Ion exchange cycles

FIGURE 11.1 The ion-exchange cycle from top to bottom sulfonation of some of the phenyl rings formation of the sodium form and metal chelation and regeneration. 

In the case of the Canadian yttrium-heavy rare earth concentrate, this is leached with HN03, causing all rare earths to go into solution. Solvent extraction separates yttrium from the other heavy rare earths, each of which can eventually be separated by further solvent extraction. In the case of xenotime, this is leached with hot H2S04 and separation of yttrium and the heavy rare earths completed in ion-exchange columns. The liquid-liquid organic solvent extraction cycle is complete within 5-10 days and is a continuous process. The resin ion-exchange cycle requires 60—90 days and is a batch process. Both processes result in pure rare earth oxides and chemicals. 

In laboratory tests using simulated HLW solution spiked with fission product tracers, Am and Cm, the denitration step proved to be a sensitive process, but Am/Cm recoveries of ca. 90% in the aqueous supernate could be realized under optimized conditions. Decontamination factors (DF) > 1000 for Zr, Nb, Mo, and 100 for Ru and Fe were obtained in the precipitation step. The solvent extraction cycle gave > 98% recovery of Am/Cm and DF > 10 for rare earths, Sr and Cs. Appreciable decontamination was also obtained for Zr/Nb (DF = 20), Ru (50), U (650), Pu (250), Np (800) and Fe (420). The ion exchange cycle served mainly for Am-Cm concentration and for removal of DTPA and lactic acid based on tests with europium as a stand-in for trivalent actinides, concentration factors of about 50 could be expected under optimized conditions. 

Irreversible swelling is a phenomenon principally observed with acrylic strong base anion exchange resins whereby upon undergoing their first few aqueous ion exchange cycles an irreversible expansion occurs of around 7—10% over and above the reversible volume changes which thereafter apply. 

Fig. 3 Schematic of a conventional ion-exchange cycle. (View this art in color at www.dekker.com.)...

Model for transport of Na+ and K+ by Na+,K+-ATPase. Inside the cell, Na+ initiates the one-way ion exchange cycle by the phosphorylation of an aspartate residue at the active site by ATP, which eventually leads to the translocation of Na" " and K+. Conformational changes of the enzyme occur during the exchange of ions. [Reproduced, with permission, from K. J. Sweadner and S. M. Goldin, Active transport of sodium and potassium ions. New Engl. J. Med. 302, 111 (1980).]... 

At reaction conditions where the rates of mass transfer are much higher than the rate of reaction, the mass transfer steps can be ignored. Thus, we assume that the reaction mechanism consists of an ion-exchange reaction step between OAc and Q+X (for the first ion-exchange cycle, X = CL and for all subsequent cycles, X = Br ) to form an active site, Q+OAc, followed by reaction of RBr at this site to form the final product ROAc and an inactive site Q+Bf. These steps may be expressed as follows ... 

After only one ion-exchange cycle, the rainbow colors were observed, indicating that this 400-900X distance had increased to at least 1200A. 

Thus, both anionic and nonionic emulsifiers in concentrations equal to that of the polymer are removed from the latex completely by one or two ion-exchange cycles. 

After the second extraction/stripping cycle, the plutonium is concentrated by evaporation or by preferential adsorption (qv) on ion-exchange resins. As in the case for uranium, the newer faciHties, such as THORP, use only a single purification step. 

Table 16-1 classifies sorption operations by the type of interaction and the basis for the separation. In addition to the normal sorption operations of adsorption and ion exchange, some other similar separations are included. Applications are discussed in this section in Process Cycles. ... 

Anion exchange resins. Should be conditioned before use by successive washing with water, EtOH and water, and taken through two OH"—H" "—OH" " cycles by successive treatment with N NaOH, water, N HCl, water and N NaOH, then washed with water until neutral to give the OH" form. (See commercial catalogues on ion exchange resins). 

Cycle A complete course of ion-exchange operation. For instance, a complete cycle of cation exchange would involve regeneration of the resin with acid, rinse to remove excess acid, exhaustion, backwash, and finally regeneration. 

Finally,Captopril is produced. Thethioester (0.85g) isdissolved in5.5N methanolicammonia and the solution is kept at room temperature for 2 hours. The solvent is removed in vacuo and the residue Is dissolved in water, applied to an ion exchange column on the H cycle (Dowex 50, analytical grade) and eluted with water. The fractions that give positive thiol reaction are pooled and freeze dried. The residue Is crystallized from ethyl acetate-hexane, yield 0.3 g. The 1 -(3-mercapto-2-D-methylpropanoyl)-L-proline has a melting point of 103°C to 104°C. 

Maximum values of specific conductance are often not achievable without exceeding maximum T alkalinity values, especially in boilers below 900 psig (6.21 MPa) with greater than 20.0% MU water whose alkalinity is >20% of TDS naturally or after pretreatment by lime-soda or sodium cycle ion exchange softening. Actual permissible conductance values to achieve any desired steam purity must be established for each case by careful steam purity measurements. The relationship between conductance and steam purity is affected by too many variables to allow its reduction to a simple list of tabulated values. 

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