Extraction processes pulsed columns

A recent and extremely important development lies in the application of the technique of liquid extraction to metallurgical processes. The successful development of methods for the purification of uranium fuel and for the recovery of spent fuel elements in the nuclear power industry by extraction methods, mainly based on packed, including pulsed, columns as discussed in Section 13.5 has led to their application to other metallurgical processes. Of these, the recovery of copper from acid leach liquors and subsequent electro-winning from these liquors is the most extensive, although further applications to nickel and other metals are being developed. In many of these processes, some form of chemical complex is formed between the solute and the solvent so that the kinetics of the process become important. The extraction operation may be either a physical operation, as discussed previously, or a chemical operation. Chemical operations have been classified by Hanson(1) as follows ... 

Irradiated UO2 is dissolved in nitric acid, resulting in a dissolver solution with the approximate composition listed in Table 12.7. This is treated by the Purex process. The main steps in the conventional Purex process are shown schematically in Fig. 12.5. All existing plants listed in Table 12.8 use some variation of the Purex process. Typically, the extractant composition (percentage TBP, diluent) and the extraction equipment (i.e., pulse columns, mixer-settlers, etc.), vary from plant to plant. However, the upper concentration limit is 30% TBP to prevent a phase reversal due to the increased density of the fully loaded solvent phase. 

A pilot plant, containing two pulsed columns, one for extraction and one for stripping, and batchwise evaporation was in operation in Sweden during 1981. Pilot plant operations have also been performed in Holland (Fig. 14.6) and Germany. The experience from these tests shows that the process concept is technically practicable and well proven. The economics of the process, however, are strongly dependent on the cost for disposal of spent pickling liquors. 

In the tests carried out in centrifugal extractors, the extraction and recovery of cesium higher than 99.99% were obtained on simulated effluents, with a very good coherence between calculated flowsheets and experimental results.102 Tests confirmed the feasibility of the implementation of the cesium process in pulsed columns, the latter representing the most adapted contactors for the industrial implementation to overcome the drawback due to the presence of solid matter in waste to be treated. 

The uranium and plutonium are recovered for further use by first dissolving the spent fuel in nitric acid and subjecting the resulting solution to a solvent extraction process. Several different processes exist, the best known being the Purex process (Fig. 18), in which tributyl phosphate (TBP) (30% solution in kerosene) is the extractant. Extraction is carried out in compact mixer-settlers or air-pulsed columns fabricated of stainless steel, with about 99.9% removal of uranium and plutonium in the extract. 

Nickel and cobalt often occur with copper, and must be separated in pure form from hydrometallurgical leach liquors. Organic acid extractants can quite readily separate copper from cobalt and nickel, but the separation of cobalt from nickel is rather difficult. In one Ni/Co separation process, di-2-ethyl hexyl phosphoric acid (D2EHPA) is used as extractant, with strict control of the pH of the aqueous phase to take full advantage of the slightly different equilibrium constants for the Co and Ni reactions. Pulsed column contactors are used rather than mixer-settlers, and nickel impurity is removed from the loaded organic phase by scrubbing it with a cobalt-rich phase. 

In the Purex process, irradiated UO2 is dissolved in nitric acid under such conditions that uranium is oxidized to uranyl nitrate and plutonium to Pu(N03)4. The resulting aqueous solution of uranyl, plutonium, and fission-product nitrates is fed to the center of countercurrent solvent extraction contactor I, which may be either a pulse column or a battery of mixer-settlers. This contactor is refluxed at one end by clean solvent and at the other by a dilute nitric acid scrub solution. The solvent extracts all the uranium and plutonium from the aqueous phase and some of the fission products. The fission products are removed from the solvent by the nitric acid scrub solution. Fission products leave contactor I in solution in aqueous nitric acid. 

Variants of this basic process are used in other plants. For example, the Comurhex plant at Malvesi [B5] filters the output from the dissolver, uses pulse columns in the extracting section, and dilutes TBP with n-dodecane instead of n-hexane. 

The other detrimental effect of TBP degradation is its complexing of zirconium. This increases the zirconium distribution coefficient and consequently decreases the decontamination coefficient. Moreover, solvent residual radioactivity is increased because of incomplete zirconium reextraction. Another and even more troublesome consequence of zirconium complexing is the formation of precipitates known as crud. This is a severe problem, particularly in mixer-settlers, and has led to a preference for pulsed columns or centrifugal contactors in the first extraction cycle when high-burnup fuel is to be processed. 

Codecontamination and partition cycle. Because the codecontamination and partition cycle is the critical step in the acid Thorex process, it will be described in more detail. In this cycle, shown in Fig. 10.21, most of the fission products were separated from the uranium and thorium, which were then separated from each other. The four solvent extraction units, HA, IBX, IBS, and 1C, were pulse columns with dimensions given in the figure. 

The Purex process is presented schematically in Figure 21.11, where the solvit extraction steps are within the dotted frame. Three purification cycles for both uranium and plutonium are shown. High levels of beta and gamma activity is presort only in the first cycle, in which > 99 % of the fission products are separated. The principle of the first cycle is shown in Fig. 21.13. The two other cycles are based upon the same chemical reactions as in the first cycle the purpose is to obtain additional decontamination and overall purity of the uranium and plutonium products. Each square in Figure 21.13 indicates a number of solvent extraction stages of the particular equipment used pulsed columns, mixer-settlers, etc. (see Appoidix A). 

Chemical Separation. Both the Magnox and THORP plants employ solvent extraction methods using TBP/OK (tributyl phosphate/odorless kerosene) to separate out the fission products and uranium and plutonium components of the dissolver liquor. In the case of the Magnox plant, this is done entirely using mixer settlers in THORP, a combination of pulsed columns and mixer settlers are employed. In both plants the overall process produces separate purified aqueous streams of uranyl nitrate and plutonium nitrate, which are subsequently treated to produce the respective oxide forms ... 

In order to avoid the expensive filtration stage immediately prior to solvent extraction, the extraction is sometimes carried out directly on the leach liquor slurry, where conditions are favourable. The Dapex process is then used, since the amine solvents are sorbed readily on to solid surfaces, and entrainment of amines is excessive. Pulsed columns of the sieve-plate type are considered ideal extraction plant in this case, although mixer-settlers have also been employed. 67, ss. s ... 

The advantage claimed for the U.S. pulse column process is that it operates with a high proportion of solids in the feed, i.e. a slurry with up to 15 per cent solids still extracts satisfactorily. This avoids a filtration stage for the feed liquor. However, a small proportion of finely divided solids passes through the whole extraction system by entrainment and makes it necessary to incorporate micro-filters in the system for the product liquor. These are pressure filters made from sintered stainless-steel of 20 /i pore size. Three filter units are necessary, one on load, one being cleaned and one installed spare. 

Extraction can be performed in stirred tanks if the process proceeds fast and separation of phases is ea.sy, but column extractors are most commonly used. The column can be filled with a particulate material. The liquids flow countercurrently whereby the flow can be uniform or pulsed. Reciprocated and rotary agitators are often used to enhance mass transfer. An example of the latter type is shown in Fig. 7.2-13 (asymmetric rotating disk (ARD) extractor). 

Samples were eluted in the reverse direction by using the Milton-Roy pump with the pulse dampener removed. The eluant flow (50-75 mL/min at 200-300 lb/in.2) was monitored at 254 nm by using an Altex 153 detector with a biochemical flow cell. Elution with each solvent was continued until the detector response returned to base line. All columns were eluted with acetonitrile this solvent was preceded by 4.5 M NaCl/0.04 M HC1 and 0.04 M HC1 elutions on the MP-1 column and by 4.5 M NaCl and distilled water elutions on the MP-50 column. The aqueous column effluents were adjusted to pH 2 (MP-1) or pH 11 (MP-50) and then extracted three times with dichloromethane. The acetonitrile column effluents were saturated with NaCl to separate the water, which was extracted twice more with acetonitrile. Fifty percent aliquots of the processed organic solvents from each respective column were concentrated in Kudema-Danish evaporators to a final volume of about 10 mL (any remaining water was removed as the low-boiling azeotrope in the process) to give 25,000 1... 

Fig. 13. (a) Observed and (b) computed formation of a droplet at an orifice in a pulsed sieve-plate extraction column. (Reprinted from Chemical Engineering Science, Volume 50, Ohta M., et al. Numerical analysis of a single drop formation process under pressure pulse condition, pp. 2923-2931, copyright 1995, with permission from Elsevier Science.)... 

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