Raffinate scrub

Stream Feed Raffinate Scrub Strip Product... 

W scrubs the extract free from the unwanted second solute. The second solute leaves the contac tor in the raffinate stream. 

After extraction, the loaded solvent contains 6 g T1 zirconium as zirconium oxide with 0.2% hafnium oxide. The raffinate is left with 0.2 to 0.3 g l l of the oxides of zirconium and hafnium of this, 70-90% is hafnium oxide. This raffinate can act as a feed solution for the recovery of pure hafnium oxide. The loaded extractant, on the other hand, is subjected to a scrubbing operation with pure zirconium sulfate solution to eliminate any co-extracted hafnium. This scrubbing operation is essentially a displacement reaction ... 

It is useful to treat the raffinate to recover the two acids it contains. The raffinate can first be subjected to a thermal treatment which decomposes it into insoluble metallic oxides, hydrofluoric acid and sulfuric acid. In a first scrubbing, sulfuric acid is recovered, whilst in a second stage, hydrofluoric acid is absorbed. When put into operation, this process not only causes a reduction in the amount of sludges but also leads to acid production for return to the process. 

The nomenclature used in solvent extraction has been defined in Chapter 1 and is illustrated in Fig. 8.1. Not all of the steps shown in this figure will be found in every extraction process, but equally there may be occasions where it is necessary to add additional steps for example, to recover the extractant from the scrub raffinate. So while Fig. 8.1 is not a completely general flow diagram it covers most of the processes likely to be found in practice. Variations of this flow sheet will become apparent during the remaining chapters. 

Where a high-purity product is sought, the same philosophy of saturating the aqueous phase with the unwanted component is used in scrubbing the extract. A recycle stream of the pure product is used to scrub the impure extract. In the process, the extract is saturated with the desired product, and the impurities are removed. The final raffinate from scrubbing is then recycled to the aqueous feed so that the now-contaminated desired product can be recovered. This is shown in Fig. 8.1. 

Four stages are needed. The strip raffinate has a concentration of 1.71, and the stripped organic contains only 2% (0.0073/0.35) of the component A in the scrubbed extract, so the stripping efficiency is 98%. The less extractable (more readily stripped) component B was completely stripped in three stages. 

The Flow Sheet - Extraction and Scrubbing. The amount of solute that can be recovered from a particular feed solution by equilibration with a solvent depends on both the distribution coefficient and the volumetric ratio of extract to raffinate phase. 

HDEHP. The total time typically consumed in the extraction step of the process is on the order of 2 shifts, or 16 hours. Following extraction, the aqueous raffinate is transferred to a separate tank and the pregnant organic phase is subsequently scrubbed and stripped in the same vessel. 

The pregnant organic phase is usually scrubbed twice with 0.03 N HCl (the first scrub is 0.03 N HNO3 if the feed solution had been in the nitrate form). The volume of each scrub is one-third of the organic-phase volume. Each scrub is mixed with the organic phase by air-sparging at least 30 minutes. The scrub raffinates are usually combined with the extraction raffinate to provide a composite feed for a second-stage extraction. 

DF Am/Cm increases as anticipated when scrub stages are used, while the drop in DF Cm/Am is greater than calculated and results mainly from the presence of hexavalent americium in the raffinate, instead of the AmO Am3+ mixture prevailing without a scrub section. 

A comparison of runs la and 2a shows that DF Am/Cm are nearly equal while DF Cm/Am is higher with HDEHP, although the aqueous raffinate still contains traces of hexavalent americium. Hence to obtain purer americium and to reduce americium loses, it was decided to increase the number of scrub stages and, in the extraction section, the number of stages and the extraction factor (Ex= D Fo)... 

The synergistic mixture (HDEHP + TOPO) was used in Experiment 3 (runs 3a-3b). The DF Cm/Am values obtained were very similar (7 and 7.5) and not higher than previously, while DF Am/Cm are respectively 1,300 and 6,100 with two and three scrub stages. The aqueous raffinate was found to contain hexavalent and trivalent americium (the apparent distribution coefficient in the final extraction stage was 0.2). 

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