Characterization of Coals from the Candiota, Butiá-Leão and Santa Terezinha coal deposits, Rio Grande do Sul, Brazil
Poster Presentation

By Jason C. Willett¹, Robert B. Finkelman¹, and Wolfgang Kalkrueth²

¹U.S. Geological Survey, Mail Stop 956, Reston, VA 20192, USA.
²Departamento de Geologia, Instituto de Geociências Universidade Federal do Rio Grande do Sul Av. Bento Gonçalves, 9500, 91501-970 Porto Alegre, RS, Brasil.

Introduction

The southern region of Brazil, comprising Paraná, Santa Catarina and Rio Grande do Sul states (fig. 1), has been known for its abundant and economically important coal beds since the beginning of the 20th century. The majority of the total coal resources occur in Rio Grande do Sul (89 percent).

Figure 1. Map showing coal deposits and a chart of stratigraphic nomeclature of the coal-bearing and associated strata for the study area.

This paper discusses recently established models of coal formation for these deposits within a sequence stratigraphic framework and related coal properties based on petrographic and chemical analyses. The coal deposits in this paper are Candiota, Leão–Butiá and Santa Terezinha (fig. 1).

Geologic Setting

Coal occurrences in Rio Grande do Sul are historically assigned to the Rio Bonito Formation (fig. 2), a fluvial to marine sandstone and shale-prone lithostratigraphic unit of Early Permian age (Artinskian/Kungurian). Coals in Rio Grande do Sul were deposited in a back-barrier depositional setting, an interpretation based on regional sequence stratigraphy analysis (for example, Holz, 1998) and tissue preservation and gelification indices derived from maceral analysis (for example, Alves and Ade, 1996; Holz and Kalkrueth, in press). The study area is part of a tectonic unit in southwestern Gondwana known as the Paraná Basin, a large intracratonic basin (for example, Milani and others 1994). This basin is located in the central-eastern part of the South American Platform (fig. 1). The sedimentary fill of the basin is divided by Milani and others (1994) into six second-order depositional sequences from Ordovician-Silurian to Late Cretaceous ages. The coal-bearing Rio Bonito Formation is located at the base of the Carboniferous/Early-Triassic Sequence (Milani and others 1994), which forms the thickest sedimentary sequence of the basin (2800 m).

Figure 2. Stratigraphy of the study area.

An overview of the general stratigraphy of the coal-bearing succession is given in figure 2, which shows the entire Early Permian interval in southernmost Brazil (Sakmarian to Kungurian/Ufimian). This interval comprises the lithostratigraphic units Itararé, Rio Bonito, Palermo, and Irati (base) and records a second-order transgressive cycle, which began at the time of deposition of the topmost Itararé unit and has its maximum flooding surface within the Palermo Formation (for example, Milani and others, 1994; Holz, 1999). This second-order cycle is punctuated by important third-order base-level falls, with generation of several third-order depositional sequences. The two coal-bearing intervals of the Rio Bonito Formation are linked to third-order sequence 2 and the base of third-order sequence 3 (fig. 2). In Rio Grande do Sul state, most of the coals occur within the transgressive systems tract of sequence 2, as detailed by Holz (1998) and Holz and others (2000), with the recognition of four main depositional systems— alluvial fan, delta, lagoonal estuary, and barrier/shoreface. According to these studies, the accumulation of the Permian peats were linked to swamps and marshes in a lagoonal estuary setting.

Coal Petrography

Coal beds were collected as full-bed channel samples. Samples were analyzed by using vitrinite reflectance to establish coal rank and maceral analysis to determine petrographic composition following standardized procedures.

At Candiota and Leão–Butiá, vitrinite reflectances vary from 0.41 to 0.52 percent R_random, indicating subbituminous rank for the entire coal-bearing successions at the two locations. At Santa Terezinha, where the coal beds occur in a depth interval from 400 to 1000 m, coal rank is significantly higher (0.55–1.02 percent R_random), indicative of high volatile bituminous coals. In this area, the coal beds have been altered in many places by diabase intrusions that locally have raised the rank level to semi-anthracite and anthracite.

The petrographic composition of the coal beds is shown in terms of maceral groups (fig. 3). The ternary diagram shows that the coals are dominated by vitrinite and inertinite macerals, with liptinite macerals not exceeding 20 percent volume. At Leão–Butiá, vitrinite is dominant in all beds (fig. 3), whereas, at Candiota and Santa Terezinha, a number of coal beds are characterized by high concentrations of inertinite macerals (>50 percent volume).

Figure 3. Ternary diagram showing petrographic composition of the coal seams.

Coal Chemistry Methods

Thirty-nine coal samples from the three basins were analyzed for major, minor, and trace elements using the U.S. Geological Survey's analytical protocol illustrated in figure 4. The analytical results are presented in tables 1, 2, and 3. A statistical summary for those elements that may be of environmental concern is shown in table 4.

Figure 4. Flow diagram of procedures used after December 1994 for the analysis of coal samples collected. (Current ASTM procedures effective July 1991). ASTM, American Society for Testing and Materials; USGS, U.S. Geological Survey.

In addition to the chemical analysis, several samples were selected for semi-quantitative X-ray diffraction (XRD) analysis of the low-temperature ash (LTA) using the procedures described by Hosterman and Dulong (1985). Polished pellets of crushed coal less than 200 mesh were examined by scanning electron microscopy (SEM) with an energy dispersive X-ray analyzer (EDX).

A selective leaching procedure (fig. 5) was used on one sample (WK-38-98) to semi-quantitatively determine the modes of occurrence of 45 elements. The sequential selective leaching procedure used was similar to that described by Palmer and others (1993), and modified from Finkelman and others (1990). Duplicate 5-g coal samples were leached sequentially with 35 ml each of 1N ammonium acetate (CH₃COONH₄), 3N hydrochloric acid (HCl), concentrated hydrofluoric acid (48 percent HF) and 2N (1:7) nitric acid (HNO₃) in 50-ml polypropylene tubes. Each tube was shaken for 18 hours by a motorized shaker. Because of the potential for gas formation during the leaching procedure, each tube was enclosed in double polyethylene bags so that gas can escape but liquid cannot be released. After each leaching step, the coal slurries were centrifuged and the resulting solutions analyzed by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) and inductively coupled plasma-mass spectroscopy (ICP-MS). Each sample was washed at least five times with distilled water, using an ultrasonic cleaner to remove the solvent. After removing the solvent and drying the residual solid, about 0.5 g of this material were removed from each tube for the determination of 28 elements by instrumental neutron activation analysis (INAA) and the determination of Hg by cold vapor atomic absorption analysis (CVAA).

Figure 5. Selective leaching protocol.

Data for the leachates and residues were processed to derive the percentages of each element leached by each of the four leaching agents. The calculated percentages were then used as an indirect estimate of the modes of occurrence of 45 elements. We estimate an error of up to ±20 percent for the leached percentages reported by comparing data for the residual fractions with data for the solutions.

Chemistry Results and Discussion

All of the samples had high ash yields; most yields were greater than 35 percent (table 2). The mean for all three coal fields was about 48.5 wt percent on a remnant moisture basis with a range of 27.4 to 83.1 wt percent. Despite the relatively high ash yields, the concentrations of most trace elements are not enriched when compared with data for U.S. coals (table 4), which have considerably lower ash yields (about 15 wt percent). The Brazilian coals, which have ash yields three times the average for U.S. coals, have trace-element concentrations double the average for U.S. coals when compared on an equal energy basis.

The oxide sums (table 2) for all of the Brazilian coal samples were near 100 percent. For two samples (99-182 and 99-190), adjustment of the oxide sums for SO₃ complexed by CaO and MgO was substantial. Only two samples (99-163 and 99-341) had low oxide sums of 83 and 89 percent, respectively, after adjustment for SO₃. Proximate, ultimate, and forms of sulfur data were obtained for 21 samples (table 1). The range for total sulfur was 0.28 to 11.46 wt percent, with an average of 2.15 on an as-received basis. Most of the variation is owing to pyritic sulfur that ranged from 0.05 to 10.5 wt percent with an average of 1.66 percent.

The SiO₂ content of the ash is remarkably uniform (60–75 wt percent), with only a couple of exceptions. Two samples (99-163 and 99-341) have low oxide sums (51.7 and 50.6 percent SiO₂, respectively). Samples 99-175 and 99-226 (47.6 and 54.3 percent, respectively) have high Fe₂O₃ concentrations, and samples 99-182 and 99-190 (33.7 and 52.4 percent respectively) have high CaO and MgO concentrations. One sample 99-224 (50.5 percent) has high concentrations of Al₂O₃, CaO, and Fe₂O₃. The uniformly high SiO₂ concentrations and high SiO₂/Al₂O₃ ratios (table 2) indicate that quartz is a major component of all of the coal samples. The XRD results (table 5) of selected samples indicate quartz contents up to 60 percent of the LTA. The high quartz contents may explain the relatively low concentrations of most trace elements in the coals from Brazil compared to U.S. coal (table 4).

Most samples had ash yields greater than 35 percent, but most of the major oxides were in small percentages of the total ash yield. The Na₂O percent in the ash is very low, generally less than 0.4 wt percent. Only six samples, all from the Santa Terezinha coal field, have more than 0.42 wt percent Na₂O. P₂O₅ is also very low; only three samples having 0.1 wt percent or more. CaO contents are low, less than 4 wt percent, with only three exceptions (samples 99-182, 99-190, and 99-223). K₂O, MgO, and TiO₂ contents are also relatively low, with few exceptions. Only Fe₂O₃ shows significant variation with a range of about 1 to more than 30 wt percent in the coals from Santa Terezinha and Leão and about 3 to 19 wt percent in the coals from Candiota.

Scanning Electron Microscopy-Energy Dispersive X-Ray Analysis

Results from the SEM-EDX analysis of the polished pellets are broadly consistent with the bulk chemical and mineralogical analyses. The high ash yields were reflected in abundant mineral clusters and dispersed grains in the coal matrix. Quartz and clays are common in all of the samples studied. Texturally, most of the minerals appear to be detrital, although siderite and calcite nodules and kaolinite blebs are probably diagenetic. Several epigenetic calcite vein fillings were observed in the polished blocks of sample WK-38-98. Both syngenetic and diagenetic pyrites were observed. Typical mineral textures are depicted in figures 6A–D. No attempt was made to quantify the minerals or the elements detected in the sulfides and accessory mineral phases.

Figure 6A. Bright spots are siderite found in sample 99-036 from the Candiota coal field.	Figure 6B. Example of the amount of inorganics found in sample WK-38-98 from the Leão coal field.
Figure 6C. Carbonate mineral found in sample 99-223 from the Leão coal field.	Figure 6D. All of the bright spots are pyrite found in sample 99-226 from the Leão coal field.

Six samples from the Leão coal fields were examined. Sample WK-38-98 was the first sample received, so it was studied intensively and was the only sample that was selectively leached according to the USGS leaching protocol. All had abundant pyrite, some with detectable copper. Other minerals observed included zircon (some with Hf, Th, or U), mica, sphalerite, the rare-earth phosphates monazite and xenotime, siderite (with Mn), calcite, ilmenite, rutile, feldspar, barite, chalcopyrite, and one cube of sylvite.

Five samples from the Candiata coal field were studied. The minerals and accessory elements observed included siderite nodules containing traces of Mn, sphalerite containing traces of Cd, pyrite (some with traces of As), rare-earth phosphates containing traces of Th, zircon, apatite, galena, rutile, feldspar, chromite, chalcopyrite, and a unidentified tin oxide.

The minerals observed in four samples from the Santa Terezinha coal field include rutile, siderite nodules containing traces of Mn, calcite, pyrite, feldspar, barite, rare-earth phosphates, zircon, chalcopyrite, and clausthalite (PbSe).

Selective Leaching Results

Results from the selective leaching protocol on sample 98-38 showed good correlation with the mineralogical analyses. Results indicate that many elements (Li, Be, Mg, Al, K, Sc, V, Cr, Rb, Zr, Hf, Sn, and U) are primarily (>50 percent) leached by the hydrofluoric acid and are probably associated with silicates such as the clays. Other elements (Ti, Ta, W) leached by the hydrofluoric acid are likely to also occur as oxides (titanium oxides were commonly observed with the SEM). Fe, Cu, Zn, As, Se, Sb, Tl, and Pb are primarily leached by the nitric acid and are undoubtedly associated with the sulfides (pyrite and chalcopyrite were observed by SEM). Ca and Mn, leached by the hydrochloric acid, are associated primarily with the carbonate minerals (calcite and siderite were observed by SEM). Substantial amounts of several elements (Co, Ni, Mo, Ba, Sr, and Cs) were leached by several solvents, indicative of multiple associations. P, Th, and most rare-earth elements were leached by hydrochloric acid, consistent with their being in phosphate minerals such as monazite and xenotime that were observed by SEM. Sodium was the only element that was primarily leached by the ammonium acetate, indicating that it is in ion-exchangeable sites. Table 6 contains the summary of the selective leaching results; figure 7 illustrates the selective leaching results for several elements.

Figure 7. Leaching results from sample WK-38-98 for selected elements.

Environmental Considerations

Compared to U.S. coals, Brazilian coals have relatively low concentrations of most trace elements considering their higher ash yields, with a mean of 48.5 wt percent. In direct comparison on a whole-coal basis (table 4), Brazilian coals are lower in As, Cd, and Se. On an equal-ash basis, Brazilian coals are higher only in Mn, V, and possibly U.

The average concentrations for elements of environmental concern (As, Se, Pb, Cd, Hg, Tl) are similar to or less than the mean for U.S. coal (table 4). The sample (99-226) with the highest pyritic sulfur content (10.5 wt percent) also had the highest concentrations of As, Mo, Tl, Cr, Co, and Ni and the second highest concentrations of Hg and Sb. The mercury values of three samples from the Leão coal field exceed 0.4 ppm, more than twice the Hg average for coals from the United States and Brazil. The calorific value of the 39 coals from Brazil is about half that of the average for U.S. coals. Recasting the element concentrations into an equal-energy basis, table 4 indicates that combustion of the Brazilian coals would produce 5 to 10 times the amount of most elements in the combustion products compared to an equal-weight U.S. coal. Most of the elements, Hg and Se being exceptions, would be concentrated in the bottom ash or fly ash.

Regional and Stratigraphic Variations

The coals from the Santa Terezinha area have the highest mean concentration of CaO, MgO, Mn, and Sr, reflecting the presence of carbonates. XRD analysis of the LTA from sample 99-182, the sample with the highest CaO (17.3 wt percent) and MgO (4.8 wt percent) values, indicated the presence of at least 30 percent calcite, ankerite, and dolomite (table 5). The LTA from sample 99-175 had 15 percent siderite, reflecting the high Fe₂O₃ (34.4 wt percent) content. In contrast, sample 99-226 from the Leão coal field had a similar Fe₂O₃ (33.0 wt percent) content, but, in this coal, the iron is present as pyrite (25 percent) rather than siderite. In general, the Leão coals have the highest Fe₂O₃ contents, as well as the highest As, Se, Mo, Pb, Hg, Tl, and Bi contents, reflecting the higher sulfide content of these coals. The coals from the Candiota coal field have the lowest values for 27 of the 40 elements considered (table 3).

Fourteen samples (99-250–9-345) from the Candiota Coal Field are benches from a core and represent the full thickness of the 12.4-m thickness of the coal bed. There is little systematic vertical variation of major or trace elements in the coal bed. Only CaO and Fe₂O₃ appear to show a trend with higher values in the bottom half of the bed (fig. 8).

Figure 8. Borehole SGQ 26–calcium and iron oxide trends.

Conclusions

Despite the high ash yields of the Brazilian coal samples, the concentrations of most trace elements were well within the range typical for world coals (Swaine, 1990) and for U.S. coal (Bragg and others, 1997). The concentrations of elements that may have potential environmental or health hazards are similar to the concentrations found in U.S. coals. However, because of the lower calorific value, combustion byproducts from Brazilian coal will have 5 to 10 times the concentration of non-volatile elements than U.S. coal combustion byproducts. The modes of occurrence of the elements in the Brazilian coal samples are consistent with most coals that have been analyzed using the USGS approach over the last 10 to 15 years (Finkelman 1995).

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