Coal Age Magazine - For nearly 100 years, Coal Age has been the magazine that readers can trust for guidance and insight on this important industry.
Issue link: https://coal.epubxp.com/i/144796
o p e r at i n g i d e a s Innovative Design of Fine Coal Cleaning Circuits for Improved Sulfur Rejection BY Z. ALI, R. BRATTON AND G. LUTTRELL, VIRGINIA TECH; M. MOHANTY, SOUTHERN ILLINOIS UNIVERSITY CARBONDALE; A. DYNYS AND L. WATTERS, TAGGART GLOBAL; AND C. STANLEY, KNIGHT HAWK COAL Modern processing plants incorporate a complex array of solid-solid and solid-liquid separation processes that remove unwanted impurities from run-of-mine coals. The most popular processes used for coal cleaning include dense medium separation, gravity concentration and froth flotation. Unfortunately, the processes traditionally used to upgrade the finest fractions of coal are generally inferior in terms of separation performance. In particular, these circuits often perform very poorly in reducing sulfur levels in the fine coal products. Although coal pyrite particles are often well liberated at particle sizes down to nominal flotation feed sizes (minus 0.15 mm), this unwanted mineral often reports to the froth product due to entrainment and/or the inherent hydrophobic nature of coal pyrite. Thus, froth flotation, which otherwise provides excellent ash separation performance, often performs poorly in terms of sulfur rejection. In fact, sulfur contents from industrial froth flotation circuits are often found to be higher in sulfur than the original feed due to the flotability of pyrite and the removal of other low-sulfur minerals from the floated product. One of the most interesting approaches for enhancing the removals of both ash- and sulfur-bearing mineral matter from coal is the multiproperty processing strategy developed and patented at Virginia Tech in the 1990s. In this approach, particles containing both valuable components and undesirable impurities are passed through two or more processes, each of which separate based on a different material property. Common properties that may be exploited include size, shape, density or surface wettability/hydrophobicity. By using more than one property, a significantly broader range of impurities can be removed at a higher overall separation efficiency. In the 1990s, this unusual concept was demonstrated using a pilot-scale coal cleaning circuit that combined column flotation with an enhanced gravity concentrator. Flotation is a surface-based process that is effective in removing ash-forming minerals such as clay, while enhanced gravity con- 50 www.coalage.com centration is a density-based process that is extensively evaluate the ash and sulfur partimore efficient in rejecting high-density min- tioning performance of different fine coal erals and middlings that contain pyrite. By sizing and cleaning operations, to use this combining two processes into a single cirfundamental insight in the design of fine coal cuit, high rejections of both ash-forming circuitry that provides the highest coal recovminerals and pyrite were obtained as comeries and mineral rejections, and to demonpared to conventional single-stage coal strate the improved circuitry at a full-scale cleaning processes. For some eastern U.S. level at an industrial coal preparation plant. coals, the two-stage circuitry was shown to In 2009, Taggart Global commissioned be capable of doubling the rejection of minits academic partners to investigate eral matter and pyritic sulfur with minimal improved processing strategies for fine coal loss of heating value. processing at the Prairie Eagle preparation Unfortunately, the multiproperty proplant. The university facilities were used to cessing concept failed to be commercially conduct laboratory and pilot-scale test work accepted due to a variety of operational necessary to fully delineate the extent of issues. One of the greatest barriers to indus- improvements that could be realistically trial implementation was the inability of achieved at this site in a cost-effective mancommercially available density separators to ner. This article presents some of the imporhandle the large volumetric flows of flota- tant data and key findings obtained from tion-feed slurry. This issue was particularly this investigation. serious for enhanced gravity separators that typically have low throughput capacities. Experimental Reversing the cleaning stages, i.e., placing The fine coal optimization tests commisthe density separator after flotation, was also sioned by Taggart Global were conducted unsuccessful due to the presence of residual using samples of fine coal (<1 mm) colair-bubble aggregates and uncontrollable lected from the Prairie Eagle prep plant. froth handling problems. The plant complex is located in Perry In addition, the conservative nature of County near Cutler, Ill. The facility, which the highly competitive coal industry made it is operated by Knight Hawk Coal, was built difficult for operators to invest in the new in 2005 to upgrade coal from surface, highand unproven enhanced gravity separation technologies. Consequently, the multiproperty processing strategy was never implemented commercially and the associated patent was allowed to expire. During the past decade, research and development programs instigated by Taggart Global have created renewed interest in developing and designing high-efficiency multiproperty processing circuits for the coal industry. Figure 1: Simplified schematic of the original flowsheet layout used at the The specific goals of this Prairie Eagle prep plant. initiative have been to July 2013