Renato G. Bautista

Leaching Kinetics of Platinum and Palladium from Spent Automotive Catalysts

Abstract Empirical rate expressions for both platinum and palladium were obtained from mass balances and concentration-time data in a packed bed reactor using different (HCl):(HNO3) leaching solution concentration ratios. The spent catalysts used in this study (-60+70 mesh to -120+140 mesh) were analyzed to be 3791 ppm platinum and 1306 ppm palladium. High initial rates for both platinum and palladium were obtained for the early stages of leaching and were followed by rapid decay up to about 110 minutes and 50 minutes respectively. Typical concentrations in the leached solution were about 40 ppm Pt and 20 ppm Pd after about 5 hours for the experimental conditions used. The extent of reaction for platinum was in the 90 percent range while that of the palladium was in the 70 percent range.

Extraction equilibria in the system GaCl3-AlCl3-HCl-H2O-tributyl phosphate

Partition equilibria and solution chemistry of the system GaCl3-AlCl3-HCl-H2O-tributyl phosphate is characterized by a simultaneous extraction of gallium chloride, hydrochloric acid, and water by a solvating mechanism. The salting-out power of the aqueous phase was explained by taking into account the stability of aluminum chloride and gallium chloride complex species and the calculated concentration of free chloride ions actually available for the extraction of gallium. Three gallium extraction mechanisms in the range of compositions studied are the extraction of GaCl3 at low H+ and salt concentrations, the extraction of GaCl-4 at moderate H+ and salt concentrations, and the competition between the extraction of GaCL-4 and an HC1-TBP complex at high H+ and salt concentrations. The extraction of gallium over a wide range of distribution ratios (D = 1 to 1 x 104) could be represented by a chemically based model taking into account the free chloride concentration and the activity coefficient of the total chloride ions.

Extraction of Cobalt from Hydrotreating Catalysts Using Supercritical Ammonia

Abstract Experiments were conducted to determine the feasibility of leaching cobalt from a hydrotreating catalyst material using supercritical aqueous ammonia solvents. The effects on cobalt extraction caused by variations in solvent composition, pressure and temperature, including subcritical conditions, were investigated. Cobalt in the catalyst material was leachable at supercritical and subcritical solvent phase conditions. Cobalt extraction at supercritical phase conditions was generally higher than extraction obtained at any of the other pressure - temperature conditions tested. Leaching enhancement at supercritical conditions was determined not to be solely the result of simple pressure or temperature effects. Rather, leaching enhancement is probably caused by the improved transport properties exhibited by supercritical fluid solvents.

Simulation ofin situ uraninite leaching-part III: The effects of solution concentration

The effects of variations in the concentrations of leaching reagents have been simulated forin situ leaching of UO2 by H2O-(NH4)2CO3-NH4HCO3. The model used in the simulations incorporates rate laws for the mineral reactions, equilibrium reactions among the solution species, and a mixing cell representation of solution flow. Of the component concentrations, the major factor affecting the rate of uraninite dissolution is the oxidant concentration. High peroxide concentrations lead to more rapid reaction with an early maximum in the U(VI) concentration. If lower oxidant concentrations are used, the reaction is under mixed kinetic and mass transfer control and the U(VI) concentration is lower but approximately constant for an extended period. Because they increase the concentration of the HCO3/- anion, high ammonium carbonate and ammonium bicarbonate concentrations also result in some enhancement in the rate of U leaching; the reaction is known to be half-order in both HCO3- and H2O2. A 10:1 ratio of (NH4)2CO3 to NH4HCO3 concentrations was found to result in a nearly constant pH during most of the leaching process. Calcite-containing gangue causes an immediate pH increase from about 8.9 to 9.4. The rate of the calcite reaction, calcite saturation index, and porosity are all independent of the lixiviant concentrations. Detailed calculations of solution speciation are necessary to predict the concentrations of individual species from those of components.

Chapter 139 Separation chemistry

Publisher Summary This chapter discusses separation chemistry. The separation chemistry and technology of rare earths have developed to a sufficiently high degree of sophistication that very high purity products can generally be produced when required. The chemistry of the rare earths is characterized by the similarity in the properties of the trivalent ions and their compounds. The chapter also explains solvent extraction. The conventional separation scheme is to leach the primary ore or concentrates and use the resulting solution containing the rare earth mixtures as the feedstock to the solvent extraction plant. Solvent extraction of the rare earth mixture in the leached solution separates them into bulk concentrates of light (La, Ce, Pr, Nd, etc.), middle (Sm, Eu, Gd, etc.) and heavy (Tb, Dy, Ho, Er, Tm, Yb, Lu, Y) rare earths. A typical solvent extraction of rare earths in a HC1 medium is with di-2-ethylhexyl phosphoric acid (HDEHP) in a kerosene diluent. This chapter discusses several of these extractants for their interesting chemistry and potential future development, in addition to the available industrial extractants and proposed for the separation of rare earths.

Simulation of in situ uraninite leaching. part I: A partial equilibrium model of the NH4HCO3-(NH4)2CO3-H2O2 leaching system

In situ leaching of uraninite and calcite by H2O2-NH4HCO2-(NH4)2CO3 solutions has been simulated using a partial equilibrium model which incorporates a one-parameter mixing cell model of solution flow. Rate laws for UO2 dissolution and for CaCO2 dissolution/precipitation were taken from the literature, as were equilibrium constants for solution phase reactions. Parameters of the model include the UO2 and CaCO3 ore grades, the concentrations of the H2O2, NH4HCO3, and (NH4)2CO3 components, porosity, exit solution flow rate, ore and mineral densities, and mineral rate constants and surface areas. Mineral conversions, component and species concentrations, and porosity are among the time-dependent quantities calculated using the model. For the conditions simulated, calcite dissolved somewhat faster than uraninite. The results emphasize the importance of the coupling between the mineral reactions and solution flow. Changes in the concentrations of the CO32- and HCO3- species are particularly complicated and not predictable from the calcite kinetics alone or from a purely equilibrium model; although the simulations did not reveal any conditions under which the solution would become saturated with CaCO3, the pH continued to change throughout the calcite dissolution and is buffered only after calcite has been consumed.

The Aggregation and Interactions of Tributyl Phosphate and Tricaprylmethylammonium Nitrate in Hexane by Osmometry

Abstract The dimerization constant of tributyl phosphate in hexane has been determined by vapor pressure osmometry and was found to be in agreement with the values in the literature obtained by infrared spectroscopy. The tributyl phosphate-tricapryl-methylammonium nitrate extractants strongly associate with each other. The addition of varying concentrations of tricaprylmethylammonium nitrate to a constant concentration of tributyl phosphate results in a linear relationship of the average degree of aggregation of the mixture with the initial concentration of tricaprylmethyl-ammonium nitrate. Varying the tributyl phosphate concentration results in a series of straight lines intersecting at one point. Extrapolation of the tributyl phosphate concentration to zero allows estimation of aggregation constants of tricaprylmethyl-ammonium nitrate in hexane by the linear regression technique.

The process synthesis of bulk superconductors

Using a variety of different processes, it is possible to produce rare earth-Ba/Sr-Cu-O type bulk superconductors with critical temperatures (Tc)in the 90K range, well above the liquid nitrogen temperature of 77K. The techniques that produce powder material superconductors need to have particle sizes in the submicrometer range in order to have nearly theoretical densities after compaction and shape forming. Major breakthroughs also are needed in the preparation of high-Tc and high critical current density material superconductors in order for the applications to be viable.

Mass Transfer Model of Chromium Reduction in a Fluidized Bed Electrochemical Reactor

A batch recycling fluidized bed electrochemical reactor system was used to recover Cr(VI) from a very dilute solution containing 2.5 g Cr/L at pH 2. It was found that the electrowinning rate is controlled by mass transfer and that the reduction process of Cr(VI) is interrupted by the formation of Cr(H 2 O) 6 3+ . The Cr 2 O 7 2- and Cr(H 2 O) 6 3+ concentration-time relationship can be predicted by a plug flow model. A finite difference method and a complex optimization technique were used to estimate the mass transfer coefficient. At Re p between 10 and 30, the Cr 2 O 7 2- and Cr(H 2 O) 6 3+ mass transfer coefficients lie between 1.52-3.20x10 -6 and 1.17-2.50x10 -6 cm/s, respectively, and increase with the Reynolds number based on the particulate cathode.

Simulation ofin situ uraninite leaching-part II: The effects of ore grade and deposit porosity

A combined partial equilibrium-mixing cell model has been used to investigate the effects of fluid flow, mineral content, porosity, and lixiviant concentrations onin situ leaching of uraninite. The model couples the rate processes of reactive transport (uraninite and calcite dissolution kinetics and leach solution flow) with solution phase equilibria (acid-base and complexation equilibria). Solution circulation and porosity changes have been explicitly treated in the following way: reacted solution was assumed to be pumped from the system at a constant rate and replaced by fresh lixiviant; the additional void volume resulting from CaCO3 or UO2 dissolution was immediately filled with lixiviant. A solution volume of 1 cm3 was taken for the base, and it was assumed that on each 1200-second increment, loaded solution was removed at the rate of 1.67 × 10-5 cm s-1, equivalent to removal of 2.0 pct of the base volume. The lixiviant considered was NH4HCO3 (NH4)2CO3-H2O2 with reference case concentrations of 1.0 × 10-4, 1.0 × 10-4, and 2.2 × 10-5 mol cm-1. The parameters that were varied in this investigation were the mass fractions of UO2 (0.000 to 0.015) and CaCO3 (0.00 to 0.40) and the initial porosity of the deposit (0.20 and 0.30). Major factors found to affect the uranium content of the solution were UO2 content and initial porosity. Higher UO2 grades were associated with higher U(VI) concentrations, and these were maintained for much longer periods; the consumption of the peroxide oxidant was under mass transfer control. As the leaching reaction slowed, solution replacement began to control the component concentrations, causing decreasing U(VI) concentrations. Higher porosity caused reduced maximum U concentrations and a faster decline. The calcite content had a slight effect on the rate of U leaching; this occurred because high CaCO3 mass fractions led to increased HCO3- concentrations. Early in the leaching process, a lower initial porosity or a higher calcite content led to a higher (less negative) value of the CaCO3 saturation index; however, for the conditions simulated, the solution did not actually become saturated. Also, decreases in the saturation index occurred sooner for higher initial porosities or lower calcite grades. The final porosity was effectively determined by the initial calcite content; dissolution of calcite continued until it had completely reacted, and the uraninite content was too low for it to contribute significantly. Changes in concentrations of the various solution species occurred more rapidly if the ore was more porous, but there were no other significant differences attributable to initial porosity. The H+ concentration was virtually constant throughout leaching if the ore did not contain any calcite; with high calcite contents (40 pct), it remained constant for an extended period following an initial sharp decrease. Changes in the OH-, NH4/+ and NH3 concentrations could be readily predicted from those of H+, and changes in the Ca species concentrations were closely related to those of the Ca and CO3 components. Total U and total H2O2 concentrations behaved oppositely (as required by the reaction stoichiometry), but changes in the concentrations of the minor U(VI) and peroxo species were more complicated. The concentrations of the CO32- and HCO3- species could not readily be predicted from the reaction kinetics, and variations in their concentrations did not reliably indicate pH.