Leaching liquid-solid equilibria

Other. Leaching, or solid extraction, is a similar process in which solute(s) are removed from a solid by a liquid mass-separating agent for which they have an affinity. Extraction is typically analyzed as an equilibrium-limited separation. 

EQUILIBRIUM. In leaching, provided sufficient solvent is present to dissolve all the solute in the entering solid and there is no adsorption of solute by the solid, equilibrium is attained when the solute is completely dissolved and the concentration of the solution so formed is uniform. Such a condition may be obtained simply or with difficulty, depending on the structure of the solid. These factors are considered when stage efficiency is discussed. At present, it is assumed that the requirements for equilibrium are met. Then the concentration of the liquid retained by the solid leaving any stage is the same as that of the liquid overflow from the same stage. The equilibrium relationship is, simply, —... 

There are two types of extraction processes, namely, leaching where solids are extracted and liquid-liquid extraction. The commonly used leaching equipment is the extraction battery where counter current flow is established resulting in a number of extracting stages. This can be modeled as equilibrium process. The McCabe-Thiele procedure can be used to model this process. In liquid-liquid extraction, ternary diagrams and tie lines are used to determine the extent of extraction. Batch extractive distillation is a commonly used extractive method for liquid-liquid extraction. 

Equilibrium relations in leaching usually are simpler than in liquid-liquid equilibria, or perhaps only appear so because few measurements have been published. The solution phase normally contains no entrained solids so its composition appears on the hypotenuse of a triangular diagram like that of Example 14.8. Data for the raffinate phase may be measured as the holdup of solution by the solid, K lb solution/lb dry (oil-free) solid, as a function of the concentration of the solution, y lb oil/lb solution. The correspond-... 

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

As previously stated, leaching is another extraction process in which a liquid is used to remove soluble matter from its mixture with an inert solid. With a few extra considerations, the equilibrium analysis of leaching is the same as for liquid extraction. Several assumptions are made in designing leaching processes. These can be rendered correct with the proper choice of solvent. It is assumed that the solid is insoluble in the solvent (dirt will not dissolve in water) and the flowrate of solids is essentially constant throughout the process. The solid, on the other hand, is porous and will often retain a portion of the solvent. 

Solid-phase microextraction (SPME) is a technique that was first reported by Louch et al. in 1991 (35). This is a sample preparation technique that has been applied to trace analysis methods such as the analysis of flavor components, residual solvents, pesticides, leaching packaging components, or any other volatile organic compounds. It is limited to gas chromatography methods because the sample must be desorbed by thermal means. A fused silica fiber that was previously coated with a liquid polymer film is exposed to an aqueous sample. After adsorption of the analyte onto the coated fiber is allowed to come to equilibrium, the fiber is withdrawn from the sample and placed directly into the heated injection port of a gas chromatograph. The heat causes desorption of the analyte and other components from the fiber and the mixture is quantitatively or qualitatively analyzed by GC. This preparation technique allows for selective and solventless GC injections. Selectivity and time to equilibration can be altered by changing the characteristics of the film coat. 

Leaching is controlled by the rate at which liquid flows past individual particles. For situations and materials where the liquid flows around the particles in the solid matrix, the rate of flow is driven by the hydraulic head of the liquid above the solid material and the efficiency of solute transfer is controlled by the contact between the surface area of the particles and the liquid. Where the rate of flow is very fast and the time over which a state of solute transfer equilibrium is established is slow, leaching may not reach true chemical equilibrium hence not all the leachable components will be leached as quickly as in the situation where the rate of flow is slower. Leaching tests are all based on the assumption that equilibrium or semi-equilibrium is reached in the timescale of the test. 

Generally, the solids are not structurally homogeneous, but the solid and liquid nevertheless will be called phases and leaching will be treated as a two-phase, mass transfer process- The solid consists of a matrix of insoluble solids, the mare, and the occluded solution. It may also contain undissolved solute and a nonextractable secondary phase, for example, coffee oil in water-soaked coffee grounds. This secondary phase is treated as part of the mate. Dimensionless parameters that can affect solnta transfer include the solute equilibrium distribution coefficients, m and M tha Pick number, v the strippirg factor, a the Biot number, Bi and the Peclet number, Be. These parameters are defined more precisely in the Notation section. 

D8. In the leaching of sugar from sugar cane, water is used as the solvent. Typically about 11 stages are used in a countercurrent Rotocel or other leaching system. On a volumetric basis liquid flow rate/solid flow rate = 0.95. The effective equilibrium constant is i% = 1.18, where m = (concentration, g/liter, in liquid)/(concentration, liter, in solid) fSchwartzberg. 19801. If pure water is used as the inlet solvent, predict the recovery of sugar in the solvent. 

D14. Barium sulfide is produced by reacting barium sulfate ore with coal. The result is barium black ash, which is BaS plus insoluble solids. Since BaS is soluble in water, it can be leached out with water. In thickeners the insoluble solids in the underflow typically carry with them 1.5 kg liquid per kg insoluble solids. At equilibrium the overflow and underflow liquids have the same BaS concentrations fTreybal. 19801. We want to process 350 kg/h of insoluble solids plus its associated underflow liquid containing 0.20 mass fraction BaS. Use a countercurrent system with 2075 kg/h of water as solvent. The entering water is pure. We desire the outlet underflow liquid to be 0.00001 mass fraction BaS. Find ... 

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. 

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