Selected Papers on Coal Petrology and Coal Geochemistry in Ren Deyi
China is rich in coal resources, ranking among the top in the world. Proved reserves can be used for hundreds of years, which is superior to oil and natural gas resources. In recent years, the proportion of coal in China's disposable energy structure is about 75%. Coal contains more than 60 kinds of trace elements, among which germanium, gallium and vanadium can be used as associated minerals. Other trace elements, such As As, F, Cr and Hg, are harmful elements or potentially harmful elements, which can enter the atmosphere, soil and water in various forms during storage, stacking, transportation, combustion and processing and utilization, causing pollution. During the period of 1997, SO2 produced by coal combustion reached 23.46 million tons, which caused serious environmental pollution. This situation is attracting people's great attention, and governments at all levels have also formulated corresponding policies to limit the exploitation of ω( St) > 3% coal seams, and formulated the standards for cities to discharge SO2 into the atmosphere. Therefore, according to the allocation of energy resources in various regions, a new technology of deashing and desulfurization before burning coal is adopted to make it clean; At the same time, it is necessary to deeply study the content, occurrence, causes and distribution of sulfur and harmful elements in coal, so as to coordinate resources with the environment and ensure the sustainable development of the national economy.
There are 22 kinds of harmful trace elements in coal: silver, arsenic, barium, beryllium, cadmium, cobalt, chlorine, copper, chromium, fluorine, mercury, manganese, molybdenum, nickel, lead, selenium, antimony, thorium, thallium, uranium, vanadium and zinc. Beryllium, cadmium, mercury, lead and thallium are toxic elements, while arsenic, beryllium, cadmium, chromium, nickel and lead are carcinogenic elements. The research on harmful elements in coal started late in China. Since 1980s, the research on the distribution, occurrence and environmental pollution of harmful elements in coal and its combustion products has been strengthened. It is preliminarily found that the contents of elements such as chromium, fluorine, mercury, molybdenum, selenium, uranium and vanadium in coal in China are higher than the average values in coal in the United States and the world, and arsenic is abnormally high in some areas. Compared with developed countries, China has less systematic analysis data and less research on the genetic types and geological background of harmful elements.
The enrichment of trace elements in coal is controlled by many factors and multi-stage actions, and is often the result of the superposition of many factors. In the peat stage of coal formation, the main controlling factors are the nature of parent rock, sedimentary environment, coal-forming plant types, microbial action, climate and hydrogeological conditions. In the coalification stage, sedimentary diagenesis, microbial action, tectonism, magmatic hydrothermal activity and groundwater activity are the main controlling factors. When the coal-bearing basin is reformed in the later stage and the coal seam enters the supergene stage, wind oxidation can further enrich or leach trace elements in coal.
According to the dominant factors of harmful elements enrichment in coal, the following genetic types can be preliminarily distinguished.
First, the land-based enrichment type
The nature of the parent rock in the terrigenous region determines the content of trace elements in the ancient soil of peat swamp, and also determines the content of trace elements in coal-forming plants and peat swamp media to a certain extent. Small and medium-sized coal-bearing basins are close to terrigenous sources, and the transportation distance of terrigenous debris is short, and sometimes the subsidence rate and filling rate of basins are high. The unusually high content of trace elements in coal has a good correlation with the high content of this element in parent rocks, which can be used as a typical example of terrigenous enrichment type. Taking Shenbei coalfield in Liaoning Province as an example, the potential harmful elements such as Cr, Ni, Zn, Cu and Co are highly enriched in the lignite in the middle and late Eocene in Shenbei coalfield, and the geometric average values of their mass fractions (ω b/ 10-6) are 58.53, 73.99, 7 1.23 and 53.23 respectively. Among all kinds of magmatic rocks, the contents of Cu and Zn in basic rocks are the highest, followed by Cr, Ni and Co, second only to ultrabasic rocks [1]. The bedrock of Shenbei coal-bearing basin is olivine basalt. The analysis of bedrock with different weathering degrees shows that with the increase of weathering degree, the mass fractions of Cr, Zn, Co, Ni and other elements decrease in different degrees, which means that these elements can be dissolved from the parent rock and enter the coal-accumulating basin under the condition of strong oxidation in the supergene zone (table 1).
Table 1 ICP-AES analysis results of harmful elements in coal and olivine basalt
Note: The mass fraction of elements in coal from China Coal Mine and Shenbei Coalfield is geometric average; (1) is relatively fresh, and WI (weathering index, according to Qiu Jiaxiang and Lin, 199 1 [2]) is 84.53; ② Moderate weathering with WI of 65.23; ③ Weathering, WI is 40.25.
Coal-accumulating basins are usually rich in humus. In order to observe the extraction effect of humic acid on trace elements in parent rock, olive basalt was soaked in distilled water and humic acid solution for 3 months, and the content of trace elements in it was determined (Table 2). It is found that humic acid solution has stronger ability to extract harmful elements from olivine basalt than water, especially zinc, copper and chromium. It can be seen that in the coal-accumulating basin rich in humus, potentially harmful elements such as Cr, Ni, Zn, Cu and Co are obviously enriched in sedimentary rocks such as coal and variegated mudstone in the coal floor, which is mainly related to the fact that the parent rock of the continental source area of the basin is olivine basalt.
Analysis results of harmful elements extracted from olive basalt by water and humic acid solution
Jurassic coal in Beipiao, Liaoning Province is rich in harmful elements such as Cr and Ni, and is also related to basement basalt, belonging to this type [3]. There are also many examples abroad. Ruppert et al. (1996) found that the average mass fraction ω(Ni) of lignite in Kosovo basin of Serbia is 100× 10-6, and the average mass fraction ω(Cr) is 58× 10-6, mainly because the terrigenous areas are serpentine and olivine. The high content of Cr and Ni in coal from South Ural Basin and Chelyabinsk Basin in Russia is also related to the widespread distribution of basic rocks and ultrabasic rocks around the basins [5].
The sedimentary basement of Neogene coal-accumulating basin in western Yunnan is mostly granite and granite gneiss. When the coal seam at the bottom of coal-bearing construction accumulates, there are rich sources of U and Ge, so the bottom coal seam is often rich in U and Ge, and some even form super-large germanium-uranium deposits.
Two. Sedimentary-biological enrichment type
Sedimentary environment is one of the most important factors to control the distribution of trace elements in coal. Generally, the content of trace elements closely related to marine sediments is high, which is not only because the content of trace elements such as B, Mo and V in seawater is higher than that in fresh water, which can provide rich material sources, but more importantly, seawater changes the pH value, Eh value and H2S content of peat bogs, resulting in a specific geochemical barrier, which is conducive to the enrichment of trace elements. Humic acid and brown humic acid can strongly complex U and other metals to form uranphthalein organic complexes. There are many dissociable charged groups in algae cell composition, which can adsorb metal ions. The enrichment degree of some lower algae to trace elements such as U is considerable. This type of enrichment formed by the combination of sedimentary environment and biology is the most characteristic in the coal seam formed by tidal flat environment of limited carbonate platform.
The late Permian Longtan Formation coal in Shanjiaoshu, Panxian County, Guizhou Province was formed in the upper delta plain environment, the Liuzhi Longtan Formation coal was formed in the lower delta plain environment, and the late Permian Changxing Formation coal in Guiding was formed in the tidal flat environment of limited carbonate platform. As can be seen from Table 3, from Panxian, Liuzhi to Guiding, with the increasing influence of seawater on peat swamp, harmful elements such as U, V and Mo in coal increased obviously. On the other hand, the roof and floor of guiding coal seam are all algal limestone, and the microscopic composition of coal is mainly hydrogen-rich matrix vitrinite, and its reflectivity Ro has reached 65438 0.48%, but there is still dark orange fluorescence. The transmission electron microscope study shows that it is rich in ultra-fine lipids, contains a large number of fungal fossils such as pyrite-reducing sulfate bacteria and sulfur bacteria, and the mass fraction ω(St) of sulfur in coal is as high as 8.89. This shows that bacteria, algae and other plants actively participate in coal formation, forming a geochemical barrier rich in H2S and S reduction, which is conducive to the enrichment of harmful elements such as U, V and Mo. Late Permian coals from Yanshan, Yunnan and Ziyun, Guizhou also belong to this enrichment type.
Table 3 Analysis results of trace elements in late Permian coal in Guizhou (INAA method)
Note: *-B value is analyzed by ICP-AES.
Three. Magmatic hydrothermal enrichment type
Mesozoic and Cenozoic magmatic activities were frequent in eastern China, and superimposed metamorphism of coal developed, among which regional magmatic thermal metamorphism of coal was the most important and had the widest influence [8]. The enrichment of harmful trace elements in middle and high rank coal is related to the hydrothermal properties of magma.
The enrichment of U, Th, W, REE and other elements in the late Triassic coal in Jian 'ou, Fujian Province, and the partial enrichment of U, Th, Zn, As, Sb and other elements in the late Triassic coal in Zixing, Hunan Province are all related to the Yanshanian granite magmatic hydrothermal activity. The late Permian coal in Mei Tian mining area, Hunan Province was affected by mica granite intrusion, and the harmful trace elements of mercury, cadmium, molybdenum and copper in coal increased obviously. The Yanshanian alkaline and weakly alkaline magmatic hydrothermal processes in western Gujiao, Shanxi Province led to the increase of Cl, se, Pb, Zn and Br contents in coal. The late Jurassic-early Cretaceous coal in Yimin Wumuchang, Inner Mongolia was influenced by subvolcanic hydrothermal metamorphism. Coal contains orpiment and realgar, and the highest arsenic content in coal is 768× 10-6 [9].
4. Deep fault-hydrothermal enrichment type
This type is usually typical in coal-accumulating basins near deep faults. The unusually high content of harmful elements in coal is related to the hydrothermal solution and volatile matter migrated from the fault zone. Zhou Yiping (1992) comparatively studied the arsenic content in the coal of Sanjiang fault zone in Yunnan and its nearby tertiary lignite basin, and found that the arsenic enrichment in coal was closely related to Sanjiang fault zone [10].
Late Permian and Late Triassic coal-bearing measures are developed in southwestern Guizhou fault depression, and the late Permian coal accumulation and its later changes are strictly controlled by Shuicheng-Ziyun fault, Shizong-Guiyang fault, Panxian fault and Nanpanjiang fault. There are many kinds of gold mines, antimony mines, arsenic mines and mercury mines in the fault depression area, especially gold mines. Harmful elements such as mercury and arsenic are enriched in coal, and low-temperature hydrothermal pyrite, calcite and chronological veins are developed. The inclusion temperature is 160 ~ 200℃. The analysis shows that the contents of harmful elements As, Cd, Hg, Mo, Pb, Se, Tl and Zn in vein pyrite caused by low-temperature hydrothermal solution are relatively high, with the mass fraction of As reaching 255× 10-6, that of Hg reaching 22.5× 10-6, and that of Se also being relatively high. The mass fraction of Zn w(Zn) can reach 326× 10-6, and the sulfur isotope value of low-temperature hydrothermal pyrite accounts for 1.8 ‰ ~-9.8 ‰. The mass fraction of mercury (w(Hg)) and zinc (w(Zn)) in calcite veins at low temperature can reach1.9×10-6 and 282× 10-6 respectively, and the contents of Sr, Ni, Ag and Pt are also high. However, the average mass fraction w(Hg) of late Triassic coal in Zhenfeng is 0.233× 10-6, and that of late Permian coal in Qinglong is 0. 127× 10-6, which is different from that of Guo Yingting and others (196) Individual samples can reach 10.5× 10-6. The enrichment of harmful elements in coal in southwest Guizhou is mainly controlled by deep faults and their derived faults, and multi-stage low-temperature hydrothermal pyrite and calcite veins become the main carriers of harmful elements.
Five, groundwater enrichment type
The rich elements in coal are related to the chemical properties of groundwater and the relative relationship between water level and coal seam, as well as the properties of surrounding rock and overlying strata of coal seam. The mass fraction of chlorine in Carboniferous coal in Illinois, USA is 0. 13% ~ 0.58%, some of which are high chlorine coal, and the chlorine content gradually increases to the deep. Chou et al. (19 1) and Shao et al. (1994) think that chlorine is related to groundwater [some coals in the former Soviet Union, Britain, eastern Germany and western Donetsk coalfield in Poland are called "high salt coals" [5]. The w(Cl) value of Staffordshire coal in England is 0.74%, that of eastern Germany is 0.43% ~ 0.77%, and that of Donets coal is 0. 1 1%. There are different views on its genesis, but most people think that in the process of diagenesis, groundwater flows through the Permian gypsum-salt layer of the overlying strata, which increases the salinity, and the chlorine content in coal increases after infiltrating into the coal-bearing measures [5].
In this paper, the genetic types of harmful trace elements enrichment in coal are preliminarily discussed. In-depth study on the geological and geochemical background of the source, migration and enrichment of harmful trace elements in coal is helpful to develop the basic theory of coal geochemistry in China, provide scientific basis for mining, washing, processing and utilization and environmental protection, and make full and rational use of China's rich coal resources.
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Preliminary study on genetic types of harmful trace elements enrichment in coal
Ren Deyi Zhao Fenghua Zhang Junying Xu Dewei
(China University of Mining and Technology, Beijing, 100083)
Abstract: The enrichment of harmful trace elements in coal is controlled by many factors and geological processes in different periods, which are often the superposition of many factors. This paper gives the preliminary results of the study on the genetic types of trace elements enrichment in coal. According to the main controlling factors, five genetic types of harmful trace elements enrichment in coal are put forward, namely: ① source rock controlling type; ② Sedimentary-biological control type; ③ Magmatic hydrothermal control type; ④ Hydrothermal control type of deep fault and ⑤ groundwater control type. In-depth study on the geological and geochemical background of the source, migration and enrichment of harmful trace elements in coal will enrich the basic theory of coal geochemistry and provide scientific basis for the harmless utilization of coal.
Keywords: coal, harmful trace elements, genetic types
(This paper was co-authored by Ren Deyi, Zhao Fenghua, Zhang Junying and Xu Dewei, originally published in the supplement of Frontier of Earth Sciences, Volume 6 1999).