Scrubbed organic phase

In some flow sheets, a scrub circuit is introduced between the extraction and stripping circuits. A scrub liquor is introduced to scrub off unwanted coextracted species from the loaded organic phase by displacing the impurities with the more strongly complexed main element. This produces a scrubbed organic phase. For systems in which the extractant is completely selective for the species of interest or when high selectivity of extraction is not required (such as for bulk extractions), a scrub circuit is not necessary. Usually, however, some coextraction of other species occurs and a scrub circuit is anployed to reduce the transfer of these species to the LSL. 

In both cases, the extraction process includes washing (scrubbing) of the extract, a stage that is not shown separately. The organic phase that results from the stripping process is returned to the beginning of the extraction process for reuse as an extractant. 

Scrubbing the loaded organic phase with water or ammonia removes HC1, transferring the copper to the pH-swing extractant, e.g., as in,... 

Data obtained in scrubbing tests can be presented graphically, such as by plotting the concentration of metals in the organic phase against A/O ratio, salt concentration in the scrub solution, temperature, etc. Typical plots for the removal of nickel from cobalt in a DEHPA-containing solvent are shown in Figs. 7.3 and 7.4 [3]. 

The data obtained from small-scale continuous operations will be required to determine whether the process should be investigated on a larger scale, such as a pilot plant. These data should be sufficient to draw a conceptual flow sheet, which will include a number of stages for extraction, scrubbing [10], and stripping the flow rates, size and type of equipment, and the various parameters considered earlier in this chapter. Another important aspect should also be considered in the continuous test work, and that is chemical analysis of both the aqueous and organic phases for their various components. 

The uranium is separated, after dissolving the sample as described for lead, by extraction with tributyl phosphate (TBP) from 4M nitric acid. After the organic phase is scrubbed with 4M nitric acid, the uranium is back-extracted into distilled water and evaporated to dryness. The uranium is loaded on a rhenium filament for analysis by dissolving the purified sample in a small volume of 0.05M nitric acid. 

The raw precious metal concentrate is totally dissolved in hydrochloric acid—chlorine solution to form the soluble chloride ions of each of the metals. Silver remains as insoluble silver chloride and can be filtered off. Gold, in the form of [AuClJ, is extracted with, eg, tributyl phosphite or methyl isobutyl ketone. Base metals are also extracted in this step, and are removed from the organic phase by scrubbing with dilute hydrochloric acid (HC1). Iron powder is then used to reduce the gold species and recover them from the oiganic phase. 

Concentration/purification by solvent extraction usually involves four steps (a) extraction of uranium from the leach liquor in a solvent, (b) scrubbing to remove impurities from the solvent, (c) stripping to remove uranium from the solvent, and (d) regeneration of the solvent. The solvent phase in solvent extraction will contain the extractant that complexes uranium to make it soluble in the organic phase, a diluent, an inexpensive material to dilute the extractant, and a modifier to improve the solubility of the extractant in the diluent. Typical extractants are amines with isode-canol acting as a modifier to improve the amine solubility in a diluent such as kerosene. The typical chemistry of the extraction would involve the reactions... 

Nickel and cobalt often occur with copper, and must be separated in pure form from hydrometallurgical leach liquors. Organic acid extractants can quite readily separate copper from cobalt and nickel, but the separation of cobalt from nickel is rather difficult. In one Ni/Co separation process, di-2-ethyl hexyl phosphoric acid (D2EHPA) is used as extractant, with strict control of the pH of the aqueous phase to take full advantage of the slightly different equilibrium constants for the Co and Ni reactions. Pulsed column contactors are used rather than mixer-settlers, and nickel impurity is removed from the loaded organic phase by scrubbing it with a cobalt-rich phase. 

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