FINAL TECHNICAL REPORT

September 1, 1996, through August 31, 1997

Project Title:  Advanced Fine Coal Processing for Improved Profitability

ICCI Project Number:  96-1/1.1A-1

Principal Investigator:  R. Q. Honaker, Southern Illinois University

Other Investigators:  H. Sevim, M. K. Mohanty, A Patwardhan and B. G. Rajan, Southern Illinois University

Project Manager:  K. Ho, ICCI

ABSTRACT

The techno-economic benefits of using advanced fine coal cleaning (AFCC) technologies have been evaluated for two operating Illinois coal preparation plants treating the Illinois No. 5 and 6 seam coals.  Using separation performance data collected from the plants, the overall impacts of the AFCC technologies were evaluated after optimization of the conventional coal cleaning processes.  The plant treating the Illinois No. 6 seam coal utilizes conventional flotation cells fro cleaning the -28 mesh particle size fraction, for which, column flotation and enhanced gravity separation were considered as the AFCC technologies for replacement purposes.  In addition to these AFCC technologies, a hindered-bed classifier was evaluated for replacing the spiral-conventional flotation cell arrangement in the plant treating Illinois No. 5 coal.  The analysis was based on the maximization of plant clean coal yield and overall mine profitability while ensuring all product quality constraining values (e.g., ash, sulfur and moisture contents).

Prior to the consideration of the AFCC technologies, simple optimization of the existing cleaning circuits indicated the potential for improving the plant clean coal yield by 0.79% and 1.63% weight units for the Illinois No. 6 and No. 5 plants, respectively, while maintaining the current product qualities.  However, after replacing the conventional flotation cells with flotation columns and enhanced gravity separators, the maximum plant clean coal yield improvement of the Illinois No. 6 plant was 3.06% weight units.  A detailed economic analysis revealed that the plant yield improvement translates to a $1.10 million increase in annual income after discounting the capital cost invested at a rate of 12%.  The maximum yield improvement realized for the Illinois No. 5 processing plant, which has an annual production rate that is three times the Illinois No. 6 plant, was 4.47%.  This yield improvement equates to an increase in annual profit of nearly $6.74 million.  The significant improvements in clean coal yield predicted from the addition of advanced circuits in both plants are believed to be the result of the excellent ash cleaning performance of column flotation and hindered-bed classifiers. In coal containing a significant amount of fine (-100 mesh) coal pyrite and middling particles, enhanced gravity separation provides a significant economic benefit in achieving the required product grade.  However, the techno-economic benefits is dependent on the feed coal characteristics and plant related parameters, which vary from site-to-site.

EXECUTIVE SUMMARY

Research projects funded by the Illinois Clean Coal Institute, the U. S. Department of Energy and other funding agencies have resulted in the development of several advanced fine coal cleaning technologies (AFCC).  These technologies include flotation columns and enhanced gravity concentrators, which have been successfully commercialized into operating preparation plants.  However, their acceptance by the coal industry has been limited due to the lack of a method to evaluate the impact of the technologies on the overall plant separation performance and mine profitability.  The goal of this project was to determine the improvements in the efficiency of coal processing plants and mine profitability that can be provided from the use of overall optimization of the plant using AFCC technologies.

It is well understood that the liberation characteristics for a given run-of-mine coal improve with decreasing particle size, which allows the finest fractions to have a superior cleanability potential.  Thus, by achieving a superior cleaning in the fine coal circuit of a coal preparation plant, a relatively high specific gravity separation in the coarse coal circuits can be tolerated, which would result in an increase in the overall plant clean coal yield.  However, due to the inefficiencies of conventional fine coal cleaning technologies, superior separation performances are typically achieved from the intermediate and/or coarse circuits of a coal processing plant.  To compensate for the relatively poor performance form the fine circuit, it is common practice to decrease the gravity cut-point in the coarse and intermediate circuits to achieve a given product quality, which is detrimental to the overall plant mass yield.  In fact, for this reason, several coal companies have elected to forego the treatment of the minus 100 mesh size fraction, which creates an inherent inefficiency.  After testing advanced technologies in the same plants, decisions have been made against the installation of the technologies due to product quality issues such as moisture, despite the fact that the technology performed well in pilot-scale demonstrations.  One of the most important issues overlooked in these studies was the improvement in overall economics.  The economic benefits provided by the superior performance of AFCC technologies were not evaluated with respect to the entire plant and thus, the additional capital costs were not justified.

In this project, the benefits realized by the optimization of the overall plant using the conventional and advanced fine coal cleaning technologies have been evaluated and compared.  To achieve this goal, Southern Illinois University teamed with two coal companies to obtain separation performance data from the conventional coal cleaning technologies, which were used to evaluate the effect of overall plant optimization and implementation of AFCC technologies.  Plant 1, which treats the Illinois No. 6 coal seam, is a three-circuit operation using heavy media vessels, heavy media cyclones and conventional flotation cells.  Plant 2 is a four-circuit plant treating the Illinois No. 5 seam coal whereby the only difference with Plant 1 is the combined use of spiral concentrators and conventional flotation cells to treat the -16 mesh size fraction.  For both plants, two AFCC circuits were evaluated for their potential use as a replacement for the conventional fine coal cleaning technologies.  For Plant 1, Advanced I circuit utilized flotation columns, whereas, Advanced II circuit used both enhanced gravity concentrators and flotation columns in a rougher-cleaner arrangement to clean the -28 mesh particle size fraction.  Since the fine circuit in Plant II treats a coarser particle size fraction, i.e., -16 mesh, a hindered-bed classifier was used as a rougher in both AFCC circuits.

An extensive plant sampling exercise was undertaken to obtain the relevant technical information from both plants.  The particle size-by-size partition curve data needed for the gravity-circuits have been obtained by collecting samples of the feed, product, and tailings from each unit operation at different specific gravity cut-points.  Using the data collected, a global plant optimization was achieved trough the utilization of a plant optimization model developed by the principal investigators.  The goal of the optimization procedure was to maximize the plant clean coal yield as a function of incremental changes in product grade, i.e., ash, total pyritic sulfur, and moisture contents.  The general optimization approach of equalizing the incremental quality from each circuit was used to maximize the plant yield at a given overall product quality value.  Based on the maximum clean coal yield values obtained considering the individual product assays as constraints, an overall clean coal yield maximization was achieved while simultaneously satisfying multiple product quality constraints, such as the ash, total sulfur and moisture contents of the clean coal product.  A summarized list of improved yield values obtained from the optimization procedure is provided in Table 1.

Table 1:  Improvements in the plant clean coal yield due to optimization based on individual product quality constraints and the implementation of advanced fine coal circuits for cleaning the Illinois No. 6 and No. 5 seam coals; current clean coal yields from Plant 1 and 2 are 72.3% and 76.8% respectively.

Constraining Variable	Feed Assays	Current Plant Target Assays	Increase in Plant Clean Coal Yield (%)
			Optimized Plant	Advanced I	Advanced II
Illinois No. 6 (Plant 1)
Ash Total Sulfur Pyritic Sulfur	28.3% 1.08% 0.57%	7.44% 1.08% 0.44%	0.99 1.60 0.87	3.23 1.83 2.04	3.43 2.42 5.24
Illinois No. 5 (Plant 2)
Ash Total Sulfur Pyritic Sulfur	22.4% 1.40% 0.56%	8.00% 1.12% 0.45%	1.77 1.27 0.75	4.53 2.26 1.07	4.60 2.49 4.60

It is obvious from the Advanced I circuit data in Table 1 that the most significant increase in overall plant clean coal yield occurs on the basis of the product ash content constraint from the installation of flotation columns.  For both plants, greater than 3% weight unit increase in the amount of coal recovered from the plant can be realized if product ash content is the only critical product quality constraint.  However, the application of the enhanced gravity concentrator has an apparent importance in determining the overall plant yield when pyritic sulfur rejection is a goal as indicated by the Advanced II circuit results.  Although the yield improvement values associated with the pyritic sulfur constraint were significant for both plants, the impact on total sulfur content was only realized for Plant 1.  It is believed that this finding is due to the difference in the relative concentrations of the organic and pyritic sulfur contents in the feed coals, i.e., the organic-to-pyritic sulfur ratio is greater in the Illinois No. 5 seam coal.

The most significant clean coal yield improvements resulting from the implementation of the AFCC technologies can be realized when all current product quality requirements are simultaneously satisfied.  From a comparison of the results in Table 1 and 2, total sulfur content was found to be the most important constraint for determining potential plant yield improvements in Plant 1.  The use of the enhanced gravity concentrator, which is known to provide excellent sulfur rejection, allowed an increase in plant mass yield of nearly 1.00% weight units compared to the sole use of flotation columns.  However, for Plant 2, the improvement in clean coal yield is less constrained by the product sulfur content requirements.  Thus, the clean coal yield difference between Advanced I and II circuits is less significant in the case of Plant II.

Table 2:  Improvements in the plant clean coal yield due to optimization based on the need to simultaneously satisfy all product quality constraints and the implementation of advanced fine coal circuits for cleaning the Illinois No. 6 and No. 5 seam coals; current clean coal yields from Plant 1 and 2 are 72.3% and 76.8% respectively.

Constraining Variable	Feed Assays	Current Plant Target Assays	Increase in Plant Clean Coal Yield (%)
			Optimized Plant	Advanced I	Advanced II
Illinois No. 6 (Plant 1)
Multiple Ash Total Sulfur Moisture	28.3% 1.08%	7.44% 1.08% 11.6%	0.79	2.07	3.06
Illinois No. 5 (Plant 2)
Multiple Ash Total Sulfur Moisture	22.4% 1.40%	8.00% 1.12% 12.0%	1.63	4.47	4.52

Based on the predicted clean coal yield improvements, a detailed economic analysis was conducted to evaluate the increase in overall revenue and net annual income resulting from teh replacement of the existing conventional fine coal circuits with the AFCC circuits.  The economic analysis included the additional revenue resulting from the improved clean coal yield of the plant in the cash-inflow based on a coal-selling price of $20 per ton of clean coal.  This analysis considered the base mining and processing costs of $10 and $3 per ton of raw coal, respectively.  The annualized capital costs of the advanced fine coal technologies based on a depreciation period of 10 years and an annual rate of return of 12% have been included in the cash-outflow.  Based on the economic analysis, the overall production (including mining and processing) cost as a function of incremental changes in product quality was evaluated.

A summarized list of improved annual income realized while maintaining the required product quality is provided in Table 3.  Due to the difference in annual plant production and the projected improvements in clean coal yield shown in Table 2, significantly higher revenue was estimated for Plant 2.  After discounting the annual capital cost, a $1.1 million revenue increase was projected for Plant 1 from the implementation of the enhanced gravity and flotation column circuit (Advanced II circuit).  However, for reasons previously discussed, the enhanced gravity-based circuit was not found to provide the maximum revenue improvement for Plant 2.  In face, Advanced I circuit, which utilizes hindered-bed classification and column flotation was predicted to provide the maximum increase in annual revenue of approximately $6.7 million.

Table 3:  Increase in annual income due to the optimization of the existing conventional plant and application of the advanced fine coal cleaning circuits for cleaning the Illinois No. 6 and No. 5 seam coal.

Constraint Variable	Feed Assays	Current Plant Target Assays	Increasing Annual Income (million $)
			Optimized Plant	Advanced I	Advanced II
Illinois No. 6 (Plant 1)
Multiple Ash Total Sulfur Moisture	28.3% 1.08%	7.44% 1.08% 11.6%	0.47	0.97	1.10
Illinois No. 5 (Plant 2)
Multiple Ash Total Sulfur Moisture	22.4% 1.40%	8.00% 1.12% 12.0%	2.67	6.74	6.65

The results in Table 3 were based on the requirements to achieve the current product quality requirements.  However, more stringent product quality requirements will necessitate the use of the most efficient AFCC circuit.  The findings of this study indicate that the Advanced II circuit, which utilizes all three aforementioned AFCC technologies, provides the most efficient separation performance based on both ash and pyritic sulfur rejection.  As such, the economic benefits associated with the use of the Advanced II circuit will improve as the product quality requirements become more constrained.  In addition, the techno-economic benefits of all AFCC technologies/circuits are a function of the feed coal characteristics and the operating parameters of each coal processing plant.