The CEDA has potential to replace existing thickening equipment.

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

Electrokinetic dewatering, a promising way to separate the clay particles from water in suspensions, has been researched for many years on topics like equipment design, parametric optimization, and ion migration.1 However, little work has been done on the clay fabric which affects the performance to a considerable degree during electrokinetic dewatering since different fabrics have different electrokinetic properties. Therefore, if we could figure out fabric and control it intentionally, a breakthrough in improving the productivity and lowering the energy consumption will be possible. Suspensions come from four corners mining plant in central Florida, which contains approximately one-third phosphate, one-third sand and one- third clay. Clay, mainly smectite here, plays the key role in dewatering due to the relatively small particle size and large surface area. Smectite plate is a 2:1 structure with permanent negative charge on the two faces, and pH-dependent charge on the edge.2 It is this property that makes the behavior of smectite complex in electrokinetic dewatering under the effect of both pH and electrical force simultaneously. To dissociate the effect of the two mechanisms, a natural settling experiment at different pH and an electrokinetic dewatering experiment were conducted. In the natural settling experiment, suspensions at acid environment reach stable state very fast and show a “gel” like behavior. However, suspensions in neutral and alkaline environment remains to be liquid like ones. The SEM results (Fig 1) of the sediments samples prepared by freeze-drying show that smectite particles in the presence of acid show edge-face fabric and flocculate into “honeycomb” structure as a result of the edge-face attraction and face-face repulsion (Fig 1a), while particles in neutral suspensions mainly display edge-edge, and in alkaline environment show all fabrics (edge-edge, edge-face, face-face) without dominating part. In the electrokinetic dewatering experiment, an electric field of 4V/cm was applied for 13 minutes. The SEM results (Fig 1b) elucidates that honeycomb forms at lower layer near anode where H+ exists due to electrolysis of water during the separation process, but appears to be more compressed compared with that of natural settling (Fig. 1a), which should be due to the electrical force. The middle and upper layer with a slightly alkaline environment looks much more disordered than the lower layer. Intuitively, we could infer that the rigid “gel” like honeycomb is not desirable for separation of particles compared with “soft” suspensions. In addition, the cells of the honeycomb are likely to trap water, which weakens electro-osmosis. The significance of this work exists in the difference of the electrokinetic properties between disordered plates and flocculated honeycomb. If the difference is quantified, then measures could be taken to control the fabric intentionally. Thus, we can predict the process more accurately and achieve a better performance. References A. Mahmoud, J. Olivier, J. Vaxelaire, A. F. Hoadley, Electrical field: a historical review of its application and contributions in wastewater sludge dewatering,Water research, 44(2010), 8. G. Lagaly, S. Ziesme . Colloid chemistry of clay minerals : the coagulation of montmorillonite dispersions. Advances in Colloid and Interface Science, 100-102(2003), 105–128.