Determination of Pyrite in Coal and Lignite by Thermomagnetic Analysis M. Hyman and M. W. Rowe Texas A 8 M University, College Station, TX 77843 Recently, we described in THISJOURNAL the construction of an inexpensive Faraday balance which is suitable for use in undergraduate chemistry laboratories.' Now we present another undergraduate experiment, the determination of pyrite (FeSz) in coal or lignite, which utilizes the same apparatus, which is reproduced in Figure 1. T o understand the technique we are proposing here, it is necessary to realize the combined effect of temperature and a strong magnetic field on the apparent weight of a ferromagnetic material suspended from a spring or balance beam. Fieure 2 shows that the annarent weieht .. " (or . s.~ r i-n gdeflection) of a ferromagnetic material is many times its actual weight in the Dresence of a strone maenetic field. Note that this large increase in apparent weight, the saturation magnetization, slowly decreases as the temperature is increased until the apparent weight finally becomes equal to actual weight of the iron a t about 770°C, the Curie point of the iron. The Curie point (also called Curie temperature) is distinctive for the particular ferromagnetic material under question. For example, i t is 390°C for Ni, 5 9 5 T for FesOl (magnetite), etc.
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Experiment The method for determination of pyrite is perhaps best demonstrated by examination of Figure 3 which is a schematic representation of the data tracing of an analysis of a hypothetical sample of coal or lignite. A run is started by weighing out a sample (region A on Figure 3) with the gas inlet open to air. The presence of initial Fez03 can he checked by inserting the magnet ( B ) .Normally only a very slight change in weight is seen a t B. The heater is then set to 90 f 10°C which drives off the water and a weight reduction is observed ( C ) .After the water has been removed, the weiaht becomes constant (D). At that point, the temperature raised to 400°C. This is sufficient to oxidize the organic material in the coal which again results in a rather extreme loss in weight ( E ) as the gaseous reaction products (CO, COz, HzO, etc.) are driven off. When the oxidation is complete, the weight is once again ohserved to level off ( F ) and the iron sulfides are oxidized to Fe7Ox. At this point. the furnace is turned off and 20% H?. -. diluted in NJcarrier for increased safety, is fed into the syswm at $1rate of 65 mllminute. Ten minutes has been found to he adequate to flush the sample area with Hz. The magnet is re-inserted which results in an apparent weight increase due to the saturation magnetization of the FezOs (G). For convenience in presentation of Figure 3, we have shown a scale expansion to accommodate the large increase in saturation magnetization as metallic iron is formed (G). The temperature is then turned to 400°C with the H9 flowine. ... and the F e 9 0..~ begins to reduce to mctallir Fe which results in a large increase f hen all the Fe,O., is reduced to Fe. in apparent weieht (H,. the &.uration m&&ation due to the irknbecomes constant ( I ) and the furnace is turned off. An increase in apparent weight is noticed as the sample cools due to the increase of saturation magnetization of the iron with decreasing temperature (Fig. 2) as exhibited by region J in Figure 3. Once again the apparent weight will become constant as the temperature approaches room temperature (K). The magnet is
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' Hyman, M., and Rowe, M. W., J. CHEME~uc.,56,835 (1979). 424
Journal of Chemical Education
THERMOCOUPLE Figure 1. Schematic diagram of Faraday balance used far the measurement of coal and lignite.
Figure 2. Variation of the saturation magnetization of 1 mg iron metal with temperature as recorded by a Faraday balance. Note that the apparent weight (saturation magnetization)of iron is much greater than its actual weight.
then removed and the final weight of the residue is recorded (L). For purposes of determining the pyrite content, only three regions of Figure 3 are of critical importance. They are (1)the initial weight, Wi, of region A , (2) the final apparent weight with the magnet in place, Wf,, of region K, and (3) the final weight, Wr, of region L. The pyrite content is then calculated using stoichiometry and the saturation magnetization of the iron: Wf,,, - Wr M Wt. FeSZ 100 % FeSz = X X218 M. Wt. Fe Wj If the percent pyrite per dry weight were desired, Wi could he replaced by the constant weight of region D, Wd.
in the sample pan. The weight corresponding to an observed spring deflection can then be read directly from the graph. I t is rather difficult to assess accurately the uncertainty to be expected when utilizing this technique. Using a commercial Cahn electrobalance which restricted us to a maximum coal samnle of -50 me-. Der run. we have examined the standard devi'ation in a number of ways. Previouslv we had ascertained that the Oraueil and Ivuna meteorites ( h e I carbonaceous cbondrites) contained Fen04 in a very homogeneous distribution for samples on the order of 50 mg.2 We therefore reduced FeaOl in the meteorite samples to iron in a manner analogous to the reduction of FeS2 to iron in this experiment and estimated the standard deviation to he f5-7%. In --- addition. w e obtained five coal samnles from Dr. P. I X 1-20IXII Dolsen of the coal Research Laboratory of the Pennsylvania T"4E State University. These samples, each weighing about one Figwe 3. Schematic diagram of weight change observed wntinuausiy as pyrite pound, ranged from finely powdered to chunks -1 mm in size. in coal or lignite is analyzed. The change in weight scale factor hom X1 to XZO Repeated analyses of 5 0 mg samples of these five samples is due to the large increase in apparent weight (saturation magnetization) as yielded standard deviations of f15, 39, 33, 23, and I%, reFe2O3present at @and 0) is reduced to ferromagnetic iron. ~pectively.~ This indicates to us that the heterogeneity of the coal samples with regard to pyrite content on -50 mg samples It is also possible to obtain the percent moisture and percent is the limitina factor for determining the uncertainty. Two of ash in the coal or lignite samples. The moisture and ash smaller ( ~ . 5 ~and ) ,finely powdered; coal samples obtained contents are, of course, easily obtainable from Figure 3. The from Dr. J. Zuckerman of the University of Oklahoma Demoisture is simply given by partment of Chemistry led to standard deviations 510%. Thus, it appears that the pyrite in carefully homogenized Wi - Wd % moisture = X 100 samples should be measurable with an uncertainty of about Wi f15% in a routine laboratory situation. and the ash is given by ~~~~~~~~
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Conclusion
depending on whether % ash in the original sample or for dry weight, respectively, is desired. Since each of the above equations involves a ratio of weights, i t is apparent that the deflections of the spring a t the various positions corresponding to Wi, Wd, W,, Wfm, and W r can be suhstituted into the eouations. That is tosav. it is the relative changes which are of importance and it is not necessary to know the actual weight in order to calculate the % pyrite, % ash, or %moisture in the coal or lignite samples. The spring deflection can be calibrated readily so that deflections can he converted to weights. One simply constructs a curve of spring deflection noted when various calibrated weights are placed
We have presented a straiahtforward experiment for the determinatibn of pyrite in cok and lignite;sing a Faraday balance. The simplicity of the experiment makes it suitable for routine operation in an undergraduate laboratory. Students could thus be exposed to principals of magnetism in materials while conducting an experiment of current relevance. Acknowledgment .
We appreciate the support of the Center for Energy and Mineral Resources of the Texas A & M University. H y m n , M., Rowe, M. W., and Herndon, J. M., Geochem. J., 13,37 119791. ' &man. M.. and Rowe. M. W.. Chapter 22 in ACSSym. Series 169: N e w Approaches in Coal Chemistry, 3 8 9 (1981).
Volume 59
Number 5 May 1982
425
