Sixty-five bauxite samples of different ages and origins were studied by scanning electron microscopy. Only broken surfaces of the specimens were investigated. Size and form of individual crystals and of grain aggregates were studied as were different types of microtextures and space-fillers.

Grain size varies from 0.05 μm to 1 mm. Smallest is the grain size of young karstic bauxite deposits that is explained by a physicochemical retardation effect of the carbonate environment. Significant differences were found by comparing the space-filling of karstic and lateritic bauxite deposits. High-level and low-level lateritic deposits show differences as well.

A combined use of macroscopic observations, petrographie microscopy, electron microprobe, SEM, and TЕM furnishes the best clues for any genetic interpretation of bauxites. SEM studies are useful in solving technological problems of bauxite processing.

An X-ray thermodifferential powder diffraction method for the quantitative determination of goethite and hematite in lateritic bauxites has been developed and consists of measuring the integrated intensities of the 012 line of hematite before and after heating the sample at 900°C and of correcting the obtained values by the X-ray mass absorption coefficient of either the untreated or heated matrix. From the corrected line intensities and the chemical analyses, the amounts of iron to be allocated to goethite and hematite in the untreated samples can be estimated. The actual content of goethite and hematite in a sample is calculated by taking into account the degree of Al substitution in each of these minerals. The method was tested on artificial mixtures of goethite and hematite and subsequently used to analyze 98 auger drill samples from lateritic bauxites of Guinea Bissau. The estimated precision of the determination of goethite and hematite content was ±2% (absolute). The method can not be applied to samples containing < 10% Fe2O3 (on a whole weight basis) unless preconcentration is carried out.

Manganiferous karst bauxites are rare on a worldwide scale. One such body, recently mined at Kincsesbánya, Hungary, has been studied by chemical, petrographic, X-ray powder diffraction, scanning electron microscopic, and energy dispersive X-ray analytical techniques. The bauxite deposits of Kinesesbánya are of Paleocene to Lower Eocene age; however, the enrichment of manganese in them was a much later, epigenetic process. Lithiophorite is the main Mn mineral in this bauxite and occurs chiefly in dusters of < 1-μm size crystallites. Well-developed crystallites, however, 5–10 μm in size, line the walls of many microfissures and voids.

The oxidation of pyritic bauxite and lignitic clays in the overlying beds apparently mobilized finely disseminated Mn and Fe. Downward-migrating acidic solutions were gradually neutralized, and Mn and Fe minerals precipitated. The manganiferous bauxite was found only along the eastern rim of heavily eroded Middle Eocene sedimentary rocks. Here, epigenetic oxidation and mobilization were optimum. Farther to the east, pyrite-rich overburden and bauxite were apparently eroded away before Fe and Mn could be mobilized.

Alumina is produced from bauxite, which contains a mixture of various oxides, such as aluminum (Al), iron (Fe), silicon (Si), and titanium (Ti). Bauxite can also be considered a source of several other valuable metals, such as scandium (Sc), vanadium (V), and gallium (Ga). The composition and mineralogy of alumina determine their economic value, but their characteristics vary by locality. The physicochemical characteristics of bauxites can also be influenced largely by weathering processes, even within the same locality. For this reason, the present study was undertaken with the objective of comparing the characterization data of three bauxite samples collected, which will be referred to as D, E, and F, from the Cruz Alta do Pará plateau in northern Brazil. The samples were solubilized by multi-acid digestion and fusion with lithium metaborate to quantify their metal compositions by inductively coupled plasma optical emission spectrometry (ICP-OES). The mineralogical characterization was conducted by X-ray diffraction (XRD), and the phase changes of minerals in bauxite were detected by thermogravimetric analysis (TGA/DTG). The total organic carbon (TOC) technique was used to quantify the C in the samples, and the moisture content was also measured. Alumina was 30 wt.% on average for all samples, good for producing high-purity alumina by hydrometallurgical processes. The results, however, showed high (~20 at.%) silica concentrations in two samples and ~3 wt.% Fe in one sample, which can pose a challenge in the Bayer process. The X-ray diffraction (XRD) analysis showed that gibbsite (Gbs), kaolinite (Kln), anatase (Ant), and hematite (Hem) were the major mineral phases in these samples. The study showed that the samples from the same mine vary in their metal content, especially with regard to Si, and they, thus, need to be processed selectively to maximize their economic value.

Resilication of bauxite produced kaolin at and beneath an old erosion surface on bauxite at the Alabama Street Mine of ALCOA in Saline County, Arkansas. The transitional alteration can be traced in morphology by scanning electron microscopy, (SEM) and in Al:Si ratio by energy dispersive analysis. In one illustrated example, the sequence of resilication took place within 1 mm thickness; in another, across 80 mm. The first morphologic alteration of gibbsite (bauxite) appears to be to allophane that occurs in micrometer-size plates which show elongate cracking and/or straight to highly curved elongate edges. The next phase is kaolinite, first in micrometer-size flakes followed by coarser flakes that grade into a zone of typical stacked kaolinite, likewise identified by X-ray powder diffraction. Notably large stacks and small flakes of kaolinite are intimately mixed in the SEMs, thus suggesting that unequal sizes of kaolinite crystals can grow during one episode of an in-situ genesis.

During the epigenetic alteration of boehmitic karstic bauxite in the Vlasenica region of Yugoslavia, which was caused by siliceous water descending through the deposit by means of cracks, fissures, etc., oolites were less altered compared with the coexisting matrix material, which was intensely kaolinized. The following zonal alteration pattern of the oolites was identified: a diaspore enrichment zone, followed by a diaspore-boehmite enrichment zone, and then the original bauxite. Considering this alteration pattern and that determined previously for the matrix material, the following overall pattern of alteration was established: a kaolinite-diaspore zone, followed by a boehmite-diaspore zone, and then the original bauxite. The alteration solutions reacted with the bauxite to form solution-matrix and solution-oolite subsystems, which were characterized by different types of diffusion and geochemistry.

In the kaolinite-diaspore zone Si-metasomatism kaolinized the matrix, and excess Al partly migrated into the oolites, forming diaspore, and outward into the bauxite. The latter Al-remobilization resulted in a transition zone, in which new boehmite was formed in both the matrix and the oolites, and diaspore only in the oolites. Thermodynamic models of oolites and whole bauxite alterations were established, on the basis of stability diagrams calculated for the mineral assemblages of both the oolites and the matrix, in the alteration zones and in the deposit as a whole. Based on these models, the genesis of the diaspore and the relationship of diaspore-boehmite-kaolinite assemblages in natural systems at low temperature and low pressure can be described.

The kaolinization of bauxite has generally been thought to be a simple process of epigenetic resilification. A study of the karstic boehmitic bauxites in the Vlasenica region of Yugoslavia, however, shows that kaohnization took place in a rather complex manner, in which the alteration was caused by the percolation of siliceous water descending through the deposit by means of cracks, fissures, etc. The matrix of one such Vlasenica deposit was found to be more highly kaolinized than the oolitic fraction. Based on a mineralogical and geochemical examination of matrix material, the following pattern of zoned alteration was identified: kaolinitic zone-boehmite enrichment zone-original bauxite. In the kaolinitic zone, well-crystallized kaolinite, formed by the reaction of dissolved silica with boehmite, has replaced all other minerals in the matrix. This Si metasomatism was accompanied by an outward migration of Al and resulted in the formation of a transition zone in which new boehmite partly replaces both kaolinite and hematite (Al remobilization). A thermodynamic model of the process has been established on the basis of stability diagrams calculated for the mineral assemblages in the alteration zones and in the deposit as a whole.

Fifty-two Bidi-Koum bauxites from Guinea, Africa, of diversified chemical composition were characterized for their mineral composition. First, 14 element oxide concentrations were determined by X-ray fluorescence (XRF) using a fusion sample preparation technique. Loss of Mass (LOM) and organic carbon (OC) concentrations were also determined. The initial X-ray diffraction (XRD) phase quantification was carried out employing XDB software. This software allows for full interpretation of a sample diffractogram and helps generate initial concentrations of identified minerals based on a standardless approach. In the stage that followed, the mass balance procedure on the XDB software helped refine the final phase composition. Then, gibbsitic Al2O3 concentrations obtained by wet chemistry for all samples and kaolinitic SiO2 concentrations obtained for selected samples were compared with the concentrations obtained using the XDB software. Phases that were quantified are: gibbsite, boehmite, kaolinite, wavellite, goethite, hematite, quartz, anatase, rutile and illite. Phase concentrations were obtained for illite from K2O and for wavellite from P2O5 concentrations. The alumina substitution in the goethite lattice was also estimated.

Approximately 20 wt.% of the bauxite from Andoom in northern Queensland, Australia is composed of material that cannot be accounted for by identifiable well-crystallized phases. This poorly-diffracting material (PDM), found within the core of bauxitic pisoliths, has similar characteristics to that of eta-alumina (η-Al2O3); a cubic form of alumina. A differential XRD pattern of the PDM displayed a series of broad diffraction maxima attributed to eta-alumina with a mean crystal size of 9 nm. Unit cell refinement, on the basis of a cubic cell, gave a lattice parameter of a = 7.98 Å for Andoom eta-alumina. TEM and selected-area electron diffraction revealed the PDM to be composed of minute (10 nm wide), randomly oriented crystals of eta-alumina in close association with Al-hematite. Chemical analysis using a nanoprobe showed Andoom eta-alumina to be almost pure alumina with <2 M% Fe, <1 M% Si and <1 M% Ti. The closely associated Al-hematite may contain as much as 22 M% Al, however a value closer to the theoretical limit of 17 M% is more likely. A broad absorption band at 3450 cm−1 and 1630 cm−1 in the infra-red spectrum of the PDM indicates the presence of a substantial quantity of H2O, strongly adsorbed onto the surface of the crystals. This is presumably due to η-Al2O3's large surface area of approximately 2200 m2/g. The natural occurrence of η-Al2O3 in bauxite may be the result of low H2O activities within the micro-environment of pores at the time of crystallization. The epigenetic replacement of kaolinite with η-Al2O3 and Al-hematite is put forward as an explanation for the formation of bauxitic pisoliths at Andoom.

Nordstrandite from Lages represents the first occurrence of this mineral in an oxisoil that does not involve the influence of a calcareous substrate. This is also the first reported occurrence in Brazil. Nordstrandite occurs near the city of Lages only in the weathered bedrock facies, mostly where the circulation of meteoric water is restricted. There is also a close association between nordstrandite and the weathering of modified pseudoleucite structures. The interaction of meteoric water and feldspathic alkaline country rock with uncommon texture and mineralogy (rich in nepheline, sodalite, nosean, analcime and natrolite) promotes the development of solutions enriched in alkaline ions with basic pH, necessary to form nordstrandite. A detailed study by X-ray diffraction (XRD) and infrared (IR) spectroscopy in natural or chemically leached samples was performed to identify the Al(OH)3 polymorphs of gibbsite and nordstrandite.

The collector for separating diasporic bauxite serves as a type of flotation reagent by adsorbing selectively on diaspore to make it hydrophobic enough to separate it from the aluminosilicates. Although the flotation process is considered economical in the desilication of Chinese diasporic bauxite, the existing collectors fail to separate these ores because of their poor adsorption selectivity over other minerals. The present study was an attempt to seek a collector for selective flotation of diaspore over aluminosilicates. A novel carboxyl hydroxamic acid compound, 2,2-bis(hydroxycarbamoyl) decanoic acid (BHDA), was designed and synthesized, and the flotation behavior of diaspore, kaolinite, and illite was investigated by flotation tests with BHDA. The interactions between the BHDA and the minerals were also explored by Fourier-transform infrared spectroscopy (FTIR), zeta-potential measurement, and density functional theory (DFT) calculation. Using BHDA as the collector, the pulp pH value affected the floatability of diaspore significantly while the floatability of kaolinite or illite was unaffected or only slightly affected. The dosage of BHDA had little effect on the floatability of the three minerals. The greatest difference in floatability between diaspore and aluminosilicates occurred at mid-range pulp pH (7). Large shifts in characteristic absorption peaks and new absorption peaks were observed for BHDA-treated diaspore but were absent from BHDA-treated aluminosilicates. The change in the negative zeta potential of diaspore was also greater than those of aluminosilicates in the presence of BHDA. The O atoms in the carboxyl and hydroxycarbamoyl of BHDA have highly negative charges, and favorable stereo conditions existed to form five- or six-membered rings, resulting in their coordination with the Al atoms of diaspore, leading to chemisorption in chelate rings; the adsorption of BHDA on kaolinite or illite, on the other hand, was mainly physical in nature. The BHDA was, therefore, highly selective in the flotation between diaspore and aluminosilicates and possibly suitable for the separation of diasporic bauxite.

The negative effects of dye-contaminated wastewater on humans and the environment are well known, so the wastewater must be treated carefully before discharge into the environment. To overcome those impacts, the search for environmentally friendly and low-cost materials is essential, especially in developing countries The objective of the present study was to determine the feasibility of using bauxite from Malaysia as a new and efficient ceramic, hollow-fiber membrane for the degradation of reactive dyes in wastewater. A porous, hollow-fiber membrane was fabricated from bauxite (BHFM) using a phase-inversion technique, followed by sintering at various temperatures. The BHFM consisted of two types of voids, having either a finger-like or a sponge-like structure. As the sintering temperature was increased, the porosity of the BHFM decreased from 46.5 to 9.5%. The greatest mechanical strength of 308.1 MPa was achieved when the BHFM was loaded with 55 wt.% of bauxite and sintered at 1450°C. The remaining 45 wt.% consisted of solvent, polymer binder, and dispersant. The BHFM functioned well as a membrane for microfiltration and a support membrane for ultrafiltration. BHFM with loading of 45 wt.%, 50 wt.%, and 55 wt.% successfully eliminated 90%, 94%, and 98% of 10 ppm reactive dye (RB5) when sintered at the highest temperature.

Processes of mineral alteration involving the mobilization and deposition of more than 30 chemical elements during bauxite formation and epigenesis have been studied on specimens from the Devonian Schugorsk bauxite deposit, Timan, Russia. Chemical analyses of the minerals were obtained by electron microprobe and element distribution in the minerals was studied by element mapping. Interpretation of these data also utilized high-resolution BSE and SE images.

The main rock-forming minerals of the Vendian parent rock are calcite, dolomite, feldspar, aegirine, riebeckite, mica, chlorite and quartz; accessory minerals are pyrite, galena, apatite, ilmenite, monazite, xenotime, zircon, columbite, pyrochlore, chromite, bastnaesite and some others. Typically, the grainsize of the accessory minerals in both parent rock and bauxite is from 1 to 40 µm. However, even within these rather small grains, the processes of crystal growth and alteration during weathering can be determined from the zonal distribution of the elements. The most widespread processes observed are: (1) Decomposition of Ti-bearing minerals such as ilmenite, aegirine and riebeckite with the formation of ‘leucoxene’, which is the main concentrator of Nb, Cr, V and W. Crystal growth can be traced from the zonal distribution of Nb (up to 16 wt.%). Vein-like ‘leucoxene’ is also observed in association with organics. (2) Weathering of columbite and pyrochlore: the source of Nb in ‘leucoxene’ is now strongly weathered columbite, while the alteration of pyrochlore is expressed in the growth of plumbopyrochlore rims around Ca-rich cores. (3) Dissolution of sulphide minerals and apatite and the formation of crandallite group minerals: ‘crandallite’ crystals of up to 40 µm size show a very clear zonation. From the core to the rim of a crystal, the following sequence of elements is observed: Ca → Ba → Ce → Pb → Sr → Nd. Sulphur also shows a zoned but more complicated distribution, while the distribution of Fe is rather variable. A possible source of REE is bastnaesite from the parent rock. More than twelve crandallite type cells can be identified in a single ‘crandallite’ grain. (4) Alteration of stoichiometric zircon and xenotime with the formation of metamict solid solution of zircon and xenotime: altered zircon rims also bear large amounts of Sc (up to 3.5 wt.%), Fe, Ca and Al in the form of as yet unidentified inclusions of 1–2 µm. Monazite seems to be the least altered mineral of the profile.

In the parent rock, an unknown mineral of the composition (wt.%): ThO2 – 54.8; FeO – 14.6; Y2O5 – 2.3; CaO – 2.0; REE – 1.8; SiO2 12.2; P2O5 – 2.8; total – 94.2 (average from ten analyses) was determined. In bauxite, another mineral was found, which has the composition (wt.%): ThO2 – 24.9; FeO – 20.5; Y2O5 – 6.7; CaO 2.0; – ZrO – 17.6; SiO2 – 8.8; P2O5 – 5.4; total – 89.3 (F was not analysed; average from nine analyses). Presumably, the second mineral is the result of weathering of the first one. Although the Th content is very high, the mineral is almost free of Pb. However, intergrowths of galena and pyrite are observed around the partially decomposed crystals of the mineral. Another generation of galena is enriched in chalcophile elements such as Cu, Cd, Bi etc., and is related to epigenetic alteration of the profile, as are secondary apatite and muscovite.

A Th-rich mineral of the crandallite group has been investigated from the weathering profile of the Schugorsk bauxite deposit, Timan, Russia. It occurs within thin (up to 0.5 mm) organic-rich veinlets together with ‘leucoxene’ in the form of small shapeless grains which vary in size from 1—2 mm to 60—70 mm. Rare grains disseminated among boehmite crystals were also found. Microprobe analyses determined that the ThO2 content can be as high as 18 wt.%. The mineral composition is intermediate between crandallite CaAl3H(PO4)2(OH)6, goyazite SrAl3H(PO4)2(OH)6, Th-crandallite and svanbergite SrAl3PO4SO4(OH)6 in the beudantite group.

Comparatively high contents of Fe and Si and a very high positive Th and Fe content correlation (r = +0.98) suggest that the formula of the hypothetical Th-bearing end-member is ThFe3(PO4,SiO4)2(OH)6 with Th and Si substituting for REE and Prespectively (woodhouseite-type substitution). Another possible substitution is Th4+ + Ca2+ ⇋ 2REE3+ (florencite-type). A deficiency of cations in the X site can be explained by either the presence of carbon, undetectable by microprobe, in the crystal lattice or a lack of X-site cations due to radiation damage induced by Th. Some excess of cations in the B site (Al and Fe3+) can be explained by the presence of very small boehmite and hematite inclusions on the crandallite grain surfaces. Th-rich crandallite may be the result of alteration of an unidentified silicate mineral from the parent rock with a composition close to the simplified formula Fe2+ThSiO4(OH)2.

A comparison of Rietveld refinement results for a bauxite-bearing regolith and its clayey cover in the Amazon region was made with stoichiometric calculations from chemical analysis and partly from thermogravimetric results. For this investigation a profile in the bauxite-bearing regolith occurrence in the ALCOA bauxite mine at Juruti, Brazil was studied. The different minerals, their compositions and their low crystallinity in the different horizons were investigated and the contents determined. It is evident that some minerals show several generations and some chemical composition changes that must be included in the Rietveld refinement. Al-rich hematites and goethites are common along the bauxite profile. Amorphous contents were determined with rutile added as an internal standard and shown to have gibbsite- or kaolinite-like composition. The minerals could be quantified in the different horizons and the difficulties were mainly related to variable crystalline aspects of the phases. Rietveld refinement can be a powerful tool in bauxite prospecting, quality control and during mining and beneficiation of ore minerals using the adapted refinement strategies.

Bauxite, the raw material of aluminum, has been one of the economically vital natural resources for Suriname. Mining operations started about a century ago, and subsequent development of a refinery industry and hydro-electric power made Suriname one of the foremost bauxite and alumina producers worldwide for a long period of time. This paper presents a concise survey of the main geological attributes of its bauxite deposits and examines significant aspects in the development of mining in the country where alumina dominated the export revenues until a decade ago. The lateritic bauxite deposits are spread across the northern part of the country and developed on various parent rocks during Late Cretaceous–Early Tertiary times. Bauxites in the coastal lowlands formed on Cenozoic sedimentary deposits, whereas plateau bauxites originated on various crystalline rocks in inland regions of the Precambrian Guiana Shield. The composition of parent rocks and timing of bauxitisation point to a genetic correspondence with West African bauxites and a strong control of paleoclimatic conditions on the distribution and properties of bauxite in both regions. The more accessible bauxite deposits in the coastal lowlands are almost mined out, whereas the plateau bauxites have been extensively explored but have not been brought into production to date. For economic and environmental reasons, the future of the bauxite industry in Suriname is currently uncertain.

In the last few decades, X-ray diffraction (XRD) systems have been paramount and irreplaceable in controlling bauxite exploration, as well as Bayer and reduction processes. XRD quantitative phase analysis in the aluminum industry witnessed a steady deployment of the Rietveld method, which at present progressively replaces existing methodologies in research and plant laboratories. Rietveld analysis not only helped to surpass traditional XRD calibration methods, it also opened the door for new applications previously not possible. The use of the Rietveld method to characterize selected materials unique to the aluminum industry, such as bauxite, red mud, and alumina is demonstrated and discussed. This paper also presents how synchrotron-based diffractograms obtained for bauxite and red mud samples allowed a much better understanding of mineralogical representation, and made it possible to leverage their Rietveld quantification. Despite clear advantages, the Rietveld method also has limitations that are revealed. For alumina phase quantification, a dedicated Rietveld analytical program was built with structure data for eight alumina mineralogical phases: alpha, beta (β-Al2O3 = Na2O•11Al2O3), delta, gamma (2), kappa, sigma, and theta. The paper gives unique examples of phase quantification in aluminas of various origins and phase composition.

The application of the Rietveld method to quantify mineral components of bauxite and lateritic samples was carried out in order to determine the ability of the method to obtain accurate mineralogical abundances for these materials. The method was initially applied to synthetic mixtures using both Cu and Co Kα radiations, and it was shown that Rietveld-derived data compared favourably with the weighed compositions. Application to two types of natural bauxite resulted in a high correlation between Rietveld predicted values and those calculated by proportioning peak intensities with chemical assays. The use of the whole pattern rather than selected peak intensities gives greater accuracy, confirmed by a strong correlation between derived oxide concentrations from XRF assays. Accuracy and precision were improved by the determination of isomorphous substitution of aluminum in goethite and hematite by refinement of unit cell dimensions. Importantly, the ability of the Rietveld program to successfully model several goethites with different levels of isomorphous substitution improved the correlation between predicted and calculated values. In addition, crystallinity and crystallite size that influence the reactivity of the mineral components can be derived from refined peak profiles.