Determination of Factors Affecting the Separation of Potentially Hazardous Trace Elements and their Behavior in Coal Tailings Impoundments

PUBLIC ABSTRACT Froth flotation is a process that separates particles based on their differences in physical and surface chemistry properties. For fine feed material that is comprised of particles having wide degrees of floatability, selectivity is often optimized by maximizing the differences in the flotation rates between the particles targeted for recovery in the froth concentrate and those needed to report to the underflow stream. The benefits of differential flotation rates are realized in the collection zone of a flotation system where bubble-particle collision and attachment occurs. However, selectivity between particles of varying floatability can be significantly enhanced in the froth zone through a reflux mechanism that circulates detached particles back to the collection zone. Particle detachment in the froth zone occurs due to bubble coalescence, which leads to an insufficient amount of bubble surface area to carryall ofthe material recovered in the collection zone. Based on previous research, the detachment process is selective in that particles having a lower degree of floatability (or hydrophobicity) are preferentially released from the bubble surfaces. Selectivity through the detachment process can be improved by the addition of a more hydrophobic material into the froth zone and possibly by recycling a portion of the flotation concentrate, thereby enriching the flotation froth. In a recent study, froth zone and overall flotation recovery values were quantified for particles in an anthracite coal that were characterized as having wide differences in floatability potential. The unique aspect of the coal was the presence of 'bone' material in the high density fractions which had a relatively high degree of floatability. As a result, the minimum product grade achieved by froth flotation was about 15% despite washability data indicating the potential for achieving a product containing nearly 3% ash. Highly floatable material was added directly into the froth zone while treating the anthracite coal and then removed from the product and tailing samples using density fractionation. The enriched froth phase reduced the product ash content of the anthracite product by 5 absolute percentage points while maintaining coal recovery at the same level. These results support the findings of previous fundamental research conducted on hematite flotation with the addition of hydrophobic silica directly into the froth phase. The proposed two-year project will further investigate the fundamentals of the detachment process as well as develop unique methods of commercial application that may be more useful for sulfide and precious mineral industries. Plastic material that contains an amount of magnetite that makes the plastic easily recoverable by a low-intensity magnetic separator will be directly added in the flotation froth through the wash water distributor of a flotation column. By varying the fonnulations of the plastic, surface hydrophobicity and thus floatability can be controlled. Initial tests will involve the flotation of silica that will be methylated to achieve varying degrees of floatability. External refluxing of a portion of the flotation froth will also be investigated using the methylated silica. To evaluate the commercial benefits, the improved selectivity achieved by froth enrichment using external refluxing and the addition of the magnetic material will be accessed for the flotation of coal, sulfide minerals and phosphate. 2