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COMPREHENSIVE INVESTIGATION OF THE LIBERATION CHARACTFXISTICS OF PYRITE AND OTHER

MINERAL MATI'ER FROM COAL

DE-FG22-95PC95220

University of Utah

Reporting Date: 07/30/96 Grant Date: 07/0 1 /95 Completion Date: 06/30/98 Award 1995/1996: $100,914

Program Director: Principal Investigator: Contracting Officer's Representative:

Professor R. Peter King Professor R. Peter King J o h C. Zysk

Report for Period 01/01/96 -06/30/96

BiSTRIBUTiON OF M1S.DOCUMEhR IS UNLIMITED 27

T U

DISCLAIMER

This report was prepared as an a w u n t of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied. or assumes any legal liability or responsibility for the accuracy, completeness, or use- fulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any spe- cific commercial product, process, or service by trade name, trademark, manufac- turer, or otherwise does not necessarily constitute or imply its endorsement, ream- mendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not neassarily state or reflect those of the United States Government or any agency thereof.

DISCLAIMER

Portions of this document may be illegible in electronic image products. Images are produced from the best available original document.

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Objectives

The objectives of the project are:

1.

2.

3.

Develop a set of laboratory techniques for the quantitative measurement of the liberation spectra in samples of coal particles in the size range 50 pm - 1 mm. Three- dimensional spectra will be required to account for the mineral matter and pyrite separately . Establish, experimentally, the Andrews-Mika diagram for a number of typical U.S. coals and to develop an appropriate parameterized description for the Andrews-Mika diagram so that it may be determined easily and quickly for coals of different origin and different type.

Establish an effective and reliable simulation technique so that liberation of both pyritic sulfur and ash during comminution operations can be modeled and the operation of coal preparation facilities-simulated. These models will be incorporated into a computer simulation system for coal cleaning plants.

Summary of Technical Progress

1. Experimental

A 710-1000 micron size fraction of a sample of Pittsburgh #8 coal was carefully fractionated using precise dense-liquid separation techniques. The resulting washability curve is shown in Figure 1. The sample was fractionated at 1.25, 1.27, 1.29, 1.31, 1.33, 1.35, 1.37, 1.39, 1.50 and 1.60 grams per cc. The 1.29-1.31 fraction was then crushed in the ultrasonic miU in the Comminution Center laboratory and progeny larger than 38 microns were fractionated at 1.25, 1.27, 1.29, 1.31, 1.33, 1.35, 1.37, 1.39, 1.45, 1.50 and 1.60 grams per cc. Each of these fractions was then screened at 500, 355, 251, 180 and 106 microns-so that the complete disposition of the progeny particle population could be established. These data define what may be regarded as a coarse-grained experimental version of the Andrews-Mika diagram for parent particles at 710-1000 microns and 1.29-1.31 g/cc. The data set is recorded in Tables 1 and 2 and is shown in Figure 2. Figure 2 represents the first ever evaluation of an Andrews-Mika diagram for coal and, while it is qualitatively similar to Andrews-Mika diagrams established for other mineral systems, it illustrates that ash is liberated comparatively slowly during comminution.

Proximate analysis was done on each of the progeny fractions, together with sulfur and average particle density. The average particle densities are particularly important in establishing the precision of the fractionation procedure. The average solid densities of the air-dried coal were measured using helium pycnometry. The results are shown in Figure 3. The density measured by helium pycnometry is, in all cases, slightly higher than the mid-point of the fractionation interval and this is consistent with the observation that helium pycnometry registers a higher solid density because much smaller pores are accessible to the helium and are not accessible to liquids because of capillary pressure.

The approximate Andrews-Mika diagram, illustrated above, does not reflect the separate

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liberation characteristics of pyritic sulfur and the other ash-forming minerals. In order to construct separate diagrams for these two components, it is necessary to separate, at least approximately, mineral matter excluding pyrite and pyrite. In order to do so, a simple model for the three-component system has been constructed based on the following assumptions: a constant ratio, y, relates mineral matter in the coal to measured ash; and the coal contains a fraction constant, s, of organic sulfur and constant fraction, c, of inherent mineral matter. In addition, all pyrite is assumed to convert to Fe203 on ashing, and pyrite contains 49.4 percent of sulfur. A simple mass balance allows the calculation of the fraction of pyrite and mineral matter-free coal in any sample in which ash and sulfur are determined. The model is described by:

A = 0.72 w2 + $cwl + w;) S = 0.494 W Z + S W ~

w1 +w,+w3 = 1

where A and S are measured ash and sulfur fractions respectively and wl, w2 and w3 are the calculated mass fractions of koal," pyrite and mineral matter. The efficacy of this simple model can be tested by comparing the calculated density of the separated fractions to the mid-point of the separted fraction using

where pc, p and pm are the densities of the "coal," the pyrite and the mineral matter respectively. The data usmg pc = 1.25, pp = 5.02, pm = 3.1, y = 0.9, S = 0.015 and c = 0.0135 are shown in Figure 4. The effectiveness of this simple model can be established by noting that the specific volume of the sample is related to these quantities by Equation 2. The predicted specific volumes are compared to the mid-point of the separation densities in Figure 4.

p.

A mounting, polishing and imaging technique has been developed to examine particles of coal using scanning-electron microscopy. A typical image is shown in Figure 5. The mounting technique was adapted from one developed in Australia and the images show good, clear discrimination between the particles of coal and the mounting medium. Pyrite is easily identified in the images and areas of high mineral content also show clearly. Suitable image processing and image analysis techniques will be developed to characterize the geometrical structure of the various component regions in the coal. Very careful calibration will obviously be required.

2. Theoretical

Two approaches are available to model the Andrews-Mika diagram. The first establishes the characteristics of the entire three-dimensional surface of the diagram including the constraint envelope. This approach has been successfully used to characterize the liberation characteristics of sphalerite from a dolomitehphalerite ore in a previous study. In essence, the model assumes

. that the distribution of progeny at any size across the composition coordinate is described by a beta distribution. When the range of the beta distribution extends beyond the liberated boundaries, the extended area is concentrated at the appropriate boundary. A good model that

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uses a minimum of parameters is available for this description and will be applied to the coal data.

A second method, which has been developed by Professor L. G. Austin, regards the Andrews-Mika diagram as resulting from a two-stage process. First, the parent particle' distributes across the size coordinate of the progeny spectrum according to the conventional breakage functions with a separate breakage function determined for each component. The three- component breakage functions for the one parent that has been investigated are shown in Figure 6. These data show clearly that both ash,and sulfur-bearing constituents generate finer progeny than the less brittle coal components. The next step in the development of a quantitative description of the Andrews-Mika diagram is to determine how each component is distributed within the three-dimensional coal, mineral-matter, pyrite space at each progeny size. Calculated distribution curves are not yet available for the first sample data set.

Personnel

Personnel who have contributed to this project during the reporting period are:

Professor R. P. King Principal Investigator Professor L. G. Austin Visiting Professor (June 1 - July 31, 1996) Dr. C. L. Schneider Postdoctoral Associate Mr. D. fitzGerald Technician Mrs. K. Haynes Program Administrator

Mr. Kerning Wu, currently at the University of Concepcion Chile, has been offered a post on the project team so that he can undertake work toward a Ph.D. degree. He has not yet been formally admitted to the University of Utah because some of his personal information is not yet available in documented form. An offer was made to Mr. Naiyang Ma, currently at the University of Birmingham, England. He has satisfied all of the requirements for entry to the University of Utah but he has been refused a visa by INS because he has not been able to return to his home country to make the visa application. Negotiations are currently underway with additional students who have been admitted for graduate study at the University of Utah. In particular, Mr. Suraj Jain, who is presently in India, appears to be particularly suitable for this project. Mr. Kendall Oliphant, an undergraduate student at the University of Utah, will rejoin the project at the end of summer. Mr. Oliphant is particularly suited because he is already well-trained in all of the experimental techniques that are being used in this project.

Activities Planned for Next Six Months

1. Each of the parent samples will be crushed using ultrasonic milling and the progeny fractionated and analyzed according to the methods that are now being developed.

2. Three-component Andrews-Mika diagrams will be constructed for each parent particle type. using both modeling techniques.

3. The image-analysis procedure wiU be further developed and calibrated against the samples prepared by density fractionation.

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Table I Measured data for parent particles

0.00 0.00 0.00 41.10 0.00 0.00 0.00 100.00 0.00 0.00 0.00 55.72 19.46 11.68 9.25 3.89 0.00 weight %

0.49 0.37 0.57 0.54 0.25 5.65 7.89 9.33 11.10 13.78

Carbon % 56.n 55.64 54.61 53.40 52.04 37.09 36.1 0 35.49 34.96 33.93 Volatile %

Sulfur % I .98 2.62 3.01 3.52 4.21

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Table I I Measured data for progeny

IAnalysis of Progeny after I Fracture I I I I I I I I 1 I I

Table I I Measured data for progeny

I

.. - .

v) v)

i!

100

90

80

70

60

50

40

30

20

10

0 - 1.2 1.3 1.4 1.5 I .6

Density g/cc Figure 1. Measured washability in the Pittsburgh #8 coal used to produce parent particles for the determination of the Andrews-Mika diagram.

8

Y

Figure 2. Approximate Andrews-Mika diagram for Pittsburgh #8 coal.

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1.6 -

1.5 -

1.4 -

1.3 -

I I

1.2 1.3 1.4 1.5

Midpoint of separation density interval, g/cc

Figure 3. Confirmation of fraction densities.

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1.6

1.41

1.40

1.39

1.38

1.37

1.36

1.35

1.34

1.33

1.32

1.31

1.30

1.30 1.31 1.32 1.33 1.34 1.35 1.36 1.37 1.38 1.39 1.40 1.41

Mid point of separation interval g/cc Figure 4. Confirmation of the three-component density model for Pittsburgh #8 coal.

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Figure 5 . SEM image of s e c t i o n e d c o a l p a r t i c l e s .

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1 02

6 5 4

3

2

IO'

6 5 4

L.. .

3 1 00

Component breakage functions

. . . . . . . . . . ....... , .. ......... , .. , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 .................. .................. -.-. .__-__. ._. . _. . . . . . . . . . . . . . . . . . . . . . .

~~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ............... . . . . . . . . . . . . ............. : : : / ........... .............. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 'd'.... ....... . . . I . . . . . , ....... . . . . . . . . . ................... . . . . . . . . . . ......,... ....... . . . . . . . . . . ................... . . . . . . . . . . ............ \ ,...... . . . . . . . . . .

...... ............ 1 ..... . . . . I;:: 1.: 1: ::. ...... : . . I ......

::; 1: :: . . . . ................. . . . . . ................. I , ....... , . . I

:: . . . .......... L ..... . . . . I . " "

. . . . I

..... - ..... , .....

..... Z... . . I . . . . . . . . . I 1 I ........ . . I . . . . . . . . .

..................... ........................ . . . . I . . . i...% . . I . . i..... . . . . . . . . . . . . ....................... ....:........ i . . i . . i ..... ........................ . . . . . . .... , ... , ... , .. , ........ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ....................... ....................... ........................ . . . . . . . . . . . . . . . . . . . . . . . . . I . . .

........... ........... . :. ... -. .: ... . . . . . . . . . . . . .......... ..... i.i... ........... . . . . . . , .... , . , ... . . . . . . . . . . . . . . . ..........

.......... . . . . . . . . . . ........... . . . . . . . . . 4 5 6 '7 8 102 2 3 4

Sieve size microns

..................... ..................... ... I ..... i : .. z Z i . . . , .. . . . . . . . . . . . . . . . . . . . . . . .................. ...,..... i , i ............ ..................... . . . . . . . . . . .

...a ....., I \\.. .....,.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..................

.................. . . . . . . . . . . . . . . . . . . . . . . ................... . . . . . . . . . . . . . . . . . . 5 6 7 8

Figure 6. Breakage functions for three separate components with ultrasonic mill.

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103 .