Process for preparing nitrites

Nitrates, such as alkali metal nitrates and ammonium nitrate, are electrolytically reduced to the corresponding nitrites by a process wherein an aqueous solution containing a nitrate is supplied into a cathode chamber of an electrolytic cell including cathode and anode chambers separated by a cation exchange membrane and an electric current is applied to the electrolytic cell, while maintaining the pH of the aqueous solution at a value of at least about 4. This process can be advantageously applied not only to the manufacture of nitrites but, also, to the treatment of waste nitrates. In the treatment of waste ammonium nitrate, the ammonium nitrite so formed is conveniently further treated by subjecting the electrolytically reduced catholyte to thermal decomposition outside the electrolytic cell, and removing the so formed nitrogen and water from the reaction system.

BACKGROUND OF THE INVENTION 
(1) Field of the Invention 
The present invention relates to a process for the preparation of nitrites. 
More particularly, the invention relates to a process for preparing an 
alkali metal nitrite or ammonium nitrite by electrolytic reduction of an 
aqueous solution containing a nitrate of an alkali metal, such as sodium 
nitrate, potassium nitrate or ammonium nitrate. 
(2) Description of the Prior Art 
Conventional processes for preparing nitrites will now be described by 
reference to the production of sodium nitrite as a typical instance of the 
nitrite. According to one known process, sodium nitrate is reduced by 
lead, whereby sodium nitrite is formed by the reaction represented by the 
formula: 
EQU NaNO.sub.3 +Pb=NaNO.sub.2 +PbO. 
According to another known process, gaseous nitrogen oxide is absorbed in a 
solution of sodium hydroxide or sodium carbonate, whereby sodium nitrite 
is formed by the reaction represented by the formula: 
EQU Na.sub.2 CO.sub.3 +2NO+1/2O.sub.2 =2NaNO.sub.2 +CO.sub.2. 
In the process using lead as the reducing agent, in order to remove lead 
incorporated in sodium nitrite, crystallization should be repeated several 
times. The process using gaseous nitrogen oxide is defective in that, if 
the absorbing liquid is acidic, nitrous acid and nitric acid are formed as 
by-products. 
The inventors conducted research with a view to developing a process in 
which nitrites can advantageously be prepared industrially and found that, 
when an aqueous solution containing a nitrate is subjected to electrolytic 
reduction under specific conditions, the corresponding nitrite can 
advantageously be prepared industrially. 
It is known that electrolytic reduction of nitrate ions is difficult to 
effect under ordinary electrolysis conditions. For example, in "LES 
REACTIONS ELECTROCHIMIQUES METHODS ELECTROCHIMIQUES D'ANALYSE", G. Charlot 
states that on a mercury electrode, direct reduction of nitrate and 
nitrite ions is very slow and is caused only at an electric potential 
which is approximately that at which supporting electrolytes are reduced, 
that is, in a solution containing Na.sup.+ or K.sup.+ ions, nitrate and 
nitrite ions are not electrically active. In short, G. Charlot feels that 
electrolytic reduction of nitrates is difficult. 
Electrolytic reduction of an aqueous solution containing ammonium nitrate 
has been attempted on a laboratory scale. For example, in Research Group 
Report AERE-R4393(1963), of the U.K. Atomic Energy Authority, it is taught 
that ammonium nitrate can be decomposed to nitrogen and water according to 
reactions represented by the formulae (1) and (2): 
EQU NH.sub.4 NO.sub.3 +2e+2H.sup.+ .fwdarw.NH.sub.4 NO.sub.2 +H.sub.2 O (1) 
EQU NH.sub.4 NO.sub.2 .fwdarw.N.sub.2 +2H.sub.2 O (2) 
In this reference, results of experiments conducted for finding conditions 
enabling decomposition of ammonium nitrate to nitrogen and water at 
temperatures approximating the boiling point are disclosed. In this 
reference, it is concluded that in the electrolytic reduction of the 
formula (1), the pH of the catholyte is gradually elevated with the 
advance of the electrolysis and the current efficiency is reduced 
substantially to zero if the catholyte is neutral or alkaline, and 
therefore, it is necessary to maintain the pH of the catholyte not higher 
than 1 during the electrolysis. 
Japanese Patent Laid-Open Application No. 56375/75 discloses a process for 
the electrolytic reduction of an aqueous solution containing ammonium 
nitrate. This process is characterized by no use of a permeable membrane, 
and as clearly described in the specification of this application (page 4, 
last line to page 5, line 4), the production of nitrites is not intended 
and conditions for preparing nitrites are not disclosed at all. 
SUMMARY OF THE INVENTION 
It is a primary object of the present invention to provide a process in 
which an alkali metal nitrite or ammonium nitrite can be selectively and 
stably formed while maintaining a high current efficiency and reducing the 
formation of by-products, such as NO.sub.x, H.sub.2, NH.sub.3 and N.sub.2, 
on the cathode to as low a level as possible. 
Another object of the present invention is to provide a process in which 
waste nitrates formed in the process using nitric acid as a solvent, such 
as the surface treatment in the metal industry or the nuclear fuel 
treatment, can be treated at a high efficiency. 
More specifically, in accordance with the present invention, there is 
provided a process for the preparation of nitrites, comprising supplying 
an aqueous solution containing a nitrate into a cathode chamber of an 
electrolytic cell including cathode and anode chamber separated by an ion 
exchange membrane and applying an electric current to the electrolytic 
cell while maintaining the pH of the aqueous solution above 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The pH value of an aqueous solution of a nitrate, that is used as the 
starting material in the present invention, is in the range of from about 
4 to about 5, unless the concentration is extraordinarily high or low. 
When this aqueous solution is introduced into a cathode chamber of an 
electrolytic cell and a mineral aid is introduced into an anode chamber of 
the electrolytic cell, the mineral acid is diffused through a cation 
exchange membrane. Accordingly, the pH of the aqueous solution of the 
nitrate is reduced. If an acid is contained in this aqueous solution, the 
pH of the aqueous solution should naturally be reduced below about 4. 
However, when application of an electric current to the electrolytic cell 
is commenced, the pH value of the aqueous solution is elevated by 
formation of an alkali metal hydroxide or dissociation of ammonium 
hydroxide, as represented by the formulae: 
EQU NaNO.sub.3 +9H.sup.+ +8e.fwdarw.NH.sub.4.sup.+ +NaOH+2H.sub.2 O (3) 
EQU NaNO.sub.2 +7H.sup.+ +6e.fwdarw.NH.sub.4.sup.+ +NaOH+H.sub.2 O (4) 
The reactions of the formulae (3) and (4) are very vigorous when the pH 
value is below about 4. In each of the reactions of the formulae (3) and 
(4), in order to perform the electrolytic reduction while maintaining the 
pH value below a certain level, it is necessary to conduct the 
electrolytic reduction while adding an acid such as nitric acid. For 
example, in experiments described in the above-mentioned Report 
AERE-R4393, ammonium nitrate is electrolytically reduced to ammonium 
nitrite, and it is simultaneously converted to nitrogen and water by 
thermal decomposition, and it is taught in this report that it is 
necessary to maintain highly acidic conditions exceeding at least about 
0.5 M concentration. It also is taught that under neutral or alkaline 
conditions, the current efficiency is reduced substantially to zero and 
hydrogen alone is formed. However, addition of an acid, for example, 
nitric acid, results in an increase in the amount of the substance to be 
electrolytically reduced, contrary to the objects of the present 
invention, and this method is not advantageous from the economical 
viewpoint. Accordingly, a process not comprising addition of an acid is 
preferred from the economical viewpoint. One of the characteristic 
features of the present invention is that a nitrate is electrolytically 
reduced, while maintaining the pH value at a level of at least about 4, 
preferably at least about 7, without addition of an acid or by adding a 
minute amount of an acid. 
In some cases, the pH of the catholyte is limited depending upon the 
composition of the cathode material used. For example, if lead is used as 
the cathode material, it is preferred from the practical viewpoint that 
the pH value of the catholyte be about 7 or more. Moreover, when ammonium 
nitrate is electrolytically reduced, it is preferred that the pH value of 
the aqueous solution of ammonium nitrate be adjusted within the range of 
from about 7 to about 10. 
It is preferred that the electrolytic reduction be carried out at a 
temperature of about 5.degree. to about 95.degree. C., particularly about 
15.degree. to about 70.degree. C. With an increase of the temperature, 
side reactions for forming by-products, such as ammonia and nitrogen 
monoxide, become vigorous, and the current efficiency for formation of the 
intended nitrite is accordingly reduced. 
The concentration of the aqueous solution of the nitrate that is used in 
the present invention is not particularly critical, as long as the aqueous 
solution retains a stable state. Generally, the concentration is selected 
in the range of from about 1% to about 50% by weight, while taking into 
account the viscosity and conductivity of the solution, and the 
electrolysis efficiency. Aqueous solutions containing compounds such as 
heavy metal salts and ions in addition to the nitrate can be conveniently 
reduced as long as these impurities have no bad influences on the 
electrolysis. 
In the process of the present invention, it is observed that oxygen gas is 
generated in the anode chamber in an electrochemically stoichiometric 
quantity according to the current applied. In order to maintain the 
conductivity within an appropriate range and facilitate the supply of 
protons to the cathode chamber, it is preferred to use an aqueous solution 
of a mineral acid, such as nitric acid, sulfuric acid, hydrochloric acid 
or other hydrohalogenic acid, as the anolyte. If the composition of the 
catholyte and formation of a gaseous by-product on the anode are taken 
into account, nitric acid is especially preferred as the mineral acid. The 
molar concentration of the mineral acid as the anolyte is ordinarily 
adjusted in the range of from about 0.1 to about 2 M. The appropriate 
concentration is determined after due consideration of influences of the 
liquid resistance on the electrolytic cell voltage, the specific 
conductivity, the corrosion resistance of the electrode and the durability 
of the ion exchange membrane. 
An ion exchange membrane having a sufficient corrosion resistance against 
the mineral acid and formed oxygen is used as the ion exchange membrane 
separating the electrolytic cell into the cathode and anode chambers. For 
example, there can be used cation exchange membranes composed of 
styrene/divinylbenzene copolymers having sulfonic acid or carboxylic acid 
groups introduced therein as exchanging groups, and ion exchange membranes 
composed of sulfonation products of chemically stable polymers, such as 
divinylbenzene/acrylic acid copolymers and homopolymers, and copolymers of 
ethylene. Generally, it is preferred that these membranes be used in the 
state reinforced by synthetic fibers or glass fibers. 
An ion exchange membrane composed of a fluorine-containing polymer is 
especially preferred because it can be used stably for a very long time. 
For example, there are preferably employed fluorine-containing copolymers 
containing pendant type sulfonic acid groups or derivatives thereof and 
having recurring units represented by the formulae: 
##STR1## 
and 
EQU --CXX'--CF.sub.2 -- (II) 
wherein R stands for a group represented by the formula 
##STR2## 
in which R' stands for a fluorine atom or a perfluoroalkyl group having 1 
to 10 carbon atoms, Y stands for a fluorine atom or a trifluoromethyl 
group and m is 1, 2 or 3, n is 0 or 1, X stands for a fluorine, chlorine 
or hydrogen atom or a trifluoromethyl group, and X' is the same as X or a 
group 
EQU CF.sub.3 --CF.sub.2).sub.z 
in which Z is 0 or an integer of from 1 to 5. In the fluorine-containing 
copolymers of this type, it is preferred that the recurring units of the 
formula (I) be present in an amount of 3 to 20 mol %. The process for 
preparing membranes of these copolymers is described in detail in the 
specification of U.S. Pat. No. 3,282,875, and these membranes are marketed 
under the tradename "Nafion membrane" by Du Pont, U.S.A. Furthermore, 
there may be used various fluorinated copolymers having weakly acidic 
functional groups as ion exchanging groups. For example, there can be 
mentioned ion exchange membranes composed of fluorinated copolymers having 
at least one functional group selected from carboxylic acid, phosphonic 
acid and phosphoric acid groups, and derivatives thereof, which is stably 
bonded to the main chain or side chain, such as copolymers having ion 
exchanging groups of the OCF.sub.2 COOM type, which are disclosed in U.S. 
Pat. No. 4,151,053, and copolymers having ion exchanging groups of the 
EQU --O--CF.sub.2).sub.n COOM 
type, which are disclosed in Japanese Patent Laid-Open Application No. 
48598/77. It is preferred that these ion exchanging groups be present in 
laminar form on the surface of the membrane, though applicable membranes 
are not limited to such membrane. Still further, there may be used a 
membrane of the amide type formed by reacting a diamine or polyamine with 
a fluorinated copolymer, which is disclosed in U.S. Pat. Nos. 3,969,285; 
4,026,783; and 4,030,988, a membrane of a fluorinated copolymer of the 
sulfonamide type, which is disclosed in U.S. Pat. No. 3,784,399 and a 
membrane composed of a fluorinated copolymer having N-mono-substituted 
sulfonamide groups, which is disclosed in British Pat. No. 1,484,611. When 
ion exchange membranes having amide groups or weakly acidic groups are 
employed, it is necessary to adjust the acid concentrations in both the 
catholyte and anolyte so as to set up such conditions as will not cause 
degradation of the membranes. 
These cation exchange membranes exert functions of separating the catholyte 
and anolyte by defining the cathode and anode chambers, supplying protons 
to the cathode chamber while preventing re-oxidation in the anode, and 
separating gases generated in both the chambers. If a partition plate 
composed of a diaphragm or porous plate, having no ion exchange capacity, 
is employed, the foregoing functions cannot be sufficiently exerted and no 
economical advantage is attained by electrolytic reduction. When an anion 
exchange membrane is used, since nitrate and hydroxide ions are permeated 
to the anode chamber, reduction to the nitrite cannot be accomplished at a 
high efficiency. 
In the present invention, a known corrosion-resistant anode material can be 
used. For example, there may be used anodes composed of platinum group 
metals, anodes formed by coating one platinum group metal or an alloy of 
at least two platinum group metals on the surface of a corrosion-resistant 
metal, such as titanium, tantalum, zirconium or niobium, and anodes formed 
by coating a mixture or mixed crystal (solid solution) of a platinum group 
metal and a corrosion-resistant metal on the surface of the 
above-mentioned corrosion-resistant metal. An anode containing iridium as 
the platinum group metal at a ratio higher than the ratios of other metals 
is especially preferred. 
In the present invention, in order to obtain the nitrite at a high current 
efficiency, it is important to select a cathode in which generation of 
hydrogen is reduced. Noble metals, such as platinum, are excellent in the 
corrosion resistance, but since the overpotential of hydrogen is low and 
generation of hydrogen is vigorous, the current efficiency for formation 
of the nitrite is very low. When iron, stainless steel, titanium and 
carbon are used, generation of hydrogen is observed, but the nitrite can 
be formed at a higher efficiency than in the case of platinum. 
A cathode composed of mercury, indium, cadmium, zinc, lead or tin, or an 
alloy of at least two of these metals, or an alloy of such metal with 
another metal, is preferred because the nitrite can be obtained at an 
enhanced current efficiency. From current-potential curves, it was 
confirmed that in such cathode material, there is an especially great 
difference between the potential for generation of hydrogen and the 
potential for reducing nitrate ions to nitrite ions. The measurement 
results of these current-potential curves are completely in agreement with 
results of the actual electrolytic reduction. The measurement of 
current-potential curves was carried out by using a measurement device 
illustrated in FIG. 1. More specifically, an anolyte 12 and a catholyte 13 
are introduced into two chambers of an electrolytic cell 9 partitioned by 
a cation exchange membrane 8, and; an anode 7 and a cathode 5 are inserted 
into these chambers, respectively, and they are connected to a direct 
current source 1 through an ammeter 2. The cathode potential and anode 
potential are measured by using reference electrodes 10 and 16 inserted in 
saturated potassium chloride solutions 11 and 17, respectively, which 
solutions 11 and 17 are connected to the respective chambers through salt 
bridges 6 and 15, respectively, and by using potentiometers 3 and 14 
located between every adjacent electrode and reference electrode. The 
potential difference between the anode 7 and cathode 5 is measured by a 
potentiometer 4 located between the anode 7 and cathode 5. For example, a 
calomel electrode for a pH meter is used as the reference electrode. The 
potential for reduction to the nitrite ion in the corresponding 
composition of the catholyte is measured under conditions to be actually 
adopted for the electrolytic reduction, while changing the electric 
current. Conditions very close to the actual electrolytic conditions can 
be set up and behaviors can be examined while changing optionally the 
compositions of the anode, anolyte, cathode and catholyte. It is difficult 
to measure the hydrogen-generating potential of the cathode to be 
examined, because reduction to nitrite ions preferentially occurs in the 
aqueous solution of the nitrate. Accordingly, an aqueous solution of a 
hydrochloride containing the same alkali metal or ammonium is used as the 
reference solution and the hydrogen-generating potential is measured by 
using the same cathode. Common electrochemical conditions are employed as 
much as possible in the respective experiments. It was found that, as the 
potential difference observed at the same current in the so obtained two 
current-potential curves is large, generation of hydrogen is reduced and 
the current efficiency for generation of the nitrite is high. This 
potential difference varies to some extent depending on the composition 
and pH of the anolyte, that is, the composition of the aqueous solution of 
the nitrate to be electrolytically reduced, and therefore, the value of 
this potential difference cannot be specifically described. However, when 
the measurement is carried out by using a catholyte containing a sodium 
nitrate solution at a molar concentration of 2.7, and having a pH value of 
9, and an anolyte containing a 0.5 M nitric acid solution, the potential 
difference .DELTA.V at a current of 1A (current density=7 A/dm.sup.2) is 
0.68 V in the case of Pb, 0.30 V in the case of Sn, 0.44 V in the case of 
Cd, 0.68 V in the case of zinc, 0.73 V in the case of In and 0.32 V in the 
case of Pb(90)-CU(10) alloy and 0.32 V in the case of Cu. When the 
measurement is carried out under the same conditions for a Pt cathode, the 
potential difference is -0.20 V. In order to perform the electrolytic 
reduction without generation of hydrogen, it is generally necessary to use 
a cathode characterized by a potential difference .DELTA.V of at least 
about 0 V, preferably at least about 0.1 V. The potential difference of 
mercury had to be measured by using a special electrolytic cell. It was 
found that mercury provides the largest .DELTA.V value. Each of the 
cathodes of mercury, cadmium, zinc, indium, lead and tin provides a 
.DELTA.V value of at least 0.1 V. An appropriate cathode material is 
selected from these metals according to the kind of nitrate and the pH 
value of the nitrate solution. For example, since cadmium and zinc are 
exceptionally easily dissolved in an aqueous solution of ammonium nitrate, 
they cannot be used in the elementary form. Furthermore, each metal has a 
preferred stable pH range. For example, a pH value of about 7 to about 13 
is preferred for lead and a pH value of about 2 to about 13 is preferred 
for tin. Accordingly, it is important to select an appropriate cathode 
material depending on the pH value of the catholyte to be electrolyzed. In 
order to effectively use cathodes of these metals as industrial 
electrodes, and improve the mechanical strength and corrosion resistance, 
minute amounts of other metals may be incorporated in these cathode 
materials. For example, minute amounts of antimony, silver, copper and 
selenium may be incorporated. The amount and kind of such additive metal 
should be selected so that a decrease in the difference between the 
potential for the reduction reaction and the potential for generation of 
hydrogen can be substantially avoided, and the .DELTA.V value can be 
maintained at a level of at least about 0.1 V. 
The shapes of the anode and cathode are optionally determined according to 
the shape or structure of the electrolytic cell. For example, perforated 
plates, porous plates, net plates, plain plates or cylinders may be used. 
If an electrode of a perforated or expanded mesh plate is employed, the 
amount used of an expensive valve metal as a core metal can be reduced, 
and further, there can be attained an advantage that both the back and 
side faces of the plate can be effectively used as the electrode surface. 
When mercury is used as the cathode, a method can be effectively adopted 
in which an anode and an anode chamber are arranged in parallel to the 
plane formed by mercury. 
A multi-electrode type electrolytic cell having a structure capable of 
clamping and supporting an ion exchange membrane therein is preferably 
used as the elctrolytic cell in the present invention. As the material of 
the electrolytic cell, there can be mentioned corrosion-resistant metals, 
polyvinyl chloride resins, polypropylene resins, polyethylene resins, 
fluorine-containing polymer resins and other corrosion resistant plastics. 
An electrolytic cell disclosed in U.S. Pat. No. 4,111,779, which is formed 
of a material prepared by explosive clad of titanium alloys and stainless 
steel, having a corrosion resistance against the catholyte and anolyte, 
and hot-rolling the bonded metals, is especially preferred. In this 
multi-electrode type electrolytic cell, gases generated escape to the back 
of the electrodes and shielding of the current by the generated gases can 
be prevented. Furthermore, since the ohmic drop can be reduced by 
minimizing the distance between electrodes, the low electrolysis voltage 
can be obtained and the unit of the power consumption can be decreased. 
The electrolytic process of the present invention will now be described in 
detail with reference to FIG. 2. 
An aqueous solution containing a nitrate, in which the pH value and nitrate 
concentration have been adjusted in advance, is introduced into an 
electrolytic reduction system illustrated in FIG. 2 from a nitrate 
solution supply opening 21 through a catholyte tank 30. A catholyte in the 
catholyte tank 30 is fed to a cathode chamber 28 of an electrolytic cell 
26 at a predetermined feed rate by a pump. The catholyte containing the 
formed nitrite and unreduced nitrate is returned to the catholyte tank 30 
from the cathode chamber 28. At this point, minute amounts of gases formed 
by side reactions, such as N.sub.2, H.sub.2 and NO, are discharged into 
the open air from a cathode gas outlet 23 through a gas-liquid separator, 
mist separator or trap (not shown). By repeating the above mentioned 
procedures, the concentration of the nitrite in the catholyte can be 
gradually increased. When the nitrite concentration reaches a 
predetermined level, the catholyte is taken out from the reduction system 
through a discharge opening 32, and fed to the subsequent steps where such 
operations as concentration, separation and decomposition are conducted. 
Generally, the nitrite concentration in the catholyte is continuously 
measured and recorded by a photospectrometer (not shown). Furthermore, the 
pH value of the catholyte is continuously measured by a pH meter (not 
shown). When maintenance of the predetermined electrolysis conditions 
becomes impossible due to the influences of the formed ammonium hydroxide, 
nitric acid is supplied from a nitric acid supply inlet 22 to adjust the 
pH value. 
The electrolytic cell 26 is separated into the cathode chamber 28 and an 
anode chamber 29 by a cation exchange membrane 27. An aqueous solution of 
a mineral acid is used as the anolyte and is recycled between an anolyte 
tank 31 and the anode chamber 29 by means of a pump (not shown). Oxygen 
gas formed in the anode chamber 29 is discharged from the reduction system 
through an anode gas outlet 24. A part of the water in the anolyte is 
permeated to the cathode chamber 28 through the cation exchange membrane 
27 with migration of protons from the anode chamber 29 to the cathode 
chamber 28. A water supply inlet 25 is provided to supply water to 
compensate for the so permeated water. 
It is preferred that the electrolysis be carried out while pH, temperature, 
flow rate and current density conditions are maintained at optimum levels. 
There can be adopted a method in which the electrolysis is conducted by 
recycling the catholyte until the concentration of the nitrite reaches a 
predetermined level and a method in which the electrolysis is carried out 
by continuously withdrawing the catholyte containing the nitrite while 
continuously feeding an aqueous solution of the nitrate at a predetermined 
rate. 
As will be apparent from the foregoing illustration, according to the 
present invention, nitrates can advantageously be converted to nitrites 
industrially. Therefore, the present invention can be advantageously 
applied to the manufacture of nitrites as industrial chemicals. Moreover, 
the present invention can be effectively applied to the treatment of waste 
nitrates formed in processes using nitric acid as a solvent, such as the 
surface treatment in the metal industry and nuclear fuel treatment. 
More specifically, the process of the present invention can be effectively 
applied to: the treatment of a solution formed by treating nitrogen oxides 
and then washing the treated solution with aqueous ammonia; the treatment 
of a liquid formed when a nitrate is hydrolyzed by ammonia; the treatment 
of a liquid formed by neutralizing nitric acid used for dissolution of 
metals by ammonia, and; the treatment of an aqueous solution containing 
ammonium nitrate, which is formed in the step of the nuclear fuel 
treatment. The present invention is valuable also from the point of view 
that the above-mentioned processes prevent environmental pollution. When 
the process of the present invention is applied to the above-mentioned 
treatment, ammonium nitrate is converted to ammonium nitrite and the 
formed ammonium nitrite is decomposed to nitrogen and water. This 
embodiment will now be described in detail. 
The reaction of thermally decomposing ammonium nitrite to nitrogen and 
water is known. On page 600, chapter 13, of "Inorganic Chemistry", written 
by Toshizo Chitani, as an instance of the processes for preparing pure 
chemical nitrogen, a process utilizing the following reaction is 
described. 
EQU NH.sub.4 NO.sub.2 .fwdarw.N.sub.2 +2H.sub.2 O 
In this reference, it is stated that, when a concentrated aqueous solution 
of ammonium nitrite is heated at about 70.degree. C., decomposition 
occurs. 
Further, in the above-mentioned Research Group Report AERE-R4393, of the 
U.K. Atomic Energy Authority, there is described a process in which 
ammonium nitrate is electrolytically reduced to ammonium nitrite, the 
ammonium nitrite formed is immediately thermally decomposed to nitrogen 
and water in the same cell, and nitrogen gas is discharged. The reaction 
per se of decomposition of ammonium nitrite is known, but this method has 
not been industrially used. The reason for this is that this method is 
very unstable and no particular device is made for the industrial 
production using this method. As typical defects of this method, the 
following defects can be mentioned. In the first place, since it is 
indispensable to perform the electrolytic reduction at a pH value of not 
higher than 1, in order to maintain such a strongly acidic state, it is 
necessary to continuously add a mineral acid, such as nitric acid, during 
the electrolysis, and selection of an appropriate cathode material is very 
difficult. In the second place, since the electrolysis is carried out at a 
temperature which is close to the boiling point of the aqueous solution, 
such by-products as NO.sub.x and NH.sub.3 are formed by side reactions and 
an appropriate secondary treatment should be conducted for removing these 
by-products. Furthermore, relatively large quantities of electrons are 
consumed for formation of these by-products and the current efficiency is 
drastically reduced. 
The inventors have found that, if the electrolytic reduction of ammonium 
nitrate and the thermal decomposition of ammonium nitrite are carried out 
in two stages, the above-mentioned known process can advantageously be 
conducted industrially. 
More specifically, according to the present invention, a catholyte 
containing ammonium nitrite, which is prepared according to the 
above-mentioned electrolytic reduction process, is taken out from the 
electrolytic cell, and the catholyte is heated to effect thermal 
decomposition of ammonium nitrite. The decomposition product is removed 
from the resulting mixture, and the residual nitrate in the liquid is 
returned to the electrolytic cell and used for the electrolytic reduction. 
Finally, the majority of the ammonium nitrite is decomposed to nitrogen 
and water. 
The concentration of the aqueous solution of ammonium nitrate is ordinarily 
chosen in the range of about 1 to about 50% by weight. The pH value of the 
catholyte is at least about 4 and preferably in the range of from about 7 
to about 10. The pH value is adjusted by using aqueous ammonia. However, 
because of the state of dissociation of ammonium hydroxide in a solution 
of ammonium nitrate, it is difficult to increase the pH value to more than 
about 10. Ammonium nitrite is stable in an alkaline region, and when the 
pH value is higher than about 9, thermal decomposition of ammonium nitrite 
is only partially caused, even if the temperature is elevated to a level 
close to the boiling point (about 102.degree. C., under atmospheric 
pressure, in the case of a 20% aqueous solution of ammonium nitrite). 
Accordingly, when the thermal decomposition is carried out on an 
industrial scale, adoption of the pH value included in the alkaline region 
is not preferred. On the other hand, if the pH value is lower than about 
7, the thermal decomposition is conducted very smoothly. 
As will be apparent from the foregoing illustration, a preferred pH range 
for formation of ammonium nitrite is quite different from a preferred pH 
range for decomposition of ammonium nitrite. Accordingly, special devices 
are required in order to conduct these two reactions in one continuous 
process. 
As practically applicable embodiments, the following two methods can be 
mentioned. 
(A) Batchwise Method: 
The electrolytic reduction is continuously carried out for a certain time, 
and then, the catholyte is taken out from the electrolytic cell. After the 
pH value has been adjusted to a level suitable for thermal decomposition, 
the catholyte is fed to a thermal decomposition apparatus and is thermally 
decomposed in this apparatus. Nitrogen and water formed by the thermal 
decomposition are removed, and also, water permeating from the anode 
chamber and water to be used for adjusting the concentration of the 
solution of ammonium nitrate are removed. The residual aqueous solution 
containing unreduced ammonium nitrate is returned to the electrolytic cell 
after the pH value has been adjusted to a level suitable for the 
electrolytic reduction, and the returned solution and a newly supplied 
aqueous solution of ammonium nitrate are subjected to the electrolytic 
reduction. 
(B) Continuous Method: 
The electrolytic reduction and the thermal decomposition are conducted by a 
continuous recycling process wherein a circuit system such as shown in 
FIG. 3, illustrated below, is utilized. It is possible and convenient in 
such a continuous recycling process for the electrolytic reduction and the 
thermal decomposition to be carried out at approximately the same pH 
value, which deviates from optimum pH ranges used in electrolytic 
reduction and thermal decomposition but is suitable for industrial 
production. 
The treatment can be conducted according to either of the above-mentioned 
methods. When the treatment is carried out on an industrial scale, 
limitations imposed on the thermal decomposition speed, the material 
balance, the heating-cooling cycle, the process control and other factors 
should be carefully examined. 
It is preferred that the thermal decomposition of an aqueous solution of 
ammonium nitrite be carried out at a temperature higher than the 
temperature adopted for the electrolytic reduction. However, it is 
possible to thermally decompose ammonium nitrite to nitrogen and water in 
the electrolytic cell. It was confirmed by the inventors of the present 
invention that partial decomposition is caused during the electrolytic 
reduction and nitrogen is formed. However, it is not preferred to conduct 
the electrolytic reduction at a temperature high enough to advance the 
thermal deomposition completely, because formation of by-product gases, 
such as NO and NH.sub.3, is enhanced, resulting in a reduction of the 
current efficiency. Accordingly, it is generally preferred to perform the 
electrolytic reduction at an optimum temperature for the electrolytic 
reduction and to elevate the temperature to a level suitable for the 
thermal decomposition in the second stage. 
At a solution pH value of about 7 and under atmospheric pressure, formation 
of N.sub.2 by thermal decomposition is observed if the temperature is 
about 70.degree. C. or higher, but in order to cause complete 
decomposition, it is necessary to heat the solution at a temperature 
higher than 85.degree. C. If the temperature is elevated to the boiling 
point of the solution, the decomposition speed is remarkably increased. 
Parts of ammonium hydroxide formed as a by-product during the electrolytic 
reduction and ammonium hydroxide added for the adjustment of the pH value 
can be recovered as ammonia in the condensed water. Accordingly, the pH 
value can be reduced to a level approximating the neutral point during the 
thermal decomposition. Incidentally, the so recovered ammonia can be used 
for adjusting the pH value of the catholyte at the step of the 
electrolytic reduction. 
In short, in the above-mentioned emdodiment, it is preferred that the 
electrolytic reduction be carried out at a temperature of from about 
15.degree. to about 70.degree. C. and the thermal decomposition be carried 
out at a temperature of about 70.degree. C. or higher. 
Incidentally, the reaction of the second stage can be effectively promoted 
by a method in which decomposition is accomplished by continuing the 
heating under a pressure lower than the atmospheric pressure. 
The inventors have found that the speed of the thermal decomposition can be 
effectively increased by using a certain catalyst. As a catalyst capable 
of providing good results, there can be mentioned catalysts containing 
about 2 to about 5% of at least one platinum group metal selected from 
platinum, ruthenium, rhodium, iridium and palladium supported on alumina 
or active carbon; granular active carbon; and porous activated alumina and 
zeolite catalysts. 
In the above-mentioned continuous method (B), where both the electrolytic 
reduction and the thermal decomposition are carried out at substantially 
the same pH values and it is intended to reduce the amount of ammonium 
nitrate formed by the adjustment of the pH value or completely prevent 
formation of such ammonium nitrate, it is impossible to avoid conducting 
the thermal decomposition at a relatively low speed. Accordingly, if a 
catalyst such as mentioned above is used in this continuous method (B), an 
especially prominent effect of increasing the decomposition speed can be 
obtained. Also, in the batchwise method (A), the use of the catalyst is 
preferred because a high decomposition speed can be maintained at a 
relatively low decomposition temperature. In any event, the solution which 
has been subjected to the thermal decomposition by heating should be 
cooled and, then, returned to the electrolytic cell. Accordingly, the 
presence of an effective catalyst is preferred from the point of view of 
energy economy. 
The process created through use of the catalyst has not been sufficiently 
elucidated. Generally, the catalyst shows a prominent effect when the 
thermal decomposition is carried out at a pH value included in the 
alkaline region, that is, a pH value of from about 8 to about 9. In the 
neutral or acidic pH region, the thermal decomposition is vigorously 
advanced, and therefore, no prominent effect can be attained by the use of 
the catalyst. It is often observed that, when the thermal decomposition is 
carried out in the alkaline pH region, the speed of the decomposition is 
drastically reduced when the thermal decomposition is advanced to a 
certain stage (for example, when the degree of decomposition arrives at a 
level of about 25 to about 30%). When the catalyst is added, this 
undesirable reduction of the decomposition speed can be effectively 
prevented and the thermal decomposition can be promoted until the degree 
of decomposition is elevated to approximately 100%. 
The catalyst can be added to the reaction system by various methods. For 
example, the catalyst may be added to the thermal decomposition column and 
suspended in the solution contained in the column. Furthermore, there may 
be adopted a method in which a catalyst layer is formed in a pack-type 
column and the solution is passed through this catalyst layer. 
A neutralization product formed by neutralizing nitric acid which has been 
used for the surface treatment of metals, the metal-dissolving treatment 
or the waste gas treatment, with ammonia or an ammonium nitrate solution 
formed in the nuclear fuel treatment, often contains metals in the form of 
compounds or ions dissolved or suspended therein. In order to prevent 
these impurities, such as metal ions, from having undesirable influences 
on the electrolytic reduction, it is preferred that these impurities be 
positively precipitated and removed as much a possible. If such separation 
operations as precipitation of metal hydroxides, growth of colloids and 
reduction of the solubility are carried out before the starting solution 
containing ammonium nitrate is fed to the electrolytic cell, or before the 
liquid left in the thermal decomposition column is recycled to the 
electrolytic cell after adjustment of the concentration and/or the pH 
value, considerable quantities of impurities can be separated and removed. 
Devices customarily used in the chemical industry may be used for 
performing these precipitation, sedimentation and separation operations. 
Namely, appropriate devices are chosen from a continuous precipitating and 
concentrating device, a thickener, a continuous clarifying device of the 
countercurrent contact type, an accelerator, a centrifugal separator, a 
cake filtering device, a filter, a pressure filter and a vacuum filter 
according to the kinds and quantities of impurities to be removed. 
When the substance to be removed is (1) a fine colloid, (2) a substance 
which does not tend to agglomerate and does not form coarse particles, (3) 
a substance to which a coagulant or precipitant cannot be applied or (4) a 
nuclear fuel, even separation of a minute amount of which involves 
problems, adoption of an ultrafiltration system yields especially good 
results in the present invention. For example, when an ultrafiltration 
system using a synthetic polymer membrane composed of polyacrylonitrile or 
an acrylonitrile copolymer (for example, a module composed of a membrance 
of the HXB type manufactured by Asahi Kasei Kogyo) is adopted, the 
majority of the colloidal particles of uranium, chlorium, iron, etc., can 
be removed, as demonstrated in the Examples set forth hereinafter. The 
above-mentioned system has a very simple structure including a liquid 
reservoir tank, a pump, a pressure valve, a pressure gauge and a 
non-return valve, and this system is effectively used when the operation 
is carried out under such conditions that washing of the electrolytic 
cell, electrodes and ion exchange membrane is not allowed for a long 
period or when the electrolytic cell is continuously used for a long time. 
This embodiment will now be described in detail with reference to FIG. 3. 
A catholyte containing an aqueous solution of ammonium nitrate to be 
electrolytically reduced in an electrolytic cell 42 is recycled between a 
catholyte reservoir tank 43 and the electrolytic cell 42, and is 
continuously subjected to electrolytic reduction until the ammonium 
nitrite concentration reaches a predetermined level. The electrolytic cell 
42 is separated into an anode chamber and a cathode chamber by a cation 
exchange membrane, and an anolyte composed of an aqueous solution of a 
mineral acid is recycled between an anolyte reservoir tank 41 and the 
electrolytic cell 42 by means of a pump (not shown) or the like. This 
electrolytic reduction system is substantially the same as the system 
described hereinbefore with reference to FIG. 1. The catholyte in which 
the nitrite ion concentration is elevated to a predetermined level is 
transferred to an electrolyzed liquid reservoir tank 44 by means of a pump 
(not shown) or the like. The electrolyzed catholyte is then fed to a 
thermal decomposition column 45 and heated at a temperature of up to the 
boiling point of the aqueous solution to decompose ammonium nitrite to 
nitrogen and water. Ammonia contained in the catholyte is recovered in the 
form of aqueous ammonia by cooling and stored in an ammonia tank 48. This 
ammonia includes ammonium hydroxide added for the pH adjustment at the 
electrolytic reduction and ammonia formed as a by-product at the 
electrolytic reduction. Unreduced ammonium nitrate is contained in the 
thermally decomposed solution coming from the thermal decomposition column 
45. Water formed by the thermal decomposition, water which has permeated 
from the anode chamber and water for the adjustment of the concentration 
are removed from this solution by distillation, and the residual liquid is 
fed to a pH adjusting tank 49 through a heat exchanger 46 by means of a 
pump (not shown) or the like. At this point, the starting material in an 
amount corresponding to the amount of reduced and decomposed ammonium 
nitrate is supplied to the liquid from a starting material tank 47. When 
the pH value is adjusted in the pH adjusting tank 49, ammonium hydroxide 
recovered by evaporation in the thermal decomposition column 45 is fed 
from the ammonia tank 48 and used for the pH adjustment. Thus, the pH 
value of the liquid is adjusted to a predetermined level suitable for the 
electrolytic reduction. Then, the liquid is passed through a filter or 
ultrafiltration system 50 to remove precipitates composed of hydroxides 
and the like. The majority of metal ions contained in the liquid are 
conveniently precipitated at a pH value of at least 7, which is ordinarily 
adopted for the electrolytic reduction. The filtered liquid, that is, the 
catholyte, is transferred to a thermally decomposed liquid reservoir tank 
51 and is recycled to the electrolytic cell 42 for the electrolytic 
reduction. When the electrolysis is carried out under a certain 
predetermined pH condition, an acid supply tank 52 is installed to supply 
nitric acid in an amount corresponding to the amount of formed ammonium 
hydroxide. It is preferred that the foregoing operations be conducted 
repeatedly under such conditions that the current efficiency can be 
maintained at as high a level as possible. 
The present invention will now be described more in detail by reference to 
the following Examples, that by no means limit the scope of the invention. 
In these Examples, a direct current was used for the electrolysis, the 
compositions of the produced gases were determined according to gas 
chromatography and the amounts of the gases were measured by using a wet 
type gas meter. Generally, water is formed during the electrolytic 
reduction and water is permeated into the catholyte from the anode chamber 
during the electrolytic reduction. Accordingly, the concentration of the 
catholyte solution before the electrolytic reduction was different from 
the concentration after the electrolytic reduction. 
EXAMPLE 1 
A highly acidic cation exchange membrane composed of a fluorinated polymer 
having sulfonic acid exchange groups was used as the ion exchange 
membrane, an electrode prepared according to the method disclosed in 
Example 1 of U.S. Pat. No. 4,005,004 was used as the anode and a lead 
electrode was used as the cathode. In an anode chamber of an electrolytic 
cell of the clamping type (having a membrane area of 0.3 dm.sup.2 /cell) 
provided with the above-mentioned membrane, anode and cathode, an aqueous 
solution containing 31.5 g/l of nitric acid was fed at a flow rate of 22.8 
l/hr. Simultaneously, an aqueous solution containing 230 g/l of sodium 
nitrate and 3.8 g/l of sodium hydroxide (the pH value being 12.9) was fed 
to a cathode chamber of the electrolytic cell at a flow rate of 22.8 l/hr. 
An electric current was applied to the electrolytic cell at a current 
density of 30 A/dm.sup.2, for 2 hours, while maintaining the liquid 
temperature at 40.degree. C. In the normal steady operation, oxygen gas 
was formed on the anode, and formation of a small quantity of the gas and 
coloration of the aqueous solution to a slightly yellowish hue were 
observed in the cathode chamber. The amount of oxygen generated on the 
anode was 0.167 mol, and the gas generated on the cathode comprised 0.0181 
mol of nitrogen, 0.0034 mol of nitrogen monoxide and 0.0005 mol of 
hydrogen. The electrolyzed catholyte comprised 194 g/l of sodium nitrate, 
17.7 g/l of sodium nitrite, 5.2 g of sodium hydroxide and 0.8 g/l of 
ammonium hydroxide. The current efficiency for fomation of sodium nitrite 
was 76.6%. 
EXAMPLE 2 
In an anode chamber of an electrolytic cell of the clamping type (having a 
membrane area of 0.3 dm.sup.2 /cell) provided with a highly acidic cation 
exchange membrane composed of a styrene-divinylbenzene copolymer having 
sulfonic acid groups introduced therein, an anode composed of a platinum 
plate and a cathode composed of lead, an aqueous solution containing 18.3 
g/l of hydrochloric acid was fed at a flow rate of 22.8 l/hr, and an 
aqueous solution containing 230 g/l of sodium nitrate and 3.8 g/l of 
sodium hydroxide (the pH value being 12.9) was simultaneously fed to a 
cathode chamber of the electrolytic cell at a flow rate of 22.8 l/hr. The 
electrolytic reduction was carried out in the same manner as described in 
Example 1. Other conditions were the same as those adopted in Example 1. 
The sodium nitrite concentration in the electrolyzed catholyte was 16.3 
g/l and the current efficiency for formation of sodium nitrite was 70.5%. 
Chlorine gas was generated in the anode chamber. 
EXAMPLE 3 
The electrolytic reduction was carried out under the same conditions as 
described in Example 2, except that an aqueous solution containing 24.5 
g/l of sulfuric acid was used as the anolyte. The sodium nitrite 
concentration in the electrolyzed catholyte was 16.6 g/l and the current 
efficiency for formation of sodium nitrite was 72.0%. 
EXAMPLE 4 
In an anode chamber of an electrolytic cell of the clamping type (having a 
membrane area of 1.0 dm.sup.2 /cell) provided with a weakly acidic cation 
exchange membrane of a fluorinated polymer having carboxyl groups, which 
was prepared according to the method disclosed in U.S. Pat. No. 4,151,053, 
an anode having the same composition as that of an electrode of Run No. 4 
prepared according to the method disclosed in Example 2 of U.S. Pat. No. 
4,005,004 and a cathode composed of lead, an aqueous solution containing 
6.3 g/l of nitric acid was fed at a flow rate of 75.2 l/hr, and an aqueous 
solution containing 400 g/l of sodium nitrate (the pH value being 5.2) was 
simultaneously fed to a cathode chamber of the electrolytic cell at a flow 
rate 75.2 l/hr. An electric current was applied to the electrolytic cell 
at a current density of 30 A/dm.sup.2, for 2 hours, while maintaining the 
liquid temperature at 90.degree. C. In the normal steady operation, oxygen 
gas was formed on the anode, and formation of a small quantity of the gas 
and slight yellowing of the aqueous solution were observed in the cathode 
chamber. 
The amount of oxygen gas generated on the anode was 0.559 mol, and the gas 
generated on the cathode comprised 0.50 mol of nitrogen, 0.027 mol of of 
nitrogen monoxide and 0.042 mol of hydrogen. The electrolyzed catholyte 
comprised 309 g/l of sodium nitrate, 50.5 g/l of sodium nitrite, 6.0 g/l 
of sodium hydroxide and 0.9 g/l of ammonium hydroxide. The current 
efficiency for formation of sodium nitrite was 65.1%. 
EXAMPLES 5 through 9 
In an anode chamber of an electrolytic cell provided with the same cation 
exchange membrane as used in Example 1, an anode prepared according to the 
method described in Example 4 of U.S. Pat. No. 4,005,004 and a cathode 
composed of indium, an aqueous solution containing 31.5 g/l of nitric acid 
was fed at a flow rate of 20 l/hr, and an aqueous solution containing 230 
g/l of sodium nitrate and 3.8 g/l of sodium hydroxide (the pH value being 
12.9) was simultaneously fed to a cathode chamber of the electrolytic cell 
at a flow rate of 20 l/hr. The electrolytic reduction was carried out 
under the same conditions as adopted in Example 1. 
Furthermore, the electrolytic reduction was repeated in the same manner as 
described above by using a platinum-plated titanium electrode, a carbon 
electrode, a mercury electrode and an electrode of a lead-tin alloy (the 
tin content being 10% by weight) instead of the indium cathode. 
In each run, the concentration of sodium nitrite in the electrolyzed 
catholyte and the current efficiency for formation of sodium nitrite were 
measured to obtain the results shown in Table 1. 
TABLE 1 
______________________________________ 
Current 
Example Sodium Nitrite Con- 
Efficiency 
No. Cathode Material 
centration (g/l) 
(%) 
______________________________________ 
5 indium 15.7 68 
6 platinum-plated 
3.2 14 
titanium 
7 carbon 9.2 40 
8 mercury 19.2 83 
9 lead-tin alloy 
16.8 73 
______________________________________ 
EXAMPLES 10 through 16 
In an anode chamber of an electrolytic cell of the clamping type (having a 
membrane area of 1 dm.sup.2 /cell) provided with the same cation exchange 
membrane as used in Example 1, an anode prepared according to the method 
disclosed in Example 5 of U.S. Pat. No. 4,005,004 and a cathode composed 
of tin, an aqueous solution containing 31.5 g/l of nitric acid was 
supplied at a flow rate of 50 l/hr, and an aqueous solution containing 217 
g/l of ammonium nitrate was simultaneously fed to a cathode chamber of the 
electrolytic cell. An electrolytic current was applied to the electrolytic 
cell at a current density of 30 A/dm.sup.2, for 2 hours, while maintaining 
the liquid temperature at 40.degree. C. during the electrolysis. The pH 
value was maintained at a level indicated in Table 2 by continuously 
feeding a minute amount of nitric acid to the cathode chamber. In the 
normal steady operation, oxygen gas was generated on the anode, and a 
small amount of the gas was generated on the cathode. 
The ammonium nitrite concentration in the electrolyzed catholyte and the 
current efficiency for formation of ammonium nitrite were measured to 
obtain the results shown in Table 2. 
TABLE 2 
______________________________________ 
Example 
pH of Ammonium Nitrite 
Current Efficiency 
No. Catholyte Concentration (g/l) 
(%) 
______________________________________ 
10 4 8.5 40 
11 4.8 9.4 44 
12 6 10.4 49 
13 7 11.5 54 
14 8 13.0 61 
15 9 14.9 70 
16 9.7 16.4 77 
______________________________________ 
EXAMPLE 17 
In an anode chamber of an electrolytic cell provided with the same cation 
exchange membrane as used in Example 1, a rhodium-plated titanium anode 
and a lead cathode, an aqueous solution containing 31.5 g/l of nitric acid 
was supplied at a flow rate of 30 l/hr, and an aqueous solution containing 
187 g/l of lithium nitrate and 2.4 g/l of lithium hydroxide (the pH value 
being 13) was simultaneously fed to a cathode chamber of the electrolytic 
cell at a flow rate of 30 l/hr. An electric current was applied to the 
electrolytic cell at a current density of 10 A/dm.sup.2, for 6 hours, 
while maintaining the liquid temperature at 70.degree. C. during the 
electrolysis. In the normal steady operation, oxygen gas was generated on 
the anode, and a small amount of the gas was formed on the cathode. 
The nitrite concentration in the electrolyzed catholyte was 12.3 g/l and 
the current efficiency for formation of the nitrite was 68%. 
EXAMPLE 18 
The electrolytic reduction was carried out in the same manner as described 
in Example 17, except that an aqueous solution containing 274 g/l of 
potassium nitrate and 5.6 g/l of potassium hydroxide (the pH value being 
13) was fed to the cathode chamber instead of the aqueous solution of 
lithium nitrate and lithium hydroxide used in Example 17. The nitrite 
concentration in the electrolyzed catholyte was 24.0 g/l and the current 
efficiency for formation of the nitrite was 81%. 
EXAMPLE 19 
Electrolytic reduction and thermal decomposition were conducted according 
to the process shown in the flow sheet of FIG. 3. 
An aqueous solution containing 217 g/l of ammonium nitrate and 0.35 g/l of 
ammonium hydroxide was fed to a cathode chamber of an electrolytic cell 42 
(having a current-applied area of 3 dm.sup.2), and the electrolytic 
reduction was carried out for 2 hours, while continuously feeding nitric 
acid in an amount of 0.54 equivalent weight, which is sufficient to 
neutralize ammonium hydroxide that is formed as a by-product. 
An electrolytic cell of the clamping type, which was separated into cathode 
and anode chambers by a strongly acidic cation exchange membrane composed 
of a fluorinated copolymer having sulfonic acid exchange groups introduced 
therein, was used as the electrolytic cell. An aqueous solution containing 
30 g/l of nitric acid was supplied as the anolyte. The current density was 
20 A/dm.sup.2. Each of the catholyte and anolyte was fed at a flow rate of 
300 l/hr. The electrolyzed catholyte comprised 65.4 g/l of ammonium 
nitrate, 78.6 g/l of ammonium nitrite and 0.35 g/l of ammonium hydroxide. 
This solution was fed to a thermal decomposition column 45 at a certain 
feed rate while maintaining the liquid temperature at 100.degree. C., and 
the thermal decomposition was conducted for 30 minutes. About 46.9 l of 
gaseous nitrogen was formed and discharged from the head of the thermal 
decomposition column 45. The ammonium nitrite concentration in the 
thermally decomposed solution was lower than the measurement limit 
according to oxidation-reduction titration. Thus, it was confirmed that 
ammonium nitrate was decomposed at a decomposition ratio of approximately 
100%. The solution left after the thermal decomposition was passed through 
a vacuum filtering device to remove suspended solids (such as metal 
hydroxides), and then, the filtrate was returned to the cathode chamber of 
the electrolytic cell 42 together with a newly supplied solution of 
ammonium nitrate and was subjected to the electrolytic reduction again. 
EXAMPLES 20 through 22 
Electrolytic reduction and thermal decomposition were conducted according 
to the process shown in the flow sheet FIG. 3. 
In a cathode chamber of an electrolytic cell 42 (having a current-applied 
area of 3 dm.sup.2), 3 l of an aqueous solution containing 217 g/l of 
ammonium nitrate and 22.7 g/l of ammonium hydroxide (the pH value being 9) 
was supplied and the electrolytic reduction was carried out for 2 hours. 
The structure of the electrolytic cell, the anolyte, the current density 
and the flow rates of the catholyte and anolyte were the same as described 
in Example 19. The resulting electrolyzed catholyte comprised 155.5 g/l of 
ammonium nitrate, 30.7 g/l of ammonium nitrite and 22.4 g/l of ammonium 
hydroxide. 
Nitric acid was added to 1 l of the recovered solution to adjust the pH 
value as indicated in Table 3, and the solution was fed to a thermal 
decomposition column 45 and the thermal decomposition was carried out for 
30 minutes, while maintaining the liquid temperature at 100.degree. to 
102.degree. C. During the thermal decomposition, generation of nitrogen 
and 0.63 equivalent of ammonia from the head of the thermal decomposition 
column was observed. The amount of nitrogen generated by the thermal 
decomposition and the percentage thermal decomposition were determined to 
obtain the results shown in Table 3. 
TABLE 3 
______________________________________ 
pH of Solution 
Amount (l) of 
at Start Nitrogen Generated 
Example 
of Thermal (as measured Percentage 
No. Decomposition 
at 20.degree. C.) 
Decomposition 
______________________________________ 
20 8.7 2.9 25.4 
21 7.8 9.5 82.0 
22 7.0 11.5 100 
______________________________________ 
EXAMPLES 23 through 25 
Electrolytic reduction and thermal decomposition were conducted according 
to the process shown in the flow sheet of FIG. 3. 
An electrolytically reduced catholyte comprising 251 g/l of ammonium 
nitrate, 14 g/l ammonium nitrite and 2.5 g/l of ammonium hydroxide and 
having a pH value of 7.9 was fed to a thermal decomposition column 45, and 
the thermal decomposition was conducted for 30 minutes, while maintaining 
the liquid temperature at a level shown in Table 4. The ammonium nitrite 
concentration in the thermally decomposed solution and the percentage 
decomposition were determined to obtain the results shown in Table 4. 
TABLE 4 
______________________________________ 
Thermal Decom- 
position Concentration 
Example 
Temperature (g/l) of Percentage 
No. (.degree.C.) NH.sub.4 NO.sub.2 
Decomposition 
______________________________________ 
23 70 13.9 1 
24 80 13.0 6.8 
25 102 2.8 80.2 
______________________________________ 
EXAMPLES 26 to 33 
Electrolytic reduction and thermal decomposition were conducted according 
to the process shown in the flow sheet of FIG. 3. 
About 16 l of an electrolytically reduced catholyte containing 241 g/l of 
ammonium nitrate, 15.9 g/l of ammonium nitrite and 20.1 g/l of ammonium 
hydroxide and having a pH value of 8.9, was divided and separately fed to 
small-size thermal decomposition columns, each including a tubular 
condenser and having a catalyst shown in Table 5 packed therein. In each 
run, the reduced catholyte was subjected to thermal decomposition at a 
temperature of 102.degree. C., without performing the pH adjustment. For 
comparison, the thermal decomposition was conducted in a catalyst-free 
system. 
The ammonium nitrite concentration in the solution thermally decomposed for 
2 hours and the percentage of decomposition was determined to obtain the 
results shown in Table 5. 
TABLE 5 
______________________________________ 
Ex- Concentra- 
Percentage 
ample Thermal Decomposition 
tion (g/l) 
Decom- 
No. Catalyst of NH.sub.4 NO.sub.2 
position 
______________________________________ 
26 not added 12.0 24.5 
27 5% by weight of platinum 
supported on granular carbon 
9.5 40.3 
28 2% by weight iridium 
supported on carbon powder 
10.7 32.7 
29 2% by weight of ruthenium 
supported on carbon powder 
8.7 45.2 
30 2% by weight of iridium- 
platimum (1:1) 
supported on carbon powder 
9.7 39.0 
31 Zeolite 5A 10.3 35.2 
32 active carbon 10.9 31.4 
33 activated alumina 11.1 30.2 
______________________________________ 
EXAMPLE 34 
Electrolytic reduction and thermal decomposition were carried out according 
to the process shown in the flow sheet of FIG. 3. 
The pH value of a solution containing 217 g/l of ammonium nitrate and about 
0.5 g/l each of iron, chromium and uranium was adjusted to 8.8 by using a 
solution of ammonium hydroxide. Precipitates of metal hydroxides formed in 
the solution were separated by filtration. The filtrate comprised 215 g/l 
of ammonium nitrate, 7.8 g/l of ammonium hydroxide, 2 mg/l of iron, 8 mg/l 
of chromium and 2 mg/l of uranium. Then, 3 l of this solution was fed to 
an electrolytic cell (having a current-applied area of 1 dm.sup.2) and the 
electrolytic reduction was carried out for 3 hours. The cation exchange 
membrane, the anolyte, the current density and the feed rates of the 
catholyte and anolyte were the same as those described in Example 19. The 
pH value of the electrolyzed catholyte was adjusted to 7.8 by adding 
nitric acid, and the solution was fed to a thermal decomposition column 
packed with a composite catalyst containing 1% by weight each of ruthenium 
and iridium supported on an alumina, and ammonium nitrite contained in the 
solution was thermally decomposed. Prior to recycling the thermally 
decomposed solution to the electrolytic cell, the pH value was adjusted to 
8.5 by using a solution of ammonium hydroxide, and a solution containing 
minute amounts of the so formed metal hydroxide precipitate was fed to an 
ultrafiltration module composed of an acrylic high polymer (manufactured 
and sold under the tradename "XBL-100" by Asahi Kasei Kogyo) at a flow 
rate of 29 l/hr. The filrate was recycled to the electrolytic cell and 
used as the catholyte. 
The ammonium nitrite concentration in the solution fed to the thermal 
decomposition column was 14.9 g/l and the ammonium nitrite concentration 
in the thermally decomposed solution was 0.3 g/l. The decomposition ratio 
was 98%. 
The concentrations of metal impurities in the filtrate which had been 
passed through the ultrafiltration module were less than 0.5 mg/l of iron, 
less than 1 mg/l of chromium and less than 0.1 mg/l of uranium. 
EXAMPLE 35 
A solution obtained by conducting filtration separation in the same manner 
as described in Example 34 was subjected to electrolytic reduction under 
the same conditions as described in Example 34. The electrolyzed catholyte 
comprised 182 g/l of ammonium nitrate, 15.4 g/l of ammonium nitrite, 8.7 
g/l of ammonium hydroxide and minute amounts of metal salts, and had a pH 
value of 8.5. 
While the above-mentioned solution was continuously fed to an electrolytic 
cell and the electrolytic reduction was continued, the electrolyzed 
catholyte was fed to a thermal decomposition column packed with a platinum 
catalyst (supported on granular carbon) from a catholyte reservoir tank. 
The thermal decomposition was conducted for 6 hours, during which 38.5 l 
of nitrogen was generated from the head of the thermal decomposition 
column. Condensed water obtained from the thermal decomposition column 
contained ammonia in an amount of about 0.7 equivalent weight as a whole. 
The pH value of the solution which had been passed through the thermal 
decomposition column was 7.7. This solution was cooled and the pH value of 
the solution was adjusted to 8.5 by using ammonium hydroxide. This 
solution was recycled to the cathode chamber of the electrolytic cell. The 
ammonium nitrate concentration in the electrolytically reduced and 
thermally decomposed solution was 126 g/l.