Hexone process

Redox [Reduction oxidation] A process for separating the components of used nuclear fuel by solvent extraction. It was the first process to be used and was brought into operation at Hanford, United States, in 1951, but was superseded in 1954 by the Purex process. The key to the process was the alternate reduction and oxidation of the plutonium, hence the name. The solvent was Hexone (4-methyl-2-pentanone, methyl isobutyl ketone), so the process was also known as the Hexone process. The aqueous phase contained a high... 

A modification of the Redox process, the U-hexone process, was used at the Idaho Chemical Processing Plant of the U.S. AEC, to recover highly enriched uranium from U-A1 alloy fuel elements irradiated in the Materials Testing Reactor. The aluminum nitrate needed as salting agent was provided when the fuel was dissolved in nitric acid. The plutonium content of the fuel was too low to warrant recovery. Plutonium was made trivalent and inextractable before solvent extraction and thus routed to the aqueous high-level waste. 

Historically, the Redox process was used to achieve the same purification as in the Purex process (97,129). The reagents were hexone (methyl isobutyl ketone) as the solvent, dichromate as an oxidant, and A1(N02)3 as the salting agent. The chief disadvantages of hexone are its flammability and its solubihty in water. However, because A1(N03)3 collects in the highly radioactive waste, thereby impeding the latter s further processing, the Redox process was abandoned in favor of the Purex process. 

When an extractable cation, such as Zi, is readily hydrolyzed, reduction of hydrogen ion concentration will reduce the distribution coefficient by increasing the proportion of the element in the form of partially hydrolyzed, nonextractable ions such as ZrO . This principle was used in the Redox process [B2, C7, C8] for the hexone extraction of plutonium from irradiated uranium, wherein the aqueous phase was made sUghtly acid-deficient with ammonium hydroxide, to reduce the extraction of zirconium and rare-earth fission products. 

The solvent used in the Redox process was hexone, methyl isobutyl ketone, an extractant already in use for purifying uranium ore concentrates (Chap. 5)., Hexone is immiscible with water and will extract uranyl nitrate and plutonyl nitrate selectively from fission-product nitrates if the aqueous solution has a sufficiently high nitrate ion concentration. In the Redox process, aluminum nitrate was used as salting agent because high concentrations of nitric acid would decompose the hexone solvent. 

Figure 10.1 is a material flow sheet for the first cycle of one form of the Redox process [F3]. Rutonium in the feed was converted to hexavalent plutonyl nitrate Pu 02(N0s)j, by oxidation with dichromate ion Cr2 07 ", as this is the plutonium valence with highest distribution coefficient into hexone. In the decontamination contactor, hexavalent uranium and plutonium nitrates were extracted into hexone solvent, and fission-product nitrates were removed from the solvent by a scrub solution containing aluminum nitrate, sodium nitrate, and sodium dichromate. 

Disadvantages of the Redox process were the volatility and flammability of the hexone solvent and the large amount of nonvolatile reagents such as A1(N03)3 added to the radioactive wastes. 

The Purex process has four significant advantages over the Redox process (1) Waste volumes can be made much lower, as the nitric acid used as salting agent can be removed by evaporation. (2) The solvent, TBP, is less volatile and less flammable than hexone. (3) TBP is more stable against attack by nitric acid. (4) Operating costs are lower. 

F3. Flagg, J. F. Solvent Extraction Processes Based on Hexone, in Chemical Processing of Aqueous Fuels, J. F. Flagg (ed.). Academic, New York, 1961, chap. 6. 

The corresponding adduct compounds for 3- and 6-valent actinides are An(N03)3S3 and An02(N03)2S2. These chemically saturated neutral conqx>unds are soluble to differmt extent in organic solvents like kerosene, and — in the case of hexone — by hexone itself. The process using hexone is referred to as the Redox process. 

The chemical process we have chosen is solvent extraction using 1.5% TBP in Amsco. We chose this sytem over the Hexone system presently used at Arco because it is more flexible and does not require the precise concentration adjustment typical of Hexone. Laboratory studies and pilot experience show that adequate decontamination can be obtained with TBP iii one extraction cycle. 

Pu extraction by hexone has received the most attention, undoubtedly because of its use in processing. The extraction of Pu(IV) and Pu(VI) as a ftmction of nitric acid concentration and the concentration of various salts has been measured by several groups. 

Two principal solvent-extraction methods are used for the production of high-grade zirconium, based upon the solvents hexone (methyl-isobutyl-ketone) and tributyl phosphate. In both cases, the aim is to purify the zirconium from its chemical homologue, hafnium, but in some variants of the process the other metallic impurities are also removed. It is also possible to obtain the hafnium in a pure condition if required, and to obtain fairly good yields of both zirconium and hafnium. 

Fig. 4.32. U.K. process for zirconium purification by hexone-thiocyanate route. Solvent A, bexone 2-7M in thiocyanic acid inititdly solvent B, texone...

The loss of hexone in the process is fairly high since its solubihty in water is about 1 -5 per cent and therefore losses of this order occur into all the aqueous streams leaving the process, in addition to any other losses due to its volatility, or to entrainment. 

A similar hexone-thiocyanate process has been operated in the U.S.A., firstly by the Northwest Electrodevelopment Laboratories at Albany since 1952 and the Carborundum Metals Corporation plant near Akron, New York, since 1953. They use packed and unpacked columns and the difficulties associated with polymerization of thiocyanic acid were alleviated by using a solvent consisting of 80 per cent hexone and 20 per cent butyl acetate instead of solely hexone. > The zirconium product is precipitated as the acid sulphate, phthalate or salicylate, and a certain additional degree of purification is obtained at this stage. A typical analysis of pure zirconium oxide after hexone-thiocyanate extraction and salicylate precipitation is as follows ... 

Although the solvent purification process is adequate for the production of say reactor-grade zirconium, it is possible to modify it so that pure hafnium may also be obtained. Distribution data are available for various solutions containing thiocyanate, sulphate and chloride from which it is possible, for example, to deduce that both hafnium and zirconium will extract into hexone provided the aqueous phase has a high thiocyanate concentration and a low chloride concentration. The zirconium may then be selectively backwashed in a second extractor using say an aqueous phase of high thiocyanate concentration and moderate sulphate concentration, where the separation factor of the system is high. The hafnium can then... 

Niobium and tqntalum have similar chemical properties and the normal methods of niobium extraction do not remove the tantalum. Separation of these two elements is necessary, for example, when the niobium is required for nuclear purposes, and several solvent extraction processes are suitable. Hexone i (methyl isobutyl ketone) and tributyl phosphatei are the two principal solvents which are used, although processes have also been developed using methyl dioctylaminei and di-isopropyl ketone.i ... 

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