Effects of Doped Ions on Mixed Potential

Marmatite has a narrower band gap than (Zn, Cu)S, thus it should be more easily oxidized than (Zn, Cu)S. But in fact, the sphalerite after Cu activation has the most excellent flotation response using xanthate. These phenomena can be explained by the mixed potential theory. 

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. 

A lot of work show that the existence of Fe ions on solid surface can catalyze the reduction of oxygen. It indicates that the rate of oxygen reduction on marmatite is larger than rate of reduction on (Zn, Cu)S, this will cause the mixed potential of marmatite to be higher than that of (Zn, Cu)S and ZnS. This process can be illustrated in Fig. 9.17. 

---