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Mineralogical and Elemental analysis of Some High Sulfur Indian Paleogene Coals: A Statistical Approach Binoy K Saikia, Peipei Wang, Ananya Saikia, Hongjian Song, Jingjing Liu, Jianpeng Wei, and Upendra N. Gupta Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/ef502511t • Publication Date (Web): 29 Jan 2015 Downloaded from http://pubs.acs.org on February 5, 2015
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Mineralogical and Elemental analysis of Some High Sulfur Indian Paleogene Coals: A Statistical Approach Binoy K Saikia
a*
, Peipei Wang b, Ananya Saikia a, Hongjian Song b, Jingjing Liu b,
Jianpeng Wei b , Upendra N Gupta a, a
Coal Chemistry Division, CSIR-North East Institute of Science & Technology, Jorhat-
785006, India b
State Key Laboratory of Coal Resources and Safe Mining, China University of Mining
and Technology (Beijing), D11, Xueyuan Road, Haidian District, Beijing 100083, P.R. China
Abstract The northeast (NE) of India has a prominent reserve of Oligocene to Eocene coals of significant interest. An extensive study on the mineralogical and elemental composition of nine high-sulfur coal samples from the various Paleogene coalfields of the northeast (NE) of India was conducted to understand the modes of occurrence and distribution patterns of the minerals as well as the trace and rare earth elements (REEs) in these coals. The majority of minerals associated with the NE region (NER) coals mainly includes, kaolinite, illite, mixed-layer illite/smectite, zircon, pyrite, goyazite, chlorite, xenotime, siderite, Ti-oxide (anatase or rutile), Fe-bearing gahnite, rhabdophane, carbonate and jarosite. The minerals are of different origins such as authigenic, syngenetic and epigenetic. The presence of ferropseudobrookite is observed for the first time in any Paleogene Indian coals. Calcite is dominant in the coals. The NER coals are enriched in F (70.40 µg/g), Pb (21.84 µg/g), Sr (125.69 µg/g), Zr (285.93 µg/g) and Ba (117.80 µg/g).
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However, NE coals contain lower REEs such as Ce (17.92 µg/g), Eu (0.30 µg/g) and Y (8.51 µg/g), La (30.82 µg/g), Ce (65.61 µg/g), and Nd (27.55), as well as Th (18.68 µg/g). The high Zr and Sr contents in the northeast (NE) Indian coals are attributed to the presence of zircon and goyazite in the coals. The statistical analysis of the major oxides, sulfur and trace elements in the coals is also performed to know their co-relations. Alumino-silicate group minerals are prominent in NE Indian coals.
Kew words: Indian coals; High sulfur coal; Elements in coals; REEs; PCA analysis
*Corresponding author:
[email protected];
[email protected] (Tel. No. +91 376 2372851)
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Introduction Indian coal deposits generally occur in two main geological horizons: a) the Lower Gondwana sediments (Permian); and b) the early Paleogene sediments (Oligocene to Eocene). Most of the major coal deposits of India (about 99%) are Gondwana coals scattered in the eastern and southeastern parts of the country. The Paleogene coals are located in the northeastern states, as well as in Jammu and Kashmir. The Indian coal occurrences have the general properties of the Southern Hemisphere Gondwana coal (high ash yield, high sulfur), whose seams are interbanded with mineral sediments.1 The Cenozoic coals in the northeast states of India are of interest to the coal geologists due to their unusual physico-chemical attributes with high organic sulfur (75-90%), with lesser amounts of pyritic and sulfate sulfur. Some researcher described these northeast Indian coals as abnormal coals because their physico-chemical characteristics and behaviour are not commensurate with properties adopted for rank classification.2 Mineralogical and elemental studies in coal samples provide information about the paleoenvironment of peat accumulation and coal formation along with formational3 and regional history4, identification of the mode of occurrence and potential mobility of particular trace elements, understanding of potential barriers to gas drainage, and evaluation of the behaviour of different coals in the preparation and utilization processes.5 The mineralogical studies on coals revealed the presence of about 201 minerals in their crystalline matter such as quartz, kaolinite, illite, calcite, pyrite, plagioclase, K-feldspar and gypsum, and occasionally dolomite, ankerite, siderite, Fe oxyhydroxides, sulphates and aluminum-hydroxides5-17. The primary motivation for the study of coal-bound mineral matters can also be attributed towards utilization of coals 18-
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, environmental
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, and health interests
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. Improper utilization of coals having
higher concentration of hazardous trace elements may cause severe health problems 24-25. Thus, the knowledge of the distribution patterns of the trace elements in coal is essential in coal processing 26, environmental impact assessment of coal and some aspects of coal formation. Most of the deleterious effects from coal are caused by the emissions of toxic elements such as arsenic, selenium and fluorine causing toxicological and environmental effects on the ecosystem and human health 27. So, the investigation of the distribution of mineral matter and elements present in any coals are of prime importance not only for its geochemical study but also for clean coal initiatives. Various authors have reported the geochemistry of coals by assessing both the hazardous air pollutants (HAPs) and the high value elements or metals that would prove beneficial even if present in significant quantities 8-10,24, 28-29 . The lanthanides and yttrium (REY, or REE if Y is not included) play a key role in the manufacture of materials and products in the modern world. They have now become a “corner stone” for modern technologies with high prices. Thus, the investigation for REY in coals is crucial from an economic point of view. Seredin and Dai (2012)30 published an extensive review paper on the distribution of REY in coals around the world. However, there is a lack of knowledge on the REY contents in the NE Indian high sulfur coals. The NE Indian coals have a high sulfur content, which retards its direct industrial utilization. Most of the sulfur is organically bound in coals (75–90%) 31. The high sulfur content in the Northeast Indian coals resulted from the increased availability of sulfate ions in sea water coupled with the activity of anaerobic bacteria
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. The geochemical
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and detailed elemental studies of these high organic sulfur northeast Indian coals are scanty as per literature survey. Geologic Background The Paleogene NE coals are distributed in the states of Meghalaya, Assam, Arunachal Pradesh and Nagaland of India (see Figure 1). The geological setting of these coalfields is described by several authors
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. The coalfields of Assam, Nagaland and
Arunachal Pradesh are aligned along an active mobile belt, which experienced intense tectonic disturbance resulting in a series of imbricate overthrusts known as ‘zone’ or ‘Belt of Schuppen’ 37. The Tikak Parbat Formation of the Barail Group of Oligocene age is the storehouse of the major resources of coal in this belt. The coal- bearing sediments of Tikak Parbat Formation in the ‘Belt of Schuppen’ are preceded by marine and followed by fluvial sedimentary sequences. The coal deposits of Meghalaya occur within a narrow belt extending from the Garo Hills in the west through the Khasi Hills in the middle of the Jaintia Hills in the east and range in age between the Palaeocene and Eocene. The Sylhet Limestone Formation, especially its Lakadong sandstone is the principal coal-bearing horizon in Garo, Khasi and Jaintia Hills. The Paleogene coal deposits of Assam and Arunachal Pradesh occur within a long, narrow belt trending northeast-southwest (NE-SW), ranging in age between Upper Eocene to Oligocene. These coal deposits are thought to have originated in the foreland basins, especially in the zone of Schuppen38 and are found in the Barail Group and are divided into three Formations. The different coal bearing patches are: (i) Desai Valley coalfield, (ii) Jhanji Valley coalfield, (iii) Nazira coalfield, (iv) Safari Valley coalfield, (v) Jeypore coalfield, (vi) Makum coalfield, (vi) Namchik-Namphuk coalfield. The
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thicker, economically viable seams are particularly confined to the Nazira, Jeypore, Makum and Namchik-Namphuk coalfields 39. The Nagaland coalfields are along the steep slopes of the western flanks of the Naga Hills. Coal seams occur in the Tikak Parbat Formation of the Barail Group of Oligocene age with two persistent seams occurring as the lower and upper members of the Tikak Parbat Formation and these coals are weakly coking 39. The mineralogical and elemental compositions, including REYs of nine medium to high sulfur coal samples from various Paleogene coalfields of the NE India are discussed in this present paper. The prime aim for this study is to have knowledge on the distribution pattern of minerals as well as degree of occurrence of individual elements (including REYs) in these coals. The data may be useful in the geochemical studies of these coals as well as in formulating their clean coal technology. The investigation of the REY contents also aims for the possibility of their potential economic sources from Indian coals. The statistical analysis was performed to better understand their mutual relationships.
Experimental Sections Coal samples Nine representative run-of-mine coal samples were obtained from the industrially important coalfields (Figure 1) in Assam (T1, T2, LD, TK and TP), Meghalaya (B and S) and Nagaland (MK and NAG) of NE India by using standard methods40. Approximately nine representative bulk samples with an approximate weight of 100kg each from a 12 m thick seam for each field was collected. The samples were
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reduced to ~1kg weight by the coning and quartering method. Samples were then crushed and sieved to 75%). The maximum and minimum ash yields are observed to be 19.80 and 3.68% for B and TIP coals respectively. The carbon and hydrogen contents of the coals fall in the range of 70-79% and 5-8% respectively. The proximate as well as the ultimate analysis indicate the rank of the coals to be sub-bituminous. The quality parameters of the coal samples are found to be different to each other, which may be due to the difference in geological age and depositional environment during coal formation. XPS analysis of sulfur in coals The XPS (Figure 2) shows the presence of sulfur in different forms in the samples studied due to the variable chemical environments resulting from the binding energies involved. However, all the coals show two peaks with a higher intensity at around 160161 and 165-166.5 eV apart from TK coal, which shows only one peak at around 166 eV. The first region indicates the presence of metal sulfides (e.g. pyrite), whereas the second region indicates the presence of organic sulfur (e.g. thiols). The XPS of these coals revealed that these coals are high in organic sulfur with less amount of inorganic sulfur,
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which is also reflected in the chemical analysis shown in Tables 1 and 2. However a detailed peak analysis of the XPS results is needed to better understand the organic sulfur functionalities in these coals. XRD analysis of minerals in coals Table 3 shows the crystalline phases recognized in each low temperature ash (LTA) of B, TK, T2, MK and LD coal samples. In the LTAs of the coal, quartz, illite, kaolinite, pyrite, jarosite, dolomite, dawsonite, ankerite and calcite were also observed. The LTA of B and MK coals have the highest concentrations of calcite and kaolinite with 61.3 and 60.3% respectively. These data are consistent with the ash chemistry as indicated in Table 4. Jarosite may probably be derived from the oxidation of pyrite in the coals. SEM-EDS analysis of minerals in coals The minerals in the coal samples (B and NAG) were identified by using a SEMEDS. The minerals in sample B mainly include kaolinite, illite, mixed-layer illite/smectite, zircon, pyrite, goyazite, chlorite, xenotime, siderite, Ti-oxide (anatase or rutile), Fe-bearing gahnite and rhabdophane (Figures 3a and b). Kaolinite occurs as fusinite cell-fillings in the B coal indicating an authigenic origin (Figure 3a-A). Kaolinite occurs along plane beddings, indicating a syngenetic (Figure 3a-B). Kaolinite also occurs along plane beddings (Figure 3a-C). The Ti-oxide, rhabdophane and goyazite in B coal are distributed in collodetrinite. Rhabdophane and goyazite in B coal are probably of authigenic origin (Figure 3a-C). Kaolinite, mixed-layer I/S, and Ti-oxides are distributed in collodetrinite in B coal (Figure 3a-D). Kaolinite along with the Fe-bearing gahnite, zircon, and Ti-oxides are distributed in the collodetrinite (Figure 3a-E). The framboidal
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and dispersive pyrite are also seen in collodetrinte (Figure 3a-F). Figure 3b shows the pyrite distributed in kaolinite (Figure 3b-A). Pyrite occurs as fracture-filling, massive, and dispersive forms in B coal (Figure 3b-B). The fracture-filling mode of occurrence suggested epigenetic origin. Xenotime, goyazite, illite, chlorite, and pyrite in kaolinite are also seen in B coal (Figure 3b-C). Figure 3b-D shows the siderite in B coal. The relation between pyrite and kaolinite (kaolinite distributed along the pyrite) showed that both the minerals are of syngenetic origin. The minerals in the NAG coal include kaolinite, occurring mainly as cell-fillings and a small proportion in collodetrinite (Figure 3c). However, other minor minerals were also found to be present including Ti-oxides, illite, ferropseudobrookite (Fe2+Ti2O5). The origin and mode of occurrence of ferropseudobrookite in NE Indian coals needs further research. Elemental composition of the coals The major and minor and elements were determined in the coal samples by using XRF and ICP-MS techniques (Table 4 and 5). There are differences in trace element concentrations among these Eocene and Oligocene coals of NE India. It is also found that the concentrations of the elements in these northeast Indian coals significantly differ from the other coals of the world. Compared to the average values for world hard coals with the elimination of sample B as an outlier, elements such as Ba, Be, Co, Cu, F, Ga, In, Mo, Pb, Rb, Sb, Tl, V and W are depleted in comparison with the average found in world coal samples as they fall in the range of concentration coefficient (CC)
