Liquid purification or separation – Processes – Chemical treatment
Reexamination Certificate
2001-01-22
2002-05-07
Hoey, Betsey Morrison (Department: 1724)
Liquid purification or separation
Processes
Chemical treatment
C210S903000
Reexamination Certificate
active
06383400
ABSTRACT:
The present invention relates to a process to reduce the nitrate and/or nitric acid concentration of an aqueous solution—referred to as a denitration process—by means of a reaction with formic acid or formol.
More specifically, the present invention relates to a process used to reduce the nitrate and/or nitric acid concentration or eliminate nitrates and/or nitric acid from aqueous effluents produced by various processes implemented equally well in the nuclear industry as in other industries, by means of a catalysed reaction with formic acid or formol.
Therefore, the technical field of the invention may be defined generally as that of the treatment of aqueous solutions, particularly aqueous industrial effluents, with a view to reducing their nitrate and/or nitric acid concentration, or eliminating said substances completely.
Of the various existing denitration processes, the chemical destruction of nitrates and/or nitric acid by means of a reaction with formic acid is the most frequently used due to its efficiency, simplicity and profitability.
The reaction of formic acid and nitric acid is for example widely used in the field of the nuclear industry by the denitration of the nitric acid solutions used in the treatment of radioactive waste.
On this subject, it is possible to refer to the work “Denitration of radioactive liquid waste” edited by L. Cecille and S. Halaszovich—Radioactive Waste Management Series; Graham and Trotman Ltd; Commission of the European Communities.
The chemistry of the homogeneous phase interaction between nitric acid and formic acid has been studied in detail and denitration installations have been developed and installed.
It has been demonstrated that denitration with formic acid is a very complex chemical process, which depends on numerous factors such as acidity, temperature, reaction mixture composition, pressure, order in which the reagents are mixed, reaction vessel's shape, etc.
The fundamental chemical reactions which take place during denitration with formic acid are as follows depending on the concentration of each of the two essential reagents:
nitric acid concentration greater than 8M
2HNO
3
+HCOOH→2NO
2
+CO
2
+2H
2
O (1)
nitric acid concentration between 0.5 and 8 M:
2HNO
3
+2HCOOH→NO+NO
2
+2CO
2
+3H
2
O (2)
2HNO
3
+3HCOOH→2NO+3CO
2
+4H
2
O (3)
slight excess of HCOOH
2HNO
3
+4HCOOH→N
2
O+4CO
2
+5H
2
O (4)
significant excess of HCOOH
2HNO
3
+5HCOOH→N
2
+5CO
2
+6H
2
O (5)
It is important to note that the denitration operations may also be carried out using formol to replace formic acid.
In this case, the reaction mechanism using formic acid as an intermediate compound is specified below.
The reactions involved during the denitration of nitric acid with formol, as a function of the nitric acid concentrations are as follows:
for nitric acid concentrations greater than 8M:
4HNO
3
+HCHO→4NO
2
+CO
2
+3H
2
O (6)
for nitric acid concentrations between 0.5 and 8M:
4HNO
3
+2HCHO→2NO
2
+2NO+2CO
2
+4H
2
O (7)
4HNO
3
+3HCHO→4NO+3CO
2
+5H
2
O (8)
in the case of a moderate excess of formol:
4HNO
3
+4HCHO→2N
2
O+4CO
2
+6H
2
O (9)
in the case of large excesses of formol:
4HNO
3
+5HCHO→2N
2
+5CO
2
+7H
2
O (10)
Therefore, it is observed that, in the non-catalysed denitration reaction mechanism with formol, formol is first oxidised into formic acid by nitric acid and then formic acid is oxidised by nitric acid into carbon dioxide.
The homogeneous phase oxidation mechanism of HCOOH with HNO
3
was studied in the document by LONGSTAFF J. U. L. and SINGER K., J.Chem.Soc, 1954, p 2604-2617, on the basis of the overall kinetic analysis of the reaction.
It was demonstrated that formic acid does not react directly with nitric acid but reacts simultaneously with nitrous acid or with NO
+
which are present at equilibrium with nitric acid HNO
3
. According to this mechanism, the role of HNO
3
is to oxidise the intermediate products of the reaction. These secondary reactions result in the formation of HNO
2
, either directly, or by the series of equilibria described in the document by ABEL E., SCHMID H; WEISS J.; Z. Phys. Chem 1930, V. 147 [A], p. 69, according to the reaction:
HNO
3
+HNO
2
⇄N
2
O
4
+H
2
O⇄2NO
2
+H
2
O (11)
In other words, the denitration reaction is fundamentally autocatalytic in nature.
One of the main characteristics of this type of reaction is the existence of an induction period during which nitrous acid HNO
2
accumulates in the solution, up to a threshold concentration from which the reaction develops rapidly.
The two elements shown above, i.e. the induction period and the autocatalytic breakdown of nitric acid are the essential causes of the problems involved in this process, which require measures to ensure safety, as described in pages 51 to 57 of the work by L. CECILLE and S. HALASZOVICH already mentioned above.
The main measures taken to ensure the start of the reaction and enable it to be carried out uninterruptedly are the addition of sodium nitrite NaNO
2
, the maintenance of the solution at boiling point, and rigorous monitoring of the reagent flow rate. The essential aim is in fact to prevent the accumulation of formic acid in the reaction mixture as described in page 168 to 177 of the work by L. CECILLE and S. HALASZOVICH mentioned above.
Failure to apply any of these measures may result, under certain conditions, to the violent, practically uncontrollable, development of the process and may induce excess pressure in the denitration apparatus and the formation of explosive gaseous mixtures.
In addition, the presence, even at trace levels, of anti-nitrous agents such as hydrazine, urea or others in the solution may prevent the reaction from starting in a satisfactory manner and induce a hazardous accumulation of formic acid in the reaction mixture, which then induces the sudden and violent development of the process.
Therefore, there is a need for a process to reduce the nitric acid and/or nitrate concentration of an aqueous solution, by means of formic acid or formol, which offers, among other things, safety and improved simplicity compared to the processes of the prior art, which is governed by perfectly controlled kinetics and makes it possible to achieve excellent denitration yields—up to the complete elimination of nitrates and/or nitric acid—irrespective of the solutions treated.
In other words, there is a need for a process to reduce the nitric acid and/or nitrate concentration of an aqueous solution, by means of formic acid or formol which does not have the shortcomings, limitations, defects and disadvantages of the processes of the prior art, in particular the homogeneous denitration process, and which solves the problems involved with these processes, and in particular the problems involved with the homogeneous denitration process.
The purpose of the present invention is to provide a process which, among other things, meets these needs.
This purpose and others are achieved according to the invention by a process to reduce the nitric acid and/or nitrate concentration of an aqueous solution, by means of formic acid or formol, wherein said reaction is carried out in heterogeneous catalysis, said process comprising the following successive steps:
a) Placing of the aqueous solution to be treated in contact with a catalyst;
b) Heating of the reaction mixture obtained—from step a)—to a predetermined temperature, in the range from 60° C. to 100° C.;
c) Addition of formic acid or formol to the reaction mixture;
d) Maintaining the reaction mixture at said temperature for a sufficient time to reach the required final nitric acid and/or nitrate concentration.
In the entire disclosure of the present invention, formic acid is referred to most of the time, but, natural
Ananiev Alekseï
Brossard Philippe
Broudic Jean-Charles
Burns Doane , Swecker, Mathis LLP
Commissariat a l'Energie Atomique
Hoey Betsey Morrison
LandOfFree
Method for reducing nitrate and/or nitric acid concentration... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for reducing nitrate and/or nitric acid concentration..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for reducing nitrate and/or nitric acid concentration... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2845883