Mixer-settler studies

Waldrop, F. B. Zirconium-Hafnium Separation Mixer-Settler Studies. Fiiul Report. U.S.A.E.C. Report, Y-612 (1950). 

The development of the novel Davy-McKee combined mixer—settler (CMS) has been described (121). It consists of a single vessel (Fig. 13d) in which three 2ones coexist under operating conditions. A detailed description of units used for uranium recovery has been reported (122), and the units have also been studied at the laboratory scale (123). AppHcation of the Davy combined mixer electrostatically assisted settler (CMAS) to copper stripping from an organic solvent extraction solution has been reported (124). 

Laboratory Extractors. Pilot-Scale Testing, and Scale-Up. Several laboratory units arc useful in analysis, process control, and process studies. The AKUFVE contactor incorporates a separate mixer and centrifugal separator. It is an efficient instrument for rapid and accurate measurement of partition coefficients, as well as for obtaining reaction kinetic data. Miniature mixer-settler assemblies set up as continuous, bench-scale, multistage, countercurrent, liquid-liquid contactors are particularly useful Tor the preliminary laboratory work associated with flow-sheet development and optimization because these give a known number of theoretical stages. 

Pertraction (PT) can be realized through a liquid membrane, but also through a nonporous polymeric membrane that was applied also industrially [10-12]. Apart from various types of SLM and BLM emulsion liquid membranes (ELM) were also widely studied just at the beginning of liquid membrane research. For example, an emulsion of stripping solution in organic phase, stabilized by surfactant, is dispersed in the aqueous feed. The continuous phase of emulsion forms ELM. Emulsion and feed are usually contacted in mixed column or mixer-settlers as in extraction. EML were applied industrially in zinc recovery from waste solution and in several pilot-plant trials [13,14], but the complexity of the process reduced interest in this system. More information on ELM and related processes can be found in refs. [8, 13-16]. 

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. 

As for the Pu purification process, a modified method is proposed for the effective stripping of plutonium from TBP (27). The nitrous acid acts as "redox reagent for Pu (IV) in the stripping and in the extraction feeds. The addition of 0.1 mol/1 of nitrous acid to the extraction feed is sufficient to the effective stripping of plutonium up to 99.9 %. This modified flow-sheet was demonstrated through the process studies with miniature mixer-settlers. ... 

For the purpose of improving the decontamination factor (DF) of FPs from U or Pu in the reprocessing of highly irradiated fuels such as those from FBR, a modified method adding inactive zirconium or hafnium ion is proposed. The feasibility of this concept has been experimentally demonstrated by both batchwise extraction and process studies with miniature mixer-settlers. 

A laboratory study was undertaken to determine the behaviour of neptunium in the WAK flowsheet, and to devise a procedure for its recovery. Based on static ( ) and counter-current experiments (J5), the conclusion was reached that about half of the Np is co-extracted with the U and Pu in the HA-HS mixer-settlers of WAK while the other half is rejected to the HAW, see Fig.1. It could also be shown that an increase of the aqueous acidity, or the addition of pentavalent vanadium as an oxidant into the lower stages of the HA mixer-settler (6), would increase the Np yield in the organic solvent. In the 1BX-1BS mixer-settlers where the partitioning of U and Pu is carried out by use of uranium (IV)nitrate - hydrazine nitrate, a splitting of the coextracted Np between the two product streams was observed the proportions of the (co-extracted) Np which ended up in the 1CU (uranium product) stream fluctuated from 30 to 93 % while the difference amount (from 7 to 70 %) ended up in the 1 BP (plutonium product) stream. 

Ito, Y. Zhang, T. Y. "Multistage mixer-settler planet centrifuge Preliminary studies on partition of macromolecules with organic-aqueous and aqueous two-phase solvent systems" /. Chromatography. 1988, 437, pp 121-129. 

Another application of solvent extraction to redox speciation studies is the method developed for iodine (Malmbeck and Skamemark 1995). It was used for online speciation of iodine in the Forsmark BWR power plant in Sweden. Iodine can enter the reactor water in two ways, either by fission of tramp uranium (uranium that is adsorbed on the outer surface of the fuel pins) or via leaking fuel pins. In this method, a tiny stream of water was withdrawn fi-om the reactor system and used as feed for a 20 stage mixer-settler battery. The mixer-settlers were arranged in four batteries with 4, 4, 6, and 6 mixer-settler units. Part of such a mixer-settler battery is shown in O Fig. 52.4. 

Batch Extractions. In nearly all commercial scale operations, a continuous extraction process, either mixer-settlers or colunm units, would be used. However, batch extraction experiments are useful for assessing overall feasibility and for optimizing the many process variables such as emulsion formulation and volume ratios of the internal, membrane, and external phases. Consequently, the most common experiment in this study was the batch extraction. In these experiments, 500 ml of a selenium solution (1 mg/L) were prepared in the extraction vessel, either in the presence or absence of other competing anions. The prepared emulsion (50 ml) was added and the mixture was stirred at a speed of 150 rpm. In this manner the emulsion drops were uniformly dispersed in the external phase while extraction proceeded. Samples of the external aqueous phase were taken at appropriate intervals and the concentrations of Se(IV), Se(VI), and sulfate were determined. 

Often solvent extraction is carried out continuously in a countercurrent multistage device/cascade. The sieve-plate tower is an example of one multistage device, whereas Figure 8.1.37(a) illustrates a multistage arrangement of N mixer-settler devices (one such device is studied in Section 6.4.1.2). First, we analyze a dilute solution of solute i in a feed-extract phase system assumed to be essentially insoluble in each other (Cussler, 1997). Then we will analyze extraction systems with some mutual solubility of the feed and the extraction solvent. 

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