METHODS FOR SAMPLING AND
INORGANIC ANALYSIS OF COAL
U.S. Geological Survey Bulletin 1823
Edited by D.W. Golightly and F.O. Simon
THE DETERMINATION OF FORMS OF SULFUR IN COAL
By Norma Rait and Philip J. Aruscavage
Total sulfur, monosulfide (S=), pyritic sulfur (S2=), and sulfate sulfur (S04=) concentrations are determined in whole-coal samples. Total sulfur, monosulfide, and sulfate sulfur are determined by infrared absorption of the SO2 produced by combustion of whole coal or of the separated forms of sulfur in an oxygen atmosphere. The pyritic sulfur concentration is measured indirectly by determining the iron concentration in a separated fraction by flame atomic absorption spectrometry. The organically bound sulfur concentration is calculated by subtracting the sum of the concentrations of monosulfide, sulfate sulfur, and pyritic sulfur from the total sulfur content. Coke and coal standards are used as control samples for measuring the accuracy and precision of the analytical procedures. The precision of the method for each form of sulfur, as determined from replicate analyses of standards, is approximately 5- to 10-percent relative standard deviation. The determination limit for sulfur is approximately 50 m g (30-percent relative standard deviation), which is equivalent to a 0.01 percent sulfur in 0.5 g of coal.
Methods are described for the determination of total sulfur and of four forms of sulfur in coal (fig. 16): monosulfide (S=), sulfate sulfur (S04=), pyritic sulfur (S2=), and organically bound sulfur. The methods are modifications of those proposed by the American Society for Testing and Materials (ASTM) (1984).
EXPERIMENTAL EQUIPMENT AND METHODS
The whole-coal samples are received as air-dried, 80-mesh powders. Typically, 500 mg of sample is required for measurements of total sulfur and the four forms of sulfur.
Laboratory, Supplies, and Equipment
A steam bath in a fume hood is required. All chemicals and gases are of reagent-grade quality. Doubly distilled water is required for the preparation of all solutions.
1. Concentrated acids: hydrochloric and nitric.
2. 1- and 5-percent hydrochloric acid solutions.
3. 10 percent (w/v) BaCl2 solution, approximately 1 L.
4. 20 percent (w/v) zinc acetate solution, approximately 1 L.
5. 50 percent (w/v) NaOH solution, for cleaning glassware.
6. 30 percent H2O2 solution, for cleaning glassware.
7. Cylinder of nitrogen gas and two-stage pressure regulator.
8. Cylinder of oxygen gas and two-stage pressure regulator.
10. Anhydrous magnesium perchlorate (Anhydron-eTM)
11. LECOR model SC-132 sulfur determinator, combustion boats, and accessories.
12. LECOR coal standards used for calibration: 0.52, 1.10, 1.77, and 2.97 percent sulfur.
13. Alpha Resources, Inc., coke and coal standards numbered 771, 772, 774, 775, 780, and 782. These materials are analyzed along with samples as control standards.
14. Flame atomic absorption spectrometer, Perkin-ElmerR model 370.
15. Iron standard solutions in 5 percent nitric acid: 0.5, 1, 3, 5, 7, 10, 20, 40, 60, and 100 m g/mL iron.
16. Two laboratory hot plate--magnetic stirrer combinations.
17. Support-lattice frame to be used as a rack for equipment for forms of sulfur equipment (fig. 17).
18. Metal tube with five outlets and one inlet for nitrogen and associated Tygon tubing.
19. Round-bottom flasks, each having a volume of 200 mL and three necks. The two outside necks have ground-glass joints 19/38; the center neck has a ground-glass joint 24/40 (fig. 18). These flasks are cleaned first with a solution of 50 percent NaOH and then with a 30 percent hydrogen peroxide solution. Finally, they are rinsed with tapwater and then with distilled water.
20. Heating mantel to fit 200-mL round-bottom flask, with accompanying Variac to regulate the temperature.
21. 70-mL test tubes and supporting laboratory jacks to raise and lower test tubes (fig. 17).
22. Thermometer (150° C) having a ground-glass joint 10/30 (fig. 18).
23. Condenser (fig. 18) with ground-glass joint 14/35.
24. PyrexTM ground-glass bushing-type reducing adapters, outer joint 24/40, inner joint 14/35 (fig. 18).
25. Pyrex reducing adapters: 19/38 to 10/30.
26. Vacuum flasks having volumes of 4 L and 250 mL, each fitted with a fritted-glass filter and a filter funnel and clamp. Each flask can be attached to a small vacuum pump.
27. Vacuum filters with diameters of 25 and 47 mm and a pore size of 0.45m m and made of cellulose triacetate containing a small amount of wetting agent.
28. PyrexTM standard-taper 10/30 inner ground-glass joint with a tube extension (outer diameter of 8 mm; length of 130 mm) (fig. 18).
29. PyrexTM ground-glass stoppers 19/38.
30. Graduated cylinders: 5, 10, and 50 mL.
31. Drying oven, maintained at 110° C.
32. Volumetric flasks, 200 mL.
33. Beakers, 250 mL, with 9-cm-diameter SpeedivapTM watch-glass covers.
34. Erlenmeyer flasks, 250 mL, and 5-cm-diameter watch-glass covers.
35. Balance with a sensitivity of 0.1 mg.
The total sulfur content of a sample is determined by use of the LECOR SC132 sulfur analyzer (LECO Corporation, 1980; Kirschenbaum, 1983). Calibration of the sulfur analyzer is based on one of the LECO coal standards (LECO Corporation, 1980). After calibration, 200 mg of sample is weighed into a combustion boat then is placed in the combustion tube. This tube operates at a temperature of 1,370° C in an oxygen atmosphere. The oxygen, which flows through the tube and over the boat, reacts with sulfur to form SO2. The released SO2 passes through the infrared absorption detector of the sulfur analyzer. After integrating the absorption signal over a period of 1 to 2 min, the instrument extrapolates and prints out the concentration of sulfur.
Laboratory Setup for the Separation of Forms of Sulfur
Within U.S. Geological Survey laboratories, the forms of sulfur are separated from 10 coal samples simultaneously. Ten three-neck, round-bottom flasks are placed in heating mantels. The first and fifth flasks have thermometers in one side neck (fig. 17), whereas the other eight flasks have stoppers in that side neck. Each flask has an air-cooled condenser attached to the center neck. The spout at the end of the condenser is placed in a 70-mL test tube that rests on a laboratory jack. Each of the 10 flasks has a bubble tube in one side neck. Nitrogen bubbles through each of the solutions in the round-bottom flasks and then passes through the condensers and into the test tubes. The rate of bubbling in a test tube is regulated by the height of the laboratory jack.
Monosulfide Sulfur Separation and Determinatlon
1. Place 500 ± 1 mg of sample into each round-bottom flask. Use of a larger quantity of sample may cause difficulty in later filtering procedures.
2. Add 25 mL of distilled water to each sample.
3. Pour 15 mL of 20 percent zinc acetate solution and 285 mL of distilled water into a beaker. Place a magnetic stirring bar in this solution, and set the beaker on a hot plate. While stirring the diluted zinc acetate solution, add 3 mL of 50 percent NaOH solution. Warm, but do not boil, the resulting mixture.
4. Place 40 mL of this alkaline zinc acetate mixture into each test tube.
5. Bubble nitrogen through the system for 15 min to remove oxygen. Set the flow of nitrogen in each of the test tubes to approximately one bubble per second by adjusting the heights of the jacks.
6. Add 5 mL of concentrated hydrochloric acid to each flask.
7. Switch on the electrical power to the heating mantels, and maintain the temperature in the flasks at 80° C. Continue to bubble the nitrogen through the system for 90 min. As the solution evaporates from a flask, add 5 percent hydrochloric acid to maintain the solution level.
8. Vacuum filter each alkaline zinc acetate mixture through a 47-mm filter.
9. Wash each precipitate thoroughly with doubly distilled water.
10. Discard the filtrate.
11. Place each filter on a marked watch glass and then transfer the watch glass into a drying oven that is maintained at 110° C. Dry the filters overnight.
12. Transfer the dry precipitate on each filter to a separate combustion boat.
13. Place 500 mg of V205 over each precipitate.
14. Determine the percent sulfur by use of the LECOR sulfur analyzer as described previously.
15. To determine the concentration of sulfur in a blank, place 500 mg of V205 in a combustion boat and measure the percent sulfur. Subtract this concentration from the previous result to obtain the percent of monosulfide sulfur in the sample.
Sulfate Sulfur Separation and Determinatlon
1. Vacuum filter the solution in each round-bottom flask through a 25-mm filter.
2. Wash the residue thoroughly with 100 mL of 1 percent hydrochloric acid.
3. Place the residue in a 250-mL Erlenmeyer flask, and save it for the determination of pyritic sulfur.
4. Place the filtrate and washings in a 250-mL beaker.
5. Add distilled water until the volume of liquid in the beaker is 200 mL.
6. Place the beaker on a hot plate, and bring the solution to a boil.
7. Add 5 mL of 10 percent BaCl2 to the boiling solution.
8. Place a SpeedivapTM watch glass on the beaker. Set the beaker on the large hot plate, and evaporate the solution to dryness (overnight).
9. Add 150 mL of 1-percent hydrochloric acid to the beaker containing dried residue, and bring the mixture to a boil. This prevents the formation of colloidal BaSO4, which causes difficulty in filtering.
10. Filter the mixture through a 25-mm vacuum filter. Wash the beaker with 1 percent hydrochloric acid, and filter the wash solution.
11. Place the filter paper and precipitate on a marked watch glass, and dry them overnight in a drying oven at 110° C.
12. Subsequently, place the filter paper and precipitate in a combustion boat. Spread approximately 500 mg of V205 over the precipitate, and determine the sulfur concentration by the sulfur analyzer. Subtract the blank from this result to give the concentration of sulfate sulfur.
Pyritic Sulfur Separation and Determination
1. Add 45 mL of distilled water to each of the residues in the Erlenmeyer flasks.
2. Add 5 mL of concentrated nitric acid to each flask.
3. Place a watch glass on top of each flask, and set the flask on a steam bath. Leave the flask on the steam bath for 90 min.
4. Vacuum filter the mixture in each flask through a 25-mm filter.
5. Wash the residue with distilled water; discard the residue.
6. Place the filtrate and washings in a 200-mL volumetric flask, and dilute to volume with distilled water.
7. Determine the iron leached from the sample by the nitric acid by flame atomic absorption spectrometry at the following conditions: 248.3 nm, 0.2-nm slit width, hollow cathode lamp, and an air-acetylene flame (oxidizing lean, blue).
8. Calculate the percent pyrite sulfur as follows:
Calculation of Organically bound Sulfur
Organically bound sulfur is not determined by direct analysis but is determined by difference, as is shown in the following equation:
Tables 27 through 33 present results from determinations of forms of sulfur in coal standards; also shown are accepted values and standard deviations for replicate measurements.
Total sulfur concentrations from replicate determinations on nine coke and coal standards are shown in table 27. The accepted values (Alpha Resources, Inc., 1985) were obtained by using the ASTM recommended method for analysis for forms of sulfur (ASTM, 1984). The determination of total sulfur, from initial weighing of the sample to the result, requires about 3 min. This measurement time is comparable to those of other instrumental methods; moreover, the time is much less than that required for classical methods (Elliot, 1981). The determination limit for total sulfur is 0.02 percent for a 200-mg sample. The measured concentrations for total sulfur are in good agreement with the published values, and the precision is approximately 7-percent relative standard deviation.
The mean concentrations and standard deviations of monosulfide sulfur obtained for six Alpha ResourcesR coal standards are shown in table 28. The results of monosulfide sulfur determinations for the coal standards numbered 774, 775, 780, and 782 are below the determination limit of the procedure, which is 0.01 percent sulfur.
The sulfate sulfur concentrations determined for standards numbered 772, 780, and 782 (table 29) are in good agreement with accepted values (Alpha Resources, Inc., 1985). Concentrations of sulfate sulfur in standards 771, 774, and 775 are below the determination limit of the method. The standard deviations for the concentrations measured for standards 772, 780, and 782 reflect the low precision of measurements in the proximity of the determination limit.
The pyritic sulfur concentrations determined for standards 771, 772, 774, 775, 780, and 782 are presented in table 30. The agreement between measured and accepted values of pyritic sulfur for standard 782 is not particularly good. The sample may contain an iron-bearing mineral that is incompletely dissolved in hydrochloric acid; thus, the iron appears as pyrite iron in a subsequent digestion with nitric acid. Iron-bearing minerals likely to occur in coal include siderite, hematite, marcasite, and goethite (Elliot, 1981). Pyritic sulfur, after oxidation by nitric acid and precipitation with BaCI2, is determined as previously described for sulfate sulfur (table 31). Results for organically bound sulfur, which is determined on the filtered residue, appear in table 32. Calculations of organically bound sulfur (table 33) are in good agreement with the accepted values.
As illustrated by data in table 31, the concentrations determined for pyritic sulfur in standard 782 is again higher than the accepted value, but the value for organically bound sulfur in standard 782 (table 32) is lower. This observation indicates that the accepted concentration for pyritic sulfur may be low. However, organically bound sulfur may have been oxidized in the nitric acid digestion, thus, giving high values for pyritic sulfur in our determinations. Because the measurements of sulfur in standards 771 and 772 are near the determination limit of the method, the precision is low.
Alpha Resources, Inc., 1985, Certification of ultimate coal and coke standards: Stevensville, Michigan, Alpha Resources, Inc.
American Society for Testing and Materials (ASTM), 1984, 1984 annual book of ASTM standards, petroleum products, lubricants, and fossil fuels, sect. 5, v. 05.05: Gaseous fuels, coal, and coke: Philadelphia, ASTM, p. 350-354.
Elliot, Martin, ed., 1981, Chemistry of coal utilization, (2d suppl. volume): New York, Wiley-lnterscience, p. 386-391, 415, 416.
Kirschenbaum, Herbert, 1983, The classical analysis of silicate rocks--the old and the new: U.S. Geological Survey Bulletin 1547, p. 48-49.
LECO Corporation, 1980, Instruction manual SC-132 sulfur system: St. Louis, Missouri.
NBS Standard Reference Materials Catalog 1984-1985, National Bureau of Standards (NBS), 1984, Special Publication 260, Washington, D.C., U.S. Government Printing Office.
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