--A problem worthy of attention
Abstract Coal is an organic rock and mineral with high reduction barrier and adsorption barrier properties, which can be enriched with some beneficial metal elements and reach the scale of mineralization under specific geological conditions. Synthesizing some research data at home and abroad, the abundance, endowment state, geological genesis, and possibility of utilization of beneficial metals niobium, gallium, rhenium, and scandium in coal and coal-bearing rock systems are discussed. The study of the enrichment or mineralization of rare metal elements in coal is one of the important contents of coal geochemistry and ore deposit geochemistry, which is worth further strengthening.
Selected Papers on Coal Petrology and Coal Geochemistry of Ren Deyi
There are some precious and beneficial elements in the trace element composition of coal, and some of them have been enriched into sizable ****-associated deposits, which have been increasingly emphasized. For example, in Kazakhstan, Kyrgyzstan and Xinjiang Yili, Tuha and other Jurassic coal-bearing basins, are found in the coal seam top plate sandstone layer and part of the coal seam *** born of large uranium deposits, some of which have formed production capacity. Another example is the large lignite-germanium deposits of the Middle and Cenozoic era found in Lincang, Yunnan, Ulaantuga Mining District of Inner Mongolia, and Primorsky Border Region of Russia, the main features of these deposits are found in numerous literatures [1-8].
In recent years, highly enriched rare metal elements such as gallium, niobium, rhenium, scandium, as well as rare earth elements and precious metal elements such as silver, gold and platinum group elements have been found in coal. Many of these high content of trace elements in coal are potentially important strategic mineral resources, or economically recoverable by-products of coal processing. Strengthening the investigation of them, in-depth study of their endowment state and enrichment law, is conducive to the full and rational use of coal resources and *** accompanying mineral resources, the development of circular economy.
This paper synthesizes the literature and known information, only on niobium, gallium, rhenium, scandium and other elements, briefly described as follows.
One, niobium (Nb)
Niobium is a corrosion-resistant high melting point of rare metals, their alloys are super heat-resistant, ultra-light, can be used as missiles, rockets and aerospace engines, but also important superconducting materials, is the world's demand for more rare metals. The Clark value of niobium in the earth's crust is 21 μg/g, and according to Ketris and Yudovich [9], the global average content of niobium in coal is 3.7 μg/g. The Russian scholar Середин suggests that when the niobium content of coal is ≥300 μg/g, it can be evaluated as an associated useful mineral [6].
The anomaly of niobium in coal may be synbiotic, mainly with the weathering crust *** born of coal tends to be rich in niobium, in the epiphytic zone conditions, niobium can be combined with organic acids, such as in the solution containing xanthate with niobium-containing minerals in the powder, in 4 or 5 months can make the solution containing niobium up to 1mg/L that is hundreds of times higher than the natural water.
Secondly, when there is acidic volcanic debris alteration of tonstein in the coal seam, niobium will also be enriched with its neighboring coal, Hower et al. reported that the niobium content in the upper and lower layers of the tonstein interlayer of the FireClay coal seam in eastern Kentucky, USA, was abnormally high, and it reached 55-88 μg/g and 76-150 μg/g, respectively [10].
The anomaly of niobium in coal may also be affected by metal-bearing hydrothermal fluids, Seredin reported [11], a graben-type Eocene lignite in the Russian Far East, due to the modification of niobium-rich carbonate hydrothermal fluids, so that the niobium content in the coal up to 60 μg/g.
The niobium-rich coals of some coals around the world, the Kuznets Coalfield, Russia, the Permian coal niobium content of up to 30 ~ 50 μg/g, and the niobium content of the coal is up to 30 ~ 50 μg/g. 50 μg/g in the Kuznets coalfield, Russia, and 180-360 μg/g in the coal ash; the niobium content in the 30th coal seam of the Issyk-Ks coal production area in the Minusinsk Carboniferous-Permian coalfield is 90 μg/g, while the niobium content in the ash is 580 μg/g. The niobium content of the two 90m and 22m thick Miocene lignite seams of the ?ytaw coalfield, Poland, is niobium rich, and the niobium content of the ash in the seams is more than 200 μg/g [6, 6, 7]. μg/g [6, 12, 13].
The average value of niobium content in the Permian coal in Heshan, Guangxi was 50 μg/g, of which the 1.1m-thick upper sub-seam of the lower coal seam of Liuhualing Mine 4 contained niobium at 126 μg/g, which was converted to niobium in coal ash at 689 μg/g [14]. According to Dai et al, the mean value of niobium content in the Upper Permian 34 coal seam in the Zhijin Coalfield, Guizhou was 64 μg/g, and the niobium content in the Upper Permian 3 coal seam in the Dafang Coalfield was 80 μg/g [15-17].
Spears and Zheng [18] analyzed coals from major UK coalfields and showed that illite is the main carrier of niobium in coal. Similar conclusions were reached by D. Manganese Liu et al [19] in their analysis of the Xiantaibao mine in Shanxi. Niobium in coals from the Kuznets coalfield in Russia is mainly enriched in pyrochlore and tantalite.Palmer et al [20] confirmed that 66% of the niobium in the studied coals was in the organic state by using a six-step step-by-step chemical extraction method.The study of the Neoproterozoic sulphur-bearing lignite of Beypazary, Turkey, by Querol et al [21] showed that niobium in the coal was predominantly in the organic state. It can be seen that in different coals, the niobium endowment state varies from place to place.
Dai Shifeng et al [22] and Zhou Yiping [23] reported that Nb is highly enriched in the alkaline volcanic ash-influenced coals and alkaline volcanic ash-eroded claystone interbedded gangue (Tonstein) in southwestern China.Alkaline Tonstein can not only be used as an isochronous marker layer but also, based on the spatial distribution pattern of the number of layers and thickness of alkaline Tonstein in the coal-bearing rock systems. It is possible to search for the location of ancient craters, which is of great significance for rare element mineral search related to the construction of alkaline volcanic rocks.
Two, gallium (Ga)
Gallium is a typical dispersed elements, is used in fiber optic communications equipment, computers and color TV display materials. The Clark value of gallium is 16 μg/g [24]. It is difficult to form independent gallium deposits in nature, but mainly from bauxite and sphalerite deposits mining integrated recovery. The global gallium content in coal is 5.8 μg/g, while the average value of gallium content in coal ash is 33 μg/g [9]. The average value of gallium content in coal in China is 6.5 μg/g [7].
Some of the world's coalfields have relatively high gallium content in coal, and the gallium content in the coal ash of some coals is as high as several hundred μg/g. Therefore, the combustion by-products of gallium-rich coals have the potential to extract gallium. According to the National Mineral Reserves Committee in 1987, the standard of industrial utilization of gallium in various types of gallium-containing deposits:bauxite ore gallium is 20μg/g, while coal is 30μg/g.
Zhou Yiping and Ren Youlang [25] showed that the gallium content of coal ash from the Upper Permian in the Southwest region can be up to 63.7～401.5μg/g, which is mainly in the organic state, and is found in the & lt;; 1.3g/cm3 density density of coal ash in the coal fields. 1.3g/cm3 density level of coal samples in the ash is more enriched. The average value of gallium content in the Permian coal on the top of Ziyun sedan chair in Guizhou was 375 μg/g. Coal No. 34 at the bottom of Longtan Formation in Zijin, Guizhou, contained 100 μg/g of gallium. 32 μg/g of gallium was found in coal of No. 11 seam in Songzao Coalfield in Chongqing [22]. In addition, several coals from the Upper Permian in Changxing, Zhejiang Province, and the Late Paleozoic in the Shikanjing and Shizuishan mines in Ningxia Province contain more than 30 μg/g of Ga.
The No. 6 coal of the Heidaigou Giant Coal Seam of the Jungar Coalfield in Inner Mongolia is a typical example of Ga enrichment in coal [26, 27]. The average value of Ga content in this coal seam is 44.6 μg/g, and some seams can reach 76 μg/g. Microzonation analysis shows that the main carrier of gallium is boehmite in the coal, and part of it is distributed in the organic matter [26, 27]. Not only that, the coal is also over-enriched in Al, resulting in the coal combustion products of this seam are highly enriched in Al2O3, and the content of Al2O3 in fly ash is more than 50%, so the Heidaigou No. 6 coal seam is a gallium-aluminum deposit with the coal *** (associated). Although gallium is enriched in the Halwusu and Guanbanwusu coals in the south and north of Heidaigou, it has not yet reached industrial grade. With the increase of coal production in recent years, the amount of gallium-rich and aluminum-rich coal resources in Heidaigou is decreasing year by year, which should cause the relevant departments to pay great attention to protect this rare coal resource. In addition, the fly ash discharged from the power plant burning the No. 6 coal seam in the area has formed an artificial deposit rich in Al and Ga after years of accumulation, and the distribution pattern, storage pattern and migration characteristics of Al and Ga in this artificial deposit deserve further in-depth study.
Russia Minusinsk Coalfield Chernogore coal origin "two Russian feet" coal seam coal containing gallium 30μg/g, coal ash containing gallium 375μg/g; Russia's Far East Lakovsk coal origin in the Miocene germanium-containing coal containing gallium 30 ~ 65μg/g, coal ash containing gallium 100 ~ 300μg/g. Low-ash coal from the Carboniferous seam "Amos" in northwestern Kentucky, USA, contains 140-500 μg/g of gallium in the coal ash [28].
Affolter (1998) showed that a large-scale power plant in Kentucky, USA, the raw coal ash contains gallium 70μg/g, slag contains gallium <22μg/g, 67μg/g in the coarse-grained fly ash, and gallium is relatively enriched in fine-grained fly ash, which is 110μg/g. Mar-don and Hower [29] showed that the coal-fired coal in southeastern Kentucky, USA, contains gallium 140～500μg/g. In all levels of products from coal-fired power plants in southeastern Kentucky, the raw coal ash contained 61 μg/g of gallium, 26 μg/g in ash, and 169 μg/g of gallium was fairly enriched in the fly ash obtained from the electrostatic precipitator. According to Fang Zheng and Gesser [30], the gallium content of coal soot taken from Canada, Israel and China amounted to more than 100 μg/g.
It can be seen that coal combustion by-products, mainly fine-grained fly ash, have become the third major source of gallium for integrated recovery from minerals in the world.
Three, rhenium (Re)
Rhenium is a rare metal with super heat resistance, is a new generation of aerospace engine materials, is a strategic mineral resources, but also high-efficiency catalysts and materials for the manufacture of new medical devices. Rhenium is an extremely dispersed element, and the Clark value of rhenium in the earth's crust is only 0.6ng/g [24]. When utilized as an associated metal, it is required that the content of rhenium in the mineral is not less than 2 ng/g. Rhenium locally reaches industrial grades in the copper-bearing sandstone-type copper deposits of Zhezkazgan, Kazakhstan. Russian Середин [6] suggests that when the rhenium content in coal exceeds 1μg/g, it can be evaluated as a useful associated rhenium mineral resource.
According to Клер and Неханова 1981 report, Jurassic coals of Angren, Uzbekistan, contain rhenium 0.2-4 μg/g, which originated from the parent rocks around the basin. According to Валиев et al. (1993), in the anthracite coal from the Nazar-Aylok Jurassic coal source in Tajikistan, the low-ash coal (Ad=3.2%) contained 2.1 μg/g rhenium, while the higher ash coal (Ad=17.9%) contained 3.3 μg/g rhenium, which suggests that the coal at this site contains both organic and mineralized forms of rhenium.
The lignite in the carbonate system of the Ebro Basin in northern Spain contains 9μg/g rhenium, and this kind of "lignite" is rich in asphaltene, with high ash content, and its characteristics are close to those of oil shale.
Rhenium is often enriched in the coal of leachate-type uranium-coal deposits. The average rhenium content of the uranium-rich zone in the upper part of the reduction zone of the 4m-thick coal seam in the Lower Ili uranium-coal deposit in Kazakhstan is 9.5 μg/g, and the average rhenium content of the lower part of the transition zone of the coal seam is 4.2 μg/g. Coal acts as a reducing barrier to the reduction and enrichment of perrhenate in solution.
According to the report of Юровский in 1968, the concentrate of long-flame coal (Ad=8%) from the South Plevolnyan Mine in the Donets Coalfield contained rhenium at 4 μg/g.
Determination of rhenium content in coal by high-resolution ICP-MS did not detect rhenium in most of the samples from our country but in the coals of Taiyuan Formation of the individual mines of Kailuan in Hebei Province, Jining in Shandong Province, and Jincheng in Shanxi Province. In individual coal seams of the Upper Permian in Xingren, Guizhou and individual coal samples of the Upper Triassic in Anyuan, Jiangxi, the rhenium content of 0.106-0.39 μg/g was measured, and these values, although lower than those required for the evaluation of associated minerals, have been more than a hundred times higher than the Clarke's value of rhenium to a few hundred times higher than that of rhenium, which is relatively rich and deserves further attention in the future. Rhenium in the early and middle Jurassic leachate-type uranium-coal bed coal in Xinjiang should be emphasized.
Four, scandium (Sc)
Scandium is a kind of super heat-resistant manufacture of light alloys of rare metals, expensive, at present, mainly from the refining of tungsten, titanium, uranium and other metals in the waste residue (scandium content of 80-100μg/g), out of the rate of quite low. Середин proposed that, when the content of scandium in the coal ash exceeds 100 μg/g, it can be used as a useful coal combustion by-products be evaluated [6]. According to Ketris and Yudovich, the global average value of scandium content in coal is 3.9 μg/g, and the average value of scandium content in coal ash is 23 μg/g [9].
Recent studies have shown that the scandium content in coal ash is quite high in some coal-producing regions. The coal ash of individual coal seams of Chernigov open-pit mine, Kartan open-pit mine and South Gilgayi mine in the Kuznets coalfield, Russia, contains scandium in the range of 100-200 μg/g [31]. Юровский found that scandium content of 400 μg/g was found in low-density fine coals of the Chernogol coal-producing area of the Kuznets coalfield after heavy-liquid separation of the coals, and thus scandium-enriched fine coals can be extracted at the stage of coal beneficiation. Coal ash from some seams of the Minusinsk coalfield in Russia contains scandium in the range of 95-175 μg/g, and the scandium content reaches 400 μg/g in the low-density class of the coal.The upper sub-seam of the No. 1 seam of the Bereozov coal production area of the Jurassic coalfield of the Kansk-Achinsk, Russia, contains scandium at a level of 230 μg/g, and the scandium content of the ash of the seam is up to 870 μg/g [32].
The Amos coal seam in northwestern Kentucky, USA, is very thin (<0.5m), and the scandium content in the coal ash reaches 560 μg/g in its bottom 8.2-cm-thick sub-bed [28].
The average value of scandium content in the Permian coalfield in Heshan, Guangxi was higher at 42.2 μg/g, while the scandium content in the coal ash in the middle of the No. 4 seam of its Suhe Mine amounted to 221 μg/g [14].
Other elements in coal with abnormally high content and possible recovery of by-products are V, Sb, Cs, Mo, W, Be, Ta, REEs, Zr, Hf and so on.
The exploration and evaluation of beneficial mineral resources associated with coal **** is very meaningful. It is difficult to make up for the lack of this work in the exploration of coal resources. When engaged in this work, need to pay attention to the following matters.
(1) Prefer the best testing method of beneficial elements to ensure the reliability of the test results.
(2) Since the *** associated beneficial elements in coal are often enriched in the local layers and specific space of the coal seam, attention should be paid to the reasonable arrangement of sampling points to grasp the law of its enrichment and mineralization.
(3) The best way to utilize beneficial metal elements in coal is to extract them from fly ash. Therefore, it is very important to study the habits of beneficial elements in coal combustion and other processing and utilization processes, and the degree of enrichment of beneficial elements in coal by-products and the possibility of their recovery.
(4)The *** associated beneficial minerals in coal are often polymetallic, and in addition to the beneficial elements, there are often potentially harmful elements, so a comprehensive techno-economic and environmental assessment must be carried out to ensure that the development of the potentially harmful elements to minimize the impact on the environment and human health.
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Potential Coexisting and Associated Mineral Resources in Coal and Coal-bearing Strata---An Issue Should Pay Close Attention to
Ren Deyi, Dai Shifeng
( Key State Laboratory of Coal Resources and Safety Mining, National Center for the Study of Coal and Coal-Bearing Materials). Coal Resources and Safety Mining, CUMT ( Beijing ), Beijing 100083;
School of Earth Science and Surveying and Mapping Engineering, CUMT ( Beijing ), Beijing 100083). ), Beijing 100083)
Abstract: Coal is a kind of organolite and mineral deposit with high reducing barrier and absorbing barrier performances. under specific geological conditions, it can enrich some useful metal elements and amount to the ore-forming scale. integrated some literatures both home and abroad, w e have discussed the abundance, hosting state, geologic genesis and possibility of utiliza- tion of useful metals such as niobium, gallium, rhenium and scandium. The research of rare metal elements enrichment or ore-forming is one of major subjects in coal geochemistry and ore deposit geotechnology. The research of rare metal elements enrichment or ore-forming is one of the major subjects in coal geochemistry and ore deposit geochemistry, and thus w orthw hile to be further strength- ened.
Key words: coal; coal-bearing strata; rare metal; coexisting and associated ore deposits coexisting and associated ore deposits
( This paper was co-authored by Ren Deyi and Dai Shifeng, and originally published in China Coal Geology, Vol. 21, No. 10, 2009)