The solvent extraction process

Solvent Recovery System (Hexane-Steam Condensing) ----------WT  

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Screw-pressed oil is aUowed to stand to settle out suspended soUds, filtered through plate filter presses, and then pumped to storage. The oil-rich solvent (miscella) from the solvent-extraction process is filtered or clarified, and most of the solvent is removed in a long tube evaporator. FinaUy, the concentrated oil passes through a stripping column where sparging steam is injected to remove the residual solvent. A metric ton of cottonseed yields ca 91... 

Tantalum Compounds. Potassium heptafluorotantalate [16924-00-8] K TaF, is the most important tantalum compound produced at plant scale. This compound is used in large quantities for tantalum metal production. The fluorotantalate is prepared by adding potassium salts such as KCl and KF to the hot aqueous tantalum solution produced by the solvent extraction process. The mixture is then allowed to cool under strictiy controlled conditions to get a crystalline mass having a reproducible particle size distribution. To prevent the formation of oxyfluorides, it is necessary to start with reaction mixtures having an excess of about 5% HF on a wt/wt basis. The acid is added directiy to the reaction mixture or together with the aqueous solution of the potassium compound. Potassium heptafluorotantalate is produced either in a batch process where the quantity of output is about 300—500 kg K TaFy, or by a continuously operated process (28). 

The products of the solvent extraction process are tantalum strip solution, niobium strip solution and raffinate - liquid wastes containing impurities and residual acids. 

The various equilibria involved in the solvent-extraction process are expressed in terms of the following thermodynamic constants ... 

In the solvent-extraction process, the platinum metal concentrate is solubilized in acid using chlorine oxidant. Ruthenium and osmium are separated by turning them into the volatile tetroxides. 

Phase ratio is given by the volume ratio of the two phases, and its significance is in the solvent extraction process. A small organic to aqueous ratio although beneficial ideally, it is sometimes unwelcome because it may result in high solvent losses. A raised organic to aqueous ratio, on the other hand, needs a large reservoir of solvent, and this may imply a financial burden. 

A knowledge of the extraction equilibria between the organic and aqueous phases helps to identify the operational variables that can control the solvent extraction process. An example - the extraction of copper from a copper sulfate solution using a chelating reagent (HR) - is considered. This is one of the best studied examples of solvent extraction. Normally, the system would not be described as a water-hydrocarbon dual-phase system, as it is in fact the Cu2+, SO-, H+, R-, and R-, and the equation... 

Solvent extraction is often applied to separate two chemically similar metals such as nickel/ cobalt, adjacent rare earths, niobium/tantalum, zirconium/hafnium, etc. For the purpose of elaboration, the example of the separation of two chemically similar elements such as zirconium and hafnium from their nitrate solution, using TBP as an extractant is considered. The solvent extraction process in this case is chemically constant (K) is given by ... 

Like the solvent extraction process, extractive distillation relies on the intimate contact of the liquid solvent and the aromatics concentrate vapors to allow the aromatics to be preferentially dissolved in the solvent. The usual list of solvents includes DEG (Diethylene glycol), TEG (Triethylene glycol), NMP (N-methyl pyrrolidone), or methyl formamide. 

None of the authors of this book is an expert in all the aspects of solvent extraction, nor do we believe that any of our readers will try to become one. This book is, therefore, written by authors from various disciplines of chemistry and by chemical engineers. The scientific level of the text only requires basic chemistry training, but not on a Ph.D. level, though the text may be quite useful for extra reading even at that level. The text is divided in two parts. The first part covers the fundamental chemistry of the solvent extraction process and the second part the techniques for its use in industry with a large number of applications. In this introductory chapter we try to put solvent extraction in its chemical context, historical as well as modem. The last two chapters describe the most recent applications and theoretical developments. 

In principle, solvent extraction is an environmentally friendly process with no air or water pollution, provided the plant flows are properly designed. It could, therefore, replace many of the present polluting processes. A particular problem, however, is the solvent extraction effluents, which may contain biochemically active substances posing new hazards to the environment. These can be handled by various solid sorbents, which then can be incinerated, but the advantage of the solvent extraction process may be lost. There is, therefore, a demand for biodegradable and environmentally benign solvent phases. In the future, additional attention is required to this field. 

From plots of the distribution ratio against the variables of the system— [M], pH, [HA] , [B], etc.—an indication of the species involved in the solvent extraction process can be obtained from a comparison with the extraction curves presented in this chapter see Fig. 4.3. Sometimes this may not be sufficient, and some additional methods are required for identifying the species in solvent extraction. These and a summary of various methods for calculating equilibrium constants from the experimental data, using graphical as well as numerical techniques is discussed in the following sections. Calculation of equilibrium constants from solvent extraction is described in several monographs [60-64]. 

The development of a solvent extraction process from a given aqueous feed solution may be confined by several restraints. For example, the temperature, flow rate, acidity, and so forth may be (essentially) fixed, and the solvent extraction process developed must then be capable of accommodating these restrictions. 

Soluble loss of a reagent (extractant, modifier, or diluent) from the solvent phase is an inherent part of the solvent extraction process, since all organic compounds are soluble, to some extent, in water. The conditions prevailing in the system can also promote solubility, which can be a particular problem if the composition and properties of the aqueous phase are inflexible. For example, the solubility of alkylphosphoric acid and carboxylic acid extractants is dependent on temperature, pH, and salt concentration in the aqueous phase. 

The economics of the solvent extraction process are very dependent upon upstream and downstream portions of the plant. Integration of the total processing step is essential to obtain maximum return. Variables, such as tonnage rates and changes in solution composition, can have a most significant result on the economics of solvent extraction. Generally, economic considerations may be divided into two major areas (1) capital investments and (2) operating costs. 

In the optimization of the solvent extraction process for the recovery of copper using LIX 64N, Robinson [77] described the eost funetion in terms of the sum of the operating and capital costs. The operating eosts were taken as resulting from losses of eopper and solvent ... 

The design criteria necessary for optimization of the engineering and design of the solvent extraction process can probably be summarized as follows ... 

This chapter therefore outlines the methods for answering these types of questions, so that the solvent extraction process may be applied in practice and desired components may be recovered in an energy-efficient, environmentally safe, and economical way. 

These general differences in chemical behavior have been exploited to provide the solvent extraction processes currently used or proposed for cobalt-nickel separation. All these processes remove cobalt from nickel... 

The solvent extraction process that uses TBP solutions to recover plutonium and uranium from irradiated nuclear fuels is called Purex (plutonium uranium extraction). The Purex process provides recovery of more than 99% of both uranium and plutonium with excellent decontamination of both elements from fission products. The Purex process is used worldwide to reprocess spent reactor fuel. During the last several decades, many variations of the Purex process have been developed and demonstrated on a plant scale. 

Since the solvent-extraction process is not selective for tungsten over molybdenum, any of the latter metal present is removed by the addition of sodium sulfide at a pH value of about 10... 

The complexity of the problem and the diversity of operating conditions in saline water conversion make it unlikely that any process based on one principle or phenomenon will provide the most efficient conversion in all operating situations encountered. The art of saline water conversion has now reached a level at which one can begin to take stock with respect to the particular advantages of the many different processes in any given situation. The final selection of a process will only be possible after careful consideration of process operation data as applied to the conversion problem at hand. Since such data are available on but very few processes at the present time, it is only possible to project on the basis of theory and experience those points which set apart one process from another. The purpose of this paper is to present information now available which may help to locate the solvent extraction process in its rightful position in the saline water conversion field. 

The solvent extraction process has not yet undergone pilot plant investigation, and all the above estimates are based on small laboratory or bench scale experiments. If further testing under practical conditions substantiates the laboratory observations, it appears that the solvent extraction process definitely has an area of specialization in the over-all saline water conversion program. 

Hydrolytic and radiolytic degradation of TAP solution in normal paraffinic hydrocarbon (NPH) in the presence of nitric acid was investigated. Physicochemical properties such as density, viscosity, and phase-disengagement time (PDT) were measured for undegraded and degraded solutions (197). The variations in these parameters were not very different from those obtained with degraded TBP. Thus, the hydro-dynamic problems expected during the solvent-extraction process with TAP would be similar to those encountered with TBP/NPH system. The influence of chemical... 

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