Leaching phase equilibria

The first phase-equilibrium diagrams discussed are for two-component liquid-vapor systems. Next, three-component diagrams used in extraction, absorption, leaching, and ion exchange are developed. Finally, enthalpy-composition diagrams, which include energy effects, are constructed. 

If (1) the carrier solid is completely inert and is not dissolved or entrained in the solvent, (2) the solute is infinitely soluble in the solvent, and (3) sufficient contact time for the solvent to penetrate the solute completely is permitted, ideal leaching conditions exist and the phase equilibrium diagrams will be as shown in Fig. 3.19a. Here the following nomenclature is employed. 

A typical system suited for leaching is depicted in Fig. 6.1-3. The phase equilibrium of such a system can be understood from the following model. 

Equilibrium diagrams for leaching. The equilibrium data can be plotted on the rectangular diagram as wt fraction for the three components solute A), inert or leached solid (B), and solvent (Q. The two phases are the overflow (liquid) phase and the underflow (slurry) phase. This method is discussed elsewhere (B2). Another convenient method of plotting the equilibrium data will be used, instead, which is similar to the method discussed in the enthalpy-concentration plots in Section 11.6. 

Composition Uiagrants In its elemental form, a leaching system consists of three components inert, insoluble solids a single non-adsorbed solute, which may be liqmd or solid and a single solvent. Thus, it is a ternaiy system, albeit an unusual one, as already mentioned, by virtue of the total mutual Mnsolubility of two of the phases and the simple nature of equilibrium. 

For the case, where the soluble component is leaching from an inert solid carrier, a separate solid phase component balance would be required to establish the solute concentration in the solid phase and hence the time dependent value of the equilibrium concentration. Cl. ... 

While it is expected that the source rocks for the radionuclides of interest in many environments were deposited more than a million years ago and that the isotopes of uranium would be in a state of radioactive equilibrium, physical fractionation of " U from U during water-rock interaction results in disequilibrium conditions in the fluid phase. This is a result of (1) preferential leaching of " U from damaged sites of the crystal lattice upon alpha decay of U, (2) oxidation of insoluble tetravalent " U to soluble hexavalent " U during alpha decay, and (3) alpha recoil of " Th (and its daughter " U) into the solute phase. If initial ( " U/ U).4 in the waters can be reasonably estimated a priori, the following relationship can be used to establish the time T since deposition,... 

Two situations are found in leaching. In the first, the solvent available is more than sufficient to solubilize all the solute, and, at equilibrium, all the solute is in solution. There are, then, two phases, the solid and the solution. The number of components is 3, and F = 3. The variables are temperature, pressure, and concentration of the solution. All are independently variable. In the second case, the solvent available is insufficient to solubilize all the solute, and the excess solute remains as a solid phase at equilibrium. Then the number of phases is 3, and F = 2. The variables are pressure, temperature and concentration of the saturated solution. If the pressure is fixed, the concentration depends on the temperature. This relationship is the ordinary solubility curve. 

For the supported catalyst it is expected that the ligand does not leach since it is chemically bonded to the carrier. In contrast, the rhodium metal bound to the ligand is subject to leaching due to the reversible nature of the complex formation. The amount will depend on the equilibrium between rhodium dissolved in the organic phase and that bound to the ligand. When an equilibrium concentration of 10 ppb Rh is attained, the yearly loss of Rh for a 100 kton production plant will be about 1 kg Rh per year. Compared to the reactor contents of rhodium (see Table 3.9, 70 kg Rh) this would result in a loss of 1.5% of the inventory per year, which would be acceptable. 

It is necessary to consider a number of equilibrium reactions in an analysis of a hydrometallurgical process. These include complexing reactions that occur in solution as well as solubility reactions that define equilibria for the dissolution and precipitation of solid phases. As an example, in analyzing the precipitation of iron compounds from sulfuric acid leach solutions, McAndrew, et al. (11) consider up to 32 hydroxyl and sulfate complexing reactions and 13 precipitation reactions. Within a restricted pH range only a few of these equilibria are relevant and need to be considered. Nevertheless, equilibrium constants for the relevant reactions must be known. Furthermore, since most processes operate at elevated temperatures, it is essential that these parameters be known over a range of temperatures. The availability of this information is discussed below. 

Generally, there is no simple and easy theoretical procedure which can provide exact or nearly precise quantitative predictions of what and how much will be adsorbed/desorbed by any solid phase over a period of time [9, 136-139]. Understanding sorption/desorption characteristics of any solid phase materials requires two main laboratory experimental techniques (a) batch equilibrium testing, and (b) continuous solid phase column-leaching testing. These involve... 

Whereas batch equilibrium tests are designed to study equilibrium sorption of solid phase particles with various pollutants, singly or in combination with other pollutants, solid phase column-leaching tests study both sorption and diffusion of organic pollutants through the subsurface environment [10,11,127, 141,142]. 

Figure 3 shows an equilibrium phase diagram of Al-Cu system (6). This is the basis for the phase transformation. The skeletal fee Cu was finally formed from the AI2CU ( Q ) phase by leaching at a constant temperature. The process of the... 

Two different approaches have been taken by researchers to determine the secondary mineralogy of CCBs (1) direct observation, which is accomplished via analysis of weathered ash materials, and (2) prediction, based on chemical equilibrium solubility calculations for ash pore-waters and/or experimental ash leachate or extractant solutions. Because the secondary phases are typically present in very low abundance, their characterization by direct analysis is difficult. On the other hand, predictions based on chemical equilibrium modelling or laboratory leaching experiments may not be reliable indicators of element leachability or accurately indicate the secondary phases that will form under field conditions (Eighmy et al. 1994 Janssen-Jurkovicova et al. 1994). 

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