Xanthate ions

Activators enhance the adsorption of collectors, eg, Ca " in the fatty acid flotation of siUcates at high pH or Cu " in the flotation of sphalerite, ZnS, by sulfohydryl collectors. Depressants, on the other hand, have the opposite effect they hinder the flotation of certain minerals, thus improving selectivity. For example, high pH as well as high sulfide ion concentrations can hinder the flotation of sulfide minerals such as galena (PbS) in the presence of xanthates (ROCSS ). Hence, for a given fixed collector concentration there is a fixed critical pH that defines the transition between flotation and no flotation. This is the basis of the Barsky relationship which can be expressed as [X ]j[OH ] = constant, where [A ] is the xanthate ion concentration in the pulp and [Oi/ ] is the hydroxyl ion concentration indicated by the pH. Similar relationships can be written for sulfide ion, cyanide, or thiocyanate, which act as typical depressants in sulfide flotation systems. 

The chemisorption mechanism can be well explained with the mineral, galena. The collector ion used is xanthate ion (C ). The mechanism of its adsorption occurs in the following steps ... 

This step is characterized by the replacement reactions between the xanthate ion (C ) and the carbonate and sulfate ions (and hydroxyl ion, depending on pH) as represented by the following ... 

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 ... 

Reactants Galena Pyrite Common oxygen Activated oxygen HS ion Ethyl xanthate ion... 

A similar structure is observed for the Pd(p-/ -PrPhDta)2PPh3 complex, where a PdS3P core is attained and both uni- and bidentate dithiocumate ligands are found (587). A mechanism involving nucleophilic attack on the OR group of the xanthate ligand by free xanthate ion is proposed (10) for the reaction of M(RXant)2 complexes with tertiary phosphines ... 

Thus, were the xanthate ion itself to adsorb and retain its charge, lateral repulsion would make it impossible for the surface coverage on the mineral to be a high one, and the desired hydrophobicity of the surface would not be achieved. In the electrochemical mechanism described by Salami and Nixon, the adsorption can become a charge-transfer reaction, continuing by the participation of oxygen until the surface is fully covered with dixanthate (and hence wettable). The mechanism is thus an electrochemical oxidation. 

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. 

Other sulphur(II) compounds also form self-assembled layers on gold and silver surfaces. Alkyl xanthate ions ROCS2 are coordinated to the surface through both S atoms and therefore have good stability. FT-IR analysis revealed that alkane chains in these monolayers are well-packed and ordered. Alkyl xanthates and dithiocarbamates were also reported to form monolayers on the surface of Ag coUoids . Thioalkanoic acids RCOSH form well-packed monolayers on both Au(lll) and Ag(lll) surfaces. However, these monolayers exhibit only short-term stability due to hydrolysis of the thiocarboxyl group. ... 

The best-known bubble separation technique is probably ore flotation, which is used for the recovery of valuable minerals from suspensions of crushed ores. For example, sulfide particles are selectively rendered hydrophobic with xanthate ions and floated by bubbling, leaving oxides and silicates in the solution. Another well-known application is the separation of surface-active materials such as surfactants and proteins by simple bubbling. 

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