Method of separating electrochemically produced dinitrogen pento

Details Method of separating electrochemically produced dinitrogen pento Method of separating electrochemically produced dinitrogen pento

1991-08-27

1992-07-07

Niebling, John

Chemistry: electrical and wave energy

Processes and products

Electrostatic field or electrical discharge

204103, 423400, C25B 100

Patent

active

051280019

DESCRIPTION:

BRIEF SUMMARY
This invention relates to a method of separating dinitrogen pentoxide (N.sub.2 O.sub.5) from its solution in nitric acid.
It is well known that dinitrogen pentoxide (N.sub.2 O.sub.5) can be prepared at relatively low cost by the electrochemical oxidation of dinitrogen tetroxide (N.sub.2 O.sub.4) dissolved in concentrated nitric acid. Typically, the solution of N.sub.2 O.sub.4 in nitric acid is situated in the anode compartment of an electrochemical cell with a second solution of N.sub.2 O.sub.4 in nitric acid present in the cathode compartment. An acid-resistance ion-exchange membrane separates the two compartments. When current is passed through the cell, N.sub.2 O.sub.5 is formed in the anode compartment and water in the cathode compartment.
It is also known from published USSR Inventor's Certificate SU-1089047A that solid N.sub.2 O.sub.5 can be separated from strong, 35-40 wt % solutions of N.sub.2 O.sub.5 in nitric acid produced electrochemically, by the subsequent addition to the solution of moderate amounts of liquid N.sub.2 O.sub.4 at a temperature of -5.degree. C. to 0.degree. C. A 1:1 by weight solvate of N.sub.2 O.sub.5 with nitric acid precipitates from the solution, from which pure N.sub.2 O.sub.5 can be recovered by low pressure sublimation.
One of the disadvantages of this method of N.sub.2 O.sub.5 separation is that it requires a starting solution of N.sub.2 O.sub.5 in nitric acid which is inefficient and costly to produce electrochemically. SU-1089047A teaches that at the temperature (typically 10.degree. C. to 15.degree. C.) of electrochemical preparation of the 35-40 wt % N.sub.2 O.sub.5 starting solution, the total (N.sub.2 O.sub.4 +N.sub.2 O.sub.5) concentration in the solution must not exceed 38-40 wt % in order to suppress the unwanted formation of precipitate in the electrochemical cell itself. By simply subtraction, it can be seen that the solution from which the N.sub.2 O.sub.5 is to be separated must contain less than 5 wt % N.sub.2 O.sub.4. However, in practice the electrical efficiency of electrochemical N.sub.2 O.sub.5 production decreases sharply with increasing N.sub.2 O.sub.5 concentration and decreasing N.sub.2 O.sub.4 concentration (especially at N.sub.2 O.sub.4 concentrations at or below 5 wt %) in the anolyte. This is due in part to leakage of N.sub.2 O.sub.4 and water through the cell membrane from the catholyte to the anolyte and also in part to the reduced electrical conductivity of the anolyte at low N.sub.2 O.sub.4 concentrations which decreases electrical efficiency. Such leakage can (in the case of N.sub.2 O.sub.4 leakage) be suppressed by employing a low N.sub.2 O.sub.4 concentration in the catholyte, but this reduces catholyte conductivity. Water leakage can be reduced but at the expense of removing it at frequent intervals from the catholyte in order to keep its concentration low.
It has now been discovered that provided the anolyte is maintained at a temperature of at least 10.degree. C., the anolyte can contain a much higher dissolved nitrogen oxide content than has hitherto been realised so enabling more efficient electrochemical methods of N.sub.2 O.sub.5 generation to be utilised by virtue of the anolytes' higher N.sub.2 O.sub.4 content.
Accordingly, the present invention provides a method of separating N.sub.2 O.sub.5 from its solution in nitric acid which comprises the steps of: nitric acid, a nitric acid solution at a temperature of at least 10.degree. C. containing at least 45 wt % of dissolved (N.sub.2 O.sub.4 +N.sub.2 O.sub.5) and having a dissolved N.sub.2 O.sub.5 :nitric acid ratio by weight of at least 1:3. O.sub.5 solvate precipitates from solution.
The method preferably further comprises the subsequent step of preferably by filtration.
In order to enhance the yield of solid precipitated, the acid solution prepared in step (a) preferably contains at least 50 wt % of dissolved (N.sub.2 O.sub.4 +N.sub.2 O.sub.5). A maximum dissolved (N.sub.2 O.sub.4 +N.sub.2 O.sub.5) concentration of up to about 60 wt % in step (a) is achievable without the formati

REFERENCES:
patent: 4432902 (1984-02-01), McGuire et al.
patent: 4443308 (1984-04-01), Coon et al.
patent: 4525252 (1985-06-01), McGuire et al.

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