Coal is a resource primarily used for electric power generation, and currently supplies 41% of global electricity needs. Coal can also be considered as an economic source of strategically important ...elements, such as Ge, Ga, U, V, Se, rare earth elements, Y, Sc, Nb, Au, Ag, and Re, as well as base metals Al and Mg. The extraction and utilization of these critical elements from coal could result in a number of benefits, which will make this source an economically and environmentally attractive option especially for China, the U.S., Russia, India, and other countries that will remain major coal users for the foreseeable future.
•Coal is an economic source of strategically important elements.•The industrial utilization of critical elements from coal ash could result in many benefits.•Uranium had been and Ge, Se, and V are being currently industrially extracted from coal ash.•Laboratory-scale technologies have been devised to extract REY from coal ash and will shift towards industrial recovery.
Coal deposits have attracted much attention in recent years as promising alternative raw sources for rare earth elements and yttrium (REY), not only because the REY concentrations in many coals or ...coal ashes are equal to or higher than those found in conventional types of REY ores but also because of the world-wide demand for REY in recent years has been greater than supply. In addition to anomalies of enrichment or depletion of light-, medium-, and heavy-REY in coal deposits (normalized to Upper Continental Crust, Post-Archean Australian Shale, or North American Shale Composite), anomalies of redox-sensitive Ce and Eu, and, in some cases, of non-redox-sensitive La, Gd, and Y, could be used as geochemical indicators of the sediment-source region, sedimentary environment, tectonic evolution, and post-depositional history of coal deposits. Factors controlling REY anomalies in coal deposits include the geochemistry of terrigenous source rocks, ingress of hydrothermal fluids, influence of marine environments, percolating natural waters, volcanic ashes, and sedimentary environments of peat formation. Additionally, the smoothness of a normalized REY distribution pattern provides a simple but reliable basis for testing the quality of REY chemical analyses for coal and other sedimentary rocks.
This paper presents data on widespread abnormal accumulations of lanthanides and yttrium (REY) in many coal deposits worldwide. High REY contents (>0.1%) have been found in coal seams and coal ashes, ...as well as in the host and basement rocks of some coal basins.
For a preliminary evaluation of coal ashes as an REY raw material, not only the abundance but also the individual REY compositions were taken into account in this paper. Three REY distribution patterns for high-REY coal ashes are fixed, with LREY- (LaN/LuN>1), MREY- (LaN/SmN<1, GdN/LuN>1), and HREY- (LaN/LuN<1) enrichment. Four genetic types of REY enrichment in coal basin can be identified: 1) terrigenous type, with REY input by surface waters; 2) tuffaceous type, connected with falling and leaching of acid and alkaline volcanic ash; 3) infiltrational or meteoric ground water driven type, and 4) hydrothermal type, connected with ascending flows of thermal mineral water and deep fluids. It is shown that the main modes of REY occurrence in high-REY coals are in fine-grained authigenic minerals (REY-bearing aluminum phosphates and sulfates of the alunite supergroup, water-bearing phosphates and carbonates) and organic compounds. Stratabound and cross-cutting REY mineralization may occur in the host and basement rocks of some coal basins. There are tuffaceous and hydrothermal types of REY mineralization outside coal seams that are significantly different in geological settings, ore body shapes, and ore compositions, as well as in REY contents and distribution patterns. The data presented indicate that coal deposits should be regarded as promising objects for recovery of REY as economic by-products of coal mining and combustion. As REY are crucial metals for alternative power and energy-efficient technologies, identification of these resources during coal exploitation and utilization may not only increase beneficiation of coal deposits themselves but also will promote humanity's further movement on the “green road”.
► There are many coal deposits worldwide with high REY content (>0.1%) in coal ashes. ► Some of coal deposits contain high REY (0.1–14%) in host and basement rocks also. ► REY in high-REY coals occur mainly as authigenic minerals and organic compounds. ► Coal deposits may be considered as new sources for REY recovery as a by-product.
Coal is a complex geologic material composed mainly of organic matter and mineral matter, the latter including minerals, poorly crystalline mineraloids, and elements associated with non-mineral ...inorganics. Among mineral matter, minerals play the most significant role in affecting the utilization of coal, although, in low rank coals, the non-mineral elements may also be significant. Minerals in coal are often regarded as a nuisance being responsible for most of the problems arising during coal utilization, but the minerals are also seen as a potentially valuable source of critical metals and may also, in some cases, have a beneficial effect in coal gasification and liquefaction.
With a few exceptions, minerals are the major hosts of the vast majority of elements present in coal. In this review paper, we list >200 minerals that have been identified in coal and its low temperature ash, although the validity of some of these minerals has not been confirmed. Base on chemical compositions, minerals found in coal can be classified into silicate, sulfide and selenide, phosphate, carbonate, sulfate, oxide and hydroxide, and others. On the basis of their abundance, they can be classified into common, uncommon, and rare. Elements associated with silicates are largely benign, but many of those associated with sulfides and selenides are toxic to the environment and human health (e.g., S, As, Hg, Tl, Se, and Pb). Critical elements, e.g., rare earth elements and Y, Ga, and Al, are mostly associated with clays, phosphate, and carbonate minerals. There are many unusual mineral phases, such as native W, Au, Ag, and various Pt phases, which may have economic and geochemical significance in coal. Although the modes of mineral occurrence of a number of elements have been widely investigated, there are some elements whose associations, and, in particular, association mechanism with minerals are, to a degree, uncertain or even largely unknown and deserve further attention.
•Minerals are often a nuisance responsible for many problems arising during coal utilization.•Minerals in coal are also potentially valuable source of some critical metals.•Minerals are the major hosts of the vast majority of elements present in coal.•More than 200 minerals have been identified in coal and its low temperature ash.•In terms of their abundance, minerals in coal can be classified into common, uncommon, and rare.
Coal, containing all the elements that are present in nature and more than 200 minerals, has a complex chemical structure, making it one of the most complex geological materials. Inorganic matter in ...coal includes minerals (in which element concentration may vary from trace to major), non-crystalline mineraloids, and elements with non-mineral associations such as those occurring in pore waters, organically bound, or in an organic association. Understanding the modes of occurrence of elements in coal is important because, theoretically, they provide useful information on peat deposition, diagenesis and epigenesis of coal, coal-hosted basin formation, and the regional geological background or evolution. Practically, the modes of occurrence of elements play a significant role in affecting coal mining, coal preparation, coal combustion, and coal utilization, and in exerting adverse effects on both the environment and human health. The modes of occurrence of critical elements in coal and coal ash are key factors for designing the method and technology required for extracting critical metals from coal or coal ash. In this paper, the following aspects are reviewed, including the modes of occurrence of 73 elements and rare gases that occur in coal (with the exceptions of organically associated C, H, O, and N), the definition of modes of occurrence and their practical and academic significance, analytical methods for determining modes of occurrence of elements in coal and their advantages and limitations, and reported modes of occurrence of elements in coal and their likely associations.
Overall, the modes of occurrence of elements in coal are classified into inorganic, organic, and intimate organic associations. Although there are common modes of occurrence of many elements in coal, there are many exceptions and most, if not all, elements have multiple modes of occurrence. Each mode of occurrence of an element may also show different levels of confidence, namely, certain, probable, possible, doubtful, unlikely, and may occur in coal with different frequencies, namely abundant, common, uncommon, rare, and unlikely. For each element, the authors present concluding comments viewpoints on the modes of occurrence of almost each element in coal that is listed in the old literature.
The different modes of occurrence for each element in different coals depend on the geological conditions of coal formation, and do not necessarily indicate inconsistency in the reported results. However, due to limitations of the analytical methods used, some data relating the modes of occurrence of elements in coal are not convincing, and in some cases are invalid or even misleading. Overall, while precisely determining the concentrations of many elements in coal is not difficult, determination of the modes of occurrence of some elements, particularly those with low concentrations and high volatility, is still a challenge. Although analytical methods certainly play critical roles in determining the modes of occurrence of elements in coal, in-depth understanding of the nature of the coal and host-rocks and the geological background of coal formation is very useful in investigating when and how these modes of occurrence of elements were formed.
Coal ash is considered as a promising alternative resource for rare earth elements and yttrium (REE + Y or REY) recovery. Coal ash samples (fly ash and bottom ash), as well as corresponding feed ...coals, collected from the Luzhou coal-fired power plant in Sichuan, southwestern China were analyzed for their chemical and mineralogical characteristics. The fly ash was further tested for its feasibility of REE extraction. The feed coals are characterized as a low-volatile bituminous, medium‑sulfur, and high-ash coal, sourced from the Guxu coalfield (Late Permian Longtan Formation) in Sichuan, SW China. REE and Y, along with high-field-strength elements Zr(Hf) and Nb(Ta), and Li, F, Sc, V, Cr, Co, Cu, Zn, Ga, Ge, Se, Sr, Cd, In, Sn, Hg, Th, U, are all enriched in the feed coal. The mineralogical compositions of the feed coals are dominated by kaolinite and illite/smectite (I/S) mixed layers, followed by the carbonate minerals including calcite, siderite, ankerite, and minor amounts of anatase and jarosite.
The combustion of the feed coals produces a Class F fly ash, characterized by an aluminosilicate composition with a low CaO content. The bottom ash is compositionally similar to the fly ash, except that it is more enriched in high-density elements (Fe and Mn), and more depleted in volatile elements. REY are enriched in both the fly ash and bottom ash, although heavy REY (HREY) are notably more enriched in the fly ash. Mineralogical analysis shows that the fly ash consists of >70% amorphous glass and <30% mineral phases such as mullite, quartz and iron oxides. The bottom ash is composed of ~60% amorphous glass but with a more complex mineralogical composition than the fly ash. Dissolution of the fly ash and bottom ash with 4% hydrofluoric acid (HF) showed that ~90% of all the REY are associated with the amorphous glass in the fly ash, whereas <50% of REY in the bottom ash is contained in the glassy component. An alkaline-acid-combined (NaOH-HCl) sequential leaching process was employed to test the extractability of REY in the Luzhou fly ash. The experimental variables, including extractant concentration, liquid-to-solid ratio, leaching temperature and leaching time, were optimized by an orthogonal array design. The optimal NaOH leaching conditions results in 41.10% of active silica removal from the fly ash and 39.43% of REY enrichment. The HCl leaching of desilicated fly ash achieves 88.15% of REY extraction efficiency under the optimal conditions, which is a dramatic increase as compared to the same leaching of raw fly ash. Even though the extraction efficiency may vary between different coal ashes from various sources, a complete REY extraction procedure from fly ash is suggested for future use.
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•REY are enriched in both the fly ash and bottom ash presented in this study.•90% REY in the fly ash are associated with the amorphous glass.•A NaOH-HCl sequential leaching process was used to test the extractability of REY in the fly ash.
Coal, one of the most complex geological materials, consists of organic and mineral matter, the latter including crystalline minerals, non-crystalline mineraloids, and elements with non-mineral ...associations. Overall, the modes of occurrence of elements in coal are classified into organic, mineral, and intimate organic associations, the latter including those adsorbed on to the surface of organics, dissolved in pore waters, and hosted in very fine-grained minerals (sub-micro- or nano-minerals) encased in or shielded by the organic matter of coal. Mineral associations, defined as elements associated with minerals are straightforward; however, confusion about organic and intimate organic associations of elements usually arise in the literature. Understanding organic, mineral, and intimate organic associations of elements is important not only because non-mineral elements and, to a lesser extent, elements associated with fine-grained minerals, play a significant role in affecting the utilization of coal, but also such modes of occurrence of elements provide useful geochemical information on coal formation and coal-bearing basin evolution. With a few exceptions (such as Cd, Nb, Ta, Zr, and Hf), most elements determined in coal, particularly in low-rank coal, have varying-degrees of organic association. In this paper, we review the definition of associations of non-mineral elements in coal, as well as their methods of determination, and then review the associations of selected elements including environmentally-sensitive (e.g., S, As, U, and Hg) and critical elements, the latter of which drive some of the significant advancements in technology and energy efficiency in the world today (e.g., rare earth elements and Y, Ge, and U), and some major elements (e.g., Ca, Mg, Fe, Al, and Ti) that largely occur in non-mineral forms in low-rank coals.
•The modes of occurrence of elements in coal are classified into organic, mineral, and intimate organic associations•The intimate organic associations include those adsorbed on to the surface of organics, dissolved in pore waters, and hosted in very fine-grained minerals.•Most elements determined in coal have varying-degrees of organic association.
Peat depositional environments, the sites where and conditions under which peat accumulates, significantly influence a resultant coal's physical properties, chemical composition, and coal utilization ...behavior. Recognition of peat depositional environments for coal is a challenging endeavor because coal's observed compositional properties not only result from a variety of geological processes operating during peat accumulation, but also reflect the influence of adjoining or external depositional sedimentary environments and alteration during later diagenesis and/or epigenesis. The maceral or microlithotype composition of any one layer of peat can be the product of years or decades of plant growth, death, decay, and post-burial infiltration by roots in addition to the symbiotic, mutualistic, parasitic, and saprophytic relationships with non-plant biota, such as arthropods, fungi, and bacteria. The overprint of increasing thermal maturation and fluid migration through time on the resulting coal can make these relationships difficult to recognize. Therefore, published models based on maceral composition alone must be used with great caution. Lipid compositions, even from lipid-poor low-rank coals, can provide important information about depositional environments and paleoclimate, especially if combined with the results of organic petrography and paleontological studies. Just as sulfur derived from seawater provides environmental clues, the ratios of two particularly relevant trace elements rather than a single trace element can be used to interpret peat depositional environments. Epigenetic minerals, as well as their corresponding chemical compositions should not be used for such a purpose; similarly, resistant terrigenous minerals deposited during peat accumulation in many cases should be used with considerable caution. The interactions of the biota present in the peat-forming ecosystem, often determined using palynological and geochemical proxies, and their interpretation in the context of geography and paleoclimate are important means for deciphering peat depositional environments. Overall, a combination of evidence from geochemistry, mineralogy, palynology, and petrology of coal and from stratigraphy, sedimentology, and sedimentary facies of related rocks is necessary for accurate and comprehensive determination of depositional environments. The need for interdisciplinary studies is underscored by peat compositional properties, which have been greatly affected by various processes during the syngenetic, diagenetic or epigenetic stages of coal formation.
China will continue to be one of the largest coal producers and users in the world. The high volume of coal use in China has focused attention on the amounts of toxic trace elements released from ...coal combustions and also the valuable trace elements extracted or potentially utilized from coal ash.
Compared to world coals, Chinese coals have normal background values for most trace elements, with the exception of higher Li (31.8μg/g), Zr (89.5μg/g), Nb (9.44μg/g), Ta (0.62μg/g), Hf (3.71μg/g), Th (5.84μg/g), and rare earth elements (∑La-Lu+Y, 136μg/g). This is not only due to the higher ash yields of Chinese coals but also to alkali volcanic ashes found in some southwestern coals. The background values of toxic elements of Hg (0.163μg/g), As (3.79μg/g), and F (130μg/g) in Chinese coals are comparable to coals from most other countries.
The genetic types for trace-element enrichment of Chinese coals include source-rock- controlled, marine-environment-controlled, hydrothermal-fluid-controlled (including magmatic-, low-temperature-hydrothermal-fluid-, and submarine-exhalation-controlled subtypes), groundwater-controlled, and volcanic-ash-controlled. The background values of trace elements were dominated by sediment source regions. Low-temperature hydrothermal fluid was one of the major factors for the local enrichment of trace elements in southwestern China.
Serious human health problems caused by indoor combustion of coal in China include endemic fluorosis, arsenosis, selenosis, and lung cancer. Endemic fluorosis, mainly occurring in western Guizhou, was mostly attributed to the high fluorine in clay that was used as a briquette binder for fine coals, in addition to a small quantity of fluorine from coal. Fluorine in the coal from endemic-fluorosis areas of western Guizhou is within the usual range found in China and the world. Endemic arsenosis in southwestern Guizhou is attributed to indoor combustion of high-As coal. Endemic selenosis in Enshi of Hubei was due to high Se in carbonaceous siliceous rocks and carbonaceous shales. Fine particles of quartz, released into air during coal combustion, are hypothesized as a possible cause for the lung cancer epidemic in Xuanwei, Yunnan, China.
Valuable elements, including Ge, Ga, U, REE (rare earth element), Nb, Zr, and Re are concentrated to levels comparable to conventional economic deposits in several coals or coal-bearing strata in China. The Ge deposits at Lincang, Yunnan province and Wulantuga, Inner Mongolia have been exploited and industrially utilized. The enrichment of Ge in the two deposits was caused by hydrothermal fluids associated with adjacent granitoids. The Ga (Al) ore deposit in the Jungar Coalfield, Inner Mongolia, was derived from the neighboring weathered and oxidized bauxite of the Benxi Formation (Pennsylvanian). The Nb(Ta)–Zr(Hf)–REE–Ga deposits in the Late Permian coal-bearing strata of eastern Yunnan and Chongqing of southwestern China were attributed to ashes of the alkali volcanic eruptions.
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