Energy & Fuels 1992,6, 720-742
720
An Investigation into the Process of Centrifugal Sink/Float Separations of Micronized Coals. 1. Some Inferences for Coal Maceral Separations Gary R. Dyrkacz,' Ljiljana Ruscic, and John Fredericks Chemistry Division, Argonne National Laboratory, 9700 South Cuss Avenue, Argonne, Illinois 60439 Received March 5, 1992. Revised Manuscript Received July 7, 1992
The process of maceral separation using sink/float centrifugal methods was investigated for several bituminous coals with particle sizes 1h) with large grinding media can also lead to accretion. Particle Size Measurements. The particle size distributions were all measured using a Coulter LS laser particle size analyzer. This device directly measures the volume distribution. Comparison with volume data from a Coulter Multisizer gave fairly close agreement, when compared on a volume basis only. The data should be considered as approximate, because there has been little investigation of how accurately coal particle distributions are determined by such equipment.* Centrifugal Separations. Standard, nonscrew cap, 50-mL centrifuge tubes were used for all of the centrifugal separations. The straight tubes were used because they allowed easier access to the float phase. All of the aqueous solution separations were done in polycarbonatetubes, and the organicsolution separations were done in either polypropylene or Tefzel tubes. Runs were usually done in duplicate. The tubes were filled immediately after ultrasonic treatment with about 42 mL of the slurry and then placed into the centrifuge rotor. Beckman JS-13, fourposition, or JS-13.1, six-position, swinging bucket rotors were used for most of the work (at 10 OOO rpm with r- = 140 mm; the relative centrifugalforce is 15 700g.) A Beckman JA-20eightposition angled rotor was used for some early work. The centrifuge used for all the separations was a Beckman high-speed 52-21refrigerated centrifuge,having a maximum operating speed of 21 OOO rpm. The centrifuge was also equipped with a slow acceleration/slowdeceleration control unit. After the solutions were centrifuged for an hour, the speed was reduced without braking to 1OOO rpm. The deceleration unit then automatically engaged to stop the rotor in a 30-min period, reducing the possibility of mixing due to rapid rotor braking. The tubes were then removed from the centrifuge, and the float phase was segregated from the sink phase in a manner similar to that described by Bird and Messmore? or Palowitch and NasiatkaSDbThe float material was sucked from the centrifuge tube using a piece of flexible tubing and a simple vacuum flask. A rigid piece of polypropylene tubing, connected to the flexible tubing, was used as a skimmer. The vacuum was adjusted so that there was just sufficient action to gently remove the solution. The technique of removing the float fraction does require some care. Phase mixing is an ever present problem. The solutions should be subjected to as little shaking or vibration as possible. Often, a hard pellet of floating coal would form on top of the solution. This pellet could be removed by very carefully running a spatula around the sides of the centrifuge tube and lifting out the freed pellet. The skimmer tube should not be jammed into or below the surface of the solution. The best technique is to very gently skim over the receding liquid surface. The float and sink fractions were filtered using 0.8-pm polycarbonate membrane filters (Nucleopore) and then washed with water and finally ethanol. The weight error between duplicate runs was generallyunder 5 % . However, with fractions where only a small amount of coal was expected (10 pm is anomalous. It is not observed in electrozone particle size measurements.
Grinding and Demineralization. Grinding is a critical phase of any particle separation operation, and maceral separation is no exception. Dyrkacz and Horwitz have previously discussed the problems of fine grinding to liberate macerals. The current discussion is intended only to reinforce and amplify this discussion.6 Often a major problem when ultrafine grinding coal is the fusing of very fine particles into larger particles. This complication is due to the large instantaneous localized temperatures and pressures generated when small coal particles are impacted by massive grinding media. Since the purpose of the grinding operation is to liberate the macerals from one another, this phenomenon must be avoided. Fluid energy mills (FEM) (jet mills), either of the centrifugal type (e.g., Sturtevant) or the opposed jet type (e.g., Trost/Garlock), seem to be one class of mills that avoid this problem. We have never microscopically observed any indication of particle reagglomeration due to grinding with either of these mills. Figure 2' shows a size distribution curve for the FEM ground coal used in this work. We have also found that slurry grinding in a planetary ball mill also can lead to fairly effective maceralliberation.13 (13)Dyrkacz, G. R. Unpublished work.
A planetary ball mill is similar to a rolling ball mill, but in the former case the bowl is revolved eccentrically at higher speeds, producing a much more violent and faster grinding action. With slurry grinding, it is relatively easy to obtain a particle size range quite similar to that produced in jet milling. (See Experimental Section for details.) However, comparisons of density gradient distributions for coal samples ground by either ball milling and jet milling indicated that jet milling provided somewhatbetter overall liberation of macerals, although both particle size distributions were nearly the same. Inefficient liberation was particularly evident in the liberation of liptinites. Even with fluid energy milling, we have consistently seen that the liptinites have a lower liberation rate from vitrinite compared to inertinites. All of the coals used in this study were subjected to chemical demineralization to remove mineral matter (except pyrite). There are always objections to this procedure, but we still maintain that high-efficiency separation of macerals is hampered by the presence of the much higher density inorganic componentsP There is still no definitiveevidenceavailable on just how much chemical damage to bituminous coals results from contact with the concentrated acids. On the other hand, our fine grinding procedure can liberate much of the mineral matter, while the density gradient separation is quite effectiveat separating it. Other than alteration of the density patterns, there is no agglomerationevident in the CsCl/Brij-35gradients when using natural coals. This is also true for undemineralized Victorian brown ~ 0 a l s . l Thus, ~ other than achieving optimal maceral separations, there is no inherent reason why mineral matter containing coals cannot be separated. This conclusion is a particularly relevant point for washability studies. Analytical Density Gradient Centrifugation (ADGC). There are two ways to approach the problem of determining the efficiencies of sink/float separations. One is by using maceral analysis, but, as Dyrkacz and co-workers have shown, there is a natural density overlap between maceral gr0UpS.6*'5 In addition, there is always some overlap of macerals due to the presence of multimaceral particles from imperfect liberation. These prob lems conspire to make understanding of the sink/float separations based on maceral analysis confusing. An alternate analytical method is to monitor the density variation of the sink and float fractions. This technique is often used in washability studies, where the float or sink fraction from a separation is subjected to a series of further sink/float density cuts to determine the efficiency of a process. But this assumes that the analytical sink/ float operation is optimal, which is what we wish to study. (High S/F separation efficiencies may be true for large particle sizes, but they are rarely confirmed for small particle^.)^ Density gradient centrifugation (DGC) techniques offer us a way around the problem. These techniques have the highest density resolving power of any density separation technique. For most coals,we have found that at least 95 % of the particles report to the proper density position. Agglomeration, a chief cause of concern in fine particle separation, can be easily observed in small(14) Anderson, K. B. Thesis. The Chemical Characterization of Macerala Isolated from Australian Coals and Sediments; University of Melbourne, 1989; pp 29-39. (15) Dyrkacz, G. R.; Bloomquiet, C. A. A.; Ruscic, L. Fuel 1984, 63, 1367-1374.
726 Energy & Fuels, Vol. 6, No. 6, 1992 -””
4
Dyrkacz et al.
I
100
“1
Vltrlnlte Exlnlta
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0
1.0
1 1.1
1.2
1.3
1.4
1.6
Density (g cmm3)
Figure 3. Preparative separation of PSOC-732 showingthemaceral distribution. Inset bar graph represents the original maceral concentration from Table 11.
scale density gradient separations. Thus, since its introduction into the area of maceral separation, we have built up a great deal of confidence in density gradient techniques. We have chosen to use what we have termed analytical density gradient centrifugation (ADGC)to study the S/F variations,because of ita convenienceand rapid turnaround as an analytical tool. Only a few milligrams of material are needed for an individual analysis. The concentration of separated coal material in the gradient is monitored by an absorbance monitor and the density by a density monitor which directly measures the density.6J0 This technique contrasts with “preparative” DGC where multigram amounts of coal can be separated in a special zonal rotor. One aspect of the analytical DGC technique that was important for our analysesis the relationship between the measured parameter, absorbance, and the amount of coal. Strictly speaking, the absorbance is proportional to the shadowarea of each particle (A, = r R 2 ,assumingspherical particles) and the number of particles per volume of the cell through which the light is passing. The shadow area is, in turn, related to the volume of the particle ( T R A ~ ) . Because a weight basis is more useful for comparison purposes, the absorbance data are converted to a weight distribution by multiplying each absorbance point by ita corresponding density. Two complications can arise in the analytical density data that might affect the accuracy of the results: (1)The macerals may have different absorptivities at different densities,and (2) there also may be a particle size variation within the S/F fractions analyzed. Considering the first complication, comparisons of analytical density gradients and the corresponding preparative density gradients for the same coal sample show little definitive differencesin the patterns.1° Preparative DGC distributions are based on direct weight measurementa of fractions. ADGC and preparative separation data for PSOC-732 can be compared from Figure 1and Figure 3. There are noticeable differences in density resolution between the two types of density gradient experiments. We believe the differences are caused by interactions of the particles with the walls of the centrifuge tube, both upon centrifugation and upon subsequent pumping of the gradient out of the centrifuge tube. The result is a slight smearing of the density band as observed for the ADGC pattern in Figure 1. Due to recent changes in our ADGC
technique, there can also be problems arising from differences in resolution between the different types of density gradient methods. Preparative runs are based on a limited number of fractions with -0.01 g cm-3resolution, compared to -0.001 g cm-3 resolution for the ADGC data. The second complicationwith the DGC distributions is a nonlinear variation in mass with absorbance due to particle size differences at different densities. This condition can arise either because of a natural difference in particle size as a function of density in the original coal or separated fraction, or because there was insufficient centrifugation time for all particle sizes to report to their respective position. In the latter case, there are two subcases: one, in which insufficient time is allowed for the S/F separation and, second, in which insufficient time has been allowed during the ADGC separation. In the latter separation, gradient runs with 1and 3 h of centrifugation time showed no significant differences in density distribution. If insufficient time is allowed for complete S/F phase disengagement,we will find that, in the float fraction, the sink contaminant will have a much higher proportion of fines than the bulk of the true float phase. To find how this particle size differentiation would effect the absorbance, two different types of experimentswere done. From a midfraction (vide infra), where differentiation of large particles and fines was expected, the absorbance pattern was obtained from an ADGC run using a 10-mg sample. The fractions of this ADGC separation corresponding to the liptinite and vitrinite were separatelycollected,filtered, and weighed. An average absorbance for each fraction was obtained from the density gradient pattern. Dividing the absorbance by the weight data, for both the liptinite and vitrinite data, gave “absorptivity” values, which differed by 2% from the average value. In a different experimental approach, we previously compared the absorbance data for individual fractions with the individual weight fractions from a preparative run.1° We found fairly close agreement between the two separations. However, generating such weight/absorbance comparison data had severalinherent difficulties. The most important problem was accurately collectingand weighingmilligram amounts of material, at either end of a density distribution. Thus, it is difficult to provide an absolute error for the ADGC technique. We believe the error between absorbance and weight distribution is probably no greater than 5 % . Nevertheless, the results indicate that the ADGC absorbance curves, when weighted by the density data, do represent the weight distribution of the coal sample. Treatment of the ADGC Absorbance Data. The raw absorbance data, which we obtained from computerized data acquisition, were scaled, and the corresponding densities were converted to density values at 25 “C. The absorbance scaling was necessary because the absorbance range of the absorbance monitor could be varied over 3 orders of magnitude, although, in practice, a range of 20 was more usual. All the purity data presented here are based on converting the absorbance data to ita weight equivalent using the densities. After baseline correction, the distribution data were then interpolated to a constant density interval of 0.001 g ~ m - ~The . resulting density distribution was then normalized to the total weight of the sink or float fraction. Integration to find the floatand sink-phase purities was done using the one-third Simpson’s rule method. The phase purity was defined as the
Separation of Micronized Coals. 1
integrated area under the density distribution curve to the low- or high-densityside of the solutiondensity divided by the integrated area for the entire phase curve. This method directly provided a measure of the amount of sink material in the float phase or float material in the sink phase. Meaning of Density for Maceral Separations. Our coal density distributions are all defined in a phenomenological sense. The pattern of the density distributions may be specific to the suspension media used and, thus, an apparent density distribution is what is being observed. The ‘true” density of coal, defined as the weight per unit volume of the pore free solid, is most closely approached by the helium density.l6J7 Our potentially sliding density scale in terms of the density gradient analysis has been previouslydiscussed.la Becauseour reported coal densities may be based, in part, on complicated particle/solute or particle/solution interactions, the problem may arise that an S/F separationmay occur at a different apparent density than the nominal density we use for the separation. Under such circumstances, we expect to see a drop off or break in the phase density distribution data at a point different from the nominal S/F (CsCVBrij-35)density. Although the actual ADGC data for all the separations are too numerous to display conveniently,we were careful to watch for this potential problem. General Sink/Float Separating Conditions. Although S/F centrifugation is, in principle, a simple technique, there are many variations that can be used. The better the separation that is anticipated, the more rigorouslythe separation conditionsneed to be controlled. Nevertheless, we chose to keep our separation system as simple as possible, basing our work on uncomplicated techniques and using equipment that should be widely available. For simple centrifugal S/F work on very fine particles ( Rij > R m i , will result in contamination of the float phase. In a uniform
The calculation of the amount of sink in the float phase and the amount of float in the sink phase is then carried out over a suitable number of density difference intervals (O.OOl-O.5OOgcm-3). (Viscosity is assumed to be constant with the value for water. This is approximately true for CsCl solutions.) The calculation results in two curves as a function of the difference in density between particles and the solution,which we designate as mass decay curves. The curves are independent of the density distribution of the coal, and are only a function of the particle size and the density difference interval. Figure 23 shows a typical set of mass decay curves for the function of float material in the sink phase and sink material in the float phase. We can now use the mass decay curves with the experimental density distribution curves, to find the fraction or percent of sink material that will contaminate the float phase or, conversely, the amount float material that will contaminate the sink phase. From mass balance considerations, the amount of sink in the float phase at a density j is
f (PjlxFj
(14)
and the fraction of sink material in the float phase is
Dyrkacz et al.
742 Energy & Fuels, Vol. 6,No.6,1992
to just balance the amount of float material: PT
=
mF
+ mS
m,+m, PF
where k is the index of the solution density and j runs from 1 to k - 1. Likewise, the fraction of float material in the sink phase is
PS
mF f=mF+ms
Combining these two equations in terms of the mass of sink material we find
where j runs from k
+ 1 to the last density point. Appendix B
The reason that a randomly aggregating system of particles will show a higher purity for the low-density material can be shown using the definitions of density and the definition of the fractional purity. For the present discussion we will only consider how the purity of the float phase will vary. The aggregate phase is composed of two componente: the float material which will be considered as all going to the proper phase, and the contaminating sink material, which will be added to the float phase in the proper proportion to just match the solution density. The density of the pure float phase can be found by integrating the density distribution from the first density point to the solution density. The density of the sink material can be found by integrating the distribution from the solution density to the last density point. The problem is to find how much sink material is needed
where mF is the mass of the float phase, ms the mass of the sink phase, p~ the density of the float phase, ps the density of the sink phase, p~ the density of aggregate (solution density), and f the fractional purity of the float phase. If for discussion purposes we hold the solution density and the density of the float phase constant, then (B-3) becomes f = K( 1 -
2)
(B-4)
If the density of the sink material increases, the purity will then increase. Increasing the sink material density
is equivalent to saying there is more mass distributed at higher density. Less sink mass is then needed to achieve the solution density, and the purity is correspondingly higher. Regirtry No. CsCl, 7647-17-8;CaN03, 10124-37-5; ZnClz, 7646-85-7;Brij-35, 9002-92-0.
