Flow sheets feed solution

Hollow fiber assembhes function as one-ended shell-and-tube devices. At one end the fibers are embedded in an epoxy tube-sheet and at the other end they are sealed. Overall flows of feed solution and permeate thus are in counterflow. Flow of permeate is into the tubes, which takes advantage of the great crushing strengths of the small diameter fibers. This also constitutes a... 

By contrast, HLW from LWR fuel reprocessing is stored ia cooled, well-agitated, stainless steel tanks as an acidic nitrate solution having relatively few sohds. Modem PUREX flow sheets minimise the addition of extraneous salts, and as a result the HLW is essentially a fission-product nitrate solution. Dissolver soHds are centrifuged from the feed stream and are stored separately. Thus the HLW has a low risk of compromising tank integrity and has a favorable composition for solidification and disposal (11). 

FIG. 22-59 Schematic of two ways to pass solution across an ED membrane. Tortuous flow (left) uses a special spacer to force the solution through a narrow, winding path, raising its velocity, mass transfer, and pressure drop. Sheet feed (right) passes the solution across the plate uniformly, with lower pressure drop and mass transfer. Coutiesy Elsevier.)... 

Cobalt and nickel can be separated by several solvent extraction processes, the choice depending on the total flow sheet requirements, on the ratio of the metals in the feed solution, and on the required purity of the products. 

In the commercial flow sheets, these elements are left in the aqueous raffinate after platinum and palladium extraction. Indium can be extracted in the -l-IV oxidation state by amines (see Fig. 11.11), or TBP (see Figs. 11.10 and 11.12). However, although the separation from rhodium is easy, the recovery of iridium may not be quantitative because of the presence of nonextractable iridium halocomplexes in the feed solution. Dhara [37] has proposed coextraction of iridium, platinum, and palladium by a tertiary amine and the selective recovery of the iridium by reduction to Ir(III). Iridium can also be separated from rhodium by substituted amides [S(Ir/ Rh) 5 X 10 ). 

Liquid-liquid extraction provides one of the easiest methods for the separation of these commercially important elements. It is more convenient than ion exchange, allowing both semicontinuous operation and the use of more concentrated feed solutions. However, the large number of mixer-settlers required imposes a considerable capital investment in the plant, and their use in the reflux mode entails that some elements are locked into the process for long periods, which also has economic implications. Therefore, flow sheets are developed to provide elements with a ready market at the earliest opportunity, leaving the remaining elements as intermediates for separation at a later stage or for sale as mixed lanthanides. 

Long-lived ty = 2.1 x 10 years) Tc, present as TCO4 in Purex process HNO3 feed solutions, is partially coextracted with uranium and plutonium in the first cycle. Unless separated in the Purex process, Tc contaminates the uranium product subsequent processing of the U02(N03)2 solution to UO2 can release some of the technetium to the environment. The presence of technetium in the purification steps as well as in the uranium product causes several other complications. Thus it is desirable to route all Tc into the high-level waste. Efforts in this direction have been described in some recent flow sheets [37]. 

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. 

Fig. 7. Flow sheet of combined forward and back extraction processes in continuous mode using two mixer/settler units with reverse micellar phase recirculation. Wl feed (first) aqueous solution, W2 stripping (second) aqueous phase, RM reverse micellar phase. (Reproduced from [139] with permission of Elsevier Science)...

Figure 8.3-2 shows the simplified flow-sheet used in the simulation program. For plantmodelling, the solution was considered to be constituted of only CO2 and THF. The dissolved polymer and drug were neglected. Because of the low concentration of these compounds in the feed, this approximation should not have significant consequences. 

Fluoride and aluminum were also included in the feed because they would be needed in an actual dissolution step for ThC (fluoride catalyzes the dissolution, and aluminum counteracts the corrosiveness of the fluoride). For efficient Th extraction, the acidity of the feed solution should be in the range of 2 to 3 mol/L in HNO3. This is a significant departure from the acid Thorex process which uses an acid-deficient feed solution and is reported to. achieve improved decontamination from fission products (8). However, acid-deficient feed solutions were considered undesirable in our flow sheet because Pu hydrolyzes and tends to polymerize at low acidity (9). The effect of the higher feed acidity used here on fission product decontamination has not yet been established but will be assessed in later experiments. 

Before being fed to Contactor III, the aqueous feed solution was treated to reduce Pu to the Pu(III) state, to prevent its extraction and ensure its recovery in the aqueous Pu product stream (3PP). The reference reductant for the flow sheet was hydroxyla-mine nitrate (HAN) at 0.3 mol/L with hydrazine nitrate (0.1 mol/L) as a holding agent or HNO2 scavenger (10). Use of HAN as Pu reductant was considered desirable because in reaction with Pu, as well as in subsequent reactions where the reductant is destroyed, only gaseous products are formed (equations 1 and 2). 

The actinides as trace components will not take any penulty on the already tested waste management procedures (denitration, calcination, vitrification), provided that Np and Pu can be retained in the aqueous waste stream. Without changes of the THOREX flow-sheet, the 1st feed solution contains Np(V) and Pu(lV). Consequently, Np ends up in the waste stream, while Pu following the heavy metal, can be withdrawn with the aqueous phase of the 2nd coextraction step with reducing agents and thus combined with Np (hatched arrow) for the further waste treatment. 

U.S. Bureau of Mines plant. Figure 7.6 is a process flow sheet for the zirconium-hafnium separation portion of the U.S. Bureau of Mines zirconium plant at Albany, Oregon [Ml]. Commercial-grade zirconium tetrachloride containing about 2 w/o hafnium was dissolved in water together with ammonium thiocyanate (NH4CNS) and NH4OH, to make a feed solution... 

Plutonium trifluoride. Plutonium trifluoride can be converted directly to plutonium metal, or it is an intermediate in the formation of PUF4 or PUF4 -PUO2 mixtures for thermochemical reduction, as described in Sec. 4.8. The stabilized Pu(III) solution, produced by cation exchange in one of the Purex process options for fuel reprocessing, is a natural feed for the formation of plutonium trifluoride, as is shown in the flow sheet of Fig. 9.9 [03]. A typical eluent solution from cation exchange consists of 30 to 70 g plutonium/liter, 4 to 5 Af nitric acid, 0.2 Af sulfamic acid, and 03 Af hydroxylamine nitrate. The sulfamic acid reacts rapidly with nitrous acid to reduce the rate of oxidation of Pu(III) to about 4 to 6 percent per day. Addition of ascorbic acid to the plutonium solution just before fluoride precipitation reduces Pu(IV) rapidly and completely to Pu(III). 

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. 

While the previously described three membrane modules required flat sheet membrane material for their preparation, special membrane configurations are needed for the preparation of the tubular, capillary, and hollow fiber modules. The tubular membrane module consists of membrane tubes placed into porous stainless steel or fiber glass reinforced plastic pipes. The pressurized feed solution flows down the tube bore and the permeate is collected on the outer side of the porous support pipe, as indicated in Figure 1.33 (d). The diameters of tubular membranes are typically between 1-2.5 cm. In some modules, the membranes are cast directly on the porous pipes and in others they are prepared separately as tubes and then installed into the support pipes. 

The gaskets not only separate the membranes but also contain manifolds to distribute the process fluids in the different compartments. The supply ducts for the diluate and the brine are formed by matching holes in the gaskets, the membranes, and the electrode cells. The distance between the membrane sheets, i.e. the cell thickness, should be as small as possible to minimize the electrical resistance. In industrial size electrodialysis stacks membrane distances are typically between 0.5 to 2 mm. A spacer is introduced between the individual membrane sheets both to support the membrane and to help control the feed solution flow distribution. The most serious design problem for an electrodialysis stack is that of assuring uniform flow distribution in the various compartments. In a practical electrodialysis system, 200 to 1000 cation- and anion-exchange membranes are installed in parallel to form an electrodialysis stack with 100 to 500 cell pairs. 

---