Climate change caused by excessive greenhouse gas emissions (CO2
O, CFCs—chlorofluorocarbons, and others) and environmental degradation are the greatest contemporary threats to Europe and the world [1
]. The mining and mineral processing sector has a particularly negative impact on the ecosystem and the loss of biodiversity. It is estimated that they are responsible for half of all global greenhouse gas (GHG) emissions [3
]. Within the European Union (EU, 27 countries) the greatest greenhouse gas emissions are connected to burning fossil fuels and the power industry. Despite significant decreases in GHG emissions (change of −22.04% since 1990), ca. 4 Mt CO2
eq is being released into the atmosphere [4
Economic growth, especially in emerging and developing countries (China, Indonesia, Brazil, Central and Eastern European countries) increases the demand for mineral resources and energy. According to forecasts, the global consumption of mineral resources by 2060 will increase to 167 Gt from the current 79 Gt [5
]. As a result of increased exploitation and processing of mineral raw materials, the volume of anthropogenic waste (mine waste-rock, tailing, and slags), GHG emission, leaching of acid mine waters (AMD), pollution of soil and surface and underground waters, among other things, through the failure of tailing ponds [7
], will increase. However, anthropogenic waste can be a valuable source of many metals, including rare earth elements (REEs); Nb, Ta, Co and non-metallic minerals.
Increasing threats from ongoing climate change require action to reduce their future negative impacts on the biosphere. In 2019, the European Commission adopted a set of initiatives with the overarching goal of achieving climate neutrality by 2050. The European Green Deal [2
] and related strategies (A New Industrial Strategy for Europe [8
], the Digital Transition [10
]) indicate the need for the efficient use of renewable resources, the reduction in pollution levels, fostering innovation and increased participation in a closed-loop economy.
The sustainable economic development of the European Union requires permanent and secure access to a mineral supply chain. “Critical metals” are of particular importance for the development of European strategies, including the European Green Deal [11
]. The critical raw materials (CRMs) group consists of metals with unique chemical and physical properties used in technologies necessary for the development of a low-carbon economy. The supply of these metals shows a high risk of limitations; at the same time, these metals are characterized by a low degree of substitution. The European Commission updates the list of CRMs every 3 years [11
]. The current 2020 list includes 30 raw materials [11
Rare earth elements (REEs) are important for the development of a modern, low-carbon economy. The REE group consists of 17 elements belonging to the lanthanide group, plus scandium (Sc) and yttrium (Y), as defined by the International Union of Pure and Applied Chemistry (IUPAC). Traditionally, REEs are divided into light rare earth elements (LREE, La-Eu) and heavy rare earth elements (HREE, Gd-Lu and Y). The unique properties (magnetic and electrochemical) of REEs make them useful in many manufacturing technologies of, for example, wind turbines (Nd, Sm, and Tb), solar panels (Yb), and electric cars (Nd and Dy). Currently, China supplies 99% of LREE and 98% of HREE to the European Union [11
]. China’s monopoly is connected with the extraction and processing of rare earth elements at every stage of the production chain. The reduction in export quotas introduced by the Chinese government on REEs at the beginning of the 21st century has led to drastic price increases and reduced their supply on the global market [15
]. Additionally, the COVID-19 pandemic has caused significant supply chain disruptions for many raw materials, including the REEs. The consequence of a limited supply of REEs will be a steady increase in the number of new raw material projects in Southeast Asia and offshore areas, among others [16
Like REEs, a regular and uninterrupted supply of natural uranium, needed for the production of nuclear fuel, medical radioisotopes and to power research reactors, is important to the European Union’s energy security. Most uranium for the production of nuclear fuel is imported from Russia (19.8%), Kazakhstan (19.6%), Niger (15.3%), Australia (14.4%) and Canada (11.6%). These countries supply almost 90% of all natural uranium to the European Union, and three of them can be classified as politically unstable countries [17
Ensuring raw material and energy security for the European Union requires measures aimed at permanent access to raw materials through the diversification of their sources. Limited internal sources of metals indicate the need to search for and obtain them outside of Europe. In the countries of South East Asia, such as Indonesia, which is characterized by a high raw material potential, primary and secondary metal deposits may be of particular importance [18
]. Mining wastes represent a significant raw material potential. It is estimated that about 8–10 billion tons of tailings are generated annually in the world as a result of the mining and processing of, among others, copper, cassiterite, zinc and lead ores and other minerals [21
In the framework of the project “New Potential sources of Rare Earth Elements”, the potential of tailings generated from the processing of cassiterite-bearing sands on the Indonesian island of Bangka was evaluated [19
]. Until now, the waste generated on Bangka Island was stored in dumps. This material was considered useless. Until 2013, there was no comprehensive study of the generated waste. Prospecting work was carried out on this island, during which 35 samples of tailings were collected [22
]. During the investigations [19
], it was found that the resulting waste is characterized by high concentrations of many valuable metals, including REEs, U, Th, Sn, Ti, Nb, and Ta. So far, there is no information in the literature about such rich metal contents in waste.
In particular, we investigated the source of rare earth elements and uranium. On the basis of the studies obtained, the usefulness and direction of the use of the waste was determined. The high contents of these metals may be important for obtaining a new supply source for the European Union. Ensuring sustainable development of Europe through the diversification of CRM sources and supplies is becoming a new challenge of the 21st century.
2. Geological Setting
A detailed description of the geological structure of Bangka Island (Figure 1
) was presented by [23
]. The weathering of granitoid rocks from the late Miocene to Holocene led to the formation of numerous on-shore and off-shore placer deposits [28
]. The genesis of these deposits is linked to the region’s strong tectonic activity, the erosion and subsidence of the Sundland area, and eustatic changes of the world ocean in the Cenozoic [23
]. The sediments of the Ranngam Group (Paleogene-Neogene), which includes sediments of the Ranngam Formation and Residual Gravel [23
], are of greatest economic importance in the recovery of heavy minerals, mainly cassiterite. The placer deposits are subject to intensive mining by state-owned companies: PT Timah, PT Koba Tin, and by artisanal mining (Figure 2
The mining activity conducted on the Bangka Island generates huge amounts of tailings. The tailings generated by state-owned companies are collected in closed landfills (Figure 3
A), while the material from artisanal mining is used to infill illegal mining sites (Figure 3
B,C). In the case of middlemen/sellers of the mineral (compradores
), the mixed tailings are deposited in domestic landfills (Figure 3
Macroscopic studies of the tailings indicate that they are similar to previously analyzed samples [19
]. Depending on the separation technique used, the tailing forms fine to medium-grained sediments (Figure 4
). In the fine-grained samples, the dominant grain fraction is in the range of 0.10–0.25 mm (Bngk 1, 3, 15) (Figure 4
A,B). Medium-grained samples are rarely dominated by grain sizes of 0.50 mm or more (Bngk 19) (Figure 4
C,D). Two groups of grains can be distinguished in the tailings: (1) very well-rounded spherical and ellipsoidal grains; (2) very poorly rounded spindle and ellipsoidal grains. The specific density of the tailings ranges from 2.82 g/cm3
to 3.88 g/cm3
. The tailings are generally medium sorted with varying statistical coefficients.
Microscopic observations and XRD analyses reveal significant mineralogical variation in the tailings (Table 2
). The main REE-carrier phases are present in the samples: monazite [Ce(PO4
)] (1.55–9.30 wt%) and xenotime [Y(PO4
)] (0.79–2.89 wt%). Other minerals consist of zircon [Zr(SiO4
)], ilmenite [Fe2+
], rutile [TiO2
], anatase [TiO2
], pseudorutile [Fe2
], cassiterite [SnO2
], quartz [SiO2
], and others. Anomalously high concentrations in samples show zircon up to 64.35 wt% (Bngk 30). The current results are for samples mentioned in Table 2
. Samples from columns Szamałek et al., 2013 [19
] and Zglinicki et al., 2020 [35
] were taken from these cited publications for comparison with the current analyses.
The mineralogical characterization of the samples by XRD identified a major presence of REE-bearing minerals. Xenotime (0.79–17.55 wt%) and monazite (1.55–21.23 wt%) are present in almost all of the analyzed samples, making the mining waste on the whole of Bangka Island economically important. The presence of titanium, tin, and zirconium phases increases the economic value of the mining waste as a secondary resource. Tailings from Bangka Island should be considered multi-component raw material, a source of many valuable metals. In addition, variable contents of quartz (2.20–40.28 wt%), tourmaline (1.96–4.65 wt%) and topaz (3.58–6.53 wt%) were found in all samples. The presence of marcasite (up to 11.15 wt%) and pyrite (up to 9.95 wt%) is also particularly important. Undergoing weathering, these minerals are destroyed, which leads to the formation of acid drainage, thereby posing a threat to the local community.
Xenotime contents in tailings from Bangka Island are variable and depend on the sampling location and processing technology of the cassiterite-bearing sands. Anomalously high amounts of xenotime, up to 17.55 wt% in tailings, were described by [19
]. However, mostly, the amounts reach up to 2.89 wt% (Table 2
). Xenotime grains are usually found in coarser fractions above 100 µm. The samples are dominated by poorly coated and sharp-edged grains. Xenotime occurs in association with monazite (Figure 5
A) and cassiterite (Figure 5
B). Twinned xenotime crystals appear in the samples (Figure 5
Xenotime forms euhedral and anhedral crystals ranging in size from 50 to 350 μm. Grains are characterized by numerous internal cracks (Figure 5
D), usually along the schistosity surface. Xenotime exhibits a varied internal structure from heterogeneous (Figure 5
E) to zonal (Figure 5
F), defined by concentric bands characteristic of magmatic-derived xenotime. Zoning in BSE imaging reveals variations in chemical composition (HREE, Y, Si, and U), across xenotime sectors [36
]. Xenotime with relatively homogeneous internal structure is very rare.
In zonal xenotimes, a characteristic feature is the change in chemical composition occurring from outside to inside the grain (Table 3
). The outer sectors contain an increased proportion of Y2
(52.52 wt%), P2
(35.29 wt%) and lower contents of HREE (10.84 wt%), UO2
(0.21 wt%). In contrast, inner zones are enriched in HREE (22.73 wt%), UO2
(2.43 wt%) and ThO2
The dominant group of grains shows a complex internal structure (Figure 6
B) resulting from resorption (solid–fluid interaction) during metasomatic processes. The change in chemical composition results from the scale of substitution of HREE, U, and Th by Y [36
]. The results of the samples obtained with EPMA are presented in Table 4
. In the studied xenotime, the P2
content is in the range of 30.50–37.10 wt%, mean 34.78 wt%, and minor admixtures are SiO2
(up to 2.45 wt%), CaO (up to 0.20 wt%), and PbO (up to 0.03 wt%). In many samples, light rare earths (LREE) occur below the EPMA detection limit (Table 4
). Depending on the EPMA analysis site, the HREE content is variable (Table 4
). The analyzed xenotime is characterized by relatively constant YPO4
molar fraction values ranging from 0.72 to 0.81, with HREEs making up the remainder (Figure 6
A,B). High contents are indicated for Dy, Er and Yb, the most critical metals of the HREE group. The Th/U ratio is high, indicating Th incorporation, mainly due to substitution in the crystallographic lattice of the mineral, according to the mechanism: (Th,U)4+
The diagram normalized to the chondrite (Figure 7
) shows a depletion in LREE (Ce, Nd) relative to HREE. Clear negative anomalies for Eu, Ho and positive anomalies for Sm, Dy, Er are seen in all patterns. Variable values, positive and negative are shown by Lu.
A significant proportion of the xenotimes with heterogeneous internal structure are characterized by high porosity (Figure 8
A–D) in which inclusions (up to 25 μm) of Th(SiO4
are present. The size and distribution of the inclusions vary, with no preferential alignment to the crystallographic axis of the xenotime. Larger inclusions form subhedral and anhedral forms, internally heterogeneous and porous (Figure 8
C,D). Around the larger inclusions, cracks and fissures are filled with material enriched in U and Th. Inclusions typically occur in the inner xenotime altered zones. Around the inclusions, alteration halos are formed that are visible during BSE imaging. Chemical composition studies show that the inclusions contain SiO2
ranging from 7.08 to 17.02 wt%, ThO2
34.68 to 71.04 wt%, and UO2
2.68 to 39.63 wt% (Table 5
). The LREE content is low or below the detection limit, while there is an enrichment in Y and HREE.
4.3. Geochemical Analysis
A geochemical investigation of the samples (Table 6
) revealed variable amounts of oxides and some correlations in the case of REE concentrations. The highest values were obtained for SiO2
in the range 2.81–55.65 wt% (mean 22.75 wt%, median 25.19 wt%), TiO2
3.74–48.51 wt% (mean 21.00 wt%, median 20.89 wt%) and Fe2
0.85–22.51 wt% (mean 13.20 wt%, median 14.93). These oxides are characterized by the greatest variation between the lowest and highest values. Among oxides with significantly lower contents, one should point out Al2
occurring in the content range between 0.32 and 10.14 wt% (mean 2.63 wt%, median 1.64 wt%) and P2
with measured content between 0.20 and 17.23 wt% (mean 2.91 wt%, median 1.4 wt%). The results of the content of other oxides are in the range < 0.01–2.06 wt%. The variation in the content of particular oxides in the samples results from differences in the amounts of the main tailings minerals, i.e., monazite, xenotime, rutile, ilmenite, zircon and others. Loss on ignition (LOI) is relatively high and reaches up to 11.10 wt%.
Chemical analyses (Table 6
) showed high contents of radioactive elements: U and Th. Uranium in the samples ranges from 0.01 to 0.16 wt% (mean 0.06 wt%, median 0.04 wt%), and thorium 0.04–1.0 wt% (mean 0.32 wt%, median 0.17 wt%).
Tailing due to its mineral composition (presence of REE-bearing minerals) contains high concentrations of REE: Ce (up to 3.51 wt%, mean 0.94 wt%), La (up to 2.6 wt%, 0.60 wt%), Gd (up to 0.75 wt%, mean 0.16 wt%), Dy (up to 0.74 wt%, mean 0.17 wt%), Pr (up to 0.64 wt%, mean 0.14 wt%), Nd (up to 0.58 wt%, mean 0.28 wt%), Eu (up to 61.8 ppm, mean 10.12 ppm), Er (up to 0.52 wt%, mean 0.11 wt%), and Yb (up to 0.52 wt%, mean 0.11 wt%). Concentrations of other REEs did not exceed 0.25 wt%. High contents were also found for Sc 132 ppm (mean 71.40 ppm) and Y maximum at 4.83 wt% (mean 0.99 wt%). Significant concentrations of rare earth elements form the most “critical” elements in the REE group (Gd, Dy, Eu, Er and Yb).
Several samples analyzed (Table 6
) showed higher total LREE content (LREE/HREE + Y ratio value above 1), while two (Bngk 1 and Bngk 3) samples showed higher HREE content (LREE/HREE + Y below 1). This indicates a dominant LREE-bearing mineral in most samples and an increased HREE concentration in single samples (Bngk 1 and Bngk 3). The sum of all REE + Y ranges from 0.96 wt% in the samples examined to as high as > 16.89 wt% in sample Bngk 22.
The correlation matrix of the elements and oxides content results is presented in Table 7
. In the correlation, the absolute values of the correlation coefficient ≥ 0.3 were considered relevant. Missing data were removed by accident. The values of the analyses below the detection limit of the method were changed to half the value of the detection limit for statistical purposes. The results were interpreted on the basis of the linear correlation coefficient value. The absolute values of the correlation coefficient above 0.3 are highlighted in red in the table. In the oxides group, there are clearly visible positive correlations between SiO2
O and MgO, indicating the presence of certain amounts of rock-forming minerals, such as silicates, in the tested samples. In turn, a strong correlation of iron and titanium oxides results from the presence of ilmenite and rutile. P2
and CaO show not only a high correlation with each other, but also with the group of REE elements (Table 7
). This relationship was confirmed mineralogically by the presence of REE carrier minerals: xenotime, monazite and fluorocarbonates. In addition, P2
shows a clearly negative correlation with SiO2
. Radioactive minerals U and Th show a strong positive correlation with CaO, P2
and the entire REE group, which was confirmed in the EMPA mineralogical studies. Elements within the REE group show strong correlation with each other, which indicates the obvious fact of their presence in the same minerals. Moreover, these elements (along with U and Th) show a clearly negative correlation with SiO2
, MgO or TiO2
). These results clearly confirm that the rock-forming and heavy minerals present in the tested samples do not contain rare earth elements. Scandium, despite belonging to the REE group, shows no correlation with any of these elements, probably due to its low concentration in the tested samples.
Sustainable development in the European Union requires constant access to mineral resources, especially those used for modern, low-carbon technologies. The limited number of CRM deposits forces the European Union to take action to ensure raw material security through diversification of metal supply sources. China’s monopoly on rare earth elements and the unstable economic situation due to COVID-19 require a search for the best and least expensive solutions. Tailings may be of particular importance for obtaining metals. Each year, about 8–10 billion tons of tailings are produced, which can be a source of many metals.
The Indonesian island of Bangka is one of the world’s major suppliers of tin to the global market. The production of cassiterite concentrates leads to the formation of a polymineral tailings containing monazite (1.55–21.23 wt%), xenotime (1.55–21.23 wt%), zircon (1.87–64.35 wt%), rhytulite (1.82–19.97 wt%), ilmenite (1.88–22.20 wt%) and others. Research conducted by Polish geologists revealed high contents of metals, including the most critical HREEs, Gd2O3 (1.42–7.16 wt%), Dy2O3 (2.28–11.21 wt%), Er2O3 (2.44–7.85 wt%), and Yb2O3 (1.71–7.10 wt%), as well as U (up to 0.11 wt%), Th (0.75 wt%), and Sc (132 ppm). Despite the high deposit potential of the tailings, this material is irretrievably lost. It should be noted that tin production on many other Indonesian islands (e.g, Belitung, and Singkep) indicates that post-cassiterite tailings could be a significant source of HREE.
A preliminary conceptual framework was developed for Bangka Island mine tailings, taking into account geometallurgical domains (e.g., density and mineralogical and chemical variability, presence of inclusions and overgrowths and others). The results obtained may be helpful for the systematic development of a technological, economic and environmental model for CRM acquisition. The new data fill another gap for the recovery of metals and critical minerals from tailings as a source of secondary raw materials.
This requires further detailed research. The example of the Bankga Island tailings highlights the extent and significance of such a potential REE resource base. The development of a technology based on a circular economy and zero-waste principle can contribute to the acquisition of rich mineral concentrates. Improving the efficiency of the technological process will improve environmental safety by reducing emissions of radioactive substances and will reduce the poverty level of the local mining community. The diversification of critical metals sources is one of the goals of the Green Deal and of the energy transition of Europe. EU–Indonesian bilateral cooperation can bring mutual benefits by ensuring the security of raw materials, increasing awareness among local communities of radiogenic risks and environmental protection, increasing the supply of metals on the world market and boosting Bangka’s GDP.