Types of Continuous Mixers

The need to use multiple extraction to achieve efficient extraction required the development of new types of continuously working extractors, especially mixer-settlers and pulsed columns, which were suitable for remotely controlled operations. These new extractors could be built for continuous flow and in multiple stages, allowing very efficient isolation of substances in high yield. A good example is the production of rare earth elements in >99.999% purity in ton amounts by mixer-settler batteries containing hundreds of stages. These topics will be further developed in Chapters 6 and 7. 

Fig. 7.31 (a) Schematic view of a plug-type flow continuous mixer with a recirculating stream Aq... 

Tubular Precipitator. This type of continuous operation may be employed to reduce polydispersity of precipitates (Raphael et al. 1997 Raphael and Rohani 1999). The tubular precipitator may operate either under the turbulent flow or laminar flow regime. The reactants are added into the inlet section equipped with static mixers and may also enter as a multi-port feed along the length of the tubular precipitator. If the reactant feeding streams are too concentrated or if too excessive formation of precipitate occurs in the inlet section of the precipitator, a third stream of solvent is also fed to dilute the flowing suspension. The latter may contain a protective colloid or surfactant that prevent agglomeration of precipitate. 

FIG. A.l. Different types of continuous extraction equipment, (a) Mixer-settler, (b) Spray column, (c) Pulsed column, (d) Tubular centrifugal contactor. 

Small-scale solvent extraction is often done batchwise in separation funnels or test tubes. However, for more elaborate extraction studies and industrial processes continuous extraction is usually preferred. All continuous solvent extraction equipment consists of a mixer for phase contact and mass transfer and a separation part for phase disengagement. The main types of continuously working apparatus are ... 

The earliest large-scale continuous industrial extraction equipment consisted of mixer—settlers and open-spray columns. The vertical stacking of a series of mixer—settlers was a feature of a patented column in 1935 (96) in which countercurrent flow occurred because of density difference between the phases, avoiding the necessity for interstage pumping. This was a precursor of the agitated column contactors which have been developed and commercialized since the late 1940s. There are several texts (1,2,6,97—98) and reviews (99—100) available that describe the various types of extractors. 

The muller is useful for mixing problems requiring certain types of aggregate breakdown, fricdional anchorage of particles to one another, and densification of the final mix. Materials which are excessively fluid or sticky should be avoided. The muller mixer is generally used for batch operations (Fig. 19-9g), although Fig. 19-9/j shows a continuous muller. 

Alkaline phenolic resin, ester hardened. The binder is a low viscosity, highly alkaline phenolic resole resin (1.2 to 1.7%). The hardener is a liquid organic ester (18 to 25%). Sand is mixed with hardener and resin, usually in a continuous mixer. The speed of setting is controlled by the type of ester used. Low sand temperature slows the cure rate, but special hardeners are available for cold and warm sand. In 24 h compression strength can reach 4000 kPa (600 psi). 

Extmders with modified barrels and screws have been used to carry out the function of a continuous mixer of mbber compounds. The early machines of this type had both barrels and screws fluted, but at different parts of the chamber. One of these machines is the Transfermix. The mbber commences travel down the barrel of the machine in the flights of the screw, but transfers to... 

In-line mixers manufactured by, for example, Kenics, Lightning, and Sulzer are also applicable for continuous small-scale testing of a solvent extraction process, and 1 inch diameter models are available. This mixer system can be used either horizontally or vertically. However, few data are available for this type of contactor, although they would appear to offer many possibilities, not only for liquid-liquid systems, but also for use in... 

Because of the diversity of contacting equipment available, it is unlikely that all these contactors will be available in any one laboratory or pilot plant. Consequently, unless test work is carried out on similar contactors, the system may not be optimized. Since mixer-settlers are the easiest to construct, are simple to operate, and require little room and low-flow rates, these contactors are, in many cases, the only ones used to investigate a continuous solvent extraction process. This is by no means ideal and may result in abandonment of a process that, using another type of contactor, could be found to be entirely satisfactory. 

In order to combine the advantages of a batch type and continuous production, a prototype for a quasi-continuous production line was developed (15-18,33-35). The principle of this quasi-continuous production line is based on a semi-continuous production of mini-batches in a specially designed high-shear mixer/granulator which is connected to a continuous multicell-fiuidized (Glatt Multicell ) bed dryer (Fig. 10). 

---