Solid-Liquid Extraction Leaching

Distillation is probably the most widely used separation technique in the chemical process industries, and is covered in Chapter 11 of this volume, and Chapter 11 of Volume 2. Solvent extraction and the associated technique, leaching (solid-liquid extraction) are covered in Volume 2, Chapters 13 and 10. Adsorption, which can be used for the separation of liquid and gases mixtures, is covered in Chapter 17 of Volume 2. Adsorption is also covered in the books by Suziki (1990) and Crittenden and Thomas (1998). 

Many industrial processes begin with a leaching step, yielding a slurry that must be clarified before solvent extraction. The solid-liquid separation is a costly step. The solvent extraction of unclarified liquids ( solvent-in-pulp ) has been proposed to eliminate solid-liquid separation. The increased revenue and reduced energy cost make this an attractive process, but many problems remain to be solved loss of metals and extractants to the solid phase, optimization of equipment design, effluent disposal, etc. 

Leaching Solid Liquid Liquid solvent Extraction of sucrose from sugar beets with hot water... 

Among engineers, population balance concepts are of importance to aeronautical, chemical, civil (environmental), mechanical, and materials engineers. Chemical engineers have put population balances to the most diverse use. Applications have covered a wide range of dispersed phase systems, such as solid-liquid dispersions (although with incidental emphasis on crystallization systems), and gas-liquid, gas-solid, and liquid-liquid dispersions. Analyses of separation equipment such as for liquid-liquid extraction, or solid-liquid leaching and reactor equipment, such as bioreactors (microbial processes) fluidized bed reactors (catalytic reactions), and dispersed phase reactors (transfer across interface and reaction) all involve population balances. 

We now turn from liquid-liquid extraction to solid-liquid extraction. In more casual terms, we turn from extraction to leaching. As expected, we can use the same ideas of differential and staged separations to handle this case as well. We begin with target separations, we list separation equipment, and we then describe differential and staged leaching. The summary leads to the case of unsteady-state leaching, a transitional case best handled by parallels with the analysis of adsorption in the next chapter. 

Extraction (sometimes called leaching) encompasses liquid-liquid as well as liquid-solid systems. Liquid-liquid extraction involves the transfer of solutes from one liquid phase into another liquid solvent it is normally conducted in mixer settlers, plate and agitated-tower contacting equipment, or packed or spray towers. Liquid-solid extraction, in which a liquid solvent is passed over a solid phase to remove some solute, is carried out in fixed-bed, moving-bed, or agitated-solid columns. 

In most unit operations it is of considerable importance that material is transferred from one phase to another across a boundary. The transfer of material from a solid phase to a liquid phase (as typically in leaching), or the transfer of material between one liquid phase to another liquid phase (as typically in molten metal and molten slag phases), extraction or between liquid and vapor phases (as typically in distillation) are well-known examples encountered in practice. 

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. 

Liquids can be extracted from solids by leaching. As the name implies, the soluble liquid contained in a solid is leached out by contacting the solid with a suitable solvent. A principal application of leaching is in the extraction of valuable oils from nuts and seeds such as, palm oil and rape seed oil. 

Before considering particular applications, the potential for the recovery of values from wastes is discussed. In other chapters the various applications of solvent extraction have been presented and the technology has been shown to be highly selective, easy to operate and control, and versatile in terms of scale. Therefore, it seems to be ideal for application in the treatment of wastes, both solid (after leaching) and liquid. As noted earlier, however, there are environmental limitations mainly caused by unintentional release of the solvent into the environment. These problems can be minimized by ensuring that the treated solutions are recycled within the plant and that any solutions discharged into the environment are treated appropriately to... 

For liquid or solid samples more complex than water, a combination of techniques is commonly required. Certainly a first step involves a need to obtain the components of interest in a solution phase. This may either involve leaching of a solid or extraction of a liquid sample with or without concurrent concentration. If the components of interest are then obtained in a water system, the techniques applicable to water analyses are immediately available. Conversely, if the extraction is into a non-miscible organic solvent and the components sought can be reextracted into water by appropriate choice of pH, then again the techniques of water sample processing can be used. 

Numerous extraction methods and techniques have been developed and reported, especially if one takes into account the variety of modifications. The most common and simple general classification of these methods is similar to that introduced in chromatography and based on the kind of phase to which the analyte is transferred. One can distinguish the extractions as liquid, solid, gas, and supercritical fluid phase extractions. More precise description specifies the two phases between which the analyte is distributed (e.g., liquid-liquid or solid-liquid [leaching] extractions). The latter methods are all called solvent extraction. 

Ultrasound-assisted extraction (USE) is an effective method for leaching many analytes from different kinds of samples [52-55]. It is simple, fast, efficient, and inexpensive in comparison with conventional extraction techniques such as solvent extraction in the Soxhlet apparatus. Ultrasound-assisted solid-liquid extraction is an effective and time-saving extraction method. Sonication accelerates the mass-transfer process between two phases. Use of ultrasound results in a reduction in operating temperature, allowing the extraction of temperature-sensitive components. The ultrasound apparatus is cheaper and its operation is easier in comparison with other novel extraction techniques such as MAE. 

Extraction is the process of transferring a substance from a solid to a liquid phase or from a liquid to another liquid phase (immiscible with the former). From a practical viewpoint, the process can be achieved by leaching, which is transfer of compoimds from a solid phase to a solution (solid-liquid extraction, SEE) or by extraction via direct (liquid-liquid extraction, LEE) or indirect (SPE or solid phase microextraction, SPME) transfer of a substance from one liquid phase to another [75]. The efficiency of the extraction process is expressed as the percentage of extraction, which takes into accoimt the affinity of the investigated compoimds for both phases. In practice, a commonly used concept is that of recovery, understood as the degree of transition of a substance from one phase to another, expressed as a percentage. There are multiple methods for determining recovery. They can be divided into two classes ... 

Leaching or solid-liquid extraction are terms that describe the extraction of soluble constituents from a solid or semisolid by means of suitable solvents. The process, which is used whenever tea or coffee is made, is an important stage in the production of many fine chemicals found naturally in animal and vegetable tissue. Examples are found in the extraction of fixed oils from seeds, in the preparation of alkaloids, such as strychnine from Nux vomica beans or quinine from Cinchona bark and in the isolation of enzymes, such as rennin, and hormones, such as insulin, from animal sources. In the past, a wider importance attended the process because the products of simple extraction procedures, known as galenicals, formed the major part of the ingredients used to fulfill a doctor s prescription. 

Whatever the scale of the extraction, leaching is performed in one of two ways. In the first, the raw material is placed in a vessel, forming a permeable bed through which the solvent or menstrum percolates. The wanted constituents are dissolved, and the solution issues from the bottom of the bed. This liquid is sometimes called the miscella and the exhausted solids, the marc. The process is called leaching by percolation. The second process employs immersion and consists of immersing the solid in the solvent and stirring. After a suitable period of time, solid and liquid are separated. 

Comparison of ASE with other solid-liquid extraction (leaching) techniques... 

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