Solid-liquid leaching factors

The separation of solids from liquids forms an important part of almost all front-end and back-end operations in hydrometallurgy. This is due to several reasons, including removal of the gangue or unleached fraction from the leached liquor the need for clarified liquors for ion exchange, solvent extraction, precipitation or other appropriate processing and the post-precipitation or post-crystallization recovery of valuable solids. Solid-liquid separation is influenced by many factors such as the concentration of the suspended solids the particle size distribution the composition the strength and clarity of the leach liquor and the methods of precipitation used. Some important points of the common methods of solid-liquid separation have been dealt with in Chapter 2. 

Leaching of chemicals from complex materials or matrices is a complicated phenomenon in which many factors may influence the release of the specific organic compounds and inorganic ions. Important factors include major element chemistry, pH, redox, complexation, liquid to solid ratio, contact time, and biological activity. To describe fully the leaching of SWMs/COMs under field conditions, a battery of leaching tests was specifically designed to simulate various physical and chemical release mechanisms. 

Most leaching tests are performed in a laboratory. Whilst leaching tests are designed and intended to reflect reality, there is a limited amount of data available from field tests which can be used to correlate with those from laboratory tests hence validate their performance in relation to the situation in the field. One of the most common errors in the interpretation of results from leaching tests is to assume that, based on the commonly used shaking test at a liquid to solid ratio of 10 1, the resultant leachant is representative of the concentration of contaminants that will emerge from the base of a deposit of the material tested. Even if it is assumed that the factors such as leachant used, pH and redox are correctly applied in the test, the concentrations of contaminants in the leachate represent an average of the concentration which will be leached... 

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. 

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, —... 

Marked axial dispersion in both the liquid and solid phases has been observed in continuous countercurrent leaching systems. The solid-phase dispersion is probably caused by nonuniform conveying and by backmixing caused by the baffles which are used to prevent solid beds from turning en masse. VnliD n values of 16.1 m and 20 m, respectively, have b n repotted for sugar beet extraction in tower and slope extractors. Local fiow nonuniformity and larger-scale flow maldistribution are the primary factors that cause axial dispersion in the extract. 

Rate of leaching when dissolving a solid. When a material is being dissolved from the solid to the solvent solution, however, the rate of mass transfer from the solid surface to the liquid is the controlling factor. There is essentially no resistance in the solid phase if it is a pure material. The equation for this can be derived as follows for a batch system. The following can also be used for the case when diffusion in the solid is very rapid compared to the diffusion from the particle. 

Surprisingly, it is in this area of liquid-solid contact that the stage efficiency comes closest to being quantifiable. This is due to two factors (1) The principal resistance to mass transfer in a stirred vessel usually resides in the solid phase, and (2) one can, as a consequence, apply the "long-time" solution to Pick s equation, outlined in Chapter 4 (Section 4.1.2.2). Solution of the relevant ordinary differential equation (ODE) and conversion to efficiency results in the following expression for both adsorption and leaching (or desorption) ... 

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