Current efficiency chlorine

Membrane cell chlorine current efficiency Chlorine efficiency losses from the formation of HOCl,... 

Mixing of the electrode products causes hydrolytic precipitation of the nickel and, after separation of the nickel hydroxide, the filtrate was returned to the cells. The sequence of the electrolytic purification steps is outlined in Figure 6.28. Nickel hydroxide slurry is first added to the anolyte for the purpose of raising the pH to 3.7 (2 H+ + Ni(OH) = Ni2+ + 2 H20), and iron(II) is oxidized by introducing chlorine. This causes hydrolytic precipitation of the iron(III) and corrects the nickel ion deficiency by the low anodic current efficiency. The iron(III) hydroxide is removed by filteration. The clarified solution is then treated with nickel carbonate and further chlorine to oxidize the cobalt(II) and allow its separation as cobalt(I II) hydroxide. 

The use of electrochemical methods for the destruction of aromatic organo-chlorine wastes has been reviewed [157]. Rusling, Zhang and associates [166, 167] have examined a stable, conductive, bicontinuous surfactant/soil/water microemulsion as a medium for the catalytic reduction of different pollutants. In soils contaminated with Arochlor 1260, 94% dechlorination was achieved by [Zn(pc)] (H2pc=phthalocyanine) as a mediator with a current efficiency of 50% during a 12-h electrolysis. Conductive microemulsions have also been employed for the destruction of aliphatic halides and DDT in the presence of [Co(bpy)3]2+ (bpy=2,2 -bipyridine) [168] or metal phthalocyanine tetrasulfonates [169]. 

On this basis a demonstration plant having a capacity of 10 000 tonnes a-1 of chlorine was built in the Bayer production plant at Leverkusen. The plant was successfully commissioned on 4 January 2000. Figure 4.8 illustrates the electrolyser section of the plant, with the peripheral apparatus arranged mostly outside this building. The 76-element electrolyser was found to behave very smoothly and could be immediately operated up to 5 kA m-2 without any problems. Permanent operation is performed at 4 kA m-2. The power consumption was found to be about 1080 kWh tonne-1 CI2 with a typical current efficiency of nearly 100%. Chlorine purity is found to be 99.9%, which obviates the need for a chlorine liquefaction purification and therefore simplifies the plant drastically. 

The disadvantage of the hypochlorite recycling process is the small increase of chlorate and bromide concentrations in the cell-liquor. However, this is offset by higher chlorine production (0.1% more), resulting in a higher current efficiency. 

The electrolyte in these baths is robust and the throwing power of the bath is excellent however, current efficiency falls with increasing current density as hydrogen evolution increases. The bath also presents significant effluent disposal problems since cyanide must be destroyed by chlorine or hypochlorite oxidation, thus adding to the capital costs of the plant. 

Also is claimed the electrolytic conversion of sodiumchloride in sodiumhydroxide and chlorine (18). In all these instances, the selective membrane is applied in order to increase the current efficiency by either impeding the disappearance of OH- ions from the cathode cell to the anode cell, or that of H+ ions in the reverse direction. 

Zhang and Rusling [66] employed a stable, conductive, bicontinuous microemulsion of surfactant/oil/water as a medium for catalytic dechlorination of PCBs at about 1 mA cm-2 on Pb cathodes. The major products were biphenyl and its reduced alkylbenzene derivatives, which are much less toxic than PCBs. Zinc phthalocyanine provided better catalysis than nickel phthalocyanine tetrasulfonate. The current efficiency was about 20% for 4,4 -DCB and about 40% for the most heavily chlorinated PCB mixture. A nearly complete dechlorination of 100 mg of Aroclor 1260 with 60% Cl was achieved in 18 hr. Electrochemical dehalogenation was thus shown to be feasible in water-based surfactant media, providing a lower-cost, safer alternative to toxic organic solvents. 

Current efficiency is here not so high as during the electrolysis of water because it is lowered by certain side reactions which occur simultaneously while the main process takes place (see equation XI-9) and (XI-10). First of all, it is chlorine which is not entirely removed from the electrolytic cell. It is partially dissolved in the electrolyte and reacts according to the nature of the medium and gives either a very slightly dissociated hypochlorous acid or a considerably dissociated hypochlorite. 

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. 

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. 

As well the bell-j ar type electrolyzer as the cell just described work with a current efficiency which is dependent on the rate of flow of the brine at a given current density. If the flow is too fast the efficiency is low as excessive amounts of chlorine enter the cathode chamber and react with the alkali hydroxide A too low rate of flow exerts also a negative influence on current efficiency as the salt concentration in the brine considerably decreases and the concentration of caustic rapidly increases this means that the hydroxyl ions migrate to the anode in greater quantities. 

When using diaphragms there is no risk of the anolyte being mixed by thermal convection so unlike the bell-jar electrolyzer higher temperatures may be used which lower the specific resistance of the electrolyte. The increased temperature has also a positive effect upon the current efficiencies as both, migration of hydroxyl ions from catholyte to anolyte and solubility of chlorine in brine, are reduced. By this, formation of hypochlorite is limited and caustic and chlorine losses are reduced. 

The first Vorce electrolyzer was built for a load of 1000 A electrolysis was performed at 7 A/sq. dm, 75 °C, and some 3.8 V. Current efficiency was 91—95 per cent and one litre of solution produced contained about 100 grams of NaOH, 190 grams of NaCl and 0.8 grams of NaC103. The chlorine was 95—96 per cent pure and contained between 0.8 and 1.5 per cent of carbon dioxide. 

The secondary reactions mentioned occur to a comparatively small extent when platinum anodes are used then current efficiency is between 96 and 97 per cent and the amount of hydrogen contained in the chlorine is small (0.1 to 0.2 per cent). 

In this reaction, which is responsible for the losses of active chlorine and a lowering of current efficiency, the hypochlorite acts as a depolarizer of the hydrogen evolution. 

Also in an alkaline solution chlorate is the product of an electrochemical reaction. In this case hypochlorous acid formed by hydrolysis of the dissolved chlorine is neutralised in the immediate vicinity of the anode and the resulting hypochlorite ions are oxidized at the electrode to chlorate ions, as soon as formed (see equation (XVII-11)). Therefore, the concentration ot hypochlorite ions in the bulk of the solution with an alkaline electrolyte will be lower than in a neutral one. The current efficiency in a slightly alkaline solution may reach 66.67 per cent, but it decreases with rising alkalinity as a result of increasing hydroxyl ions discharge. However, if current efficiency approximating 60 per cent, which was normal in the first plants for electrochemical manufacture of chlorates, is acceptable, work with a moderately alkaline electrolyte will be the easiest. 

Measurements of the characteristics of various electrolyte compositions and the pure salts are reported by some authors [266,267,282], At 750°C the standard decomposition potentials of the electrolyte components are [282] MgCl2 -2.51 V NaCl -3.22 V KC1 -3.27 V LiCl -3.30 V CaCl2 -3.33 V BaCl2 -3.40 V. Codeposition of sodium or calcium will thus occur only by depletion of MgCl2 (<3%). This lowers the current efficiency and causes a temperature increase due to the recombination of sodium and chlorine. 

The current efficiency would be 100% if the recombination reaction of sodium with chlorine did not occur. Although the diaphragm prevents sodium droplets formed at the cathodes to react with the chlorine bubbles formed at the anodes, another phenomenon causes the decrease in current efficiency. Thus, the slight solubility of sodium in the melt causes the melt to become a partial electronic conductor (see Section I.B.4 Electrical Conductivity of Metal-Molten Salt Mixtures ). This electronic conductivity and the recombination reaction of sodium and chlorine dissolved in the electrolyte decrease the current efficiency. 

Own experiments in divided cells using Nation membrane separators and hypochlorite solutions in the ppm range of concentration resulted in current efficiency values for active chlorine reduction of a few percent. Shifting the pH to higher values complicated the experiments. A buffer stabilised the pH but the relatively high concentration of buffer ions hindered the electrochemical reaction. Thus, quantification is difficult. Kuhn et al. (1980) showed reduction inhibition when calcareous deposits were precipitated on the cathode, but practical experiments showed the decrease of chlorine production in this case. 

Atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine), a photosynthetic inhibitor that is used in large quantities for weed control in com and sorghum, has been treated electrochemically in aqueous solution on reticulated vitreous carbon cathode in the presence of noble-metal catalysts (Stock and Bunce 2002). Elec-trocatalytic hydrogenolysis to 2-ethylamino-4-isopropylamino-s-triazine occurs in quantitative yield, and is most efficient with Pd-based catalysts. Current efficiency increases with increasing atrazine and catalyst concentration, and decreasing current density. The Authors observed a time lag between the start of the electrolysis and the appearance of the dechlorinated products, which was attributed to the absorption of hydrogen by the palladium lattice. As alternative to the electrochemical treatment, the degradation of chlorinated triazines by zero-valent-iron was already mentioned (Dombek et al. 2004). 

The same relative oxidation power was found for the above methods in the degradation of all aromatics, as deduced from the percentages of TOC removal after 3 h of electrolysis of solutions with lOOmgdm-3 TOC of 4-CPA, MCPA, 2,4-D, and 2,4,5-T at pH 3.0 and 100 mA given in Table 19.2. All initial chlorine of these compounds was released as Cl-, which remained stable in solution. The apparent current efficiency (ACE) for these trials was then calculated from the following equation ... 

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