Mineral-solution equilibria Subject

The present description pertaining to copper refers to solvent extraction of copper at the Bluebird Mine, Miami. When the plant became operational in the first quarter of 1968 it used L1X 64, but L1X 64N was introduced in to its operation from late 1968. The ore consists of the oxidized minerals, chrysocolla and lesser amounts of azurite and malachite. A heap leaching process is adopted for this copper resource. Heap-leached copper solution is subjected to solvent extraction operation, the extractant being a solution of 7-8% L1X 64N incorporated in kerosene diluent. The extraction process flowsheet is shown in Figure 5.20. The extraction equilibrium diagram portrayed in Figure 5.21 (A) shows the condi-... 

In the case of Al(OH)3 freshly precipitated from solution, there is a slow recrystallization from the amorphous to the crystalline form, with the result that the ion activity product, (aAl3+) (aoH-) of the aqueous solution decreases over time, approaching the value for the crystalline mineral. Here, as in many chemical processes, activities of ions and molecules in solution are subject to control by reaction rates (kinetics) as well as equilibrium constants. The important role of kinetics in chemical reactions will be discussed later in this chapter. 

It is certainly more constant than that of sediments being introduced into the basin. This fact is due to the greater mobility of material in solution which tends to even out local fluctuations in concentration through the action of waves and currents. The sediment is much less subjected to such a mechanical homogenization process and tends, therefore, to attain equilibrium by localized mineral reaction. The type of thermodynamic system operative is most likely to be "open", where each point of sediment has some chemical variables fixed by their concentration in the sediment (inert components due to their low solubility in the solution) and other chemical components, which are soluble, have their concentration in the sediment a function of their activity in the aqueous solution. The bulk composition of the resulting sediment will be largely determined by the composition of the waters in which it is sedimented and the length of time it has reacted with this environment. The composition of the aqueous solution is, of course, determined to a minor extent by these reactions. 

As outlined above (p. 3), a reaction can be subject to microscopic diffusion control only if one of the reactive intermediates is formed from an inactive precursor in the reaction mixture. There are two sets of conditions which have provided evidence for microscopic diffusion control in nitration. One concerns solutions of nitric acid in aqueous mineral acids or organic solvents for, in most of these solutions, the stoicheiometric nitric acid is mainly present as the molecular species in equilibrium with a very small concentration of nitronium ions. A reaction between a substrate and a nitronium ion from this equilibrium concentration can, in principle, be subject to microscopic diffusion control. The other set of conditions is when the substrate is mainly present as the protonated form SH+ but when reaction occurs through a very small concentration of the neutral base S. A reaction between the neutral base and a nitronium ion can then, in principle, be subject to microscopic diffusion control even if the nitronium ions are the bulk component of the HN03/N0 equilibrium. In considering the evidence for microscopic diffusion control it is convenient to consider separately the reactions of those species involved in prototopic equilibria. 

Water held in the interstices of solids, as liquid covering the surface and as free water in cell cavities, is subject to movement by gravity and capillarity, provided passageways for the continuity of flow are present. Water flow due to a capillarity applies to water not held in solution and to all water above the fiber saturation point (as in textiles, paper, and leather) and to all water above the equilibrium moisture concentration at atmospheric saturation as in fine powers and granular solids, such as paint, pigments, minerals, clays, soil, and sand (H6). 

The release of uranium and thorium from minerals into natural waters will depend upon the formation of stable soluble complexes. In aqueous media only Th is known but uranium may exist in one of several oxidation states. The standard potential for the oxidation of U in water according to equation (2) has been re-evaluated as E° - 0.273 0.005 V and a potential diagram for uranium in water at pH 8 is given in Scheme 3. This indicates that will reduce water, while U is unstable with respect to disproportionation to U and U Since the Earth s atmosphere prior to about 2 x 10 y ago was anoxic, and mildly reducing, U " would remain the preferred oxidation state in natural waters at this time. A consequence of this was that uranium and thorium would have exhibited similar chemistry in natural waters, and have been subject to broadly similar redistribution processes early in the Earth s history. Both U " and Th are readily hydrolyzed in aqueous solutions of low acidity. A semiquantitative summary of the equilibrium constants for the hydrolysis of actinide ions in dilute solutions of zero ionic strength has been... 

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