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Pilot Plant Demonstration of Sand-Clay-Overburden Mix for Accelerated Reclamation



Dewatering of various types of fine wastes has been a subject of intense research for many years due to the economic and environmental impacts of their disposal. These wastes include fine phosphatic clays generated by phosphate mining, tailings from the kaolin industry, red mud from processing bauxite, and many other chemical processing wastes. The phosphate industry in Florida generates approximately 100,000 tons per day of phosphatic waste clay. This waste containing about 3% solids has historically been pumped into large, above-ground impoundments, where clarified water is decanted through spillways as the accumulated clays slowly consolidate to about 20% solids. After water is removed from the filled ponds the exposed clays slowly dehydrate and form a crust on their surface which hinders further surface evaporation. Without additional physical treatment to dewater the mass, it may take several decades for the clays to consolidate to a solids content of 25-35%. Because these clay ponds occupy up to 40% of the mined area, they represent a considerable economic penalty to the industry and limit the re-use of tens of thousands of acres of central and north Florida land. This conventional practice also ties up tremendous amounts of water and causes loss of water through evaporation. The economic impact of this conventional disposal practice, coupled with the difficulty of obtaining new mining permits due to this issue, has prompted the mining industry to seek new methods for rapid dewatering of the waste clays. In this report, the results of pilot-plant testing of a novel process using a cyclone, static screen, and a screw classifier in series to rapidly dewater slurries containing dilute clay and tailings sand are discussed. The dewatered sand:clay mix produced by the pilot plant was blended with overburden and further consolidation was measured. Results indicate that the mixture of tailings sand and clay mix could be dewatered to 50% solids or more in minutes. The solids content of the mixture of overburden, sand, and clay discharged from the pilot plant averaged 67% solids. A sample of the mixture placed in an unlined trench drained to 80% solids in 10 days. Standard soil tests performed on a dehydrated mixture of overburden, sand, and clay gave positive results, indicating that the permeability was unexpectedly high and that the Plasticity Index was unexpectedly low.
Producer soil tests performed on a dehydrated mixture of overburden, sand, and clay also gave positive results, indicating that lime would not be required for pH adjustment of the soil and that the soil contained extractable levels of phosphorus, potassium, magnesium, and calcium nutrients.