Fine coal cleaning for improved sulfur rejection – Part 4

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.

Industrial demonstration
In light of the promising data obtained from test programmes, a new plant flowsheet was designed by engineers at Taggart Global. A simplified schematic of the upgraded flowsheet is provided in Figure 10.

Figure 10. Simplified schematic of the upgraded flowsheet utilised at the Prairie Eagle preparation plant.

The new circuitry continued to utilise the existing water-only cyclones and clean coal classifying cyclones for their primary functions of cleaning and sizing coal. In addition, the sulfur partitioning capabilities of these units were exploited to ensure that nearly all of the high-density pyritic sulfur was captured in the cyclone underflows. The overflow from the clean coal cyclones, which was now depleted of pyritic sulfur, was then passed to a bank of deslime cylones where high-ash ultrafines were rejected as overflow.

The low-sulfur underflow was then upgraded by column flotation cells, which utilised froth washing to minimize the recovery of ultrafine mineral impurities, such as clays, misplaced in the water reporting to the underflow of the deslime cyclones. The pyrite-enriched underflow from the clean coal classifying cyclones was permitted to flow to the fine wire sieves and pass as an underflow effluent into a newly installed bank of fine desulfurisation spirals. The high-sulfur reject from the spirals was allowed to pass to the thickener for disposal, while the pyrite-depleted fines passed to column flotation along with the overflow from the clean coal classifying cyclones.

One additional upgrade to the plant was the installation of a screenbowl centrifuge to maximise the amount of moisture removed from the fine clean coal products. Simulations indicated that the improved flowsheet would provide an additional 10 short t of clean coal from the flotation circuit at ash and sulfur levels below 10% and 3%, respectively.

Plant sampling campaigns conducted by Taggart Global shortly after plant commissioning indicated that the new flowsheet did meet expectations. The new fine coal flotation column provided approximately 9 short tph of additional clean coal at ash and sulfur contents of 10.8% and 3.1%, respectively (Bethell et al., 2013). A much more extensive sampling of the new circuitry was undertaken now that the plant operations have been fully optimised; however, the laboratory analysis of the samples was not completed at the time this manuscript was prepared. The results obtained to date indicate that the two-stage circuit has performed extremely well and that all projected targets have been successfully met in terms of both technical performance and financial returns.

Conclusions
A new flowsheet for improving the rejections of ash and sulfur from fine coal was developed, designed and implemented as a cooperative effort between an engineering firm, university research groups and a coal producer. The new flowsheet was developed based on extensive test work conducted using both laboratory and pilot-scale equipment. The primary feature of the new circuitry is the utilisation of existing sizing units such as classifying cyclones and sieves to preferentially partition high-density pyrite into a high-volume sulfur-depleted stream and a low- volume sulfur-enriched stream. The former is ideally suited for direct processing by a deslime column flotation circuit, while the latter requires a combination of both density-based separation (desulfurisation spirals) and surface-based separation (column flotation) to achieve acceptable rejections of ash and sulfur. The new circuity has been successfully implemented on an industrial scale and has already met expectations in terms of production, quality and financial returns.

Note
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.

References
BETHELL, P.J., WATTER. L.B. and WOLFE, E. (2013). “Optimising fine circuit design to maximise pyrite rejection and enhance coal marketability” Proceedings of the XVII International Coal Preparation Congress and Exhibit (Istanbul, Turkey; 1 – 6 October 2013).

Read the article online at: https://www.worldcoal.com/handling/02012014/fine_coal_cleaning_for_improved_sulfur_rejection_part_4_-preparation01d/