FINAL TECHNICAL REPORT

September 1, 1995, through August 31, 1996

Project Title: A MODIFIED RELEASE ANALYSIS PROCEDURE USING ADVANCED FROTH FLOTATION MECHANISMS

DOE Cooperative Agreement Number: DE-FC22-92PC92521 (Year 4)

ICCI Project Number: 95-1/1.2B-1P

Principal Investigator: R. Q. Honaker, Department of Mining Engineering, Southern Illinois University at Carbondale

Other Investigator: M. K. Mohanty, Department of Mining Engineering, Southern Illinois University at Carbondale

Project Manager: K. Ho, ICCI

ABSTRACT

Recent studies indicate that the optimum separation performances achieved by multiple stage cleaning using various column flotation technologies and single stage cleaning using a Packed-Flotation Column are superior to the performance achieved by the traditional release procedure, especially in terms of pyritic sulfur rejection. This superior performance is believed to be the result of the advanced flotation mechanisms provided by column flotation technologies. Thus, the objective of this study was to develop a suitable process utilizing the advanced froth flotation mechanisms to characterize the true flotation response of a coal sample.

This investigation resulted in the development of a modified coal flotation characterization procedure, termed as the Advanced Flotation Washability (AFW) technique. The apparatus used for this procedure is a batch operated Packed-Column device which provides enhanced selectivity due to a plug-flow environment and a deep froth zone. The separation performance achieved by the AFW procedure was found to be superior to those produced by the conventional tree and release procedures for three nominally -100 mesh coal samples and two micronized samples. The largest difference in separation performance was obtained on the basis of product pyritic sulfur content. A comparison conducted between the AFW and the release procedures at an 80% recovery value showed that the AFW technique provided a 19% improvement in the reduction of pyritic sulfur. For an Illinois No. 5 coal sample, this improvement corresponded to a reduction in pyritic sulfur content from 1.38% to 0.70% or a total rejection of 66%. Micronization of the sample improved the pyritic sulfur rejection to 85% while rejecting 92% of the ash-bearing material. In addition, the separation performance provided by the AFW procedure was superior to that obtained from multiple cleaning stages using a continuous Packed-Column under both kinetic and carrying-capacity limiting conditions.

U. S. DOE Patent Clearance is NOT required prior to the publication of this document.

EXECUTIVE SUMMARY

The goals of this project are to evaluate the current status of the coal flotation characterization procedures, such as release and tree analyses with respect to the advanced froth flotation technologies presently being introduced and to modify the traditional procedures so that a true theoretical optimum recovery-grade curve for any froth flotation process can be obtained from the analysis.

The traditional release and tree analysis procedures are recognized internationally as the analyses which provide the ultimate recovery-grade relationship that can be achieved by any flotation process for the treatment of a given coal. An analogous to release analysis is the washability analysis for gravity-based separations. Dell introduced the concepts of release analyses in 1953 and refined the procedure in 1964 and 1972. To date, release analysis, which is conducted using a Denver flotation device, has been successfully used as a tool by preparation plant operators and researchers for evaluating the efficiency of new flotation technologies and for optimizing current flotation systems. However, Dell et al. (1972) recognized the fact that potentially better performances could be achieved by other flotation devices. "The (release) approach towards this absolute measurement is, however, a function of cell design, and it is yet impossible to say whether results even better than those with the Denver unit are possible."

In agreement with Dell's statement, the introduction of advanced flotation technologies has resulted in separation performances superior to that predicted by the traditional release analysis procedure. This fact was found to be especially true on the basis of pyritic sulfur rejection with single stage cleaning using the Packed-Column and multiple stage cleaning using other flotation column technologies. It is believed that the superior performance is due to an improvement in the hydrodynamic conditions in the flotation cell and to the utilization of selectivity mechanisms in the froth zone. Due to the inherent constraints associated with the Denver cell which prevent the use of deep froth depths, the phenomena, such as reflux, selective detachment and froth washing of entrained materials, are not easily achieved in the traditional release analysis process. It is believed that these deficiencies have resulted in several steady-state column flotation results being superior to the corresponding release data.

A theoretical simulation of the release analysis procedure conducted by the principal investigators supports the above statements that release analysis should be conducted with a flotation device providing a plug-flow hydrodynamic environment and a deep froth depth. Plug-flow conditions were found to provide a higher recovery of particles to the froth zone when compared to perfectly-mixed conditions, which is characteristic of the Denver conventional cell used in the release analysis procedure. It was also found that the selectivity between particles of varying hydrophobicity is best achieved in the froth phase where the selective detachment mechanism can be utilized. Deep froth depths provide more reflux and a separation performance approaching the optimum separation performance. Unfortunately, conventional cells do not support deep froth depths and, therefore, have limited ability to provide sufficient reflux. Since release analysis is based on selectivity, a flotation column apparatus is the desired separation device to replace the Denver cell for obtaining the optimum separation performance. Thus, the project objectives are: 1) to develop a new release analysis apparatus which will provide plug-flow conditions while allowing a significant froth reflux action; 2) to compare the optimum separation performance predicted by the modified release analysis procedure for the treatment of fine coal sample with that obtained by the traditional release and tree analyses, and washability analysis; 3) to compare the separation performances obtained for several coal samples using the proposed release procedure with the optimum recovery-grade curves obtained from the single-stage treatment provided by, Packed-Column flotation technology, which was found to produce the best separation performance among the six different flotation column technologies studied in a recently completed ICCI project. Achieving these objectives will result in a modified release analysis procedure which utilizes the advanced flotation mechanisms common to the modern flotation technologies and strategies being used today by coal preparation plant operators and researchers. Thus, a true optimum theoretical separation performance for any froth flotation process could be obtained for fine coal characterization.

During this reporting period, a modified coal flotation characterization procedure, termed as Advanced Flotation Washability (AFW) technique was developed. The new apparatus used for this procedure is essentially a batch operated 2-inch diameter, 5 ft tall packed-column, in which the feed slurry was continuously recirculated to avoid deposition of solid particles in the cell and to provide a feed flow counter-current to that of the air bubbles to effect superior bubble-particle collision. The cell was equipped with a PID controlled wash water system, which was mainly used to mobilize the deep froth in the cell and to adjust the pulp level to operate the cell at a desired froth depth to facilitate column reflux action.

The general approach used in the traditional release analysis for removing the hydrophilic mineral particles in the first stage and collecting the concentrate samples having varying degrees of hydrophobicity in the second stage was pursued in the AFW procedure to obtain an ultimate combustible recovery-grade curve representing the true flotation response of a coal sample. Several experiments were conducted using the AFW device on a relatively high sulfur, -100 mesh Illinois No. 5 run-of-mine coal sample collected from a local coal preparation plant. Initial experiments, which were conducted without the wash water system, concentrated on determining the optimum aeration rate and reagent dosages required for an efficient operation of the cell. In the subsequent experiments, a wash water system was integrated to the AFW device, in which the flow rate of the wash water was controlled using a pulp level controller. In other words, the wash water rate was equal to the water flow rate leaving the cell with the froth concentrate.

The performance curve improved by operating the device at a reduced aeration rate, which resulted in a very controlled flow of product from the lip of the column and thereby reduced the amount of pulp water recovered to the product launder. The wash water addition mobilized the froth zone and allowed an efficient operation of the cell. However, even three flotation steps (i.e., rougher-cleaner-cleaner) conducted with a froth depth of about 3 ft in the first stage of the procedure were not sufficient to produce significant improvement in the performance curve generated in the second stage of the procedure. In addition, since the flotation steps in the first stage were conducted at a very low aeration rate to minimize the recovery of pulp water to the product launder and thereby reduce the problem of hydraulic entrainment, each step took about 2 to 3 hours for complete flotation, which is much longer than that required for steps followed in the traditional release procedure. Therefore, in a few of the later experiments, the flotation steps of the first stage were conducted using the conventional cell. The experiment completed with four flotation steps conducted at high solids content in the first step produced the best performance curve in the second stage conducted using the AFW device.

Similar coal characterization experiments were also conducted using the traditional release and tree analysis procedures. The best performance curve generated using the AFW technique was superior to those of the traditional procedures. While treating an Illinois No. 5 coal having an ash, total sulfur and pyritic sulfur contents of about 20%, 2.7% and 1.55%, respectively, the AFW procedure produced a product of 4.8% ash, 1.75% total sulfur and 0.97% pyritic sulfur at a combustible recovery value of 80%. In comparison, the optimum performance results obtained to date from the traditional procedures at the same combustible recovery are 5.6% ash, 2.05% total sulfur and 1.28% pyritc sulfur.

Several experiments are on-going and will be conducted in the next reporting period to solidify the AFW procedure and verify the above finding by conducting Anova analyses to determine the statistical significance of the difference in the performance achieved from the traditional and modified coal characterization procedures. The AFW procedure will be evaluated on a micronized sample obtained from the bulk Illinois No. 5 coal sample used in this investigation. In addition, multiple-stage cleaning tests will be conducted using a continuously operated Packed-Column to determine the significance of the results obtained from the AFW procedure.