Comments on paper" Geochemical Characteristics of Rare-Metal

Jul 6, 2018 - K2) coal from the Moxinpo Mine, Chongqing, China. However ... and geological interpretation, some of which at least may need careful re-...
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Comments on paper "Geochemical Characteristics of Rare-Metal, Rare-Scattered, and Rare-Earth Elements and Minerals in the Late Permian Coals from the Moxinpo Mine, Chongqing, China" Shifeng Dai, Colin R. Ward, David French, and James C. Hower Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00976 • Publication Date (Web): 06 Jul 2018 Downloaded from http://pubs.acs.org on July 8, 2018

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Comments on paper "Geochemical Characteristics of Rare-Metal, Rare-Scattered, and Rare-Earth Elements and Minerals in the Late Permian Coals from the Moxinpo Mine, Chongqing, China" Shifeng Daia,b, Colin R. Wardc, David Frenchc, James C. Howerd a

State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology, China

b

School of Resources and Geosciences, China University of Mining and Technology, Xuzhou 221116, China

d

PANGEA Research Centre, School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney, NSW 2052, Australia c

University of Kentucky, Center for Applied Energy Research, 2540 Research Park Drive, Lexington, Kentucky 40511, United States

Abstract The paper “Geochemical Characteristics of Rare-Metal, Rare-Scattered, and Rare-Earth Elements and Minerals in the Late Permian Coals from the Moxinpo Mine, Chongqing, China”, by Qin et al. (2018) in Energy & Fuels provides potentially useful data on the proximate analysis, geochemical, and mineralogical properties of the Late Permian (No. K2) coal from the Moxinpo Mine, Chongqing, China. However, after a careful examination of the whole paper, we regrettably found that the paper by Qin et al. is affected by issues of coal rank determination, mineralogical identification, geochemical composition, and geological interpretation, some of which at least may need careful re-consideration.

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The paper “Geochemical Characteristics of Rare-Metal, Rare-Scattered, and Rare-Earth Elements and Minerals in the Late Permian Coals from the Moxinpo Mine, Chongqing, China”, by Qin et al.1 provides potentially useful data on the proximate analysis, geochemical, and mineralogical properties of the Late Permian (No. K2) coal from the Moxinpo Mine, Chongqing, China. We read it with great interest because (1) we have recently investigated the same coal in the same area;2 (2) this coalfield is one of the important coal producing areas in southwestern China; and (3) the coals in the this coalfield and surrounding coal deposits have highly elevated concentrations of critical elements such as U, Re, V, Cr, Se, and rare earth elements and Y (REY, or REE if Y is not included), making them potential raw material sources for these important elements.2,3,4 Regrettably, the paper by Qin et al. 1 is affected by issues of coal rank determination, mineralogical identification, geochemical composition, and geological interpretation, some of which at least may need careful reconsideration. 1. Coal rank and calorific value With respect to the coal rank, the dry, ash-free percentage of volatile matter (VMdaf) calculated from values in Table 1 of Qin et al. ranges from 17.1% to 25.4%.1 If the highest-ash sample, with 41.2% ash (air dried basis), is excluded, the range only extends to 23.0% VMdaf. This is similar to the range of 19.9% to 21.9% (daf) reported for the K2 seam by Dai et al. 2 Both sets of data indicate a low volatile bituminous coal rank. However, value of 0.97% for vitrinite random reflectance given for the coal by Qin et al.1 indicates a high volatile A bituminous rank level. This is well below the values of 1.28% to 1.40% for vitrinite random reflectance reported for the K2 seam by Dai et al., 2 which are also consistent with low volatile bituminous material. In the absence of notes on the methods used for the coal petrology (except for citation of ASTM Standard D388-12 5

), or on the source of the reflectance data, the validity of this single reflectance measurement is

clearly open to question. In Table 1, and also in the text of section 4.1 of Qin et al. 1, the units of calorific value are referred to as “KJ/kg”. Despite the capital “K” (referring, if properly used, to “Kelvin”), these are probably meant to represent “kilojoules per kilogram” (kJ/kg). Even so, the magnitude of the values indicates that they should be given as MJ/kg, or megajoules per kilogram. In addition, only by reference to the (inadequate) footnote in Table 1 is the reader made aware that these represent net and not gross calorific values. 2

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2. Mineralogy Section 4.6 in Qin et al.’s paper indicates that the main minerals in the coals are “clay minerals (kaolinite), pyrite, calcite, and quartz along with traces of rutile”. 1 This is supported by an X-ray diffractogram (Figure 8 of Qin et al. 1) showing peaks derived from those minerals. However, despite section 3.3. of the text indicating that the samples were scanned from 5 to 70 degrees two-theta, this plot extends only from 10 degrees, and misses the opportunity to prove or disprove the presence of clay minerals such as illite or interlayered illite/smectite, which have their strongest peaks at around 8.4 to 8.7 degrees and between 5 and 8 degrees, respectively. This omission is potentially significant, as Dai et al. also note the presence of both potassian and ammonian illite in low-temperature ashes of the K2 seam. 2 Unlike the work of Dai et al., 2 the evaluation of the minerals in the coal by Qin et al.1 was not quantified, nor do the results from the full suite of samples appear to have been evaluated to establish whether any other species might also be present. Since the relative proportions of the individual major element oxides vary significantly (Table 2 of Qin et al. 1 ), especially if they are recalculated to give a total in each case of 100%, some variation may be expected in the mineral assemblages occurring in the coals as well. In section 4.6.1 Qin et al. note that energy-dispersive X-ray spectroscopy (EDX) data indicate that the kaolinite in the K2 coal contains Ti, Na, Mg, S, and Ca. 1 The crystal lattice of kaolinite may accommodate a very small proportion (around 0.2%) of Ti,6 but otherwise consists only of Al, Si, O and H. Re-interpretation of the statement in this light suggests that either the minerals analysed by EDX spectroscopy were not kaolinite, or that the kaolinite was admixed with other phases in the areas analysed. For example, the high percentage of Ti indicated by the EDX data in Figure 9C of Qin et al. 1 may represent fine particles of a TiO2 mineral, such as anatase, intimately associated with the kaolinite component (cf. Ward et al.,7), while the mixture of Al, Si, Ca and S, plus minor Na and Mg, in Figure 9B may possibly represent a mixture of kaolinite, gypsum, and illite or illite/smectite. Although the co-occurrence of pyrite and rutile (or, more likely, anatase) in Figure 10H is noted in the text by Qin et al., 1 no mention is made of the Ti-bearing phase in the image itself, nor in the caption of the figure. Despite being described in the text only as xenotime, and not described at all in the figure caption, the material in Figure 12C appears from the EDX data to be a mixture of 3

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xenotime and kaolinite. Ideally the presence of yttrium should be further confirmed by identification of the Yk peaks in the EDX spectrum. Section 4.6.1 Qin et al. 1 suggest that: “… the peat bog of Moxinpo coal mine was possibly not far from the provenance of the Emeishan, which mainly comprises gneiss and basalts. Thus, it can be inferred that kaolinite in the K2 coal is mostly derived from feldspar and quartz, which have been weathered and transported into the peat bog during the depositional and diagenetic stages”. 1 This appears to suggest a detrital origin for the kaolinite component. However, Figure 9C shows kaolinite, probably intimately mixed with a TiO2 mineral such as anatase, occurring in the cell lumens of the coal, a mode of occurrence that is unlikely for detrital sediment but is consistent with authigenic precipitation from solution in the pores of the original peat deposit (cf. Ward 8). While the elements that formed the kaolinite may have been derived from weathering in a nearby hinterland and transported to the peat bog in solution, they could also have been derived from in-situ alteration of contemporaneous volcanic ash or introduced in solution with groundwater from a more distal element source. In section 4.6.2 Qin et al. state, without support or reference, that: “Pyrite has a strong selectivity to maceral components, especially gelovitrinite, which is the preferred host for sulfide. 1 It is unclear whether this is intended to be a generic statement, covering coals in general, or whether it is meant to be a summary of the findings represented in Figure 10 and discussed in the text immediately preceding the sentence in question. If the former, it is not supported by wider experience (e.g., Chou 9; Dai et al. 10); if the latter, there are several different reasons for an association of pyrite with vitrinite, ranging from the conditions of vitrinite formation (e.g. organic gels supporting the growth of pyrite framboids) to the mechanical brittleness that facilitates fracturing and fluid migration paths. A simplistic assertion of “strong selectivity” implies a limited understanding of the different processes by which pyrite might be formed. In the conclusions to the paper 1, Qin et al. extend the statements in the discussion above to indicate that (all of) the “minerals in the K2 coal including kaolinite, pyrite, quartz, calcite, rutile, xenotime, and bastnäsite, were dominantly derived from the weathered and oxidized felsic-intermediate rocks of the Emeishan basalt”. 1 In addition to the authigenic rather than detrital formation of kaolinite as discussed above, it is impossible for pyrite and calcite in coal to represent detrital material from the sediment source region. Almost all pyrite found in coal, as well as most of the carbonate minerals, such as calcite, dolomite, ankerite, and siderite, are formed by crystallization in place, either within the original peat 4

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deposit; shortly after burial of the peat; or in the cleats, pores, and fractures of the coal after compaction and rank advance.8,11 Rare instances have been noted of detrital calcite, ankerite, and dolomite in coal, 12-14 but these are unusual because such carbonate minerals are expected to decompose in the acidic environment of the peat deposit. Dai et al. reported discrete calcite particles in the lowermost portion of the other coal (K1 Coal) in this coalfield, but suggested that they were derived from the mafic tuff associated with the coal rather than from the Emeishan basalt, and were transported only a short distance into the peat-forming mire.2 The mafic tuff underlying the K1 Coal in the area was deposited on the eroded limestone surface of the middle-Permian Maokou Formation, and then served as the substrate for the coal deposits. 2 Residual breccias made up mainly of limestone fragments from the Maokou Formation generally occur in the mafic tuff layer. 15 3. Geochemistry Qin et al. have used the ratio (Fe2O3+CaO+MgO)/(SiO2+Al2O3) in the coals as an indicator of the peat-forming environment, distinguishing between marine (> 0.23), terrestrial (< 0.23), and marine-terrestrial deposits. 1 Although one Chinese reference16, as presented in Table 2 of the paper by Qin et al.1 is cited to support this categorization, that index as an environmental indicator is not appropriate for peat-forming. Although Ca and Mg are richer in seawater than in river water,17 the ratio of Fe concentration in seawater to that in river water is only 0.1. 17 Even so, the water chemistry of the depositional environment is not necessarily linked to the ultimate mineral-matter chemistry, and a more searching evaluation is necessary if such a chemically-based environmental classification of coal formation is to be regarded as plausible. The major carriers of Ca and Mg observed in coals from southwestern China are generally epigenetic carbonate minerals, such as calcite, dolomite, and ankerite.12-14 Epigenetic minerals and their constituent elements form at a much later stage, possibly from fluids of quite different origin, and do not necessarily reflect the original peat-forming environment. Indeed, Qin et al. have also observed calcite as epigenetic fracture-fillings in their Figure 12A.1 Gypsum, another possible host for Ca, has also been observed in coal, especially lower-rank materials. This mineral may be formed syngenetically during peat accumulation,18 but in many cases is derived from organically-associated Ca in the maceral components.8,11,19,20 Such Ca may be mobile within the peat bed,21 or precipitated from dissolved Ca2+ and SO42− ions with evaporation of pore water. As noted by Qin et al. 1 and others8,22-24), gypsum may also be produced by reactions between calcite and sulfuric acid derived from oxidation of pyrite in the coal. The Ca associated with gypsum 5

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may thus have many different modes of origin, and cannot be used as indicator of peat-forming environment. This applies particularly to the calcite of epigenetic origin as reported by Qin et al. 1 Qin et al. determined the concentrations of rare earth elements and Y in the samples using inductively coupled plasma mass spectrometry (ICP-MS) after microwave-assisted digestion. 1 Although the standards used and related details are not provided by Qin et al.,1 such methods have been extensively used for trace element determination by many other authors. However, the zigzag normalized distribution patterns for the REY in most samples (with a few exceptions, such as K2-7 and K2-13) suggest that the REY data given by Qin et al.1 may be questionable. As pointed out by Dai et al.,25 normalized to the UCC (Upper Continental Crust), anomalies of REY in coal generally exist for redox-sensitive Ce and Eu, and, in some cases, non-redox-sensitive La, Gd, and Y. The other REY in the distribution patterns would not be expected to have anomalies. The smoothness of a normalized (e.g., UCC) REY distribution pattern provides a simple reliable basis for testing the quality of REY data from coals and other sedimentary rocks.25 Based on the distribution pattern of REY with negative Ce−Eu anomalies given by Qin et al., 1 and assuming that these data are reliable, the conclusion that “the sediment source region for the Late Permian coals from the Moxinpo mine was considered to be the basaltic Emeishan rather than the basaltic Kangdian Upland” 1 is not consistent with the previous statement in the same paper that felsic-intermediate terrigenous rocks at the top of the Emeishan basalt sequence are the dominant terrigenous materials. In addition to confusing relation between the basaltic Emeishan and the basaltic Kangdian Upland as used by Qin et al.,1 inferring the sediment source region for the coals as the basaltic Emeishan, based on Ce and Eu anomalies, is incorrect. Dai et al.26 and other subsequent studies,22,27-30 showed that coals with input from the same Emeishan basalts have large positive Eu anomalies. Based on the Al2O3/TiO2 ratios (12.33-25.44), the low concentrations of V, Sc, Co, Ni, Cu, and Zn, and the distinctively negative Eu anomalies for the K2 Coal, Dai et al.2 further concluded that inorganic materials in the K2 Coal were dominantly derived from the felsic-intermediate terrigenous rocks at the top of the Emeishan basalt sequence, rather than the mafic Emeishan basalts themselves. References (1) Qin, S., Gao K., Sun, Y., Wang, J., Zhao, C., Li, S., Lu, Q. Geochemical Characteristics of Rare-Metal, RareScattered, and Rare-Earth Elements and Minerals in the Late Permian Coals from the Moxinpo Mine, Chongqing, China. Energy Fuels 2018, DOI: 10.1021/acs.energyfuels.7b03791. (2) Dai, S., Xie, P., Jia, S., Ward, C.R., Hower, H.C., Yan, X., French, D. Enrichment of U-Re-V-Cr-Se and rare 6

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earth elements in the Late Permian coals of the Moxinpo Coalfield, Chongqing, China: Genetic implications from geochemical and mineralogical data. Ore Geol. Rev. 2017, 80, 1–17. (3) Dai, S., Yan, X., Ward, C.R., Hower, J.C., Zhao, L., Wang, X., Zhao, L., Ren, D., Finkelman, R.B., 2016. Valuable elements in Chinese coals: A review. Int. Geol. Rev. 60, 5-6, 590-620. (4) Dai, S., Finkelman, R.B. Coal as a promising source of critical elements: Progress and future prospects. Int. J. Coal Geol. 2018, 186, 155-164. (5) ASTM Standard D388-12. Standard Classification of Coals by Rank. ASTM International, West Conshohocken, PA. 2012. (6) Weaver, C.E., Pollard, L.D. The Chemistry of the Clay Minerals. Elsevier, Amsterdam, 213 pp. 1973. (7) Ward, C.R., Spears, D.A., Booth, C.A., Staton, I., Gurba. L.W. Mineral matter and trace elements in coals of the Gunnedah Basin, New South Wales, Australia. Int. J. Coal Geol. 1999, 40, 281-308. (8) Ward, C.R. Analysis, origin and significance of mineral matter in coal: An updated review. Int. J. Coal Geol. 2016, 165, 1–27. (9) Chou, C-L. Sulfur in coals: A review of geochemistry and origins. Int. J. Coal Geol. 2012, 100, 1-13. (10) Dai, S., Ren, D., Tang, Y., Shao, L., Li, S. Distribution, isotopic variation and origin of sulfur in coals in the Wuda coalfield, Inner Mongolia, China. Int. J. Coal Geol. 2002, 51, 237–250. (11) Ward, C.R. Analysis and significance of mineral matter in coal seams. Int. J. Coal Geol. 2002, 50, 135–168. (12) Shao, L., Zhang, P., Ren, D., Lei, J. Late Permian coal-bearing carbonate successions in southern China: coal accumulation on carbonate platforms. Int. J. Coal Geol. 1998, 37, 235–256. (13) Dai, S., Seredin, V.V., Ward, C.R., Hower, J.C., Xing, Y., Zhang, W., Song, W., Wang, P.. Enrichment of U– Se–Mo–Re–V in coals preserved within marine carbonate successions: geochemical and mineralogical data from the Late Permian Guiding Coalfield, Guizhou. China. Miner. Deposita 2015, 50, 159–186. (14) Wang, X., Zhang, M., Zhang, W, Wang, J., Zhou, Y., Song, X., Li, T., Li, X., Liu, H., Zhao, L. Occurrence and origins of minerals in mixed-layer illite/smectite-rich coals of the Late Permian age from the Changxing Mine, eastern Yunnan, China. Int. J. Coal Geol. 2012, 102, 26–34. (15) China Coal Geology Bureau. Sedimentary Environments and Coal Accumulation of Late Permian Coal Formation in Western Guizhou, Southern Sichuan, and Eastern Yunnan, China. Chongqing University Press, Chongqing, 1996, pp. 156–216 (in Chinese with English abstract). (16) Ye, D. M.; Luo, J. W.; Xiao, W. Z. Origin and application of maceral in southwestern China; Geological Publishing House: Beijing, 1997; pp 20−23. (in Chinese) (17) Reimann, C., de Caritat, P. Chemical Elements in the Environment. Springer-Verlag New York, Inc., New York, 1998, 397 pp. (18) Liu, J., Ward, C.R., Graham, I.T., French, D., Dai, S., Song, X.. Modes of occurrence of non-mineral inorganic elements in lignites from the Mile Basin, Yunnan Province, China. Fuel 2018, 222, 146-155. (19) Li, Z., Ward, C.R., Gurba, L.W., Occurrence of non-mineral inorganic elements in macerals of low-rank coals. Int. J. Coal Geol. 2010, 81, 242-25. 7

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