EP3478877A1

EP3478877A1 - Electrolyte system for the synthesis of sodium perchlorate, comprising an anode having an outer surface of boron-doped diamond

Info

Publication number: EP3478877A1
Application number: EP17740459.7A
Authority: EP
Inventors: Olivier DABARD; Angeline AUMELAS; Guillaume GOTTI; Karine Groenen Serrano; André SAVALL
Original assignee: Centre National de la Recherche Scientifique CNRS; Universite Toulouse III Paul Sabatier; ArianeGroup SAS
Current assignee: Centre National de la Recherche Scientifique CNRS; Universite Toulouse III Paul Sabatier; ArianeGroup SAS
Priority date: 2016-06-30
Filing date: 2017-06-29
Publication date: 2019-05-08
Also published as: WO2018002548A1; JP2019520482A; FR3053363A1; FR3053363B1

Abstract

The invention relates to an electrolyte system (1) for the synthesis of sodium perchlorate by means of the electrochemical oxidation of sodium chlorate, said system (1) including at least: an electrolyte (10) comprising sodium chlorate; an anode (15) in the electrolyte (10), the anode (15) having an outer surface of boron-doped diamond; a cathode (17) in the electrolyte (10); and an electrical generator (20) connected to the anode (15) and to 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 chlorate. 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 by electrochemical oxidation of sodium chlorate (NaCIOs). The electrochemical oxidation of sodium chlorate to sodium perchlorate is carried out with an electrolytic system comprising an anode and a cathode present in a bath comprising sodium chlorate. In existing electrolytic systems, the oxidation reaction of the chlorate ion at the anode is accompanied by parasitic reactions at the cathode generating for example the formation of chlorine.

It would be desirable to limit such parasitic reactions and improve the efficiency of the electrochemical oxidation of sodium chlorate to sodium perchlorate.

The invention aims specifically to meet the aforementioned need.

Object and summary of the invention

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

an electrolyte comprising sodium chlorate,

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 advantageously makes it possible to improve the efficiency of the electrochemical oxidation of sodium chlorate to sodium perchlorate.

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 the techniques of the prior art, it is known, 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, a step for separating the sodium perchlorate formed from the dichromate or fluoride additive. This separation step lengthens and complicates the process for obtaining sodium perchlorate. The use of a cathode whose external surface is formed of a metal alloy with a chromium content of between 5% and 25% advantageously makes it possible to dispense with the use 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 possible, therefore, to render superfluous the step of separating sodium perchlorate from the dichromate or fluoride additive and thus to simplify the process for obtaining sodium perchlorate by electrochemical oxidation of sodium chlorate. 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.

Alternatively, a cathode having an outer surface of nickel or zirconium could be used. In particular, a cathode made entirely of nickel or zirconium could be used.

In an exemplary embodiment, the concentration of sodium chlorate in the electrolyte may be greater than or equal to 0.1 mol / L. In particular, the concentration of sodium chlorate in the electrolyte may be greater than or equal to 0.5 mol / L. The concentration of sodium chlorate in the electrolyte may for example be between 0.5 mol / L and Cs where Cs denotes the saturation concentration of sodium chlorate in the electrolyte, for example between 0.5 mol / L and 5 mol / L.

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 being able to further comprising an electrolyte circulation circuit configured to flush the electrolyte between the first and second chambers.

According to a second aspect, the invention provides a process for the manufacture of sodium perchlorate comprising a step of electrochemical oxidation of sodium chlorate 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 further accelerate the formation of sodium perchlorate. 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;

FIG. 6 is a graph obtained experimentally comparing the formation kinetics of sodium perchlorate obtained by implementing a system according to the invention with that obtained by implementing a system outside the invention, and

FIG. 7 is a graph obtained experimentally comparing the gas flow rates generated during the use of a system according to the invention and a system outside 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 perchlorate by electrochemical oxidation of sodium chlorate.

System 1 comprises a first chamber 3 in which a liquid electrolyte is present. The electrolyte 10 comprises sodium chlorate. The concentration of sodium chlorate in the electrolyte 10, before initiation of the electrochemical oxidation, may be greater than or equal to 0.1 mol / L, for example 0.5 mol / L and for example be between 0, 5 mol / L and 5 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 chlorate 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, palladium, molybdenum or an alloy thereof. The material forming the substrate 28 may for example be chosen from: titanium, zirconium, hafnium, vanadium, niobium, tantalum, palladium, molybdenum 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 zirconium or nickel. This outer 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 316L 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 consists entirely of a metal alloy with a content mass of 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.

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 at 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 second chamber is maintained at a substantially constant temperature by a thermostat 23 in which a temperature control fluid circulates (arrows F2). The thermostat 23 can for example, maintain the temperature of the second chamber at about 20 ° C.

The system 1 may further include a condenser 25 for condensing at least a portion of the vapors produced during the electrochemical oxidation. 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 method of electrosynthesis of sodium perchlorate from sodium chlorate 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 which electrochemical oxidation of sodium chlorate to sodium perchlorate in the first chamber 3. Electrochemically, sodium perchlorate is synthesized according to the overall reaction: NaClO ₃ + H ₂ O → NaClO ₄ + H _{2 (g)}

The formation of sodium perchlorate is obtained by anodic oxidation of sodium chlorate according to the reaction: CIO ₃ ^" + H ₂ O → CIO 4 ^" + 2 H ⁺ + 2 e

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

The experimental setup that has been used is of the type shown in Figure 1. The assembly has been equipped with a flow meter at the refrigerant outlet which quantifies the flow (in liters per hour: L / h) and the quantity of gases produced during electrolysis.

In this example, the electrolytic system included a stainless steel cathode (316L steel having a chromium content of 18%). Two separate anode materials were tested. A first test according to the invention was carried out with a Nb / DDB anode having a niobium substrate coated with a boron-doped diamond coating. A second test outside the invention was carried out with a Ti / Pb0 ₂ anode having a titanium substrate covered with a coating of lead dioxide.

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

anode in Nb / DDB or Ti / Pb0 ₂ ,

- 316 L stainless steel cathode,

imposed current between the anode and the cathode of 15A (current density of 2 kA / m ² ),

- volume of the electrolyte equal to 1 liter,

initial mass of sodium chlorate in the electrolyte of about 550 grams,

flow rate of the electrolyte between the first and second chambers of 300 liters / hour, and

initial temperature of the electrolyte of 20 ° C.

Table 1 below and Figure 6 highlight the advantages conferred by the system according to the invention. It is found that the efficiency and the formation kinetics of sodium perchlorate are improved thanks to the system according to the invention in comparison with the system outside the invention using a Ti / Pb0 ₂ anode.

Analysis of the gas flow rates recorded during the electrosynthesis showed additional gas production (s) for the system outside the invention in comparison with the system according to the invention (see FIG. 7). Oxygen is produced at the anode in a larger quantity in the system outside the invention, which contributes to decrease the yield. In addition there is, in the system outside the invention, a parasitic reaction for the production of a chlorine gas also participating in lowering the yield of the reaction. The system according to the invention thus evaluated limits the importance of the parasitic reactions, even though the electrolyte used is devoid of additive dichromate or fluoride.

Table 1

The amounts of chlorate ion and perchlorate ion material 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 chlorate ions to perchlorate ions and the total load applied to the system.

Q (C10â ->CIO;) 2 xn _cl0T x F

Yield, current ~ X 100 = x 100

Qtotal I xt in the above formula n io4- _C designates the number of moles of ions perchlorates end of the synthesis, F is the Faraday constant (96485 C.mol ^"1), the impressed current I (A) and t electrolysis time (s).

The chemical yield (Rdtchim.) Of the synthesis is calculated by relating the number of moles of perchlorate ions formed to the number of moles of chlorate ions consumed. The progress of the synthesis (Av) is in turn determined by making the ratio between the number of moles of chlorate ions consumed and the number of moles of initial chlorate ions. n C107 (consumed) """

Av = - X 100

nCIOJ (t = 0)

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 chlorate, said system (1) comprising at least:

an electrolyte (10) comprising sodium chlorate,

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; 117) present in the electrolyte (10), the cathode having an external surface (S3; _S4) of metal alloy having a mass content of chromium of between 5% and 25%, 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).

The system of claim 1, wherein the anode (115) is entirely formed of boron doped diamond.

4. System (1) according to any one of claims 1 to 3, wherein the metal alloy of the cathode (17; 117) has a chromium mass content of between 10% and 20%.

5. System (1) according to any one of claims 1 to 4, wherein the metal alloy of the cathode (17; 117) is a steel, a nickel alloy or a zirconium alloy.

6. System (1) according to any one of claims 1 to 5, wherein the concentration of sodium chlorate in the electrolyte (10) is greater than or equal to 0.1 mol / L.

7. System (1) according to any one of claims 1 to 6, wherein the electrolyte (10) is free of fluoride or alkali metal dichromate.

8. System (1) according to any one of claims 1 to 7, comprising a first chamber (3) in which are present the electrolyte (10), the anode (15) and the cathode (17) and a second chamber (5), separate from the first chamber (3), wherein the electrolyte (10) is present, the system (1) further comprising a circuit (7) for circulating the electrolyte configured to flow the electrolyte (10) between the first (3) and second (5) chambers.

9. A method of manufacturing sodium perchlorate comprising a step of electrochemical oxidation of sodium chlorate sodium perchlorate by implementing a system (1) according to any one of claims 1 to 8.

10. The method of claim 9, wherein the imposed current density during the electrochemical oxidation can be between 1000 A / m ² and 20000 A / m ² .