Process for preparing hydroxylamine from NO.sub.x -containing flue gases

A process and device are disclosed for preparing hydroxylamine from NO.sub.x -containing flue gases. Nitrogen removal from the NO.sub.x -containing flue gases by absorption in an aqueous solution of FeII-EDTA and by desorption through vapor stripping of the NA from the thus obtained FeII(NO)-EDTA solution, with admixture of hydrogen, leading to the recovery of an NO/H.sub.2 gaseous mixture, is combined with hydroxylamine synthesis by catalytic reduction of NO with hydrogen.

A process and device are disclosed for preparing hydroxylamine from 
NO.sub.x - and, possibly, O.sub.2 -containing flue gases. Nitrogen removal 
from the flue gases is combined with hydroxylamine synthesis by catalytic 
reduction of NO with hydrogen. 
This invention makes it possible to combine nitrogen removal from flue 
gases, which is laid down by law in many countries for environmental 
reasons, with the commercial production of hydroxylamine, which represents 
a valuable intermediate product for the production of nylon-6. 
Nitrogen removal from flue gases, especially from those found in industrial 
combustion installations, coal and crude petroleum power stations as well 
as in the preparation of azotic acids, is very important these days for 
environmental reasons and is in many countries subjected to strict 
statutory regulations. The EC guidelines, for instance, lay down that the 
nitrogen oxides content in the flue gases of industrial power stations and 
in industrial flue gases are not to exceed 200 to 400 mg/m.sup.3. These 
low limits cannot be observed by proper measures during the combustion 
process, the so-called primary measures, alone. It is therefore necessary 
to use a special process for the removal of undesirable nitrogen in 
so-called nitrogen removal processes in order to fulfil these high 
demands. The nitrogen removal process that is used the most at present is 
the nitrogen removal by selective catalytic reduction (SCR), in which the 
NO.sub.x (i.e. different nitrogen oxides, especially NO, possibly mixed 
with NO.sub.2) nitrogen oxides in the flue gases are catalytically reduced 
to N.sub.2 and H.sub.2 O in reaction with ammonia (NH.sub.3). This 
reaction usually happens at a temperature of 300.degree. to 400.degree. C. 
There are several other alternative industrial processes for nitrogen 
removal, but none of these processes transforms the undesirable nitrogen 
oxides in a technically acceptable product. 
DE-PS 3 406 085 describes an attempt to produce an acceptable product, NO 
rich gas, during the purification of exhaust gases. It explains a process 
for nitrogen removal from NO.sub.x -containing flue gases by absorption of 
NO.sub.x nitrogen oxide in aqueous solutions of iron (II) salt with pH 
values of 0 to 1. However, since the solubility of NO.sub.x in such a 
solution is known to be extremely low, this process is not workable enough 
for industrial nitrogen removal. 
Other processes have also been suggested in which SO.sub.x and NO.sub.x are 
removed from flue gases or other absorbent, like Mg(OH).sub.2, Na.sub.2 
SO.sub.3, citrates and suchlike, are used as absorbing agents (cf. H. 
Hasui and H. Omichi, "The Mitsui Wet Process for SO.sub.2 and NO.sub.x 
Removal," Nenryo Kyokai-Shi 55 (1979) 4, 264 to 269; E. Sada, H. Kumazawa, 
I. Kudo and T. Kondo, "Ind. Eng. Process Des. Dev.," 20 (1981) 3, 46-49; 
E. Sada, H. Kumazawa, Y. Sawada and T. Kondo, Ind. Eng. Process Des. Dev., 
21 (1982) 4, 771-774, and W. Weisweiler, B. Retzlaff and L. Raible, Chem. 
Eng. Process, 18 (1984) 85-92). 
When using absorbents like those the chemical equilibrium is moved to the 
desired side. At the same time, however, an undesirable oxidation of iron 
(II) to iron (III) occurs on the basis of the oxygen contents found in all 
flue gases, which strongly reduces the absorbing power of the absorbent. 
In addition sulphates form, and their removal is problematic. Normally 
unhydrated lime is added and potassium sulphate is obtained by 
precipitation. The Fe(EDTA) complex is lost, however. 
On the other hand hydroxylamine (NH.sub.2 OH) is generally recognized to be 
a valuable intermediate product for nylon-6 synthesis. 
Hydroxylamine is used to prepare cyclohexanonoxim out of cyclohexanone, in 
which the resulting cyclohexanonoxim is transferred to caprolactam through 
the Beckmann rearrangement. This caprolactam can be polymerized into a 
polyamide, namely nylon-6, an extraordinarily valuable synthetic 
substance. 
Therefore it has been a long-lasting endeavor to prepare the hydroxylamine 
necessary as intermediate material for the preparation of nylon-6 as 
economically as possible on a commercial scale. One of the most famous 
synthesis processes is the so-called Raschig process, based on the 
reduction of ammonium nitrate with a solution of bisulfite and sulphur 
dioxide. Another famous synthesis process is the so-called BASF process, 
in which nitrogen oxides NO with gaseous hydrogen are directly reduced to 
hydroxylamine (cf. DE-PS 1 177 118 and K. Jockers, "Nitrogen" No. 50, 
November/December 1967, 27-30). In this process a mixture of NO and 
H.sub.2 reacts in an aqueous acid medium in the presence of a platinum or 
other noble metal catalyst that has been reduced to slurry. To carry out 
this BASF process industrial quality chemicals are necessary and the NO 
that is used as a starting material has to be prepared in-situ by 
oxidation of ammonia. Using already obtained, NO.sub.x -containing flue 
gases is not possible according to the above-cited prior publications. 
Other efforts were also made to enable hydroxylamine synthesis from flue 
gases. For instance, in IT-PS 1 152 229 the use of NO.sub.x and SO.sub.2 
-containing process flue gases as starting material is suggested. This 
process, however, is restricted to the use of comparatively high NO.sub.x 
concentrations in the 1% dimension. This means that in order to carry out 
the process only flue gases of low pressure azotic acid productions can be 
used. 
The U.S. Pat. No. 4 115 523 introduces a process for the preparation of 
hydroxylamine from NO and H.sub.2 S from industrial flue gases. 
But also in this process high concentrations of NO.sub.x of more than 10 
percent by volume are necessary, because only with these concentrations 
the hydroxylamine synthesis can be satisfactory. 
The invention therefore had to find a way for hydroxylamine synthesis from 
flue gases that are already obtained to enable the commercial and 
industrial hydroxylamine preparation as a valuable intermediate product 
for nylon-6 synthesis. 
It was determined that according to invention this task can be solved by 
combining the nitrogen removal from NO.sub.x - and possibly 
oxygen-containing flue gases, which is already necessary for environmental 
reasons, with the catalytic reduction of the thus gained NO by means of 
hydrogen in the framework of a commercially feasible overall process. 
Important is here that the nitrogen removal from the NO.sub.x - and also 
possibly from the oxygen-containing flue gases by absorption of NO.sub.x 
in an aqueous solution of FeII-EDTA is carried out at a comparatively low 
temperature and that the complexly absorbed NO is desorbed and 
concentrated from the aqueous solution of FeII(NO)-EDTA at an increased 
temperature by vapor stripping under induction of electrolytically 
generated hydrogen, so that the thus obtained gaseous mixture after 
removal of the vapor by condensation contains NO and H.sub.2 in the for 
the catalytic synthesis necessary proportion of 35 to 65 percent by 
volume. This gaseous mixture is directly suitable as charging gas for the 
commercial preparation of hydroxylamine by catalytic reduction. 
Subject of the invention is a process for preparing hydroxylamine from 
NO.sub.x - and possibly O.sub.2 -containing flue gases by combining 
nitrogen removal from the NO.sub.x - as well as the possibly O.sub.2 
-containing flue gases with hydroxylamine synthesis by catalytic reduction 
of NO with hydrogen. This process is characterized by its steps, which are 
the following: 
(a) a NO.sub.x -containing flue gas is introduced into the bottom section 
of an absorber, where the NO.sub.x which is contained in the flue gas is 
absorbed at a comparatively low temperature in countercurrent contact with 
an aqueous FeII-EDTA solution which was inserted into the head of the 
absorber, a FeII(NO)-EDTA complex being produced thereby which is 
dissolved in the aqueous solution and which is removed as bottom product 
from the absorber together with the aqueous solution that contains it and 
after going through a heat regenerator to increase its temperature it is 
inserted as head product in a desorber, while the flue gas, now free from 
NO.sub.x, is removed overhead from the absorber; 
(b) from the aqueous solution of the FeII(NO)-EDTA complex with increased 
temperature, which is inserted in the head of the desorber, the NO is 
desorbed in countercurrent contact with the vapor that was transferred 
from a reboiler into the bottom section of the desorber, and with hydrogen 
that was transferred from an electrolyzer. The thus obtained aqueous 
solution containing the dissolved FeII-EDTA complex is removed as bottom 
product from the desorber and led back into circulation in the upper 
section of the absorber via an electrolyzer to reduce the possibly 
contained FeIII-EDTA to FeII-EDTA and via a heat regenerator as well as a 
condenser for the step-by-step decreasing of its temperature, and 
(c) the gaseous mixture of NO, H.sub.2 and water vapor which was removed 
overhead from the desorber is led to the reboiler of the direct catalytic 
hydroxylamine synthesis after the water vapor has been removed in a 
condenser and the condensed vapor has been returned to the reboiler. 
According to the invented process it is surprisingly easy and economical to 
effectively remove nitrogen from NO.sub.x - and possibly oxygen-containing 
flue gases by absorption and by complex absorption of NO.sub.x in an 
aqueous FeII-EDTA solution, without the occurrence of a reduction to N2, 
as is the case in most known nitrogen removal processes. On the other 
hand, according to the invented process the NO.sub.x that is obtained 
after the desorption can be directly catalytically reduced to 
hydroxylamine after the concentration of NO.sub.x is mixed with 
proportionally added amounts of hydrogen, without having to use some 
valuable starting material like ammonia and suchlike. Therefore the total 
process according to invention can be carried out very economically. The 
process according to invention therefore can also be carried out 
successfully when the flue gases that are used as starting material 
contain further oxygenating components like oxygen and/or NO.sub.2, which 
cause an at least partial oxidation of the aqueous FeII-EDTA solution into 
an aqueous FeIII-EDTA solution. 
According to the invention this circumstance is taken into account by 
electrolytically regenerating the partially oxygenated aqueous absorbing 
solution while at the same time hydrogen is absorbed, which is directed in 
such a way that there is a right amount of hydrogen for the commercial 
production of hydroxylamine for the desorbed NO. According to the invented 
process even very low concentrations of NO.sub.x of even less than 500 
mg/m.sup.3 can be concentrated for a workable hydroxylamine synthesis. 
As mentioned above the invented process offers many advantages over the 
comparable processes of the state of art that were known until now. Since 
some types of flue gases from which nitrogen has to be removed contain 
considerable amounts of nitrogen oxides (NO.sub.2) next to 3 to 5 percent 
by volume of oxygen, it is very important that the invented process can 
also be used with flue gases which contain NO.sub.2. This ecologically 
harmful component is produced in particular in the preparation of azotic 
acids and the flue gases contain considerable concentrations of NO.sub.2. 
The typical concentrations of NO.sub.x in the flue gas leaving the last 
absorber of an installation for the preparation of azotic acids can be as 
high as 4000 ppm, while the NO.sub.2 content can be 50% of the total 
concentration of NO.sub.x. 
These amounts of NO constantly cause problems, which manifest themselves in 
brown NO.sub.2 plumes coming out of the chimney of an azotic acids 
production plant. The total NO.sub.x content in the flue gas has to be 
reduced to 200 ppm before being let off into the atmosphere. It is 
generally accepted that the NO.sub.2 content has to be reduced to less 
than 75 to 100 ppm in order to produce a colorless smoke trail. 
When using the process according to invention on such flue gases, the 
nitrogen dioxide (NO.sub.2) contained in the flue gas is transformed into 
nitrogen oxide NO according to the following equation during the 
absorption-complex-building process: 
EQU NO.sub.2 +3 FeII-EDTA+2H.sup.+ .fwdarw.FeII(NO)-EDTA+2 FeIII-EDTA+H.sub.2 O 
The nitrogen dioxide (NO.sub.2) is dissolved in the scouring solution and 
then reduced to nitrogen oxide (NO) by FeII, which builds the 
corresponding nitrosyl complex FeII(NO)-EDTA. The NO can be released from 
this complex in a high concentration. The thus produced FeIII-EDTA is 
reduced to the active FeII-EDTA in an electrolyzer. This has as a result 
that the NO.sub.2 content, which was originally present in the flue gas, 
is transformed into NO, which in highly concentrated form is very valuable 
for the preparation of the NO/H.sub.2 mixture for hydroxylamine synthesis. 
That this reaction really occurs when the process is carried out according 
to invention is demonstrated in the laboratory tests described below in 
examples 2 and 3. 
Since the absorption of NO.sub.x in the absorber is more effective when the 
absorption solution is at a lower temperature in the process according to 
invention, the absorption solution is cooled down to a temperature of 
20.degree. to 40.degree. C., preferably 30.degree. C., before being 
inserted into the absorber. This increases the absorbing power of the 
absorption solution with respect to the workings at a working temperature 
of 50.degree. C. at least by a 3.3 factor (at 50.degree. C. the 
equilibrium constant is 287 bar.sup.-1 and at 35.degree. C. the 
equilibrium constant is 929 bar.sup.-1). While passing through the 
absorber the NO content of the inserted flue gas decreases from an initial 
500 ppm, for instance, to 100 ppm, for instance, and the concentration of 
the FeII(NO)-EDTA complex in the enriched absorption solution is, for 
instance, 12.5 mMol/l, i.e. there is a 0.25 transformation degree. 
On the other hand the desorption of NO from the FeII(NO)-EDTA complex in 
the desorber is determined by the value of the equilibrium constants at 
higher temperatures, which are defined by the following equation: 
##EQU1## 
in which y is the transformation degree and p the partial pressure of NO 
(in bar). 
At 100.degree. C. the equilibrium constant is 11.3, i.e. the maximum 
reachable partial pressure of NO during desorption is restricted to a 
value of less than 0.05 bar. By using a condensable inert gas, like e.g. 
water vapor, high concentrations of NO can be reached after removal of the 
inert gas by condensation. By using this method of vapor stripping 
according to invention high concentrations of NO can be reached after 
desorption, which are extremely advantageous for the practical 
hydroxylamine synthesis. 
Subject of the invention is further a device for carrying out the process 
described above. This device is characterized by the fact that it contains 
an absorber in the bottom section of which the NO.sub.x - and possibly 
O.sub.2 -containing flue gas is inserted and from which the flue gas, 
freed from NO.sub.x, is removed overhead. 
a heat exchanger in the bottom section of which the FeII(NO)-EDTA 
complex-containing aqueous solution is inserted in order to increase its 
temperature to a value close to its boiling point, this solution being 
then removed overhead and inserted into a desorber, while the hot 
regenerated aqueous FeII-EDTA solution which was extracted from an 
electrolyzer and inserted into the head of the heat regenerator, is 
removed as bottom product after cooling down in the heat exchanger and is 
led via a cooler to the head of the absorber; 
a desorber in the top section of which the heated aqueous FeII(NO)-EDTA 
complex solution is inserted and in countercurrent contact with the 
hydrogen and water vapor introduced in the bottom section of the desorber 
is freed from the complexly absorbed NO at an increased temperature. 
Afterwards the NO, H.sub.2 and water vapor mixture that is created in the 
absorber is removed overhead and--after removal of the vapor in a 
condenser and returning the condensed water vapor into a reboiler in which 
the vapor is produced that is led into the desorber--is directly led to 
the catalytic hydroxylamine synthesis, while the aqueous FeII-EDTA 
solution which was removed from the desorber as bottom product is led to 
an electrolyzer, and 
an electrolyzer which consists of an anode section and a cathode section. 
The aqueous FeII-EDTA solution which is removed from the desorber is 
inserted into the cathode section to reduce the possibly contained 
FeIII-EDTA to FeII-EDTA while at the same time building hydrogen which is 
inserted in the bottom section of the desorber. The regenerated aqueous 
FeII-EDTA solution is extracted from the cathode section at increased 
temperature in view of step-by-step cooling and is led to the heat 
exchanger and finally via the cooler into the absorber, while the oxygen 
that is produced in the anode section is removed.

The invention is explained in more detail below with an exemplary form of 
execution, as diagrammatically shown in FIG. 1. This form of execution 
shows a typical example for a possible industrial use for a pilot plant 
industry. 
The invention is explained in more detail in the following examples with 
reference to the enclosed drawings with preferred forms of execution, 
without being limited to them. 
EXAMPLE 1 
In this example 10,000 Nm.sub.3 /h of flue gas with a 500 mg/m.sup.3 
NO.sub.x content are treated. The inserted flue gas has already been 
desulfurized according to one of the usual wet FGD processes. The 
temperature of the inserted flue gas is 50.degree. C. and it contains less 
than 100 mg/m.sup.3 SO.sub.2. 
The absorber (2) used consists of a filled column with a 1.4 m diameter. 
The flue gas current (1) is inserted in the bottom of the absorber (2) and 
is put in countercurrent contact with the reactive absorption solution 
which was inserted at the head of the absorber. This solution consists of 
an aqueous solution that contained 50 mMol/l FeII-EDTA complex 
(EDTA=ethylene diamine tetracids). The pH value of the solution is kept 
constant between 2.8 and 3.0. The solution is inserted into the head of 
the absorber (2) with a 14 m.sup.3 /h flow rate through a suitable flow 
distribution system. 
It is advantageous to cool down the absorption solution to 30.degree. C. 
before inserting it into the absorber (2). This causes the working 
temperature to decrease from 50.degree. C. to 35.degree. C., which causes 
the absorbing capacity of the solution to increase by a 3.3 factor (at 
50.degree. C. the equilibrium constant is 287 bar.sup.-1 and at 35.degree. 
C. the equilibrium constant is 929 bar.sup.-1). 
The NO.sub.x content of the flue gas decreases from 500 to 100 ppm while 
the gas passes through the absorber (2), i.e. the purified flue gas (3) 
that was removed from the absorber (2) now contains only 100 ppm NO.sub.x. 
The absorption solution that was inserted into the head of the absorber 
(2) is free of NO, while the enriched FeII(NO)-EDTA complex solution at 
the bottom of the absorber (2) contains a 12.5 mMol/l concentration, i.e. 
there is a 0.25 conversion degree. 
After the enriched extraction solution is removed from the absorber (2) it 
is inserted into the bottom of the heat exchanger (5) through the main 
(4). A liquid-liquid heat exchanger is preferred to bring the temperature 
of the enriched absorption solution to a value which is as close as 
possible to the boiling point of the solution. At the same time the hot 
absorption solution, free of NO, is cooled down to the lower temperature 
of the absorber (2) in this heat exchanger (5) before returning to the 
absorber (2). This operation can preferably be carried out in a classical 
shell and tube heat exchanger, in which the cool solution flows through 
the shell while the hot solution flows through the tubes. The temperature 
difference (.DELTA.T) between the hot and cool side of the heat exchanger 
(5) is usefully in the range of 5.degree. to 15.degree. C., preferably 
10.degree. C. 
The heat transmission is considerable in this case, since 14 t/h of an 
aqueous solution have to be heated from 35.degree. C. to 90.degree. C. 
This means that there is a 1060 kW heat transmission rate. To reach this 
heat transmission rate a heat exchanger surface of about 120 m.sup.2 is 
necessary. 
The cool, of NO depleted solution leaves the heat exchanger at a 
temperature of 45.degree. C. This liquid flow has to be cooled down 
further to 30.degree. C. This cooling occurs in a cooler 13, which is 
preferably a second heat exchanger. The cooler side of this unit is 
supplied with coolant almost at room temperature. In this case there is a 
240 kW heat transmission rate, the temperature difference .DELTA.T is 
smaller than the temperature difference used in the main heat exchanger 5, 
and it can lie in the 3.degree. to 8.degree. C. range, preferably at 
5.degree. C. The heat exchanger surface necessary in the cooler 13 is 
therefore only about 50 m.sup.2. 
The NO enriched absorption solution, heated to 90.degree. C. in the heat 
exchanger (5), is inserted through the main (6) into the head of a 
desorber (7). In this desorber (7) the FeII-EDTA complex is regenerated 
and the complexly absorbed NO is released in concentrated form. The 
desorber (7) also preferably consists of a filled column. The diameter of 
this column can be a lot smaller than that of the absorber (2) and it can, 
for instance, be 0.3 m. 
The NO enriched absorption solution, which has been heated while passing 
through the liquid-liquid heat exchanger (5) at, for instance, 90.degree. 
C., is led into the desorber (7) in countercurrent to the hydrogen which 
is inserted through the main (10) in the bottom section of the desorber 
(7) with a flow rate of, for instance, 7.2 Nm.sup.3 /h. This hydrogen is 
produced in an electrolyzer (9). Furthermore, water vapor is inserted 
through the main (20) into the bottom section of the desorber (7). The 
water vapor is produced in a reboiler (19). The volumetric flow rate of 
the water vapor has to be guided or controlled with care, and in this case 
it cannot be lower than 195 Nm.sup.3 /h, corresponding to 160 kg/h. 
In this way a gaseous mixture is produced at the head of the column (7), 
which has a temperature of close to 100.degree. C. The NO content of this 
gaseous mixture is 1.9 percent by volume, the H.sub.2 content is 3.5 
percent by volume and the rest is water vapor. This gaseous mixture is 
removed from the head of the desorber (7) through the main (15) and led 
through a condenser (16) in which the total amount of water vapor 
condenses, which is led through the main (18) into the reboiler (19). The 
gaseous mixture that is removed from the condenser (16) through the main 
(17) at a flow rate of 11.1 Nm.sup.3 /h consists of 35 percent by volume 
of NO and 65 percent by volume of H.sub.2. This gaseous mixture can be 
used directly for the commercial production of hydroxylamine by catalytic 
reduction of nitrogen oxide with hydrogen according to the BASF process. 
Because most of the industrial flue gases contain of 3 to 5 percent by 
volume of oxygen, an undesirable oxidation from FeII-EDTA to FeIII-EDTA 
occurs when the FeII-EDTA complex comes in contact with the flue gas which 
contains this oxygen in the absorber (2). Therefore it is imperative that 
this oxygenated complex be reduced to FeII-EDTA complex before it is 
inserted in the absorber (2). This reduction preferably occurs by 
electrolytic cathodic reduction. 
The aqueous absorption solution, free of NO, which is removed from the 
bottom of the desorber (7) is therefore properly inserted through the main 
(8) into the cathode section of an electrolytic cell (electrolyzer (9)). 
It is advantageous to carry out the electrolysis at a higher temperature, 
because that makes it possible to keep the tension of the electrolytic 
cell lower. For this reason the electrolytic cell, i.e. the electrolyzer 
(9), is arranged between the bottom of the desorber (7) and the hot side 
of the heat exchanger (5). 
The oxidation rate of the FeII-EDTA complex in the absorber (2) can be 
estimated as follows: laboratory tests show that the oxidation rate is 
about 10%/h. In this example this corresponds to an FeII-EDTA production 
rate of 70 Mol/h. Therefore a current of about 2 kA is needed for the 
electrolytic reduction of this liquid flow in the electrolyzer (9). 
The hydrogen that is needed for the preparation of the hydroxylamine 
synthesis gaseous mixture in the desorber (7) is also produced in the 
electrolyzer (9). Since the NO removal rate from the aqueous absorption 
solution in the desorber (7) was 175 Mol/h (3.9 Nm.sup.3 /h) in this 
example, this means that for the preparation of a 65 percent by volume 
H.sub.2 /35 percent by volume of NO mixture 323 Mol/h (7.2 Nm.sup.3 /h) 
hydrogen have to be produced. The current which is necessary for this 
hydrogen production (at a current efficiency of 90%) is 20 kA. 
The total capacity of the electrolytic cell (9) is thus 22 kA. If one 
assumes that the current density is 2 kA/m.sup.2, it follows that the 
electrode surface in the electrolyzer (9) has to be 11 m.sup.2. The 
electrolyzer used has to consist of an anode section and a cathode 
section, and it has to contain a membrane of a diaphragm made out of a 
suitable material as a separative element. 
In the anode section of the electrolyzer (9) a total amount of 4.1 Nm.sup.3 
/h of oxygen is produced out of the hydroxonium ions. This hydrogen is 
removed from the anode section of the electrolyzer. 
The energy consumption of an electrolytic cell is about 66 kW, resulting 
from a 22 kA current and a 3 V cell tension. 
EXAMPLE 2 
A 300 ml washing bottle contains 100 ml of a solution which contains a 60 
mMol/l concentration of FeII-EDTA. The initial pH value of the solution is 
2.54, the temperature is 21.degree. C. A flow of gas of 47 l/h is led 
through the solution. The gas consists of nitrogen with 170 ppm NO.sub.2, 
60 ppm NO and 3 percent by volume of oxygen. The total amount of NO.sub.x 
of the gases that leave the washing bottle is continuously monitored. 
After a very short period of time the NO.sub.x concentration at the 
exhaust decreases to a value of close to 0. The flow of gas is maintained 
for 30 minutes, the NO.sub.x content of the escaping gas remains at a 
value of close to 0. Then the flow of gas is stopped. 
The washing bottle is then placed in an oil-bath of 100.degree. C. and 47 
l/h of nitrogen (free of nitrogen oxides) is led through the solution 
inside the washing bottle. The escaping gas shows a very high NO peak with 
a maximum concentration of 1200 ppm. No NO.sub.2 could be detected. The 
integration of the established peak shows that the total amount of removed 
NO corresponded to the total amount of absorbed NO+NO.sub.2. The pH value 
of the solution increased from 2.54 to 2.76 during the test. The course of 
the NO and NO.sub.2 concentrations in the exhaust gas during this test has 
been diagrammatically depicted in FIG. 2 of the enclosed drawings. 
EXAMPLE 3 
A 60 mMol/l FeII-EDTA-containing solution was inserted into the head of a 
laboratory sieve plate column with a height of 50 cm and an inside 
diameter of 3 cm. In countercurrent to the solution a gas, flowing from 
bottom to top, was inserted with a constant rate of 60 l/h. The gas 
consisted of nitrogen with 3 percent by volume of oxygen and 1000 ppm 
nitrogen oxides. The NO.sub.2 content was 300 ppm, the NO content was 700 
ppm. 
The flow rate of the solution was kept at 1 l/h. The gas that escaped from 
the column was continuously monitored for its NO.sub.x content. The 
escaping gas was free of NO.sub.x. All nitrogen oxides had been absorbed. 
Determining the reducible nitrogen content of the solution showed a 2.3 
mMol/l concentration. 
The reduction of the liquid flow to 500 ml/l did not lead to a change in 
the NO.sub.x content of the escaping gas, which remained at a value of 
close to 0. The reducible nitrogen content of the solution increased to 
4.0 mMol/l.