96/01237 Recovery of anthracite from coal preparation-plant tallings

Abstract

Comparison of gravity-based washability curves with flotation results indicates that gravity-based processes are always more efficient, especially at treating coals with a significant yield of middlings. Because of bubble attachment to the hydrophobic (coal) portion of the particle, a particle containing as little as 5% coal may still report to the froth product. Therefore, for coal fines having high middlings content, a high combustible recovery commonly results in high ash and sulfur levels in the flotation products. Although the gravity methods are not very efficient in handling very fine particles, incorporation of gravity separators along with flotation into the fine-coal processing circuit is still the best solution. The coal flotation process is based on differences in surface properties between hydrophobic coal particles and hydrophilic high-ash refuse. Coals of various ranks have different chemical compositions and physical structures and as a result their surface properties and floatability change with coalification. While metallurgical coals float easily and may only require a frother, flotation of lower-rank bituminous and subbituminous coals may be difficult. A new multiunit fine-coal cleaning circuit has been developed at the Center for Coal and Minerals Processing (CCMP at Virginia Tech, Blacksburg, Va.). The circuit consists of a Microcel flotation column and a Mozley Multi-Gravity flowing film separator (MGS). The Microcel column uses small air bubbles to improve the flotation recovery of fine coal, while the MGS utilizes centrifugal forces to enhance the gravity separation of fine pyrite.