WARREN
H. ADAMS, ERIC B. FOWLER, and C. W. CHRISTENSON
Los Alamos Scientific Laboratory, Los Alamos, N. Mex.
A Mefhod for.
Treating Radioactive Nitric Acid Wastes Using Paraformaldehyde This method of nitric acid destruction will be of special interest to those workers concerned with disposal of radioactive wastes first step in storage or treatment of high level liquid radioactive wastes is usually neutralization or destruction of the nitric acid present. Destruction by electrolytic methods is expensive. Neutralization results in an increase in volume and a high solids content which often interferes with subsequent treatment, such as fixation in a ceramic body or concrete block. Solids resulting from neutralization also limit the extent to which the solution can be concentrated. The use of formaldehyde (35% aqueous solution) for the destruction of proposed nitric acid in radioactive wastes has been (3-5) and is being developed at Hanford Atomic Products Operation (7, 2 ) . An aqueous solution of formaldehyde produces a relatively large increase in volume of liquid waste, depending upon the original acid concentration. However, one important advantage to the use of formaldehyde is that solids content of the waste solution does not increase. If paraformaldehyde (a linear polymer with a formula H .(HCHO),OH, where x is approximately 10 to 100) is added directly to the waste solution, the nitric acid is destroyed, and the volume of waste is reduced substantially. Gases evolved can be exhausted through suitable filters to the atmosphere or alternatively the nitrogen dioxide may be converted to nitric acid and re-used. This work was undertaken to show that the use of paraformaldehyde for the destruction of nitric acid is feasible. Paraformaldehyde was found to offer several advantages over formaldehyde. Waste volume can be reduced substantially and most of the nitric acid easily removed. Paraformaldehyde is essentially 1OOyoactive reagent, but in formaldehyde u p to 65y0 of the reagent is water, which adds to the final waste volume. Volume reduction when formaldehyde is used can be accomplished
T H E
I.
.
by distillation, but this may require a more extensive equipment installation. The reaction can be controlled easily by the rate of addition of paraformaldehyde. Residual formic and nitric acids can be neutralized with a minimum of caustic, if necessary, before storage or subsequent treatment by fixation in clay or concrete. Adequate equipment is available for the addition of solids to reaction vessels accurately without difficulty.
Experimental A waste solution, 8N in nitric acid with 400 p.p.m. of iron, was used, as it is expected to be produced a t Los Alamos soon. Small amounts of paraformaldehyde were added directly to the nitric acid solution in a beaker. After a short induction period, the reaction proceeded with evolution of large volumes of carbon dioxide and nitrogen oxides. With continued additions of paraformaldehyde, solution temperature increased to 87' to 90' C. The molecular ratio of nitric acid to paraformaldehyde was 96
es I
60
120
100
e40
300
J
360
INDUOTION TIME 18ECl
Figure 1. Increasing the temperature from 30' to 90" C. produces a 30-fold decrease in induction time Each point represents 50 ml. of 8N nitric acid and 0.75 grams of paraformaldehyde
about 1.7 to 1, and final volume was about 60% of the original. Volume reduction is due to destruction of nitric acid as well as to evaporative loss of water. Because of water loss the reaction proceeds somewhat more favorably; the resulting higher acid concentrations react more rapidly with paraformaldehyde, and slightly less paraformaldehyde is consumed per mole of nitric acid destroyed. When starting with 8N nitric acid, the final solution is about 2N in total acid, of which about 0.6N is nitric. If the solution is refluxed for 2 or 3 hours, total acidity is reduced to about 0.8N, of which 0.1N to 0.2N is nitric; the balance is formic acid. These acid concentrations are "as analyzed" in the reduced volume (GOT0 of the original volume) and are not corrected to the original volume. Total acid was determined by titration with standard sodium hydroxide solution; nitric acid was determined bv the nitron method. Results
Induction Time. If paraformaldehyde is added to a solution of 8N nitric acid at room temperature, there is an appreciable induction period before the reaction begins, which may be 30 to 40 minutes for an 8 N solution made of asreceived reagent grade nitric acid. However, heating decreases the induction period; a t 40' C. it is only 2 minutes (Figure 1). Further, reaction rate decreases as the nitric acid concentration decreases. Healy ( 3 ) found that the reaction may be initiated by addition of small amounts of nitrogen peroxide or sodium nitrite and that reaction rate depends, to an extent, upon the nitrous acid content of the nitric acid. Thus the induction period is dependent upon nitric acid concentration, the VOL,,5?. NO. 1
JANUARY 1960
55
presence or absence of nitrous acid, and solution temperature. As no attempt was made to remove nitrous acid, which is probably in the nitric acid, slightly different curves could be expected for different batches of nitric acid. Paraformaldehyde Requirements. Although this work was done primarily to develop a method for disposing of 8N nitric acid wastes, the method can be applied to other nitric acid concentrations. A study was made of the change in acid concentration as a function of the amount of paraformaldehyde added to solutions originally ICN, 8Ar, and 4N in nitric acid (Figure 2). Here, 500 ml. of nitric acid were added to a 1000-ml. graduated cylinder immersed in a water bath at 60’ to 70’ C. This temperature was chosen to shorten induction time and facilitate volume measurements during the reaction. A reflux condenser was attached to the cylinder to prevent loss of water during the experiment. Paraformaldehyde was added a t a rate at which the reaction could be controlled easily. T h e solution was sampled after each 10-gram addition of paraformaldehyde when the reaction had apparently neared completion for the particular addition. For the more concentrated (16N, 8 N ) solutions, the paraformaldehyde was added in approximately 0.75 gram amounts until 10 grams had been added ; the reaction was permitted to proceed to apparent completion, and the solution was sampled. When acid concentration had been reduced to about 4N, paraformaldehyde was added in two equal amounts of 5 grams each. For comparison, acid concentrations were corrected to the original solution volume in this instance
T O T A L ACID
A
N I T R I C ACID
and the results were plotted us. grams of paraformaldehyde added. The curves show that it may be advantageous to concentrate dilute acid wastes to decrease the consumption of paraformaldehyde. For example, 100 grams of paraformaldehyde will destroy 480 grams of 1 6 N nitric acid but only about 320 grams of 8 N acid. If the reaction takes place in an open vessel, the results are somewhat more favorable, as water loss has a slight concentrating effect. For example, if 500 ml. of 16N nitric acid are treated with a total of 100 grams ofparaformaldehyde, a final solution volume of 200 ml., containing 0.3Ar nitric acid and about 2.7N formic acid, is obtained. When water was not allowed to escape, a final volume of 300 ml. containing 1.2N nitric acid and 0.4N formic acid was obtained.
Healy (3) reports the reaction between formaldehyde and nitric acid to be as follows : 4HN03 4HN03
3HCHO + 4N0 5Hn0
+ + HCHO
-+
+ 3C02
4x02
+ Con
+ 3Hz0
(I)
(2)
The first reaction occurs when nitric acid concentration is 2N to 6.V, and the second when the acid is above 12-V. Between 6JV and 1I-Y, a combination of the two reactions occurs. The same reactions are assumed to occur when paraformaldehyde is used (Figure 2). The molecular ratio of nitric acid to paraformaldehyde was approximately 2.3 to 1, 1.4 to 1, and 1.05 to 1 for solutions originally 16N, 8.11, and 4N in nitric acid, respectively. These are for experiments in which the reaction vessel was equipped with a reflux condenser. At high acidities, the reaction between nitric acid and paraformaldehyde is vigorous but becomes slower as acidity is decreased. At 2,V or less, formaldeh) de is partially oxidized to formic acid : + HCOOH
+
2NOn $- Ha0 (3) Formic acid then reacts with dilute nitric acid 2“03
f 3HCOOH + 2 N 0 3C02
e0
40
e0
80
100
QRAMS OF PARAFORMALDEHYDE ADDED
Figure 2. Efficiency of the destruction of nitric acid with paraformaldehyde decreases as nitric acid normality decreases
56
y
1
60
30 RATE OF ADOiTlDN
20
>
’
0
Reactions and Equations
H C H O 4- 2”03
0
80
+
+ 4Hn0
(4)
but at a slower rate, so that there is a build-up of formic acid in the final product. In these experiments, rate of temperature increase as paraformaldehyde is added is dependent upon the rate of addition of paraformaldehyde (Figure 3). Reaction rate can be controlled easily by addition rate of paraformaldehyde. However, the reaction between 8N nitric acid and 40 grams of paraformaldehyde added at once was not violent.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Discussion
The formic acid concentrations reported here are somewhat higher than those reported by Healy (5). However, if paraformaldehyde were used under the same experimental conditions as in formaldehyde experiments, comparable formic acid concentrations would be obtained in the final product. At this time, the authors have no knowledge of the radiation stability of formic acid. The decomposition products, however, would be carbon dioxide and water and both of these products should diffuse through the pores of concrete or ceramic bodies. Other gaseous products are formed in wastes containing large quantities of radioactive elements in the absence of paraformaldehyde. The problems associated with the formation of gaseous decomposition products in wastes fixed in concrete cannot be resolved without more experimental work. From an economic standpoint, the paraformaldehyde costs about 1 cent per pound more in “active reagent” than does formaldehyde. literature Cited
(1) Barton, G. B., “Removal of Xtric Acid from Purex Plant First Cycle Waste (IWW) b y the Reaction with Formaldehyde,” HW-55941 (May 2, 1958) (confidential). (2) Evans, T. F., “Pilot Plant Denitration of Purex Wastes with Formaldehyde,” HW-58587 (Feb. 23, 1959) (unclassified). (3) Glueckauf, E., Healy, T. V., Atomics 6, 370 (1955). (4) Goldschmidt, B., Regnault, P., Prevot, I., Conf. Peaceful Uses Atomic Energy, Geneva, 8/P/849 (1955). (5) Healy, T. V., J . Appl. Chem. (London) 8,553-61 (1955).
RECEIVED for review June 5, 1959 ACCEPTED October 19, 1959
