ENVIRONMENTAL ASPECTS OF PHOSPHATE MINE TAILINGS IN FLORIDA: A REVIEW

A review of the literature on the history and environmental geology of waste generation associated with phosphate mining in central Florida highlights long standing problems. The overall nature of tailings, reoccurring environmental problems, and attempted solutions are discussed. Phosphate mining has impacted a significant portion of Florida since it began in 1888, although current operations are concentrated in central and northern Florida. During the strip mining process, the matrix is mixed with water forming a slurry, which then undergoes washing and flotation in order to separate the phosphate from waste. One ton each of waste sand and clay is produced for each ton of phosphate product. The sand tailings can be worked in with the overburden as fill, but the clay tailings present a more serious waste disposal problem. The clay tailings are pumped to clay settling areas with an initial percent solids as low as 2%. Settling slows over time so that after 10 years, the clays may only reach 20% solids. High levels of cations along with a high cation exchange capacity make the clays very fertile, however, they have extremely low permeability, limiting agricultural use. Attempts have been made to use the slimes as a soil amendment, but transportation of the clays for that purpose is not cost effective and the clays are difficult to work into existing soil. The phosphate that remains with the clay tailings after benefication is considerable; some workers estimate that one third of the phosphate in the original ore matrix lies in the clay tailings. Several researchers have investigated processes by which the phosphate can be recovered from the clay tailings. As of yet, no process has proved to be economically feasible. One failed method involves leaching the clays with sulfuric acid, then recovering the phosphate from the acid leachate. The main problems with this method is the huge acid consumption and the leaching of aluminum and iron along with the phosphate. Another method of recovering the phosphate requires the dewatering of the clays followed by calcining, but both dewatering and calcining are extremely energy intensive. The most promising investigations to date involve hydrocyclones whereby the clays are segregated into size fractions and then flotation is performed on the coarser material.