Knona C. Liddell

Leaching of CuFeS2 by aqueous FeCl3, HCl, and NaCl: Effects of solution composition and limited oxidant

Batch leaching experiments were performed in which the initial amounts of chalcopyrite and ferric chloride were selected to ensure that the oxidant was significantly depleted over the course of an experiment. Solution samples were analyzed for Cu(II) and Fe(III) by visible spectrophotometry and for total copper and total iron by atomic absorption, making it possible to measure changes in the solution component concentrations as leaching progressed. For selected samples, the solution potential was also measured. In all experiments, the Cu(II) concentration passed through a maximum and, simultaneously, the Cu(I) concentration increased very sharply. An acceleration in the total rate of leaching was normally observed at the same time. Early in a leach, the solution potential was too high for the reduction of Cu(II) to Cu(I) to take place at the time of the increase in the overall leaching rate, however, the solution potential dropped sharply during a span of a few hours, reaching a value low enough that reduction of cupric ion became possible. The amount of Cu(I) present at the completion of a leach was dependent on the total chloride concentration of the system. The highest Cu(I)/Cu ratios were observed in systems with the highest chloride concentrations. The ultimate extent of CuFeS2 leaching was dependent on the initial FeCl3 and total chloride concentrations; the FeCl3 was virtually completely consumed and the total chloride concentration controlled the extent to which Cu(II) was reduced by reaction with chalcopyrite.

A mathematical model for calculation of equilibrium solution speciations for the FeCl3-FeCl2-CuCl2-CuCl-HCl-NaCl-H2O system at 25 ‡C

Equilibrium solution speciation computations were performed for the FeCl-FeCl3-CuCl2-CuCl-HCl-NaCl-H2O system at 25 ‡C. In dilute solutions, complexation of Fe(III), Fe(II), and Cu(II) is insignificant but the major Cu(I) species is CuCl2-. In concentrated solutions, FeCl30, FeCl20, and CuCl20 are the major Fe(III), Fe(II), and Cu(II) species, and CuCl32- is the most important cuprous complex. High Cu(I)/Cu(II) ratios are apparently more readily attainable in CuCl2 than in FeCl3 media. The Cu(I)/Cu(II) ratio is increased by making the solution more concentrated in any component except FeCl3 or CuCl2. Neither the ionic strength nor the total chloride concentration is a good predictor of the Cu(I)/Cu(II) ratio.

Plasma-enhanced metal-organic chemical vapor deposition (PEMOCVD) of catalytic coatings for fuel cell reformers

Fuel cells have the potential to solve several major challenges in the global energy economy: dependence on petroleum imports, degradation of air quality, and greenhouse gas emissions. Using catalyst-based reformer technology, hydrogen for fuel cells can be derived from infrastructure fuels such as gasoline, diesel, and natural gas. Platinum is one catalyst that is known to be very effective in hydrogen reformers. Reformer size can be reduced when there is more efficient catalyst loading onto the substrate. In this experimental work, platinum was loaded onto /spl gamma/-alumina coated substrates by plasma-polymerization followed by heat treatment. Vapor from a platinum-containing organic precursor was converted to plasma and deposited onto the substrate. The plasma-polymerized film was then calcined to drive off organic material, leaving behind a catalyst-loaded substrate. The plasma-polymerized organic film and the final heat-treated catalyst-loaded substrate surface were characterized by scanning electron microscopy (SEM) and impedance spectroscopy. Energy dispersive spectroscopy (EDS) was used to detect the presence of the catalyst on the substrate.

A Partial Equilibrium Chemical Model for the Dump Leaching of Chalcopyrite

The chemistry of the dump leaching of chalcopyrite has been studied by means of a chemically based model that makes it possible to calculate concentration changes for solution species during mineral dissolution. In a dump, chalcopyrite can dissolve in two ways: CuFeS2 + 4Fe3+ → Cu2+ + 5Fe2+ + 2S° CuFeS2 + O2 + 4H+ → Cu2+ + Fe2+ + 2S° + 2H2O CuFeS2 dissolution is not at equilibrium in a leach dump. However, there are a large number of homogeneous reactions taking place in the leach liquor. These may be treated as rapid equilibrium reactions in this “partial equilibrium” model. The model equations are linear and consist of an equation for each aqueous phase reaction, mass and charge balances, and a kinetic equation. Dissolution by O2 and acid should be prevented if possible. The acid consumed is partly replaced by shifts in the solution equilibria. However, the net pH increase that occurs leads to precipitation of ferric ion. The total amount of copper dissolved without precipitation is highest if high Fe(III) concentrations are used and oxygen is excluded. High H2SO4 concentrations are beneficial and high FeSO4 concentrations deleterious because of their influences on the precipitation equilibria.

A partial equilibrium model to characterize the precipitation of ferric ion during the leaching of chalcopyrite with ferric sulfate

A partial equilibrium model has been developed and used to characterize the conditions under which precipitation of ferric ion occurs during the dump leaching of chalcopyrite ores. The precipitates which have been considered include amorphous Fe(OH)3, α-FeOOH (goethite), and Na+, K+, Ag+, Pb2+, and H3O+ jarosites. Solution of the model equations makes possible the determination of the concentrations of the solution species during leaching of the mineral. The concentration product for Fe(OH)3 (am) and α-FeOOH was calculated for changing solution concentrations and compared with the solubility product constants to determine when precipitation would be expected thermodynamically. The K+, Na+, Ag+, and Pb2+ concentrations that would be necessary to satisfy the solubility product constants for the corresponding jarosites were calculated for various initial concentrations and varying amounts of O2 consumption.

Synthesis of Pt/ZrO/sub 2/ catalyst on Fecralloy substrates using composite plasma-polymerized films

In a hydrogen-based energy system, fuel cells will utilize hydrogen to produce electricity while reformers produce hydrogen from infrastructure fuels, such as gasoline, diesel and natural gas. Reformers based on microchannel technology require a catalyst dispersed throughout a porous support, and the support must adhere firmly to the substrate. In this work, catalyst and support precursors were deposited via plasma enhanced chemical vapor deposition onto Fecralloy substrates, in alternate layers of plasma-polymerized platinum acetylacetonate and zirconium acetylacetonate. Non-equilibrium, inductively-coupled plasma was generated by applying radio frequency fields to a precursor vapor plume emanating from a heated sublimator crucible. After calcining the composite organic film to volatilize organic constituents, catalytically active platinum agglomerates remained supported by a matrix of zirconia. Plasma-processing took place directly in precursor vapor without added carrier gas. The intermediate organic composite film and the final synthesized platinum-loaded support adhering to the Fecralloy have been evaluated with profilometry, scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction, and inductively coupled plasma-mass spectrometry. Cubic phase platinum and cubic phase zirconia have been detected on the Fecralloy. This material catalyzes conversion of carbon monoxide to carbon dioxide in a water gas shift (WGS) reactor in the temperature range 400/spl deg/C-500/spl deg/C.

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.

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.

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.

ADVANCES IN THE EXTRACTION, PROCESSING AND APPLICATION OF REFRACTORY METALS

ConclusionRefractory metals, with their unique properties and processing requirements, continue to challenge the metallurgical community as means are sought to improve their capabilities. As the articles gathered for this issue indicate, metallurgists and materials scientists are meeting that challenge and advancing the state of the art in the processing of high-temperature materials across a broad front. As our knowledge of the extraction, processing and application of refractory metals continues to advance, so too will our progress in propulsion, space exploration and transportation. Thus, the rewards for the refractory metallurgist are commensurate with the challenges that these unique materials present.