Chapter 6 Chemical leaching of mechanically activated minerals

Abstract

The traditional scheme of metals extraction from minerals involves some processes of mechanical character ameliorating the accesibility of the valuable component by the leaching agent. Leaching represents the key stage in the extraction scheme and its course may be affected by selection and choice of the method leaching and/or by convenient pretreatment of the solid phase. Thermal and mechanical activation belongs among the most important pretreatment methods which influence solid phase leachability. The thermal activation of sulfidic ores aims at transforming the poorly soluble minerals into more soluble forms. That enables better selectivity in transfer of usable metal into solution, nevertheless it appears that some new problems concerning exploitation of the sulfur emissions arise. In the past three decades enhanced public awareness and governmental pressure have focussed on the problem of sulfur oxide pollution. Sulfidic minerals account for a large fraction of the sulfur oxides. The special problem of the minerals is the presence of small amounts of As, Hg, Te, Se which may be emitted together with sulfur in form of oxides by the thermal activation. The mechanical activation of minerals makes it possible to reduce their decomposition temperature or causes such a degree of disordering that the thermal activation may be omitted entirely. In this process, the complex influence of surface and bulk properties occurs. The mineral activation leads to a positive influence on the leaching reaction kinetics, to an increase in the measured surface area and to further phenomena, especially the potential mitigation of environmental pollutants which is becoming increasingly important with time. At present, it is not known whether the kinetics of heterogeneous reactions are determined by the contact area, the structure of the mineral, or both. The required modification of the structure can be achieved by mechanical activation of the mineral, typically by intensive grinding. The breaking of bonds in the crystalline lattice of the mineral brings about a decrease (ΔE*) in activation energy and an increase in the rate of leaching [6.1] Δ E* = E - E* (6.1) where E is the apparent activation energy of the non-disordered mineral and E* is the apparent activation energy of the disordered mineral. The relationship between the rate of leaching and temperature is usually described by the Arrhenius equation k = Z exp (- E / R T) (6.2) where k, Z, R and T stand for the rate constant of leaching for the non-disordered mineral, pre-exponential factor, gas constant and reaction temperature, respectively. For the disordered mineral we can write k = Z exp (- E* / R T) (6.3) and after substituting for E* from (6.1) we obtain k* = k exp (Δ E* / R T) (6.4) From (6.1) it is clear that exp (ΔE*/RT) > 1 and thus it follows from eq. (6.4) that k* > k, i.e., the rate of leaching of a disordered mineral is greater than that of an ordered mineral. It was Senna who analysed the effect of surface area and the structural disordering on the leachability of mechanically activated minerals [6.2]. In order to solve the problem - whether surface area or structural parameters are predominant for the reactivity, the rate constant is divided by the proper surface area and plot against the applied energy by activation (Fig. 6.1). If the rate constant of leaching divided by the surface area remains constant with respect to the applied energy, as shown in Figure 6.1a, then the measured surface area may be the effective surface area and at the same time, the reaction rate is insensitive to structural changes. If, on the other hand, the value k/Si decreases with applied energy, as shown in Figure 6.1b, then the surface area is probably not the effective surface area. In the third case where k/Si increases with increasing applied energy, as shown in Figure 6.1c, the surface area Si, may be again the effective surface area, with an overlapping effect of the structural imperfection, as a result of mechanical activation. Alternatively, when k/Si and X vary parallel to each other with E, as shown in Figure 6.1d, or the value k/Si is proportional to X, as shown in Figure 6.1e, it seems more appropriate to accept the chosen Si as an effective surface area. © 2000 Elsevier B.V. All rights reserved.