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Development and Pilot-Scale Demonstation of Deep Cone Paste Thickening Process for Phosphatic Clay Disposal



Phosphate mining and beneficiation produces huge amounts of phosphate clays in central Florida. There are more than 85,000 acres of phosphatic clay ponds and clayfilled mine cuts, with approximately 5,000 acres of additional ponds created each year by ongoing phosphate mining and beneficiation. Waste clay disposal represents one of the most challenging problems for the phosphate industry. Because of their colloidal nature and ultrafine size (91.5% by weight -325 mesh or 0.044 mm), the phosphatic clays are very stable in water suspensions and settle extremely slowly. It takes several years for waste clay slurry to thicken from about 3% to 20% solids by gravity settling. The most widely used method for disposing of phosphatic waste clays is to store them in clay ponds. This impounding approach is not preferred by the industry since a tremendous volume of water is tied up, large areas of land are occupied, potential dam failures may cause environmental disaster and public outrage, etc.

The purpose of this research project was to develop in the laboratory and demonstrate on pilot scale the feasibility of the Deep ConeTM Thickening (DCT) process to thicken phosphatic clay waste to a paste, without filtration, for mine backfilling or surface stacking. A 2 tph Deep ConeTM paste thickener from Dorr-Oliver Eimco was employed at the South Fort Meade mine of The Mosaic Company in central Florida for a pilot-scale study to investigate the effects of key operating parameters, including feed rate, sand addition rate, flocculant type and dosage, bed depth, etc. The pilot-scale field testing successfully demonstrated the simultaneous production of an underflow paste product and a clear overflow water stream. Typical overflow water recovery and underflow solids recovery were more than 88% and 98%, respectively, with a residence time of as low as 2 hours. The highest clay content and total solids content in the paste were more than 25% and 35%, respectively, with a clay/sand ratio of 2:1.