Ind. Eng. Chem. Process Des. Dev. 1981, 20, 361-366
38 1
Conversion of Hydroxylamine Hydrochloride to Hydroxylamine Nitrate by Electrodialysis and Water-Splitting Processes Yuehslung Chang' and Harry P. Gregor Depatiment of Chemical Engineering and Applied Chemistty, Columbla Univers&, New York, New York 10027
Electrodialysis and electrodialytic water-splltting constitute an inexpensive process for the conversion of relatlvely inexpensive salts into expensive ones; the example described is the conversion of hydroxylamine hydrochloride into expensive hydroxylamine nitrate (HAN), which is an efficient reductant for plutonium for many processing applications. Laboratory studies showed that HAN at a suitably high concentration and low free acid content was obtained directly. The apparent current efficiency was a function of the current density (due to the diffusional transport), and was primarily controlled by the HAN concentration In the product stream. The concentration of product vs. process time relationship could be predicted by a batch-recirculating plug flow cell model.
Introduction Hydroxylamine nitrate (HAN) is a rapid, efficient reductant in many processing applications, but one having a relatively high cost of production. One large-scale process in which it is used involves the purification of plutonium. A number of plutonium reductants have been considered, including hydrazine-stabilized ferrous nitrate (Horner, 1969), uranium(1V) nitrate (Schlea et al., 1969), platinum-catalyzed hydrogen (Rainey, 1965), hydrazine (Swanson, 1971),ferrous sulfamate (Walser, 1970), and the hydroxylamine salts (Connick, 1954; Orth et al., 1971; Henry, 1972). Of these, only ferrous sulfamate and hydroxylamine sulfate have been used extensively, until recently. The former must be used in large excess, which increases the costs and the waste volume. Also, the ferrous ions introduced and the sulfate ions generated by the sulfamate eventually contribute to plutonium loss, precipitate formation, and stainless steel corrosion. Hydroxylamine sulfate does not introduce metal ions as impurities, but the reduction is slow and incomplete and the added sulfate ions are undesirable. The use of HAN for the reduction of plutonium was proposed in 1959 by Morgan et al. in a process in which plutonium was separated from uranium in a 20% tributyl phosphate system by countercurrent extraction with a 0.1 M HAN solution. Similar investigations were carried out by other investigators (Burns and Swafford, 1969; McKibben and Bercaw, 1971; Patigny et al., 1974; and Richardson and Swanson, 1975). The advantages of this reagent include a high reducing rate, more complete reduction, and a decrease in the loss of plutonium to various waste streams. Other interesting applications of HAN include the preparation of technical grade oximes and pure ammonium nitrate by reaction of appropriate ketones with HAN solutions (Fuchs et al., 1972) and the removal of radioactive iodine from fission products by adding HAN to cause precipitation of insoluble palladium iodide for long-term iodine storage (Mailen and Horner, 1977). Many processes are available for the preparation of hydroxylamine sulfate (Schultze, 1940; Korczynski and Dylewski, 1969; El-Ghatta and Forrer, 1976) and hydroxylamine hydrochloride (Mikula et al., 1966; Korczyski et al., 1966); however, relatively few have been reported for that of HAN. Catalytic reduction processes have been proposed by Aggenbach and De Rovij (1972) and Kartte * h o c 0 Chemical Research, Amoco Research Center, Naperville, Ill. 60540.
et al. (1972). Such processes are generally quite expensive. More recently, Wheelwright (1972, 1977) demonstrated that hydroxylamine sulfate could be converted into HAN by a three-step cation-exchangeprocess. In some cases this process can give rise to a high free acid content in the product; this acts to destabilize HAN and lowers its reducing power (McKibben and Bercaw, 1971; Barney, 1976). It has also been reported that the addition of the stoichiometric amount of barium nitrate to HAS is employed for making HAN. In this communication we report alternative methods for the production of HAN by a simple flexible, and efficient electrodialyticprocess. The process may be superior to some currently employed such as the cation-exchange process of Wheelwright (1972,1977) in some aspects. It can produce HAN of a quite low free acid content, a high HAN concentration, a high chemical yield without recycling, and the capacity of the operation can be varied over wide ranges by changing the current density and flow rate without a deterioration in the product quality. These concepts are generally applicable to any ionic reactions of the type: AX BY = AY + B X AX + BY H20 = AY + HX BOH; BA + H20 = HA + BOH. For example, we have used 5% sodium acetate to produce 4% sodium hydroxide and 35% acetic acid (Gregor and Kassotis, 1980). Process Description This conversion can be carried out through a double decomposition electrodialytic reaction, e.g., starting with hydroxylamine hydrochloride and nitric acid: NH,OH.HCl + HN03 = NH20H.HN03+ HC1, the general concept of which was first proposed by Kollsman (1958). With the membranes of high selectivity now available, one can obtain products of high purity. With bipolar membranes, the concept can be extended to water-splitting triple decomposition reactions; e.g., starting with hydroxylamine hydrochloride and sodium nitrate: NH20H.HC1 NaN03 = NH20H.HN03+ HC1+ NaOH. This system replaces nitric acid by the less expensive nitrate salt. A laboratory multicompartment cell capable of continuous operation for the double decomposition reaction (Figure 1) has a feed solution of hydroxylamine hydrochloride into compartments 1, 5,9, etc. The anode and cathode compartments are A and C, respectively. A solution of nitric acid passes into Compartments 3,7,11, etc. Each repeating unit consists of two anion-exchange membranes, two cation-exchange membranes, feed compartments, and product compartments, respectively.
+
+
Q19643Q5/81/112Q-Q361$O1.25/Q @ 1981 American Chemical Society
+
+
362 Ind. Eng. Chem. Process Des. Dev., Vol. 20, No. 2, 1981 NHPH.HN03
HCI
I
NH,OHHCI
I
HNO,
/-Repeating
Unit
-1
Figure 1. Multi-cell assembly for double decomposition reactions. HCI
NH,OH,HK$
NaOH
.,
b
a
c
-
o
c
b
r -
NH,OHHCI
Figure 3. Schematic of batch recirculating operations: (1)ampere meter; (2) dc power source; (3) voltmeter; (4)heat exchanger; (5) thermometer; (6) water-splitting cell; (7) reservoir; (8) oscillating pump; (9) variac.
NaN03
1-
Repeolmg U n i t
Figure 2. Multi-cell assembly for water-splitting triple decomposition reactions.
For the multicompartment cell for the water-splitting triple decomposition reaction, one adds bipolar membranes, which can be made by several different methods (Frillete, 1956; Oda and Saito, 1959; Michaels, 1965; DeKorosy and Zeigerson, 1971; Mintz, 1974). A simple and effective one (Benjamin, 1975) is to fuse together an anionand a cation-exchange membrane with heat and pressure. With this assembly (Figure 2), a solution of hydroxylamine hydrochloride is fed into compartments 2,7,12, etc., and a solution of sodium nitrate into compartments 4,9, 14, etc. When current is passed, the key product HAN is produced in compartments 3,8,13, etc., while the byproducts, hydrochloric acid and sodium hydroxide, are produced in compartments 1, 6, 11, etc., and 5, 10, 15, etc., respectively. Thus, each repeating unit consists of a bipolar membrane, two anion-exchange membranes, two cation-exchange membranes, two feed compartments, and three product compartments. Experimental Section A laboratory cell assembled from a number of polypropylene rings was used. Each ring has an exposed inner area of 58.0 cm2and a width of 2.5 cm. The electrodes were constructed from 0.004 in. thick titanium coated on one side with 100 pin. of platinum and cemented to the support plates. The cell rings along with their membranes (Figures 1 and 2 show the single repeating unit used) were held together by a press. Ionac membranes MA 3475 and MC 3470 and Asahi membranes AMV and CMV were used as monopolar membranes. The former are typical heterogeneous ionexchange membranes; the latter are of homogeneous character. The studies of Benjamin (1975) showed that Ionac membranes MA 3475 and MC 3470 could be fused
if a solvent for the membrane binder, dimethylformamide (DMF), was applied to the membranes prior to fusing in a heated press. Bipolar membranes used in the present studies were prepared according to these procedures. Immediately prior to pressing, the membranes to be fused were liberally swabbed with DMF. They were then placed together between two pieces of aluminum foil and pressed at 200 O F and 1300 psi for 3 min in a hydraulic press (Pasadena Hydraulics, Inc.). Asahi membranes AMV and CMV could also be fused, but the resultant bipolar membranes were found to be impractical, probably due to problems of water transport (Benjamin, 1975). The cell and auxiliary equipment were assembled for batch recirculating operations as shown in Figure 3. All solutions were prepared with chemicals of reagent grade and deionized water. In operation, after the solutions were charged, pumps and heat exchangers were turned on and 15 min was required to allow for the adjustment of liquid levels and the attainment of uniformity in temperature and solution composition. Then, current was passed through the cell and the temperature was controlled within 25 f 1 "C. The voltage was adjusted as necessary to keep the current constant. In a series of preliminary runs, the effects of product and feed stream concentration and current density upon current efficiency were ascertained. These experiments were run only until a concentration change of approximately 0.05 N was obtained in the product streams. In production runs, the concentration-time relationship was studied by collecting samples (10 d)from each stream at a predetermined time interval. The following methods were used for analysis: hydroxylamine by the method of Rachig (Furman, 1958); nitrate by UV spectrophotometry (APHA, 1975a); sodium by atomic absorption (APHA,1975b); chloride by Volhard (APHA, 1975~); acid-base by titration to the phenolphthalein end point. Theoretical Several processes contribute to the overall current efficiency of the production of HAN, shown in Figure 4. On the assumption that diffusional and electric transport are separable, convective and electroosmotic transport are negligible, and stirring effectivelyprevents solution phase concentration polarization, one can use a macroscopic mass balance (Chang, 1979) to show that for the hydroxylammonium (HA+) ions na 1 \
Ind. Eng. Chem. Process Des. Dev., Vol. 20, No. 2, 1981 363 a
a I
C 1
C
0 100 N
A
Figure 4. Processes which determine the overall current efficiency in the production of HAN: -, desired electric transport; ----, desired diffusional transport; -- - -, undesired electric transport; ----, undesired diffusional transport.
00
100
0
200
I/
I
300
400
M O
, cm'lamp
Figure 6. Overall current efficiency vs. reciprocal of current density. 10
v
050 N N%OHHHCI
* ' C
oa
2 A
z 06 N I
04
02
\
-
0
Asoh,
A
lonoc
00 1 00
05
15
10 CHAN
N
Figure 7. Overall current efficiency vs. concentration of Figure 5. Batch recirculating system with a plug flow electrodialytic cell and a perfect back-mix tank.
where q, 'T, i, D,d, and c denote apparent current efficiency, transport number, current density, diffusion coefficient, membrane thickness, and concentration, respectively. The effective current efficiency is defined in terms of the ultimate concentration of pure product formed; co-ion leak and electroosmotic transport both are contributing. Superscripts c and a denote cation- and anion-exchange membranes; subscripts f and p denote feed and product, respectively, and the 3 is the Faraday. If one neglects transport by diffusion and uses membranes of high fixed-charge concentration, then q
= PI - P$,2/EF2
(2)
where Pl and P2are parameters and EF is the fixed-charge concentration in the membrane. Assuming that idealized plug flow exists in the electrodialytic cell and that the reservoir is a perfect back-mix tank (Figure 5), one can show that concentration changes in the system are governed by (Chang, 1979)
and ac0
V,-
at
= nu(cf - co)
(4)
where A is the cross-sectional area of the cell, u the volumetric flow rate, a membrane area per unit length, V, volume of the reservoir, n the number of repeating units in the electrodialytic stack, and the subscript P for products is neglected. With initial conditions, co(0), ci(0), and the boundary conditions, c ( 0 , t ) = co(t) and c(L,t) = co(t), eq 3 and 4 can be solved by stepwise integration. Alter-
HAN.
natively, one may consider an approximate analysis of eq 3 and 4 and obtain the following analytic solution B+lln B-1
[
(p1/p2)1/2 + CO(T) (p1/P2)'/2 - CO(0) (p1/p2)1/2 - CO(T) (P1/P2Y2+ CdO)
1
Co2(0)5 Co2(T)< P1/P2or Co2(T)L Co2(0)> P1/P2
+ In (Pl/P2) - CO2(O) -
CO2(T)- (P,/P2) 2nT; C,2(T) > PJP2 > C&O) ( 5 ) T = ut/Vr,B = e ~ p ( 2 ( P ~ P ~ ) 'and / ~ l )1,
where Co = CO/EF, = aLi/3eFu. Results and Discussion A critical parameter of these processes is the overall current efficiency. The applicability of eq 1to the results of preliminary runs is given in Figure 6 by a plot of the current efficiency against the reciprocal of the current density for runs carried out with the cell assembly of Figure 1. The product solution was initially at 0.5 N, with feeds of hydroxylamine hydrochloride at 1.0, 0.5, and 0.25 N, respectively. The stqaight lines show that the (tc- t")term is relatively independent of current density. The variation in current efficiency with current density was due primarily to diffusional transport across the membrane. Diffusion coefficients for neutral electrolyte, really for co-ions (in this case HA+ across the anion-exchange'meqbrane) were quite small, about 1X lo-' cm2/s. The intercepG-give the value for (tc- t")which appears to be relatively independent of hydroxylamine hydrochloride concentration and indicate
364
Ind. Eng. Chem. Process Des. Dev., Vol. 20, No. 2, 1981
Table I. Production of HAN in Double Decomposition Electrodialytic Cell. Membranes: MC 3470 and MA 3475; Current Density: 18.3 mA/cm2;Circulation Rate: 6 mL/s NH,OH.HCl
HNO,
NH,OH.HNO,
time, min
N
mL
N
mL
N
mL
0 60 120 240 360 480
0.550 0.480 0.405 0.260 0.130 0.020
465 450 435 415 385 350
0.415 0.435 0.400 0.315 0.250 0.195
855 835 810 185 155 130
0.095 0.165 0.235 0.310 0.485 0.515
465 460 455 455 460 460
HCl I
N
q %
mL
0.095 0.110 0.240 0.380 0.505 0.605
92 91 85 74 58
410 460 455 450 450 445
Table 11. Production of HAN in Water-Splitting Triple Decomposition Electrodialytic Cell. Membranes: MC 3470 and MA 3475 and Bipolar Membrane; Current Density = 18.3 mA/cmz;Circulation Rate: 6 mL/s NH,OH.HCl
NaNO,
NH,OH*HNO,
time, min
N
mL
N
mL
N
mL
0 60 120 240 360 480
0.510 0.440 0.375 0.245 0.125 0.010
410 455 420 395 365 340
0.490 0.455 0.410 0.335 0.260 0.180
840 825 805 185 160 735
0.085 0.150 0.220 0.340 0.445 0.535
410 465 460 410 415 410
HCl q %
85 90
I1 69 59
NaOH
N
mL
N
mL
0.095 0.160 0.235 0.360 0.480 0.590
450 440 430 425 420 410
0.095 0.170 0.245 0.315 0.485 0.595
460 455 450 445 445 440
Table 111. Production of HAN in Double Decomposition Electrodialytic Cell. Membranes: CMV and AMV; Current Density: 40.0 mA/cm2;Circulation Rate: 6 mL/s NH,OH.HCl
HNO,
NH,OH*HNO,
time, min
N
mL
N
mL
N
mL
0 60 120 240 360 480 600
1.150 1.115
2050 2040 2035 2020 2010 2000 1985
0.550 0.515 0.415 0.410 0.335 0.265 0.205
2100 2080 2070 2050 2025 2010 1985
0.110 0.355 0.620 1.085 1.495 1.850 2.260
320 315 310 310 310 305 310
1.070 1.000 0.910 0.855 0.195
that counterion transport is controlled predominantly by the product stream (HAN) concentration. The dependence of current efficiency upon HAN concentration is evident from Figure 7, where two sets of results are presented, comparing the homogeneous membranes (CMV and AMV) and the heterogeneous membranes (MC 3470 and MA 3475). The superiority of the former for the present application is obvious, particularly at higher concentrations. Equation 2 was used to correlate the experimental data of Figure 7. Fixed charge concentrations were estimated from previous data (Kitamoto and Takasima, 1971; Benjamin, 1975) to be 2.31 N and 0.93 N for the homogeneous and heterogeneous membranes, respectively. Best fit parameters for the homogeneous membranes were found to be PI = 0.99, Pz = 0.640; for the heterogeneous films PI = 0.98 and P2 = 0.884. For more concentrated solutions, eq 2 must be replaced by a more complete and complex one (Chang, in preparation). The results of production runs are summarized in Tables I, 11, and 111. The presence of chloride in the HAN solution was not detectable within the limit of the Volhard titration (
