ELECTROCHEMICAL CONSIDERATIONS

Application of electrochemical techniques and measurements in Sulphide flotation system has established that the flotation is partly governed by electrochemical interaction between flotation agents (collectors) and the minerals. Interaction of a mineral with a sulfhydryl collector (xanthates) corresponds to a specific potential, the value of the potential depends upon the chemical reaction [1]. Two main redox reactions have been recognized. The first is adsorption of xanthate ion at the mineral surface:

• X^- + M M(X) + e^-
• ½ O₂ + H₂O + 2e^- 2OH^-

The second reaction is oxidation of xanthate to dixanthogen at the mineral surface.

• X^- Â½ X₂ + e^- 
• ½ O₂ + H₂O + 2e^- 2OH^- 

In both cases the presence of oxygen is essential as electron acceptor.

The interactions between electrode surfaces are often called galvanic. They are due to the different electrochemical activity that the conducting solids present. The combination of a cathodic surface (electron acceptor) and an anodic one (electron donor) result in the creation of a galvanic cell. The existence of a convenient oxidant, such as dissolved oxygen, enhances the creation of a galvanic current, serving as the ultimate electron acceptor [2]. Such reactions can be hindered either by lowering the activity of oxygen in water or increasing the pH of solution. The electrochemical activity of minerals is characterized by means of their rest potential (the potential reached spontaneously by the mineral electrode in a aqueous solution). The mineral with the higher rest potential acts as a cathode, while the other one with the lower rest potential is the anode.

During the flotation process, the galvanic interactions have a significance influence as there is a mixing of solids with varying electrochemical activity, such as sulphide minerals and metals originated from the corrosion of grinding media (being iron alloys). The latter are more anodic than the sulphides, consequently, affect the selectivity of the process.

The next figure (adapted from [2]) presents the mechanism of galvanic interactions and the possible reactions during the electrolytic contact of two sulphides or with grinding media (steel balls, rods). The hydroxyls that are produced on the cathodic mineral result in the depression of flotation, while the elemental sulphur that forms on the anodic sulphide can increase its hydrophobicity and even lead sometimes to collectorless flotation. The environment during grinding facilitates the above reactions [3].

The rest potential of sulphide minerals has been studied. Among all of them pyrite has the highest rest potential (According to S.R. Rao and J.A. Finch [4], 424 mV at pH 6 vs. SHE, Standard Hydrogen Electrode). This mineral can regarded as the least electrochemical active or the most cathodic one. On the contrary, sphalerite has a rest potential of 188 mV presenting high reactivity. Electrons flow from a less cathodic mineral to a more cathodic one; thus, pyrite serves as an electron acceptor. The less cathodic mineral losses electrons; for example, sulphide is oxidized to sulfur.

Generally, the galvanic interactions modify the mineral surface and affect the ability of minerals to electrocatalyze the charge transfer reactions. Therefore, they can interfere with the flotation of sulphide by sulfhydryl collectors.

Common types of Grinding Mills Magnetite Composition, Crystallization & Structure Roll Crusher Capacity Formula Crusher Feeders: Selecting, Design, Arrangement Important Cooling Process extreme in the requirement for surge capacity Crystal Notation and Crystallographic Axes Witherite Composition, Crystallization & Structure Gold Bio-Oxidation Process Alternative Secondary Crushers Feasibility: is it worth it? Froth Modifiers Arsenic Elements, Alloys and Symbols Chemical Mineralogy and Groups Oxychlorides – Atacamite and Hydrous Chlorides – Carnallite