Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Preparing inorganic compound
Reexamination Certificate
1987-04-13
2001-03-13
Miller, Edward A. (Department: 3641)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Preparing inorganic compound
Reexamination Certificate
active
06200456
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to the large scale production of anhydrous nitric acid and nitric acid solutions of dinitrogen pentoxide and more particularly to an electrolytic method and apparatus for simultaneously synthesizing water-free nitric acid and solutions of dinitrogen pentoxide in anhydrous nitric acid.
Nitric acid has become a major industrial chemical, with diverse applications and large scale industrial use in the manufacture of fertilizers, organic chemicals, explosives and the like. Generally, for most industrial and other applications, aqueous nitric acid is produced at a concentration of 50-70 wt. % HNO
3
by a standard ammonia oxidation process. In this process, ammonia is oxidized with excess oxygen over a catalyst to form nitric oxide and water. The nitric oxide is then oxidized to nitrogen dioxide, which is absorbed in water to form nitric acid and additional nitric oxide. The nitric acid is then concentrated, but since HNO
3
forms an azeotrope with water at 68.8 wt. %, it cannot be separated from the water or concentrated beyond approximately 70 wt. % by simple distillation.
While the commonly available 70 wt. % HNO
3
is suitable for the production of ammonium nitrate fertilizer and many other inorganic chemicals, more highly concentrated or completely anhydrous (water-free) nitric acid is required for use in many organic nitrations. Mixtures of nitric and sulfuric acids are also commonly used for organic nitrations, to insure a low water concentration which is favorable for these reactions. The rocket-fuel and semiconductor industries employ red fuming nitric acid, which typically consists of 15 wt. % dinitrogen tetroxide (N
2
O
4
), 2 wt. % H
2
O, and 83 wt. % HNO
3
.
Highly concentrated nitric acids are widely employed in the explosives industry. The prior known Bachman process, used commercially in the U.S. for the production of cyclonite (1-3-5-trinitro-1,3,5,-triazine or commonly known as RDX) and HMX (1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane), involves a continuous nitration of hexamethylenetetramine by reaction with strong nitric acid, ammonium nitrate, and acetic anhydride. In England, RDX is manufactured by the Woolwich process in which hexamethylenetetramine is reacted with anhydrous nitric acid.
Anhydrous nitric acid, e.g., 98 to 100 wt. % HNO
3
, has been synthesized by distillation of a weaker aqueous solution of nitric acid with sulfuric acid, the latter serving as a dehydrating agent. Typically, 60 wt. % HNO
3
is mixed with 93 wt. % H
2
SO
4
in a packed tower which is provided with a steam heated reboiler. The nitric acid vapor is distilled and condensed, and the sulfuric acid and water leave the bottom as approximately 70 wt. % H
2
SO
4
. Water is then removed from the sulfuric acid in a sulfuric acid concentrator, and the 93 wt. % H
2
SO
4
is recycled in the process. An alternative extraction medium is a 72 wt. % solution of magnesium nitrate in water. In this process, which is used in conjunction with the U.S. manufacture of RDX and HMX, the nitrate solution typically leaves the distillation column at approximately 68 wt. %, and is reconcentrated by flashing to a steam heated vacuum drum.
These methods for producing anhydrous HNO
3
require the recycling of large quantities of sulfuric acid or magnesium nitrate. This inherently presents the potential of major catastrophic accidents, as well as the production of large quantities of waste heat and energy. Thus from a cost standpoint, these processes are inherently deficient.
When water is further removed from anhydrous nitric acid, the anhydride of nitric acid, dinitrogen pentoxide (N
2
O
5
), is formed as represented by the equation:
2HNO
3
→N
2
O
5
+H
2
O (1)
Thus solutions of N
2
O
5
in HNO
3
can be prepared which can be thought of as greater than 100% HNO
3
, and which have unique properties for some chemical syntheses.
A number of organic nitration and nitrolysis reactions have been found to proceed faster, more efficiently, and in highest yield by the use of solutions of N
2
O
5
in HNO
3
. The high explosive HMX can be prepared by the reaction of a series of 1,3,5,7-tetra-azacyclooctanes with N
2
O
5
, formed in the reaction mixture in-situ by dehydration of the nitric acid. The dehydration is accomplished by reagents such as phosphorus pentoxide (P
2
O
5
), polyphosphoric acid, trifluoroacetic acid anhydride, or sulfur trioxide (SO
3
). Pure N
2
O
5
can also be synthesized by oxidation of N
2
O
4
with ozone. In a carbon tetrachloride medium, N
2
O
5
converts aliphatic secondary amines into nitramines in excellent yield. These reactions have never achieved large-scale use because of the high cost of producing N
2
O
5
, either by chemical dehydration or by ozonolysis. Chemical dehydration requires expensive recycling processes, and ozonolysis is electrically inefficient.
A third general approach to the synthesis of nitric-acid solutions of N
2
O
5
is direct electrochemical oxidation of a suitable precursor compound.
The basic reaction, the oxidation of N
2
O
4
in HNO
3
at a platinum anode in an electrolysis cell divided by a diaphragm, was first described in German Patent No. 231,546,
J. Zawadski and Z. Bankowski,
Roznicki Chemii.
22 (1948), 233, extended this work, employing the same reaction and essentially the same type of apparatus, a laboratory size, stirred electrolysis cell. The anode comprised a platinum sheet, the cathode was made of sheet lead, and the diaphragm employed was a porous ceramic. In this method also, the cell voltage was controlled, and N
2
O
5
was produced with a current efficiency of 35% and a specific energy of 5 kWH/kg.
The electrolysis reaction that produces N
2
O
5
can be written as follows:
N
2
O
4
+2HNO
3
→2N
2
O
5
+2H++2e
−
(2)
If there is water in the nitric acid at the beginning of the electrolysis, it is be consumed by the reaction:
H
2
O+N
2
O
5
→2HNO
3
(3)
At a certain point in time during the electrolysis, when all of the water has been consumed, the anolyte consists solely of HNO
3
(anhydrous) and unreacted N
2
O
4
. From that point on, excess N
2
O
5
is generated. Eventually all of the N
2
O
4
is consumed by electrolysis, and the anolyte will then consist solely of HNO
3
and N
2
O
5
. If desired, the N
2
O
5
/HNO
3
solution can be reacted with an aqueous nitric acid solution in the correct stoichiometric amount to yield pure, anhydrous HNO
3
.
In addition to Reaction 2, N
2
O
5
can also be formed by the electrolytic oxidation of HNO
3
according to the reaction:
2HNO
3
→N
2
O
5
+2H++(½)O
2
+2e
−
(4)
The oxidation of HNO
3
proceeds at a higher anode potential than Reaction 1 and may proceed concurrently with Reaction 1, if the anode potential is in a region where both can occur. Although Reaction 4 produces N
2
O
5
, it also produces oxygen as a byproduct and the current efficiency for the production of N
2
O
5
is lower.
The current efficiency for N
2
O
5
production and yield based on the use of N
2
O
4
could be substantially improved compared to that obtained by Zawadski and Bankowski by performing the electrolysis in a controlled-potential electrolysis cell, and controlling the anode potential to minimize the extent of the oxidation of HNO
3
(Reaction 4). See U.S. Pat. Nos. 4,432,902, 4,443,308 and 4,525,252. With the apparatus and methodology described in the aforementioned U.S. Patents, using a laboratory-size divided cell having a porous-glass membrane and a platinum-wire anode and cathode, a current efficiency of approximately 65% and a chemical yield of about 50% were achieved.
That anhydrous HNO
3
can be produced by the electrolytic reactions described above, is also disclosed by USSR Patents Nos. 1,059,023A and 1,089,172A, but there is no discussion of the preparation of N
2
O
5
/HNO
3
solutions per se in these patents. In the work described in the '023A patent, the oxidation of aqueous HNO
3
was carried out according to
Harrar Jackson E.
McGuire Raymond R.
Quong Roland
Rigdon Lester P.
Carnahan L. E.
Gaither Roger S.
Gottlieb Paul A.
Miller Edward A.
The United States of America as represented by the Department of
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