Economical recovery of fly ash-derived magnetics and evaluation for coal cleaning

The coal industry in Kentucky utilizes approximately 130,000 tons of magnetite annually. During recent years, China's increased demand for magnetite has generated concerns about the long term availability and cost. Over the last 6 months, the delivered cost of magnetite has increased 2.5 times over 2001 prices. A supply shortage in the future would have a significant impact on the ability to supply clean coal to utilities. High density, magnetic material can be economically recovered from coal combustion ash and be effectively utilized to address the shortage of magnetite used as heavy media for efficient coal cleaning. While this concept has been attempted at the commercial scale numerous times in the past, each attempt was unsuccessful, not for technical reasons, but for economic reasons because there is simply an insufficient amount of magnetic material present in fly ash to economically justify recovering this material as a single product. The present evaluation focused on incorporating magnetite recovery into a process flowsheet that produces an additional product from combustion ash. Approximately 0.5 tons of magnetite was recovered and evaluated as a substitute for mineral magnetite in dense medium coal preparation. The separations achieved using the magnetic bottom ash particles in dense medium separation are comparable to those achieved through the use of conventional magnetite. Probable error values ranged from 0.02 at a medium density of 1.4RD to 0.06 at 1.7RD. For the bottom ash material used in this study, the upper medium density limit appears to be around 1.6RD. Above this value, medium viscosity appears to have a significant impact.