Current efficiency, chloride

Electrolysis. Electro winning of hafnium, zirconium, and titanium has been proposed as an alternative to the KroU process. Electrolysis of an all chloride hafnium salt system is inefficient because of the stabiHty of lower chlorides in these melts. The presence of fluoride salts in the melt increases the StabiHty of in solution and results in much better current efficiencies. Hafnium is produced by this procedure in Erance (17). 

If the ECM of titanium is attempted in sodium chloride electrolyte, very low (10—20%) current efficiency is usually obtained. When this solution is replaced by some mixture of fluoride-based electrolytes, to achieve greater efficiencies (> 60%), a higher voltage (ca 60 V) is used. These conditions ate needed to break down the tenacious oxide film that forms on the surface of titanium. It is this film which accounts for the corrosion resistance of titanium, and together with its toughness and lightness, make this metal so useful in the aircraft engine industry. 

Salt that is substantially free of sulfate and other impurities is the cell feed. This grade may be purchased from commercial salt suppHers or made on site by purification of cmde sea or rock salt. Dried calcium chloride or cell bath from dismanded cells is added to the bath periodically as needed to replenish calcium coproduced with the sodium. The heat required to maintain the bath ia the molten condition is suppHed by the electrolysis current. Other electrolyte compositions have been proposed ia which part or all of the calcium chloride is replaced by other salts (61—64). Such baths offer improved current efficiencies and production of cmde sodium containing relatively Htde calcium. 

The quality of the refined metal, and the current efficiency strongly depend on the soluble vanadium in the bath and the quality of the anode feed. As the amount of vanadium in the anode decreases, the current efficiency and the purity of the refined product also decrease. A laboratory preparation of the metal with a purity of better than 99.5%, containing low levels of nitrogen (30-50 ppm) and of oxygen (400-1000 ppm) has been possible. The purity obtainable with potassium chloride-lithium chloride-vanadium dichloride and with sodium chloride-calcium chloride-vanadium dichloride mixtures is better than that obtainable with other molten salt mixtures. The major impurities are iron and chromium. Aluminum also gets dissolved in the melt due to chemical and electrochemical reactions but its concentrations in the electrolyte and in the final product have been found to be quite low. The average current efficiency of the process is about 70%, with a metal recovery of 80 to 85%. 

A number of metal porphyrins have been examined as electrocatalysts for H20 reduction to H2. Cobalt complexes of water soluble masri-tetrakis(7V-methylpyridinium-4-yl)porphyrin chloride, meso-tetrakis(4-pyridyl)porphyrin, and mam-tetrakis(A,A,A-trimethylamlinium-4-yl)porphyrin chloride have been shown to catalyze H2 production via controlled potential electrolysis at relatively low overpotential (—0.95 V vs. SCE at Hg pool in 0.1 M in fluoroacetic acid), with nearly 100% current efficiency.12 Since the electrode kinetics appeared to be dominated by porphyrin adsorption at the electrode surface, H2-evolution catalysts have been examined at Co-porphyrin films on electrode surfaces.13,14 These catalytic systems appeared to be limited by slow electron transfer or poor stability.13 However, CoTPP incorporated into a Nafion membrane coated on a Pt electrode shows high activity for H2 production, and the catalysis takes place at the theoretical potential of H+/H2.14... 

One possible strategy in the development of low-overpotential methods for the electroreduction of C02 is to employ a catalyst in solution in the electrochemical cell, A few systems are known that employ homogeneous catalysts and these are based primarily on transition metal complexes. A particularly efficient catalyst is (Bipy)Re[CO]3Cl, where Bipy is 2,2 bipyridine, which was first reported as such by Hawecker et al. in 1983. In fact, this first report concerned the photochemical reduction of C02 to CO. However, they reasoned correctly that the complex should also be capable of catalysing the electrochemical reduction reaction. In 1984, the same authors reported that (Bipy)Re[C013CI catalysed the reduction of C02 to CO in DMF/water/ tetraalkylammonium chloride or perchlorate with an average current efficiency of >90% at —1.25 V vs. NHE (c. —1.5V vs. SCE). The product analysis was performed by gas chromatography and 13C nmr and showed no other products. 

The composition of the codeposition bath is defined not only by the concentration and type of electrolyte used for depositing the matrix metal, but also by the particle loading in suspension, the pH, the temperature, and the additives used. A variety of electrolytes have been used for the electrocodeposition process including simple metal sulfate or acidic metal sulfate baths to form a metal matrix of copper, iron, nickel, cobalt, or chromium, or their alloys. Deposition of a nickel matrix has also been conducted using a Watts bath which consists of nickel sulfate, nickel chloride and boric acid, and electrolyte baths based on nickel fluoborate or nickel sulfamate. Although many of the bath chemistries used provide high current efficiency, the effect of hydrogen evolution on electrocodeposition is not discussed in the literature. 

DSA-02 oxide-coated Ti anode (DSA = Dimensionally Stable Anode) is placed, thus creating a large cathode volume. The effluent solution flows perpendicularly through the electrodes with a typical flow rate of 0.5 dm3 s-1. The flowthrough metal electrodes have an active area approximately 15 times their geometric area. The cell allows air sparging to increase the mass-transfer. The current efficiency is about 40% when the inlet concentration of the metal ions is 150 to 1500 ppm and the concentration at the out-let is about 50 ppm. The cell is currently used for the treatment of recirculated wash-waters from acid copper, copper cyanide, zinc cyanide, zinc chloride, cadmium sulphate, cadmium cyanide and precious metal plating and washwaters from electroless copper deposition. Since the foam metal electrodes are relatively expensive the electrodes... 

MacKinnon and coworkers [391-393] have studied a number of organic compounds as additives and have shown that tetrabutylammonium chloride improves surface morphology and current efficiency during zinc electrowinning from acidic chloride solutions. A similar influence was observed using other tetraalkylammo-nium compounds as additives [371, 374, 375, 394],... 

The amount of EGA (the current efficiency of EGA generation) is measured in both acetone and methylene chloride-THF. Both Figs. 1 and 2 show that a sharp increase in the concentration of EGA is observed in the electrolysis of lithium, sodium, and magnesium perchlorate solution, being consistent with the result of the oxirane ring opening reaction to ketones (Table 2). 

Effect of Cation. Of all alkali chlorides, only lithium chloride is sufficiently soluble in ethylenediamine to act as an electrolyte. On the other hand, all alkali iodides are soluble in ethylenediamine. Since the anion has a large effect on current efficiency, a common anion such as the iodide ion must be used to compare the effect of the various cations. The results of runs 6 and 7 show that the current efficiency was slightly higher and the percentage of octalin formed much greater when rubidium iodide was used instead of lithium iodide. The metallic cations Li and Rb give markedly better current efficiency than the organic cations (runs 6, 7, 8, and runs 5, 9, 10, 11). 

Perchlorate anion gives higher current efficiency than chloride anion (runs 10 and 12). The low current efficiency in these runs is caused by the presence of the organic cation. 

Based on results of electrochemical reductions of tetralin in ethylenediamine, current efficiency is highest with aluminum as cathode material and with lithium chloride as electrolyte. A substantial increase in current efficiency was obtained in reducing 1-decene by adding a proton donor. 

Like silver, Co(III) is also a powerful oxidizing agent with E° = 1.82 V. Co(II) in HNO3 has been employed to degrade different organic compounds [70,71,73,74] by using separators to prevent Co electrodeposition. In acidic aqueous media, the oxidation of Co(II) to Co(III) has less than 100% current efficiency because it occurs at a more positive potential than water. Cobalt has the advantage over silver in that cobalt chloride complexes are... 

The amount of energy consumed depends upon the voltage across the bath and the current efficiency of the electrolyticaJ process. Although three products alkali hydroxide, chlorine, and hydrogen are obtained when electrolyzing a solution of sodium or potassium chloride, current efficiency is usually assessed by the resultant caustic which is the main product. 

This equation indicates that during the first stages of electrolysis, when the concentration of caustics is Btill negligible (c2 0), current efficiency equals 100 per cent. In the course of time, however, current efficiency steadily decreases and after a longer period, when the concentration of chloride drops to zero (cx s 0), it theoretically attains the limit value rj/ = 100 (1 —< ). This theoretical result is confirmed in the table below which shows the experimental results obtained on the electrolysis of potassium chloride. 

On the whole it may be stated that due to the fact that hydroxyl ions are removed from the reaction zone before they can react with chlorine, more concentrated caustic solutions are here obtained than with electrolyzers using nonfiltering diaphragms. In spite of this improvement it is not possible to convert all chloride into hydroxide should be a satisfactory current efficiency attained and a caustic with a low content of hypochlorite and chlorate produced. 

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