One of the most compact and efficient of the mixer-settlers is the pump-mix mixer-settler, developed by Coplan et al. [C5, C6] specifically for radiochemical separations. One stage of this device is shown schematically in Fig. 4.22. A countercurrent cascade of mixer-settlers is shown in Fig. 4.23. The stage consists of a mixing chamber at the left of Fig. 4.22 and a settling chamber at the right. The rotating impeller in the mixing chamber serves to promote equilibrium contact between light and heavy phases, to pump phases between adjacent stages, and to control liquid levels. The mixing chamber is divided into an upper and a lower compartment by a horizontal baffle, pierced with a hole somewhat larger than the impeller shaft. This shaft, which is hollow, passes through the upper compartment of the mixing chamber and dips into the lower. The impeller draws liquid from the bottom compartment through the hollow shaft and discharges it through holes between the blades into the upper compartment. This pumping action of the impeller maintains an interface between mixed phases and the heavy phase at the bottom of the shaft. A free surface between mixed phases and air is maintained in the upper part of the mixing chamber. This free surface is progressively lower in adjacent stages in the direction of light-phase flow.

Mixed phases flow by gravity from the mixing chamber past a baffle into the settling chamber (closed at top) where the two phases separate. The heavy phase flows through a trapped outlet near the bottom (not shown) into the bottom compartment of the mixing chamber of the adjacent stage, where the interface controlled by the stirrer shaft is at a lower level. The light phase flows through an outlet near the top of the settling chamber directly into the mixing chamber of the adjacent stage in the opposite direction.

Flow of heavy phase in one direction is induced by the pumping action of the stirrer, coupled with the trap between settling and mixing sections; flow of light phase in the opposite direction is induced by the progressively lower level of the free surface at the top of the mixing sections.

For an improved version, shown in Fig. 4.24, the impeller is a volute-vaned pump which recirculates an emulsion of the mixed phases within the mixing section [Dl]. Interface control weirs provide flexibility for adjusting impeller speed and mixing intensity without upsetting the net interstage flow rate. Solvent from the previous stage flows into the vortex above the impeller and also into the interface weir section, resulting in pumping action to establish hydraulic gradients for the interstage flow of both solvent and aqueous streams.

Properties of two improved pump-mix mixer-settlers, one designed for a total interstage flow of 7.6 liters/min and the other for 380 liters/min, are summarized in Table 4.13 [Dl]. The holdup times per contactor are 1 and 2 min, respectively. The small unit was designed for reprocessing highly enriched uranium fuel and, by limiting its height to 7.6 cm, is critically safe up to Ms U concentrations of 400 g/liter, provided it is not located near any dense material that can reflect neutrons. The large unit is suitable only for process solutions of low fissile enrichment. Aqueous-to-solvent flow ratios of 0.1 to 1.5 can be attained without excessive

entrainment in the interstage flow streams. Stage efficiencies, i. e., the actual interphase transfer per mixer-settler stage relative to that predicted for an equilibrium stage, of greater than 80 percent have been obtained in the TBP extraction of uranium and plutonium [Dl].

The pump-mix mixer-settler is readily scaled over a wide range of throughputs, and because individual stages can be relied on to perform at high and reproducible efficiency, there is less risk in designing a production-scale separation plant than with some of the other types of solvent extraction contactors. A production plant can be designed with assurance on the basis of single-stage equilibrium data, data from a small-scale mixer-settler cascade, and hydraulic tests on a small section of a full-scale mixer-settler cascade. The horizontal arrangement of a mixer-settler cascade permits interruption of steady-state operation and shutdown for several hours without losing the concentration gradient of the cascade, so that the cascade can be restarted relatively easily.

Mixer-settler contactors of much larger scale are used in the solvent extraction operations associated with production of natural uranium (cf. Chap. 5), wherein nuclear criticality is not

Source-. M. W. Davis and A. S. Jennings, “Equipment for Processing by Solvent Extraction,” in Chemical Processing of Reactor Fuels,

J. F. Flagg (ed.), Academic, New York, 1961, by permission.

an issue. In the Kerr-McGee uranium extraction plant at Shiprock, New Mexico, where uranium-bearing leach liquor is contacted with alkyl phosphate in kerosene, there are four stages of mixer-settlers [Т2]. Each stage consists of a wood-stave settling tank 4.9 m in diameter and 2.1 m high in which a 1.2-m-diameter stainless steel mixing vessel with a 0.46-m-diameter turbine is placed. The aqueous flow of 6.3 liter/s is contacted with 1.3 liter/s of organic. The latter is pumped from one stage to the next by air-lift pumps. Interstage aqueous flow is by gravity, with elevation differences of 0.3 m between successive stages. The estimated holdup time per stage is 50 min.

The mixer-settler used at the Vitro uranium recovery operation near Salt Lake City, Utah, is shown schematically in Fig. 4.25. The contactor is a rubber-lined tank 6.1 m in diameter and

2.4 m straight height, capable of contacting 28 liter/s of aqueous solution [Т2]. Organic and aqueous streams from adjacent stages in the cascade are introduced directly into a turbo-mixer, with a 46-cm-diameter impeller, mounted at the top of the tank. The mixer phases emerge into the tank, which acts as a large settling chamber. For an aqueous-to-organic flow ratio of 6, the holdup time per stage is estimated to be 46 min.