Solvent Extraction
Solvent extraction (SX) is a liquid-in-a-liquid separation process where a certain product will concentrate more in one liquid, while another product (or the waste materials) remain in the other liquid. In its most typical form SX will use two immiscible liquids, an aqueous (water) phase and an organic (water-repelling) phase. Just like oil and water, these liquids will combine with physical agitation, but when left undisturbed they will separate back into their own phases.
In solvent extraction the selection of each of these phases is highly specialized to the starting material and the products being separated. Both the aqueous and organic phases will often be mixtures themselves, formulated to be most effective at attracting the target materials and keeping them in solution. Extraction occurs as the phases separate, resulting in the desired components being dissolved and highly concentrated in one of the two liquids, typically the aqueous phase. This is then collected and further processed to isolate the material from the aqueous solution.
It is possible that the remaining organic phase has other desired material in solution, which is processed by back-extraction, also called stripping. The organic phase will be agitated with a different aqueous mixture having a higher affinity for this material than the organic phase, so when the phases separate the material has moved from being concentrated in the organic phase to being in solution in the aqueous phase.
Solvent extraction has been elegantly implemented in mining applications, such as separating and concentrating rare earth elements (REEs). A mixer-settler device is used, where these chemically similar elements can be extracted, separated, and collected on a continuous, counter current loop. The mixing section has the stirrer, where the two liquids are brought together and mixed with an impeller. This two-phase liquid mixture then flows over into the settler section, where it is allowed to separate, and carry along its respectively dissolved agents within each phase. One mixer-settler is a single stage of extraction. To maximize the yield it is effective to undertake a series of extractions, so it is common to see an assembly in arrays of 60 units or more.
An ideal stirrer for such mixer-settler units at the pilot scale is Caframo’s petite digital stirrer, the BDC250. Its compact footprint and design for 2 L volumes allows for large numbers of extraction stages within a small amount of space, as well as minimizing chemical usage and waste. With a speed range of 50‑2500 rpm and designed for continuous use, the BDC250 has the additional versatility of including a stirring impeller with the motor, with the choice of axial or radial mixing blades.
Image courtesy of SGS Canada Inc. Used with permission.
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