Determination of Nitrogen Losses in the Sulfuric Acid Solution, Waste of the 15N Separation Plant

The most used method for production of stable isotope 15N is based on the isotopic exchange 15N/14N in the nitrogen oxides-nitric acid solution system, known as Nitrox system. At the bottom of the packed separation column, the reflux is provided by total conversion of nitric acid into nitrogen oxides by reaction with sulfur dioxide, in a packed column type refluxer. The waste of HNO3-SO2 reaction is sulfuric acid, solution of 6570%, which is evacuated at the bottom of the nitrogen oxides refluxer. The magnitude of the nitrogen losses, as NOx and/or HNO3 in the waste sulfuric acid, were determined by analysing the effluent sulfuric acid by spectrofotometric method with Nessler reagent.

At the bottom of the packed separation column, nitric acid is total converted into nitrogen oxides by reaction with sulfur dioxide: (2) 2HNO 3 + SO 2 = 2NO 2 + H 2 SO 4 The nitrogen (enriched in 15 N) losses in the waste sulfuric acid should be as much as few ppm of feeding nitric acid in order to have an efficient operation of 15 N separation plant. The magnitude of nitrogen losses indicates the operating mode of the nitrogen oxides refluxer.

Experimental part
For determination of nitrogen from waste sulfuric acid solution the calibration line was obtained by using sulfuric acid solution with known nitrogen concentration. That sample was placed in a modified Kjendahl apparatus, potasium hydroxide was added to neutralize the acid, then Dewarda's alloy, and the apparatus was heated in H 2 current for 15 min. The gaseous mixture containing ammonia, was bubled in a flask with a known volume of 0.1 N H 2 SO 4 . * email: cristina.marcu@itim-cj.ro, Phone: (+40) 264584037 To that solution was added 1 mL of alkaline solution (10 g NaOH, 10 g Na 2 CO 3 in 100 mL double distilled water) and 2 mL of Nessler reagent. The yellow colored compound immediately appeared and the absorbance of that sample was measured at 425 nm with UV-1800 Shimadzu spectrophotometer after 15 minutes. After plotting of the calibration line, the magnitude of nitrogen losses in the waste sulfuric acid have been determined.
The method requires a rigid control of the reaction conditions to prevent the turbidity of the solution [10].
The absorbance of the waste sulfuric acid samples was determined and the corresponding nitrogen concentrations were read from the calibration line.
Knowing that the 15 N separation plant is fed with 60 moles of HNO 3 /h, the nitrogen losses, calculated in ppm of the nitric acid flow, are presented (Table 1).
All waste sulfuric acid samples presented in Table 1 contain nitrogen (enriched in 15 N) showing an improper operation mode of the refluxer.

Conclusions
Any loss of nitrogen (enriched in 15 N) is an additional, uncontrolled production that diminishes the normal production of 15 N separation plant, indicating an improper operation of the nitrogen oxides refluxer.