Countercurrent Ion Exchange

Unlike fixed bed designs where scale up data may be obtained semi-empirically, continuous countercurrent ion exchange plant requires model hydrodynamic data for both the liquid and resin phases as well as predetermined equilibrium and kinetic data for a chosen system. A continuous cycle becomes particularly attractive when required to treat more highly concentrated liquors or operate at high treatment flowrates. 

In the early days of development of continuous ion exchange processes the benefits to water treatment of high efficiency and low leakage were paralleled by the rapidly advancing designs of counterflow fixed bed plant. Thus the more complex hydrodynamic requirements of CIX, resin losses due to mechanical attrition, and perhaps a conservative attitude towards availability of plant during periods of unscheduled maintenance meant that, in the UK at least, the CIX 

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FIG. 16-63 Asahi countercurrent ion-exchange process. [Gilwood, Chem. Eng., 74(26), 86 (1967) copyiight 1967 hy McGraw-Hill, Inc., New York. Exceipted with special peimission of McGi aw-Hill.]... 

Gilwood, M.E. Saving Capital and Chemicals with Countercurrent Ion Exchange, Chemical Engineering, Dec. 18, 1967, p. 83. 

Carman (C6) described the successful development and application to a uranium resin-in-pulp process of a continuous countercurrent ion-exchange pilot plant. This new technique is based on the observation that the resins at the correct level of air agitation float in close proximity to the surface of the pulp. So long as the resin beads are able to move about gently but freely in the surface layer, a satisfactory rate of ion exchange is possible. Under this condition, the mechanical damage to the resin due to attrition is negligible. 

A general scheme for the countercurrent ion-exchange separation process with flow reversal is analogous to the schemes for such two-phase separation processes as rectification, chemical exchange, etc. (Fig.l). 

Countercurrent ion exchange, besides being used for separating mbttures, may also be used to solve other practical and important problems, e.g., for recovery of valuable and/or toxic substances from solution and pulps, for concentrating solutions, for softening and demineralizing water, for separating nonelectrolytes from electrolytes, and for synthesis of different compounds. 

Countercurrent ion exchange is rather widely used for the demineralization of water. 

It should be noted that long before the advent of the Rrst countercurrent ion-exchange column (Nordel, [58]) research using a countercurrent solution and a granulated sorbent for the purification of sugar solution by activated coal had been performed. However, poor mechanical stability of the sorbent [59] discouraged wide applicability of the approach. 

Figure 7 Schematic diagram of two-sectional countercurrent ion-exchange column operating at different temperatures to provide recovery of waste-free bromine from seawater.

The number of publications involved with the recovery of rubidium from seawater is very limited. Most of the work in this field is by Russian scientists, who have proposed several schemes for the combined recovery of rubidium, strontium, and potassium with natural zeolites [15, 19, 250-253, 257]. A number of inorganic sorbents with high selectivity toward rubidium were also synthesized for the recovery of rubidium from natural hydromineral sources, including seawater. Ferrocyanides of the transition-metal ions were shown to exhibit the best properties for this purpose [258, 259]. Mordenite (another natural zeolite) has recently been proposed for selective recovery of rubidium from natural hydromineral sources as well [260]. A review of the properties of inorganic sorbents applicable for the recovery of rubidium from hydromineral sources has been published [261]. Studies of rubidium recovery fix>m seawater [15, 19, 250-253] have shown that the final processing of rubidium concentrates, especially the selective separation of Rb -K mixtures remains the major problem. A report was recently published showing that this problem can be successfully solved by countercurrent ion exchange on phenolic resins [262]. 

J. H. Smith, Modern Countercurrent Ion Exchange Plants and The Hipol Process , Chem. Ind. (London), 1980, No. 18, 718. 

The uranium industry more than any other founded the commercial realization of Continuous Countercurrent Ion Exchange (CIX) technology which has resulted in being able to treat unclarified leach liquors in a near ideal continuous manner. Several modern CIX plants in the uranium industry are based on the successful Multistage... 

Figure 10.6 The Higgins Loop continuous countercurrent ion exchange system Valve positions during cycles ...

The ionic concentration to be treated is an overriding consideration governing the cost of plant of a given design and therefore for very high ion concentrations it is foreseeable that membrane pretreatments such as reverse osmosis and electrodialysis will continue to fulfil an important role as might a more widespread revival of continuous countercurrent ion exchange. 

J. C. Milbourne and I. R. Higgins, Recovery of uranium using continuous countercurrent ion exchange (CCIX ), in Separation Processes Heavy Metals, Ions and Minerals, Ed. M. Misra, TMS, WaiTendale, PA, pp. 3-14. 

Figure 35. Sequence of operation of the Cloete-Streat countercurrent ion exchange process.

One of the more interesting methods for continuous countercurrent ion exchange is the use of fluidized bed techniques for continuous circulation of the resin. Figure 1.6 shows the Dorrco Hydro-softener. In the fluidized bed, a solid phase is suspended in a liquid or gas. Consequently, the solid behaves like a fluid and can be pumped, gravity fed, and handled very much like a liquid. The fluidized resin moves down through the softener on the right and is then picked up by a brine-carrier fluid and transferred to the regenerator on the left. 

Hiester, N. K. et al. Continuous countercurrent ion-exchange with trace components. Chem. Engng. News 50, No. 3, 139 (March 1954). 

Selke WA, Bliss H. Continuous countercurrent ion exchange. Chem Eng Prog 47 529-533, 1951. 

Nitreat A process for removing nitrates from potable water by continuous countercurrent ion exchange. Developed by Anglian Water and operated at the Keldgate reservoir, Yorkshire, since 2009. 

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