Showing papers on "Gibbsite published in 1994"

Geochemical differentiation in laterite and bauxite formation

Abstract: The geochemical differences between laterite and bauxite formation are discussed under the aspect that both weathering products are predominantly formed by residual enrichment of iron and aluminium. Residual enrichment is corroborated by various arguments and clearly proved for laterite formation on ultramafic rocks. Tropical weathering of the most widespread acidic rocks leads, in spite of their high Al/Fe ratio, generally to formation of laterites showing a high enrichment of iron whereas aluminium is not or only slightly enriched. This chemical variation indicates that high amounts of Al are removed additionally to Si in soil solutions when saprolites on acidic rocks are transformed into laterites containing goethite, hematite and kaolinite as main constituents. Dissolved free aluminium exist in equilibrium with kaolinite only in minimal contents, hence the removal of Si and Al probably takes place in form of freshly formed colloidal or subcolloidal, X-ray amorphous particles with kaolinitic or similiar composition. The frequently observed occurrence of two generations of secondary kaolinite in laterites corroborates this conclusion. There is general agreement that bauxites are formed by stronger leaching compared with laterites. Increased leaching lowers the concentration of dissolved silica, thus enabling gibbsite instead of kaolinite stability. The concentration of dissolved free aluminium, however, is increased in equilibrium with gibbsite. Free aluminium shows in contrast to iron an effective solubility atpH= 5, giving rise to downward leaching and re-precipitation at higher pH below a laterite crust which covers the bauxite zone in most of the bauxite deposits. In spite of the stronger leaching, bauxite formation implies a lower mass loss (mainly Si) during weathering than the formation of laterites (loss ofSi + Al) related to their highly increased iron content.

X-ray photoelectron spectroscopic study of hydrated aluminas and aluminas

Abstract: X-ray photoelectron spectra of hydrated aluminas (boehmite, diaspore, bayerite, and gibbsite), transition aluminas (y, δ, η, θ, X, and k –Al2O3) and corundum (α-Al2O3) have been studied for spectral characterization of each compound. The O1s spectra are shifted 0.2–1.2 eV to higher binding energy (Eb) in the order of α-Al2O3 < boehmite, diaspore < bayerite, gibbsite, and this agrees with the order of bulk OH/Al molar ratio in samples. The Eb and FWHM values of O1s spectra of transition aluminas depend on the ratio OH/O, i.e., the amount of OH− ions chemisorbed on them, and tend to decrease toward those of α-Al2O3 with increasing calcination temperature. Therefore, it is considered that an attracting effect of the proton on valence electrons in the hydroxyl oxygen causes the increased binding energy between core electrons and oxygen atomic nuclei. The broad O1s spectra of boehmite and diaspore can be deconvoluted into equal contribution from the two oxygen species in O2− and OH− ions in their structures.

Comparison of the transformation sequence from γ-AlOOH (boehmite) to α-Al2O3 (corundum) induced by heating and by ball milling

Abstract: This paper presents the X-ray diffraction investigation of ball-milling-induced transformation of boehmite (γ-AlOOH) and of gibbsite (Al(OH) 3 ) to corundum (α-Al 2 O 3 ). The results are compared with the transformation observed in heating experiments, and the conclusion is drawn that equivalence exists between the high energy ball milling and the thermal process.

The preparation of alumina fibre by sol-gel processing

Abstract: Attempts have been made to prepare alumina fibre from the colloidal sol and polymerized alkoxides. The aluminium chloride or aluminium nitrate systems were found to be potential methods for producing continuous alumina fibre: the aluminium nitrate system had a better sintering behaviour than the aluminium chloride system. The aluminium isopropoxide system, however, was unsuitable for preparing alumina fibre but was suitable for the preparation of monoliths, membranes, powders, and multicomponent ceramics. The thermal changes of these precursors were studied by transmission electron microscopy, Fourier-transform infrared spectroscopy and X-ray diffraction. The results demonstrated the different routes of phase transformation as the temperature increases. The aluminium chloride system exhibits two routes for phase transformation: (a) boehmite → γ → δ → θ → α-Al2O3, and (b) gibbsite → χ → κ → α-Al2O3.

Influence of hydrolysis temperature on the hydrolysis products of aluminium alkoxides

Abstract: The influence of hydrolysis on the hydrolytic polycondensation of aluminium alkoxides was studied. Two mechanisms of hydrolysis which give two different products are proposed for this process. Structural transformations of the resultant condensates, i.e. boehmite, (γ-AlOOH), the product of hot-water hydrolysis, and gibbsite Al (OH)3, the product of cold water hydrolysis, have been investigated in temperature range 20–1300 °C. An explanation was offered for the variations in the a-Al2O3 transformations of the condensate produced by room temperature hydrolysis.

Exchange and solution phase chemistry of acid, highly weathered soils .I. Investigation of mechanisms controlling Al release into solution

Abstract: Soil solid phase and soil solution characteristics were evaluated on surface and subsoil horizons of 60 acid, highly weathered soils in the unamended state, and on a subset of 39 surface horizons following the addition of CaSO4.2H2O. The activity of Al3+ Was found to be strongly correlated with the soil solution pH, with Al3+ activity increasing as pH decreased. For surface soils in the unamended state, and for the majority of subsoils, the Al3+ activity calculated was comparable to the theoretical activity supported by the dissolution of gibbsite (Al2O3.3H2O). The Al3+ activity in CaSO4.2H2O-amended samples tended to fall below that maintained by gibbsite dissolution and was more closely correlated with the Al3+ activity supported by the dissolution of jurbanite (AlSO4 (OH).5H2O).

Process and apparatus for the extraction of gibbsitic alumina from bauxite

Abstract: The invention provides a process for extracting gibbsite from gibbsitic alumina containing bauxite, comprising: (i) providing a thick slurry of ground bauxite containing gibbsite; (ii) mixing the thick slurry with preheated spent Bayer process liquor stream to form a preheated combined slurry-liquor mixture; (iii) passing the preheated combined liquor/slurry mixture through a reaction tube or plurality of tubes sized such that the slurry-liquor mixture remains in the reaction tube for an average total residence time of up to about four minutes, said residence time being sufficient to extract the gibbsite from the slurry; (iv) separating red mud containing solids from the resulting pregnant liquor in which the gibbsite has been dissolved in a solid-liquid separator operating at substantially the same temperature and pressure as in the reaction tube; and (v) removing dissolved silica from the pregnant liquor by seeding the liquor with Bayer process desilication product and separating the desilicated pregnant liquor from the solid desilication product. An apparatus for carrying out the above process is also described.

Phase transformations and control of habit in lyothermal synthesis of α-Al2O3

Abstract: The phase transformation of gibbsite to corundum (α-Al 2 O 3 ) in 1,5-pentanediol (glycothermal treatment) was investigated between 270 and 450°C and between 10 and 50 MPa. The kinetics of transformation to corundum in this range are hindered. Only the formation of microstalline tohdite (5 Al 2 O 3 : 1 H 2 O) can be observed. The lowering of the nucleation energy of corundum by adding α-Al 2 O 3 seeds enables the synthesis of microcrystalline corundum at 330°C. The size of the gibbsite particles and the size and quantity of the α-Al 2 O 3 seeds are important for the homogeneity and particle size of the resulting corundum. The particle size and the morphology of glycothermal α-Al 2 O 3 powders were compared with the results of hydrothermal synthesis of α-Al 2 O 3 .

Response of some soils of Galicia (NW Spain) to H2SO4 acidification

Abstract: This paper presents the results of an acidification experiment, consisting of seven consecutive equilibrations of repesentative soils of Galicia with a H2SO4 solution (pH 3). Different responses to soil acidification, such as SO4 retention and cation release, were evaluated. In soils derived from gabbro and amphibolite, SO4 retention and Al release were the principal acid neutralization mechanisms, whereas in soils derived from granite, schist, shale and sandstone Al release was the main process. The SO4 retention was significantly correlated with Al and Fe extracted with dihionite-citrate-bicarbonate and crystalline Fe. The released base cations came mainly from exchange sites, though sometimes also from other sources, probably by mineral weathering. The major sources of Al in these soils were metalorganic complexes and weatherable minerals. Solutions with pHs close to 4 are in equilibrium with gibbsite, kaolinite, jurbanite and alunite; at lower pH values, with jurbanite and alunite.

Sedimentological and Clay Mineral Studies of Kumbla-Shiriya Estuary, West Coast of India

Abstract: Four sediment facies viz ., sand, clayey sand, sandy clay and silty clay occur in Kumbla-Shiriya (K-S) estuary. Sediments have formed from bottom suspension and graded and uniform suspension as a result of variable hydraulic conditions in different parts of the estuary. Kaolinite+chlorite is the dominant clay mineral followed by montmorillonite and illite. Gibbsite, goethite and quartz also occur in minor quantities. It appears that the abundance and distribution of clay minerals in the estuary are controlled by the geology of the source area and the water circulation pattern within the estuary.

Formation of Clay Minerals by Water-Rock Interaction in the Fracture of Gneiss

Abstract: As the groundwater flows along the fractures of crystalline rocks, it will be in contact with the fracture walls mostly coated by secondary minerals which are quite different form those of host rocks. The presence of fracture-filling minerals in crystalline rocks is important on the view point of radioactive waste disposal because of their great surface reactivity. The Surichi drill hole of 200 m in depth in the Yugu area composed mainly of Precambrian gneiss was selected to study the formation process of clay minerals on the fracture wall of gneiss, and their relation with present groundwater. The water-rock interaction in fractures resulted in the formation of gibbsite and clay minerals. They are formed by two different processes : (1) Incongruent dissolution of feldspar by groundwater diffused from a fracture path into rock matrix produced smectite and illite in situ, (2) on the wall of fracture, gibbsite, kaolinite, smectite and illite are formed by precipitation of dissolved species in groundwater. They show the paragenetic sequence such as gibbsitekaolinitesmectite or illite. The paragenetic sequence of fracture-filling minerals was controlled by increase of pH of groundwater, decrease of fracture permeability by precipitation of fillings, and immobility of alkali or alkaline earths in groundwater. The groundwater from the Surichi borehole is a type with pH range of 8.6-9.2. The sodium and bicarbonate in groundwater would be supplied by the dissolution of albite and calcite, respectively. The saturation index of groundwater and surface water calculated by WATEQ4F indicates that gibbsite and kaolinite are under precipitation to equilibrium state, and that smectite and illite are under equilibrium to redissolution environment. The stability relation of clay minerals in the system shows that kaolinite is stable for all waters.

Mineralogical Study of the Granite Weathering in the Seoul Area: Water-Rock Interaction in the Namsan Granite

Abstract: The weathering of the Namsan granite was studied in terms of sorption process. The Namsan granite consists mainly of quartz, alkali feldspar, plagioclase ( and biotite with small amounts of sericite, magnetite and ilmenite. The kinetic factors for altering the granite body are the proton and hydroxyl ions derived from the reaction of water and mineral. There are two different types of pH variation curves for rocks of different mineral assemblages. when powdered granite was dispersed in distilled water under ambient condition. The sorption-process proceeds by three steps for fresh granite; (1) the initial rapid pH-rise to 10 by the uptake of proton by negatively charged mineral surfaces, (2) the gradual pH-down, and (3) the stable pH tail between 7.1-7.5. For somewhat weathered granites, the sorption proceeds; (1) the initial rapid pH-down to 4.8, (2) the slight pH-rise and slow ph-down, and (3) the stable pH tail between 5.0-5.3. The reaction rate is controlled by the density of adsorbable sites, the solubility of the mineral, pH of the system and formation of amorphous gel and gibbsite. Amorphous gel floates on the surface of the solution while stirring the powdered granite and then is transformed into gibbsite in an hour or so. The pH saturation values for -325 mesh fresh granite from 5 m depth is about pH 10 when rock/water ratio is over 10g/200 ml.

Allophane imogolite, and gibbsite in coatings in Costa Rican Andisol.

Abstract: Abstract Neoformation of allophane, imogolite, and gibbsite has been studied extensively in soils in humid climates, but little is known about their micro distribution patterns. such information may provide insight about the conditions at the micro-site scale, responsible for their formation and disappearance. Our study describes amorphous and crystalline coatings in a Melanudand on a 18,000 yr old andesitic lava in Costa Rica. Fine-textured, white to yellow coatings could be observed in the B, C, and R horizon in the field. In thin sections the coatings are translucent and isotropic in plane polarized light (PPL), indicating that they consist of amorphous material. In the B horizon, under crossed polarized light, isotropic coatings show a gradual transition towards crystalline outer margins and spots, while in PPL such transition cannot be observed, suggesting a genetic relationship. Submicroscopical analyses reveal that the amorphous coatings in the R and C horizon consist of allophanic material with a molar Al/Si ratio of 0.9, those in the B horizon consist of allophanic material and imogolite with a molar Al/Si ratio of 1.4. The amorphous coatings resulted from precipitation of Al and Si liberated upon weathering of primary minerals in an initial stage of lava weathering. The crystalline coating parts in the B horizon consist of gibbsite and represent the ultimate stage of mineral neoformation. The different coating composition in the soil horizons is the result of different leaching conditions at a macro and micro scale. Care should be taken to compare results obtained at different sampling scales with different analytical methods.