CONCURRENT MERCURY REDUCTION AND DEWATERING OF CLEANED ULTRAFINE COAL RECOVERED FROM WASTE PONDS BY HIGH INTENSITY SONICATION

摘要

In most coal preparation processes, significant amounts of coal fines (<850 μm) and ultrafines (<105 μm) are generated. Because these particles are difficult to dewater, they often are discarded in waste ponds. This translates into a major economic loss to the coal industry, not only because of the fuel value lost but also the substantial economic resources required to manage coal waste ponds in an environmentally-sound manner. Recently, we explored the possibility of using a high intensity sonication process in recovering, cleaning, and dewatering fine/ultrafine coal particles from the waste ponds. We also concurrently attempted to reduce the mercury concentration in the fine and ultrafine particles recovered from run-of-mine (ROM) coal-water slurry. We combined the sonication approach with vegetable oil agglomeration to achieve deep reduction in the moisture-, ash-, and mercury-content. In addition, we undertook differential scanning calorimetry (DSC) measurements on the recovered coal to understand the interaction between the coal particles, water, and oil. The results suggested that vegetable oil was effective in displacing water from the coal-water interface, with the enthalpy of the water-vapor transition of oil-agglomerated coal particles decreasing on sonication of the particles. Our process of sonication in the presence of a slurry chemical conditioner achieved deep reductions in the moisture and mercury contents of the coals. In fact, when we combined sonication treatment with oil agglomeration, we were able to reduce the moisture content of ROM coal and waste coal to 6 wt% and 12 wt% respectively, while we succeeded in increasing their LOI content to 91 wt% and 76 wt%.