Solid Extraction Leaching

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. 

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 liquid-solid extraction (LSE) the analyte is extracted from the solid by a liquid, which is separated by filtration. Numerous extraction processes, representing various types and levels of energy, have been described steam distillation, simultaneous steam distillation-solvent extraction (SDE), passive hot solvent extraction, forced-flow leaching, (automated) Soxh-let extraction, shake-flask method, mechanically agitated reflux extraction, ultrasound-assisted extraction, y -ray-assisted extraction, microwave-assisted extraction (MAE), microwave-enhanced extraction (Soxwave ), microwave-assisted process (MAP ), gas-phase MAE, enhanced fluidity extraction, hot (subcritical) water extraction, supercritical fluid extraction (SFE), supercritical assisted liquid extraction, pressurised hot water extraction, enhanced solvent extraction (ESE ), solu-tion/precipitation, etc. The most successful systems are described in Sections 3.3.3-3.4.6. Other, less frequently... 

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... 

C Extraction. Ba occurs in nature chiefly in two minerals, barytes BaS04 and witherite BaG03. Ba is recovered from barytes by reduction of the sulfate with carbon to give BaS. The resulting solid is leached with HOH to dissolve the BaS, and the solution is employed to prepare Ba salts. 

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 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. 

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. 

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... 

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. 

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. 

In ancient times, water was used to remove alkalies (KOH, NaOH) from wood ashes, a process known as lixiviation. Later on, the process for partially dissolving a solid material to remove a desired component was called leaching, a term still used by industry. Chemists prefer the term liquid-solid extraction to distinguish the process from liquid-liquid extraction. 

Size-reduction equipment used to prepare solids for leaching is usnaily selected so as to provide the desired mean particle size without excessive generation of fines or oversize panicles. Therefore, in many instances grinders or cutters employing sharp uniformly spaced kaives are used, in certain cases these carters also provids corrugated particle surfaces that manimize surfnce exposure to the extract. 

In liquid extraction, sometimes called solvent extraction, a mixture of two components is treated by a solvent that preferentially dissolves one or more of the components in the mixture. The mixture so treated is called the raffinate and the solvent-rich phase is called the extract. The component transferred from raffinate to extract is the solute, and the eomponent left behind in the raffinate is the diluent. The solvent in the extract leaving the extractor is usually recovered and reused. In extraction of solids, or leaching, soluble material is dissolved from its mixture with an inert solid by means of a liquid solvent. The dissolved material, or solute, can then be recovered by crystallization or evaporation. Crystallization is used to obtain materials in attractive and uniform crystals of good purity, separating a solute from a melt or a solution and leaving impurities behind. 

This chapter discusses the methods of removing one constituent from a solid or liquid by means of a liquid solvent. These techniques fall into two categories. The first, called leaching or solid extraction, is used to dissolve soluble matter from its mixture with an insoluble solid. The second, called liquid extraction, is used to separate two miscible liquids by the use of a solvent that preferentially dissolves one of them. Although the two processes have certain common fundamentals, the differences in equipment and, to some extent, in theory are sufficient to justify separate treatment. 

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