Sulfate, electrolytic oxidation

Hydrogen peroxide was first made in 1818 by J. L. Thenard who acidified barium peroxide (p. 121) and then removed excess H2O by evaporation under reduced pressure. Later the compound was prepared by hydrolysis of peroxodisulfates obtained by electrolytic oxidation of acidified sulfate solutions at high current densities ... 

Crystallization yields the commercial product, pink heptahydrate. Further oxidation of this salt in dilute H2SO4 with ozone or fluorine produces hydrated cobalt(III) sulfate, 002(804)3 I8H2O. This blue octadecahydrate, 002(804)3 I8H2O also is obtained by electrolytic oxidation of cobalt(II) chloride or any cobalt(II) salt solution in 8M sulfuric acid. 

Bottcher s accumulator — This was an - accumulator with a lead and a zinc electrode in a zinc sulfate electrolyte. Upon discharge zinc ions are reduced to the metal, and lead is oxidized to lead sulfate. These processes are reversed upon charging. 

Cobalt(III) forms few simple salts, but the green hydrated fluoride CoF3-3.5H20 and the blue hydrated sulfate Co2(S04)3-18H20 separate on electrolytic oxidation... 

Due to high T and 2 law efficiencies of sulfuric acid based cycles, to date, more than 20 sulfuric acid and/or metal sulfate decomposition based TCWSCs have been reported. Despite difficulties that challenge efficient electrolytic oxidation of sulfur dioxide (SO2), the Westinghouse hybrid cycle still remains as one of the most studied TCWSCs. The Westinghouse cycle is as follows [14] ... 

Small quantities of fluorides often have a beneficial effect in electrolytic oxidation, e.g., of iodate to periodate and of sulfate to persulfate the part played by the fluoride is not clear, although in most instances its presence is accompanied by an increase of anode potential. Perchlorates have sometimes been added to solutions to improve the oxidation efficiency they have an effect similar to fluorides. 

The use of brine as a solvent in the hydrometallurgical separation of lead from its ores was extensively studied by Lyon and Ralston (H8). Saturated sodium chloride solution and neutral ammonium acetate solutions were found to be good solvents for lead chloride and lead sulfate. Lead oxide and lead carbonate became soluble if the brine was first acidified with either sulfuric or hydrochloric acid. The dissolved lead was recovered electrolytically (S18). Marsden (M13) used this method in a process to recover lead from zinc plant residues. About 80% recovery of the lead was obtained by leaching at ambient temperature and the recovery of lead was increased to 98% when hydrochloric acid was added to the brine. 

World nickel metal production in 2002 was 678000 tons [39]. Hydrometallurgy has typically been applied to the treatment of nickel-copper mattes, anode nickel, and reduced laterite ore. The sulfide concentrates are usually oxidized by roasting and then smelted to copper-iron-nickel sulfide matte (75-80% Cu-Ni), which is refined or used directly to make M onel metal. Cathode nickel can be produced from a variety of electrolytes, including chloride, sulfate, or a mixed chloride-sulfate. The former two are acid systems used in leaching and electrowinning. Mixed chloride-sulfate electrolytes are used for electrorefining the nickel sulfide matte from the traditional matte-smelting operations. 

The advantages of using chloride electrolytes compared with sulfate electrolytes are higher electrical conductivity, lower electrolyte viscosity, lower overpotential for nickel reduction, and higher solubility and activity of nickel. An important factor is the lower anode potential of chlorine evolution compared with oxygen evolution in sulfate electrolytes using the common lead anodes. Chloride electrolytes require insoluble or dimensionally stable anodes, usually titanium coated with an electroactive noble metal or oxide, and a diaphragm system to collect the CI2 gas from the anode. The chlorine liberated at the anode is recycled for use in the leach circuits. In practice, some decomposition of water... 

Chlorine cannot be used in pure sulfate electrolyte nickel(III) hydroxide is normally used (28). Other oxidants that can be used to remove cobalt from the sulfate system include peroxide, persulfate, and ozone [5, p. 803]... 

The pH of the solution is raised by adding, for example, nickel carbonate. Iron hydroxide coprecipitates other impurities from the anolyte, particularly lead and arsenic [43], Oxidation of iron is accomplished by using air in pure sulfate electrolyte systems and by using chlorine in chloride systems. Iron can be removed from pure chloride electrolytes by solvent extraction with tributylphosphate. 

Electrolytic oxidation of anthracene in 20 per cent sulfuric acid solution with 1 per cent of vanadium pentoxide present is carried out at 80° C. with lead electrodes and a current density of 300 amperes per square meter at 1.6 volts. Good yields have been claimed 10 for this process. Air under pressure has been used for the oxidation of anthracene in the form of dispersions in aqueous ferric sulfate solutions,20 or as a solution iu pyridine or dispersion in aqueous alkaline solutions preferably in the presence of catalysts 21 of copper, cobalt, nickel or lead compounds. Vanadium compounds have been found more active than chromium compounds for use as oxidation catalysts in the form of suspensions in the liquid phase, as in the preparation of aniline black.22 Anthracene suspended in water or dilute sulfuric arid or dissolved in a solvent as acetone is oxidized with ozone, or ozonized oxygen at ordinary temperatures.28... 

Cobalt(III) sulfate has been prepared from cobalt(II) sulfate by electrolytic oxidation1 and by treatment with ozone2 or fluorine.3 The electrolytic oxidation of a saturated solution of cobalt(II) sulfate in 10 N sulfuric acid at 5 to 10° 1 gives cobalt(III) sulfate 18-hydrate. Under these conditions, this compound is insoluble and does not decompose rapidly. 

Electrolytes used are sulfuric acid, hydrochloric acid, sodium hydroxide, inorganic salts, and organic salts. Glacial acetic acid, methyl alcohol, and ethyl alcohol have also been found useful. Promoters are stannous chloride, copper sulfate, mercurous sulfate, antimony oxides, ketones, and salts of lead, titanium, molybdenum, and vanadium. 

Anions of common strong acids, such as C104, S04, CF, NOa , etc. exhibit as a rule only weak complexing interactions, if any. Nevertheless, even weak complexation may be of importance in electrode kinetics if the complex ion undergoes electrode reaction more easily than the free metal ion, as is often the case, especially with chlorides. In such cases, the complex takes the role of an electroactive species, as already discussed for the hydroxo complexes. Thus, e.g., nickel can hardly be anodically dissolved at all if chloride ions are not present in the solution. In sulfate electrolytes, the oxidation product (some oxygen-containing species) forms a passive film and further dissolution is blocked soon after an anodic overpotential is imposed upon the electrode. The phenomenon of passivity is discussed elsewhere (cf. Volume 4). At this point, one should note that passivity is absent in the presence of chlorides. 

F ure 8.20 Corrosion cell formed between oxidized and non-oxidized iron surfaces in the presence of a sulfate electrolyte [13]. 

A novel synthetic method for polyphenols was described by Inoue and co-workers [172] w-cresol, 3-methylcatechol, 4-methylcatechol and 4-ethylcatechol were polymerised using a redox reaction between phenol derivatives and gold ions. The oxidised phenol derivatives produced by the reduction of gold ions formed water-insoluble polymers with MW of approximately 1,000 g/mol. Alternatively, Wei and co-workers [173] described the electrochemical polymerisation of phenol on 304 stainless steel anodes and the subsequent coating structure analysis. Anodic oxidation was carried out using 304 stainless steel anodes in a neutral 0.1 mol/1 phenol solution with an electrolyte composed of 0.1 mol/1 sodium sulfate. This oxidation generated a yellow brown polyphenol coating on the steel surface of the anode. 

Hara [237] prepared perchlorate, sulfate, and acetate solutions containing Am02 free of Am by first extracting AmO from buffered 1m acetate (pH 3) solutions of Am(iii) and Am(v) into 0.1 M thenoyltrifiuoroacetone in isobutanol and back-extracting the Am(v) into an aqueous phase. More exotic methods for obtaining the AmOj ion in aqueous solution include dissolution of solid U3 Am04 in dilute perchloric acid or the electrolytic oxidation of Am(iii) in 2 M LilOj/O. M HIOj (pH 1.47) solution [3]. 

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