Cation-Exchange Conversion of Hydroxylamine Sulfate to Hydroxylamine Nitrate Earl J. Wheelwright Battelle Memorial Institute, Pacific Northwest Laboratones, Richland, Washington 99352
Hydroxylamine nitrate is a rapid, efficient reductant for reducing plutonium to the +3 state in many processing applications, but it is relatively expensive. Laboratory and pilot plant experiments have shown that inexpensive hydroxylamine sulfate can be converted to the nitrate by a simple three-step cation-exchange process employing only HN03 as a consumable reagent.
Introduction In large-scale processes for plutonium purification, including the partition from uranium by solvent extraction, the separation from neptunium by anion exchange and cationexchange purification, plutonium is reduced to Pu(II1). Ferrous sulfamate has been the principal reductant for many large-scale processes although hydrazine has been used in combination with ferrous sulfamate for some applications and both uranium(1V)-hydrazine and hydroxylamine sulfate have been used to a limited extent. Hydrazine serves only as a holding reductant because the reaction with plutonium is too slow. Ferrous sulfamate reduces plutonium a t a very rapid rate, even in moderately strong nitric acid, but the iron introduced into the process stream complicates further processing and waste disposal. The sulfate generated by the sulfamate increases the corrosion of stainless steel equipment and increases the plutonium process losses because of the formation of sulfate complexes. Hydroxylamine sulfate does not introduce impurity metal ions, but the rate of reduction of plutonium is moderately slow, and the added sulfate contributes to corrosion, excessive plutonium process losses, and precipitate formation. The use of hydroxylamine nitrate for the reduction of plutonium has been investigated a t both the Savannah River (McKibben and Bercaw, 1971) and Hanford (Bruns and Swafford, 1969) separation plants. The reduction has been shown to be rapid and complete in the absence of sulfate. The presence of sulfate was shown to reduce the rate of reduction of plutonium by hydroxylamine or hydrazine, but hydrazine nitrate continued to be a slower reducing agent than hydroxylamine nitrate (McKibben and Bercaw, 1971). The relatively high cost deters the extensive use of hydroxylamine nitrate. This work was undertaken to develop an inexpensive process for converting the much less expensive hydroxylamine sulfate to hydroxylamine nitrate a t the plant site. Process Conception and Development Preliminary attempts to convert hydroxylamine sulfate to the nitrate by flowing a 0.5 M hydroxylamine sulfate solution through a bed of quaternary ammonium type exchange resin in the nitrate form failed to provide satisfactory sulfate removal. Hydroxylamine sulfate can be rapidly converted to hydroxylamine nitrate by a simple three-step cation-exchange process which requires only nitric acid and demineralized water in addition to the resin. ( 1 ) Feed Absorption Step. When an aqueous solution of hydroxylamine sulfate is passed through a bed of strongly 220
Ind. Eng. Chern., Process Des. Dev., Vol. 16, No. 2, 1977
acidic cation-exchange resin, such as Dowex 50W, X8 (50-100 mesh), in the H + form, the exchange reaction is
2H+
+ 2NH,3OH+ + S03*- + 2NH30H+ + 2H+ + SO,+-
The bar overline indicates the resin phase. The thermodynamics and kinetics of this system are favorable, and hydroxylamine does not appear in the resin bed effluent solution until nearly all of the exchange capacity of the resin has been utilized for the absorption of hydroxylamine. (2) Resin Bed Wash Step. The unabsorbed hydroxylamine and nonabsorbable sulfate are removed by washing the bed of loaded resin with demineralized water. (3) Product Elution Step. Elution of the water-washed resin bed with 2 M HNO3 removes the hydroxylamine according to the exchange reaction NH3OH+
+ H + + NO:j-
F?
H+ + NH:jOH+ + NOR-
Only a modest excess of HNO:, is required. Removal of the excess acid in the resin bed by a water flush and resettling of the bed are the only steps required in preparation for a second cycle. Eight experiments were performed to examine the effects of feed concentration, resin particle size, operating temperature, and flow rate upon the efficiency of the conversion process, The resins examined were contained in a 2.54-cm i.d. glass column, water jacketed for temperature control. The column effluent solutions were collected in 100-mL fractions, and a sufficient number were analyzed to provide the data neded for an evaluation of each experiment. The feed-solution breakthrough and elution curves from experiment no. 1, shown in Figure 1, depict the exchange process. In a production process, the loss of hydroxylamine during the feed and wash cycles would be eliminated by alternately using two columns in series, the second column serving as a tailing column. The solution represented by the shaded portions of the elution curve could be recycled to increase the average concentration of hydroxylamine and decrease the excess acid concentration of the product. The total acid, designated HT+, was determined by titration with standard base solution to a phenolphthalein end point and includes both the free acid and the hydroxylammonium ion. The operating conditions and results for the eight experiments are given in Table I. With 50-100 mesh resin there is little difference in product quality when the flow rate was increased from 4 to 8 mL/min-cm*. With the larger 20-50 mesh resin, a t the same operating temperature, the product quality, in terms of concentration and excess acid, is not quite as good as that obtained with the 50-100 mesh resin, and the
Table I. Cation-Exchange Conversion of Hydroxylamine Sulfate to Nitrate Experiment no.
Resin bed: Mesh size, (Dowex 50W, X-8) Bed volume, L Operating temperature, "C Feed solution: Volume, L (NH2OH)2.H2S04 concentration, M Solution flow rate, ml/min-cm' Water wash solution, Volume, L Solution flow rate, ml/min-cm" Eluting solution: Molar concentration of HNO:j,M Flow rate, ml/min-cm' Total resin capacity, mol/L Total eluted product: Volume, L NH?OH.HNO:j concentration, M Excess HNO:j,M SO4"- concentration, M Eluted product minus recycle Volume, L NH?OH.HNO:, concentration, M Excess "03, M NH.OH.HNO:j recycled, %
1
2
3
4
50-100 0.87 10
50-100 0.87 10
20-50 0.81 10
20-50
0.81 10
6
7
8
20-50 0.81
20-50 0.81
20-50
10
10
20-50 0.81 60
5
2.0 1.0 7.8
1.0
2.0 1.0
3.8
2.0 1.0 5.7
9.7
8.0
1.0 8.0
1.00 8.0
2.00 3.8
2.00 5.7
2.00 7.8
2.00 9.7
2.00 8.0
2.00 8.0
2.00 8.0 1.89
2.03 3.8 1.82
1.98 5.7 1.84
2.03
2.05 9.7 1.84
2.01 8.0 1.82
a
7.8 1.88
1.60 0.92 0.97
