Mixers-settlers pumps

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. 

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 simple box-type mixer—settler (113) has been used extensively in the UK for the separation and purification of uranium and plutonium (114). In this type of extractor, interstage flow is handled through a partitioned box constmction. Interstage pumping is not needed because the driving force is provided by the density difference between solutions in successive stages (see Plutoniumand plutonium compounds Uraniumand uranium compounds). 

A widely used type of pump—mixer—settler, developed by IsraeH Mining Industries (IMI) (115), is shown in Figure 13a. A unit having capacity 8.3 m /min (2000 gal /min) has been used in phosphoric acid plants (116). The unique feature of this design is that the pumping device is not required to act as the mixer, and the two phases are dispersed by a separate impeller mounted on a shaft miming coaxially with the drive to the pump. 

The General Mills mixer—settler (117), shown in Figure 13b, is a pump—mix unit designed for hydrometaHurgical extraction. It has a baffled cylindrical mixer fitted in the base and a turbine that mixes and pumps the incoming Hquids. The dispersion leaves from the top of the mixer and flows into a shallow rectangular settler designed for minimum holdup. 

FIG. 15-28 Pump-Mix mixer-settler. [Coplan, Davidson, and Xehroski, Chem. Eng. Prog., 50, 403 (1954), with permission.]... 

The disadvantages of the countercurrent cascade of mixer settlers, including the necessity for large-volume settlers and interstage pumping of liquids, are not present in the vertical column. Here the liquids flow... 

An extreme case of the mixer-settler is the pump reactor, wherein the reaction occurs within the pump itself. The pump provides mixing and short contact time. These reactors are useful for fast reactions requiring high mixing and short time, as in some aromatic nitration reactions. As indicated later, pump reactors are also useful for some polymeric reactions. 

Pumps. Liquid feedrates between 0.3 and 25 mL/minute are required for the process using the mixer-settlers described previously. Liquids were fed from supply tanks placed about 1.5 m below the batteries ("inactive solutions ) or about 4 m below the batteries ("active feeds ). Different pump types were tested and an electronically controlled membrane pump with a PTFE membrane was selected as most suitable, being easily adapted to remote control, sufficiently corrosion resistant and giving flowrates in the desired range (Prominent Electronic A2001). 

Shut-down. When the end-of-feed condition occurs for 1AAF, 2AAF or 3AAF, the feed-line to the "high active" feed pump is switched over to the "inactive" feed tank, which can contain simulated HLLW solution, but more normally only HN0 of the appropriate concentration. This solution is used until most of the equipment has been roughly decontaminated. Then all feed-pumps and mixer-settlers are shut off. The mixer-motor units and level transducers are removed from the batteries and the mixer-settlers emptied from solution using a hose and pump attached to a waste tank. 

To meet the supercritical conditions more suitable a mixer-settler unit has been developed by Schaffner [2], A regenerative pump was used as mixer to achieve an intense contact between the two phases and to create a large mass transfer surface area. This type of pump is capable of conveying gases as well as liquids and creates a high pressure difference which is used to separate the phases in the following fluid cyclone (fig. 1). 

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