Anal. Chem. 1986, 58, 319-321 Wootan, D. L.; Coleman, W. M.; Dorn, H. C.; Taylor, L. T. J . Chromatogr. 1976, 123, 419. Wootan, D. L.; Coleman, W. M.; Glass, T. E.; Dorn, H. C.; Taylor, L. T. Adv. Chem. Ser. 1978, No. 170, 37. Welsh, D. J.; Hellgeth, J. W.; Glass, T. E.; Dorn, H. C.; Taylor, L. T. ACSSymp. Ser., 1978, No. 7 1 , 274. Coleman, W. M.: Wootan, D. L.; Dorn, H. C.; Taylor, L. T. Anal. Chem. 1977, 4 9 , 533. Painter, P. C.; Coleman, W. M. Fuel 1979, 58, 301. Brown, R. S.; Hausler, D. W.; Taylor, L. T.; Carter, R. C. Anal. Chem. 1081 .- - ., 53. - - , 197. . Philip, C. V.; Anthony, R. G. Fuel 1982, 6 1 , 357. Jewell, D. M.; Weber, J. H.; Bunger, J. W.; Plancher, H.; Latham, D. R. Anal. Chem. 1972, 4 4 , 1391. Jeweil, D. M.; Albaugh, E. W.; David, B. E.; Ruberto, R. G. Ind. Eng. Chem. Fundam. 1974, 13, 278.
319
Still, W. C.; Kahn, M.; Abhijlt, M. J . Org. Chem. 1978, 4 3 , 2923. Technical Bulletin IE-92, Rohm and Haas: Phliadelphia, PA, Dec 1967. Strachan, M. 0. Ph.D. Thesis, University of Melbourne, Australia, 1985. Middleton, W. R. Anal. Chem. 1967, 3 9 , 1839. Strachan, M. G.; Chaffee, A. L.; Esdaile, R.; Johns, R. 6.; Yost, R. S. Proc. Aust. Coal Hydrogenation Workshop 6th 1981, 7-16. (291, Curtis, C. W.; Hathaway, C. D.; Guin, J. A,; Tarrer, A. R. Fuel 1980, 59. 573. ~~
RECEIVED for review December 21,1983. Resubmitted July 1,1985. Accepted September 19,1985. M. G. Strachan acknowledges the financial support of a Melbourne University Postgraduate Scholarship.
Determination of Total Sulfur by Ion Chromatography following Peroxide Oxidation in Spent Caustic from the Chemical Cleaning of Coal Colin
D;Chriswell, David R. Mroch, and Richard Markuszewski*
Ames Laboratory, USDOE, Iowa State University, Ames, Iowa 50011
Total sulfur In samples of spent caustlc arlslng from the chemlcal cleanlng of coal has been determlned by Ion chromatography after oxldatlon of aU sulfur species to sulfate. Oxldatlon wHh hydrogen peroxlde first under basic condltlons and subsequently under strongly acldlc conditions was requlred for quantltatlve converslon of all sulfur specles to sulfate. The effects of pH, sample slze, and time of oxldatlon have been studied. The subsequent determlnatlon of sulfate by Ion chromatography was stralghtforward, using well-established procedures. The results for total sulfur were reproducible, and the accuracy, based on total sulfur balance, was good. Previously low material balances for total sulfur have been corrected by this modlfled procedure to acceptable values.
The chemical cleaning method of coal using molten caustics has been studied by several researchers (1-5). Recent results have shown that it provides an effective means for reducing ash and sulfur contents to levels meeting all existing and anticipated standards (6). The first step in this process consists of treating ground coal with a molten mixture of sodium hydroxide and potassium hydroxide. Sulfur species in the coal are believed to be converted predominantly to sulfide, and ash-forming mineral components are converted to species such as silicates, aluminates, and ferrates during the treatment (7). Following the molten caustic treatment, the coal is separated from the spent caustic and washed with water, acid, and again with water. Until recently, attempts in our coal cleaning laboratory a t material balance for total sulfur deviated by as much as 24%. Less sulfur could be accounted for after the treatment in the cleaned coal, in the spent caustic, and in other process streams than was found in the coal entering the molten caustic reactor. Preliminary studies have shown that the "missing" sulfur was likely to be present in the spent caustic and was in a form not 0003-2700/86/0358-0319$0 1.50/0
oxidized by hydrogen peroxide prior to its determination as sulfate. In the present work, a procedure was developed to oxidize all the sulfur forms present in spent caustic to sulfate for subsequent determination by ion chromatography. An attempt was also made to characterize the sulfur species present in spent caustic that was not being oxidized in sulfate by hydrogen peroxide.
EXPERIMENTAL SECTION Apparatus and Equipment. All chemicals used were of ACS reagent grade. Sulfur-free deionized water was used for all dilutions. Only standard laboratory glassware was used. Determinations of sulfate were performed by using a Model 2000i ion chromatograph (Dionex, Sunnyvale, CA) using a conductivity detector, an AS3 separation column, and an AFS fiber suppressor. The eluent was 0.0023 M Na2C03/0.0029M NaHC03 at a flow rate of 3 mL/min. Further details of the procedures used are described in the Dionex Application Note 30 (8). Analytical Procedures. Total sulfur determinations on raw coal, cleaned coal, and powdered spent caustic were performed by the Ames Laboratory Analytical Services Division using a Fisher total sulfur analyzer. The analyzer was calibrated by using National Bureau of Standards (NBS) Standard Reference Materials (SRM) coal samples. Total sulfur determinations were performed on aqueous solutions of spent caustic and other process streams by first oxidizing the sulfur species to sulfate with hydrogen peroxide and then determining sulfate either by titration with barium perchlorate (9) in the initial part of this work or by ion chromatography (8) in the final recommended procedure. Recommended Procedure for Determining Total Sulfur in Spent Caustic. Accurately weigh a sample of spent caustic anticipated to contain about 3-4 mg of sulfur. Dissolve the spent caustic in sulfur-free deionized water and dilute to exactly 100 mL. Add 25-mL aliquob of this solution to each of three 500-mL Erlenmeyer flasks and add 25 mL of 30% hydrogen peroxide to each flask. Gently swirl the flasks as the hydrogen peroxide is added. If a vigorous reaction commences, stop the addition until the foaming subsides. Allow the flasks to sit for approximately 1h at room temperature (about 23 "C). After an induction period, which may range from a few minutes to- half an hour, a vigorous 0 1986 American Chemlcai Society
320
ANALYTICAL CHEMISTRY, VOL. 58, NO. 2, FEBRUARY 1986
Table I. Sulfur Balance for Caustic Desulfurization of 50 g of Illinois No. 6 Coal found in various fractions, g H2O
sample
experimental conditions
raw coal
spent caustic
1 2 3 1 3 4 5 6 7 8 9 10
standard standard standard reanalysis of sample after several months reanalysis of sample after several months reanalysis of sample after several months reanalysis of sample after several months reanalysis of sample after several months Feo added to reactor during coal cleaning Fe203added to reactor during coal cleaning Fe203added to spent caustic after reaction Fe304added to reactor during coal cleaning
2.01 2.01 2.01 2.01 2.01 2.01 2.01 2.01 2.01 2.01 2.01 2.01
0.76 0.76 0.90 1.30 1.28 1.29 1.22 1.19 0.59 0.78 0.76 1.04
wash of treated coal
residue from caustic
0.52 0.52 0.52 0.58 0.56 0.48 0.52 0.52 0.52 0.65 0.58 0.52
0.04 0.01 0.01 0.04 0.01
%S cleaned coal
total
accounted for
0.27 0.23 0.30 0.27 0.30 0.32 0.35 0.38 0.61 0.34 0.29 0.29
1.59 1.52 1.73 2.19 2.15 2.09 2.09 2.09 1.94 1.99 2.10 1.90
79 76 86 109 107 104 104 104 97 99 104 95
a a a
0.61 0.34 0.47 0.05
Not determined.
reaction may occur. Addition of a small amount of water will suppress this reaction. After 1 h, insert a pH electrode into each solution and add 50% hydrochloric acid to each solution until a pH of 1 0.5 is reached. Place a watch glass or other cover on the flasks and allow the reaction to proceed for at least 16 h. At the conclusion of the acidic oxidation period, place the flasks on a hot plate and boil the solution for about 5 min to destroy residual hydrogen peroxide. Transfer the solutions t o 100-mL volumetric flasks and dilute to volume with sulfur-freedeionized water. Determine the sulfate content by ion chromatography using standard procedures (8). Caution. Commercial 30% hydrogen peroxide and its dilute solutions can cause severe burns. Proper protective clothing, eye protection, and plastic gloves should be worn at all times when working with this reagent. If hydrogen peroxide comes in contact with the unprotected skin, it should be washed with copious amounts of water. Application of a paste of ascorbic acid to minor peroxide burns on the skin will relieve the burning sensation. If serious burns occur or if contact is made with the eyes, flushing with copious amounts of water is required and medical help is recommeded.
Table 11. Sulfur Distribution among Fractions of Spent Caustic after Acidic and Basic Oxidation
*
RESULTS AND DISCUSSION Uncertainties in Total Sulfur Determinations on Spent Caustic Samples. Most material balances of sulfur obtained previously were usually low by 20-25% (Table I). This discrepancy obviously could have occurred due to inaccuracies in the determinations performed on the coal or on any of the various process streams. However, it was observed that sulfur determinations performed on retained samples of aqueous solutions of the spent caustic after periods of storage ranging from a few weeks up to several months consistently yielded sulfur results much higher than those originally obtained, as shown in Table I. Thus, it was assumed that the “missing” sulfur was present in the spent caustic and that it was in some form that was not readily oxidized to sulfate. Additional evidence that the missing sulfur was contained in the spent caustic was obtained when iron salts were added to the molten sodium hydroxide-potassium hydroxide mixtures used to leach the ash and sulfur from coal. A voluminous precipitate formed in aqueous solutions of spent caustic recovered from these runs. Sulfur determinations performed on filtered samples of the spent caustic solutions yielded results consistent with those obtained on samples arising from runs with no added iron salts. But, the precipitate of hydrated iron oxide, when analyzed with a Fisher total sulfur analyzer as shown in Table I, was found to contain a sufficient amount of sulfur to give quantitative material balances. The same phenomena were found to occur if iron salts were added to spent caustic after it had been separated from cleaned coal. Thus, it was assumed that the missing sulfur was present in
70 S found after after acidic basic oxidation oxidation
spent caustic fraction total fraction after purging of hydrogen sulfide humic materials phenolic materials solution remaining after removal of H2S, humics. and Dhenolics ~~
~~
~~
~~
0.32 0.18 0.03 0.03 0.12 ~
0.24 0.09 0.03 0.03 0.03 ~
the spent caustic in some form that would react with iron salts to form an insoluble sulfur-containing species. Modifications in Oxidation Conditions. Once it was concluded that the spent caustic contained additional sulfur that was not being oxidized to sulfate, means were sought for making the oxidation more complete. Allowing solutions to stand for several weeks was not an efficient procedure. Adding iron salts to samples during oxidation with hydrogen peroxide led to somewhat higher sulfur results, but the values were erratic. The inconsistencies presumably arose from difficulties in separating the voluminous precipitates of hydrated ferric oxide from the solution prior to analysis by ion chromatography. The difficulties were not insurmountable, but simpler procedures were desired. Various procedures were attempted such as increasing the reaction time, increasing the reaction temperature, adding materials to the oxidation mixture, and reoxidizing samples several times. Only one modification of the procedure resulted in consistently increased sulfur levels and this was simply to reoxidize the sample with hydrogen peroxide after adding acid. When acid was added soon after the addition of hydrogen peroxide, hydrogen sulfide was evolved. Although some sulfur was lost, the total sulfur values were still higher than those obtained when no acid was added, indicating a significant increase in the total amount of sulfate formed. It was found that if samples were oxidized first with no added acid for a period of about 1 h and then acidified, hydrogen sulfide was not evolved, and much higher and reproducible total sulfur results were obtained. Thus, a standard analytical protocol was adopted in which a sample of spent caustic was first oxidized under basic conditions for 1h and then acidified and the oxidation continued for an additional period of time. Attempts to characterize the sulfur species that was oxidized only under acidic conditions were unsuccessful. However, it was found that the species was not sulfate, sulfide, polysulfide,
ANALYTICAL CHEMISTRY, VOL. 58, NO. 2, FEBRUARY 1986 T a b l e 111. S u l f u r B a l a n c e during t h e C a u s t i c D e s u l f u r i z a t i o n o f P i t t s b u r g h D e t e r m i n a t i o n of S u l f u r
No. 8 C o a l U s i n g Acidic
321
Oxidation f o r the
f o u n d in various fractions, g sample
r a w coal
spent caustic
1
1.53 1.53 1.53 1.53 1.53
1.17 1.13
2 3 4 5 av.
" Relative standard deviation = 9.8%.
H 2 0 wash of
0.98 0.96 0.96
treated coal
0.30 0.30 0.30 0.30 0.30
1.04"
cleaned coal
total
% S accounted for
0.20 0.20 0.20 0.20 0.20
1.67 1.63 1.48 1.46 1.46
109.2 106.5 96.7 95.4 95.4
1.54
100.7*
Relative standard deviation = 6.6%
elemental sulfur, or sulfur-containing phenols or humates. As shown in Table 11, the species remained in acidified caustic samples after purging of H2S, precipitation of humics, and extraction of phenolics. Accuracy and Preaision. Because the species being determined has not been characterized and the basic oxidation followed by the acidic oxidation is the only known procedure that leads to its quantitative conversion to sulfate, the absolute accuracy of the procedure is not known. The only basis for estimating accuracy is material balance for sulfur in all fractions during the caustic cleaning of coal (see Table 111). By use of the modified oxidation procedure, material balances ranging from 95 to 109% have been obtained for total sulfur. The average sulfur balance is 101% with a relative standard deviation of 7 % . By use of the modified oxidation procedure, eight replicate determinations were performed on a spent caustic recovered from the cleaning of Pittsburgh No. 8 coal. Sulfur values from 0.34 to 0.41% with an average of 0.36% and a relative standard deviation of 6.4% were obtained. Effect of Experimental Variables. Samples of spent caustic from Pittsburgh No. 8 coal, ranging in size from 100 to lo00 mg, were oxidized by this new procedure to determine the effects of the amount of sulfur present on the efficiency of the oxidation. Sulfur values ranging from 0.34 to 0.38% were found with no pattern of dependence upon sample size. A sample size anticipated to contain about 3-4 mg of sulfur is recommended simply because in the final measurement of sulfate it gives an ion chromatographic peak large enough to make the reagent blank insignificant, and no dilutions are required to keep the peak on scale. The effects of pH upon the oxidation efficiency are pronounced. Aliquots of a single sample were oxidized at a pH of 12.6 for 1 h, and the pH of the aliquots was then adjusted to 9, 7, 5, 2.5, and 1. Oxidation of each sample was then continued for an additional 16 h. Values of sulfur obtained were 0.24% at pH 9, 0.24% at pH 7, 0.29% at pH 5, 0.30% at pH 2.5, and 0.36% at pH 1. Aliquots of a sample were oxidized under basic conditions for 1h and then acidified to pH 1and the oxidation allowed to continue for varying times. It was found that a time interval of at least 16 h is sufficient for complete oxidation. Values for sulfur obtained after various periods of oxidation under
acidic conditions were 0.24% at time 0; 0.28% after 1h; 0.28% after 2 h; 0.28% after 3 h; 0.27% after 4 h; 0.36% after 16 h; and 0.34% after 72 h. It is interesting to note that 0.24% sulfur was determined in this same sample when it was oxidized under only basic conditions for 16 h. It would appear that approximately two-thirds of the sulfur contained in this sample is oxidized readily to sulfate under basic conditions, some additional amount is readily oxidized under acidic conditions, and a little over 20% of the sulfur is oxidized to sulfate only slowly under acidic conditions.
ACKNOWLEDGMENT We greatly appreciate the assistance of Richard G. Richardson and James L. Hofer, who performed many of the sulfur determinations reported herein, and the efforts of Robert Z. Bachman and the Ames Laboratory Analytical Services group who performed all sulfur determinations on whole coal samples. Pertinent discussions with Chris McGowan, visiting scientist from Tennessee Technological University, were also greatly appreciated. Registry No. Sulfur, 7704-34-9;sodium hydroxide, 1310-73-2. LITERATURE CITED (1) Masciantonio, P. X. Fuel 1985, 44, 269-275. (2) Meyers, R. A. Hydrocarbon Process. 1979, 123-126. (3) TRW Energy Development Group "Gravimelt Process Development"; Final Report, DOE/PC/42295-T7(DE84013743); Redondo Beach, CA, June 1983, (4) MalJgren, B.; Hubner, W. "Coal Cleaning by Molten Caustic"; Proc. 1983 International Conference on Coal Science; Pittsburgh, PA: pp 256-259. (5) Markuszewskl, R.; Mroch, D.; Norton, 0. A.; Straszhelm, W. E. Am. Chem. Soc. Div. Fuel Chem. Prepr. Pap. 1985, 30, 41-48. (6) Meyers, R. A. "Gravimelt Process Applications and Economics"; Proceedings First Annual Pittsburgh Coal Conference; Pittsburgh, PA, Sept 17-21, 1984: pp 381-384. (7) Chlotti, P.; Markuszewskl, R. "Reaction of Pyrite with Fused NaOH," Ind. Eng. Chem. Process Des. Dev. 1985, 24, 1137-1140. (8) Dionex, Applicatlon Note 30; Dionex Corp.: Sunnyvale, CA, 1981. (9) Fritz, J. S.;Yamamura, S. S. Anal. Chem. 1955, 27, 1461-1464.
RECEIVED for review August 26, 1985. Accepted October 3, 1985. Ames Laboratory is operated for the U.S. Department of Energy by Iowa State University under Contract W7405-Eng-82. This work was supported by the Assistant Secretary for Fossil Energy, Office of Direct Coal Utilization, through the Pittsburgh Energy Technology Center.
