Energy & Fuels 1993, 7, 655-660
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Investigations into the Process of Maceral Separation by Centrifugal Techniques. 3. Continuous Flow and Sink/ Float Maceral Separation of -100-mesh Coal? Gary R. Dyrkacz,*C. A. A. Bloomquist, and Ljiljana Ruscic Chemistry Division, Argonne National Laboratory, 9700 South Cuss Avenue, Argonne, Illinois 60439 Received March 31, 1993. Revised Manuscript Received June 30, 199P
Centrifugal sink/float and continuous flow centrifuge separations were conducted on a -100-mesh high-volatile bituminous coal. Using CsCl/Brij-35 as the separation media, float and sink phases could be enriched to greater than 75 % pure phases. This enrichment was less than expected under the separating conditions employed. The reasons for this behavior were not clear. The float and sink phases were monitored by analytical density gradient centrifugation. The gradient was documented by both the standard flow cell method and direct imaging of the density distributions.
Introduction In two previous series of papers, we investigated two types of biphasic density separations of macerals: continuous flow centrifugation (CFC) and simple centrifugal sink/float (S/F).14 In both series, the feed coals were ground in a fluid energy mill and subsequently chemically demineralized. The mean particle size of the volume distributions were about 3 pm, with a top size of 10 pm. Such ultrafine particles place severe constraints on separations. The liquid carrier media must be very efficient at not only wetting the coal but also maintaining a good dispersion over the time needed for separation. Moreover, the slow sedimentation of very fine particles demands high rotor speeds to transport particles in a practical time. In addition, fine particles are difficult, or, for the ever present submicrometer material, impossible to identify by traditional optical microscopic methods. Even when the analyses can be done, the morphology has usually been lost. Only the three maceral groups can then be identified. Thus, large particles (>lo pm) would be much easier to work with. Opposing the advantages of separating large maceral particles is the problem of achieving efficient maceral liberation. Our experience has been that inertinites readily break away from vitrinites during comminution. However, the liptinites often remain firmly attached.s76 As a consequence, the yield of monomaceral particles will be quite small. Only severe mechanical grinding appears to liberate the liptinites from the vitrinites. Other commit Thie work was performed under the auspices of
the Office of Basic Energy Sciences, Division of Chemical Sciences, U.S. Department of Energy, under contract no. W-31-109-ENG-38. Abstract published in Advance ACS Abstracts, August 15, 1993. (1) Dyrkacz, G. R.; Bloomquiet, C. A. A. Energy Fuels 1992,6,357374. (2) Dyrkacz, G. R.; Bloomquiet, C. A. A. Energy Fuels 1992,6, 374386. (3) Dyrkacz, 0 . R.; Ruscic, L. R.; Fredericks,J. Energy Fuels 1992,6, 720-742. (4) Dyrkacz,G. R.; Ruscic, L. R. Energy Fuela 1992,6,743-752. (5) Dyrkacz, G. R.; Horwitz, E. P. Fuel 1982, 61, 3-12. (6) Choi, C.-C.;Dyrkacz, G. R. Energy Fuels 1987,3,579-585.
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nution methods such as chemical comminution may work, but they are not sufficiently developed for routine use.697 If pure macerals are desired from large particle feeds, there are still compromises on particle size that must be made. Some grinding must be done. How much is necessary will depend on the type and size of the macerals in the coal sample. For example, a coal sample containing microspores will require more comminution than a coal containing macrospores. With a trimacerite coal, maceral separation of 1mm coal particles will likely produce very little free liptinite and only a small amount of free inertinite. Microlithotypes rather than individual macerals are more likely to be liberated. The individual macerals will be liberated only with further comminution. Taulbee and co-workers separated -100-mesh (149 pm) particles into a series of fractions and recovered about one-third of the original liptinite in high purity (>89%).* In this -100-mesh size range, many macerals will retain enough morphology to be identified with certainty. Nevertheless, there will always be some fraction of particles whose size is too small to clearly identify the maceral type. In spite of this, a -100-mesh coal seems to be reasonable starting point to obtain some pure maceral species. Centrifugation to promote separation is still advisable in this size range. To extend our effort in this area we felt that an investigation of centrifugation separations of -100mesh coal would be an interesting adjunct to our ultrafine separation work.
Experimental Section Separations. The details of both the continuous flow separation and sink/float separation procedures have been previously published.I4 There were no changes to these procedures except in the manner the coal was dispersed in the aqueous solutions. The coal samples were dispersed by gentle stirring overnight. This avoided any further particle size reduction. However, no separation differenceswere noted when (7) Fremont, W. J. J.; Chermin, H. A. G.; Joziasse, J.; Nater, K. A. Proc., 1989Int. Conf. Coal Sci., Oct. 23-27, Tokyo, Jpn. 1989,113-116. (8)Taulbee, D.; Poe, S. H.; Robl, T.; Keogh, B. Energy Fuels 1989,9, 662-670.
0 1993 American Chemical Society
Dyrkacz et al.
656 Energy & Fuels, Vol. 7,No. 5, 1993 this dispersion method was compared to mild ultrasonic dispersion of the particles. The general operation was as follows: All the sink/float runs were performed in 50-mL centrifuge tubes using CsCl/Brij-35(polyoxyethylene-23-laurylether). The concentration of Brij-35 was 8 g/L. The coal was dispersed in the solution, and centrifuged for one hour at 10 000 rpm (15 700g). A t least one-half of the tube contents was removed as the float material. Analytical density gradient centrifugation (ADGC) of the separated coal phases was as previously described with only one significant change.' Whereas usually 2 mg of coal was layered on a gradient, the larger particle size material required at least 4 mg of coal. Sample sizes from 4 to 10 mg per centrifuge tube were found not to have any effect on subsequent purity values. The ADGC procedure consists of mechanically forming an aqueous CsCl gradient in 50-mL centrifuge tubes. Brij-35 (8 g/L) is present to maintain a good particle dispersion. The coal is layered on top of the gradient and the tube centrifuged for 1 h at 10 000 rpm (15 700g). The gradient is then pumped out of the centrifuge tube through an absorbance monitor and a flowthrough density monitor. The absorbance has been shown to be proportional to the mass of material. The resulting density distributions can be integrated to provide the purity of the separated phase being analyzed. Particle size measurements were made with a Model ll2LTSD Elzone particle size instrument (Particle Data Inc.) and a 95-pm orifice. Photography and Measurements of the Gradients. Besides the standard method for analyzing the gradients,' two related methods were used to monitor the density distribution after density gradient centrifugation of the material: In the first, the centrifuge tube was placed in front of a light table and the gradient photographed with a Polaroid MP-3 camera system; Type 55 4 X 5 positive/negative Polaroid film was used. The resulting negatives were manually scanned using a Leitz MPV-3 microphotometer with a 3.2X objective. The transmitted light was masked such that a 1cm long X 1mm wide slit was used to scan the bands (1 mm in the direction of changing density). Transmission values were read at 1-mmintervals, which roughly correspondsto 0.01 g cma. The resulting data were then corrected against a blank negative of the light source and converted to absorbance values using photographs of neutral-density filters. In a second technique, the centrifuge tube was directly photographed against the light box with a CCD camera (Javelin, ModelJE3462RGB)coupled to a frame grabber (Univision,Model Scorpion) installed in a PC and driven by appropriate imaging software (Media Cybernetics, Image-Pro+). Image resolution was 640 X 480 pixels, with 256 gray levels. After background correction, a standard thick profile routine was used to display the coal distributions. This routine generated averaged pixel intensities perpendicular to the gradient as a function of length parallel to the gradient. The area taken for measurement was about 80%of the centrifugetube width. This operationsmoothed out the noise due to individual large particles. In both types of optical measurements, because the image was not scanned there was some distortion of the actual density distribution due to optical parallax. To minimize this, all images were taken with the high-density edge of the coal distribution colinear with the camera lens axis.
Results and Discussion Coal Sample. The coal chosen for this work was APCS 7, a high-volatile bituminous coal from the Argonne Premium Coal Sample Program. The -100-mesh coal was used as received for all separations; no further grinding or demineralization was done. This coal was chosen because it represents a reasonable maceral content for most coals that might be considered for maceral separation. It has also been used in our previous fine particle investigations.
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Figure 1shows the analytical density gradient centrifugation (ADGC) pattern for this coal. For comparison, the fluid energy mill (FEM) ground, chemically demineralized version of this coal is also shown. Note that the free liptinite which should be in the 1.10-1.24 g cm4 region is almost nil in the large particle sample, compared to the processed material. It is also clear from the data that the mineral containing vitrinite must be mixing into the inertinite area. The particle size distribution is shown in Figure 2. The data were obtained from the original APCS 7 coal making sure to rotate the ampule to obtain arepresentative sample. The data differ significantlyfrom that reported by Vorres? The volume average particle size was 32.1 pm, while the number average was 16.3 pm. The number average is expected to be on the high side; electronic noise problems prevented us from obtaining reliable size data below about 10 pm. However, the volume data were less affected by this problem. From Figure 2, nearly all the material is less than 100 pm in size. For the present study we felt that this -100 mesh coal would be reasonable for testing separations of larger particles. The average particle size of this material is in a range where some maceral morphology would still be preserved. S/F Separations. Based on our previous sink/float separations we selected CsCl/Brij-35 for the S/F separa(9) Vorres, K. S . Users Handbook for the Argonne Premium Coal Sample Program, Oct 1989.
Maceral Separation by Centrifugation Table I. Separation of -100-mesh APS 7 Coal at 1.280 g cmJ yield ( % )C purity ( % ) run concn,agJL float sink float sink 1 50 16.8 91.8 8.1 82.8 2 50 11.0 94.2 1.7 81.2 ~RF*I~ 81.9 62.9 92.8 1.1 3 50 16.2 93.1 6.3 81.1 3RF 81.9 51.4 12.2 26.1 4d 60 15.2 94.1 13.4 18.9 5 10 82.6 92.2 9.0 85.0 6 200 64.6 82.5 14.1 85.3 a Starting concentration of coal. RF = recycled float phase at same density. c 4 h centrifugation. d -74 pm, +20 pm size fraction. a Yields are based on amount of feed coal used.
ti0ns.39~Centrifuge conditions were 10K rpm for 1h. Under these conditions nearly all the particles would have ample time to report to their proper phase. Although there were particles smaller than we could measure with our instrumentation, on a volume basis they constituted only a small mass fraction of material (Figure 2). We expected that any spherical particles over 2 pm in size would reach their respective phase. After centrifugation, S/F separations done at high coal concentrations (150 g/L) always resulted in a nearly clear liquid. The coal pelleted at both the top and bottom of the centrifuge tube. The clear solutions made it easy to avoid phase contamination during removal of the float phase. Often some coal particles were distributed throughout the solution, but this distribution was quite uniform. Particles in this liquid region must have densities so close to the solution density that their motion during centrifugation was negligible. The amount of this material could not have been more than a few percent of the coal material. No indication of mixing due to “stringers” was evident as observed for the ultrafhe particle S/F separations. Most of the particles apparently were large enough to effectively resist destabilizingforces during the centrifugationprocess. Alternately, the particles may have been prevented from mixing by being very tightly compacted in the float or sink pellets. At low coal concentrations (10 g/L), the solutions were often not as clear and streaming was observed. This is in line with what we found with the ultrafine particle separations.3 Formation of a coal pellet may have a strong influence on the contamination of the sink material. The concentration of coal was probably too low to form a pellet. Thus, mixing occurred. Consequently, in all runs approximately 80% of the fluid phase was removed as the float phase. The analysis of the separated fractions was done by ADGC before the fractions were filtered and dried. Purities were derived from the ADGC data by integration of the appropriate areas under the density distribution curves. These purity values are not maceral purities but represent a measure of the contamination of float material in a sink phase or sink material in a float phase. Table I shows the S/F separation results at a density of 1.280 g cm-3. From Figure 1,this density is on the lowdensity edge of the distribution. Such a separation approximately mimics the separation of liptinite from vitrinite. From our previous work, we also knew that this area would be near the lowest expected purity. However, simple calculations showed that phase purities of greater than 99% could be e ~ p e c t e d . ~
Energy & Fuels, Vol. 7, No. 5, 1993 657 Table 11. Fraction of Material Expected at Various Densities yield ( % )a density (g cma) float sink 1.260 2.1 97.9 1.280 8.O(19.gb) 92.0 1.300 24.8 15.2 1.330 41.6 58.4 1.400 74.3 25.1 a Data is derived by integration of the APCS I data in Figure 1. For +20 pm, -150 pm fraction. ~
Because of the monitoring difficulties with large particles, several runs were done to check the repeatability of the data. As can be seen from runs 1-3, the phases showed excellent reproducibility. In contrast to our earlier work on fine coal particles (
