Dithiolates flotation

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. 

These collectors are effective only under oxidizing conditions, and it is generally accepted that the species that confers hydrophobicity on the mineral surface is either a chemisorbed metal thio compound or the oxidized form of the collector, dithiolate. The amounts of each species formed will depend on the relative stabilities of the metal—sulfur and sulfur—sulfur bonds. The formation of four-membered chelate rings is also possible with soft metal ions such as copper(I) because the largely covalent character of the bond in this instance is able to overcome the strain within the ring by extensive electron delocalization. This could account for the >artial selectivity of some of these reagents for the copper minerals, which has been put to good use in the sequential flotation of copjrer, lead and zinc from complex sulfide ores. ... 

Nixon also suggested that prominent theories of flotation could be reconciled by the electrochemical approach. This is illustrated in Fig. 1, in which the requirement of a neutral surface species in Cook and Nixon s model can be met in different ways. The anodic reaction can be adsorption as proposed by Wark and Cox (Fig. la). Also, it can be the formation of a metal thiol compound as proposed by Taggart and co-workers. The latter process can occur as a single step as in Fig. lb or through separate surface oxidation and ion-exchange processes as shown in Figs. Ic and Id. In addition, the anodic process can be the formation of the dithiolate as suggested by Nixon and illustrated in Fig. le. 

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