Xanthates, oxidation

The influence of pulp potential on the floatability of chalcopyrite is shown in Fig. 4.4 for an initial concentration of 2x 10 mol/L ethyl XMthate and butyl xanthate. The lower flotation potential is -O.IV for KBX and OV for KEX. The hydrophobic entity is usually assumed to be dixanthogen (Allison et al., 1972 Woods, 1991 Wang et al, 1992) by the reaction (1-3). The calculated potential in terms of reaction (1-3), are, however, 0.217 V and 0.177 V, respectively, for ethyl and butyl xanthate oxidation to dixanthogen for a concentration of 2 x lO" mol/L, which corresponds to the region of maximum recovery but not to the lower limiting potential for flotation, indicating that some other surface hydrophobicity to the mineral. Richardson and Walker (1985) considered that ethyl xanthate flotation of chalcopyrite may be induced by the reaction ... 

Janetski et al. (1977) used voltammetry to study the oxidation of pyrite electrode in solution at different pH in the absence and presence of ethyl xanthate to demonstrate that the oxidation of pyrite itself increases as the pH is increased. At high pH condition, the oxidation of pyrite occurs at a potential cathodic to that for xanthate oxidation and hence, only the mineral will be oxidized at the mixed potential and flotation will be depressed. 

Janetski et al. (1977) used voltanunetric method to study the electrochemical behavior of a pyrite electrode in ethyl xanthate solution containing various concentration of sodium sulphide. They observed an additional anodic wave due to the oxidation of the dissolved sulphide species present and that the wave appeared at potential cathodic to xanthate oxidation. Therefore, they concluded that the presence of sulphide in solution introduced an anodic process which occurred in preference to xanthate oxidation and hence dixanthogen would not be formed and the pyrite would not be rendered floatable. 

It is evident that h is decreased and the oxidation of pyrite itself increased as the pH is increased. At potentials lower than that of xanthate oxidation, the reaction (4.168) will take place preferentially and oxidation of xanthate will be suppressed. The critical depression pH value could be obtained from Eqs. (4.168) and (4.169) ... 

At pH > pH, the process of xanthate oxidation into dixanthogen will be hindered and the flotation pyrite will be depressed as shown in Fig. 4.56 due to competitive adsorption between hydroxyl ions and xanthate ions. 

T.3.2.2. Hydrosulfide ions The depression by hydrosulfide ions is based on critical concentration of HS ions relevant to xanthate oxidation, such that dixanthogen will not be formed and the mineral will not be rendered floatable. 

A prewave is also evident on voltammograms for ethyl xanthate oxidation at a lead metal electrode,and its characteristics are indicative of a reversible chemisorption process. The prewave commences at -0.7 V under the same conditions as those for Fig. 2, that is, at an ethyl xanthate concentration of 10 mol dm". This corresponds to an underpotential to the formation of lead xanthate on lead of -0.2 V, which is the same as that observed for galena. 

An analogous correspondence between the underpotential for xanthate oxidation on the metal and the metal sulfide is observed for copper. There have been a number of voltammetric investigations of the ethyl xanthate/copper and ethyl xanthate/chalcocite (Cu2S) " systems, and similar findings have been reported. A single prewave peak is observed for copper, and this occurs at a potential -0.28 V more negative than that for chalcocite. As with galena, this shift corresponds to the difference in the activity of the metal in the element and in its sulfide. 

It was considered that hydrogen peroxide is formed at silver and chalcocite surfaces when oxygen is present as this compound is an intermediate in the cathodic reduction of oxygen. Clearly, then, there is interaction between the two reactions that make up the mixed potential system, namely, xanthate oxidation and oxygen reduction. Thus, care must be taken in considering the individual processes in isolation. 

The examples in the preceding section, of the flotation of lead and copper ores by xanthates, was one in which chemical forces predominated in the adsorption of the collector. Flotation processes have been applied to a number of other minerals that are either ionic in type, such as potassium chloride, or are insoluble oxides such as quartz and iron oxide, or ink pigments [needed to be removed in waste paper processing [92]]. In the case of quartz, surfactants such as alkyl amines are used, and the situation is complicated by micelle formation (see next section), which can also occur in the adsorbed layer [93, 94]. 

The next significant strength improvement followed the 1950 Du Pont (19) discovery of monoamine and quaternary ammonium modifiers, which, when added to the viscose, prolonged the life of the ziac cellulose xanthate gel, and enabled even higher stretch levels to be used. Modifiers have proliferated siace they were first patented and the Hst now iacludes many poly(alkylene oxide) derivatives (20), polyhydroxypolyamines (21—23), and dithiocarbamates (24). 

Xanthates. These compounds (12) are relatively fast accelerators which are used at low temperature because most examples decompose without cross-linking at higher temperature. Xanthates (qv) are produced by reaction of equimolar amounts of alcohol and carbon disulfide in the presence of caustic. The sodium salt is then converted to the 2inc compound or oxidized to the disulfide. 

Dithiophosphates. These compounds (13) are made by reaction of an alcohol with phosphoms pentasulfide, then neutralization of the dithiophosphoric acid with a metal oxide. Like xanthates, dithiophosphates contain no nitrogen and do not generate nitrosamines during vulcanization. Dithiophosphates find use as high temperature accelerators for the sulfur vulcanization of ethylene—propylene—diene (EPDM) terpolymers. 

Esters derived from the primary alcohols are the most stable and those derived from the tertiary alcohols are the least stable. The decomposition temperature is lower in polar solvents, eg, dimethyl sulfoxide (DMSO), with decomposition occurring at 20°C for esters derived from the tertiary alcohols (38). Esters of benzyl xanthic acid yield stilbenes on heating, and those from neopentyl alcohols thermally rearrange to the corresponding dithiol esters (39,40). The dialkyl xanthate esters catalytically rearrange to the dithiol esters with conventional Lewis acids or trifluoroacetic acid (41,42). The esters are also catalytically rearranged to the dithiolesters by pyridine Ai-oxide catalysts (43) ... 

Many oxidizing agents, including sodium nitrate, convert the alkaU metal xanthates to the corresponding dixanthogen ... 

Zinc ores are generally floated at the mine (18). In the case of simple zinc sulfide ores, flotation is carried out by treatment with copper sulfate to activate the sphalerite causing it to be wet by the organic collector (eg, xanthate). The now-hydrophobic zinc ore particles attach themselves to the rising bubbles. Oxidized ore particles present must be sulftdized with sodium sulfide to be floated (19). Flotation produces concentrates which are ca 50—60% zinc. In mixed ore, the lead and copper are usually floated after depressing the sphalerite with cyanide or zinc sulfate. The sphalerite is then activated and floated. 

Curing Systems. Polychloroprene can be cured with many combiaations of metallic oxides, organic accelerators, and retarders (114). The G family of polymers, containing residual thiuram disulfide, can be cured with metallic oxides alone, although certain properties, for example compression set, can be enhanced by addition of an organic accelerator. The W, T, and xanthate modified families require addition of an organic accelerator, often ia combination with a cure retarder, for practical cures. 

Pyridazines form complexes with iodine, iodine monochloride, bromine, nickel(II) ethyl xanthate, iron carbonyls, iron carbonyl and triphenylphosphine, boron trihalides, silver salts, mercury(I) salts, iridium and ruthenium salts, chromium carbonyl and transition metals, and pentammine complexes of osmium(II) and osmium(III) (79ACS(A)125). Pyridazine N- oxide and its methyl and phenyl substituted derivatives form copper complexes (78TL1979). 

Aetivators. These are used to make a mineral surface amenable to collector coating. Copper ion is used, for example, to activate sphalerite (ZnS), rendering the sphalerite surface capable of absorbing a xanthate or dithiophosphate collector. Sodium sulfide is used to coat oxidized copper and lead minerals so that they can be floated by a sulfide mineral collector. 

Diazo compounds Diazoniiim sulfides and derivatives, Xanthates 1,2-epoxides Halo-aryl metals Haloarenemetal TC-complexes Halogen oxides Hydraziniiim salts Hyjiohalites... 

The first step in the manufacture of the foil involves the production of alkali cellulose. This is then shredded and allowed to age in order that oxidation will degrade the polymer to the desired extent. The alkali cellulose is then treated with carbon disulphide in xanthating chums at 20-28°C for about three hours. 

The described method of preparation of w-nitrophenyl disulfide is essentially that of Foss and co-workers and is a modification of that reported by Ekbom. The disulfide has been prepared by reaction of potassium ethyl xanthate with w-nitrobenzenedi-azonium chloride solution, followed by hydrolysis to yield the mercaptan, which is subsequently oxidized with potassium ferro-cyanide or dilute nitric acid to the disulfide. ... 

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