Partitioning and behavior of coal macerais during dry coal cleaning

Partitioning and behavior of coal macerais during dry coal cleaning

Coal Science J.A. Pajares and J.M.D. Tasc6n (Editors) 9 1995 Elsevier Science B.V. All rights reserved. 1549 Partitioning and behavior of coal mace...

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Coal Science

J.A. Pajares and J.M.D. Tasc6n (Editors) 9 1995 Elsevier Science B.V. All rights reserved.

1549

Partitioning and behavior of coal macerals during dry coal cleaning J.M. Stencel, H. Ban, J.L. Schaefer and J.C. Hower Center for Applied Energy Research, University of Kentucky, Lexington, Kentucky 40511, USA

1. ABSTRACT Fine coal cleaning has been studied using dry, triboelectrostatic experimentation. The 45-75 tun coals were pneumatically transported through a Cu-loop tribocharger, and then transported through a 100-200 kV/m electric field. Positively charged coal was deflected to the negative electrode and negatively charged minerals were deflected to the positive electrode. Samples were retrieved and subjected to petrographic analysis. Petrographic analysis of these samples showed that, for high volatile A and B bituminous coals, vitrinite macerals were significantly enhanced in the clean-coal, whereas the fusinite + semifusinite + exinite maceral were enhancexl in the tailings/minerals. For a high volatile C bituminous coal, this trend in enhancement was not observed. The vitrinite partitioning, greater for dry processing than for wet processing, may be related to differences in surface chemical and physical properties of the coals. 2. INTRODUCTION Triboelectrostatic separation works because differential charge can be imparted on carbon and mineral constituents in coal, the primary reason for which is a difference in the surface work function of the carbon and mineral constituents. The fundamentals oftriboelectrostatic separation are not fully understood, nor is dry electrostatic processing commercially practiced. There is renewed interest in the potential of such dry cleaning technology [ 1-5]. Wet processing of fine coal is commercially practiced and fundamentally understood [6, 7]. This type of information is not readily available for dry coal cleaning. As a consequence, laboratory-scale experiments were performed to compare dry triboelectrostatic separation with oil agglomeration cleaning. Cleaned fractions were examined petrographically to determine whether differences exist between the effects of dry and wet beneficiation. 3. EXPERIMENTAL Table 1 describes the coals that were used. Three eastern Kentucky samples (2180, 31011, 4977) provided a rank series ofpetrographic~y comparable coals. The Illinois Basin coals were lower rank and had high vitrinite and sulfur contents.

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Table 1. Raw coal proximate and petrographic analyses. Sample Coal Bed Location 2180 PeachOrchard MagoffinCo., KY 31011 Elswick Pike Co.,KY 4977 Leatherwood HarlanCo., KY 71714 Springfield (W. Ky. 9) Henderson Co., KY 8377 Herrin(Illinois No. 6) SalineCo., IL

I Macerals (vol. %) tools, ash sulfur[ V'd: Fus Sfs 4.77 8.94 0.76 149.0 10.1 24.8 0.71 5 . 9 0 2.21 158.1 10.8 15.4 2.63 17.27 0.93 160.8 14.3 12.8 10697 10.68 5.09 179.1 11.6 4.8 . 21.55 6.44 188.0 6.0 2.4

Figure 1 shows the triboelectrostatic experimental setup [1,2]. Coal samples were wet ground and sieved to a size fraction 45-75 Ixm, dried, and fed (rate: 0.1 - 1 g/s) into a carrier nitrogen gas (5-20 m/s) through a Cu triboeharger. Charged particles were deflected by an electric field. A 400 mesh screen was installed in the separator about 10 cm downstream of the electrodes to capture uncharged particles. Clean coal reported to the negative electrode, whereas mineral matter reported to the positive electrode. Five fraetions were collected from each separation test [3]. From the tube exit, deposits on the first 3 em downstream were labelled as the first dean and tailing fractions, from 3 em to 28 em were labelled the second clean and tailing fractions, and the particles captured by the screen were labelled the center fraction.

Mic 4.4 5.2 2.4 0.6 0.1

Mac 0.0 0.0 0.0 0.2 0.1

Exn 11.6 10.3 9.5 3.7 2.8

Res 0.1 0.2 0.2 0.0 0.6

Rmax] 0.73 0.95 0.87 0.45 o.69

Feeder \-,/

Carder Gas

Figure I A schematic diagram of the triboelectrostaticseparation system.

Petrographic analysis was a combined maeeral and microlithotype analysis (Table 2). The procedure used here differs slightly from the conventional microlithotype analysis. Microlithotypes are defined on the basis on the maceral composition within the 50 prn diameter circle surrounding the center point in the field of view. Many of the particles examined in studies o f fme coal cleaning, such as this investigation, are considerably smaller than 50 gin. The area counted as the mierolithotype therefore is an entire particle if less than a 50 m diameter particle is exposed at the surface. Table 2. Maceral group composition ofmicrolithotypes Monomaceral microlithotypes Vitrite (Vt) Vitrinite CV~ 95% Liptite (Lp) Liptinite (L)> 95% Inertite (In) Inertinite (I)> 95%

Bimaceral micr01itho_types Clarite (CI) V + L> 95% Vitrinertite (Vi) V + I> 95% Durite (Du) I + L> 95%

Trimaceral microlithotypes Duroclarite (D O V~ L, I; each> 5% Clarodurite (Cd) I> V, L; each> 5% Vitrinertoliptite (VI) L> V, I; each> 5%

Carbominerite (Cm) 20% < silicates, carbonates < 60% (volume) 5% < sulfides < 20% (volume)

I

I

I I 1

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Extensive studies have been conducted to investigate particle charging and the effects of parameters such as velocity, temperature and relative humidity of the carrier nitrogen, as well as the coal type and particle size. Coal samples deposited on the two copper plates and those passing through the separator were collected, weighed and subjected to proximate, elemental and petrographic analyses. 4. RESULTS AND DISCUSSIONS The petrographic analyses of two beneficiated coals (4977 and 8377) is given on Tables 3. In comparing the clean vs. tails (A vs. B, C vs. D) for the feed (F) from each sample, with the exception of sample 71714, there is a partitioning of the macerals between the clean and tails. Vitrinite was concentrated in the clean product whereas fusinite + semifusinite + macrinite and exinite were concentrated in the tailings. As expected, in the railings there is also an increased concentration of mineral matter, including silicates, sulfides, and carbonates. Table 3. Maceral and microlithotype analyses for coals 4977 and 8377. macerals and minerals (vol.

%)

Sample item Type mois. ash sulfur Vit Fus Sfs 4977 A 1st clean 3.10 4.43 0.77 83.6 7.6 3.0 200x32 B 1st tails 0.83 44.05 0.95 49.0 10.0 5.0 C 2nd clea 2.95 5.22 0.78 78.8 7.6 3.2 D 2nd tails 2.36 15.81 1.14 46.7 24.0 10.0 E center 1.20 11.37 52.7 23.3 6.3 F feed 2.14 10.87 0.87 65.8 14.2 5.4

Mic Mac Exn Res Min* Sil 2.0 0.2 3.6 0.0 0.0 0.0 0.7 0.0 5.7 0.0 6.3 ~ . 7 3.2 0.0 7.0 0.0 0.2 0.0 3.7 0.0 8.0 0.0 2.3 5.0 6.3 0.0 8.0 0.0 0.7 1.0 1.6 0.2 7.8 0.0 1.6 3.4

Sul Car 0.0 0.0 0.0 0.7 0.0 0.0 0.0 0.3 1.0 0.0, 0.0 0.0~

8377 200x32

0.2 0.0 0.0 0.0 0.0 0.2

0.0 1.2 0.2 7.4 2.0 1.0

A B C D E F

1st clean 1st tails 2nd clea 2nd tails center feed

3.67 1.73 3.52 2.07 1.75 3.31

4.27 2.20 95.8 39.66 58.2 9 . 3 1 3.56 93.4 26.54 11.40 73.1 20.46 8.08 75.8 15.07 5.31 85.8

monomacerites

1.6 4.4 1.4 8.0 6.0 5.2

0.6 5.6 0.8 3.4 2.0 1.8

0.0 0.0 0.0 0.0 0.0 0.0

1.6 4.0 1.4 1.7 3.0 1.4

0.2 0.0 0.0 17.6 0.4 1.8 0.0 4.9 0.0 8.0 0.0 2.8

Vt 49.6 34.8 43.4 21.5 24.1 36.2

Lp 0.0 0.4 0.0 0.4 0.0 0.4

In 5.6 12.2 4.0 17.6 10.5 10.1

CI 4.0 4.8 2.8 2.5 0.7 3.7

Du 4.6 7.8 4.8 13.7 17.3 7.5

Vi 6.8 1.3 7.2 5.3 8.5 6.0

Dc 25.0 12.6 29.8 16.5 22.4 21.1

Cd 1.8 2.2 3.2 8.1 6.1 4.1

Vl Cm 0.6 2.0 1.7 22.2 0.4 4.4 2.1 12.3 1.0 9.2 0.8 9.9

8377 200x32

69.4 37.3 55.6 46.4 43.2 56.4

0.0 0.4 0.0 0.3 0.4 0.0

1.4 4.4 1.0 8.8 5.9 4.5

10.4 5.3 15.5 9.7 8.6 10.1

0.2 0.9 0.0 0.3 0.2 0.0

3.6 1.3 3.6 3.4 3.8 3.1

8.4 4.0 7.7 5.3 8.6 7.4

0.2 0.0 0.2 0.3 0.2 0.2

0.2 6.2 0.4 45.8 0.0 16.3 0.0 25.4 0.2 28.7 0.2 18.1

1st clean 1st tails 2nd clea 2nd tails center feed

0.0 0.8! 0.0 0.3 1.0 0.2

mineral free) bimacerites tdmacedtes

Sampl e Item Type 4977 A 1st clean 200x32 B lsttails C 2nd clea D 2nd tails E center F feed A B C D E F

0.0 8.0 0.6 1.1 2.2 1.6

Some of the partitioning is a consequence ofmineral-maceral associations and not necessarily a consequence of maceral properties, similarly, not all of the partitioning can be ascribed to mineral association. For example, much of the inertinite in the tailings is present as a monomaceral with no apparent mineral matter. Some inertinite is associated with

1552 increased proportions of durite and, in some cases, clarodurite and vitrinertoliptite. The latter two associations are not as well established as the durite association. Mineral matter reported rather efficiently to the tailings fractions. The high pyrite coals 31011, 8377, 71714), in particular, demonstrated a high partitioning of pyrite. Sample 71714, the high volatile C Springfield coal behaved differently than the higher rank coals in that its maceral partitioning was minimal and tailings ash content was considerably lower than from the higher rank coals. Rank, and moisture content, may govern the behavior of this coal. Comparing the petrographic analysis of samples from the dry experiments to the petrographic analysis from the same coals beneficiated by oil agglomeration, we note that the triboelectric separation produced a consistently greater vitrinite enhancement in the clean fraction for all the coals except coal 4977. The fusinite + semifusinite + macrinite and exinite concentrations were decreased in the wet tailings, and were increased in the dry tailings. 5. CONCLUSIONS When using dry triboelectrostatic separation and for high volatile A and B coals, vitrinite maeeral concentrations were significantly enhanced in the cleaned fraction, whereas fusinite + semifusinite + exinite maceral concentrations were enhanced in the tailings. This partitioning is greater than that observed for the same coals subjected to wet processing. For a high volatile C coal, this enhancement was not observed in the dry separation results. Coal characteristics and process parameters leading to efficient dry coal cleaning techniques are continuing to be investigated. 6. ACKNOWLEDGEMENTS Partial financial support of the U.S.D.O.E through Pittsburgh Energy Technology Center (DE-FG22-91PC290) and the Commonwealth of Kentucky is acknowledged. REFERENCES

1. J. Schaefer, J. Stencel and H. Ban, Pro~ Ninth Annual International Pittsburgh Coal Conference, Oct. 12-16, 1992, Pittsburgh, PA, USA, pp. 259-264. 2. H. Ban, J. Schaefer and J. Stencel, Pro~ of Tenth Annum International P~ttsburgh Coal Conference, Sept. 20-24, 1993, Pittsburgh, PA, USA, pp.138-143. 3. J.L. Schaefer, H. Ban and J.M. Stencel, Pro~ of llth Annum Intn'l Pittsburgh Coal Conference, Sept. 12-16, 1994, Pittsburgh, PA, USA, Vol. 1, pp. 624-629. 4. H. Ban, J. Yang, J. Schaefer, K. Saito and J. Stencel, Pro~ Seventh International Conference on Coal Science, Sept. 12-17, 1993, Banf, Alberta, Canada, Vol. 1, pp.615-618. 5. H. Ban, J. Schaefer and J. D. Stencel, Fuel, 1994, Vol. 73, No. 7, pp. 1108-1115 6. K.W. Kuehn, J.C. Hower, G.D., Wild, and B.K. Parekh, Abstracts, lOth Ann. Meeting of The Society for Organic Petrology, v, 10, pp. 65-66. 7. J.C. Hower, K.A. Frankie, G.D. Wild, and E.J. Trinkle, Fuel Processing Technology, v. 9, pp. 1-20.