Zirconium scrubbing

After extraction, the loaded solvent contains 6 g T1 zirconium as zirconium oxide with 0.2% hafnium oxide. The raffinate is left with 0.2 to 0.3 g l l of the oxides of zirconium and hafnium of this, 70-90% is hafnium oxide. This raffinate can act as a feed solution for the recovery of pure hafnium oxide. The loaded extractant, on the other hand, is subjected to a scrubbing operation with pure zirconium sulfate solution to eliminate any co-extracted hafnium. This scrubbing operation is essentially a displacement reaction ... 

The heavy mineral sand concentrates are scrubbed to remove any surface coatings, dried, and separated into magnetic and nonmagnetic fractions (see Separation, magnetic). Each of these fractions is further split into conducting and nonconducting fractions in an electrostatic separator to yield individual concentrates of ilmenite, leucoxene, monazite, rutile, xenotime, and zircon. Commercially pure zircon sand typically contains 64% zirconium oxide, 34% silicon oxide, 1.2% hafnium oxide, and 0.8% other oxides including aluminum, iron, titanium, yttrium, lanthanides, uranium, thorium, phosphorus, scandium, and calcium. 

Distillation under reduced pressure allowed separation of the TBP-diluent mixture components into three fractions diluent, 60% TBP, and distillation residue. The first two fractions can be reused in the process, but the residue contains high-molecular-weight degradation products, which are not eliminated by alkaline scrubbing. Distillation removes the degradation products that are responsible for poor hydrodynamic behavior and for the retention of radioactive products such as plutonium, zirconium, and ruthenium (143). 

The product gases are first cooled below 200°C to selectively condense so-called zirconium tetrachloride snow in a large space condenser. The sihcon tetrachloride subsequently is condensed in a quench condenser wherein the warm gases are countercurrendy scrubbed with liquid silicon tetrachloride at —20° C. The sihcon tetrachloride is purified by stripping and distillation. 

It is of interest to note that addition of 0.001 fluoride to the extraction scrub solution did not improve the zirconium-thorium separation significantly in the scrub section. A large improvement in zirconium-uranium separation has been observed by addition of fluoride to scrub streams in the Purex process. This difference is probably due to the thorium complexing the fluoride and lowering the free fluoride to a level which is ineffective in altering zirconium distribution. 

Zr-Al coprocess waste test, the feed, extractant, and scrub flows were 1, 0.5, and 0.1 mL/min, respectively. For the high sodium concentration waste, the feed, extractant, and scrub flows were 0.75, 1, and 0.25 mL/min, respectively. Samples of raffinate were drawn for analytical analysis approximately five hours after equilibrium had been reached. The resultant decontamination factors agreed reasonably well with our calculations. For the coprocess waste run, we expected an americium decontamination factor of 200. We purposely built in a large, overkillM in the sodium waste run by increasing the organic to aqueous flow rates. The sodium waste run produced a raffinate that, when calcined, would be well below the guideline for alpha-free waste with no allowance for decay. Analytical analysis of feeds and raffinates confirmed our batch results in that actinides were fractionated from major waste constituents such as aluminum, zirconium, sodium, and fluoride. 

The calculation of the concentration of extractable components in a countercurrent cascade of equilibrium solvent extraction stages is first developed for the simple countercurrent extraction section of Fig. 4.3. The theory is then extended to the extracting-scrubbing system of Fig. 4.4 for fractional extraction and is illustrated by a numerical calculation for the separation of zirconium from hafnium, using TBP in kerosene as solvent. 

Suppose that we wish to recover 98 percent of the zirconium and to obtain a zirconium-hafnium decontamination factor of 200. The limiting ratio of scrub to solvent, from... 

Figure 4.17 Flow sheet for zirconium-hafnium extracting-scrubbing example.

As an example of the use of these equations for an extracting-scrubbing cascade, consider the addition of a scrubbing section to the hafnium-zirconium separation example, which was first analyzed in Sec. 6.2 as a simple extraction problem. The modified flow sheet is shown in Fig. 4.17, and desired recoveries and decontamination are given in Table 4.7. 

Table 4.7 Specifications for zirconium-hafnium separation example in an extracting-scrubbing cascade...

Table 4.10 Concentrations in scrubbing section, zirconium-hafnium separation example... 

Figure 4.19 is a plot of zirconium concentration versus hafnium concentration in the organic phase, with points for the extracting section from Table 4.9 and points for the scrubbing section from Table 4.10. The point of intersection occurs at... 

Thus, six theoretical stages in the extracting section and four in the scrubbing section would result in higher values of zirconium recovery and hafnium decontamination than those specified. 

The feed conditions have been chosen so that nitric acid in the organic feed is nearly in equilibrium with the nitric acid in the aqueous feed and scrub solution, so that little change in nitric acid concentration occurs through the cascade. The small changes in nitric acid concentration are due principally to variations in amount of TBP complexed by zirconium. 

Hexone flowing from the extracting section to the scrubbing section B contained almost all the hafnium in the feed and about 30 percent of the zirconium. The scrubbing section consisted of three columns of 10.2 cm Pyrex pipe 45.4 m in total length. Countercurrent flow... 

A rough estimate of the number of theoretical plates in the scrubbing and extracting sections can be obtained from the material flow sheet for these sections, Fig. 7.7. The distribution coefficients for zirconium and hafnium assigned to the two sections were obtained from the following conditions ... 

The fraction of feed zirconium recycled through the scrubbing section is (14.73 g/literX530 liters)/(117.6 g/literX189 liters) = 0.35 McClain and Shelton state that about 30 percent was recycled. 

Production of ZrCl4. Zirconium oxide from the hafnium-separation step was mixed with carbon black, dextrin, and water in proportions 142 Zr02, 142 C, 8 dextrin, and 8 water. The mixture was pressed into small briquettes (3.8 X 2.5 X 1.9 cm) and dried at 120°C in a tray drier. The oxide briquettes were charged to the reaction zone of a vertical-shaft chlorinator lined with silica brick. The charge was first heated by carbon resistance strips until it became conductive. During production, the bed temperature was maintained at 600 to 800 C by an electric current passed directly through the bed. After steady conditions were reached, a reactor 66 cm in diameter produced about 25 kg ZrCLt/h. The ZrCU was condensed from the reaction products in two cyclone-shaped aftercondensers in series, and the chlorine off-gas was removed in a water scrubbing tower. 

Refer to Fig. 7.7 and show that the number of theoretical plates in the scrubbing and extracting sections of the zirconium-hafnium separation plant are 15.4 and 19.5, respectively, for the flow rates and distribution coefficients stated in the figure. Show that the zirconium and hafnium concentrations in the aqueous feed to the extracting section and the solvent phase leaving that section are as stated. 

In this example, uranium and ruthenium are the key components whose compositions in feed, aqueous waste, and organic extract are specified. Nitric acid concentration in feed is specified, but its distribution between the two product streams must be found by trial, as in the zirconium-hafnium separation example in Sec. 6.5 of Chap. 4. The HNO3 concentrations of 0.02 M in organic extract and 1.658 M in aqueous waste were found by trial to require the same number of scrubbing and extracting stages as the specified uranium and zirconium separation, as will now be shown, and hence represent the calculated distribution of nitric acid. 

In the scrubbing section of the HA column, fission products were scrubbed from the organic phase leaving the extraction section by the HAS scrub stream. It contained 0.01 Af H3PO4 to complex protactinium and zirconium-niobium and reduce their extraction. It also contained 0.01 Af ferrous sulfamate to reduce plutonium and chromium corrosion product to inextractable species. 

CodecontaminatioiL The codecontamination section consists of the HA extraction section equipped with short-contact-time centrifugal contactors and the HS scrubbing section equipped with pulse columns. In the HA section, uranium and plutonium in the aqueous feed and reflux from the HS section are extracted into the organic stream containing 30 v/o TBP. In the HS section any ruthenium extracted by TBP is scrubbed into the aqueous phase with 3 M HNO3. Then any zirconium-niobium in the TBP is scrubbed with 0.3 M HNOa. Scrubbing is at 50°C to enhance decontamination of ruthenium. 

Swanson, J. L Neptunium and Zirconium Extraction under Purex HA Column Scrub Conditions, Report BNWL-1588, 1971. 

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