Alkali and earth alkaline metal contents in the aluminium cathode in aluminium electrolysis

The contents of sodium, lithium, calcium and magnesium in aluminium in contact with NaF–AlF₃-based melts in laboratory and in industrial aluminium cells were investigated in the temperature range 950–1030 C. The experimental data were compared with a thermodynamic model. It was found that the addition of alumina or CaF₂ to the NaF–AlF₃ melts has only a minor effect on the equilibrium content of sodium in aluminium. Cathodic polarization enhances the content of sodium in aluminium. However, polarization has a smaller effect on the concentrations of lithium, calcium and magnesium in aluminium in industrial cells.     

Oxidative coupling of methane to C2-hydrocarbons over La-promoted CaO catalysts

La₂O₃ promoted CaO [La/Ca (mol/mol) = 0.05] catalyst shows very high activity and selectivity (methane conversion: 25%, C₂-selectivity: 66% and C₂-space-time-yield: 864 mmol ·g^–1 (cat.)·h^–1) with no catalyst deactivation in oxidative coupling of methane to C₂-hydrocarbons at 800 ° C.     

Properties of RO-Y2O3-Al2O3-SiO2 glasses

The coefficient of linear expansion, glass-transition temperature, temperature at the orset of deformation (strain point), density. Young modulus, microhardness, crystallizability, and contact angle are studied as a function of the composition in RO–Y₂O₃–Al₂O₃–SiO₂ (R=Ca and/or Mg) glass systems. The composition ranges for glasses (with strain point >900°C and coefficient of linear expansion of (32–45)×10^–7°C^–1) that can be used for soldering silicon-nitride ceramics were established.Translated from Steklo i Keramika, No. 12, pp. 5–7, December, 1996     

Effect on discharge of the heat treatment of graphite fluoride under a hydrogen atmosphere

Thermal decomposition of two types of graphite fluorides (CF) _n and (CF) _n , has been carried out in a hydrogen atmosphere at several temperatures between 100 and 500°C, with the object of improving the initial discharge behaviour of the Li/graphite flouride cell. The main reaction was the C-F bond rupture to form graphite-like carbon around the particle surface. The drop in cell voltage at the beginning of discharge could be minimized, and the polarization during discharge reduced by heat treatment under a hydrogen atmosphere. (CF) _n , heat treated at 400°C for 1 h, yielded a discharge capacity of 730–800 mA h per g of active material, corresponding to the discharge efficiency of 8390% at 25°C, and (C₂F) _n , heat treated at 350°C, for 10h, gave 670 mA h g^–1, corresponding to 91 % at 25°C.     

Alumina supported Pt(1%)/Ce0.6Zr0.4O2 monolith: Remarkable stabilization of ceria–zirconia solution towards CeAlO3 formation operated by Pt under redox conditions

A structured Pt(1 wt%)/ceria–zirconia/alumina catalyst and the metal-free ceria–zirconia/alumina were prepared, by dip-coating, over a cordierite monolithic support. XRD analyses and Rietveld refinements of the structural data demonstrate that in the Pt supported catalysts ceria–zirconia is present as a Ce_0.6Zr_0.4O₂ homogeneous solid solution and that the deposition over the cordierite doesn’t produce any structural modification. Moreover no Pt sintering occurs.By comparing the XRD patterns recorded on Pt/ceria–zirconia/alumina and ceria–zirconia/alumina after three redox cycles, it results that Pt, favouring the structural reorganization of the ceria–zirconia into one cubic solid solution, prevents any CeAlO₃ formation. On the contrary, such phase due to the interaction between Ce³⁺ and the alumina present in the washcoat is detected when redox cycles are carried out on the ceria–zirconia metal free.Transmission electron microscopy (TEM) investigations of the redox cycled Pt/ceria–zirconia/alumina catalyst detected ceria–zirconia grains with diameter between 10 and 35 nm along with highly dispersed Pt particles (2–3 nm) strongly interacting with ceria.Scanning electron microscopy (SEM) and EDX analyses, recorded on the redox cycled Pt/ceria–zirconia/alumina washcoated monolith evidence a homogeneous distribution of the active components through the channels even after redox aging.Reduction behaviour and CO oxidation activity are in good agreement with the structural modification of the solid solution induced by the redox cycles and reflect the positive effect of Pt/ceria interaction on the catalytic performances.The effect of redox aging on the NO reduction by C₃H₆, in lean conditions, was investigated over the Pt/ceria–zirconia/alumina monolith. The catalyst shows at low temperature (290 °C) good NO removal activity and appreciable selectivity to N₂.     

Selective Decomposition of NO in the Presence of Excess O2 in Electrochemical Cells

NO decomposition in solid electrolyte cells was investigated in the presence of excess O₂. The results show that NO is decomposed via an electrocatalytic mechanism rather than electrolysis in the range of 1–4 V of applied voltage. The NO is catalytically decomposed to N₂ on the cathode surface and O^2– produced in situ is transferred through the yttria-stabilized zirconia (YSZ) to the anode by direct current (d.c.) and then is evolved in the form of O₂, which helps to maintain the active state of the cathode. In a Pd/YSZ/Pd cell, the palladium metal surface is the active site for NO decomposition, while in the RuO₂/Pd/YSZ/Pd cell, the partially reduced RuO _x (0 < x < 2) is the main active site for NO decomposition. At 600 °C, the rate-determining step for the overall transportation of O^2– from cathode to anode in the RuO₂/Pd/YSZ/Pd cell is the transportation of O^2– at the cathode Pd/YSZ interface. The transportation rate of O^2– at the cathode M/YSZ interface decreases in the order of Ag > Au > Pd > Pt. Substitution of the Pd cathode by Ag leads to an increase in current density by a factor of 3.5. A higher NO decomposition parameter (=13.4) is also achieved at a lower temperature of 500 °C.     

Catalytic activity of (CaO)1−x(ZnO)x solids for the N2O decomposition: relation with photoluminescence

(CaO)_1–x (ZnO) _x mixed oxides (x=0–1), heated at 1423 K under atmospheric conditions, were checked for their catalytic activity in the N₂O decomposition in the temperature range of 450–650°C. Although the catalytic activity was measured in the dark, it was found to be linearly related with the photoluminescence intensity of the catalysts.     

Studies on the redox behaviour of La1.867Th0.100CuO4 and its catalytic performance for NO decomposition

La_1.867Th_0.100CuO₄ was prepared by means of the citric acid complexing method. The reduction–oxidation (redox) properties of this composite oxide have been investigated by using the XRD, TGA, EPR, TPD, and SEM methods. The fresh (non-reduced) La_1.867Th_0.100CuO₄ catalyst is single phase with tetragonal K₂NiF₄-type structure. There were three reduction steps observed over La_1.867Th_0.100CuO₄ in the temperature ranges of 25–100, 100–300, and 300–500 °C, respectively. After reduction at 300 °C, the material still retained its original single phase but there were oxygen vacancies generated in the lattice. After reduction at 500 °C, it decomposed to a mixture of oxides. In the course of reduction, trapped electrons were generated. During the oxidation of the reduced sample, O ₂ ^– was detected. Apparently, oxygen vacancies are able to stabilise O ₂ ^– on the surface of the -1ptcatalyst. NO adsorption on both the fresh and reduced La_1.867Th_0.100CuO₄ samples generated NO radicals and O ₂ ^– species. On a La_1.867Th_0.100CuO₄ sample reduced at 300 °C, [O₂NO₂]^2– was generated in NO adsorption and decomposed to N₂ and O^2– at ca. 730 °C. After reduction, the O ₂ ^– inside the La_1.867Th_0.100CuO₄ lattice became more mobile and participated in the decomposition of [O₂NO₂]^2–. The fresh (non-reduced) La_1.867Th_0.100CuO₄ sample with cation defects in its lattice shows higher NO decomposition activity than the fresh La₂CuO₄ sample in which there are no cation defects. The 300 °C-reduced La_1.867Th_0.100CuO₄ with cation defects and oxygen vacancies is more active than the fresh one for NO decomposition. The redox action between Cu⁺ and Cu²⁺ is an essential process for NO decomposition.     

Influence of CaF2 and AlF3 on the kinetics and mechanism of the Al electrode reaction in cryolite melts with various alumina contents

Electrochemical techniques were used to study the kinetics and mechanism of the aluminium electrode reaction in two cryolite-based melts containing cryolite with either 11 wt % AlF₃ or 5 wt % CaF₂ additions and variable alumina contents at 1000 °C. A three step electrode process was observed in both melts, comprising a preceding chemical reaction followed by two charge transfer steps. The exchange current density of the cathodic reaction was found to be dependent on the concentration of aluminium fluoride. By a combination of electrochemical impedance spectroscopy (EIS) and galvanostatic relaxation methods (GRM), the exchange current density of the first (slower) charge transfer step, the Warburg diffusion impedance, the double layer capacitance of the aluminium electrode and the rate of the preceding chemical step, were evaluated in the range of 2–8 wt % alumina. The role of the two additives, AlF₃ and CaF₂, was evaluated.     

Mechanical Activation of Perovskite-Type Oxides for Catalytic Combustion

The effect of grinding on the catalytic properties of La_0.8Sr_0.2MnO_3± powders prepared via a ceramic or sol–gel process was studied with respect to the carbon black combustion temperature (T ^C). For the ceramic process, a milling time of 5 h led to a decrease of 77°C in the T ^C (from 615 to 538°C) in relation with increasing BET surface area. Regarding the sol–gel process, the T ^C decreased from 535 to 505°C after 1 h of grinding. Nevertheless, upon further milling (10 h), the benefit of the grinding effect disappeared due to a continuous decrease in the BET surface area. Finally, grinding (when well controlled) enabled one to obtain ceramic powders showing performances as good as sol–gel materials (T ^C540°C).     

Interaction of methane with surface of alumina studied by FT-IR spectroscopy

Methane adsorption on alumina was investigated by FT-IR spectroscopy at 173 K. Adsorbed methane gives four distinct IR bands at 3008, 3000, 2900 and 1305 cm^–1 which are attributed to v₁ (2900 cm^–1), v₃ (3008, 3000 cm^–1), and v₄ (1305 cm^–1) modes of methane respectively. The appearance of the v₁ mode indicates that the T_d symmetry of methane is distorted by the adsorption. The intensities of these bands increase significantly with outgassing temperatures of alumina, reach their maxima at an outgassing temperature near 773 K, and then decrease with further higher outgassing temperatures. Two hydroxyls with IR bands at 3750 and 3665 cm^–1 are perturbed evidently by the adsorbed CH₄ thereby resulting in two redshifted bands at 3707 and 3640 cm^–1. Coadsorbed CO slightly affects the adsorbed CH₄ indicating the very weak interaction between CH₄ and surface cations of alumina. It is proposed that the adsorbed CH₄ on alumina is formed mainly via the interaction of CH₄ with both surface hydroxyl and c.u.s. oxygen anion.     

Electrolytic process optimization by simultaneous-modular flowsheeting

A procedure is described for computer-assisted optimization of an electrolytic process flowsheet. Material, energy, and economic balances for all process units were incorporated in a nonlinear optimization routine for predicting the minimum selling price based on a discounted cash flow rate of return on investment. The optimization utilized a simultaneous-modular approach which was incorporated into the public version of the Aspen flowsheeting package, and used an infeasible path convergence method based on successive quadratic programming procedures. Electrolyte vapour-liquid equilibrium data were estimated by the non-random two-liquid model. The Lagrangian multipliers of the constraint equations were used to determine the sensitivity of the optimum to key process variables. The method was illustrated by evaluation of two process flowsheets for electrosynthesis of methyl ethyl ketone (MEK) from 1-butene based on pilot-plant performance reported in the patent literature.List of symbols A _c cell cost factor ($ cell^–1) - A _H heat exchanger cost factor ($ m^–2) - A _p pump cost factor ($ sl^–1) - A _R rectifier cost factor ($ kVA^–1) - A _T tank cost factor ($l ^–0.5) - A _cm cell maintenance factor ($ A^–1 y^–1) - A _cl cell labour ($ cell^–1 y^–1) - A _cw cooling water cost ($ m^–3) - A _e electricity cost ($ kWh^–) - A _m membrane cost ($ cell^–1 y^–1) - A _om other maintenance factor, fraction of plant capital less cell cost - C _p cooling water heat capacity (kJ kg^–1 °C^–1) - H operating hours per year - I _C current to each cell (A) - I _TOT total current to all cells (A) - L _A Lang factor for auxiliaries - L _C Lang factor for cells - L _R Lang factor for rectifiers - N number of cells in plant - Q heat removal load (kJ h^–1) - R production rate (kgh^–1) - T _cw cooling water temperature rise (°C) - T _LM cooler log mean temperature difference (°C) - U heat transfer coefficient for cooler (kW m^–2 °C^–1) - v _c electrolyte flow to each cell (l ^-1) - v _C cell voltage (V) - _R rectifier efficiency - cooling water density (kg m^–3) - _T surge tank residence time (s)     

In situ controlled electrochemical promotion of catalyst surfaces: Pd-catalysed ethylene oxidation

The catalytic activity of polycrystalline Pd films deposited on 8 mol% Y₂O₃-stabilized–ZrO₂ (YSZ), an O^2–-conductor, can be altered reversibly by varying the potential of the Pd catalyst film via the effect of nonfaradaic electrochemical modification of catalytic activity (NEMCA) or electrochemical promotion. The complete oxidation of ethylene was investigated as a model reaction in the temperature range 290–360 °C and atmospheric total pressure. The rate of C₂H₄ oxidation can be reversibly enhanced by up to 45% by supplying O^2– to the catalyst via positive current application. The steady-state rate change is typically 10³–10⁴ times larger than the steady-state rate I/2F of electrochemical supply or removal of promoting oxide ions. The observed behaviour is discussed on the basis of previous NEMCA studies and the mechanism of the reaction.     

A rechargeable battery of the type polyaniline/propylene carbonate-LiClO4/Li-Al

A secondary battery of the type polyaniline/propylene carbonate-LiClO₄/Li–Al is described. The polymer is made by aniline oxidation with ammonium persulphate in NH₄F, 2.3 HF as solvent. The discharge capacity of the polymer is 100 Ah kg^–1 at 25°C and 140 Ah kg^–1 at 40°C for current densities of 0.5 mA cm^–2 and for an amount of material giving a capacity of 10 mAh. The voltage in open circuit for the fully charged battery is 3.6 V. The average usable potential is 2.8–3 V. The energy density for the polymer lies between 280 and 420 Wh kg^–1. The ratio of the amounts of electricity in discharge and charge is one for several hundred deep cycles. The behaviour with regard to self discharge and to constant applied voltage (floating life) is excellent.     

Effect of fluoride mineralizers on alumina sintering

Conclusions Additions to ground commercial alumina of fluorides (AlF₃ or CaF₂) in amounts of 1% exert a retarding and refining action during the sintering of alumina fired in the temperature range 1200–1700°C. The AlF₃ is the most effective additive. Retardation of sintering is explained by the healing of the surface defects in the grains of Al₂O₃ under the influence of the mineralizers.Translated from Ogneupory, No. 1, pp. 37–40, January, 1970.     

Double-layer capacitance and polarization potential of baked carbon anodes in cryolite-alumina melts

Baked carbon anodes with varying apparent densities and baking temperatures were tested in Na₃AlF₆–Al₂O_3(sat) melts at 1010° C. The double-layer capacitance (C _dl) was used as an indicator of the wetted surface area. For unpolarized anodes,C _dl increased with increasing time of immersion and reached a constant level after 1.5–2h. The values decreased with increasing polarization potential in the range 1–1.5 V positive to aluminium. TheC _dl of polished samples increased markedly during electrolysis, particularly at low current densities. No clear correlation was found betweenC _dl and apparent density. Semi-logarithmic plots of potential versus current could be divided into three segments. The lower two were linear, the ranges and slopes being 0.01–0.1 A cm^–2, 0.20–0.44 V per decade and 0.1–0.5 A cm^–2, 0.18–0.24 V per decade, respectively. At higher current densities the curves bent upwards. The current density corresponding to an overpotential of 0.5 V increased slightly with increasing apparent density, whereas the ohmic voltage drop. at constant current density decreased. The current densities were corrected for differences in wetted surface area on the basis of theC _dl data. The change in baking temperature from 970 to 1100°C had no appreciable effect on the overpotential, whereas samples baked at 1250°C showed a somewhat lower overpotential.     

Aluminium dissolution in the NaF-AlF3-Al2O3 system: Effects of alumina,temperature, gas bubbling and dissolved metal

Current reversal chronopotentiometry, with and without a delay time between the forward and reverse current pulses, was employed to evaluate the effects of temperature, alumina content, gas bubbling (argon and carbon dioxide) and dissolved metal on the rate of aluminium dissolution in NaF–AlF₃–Al₂O₃ molten bath. The working electrode was a tungsten wire electrode and the temperature range studied was 824–1040°C. The effect of the alumina content was determined in melts with CR=1.45 and CR=4.3 at 1029±3°C (CR = mol NaF/mol AlF₃). The experiments involving gas bubbling and dissolved metal were carried out in melts similar to industrial compositions, i.e. CR=2.4, 4.8 wt. % Al₂O₃ at 980°C. In general, the dissolution rate of aluninium increased with increasing temperature, decreased slightly with increasing alumina content in acidic melts (CR<3) but changed little in basic melts (CR>3), increased with bubbling and decreased in the presence of dissolved metal. The rate of Al dissolution is thus mass transport controlled.     

An apparatus for rapid measurements in aqueous electrolytes at high temperatures and pressures

An apparatus is described to measure the initial polarization of freshly prepared surfaces of zirconium and its alloys in water under controlled conditions at high temperatures and pressures. The main feature is a working electrode capable of being pneumatically inserted into a replenished autoclave at high temperature and pressure. The autoclave is fitted with an external reference electrode at ambient temperature and pressure. Using this, the hydrogen electrode in 0.1 M NaOH has been shown to give an apparent temtemperature coefficient of about 1.0 mV° C^–1 over the range 20–200° C.     

Electrical conductivity of low-melting electrolytes for aluminium smelting

To determine the electrical conductivity of low-melting electrolytes (AlF₃-rich, e.g. NaF/AlF₃ molar ratio = 1.2), a tube-type cell was used, applying ac-techniques with a sine wave signal with small amplitude in the high frequency range.One melt tested contained 55 mol% NaF and 45 mol% AlF₃ with and without addition of 2 wt.% alumina. Another melt tested contained 55 mol% KF and 45 mol% AlF₃ with and without addition of 2 wt.% alumina. The electrical conductivity data in the molten system can be described by a simple equation of the Arrhenius type:

κ=A e^−B/T