WO2018015626A1

WO2018015626A1 - Electrolyte system for the synthesis of sodium perchlorate, comprising an anode having an outer surface made of boron-doped diamond

Info

Publication number: WO2018015626A1
Application number: PCT/FR2017/051761
Authority: WO
Inventors: Olivier DABARD; Angeline AUMELAS; Guillaume GOTTI; Karine Groenen Serrano; André SAVALL
Original assignee: Airbus Safran Launchers Sas; Centre National D'etudes Spatiales Cnes; Centre National De La Recherche Scientifique; Universite Paul Sabatier - Toulouse Iii
Priority date: 2016-07-22
Filing date: 2017-06-29
Publication date: 2018-01-25
Also published as: FR3054243A1

Abstract

The invention relates to an electrolyte system (1) for the synthesis of sodium perchlorate using electrochemical oxidation of sodium chloride, said system (1) at least comprising: - an electrolyte (10) containing sodium chloride at a minimum concentration of 0.1 mol/L; - an anode (15) in the electrolyte (10), the anode (15) having an outer surface made of boron-doped diamond; - a cathode (17) in the electrolyte (10); and - an electrical generator (20) connected to the anode (15) and the cathode (17).

Description

Electrolytic system for synthesis of sodium perchlorate with boron-doped diamond outer surface anode

Background of the invention

The invention relates in particular to an electrolytic system for the synthesis of sodium perchlorate by electrochemical oxidation of sodium chloride.

Ammonium perchlorate (NH ₄ CIO ₄ ) is used in the space industry as a constituent of propellant shipments and is typically synthesized from sodium perchlorate (NaClO ₄ ).

Sodium perchlorate can, for its part, be obtained from sodium chloride (NaCl) by carrying out two separate operations of electrochemical oxidation. In this type of process, the first operation consists of the electrochemical oxidation of sodium chloride to sodium chlorate (NaClOa). This first operation can implement an anode having an external surface made of graphite, magnetite or ruthenium dioxide. The second operation consists of the electrochemical oxidation of sodium chlorate to sodium perchlorate. This second operation can implement an anode having an outer surface of lead dioxide. Between these two operations, the electrolyte obtained in the first operation is treated to separate the sodium chlorate formed from the residual sodium chloride.

It would be desirable to have new electrolytic systems which would make it possible to synthesize in a single operation, without having to interrupt the process, sodium perchlorate by electrochemical oxidation of sodium chloride.

The invention aims specifically to meet the aforementioned need.

Obiet and summary of the invention

For this purpose, the invention proposes, in a first aspect, an electrolytic system for the synthesis of sodium perchlorate by electrochemical oxidation of sodium chloride, said system comprising at least:

an electrolyte comprising sodium chloride at a concentration greater than or equal to 0.1 mol / L,

an anode present in the electrolyte, the anode having a boron-doped diamond outer surface,

a cathode present in the electrolyte, and

an electric generator connected to the anode and to the cathode.

The electrolytic system according to the invention implements an anode having an electrically active outer surface formed of a particular material and thus advantageously makes it possible to obtain, in a single operation, sodium perchlorate by electrochemical oxidation of sodium chloride. The electrolytic system according to the invention thus makes it possible to avoid having to interrupt the process or modify the operating conditions to form sodium perchlorate from sodium chloride. The invention thus makes it possible to significantly simplify the production of sodium perchlorate by electrochemical oxidation of sodium chloride.

In an exemplary embodiment, the anode may comprise an electrically conductive substrate coated with a boron-doped diamond coating.

Such a configuration is advantageous insofar as it implements an anode having a limited amount of boron doped diamond and therefore reduced cost.

Alternatively, an entirely boron-doped diamond anode may be used.

In an exemplary embodiment, the cathode may have an outer surface of a metal alloy having a mass content of chromium of between 5% and 25%.

In certain techniques of the prior art, it is known, in particular in order to limit the parasitic reactions, to add to the bath an additive of the dichromate or alkali metal fluoride type. When this additive is present, it is necessary to carry out, after the electrochemical oxidation, an operation for separating the sodium perchlorate formed from the additive dichromate or fluoride. This separation operation lengthens and complicates the process for obtaining sodium perchlorate. The combination of a cathode whose external surface is formed of a metal alloy with a chromium content of between 5% and 25% and of an anode having a boron-doped diamond outer surface advantageously makes it possible to overcome the implementation of additive dichromate or alkali metal fluoride in the electrolyte while limiting the amount of impurities generated during electrosynthesis. The implementation of such a cathode makes it therefore possible to make the operation of separating sodium perchlorate from the dichromate or fluoride additive superfluous and thus to further simplify the process for obtaining sodium perchlorate by oxidation. electrochemical sodium chloride. Thus, in an exemplary embodiment, the electrolyte may be free of fluoride or alkali metal dichromate.

In an exemplary embodiment, the metal alloy of the cathode may have a chromium mass content of between 10% and 25%, for example between 10% and 20%.

In an exemplary embodiment, the metal alloy of the cathode may be a steel, a nickel alloy or a zirconium alloy.

It would be possible alternatively to use a cathode having an external surface made of zirconium or zirconium alloy (this zirconium alloy does not necessarily comprise chromium at a mass content of between 5% and 25%). In particular, a cathode made entirely of zirconium or zirconium alloy could be used.

As for the cathode having a metal alloy outer surface comprising chromium, the use of a cathode having an external surface made of zirconium or zirconium alloy makes it possible to dispense with the use of dichromate or fluoride additive of alkali metal in the electrolyte while limiting the amount of impurities generated during electrosynthesis.

In one exemplary embodiment, the concentration of sodium chloride in the electrolyte may be greater than or equal to 0.5 mol / L. The concentration of sodium chloride in the electrolyte may for example be between 0.5 mol / L and Cs where Cs denotes the saturation concentration of sodium chloride in the electrolyte. The concentration of sodium chloride in the electrolyte may for example be between 0.5 mol / L and 5 mol / L. In an exemplary embodiment, the system may include a temperature control device configured to maintain the electrolyte at a temperature below a predetermined temperature.

Such a characteristic is advantageous because it participates in the electrosynthesis of sodium perchlorate at a moderate temperature, which advantageously makes it possible to improve the service life of the electrodes of the electrolytic system.

In an exemplary embodiment, the system may comprise a first chamber in which the electrolyte, the anode and the cathode are present and a second chamber, distinct from the first chamber, in which the electrolyte is present, the system comprising in addition to an electrolyte circulation circuit configured to flow the electrolyte between the first and second chambers.

According to a second aspect, the invention provides a method for manufacturing sodium perchlorate comprising a step of electrochemical oxidation of sodium chloride to sodium perchlorate by implementing a system as described above.

In an exemplary embodiment, the current density imposed during the electrochemical oxidation can be between 1000 A / m ² and 20000 A / m ² , for example between 1000 A / m ² and 5000 A / m ² .

The implementation of such current density values is advantageous in order to accelerate the formation of sodium perchlorate.

In an exemplary embodiment, the electrolyte can be maintained at a temperature of less than or equal to 40 ° C, or even less than or equal to 30 ° C, during the electrochemical oxidation.

As mentioned above, maintaining the electrolyte at a moderate temperature advantageously makes it possible to improve the life of the electrodes of the electrolytic system.

Brief description of the drawings

Other features and advantages of the invention will emerge from the following description, given in a non-limiting manner, with reference to the appended drawings, in which:

FIG. 1 schematically represents an example of a system according to the invention, FIGS. 2 and 3 schematically represent anode structures that can be used in the system of FIG. 1,

FIGS. 4 and 5 schematically represent cathode structures that can be used in the system of FIG. 1, and

FIG. 6 is a graph obtained experimentally showing the evolution of the concentrations of the species contained in the electrolyte during electrosynthesis of sodium perchiorate in a single operation conducted with two electrolytic systems according to the invention.

Detailed description of embodiments

Figure 1 shows schematically an example of electrolytic system 1 according to the invention. This system 1 is configured to manufacture sodium perchiorate by electrochemical oxidation of sodium chloride.

System 1 comprises a first chamber 3 in which a liquid electrolyte is present. Before initiation of the electrochemical oxidation, the electrolyte 10 comprises sodium chloride at a concentration greater than or equal to 0.1 mol / l. The concentration of sodium chloride in the electrolyte 10, before initiation of the electrochemical oxidation, may be greater than or equal to 0.5 mol / L, for example 1 mol / L, or even greater than or equal to 2 mol / L. The concentration of sodium chloride in the electrolyte 10, before initiation of the electrochemical oxidation, may for example be between one of the lower limits previously listed and the saturation concentration of sodium chloride in the electrolyte. The concentration of sodium chloride in the electrolyte 10, before initiation of the electrochemical oxidation, may for example be between 0.1 mol / L and 5 mol / L, or even between 0.1 mol / L and 3 mol / L.

The electrolyte 10 may be an aqueous electrolyte. The electrolyte 10 is, moreover, free of fluoride or alkali metal dichromate. In particular, the electrolyte 10 is devoid of the following compounds: sodium fluoride (NaF), potassium fluoride (KF), sodium dichromate (Na ₂ Cr ₂ O ₇ ) and potassium dichromate (K ₂ Cr ₂ O ₇ ) .

The system 1 further comprises an anode 15 and a cathode

17 which are each present in the electrolyte 10 in the first Chamber 3. The electrochemical oxidation of sodium chloride to obtain sodium perchlorate is intended to be carried out in the first chamber 3.

The anode 15 has an outer surface, corresponding to its active surface, diamond doped with boron. The boron-doped diamond may typically have a boron mass content of between 0.005% and 0.55%, the remainder being diamond. This external surface is present in the electrolyte 10. FIG. 2 illustrates the structure of an exemplary anode 15 that can be used in the system 1 of FIG. 1. In this example, the anode 15 comprises a conductive substrate of the Electricity 28 covered with a diamond coating doped with boron. The coating 26 defines the outer surface Si of the anode 15. The substrate 28 is formed of a material different from that constituting the coating 26. The material forming the substrate 28 may comprise titanium, zirconium, hafnium, vanadium , niobium, tantalum, molybdenum, palladium or an alloy thereof. The material forming the substrate 28 may for example be chosen from: titanium, zirconium, hafnium, vanadium, niobium, tantalum, molybdenum, palladium and their alloys. The material forming the substrate 28 may still be graphite. The thickness ei of the coating 26 may for example be greater than or equal to 0.5 μm, and for example be between 1 μm and 5 μm. Alternatively, an anode 115 made entirely of boron-doped diamond may be used. This anode 115 defines an outer surface S2.

The cathode 17 has, for its part, an external surface, corresponding to its active surface which may, for example, be formed of a metal alloy having a mass content of chromium of between 5% and 25%. The outer surface of the cathode may alternatively be formed of pure zirconium with unavoidable impurities or a zirconium alloy. This external surface is present in the electrolyte 10.

In particular, the cathode may have an outer surface formed of a metal alloy having a chromium mass content of between 10% and 25%, for example between 10% and 20%. In this case, the metal alloy of the cathode 17 may be a stainless steel, for example 316 L stainless steel, a nickel alloy or a zirconium alloy. FIG. 4 illustrates the structure of an exemplary cathode 17 that can be used in the system 1 of FIG. 1. In this example, the cathode 17 is entirely made of a metal alloy with a mass content in chromium of between 5% and 25% (massive metal alloy cathode with a mass content in chromium of between 5% and 25%). This cathode 17 defines an outer surface S ₃ . Alternatively, as shown in FIG. 5, it would be possible to use a cathode 117 comprising an electrically conductive substrate 119 coated with a metal alloy coating 118 having a mass content in chromium of between 5% and 25%. The coating 118 defines the outer surface S ₄ of the cathode 117. The substrate 119 is formed of a material different from that constituting the coating 118. Similar cathode structures are usable when using a cathode having an outer surface of zirconium or zirconium alloy.

The system 1 illustrated in FIG. 1 furthermore comprises an electric generator 20 connected to the anode 15 and to the cathode 17. The anode 15 is connected via the conductor 16 to the positive terminal 18 of the generator 20. cathode 17 is connected via the conductor 19 to the negative terminal 21 of the generator 20.

The system 1 further comprises a second chamber 5, separate from the first chamber 3, wherein the liquid electrolyte is present. The presence of this second chamber 5 is optional. The second chamber 5 is in communication with the first chamber 3. The electrolyte 10 is intended to flow between the first 3 and second 5 chambers during the electrochemical oxidation. In order to allow this circulation, the system 1 comprises a circulation circuit of the electrolyte 7 which comprises in the illustrated example:

a first channel 12 connecting an outlet 3s of the first chamber 3 to an inlet 5e of the second chamber 5,

a second channel 8 connecting an outlet 5s of the second chamber 5 to an inlet of the pump 9, the second channel 8 can, as illustrated, be provided with a valve 13, and

a third channel 11 connecting an output of the pump 9 to an inlet 3e of the first chamber 3.

When the pump 9 is actuated, the electrolyte 10 flows between the first 3 and second 5 chambers according to the path shown by the arrows F1. During its circulation, the electrolyte 10 passes through the first channel 12, the second channel 8 and the third channel 11. The system 1 further comprises a temperature control device 23 which is configured to maintain the electrolyte 10 at a temperature below a predetermined temperature. The predetermined temperature may for example be 40 ° C or 30 ° C. In the illustrated example, the thermostat 23, in which a temperature control fluid circulates (arrows F2), is positioned at the second chamber 5 which is maintained at a substantially constant temperature. It could alternatively position the thermostat 23 at the first chamber 3. The thermostat 23 could still be positioned at at least one of the channels 8, 11 and 12, and preferably at the first channel 12.

The system 1 may further include a condenser 25 for condensing at least a portion of the vapors produced during the electrochemical oxidation. The arrow F4 shown in Figure 1 at the outlet of the refrigerant 25 corresponds to a flow of gaseous hydrogen produced during the electrosynthesis. A cooling fluid circulates in the refrigerant 25 to achieve this condensation (arrows F3). The cooling fluid is for example before heat exchange at a temperature of -5 ° C.

The electrosynthesis method of sodium perchlorate from sodium chloride will now be described in connection with the system of FIG.

The circulation of the electrolyte 10 between the first 3 and second 5 chambers is first initiated by actuation of the pump 9. A potential difference is then imposed between the anode 15 and the cathode 17 using the generator wherein electrochemical oxidation of sodium chloride to sodium perchlorate in the first chamber 3 without having to separate the intermediate chlorate that is formed during the oxidation process. Electrochemically, sodium perchlorate is synthesized according to the overall reaction:

NaCl + 4H ₂ 0 * NaClO ₄ + 4H ₂ (1)

The main steps in the synthesis of sodium perchlorate are detailed below. At the anode:

2 Cl Cl ₂ (aq) + 2 e (E = 1.396 V / ESH at pH = 0) (2)

In solution, there is hydrolysis of the chlorine in the vicinity of the anode: Cl ₂ (aq) + H ₂ 0 HOCl + Cl + H ⁺ (K ° = 2 x W ⁴ ) (3)

The hypochlorous acid dissociates within the solution:

HOCl + H ₂ 0 OC 1 + H ₃ O ⁺ (K _A = 2 x 10 ⁸ ) (4)

Depending on the temperature, the chlorate may be formed (i) electrochemically (for temperatures up to 40 ° C) or (ii) chemically for higher temperatures.

(i) The chlorate can be formed by electro-oxidation of hypochlorite ions or hypochlorous acid for temperatures of less than or equal to 40 ° C: 6 OC1 + 3H ₂ 0 * ~ 2 CIO, + 4 Cl + 6H ⁺ + 3/2 0 ₂ + 6 th (5)

6 HOCl + 3 H ₂ 0 * ~ 2 CIO, + 4 Cl + 12 H ⁺ + 3/2 0 ₂ + 6 e

(5a)

(ii) For higher temperatures, the chlorate is formed by chemical reaction between hypochlorous acid and hypochlorite ion: 2 HOCl + OCl ^ == ^ CIO, + 2 Cl ^' + 2 H ⁺ (6)

Once formed, the chlorate ion is then electro-oxidized to perchlorate. The sequence of reactions described above takes place during the same electro-oxidation operation without having to interrupt the process in order to carry out an operation for separating the different species in solution or for modifying the operating conditions (nature of the electrodes, temperature, etc.). .).

At the cathode, there is mainly reduction of the water with release of hydrogen:

2 H ₂ O + 2 e- * ~ H ₂ + 2 OH (H 0 = - 0828 V / ESH at pH = 14) During electrosynthesis, the electrolyte 10 can flow continuously. It is possible to impose a flow rate of the electrolyte in the circuit 7 of between 250 liters / hour and 350 liters / hour, for example. The current density imposed during the electrochemical oxidation can be between lkA / m ² and 20 kA / m ² , for example between 1 kA / m ² and 5 kA / m ² .

In a variant not illustrated, sodium perchlorate is obtained using a system without the second chamber 5. In the latter case, there is not necessarily movement of the electrolyte in the system.

Example

In this example, two electrolytic systems according to the invention were evaluated. The experimental setups that were used are of the type shown in Figure 1.

Each of the systems tested included a Nb / DDB anode having a niobium substrate coated with a boron-doped diamond coating. The first system tested included a zirconium cathode and the second system tested included a stainless steel cathode (316 L steel with a chromium content of 18%).

The experimental conditions imposed during the tests that have been carried out are reported below:

- anode in Nb / DDB,

- cathode made of zirconium or 316 L stainless steel,

- impressed current between the anode and the cathode 5A (current density of about 7 kA / m ^2),

- volume of the first chamber equal to 70 mL,

initial concentration of chloride ions in the electrolyte of about 2 mol / L, and

- duration of the test of 8 hours.

During these two tests, the temperature of the electrolyte was kept below 30 ° C. The temperature of the electrolyte noted at the end of the test is 28.5 ° C and 29.4 ° C when using cathode 316 L and zirconium respectively. In both tests, no fluoride additives or alkali metal dichromates were added to the electrolyte.

The evolution of the concentrations of the different species present in the electrolyte is given in FIG. 6. In this figure:

curves 1 and 2 represent the chloride ion concentration respectively with the 316 L steel cathode and with the zirconium cathode,

curves 3 and 4 represent the concentration of hypochlorite ions respectively with the 316 L steel cathode and with the zirconium cathode,

curves 5 and 6 represent the concentration of chlorate ions respectively with the 316 L steel cathode and with the zirconium cathode, and

curves 7 and 8 represent the concentration of perchlorate ions respectively with the 316 L steel cathode and with the zirconium cathode.

It can be seen that the evolution of the concentrations is very close with the two systems according to the invention.

Table 1 below gives the yields obtained for the two electrosyntheses that were carried out:

Table 1

The yields obtained for the two electrolytic systems tested are very close and have satisfactory values. The amounts of material of the species present in the electrolyte were determined by ion chromatography during the tests carried out.

The current yield (Rdtcurrent) of the reaction is calculated as the ratio of the feedstock used for the oxidation of the chloride ions to the total load applied to the system.

" _ 3_F _ (2x [ClO -] + 6x [CLQ3 -] + 8 [04 -]) x96485 XV50f

Current - _{/ f} - ixt (seconds) In the above formula, Vsol refers to the total volume of the electrolyte and the intensity of the current applied during the electrosynthesis.

The chemical yield (Rdtchim.) Of the synthesis, which corresponds here to a degree of conversion of the chloride ions into perchlorate, chlorate and hypochlorite ions, is calculated by comparing the sum of the numbers of moles of perchlorate ions, chlorates and formed hypochlorites and the number of moles of chloride ions consumed.

K Chemic -----; * lût)

The progress of the synthesis (Av) is in turn determined by making the ratio between the number of moles of chloride ions consumed and the number of moles of initial chloride ions.

_{Av =} [CHO [cH _t, _10Q

The expression "understood between ... and ..." must be understood as including boundaries.

Claims

An electrolytic system (1) for the synthesis of sodium perchlorate by electrochemical oxidation of sodium chloride, said system (1) comprising at least:

an electrolyte (10) comprising sodium chloride in a concentration greater than or equal to 0.1 mol / L,

an anode (15; 115) present in the electrolyte (10), the anode (15; 115) having an outer surface (Si; S ₂ ) of diamond doped with boron; - a cathode (17; in the electrolyte (10) and having an outer surface of zirconium or zirconium alloy, and

an electric generator (20) connected to the anode (15; 115) and to the cathode (17; 117).

The system (1) of claim 1, wherein the anode (15) comprises an electrically conductive substrate (28) coated with a boron-doped diamond coating (26).

3. System (1) according to any one of claims 1 or 2, wherein the cathode has an outer surface of zirconium alloy having a mass content of chromium of between 5% and 25%.

4. System (1) according to any one of claims 1 to 3, wherein the concentration of sodium chloride in the electrolyte (10) is greater than or equal to 0.5 mol / L.

5. System (1) according to any one of claims 1 to 4, wherein the electrolyte (10) is free of fluoride or alkali metal dichromate.

6. System (1) according to any one of claims 1 to 5, further comprising a device for regulating the temperature (23) configured to maintain the electrolyte (10) at a temperature below a predetermined temperature.

7. A method of manufacturing sodium perchlorate comprising a step of electrochemical oxidation of sodium chloride to sodium perchlorate by implementing a system (1) according to any one of claims 1 to 6.

8. The method of claim 7, wherein the imposed current density during the electrochemical oxidation can be between 1000 A / m ² and 20000 A / m ² .

The process of any one of claims 7 or 8, wherein the electrolyte (10) is maintained at a temperature of 40 ° C or lower during electrochemical oxidation.