A.A. Wragg

The modelling of concentration—time relationships in recirculating electrochemical reactor systems

Abstract Approximate and rigorous models are presented and compared for the prediction of concentration—time and current—time relationships for electrochemical reactor systems operating with continuous recirculation of the electrolyte. The reservoir is considered as a well mixed tank while the reactor is considered both as a plug and as a perfectly back mixed system. Application to the depletion in concentration of Cu 2+ containing electrolytes using fluidized bed reactors is discussed.

Influence of electrolyte composition on current yield during ferrate(VI) production by anodic iron dissolution

The current efficiencies for ferrate(VI) formation under conditions of bubble induced convection with different anolyte compositions were compared. Results using 14 M KOH, 5 M NaOH, 5 M LiOH and a mixture of LiOH and NaOH of constant OH− concentration of 5 M at various temperatures and current densities were compared to previous data for 14 M NaOH solution. NaOH gave the best results under all conditions studied.

Electrochemical production of ferrate(vi) using sinusoidal alternating current superimposed on direct current. Pure iron electrode

The current yield for the anodic oxidation of a pure iron (99.95%) electrode to ferrate(VI) ions in 14 M NaOH between 30 and 60 °C using a sinusoidal alternating current (a.c.) at amplitudes in the range 38–88 mA cm−2 and frequencies in the range 0.5 mHz to 5 kHz superimposed on direct current (d.c.) of 16 mAcm−2 was measured under conditions of bubble induced convection in a batch cell. The current yield for ferrate(VI) synthesis exhibited a complex dependence on temperature and a.c. frequency, but generally a maximum was observed in a frequency range 2–50Hz depending on the a.c. amplitude. A global maximum current yield after 180 min of electrolysis of 33% was reached at the following conditions: a.c. amplitude of 88 mA cm−2, a.c. frequency of 50 Hz and temperature of 40 °C. At the optimum conditions the highest d.c. electrolysis yield was 23%. Thus, operation with the a.c. component leads to an increase in the yield by 43% with respect to d.c. electrolysis alone.

Ionic mass transfer by free convection with simultaneous heat transfer

Abstract Rates of electrochemical mass transfer by free convection under the influence of simultaneous thermal free convection have been measured for upward facing horizontal disc electrodes. The electrode reaction was the cathodic deposition of copper from cupric sulphate solutions containing H 2 SO 4 as swamping electrolyte. Results have been correlated by the equations. and where GR m is a combined Grashof number for concentration and thermal buoyancy effects. . Mass transfer has also been measured for the novel situation of nucleate boiling at the electrode surface and diffusion layer thicknesses as small as 1·7 × 10 −3 mm have been obtained.

Influence of anode material composition on the stability of electrochemically-prepared ferrate(VI) solutions

The stability of electrochemically-prepared ferrate(VI) solution in 14M NaOH solution was studied. White cast iron and pure iron were used as anode materials and the anodic current density during the ferrate(VI) preparation was varied in the range 4.4 to 42.8 mA/cm 2 . The solution temperature ranged from 20 to 50°C. The ferrate(VI) decomposition rate was found to depend strongly on solution temperature, anodic current density and also on the anode material composition. The decomposition rate was higher for white cast iron ie the anode material with greater iron carbide content.

Mass transfer and free convection associated with electrodeposition at long narrow upward facing tracks

Abstract This paper describes an investigation of free convection and mass transfer at narrow upfacing tracks 100 mm long, associated with copper deposition from a sulphate/sulphuric acid bath. The electrode width was varied from 0.8 to 7.5 mm, and the copper sulphate concentration from 0.01 to 0.28 M thus allowing a broad range of Rayleigh number to be investigated. The onset of free convection was identified by measurement of the transient current and use of an interferometric (schlieren) technique. The critical time corresponding to the onset of free convection was shown to depend on the technique used. After one or two minutes the current reached a steady value for high Ra and exhibited regular oscillations of appreciable amplitude for Ra below 106. Determination of mass transfer rates from the steady state currents gave a dependence of Sherwood number on Ra0.33 for Ra over 2×05, indicating turbulent natural convection. The low value for the transitional Ra is attributable to the peculiar aspect ratios of the electrodes.

Free convective mass transfer at up-pointing truncated cones

Free convective mass transfer at the individual surfaces and total surface of up-pointing truncated cones of different height and base diameter were experimentally studied using the limiting diffusion current technique. It was found that the mass transfer rate is highest for the upward-facing horizontal surface. The experimental mass transfer coefficient for the combination of downward-facing horizontal and conical surface is lower than that obtained from the equivalent summed separate surfaces due to the fact that the conical surface is exposed to solution which has already been depleted in cupric ions. The total mass transfer was successfully correlated using a single characteristic dimension (defined as a surface area divided by perimeter projected onto horizontal plane) which takes into account all the relevant dimensions of a truncated cone, i.e. the height and two diameters.

Electrochemical study of mass transfer in decaying annular swirl flow Part II: Correlation of mass transfer data

This paper presents correlations of local mass transfer at the inner rod and the outer wall in annular decaying swirl flow generated by axial vane swirl generators. Four swirl generators with vane angles in the range 15–60° to the duct axis were used and experiments were carried out in a Reynolds number range 3300–50000 and at a Schmidt number of 1650. The results were correlated in the general form Shx = 0.0204 Rex0.86 (1 + tan θi)0.53Sc1/3, for the inner rod, and Shx = 0.0224 Rex0.86 (1 + tan θo)0.55Sc1/3, for the outer pipe. Comparison is made with heat transfer data for work with a similar entry configuration.

Local mass-transfer distribution in the channels of a serpentine flow baffled parallel plate cell

Mass-transfer measurements were made in a parallel plate cell equipped with baffles which produced a three-channel serpentine flow pattern and three-dimensional mass-transfer distribution effects. The entry and exit configurations were in the form of slots of rectangular cross-section. Local mass-transfer coefficient values obtained in the different channels, using surface-flush microelectrodes, reflected the complex hydrodynamics associated with phenomena such as the cell inlet and exit effects, the flow reversal effects at the baffles and preferential flow phenomena in the channels. Mass-transfer measurements obtained by averaging point measurements over various zones of the cell compared favourably with those of other workers. A hydrodynamic model of flow in the cell corresponded well to the measured mass-transfer distribution.

Modelling of time-dependent performance criteria in a three dimensional cell system during batch recirculation copper recovery

A three-dimensional electrode cell with cross-flow of current and electrolyte is modelled for galvanostatic and pseudopotentiostatic operation. The model is based on the electrodeposition of copper from acidified copper sulphate solution onto copper particles, with an initial concentration ensuring a diffusion-controlled process and operating in a batch recycle mode. Plug flow through the cell and perfect mixing of the electrolyte in the reservoir are assumed. Based on the model, the behaviour of reacting ion concentration, current efficiency, cell voltage, specific energy consumption and process time on selected independent variables is analysed for both galvanostatic and pseudopotentiostatic modes of operation. From the results presented it is possible to identify the optimal values of parameters for copper electrowinning.