Emulsions mixer-settlers

Each cell in the extraction system presented in Fig. 122 is called a mixer-settler extractor and is made up of two parts. The role of the first part, the mixer, is to emulsify the incoming aqueous and organic phases and to transfer the emulsion to the second part of the extractor-settler cell. The purpose of the settler is to stratify the phases and enable the separation of the two liquids. 

The archetypal, stagewise extraction device is the mixer-settler. This consists essentially of a well-mixed agitated vessel, in which the two liquid phases are mixed and brought into intimate contact to form a two phase dispersion, which then flows into the settler for the mechanical separation of the two liquid phases by continuous decantation. The settler, in its most basic form, consists of a large empty tank, provided with weirs to allow the separated phases to discharge. The dispersion entering the settler from the mixer forms an emulsion band, from which the dispersed phase droplets coalesce into the two separate liquid phases. The mixer must adequately disperse the two phases, and the hydrodynamic conditions within the mixer are usually such that a close approach to equilibrium is obtained within the mixer. The settler therefore contributes little mass transfer function to the overall extraction device. 

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. 

In the early 1970s Li [13] proposed a method that is now called Emulsion (surfactant) Liquid Membrane (ELM) or Double Emulsion Membrane (DEM) (Fig. 3). The name reveals that the three liquid system is stabilized by an emulsifier, the amount of which reaches as much as 5 % or more with respect to the membrane liquid. The receiving phase R, which usually has a smaller volume than the donor solution, F of similar nature, is finally dispersed in the intermediate phase, M. In the next step the donor solution F is contacted with the emulsion. For this purpose, the emulsion is dispersed in the donor solution F by gentle mixing typically in a mixer-settler device. After this step, the emulsion is separated and broken. The enriched acceptor solution is further processed and the membrane liquid M is fed back for reuse. 

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 advantages and disadvantages of membrane based processes and pertraction through various types of liquid membranes are summarized in Table 23.5. HF contactors are supposed in these processes with the exception of pertraction into stable emulsions (ELM) where mixed column contactors or mixer-settlers are used. 

Extraction can be enhanced by the application of a dc or pulsed electric field, typically on the order of 1 kV/cm. This requires that the aqueous phase be dispersed and the organic phase be of low conductivity. The improvement in mass transfer rate is due to the breakup of large drops by the action of the field and to the increase of drop velocity resulting in increased mass transfer coefficients. It has also been found that low-frequency pulsed fields are effective in breaking up emulsions in the settler stage of mixer-settler units. 

Although the process proved satisfactory from the chemical standpoint, practical problems emerged in that the hydraulic operation of the mixer-settler batteries was extremely poor. In effect, as soon as the aqueous solutions from the dissolution of irradiated targets were placed in contact with the organic extraction phases, a stable emulsion was formed, produced by the appearance of extensive precipitates at the aqueous solution/organic solution interface. As no chemical remedy was found to solve this problem, we attempted to adapt this type of process to extraction chromatographic techniques. 

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. 

More often a mechanical agitator is required to form the emulsion of the two immiscible liquids at each stage. One common design is the mixer-settler (shown above). The two liquid phases are fed to the tank where the mixer imparts mechanical energy and thus disperses one of the phases as fine droplets in the second phases (usually continuous). This state of mixtures is called an emulsion. 

Wardius [51] extended the advancing front model to be employed to multistage mixer-settler systems for liquid membrane operations. They presented a zero order solution to the perturbation equations based on the model developed by Ho et al. [29]. The emulsion globule residence time distribution in each mixer was assumed to be exponential and the fractional utilization of internal reagent was given by... 

Co and Ci are the continuous phase solute concentrations in teed and inside the mixer, respectively, and are the feed rates of the continuous and emulsion phases, respectively, Cir is the internal reagent concentration (based on volume of emulsion), R is the emulsion globule radius, Kr is the emulsion phase holdup volume in the mixer, a is the partition coefficient for solute between external phase and emulsion, is the effective solute diffusivity in the emulsion, and % is the dimensionless reaction front position. Hatton and Wardius [51] also extended their analysis to develop simple graphical and numerical procedures for the prediction of multistage extraction performance of mixer-settler trains operating either cocurrendy or countercurrently without any external recycle over individual stages. For a typical stage i in a cocurrent mixer-settler, they defined the parameter 6 as... 

As a standard appliance in the extraction of uranium, a multistage mixer-settler arranganent with concurrent flow of two phases, water and organic, is used. It can be replaced by novel design with membrane contactors that avoid the constraints of conventional systems. Manbrane extraction used for uranium recovery has many advantages over conventional methods, like no fluid-fluid dispersion, no emulsion... 

Columns are useful for processing low flow rates and for systems that exhibit a tendency to form emulsions. An important benefit of a column contactor is the large number of possible theoretical stages and the ability to operate closer to the operating line rather than the equilibrium curve, thereby maximising mass-transfer kinetics. The settling volume is considerably lower than for the corresponding mixer-settler, so columns are preferred for systems in which solution lock-up and low solvent inventories are important (such as in precious metal extraction systems). Columns take up very little floor space, but require considerable headroom mixer-settler requirements are the opposite (Movsowitz et al. 2001 Fox et al. 1998). 

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. 

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