Current efficiency, chloride electrolysis

Refractory metals and particularly niobium and tantalum are obtained by molten salt electrolysis. Several authors 71-6/ investigated the electrochemical reduction of niobium ions in chloride or chloride + fluoride melts. However the current efficiency during electrolysis is strongly dependent on the average valence of the bath. Different reduction steps are proposed with niobium valences ranging from V to I. This is supported by the existence of many stoichiometric compounds. However, very few if not nothing is known on thermodynamics. 

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). 

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. 

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... 

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). 

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. 

Fig. 125. Dependence of current efficiency upon time during electrolysis of a neut ral solution of sodium chloride without the mo of a diaphragm.

It follows from the above explanation that electrolysis of alkali chlorides in an electrolyzer without a diaphragm must be interrupted before curve h which represents the concentration of hypochlorite oxygen changes into a horizontal line only under this condition is the process economical, as a prolonged electrolysis would result in a waste of current without any further increase in th<) hypochlorite content. Moreover, care should be taken to prevent the hypochlorite ions formed from being electrochemically oxidized, as this would result in lower current efficiency and lower hypochlorite concentration in the liquor produced. This process is influenced by a number of factors, e. g. brine concentration, hydrogen ion concentration, anode material, current density, temperature, and last but not least a suitable design of the electrolyzer. 

The physico-chemical properties such as density, conductivity, and viscosity of the magnesium chloride electrolyte are of substantial importance for the current efficiency of the magnesium electrolysis. The solubility of the reaction products, Mg and CI2, in the electrolyte is also important to attain the high current efficiency. Interfacial properties between the Mg electrolyte and the metal may depend significantly on the oxide concentration of the bath. 

Though electrolyte impurities, such as oxides, hydroxides, chlorides, bromides, and iodides, were formerly considered to be undesirable, later it was found that the presence of small amounts of oxides in the melt increases the current efficiency of the electrolysis. Christensen et al. (1994) obtained the highest current efficiency in melts with O/Nb molar ratios in the range 1 < no/ Nb < 0.5. 

Using the treated membrane electrolysis of sodium chloride solution was carried out under the same electrolysis conditions as 1). The treated surface of the membrane was faced to the cathode side in the electrolyzer. When 6.5 N sodium hydroxide solution was obtained as catholyte, the current efficiency was 93% and the cell voltage was 3,85v. On the other hand, the ion exchange membrane not treated by phosphorous pentachloride and triethylamine showed the current efficiency of 52% and the cell voltage of 3.68v when 6.5 N sodium hydroxide was obtained as catholyte. 

In electrolysis of sodium chloride solution under the same conditions as mentioned before, 8,0 N sodium hydroxide was obtained as catholyte at the current efficiency of 95 % and the cell voltage of 4.1 V. 

Performance of NEOSEPTA-F in Sodium Chloride Solution Electrolysis. Figure 5 shows the relationship of the cell voltage and the current efficiency respectively with the concentration of sodium hydroxide in catholyte when electrolysis of sodium chloride solution was carried out at the current density of 30 A/cm. From the economical viewpoint, i.e, the electrolysis power cost, depreciation of equipment cost, membrane cost and so on, the optimum concentration of sodium hydroxide for NEOSEPTA-F C-1000 is about 20 % and that for NEOSEPTA-F C-2000 is about 27 %. 

Figure 2.1 Change in current efficiency (transport number) with concentration of sodium hydroxide in catholyte. (1) Perfluorocarbon membrane NEOSEPTA-F C-1000 (2) Perfluorocarbon membrane NEOSEPTA-F C-2000. Electrolysis at 20 A dm 2 at 80 °C using 3.5 N sodium chloride solution as anolyte and sodium hydroxide solution of various concentrations as catholyte.

The concentration of sodium hydroxide at the cathode surface is higher than that of the bulk solution in electrolysis of sodium chloride because 1 mol of water decomposes at the cathode surface per Faraday. When the solution at the cathode surface is separated from the bulk solution with a suitable separator, sodium hydroxide of higher concentration can be obtained from the cathode surface.171 The concentration of caustic soda produced from an electrolyzer is generally about 32-35%, of which 42-54% is directly and economically produced from an electrolyzer by forming a specific, thin membrane layer on the cathode side of the membrane.172 The current efficiency for caustic soda production is more than 95% in commercial production. 

Permselectivity of counter-ions through the ion exchange membrane depends on the fixed ion concentration of the membrane (Chapter 2.3). Many attempts have been made to increase the fixed ion concentration of the membrane to increase the ion exchange capacity and to decrease the water content of the membrane, namely, to increase the fixed ion concentration without increasing the electrical resistance of the membrane. Figure 4.8 shows an example of the relationship between current efficiency to produce sodium hydroxide and the fixed ion concentration of the membrane for the electrolysis of sodium chloride solution.17 It is apparent that the current efficiency increases with increasing fixed ion concentration of the membrane. 

Figure 4.8 Current efficiency versus fixed ion concentration of a cation exchange membrane in the electrolysis of a sodium chloride solution. Cation exchange membrane sulfonated styrene—divinylbenzene type. Anolyte saturated NaCl catholyte 3.0 N NaOH current density 10Adm 2 at 70 °C.

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