Extraction-precipitation technique

An overview is presented of plutonium process chemistry at Rocky Flats and of research in progress to improve plutonium processing operations or to develop new processes. Both pyrochemical and aqueous methods are used to process plutonium metal scrap, oxide, and other residues. The pyrochemical processes currently in production include electrorefining, fluorination, hydriding, molten salt extraction, calcination, and reduction operations. Aqueous processing and waste treatment methods involve nitric acid dissolution, ion exchange, solvent extraction, and precipitation techniques. 

Tewes et al., using both precipitation and extraction separation techniques, observed essentially complete exchange within the separation time (1 min) during an isotopic study ( V) of this exchange. The media ranged from 0.3 M perchloric acid to 7 M hydrochloric acid reactant concentrations were 10 M. 

Various methods ofachieving preconcentration have been applied, including Hquid -hquid extraction, precipitation, immobihzation and electrodeposition. Most of these have been adapted to a flow-injection format for which retention on an immobihzed reagent appears attractive. Sohd, sihca-based preconcentration media are easily handled [30-37], whereas resin-based materials tend to swell and may break up. Resins can be modified [38] by adsorption of a chelating agent to prevent this. Sohds are easily incorporated into flow-injection manifolds as small columns [33, 34, 36, 39, 40] 8-quinolinol immobilized on porous glass has often been used [33, 34, 36]. The flow-injection technique provides reproducible and easy sample handhng, and the manifolds are easily interfaced with flame atomic absorption spectrometers. 

Since SDS is practically not dialyzable, it must be removed by other techniques extraction, precipitation, electroelution, and washing out from immobilized proteins. 

Unlike clenbuterol, salbutamol is a difficult compound to analyze due to its particular chemical attributes. It is a basic compound subjected to protein binding poor recoveries are obtained especially when protein precipitation techniques are used to prepare the extracts (145). In addition, salbutamol is charged at all pH values and does not readily lend itself to simple, specific back-extracting procedures. This severely restricts the options of sample cleanup. However, a Subtilisin protease digestion step followed by acid clarification and solid-phase extraction has been suggested (146) as an adequate extraction and cleanup procedure prior to the end-point determination of salbutamol by an enzyme immunoassay (139) based on the cross-reactivity of anticlenbuterol antibodies. 

One of the key steps in any isotope dilution analysis concerns the isolation and purification of the diluted activity, plus the measurement of its specific activity. Two techniques are usually preferred for the separation precipitation and solvent extraction. As a purification step, precipitation has the advantage that the precipitate can easily be weighed at the time of separation, thereby allowing a quick determination of the specific activity. The main problem with the use of precipitation techniques involves the occurrence of co-precipitation phenomena, in which unwanted materials are precipitated along with the desired substance, thus altering the sample specific activity. Precipitation techniques are used for the isolation of inorganic components. 

Cleanup or fractionation procedures that have been used in the more recent fat-soluble vitamin assays include sterol precipitation, open-column chromatography, solid-phase extraction, and high-pressure gel permeation chromatography. High-performance LC has been used on a semipreparative scale in vitamin D and vitamin K assays to obtain purified fractions of sample extracts. This technique is discussed in Sec. V.B.3. 

Even though in the THOREX process 233U can be preferentially recovered from irradiated thorium fuel by using an extraction flowsheet based on 5% TBP n-dodecane as the extractant, further lowering of the concentration of TBP in the solvent has certain advantages in terms of reduced co-extraction of thorium and fission products (195, 196). Ramanujam et al. reported a sequential precipitation technique... 

Although the process proved satisfactory from the chemical standpoint, practical problems emerged in that the hydraulic operation of the mixer-settler batteries was extremely poor. In effect, as soon as the aqueous solutions from the dissolution of irradiated targets were placed in contact with the organic extraction phases, a stable emulsion was formed, produced by the appearance of extensive precipitates at the aqueous solution/organic solution interface. As no chemical remedy was found to solve this problem, we attempted to adapt this type of process to extraction chromatographic techniques. 

Primary recovery of the active ingredient from the solid or liquid phase to remove large quantities of unwanted waste materials, which may themselves be processed further. Suitable techniques include solvent extraction, precipitation by chemical or physical changes to the product-containing solution, and ultrafiltration or microfiltration to separate products above a particular size. Work done on combined biomass separation-primary product recovery processes such as expanded-bed adsorption are now being commercialized in the pharmaceutical industry. 

The precipitation technique is often applied in the primary step and/or the final step of a procedure in which another technique of rapid separation, such as recoil, solvent extraction, or distillation, is used. 

Toxins and a large group of compounds (e.g. quaternary ammonium bases and dioxins) that require special isolation techniques, using ion pairs or ion exchange resins, formation of derivatives, continuous extraction, precipitation and concentration... 

This involves the use of tertiary amine extraction of the An ions from acidic 11 M LiCl solutions. Spectroscopic studies have indicated that, in the cases of Am and Nd at least, the octahedral trianionic hexachloro complexes are extracted from 11 M LiCl. Stability constant data for the chloride complexing of Am , and Cfin media of ionic strength 1,0 have been reported. Tertiary amines also extract Pu and a study of extraction from nitrate media by trilaurylamine (TLA) in xylene has been reported. " This showed that the mass transfer rate was controlled by the reactions between Pu from the bulk phase and interfacially adsorbed TLA-HNOs. The separation of individual transplutonium elements from the Tramex actinide product may be achieved using ion exchange or precipitation techniques." ... 

Enrichment More sophisticated separation techniques such as solvent extraction, precipitation, adsorption, and ultrafiltration (UF) are commonly used to concentrate and partially purify the product. The purity of the product usually reaches 10-80% (w/v) at the end of these steps. 

Ruthenium precipitation techniques are useful in head-end treatments to solvent extraction processes. Scavenge cakes resulting from such operations often constitute major activity fractions of high-level wastes. 

Metabolite/residue analysis. Milk, urine and plasma samples were first analyzed by a microbiological cylinder/plate procedure against M.luteus which has a limit of detection of 0.02 ppm. A sub-sample of the milk was prepared for this assay by a centrifugation step followed by a pH adjustment to 8.5. In addition, an HPLC/RAM analysis was conducted after treating another sub-sample with FTSH (10% formic acid, 30% trifluoroacetic acid, 2% sodium chloride, 2N hydrochloric acid) followed by centrifugation to precipitate the proteins. The supernatant was basified and concentrated by C-18 solid phase extraction (SPE) techniques. The HPLC conditions were Column - 20 cm x 4.8 mm C-8 Mobile-phase -linear gradient at 5%/minute from 90 10 O.IM pH 7 phosphate buffer methanol to 20 80 Detectors - UV operated at 214 nm and a radioactivity flow detector operated in the DPM mode. 

The precipitation technique is not suitable for large-scale, continuous remote operations in which both uranium and plutonium have to be isolated in a very pure state from the fission products. It was therefore replaced in the late 1940 s by solvent extraction in which the fuels were dissolved in nitric acid and contacted with an organic solvent which selectively extracted the desired elements. The technique has been mentioned in 9.2.6, 9.4.3 and 16.3.3 but is described in more detail in Appendix A. 

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