Brazilian Coal Tailings Projects: Advanced Study of Sustainable Using FIB-SEM and HR-TEM

Abstract

The objective of this study is to obtain a more detailed assessment of particles that contain rare-earth elements (REEs) in abandoned deposits of Brazilian fine coal tailings (BFCTs), so as to aid current coal mining industries in the identification of methodologies for extracting such elements (Santa Catarina State, Brazil). The BFCT areas were sampled for traditional mineralogical analysis by X-ray Diffraction, Raman Spectroscopy and nanomineralogy by a dual beam focused ion beam (FIB) coupled with field emission scanning electron microscopy (FE-SEM) and high-resolution transmission electron microscopy (HR-TEM) coupled with an energy dispersive X-ray microanalysis system (EDS). The results show that the smaller the sampled coal fines were, the higher the proportion of rare-earth elements they contained. Although the concentration of REEs is below what would normally be considered an economic grade, the fact that these deposits are already ground and close to the surface negate the need for mining (only uncovering). This makes it significantly easier for REEs to be extracted. In addition, owing to their proximity to road and rail transport in the regions under study, the opportunity exists for such resources (BFCTs) to be utilized as a secondary market as opposed to simply being discarded as has been done in the past.

1. Introduction

Although coal mining activities exert harmful impacts on the local environment and cause problems for public health, coal mining remains an important economic activity related to the production of electricity globally [1,2,3,4]. Open pit coal mining, conducted without environmental safeguards, causes contamination of the soil and water sources by introducing the following hazardous elements which are harmful to human health: arsenic, cadmium, lead, mercury, radionuclides, thorium, and uranium, among others [5,6,7,8,9,10,11,12,13].

According to Zhao et al. [14], Saikia et al. [15], Hu et al. [16], George et al. [17] and Guo et al. [18], concentrations of hazardous chemical element contaminants derived from coal can vary significantly from region to region, taking into account both micro and macro scales. This justifies the need for studies aimed at understanding contaminants originating in coal tailings and involving possible contaminants derived from coal mining activities throughout the world [19,20,21]. A thorough scientific understanding of the dangers coal presents is necessary in order to create public policies that can subsidize environmental restoration projects, capable of mitigating harmful impacts exerted on the environment through deposition of mineral coal tailings containing dangerous elements, thus contributing to the sustainability of the environment [18,20,22,23,24].

Three southern Brazilian states, Paraná, Santa Catarina and Rio Grande do Sul, are responsible for the majority of mineral coal mining that occurs in Brazil and that is aimed at feeding thermoelectric power plants generating Brazilian electricity [25,26,27,28]. These studies [25,26,27,28], carried out with coal in the southern region of Brazil, raise concerns related to the negative impacts attributed to environmental quality that are capable of covering large regions, generating environmental unsustainability. Of the three states mentioned above (Paraná, Santa Catarina and Rio Grande do Sul), the state of Santa Catarina contains 80% of Brazil’s known coal reserves, totaling approximately seven layers of coal in areas destined for mining activities [27,29]. The majority of Santa Catarina 200+ coal mines are open pit mines, accounting for 3,643,000 tons of coal extracted annually, which represents a total of 50% of the mineral coal extracted in Brazil [27,30]. Globally, coal-fired power plants account for 38% of the world’s energy generation [28,30].

The chemical composition of coal tailings depends on their geological formation [31]. Not all coal is created equal and geographic disparities result in different elemental concentrations in different coal deposits [27,28,30]. Several authors who have studied coal tailings in different countries have reported enormous variability in chemical and mineralogical compositions exhibited by mineral coal [21,27,30,32,33]; therefore, identification of possibilities for improvements in the extraction of high-value materials in coal areas is of paramount importance [34,35].

Brazilian fine coal tailings (BFCTs) from abandoned areas may contain non-coal rocks that have fallen during the mining of BFCT deposits, ultra-fine mixed coal/rock particles that have been rejected due to density, and coal particles that have accidentally been misplaced in the separation of coal during mining activities [21,27,30,33]. In general, over the past few decades, all BFCTs have been deposited in large containment basins or trenches that, with time, will end up overflowing or backfilled with soil or rocks present in coal mines (see Figure 1 example). Given this scenario, it is crucial that new studies are conducted, not only in Brazil but on a global level, as coal is utilised in almost all countries, developed and non [9,11,12,13,36].

It is worth remembering that scientific studies aimed at identifying nanoparticles (NPs, minerals and/or amorphous phases) present in coal tailings material after the mining process are necessary to understand the mechanisms of geological and environmental evolution and possible health risks [11,36,37,38,39] when assessing provenance, migration, deposition and coal breakdown after the mining process. Natural NPs in coal mining areas are often neglected compared to traditional mineralogical materials [33,36,40,41]. Understanding the origins of natural NPs in coal areas affords opportunities for environmental scientists to appreciate NP sources of pollution, as well as providing information on hidden mineral resources within coal formations [42,43].

The objective of this study is to obtain a more detailed assessment of particles that contain REEs in abandoned areas containing BFCTs, so as to aid coal mining industries in the identification of methodologies for extracting such elements. The authors also wish to contribute to the scientific knowledge about coal tailings on a global level, motivating and stimulating new studies on and interest in REEs in BFCTs to achieve more sustainable mining operations. This study is of fundamental importance as it implements one of the most advanced technological techniques globally, such a technique consisting of a combination of mineralogical analysis by X-ray Diffraction, FE-SEM and HR-TEM, in addition to the usage of X-ray microanalysis of dispersive energy. Said techniques are aimed at determining the most abundant mineral particles in the crystalline phases present in sediments in abandoned BFCT deposits. This enables a better and more efficient use of coal tailings, capable of contributing to local sustainability.

2. Materials and Methods

2.1. Studied Areas and Sampling Procedures

Coal in Brazil has never enjoyed federal subsidies or investment on a scale similar to renewable energies such as hydro, solar and wind, all of which are abundant in Brazilian territory [44,45]. However, in periods of drought, coal and other non-renewable fuels are routinely encouraged in order to meet a diversified mix of power generation for the country’s energy needs [45]. Brazil has experimented with governmentally operated mining operations, especially during military dictatorship from 1964 to 1985; the largest scale of production is private, supplying coal to coal power plants [27,46].

In the Brazilian state of Santa Catarina, coal is of great importance due to the energy characteristics of this state, which does not have large hydroelectric power plants which, conversely, dominate the states of Paraná and Amazonas [27,30]. The impacts of coal mines are more visible in relation to both the environment and the health of the population, especially those residing in the vicinity of coal mines, along the routes of coal transport and in proximity to the coal power plant itself [47,48]. The state of Santa Catarina, where the cities of Lauro Muller and Siderópolis are located (Figure 1), is characterized by a subtropical climate, with average temperatures ranging from 6 °C in winter to 25 °C in summer [49,50]. The region surrounding the cities of Lauro Muller and Siderópolis is characterized by high numbers of deactivated, abandoned open pit coal mines [50].

A total of twenty-six BFCTs samples were collected from abandoned coal mining areas in the cities of Lauro Muller (Figure 2A,B) and Siderópolis (Figure 2C,D). As it can be evinced from Figure 2A–D, the areas are highly heterogeneous and not only contain BFCTs, but also coal mine rocks and even abandoned mining machinery. It is important to bear in mind that abandoned coal mining areas contain material that is extremely heterogenous in size, ranging from giant rocks to nanoparticles of coal. The sampled material was excavated with shovels and a backhoe and later stored in plastic containers. The BFCT samples were transported to laboratories to be dried, ground and divided into different fractions, depending on the analyses to be performed (e.g., <20 mesh to mineralogy; <60 mesh to chemical composition). This sample preparation procedure was based on ASTM D-2013 standards [51]. Studies carried out on Brazilian coal tailings [27,36,46,52] were also used as a theoretical basis for the elaboration of this study with regard to the structuring of methodological procedures.

2.2. Laboratory Analysis Procedure

In each of the laboratories (e.g., chemical analysis, XRD, and electron microscopies), all twenty-six obtained BFCT samples were dried in ovens with ventilation at a maximum of 40 °C for 48 h to avoid volatilization of volatile elements (e.g., halogens and Hg) [27,36,46,52,53]. Subsequently, the samples from Lauro Muller and Siderópolis were homogenized to evaluate the two areas of interest so that they are truly representative, thus ensuring greater reliability of the results obtained [53,54,55,56].

X-ray diffraction determines the most abundant minerals in the sediments. For the XRD the PANalytical X’Pert PRO powder diffractometer was used (Malvern, UK), which contains a tube composed of copper (λCuKαmedia = 1.5418 Å; λCuKα1 = 1.54060 Å; and λCuKα2 = 1.54439 Å) with a vertical goniometer (Bragg-Brentano geometry) for automatic sediment exchange in addition to a secondary monochromator with a PixCel detector (New York, NY, USA) and graphite. These measurements were 40 kV and 40 mA, in angular range (2θ) with possible variations of 5 and 70° (Figure 3). The sample data were treated in diffractograms, with mineral phases being identified using the specific software X’Pert HighScore (PANalytical), version 2016, combined with the powder diffraction file database (International Center for Diffraction Data—ICDD, Pennsylvania, USA) originated from the analysis of the sediments collected in abandoned coal piles at points sampled in the cities of Lauro Muller and Siderópolis.

Chemical analysis was completed via inductively coupled plasma of mass spectrometry (ICP-MS)—Elan 9000 from Perkin Elmer (Woodbridge, ON, Canada). Following completion of the drying process, 0.5 g of sediment was used. These dried sediments were placed in a sterilized glass container. Extraction was completed through the addition of 20 mL of HNO₃/HCl (Tracepur, Meck, Rahway, NJ, USA) to each container, diluted to internal standards Sc45, Be9, Bi209 and In115 and (10 μg L⁻¹) for 6 min. A HD2070—Sonopuls Ultrasonic Homogenizer (Bandelin, Berlin, Germany), containing a glass probe with a thickness of 6 mm, was successively utilized. The sampled material was filtered using a 0.45 μm filter in an aqueous base, with HNO₃ concentration adjusted to 1%. Following preparation, the following trace elements were quantified: Al, Ca, Fe, K, Mg, Na, Na, P, Ti, As, Ba, Bi, Cd, Co, Cr, Cs, Cu, Ga, Li, Mn, Mo, Nd, Ni, Pb, Sb, Sn, Sr, Ta, Th, U, V, W, Y and Zn utilizing ICP/MS through the external calibration method, with samples in a clean room (class 100), in addition to Argon gas of 99.999% purity (Praxair, Madrid, Spain) used as carrier between ICP/MS measurements. Extraordinary data containing crystal structure (including at the atomic scale), morphology, size and composition and geochemical distribution of chemical elements distributed in NPs were provided by the combination of advanced techniques [27,57], summarized below, thus offering insights into the processes of the geological evolution of BFCTs. For mineralogical analyses, the most abundant minerals were evaluated by XRD, while electron microscopes were used especially for the detection of minority minerals (<5%) or to evaluate the associations between abundant and minority minerals. To better understand the modes of occurrence of REEs used in this study, a dual beam focused ion beam (FIB) coupled with field emission scanning electron microscopy (FE-SEM) and an energy dispersive X-ray microanalysis system (EDS) were utilized [27,36,46,52,53]. With this, it is possible to evaluate, for example: (1) presence of minerals in grains that contain REEs such as monazite; (2) mode of occurrence of REEs agglomerated with clays; (3) REEs associated with the organic fractions of the coal; (4) other modes of occurrence of REEs. The FIB ability to prepare particles for analysis by a high resolution transmission electron microscope (HR-TEM) coupled with an energy dispersive X-ray microanalysis system (EDS) facilitates the study of particles <50 nm; such particles are difficult to analyze robustly with the FE-SEM, due to the low efficiency of this equipment for such small particles [15,58,59].

Routine XRD inspections for sediment samples and sulfate salts were conducted with 0–60° 2θ scans with a count time of 0.5/s per step. Fluorite as an internal reference material was used to determine the semiquantitative mineralogical composition [58,59]. The mineral species, when analyzed in FE-SEM, involved particles with thicknesses between 0.5 and 80 μm, while, when analyzed using HR-TEM, particulate minerals exhibited a thickness between 0.1 and 500 nm. The HR-TEM equipment used is equipped with a M4 TORNADO (supplier: Bruker Nano GmbH, Berlin, Germany), manufactured in 2018, equipped with a Rh X-ray tube and an air-cooled HV generator (Figure 3). The ultrafine crystal morphology and structure were quantified using the methodological model of Zeiss ULTRA Plus FE-SEM, through conductive charge compensation applied to sediment samples, in addition to the use of JEOL-2010F 200-keV—HR-TEM (Tokyo, Japan) with detection scanning X-rays (STEM). The sample measurements by HR-TEM respected a phase shift of π/4 in a probe containing 14.5 mrad. Consequently, the EDS spectra in TEM image mode were quantified using the ES Vision software, version 2018, suitable for identifying the crystalline phases present in the sediments sampled in this study. The chemical composition of the sampled BFCTs were determined using inductively coupled plasma atomic emission spectrometry (ICP-AES), followed by inductively coupled plasma mass spectrometry (ICP-MS) [27,36,46,48,60,61]. The following elements were quantified as those present in the sampled BFCTs, which is currently the most advanced technique responsible for the identification of dangerous elements in mineral coal tailings [60,61]: carbon (C), Nitrogen (N), sulfur (S), oxygen (O), and hydrogen (H). As previously mentioned, mineral coal tailings are abandoned and out in the open, which increases the risk of environmental degradation in addition to compromising human health in the cities of Lauro Muller and Siderópolis.

3. Results and Discussion

3.1. Chemical Composition of the Sampled BFCTs Identified

The most abundant minerals compared to the minority phases were very heterogeneous in all BFCTs studied (Table 1). The Lauro Muller samples showed a greater degree of mineralogical variety when compared to those from Siderópolis. Similar mineralogical composition was found in previous studies by Silva et al. [27] and Oliveira et al. [36], related to coal and coal tailings in Brazil.

In both study areas a wide variety of sulfates were detected (Figure 4). This is due to the fact that BFCTs contain Fe-sulfides and carbonates, which, after being abandoned for decades, undergo several geochemical and microbiological reactions in the presence of oxygen and water [62,63], resulting in the decomposition of several primary minerals present in coals (including several clays and oxides) due to the presence of sulfuric acid (from the degradation of Fe-sulfides). It should be noted that the greater abundance of sulfates in the Lauro Muller region is not accounted for by its older age compared to Siderópolis, but rather only by the abundance of primary minerals in this region.

In the Lauro Muller region, most of the samples were located in large deposition basins abandoned for decades—these areas were flooded and many of the minerals detected are the same that were also located by authors who previously studied coal mine drainage in this region [27,47]. Toxicological studies performed by Nordin et al. [47] for mineral sulfates, as represented in Figure 4 and Figure 5, show a high negative effect on human health; therefore, it is crucial that sampled BFCTs are used to avoid problems in the environment and in the health of the population. This study highlights the great importance of seeking an application for BFCTs, one with such a great economic potential that BFCTs are no longer posing environmental and human health risks.

As it can be evinced from Table 1, the minerals containing REEs were monazite and xenotime. These were detected by FE-SEM and HR-TEM owing to the preparation conducted by FIB which allowed precise cuts of several agglomerated clays containing monazite and xenotime in their interiors. In addition to phosphate minerals, multiple inorganic amorphous phases containing REEs were detected (Figure 6). It was also possible to detect some REEs associated with the organic phases present in BFCTs, albeit in lower amounts than the minerals and associated with amorphous inorganic phases. In some cases, when preparing the 136 agglomerated grains of amorphous material containing REEs with FIB, it was possible to confirm that: most cases (98 grains) contained mozanite grains; many agglomerates (31 grains) did not contain minerals such as monazite, but rather organic material that contained smaller portions of REEs; and, in rare cases, the particles (7 grains) were just amorphous material of Al, Si, P, K that contained REEs. It is worth noting that particles were easily detected by FE-SEM/EDS since their sizes were always 1–14 µm. When prepared by FIB-SEM/EDS such particles could be easily studied by HR-TEM/SAED/EDS, as they always fell between 2–190 nm.

The chemical composition (Table 2 and Table 3) was even more complex than the mineralogical composition. This is likely due to the fact that the coal samples may have come from different geological formations, in addition to undergoing procedures that completely changed the homogeneity of BFCTs during the mining process and subsequent dumping, burial and/or backfilling. Furthermore, the different disposition modes of BFCTs also completely alter their compositions [3,64,65]. As the studied samples are from abandoned areas, information about the filling of the ponds (especially in the Lauro Muller region) and removal of engineering equipment (e.g., pumping and piping) were unavailable, in addition to details on the change of ownership of the property; the authors had no access to information about the logistical disposition of BFCTs. As a result, it is highly difficult to interpret the geochemistry of abandoned coal mining areas. The most reasonable way forward, therefore, consists of conducting a representative collection of several BFCTs disposal points and subsequently mixing them homogeneously in order to obtain representative data in relation to the chemical and mineralogical structure of the REEs.

As evidenced by previous coal studies from the Lauro Muller region of Siderópolis, the high heterogeneity of differing coal mines [66,67] also results in BFCTs which are highly variable in terms of their chemical and mineralogical composition. Almost all minor chemical elements in both the Lauro Muller region and the Siderópolis area (Table 3) are above common global average concentrations for coals [68].

This is accounted for by the fact that they are coal fines and their small size directly influences the concentration of trace elements. A similar interpretation has already been advanced by several studies that evaluated coal in seams as compared to coal in mining products and by-products [5,15,69,70]. Major elements were more abundant in the Lauro Muller region, while several minor elements, notably tin (Sn), molybdenum (Mo), and some REEs, were detected in elevated concentrations in some of the samples, especially in the Siderópolis region (Table 3). Furthermore, as described above, the chemical composition can be derived from many factors, such as the geological formation of the coal, the methods used in coal mining, handling and several other influences on the area where BFCTs have been deposited.

Elements such as arsenic (As), cadmium (Cd), copper (Cu), mercury (Hg), molybdenum (Mo), lead (Pb), antimony (Sb), and zinc (Zn) were present in higher concentrations in the region of Siderópolis. These data conform to the higher proportion of sulfides that were more abundant in Siderópolis than in Lauro Muller, and the correlation between sulfides and these potentially toxic elements can be noted, especially in sulfide particles smaller than 100 nm. Other chemical elements, such as chromium (Cr), nickel (Ni) and gadolinium (Gd) were also present in higher concentrations in Siderópolis. However, according to the data obtained by electron microscopy, such elements are associated with siderite grains between 42 and 1582 nm, and the largest proportion of such elements is found in the smallest sized nano-siderites. This could only be appreciated following the scraping of the surface of the siderite particles—which were always agglomerated with clays and amorphous phases—with FIB, so that they could be better evaluated by HR-TEM/EDS and FE-SEM/EDS.

3.2. Nanoparticles

Nanoparticles (NPs) and ultra-fine particles (natural or anthropogenic) have been increasingly studied as they have marked effects on both the environment and human health, regardless of their method of deposition or accumulation [71,72,73]. The NPs detected in this study are excellent indicators of the mechanisms existing in the geological processes and can be used for the exploration of existing resources in the BFCTs, considering that the detected NPs had geochemical, physical and/or biological information about the layers buried in the basins and areas of deposits of fine coal tailings. Similar considerations about NPs have been reported by authors who have studied natural NPs [73,74].

The multi-analytical combination used in this study enabled the understanding of the multiple associations between minerals, amorphous phases and complex chemical elements such as REEs. For example, using FIB made it possible to control the size of BFCTs prepared to analyze NPs—which were often highly heterogeneous and poorly crystallized—and thus ensured a uniform thickness suitable for a variety of FE-SEM, HR-TEM, NanoSIMS and STXM. This study shows that the combination of FIB-SEM-TEM and FIB-SEM-atomic probe tomography (APT) enabled acquisition of information from several scales and comprehensive multidimensional information about the NPs present in the regions of Lauro Muller and Siderópolis. Pokrovski et al. [75] achieved similar success in their studies to explain the redox and structural state of nanogold during interactions between sulfur-rich hydrothermal fluids containing gold and arsenic-free pyrite at high temperatures and high pressures. Therefore, it is of great importance that scientists studying natural NPs increasingly use metanalytical procedures to understand coal and coal by-products, in addition to other mining environments [72,73].

The results obtained in this study show that the NPs containing REEs and hazardous elements have different crystalline structures, geochemical readiness, thermodynamic equilibrium and physical characteristics compared to the characteristics of micro or macro-particles. When the particle (mineral, amorphous, or aggregates) is nanosized, a large proportion of atoms are located on the surface or in the near-surface environment. As a result, the atomic arrangement and surface characteristics significantly affect the properties of the NPs, attributing distinct mineralogical qualities in parallel with the macrocrystals [76]. Relevant discrepancies in adsorption performance [5,77,78], dissolution rate, thermal performance [79,80,81,82], aggregation state [22,83,84] and catalytic intensity occur between metallic inorganic elements (nanominerals) and their respective large-scale particles.

4. Conclusions

The adequate study of BFCTs using not only the chemical composition, but a detailed investigation via electron microscopy shows that, although there are several benefits to carrying out studies on the extraction of REEs and other compounds of economic importance, studies on extraction methods should be better developed for large regions. It is crucial that different methods be developed to extract the different forms of occurrence of REEs present in BFCTs, be they minerals, nanometric inorganic agglomerates or ultra-fine particles from organo-metallic associations. Therefore, caution is crucial to successfully use BFCTs, even though these materials are thin, easily excavated and without overloads, so that the elemental profiles of different fractions can allow for the profitability, or not, of deposits in Lauro Muller and Siderópolis.

In addition to technological factors, several economic factors (e.g., prices of various elements of interest, cost of the processing plant, cost of chemicals used in the separation of the elements and recovery and reuse of the elements) also influence the markets for the elements that can be recovered versus BFCT disposal costs. It should also be noted that the locations of this study are close to road and rail transport, which will ease future studies of extraction of compounds present in BFCTs. In addition, the use of secondary coal in coal power plants is a tempting prospect, especially as they are already crushed and exhibit excellent granulometry.

Author Contributions

Conceptualization, M.L.S.O., L.D.M. and D.P.; data curation, A.N.; formal analysis, M.E.N.-H. and A.N.; funding acquisition, B.W.B.; investigation, L.D.M.; project administration, G.d.V.M. and D.P.; supervision, L.D.M. and M.L.S.O.; visualization, A.N.; writing—original draft preparation, M.L.S.O. and A.N.; writing—review and editing, G.d.V.M. and A.N. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Informed consent was obtained from all subjects involved in the study.

Data Availability Statement

Not applicable.

Acknowledgments

The authors are grateful to the Center for Studies and Research on Urban Mobility (NEPMOUR+S/ATITUS), and Fundação Meridional, Brazil. We also wish to thank the Brazilian National Council for Scientific and Technological Development (CNPq).

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

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Figures and Tables

Figure 1. Location map of the cities of Lauro Muller and Siderópolis (Santa Catarina State, Brazil). Adapted from the IBGE geographic database.

Figure 2. Illustrations of abandoned coal fines areas in the state of Santa Catarina, Brazil. (A) Lauro Muller region during the sampling work; (B) example of coal mine rock that was deposited in the coal fine area; (C) a recent coal beneficiation; (D) coal fines deposit basin in the Siderópolis region.

Figure 3. Dynamics of the coal sediment sampling site and the HR-TEM and XRD in the laboratory of the Universidad de La Costa (Colombia) used in this study.

Figure 4. Classic associations between sulfates present in the study areas. This image shows clusters of melanterite and rozenite detected in Lauro Muller. Al, K and Mg contained in clays and amorphous phases were also detected as minority fractions associated with iron sulfate phases.

Figure 5. Typical jarosite detected in Siderópolis in areas where BCFRs are located in sedimentation basins flooded by water, forming large lakes contaminated by sulfuric acid and metallic inorganic elements.

Figure 6. Inorganic agglomerate containing traces of organic material that, when evaluated by FIB-SEM/EDS, it was possible to detect several REEs present in such a particle.

Table 1

Minerals detected (▲ More than 5% detected by XRD; ▼ minor proportion detected by FE-SEM and/or HR-TEM).

Minerals Detected	Lauro Muller	Siderópolis
Amorphous organic nanophases	▲	▲
Silicates
Quartz, SiO2	▲	▲
Chlorite, Na0.5Al6(Si,Al)8O20(OH)10·H2O	▲	▲
Illite, K1.5Al4(Si6.5Al1.5)O20(OH)4	▲	▲
Kaolilite, Al2Si2O5(OH)4	▲	▲
Muscovite, KAl2(AlSi3)O10(OH)2	▲	▲
Zircon, ZrSiO4	▼	▼
Suphides
Chalcopyrite, CuFeS2	▼	▼
Galena, PbS	▼	▼
Marcasite, FeS2	▼	▼
Pyrite, FeS2	▼	▼
Pyrrhotite Fe(1−x)S	▼	▼
Sphalerite, ZnS	▼	▼
Carbonates
Calcite, CaCO3	▼	▼
Dolomite, CaMg(CO3)2	▼	▼
Siderite, FeCO3	▼	▼
Phosphates
Monazite, (Ce, La, Th, Nd, Y)PO4	▼	▼
Xenotime, (Y,Er)PO4	▼	▼
Sulfates
Anhydrite, CaSO4	▼	▼
Alunogen, Al2(SO4)3·17H2O	N.D.	▼
Barite, BaSO4	▼	▼
Copiapite, MgFe4(SO4)6(OH2) 18H2O	▼	N.D.
Coquimbite, Fe2(SO4)3·9H2O	▼	N.D.
Epsomite, MgSO4·7H2O	▼	▼
Ferrohexahydrite, FeSO4·6H2O	▼	▼
Halotrichite, FeAl2(SO4)4·22H2O	▼	N.D.
Hexahydrite, MgSO4·6H2O	▼	N.D.
Gypsum, Ca[SO4]·2H2O	N.D.	▼
Jarosite, KFe3+3(SO4)2(OH)6	▼	▼
Melanterite, FeSO4·7H2O	▼	▼
Natrojarosite, NaFe3(SO4)2(OH)6	▼	▼
Pickeringite, MgAl2(SO4)4·22H2O	▼	▼
Rozenite, FeSO4·4H2O	▼	▼
Schwertmannite, Fe3+16O16(OH)12(SO4)2	▼	N.D.
Oxides and hydroxides
Anatase, TiO2	▼	▼
Hematite, Fe2O3	▲	▼
Goethite, Fe(OH)3	▼	▼
Gibbsite, Al(OH)3	▼	▼
Maghemite, Fe2O3	▼	▼
Magnetite, Fe3O4	▼	▼
Rutile, TiO2	▼	▼
Amorphous inorganic phases	▲	▲

Table 2

Major organic elements detected in Brazilian fine coal tailings.

Element	Lauro Muller (%)	Siderópolis (%)
C	45.31	51.39
N	0.82	1.92
S	4.85	4.32
O	5.34	4.21
H	3.5	4.07

Table 3

Chemical composition of fine coal tailings (Lauro Muller area and Siderópolis area).

Lauro Muller Area				Siderópolis Area
Element	Mean	Min	Max	Element	Mean	Min	Max
	(%)	(%)	(%)		(%)	(%)	(%)
Al	11.9	3.97	16.8	Al	8.9	1.37	11.8
Ca	0.9	0.03	1.7	Ca	0.7	0.01	1.1
Fe	4.1	0.5	12.9	Fe	2.1	0.1	10.2
K	4.3	0.9	4	K	2.8	0.3	3.7
Mg	0.2	0.6	0.9	Mg	0.1	0.5	0.8
Na	0.5	0.3	0.8	Na	0.4	0.1	0.5
Na	0.6	0.2	1.2	Na	0.3	0.2	0.6
P	0.8	0.1	0.9	P	0.4	0.3	0.7
Ti	0.6	0.3	0.8	Ti	0.6	0.1	0.9
	(ppm)	(ppm)	(ppm)		(ppm)	(ppm)	(ppm)
As	31.1	17.2	93	As	39.2	19.2	103
Ba	499	121	641	Ba	389	91	842
Bi	0.6	0.3	0.8	Bi	0.7	0.6	0.9
Cd	0.5	0.2	0.6	Cd	0.9	0.5	2.3
Co	5.7	0.6	10.2	Co	8.7	2.6	18.3
Cr	110	17.1	209	Cr	139	11.2	311
Cs	27.1	8.2	41.2	Cs	29.3	9.1	48.1
Cu	39.1	8.4	109.1	Cu	48.1	9.4	113.3
Ga	29.1	4.5	43.8	Ga	31.1	8.1	47
Li	142	10.1	294	Li	151	12.1	308
Mn	105	9.3	1024	Mn	111	10.3	1183
Mo	3.5	2.9	9.2	Mo	8.1	3.9	15.1
Nd	20.1	8.1	50.2	Nd	29	11.8	59.5
Ni	28.2	6.9	68.1	Ni	33.2	11.5	79.1
Pb	37.1	18.2	185	Pb	40.1	19.9	238
Sb	8.9	2.2	73.3	Sb	19.3	7.9	93.8
Sn	5.2	1.8	20.1	Sn	8.2	2.8	19.7
Sr	110	43.2	341	Sr	120	48.2	440
Ta	0.5	0.1	1.3	Ta	0.8	0.5	1.9
Th	20.6	9.6	29.8	Th	29.1	10.5	41.1
U	4.1	2.1	10.8	U	8.1	3.3	19.1
V	182	48.5	234	V	235	38.8	304
W	6.9	3.9	9.1	W	16.1	2.8	29
Y	19.3	8.2	31.8	Y	21.3	9.2	39.3
Zn	50.3	21.1	199	Zn	69.1	28	214
Zr	122	22.1	282	Zr	162	30.1	301

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