FINAL TECHNICAL REPORT
December 1, 1994, through October 31, 1995
Project Title: A FINE COAL CIRCUITRY STUDY USING COLUMN FLOTATION
AND GRAVITY SEPARATION (ADDENDUM)
ICCI Project Number: 94-1/1.1A-1
Principal Investigator: Ricky Q. Honaker, Department of Mining Engineering, Southern Illinois University at Carbondale
Other Investigators: Damin Wang, Department of Mining Engineering, Southern Illinois University at Carbondale
Project Manager: Ken K. Ho, Illinois Clean Coal Institute
ABSTRACT
In a previous ICCI project, an enhanced gravity separator commercially known as the Falcon Concentrator was found to effectively treat 28 x 400 mesh coal. In this project, a 40-inch diameter Falcon concentrator (C40) having an estimated throughput capacity of 100 tons/hour was designed and constructed by the manufacturer and tested by the principal investigators in a closed-circuit system to evaluate its potential for cleaning fine coal in operating preparation plants.
In this study, mass flow rates as high as 94 tons/hr were found to be efficiently treated by the C40 Falcon unit. This mass flow rate is four to ten times higher than the feed capacities of other enhanced gravity concentrators, most likely due to the greater applied centrifugal force of over 200 g's. In fact, test data indicate that even higher feed flow rates may actually improve the metallurgical performance by increasing the recovery of the courser coal particles. From the treatment of a low sulfur content Illinois No. 6 flotation feed sample, the ash and total sulfur contents in the 100 x 400 mesh size fraction was reduced from 11.3% to 5.4% and 1.23% to 0.87%, respectively, while recovering about 93% of the combustible material. For an Illinois No. 6 refuse pond coal sample, the C40 unit reduced the ash contents of the 16 x 100 mesh and 100 x 400 mesh size fractions from 22.1% to 8.0% and 3l.6% to l4.8%, respectively, while achieving slightly greater than 80% combustible recovery. The total sulfur content for both fractions was reduced from 7.9% to 2.7% as a result of the rejection of nearly 90% of the pyritic sulfur. Little to no separation was achieved on the minus 400 mesh size fraction. The applied centrifugal force and tailings underflow rate were found to be the main operating variables for controlling separation performance. Potential plugging of the underflow nozzles was identified as an operating problem that must be considered upon implementation of the technology into an operating preparation plant.
EXECUTIVE SUMMARY
Column flotation provides excellent recovery of ultrafine coal while producing low ash content concentrates. However, like other flotation processes, column flotation is not efficient for treating fine coal containing significant amounts of mixed-phase particles. An ICCI research project conducted during the previous funding year has shown that mixed-phased particles can be more efficiently treated using enhanced gravity separators. The Falcon Concentrator was found to be capable of producing Phase I compliance coal from high pyritic, low organic sulfur content coals while achieving energy recovery values greater than 90%. The excellent separation performance achieved by the Falcon Concentrator was also indicated by a probably error (Ep) value of 0.12 at gravity-cut points between 1.5 and 1.7. This separation performance was obtained at throughput and the ability of the Falcon Concentrator to maintain the exceptional performance upon scale-up to industrial-size units are presently unknown.
In this project, a 40-inch diameter Falcon Concentrator was evaluated to determine the scale-up effects on the optimum operating parameter values as determined previously for the 10-inch diameter Falcon Concentrator. The specific objectives of the project were: 1) to evaluate the throughput scale-up of the Falcon Concentrator; 2) to determine the scale-up effects on the optimum parameter values as compared to those obtained on the 10-inch Falcon Concentrator in the ICCI project; 3) to evaluate the feasibility of using a classifying cyclone-Falcon Concentrator circuit arrangement to obtain a final clean coal concentrate; and 4) to evaluate operating and wear characteristics of the Falcon Concentrator over a long operating period of at least 100 hours.
The 40-inch diameter (C40) Falcon Concentrator studied in this investigation utilized a vertically-mounted bowl that was sloped outward from bottom to top at an angle of 10o from vertical. The purpose of the sloped bowl is to utilize a portion of the applied centrifugal force to create an upward force parallel to the bowl wall which pushes the solids bed toward the top of the bowl. The heavy particles forming the beached particle bed near the bowl wall is removed through a series of chutes spaced at equal distances along the circumference of the bowl. The underflow material travels through 36 nozzles that are equipped with pinch valves for controlling the underflow rate. The light particles travel over the chutes and a lip to report to the overflow stream as the clean coal product. The C40 unit tested had the ability to provide a centrifugal force of about 210 times the natural gravitation pull (or 210 g's). The unit required a floor space of 10 x 10 ft2 and stood approximately 15 ft. tall.
The C40 Falcon unit was evaluated in a closed-circuit system which was capable of supplying 2000 gallons/minute of feed. In this circuit, the overflow product was directly fed to a product sump and the underflow stream was directed to a tailings sump and then pumped to the product sump. The material in the product sump was then pumped back to the feed sump. Additional water to the circuit is only added through the small amounts provided as gland water for the feed and product pumps. The feed coal samples used in this study were a flotation feed that was nominally -28 mesh and a refuse sample collected from an inactive tailings pond. Both coal samples were originally extracted from the Illinois No. 6 coal seam. Upon charging the sample into the feed sump, the slurry was passed through a trash screen of 16 mesh to remove material that may plug the underflow orifices in the Falcon unit.
Initial tests were performed to evaluate the effect of operating parameters on the separation performance provided by teh C40 Falcon Concentrator. The applied centrifugal force and the tailings underflow rate were found to have a large effect on the combustible recovery of the 16 X 100 mesh size fraction and on the ash rejection of both the 16 x 100 mesh and the 100 x 400 mesh size fractions. Increasing the centrifugal force from about 50 to 140 g's resulted in a 40% decrease in the recovery of combustibles in the 16 x 100 size fraction of the refuse pond material. However, increasing the feed flow rate at a given centrifugal force was found to increase the combustible recovery by about 20%. This is due to a decrease in the particle residence time which is insufficient to allow the courser particles sufficient time to report to the underflow ports. Increasing the centrifugal force from 50 to 200 g's resulted in a gradual reduction in the combustible recovery for the 100 x 400 size fraction. However, the ash rejection improved significantly from about 40 to 75%. Increasing the volumetric feed rate from 1400 to 1800 gallons/min was found to decrease the ash rejection by nearly 10% due to the shorter particle retention time. Little or no effect was obtained on the -400 mesh size fraction.
The C40 Falcon Concentrator was found to be effective at reducing teh ash content of the 16 x 400 mesh fraction in the flotation feed and the refuse pond samples while maintaining a high recovery of combustibles. For the flotation feed sample, the ash content in the 100 x 40 mesh size fraction was reduced from 11.3% to 5.4% while recovering 93% of the combustibles. For the high ash refuse sample, the ash contents in the 16 x 100 and 100 x 400 mesh size fractions were reduced from 22.1% to 8.0% and 31.6% to 14.8%, respectively, while maintaining recovery values slightly greater than 80%. Little or no ash reduction was achieved for the -400 mesh size fraction.
High rejections of total and pyritic sulfur were achieved from the treatment of the low sulfur content flotation feed sample and the high sulfur refuse pond sample using the C40 Falcon Concentrator. For the flotation feed sample, the total sulfur contents of the 16 x 100 and 100 x 400 mesh size fractions were reduced from 1.19% to 1.08% and 1.23% to 0.87% which equates to a sulfur rejection of 27% and 42%, respectively. This sulfur reduction was achievable due to the rejection of heavy coal pyrite to the tailings stream. For the refuse pond sample, the total and pyritic sulfur contents in the 16 x 400 mesh size fraction were reduced from about 7.9% to 2.7% and 5.5% to 1.1%, respectively, while recovering slightly greater than 80% of the combustible material. This corresponds to a pyritic sulfur rejection of nearly 90%. In general, 80% to 90% rejection of pyritic sulfur was consistently achieved throughout the test program on the 16 x 100 mesh size fraction of the refuse pond sample while recovering between 70% to 80% of the combustible material. For the 100 x 400 mesh size fraction, 70% to 80% pyritic sulfur rejection was commonly obtained while recovering 80% to 95% of the combustibles.
The metallurgical performance results obtained from the treatment of the refuse pond sample using the C40 Falcon unit were found to be nearly equal to shoe achieved by the C40 Falcon unit. This finding indicates that small scale C10 Falcon tests can be used to evaluate the potential of the C40 Concentrator.
Mass feed flow rates and feed slid contents as high as 94 tons/hr and 20% by weight, respectively, have been efficiently treated using the C40 Falcon Concentrator. This mass flow rate is approximately four to ten times the capacity of other enhanced gravity concentrators. The main reason for the high throughput capacity of the Falcon unit is its ability to apply a centrifugal force up to 300 g's as compared to the 60 g's provided by the Knelson and Kelsey Jig enhanced gravity concentrators. In fact, based on the test data, higher feed flow rates and centrifugal forces may provide enhanced energy recovery for the 16 x 100 mesh size fraction while maintaining or possibly improving the ash rejection in the 100 x 400 mesh size fraction. This may allow more efficient treatment of a feed having a wide range of particle sizes such as the 27:1 particle size ratio effectively treated in this study.
A total of 170 hours of operation has been logged on the C40 Falcon Concentrator during this investigation. During this period, no major mechanical problems have either occurred r been identified. The cumulated power consumption is 2100 kilowatt-hours. Using an estimated average of 50 ton/hr for the mass feed flow rate treated during this study, the power cost for the C40 Falcon unit is less than $0.10/ton.
Maintaining of a proper particle bed in the Falcon bowl is required to achieve effective separation. To maintain a stable particle bed requires the pairing particle feed characteristics with proper nozzle size and lip size, which is used to control the bed thickness. For example, the 2-inch wide lip providing the thickest particle bed combined with 5/32-inch nozzle orifices was required for the relatively clean flotation feed sample in order to reject a small portion of the feed material through the underflow nozzles without collapsing the particle bed. On the other hand, a 1.5-inch wide lip was required for the high-ash refuse sample. This lip size reduction was due to the large weight of the solids bed due to the increase in the amount of heavies reporting in the feed which caused a heavy load to the lower bearings located at the bottom of the rotating axis to maintain a specific vertical position. Upon nozzle plugging with the 2-inch wide lip, a severe bowl imbalance caused the bearings to actually rotate around the shaft axis and resulted in an automatic system shutdown when treating the refuse pond material. Use of the 1.5-inch wide lip resolved this problem. In addition, 7/32-inch nozzle orifices were needed to allow sufficient underflow rates to remove the large amount of heavy particles.
An operational problem associated with the Falcon Concentrator is the plugging of the underflow nozzles which is sometimes not easily detected by an operator. Nozzle plugging results in by-pass of heavy particles to the product overflow stream and may cause a vibration shutdown due to a bowl imbalance. To reduce this problem in an operating preparation plant, a two-dimensional screen will be required to remove teh oversize material, i.e., any particle greater than 12 mesh. However, even with careful screening, it should be expected that some oversize will report to the Falcon feed over a period of time. Thus, it is highly recommended that the manufacturer develop an internal nozzle cleaning system whereby the coarse material can be removed from the nozzle orifices and bowl. In addition, industrial users of this technology should be aware of this problem so that extra attention can be provided to ensure efficient operation in the plant environment.
Long term tests of 16 and 24 hours were conducted on the C40 Falcon Concentrator during which no mechanical failures were realized. However, during the test involving the treatment of the refuse pond sample, relatively large fluctuations in metallurgical performance occurred during the second operating day. These fluctuations were not observed by the operators during the tests. This finding demonstrates the need for an on-line control system to monitor and maintain highly efficient metallurgical performance. A control system for the Falcon is currently being developed as part of an ICCI project.