Xanthates dixanthogen reactions

Dixanthogen is the oxidized product of xanthate. Dixanthogen can be produced by adopting electrochemical method. Electrochemical oxidation for treating xanthate had applied in flotation industry. The electrochemical reaction can be expressed as follows ... 

In the initial formation of the cupric xanthates, soluble xanthate complexes form prior to the precipitation of the cuprous xanthate with the concurrent formation of the dixanthogen (51). The dixanthogen can be separated by virtue of its solubiUty in ether. Older samples of alkah metal xanthates contain some dixanthogen, which is thought to form by the following reaction (33) ... 

In the reaction between xanthates and sulfonyl chlorides, the xanthates convert to dixanthogens, and the sulfonyl chlorides reduce to sulfinic acids and other compounds (38) ... 

The electrochemical mechanism can be well explained with the mineral pyrite. The collector ion is xanthate ion (CT), a member in the anodic sulfydryl collectors group. Two electrochemical reactions occur on the surface of the pyrite. There is the formation of dixanthogen (C2) by anodic oxidation of xanthate ion (CT) on the surface of pyrite coupled with cathodic reduction of adsorbed oxygen. These reactions are shown below ... 

The mixed-potential model demonstrated the importance of electrode potential in flotation systems. The mixed potential or rest potential of an electrode provides information to determine the identity of the reactions that take place at the mineral surface and the rates of these processes. One approach is to compare the measured rest potential with equilibrium potential for various processes derived from thermodynamic data. Allison et al. (1971,1972) considered that a necessary condition for the electrochemical formation of dithiolate at the mineral surface is that the measmed mixed potential arising from the reduction of oxygen and the oxidation of this collector at the surface must be anodic to the equilibrium potential for the thio ion/dithiolate couple. They correlated the rest potential of a range of sulphide minerals in different thio-collector solutions with the products extracted from the surface as shown in Table 1.2 and 1.3. It can be seen from these Tables that only those minerals exhibiting rest potential in excess of the thio ion/disulphide couple formed dithiolate as a major reaction product. Those minerals which had a rest potential below this value formed the metal collector compoimds, except covellite on which dixanthogen was formed even though the measured rest potential was below the reversible potential. Allison et al. (1972) attributed the behavior to the decomposition of cupric xanthate. 

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 ... 

However, the decomposition potential of zinc xanthate into dixanthogen is above 0.3 V according to reactions (4-33) or (4-34) and the upper potential limit of flotation of marmatite extends to 620 mV, which indicates the coexistence of dixanthogen on marmatite in this condition. The difference of flotation behavior and hydrophobic entity between sphalerite and marmatite may be due to the existence of iron in marmatite. 

Persson et al. (1991) used diffuse reflection infrared Fourier transform (DRIFT) spectroscopy to study the interactions between galena, pyrite sphalerite and ethyl xanthate. They provided the evidence that the DRIFT spectrum of oxidized galena treated with an aqueous solution of potassium ethyl xanthate is practically identical with that of solid lead (II) ethyl xanthate, which can be formed as the only detectable siuface species on oxidized galena. Dialkyl dixanthogen is formed as the only siuface species in the reaction between oxidized pyrite and aqueous solution of potassium alkyl xanthate. 

According to the mixed potential theory, an anodic reaction can occur only if there is a cathodic reaction proceeding at finite rate at that potential (Rand and Woods, 1984). For the flotation systems, the cathodic reaction is usually given by the reduction of oxygen. The corresponding anodic reaction involves interaction of xanthate on the sulphide minerals in various ways, including the reaction of xanthate with the sulphide mineral (MS) to form metal xanthate and the oxidation of xanthate to dixanthogen (X2) at the mineral surface. 

The mixed potential of the sulphide mineral in the flotation pulp will determine the oxidation product on its surface. If the mixed potential of the mineral in the presence of oxygen, xanthate and other reagents is above the mixed potential for the X7X2 redox couple, then the reaction will produce dixanthogen on the surface. If the mixed potential is lower than the X /Xz redox couple, metal xanthate reaction will take place rendering the surface hydrophobic. 

In the collectors used, R is generally in the C2 to C6 range. Xanthates are readily oxidized to dixanthogens, and the extent of this reaction may have a big effect on the efficiency of the collector. 

Table 4.2 lists the values of rest potential for a few minerals in potassium ethyl xanthate solutions (6.25 X 10 mol/1, pH 7) and infrared identifications of surface reaction products (Allison et al., 1972). Only those minerals such as chalcopyrite and pyrite have surface reaction product of dixanthogen. 

The elecirochemical mechanism, also called the mixed-potential mechanism [477], assumes charge transfer within a particle from the cathodic patch, at which oxygen is reduced, to the anodic one, at which the sulfide itself and/or xanthate anion are oxidized. This mechanism describes a broad spectrum of interfacial phenomena involving, as an intermediate step, a redox reaction in which the anodic and cathodic processes are spatially separated. Some examples of this include electrocatalytic chemisorption of xanthate and synthesis of dixanthogen and precipitation of xanthate-metal complexes (nncleation of a microphase of metal xanthate). In the latter reaction, the anodic sulfide dissolution is initiated with the ionization of surface metal atoms, and the metal ions thus produced on the surface are transferred into aqueous solution to form hydrated metal ions or metal-ion complexes associated with anions [478]. The ionization of surface metal atoms is an electrochemical oxidation, whereas the hydration or complex-ation of metal ions is a chemical process (an acid-based reaction). 

Dixanthogen can also be produced from xanthate by adopting ammonium sulfate, high electrovalent metal ion, or iodine as oxidizer. The oxidation reactions of xanthate can be expressed as follows ... 

It was reported recently that some other dixanthogens can be produced by oxidizing dihydric alcohol xanthate. The oxidization reaction can be given as follows ... 

Voltammograms for copper and for galena (Fig. 2) displayed prewaves on the positive-going scans at potentials below the reversible value for the formation of the metal xanthate and dixanthogen. These prewaves are indicative of the chemisorption process, reaction (2). 

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