1:45 PM - 2:15 PM
[HRE12-01] Understanding the characteristics of REE-rich coals
★Invited Papers
Keywords:coal, rare earth elements, maceral
During the past decade, coal and coal by-products (e.g., fly ash) have attracted attention as an important source for rare earth elements (REE: La-Lu, and Y). In some case, REE concentration in coal (ash basis) reach comparable to or even higher than conventional deposits (e.g., carbonatites, weathering crust) [1]. In order to understand the characteristics of REE-rich coals, many studies have been conducted on the geology and geochemistry of coal-bearing strata rich in REE, selective leaching experiments to investigate the modes of occurrence of REE in coal, and in-situ analyses such as LA-ICP-MS.
The modes of occurrence of REE in coal are particularly important for understanding the enrichment mechanisms of REE in coal seams, and the behavior of REE during coal utilization such as coal combustion. The modes of occurrence of elements in coal are defined as 1) organic association, 2) intimate organic association, and 3) mineral association [2]. It is well known that humic acids show a high affinity for REE due to carboxyl groups. REE in brown coals have a strong organic affinity, whereas REE in hard coals and anthracites mainly occur in authigenic minerals [2, 3]. The abundance of humic acids produced during peatification and early diagenesis can be one of the most important factors constraining primary REE concentrations in coal seams. However, the decarboxylation reaction proceeds during maturation. Useful indicators are needed to understand the characteristics of REE-rich coal.
Macerals are micrometer-scale organic constituents of coal with physical and chemical properties for each origin. Therefore, maceral composition has been widely used to evaluate the characteristics of coal (e.g., hydrocarbon generation potential), and the peat-forming environments. If the differences in the REE composition of each maceral are revealed, the characteristics and formation mechanisms of REE-rich coal can be more accurately discussed based on organic petrology (maceral composition).
The maceral composition of humic coals is commonly dominated by the vitrinite group (huminite group in low-rank coal). Vitrinite (huminite) is divided into telovitrinite (telohuminite), detrovitrinite (detrohuminite) and gelovitrinite (gelohuminite) [4, 5]. Telovitrinite (telohuminite) is mainly derived from cellulose and lignin in parenchymatous and woody tissues, whereas gelovitrinite (gelohuminite) is mainly assumed to be derived from cell excretions (e.g., tannins), or humic fluids formed from plant tissues during decay and diagenesis [4, 5]. Gelovitrinite (gelohuminite) can have the relatively high REE potential, since the maceral is mainly derived from organic matter rich in carboxyl groups with a high affinity for REE (e.g., humic acids). In this study, the LA-ICP-MS analysis of telohuminite and gelohuminite (corpohuminite) in woody fragment was performed to investigate the REE concentrations of these macerals. The preliminary results suggested that gelohuminite (corpohuminite) has higher REE potential than telohuminite.
Upper Cretaceous and Cenozoic coal-bearing strata are widely distributed in Japan. Although detailed investigations on the REE potential of Japanese coals are very limited, gelovitrinite becomes more common in Cretaceous and Cenozoic coals [4]. Moreover, the tectonic disturbance during early stages of coalification is also important for the formation of gelovitrinite [4]. Based on the characteristics of organic petrology and tectonic activity, this presentation will discuss the REE potential of Cenozoic coals on East and Southeast Asian continental margins.
References
[1] Seredin and Dai, 2012. Int. J. Coal Geol., 94, 67-93.
[2] Dai et al., 2020. Int. J. Coal Geol., 218, 103347.
[3] Arbuzov et al., 2019. Int. J. Coal Geol., 206, 106-120.
[4] ICCP, 1998. Fuel, 77, 349-358.
[5] Sýkorová et al., 2005. Int. J. Coal Geol., 62, 85-106.
The modes of occurrence of REE in coal are particularly important for understanding the enrichment mechanisms of REE in coal seams, and the behavior of REE during coal utilization such as coal combustion. The modes of occurrence of elements in coal are defined as 1) organic association, 2) intimate organic association, and 3) mineral association [2]. It is well known that humic acids show a high affinity for REE due to carboxyl groups. REE in brown coals have a strong organic affinity, whereas REE in hard coals and anthracites mainly occur in authigenic minerals [2, 3]. The abundance of humic acids produced during peatification and early diagenesis can be one of the most important factors constraining primary REE concentrations in coal seams. However, the decarboxylation reaction proceeds during maturation. Useful indicators are needed to understand the characteristics of REE-rich coal.
Macerals are micrometer-scale organic constituents of coal with physical and chemical properties for each origin. Therefore, maceral composition has been widely used to evaluate the characteristics of coal (e.g., hydrocarbon generation potential), and the peat-forming environments. If the differences in the REE composition of each maceral are revealed, the characteristics and formation mechanisms of REE-rich coal can be more accurately discussed based on organic petrology (maceral composition).
The maceral composition of humic coals is commonly dominated by the vitrinite group (huminite group in low-rank coal). Vitrinite (huminite) is divided into telovitrinite (telohuminite), detrovitrinite (detrohuminite) and gelovitrinite (gelohuminite) [4, 5]. Telovitrinite (telohuminite) is mainly derived from cellulose and lignin in parenchymatous and woody tissues, whereas gelovitrinite (gelohuminite) is mainly assumed to be derived from cell excretions (e.g., tannins), or humic fluids formed from plant tissues during decay and diagenesis [4, 5]. Gelovitrinite (gelohuminite) can have the relatively high REE potential, since the maceral is mainly derived from organic matter rich in carboxyl groups with a high affinity for REE (e.g., humic acids). In this study, the LA-ICP-MS analysis of telohuminite and gelohuminite (corpohuminite) in woody fragment was performed to investigate the REE concentrations of these macerals. The preliminary results suggested that gelohuminite (corpohuminite) has higher REE potential than telohuminite.
Upper Cretaceous and Cenozoic coal-bearing strata are widely distributed in Japan. Although detailed investigations on the REE potential of Japanese coals are very limited, gelovitrinite becomes more common in Cretaceous and Cenozoic coals [4]. Moreover, the tectonic disturbance during early stages of coalification is also important for the formation of gelovitrinite [4]. Based on the characteristics of organic petrology and tectonic activity, this presentation will discuss the REE potential of Cenozoic coals on East and Southeast Asian continental margins.
References
[1] Seredin and Dai, 2012. Int. J. Coal Geol., 94, 67-93.
[2] Dai et al., 2020. Int. J. Coal Geol., 218, 103347.
[3] Arbuzov et al., 2019. Int. J. Coal Geol., 206, 106-120.
[4] ICCP, 1998. Fuel, 77, 349-358.
[5] Sýkorová et al., 2005. Int. J. Coal Geol., 62, 85-106.