УДК 552.574

Rare earth metals of coals

Портнов Василий Сергеевич – доктор технических наук, профессор Карагандинского технического университета имени Абылкаса Сагинова (Республика Казахстан).

Мусымбаева Алия Думановна – кандидат технических наук, старший научный сотрудник Карагандинского технического университета имени Абылкаса Сагинова (Республика Казахстан).

Abstract: To construct a model of the surface layer of coals of the Shubarkol deposit containing rare earth metals, to establish their role in the physical processes occurring in the pores of nano and mesostructures. To assess the influence of the nanostructure of coal matter on the formation of rare earth elements and uranium, using the patterns of thickness and energy of the surface layer that we have established, a model has been developed that determines the formation of rare earth elements, taking into account the grades of coal and their porosity. For the first time, the relationship between the thickness of the surface layer of coal matter and the radius of pores with the formation of rare earth elements and uranium in coals of the Shubarkol deposit has been calculated. Capillary condensation plays a leading role in this process. The role of multiscale evolution of defects in coal matter on the formation of REE and uranium concentrations under the influence of dynamic loads on a coal seam (tectonic disturbances, underground or open-pit mining) and the influence of the fractal dimension of coal is shown. The established patterns of the formation of rare earth elements and uranium show their connection with the nanostructure of the surface layer and porosity determined by the nano- and meso-structures of coal with the leading role of the concentration of a heterogeneous mechanism, which is associated with the tectonics of the coal massif and coal mining. The established patterns are important for understanding and taking into account the zones and areas of formation of rare earth elements and uranium in coals during their development.

Аннотация: Цель стать - построить модель поверхностного слоя углей Шубаркольского месторождения, содержащих редкоземельные металлы, установить их роль в физических процессах, происходящих в порах нано- и мезоструктур. Для оценки влияния наноструктуры угольного вещества на образование редкоземельных элементов и урана, используя установленные нами закономерности толщины и энергии поверхностного слоя, была разработана модель, определяющая образование редкоземельных элементов с учетом марок угля и их пористости. Впервые рассчитана зависимость между толщиной поверхностного слоя угольного вещества и радиусом пор при образовании редкоземельных элементов и урана в углях месторождения Шубарколь. Ведущую роль в этом процессе играет капиллярная конденсация. Показана роль многомасштабной эволюции дефектов в угольном веществе на формирование концентраций РЗЭ и урана под воздействием динамических нагрузок на угольный пласт (тектонические нарушения, подземная или открытая разработка) и влияние фрактальной размерности угля. Установленные закономерности образования редкоземельных элементов и урана показывают их связь с наноструктурой поверхностного слоя и пористостью, определяемыми нано- и мезоструктурами угля, при ведущей роли концентрационного гетерогенного механизма, который связан с тектоникой угольного массива и добычей угля. Установленные закономерности важны для понимания и учета зон и участков образования редкоземельных элементов и урана в углях в процессе их разработки.

Keywords: surface layer, rare metal, nanostructure, mesostructure, atomic volume, size effect, coal substance, fractal.

Ключевые слова: поверхностный слой, редкий металл, наноструктура, мезоструктура, атомный объем, размерный эффект, угольное вещество, фрактал.

Introduction

A large group of rare earth elements (REE) is known in coal deposits, which can be used in industry [1]. Germanium, lithium and gallium are extracted from coal on an industrial scale. Extraction of rare earth elements from coals has begun in Russia, Kazakhstan and China [2, 3]. Analysis of the list of rare metals contained in weathered coals of the Shu-Barkol deposit showed that uranium and rare earth elements are of the greatest industrial interest [4]. Its content corresponds to low-grade uranium ores; for rare earth metals, coals are an ordinary raw material, but with increased contents of yttrium group rare earth elements [5]. Weathered Shubarkol coals are characterized by a heterogeneous content of trace elements, mainly in the near-surface layers of coal. Their increased concentrations are spatially confined to anomalous zones of uranium.

During preliminary exploration of the Shubarkol deposit, higher concentrations of uranium and rare earths were established in the zone of weathered coals. In order to determine the scale of uranium and accompanying mineralization, special work was carried out. An experimental site was drilled in detail in the area of the greatest distribution of anomalies 2 km along the perimeter of the deposit, and the location of radioactive anomalies in the zone of weathered rocks at the outcrops of coal horizons was confirmed. The most intense anomalies were recorded in the western, northern and northeastern parts of the deposit, where the Upper coal horizon is characterized by the greatest thickness. Radioactive anomalies of variable intensity can be traced here in the form of a strip with a width from 20-25 m to 200-250 m (experimental area), called the zone of stable spread of anomalies. In the southern, split part of the outcrops, the width of the anomalous zone decreases (mostly 20-25 m). The depth of the anomaly from the daytime surface ranges from 6-15 m in the south to 25-50 m in the north of the trough. The genesis of mineralization is infiltration. According to the testing results, the content of uranium and rare earth elements (yttrium, scandium, lanthanum, cerium, dysprosium, gadolinium) in individual mines exceeds the clarke value determined by M.P. Ketris and Y.E. Yudovich for the coals of the world (Ketris, Yudovich, 2009). At the same time, the uranium content in weathered coals corresponds to the content in low-grade uranium ores, while the uranium will be associated with the main useful component - coal [6,7].

The purpose of the work is to construct a model of the surface layer of coal from the Shubarkol deposit containing rare earth metals, and to clarify their role in the physical processes occurring in nano- and meso-sized pores. In works [8, 9], the thickness of the surface layer of atomically pure metals is considered. It is on average 2-6 nm. In [10, 11], we determined the thickness of the surface layer d(I) of the coal substance. It turned out to be equal to an average of 150-200 nm, which is two orders of magnitude greater than the thickness of the layer of atomically pure metals.

Methods

Coal testing took place along two sections, using the furrow method, according to GOST 9815-75 “Brown coals, hard coals, anthracite and oil shale. Sampling methods”, across the strike of coal seams in the direction from the soil to the roof. The length of the furrow sample varied depending on the thickness of the formation, ranging from 0.1 to 1.0 m, the width of the furrow - 0.05 m. A total of 18 samples of weathered coals were selected, which were analyzed in the laboratory of the Federal State Budgetary Institution "IMGRE" for REE using the inductively coupled plasma mass spectrometric method (ICP-MS). In the process of performing analytical work to determine the elemental composition of coals and coal-bearing rocks using ICP MS plasma spectrometry methods, a systematic approach and a comprehensive analysis of the main factors influencing the correctness of the data obtained were used. For this purpose, studies were carried out on various methods of chemical preparation of samples for analysis, including open acid decomposition with a mixture of nitric, perchloric and hydrofluoric acids. With this decomposition, the organic matrix and silicate structure of the substance are destroyed and silicon is subsequently removed in the form of the volatile compound silicon fluoride SiF4. The selected sample opening method is most suitable when using a highly sensitive elemental analysis method (ICP MS), since removing the silicate matrix from the sample reduces the salt load on the ICP instrument and provides the lowest detection limits for trace elements.

Inductively coupled plasma mass spectrometry (ICP MS) uses an argon inductively coupled plasma as an ion source and a mass spectrometer to separate and subsequently detect these ions.

Based on the results of analytical work, the maximum contents of rare metals and uranium were found to be confined to the upper three-meter sampling intervals, which indicates the accumulation of metals in the top part of the oxidized coal seam.

Conclusion

Description of the empirical model of the surface layer of coal at the Shubarkol deposit. For comparative analysis. Let us consider the thickness of the surface layer of frame-type hydrocarbons (HCs), which, due to their highly symmetrical diamond-like structure, have a number of unique properties, which include, first of all, high thermal stability [12]. These are adamantane C10H16, diadamanta C14H20, twistane C10H16 - an isomer of adamantane, cubane C8H8. Adamantane and diadamantane are tricyclic hydrocarbons, twistane are teracyclic hydrocarbons, and cubane are pentocyclic hydrocarbons.

For framework hydrocarbons, it is quite difficult to determine the surface energy in the solid state, since their molecules on the surface practically do not move, which significantly distinguishes them from liquids. Since the beginning of the 20th century, various methods have been developed for determining the surface energy of solids. A review of these methods was carried out in [13]. Works [9, 10] propose methods for determining the surface energy of solids from the size dependence of microhardness or electrical resistance of deposited coatings. For adamantane, the surface energy we calculated is 378.7 mJ/m2, and the experimentally observed microhardness is 372 mJ/m2 and the electrical resistance is 384 mJ/m2, that is, the difference is within the experimental error.

Shubarkol coals contain small amounts of REE impurities with a size of about 3-4 nm and they enter mesopores. REE are also included in the organic (vitrinite) and inorganic parts (monazite, crandallite, etc.). The vitrinite content is more than 80% of the organic mass of coal. The vitrinite group includes three macerals (telinite, collinite and vitrodetrinit) and is part of coals. Vitrinite has a molecular size in the range of 1-3 nm and is also included in the mesopores of Shubarkol coal. The inorganic part in weathered coals of the Shubarkol deposit contains mainly REE oxides, for example, Nd2O3. The thickness of the surface layer of this crystal, calculated using formula (1), is 7.9 nm, i.e. it is also included in the mesopores of Shubarkol coal grade D. In other words, in general, in grade D coal the spectrum of rare metals is significantly wider than in coal grades G, Zh, K, which are common in the deposits of Kazakhstan. The distribution of rare metals in the oxidation zone of a coal seam will be similar to the distribution of rare metals in weathering crusts, with a characteristic removal of light REEs and a relative enrichment in heavy REEs. These features are manifested in the coals of the Shubarkol deposit. They are characterized by relatively high contents of medium-heavy rare earth metals, localized under the mudstone screen in the upper part of the layer of weathered coals [4]. Anomalies of increased activity, most likely of uranium nature, are confined to the upper parts of the coal member and overlying rocks. In general, anomalous zones of radioactive weathered coals among weathered coals of the Shubarkol deposit (uranium concentration exceeds 1000 g/t) can be considered as areas of a single complex uranium-rare earth deposit [4].

The forms of occurrence of REE in coals of the Shubarkol deposit indicate that their concentration in coal is due to the leading role of the hydrogenogenic mechanism. Regardless of the source of entry into the coal accumulation basin, REE under the conditions of the aggressive environment of the paleo-peat in the bulk transform into a mobile form and ultimately accumulate in organic matter [3].

Rare metals from Shubarkol deposit coals have a surface layer thickness of about 3 nm, i.e. represent a nanostructure. The thickness of the surface layer of the coal itself is about 0.2 microns, i.e. is a mesostructure. The pore radius in the coal substance is 30.6 nm, which corresponds to mesopores, and the specific surface area of the coal is 857 m2/g. Rare metals, either in the form of pure impurities (~ 3 nm), or in the form of oxides (~ 7 nm), or in the form of organic impurities (~ 1-3 nm), freely enter the mesic supports of coal. The occurrence of uranium and rare metals in the coals of the Shubarkol deposit indicates that their concentration in coal is due to the leading role of the hydrogenogenic mechanism. The fractal dimension of Shubarkol coal is 2.60, which is lower than that of anthracite – 2.74. In the mesoporous structure of Shubarkol coal, adsorption of rare metal impurities is highly developed due to the formation of adsorption layers on the surface of these mesopores, which lead to volumetric filling of these pores through the mechanism of capillary condensation.

This research has been funded by the Committee of the Ministry of Science and Higher Education of the Republic of Kazakhstan (Grant No. AP14972877).

References

1. Хасанов, Р.Р. (2010). Редкоземельные элементы в визейских угольных пластах волго-уральского региона / Р.Р. Хасанов, Ш.З. Гафуров, А.Ф. Исламов // Ученые записки Казанского университета. Естественные науки. –Том 152. - кн. 4. – С. 116-122.
2. Арбузов, С.И., Вергунов А.В., Ильенок С.С., Иванов В.А., Иванов В.П., Соктоев Б.Р. (2019). Геохимия, минералогия и генезис редкометалльно-угольного месторождения в пласте XI на юге Кузнецкого бассейна// Геосферные исследования. – № 2. - С. 35-61. https://doi.org/10.17223/25421379/11/3.
3. Арбузов, С.И., Финкельман Р.Б., Ильенок С.С., Маслов С.Г., Межибор А.М., Блохин М.Г. (2019). Формы нахождения редкоземельных элементов (La, Ce, Sm, Eu, Tb, Yb, Lu) в углях северной Азии (обзор) // Химия твердого топлива. – № 1. - С. 3–25. https://doi.org/10.1134/S002311771901002X.
4. Амангелдыкызы А. (2021). Исследование распространенности редкоземельных металлов в углях главных угольных бассейнов Центрального Казахстана: Диссертация PhD, 6D070600. –КарТУ, Караганда, – 202с.
5. Сафонов, А.А., Маусымбаева А.Д., Портнов В.С., Парафилов В.И., Ганеева Л.М (2018). Микрокомпонентный состав углей Центрального Казахстана // Уголь. –№9. -С.89-94. https://doi.org/18796/0041-5790-2018-9-70-75.
6. Педаш Е.Т., Ко Н.А.(1987). Отчет о детальной разведке Шубаркольского угольного месторождения. Книга 1. Текст отчета. Караганда, – 320 с.
7. Левченко Е.Н., Ключарев Д.С. (2019). Отчет по «Геолого-экономической оценке перспектив извлечения редких и рассеянных элементов из выветрелых углей Шубаркольского месторождения», ФГБУ «ИМГРЭ». Москва,–207с.
8. Юров, В.М., Гученко С.А., Лауринас В.Ч. (2018). Толщина поверхностного слоя, поверхностная энергия и атомный объем элемента // Физико-химические аспекты изучения кластеров, наноструктур и наноматериалов. – 2018. – Вып. 10. – С. 691-699. https://doi.org/10.26456/pcascnn/2018.10.691.
9. Mullagaliyeva, L.F., Baimukhametov K., Portnov V.S., Yurov V.M., Ibragimova D.A. (2022). Nanostructures of coal beds in the Sherubaynurinsky section of the Karaganda basin/ //Naukovyi Visnyk Natsionalnoho Universytetu, 4(190), 17-22. https://doi.org/10.33271/nvngu/2022-4/017.
10. Юров, В.М., Маханов К.М., Портнов В.С. (2020). Наноструктуры в тонком слое угольного вещества // Физико-химические аспекты изучения кластеров, наноструктур и наноматериалов. – Вып. 12. – С. 746-757. https://doi.org/10.26456/pcascnn/2020.12.746.
11. Portnov V., Yurov V., Reva M., Mausymbaeva A., Imanbaeva S.(2021) Nanostructures in surface layers of coal matter / //Visnyk of Taras Shevchenko National University of Kyiv, Geology, 4(95), 54-62. https://doi.org/10.17771/1728-2713.95.07
12. Брусиловский, А.И.(2002). Фазовые превращения при разработке месторождений нефти и газа. М.: «Грааль». – 575с.
13. Ролдугин, В.И.(2008). Физикохимия поверхности. –Долгопрудный: Издательский дом «Интеллект». - 508 с.