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Fine coal cleaning for improved sulfur rejection – Part 2

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.

The fine coal optimization tests commissioned by Taggart Global were conducted using samples of fine coal (<1 mm) collected from the Prairie Eagle preparation plant. The plant complex is located in Perry County near Cutler, Illinois. The operation, which is operated by Knight Hawk Coal LLC, was built in 2005 to upgrade coal from surface (initiated in March 2005), highwall (initiated in August 2005) and underground (initiated in July 2006) mining operations. The facility currently produces thermal coals containing 10% ash, 2.8 – 3.2% sulfur and 11,200 BTU/lb. The plant has a nameplate feed capacity of 500 short tph and produces approximately 2.7 million short tpa of clean coal.

The coarse coal circuitry at the Prairie Eagle plant incorporates a dense medium cyclone to upgrade particles larger than 1 mm in diameter. The original fine coal circuitry (Figure 1) was used to clean raw feed coals in the 1 x 0.15 mm size class using a combination of water-only cyclones (380 mm diameter) and secondary scavenging spirals.

Figure 1. Simplified schematic of the original flowsheet layout utilised at the Prairie Eagle preparation plant.

The coal products from the cleaning units are initially dewatered using clean coal classifying cyclones (380 mm diameter) and fine wire sieves (0.35 mm aperture) and then fully dewatered using a centrifugal dryer (EBW-40). Particles finer than 0.15 mm from the clean coal classifying cyclones are discarded as waste along with the underflow streams from the fine wire sieves and the refuse high-frequency screen. The plant flowsheet also incorporated two recycle streams: i.e., the effluent from the centrifugal dryer is recycled back to the clean coal classifying cyclones and the spiral middlings are recycled back to the water-only cyclones. These recycle streams are needed to avoid excessive coal losses that would otherwise occur if these streams were discarded.

In order for the Prairie Eagle operations to response to increased market demands, Taggart Global was contracted to design a plant expansion that would increase capacity up to 750 short tph. The plant expansion also provided an opportunity for the engineering firm to work with company management to develop more efficient circuitry for processing fine coal. Due to market demands, there was considerable interest in identifying new equipment solutions that would allow the operation to recover additional fine coal without adversely impacting the clean coal ash and sulfur levels. In particular, considerable interest was expressed in adding a froth flotation circuit to the plant flowsheet.

The third part of the article, 'Results and Discussion', 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.

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