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Fine coal cleaning for improved sulfur rejection – Part 1

World Coal,

Z. Ali, R. Bratton and G. Luttrell, Virginia Tech, M. Mohanty, Southern Illinois University Carbondale, A. Dynys and L. Watters, Taggart Global LLC, and C. Stanley, Knight Hawk Coal LLC, US, discuss the innovative design of fine coal cleaning circuits for improved sulfur rejection.

Modern processing plants incorporate a complex array of solid-solid and solid-liquid separation processes that remove unwanted impurities from ROM 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 (Mohanty et al., 2008). In particular, these circuits often perform very poorly in reducing sulfur levels in fine coal products (Shah et al., 2001). 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 (Kawatra and Eisele, 2001). 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 (Mbamba et al., 2012).

One of the most interesting approaches for enhancing the removals of both ash- and sulfur- bearing mineral matter from coal is the multi-property processing strategy developed and patented at Virginia Tech in the 1990s (Luttrell et al., 1996). 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 utilising 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. By combining two processes into a single circuit, high rejections of both ash-forming minerals and pyrite were obtained as compared to conventional single-stage coal cleaning processes. For some eastern US coals, the two-stage circuitry was shown to be capable of doubling the rejection of mineral matter and pyritic sulfur with minimal loss of heating value (Yoon et al., 1995).

Unfortunately, the multi-property processing concept failed to be commercially accepted due to a variety of operational issues. One of the greatest barriers to industrial implementation was the inability of commercially available density separators to handle the large volumetric flows of flotation feed slurry. This issue was particularly serious for enhanced gravity separators that typically have low throughput capacities. Reversing the cleaning stages, i.e., placement of the density separator after flotation, was also unsuccessful due to the presence of residual air-bubble aggregrates and uncontrollable froth handling problems. In addition, the conservative nature of the highly competitive coal industry made it difficult for operators to invest in the new and unproven enhanced gravity separation technologies. Consequently, the multi-property processing strategy was never implemented commercially and the associated patent was allowed to expire.

During the past decade, R&D programmes instigated by Taggart Global have created renewed interest in developing and designing high-efficiency multi-property processing circuits for the coal industry. The specific goals of this initiative have been:

  • To extensively evaluate the ash and sulfur partitioning performance of different fine coal sizing and cleaning operations.
  • To utilise this fundamental insight in the design of fine coal circuitry that provides the highest coal recoveries and mineral rejections.
  • To demonstrate the improved circuitry at a full-scale level at an industrial coal preparation plant.

In 2009, Taggart Global commissioned their academic partners to investigate improved processing strategies for fine coal processing at the Prairie Eagle preparation plant. The university facilities were used to conduct laboratory and pilot-scale testwork necessary to fully delineate the extent of improvements that could be realistically achieved at this site in a cost-effective manner. This article presents some of the important data and key findings obtained from this investigation.

The second part of the article, looking at current operations at the Prairie Eagle plant, can be reached here.

This article was first presented at Coal Prep International 2013 and is presented here by permission of Penton Media. Coal Prep International 2014 will take place in Lexington, Kentucky between 18 April and 1 May 2014.

KAWATRA, S.K. and EISELE, T.C. (2001): Coal Desulfurization: High Efficiency Preparation Methods (Taylor & Francis, New York, US), pp. 100 – 107.

LUTTRELL, G.H. and YOON, R.-H. (1996): “Apparatus for Improved Ash and Sufur Rejection” U.S. Patent 5,522,510, Issued 4 June 1996.

MBAMBA, C.K., HARRISON, S.T.L., FRANZIDES, J.-P. and BROADHURST, J.L. (2012): “Mitigating Acid Rock Drainage Risks While Recovering Low Sulfur Coal from Ultrafine Colliery Using Froth Flotation” Mineral Engineering, Vol. 29, pp. 13 – 21.

MOHANTY, M.K., SAMAL, A.R. and PALIT, A. (2008): “Evaluation of an Enhanced Gravity Based Fine Coal Circuit for High Sulfur Coal” Mineral & Metallurgical Processing, Vol. 25, No. 1, pp. 13 – 18.

SHAH, C.L., ABBOTT, J.A., MILES, M.J., LI, X. and XU, J. (2002):“Sulfur Reduction Evaluation of Selected High Sulfur Chinese Coals,” Fuel, Vol. 81, pp. 519 – 529.

YOON, R.H., LUTTRELL, G.H. and PHILLIPS, D.I. (1995): “Bench-Scale Testing of the Multi-Gravity Separator in Combination with Microcel”. Final Project Report, Pittsburgh Energy Technology Center, US Department of Energy, Contract No. DE-AC22-92PC92205, Vol. 1, 150.

Written by Z. Ali, R. Bratton and G. Luttrell, Virginia Tech; M. Mohanty, Southern Illinois University Carbondale; A. Dynys and L. Watters, Taggart Global LLC; and C. Stanley, Knight Hawk Coal LLC.

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