Recovery factor, solvent extraction

Because of cost factors, solvent extraction applied to large scale hydrometallurgical processes, such as the recovery of copper from acidic ore leach solutions, is carried out with the most selective reagent for e.g., copper versus iron, which is not itself a liquid solvent, in a petroleum diluent that confers on the mixture the desired physical properties. For the particular case of copper recovery, commercial hydroxyoxime reagents have been used on a very large scale, but their discussion is outside the scope of this book. 

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. 

The second part deals with applications of solvent extraction in industry, and begins with a general chapter (Chapter 7) that involves both equipment, flowsheet development, economic factors, and environmental aspects. Chapter 8 is concerned with fundamental engineering concepts for multistage extraction. Chapter 9 describes contactor design. It is followed by the industrial extraction of organic and biochemical compounds for purification and pharmaceutical uses (Chapter 10), recovery of metals for industrial production (Chapter 11), applications in the nuclear fuel cycle (Chapter 12), and recycling or waste treatment (Chapter 14). Analytical applications are briefly summarized in Chapter 13. The last chapters, Chapters 15 and 16, describe some newer developments in which the principle of solvent extraction has or may come into use, and theoretical developments. 

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... 

Step 4. Calculate the percent of analyte extracted into the organic solvent at equilibrium. The recovery factor, Rx, is the fraction of the analyte extracted divided by the total concentration of the analyte, multiplied by 100 to give the percentage recovery ... 

In laboratory tests using simulated HLW solution spiked with fission product tracers, Am and Cm, the denitration step proved to be a sensitive process, but Am/Cm recoveries of ca. 90% in the aqueous supernate could be realized under optimized conditions. Decontamination factors (DF) > 1000 for Zr, Nb, Mo, and 100 for Ru and Fe were obtained in the precipitation step. The solvent extraction cycle gave > 98% recovery of Am/Cm and DF > 10 for rare earths, Sr and Cs. Appreciable decontamination was also obtained for Zr/Nb (DF = 20), Ru (50), U (650), Pu (250), Np (800) and Fe (420). The ion exchange cycle served mainly for Am-Cm concentration and for removal of DTPA and lactic acid based on tests with europium as a stand-in for trivalent actinides, concentration factors of about 50 could be expected under optimized conditions. 

The feed to an aromatics complex is normally a C6+ aromatic naphtha from a catalytic reformer. The feed is split into Cg+ for xylene recovery and C7 for solvent extraction. The extraction unit recovers pure benzene as a product and C7+ aromatics for recycling. A by-product of extraction is a non-aromatic C6+ raffinate stream. The complex contains a catalytic process for disproportionation and transalkylation of toluene and C9+ aromatics, and a catalytic process for isomerization of C8 aromatics. Zeolitic catalysts are used in these processes, and catalyst selectivity is a major performance factor for minimizing ring loss and formation of light and heavy ends. The choice of isomerization catalyst is dependent on whether it is desired to isomerize ethylbenzene plus xylenes to equilibrium or to dealkylate ethylbenzene to benzene while isomerizing the xylenes. Para-selectivity may also be a desired... 

The result of the separation of U and Pu from the fission products and other actinides is characterized by the decontamination factor, given by the ratio of the activity of the fission products and actinides in the fuel to that in uranium and plutonium after separation. The decontamination factors should be in the order of 10 to 10, and the recoveries of U and Pu should be near to 100%. These requirements are best met by solvent extraction procedures. With respect to the high activity of the fuel, remote control of all operations is necessary. 

The solvent extraction process consists of the unit operations of solvent extraction, meal desolventizing, meal drying and cooling, miscella distillation, and solvent recovery. These unit operations are highly interrelated, primarily because of various heat recovery methods that link the operations together. A process upset in any one of the unit operations will typically cause abnormal operation in the others. The key to efficient solvent extraction operation is process consistency. Consistency of seed preparation, consistency of rate, and consistency of solvent extraction operating parameters are all important factors in operating a safe, environmentally friendly, and cost-effective solvent extraction process for oil extraction. 

The ideal solvent would be easily recovered from the extract. For example, if distillation is the method of recovery, the solvent-solute mixture should have a high relative volatility, low heat of vaporization of the solute, and a high equilibrium distribution coefficient. A high distribution coefficient will translate to a low solvent requirement and a low extract rate fed to the solvent recovery column. These factors will minimize the capital and operating costs associated with the distillation system. In addition to the recovery aspects, the solvent should have a high selectivity (ratio of distribution coefficients), be immiscible with the carrier, have a low viscosity, and have a high density difference (compared to the carrier) and a moderately low interfacial tension. 

Choice of Solvent The solvent selected will offer the best balance of a number of desirable characteristics high saturation limit and selectivity for the solute to be extracted, capability to produce extracted material of quality unimpaired by the solvent, chemical stability under process conditions, low viscosity, low vapor pressure, low toxicity and flammability low density low surface tension, ease and economy of recovery from the extract stream, and price. These factors are listed in an approximate order of decreasing importance, but the specifics of each application determine their interaction and relative significance, and any one can control the decision under the right combination of process conditions. 

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