Solid Extraction or Leaching

There are two types of extraction. Solid extraction or leaching involves solids which are leached or extracted by solvent, and liquid extraction where immiscible solvent is used to extract liquid product from the mixture. The following chapter is based on material from [86, 3, 5]. 

There are three processes involved in leaching or solid extraction. Dissolution of solids, separation of solvent from insoluble sohd material, and washing. Most of the leaching plants are operated batchwise. In a batch leaching, the solids are stationary and solvent is flowing through the bed of particles. 

Constant underflow means constant overflow as given by Eqnation 7.5 below. 

This is the case of constant underflow where slope of the operating line 

Using the equilibrium relation and the operating line given by Equation 7.3, McCabe-Thiele procedure can be performed. This is illustrated in the following example. 

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EXTRACTION I Liquid-Solid). Many substances used in modem processing industries occur in a mixture of components dispersed through a solid material, To separate the desired solute constituent or to remove an unwanted component from the solid phase, the solid is contacted with a liquid phase in Ihe process called liquid-solid extraction, or simply leaching In leaching, when an undesirable component is removed from a solid with water, the process is called washing. 

The term species generally refers to the molecular forms of an element or a cluster of atoms of different elements in a given (in this case solid) matrix (Bernhard et al., 1986a). The term form is also used to indicate uncertainty or lack of knowledge about the exact nature of the species one expects to find in an environmental sample. Conditional speciation assessment techniques like sequential extractions or leaching tests are not able to yield information at the true molecular level in solid phases thus the more general term form is used rather than species when referring to the results of such techniques commonly applied to date to soils and sediments. 

Liquid-liquid extraction is a process for separating components in solution by their distribution between two immiscible liquid phases. Such a process can also be simply referred to as liquid extraction or solvent extraction however, the latter term may be confusing because it also applies to the leaching of a soluble substance from a solid. 

Batch Percolators The batch tank is not unlike a big nutsche filter it is a large circiilar or rectangiilar tank with a false bottom. The solids to be leached are dumped into the tank to a uniform depth. They are sprayed with solvent until their solute content is reduced to an economic minimum and are then excavated. Countercurrent flow of the solvent through a series of tanks is common, with fresh solvent entering the tank containing most nearly exhausted material. In a typical ore-dressing operation the tanks are 53 by 20 by 5.5 m (175 by 67 by 18 ft) and extract about 8200 Mg (9000 U.S. tons) of ore on a 13-day cycle. Some tanks operate under pressure, to contain volatile solvents or increase the percolation rate. A series of pressure tanks operating with countercurrent solvent flow is called a diffusion battery. 

The leached solids must be separated from the extract by settling and decantation or by external filters, centrifuges, or thickeners, all of which are treated elsewhere in Sec. 18. The difficulty of solids-extract separation and the fact that a batch stirred tank provides only a single equilibrium stage are its major disadvantages. 

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. 

Leaching is concerned with the extraction of a soluble constituent from a solid by means of a solvent. The process may be used either for the production of a concentrated solution of a valuable solid material, or in order to remove an insoluble solid, such as a pigment, from a soluble material with which it is contaminated. The method used for the extraction is determined by the proportion of soluble constituent present, its distribution throughout the solid, the nature of the solid and the particle size. 

The variety of extractors used in liquid-solid extraction is diverse, ranging from batchwise dump or heap leaching for the extraction of low grade ores to continuous countercurrent extractors to extract materials such as oilseeds and sugar beets where problems of solids transport have dominated equipment and development. 

One or more components of sucli mixture are removed (extracted) by exposing the mixture to the action of a solvent in which the component to be removed is soluble. If the mixture consists of two or more solids, extraction is performed by percolation of an appropriate solvent through it This procedure is also called leaching, especially if the solvent is water coffee making is and example. Synthetic fuels can be made from coal by extraction with a coal-derived solvent followed by hydrogenation. 

A test method for the batch extraction of treated or untreated solid waste or sludge, or solidified waste, to provide an indication of the leaching potential (ASTM D-5233) is also available. The goal of this test method is to provide an extract for measurement of the concentration of various analytes and therefore may be applied to a study of the smaller molecules that reside within the coal matrix. This test method, as written, is intended to provide an extract suitable for measurement of the concentration of analytes that will not volatilize under the conditions of the test method and may appear to offer limitations on the use of coal, but the test method does describe a procedure for performing a batch extraction of a solid. Again, the sampling and handling requirements that may be associated with the analysis of coal should also be applied to the method. 

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. 

An environmental application of liquid extraction is the removal of trace organics from water. Examples are the separation of acetic acid-water mixtures and removal of solvents, insecticides, pesticides, etc., from water. It can also be applied to the separation of liquids with close boiling points or those that form azeotropes, such that distillation is not useful. In addition, zero- or low-volatility compounds, such as metals and organometallic derivatives, can be separated by liquid extraction as can mixtures of water-hydrogen bonded compounds, such as formaldehyde. Solid extraction (leaching) can be used to remove organics or heavy metals from contaminated soils, sludges or contaminated equipment. 

Besides the theoretical interest in the unusual phase behavior encountered in these systems, the principles involved can be applied in operations wherein the nonideality is intentionally created. The magnitude of solubility of a compound of low volatility in a gas above its critical temperature. .. is sufficient to consider the gas as an extracting medium, that is fluid-liquid or fluid-solid extraction analogous to liquid-liquid extraction and leaching. In this case the solute is removed and the solvent recovered by partial decompression. Thus compression of a gas over a mixture of compounds could selectively dissolve one compound, permitting it to be removed from the mixture. Partial decompression of the fluid elsewhere would drop out the dissolved compound, and the gas could be reused for further extraction. 

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