Pyritic sulfur analysis

This work has demonstrated that organically bound sulfur forms can be distinguished and in some manner quantified directly in model compound mixtures, and in petroleum and coal. The use of third derivatives of the XANES spectra was the critical factor in allowing this analysis. The tentative quantitative identifications of sulfur forms appear to be consistent with the chemical behavior of the petroleum and coal samples. XANES and XPS analyses of the same samples show the same trends in relative levels of sulfide and thiophenic forms, but with significant numerical differences. This reflects the fact that use of both XPS and XANES methods for quantitative determinations of sulfur forms are in an early development stage. Work is currently in progress to resolve issues of thickness effects for XANES spectra and to define the possible interferences from pyritic sulfur in both approaches. In addition these techniques are being extended to other nonvolatile and solid hydrocarbon materials. 

In the determination of sulfate, 2 to 5 g of the analysis sample is mixed with HC1 (2 volumes concentrated HC1 + 3 volumes of water), and the mixture is gently boiled for 30 minutes. After filtering and washing, the undissolved coal may be retained for the determination of pyrite sulfur, or it may be discarded and a fresh sample used for pyrite sulfur. Saturated bromine water is added to the filtrate to oxidize all sulfur forms to sulfate ions and ferrous ions to ferric ions. After boiling to remove excess bromine, the iron is precipitated with excess ammonia and filtered. This precipitate must be retained for the determination of nonpyrite iron if a fresh sample of coal was used for the determination of the pyrite iron. The sulfate is then precipitated with ISaCE, and the BaSC>4 is determined gravimetrically. 

The analysis of the coal used in these runs is given in table II. The coal contained 5.5 percent sulfur and 16.5 percent ash. The pyritic sulfur in coal was 3.08 percent and the organic sulfur 1.95 percent. 

A recent seven laboratory round robin analysis of the pyrite in nine coals using ASTM Method D 2492 showed a mean relative standard deviation (RSD) of 7.4% for the within-laboratory determinations (Janke, L., Canmet Energy Research Laboratory, Ottawa, Ontario, Canada, personal communication, 1986.). Since organic sulfur in coal is calculated as the difference between the total sulfur and the sum of the sulfate and pyritic sulfur, the determination of organic sulfur using ASTM Method D 2492 cannot be expected to yield a within-laboratory RSD much different from the 7.4% found in the seven laboratory round robin analysis of pyritic sulfur. 

The results of the proximate and ultimate analysis of Rasa coal appear in Table I. The results of the forms of sulfur analysis and carbon aromaticity are also included in Table I. The total sulfur on an as-received basis was 10.77 weight percent, and includes 0.02 weight percent sulfate, 0.30 weight percent pyrite, and 10.45 weight percent organic sulfur. Organic sulfur is calculated as the difference... 

We present here the preliminary results of our attempt to develop a new method for the analysis of pyrite in coal and lignite. It is well known that sulfur in coal is present in different forms. In particular, although the iron sulfide in coal is generally pyrite ( 1), other iron sulfides are frequently present. For example, iron disulfide occurs as marcasite, a rhombic crystalline form, as well as pyrite, a cubic crystalline form. Perhaps the term disulfide sulfur should be used to replace the pyritic sulfur more commonly quoted, as recently suggested by Youh (2). Since the chemical reactivity of these two disulfides of iron is similar, our method will record them equally well. Nonetheless, we will continue to refer to the pyrite determinations here, although we are really talking about the chemical species FeS2 rather than a particular crystalline structure. 

Attempts to increase pyrite removal by increasing the reaction time met with limited success under our standard conditions because reaction of the ferric ion with the coal matrix depleted the ferric ion that was needed for extraction of the pyrite. Thus, for example, increasing the coal reaction time from 2 to 12 hrs only increased pyritic sulfur removal from 60 to 80% for Pittsburgh coal. Similar results were obtained for the other three coals. The only alternatives were to increase the amount of leach solution or to use a continuous or semi-continuous (multiple-batch) reactor. A multiple-batch mode was chosen because it was a simple laboratory procedure and at the same time it could approximate conditions encountered in a commercial plant. A 1-hr-per-batch leach time was used because our 2 hr results indicated that in the early stages of removal the rate begins to decrease after 1 hr, and six leaches (or batches) per run were used to assure that any pyrite that could be removed in a reasonable amount of time was removed. The progress of removal was monitored by analyzing the sulfate content in each spent leach solution elemental sulfur was not removed until all the leaches were completed. Table VII shows pyrite extraction as a function of successive leaches as followed by sulfate analysis of the leach solution. Note that the major portion of pyritic sulfur is removed in the first two leaches or 2 hrs, followed by lesser amounts in... 

M. J. Whitaker and M. F. Bryant, The Determination of Pyritic Sulfur in Coal or Iron(III) in Aqueous Solutions by Flow Injection Analysis. J. Coal Qual., 4 (1985) 68. 

To measure the validity of the EPM technique coals were chosen in which sulfate sulfur as determined by ASTM methods equaled zero and in which pyritic sulfur was minimal as determined by ASTM methods and as observed by optical microscopy. Since inorganic sulfur contents are small, any discrepancies between EPM and ASTM organic sulfur contents due to inaccurate pyrite or sulfate analysis also should be small. As can be seen in Table II the EPM analyses very closely approach those of the ASTM. 

Depending on the prior coal treatment, the sulfur value from the ultimate analysis may include all three of them. Various methods are available for ultimate sulfur determination (e.g., ASTM D-4239 [38], ISO 334 [39]). All are based on the combustion of the sulfur to produce a sulfete, which is measured gravi-metrically or volumetrically. Even x-ray fluorescence can be used to determine the total sulfur. To differentiate between the types of sulfur (ASTM D-2492 [40], ISO 157 [41]), the coal sample is treated with dilute HCl solution in which only sulfete sulfur is soluble. The pyrite sulfur is determined by subtracting the amount of soluble iron in dilute HCl from the amount of iron soluble in HNO3. From this difference, the quantity of FeS2 can be calculated. Together with the total sulfur, the organic sulfur can be determined by difference. Also x-ray spectroscopy can proof the chemical state of the sulfur in the coal. 

Multivariate analysis demonstrated that the organic sulfur content of coal plays a stronger role in determining the sulfur content of tar or gases, whereas the pyritic sulfur content plays a key role in determining the sulfur content of the char, at the low-temperature (500°C) devolatilization conditions. With increasing peak devolatilization temperature, however, the gaseous sulfur yield increases at the expense of char sulfur. ... 

Two methods have been proposed for the analysis of sulfur in impure samples of pyrite, EeS2. Sulfur can be determined in a direct analysis by oxidizing it to S04 and precipitating as BaS04. An indirect analysis is also possible if the iron is precipitated as Ee(OH)3 and isolated as Ee203. Which of these methods will provide a more sensitive determination for sulfur What other factors should be considered in deciding between these methods ... 

Sulfide grains sampled from the Type II veins in the high gold zones from two drill cores were prepared for sulfur isotope analysis. Pyrite samples (n=5) have average 5 S of 8.0 + 0.5 %o and a range of 7.6 - 8.3 7oo, whereas arsenopyrite (n=4) averages 7.9 + 0.5 %o and ranges between 7.3 - 8.4 %o (Fig. 3). 

Thallium in Pyrite. In 1867 Dr. E. Carstanjen found that the flue dust from the pyrite-roasting kilns of L. Rohr s sulfuric acid plant at Oranienburg was unusually rich in thallium. It yielded on analysis 3.5 per cent of metallic thallium. By working up a large quantity of flue dust from several kilns, he prepared twenty or thirty pounds of the metal. 

Some general problems associated with the determination of sulfur in coal are nonuniform distribution of pyrite particles, failure to convert all the sulfur to sulfate, and loss of sulfur as sulfur dioxide during the analysis. The nonuniform distribution of pyrite necessitates the collection of many sample increments to ensure that the gross sample is representative of the lot of coal in question. Pyrite particles are both hard and heavy and have a tendency to segregate during the preparation and handling of samples. Because the particles are harder, they are more difficult to crush and pulverize and tend to concentrate in the last portion of material that remains from these processes. 

Elemental Sulfur. In 1942, Chatterjee (44) reported the presence of elemental sulfur in weathered Indian coal but not in fresh samples. He suggested that, during weathering, pyrite is first oxidized to ferrous and ferric sulfates, and that then ferric sulfate oxidizes pyrite to elemental sulfur. The presence of elemental sulfur in U.S. coals was confirmed recently by Richard et al. (45) and White and Lee (46). Duran et al. (47) used extraction and gas chromatographic analysis to determine elemental sulfur in a suite of U.S. coals. They found that elemental sulfur (0.03-0.17%) is present in coal that has been exposed to the atmosphere, but is absent in pristine samples that have been processed and sealed under a nitrogen atmosphere. These data support Chatteijee s discovery that elemental sulfur in coal is a weathering product. 

A procedure for the direct determination of the sulfate, sulfide, pyritic, and organic sulfur in a single sample of coal has been reported by McGowan and Markuszewski (17). The method uses various strengths of perchloric acid as the selective oxidizing agent. The results obtained for the analysis of three coals were comparable to ASTM results and the relative standard deviations for nine sulfate, four pyritic, six organic, and four total recovered sulfur determinations were 2.7, 3.4, 2.4, and 2.4%, respectively. 

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