November 1, 1999, through October 31, 2000

Project Title: USE OF SUBAERATION CELL AND ISGS WASHER TO PROCESS 100 LB. OF TAILINGS/HOUR IN A COMMERCIAL PLANT

ICCI Project Number: 99-1/3.1B-1

Principal Investigator: Latif A. Khan, Illinois State Geological Survey (ISGS)

CO-PI: William R. Roy, ISGS

Project Manager: Ken Ho, ICCI

ABSTRACT

The purpose of this project was to test a novel washer on a subaeration cell to produce coal with a lower sulfur and ash content than is currently possible. The washer has been developed at the ISGS to increase the throughput and cleaning efficiency of a froth flotation circuit. The ISGS froth washer has been successfully scaled up by more than a factor of ten from a 0.11-ft³ miniature cell to a 1.3-ft³ subaeration cell. The larger cell was successfully tested online in a coal preparation plant in White County on a split stream during a debugging and testing operation. Continuous tests were conducted in a plant in the presence of Dr. Ken Ho, project manager, ICCI. The modified subaeration cell was especially effective in removal of pyritic sulfur, and under some conditions, it was able to produce a cleaner product than that predicted by the Advanced Flotation Release Analysis (AFRA). Even at one third of its capacity, it handled all the froth that was generated in this cell. With regard to cleaning the tailings and recovery of combustibles, the subaeration cell equipped with the ISGS washer performed as well in the commercial plant as in the laboratory.

Without the usual multiple stages of cleaning and re-cleaning, the ISGS washer produced a product that was cleaner than that predicted by AFRA. For 90 % combustible recovery, the ash content of the product produced in the plant with the ISGS washer was almost 1/3 of that predicted by AFRA. For the same ash content, the recovery of the cleaner combustible with the ISGS washer was twice that predicted by AFRA. Irrespective of the combustible recovery, the AFRA predicted a product with higher sulfur content than that produced with the ISGS washer. Through these tests valuable experience was gained and improvements are being made to the system. A no-cost extension was granted by ICCI so that a larger, more automated unit could be tested at another plant.

The ISGS washer developed with financial assistance from ICCI has the potential to produce low-sulfur coal economically. Using this device to process the fine coals that are currently discarded (on average 25% of the coal produced), the ISGS washer will not only cut the cost of mining and reclamation drastically, but also could help produce a cleaner product. If widely adopted this device, could help Illinois coal producers produce cleaner coal at less cost, and compete with out-of-state coal producers, save mines from closing and help open new mines.

Pages 1-17 contain propriety information

EXECUTIVE SUMMARY

This project was designed to examine the performance of a pilot plant-scale version of a subaeration cell equipped with the ISGS washer during online testing on a split stream in an industrial environment, and to determine what modifications were required for its adoption by coal processing plants. The ISGS washer, when attached to a subaeration cell in single-stage flotation tests, consistently produces a product that is cleaner than that produced with a packed column using the same feed material. The rate of production of the concentrate from the modified cell is more rapid than that of flotation columns. Under optimum conditions, its performance approaches closely and in some cases exceeds the optimal performance of any flotation device predicted by Advanced Flotation Release Analysis (AFRA).

The 1990 Clean Air Act Amendments (CAAA) are compelling energy producers to pursue more efficient and environmentally sustainable energy sources. Coal is the major natural energy resource of Illinois but because of the CAAA and the inability of coal producers to compete for markets with out-of-state producers, the coal mines in Illinois have been steadily losing market share.

Illinois coal mines have been closing because they cannot produce coal that contains less sulfur and ash-forming minerals and is less expensive than their competitors. The coal produced is not of the desired quality because the processing methods used to upgrade coal to marketable products have failed to generate a marketable product at a reasonable cost. Both the inefficiency of the processing methods used and the capital and operating costs of the cleaning methods required to clean the coal thoroughly have been responsible for this dilemma. This predicament has encouraged the coal producers in Illinois to recover the easily cleanable coal from the mined-out material, and to discard the rest with the result that about 25% of the coal that is mined in Illinois is rejected. Discarded coal increases the cost of production instead of augmenting revenues in addition to posing environmental hazards.

The coal produced at coal cleaning plants contains more sulfur and ash-forming minerals than necessary because the processing methods currently used to upgrade Illinois coal cannot generate a cleaner product at a reasonable cost. As in any physical beneficiation method, separation of particles from each other by flotation is possible only if the feed components are liberated. Nearly complete liberation of the pyrite in Illinois coal would require grinding to particle sizes not exceeding a few micrometers in size. In fine size ranges, the separation of fine particles in subaeration cells and flotation columns is adversely affected by 1) non-selective adhesion of particles to air bubbles, 2) entrapment of the mineral matter in the froth and 3) mechanical carryover of the particle suspended in slurry. Unlike coarser particles, the detaching forces working on the nonselectively attached fine particles are small and thus, they are not easily mechanically dislodged once they get "hooked" on the bubbles. One way to dislodge these nonselectively attached particles and the entrapped particles is to wash the froth with enough water such that the particles are transferred back to the aqueous phase, and then to drain the froth adequately. This is accomplished in the ISGS froth washer.

A device that can help existing processing equipment produce cleaner coal at a faster rate will reduce the number of flotation machines required to process a given amount of the material. This will result in reductions of the capital and operating costs, and perhaps an increase in profits. Not only will the costs of disposing of rejected coal be reduced by recovering more fine coal, but revenues will be bolstered by the sale of a cleaner coal product generated from the material discarded earlier.

Without multiple stages of cleaning and re-cleaning, this ISGS washer can make a single subaeration flotation cell produce a cleaner product than that produced by a flotation column, but at throughput rate greater than that of a common subaeration cell. This device is more effective than the old system of subaeration-cell batteries in which the froth or tails from one cell were re-cleaned in another to generate a product of desired quality. When equipped with this device, each cell can produce the final product in a single run leading to increased capacity of the plant or a decrease in the number of cells required to process a given stream of material. This may result in considerable savings in installation and/or operational costs. The production costs will also be reduced because of the better cleaning efficiency of fines, and because of the associated saving on the disposal of the fines.

The flotation device used in this work has been successfully tested at the laboratory scale. The device, which consists of a 1.3-cu ft subaeration cell and the ISGS washer, was tested in a plant to debug and trouble shoot it under continuous industrial conditions. Improvement to the system conceived during the testing are being incorporated in the design. Figures 1 and 2 compare the performance of the pilot-plant scale version of the subaeration cell equipped with the ISGS froth washer with the results from Advance Flotation Release Analysis (AFRA). The AFRA was developed at Southern Illinois University (SIU) to determine the limits of commercially available flotation machines. The curve was used as a standard to evaluate the results achieved with the device in the laboratory and during online testing in a plant using a split stream. The online testing resulted in a cleaner product than that predicted by the AFRA for any flotation device.

For example, at a 90% recovery of combustibles, the AFR curve predicted that the product would have an ash content of about 11.5%, whereas the ash content of the products produced by the ISGS washer in the plant tests was about 4.5% under optimum conditions. Moreover, for the same ash content, the recovery of combustible material predicted by the AFR curve was about 42% whereas the ISGS washer produced about 90% recovery at the same ash content of 4%. The sulfur content of the combustible material was consistently about 0.5- 1.0% less than that predicted by the AFRA (Figures 1-2) regardless of the amount recovered.

During the intensive washing and enhanced drainage in the washer, both the particles that are unselectively attached and those that are trapped between the air bubbles are flushed out. To ensure that the flushed particles do not become trapped in the lower layers of the froth as they are transported, it is best to limit the vertical height of the froth that is being washed. In the ISGS washer, the washed-out minerals are carried a short vertical distance (1 inch) to a separate stream along the lower part of the washer, and not allowed to become entrapped in the counter current moving froth.

Figure 1. Comparison of combustible recovery vs. ash of the product produced from a slurry (5% solids) of coal refuse and coal mixture (24% ash) in AFRA test, Big Cell (1.3-ft³) in laboratory and in a plant on coal refuse (3% solids with 26% feed ash) conducted near optimum conditions.

Figure 2. Comparison of combustible recovery vs. sulfur of the product produced from a slurry (5% solids) of coal refuse and fine coal (3.44 % sulfur) in AFRA test, Big Cell (l.3-ft³) in laboratory and in a plant on coal refuse (3% solids with 2.15% sulfur) conducted near optimum conditions

The remainder of this report contains proprietary information and is not available for distribution except to the sponsor(s) of this project.