JP2019520482A - Electrolytic system for synthesizing sodium perchlorate with anode having an outer surface made of boron-doped diamond - Google Patents

Abstract

The present invention provides an electrolysis system (1) for synthesizing sodium perchlorate by electrochemically oxidizing sodium chlorate, wherein the system (1) comprises at least an electrolyte containing sodium chlorate ((1) 10) an anode (15) present in the electrolyte (10), having an outer surface made of boron-doped diamond, a cathode (17) present in the electrolyte (10), an anode (15) ) And a power source (20) connected to the cathode (17).

Description

  The invention relates in particular to an electrolysis system for synthesizing sodium perchlorate by electrochemically oxidizing sodium chlorate.

Ammonium perchlorate (NH ₄ ClO ₄ ) is used in the space industry as a component for propellant charge, which is typically synthesized from sodium perchlorate (NaClO ₄ ). Sodium perchlorate can be obtained by electrochemically oxidizing sodium chlorate (NaClO ₃ ). Sodium chlorate is electrochemically oxidized to sodium perchlorate by an electrolytic system having an anode and a cathode both present in a bath containing sodium chlorate. In current electrolytic systems, the oxidation reaction of chlorate ions at the anode is achieved by a parasitic reaction at the cathode, for example a reaction that causes dichloride to be formed.

  It is desirable to limit these parasitic reactions and improve the efficiency of the electrochemical oxidation of sodium chlorate to sodium perchlorate.

  The present invention specifically seeks to meet the above needs.

To this end, in a first aspect, the present invention is an electrolysis system for synthesizing sodium perchlorate by electrochemically oxidizing sodium chlorate, said system comprising at least
An electrolyte containing sodium chlorate,
An anode present in the electrolyte, having an outer surface made of boron-doped diamond,
A cathode present in the electrolyte,
Proposing an electrolysis system comprising a power supply connected to the anode and the cathode.

  The electrolytic system of the present invention uses an anode made of a specific material and having an electrically active outer surface which advantageously serves to improve the efficiency of the electrochemical oxidation of sodium chlorate to sodium perchlorate.

  In one embodiment, the anode may comprise a conductive substrate covered with a coating made of boron-doped diamond.

  Such a configuration is advantageous as long as it uses an anode with a limited amount of boron-doped diamond, which in turn is low in cost.

  In one variant, it is possible to use an anode made entirely of boron-doped diamond.

  In one embodiment, the cathode may have an outer surface made of a metal alloy having a chromium content in the range of 5% to 25% by weight.

  In the prior art, it is known to add alkali metal fluoride or dichromate type additives to the bath to limit parasitic reactions. If additives are present, it is necessary to carry out the step of separating the resulting sodium perchlorate from the fluoride or dichromate additive at the end of the electrochemical oxidation. The separation step prolongs the process of obtaining sodium perchlorate and makes it more complicated. Using a cathode having an outer surface made of a metal alloy having a chromium content in the range of 5% to 25% uses the alkali metal fluoride or dichromate additive in the electrolyte While helping to avoid, it limits the amount of impurities generated during electrosynthesis. As a result, using such a cathode eliminates the step of separating sodium perchlorate from the fluoride or dichromate additive, thereby electrochemically oxidizing sodium chlorate Simplifies the process of obtaining sodium perchlorate. Thus, in one embodiment, the electrolyte may not contain any alkali metal dichromate or fluoride.

  In one embodiment, the metal alloy of the cathode may have a chromium content in the range of 10 wt% to 25 wt%, such as in the range of 10 wt% to 20 wt%.

  In one embodiment, the metal alloy of the cathode may be steel, nickel alloy or zirconium alloy.

  In one variant, it is possible to use a cathode having an outer surface made of nickel or zirconium. In particular, it is possible to use a cathode made entirely of nickel or zirconium.

  In one embodiment, the concentration of sodium chlorate in the electrolyte may be at least 0.1 mole (mol / L) per liter. Specifically, the concentration of sodium chlorate in the electrolyte may be 0.5 mol / L or more. As an example, the concentration of sodium chlorate in the electrolyte may be in the range of 0.5 mol / L to Cs (where Cs represents the saturation concentration of sodium chlorate in the electrolyte), for example 0.5 mol / L to It may be in the range of 5 mol / L.

  In one embodiment, the system comprises both a first chamber in which the electrolyte, the anode and the cathode reside and a second chamber in which the electrolyte resides and which is distinguishable from the first chamber, the system possibly further comprising And an electrolyte circulation circuit configured to flow an electrolyte between the first chamber and the second chamber.

  In a second aspect, the present invention is a method of producing sodium perchlorate, wherein the method electrochemically oxidizes sodium chlorate to sodium perchlorate by using the above system We propose a process for producing sodium perchlorate comprising the steps.

In one implementation the current density applied during the electrochemical oxidation is 1000 amps per square meter ^{(A / m 2) ~20,000A /} m 2, is in the range of, for example, 1000A ^{/ m 2} ~5000A ^/ m 2 obtain.

  Such current density values are advantageous to further promote the formation of sodium perchlorate.

  Other features and advantages of the present invention will become apparent from the following description, given by way of non-limiting example, and with reference to the accompanying drawings.

FIG. 1 is a schematic view showing an example of the system of the present invention. 2 and 3 are schematic diagrams showing the construction of an anode suitable for use in the system of FIG. 2 and 3 are schematic diagrams showing the construction of an anode suitable for use in the system of FIG. FIGS. 4 and 5 are schematics showing cathode structures suitable for use in the system of FIG. FIGS. 4 and 5 are schematics showing cathode structures suitable for use in the system of FIG. FIG. 6 is a graph obtained by tests comparing the rate at which sodium perchlorate is obtained using the system of the present invention with the rate obtained using a system not based on the present invention. FIG. 7 is a graph obtained by tests comparing the gas flow rates generated when using the system of the present invention and the system not based on the present invention.

  FIG. 1 is a schematic view showing an example of an electrolysis system 1 of the present invention. System 1 is configured to produce sodium perchlorate by electrochemically oxidizing sodium chlorate.

The system 1 comprises a first chamber 3 in which a liquid electrolyte 10 resides. The electrolyte 10 contains sodium chlorate. Before electrochemical oxidation is started, the concentration of sodium chlorate in the electrolyte 10 may be 0.1 mol / L or more, for example 0.5 mol / L or more, for example 0.5 mol / L to 5 mol / L It may be in the range of The electrolyte 10 may be an aqueous electrolyte. Furthermore, the electrolyte 10 does not contain any alkali metal fluoride or dichromate. Specifically, the electrolyte 10 is formed of one of the following compounds: sodium fluoride (NaF), potassium fluoride (KF), sodium dichromate (Na ₂ Cr ₂ O ₇ ), or potassium dichromate (K ₂ Cr) It does not contain any of ₂ O ₇ ).

  System 1 also includes an anode 15 and a cathode 17. Each of these is present in the electrolyte 10 in the first chamber 3. The electrochemical oxidation of sodium chlorate, which is carried out to obtain sodium perchlorate, can be carried out in the first chamber 3.

The anode 15 has an outer surface corresponding to its active surface and made of boron-doped diamond. Boron-doped diamond may typically have a boron content in the range of 0.005 wt% to 0.55 wt%. The rest is made up of diamonds. This outer surface is present in the electrode 10. FIG. 2 shows an example construction of an anode 15 suitable for use in the system 1 of FIG. In this example, the anode 15 comprises a conductive substrate 28 covered with a coating 26 made of boron-doped diamond. The coating 26 defines the outer surface S _{1 of the} anode 15. The substrate 28 is formed of a material different from that of the film 26. The material forming the substrate 28 may include titanium, zirconium, hafnium, vanadium, niobium, tantalum, palladium, molybdenum, or alloys thereof. The material forming the substrate 28 may be selected, for example, from titanium, zirconium, hafnium, vanadium, niobium, tantalum, palladium, molybdenum or alloys thereof. The material forming the substrate 28 may be graphite. For example, the thickness e1 of the coating 26 may be greater than or equal to 0.5 micrometers (μm) and may for example be in the range of 1 μm to 5 μm. In one variant, it is possible to use an anode 115 made entirely of boron-doped diamond. The anode 115 defines the outer surface _{S 2.}

The cathode 17 has an outer surface corresponding to its active surface, which may consist of, for example, a metal alloy having a chromium content in the range of 5% to 25% by weight. In one variant, the outer surface of the cathode may be made of zirconium or nickel. This outer surface is present in the electrolyte 10. Specifically, the cathode has an outer surface made of a metal alloy having a chromium content in the range of 10 wt% to 25 wt%, for example in the range of 10 wt% to 20 wt%. Good. Under such circumstances, the metal alloy of cathode 17 may be stainless steel, for example 316L stainless steel, nickel alloy or zirconium alloy. FIG. 4 shows the structure of an example of a cathode 17 suitable for use in the system 1 of FIG. In this example, the cathode 17 is entirely formed of a metal alloy having a chromium content in the range of 5 wt% to 25 wt% (a chromium content in the range of 5 wt% to 25 wt%). Solid cathode made of metal alloy). The cathode 17 defines an outer surface _{S 3.} As shown in FIG. 5, in one variant, a cathode 117 is used which comprises a conductive substrate 119 covered with a coating 118 of a metal alloy having a chromium content in the range of 5% by weight to 25% by weight. It is also possible. The coating 118 defines the outer surface S _{4 of the} cathode 117. The substrate 119 is made of a material different from that of the film 118.

  The system 1 shown in FIG. 1 further comprises a power supply 20 connected to the anode 15 and the cathode 17. Anode 15 is connected by conductor 16 to positive terminal 18 of power supply 20. The cathode 17 is connected to the negative terminal 21 of the power supply 20 by a conductor 19.

The system 1 also comprises a second chamber 5 distinguishable from the first chamber 3. The liquid electrolyte 10 is present in the second chamber 5. The presence of the second chamber 5 is optional. The second chamber 5 is in communication with the first chamber 3. The electrolyte 10 can be circulated between the first chamber 3 and the second chamber 5 while electrochemical oxidation is taking place. In order to enable such circulation to take place, the system 1 in the example shown
A first channel 12 connecting the outlet 3s of the first chamber 3 to the inlet 5e of the second chamber 5;
A second channel 8, optionally including a valve 13 as shown, connecting the outlet 5s of the second chamber 5 to the inlet of the pump 9;
A third channel 11 connecting the outlet of the pump 9 to the inlet 3e of the first chamber 3;
The electrolyte circulation circuit 7 is included.

  During operation of the pump 9, the electrolyte 10 circulates between the first chamber 3 and the second chamber 5 following the path indicated by the arrow F1. The electrolyte 10 flows through the first channel 12, the second channel 8 and the third channel 11 during circulation.

  The second chamber is maintained at a substantially constant temperature by means of a thermostat 23 through which the temperature control fluid flows (arrow F2). For example, the thermostat 23 can maintain the temperature of the second chamber at about 20 ° C.

  System 1 may include a cooling device 25 that serves to condense at least a portion of the vapor generated during electrochemical oxidation. A cooling fluid flows through the cooling device 25 to carry out such condensation (arrow F3). For example, prior to heat exchange, the cooling fluid may be at a temperature of about -5.degree.

  A method of electrolytically synthesizing sodium perchlorate from sodium chlorate is described below with reference to the system of FIG.

First, by operating the pump 9, circulation of the electrolyte 10 between the first chamber 3 and the second chamber 5 is started. Thereafter, by using the power supply 20, a potential difference is applied between the anode 15 and the cathode 17, thereby electrochemically oxidizing sodium chlorate in the first chamber 3 into sodium perchlorate. Do. Sodium perchlorate is the whole reaction:
NaClO ₃ + H ₂ O → NaClO ₄ + H _{2 (g)}
Is electrochemically synthesized according to

Sodium perchlorate has the following reaction:
^{_{ClO 3 - + H 2 O →}} ClO 4 - + 2H + + 2e -
Formed by the anodic oxidation of sodium chlorate according to

During electrosynthesis, the electrolyte 10 may circulate continuously. The electrolyte may be circulated in the circuit 7 at a rate, for example, in the range of 250 liters per hour (L / h) to 350 L / h. The current density applied during electro-oxidation may be in one square meter per kiloampere ^{(kA / m 2) ~20kA /} m 2, within the range of, for example, ^{1kA / m 2 ~5kA / m 2} .

  In a variant not shown, sodium perchlorate is obtained by using a system which does not include the second chamber 5. Under such circumstances, no circulation of electrolyte is required in the system.

The test settings used were of the same type as shown in FIG. The setting was equipped with a flow meter at the outlet from the cooling device. The flow meter served to quantify the flow rate (liters per hour: L / h) and the amount of gas produced during the electrolysis.

In this example, the electrolysis system had a stainless steel cathode (316 L steel with a chromium content of 18% by weight). Two distinguishable anode materials were tested. The first test according to the invention was undertaken using an Nb / DDB anode having a niobium substrate covered with a boron-doped diamond coating. The second test was not based on the present invention and was undertaken using a Ti / PbO ₂ anode with a titanium substrate covered with a lead dioxide coating.

The test conditions given during the initiated test are set as follows. That is,
· Nb / DDB or anode is made of Ti / PbO _2,
· Cathode made of 316L stainless steel,
The current applied between the anode and the cathode is 15 A (current density ² kA / m ² ),
• volume of electrolyte equal to 1 liter (L),
The initial weight of sodium chlorate in the electrolyte is about 550 grams (g),
The electrolyte circulation flow rate between the first chamber and the second chamber is 300 L / h, and the initial temperature of the electrolyte is 20 ° C.

Table 1 below and Figure 6 demonstrate the advantages provided by the system of the present invention. Efficiency and speed of formation of sodium perchlorate is not based on the present invention, as compared to systems using Ti / PbO ₂ anode, it is found to be improved by the system of the present invention.

Analysis of the gas flow rates recorded during electrosynthesis shows that systems not based on the present invention generate additional gas as compared to the system of the present invention (see FIG. 7). In systems not based on the present invention, higher amounts of dioxygen are produced at the anode, which is responsible for the reduced efficiency. In addition, within systems not based on the present invention there is a parasitic reaction that produces a chlorine containing gas. This parasitic reaction is also involved in reducing the efficiency of the reaction. The system of the invention thus evaluated limits the magnitude of the parasitic response, and does so even if the electrolyte used does not contain any fluoride or dichromate additives.

  The quantities of material for chlorate and perchlorate were determined by ion chromatography during the test run.

The current efficiency of the reaction (Eff _current ) was calculated by determining the ratio of the charge used to oxidize chlorate ion to perchlorate ion divided by the total charge applied to the system.
In the above formula, n _{ClO 4−} represents the number of perchlorate ions at the end of synthesis, F is the Faraday coefficient (96,485 coulombs per mole (C. mol ⁻¹ )), I is the current added ( A) and t is the time of electrolysis (seconds (s)).

The chemical efficiency of synthesis (Eff _chemistry ) is calculated by determining the ratio of the number of moles of perchlorate ion formed divided by the number of moles of chlorate ion consumed.

The progress of synthesis (Adv) is determined by determining the ratio of the number of moles of chlorate ion consumed divided by the initial number of moles of chlorate ion.

  The term "within the range of ...-..." is to be understood as including boundary values.

The term "within the range of ...-..." is to be understood as including boundary values.
The following embodiments are also disclosed in the present disclosure.
Embodiment 1
An electrolysis system (1) for synthesizing sodium perchlorate by electrochemically oxidizing sodium chlorate, the system (1) comprising at least
An electrolyte containing sodium chlorate (10),
An anode (15; 115) present in said electrolyte (10), having an outer surface (S ₁ ; S ₂ ) made of boron-doped diamond ,
The cathode (17; 117) present in said electrolyte (10), having an outer surface (S ₃ ; S ₄ ) made of a metal alloy having a chromium content in the range 5% to 25% by weight ,
A power supply (20) connected to the anode (15; 115) and the cathode (17; 117)
Including an electrolysis system (1).
Second Embodiment
The system (1) according to embodiment 1, wherein the anode (15) comprises a conductive substrate (28) covered with a coating (26) made of boron-doped diamond.
Third Embodiment
The system (1) according to embodiment 1, wherein the anode (115) is formed entirely of boron-doped diamond.
Fourth Embodiment
The system (1) according to any one of the preceding embodiments, wherein said metal alloy of said cathode (17; 117) has a chromium content in the range 10 wt% to 20 wt%. .
Fifth Embodiment
5. The system (1) according to any one of the embodiments 1-4, wherein the metal alloy of the cathode (17; 117) is a steel, a nickel alloy or a zirconium alloy.
Sixth Embodiment
The system (1) according to any one of the preceding embodiments, wherein the concentration of sodium chlorate in the electrolyte (10) is at least 0.1 mol / L.
Seventh Embodiment
The system (1) according to any one of the preceding embodiments, wherein the electrolyte (10) does not comprise any alkali metal fluoride or dichromate.
[Eighth embodiment]
The first chamber (3) in which the electrolyte (10), the anode (15), and the cathode (17) reside, and the first chamber (3) in which the electrolyte (10) resides are distinguishable An electrolyte circulation circuit comprising both with the second chamber (5), the system (1) being further arranged to allow the electrolyte (10) to flow between the first and second chambers (3, 5) The system (1) according to any one of the preceding embodiments, comprising (7).
[Embodiment 9]
A method of producing sodium perchlorate, said method comprising
Sodium perchlorate comprising electrochemically oxidizing sodium chlorate to sodium perchlorate by using the system (1) according to any one of the embodiments 1-8. How to manufacture.
Tenth Embodiment
The current density applied during the electrochemical oxidation is obtained in the range of ^{1000A / m 2 ~20,000A / m 2} , The method of embodiment 9.

Claims (10)

An electrolysis system (1) for synthesizing sodium perchlorate by electrochemically oxidizing sodium chlorate, the system (1) comprising at least
An electrolyte containing sodium chlorate (10),
An anode (15; 115) present in said electrolyte (10), having an outer surface (S ₁ ; S ₂ ) made of boron-doped diamond,
The cathode (17; 117) present in said electrolyte (10), having an outer surface (S ₃ ; S ₄ ) made of a metal alloy having a chromium content in the range 5% to 25% by weight ,
An electrolysis system (1) comprising a power supply (20) connected to the anode (15; 115) and the cathode (17; 117).   The system (1) according to claim 1, wherein the anode (15) comprises a conductive substrate (28) covered with a coating (26) made of boron-doped diamond.   The system (1) according to claim 1, wherein the anode (115) is formed entirely of boron-doped diamond.   The system (1) according to any one of claims 1 to 3, wherein the metal alloy of the cathode (17; 117) has a chromium content in the range 10 wt% to 20 wt%. .   The system (1) according to any one of the preceding claims, wherein the metal alloy of the cathode (17; 117) is a steel, a nickel alloy or a zirconium alloy.   The system (1) according to any one of claims 1 to 5, wherein the concentration of sodium chlorate in the electrolyte (10) is at least 0.1 mol / L.   The system (1) according to any one of the preceding claims, wherein the electrolyte (10) does not comprise any alkali metal fluoride or dichromate.   The first chamber (3) in which the electrolyte (10), the anode (15), and the cathode (17) reside, and the first chamber (3) in which the electrolyte (10) resides are distinguishable An electrolyte circulation circuit comprising both with the second chamber (5), the system (1) being further arranged to allow the electrolyte (10) to flow between the first and second chambers (3, 5) A system (1) according to any one of the preceding claims, comprising (7). A method of producing sodium perchlorate, said method comprising
Sodium perchlorate comprising the step of electrochemically oxidizing sodium chlorate to sodium perchlorate by using the system (1) according to any one of claims 1 to 8. How to manufacture. The current density applied during the electrochemical oxidation is obtained in the range of ^{1000A / m 2 ~20,000A / m 2} , The method of claim 9.