Relative Dissolution Rates Research Articles

Reaction Rims on Ilmenite and Chromite: Implications for Volatile Behavior and Crystallization Conditions of Kimberlite Magma

Abstract Significant uncertainty surrounds the crystallization conditions and the composition of kimberlite melts, including the role of volatiles (H2O and CO2) due to their hybrid nature, intense alteration, and volatile loss during emplacement. In this study, we address these uncertainties by investigating the interaction between oxides (ilmenite and chromite) and kimberlite magma. During kimberlite ascent, mantle minerals react with the magma and develop dissolution textures, compositional zoning, and rims of secondary mineral phases in response to crystallization conditions and the composition of kimberlite magma. We examined oxides from several lithologies within the BK1 and AK15 kimberlites of the Orapa cluster in Botswana, where diamonds demonstrate distinct dissolution styles in each lithological unit owing to differences in magma saturation with volatiles. Here we discovered a strong correlation of the reaction products on ilmenite and chromite with the dissolution style of diamonds in the same kimberlite unit. Diamonds with glossy, low-relief surface features indicative of fluid-rich magma occur in the kimberlite units where ilmenite and chromite develop reaction rims of Ti-bearing phases. Diamonds with corrosion sculptures implying a volatile-undersaturated magma occur in kimberlite units with heavily resorbed chromite and ilmenite completely replaced by a MUM (magnesio-ulvöspinel-magnetite)–perovskite symplectite. Furthermore, the composition of ilmenite reaction rims depends on kimberlite lithology, where MUM co-exists with perovskite or its break-down product anatase in the two coherent kimberlite units, or with perovskite and titanite in the massive volcaniclastic unit. We examine how decompression, cooling, degassing, or assimilation of crustal rocks by kimberlite magma could have shifted conditions from perovskite to titanite stability in the volcaniclastic kimberlite unit. We propose perovskite replacement by anatase-calcite pseudomorphs in the top coherent unit, from which diamonds exhibit an overprint of fluid resorption with a melt resorption. Composition of ilmenite reaction rims provides estimates of kimberlite crystallization temperatures of 730–1275 °C and oxygen fugacities of +0.5 to −3.5 relative to the nickel-nickel oxide buffer, which are validated through controlled experiments. Our study shows that preservation of ilmenite, the type of Ti-phase in its reaction rim, the relative rate of chromite dissolution, and compositional re-equilibration with kimberlite can help model the eruption process as well as the style and rate of diamond dissolution.

Formulation and optimization of oral fast dissolving films loaded with nanosuspension to enhance the oral bioavailability of Fexofenadine HCL

Fexofenadine HCL (FEX) is an antihistamine that has inadequate oral bioavailability and limited aqueous solubility. The goal of this work is to develop and evaluate FEX nanosuspensions integrated into oral fast-dissolving films (OFDFs) to improve drug bioavailability. FEX nanosuspension was generated by an antisolvent-precipitation approach using a full factorial design (32). The influences of stabilizer content and the ratio of solvent-to-antisolvent on the particle size and relative dissolution rate were investigated. The optimized formula showed an average size of 57.3 nm with the relative dissolution rate of 4.42. Such formula was evaluated for FT-IR, DSC, XRD and morphology. The FT-IR studies appeared no chemical interaction between FEX and stabilizer. The crystallinity of FEX was reduced as detected by DSC and confirmed by XRD. SEM revealed that FEX-nanosuspensions had a rod-like appearance. The optimized FEX nanosuspensions were then directly loaded into OFDFs throughout a solvent casting process. The influence of the CMC and SSG concentrations on disintegration time and dissolution efficiency % after 10 min was detected using Statgraphics® software. The optimum FEX-OFDF prepared with 1% w/v CMC and 2.68% w/v SSG showed a disintegration time of 21.74 s and a dissolution efficiency of 79.23% after 10 min. The optimized formulation of FEX-OFDF increased the oral bioavailability of Fexofenadine hydrochloride by 4.15-fold when evaluated in rabbits compared to commercially available Telfast® tablets.

High-Efficiency Desulfurization of High-Sulfur Bauxite Calcined in a Conveyor Bed: Kinetics, Process, and Application

The reaction process, mechanism, and kinetics of the desulfurization of high-sulfur bauxite during calcination were investigated using thermal analysis–infrared analysis. A conveyor-bed calcination system was used to study the variations in the physical phase, desulfurization rate, and alumina dissipation rate of high-sulfur bauxite in the range of 500 °C–650 °C. The results show that sclerite monohydrate, kaolinite, rhodochrosite, pyrite, and dolomite mainly decompose during the calcination of high-sulfur bauxite, generating H2O(g), CO2, and SO2 as gaseous products. The decomposition of sclerite monohydrate and kaolinite and the dehydroxylation reactions of rhodochrosite and pyrite occur at <650 °C, with inseparable temperature overlap. High-sulfur bauxite desulfurization follows a three-dimensional spherical diffusion mechanism, with an activation energy of 181.16 kJ/mol, controlled by the diffusion rate of O2 or SO2 through the solid product layer. High-sulfur bauxite was calcined at 600 °C–650 °C for around 3.5 s in a conveyor bed, resulting in a negative divalent sulfur content of <0.03 wt.%, desulfurization rate of >0.98, and relative dissolution rate of alumina of >99%, satisfying the requirements of aluminum extraction via the Bayer method. The desulfurization rate predictions of the kinetic model were consistent with the experimental data.

Исследование возможности применения гранулированного сахаросодержащего продукта с функциональными добавками при производстве желейных начинок

Introduction. Iodine is one of the most important elements for maintaining human health and cognitive skills. Contemporary food industry is looking for new functional foods fortified with macro- and micronutrients. Confectionery products occupy a fairly large segment of human diet. The present research objective was to substitute traditional white sugar with an experimental granulated sugar-containing product that contained maltodextrin, cane molasses, and Japanese kelp extract. This substitute could reduce the calorie content of jelly and increase its functional properties while maintaining its sensory profile, e.g. consistency, increasing its shelf-life, and simplifying the tec hnological process.
 Study objects and methods. Granulated sugar-containing product is a new component for the confectionery industry, but its effect on technological processes is unpredictable. The research featured an experimental granulated sugar-containing product with maltodextrin, cane molasses, and Japanese kelp. The growth rate of granules and their structure were studied by scanning microscopy and radiography, the granulometric composition and solubility – by determining the relative dissolution rate, the effect of the granulated sugar-containing product on the sensory and physicochemical parameters of jelly – by refractometry and titrometry.
 Results and discussion. The experimental granulated sugar-containing product contained sucrose both in crystalline and amorphous state, which affected the distribution of nutrients within the product. The growth rate of granules affected the resulting structure of the product and its solubility, which decreased by 2.5 times as the size of the granules grew. The dissolution rate decreased compared to crystalline sugar. As a result, the technological process of jelly production had to be adjusted. The developed jelly had a lower energy value and better nutritional qualities. The high content of vitamins and macro- and microelements makes it a functional product. The experimental jelly received 22.3 points for sensory profile. Its hysico-chemical parameters (soluble solids < 68%, acidity > 2.2 8%) met the standard requirements for this type of product.
 Conclusion. The new granulated sugar-containing product simplified the technological process and improved the sanitary, hygienic, and working conditions.

Mechanistic study of dissolution enhancement by interactive mixtures of chitosan with meloxicam as model

To enhance dissolution rate of meloxicam (MX), a poorly soluble model drug, a natural polysaccharide excipient chitosan (CH) is employed in this work as a carrier to prepare binary interactive mixtures by either mixing or co-milling techniques. The MX-CH mixtures of three different drug loads were characterized for morphological, granulometric, and thermal properties as well as drug crystallinity. The relative dissolution rate of MX was determined in phosphate buffer of pH 6.8 using the USP-4 apparatus; a significant increase in MX dissolution rate was observed for both mixed and co-milled mixtures comparing to the raw drug. Higher dissolution rate of MX was evidently connected to surface activation by mixing or milling, which was pronounced by the higher specific surface energy as detected by inverse gas chromatography. In addition to the particle size reduction, the carrier effect of the CH was confirmed for co-milling by linear regression between the MX maximum relative dissolution rate and the total surface area of the mixture (R2 = 0.863). No MX amorphization or crystalline structure change were detected. The work of adhesion/cohesion ratio of 0.9 supports the existence of preferential adherence of MX to the coarse particles of CH to form stable interactive mixtures.

Observation of the reactions between iron ore and metallurgical fluxes for the alternative ironmaking HIsarna process

ABSTRACT This work investigates the melting behaviour of iron ore with calcium-based fluxes, including lime, limestone and basic oxygen furnace (BOF) steelmaking slag. With an aim to explore the potential kinetic benefits that can be obtained through the formation of a liquid medium on the overall productivity of HIsarna. A HT-CLSM was used to rapidly heat and observation the interface between a given pair of chosen materials in-situ. Through this method, the rate at which the reaction progressed and the possible disruptive phenomenon that may occur was observed directly. The molten interface of fluxed materials is compared against each other offering insight into relative dissolution rate. Each flux shows promise of increasing ore fluidity to a varying degree with limestone generating above a 60% liquid fraction in the 1400°C reaction temperature.

Idiomorphic oikocrysts of clinopyroxene produced by a peritectic reaction within a solidification front of the Bushveld Complex

Orthopyroxenite cumulates throughout the Critical Zone of the Bushveld Complex commonly contain prominent euhedral crystals of bottle-green augite, typically around 1 cm in size and surrounded by “haloes” of nearly pure plagioclase. On close examination, the augite grains can be seen to be oikocrysts, containing extensively resorbed chadacrysts of orthopyroxene, indicating an effective peritectic reaction relationship between the two pyroxenes. A detailed textural study of one such layer from the UG3 Unit in the Eastern Bushveld reveals some distinctive grain-scale features, including the presence of extensively 3D-interconnected chains of thousands of cumulus chromite grains that surround the orthopyroxene grains and extend through the augite oikocrysts. The high degree of interconnectivity is remarkable in view of the relatively low (5 vol.%) modal proportion of chromite in the sample. Other noteworthy features include zoning of the oikocrysts towards higher incompatible element contents in the rims and the presence of the plagioclase haloes around the augite oikocrysts. These haloes are essentially narrow zones devoid of orthopyroxene inclusions developed within large plagioclase oikocrysts that overgrow all the other phases. We propose a mechanism whereby the plagioclase oikocrysts grow early in communication with the main magma body within an initial crystal mush of orthopyroxene and chain-textured chromite. The clinopyroxene oikocrysts grew in the remaining pore space in such a way that dissolution of orthopyroxene occurred within a narrow few-mm wide chemical boundary layer ahead of the advancing oikocryst margin. The relative rates of crystallisation and dissolution were controlled by limited chemical diffusion through the boundary layer such that orthopyroxene grains more than a few grain diameters away showed no reaction at all. The plagioclase haloes developed as a result of continuing growth of the plagioclase oikocrysts to overtake the growing pyroxene oikocrysts, locking in the orthopyroxene-depleted boundary layer, and preserving euhedral cumulus morphologies away from the pyroxene oikocrysts. This texture represents a circumstance whereby oikocrysts in the same rock develop at different overlapping stages: plagioclase oikocrysts forming early in diffusive connection with the magma column, and clinopyroxene forming peritectic poikilitic textures within residual liquid pockets chemically isolated from the overlying magma body.

The impact of light on understanding the mechanism of dissolution and leaching of sphalerite (ZnS), pyrite (FeS2) and chalcopyrite (CuFeS2)

A theoretical framework is presented for understanding the interaction of light with the electronic structure of sphalerite, pyrite and chalcopyrite and how these factors affect their rate of dissolution. Their electronic structure gives rise to the colour of these minerals, primarily because of their semiconducting properties. An advanced theory of dissolution is presented that describes how the electronic structure of these minerals also impacts their mechanism of dissolution. This theory is based on the requirement that in order to dissolve these minerals under oxidative conditions, electrons must be removed from bonding orbitals. Since the electrons with the highest energy are in the upper valence band, the bonding character of the upper valence band plays a crucial role in determining the relative rate of dissolution of different minerals. These ideas are used to interpret the effect of light on the rate of dissolution of these minerals. The implications of this analysis for the order of the relative rates of dissolution of sulphide minerals is discussed.

Congruent Release of Drug and Polymer from Amorphous Solid Dispersions: Insights into the Role of Drug-Polymer Hydrogen Bonding, Surface Crystallization, and Glass Transition.

Drug loading is an important parameter known to impact the release rate of a poorly soluble drug from an amorphous solid dispersion (ASD). Recent studies have shown that small increases in drug loading can dramatically reduce the drug release rate from ASDs prepared with poly(vinylpyrrolidone-co-vinyl acetate) (PVPVA). However, the link between drug physicochemical properties and the drug loading where the release is abruptly compromised is not well understood. This study probes the role of different factors on the relative dissolution rates of drug and polymer from PVPVA-based ASDs as a function of drug loading: (1) the impact of drug-polymer hydrogen bonding interactions on the initial dissolution rate of ASDs, investigated using two structural analogues, indomethacin (IND) and indomethacin methyl ester (INDester), (2) the influence of surface drug crystallization, observed for INDester ASDs, and (3) by changing temperature, the impact of the "wet" glass transition temperature (Tg). Scanning electron microscopy (SEM), with or without energy dispersive X-ray (EDX) analysis, Fourier transform infrared spectroscopy (FTIR), and powder X-ray diffraction (PXRD) were utilized to study the solid-state phase behavior and/or drug enrichment on the partially dissolved ASD tablet surfaces. Nanoparticle tracking analysis (NTA) was utilized to study the solution-state phase behavior. It was found that, contrary to expectations, ASDs with drug-polymer hydrogen bonding exhibited poorer initial release at moderate drug loadings (15-25%) as compared to the non-hydrogen bonding analogue ASDs. Surface crystallization led to the deterioration of dissolution performance. Lastly, Tg relative to experimental temperatures also appeared to play a role in the observed dissolution behavior as a function of drug loading. These findings shed light on potential mechanisms governing ASD dissolution performance and will aid in the development of optimized ASD formulations with enhanced dissolution performance.

Optimization, in-vitro Release and in-vivo Evaluation of Gliquidone Nanoparticles.

The present work embarks upon increasing the dissolution rate and the bioavailability of model anti-diabetic drug, gliquidone, a sulfonylurea class drug used for treating diabetes mellitus type 2. The gliquidone nanoparticles were prepared by using anti-solvent precipitation technique in which, gliquidone solution in acetone was added at a controlled rate to an aqueous solution containing polyvinylpyrrolidone K25 (PVP K25) as stabilizer. The effect of drug concentration (X1), polymer concentration (X2) and solvent to anti-solvent ratio (X3) on particle size and dissolution was studied using Box-Behnken design. The results revealed that by decreasing the drug concentration and by increasing the stabilizer concentration and solvent/anti-solvent ratio, reduction in the size of the particles was observed. The mentioned parameters were optimised and particle of size about 175 nm was achieved. The relative dissolution rate of prepared gliquidone nanoparticles in phosphate buffer pH 7.4 was ~ 4.7 times faster than original drug at t = 45 min. Further, the gliquidone nanoparticles were characterized by scanning electron microscope (SEM), Fourier transform-infrared spectroscopy (FTIR), differential scanning calorimetry (DSC) and X-ray powder diffraction (XRD). The particles revealed to be oval in shape with stabilizer molecules on surface and exhibited decreased crystalline nature when compared to free gliquidone. Finally, the in vivo studies using gliquidone nanoparticles revealed ~ 2.5-fold increase in Cmax when taken orally in the form of hard gelatin capsules in comparison to free gliquidone. Thus, overall investigation suggests that the developed strategy of gliquidone nanoparticles possess a keen potential for exhibiting anti-diabetic effect.

Effect of sintering temperature of potassium feldspar‐limestone/dolomite on composition and microstructure of silicate fertilizers

AbstractAcid soils call for a fertilizer providing base cations (Ca, Mg, K, et al.) and releasing nutrients with a gradual rate. The dissolution rate from soil minerals such as potassium feldspar (KAlSi3O8) is believed to be too slow to provide agronomic benefits. Using limestone (CaCO3) and/or dolomite (CaMg(CO3)2) as the additives, crystalline structure breakdown of potassium feldspar is achieved by high‐temperature treatment and a K‐containing multi‐nutrient silicate fertilizer is thus produced. Effect of sintering temperature on the available nutrient contents, mineral phases, and microstructure was investigated. The evolution of K‐containing minerals and their relative dissolution rate were discussed. The dissolution behaviors of other calcium/magnesium silicates were also analyzed. After sintered at 1220°C, the plant‐available Si, Ca, K, and Mg contents in the fertilizer were 15.1%, 21.6%, 5.4%, and 2.4%, with the proportion of 45% potassium feldspar, 33% limestone, and 22% dolomite. Kalsilite (KAlSiO4) was proved to be the most effective one among potassium feldspar (KAlSi3O8), leucite (KAlSi2O6), and kalsilite (KAlSiO4), which was totally soluble after 30 minutes extraction in 0.5 mol/L HCl acid. Pseudwollastonite (Ca3Si3O9) was totally soluble while gehlenite (Ca2Al2SiO7) and akermanite (Ca2MgSi2O7) could only be dissolved after extraction of several times. Chemical reaction processes between KAlSi3O8 and CaO/MgO were discussed, and dicalcium silicate (Ca2SiO4, C2S) should be avoided in the reaction system.

Enhanced chemical weathering of albite under seawater conditions and its potential effect on the Sr ocean budget

Abstract Currently, knowledge about the dissolution kinetics of silicate minerals under seawater conditions in general and about feldspar minerals specifically is lacking, even though feldspars are the most common minerals in the earth’s crust. Consequently, global geochemical cycles of elements and isotopes overlook the potential effects of silicate dissolution in seawater and its contribution to mass balances. The aim of the present study is to quantify the effect of salinity on albite dissolution kinetics under seawater conditions and the possible impact of salinity on Sr concentrations and isotopic compositions in seawater. A series of albite dissolution experiments with synthetic NaCl solutions (0.0001–1 molal) at pH 5 and 25 °C were conducted using multi-point batch experiments (MPBEs). Above the Na concentration threshold (0.01 molal), the albite dissolution rate was found to increase with the NaCl concentration and with the ionic strength. Based on the present study and previous experimental studies, a combined model that describes the effects of pH, Na concentration and ionic strength on the albite dissolution rate was developed. The proposed model suggests that albite dissolution under natural seawater conditions is mainly controlled by the effect of ionic strength. Considering that seawater solutions are under far-from-equilibrium conditions, the dissolution rate of albite is more than order of magnitude faster in seawater than in the continental, abiotic environment. Hence, the potential of silicate dissolution in seawater to contribute elements to the oceans is significant. In addition, a MPBE was conducted with a Bancroft, Ontario, Canada albite sample using synthetic seawater (SSW). Based on the changes in Si, Al, Sr, Ca and 87Sr/86Sr with time in this experiment, it is suggested that the relative dissolution rates of albite and apatite inclusions in the albite sample control the Sr concentrations and isotopic compositions. The estimated contribution of Sr release due to feldspar dissolution in seawater under far-from-equilibrium conditions is 2.67∙108 mol y−1. This result is significant if considering other forms of Al-Si mineral dissolution on the ocean floor.

The electrochemical behaviour of enargite in ammoniacal solutions. I. Anodic reactions

The leaching of enargite in acidic solutions is known to be particularly slow and, as in the case of chalcopyrite, the use of alternative lixiviants has been extensively studied. In particular, the use of ammoniacal solutions has been a focus due to the faster kinetics in such media. However, there have not been any fundamental studies of the electrochemistry of enargite in such solutions. A fundamental study has therefore been made of the electrochemical behaviour of enargite in both ammonium sulfate and ammonium chloride solutions at pH values from 8 to 10 and comparisons have been drawn with the behaviour of chalcopyrite under similar conditions. Mixed potential measurements have shown that copper(II) is an effective oxidant for enargite in ammoniacal solutions and the potential region in which enargite is oxidised have been defined. Voltammetry under various conditions has shown that partial passivation also occurs in such solutions and that the anodic behaviour in chloride solutions is similar to that in sulfate. Potentiostatic measurements at potentials in the region of the mixed potentials in copper(II) solutions have been used to derive pseudo-steady-state curves that reveal Tafel-like behaviour with slopes of approximately 120 mV/decade suggesting a one-electron rate determining step. A first-order dependence of the rate of anodic dissolution on the ammonia concentration has been derived from this data. Coulometric measurements suggest that thiosulfate is the main product of anodic oxidation under these conditions. The relative rates of dissolution of chalcopyrite and enargite in ammoniacal solutions have been estimated using mixed potential theory that show that enargite dissolves more rapidly than chalcopyrite in such systems.

Effect of particle deposition density of dry powders on the results produced by an in vitro test system simulating dissolution- and absorption rates in the lungs

The surface area of the air/liquid interface in the lungs is substantial, so deposited doses of aerosol medicines per interface surface area when administered via the inhalation route is always quite low. However, in most in vitro systems used for dissolution testing of dry powder inhalables, the dose per surface area is generally much higher.The aim of this study was to investigate in one in vitro lung dissolution system, the DissolvIt, the manner in which the deposited dose per test surface area of drug particles influences the simulated dissolution- and absorption rate.Here we used the dissolution test method DissolvIt to investigate the influence on dissolution behavior by varying the deposited surface density of tested drugs. Dry powders of three different active pharmaceutical ingredients with different solubilities were used; salmeterol, budesonide and fluticasone propionate.It was found that by varying the dose density from 0.23 to 29 µg/cm2 the dissolution- and absorption rate of test particles was affected for all three substances, with decreasing relative dissolution rates above certain dose limits. The effect was much more prominent with the least soluble fluticasone propionate. In contrast, in a real lung it has been shown that a tenfold increase of the even less soluble fluticasone furoate did not affect the pulmonary dissolution- and absorption as measured in the ex vivo isolated perfused rat lung.This indicates that the deposited particle dose on the test surface used must be carefully considered in all in vitro dissolution testing apparatuses used for inhalation drugs, especially when aiming for in vitro-in vivo correlations. Conclusive data show that in the DissolvIt system consistent normalized dissolution- and absorption data can be obtained if the deposition density of test substance are kept below 1 µg/cm2 and the variability between the initial drug doses is smaller than 10–15% expressed as standard deviation.

Congruent release of drug and polymer: A “sweet spot” in the dissolution of amorphous solid dispersions

Liquid-liquid phase separation (LLPS) occurs following amorphous solid dispersion (ASD) dissolution when the drug concentration exceeds the “amorphous solubility”, and is emerging as an important characteristic of formulations that may enhance the oral bioavailability of poorly soluble drugs. The purpose of this research was to identify criteria that impact the rate and extent of drug release and hence the occurrence or not of LLPS upon ASD dissolution. Specifically, the effect of drug log P, phase behavior of the hydrated but undissolved ASD matrix and the relative dissolution rates of drug and polymer were studied as a function of drug loading, using nilvadipine (Nil) (ClogP = 3.04) and cilnidipine (Cil) (ClogP = 5.54) as model drugs. The model polymer was poly (vinylpyrrolidone-co-vinyl acetate) (PVPVA). Nil-PVPVA and Cil-PVPVA ASDs with different drug loadings were prepared. Surface area normalized dissolution rates of both the drug and the polymer from ASD tablets were studied. At a similar and relatively low drug loading (<20% w/w drug), dissolution of both Nil-PVPVA and Cil-PVPVA ASDs was found to switch from rapid, congruent (i.e., simultaneous) release of drug and polymer to incongruent release with slow release of drug. Only ASDs showing congruent release underwent LLPS, with the formation of amorphous drug-rich aggregates (~300nm). Scanning electron microscopy (SEM) and micro-computed tomography (micro-CT) showed the presence of characteristic “pits” on the surface of partially dissolved, incongruently releasing ASD tablets. These most likely arise due to faster polymer release in comparison to drug, whereby the drug-rich composition around these pits was confirmed by energy-dispersive X-ray (EDX) analysis and the surface drug enrichment on the compacts was confirmed by X-ray photoelectron spectroscopy (XPS). This study demonstrates two important findings, firstly, a link between congruent release of drug and polymer and the occurrence of LLPS and secondly, the switch between congruent and incongruent release of drug and polymer is a result of competitive kinetics between phase separation and the release rate of ASD components with minimal influence from drug hydrophobicity for two structural analogues.

Study on Bayer digestion behaviour of low grade bauxite with high sulphur

Low grade bauxite with high sulphur is rich in nature but difficult to be applied industrially. Sulphur in bauxite leads to a series of remarkable negative effects in Bayer process. This work is focused on the dissolution behaviour of sulphur, iron and aluminium from a low grade bauxite. The process parameters such as holding time, alkali concentration and temperature were optimised. Scanning electron microscope, X-ray diffraction and X-ray fluorescence spectrometer were used to characterise the morphology, phase and composition of the bauxite and residue. The results indicate that the relative dissolution rate of aluminium can approach to 75%. The increase in iron dissolution rate enhances the sulphur dissolution rate, enlightening us that it is possible to remove sulphur and iron simultaneously by the seed precipitation for high sulphur bauxite. This research is a novel technical support and theory guidelines for utilising the low grade bauxite with high sulphur.

Study on Bayer digestion behaviour of low grade bauxite with high sulphur

Low grade bauxite with high sulphur is rich in nature but difficult to be applied industrially. Sulphur in bauxite leads to a series of remarkable negative effects in Bayer process. This work is focused on the dissolution behaviour of sulphur, iron and aluminium from a low grade bauxite. The process parameters such as holding time, alkali concentration and temperature were optimised. Scanning electron microscope, X-ray diffraction and X-ray fluorescence spectrometer were used to characterise the morphology, phase and composition of the bauxite and residue. The results indicate that the relative dissolution rate of aluminium can approach to 75%. The increase in iron dissolution rate enhances the sulphur dissolution rate, enlightening us that it is possible to remove sulphur and iron simultaneously by the seed precipitation for high sulphur bauxite. This research is a novel technical support and theory guidelines for utilising the low grade bauxite with high sulphur.

Consideration of enthalpic and entropic energy contributions to the relative rates of chalcopyrite dissolution in the presence of aqueous cationic impurities

To understand the effect of aqueous impurities on chalcopyrite dissolution during acid metalliferous drainage and hydrometallurgical processes, batch dissolution experiments were carried out at 650 and 750mV (SHE), pH1 and 35–75°C in the presence or absence of aqueous cationic additives. Activation energies (Ea) for chalcopyrite dissolution in the presence of additives at 750mV, derived using a modified ‘time to a given fraction’ method, demonstrate that Ea varies with reaction extent. The overall trend of evolution from interface- to transport-controlled mechanism was independent of additive type or addition. This suggests that it is not primarily variation in the enthalpy of dissolution that controls the significant changes in relative dissolution rates on addition of additives.Rather, the relative dissolution rates on addition of aqueous cations are found to relate to the M–O framework volume (M and O being the cationic additive and oxygen within H2O in the first sphere of hydration, respectively). There is good inverse linear correlation between this volume and the relative dissolution rate per unit ionic strength which varies as K+>Al3+>no additive>Mg2+>Na+>Ca2+. It is proposed that this effect is related to the relative strength of hydration of the additives with the commensurate entropic effect on solute hydration being advantageous for chalcopyrite dissolution on K+ and Al3+ addition and detrimental for the other additives. H4SiO4, the dominant aqueous species upon Si addition under the conditions examined, has low affinity for water, resulting in a detrimental entropic contribution to solute hydration and dissolution. The effect of Fe addition on relative dissolution rate is convoluted by the role of Fe3+ as the primary oxidant for chalcopyrite dissolution. On addition of aqueous Fe to dissolution at 750mV, the effect is detrimental due to the detrimental effect on the entropy of hydration of the solute and the relative abundance of Fe3+. At 650mV the effect of the greater presence of Fe3+ due to Fe addition dominates and the dissolution rate is enhanced.

Late Cenozoic sea level and the rise of modern rimmed atolls

Sea-level records from atolls, potentially spanning the Cenozoic, have been largely overlooked, in part because the processes that control atoll form (reef accretion, carbonate dissolution, sediment transport, vertical motion) are complex and, for many islands, unconstrained on million-year timescales. Here we combine existing observations of atoll morphology and corelog stratigraphy from Enewetak Atoll with a numerical model to (1) constrain the relative rates of subsidence, dissolution and sedimentation that have shaped modern Pacific atolls and (2) construct a record of sea level over the past 8.5millionyears. Both the stratigraphy from Enewetak Atoll (constrained by a subsidence rate of ~20m/Myr) and our numerical modeling results suggest that low sea levels (50–125m below present), and presumably bi-polar glaciations, occurred throughout much of the late Miocene, preceding the warmer climate of the Pliocene, when sea level was higher than present. Carbonate dissolution through the subsequent sea-level fall that accompanied the onset of large glacial cycles in the late Pliocene, along with rapid highstand constructional reef growth, likely drove development of the rimmed atoll morphology we see today.

RELATIVE DISSOLUTION RATES OF RADIOACTIVE MATERIALS USED AT AWE.

A simple in vitro dissolution test was used to provide a semi-quantitative comparison of the relative dissolution rates of samples of radioactive materials used at Atomic Weapons Establishment in a lung fluid surrogate (Ringer's solution). A wide range of dissolution rates were observed for aged legacy actinides, freshly produced actinide alloys and actinides from waste management operations.