FINAL TECHNICAL REPORT
September 1, 1996, through August 31, 1997
Project Title: AN INNOVATIVE APPROACH FOR IN-SITU DEWATERING/HARDENING OF FINE CLEAN COAL SLURRY
ICCI Project Number: 96-1/1.1 B-1
Principal Investigator: Dr. B.K. Parekh, University of Kentucky
Center for Applied Energy Research
Other Investigators: Dr. W.W. Wen, U.S. DOE/FETC; Mr. Garen Evans, Andritz Ruthner Inc.
Project Manager: Dr. Ken Ho, ICCI
ABSTRACT
Ultra-fine clean coal slurry is difficult to dewater to a low (~20 percent) moisture level using conventional dewatering techniques. The main objective of the present research program was to develop an economical and effective dewatering process by adding fibrous waste material to coal slurry before dewatering using vacuum, pressure and centrifuge dewatering. It was also the objective of the program to obtain a dewatered product with sufficient strength to facilitate its handling.
The "as received" fine size clean coal slurry (average particle size 300 m) obtained from CONSOL Inc. Rendlake plant could be dewatered to about 22 percent moisture using vacuum and high pressure techniques and to about 15 percent moisture using centrifuge technique. Addition of fiberous material was not effective in lowering the moisture of the filter cake. The product was ground to 14 micron average particle size and fiber addition studies were conducted with this material. In laboratory studies, using vacuum filtration, addition of 1 weight percent (19.6 lb/st, 10 Kg/t) of wood powder was effective in lowering filter cake moisture from 46.5 to 41 percent. Using high pressure filter addition of 0.5 weight percent (9.8 lb/st, 5 Kg/t) of raw paper pulp was effective in lowering filter cake moisture from 22.5 to 19.5 percent. Addition of 0.1 weight percent (1.96 lb/st, 1 Kg/t) anionic surfactant further lowered the filter cake moisture to 18 percent. Pilot scale high pressure dewatering tests conducted by addition of either 1 weight percent (19.6 lb/st, 10 Kg/t) of plastics or 2 weight percent (39.3 lb/st, 20 Kg/t) of carpet fibers provided filter cake with 24.5 percent moisture. Addition of 0.1 weight percent (1.96 lb/st, 1 Kg/t) of a cationic surfactant lowered the filter cake moisture to 23 percent. Pilot scale centrifuge provided a filter cake with 34 percent moisture. Addition of orimulsion or fiberous material was not effective in lowering the filter cake moisture using the centrifuge.
Hardening properties of the dried filter cakes, such as compression strength, obtained with addition of raw paper pulp improved to 4.5 Kg/cm2 (14 lb/in2), impact resistance from 0 to 4, abrasion resistance from 0 to 70 percent and dust reduction efficiency to 75 percent. Addition of asphalt emulsion significantly improved the non-wetability of the dewatering product.
EXECUTIVE SUMMARY
Recovery of fine clean coal from waste streams offers an attractive source of low-ash, low-pyritic sulfur clean coal. In the fine state, most of the mineral matter is liberated and could be effectively removed using the advanced column flotation technology. These fine coal cleaning techniques will be particularly applicable to the Illinois high-ash, high-pyrite sulfur coals, where ultra-fine grinding is necessary to liberate the mineral matter. However, if the recovered fine clean coal cannot be dewatered to a low (less than 20 percent) moisture level, then acceptance of these newly developed techniques by the coal industry is unlikely. Thus, the objective of the research program was to develop an economical and effective dewatering process which will dewater the coal as well as re-constitute it in a single stage, thus, providing an easily handlable product. The scope of the research program involved addition of waste fibers obtained from paper, carpet and wood to the coal slurry prior to dewatering.
The clean coal slurry samples, obtained from the CONSOL Inc. Rendlake mine used for the study had about 32 percent solids, which is double than usual amount found in a froth flotation product. The average particle size of the product was 300 m (48 mesh). The ash and sulfur content of clean coal were 5.56 percent and 1.1 percent, respectively.
The average particle size of the 'as received' slurry material was large, thus, with the consent of the ICCI projector manager the material was fine ground to average particle size of 14 microns using an attrition mill. Table A shows the baseline laboratory dewatering data of both of the slurry obtained using vacuum, pressure and centrifuge techniques. For the 'as received' slurry, both vacuum and pressure filtration provided filter cakes with 23 percent moisture and the centrifuge provided a filter cake with 15 percent moisture. The fine ground slurry, because of ultra fine particles, provided filter cakes with 46.5, 23.0 and 33.5 percent moisture using vacuum, pressure and centrifuge filter, respectively.
Table A. Baseline laboratory dewatering data of "as received" and fine ground Rend Lake coal slurry
| Filter cake moisure (%) | |||
| Sample | Vacuum | Pressure | Centrifuge |
|
(1) As received slurry
Average particle size - 300 um |
23.0 |
23.0
(40 psig (2.7 bar)) |
15.0 |
|
(2) Fine ground slurry
Average particle size = 14 um |
46.5 |
23.0
(80 psig (5.4 bar)) |
33.5 |
Effect of addition of various reagents on dewatering of 'as received' slurry are summarized in Table B. Using vacuum filtration, addition of 0.025 weight percent (90.49 lb/st, 0.25 Kg/t) of a cationic surfactant lowered filter cake moisture from 23.0 to 19.8 percent. Addition of 0.075 weight percent (1.47 lb/st, 0.75 Kg/t) of anionic or non-ionic surfactant lowered filter cake moisture from 23.0 to 19.0 percent using high pressure filter (40 psig (2.7 bar) pressure). Similarly, addition of 0.001 weight percent (1.9610-2 lb/st, 10 g/t) of an anionic flocculant lowered the filter cake moisture from 23.0 to 21.0 percent using the high pressure filter. Using centrifuge filter, for all the reagents studied, no reduction in filter cake moisture was noticed.
Table B. Effect of reagent and waste fiberous material addition on dewatering of "as received"
| Filter cake moisture (%) | ||||
| Reagent |
Amount
(weight percent) |
Vacuum |
Pressure (40 psig
2.7 bar)) |
Centrifuge |
| -- | -- | 23.0 | 23.0 | 15.0 |
| cationic surfactant | 0.025 | 19.8 | 22.5 | 15.0 |
|
Anionic or non-ionic
surfactant |
0.075 | 22.0 | 19.0 | 15.0 |
| Anionic flucculant | 0.001 | 23.8 | 21.0 | 15.0 |
The dewatering results of the 'as received' slurry showed that due to the presence of a significant amount of coarse material, the currently produced froth can be dewatered to 22 percent moisture using either vacuum or pressure filtration. A 15 percent moisture filter cake could be obtained using a screen bowl centrifuge. The effect of addition of fiberous material could anly be evaluated on ultra-fine difficult to dewater coal slurry. Thus, most of the fiber addition tests were conducted with the ultra-fine ground slurry of average particle size of 14 microns.
The effect of addition of various waste fiberous material on dewatering of ultra fine ground slurry is summarized in Table C. For the vacuum filter addition of 1 weight percent (19.6 lb/st, 10Kg/t) of fine wood powder lowered the filter cake moisture from 46.5 to 41.0 percent. Of all the waste fibers tested only the wood fibers were effective in lowering filter cake moisture using vacuum filter. Using pressure filter addition of either 1 weight percent (19.6 lb/st, 10 Kg/t) of wood powder or 2 weight percent (39.3 lb/st, 20 Kg/t) of carpet fibers lowered filter cake moisture form 23.0 to 20.0 percent. Addition of 0.5 weight percent (99.8 lb/st, 5 Kg/t) of raw paper pulp fibers was effective in lowering filter cake moisture form 23.0 to 19.5 percent. Addition of 0.1 weight percent (1.96 lb/st, 1 Kg/t) of anionic surfactant along with raw paper pulp fiber lowered the filter cake moisture to 18 percent. These data shows that combination of raw paper fibers and anionic surfactant was the most efffective in lowering the filter cake moisture. With this combination an absolute moisture reduction of 5 percent or in other words 20 percent moisture reduction improvement over that obtained without the addition of fibers.
In one of the experiment, using a combination of vacuum and pressure filters and adding 3 weight percent (58.9 lb/st, 30 Kg/t) of raw paper fibers, lowered the filter cake moisture from 37.5 to 26.2 percent.
Table C. Effect of fiberous material addition on dewatering of ultra fine ground Rendlake coal
| Filter cake moisture % | ||||
| Fiberous material |
Amount (weight
percent) |
Vacuum | Pressure* | Centrifuge |
| Baseline | -- | 46.5 | 23.0 | 33.5 |
| Computer paper | 1 | 46.1 | 33.2 | 36.0 |
| Wood powder | 1 | 41.0 | 20.0 | 34.6 |
| Carpet | 2 | 42.5 | 20.0 | 35.2 |
| Raw paper pulp | 0.5 | -- | 19.5 | -- |
|
Raw paper pulp with anionic
surfactant |
0.5 + 0.1 | -- | 18.0 | -- |
*80 psig (5.4 bar) pressure
The hardening properties of the dried filter cake obtained using 3 weight percent (58.9 lb/st, 30 Kg/t) of raw paper pulp showed significant improvement. For example, the compression strength improved from 0 to 2045 lb/ft2 (4.5 Kg/cm2), impact resistance from 0 to 4, abrasion resistance from 0 to 70 percent, and dust reduction efficiency from 0 to 75 percent.
Addition of 3 weight percent (58.9 lb/st, 30 Kg/t) raw paper pulp along with 1 weight percent (19.6 lb/st, 10 Kg/t) asphalt emulsion provided a filter cake which could withstand in water for more than 60 days indicating its improved stability.
In the pilot scale hyperbaric filter test a filter cake with 26 percent moisture was obtained using 80 psig (5.5 bar) pressure. It provided solids throughput of 99.4 lb/ft2.hr (497 Kg/m2.hr) at an air consumption of 13450.5 ft3/st (427 m3/t). Using these filter conditions addition of 1 weight percent (19.6 lb/st, 10 Kg/t) of plastics or 2 weight percent (39.3 lb/st, 20 Kg/t) of carpet fibers lowered filter cake moisture from 26 to 24.5 percent. Addition of 0.1 weight percent (1.96 lb/st, 1 Kg/t) of a cationic surfactant, further, lowered the filter cake moisture to 23 percent.
Pilot scale solid bowl centrifuge dewatering of the fine clean coal provided a filter cake with 34 percent moisture. Addition of fiberous material was not effective in lowering the filter cake moisture using the centrifuge.
Recommendations are offered for conducting detailed dewatering studies using a column flotation product with the addition of raw paper fibers. Studies should also be conducted with respect to morphology of the filter cakes.