Extractant loss entrainment

In the liquid-hquid extraction area, in the mining industry, coming out of the leach tanks is normally a slurry, in which the desired mineral is dissolved in the liquid phase. To save the expense of separation, usually by filtration or centrifugation, attempts have been made to use a resident pump extraction system in which the organic material is contacted directly with the slurry. The main economic disadvantage to this proposed system is the fact that considerable amounts of organic liquid are entrained in the aqueous slurry system, which, after the extraction is complete, is discarded. In many systems this has caused an economic loss of solvent into this waste stream. 

Solvent extraction carried out in conventional contactors like mixer-settlers and columns has certain limitations, including (a) controlling optimum dispersion and coalescence, (b) purifying both phases to ensure that stable emulsions are avoided (c) temperature control within a narrow band (d) high entrained solvent losses and related environmental and process economic effects and (e) large equipment dimensions and energy requirements when the density differential or selectivity is low. 

Solubility data on the LIX and Kelex extractants indicate that these materials are poorly soluble in aqueous media. Accordingly, in plant operations at about pH 1.5, reported losses are approximately <15 ppm, which includes both soluble and entrainment losses as determined by inventory (detailed later in Table 7.6). The solubility of LIX 63, LIX 64, and LIX 64N in water at pH 4.8 has been reported at 5.8, 4.3, and 6.2 ppm, respectively [20]. 

In the absence of a solvent-recovery method, entrainment is expected to be the major solvent-loss factor in all solvent-extraction applications [94], including CSSX [89], potentially amounting to several hundred ppm of the aqueous effluent. Solvent loss is known to be the economic determinant in most commercial solvent-extraction systems... 

Cell recovery is mainly concerned with what is known as clear mash, i.e. sugar solutions without any entrained solids suchas as those in a whole-grain mash. The use of cell recovery on mash can be accomplished by clarifying the mash prior to fermentation or else incorporating a wet-milling front end and taking the saccharified starch as substrate. However the production of a clear mash directly from cereal mash is presently not feasible. This is due to excessive losses of carbohydrate in the cake or non-fermentable extract ranging from 10 to 20 per... 

Solvent extraction Podbielniak extraction Membrane solvent extraction Coupled transport Minimum emulsification and associated entrainment loss Enhanced selectivity and concentration... 

The early experiments on solvent extraction directly from leached pulp were beset with problems such as losses of solvent in the aqueous phase and the formation of emulsions. The use of mixer-settler, pump mixer, and internal mixer-settler type contactors on a laboratory scale (Gil) has demonstrated the feasibility of uranium extraction from desanded slurries with 5-1. )% solids and from high-density slurries with 48-60% percent solids. The deemulsification rate of a synthetic slurry as a function of the temperature of the system and the pH of the slurries (T12) and the effect of extractant entrainment in the aqueous effluent on solvent extraction of uranium from slurries containing more than 40% solids (E6) have been studied. 

Ordinary rectification for the dehydration of acetic acid requires many trays if the losses of acid overhead are to be restricted, so that azeotropic processes are used exclusively. Among the entrainers that have been found effective are ethylene dichloride, n-propyl acetate, and n-butyl acetate. Water contents of these azeotropes are 8, 14, and 28.7 wt %, respectively. Accordingly, the n-butyl acetate is the most thermally efficient of these agents. The n-propyl acetate has been used in large installations, in the first stage as solvent for extraction of acetic acid and then as azeotropic entrainer to remove the accompanying... 

Liquid waste streams containing an insoluble liquid can arise from extraction processes, from steam ejectors operating on solvent distillation systems, or from the loss of heat exchange fluid from a heat exchanger. These should be phase-separated before final disposal measures are undertaken. A simple settler, or a unit such as an American Petroleum Institute (API) separator can be used to accomplish this step. Coupling the initial separator to an entrained or dissolved air flotation unit can reduce the concentration of residual organics further [75]. The recovered organics can be recycled via a further cleanup if required, and the water phase more safely discarded. 

Immiscible-liquid solvent extraction is a well-established practice for recovery, concentration, and purification of organophilic solutes (e.g., antibiotics, amino acids, vitamins) present in aqueous process streams such as fermentation broths or plant or animal tissue extracts [88]. The process is, however, frequently rendered difficult or impossible by problems of emulsification, loss of entrained solvent, and contamination by particulate impurities in the feed. Integrated membrane separation with liquid/liquid extraction is iUustrated in Fig. 9.7. 

Figure 10.6 illustrates a typical extractive distillation process consisting of the extractive distillation column and the solvent recovery column. Fresh feed containing the binary AB is introduced around the middle of the extractive distillation column, and the solvent S is introduced near the top. Components A and B are close boilers and/or potentially azeotrope formers that are difficult or impossible to be separated by ordinary distillation. Whether individual component A is more volatile than B or vice versa, in the presence of the solvent, B becomes less volatile due to its higher affinity to the solvent. As a result, essentially pure A is distilled as the overhead of the extractive distillation column. Component B is entrained with the solvent in the bottoms stream, which is sent to the solvent recovery column. The solvent is substantially less volatile than component B, allowing easy separation by ordinary distillation. Practically pure B is recovered in the overhead, and pure solvent in the bottoms. The solvent is recycled to the extractive distillation column with makeup that might be required to compensate for losses. 

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