Mineral electrode potential

There existed oxidation-reduction reactions with the same reaction speed on the sulphide mineral surface in water. One is the self-corrosion of sulphide mineral. Another is the reduction of oxygen. If the equilibrium potential for the anodic reaction and the cathodic reaction are, respectively, E and, and the mineral electrode potential is E, the relationship among them is as follows ... 

Mineral static potential or mineral electrode potential is an important parameter. It plays an important role in flotation. It has been demonstrated that the floatability of minerals has direct relation with electrode potential because the grinding-flotation system is similar to the multi-electrode reaction coupled system. Therefore, it is important to study the variance rules of the mineral electrode potential under different conditions. 

At the same pH made by different pH modifiers, the different flotation response of one sulphide mineral may arise from its effect on the potential of this mineral electrode. The change of potential of mineral electrodes with time at pH= 12 modified by NaOH and Ca(OH)2 is measured and demonstrated in Fig. 10.5. It follows that a mineral electrode potential increases faster with the time at pH= 12 modified by sodium hydroxide and changes a little at the same pH modified by calcium hydroxide. When pH is adjusted by NaOH, the electrode potential of galena, sphalerite and p5oite increase rapidly, respectively, from -30, -12, and 70 mV at the initial stage to -10, 10, and 110 mV after 50 min. When pH is modified by lime, the electrode potential of galena, sphalerite and pyrite... 

As demonstrated in Section 5.2, the electrode potential is determined by the rates of two opposing electrode reactions. The reactant in one of these reactions is always identical with the product of the other. However, the electrode potential can be determined by two electrode reactions that have nothing in common. For example, the dissolution of zinc in a mineral acid involves the evolution of hydrogen on the zinc surface with simultaneous ionization of zinc, where the divalent zinc ions diffuse away from the electrode. The sum of the partial currents corresponding to these two processes must equal zero (if the charging current for a change in the electrode potential is neglected). The potential attained by the metal under these conditions is termed the mixed potential Emix. If the polarization curves for both processes are known, then conditions can be determined such that the absolute values of the cathodic and anodic currents are identical (see Fig. 5.54A). The rate of dissolution of zinc is proportional to the partial anodic current. 

The properties of the el ectrical doubl e 1 ayer (EDL) have been the subject of considerable research (1,3,5,8,10). Unlike reversible electrodes, where surface potential is controlled and charge develops in response to changes in electrode potential, mineral surfaces develop potential in response to the formation of surface charge (8). On the surface of hydrous oxides, for example, hydroxyl groups... 

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. 

Many investigators have used different techniques to study the electrochemical behavior of different sulphide mineral electrodes in solutions of different compositions. Linear potential sweep voltammetry (LPSV), and cyclic voltammetry (CV) have been perhaps, used most extensively and applied successfully to the investigation of reactions of sulphide minerals with aqueous systems. These techniques have provided valuable information on the extent of oxidation as a function of potential for various solution conditions and have allowed the identity of the surface products to be deduced. 

E and E, . represent the equilibrium potential of mineral anodic dissolution and cathode reduction of oxygen, respectively. represents the mineral mixed potential in certain system. and Zg are current density of anodic and cathode reaction, respectively. When the discharge is the controlled step of electrode reaction, according to electrochemistry theory, the equation can be described as following ... 

Figure 10.5 The change of potential of mineral electrode widi time at pH= 12 modified by NaOH and Ca(OH)2 (KNO3 0.1 mol/L BX 10" mol/L)...

Many of them are sufficiently electropositive to dissolve in mineral acids, although a few are noble — that is, they have such positive electrode potentials that they are unaffected by simple acids. 

Chromium is a white, hard, lustrous, and brittle metal (mp 1903 10°C). It is extremely resistant to ordinary corrosive agents, which accounts for its extensive use as an electroplated protective coating. The metal dissolves fairly readily in nonoxidizing mineral acids, for example, hydrochloric and sulfuric acids, but not in cold aqua regia or nitric acid, either concentrated or dilute. The last two reagents passivate the metal in a manner that is not well understood. The electrode potentials of the metal are... 

Ga, In and T1 are much less common elements, obtained in small amounts from sulfide minerals of other elements and used only in specialized applications. The metals are less reactive than aluminum Fig. 1 shows a Frost diagram in which the much larger negative slope (negative electrode potential see Topic E5) of A1 is apparent. Thallium compounds are extremely toxic but do not normally pose an environmental hazard because they are little used. 

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