Organic raffinate

Before solvent extraction, the feed was treated with manganous nitrate and less than equivalent permanganate to convert plutonium to Pu(VI). Tests in miniature mixer-settlers with synthetic feed solution but not containing Cf showed acceptably small concentrations of actinides in the aqueous waste (1AW) and organic raffinate (1CW) streams, and also acceptable separation of plutonium from curium and americium. 

The extract is vacuum-distilled ia the solvent recovery column, which is operated at low bottom temperatures to minimise the formation of polymer and dimer and is designed to provide acryUc acid-free overheads for recycle as the extraction solvent. A small aqueous phase in the overheads is mixed with the raffinate from the extraction step. This aqueous material is stripped before disposal both to recover extraction solvent values and minimise waste organic disposal loads. 

Uranium is subsequendy stripped reductively from the loaded solvent using a bleed stream of the raffinate acid to which ferrous iron has been added to reduce uranium to its nonextractable, quadravalent state. Raffinate is acid from which uranium has already been extracted. By controlling the organic-to-aqueous volume phase ratios in the extraction and stripping circuits, uranium is concentrated by a factor of approximately 70. 

Modem commercial wet-acid purification processes (see Fig. 4) are based on solvents such as C to Cg alcohols, ethers, ketones, amines, and phosphate esters (10—12). Organic-phase extraction of phosphoric acid is accompHshed in one or more extraction columns or, less frequently, in a series of countercurrent mixer—settlers. Generally, 60—75% of the feed acid P2 s content is extracted into the organic phase as H PO. The residual phosphoric acid phase (raffinate), containing 25—40% of the original P2O5 value, is typically used for fertilizer manufacture such as triple superphosphate. For this reason, wet-acid purification units are almost always located within or next to fertilizer complexes. 

The purified acid is recovered from the loaded organic stream by contacting with water in another countercurrent extraction step. In place of water, an aqueous alkafl can be used to recover a purified phosphate salt solution. A small portion of the purified acid is typically used in a backwashing operation to contact the loaded organic phase and to improve the purity of the extract phase prior to recovery of the purified acid. Depending on the miscibility of the solvent with the acid, the purified acid and the raffinate may be stripped of residual solvent which is recycled to the extraction loop. The purified acid can be treated for removal of residual organic impurities, stripped of fluoride to low (10 ppm) levels, and concentrated to the desired P2 s Many variations of this basic scheme have been developed to improve the extraction of phosphate and rejection of impurities to the raffinate stream, and numerous patents have been granted on solvent extraction processes. 

For vanadium solvent extraction, Hon powder can be added to reduce pentavalent vanadium to quadrivalent and trivalent Hon to divalent at a redox potential of —150 mV. The pH is adjusted to 2 by addition of NH, and an oxyvanadium cation is extracted in four countercurrent stages of mixer—settlers by a diesel oil solution of EHPA. Vanadium is stripped from the organic solvent with a 15 wt % sulfuric acid solution in four countercurrent stages. Addition of NH, steam, and sodium chlorate to the strip Hquor results in the precipitation of vanadium oxides, which are filtered, dried, fused, and flaked (22). Vanadium can also be extracted from oxidized uranium raffinate by solvent extraction with a tertiary amine, and ammonium metavanadate is produced from the soda-ash strip Hquor. Fused and flaked pentoxide is made from the ammonium metavanadate (23). 

After mixing, the solvent and waste are separated. The solvent with dissolved organics is called the extract. The waste remaining after extraction is called the raffinate. The extract may be sent to a distillation or steam stripping unit to separate the dissolved organics from the solvent and the solvent can be recycled back to the extraction process. The raffinate may require additional treatment or may be disposed or incinerated. 

Butyl alcohol, obtained from hydration of Raffinate 1, can be dehydrated and subsequently refined to high purity, polymer-grade isobutylene (25). Alternatively, the isobutylene from alcohol dehydration can react with methanol in the presence of an acid catalyst to give methyl /-butyl ether (MTBE) gasoHne additive (see Ethers organic). 

Multistage counter-current liquid-liquid extraction is a typical continuous process. Fig. 121 shows a schematic diagram of the process. The slope of the line depends on the volumetric ratio of the aqueous and organic phases. The aqueous and organic phases move through the system as two counter currents. The raffinate from a specific extraction stage moves on to the next stage, to be mixed with a relatively fresh portion of the extract, i.e. extractant that was previously mixed with a low-concentration aqueous solution. 

Two main schemes exist for the separation and purification of tantalum and niobium using liquid-liquid extraction. The first is based on the collective extraction of tantalum and niobium from an initial solution into an organic phase so as to separate them from impurities that remain in the aqueous media, the raffinate. The separation of tantalum and niobium is subsequently performed by fractional stripping into two different aqueous solutions. In this case, stripping of niobium is performed using relatively weak acids prior to the stripping of tantalum. Fig. 125 presents a flow chart of the process. 

Equilibrium data must be obtained for material balance showing raffinate and extracted phases. A simple separation funnel for single-stage extraction using amyl acetate as organic solvent is shown in Figure 7.13. 

Liquid-liquid extraction (also called solvent extraction) is the transfer of a substance (a consolute) dissolved in one liquid to a second liquid (the solvent) that is immiscible with the first liquid or miscible to a very limited degree. This operation is commonly used in fine chemicals manufacture (I) to wash out impurities from a contaminated solution to a solvent in order to obtain a pure solution (raffinate) from which the pure substance will be isolated, and (2) to pull out a desired substance from a contaminated liquid into the solvent leaving impurities in the first liquid. The former operation is typically employed when an organic phase is to be depleted from impurities which are soluble in acidic, alkaline, or neutral aqueous solutions Water or a diluted aqueous solution is then used as the solvent. The pure raffinate is then appropriately processed (e.g. by distillation) to isolate the desired consolute. In the latter version of extraction impurities remain in the first phase. The extract that has become rich in the desired consolute is then appropriately processed to isolate the consolute. Extraction can also be used to fractionate multiple consolutes. 

The extractability of such a complex is quite good. Thus, if the feed solution is low in its acidity only tantalum is extracted, while niobium remains in the raffinate. The niobiumbearing raffinate is then separately contacted with TBP after adjusting its acidity to higher values so that niobium is than transferred from aqueous to the organic phase. 

The only other olefin feedstock which is hydroformylated in an aqueous/organic biphasic system is a mixture of butenes and butanes called raffinate-II [8,61,62]. This low-pressure hydroformylation is very much like the RCH-RP process for the production of butyraldehyde and uses the same catalyst. Since butenes have lower solubility in water than propene, satisfactory reaction rates are obtained only with increased catalyst concentrations. Otherwise the process parameters are similar (Scheme 4.3), so much that hydroformylation of raffinate-11 or propene can even be carried out in the same unit by slight adjustment of operating parameters. 

Raffinate-II typically consists of40 % 1-butene, 40 % 2-butene and 20 % butane isomers. [RhH(CO)(TPPTS)3] does not catalyze the hydroformylation of internal olefins, neither their isomerization to terminal alkenes. It follows, that in addition to the 20 % butane in the feed, the 2-butene content will not react either. Following separation of the aqueous catalyts phase and the organic phase of aldehydes, the latter is freed from dissolved 2-butene and butane with a counter flow of synthesis gas. The crude aldehyde mixture is fractionated to yield n-valeraldehyde (95 %) and isovaleraldehyde (5 %) which are then oxidized to valeric add. Esters of n-valeric acid are used as lubricants. Unreacted butenes (mostly 2-butene) are hydroformylated and hydrogenated in a high pressure cobalt-catalyzed process to a mixture of isomeric amyl alcohols, while the remaining unreactive components (mostly butane) are used for power generation. Production of valeraldehydes was 12.000 t in 1995 [8] and was expected to increase later. 

For most exploratory work, analysis of the organic phase is not necessary. If no volume change of the phases occurs and no third phase or crud is formed, the analysis of the aqueous raffinates is sufficient, since the metal concentration in the solvent can be readily calculated from the initial metal concentration of the feed solution and the phase ratio used. 

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. 

Some consideration should be given at this point to the need to prevent loss of the organic phase in the aqueous raffinate. This loss can arise by either solubility in the aqueous phase or by entrainment of droplets not fully settled. The solvent lost in this way can offer a finite environmental hazard and be an economic cost on the process. 

The previous chapters have demonstrated that liquid-liquid extraction is a mass transfer unit operation involving two liquid phases, the raffinate and the extract phase, which have very small mutual solubihty. Let us assume that the raffinate phase is wastewater from a coke plant polluted with phenol. To separate the phenol from the water, there must be close contact with the extract phase, toluene in this case. Water and toluene are not mutually soluble, but toluene is a better solvent for phenol and can extract it from water. Thus, toluene and phenol together are the extract phase. If the solvent reacts with the extracted substance during the extraction, the whole process is called reactive extraction. The reaction is usually used to alter the properties of inorganic cations and anions so they can be extracted from an aqueous solution into the nonpolar organic phase. The mechanisms for these reactions involve ion pah-formation, solvation of an ionic compound, or formation of covalent metal-extractant complexes (see Chapters 3 and 4). Often formation of these new species is a slow process and, in many cases, it is not possible to use columns for this type of extraction mixer-settlers are used instead (Chapter 8). 

The potassium penicillin is recovered by filtration and the solvent recirculated. Recovery of the organic phase from the aqueous raffinate is also very important to minimize costs and environmental impact. Butyl acetate, being a low-boiling solvent, can be recovered easily by distillation. 

---