**7. Preparation of highly enriched 15N by chemical exchange**

The migration distance and cross linking can both affect the nitrogen isotope separation. It has relatively low separation coefficient by ion exchange chromatographic method when compared with that of NITROX method, it is very difficult to prepare 99.9% 15N directly from natural abundance of 0.366%. At the present stage of industrial factory, NITROX was used widely to produce various percentages of 15N products. It is very difficult to get highly enriched 15N by this method. In order to improve feasibility of this method, one good idea is to combine NITROX method and ion exchange chromatography. Starting from the natural abundance, NITROX has advantage to enrich 15N because of large separation coefficient; while when it reaches to relative highly enriched 15N, due to the good merit of very small HETP value, the ion exchange chromatographic method is performed to get highly enriched 15N.

In order to simulate the industry scale, large diameter column (=3.0 cm) and high speed band velocity were used in this stage. 100 g commercial NH4Cl (15N-80%) were feed into 3.0 cm diameter ion exchange columns. In order to shorten the ammonium adsorption time, the concentration of NH4Cl was adjusted to about 0.5 mol/dm3 and the length of ammonium adsorption band was 138 cm. Then 0.22 mol/dm3 NaOH was feed to the rear part of ammonium adsorption band to develop the adsorption band. The flow rate of feeding solution is 50 ml/min and the band velocity is 12 m/d. When the front boundary of ammonium adsorption band reached to twenty five meters, the three way valve was connected to the column for portion sampling. The sampling volume for each sample was about 0.5 ml for every seven minute and it's corresponding to 5.0 cm of adsorption band for each sampling. These samples were analyzed by mass spectrometer for determine the nitrogen isotopic ratio as the result of the migration distance equal to 25 meters. When the rear boundary of ammonium adsorption band passes through the three way valve, the sampling was stopped. In order to keep the length of ammonium adsorption band constant and to avoid band diffusion, total amount of 3.0 L pure water with same flow rate was feed to the top of the column to develop the inside part of free ammonium molecular of adsorption band, the free ammonium molecular was move to the front boundary and readsorb again at the front boundary. The adsorption band moved forward 22.5 cm after 3.0 L pure water washing. Five days later, NaOH solution was feed again to develop the migration distance until fifty meters. The operation condition was the same with the first stage. When the front boundary of ammonium adsorption band was reached to fifty meters, three way valve was connected to the column for portion sampling. The sampling volume for each sample was about 0.5 ml for every five minute and it's corresponding to 4.0 cm of adsorption band for each sampling. Before the rear boundary of ammonium adsorption band reach to three way valve, about 10 cm left the boundary, sampling was changed to whole fraction sampling and the volume of each sampling was 30 ml and each corresponds to 1.0 cm of adsorption band, the sampling time was 45 second. Before sampling, excess amount of HCl was added to each bottle for neutralization. After the rear boundary pass through three way valve, 3.0 L pure water was feed to the top of column.

358 Ion Exchange Technologies

(Square dot was the result of SQS-6 resin)

HETP cm

0.015 0.020 0.025 0.030 0.035 0.040 0.045 0.050 0.055 0.060

15N.

**Figure 9.** Observed HETP values and cross linking

**7. Preparation of highly enriched 15N by chemical exchange** 

 SQS-6 resin Present resin

The migration distance and cross linking can both affect the nitrogen isotope separation. It has relatively low separation coefficient by ion exchange chromatographic method when compared with that of NITROX method, it is very difficult to prepare 99.9% 15N directly from natural abundance of 0.366%. At the present stage of industrial factory, NITROX was used widely to produce various percentages of 15N products. It is very difficult to get highly enriched 15N by this method. In order to improve feasibility of this method, one good idea is to combine NITROX method and ion exchange chromatography. Starting from the natural abundance, NITROX has advantage to enrich 15N because of large separation coefficient; while when it reaches to relative highly enriched 15N, due to the good merit of very small HETP value, the ion exchange chromatographic method is performed to get highly enriched

5 10 15 20 25 30 35 40

Cross linking %

In order to simulate the industry scale, large diameter column (=3.0 cm) and high speed band velocity were used in this stage. 100 g commercial NH4Cl (15N-80%) were feed into 3.0 cm diameter ion exchange columns. In order to shorten the ammonium adsorption time, the concentration of NH4Cl was adjusted to about 0.5 mol/dm3 and the length of ammonium adsorption band was 138 cm. Then 0.22 mol/dm3 NaOH was feed to the rear part of ammonium adsorption band to develop the adsorption band. The flow rate of feeding solution is 50 ml/min and the band velocity is 12 m/d. When the front boundary of The large scale experimental conditions and results were listed in Table 3 and the chromatographic isotopic distribution curve of 25 m and 50 m migration distance were given in Figure 10 and Figure 11. During the long chromatographic operation, small amount ammonium ion exists in the tailing and may cause to the decrease the final isotope separation effect; this is the disadvantage of the long chromatographic operation. From the isotopic ratio curve of Figure 10 and Figure 11, the front boundary of ammonium adsorption band, the 14N isotopes could be continually enriched to over 99% which starting from 20%. On the contrary, 15N isotope was enriched in the rear boundary regions steadily. Compare the results by Run1 and Run2 in Table 3, both are starting from 80%-15N NH4Cl, the main differences are column diameter and flow rate. In Run3, high flow rate (50 ml/min) and 3.0 cm diameter column was used. The results show that at the rear band, both results have nearly the same values of separation coefficient (0.022) and HETP (0.16cm). At Run3, the highest percentage of enriched 15N was 99.756%, higher than the value of 99.672% by Run2, even if the migration distance in Run3 is shorten than Run2. This means the speed of ion exchange between sodium and ammonium is very fast, and high flow rate does not affect to the HETP and separation coefficient in this operation systems. It is obviously that high flow rate has advantage to get highly enriched 15N and is much suitable to industrial operation. Run 3 and Run 4 were within the same experiment in a whole experiment; here it was divided into two parts. From the results of Run3 and Run4, 15N was steadily enriched with


**Table 3.** The experimental conditions and results for nitrogen isotope separation by using 80% 15NH4Cl

the increase of the long migration distance, but the increasing trend becomes very low. For the first twenty five meter's migration, 15N has been enriched to 99.756%, after another twenty five meter's operation, the final maximum enrichment percentage reached to 99.859%. Enriched 15N is only increased 0.1% by another twenty five meter's migration, although the highly enriched regions became broaden and we can get much volume of 15N which the percentage is over 99.8%. The separation coefficient of Run3 and Run4 is same because of the same operation condition; Run4 has relatively smaller HETP value than the value of Run 3. It indicates that long chromatographic operation has advantage and can steadily enrich 15N to very high percentage. The reason why the enriched 15N could not enrich to higher percentage in the present operation system may come from the stopping during the operation between Run3 and Run4. Because of the stopping by one week, the inner ammonium adsorption band may take remixing within the middle adsorption band. Highly enriched 15N at rear band will mix with middle level enriched 15N which was located in the middle band. If this operation restarts again, it will take long distance chromatographic operation to compensate the mixing and reach to the same percentage of 15N with twenty five meter's operation. From this result, the effective migration distance may be less than fifty meters. This can be confirmed by the analysis of the isotopic curve of Run 3 and Run 4 in Figure 10 and Figure 11. In Figure 10, there is a flat regions existing in the middle ammonium adsorption band, in this region, the ratio of 15N/14N is the same with

**Figure 10.** 15N/14N isotopic ratio (25 meter)

Ammonium

Adsorption capacity

Feed solution 0.19M NH4Cl (15N-

Boundary Front 14N Rear

80%)

Temp. K 308

Run No. Run 1 Run 2 Run 3 Resin SQS-6 high porous cation exchange resin

Effluent solution 0.20M NaOH 0.23M NaOH 0.24M NaOH

Migration distance m 30 25 50 Column diameter mm 8 30 30

adsorption band m 1.0 1.38 1.38 Flow rate ml/min 1.5 50 50 Band velocity m/d 4.8 11.1 14.4

Q mmol/m 74.3 1230 1250

Nitrogen % 99.143 99.672 96.617 99.756 99.710 99.859 Separation coefficient 0.023 0.022 0.022 0.022 0.023 0.023 Slope coefficient Ks 0.6104 0.1541 0.5218 0.1569 0.4919 0.1599 HETP cm 0.039 0.158 0.044 0.157 0.048 0.145

**Table 3.** The experimental conditions and results for nitrogen isotope separation by using 80% 15NH4Cl

the increase of the long migration distance, but the increasing trend becomes very low. For the first twenty five meter's migration, 15N has been enriched to 99.756%, after another twenty five meter's operation, the final maximum enrichment percentage reached to 99.859%. Enriched 15N is only increased 0.1% by another twenty five meter's migration, although the highly enriched regions became broaden and we can get much volume of 15N which the percentage is over 99.8%. The separation coefficient of Run3 and Run4 is same because of the same operation condition; Run4 has relatively smaller HETP value than the value of Run 3. It indicates that long chromatographic operation has advantage and can steadily enrich 15N to very high percentage. The reason why the enriched 15N could not enrich to higher percentage in the present operation system may come from the stopping during the operation between Run3 and Run4. Because of the stopping by one week, the inner ammonium adsorption band may take remixing within the middle adsorption band. Highly enriched 15N at rear band will mix with middle level enriched 15N which was located in the middle band. If this operation restarts again, it will take long distance chromatographic operation to compensate the mixing and reach to the same percentage of 15N with twenty five meter's operation. From this result, the effective migration distance may be less than fifty meters. This can be confirmed by the analysis of the isotopic curve of Run 3 and Run 4 in Figure 10 and Figure 11. In Figure 10, there is a flat regions existing in the middle ammonium adsorption band, in this region, the ratio of 15N/14N is the same with

15N Front 14N Rear

0.50M NH4Cl (15N-80%)

0.50M NH4Cl (15N-80%)

15N Front 14N Rear

15N

**Figure 11.** 15N/14N isotopic ratio (50 meter)

the original feeding solution, 80%. At the rear boundary of the isotopic distribution curve, the slope of isotopic curve is still kept sharpness. In Figure 11, it was found that this flat region almost disappears and the isotopic ratio of 15N/14N was steadily increasing in the whole ammonium adsorption band. At the rear boundary region of the isotopic curve, there is a large amount of highly enriched 15N and the slope coefficient of isotopic distribution curve becomes flat. This is the evidence that by long chromatographic operation, highly enriched 15N was remixing again with the middle level enriched 15N in the middle position of ammonium adsorption band. The percentage and gram distribution of enriched 15N which along with ammonium adsorption band were listed in Table 4. 15N was mainly concentrated in the adsorption band which range from 64.5-152.03 cm in the adsorption band and 4.6 gram of highly enriched 15N(>99.82%) were successfully obtained.


**Table 4.** Percentage and gram distribution of enriched 15N along ammonium adsorption band by 50 m chromatography
