Search Results (24)

The most important bauxite deposits in Türkiye are located in the Seydişehir (Konya) and Akseki (Antalya) regions, situated along the western Taurus Mountain, with a total reserve of approximately 44 million tons. Some of the bauxite deposits have been exploited for alumina since the 1970s. In this study, bauxite samples, collected from six different deposits were examined to determine their mineralogical and chemical composition, as well as their REE content, with the aim of identifying which bauxite types are enriched in REEs and assessing their economic potential. The samples included massive, oolitic, and brecciated bauxite types, which were analyzed using optical microscopy, X-ray diffraction (XRD), X-ray fluorescence (XRF) and inductive coupled plasma-mass spectrometry (ICP-MS), field emission scanning electron microscopy (FESEM-EDX), and electron probe micro-analysis (EPMA). Massive bauxites were found to be more homogeneous in both mineralogical and chemical composition, predominantly composed of diaspore, boehmite, and rare gibbsite. Hematite is the most abundant iron oxide mineral in all bauxites, while goethite, rutile, and anatase occur in smaller quantities. Quartz, feldspar, kaolinite, dolomite, and pyrite were specifically determined in brecciated bauxites. Average oxide contents were determined as 52.94% Al₂O₃, 18.21% Fe₂O₃, 7.04% TiO₂, and 2.69% SiO₂. Na₂O, K₂O, and MgO values are typically below 0.5%, while CaO averages 3.54%. The total REE content of the bauxites ranged from 161 to 4072 ppm, with an average of 723 ppm. Oolitic-massive bauxites exhibit the highest REE enrichment. Cerium (Ce) was the most abundant REE, ranging from 87 to 453 ppm (avg. 218 ppm), followed by lanthanum (La), which reached up to 2561 ppm in some of the massive bauxite samples. LREEs such as La, Ce, Pr, and Nd were notably enriched compared to HREEs. The lack of a positive correlation between REEs and major element oxides, as well as with their occurrences in distinct association with Al- and Fe-oxides-hydroxides based on FESEM-EDS and EPMA analyses, suggests that the REEs are present as discrete mineral phases. Furthermore, these findings indicate that the REEs are not incorporated into the crystal structures of other minerals through isomorphic substitution or adsorption. Full article

The Sanandaj–Sirjan Zone and Zagros Fold–Thrust Belt in Iran host numerous Mediterranean-type karst bauxite deposits; however, their formation mechanisms and critical raw material potential remain ambiguous. This study combines mineralogical and geochemical analyses to explore (1) the formation of authigenic minerals, (2) the role of microbial organic processes in Fe cycling, and (3) the assessment of their critical raw materials potential. Mineralogical analyses of the Late Cretaceous Daresard and Middle–Late Permian Yakshawa bauxites reveal distinct horizons reflecting their genetic conditions: Yakshawa exhibits a vertical weathering sequence (clay-rich base → ferruginous oolites → nodular massive bauxite → bleached cap), while Daresard shows karst-controlled profiles (breccia → oolitic-pisolitic ore → deferrified boehmite). Authigenic illite forms via isochemical reactions involving kaolinite and K-feldspar dissolution. Scanning electron microscopy evidence demonstrates illite replacing kaolinite with burial depth enhancing crystallinity. Diaspore forms through both gibbsite transformation and direct precipitation from aluminum-rich solutions under surface conditions in reducing microbial karst environments, typically associated with pyrite, anatase, and fluorocarbonates under neutral–weakly alkaline conditions. Redox-controlled Fe-Al fractionation governs bauxite horizon development: (1) microbial sulfate reduction facilitates Fe³⁺ → Fe²⁺ reduction under anoxic conditions, forming Fe-rich horizons, while (2) oxidative weathering (↑Eh, ↓moisture) promotes Al-hydroxide/clay enrichment in upper profiles, evidenced by progressive total organic carbon depletion (0.57 → 0.08%). This biotic–abiotic coupling ultimately generates stratified, high-grade bauxite. Finally, both the Yakshawa and Daresard karst bauxite ores are enriched in critical raw materials. It is worth noting that the overall enrichment appears to be mostly driven by the processes that led to the formation of the ores and not by the chemical features of the parent rocks. Divergent bauxitization pathways and early diagenetic processes—controlled by paleoclimatic fluctuations, redox shifts, and organic matter decay—govern critical raw material distributions, unlike typical Mediterranean-type deposits where parent rock composition dominates critical raw material partitioning. Full article

Boehmite is an aluminum oxyhydroxide (AlO(OH)) and one of bauxite’s main mineral phases. This mineral is highly valued as an important source of aluminum for the metallurgical industry. However, the formation of synthetic boehmite has been observed in water treatment when aluminum anodes are used for electrocoagulation. This boehmite occurs in flocs that capture impurities from the water, but removing these flocs is a slow process. Therefore, the froth-flotation method was employed in the present study to float synthetic boehmite. This was achieved by evaluating the particle size of synthetic boehmite, generating microbubbles, and using an anionic collector system in a novel experimental setup. The results show that the surfactants sodium dodecyl sulfate (SDS) and potassium oleate (PO) favor the recovery of synthetic boehmite in different particle sizes, with the particle size favored related to the bubble size generated. It was noted that increasing the SDS concentration enabled the microbubbles to recover up to 95% of boehmite particles with diameters of less than 30 microns. Full article

Investigation of the critical metal elements in coal and coal-bearing strata has become one of the hottest research topics in coal geology and coal industry. Coal-hosted Ga-Al-Li-REE deposits have been discovered in the Jungar and Daqingshan Coalfields of Inner Mongolia, China. Gallium, Al, and Li in the Jungar coals have been successfully extracted and utilized. This paper reviews the discovery history of coal-hosted Ga-Al-Li-REE deposits, including contents, modes of occurrence, and enrichment origin of critical metals in each coal mine, including Heidaigou, Harewusu, and Guanbanwusu Mines in the Jungar Coalfield and the Adaohai Coal Mine in the Daqingshan Coalfield, as well as the recently reported Lao Sangou Mine. Gallium and Al in the coals investigated mainly occur in kaolinite, boehmite, diaspore, and gorceixite; REEs are mainly hosted by gorceixite and kaolinite; and Li is mainly hosted by cholorite. Gallium, Al, and REEs are mainly derived from the sediment-source region, i.e., weathered bauxite in the Benxi Formation. In addition, REE enrichment is also attributed to the intra-seam parting leaching by groundwater. Lithium enrichment in the coals is of hydrothermal fluid input. The content of Al₂O₃ and Ga in coal combustions (e.g., fly ash) is higher than 50% and ~100 µg/g, respectively; concentrations of Li in these coals also reach the cut-off grade for industrial recovery (for example, Li concentration in the Haerwusu coals is ~116 µg/g). Investigations of the content, distribution, and mineralization of critical elements in coal not only provide important references for the potential discovery of similar deposits but also offer significant coal geochemical and coal mineralogical evidence for revealing the geological genesis of coal seams, coal seam correlation, the formation and post-depositional modification of coal basins, regional geological evolution, and geological events. Meanwhile, such investigation also has an important practical significance for the economic circular development of the coal industry, environmental protection during coal utilization, and the security of critical metal resources. Full article

The use of reduction leaching in the production of alumina from bauxite by the Bayer process in order to decrease the amount of waste (bauxite residue) by adding elemental iron or aluminum, as well as Fe²⁺ salts and organic compounds in the stage of high-pressure leaching, requires the purchase of relatively expensive reagents in large quantities. The aim of this study was to investigate the possibility of the use of electrolytically reduced bauxite residue (BR) as a substitute for these reagents. Reduced BR was obtained from Al-goethite containing BR using a bulk cathode in alkaline suspension. The degree of deoxidation of Fe³⁺ compounds was 55% after 2 h of electrolysis with a current yield of more than 73%. The addition of reduced BR according to the shrinking core model leads to a change in the limiting stage of the high-pressure boehmitic bauxite leaching from a surface chemical reaction to internal diffusion. The activation energy decreased from 32.9 to 17.2 kJ/mol by adding reduced red mud. It was also shown that the addition of reduced BR increased the rate of thickening of the slurry after leaching by a factor of 1.5 and decreased the Na₂O losses by 15% without the addition of lime. The solid residue was examined by means of X-ray diffraction analysis and scanning electron microscopy to confirm the presence of magnetite and elemental iron. A preliminary techno-economic analysis was carried out to assess the applicability of the proposed process. Full article

In the Campania region (Southern Italy), in the Matese Mts. (Albian to Turonian/Coniacian) and Caserta district (Albian to Cenomanian), two karst bauxite deposits outcrop, consisting of flat lenses over shallow karst carbonate. Although the mineralogy and geochemistry of Campania bauxite deposits have been widely studied in recent years, new major and trace elements relationships were provided to highlight paleoclimatic and paleoenvironmental conditions that occurred during their formation. The purpose of this research is to provide for the first time information on the paleoclimatic and paleoenvironmental conditions that affected the bauxites of Campania. These deposits formed during different periods since the Matese deposit formed during intense weathering processes with more abundant precipitation while the Caserta district deposit experienced a more long-lasting exposure event. During the formation of the studied bauxites, the drier conditions favored the replacement of kaolinite by boehmite. R-mode factor analysis showed geochemical affinity among Al₂O₃, TiO_2, and Nb. REEs minerals are mainly associated with the bauxite matrix while Zr, Hf, and V were mainly concentrated in detrital minerals during the later stages of bauxitization. Parental affinity indices (Eu/Eu* vs. Sm/Nd; Eu/Eu* vs. TiO₂/Al₂O₃) assessed the origin of the protolith of the Campania bauxites by rejecting the hypothesis of the dissolution of the bedrock carbonate. The results confirmed the eolian transport of parental material with an Upper Continental Crust and an intermediate to mafic magmatic composition. Full article

The Carboniferous Benxi Formation in southern Shanxi of North China has significant bauxite resource potential; however, the source of its metallogenic material and its sedimentary environment remain unclear. The microscopy, X-ray diffraction, X-ray fluorescence spectroscopy, and inductively coupled plasma mass spectrometry methods were applied in this study to examine the mineralogical, petrographic, and geochemical characteristics. Geochemical proxies of La/Y, Sr/Ba, Al₂O₃/TiO₂, Zr/Sc, Th/Sc, La/Sc, and Th/Co were analyzed to investigate the paleo-depositional environment and provenance of the aluminum-bearing strata. The findings indicate that diaspores are the primary ore minerals in bauxite, while kaolinite and rutile are the predominant gangue minerals. Both the bauxite and claystone/aluminous rocks exhibit high enrichment in Li, Bi, and U, with relative enrichment in In, Sb, Th, Nb, and Ta. Li is notably concentrated in the claystone/aluminous rocks, reaching up to 1994.00 μg/g, primarily occurring in cookeite and boehmite, while U is highly concentrated in the bauxite. The aluminum-bearing strata were primarily formed under alkaline-reducing conditions, with changes in acidity and alkalinity of the environment during the sedimentary diagenetic process. Marine transgressions significantly impacted the sedimentary environment of the aluminum-bearing strata, and the paleoclimate was characterized as hot and humid. The principal factors contributing to enrichment of aluminum in the sedimentary basin were the in situ weathering of aluminum-rich source rocks and the transport of clastic materials from high-aluminum source rocks. The source rocks were closely associated with intermediate-acidic magmatic rocks and potentially related to the weathering of Ordovician carbonates. Full article

In recent years, great breakthroughs have been made in gas explorations of the Upper Paleozoic bauxite series in the Longdong area of the Ordos Basin, challenging the understanding that bauxite is not an effective reservoir. Moreover, studying the reservoir characteristics of bauxite is crucial for oil and gas exploration. Taking the bauxite series in the Longdong area as an example, this study systematically collects data from previous publications and analyzes the petrology, mineralogy, oolitic micro-morphology, chemical composition, and other sedimentary characteristics of the bauxite series in the study area using field outcrops, core observations, rock slices, cast slices, X-ray diffraction analysis, scanning electron microscopy and energy spectra, and so on. In this study, the oolitic microscopic characteristics of the bauxite reservoir and the significance of oil and gas reservoirs are described. The results show that the main minerals in the bauxite reservoir are boehmite and clay minerals composed of 73.5–96.5% boehmite, with an average of 90.82%. The rocks are mainly bauxitic mudstone and bauxite. A large number of oolites are observable in the bauxite series, and corrosion pores and intercrystalline pores about 8–20 μm in size have generally developed. These pores are important storage spaces in the reservoir. The brittleness index of the bauxite series was found to be as high as 99.3%, which is conducive to subsequent mining and fracturing. The main gas source rocks of oolitic bauxite rock and the Paleozoic gas series are the coal measure source rocks of the Upper Paleozoic. The oolitic bauxite reservoirs in the study area generally have obvious gas content, but the continuity of the planar distribution of the bauxite reservoirs is poor, providing a scientific basis for studying bauxite reservoirs and improving the exploratory effects of bauxite gas reservoirs. Full article

The Kolijan bauxite deposit (southeast Mahabad, northwestern Iran) mainly contains aluminum-bearing iron ores and was deposited in karstic depressions and sinkholes of the middle Permian carbonate rocks of the Ruteh Formation. Based on microscopic observations, the aluminum-bearing iron ores were allogenic in origin. According to XRD and SEM-EDS analyses, hematite and goethite are their main constituents, accompanied by lesser amounts of kaolinite, illite, amesite, boehmite, rutile, anatase, calcite, pyrolusite, crandallite, and parisite-(Ce). Chondrite-normalized REE patterns are indicative of fractionation and enrichment of LREE (La–Eu) compared to HREE (Gd–Lu), along with positive Eu and Ce anomalies (Eu/Eu* = 2.29–5.65; Ce/Ce* = 3.63–5.22). Positive Ce anomalies can be attributed to the role of carbonate bedrock as a geochemical barrier and the precipitation of parisite-(Ce). A strong positive correlation between Eu/Eu* and Ce/Ce* (r = 0.84) indicates that Eu anomalies, similar to Ce anomalies, are closely dependent on an alkaline pH. The distribution and fractionation of elements in the iron ores were controlled by a number of factors, including the pH of the environment in which they formed, wet climatic conditions, adsorption, isomorphic substitution, scavenging, co-precipitation, fluctuations of the groundwater table level, and the role of carbonate bedrock as a geochemical barrier. This research indicates that the aluminum-bearing iron ores were probably generated from the weathering of basaltic protolith. Full article

The use of aluminum-rich clays and bauxites as refractory materials is common. Upon firing, these materials form mullite crystals in the shape of needles embedded in a siliceous and vitreous matrix, with mullite being responsible for the refractory properties. In this study, bauxite samples for use in refractory applications have been characterized. Chemical analysis revealed that the alumina content varied between 34 and 40%, with silica values generally being high (around 40%), except for one sample (26%). Two samples were found to be the most suitable for use as “refractory clay” refractories. However, high silica or Fe oxide contents can affect mineralogical transformations at high temperatures. Mineralogical analysis confirmed the presence of several minerals in the bauxite materials, including kaolinite, halloysite, anatase, rutile, gibbsite and boehmite. Differential thermal analysis (DTA) showed the decomposition of gibbsite and its partial transformation to boehmite and alumina, and the dehydroxylation of kaolinite, with primary mullite crystallization observed at a high temperature. These findings provide valuable information for the selection and optimization of bauxite materials for refractory applications, considering their chemical composition and mineralogical characteristics. Full article

This study provides a characterization of materials from wastes and outcrops of two inactive bauxite mines located close to Sant Joan de Mediona and Peramola, Spain. Mineralogy was determined via powder X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). Thermal properties were measured via differential thermal analysis–thermogravimetry (DTA-TG) and gresification tests. The crystalline phases are medium-high crystalline kaolinite and variable amounts of illite, quartz, calcite, boehmite, hematite and rutile/anatase. DTA show two endothermic peaks produced by the dehydroxylation of minerals: the first peak, at 530–538 °C, belongs to boehmite; the second peak, at 535–568 °C, corresponds to kaolinite. An exothermic peak at 950–978 °C is associated with mullite crystallization. The optimal sintering temperatures obtained from the gresification curves (firing shrinkage and water absorption) were 970 °C for carbonate-poor, illite-rich clays; 1100 °C for illite- and carbonate-poor samples; and near 1190 °C for carbonate-rich materials. The carbonate-poor samples fired at 1300 °C contain sillimanite and mullite, and the carbonate-rich materials are rich in gehlenite, anorthite, and hedenbergite. The mineralogy of these materials is sufficient to obtain ceramic materials with suitable properties, but not their low plasticity. They could be used in the formulation of bricks or as part of mixtures to produce refractory ceramics. Full article

Reductive leaching in the Bayer cycle using iron (2+) allows for Al extraction to be significantly increased through the magnetization of Al-goethite and Al-hematite. However, the use of expensive iron (2+) salts or iron powder as a source of iron (2+) leads to a significant increase in production costs. In this work, the feasibility of a new method, the reductive leaching of bauxite using an electrolysis process, was investigated. The reduction of iron minerals of boehmitic bauxite in both the Bayer solution and purely alkaline solutions was carried out. Experiments were performed using a plate cathode and a bauxite suspension in an alkaline solution, as well as using a bulk cathode with a stainless-steel mesh at the bottom of a cell as the current supply. During the electrolysis process, the potential of the cathode relative to the reference electrode was measured as a function of the current at different concentrations of solid (100–300 g L⁻¹) and suspension temperatures (95–120 °C). It was shown that the current efficiency using the suspension and plate cathode with the predominant deposition of Fe did not exceed 50% even with the addition of magnetite to increase the contact of the solid phase with the current supply. With the use of a bulk cathode, the reduction of iron minerals led predominantly to the formation of magnetite with the efficiency of using the electric current at more than 80%. As a result of the preliminary desilication and electroreduction, it was possible to extract more than 98% of Al from bauxite and to increase the iron content in the bauxite residue to 57–58%. Full article

X-ray computer tomography (XRT) is a three-dimensional, nondestructive, and thus reproducible examination method that allows for the investigation of internal and external structures of objects. Due to its characteristics, the XRT technique has increasingly established itself as an alternative examination method and is also applied in the field of mineral processing. Within this work, XRT is used to investigate the influence of hydrochloric acid leaching of iron-rich bauxites on grain composition. Acid leaching is a promising method for the beneficiation of iron-rich bauxites for refractories. Many studies have already established that leaching with hydrochloric acid can reduce the Fe₂O₃ content in bauxites. However, apart from the influence of the leaching process on the composition of the bauxites, aspects such as the influence of the acid on the exact grain constitution or the porosity behavior have rarely been considered so far. To address these open questions, XRT analysis was used to examine and characterize various bauxites. By comparing identical grains before and after leaching, it was observed that in gibbsite bauxites the acid penetration is deeper, and the volume decreases significantly. In diasporic and boehmitic bauxites, clear leaching edges can be seen in which the iron content has been reduced. Full article

The Bayer process is the main method of alumina production worldwide. The use of low-quality bauxites for alumina production results in the formation of a significant amount of technogenic waste—bauxite residue (BR). The Bayer reductive method is one possible way to eliminate BR stockpiling, but it requires high-pressure leaching at temperatures higher than 220 °C. In this research, the possibility of boehmitic bauxite atmospheric pressure leaching at both the first and second stages or high-pressure leaching at the second stage with the simultaneous reduction of hematite to magnetite was investigated. Bauxite and solid residue after NaOH leaching were characterized using XRD, SEM-EDS, and Mössbauer spectroscopy methods. The first stage of leaching under atmospheric pressure with the addition of Fe(II) species in a strong alkali solution (330–400 g L^–1 Na₂O) resulted in a partial reduction of the iron minerals and an extraction of more than 60% of Si and 5–25% of Al (depending on caustic modulus of solution) after 1 h. The obtained desilicated bauxite was subjected to atmospheric leaching at 120 °C in a strong alkali solution (350 g L⁻¹) or high-pressure leaching at 160–220 °C using the Bayer process mother liquor in order to obtain a concentrate with a magnetite content higher than 83 wt. %. Full article

With the rapid development of the alumina industry and the shortage of bauxite, high-alumina coal fly ash (HACFA) has attracted more and more attention as a potential alternative alumina resource. In order to extract alumina from HACFA with newly developed technology, the investigation of the crucial step, the reaction between NH₄Al(SO₄)₂·12H₂O and NH₃·H₂O, is necessary and valuable. Thermodynamic analyses have shown that four kinds of alumina hydrate (boehmite, diaspore, gibbsite, and bayerite) might be formed at 120–200 °C, and ammonioalunite might be formed at temperatures over 180 °C. A hydrothermal reaction crystallization method was applied to this reaction. The experimental results showed that boehmite (AlOOH) could be formed at 150 °C and 200 °C after 12 h and NH₄Al₃(SO₄)₂(OH)₆, an unstable intermediate, is formed during the initial stage and transformed into boehmite, eventually. The higher temperature (200 °C) was more energetically favorable for the formation of NH₄Al₃(SO₄)₂(OH)₆, and the crystallinity of the products was better. More importantly, the sheet-like structure of boehmite (AlOOH) could be formed at 150 °C after 24 h of reaction time. The SEM results proved that the sheet-like structures evolutionary process of boehmite. Full article