Patent Number: 051005859
Section: description

DETAILED DESCRIPTION OF THE INVENTION These and other objects of the invention for the recovery of strontium and technetium values from an aqueous nitric acid feed solution containing these and other fission product values may be met by preparing an extractant of about 0.2M bis-4,4'(5')[(t-butyl)cyclohexano]-18-Crown-6 (Dt-BuCH18C6) in 1-octanol as a diluent, contacting the extractant with the aqueous solution which is up to 6M in nitric acid, maintaining the contact for a period of time sufficient for the strontium and technetium values to be taken up by the extractant, and separating the extractant from the aqueous solution, thereby separating the strontium and technetium values from the aqueous solution. The macrocyclic polyether may be any of the dicyclohexano crown ethers such as dicyclohexano-18-Crown-6, dicyclohexano 21-Crown-7, or dicyclohexano-24-Crown-8. The preferred crown ethers have the formula: 4,4'(5')[(R,R')dicyclohexano]-18-Crown-6, where R and R' are one or more members selected from the group consisting of H and straight chain or branched alkyls containing 1 to 12 carbons. Examples include, methyl, propyl, isobutyl, t-butyl, hexyl, and heptyl. The preferred ethers include dicyclohexano-18-crown-6 (DCH18C6) and bis-methylcyclohexano-18-crown-6 (DMeCH18C6). The most preferred ether is bis-4,4'(5')[(t-butyl)cyclohexano]-18-Crown-6 (Dt-BuCH18C6). The amount of crown ether in the diluent may vary depending upon the particular form of the ether. For example a concentration of about 0.1 to 0.5M of the t-butyl form in the diluent is satisfactory, with 0.2M being the most preferred. When the hydrogen form is used, the concentration may vary from 0.25 to 0.5M. Concentrations above about 0.5M of the ether in the diluent will not improve strontium recovery when R and R' are H. The diluent is an organic compound which has a high boiling point, i.e. about 170.degree. to 200.degree. C., limited or no solubility in water, is capable of dissolving from about 0.5 to 6.0M water, and in which the crown ether is soluble. These diluents include alcohols, ketones, carboxylic acids and esters. Suitable alcohols include 1-octanol, which is most preferred, although 1-heptanol and 1-decanol are also satisfactory. The carboxylic acids include octanoic acid, which is preferred, in addition to heptanoic and hexanoic acids. Ketones which meet the criteria may be either 2-hexanone or 4-methyl-2-pentanone, while the esters include butyl acetate and amyl acetate. One required characteristic of the diluent is that it must be able to dissolve a minimum amount of water. This amount varies with the particular diluent. For alcohols and carboxylic acids the amount of water may vary from about 1.0 to 6.0M, while ketones and esters should dissolve from about 0.5 to 1.0M of water. Although diluents capable of dissolving larger quantities of water are satisfactory from the process standpoint, their use may result in greater losses of extractant and diluent during the process. While we do not wish to be bound by this explanation, it appears that the reason for the low distribution values of strontium in the non water-dissolving diluents is that the nitrate anion remains hydrated on extraction because of the low charge density of Sr.sup.2+. Hydrated anions are hydrophilic and thus will not readily transfer into a lipophilic medium. By the use of a diluent which can contain dissolved water, an environment is provided in which the anion need not dehydrate to be solvated. By avoiding anion dehydration, additional free energy of extraction is achieved. It is not necessary that the diluent be saturated with water before contact of the extractant with the aqueous nitric acid containing waste solution because the water for dissolution in the diluent is obtained from the aqueous waste feed solution. The volume ratio between the organic extractant and the aqueous acid feed solution depends upon the particular extractant system, i.e. the particular crown ether and diluent system. Generally these ratios may vary from about 1:1 to 1:4. For the extractant system consisting of bis-4,4'(5')[(t-butyl)cyclohexano]-18-Crown-6 in 1-octanol, the preferred ratio is 1:3. For dicyclohexano-18-crown-6, a 1:2 ratio is preferred. The extraction process is preferably run at ambient temperature since higher temperatures have little effecttion on extraction. Contact times at ambient temperature are short, typically on the order of about 0.5 minutes to provide complete extraction of the strontium and technetium values. Stripping the extracted strontium and technetium values from the extractant can be readily accomplished by contacting the extractant with water. A major advantage of the process of the invention is the stability of the crown ether-diluent systems to radiolysis. Exposure of DCH18C6 in octanol to an absorbed dose of 10 watt-hours/liter showed no change in strontium distribution ratios on extraction and stripping and on phase disengagement time. It is believed that any radiolytic degradation products formed are water soluble and therefore would be removed during extraction, scrubbing, and stripping cycles. The following examples are given to illustrate the invention but are not to be taken as limiting the scope of the invention which is defined in the appended claims. EXAMPLES The crown ethers dicyclohexano-18-crown-6 (DCH8C6) (99% pure), and bis-4,4'(5)[(t-butyl)cyclohexano]-18-crown-6 (Dt-BuCH18C6) were used without further purification. The extraction of HNO.sub.3 by the various solvents was measured by equilibrating the organic phase four times with a 1M HNO.sub.3 solution using an organic to aqueous phase ratio of 3. The resultant organic phase was stripped of acid by repeated water washings and the washings titrated with standard sodium hydroxide. All distributions of Sr were measured radiometrically. Prior to a distribution experiment, the organic phase was pre-equilibrated by contacting it 2-3 times with twice its volume of 1M HNO,. A 1.00 mL aliquot of this pre-equilibrated organic phase was then combined with an equal volume of fresh 1M HNO, spiked with .sup.85 Sr. The two phases were mixed using a vortex mixer for one minute, then centrifuged until complete phase separation was obtained. The .sup.85 Sr activity in each phase was measured by gamma counting using a Beckman Biogamma Counter. All measurements were performed at 25.+-.0.5.degree. C. EXAMPLE 1 A series of experiments were made to correlate the distribution of strontium with the molarity of water in a variety of diluents. 1 ml portions of 0.5M DCH18C6 or 0.1M Dt-BuCH18C6 in the various organic diluents were contacted with 1.0M HNO.sub.3 spiked with .sup.85 Sr. The results, grouped by diluent types, are shown in FIGS. 1 and 2 respectively. They show the positive effect of increasing water content in the diluent on the strontium extraction constant. EXAMPLE 2 Another series of experiments were run in the manner described above in order to determine the influence of acidity on the distribution ratios of the inert constituents and fissions products contained in a synthetic dissolved waste sludge (DWS) using 0.375M DCH18C6 and 0.125M Aliquat in octanoic acid. Aliquat 336, which is tricaprylylmethylammonium nitrate, was introduced into the organic phase to create an environment more favorable for ion pair formation. The results are shown in Table 1 below. TABLE 1 ______________________________________ Effect of Acidity Upon distribution Ratios of DSW Constituents System: 0.375 --M DCH18C6/0.125 --M Aliquat 336 in OA vs. DSW @ Various HNO.sub.3) Concentrations. Distribution Ratios 1 M 3 M 6 M ______________________________________ Inert Constituents Na 0.37 0.41 0.21 Mg 0.14 0.09 0.05 Al 0.14 0.10 0.06 Ca 0.15 0.17 0.19 Cr 0.14 0.10 0.06 Mn 0.14 0.10 0.06 Fe 0.14 0.10 0.06 Ni 0.14 0.10 0.06 Cu 0.14 0.10 0.06 Fission Products Sr 2.4 3.8 5.8 Y 0.14 0.09 0.06 Zr 0.09 0.10 0.12 Mo 0.15 0.12 &lt;0.10 Ru 0.17 0.15 0.11 Rh 0.15 0.10 &lt;0.06 Pd 0.20 &lt;0.20 &lt;0.24 Ag &lt;0.20 &lt;0.20 &lt;0.20 Cd 0.14 &lt;0.11 &lt;0.13 Cs 0.27 0.16 0.03 Ba 1.4 1.9 2.2 La 0.14 0.10 0.08 Ce 0.14 0.12 0.08 Pr 0.14 0.09 0.07 Nd 0.17 0.13 0.10 Sm 0.13 0.07 0.05 Eu 0.14 0.10 0.07 ______________________________________ The data show that only Sr, Ba, and Zr distribution ratios increase with acidity: in all other cases D's decrease. EXAMPLE 3 The effect of changing diluent on D.sub.Sr was studied using solutions of DCH18C6 in using 2-ethylhexanol, n-octanol, and n-decanol. A comparison of D.sub.Sr using the above three alcohols and n-octanoic acid is shown in Table 2 below. TABLE 2 ______________________________________ Comparison of D.sub.sr using 0.1 --M DCH18C6 in octanoic acid, 2-ethylhexanol, n-octanol, and n-decanol, 25.degree. C. DSr Diluent 1M HNO3 3M HNO3 ______________________________________ octanoic acid 0.35 0.66 2-Ethylhexanol 0.57 2.0 n-octanol 0.71 3.1 n-decanol 0.36 1.6 ______________________________________ The results show that n-octanol is more effective in increasing D.sub.Sr than either 2-ethylhexanol or n-decanol and more effective than octanoic acid, especially at 3M HNO.sub.3. EXAMPLE 4 The extractant dependencies of D.sub.Sr with DCH18C6 in n-octanol from 1, 3, and 6M HNO.sub.3 at 25.degree. and 50.degree. C. are shown in FIG. 3. Under all conditions, the extractant dependencies are less than 1st power, although dependency increases as the concentration of DCH18C6 decreases. The D.sub.Sr extractant dependency is also slightly higher when the extraction takes place from 6M rather than 1M HNO.sub.3. Temperature is also shown to have little effect until the concentration of crown is less than 0.1M. EXAMPLE 5 The extractant dependencies of D.sub.Sr with DHC18C6, Dt-BuCH18C6, and bis-4,4'(5')-(methylcyclohexano)-l8-Crown-6 (DMeCH18C6) in n-octanol from 1M HNO.sub.3 at 25.degree. C. are shown in FIG. 4. As may be seen, each of the three extractants is equally effective at low contentration (about 0.01M). At concentrations high enough to yield acceptable D.sub.Sr values however, the di-t-butyl compound gives distribution ratios a factor of 5 or more greater than does DCH18C6 and at least twice those for the dimethyl compound. EXAMPLE 6 The distribution ratios of a number of inert constituents and fission products were measured at 40.degree. C. from synthetic dissolved sludge waste with 0.25M and 0.50M DCH18C6 in n-octanol using the experimental methods hereinbefore described. Measurements were also carried out using n-octanol without crown ether for comparison. The results are given in Table 3 below. TABLE 3 ______________________________________ Distribution Ratios of Inert Constituents and Fission Products from Dissolved Sludge Waste (1M HNO.sub.3 --0.5M Al) T = 40.degree. C. Distribution Ratios 0.25 --M DCH18C6 0.5 --M DCH18C6 n-Octanol in n-Octanol in n-Octanol ______________________________________ Inert Constituents Na 0.02 0.30 0.49 Mg 0.02 0.07 0.02 Al 0.02 0.07 0.02 Ca 0.03 0.15 0.18 Cr 0.03 0.07 0.02 Mn 0.03 0.07 0.02 Fe 0.03 0.07 0.02 Ni 0.03 0.08 0.02 Cu 0.03 0.08 0.03 Fission Products Sr 0.02 4.1 5.0 Y 0.02 0.08 0.02 Zr 0.04 0.10 0.06 Mo 0.15 0.26 0.18 Ru 0.25 0.52 0.53 Rh 0.03 &lt;0.05 &lt;0.05 Pd 0.21 0.47 0.71 Ag &lt;0.05 &lt;0.2 0.10 Cd 0.02 0.10 0.03 Cs -- (0.11) (0.33) Ba 0.03 1.3 1.5 La 0.03 0.08 0.02 R.E.'s 0.01-0.04 0.04-0.11 0.01-0.05 ______________________________________ () Radiometric Determination The data show that D is greater than one only for Sr and Ba. While Na, Ru, and Pd show some extractability, the distribution for the strontium is sufficiently higher to achieve complete separation. The data also show that selectivity is not significantly different for the two DCH18C6 concentrations and that n-octanol makes an appreciable contribution to the D's of all constituents except Sr and Ba. EXAMPLE 7 A series of experiments were run to study the affect of HNO.sub.3 concentration on the D's using 0.5M DCH18C6 solution. The results are shown in Table 4 below. TABLE 4 ______________________________________ Effect of Acidity Upon Distribution Ratios of DSW Constituents, 40.degree. C. System: 0.502 --M DCH18C6 in Octyl Alcohol vs. DSW @ Various HNO.sub.3 Concentrations Distribution Ratios 1 M 3 M 6 M ______________________________________ Inert Constituents Na 0.45 0.41 0.16 Mg 0.09 0.05 0.05 Al 0.09 0.06 0.06 Ca 0.15 0.23 0.24 Cr 0.09 0.06 0.06 Mn 0.09 0.06 0.06 Fe 0.09 0.06 0.06 Ni 0.09 0.06 0.06 Cu 0.09 0.06 0.06 Fission Products Sr 2.0 6.9 15.0 Y 0.09 0.06 0.06 Zr 0.10 0.12 0.21 Mo 0.20 0.23 0.27 Ru 0.40 0.41 0.23 Rh 0.12 0.09 0.06 Pd 0.67 0.58 0.29 Ag &lt;0.2 &lt;0.2 &lt;0.2 Cd &lt;0.1 &lt;0.1 &lt;0.13 Cs 0.34 0.18 0.03 Ba 0.91 2.6 3.6 La 0.10 0.07 0.08 Ce 0.10 0.08 0.08 R.E.'s Pr 0.09 0.06 0.07 Nd 0.12 0.10 0.10 Sm 0.07 0.03 0.05 Eu 0.10 0.06 0.07 ______________________________________ The data show clearly that the selectivity of Sr over every cationic constituent increases with an increase in acidity, which parallels the behavior of the corresponding octanoic acid system. EXAMPLE 8 Another series of experiments were run in the manner described above to determine the D.sub.Sr from nitric acid solutions with Dt-BuCH18C6 at several concentrations in n-octanol at 25.degree. and 50.degree. C. The results, as shown in FIGS. 5a and 5b, illustrate that Sr extraction continues to improve as the acidity of the aqueous phase increases up to 6M HNO.sub.3. The figures further illustrate that the temperature has little effect upon distribution ratios. EXAMPLE 9 The extraction of technetium from nitric acid solutions of various acidities using 0.20M Dt-BuHC18C6 in n-octanol was studied in the manner described above. The distribution ratios of Tc were measured radiometrically using liquid scintillation counting The results are given in Table 5 below. TABLE 5 ______________________________________ D.sub.TC using 0.20 D-t-BuCH18C6 in n-octanol at various acidities [HNO.sub.3 ] M D.sub.TC ______________________________________ 0.010 0.12 0.10 0.24 0.5 0.69 1.0 1.25 3.0 1.85 6.0 1.61 ______________________________________ The data from the table above show that, as with strontium, the D.sub.Tc increases with increasing nitric acid concentration. As has been shown by the preceding discussion and Examples, the process of the invention provides a safe and effective means for the recovery of Sr and Tc values from aqueous solutions up to 6 molar in nitric acid which contain these and other metal values.