RESUMO

A novel technique has been devised for the synthesis of microporous α-Cr(2)O(3) (eskolaite). The technique was based on the formation of amorphous-Cr(2)O(3) onto microporous activated carbon through adsorption-reduction of dichromate ions (Cr(2)O(7)(2-)) at the activated carbon/aqueous solution interface. Then, the Cr(2)O(3)-loaded carbon was thermally processed under oxidizing conditions to remove the carbon and recover the chromium oxide as α-Cr(2)O(3). Both the Cr(2)O(3)-loaded carbon and the synthetic product were characterized by XRD, SEM, surface area and pore volume measurements. The synthetic eskolaite assayed 97.3% Cr(2)O(3) and its specific surface area was 15.48 m(2)/g and pore size of 16.1 Å.

RESUMO

The effect of temperature and pH on the zeta potential of alpha-Al2O3 and adsorption of fluoride ions at the alpha-Al2O3/aqueous solution interface has been investigated through electrophoretic mobility measurements and adsorption studies, to delineate mechanisms involved in the removal of fluoride ions from water using alumina as adsorbent. When the temperature increases from 10 to 40 degrees C, the pH of the point of zero charge (pH(pzc)) shifts to smaller values, indicating proton desorption from the alumina surface. The pH(pzc) increases linearly with 1/T, which allowed estimation of the standard enthalpy change for the surface-deprotonation process. Fluoride ion adsorption follows a Langmuir-type adsorption isotherm and is affected by the electric charge at the alpha-Al2O3/aqueous solution interface and the surface density of hydroxyl groups. Such adsorption occurs through an exchange between fluoride ions and surface-hydroxyl groups and it depends on temperature, pH, and initial fluoride ion concentration. At 25 and 40 degrees C, maximum fluoride adsorption density takes place between pH 5 and 6. Increasing the temperature from 25 to 40 degrees C lowers the adsorption density of fluoride.

RESUMO

Arsenic removal from high-arsenic water in a mine drainage system has been studied through an enhanced coagulation process with ferric ions and coarse calcite (38-74 microm) in this work. The experimental results have shown that arsenic-borne coagulates produced by coagulation with ferric ions alone were very fine, so micro-filtration (membrane as filter medium) was needed to remove the coagulates from water. In the presence of coarse calcite, small arsenic-borne coagulates coated on coarse calcite surfaces, leading the settling rate of the coagulates to considerably increase. The enhanced coagulation followed by conventional filtration (filter paper as filter medium) achieved a very high arsenic removal (over 99%) from high-arsenic water (5mg/l arsenic concentration), producing a cleaned water with the residual arsenic concentration of 13 microg/l. It has been found that the mechanism by which coarse calcite enhanced the coagulation of high-arsenic water might be due to attractive electrical double layer interaction between small arsenic-borne coagulates and calcite particles, which leads to non-existence of a potential energy barrier between the heterogeneous particles.

Assuntos

Arsênio/isolamento & purificação , Poluentes da Água/isolamento & purificação , Purificação da Água/métodos , Arsênio/química , Carbonato de Cálcio/química , Filtração , Ferro/química , Mineração

RESUMO

Studies on the Merrill-Crowe process as applied to silver recovery have shown that one half of the used zinc powder is wasted in water reduction at high cyanide concentrations, while the other half reduces silver ions from the cyanide solution. However, the cementation mechanisms as an electrochemical process taking place on the zinc surface do not explain the split of the electric current resulting from the anodic dissolution of zinc into two equal values. This study demonstrates that the mechanism for silver precipitation at high and low cyanide concentrations differs considerably. At low cyanide concentrations cementation is essentially an electrochemically-controlled process following a shrinking-core behavior. At high cyanide concentrations, the process seems not to be electrochemically controlled. The areas for zinc dissolution and silver deposition are not connected by an electrical-conducting medium and reduction of silver-cyano complex ions takes place by hydrogen adsorbed onto silver growing outward from the cementing zinc particles. The results are based on scanning electron microscopy of solids recovered from cementations in stirred reactors and in situ observations by optical microscopy of the cementation process on the edge of thin zinc disks in cyanide solutions.

Assuntos

Biofísica/métodos , Zinco/química , Adsorção , Corantes , Cobre/química , Cianetos/química , Relação Dose-Resposta a Droga , Condutividade Elétrica , Impedância Elétrica , Eletroquímica , Hidrogênio , Íons , Modelos Químicos , Prata/química , Compostos de Zinco

RESUMO

The most important factor in the electrodialysis (ED) process is the permselectivity of the ion exchange membranes, which permit not only the separation of cations and anions in a solution, but also the separation of ions with the same sign but different valences. In this work, the mechanism of the permselectivity has been studied through the measurement of the potentials at different planes of the membrane. The experimental results have shown that there was a secondary potential inside ion exchange membranes in an electrodialysis process. At the membrane side touched with dilute solution, this secondary potential enhanced the external electrical field, and thus speeded up the passage of the corresponding ions in the dilute solution through the membranes; at the membrane side touched with concentrated solution, the secondary potential was contrary to the external electrical field and thus counteracted it, which could be very helpful by preventing the ions in the concentrated solution from entering the membranes. Obviously, the existence of the secondary potential might play an important role in the permselectivity of ion exchange membranes in ED processes.

RESUMO

The adsorption of ethyl and amyl xanthate ions on galena and sphalerite fines has been studied using electrophoretic light-scattering (ELS) measurements. It was performed on galena and sphalerite (<2&mgr;m) in aqueous solution at different potassium ethyl xanthate (PEX) and potassium amyl xanthate (PAX) concentrations. It has been observed that the presence of PEX or PAX caused the isoelectric points (IEP) of galena and sphalerite fines to shift and the electrophoretic mobility to reverse in sign, indicating that the xanthate ions chemisorbed on galena and sphalerite surfaces. This adsorption markedly broadened the electrophoretic mobility distribution of the mineral fines, suggesting that the populations of the particles have quite different adsorption densities of xanthate ions, and therefore the particle hydrophobicity was different. This phenomenon might be attributable to the effect of the hemimicelle adsorption of the xanthate ions on the minerals, the nonuniform distribution of active sites and their degree of activity, the effect of particle size and shape, etc. The nonuniform adsorption has been found to increase with increasing PEX or PAX concentration, reaching a maximum at a medium concentration followed by a decline. Also, experimental results have demonstrated that the nonuniform adsorption of the xanthate ions is much stronger on sphalerite than on galena, which may explain why sphalerite has a worse flotation response than galena when alkyl xanthates are used as collectors in flotation systems. Copyright 2001 Academic Press.

RESUMO

Computations based on the extended DLVO theory are carried out on the potential energies of interactions between air bubbles and talc particles covered by nonpolar oil. It is shown that the major role of nonpolar oil in this system is to greatly increase the depth of the primary energy valley, giving rise to a much stronger bubble-particle aggregate that can support greater aggregate-rupture force fields from turbulent flows. Also, due to nonpolar oil involvement, the energy barrier between bubbles and mineral particles sharply collapses down and further separates, indicative of a greater probability of attachment of mineral particles to air bubbles. A linear relationship is found between the primary energy valley and the contact angles of oil or bubbles, and thus a simple and approximate formula is presented to evaluate the depth of the primary energy valley. In addition, it is found that the primary energy valley and the energy barrier are directly proportional to the effective particle radius, but the barrier location is independent of the effective particle radius. Copyright 1999 Academic Press.