Robert W. Bartlett

Dissolution kinetics of magnesium silicates

Abstract Kinetic dissolution experiments on serpentine, forsterite and enstatite over a wide pH range at 25°C show that there is an initial rapid exchange of surface magnesium ions with hydrogen ions followed by a longer period of hydrogen exchange and extraction of internal magnesium and silicon, with the amount extracted proportional to t 1 2 . The parabolic exchange kinetics are consistent with either of two rate controlling mechanisms: nonsteady state diffusion of ions within the mineral and quasi-steady state diffusion of ions through a leached shell surrounding the mineral. Diffusion coefficients for magnesium are greater than for silicon for each of the minerals, leading to incongruent dissolution over moderate time periods. The diffusion coefficients decrease in the order forsterite > serpentine > enstatite. Eventually the parabolic exchange rates decrease to the rate of dissolution of all material at the aqueous interface. Hence, over very long periods the amount of silicon and magnesium dissolved is proportional to t and dissolution is congruent. In highly acid solutions dissolution rates are fast and this terminal condition is reached much sooner.

High temperature transport processes in lithium niobate

Abstract The electrical conductivity of single crystal lithium niobate (LiNbO 3 ) was determined as a function of temperature for various oxygen partial pressures. The electrical conductivity is proportional to P o 2 − 1 4 which can be explained by a defect equilibrium involving singly ionized oxygen vacancies and electrons. Measurements of electrical transport numbers at 1000°K show the electrical conductivity of LiNbO 3 to be ionic at one atmosphere of oxygen and electronic at low oxygen partial pressures. Thermoelectric measurements indicate that LiNbO 3 at low oxygen partial pressures is n -type and that the concentration of electrons at 1000°K and in an atmosphere of 50% C0/50% CO 2 a is 4 × 10 17 cm 3 with a mobility of 1.7 cm 2 V sec. The diffusion of oxygen in LiNbO 3 was determined as a function of temperature at an oxygen partial pressure of 70 Torr. by measuring O 18 /O 16 isotope exchange with the gas phase as a function of time. The diffusion data may be represented by D = 3.03 × 10 −6 exp ( −29.4 kcal mole −1 RT )cm 2 sec . Consideration of the Nernst-Einstein relation for oxygen and the variation in conductivity with Li 2 O activity indicate that the ionic conduction is caused by transport of lithium ions.

Diffusivity and solubility of oxygen in platinum and Pt-Ni alloys

The diffusivity and solubility of oxygen in metal specimens were determined from measurements of permeation through thin wall tubes containing oxygen and heated by electrical resistance. The permeating oxygen desorbed in vacuum as monoatomic oxygen and the flux was monitored mass-spectrometrically. A known helium leak rate and experimentally-determined sensitivities of the two gases were used for flow calibration. The diffusion coefficient of oxygen in pure platinum, calculated from the time lag to reach a steady state flux, is: {fx065-01} in the temperature range of 1435∮ to 1504°. The solubility of oxygen in pure platinum was obtained from the steady-state flux using the previously determined diffusivity. The solubility is proportional to p1/2O2 and at 1 atm of oxygen the solubility is: CsO = (0.2 ± 0.1) × 1012 exp {fx065-02}, wt pct. Small amounts of nickel, less than required for internal oxidation, had a negligible effect on the oxygen solubility and diffusivity in platinum alloys.

Microstructural changes in SmCo5 caused by oxygen, sinter-annealing and thermal aging

Abstract The solubility of oxygen in SmCo 5 at 1125 °C (sintering temperature) in excess of the 800 °C solubility was determined to be 3500 ± 500 ppm. The source of oxygen during sintering is the samarium oxide subscale. Submicron oxide particles precipitate within SmCo 5 grains on cooling from the sintering temperature. Oxidation also causes depletion of samarium and precipitation of Sm 2 Co 17 particles within SmCo 5 grains. Both inclusions may be sources of domain wall nucleation. The oxide inclusions can be removed by a thermal aging treatment that collects the oxide into a few large grains outside the SmCo 5 grains by a solution/reprecipitation mechanism involving grain boundary transport of samarium. The Sm 2 Co 17 inclusions are not affected by thermal aging, but they can be prevented from occurring by including excess samarium in the sintered compact to replenish samarium lost by oxidation.

Conversion and extraction efficiencies for ground particles in heterogeneous process reactors

Chemical conversion of particles and extraction from particles are encountered in many metallurgical processes. Equations characterizing particle size distributions resulting from grinding were coupled with the rate equations for quasi-spherical particles under several different commonly encountered rate controlling conditions to calculate conversion/extraction efficiencies for particulate feeds in batch, plug flow, and backmix flow reactors. Families of curves were computed expressing the fraction of feed unreacted as a function of a normalized average residence time parameter. Using these curves, the required sizes of process reactors to yield a selected conversion/extraction efficiency can be determined from limited laboratory kinetic data.

Micrograin models of reacting porous solids with approximations to logarithmic solid conversion

Abstract Several experimental studies of gaseous reactions with porous solids at slow reaction rates, where pore diffusion is not rate controlling, have shown that the heterogeneous reaction rate decreases following an apparent logarithmic law. This is superficially analogous with a first order homogeneous batch reaction. In the present study a micrograin model for a porous reacting pellet is proposed and several micrograin size distribution are examined. It is shown that dimensionless rates calculated for the expected log-normal distribution of micrograins, within a wide range of size distribution standard deviations, approximates the logarithmic law within the usual experimental error. Normal distributions and Gates-Gaudin-Schuhmann distributions for the aggregation of micrograins constituting the porous solid pellet are also considered.

Kinetics of sulfation of atlantic ocean manganese nodules

The sulfation kinetics of manganese nodules from the Blake Plateau of the Atlantic Ocean were determined using a TGA method in low concentrations of SO2, typical of power plant stack gases. Above 400°C the rate is proportional to the SO2 pressure and the unreacted solid fraction. The rate is independent of nodule particle size and the apparent activation energy is low. The sorption capacity is complete sulfation of the major constituent oxides; Mn, Fe, and Ca. Leaching completely sulfated nodules in boiling water provides Ni, Cu, and Co extractions above 80 pct. Much of the manganese but little of the iron is also extracted. At 300°C the SO2 sorption capacity is lower with most of the sulfation attributed to the manganese oxides.

Microstructure and growth kinetics of the fibrous composite subscale formed by internal oxidation of SmCo5

The oxidation of SmCo5 is an unusual example of selective internal oxidation in which the subscale formed consists of a composite microstructure containing samarium oxide fibers and β-cobalt. The oxide fiber size increases with oxidation temperature, and the orientation of the fibers is generally perpendicular to the subscale-alloy interface. The unusual structure results because of the high concentration of samarium in the intermetallic compound, coupled with a low solubility of samarium in metallic cobalt resulting in a subscale consisting of 39 vol pct oxide. Growth of the subscale follows the parabolic oxidation law, and the kinetics have been determined between 100 and 1125°C. The kinetics are too fast to be explained by lattice diffusion in either the oxide or the cobalt phases. Oxygen diffusion down the cobalt-oxide fiber interface appears to be the transport mechanism for this diffusion controlled process. The oxidation behavior of PrCo5 is identical with that of SmCo5.

Pore diffusion-limited metallurgical extraction from ground ore particles

An equation describing the extraction from quasispherical ore particles into a fluid phase when diffusion within the particle is rate limiting is coupled with the particle size distribution resulting from comminution to compute extraction efficiency curves. The fraction extracted is given as a function of a dimensionless time involving the effective diffusion coefficient and the largest particle radius in the comminuted feed. Extraction efficiency curves were computed for batch, plug flow, and single and multistage back-mix flow (continuous stirred tank) reactors. For the appropriate hydrometallurgical and vapometallurgical extraction processes these curves can be used by designers to size process reactors to yield the desired extraction efficiency.

Transport and cathode deposition of reducible impurities during electrorefining with nonpolarizable electrodes

A general equation for cathode impurity concentration is developed for steady-state electrorefining with an impurity that is more readily reduced than the primary metal. The results depend on the equilibrium cathode/electrolyte distribution coefficient and the transport properties of the electrolyte. Applicability is restricted to situations where electrode kinetics are not rate limiting, such as most fused salt systems. Experimental results for copper impurity in a Sn/fused SnCl2/Sn system are compared with calculated cathode impurities as a function of cell voltage.