Progress has been made in recent years in the science of maceral separation. However, there are many areas that can be improved and new areas investigated. Power of density gradient centrifugation to physically resolve macerals and submaceral species coupled with other instrumental techniques is attractive for defining the limits of coal heterogeneity as well as investigating the ability of other separation methods

Dyrkacz, G. R.

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_ i_ I I ................. I .... -'; - '''''_........ ----' .......... "--= ....................... [i]! ..... -.....................................................ADVENTURES IN MACERAL SEPARATIONGary IL DyrkaczChemistry DivisionArgonne National Laboratory9700 S. Cass Ave.Argonne, Illinois 60439DISCLAIMERThis report was prepared as an account of work sponsored by an agency of the United StatesGovernment. Neither the United States Government nor any agency thereof, nor any of theirempioyec_ makes any warranty, cxpre_ or implied, or assumes any legal liability or respomi-bility for the accuracy, eompletene_ or usefulness of any information, apparatuS, product, orprocess disclosed, or represents that its use would not infringe privately owned rights. Refer-ence herein to any specific eomrnorcial preduct, process, or service by trade name, trademark.manuiacturer, or otherwise do_ not nea:ssarily oonstitute or imply its endorsement, recom-mendation, or favoring by the United States Government or any agency thereof. The viewsand opinions of authors expressed hen:in do not nece_arily state or rdl_'t thore of theUnited States Government or any age_y thereof.Tl_e lul_m|t_ nu_alta_ turnbt,e, i,ath_lbY i coatrlKtof ol slut U.S. Gov_alwnont I[ lu:co,a,,,a*v. ,t_ u. s. c.o,_,,,m,,t ,et,*m • [ur,dm ¢ontrmcs No. W-31.lO_ENG.38.I.........................1mm,uduco it.* m,bl_cl form o_ m,sceat,t_tlon, o, -,low Oth_* to de m. io_O. S.Gov_llenl purpom.DISTRIBUTION OF THIS DOCUMENT IS UNLIMITED*This work was performed under the auspices of the Office of Basic Energy Sciences, Divisionof Chemical Sciences, U.S. Department of Energy, under contract number W-31-109-ENG-38.a III I III II I I E I II II • IIII IllGary R. DyrkaczChemistry Division,Argonne National Laboratory9700 S. Cass AveArgonne, Illinois 60439IntroductionUnderstanding coal structure tasks the best researchers with the latest equipment. The problems startwith the raw materials to produce coal - the profusion of plant tissues and minerals of the peatswamps. The problems are further compounded by environmental processing of this raw materialthrough a host of biochemical, chemical and thermal transformations) The resulting coal is thenphysically and chemically heterogeneous, both in terms of the organic material and the mineralcomponents. Just how heterogeneous is not even clear. Indeed, from the organic chemistryperspective, it is not even known whether any regular structure can even be assigned to coal.Physically we divide coal into macroscopic or microscopic components called macerals as onemeasure of the physical heterogeneity. Some macerals are relatively easily associated with certainliving tissues such as spores or algae. More often the identity of the original material is uncertain.Partly this is due to the accepted methods of preparing samples for microscopic observation, butoften the original peat is so extensively degraded that any remnants of the original physical structureare gone. 2Because macerals are the first level of heterogeneity that can be readily observed, separating themto obtain more pure species would appear to be worthwhile. This is true even for a single maceraltype, such as vitrinite, which can still be heterogeneous. However, it is notable that maceralseparations are not routinely done, even when fundamental structure studies are anticipated. Thisis contrary to many other fields (e.g. plant science or bioscience areas) where physical and chemicalseparations of complex organic materials is a mandatory first stage in unraveling structures. Onereason may have been a lack of maceral separation science. One of our goals has been to put maceraland, lately, submaceral separations on a more scientific basis.The methtvAology of maceral separations can be broken down into several processes which are listedin Table 1. Each of these processes, when viewed in depth, is intertwined with complex and oftenlittle understood issues of coal's physical and chemical structure. As a result viable separationprocedures are achieved mostly by guessing, faith and a lot of work. For efficient maceraiseparations to be realized, all these subprocesses must interlock in concert. We have been able toeither thread our way through or skirt a number of the problems. However, there is still a largelyunexplored vein of information within each area that is specifically important to maceral sepa:'ations,and probably generally to coal science. A complete idea of where work is needed to improveseparations cannot be fully addressed here. Only a brief overview and flavor of where work standscan be given.DiscussionLiberationCoal constituents must be liberated from each other for optimal separation even to be possible.Without sufficient liberation some macerals may not be liberated at all. At best, they can only beenriched. Incomplete liberation also affects other constituents by limiting the pure materials thatcanbe separated. This can potentially bias the type of material that is recovered as monomaceralparticle_. F,_rexample, liptinite_ are u_u'dly difficult to liberate cleanly from vilrinites. In densityseparations, these bimaceral particles will usually be found mixed in with low density monomaceralvitrinite particles. This vitrinite is different from any of the other vitrinite material. _ Eliminatingthis portion of material to obtain pure vitrinite can lead to differences in vitrinite properties that donot reflect the original coal.Mechanical grinding has been the primary method for liberation. The small size of many maceralspecies means that the coal must be ground very fine - on the order of lO microns. Fluid energyT I I i lima "i_-pl] al IN II ill] III UT-----mills appear to be particularly effective at producing coal particles in this range. They aiso appearto cause the least amount of reaccretion or rehomogenization of the coal.4What is the appropriate stage to stop grinding? This question is not only important from the aspectof maceral separation, but can extend to how we perceive the chemical heterogeneity of coal.Ideally, grinding needs to be continued until all particles are completely monomaceral. This goalis opposed by the problem of identification of the ground material. Simple optical identificationbased on reflectance differences between the macerals is not possible below about 2 microns. Inaddition, another limiting factor is reached in the submicron range (<0.5 microns). Particles in thisrange can strongly interact with separation media giving quite different densities than larger particleswith the same density. sThe preceding demands of maceral liberation critically depend on the maceral concept and maceralidentification. But what is the nature of chemical heterogeneity within even a single monomaceralpartizle? If coal is ground to submicron sizes will new distinct particulate species appear? Thiswould suggest that macerals themselves consist of chemical domains of regular structure? Oftenvitrinite will exhibit detailed substructures, when coal samples are polished and etched. '2Alternatively, what if no further resolution occurs? This would be true if coal is so heterogeneousthat it's structure varies on a nanometer scale, or if monomaceral particles are chemicallyhomogeneous. These questions are not easy to answer, but point to a level of physical separationthat has received only a small amount of attention. _DemineralizationThe common minerals found in coal have densities from - 1.5 to 4 times that of the organic coalmaterial. Thus, a small amount of included mineral can have a large influence on maceral particledensity. Optimum separation of macerals demands that the coal be as mineral free as possible.Mechanic',d demineralization by density methods is not sufficient to remove the inorganic materials._Chemical demineralization using strong acids is the alternative, but the effect of this procedure onthe chemical structure of coal is still uncertain. Fortuitously, pyrite, which has a density of-4.5 gcmt and is par_..k:alarlydifficult to remove chemically, is nearly completely liberated from the coalparticles during fluid energy mill grinding.Wetting and DispersionMost maceral separations are based on differences in maceral densities in liquid suspensions. If coalparticles extensively aggregate in a medium, then separation will be inefficient. There are tworelated components to the problem: l.The coal particles must be completely wetted by the media.2. The particles must remain isolated from one another or undergo nearly elastic collisions over theseparation time. Wetting is a necessary condition for dispersion. Just what governs the effectivenessof a particular media for maceral particle dispersion is not clear. The result is that the dispersionremains in the realm of "shake-the-shelf" chemistry. An example of the importance of dispersioncan be seen in Figure 1.This data is taken from a study of centrifugal sink/float separations for coalin a variety of commonly used suspension media. 6 SI is a separation index, which is a measure ofthe efficiency of separation. An SI value of 0 represents no separation relative to the original coalwhile 1represents a complete disengagement of the float and sink material. A negative value is dueto aggregation effer_. RI is the recovery index and represents the fraction of float or sink phasefound after separation, relative to the maximum fraction of material expected from a perfectseparation. Thus, a value of 0 represents nc desired phase found, while 1 represents all the phaseisolated. Of the five media systems investigated, only two stood out: aqueous CsCi/Brij-35 andCa(NO3)JBrij-35. (Brij-35 is the nonionic surfactant polyoxyethylene-23-1auryl ether.) Othercommonly used media, such as ZnCI z or organic systems based on CC14.were not as effective.Contrary to expectations, recycling nz.aterial in aggregating media showed no further separation.Although Brij-35 seems to be a m_gic material that fixes all dispersion problems, it is not.Separations in aqueous ZnCI z/Brij-35 solutions were only moderately better than ZnC! 2solutions.Thus, the effect c,f the mc,.tiacan _,: subtle. Much more work is needed to understand what drivesthe interactions.SeparationThe heart of maceral enrichment is the separation step. Sedimentation methods are the mostcommonly used, particularly centrifugal sink/float. A relative newcomer on the maceral separationscene is density gradient centrifugation(DGC). This technique is superior to any type of sink/floatseparation. It has the highest density resolving ability of any maceral separation method, and] II I II III -I- " / _- 711I r" F_I1111rl I....... , f , .. llrlll ll_--......... _.functions both as a .separation and characterization method. DGC methods are definitely the methodof choice lor optimum enrichment of macerals. However, DGC methods do have limitations on theamount of material than can be obtained in any single separation cycle. This can be overcome byfirst using sink/float methods to isolate desired density regions.In some studies high density resolution of macerals may not be necessary. Thus, sink/floattechniques will remain an alternate separation method or at least an adjunct to DGC separations.With this in mind, we have recently exanfined the process of centrifugal sink/float (S/F} separationsto understand its limitations and provide guidance in its use. _'7. In addition, we have explored indetail a continuous flow centrifugation (CFC) technique that allows much larger amounts of materialto be separated in a shorter time than by simple centrifugal sink/float. _'° Both studies could not havebeen done without the resolving power and speed of analytical density gradient centrifugation(ADGC) methods. Each study required over two hundred separate ADGC analyses of float or sinkphases.Several general observations emerged from the sink/float studies. The effect of media has alreadybeen discussed. Figure 2 displays the ADGC results of a complete separation sequence on a singlehigh volatile bituminous coal using CsCI/Brij-35. The vertical bars represent the nominal solutiondensities used for each fractionation. This particular separation sequence was from low density tohigh density. Note that several fractions are not very pure. This is a consequence of two effects.First, even though a density separation may have a constant separation efficiency (i.e., the fractionof mass that will report to the proper phase), the amount of contamination depends on the efficiencyfactor and the density d_stribution. Hence, separations made near large bands will be morecontaminated than separations further from the mr.m band. We were able to predict the purity offractions based on the former effect with only moderate success. The second general observationis that float phases invariably contaminates the sink phase due to solution instabilities during thecentrifugation process. Much more work is needed to optimize and predict the dynamics of suchseparations. In the course of this work, we also realized that the density interval of a fraction canalso dramatically effect the _,urity. The narrower the density interval between successiveseparations, the more contaminated will be the resulting fractions. This is due to the fact that theseparation inefficiency is constant, but the mass of a fraction will be smaller as the density intervalbetween separations decreases. DGC is .notas affected by this phenomena; this is one reason for itshigh resolving power. Generally, the S/F separations produced fractions with purities >60 %. Manyfractions were routinely better than 80% pure.Continuous flow centrifugation separations, another version of sink/float separations, had verysimilar constraints to the simple centrifugal sink/float. Even the overall fraction purities were aboutthe same. CFC separations are carried out in a special centrifuge that allows the continuous removalof liquid from the rotor. Float material is entrained with the expelled liquid, while sink material isretained in the rotor (or in more sophisticated centrifuges, separately ejected). Very dilute solutionscan be used so that particle aggregation can be minimized. Single density separations with up to300g of coal can be accomplished within a day. This is far more than can be conveniently handledby simple centrifugal sink/float methods for finely ground coal. We believe that the efficiency ofCFC separations can be substantially improved. For one the rotor dynamics appeared to be morecomplicated than generally believed. We suspect that there are regions within the separation sectionof the rotor where the particles are not subjected to the expected flow and centrifugal dynamics.Detailed studies of this device are needed to corroborate this. Modifications to the rotor may benecessary to increase the efficiency.IsolationIsolation is included in Table l mainly because of its impact on time for a complete separation cycle.The largest fraction of time in the separation of ultrafine coal is spent filtering and washing theseparated samples. It ;_soften wise to use membrane filters to avoid selective loss of fine coal, whichfurther slows the filtration process.SummaryProgress has been made in recent years in the science of maceral separation. However, there aremany areas that can improved, and new areas to be investigated. The power of DGC to physicallyresolve macerals and submaceral species coupled with other instrumental techniques is particularlyattractive for defining the limits of coal heterogeneity as well as investigating the ability of otherseparation methods.1 I IIIN I [[[ iii IAcknowledgementThis work was performed under the auspices of the Office of Basic Energy Sciences, Division ofChemical Sciences, U. S. Department of Energy, under contract number W-31-109-ENG-38.References1. Stach, E., Mackowsky, M.-Th., Teichmuller, M., Taylor, G. H., Chandra, D., Teichmuller, R.,Murchinson, D.G., and Zienke, F., 'Stach's Textbook of Coal Petrology', 2nd Edn., GebruderBorntraeger, Berlin, 1975.2. Kroger, C., Erdol und Kohle Erdgas Petrochem., 1964, 17, 802-812.3. Dyrkacz, G. R., Bloomquist, C. A. A., and Ruscic, L. R., Fuel, 1984, 63, 1166-1173.4. Dyrkacz, G. R., Bloomquist, C. A. A., in preparation.5. Dyrkacz, G. R., and Horwitz, E. P., Fuel, 1982, 61, 3 -12.6. Dyrkacz, G. R., Ruscic. L.R., and Fredericks, J., Energy andFuels, 1992, 6, 720-742.7. Dyrkacz, G. R., Ruscic. L.R., ibid., 1992, 6, 743-752..8. Dyrkacz, G. R., Bloomquist, C. A. A.,ibid, 1992, 6, 357-374.9. Dyrkacz, G. R., Bloomquist, C. A. A.,ibid., 1992, 6, 375-386.10.Dyrkacz, G. R., Bloomquist, C. A. A., Ruscic. L.R.,ibid., 1992, 7, 655-660.Table 1. Coal Maceral Separation Subprocesses.Liberation of constituentsDemineralizationWetting and dispersion of maceral particlesSeparationIsolation1.1o• 0_ • e• • •0.9• o • t-o3 • 0• • o83 • •m • CsCl/BrJJ-350.7 •I_ • C8ClBmBenzenelCCl4m¢/) 0.5- m[] _ [] e_ZnCl2[] [3Ca(NO_)=/BrlI-350.3 •@ •0.1 • • []-0.1-0.0 0]" 0:4 0:6 0]8 ,:0 112 ,14RIFigure 1. S/F Separation and recovery indices for float fractions from three high volatile bituminouscoal.PSOC.732I,11] I 1 1 I I_l-'rH I _ [ l_v,..IFI I I I I I I I 1 I I I I 1 1 i'_..JJ,_l I I I I I t I I I I I 1 I I I I%,,fFrrl I I I I I I I I I I I I I I I I l_l_.__'1 ri i i i i i 1 i i i i i LI I I I_LI I ["?_._, ,'_"__ ' 'i''_; I ' " ",' "" "," 't ll,,,,_-T ,Ir: .... ' ' _'_" .... ' .... I._: ii ! p 1.240 1.294n--F- .... _ ' ' ' I ' ' 'i,/  1.33,o1.1 1.2 1.3 1.,, _.sDensity (g cm "a)Figure 2. Sequential S/F Separation of a high volatile bituminous cool distributions are normalizcdto peak values.

Adventures in maceral separation

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Adventures in maceral separation

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