FR2493878A1 - Aq. alkaline soln. contg. hydrogen peroxide, made in electrolysis cell - esp. caustic soda soln. contg. hydrogen peroxide and used as oxidant or bleach - Google Patents

Abstract

A cell is used contg. a membrane sepg. an anode chamber from a cathode chamber, in which a porous cathode is located. The cathode is a carbon or graphite felt or fabric, which is given a hydrophilic treatment prior to its use as the cathode. An emulsion of O2 or a gas contg. O2 in an aq. soln. of alkali hydroxide is fed through the cathode to formH202 dissolved in the soln. The hydrophilic treatment pref. consists in immersing the felt or fabric in (A) an alkanol, esp. pure, boiling ethanol for 2 h, then washing in water; or (B) in a hot mixt. of 50% chromic and sulphuric acids for 0.5-5 minutes, then washing in water. The pref. cell uses a perforated sheet anode made of Ni or steel coated with Ni. The prod. is an oxidising and bleaching agent which does not cause the corrosion and pollution created by similar agents based on Cl cpds., and can be used for bleaching paper pulp.

Description

Hydrogen peroxide is an oxidizing and bleaching agent
which has the advantage of not generating corrosive waste or
pollutants unlike other oxidizing and bleaching agents
such as chlorine derivatives (bleach,
chlorine dioxide, etc.). Its selective action is easily contro-
lable and is thus appreciated in many industries where the
quality of the finished product is of great importance, particularly in
the field of whitening paper money.

However, hydrogen peroxide is a relatively
expensive, which limits its use: this high cost is explained by
the fact that, in conventional processes of water production
oxygenated, it is purified and concentrated with a view to its
keting. These operations are however not necessary for
many applications; for example, pasta bleaching
paper is made with a dilute solution of hydrogen peroxide in
an alkaline hydroxide, usually soda.

In this case, a process for producing a solution
alkaline hydrogen peroxide would be an advantage since it
would dominate the dilution operations of a commercial solution
concentrated oxygenated water and mixing it with a solution
alkali hydroxide. If, moreover, this production was reduced
on the place of use, we would save the transport
and the assurance of security of supply.

In addition, the production of hydrogen peroxide in
alkaline electrochemically by reduction of oxygen or air under pressure; presence of an aqueous alkaline solution on cathodes
porous coal, is a known technique.

These cathodes are usually made up of particles
of carbon agglomerated together by means of a hydrophobic binder.

However, these electrodes have two disadvantages
important
- a delicate realization and
- a relatively short life expectancy, by the very fact of their
principle of operation based on a triple contact (gas-electrolyte
electrode).

 The progressive penetration of the alkaline solution into the porous structure, by reducing the active surface, is at the origin of the aging of the electrode.

 These cathodes can also be constituted by a fixed bed of graphite particles. These cathodes rapidly exhibit a high heterogeneity of potentials; in addition, the electrolyte and the oxygen must be kept under pressure, which increases the cost of production of the cell and reduces the longevity of the cathode.

 The subject of the present invention is therefore a process for the production of oxygenated water in an alkaline medium by electroreduction of oxygen or of an oxygen-containing gas, in the presence of water and an alkaline hydroxide, in a cell electrolytic system comprising a cathode and an anode to overcome the disadvantages of the prior art; according to this process, a solution of oxygen, or a gas containing it, is passed through a porous cathode consisting of a felt or cloth in an electrolyte of an aqueous solution of an alkaline hydroxide. carbon or graphite, inside said cell, to form oxygenated water by reducing the oxygen in the presence of water and an alkaline hydroxide in contact with the cathode, this cathode being further separated of the anode by a semipermeable separating membrane to prevent the contact of hydrogen peroxide formed with the anode.

 The invention also relates to an electrolysis cell for the electrochemical production of hydrogen peroxide in alkaline aqueous solution, by electroreduction of oxygen or of an oxygen-containing gas in the presence of water and water. an alkaline hydroxide, for carrying out the above defined process.

The electrolysis cell according to the invention comprises for this purpose
- a porous cathode made of felt or carbon or graphite fabric>
an anode made of nickel or nickel-plated steel in the form of a perforated sheet,
a cathode compartment for accommodating said cathode,
an anode compartment following said cathode compartment for accommodating said anode,
a semi-permeable separable membrane for separating the cathode compartment from the anode compartment, and also
a device for supplying the cathode compartment with an emulsion of oxygen or of gas containing oxygen in an aqueous solution of an alkaline hydroxide,
a device for withdrawing oxygenated water in alkaline aqueous solution formed from said cathode compartment, and
a device for supplying the anode compartment with an aqueous solution of an alkaline hydroxide.

 According to a particular embodiment, the cathode compartment is partitioned by the cathode in two subcompartments, one upstream and the other downstream of this cathode, the supply of oxygen emulsion being in the sub-compartment. upstream compartment and the withdrawal of oxygenated water formed in the subcompartment downstream.

 According to an essential characteristic of the invention, the cathode is made of felt or carbon or graphite fabric previously subjected to a hydrophilization treatment.

 According to another characteristic of the invention, the separating membrane is constituted by a cation exchange membrane or by a microporous membrane of the hydrophilic type.

 According to another characteristic, the electrolysis cell comprises a circuit for recycling the contents of the downstream cathode sub-compartment to the supply of the upstream cathode sub-compartment in order to bring the concentration of hydrogen peroxide in said sub-compartment downstream at the desired level.

 According to another characteristic also, the electrolysis cell comprises a device for creating a vortex in the ~ upstream cathodic sub-partition, in order to constrain the emulsion to be swept, at a high rate, the surface of the cathode .

 In addition, a nozzle is provided to evacuate the gaseous excess of the emulsion of said upstream subcompartment and recycle it to the feed of this upstream cathode subcompartment.

 According to a new characteristic, the electrolysis cell comprises a circuit for evacuating gaseous oxygen, formed in the anode compartment, in a forced anode circulation in a non-iron circuit.

 According to another new characteristic, the concentration of the aqueous solution of alkaline hydroxide is maintained between 0.1 mol / liter and 2.5 mol / liter.

 The essential characteristic of the invention lies in the fact that the cathode is constituted by a coherent network of carbon or graphite fibers, such as a felt or fabric of carbon or graphite; a graphite felt, however, has better electrical conduction qualities and better mechanical strength than a carbon felt.

The felt or fabric used preferably has a thickness of at least 2 mm in order to obtain a large active surface. The porosity of these felts or fabrics is at least 98%. It is necessary to subject these felts and fabrics to pre-treatment so that they can be used as cathodes in electrolytic cells of the invention.
Indeed, these felts or fabrics have a clear hydrophobic tendency that would prohibit a sufficient passage of the electrolyte through its thickness. It is therefore advantageous to subject them to a prior hydrophilization treatment. This can be conducted in two different ways
the first treatment method consists in immersing the felt or fabric in a lower alcohol, for example pure ethanol, to boiling for a duration of about two hours
the second method of treatment comprises immersing the felt or fabric in about 50% sulfochromic acid under heat for about 30 seconds to 5 minutes.

 In both cases, a deep degreasing of the felt or fabric is carried out and simultaneous oxidation of the fibers which makes it possible to create hydrophilic groups thereon.

 The felt or fabric is then, in either case, washed and rinsed with water, and then used in the state as a cathode.

 The hydrophilization treatment made it possible to multiply the acceptable cathodic current densities by a factor of 2 and even, in some cases, by a factor of 3, whereas in tests carried out on samples of felts or fabrics used without Prior hydrohilization treatment, the maximum measured current densities, corresponding to the reduction of oxygen in hydrogen peroxide, were located at 15 mA / cm 2 only.

 The cathode thus produced, for use within the cell, is advantageously sandwiched between two charge collectors, each consisting of a perforated sheet of nickel or nickel-plated steel, or any other material able to withstand the effects of the medium at the working potential. .

The reaction that takes place in contact with the cathode in the cathode compartment is a reduction of oxygen (transported in dissolved form by the electrolyte), in the presence of water and an alkaline hydroxide, in hydrogen peroxide

 On the other hand, the undissolved gaseous oxygen bubbles can not pass through the felt or fabric which acts as a filter of the fact of the reciprocal dimensions of these bubbles and the pores of the felt or fabric.

 The particular characteristics of the felt or hydrophilized carbon cloth electrodes designate them to advantageously replace the conventional electrodes employed to carry out such a reaction, namely either the piercing electrodes or the triple contact electrodes, which have significant disadvantages. Percolating electrodes pose problems on several levels: by their design, they quickly have a high heterogeneity of potentials, which leads to a decrease in faradic efficiency. Moreover, the electrolyte, to cross, must be sent under pressure, which has the effect of increasing the cost of production of the cell and significantly impair the life of the electrode. does not allow, moreover, to avoid the circulation of the electrolyte along preferential paths. Finally, the percolating electrode is disadvantaged by a rather delicate implementation, at a cost higher than that of an electrode made of felt or carbon fabric. For its part, the triple contact electrode also has major drawbacks: its realization is particularly delicate because of the tendency of the carbon to be hydrophilized as the electrode is used. delicate is to find, by treatment with a binder based on "Teflon" between the risk of fast wetting and that of too much hydrophobation. The elactrode is in all cases subject to a fairly rapid aging which is manifested by a gradual wetting and the appearance of beading in the gas phase.

 As felts or fabrics of carbon, or of graphite, it is possible to use, according to the present invention, in particular the products marketed by the firm "Le Carbone Lorraine" whose characteristics are grouped in the following table.

TABLE I

<tb> TCM <SEP> 128 <SEP> TCM <SEP> 909 <SEP> TCM <SEP> 285 <SEP>
<tb><SEP> Fabric <SEP> carbon <SEP> carbon <SEP> graphite
<tb><SEP> diameter <SEP> of <SEP> threads <SEP> (mm) <SEP> 0.2-0.4 <SEP> 0.4-0.7 <SEP> 0.4-0, 5
<tb><SEP> Thickness <SEP> (mm) <SEP> 0.25 <SEP> 0.35-0.4 <SEP> 0.6-0.7
<tb><SEP> resistance <SEP> taken <SEP> on <SEP> a <SEP> 1-1,5 <SEP> 0.85 <SEP> 1.25
<tb><SEP> square <SEP> (in <SEP>#)
<tb><SEP> RVG <SEP> 1000 <SEP> 1 <SEP> RVG <SEP> 2000 <SEP> RVG <SEP> 4000
<tb><SEP> Felt <SEP> Graphite <SEP> Graphite <SEP> Graphite
<tb><SEP> diameter <SEP> of <SEP> fibers <SEP> (p <SEP>) <SEP> 9 <SEP> 9 <SEP> 9
<tb><SEP> thickness <SEP> (mm) <SEP> 1.2 <SEP><SEP> 3.5 <SEP> 9
<tb><SEP> resistance <SEP> taken <SEP> on <SEP> a <SEP> 1.7 <SEP> 1 <SEP> 0.2
<tb> square <SEP> (in <SEP> @) <SEP>
<Tb>
Of course, these examples are not limiting and other similar products, for example the products sold by Union Carbide under the trade name UCAR or Toyobo, may also be suitable.

 Moreover, in order to let the OH ions pass, while preventing the migration of HO2 ions (hydrogen peroxide) from the cathode compartment to the anode compartment, and thus avoid the reverse reaction of destruction of oxygenated water to the anode, a separator is interposed according to the present. invention between the cathode compartment and the anode compartment.

 This separator may be either a cation exchange membrane or a hydrophilic microporous membrane.

 As an indication, the following table gives the values of oxygenated water concentrations obtained with different commercial cation exchange membrane tested at target theoretical concentrations of 10 and 30 g / l.

TABLE II

<tb> Theoretical <SEP> Concentrations <SEP> (yield <SEP> 100 <SEP>%)
<tb><SEP> Membranes <SEP> 10 <SEP> g / l <SEP> i <SEP> 30 <SEP> g / l
<tb>(<SEP> names (commercial <SEP> names)
<tb> H2O2 <SEP> g / I <SEP> 11202 <SEP> H2O2 <SEP> g / l <SEP>
<tb><SEP> Ionac <SEP> MC <SEP> 3470 <SEP> 9.2 <SEP> 15.7
<tb><SEP> Nafion <SEP> 427 <SEP> 2.2 <SEP> 3.6
<tb><SEP> Nafion <SEP> 152 <SEP> E <SEP> 5.6 <SEP> 8.2
<tb><SEP> Nafion <SEP> 125 <SEP> 4.1 <SEP> 0.7
<tb><SEP> Phone <SEP> Poultry <SEP> 4.8 <SEP> 9.8
<tb><SEP> OR <SEP> 2082
<tb><SEP> Selenion <SEP> CVM <SEP> 7.1 <SEP> 11.5
<Tb>
With the condition of offering good resistance to oxidizing agents and good resistance in an alkaline medium, a cationic membrane makes it possible to maintain perfect separation of the ion species.

 However, such a membrane may be the seat of an electrodialysis reaction characterized by a change in pH, the latter decreasing in the anode compartment while it is believed in the cathode compartment.

 For paper applications, this reaction is a slight disadvantage since the concentration of sodium hydroxide increasing in the cathode compartment, the weight ratio H 2 O 2 / NaOH (hydrogen peroxide on sodium) tends to decline.

 On the other hand, the microporous membranes leave the free passage to the OH- ions, while preventing an excessive diffusion of the H02 ions towards the anode compartment, and do not provoke an electrodialysis reaction. However, the higher the concentration of oxygenated water in the cathode compartment, the more the HO 2 - ions tend to migrate to the anode compartment. The results obtained for a concentration of about 20 g / l of oxygenated water in the cathode compartment nten are no less particularly interesting, since faradic yields remain, for this high concentration2 of the order of 80%.

 The microporous membranes preferably used consist either of sintered polyvinyl chloride (PVC) scit by a microporous polypropylene film containing a hydrophilizing agent and having a very good chemical stability in alkaline solution.

 Also by way of example, the following table gives the values of oxygenated water concentrations obtained with various commercially available microporous membranes tested for theoretical target concentrations of 10 and 30 g / l.

TABLE III

<tb> Theoretical <SEP> Concentrations <SEP> (rende- <SEP>
<tb><SEP> ment <SEP> 100 <SEP> 7.) <SEP>
<tb><SEP> Membrane <SEP> Material <SEP> 10 <SEP> g / l <SEP> - <SEP> 30 <SEP> g / l
<tb><SEP> microporous
<tb><SEP> H202 <SEP> g / l <SEP> H2O2 <SEP> g / l <SEP>
<tb><SEP>"Porvic"<SEP> PVC <SEP> | <SEP> 5.1 <SEP> 1 <SEP> 7 <SEP>
<tb><SEP>"4cuma<SEP> I <SEP> PVC <SEP>! <SEP><SEP> 4.8 <SEP> 7.4
<tb><SEP>"IRCHA"<SEP> 5A <SEP> Polysulfone <SEP> 3.2 <SEP> 4.7
<tb><SEP>"Celgard"<SEP> 3501 * 1 <SEP> Polypropylene <SEP> i <SEP>8> 4 <SEP> 14.8
<tb><SEP>"Celgard"<SEP> 5511 ** <SEP> Polypropylene <SEP> 6.8 <SEP> 12
<Tb>
* wettable film with water, permeable to air, porosity 45%,
pore size 200, marketed by CELANESE COPSORATION ** stratified, wettable with water, Celgard 3501 on a non-woven
polypropylene marketed by CEL4NESE CORPORATION
The polyvinyl chloride separators have the largest porosity, resulting in a larger exchange surface between the two anode and cathode compartments.

 The pore sizes of the porous polypropylene membranes marketed are much smaller, between 200 and 400. However, the voltage across a cell equipped with such membranes remains at an acceptable level, of the order of 2 V for a current density of 30 mA / cm 2.

Furthermore, according to another essential characteristic of the invention, the oxygen (or a gas containing it) is fed into the upstream cathodic subcompartment in the form of an oxygen emulsion (or said gas) in a solution. aqueous alkali hydroxide. This emulsion contains both
dissolved oxygen until saturation and which participates in the hydrogen peroxide formation reaction,
and oxygen gaseous dispersed in the emulsion and not participating directly in this reaction in this form, but forming an oxygen reserve dissolving in said aqueous solution as the dissolved oxygen is consumed in the reaction of hydrogen peroxide formation.

 Thus, the oxygen involved in the formation reaction of oxygenated water is dissolved oxygen. As a result, all the parameters favorably influencing the solubility of oxygen in the aqueous solution of alkali metal hydroxide are of interest.

vers l'alimentation du compartiment cathodique, ce recyclage pouvant couvrir jusqu'à la moitié de la consommation cathodique d'oxygène.In particular, the creation of a vortex in the upstream cathode sub-compartment, in order to constrain the oxygen emulsion to be swept, at a high flow rate, the surface of the cathode, makes it possible at the same time to increase the solubility of the cathode. oxygen and the flow of fluid. In addition, it is economically desirable to recycle the oxygen generated at the anode following the reaction

to the supply of the cathode compartment, this recycling can cover up to half of the cathodic oxygen consumption.

 Moreover, the alkaline hydroxide concentration of the electrolyte exerts a double influence: when this concentration decreases, the solubility of oxygen increases, as well as the kinetics of the hydrogen peroxide formation reaction, but, on the other hand, the conductivity the electrolyte decreases, and the increase in the activity of the cathode is accompanied by a rise in ohmic drops and hence the voltage to be applied across the cell.

 In order to reconcile hydrogen peroxide formation rate and energy cost of the process, an alkali metal hydroxide concentration range of 0.1 to 2.5 moles / liter constitutes a valid compromise.

 A particular embodiment of the electrolysis cell according to the invention is described below, with reference to the attached drawing in which the single figure shows a schematic view of the electrolysis cell according to the invention for the production of hydrogen peroxide in alkaline solution.

 This electrolysis cell consists of an electrochemical cell 1 divided by a separating membrane 2 into a cathode compartment in which is housed a cathode 3 and an anode compartment in which an anode 4 is housed.

 The cathode 3 divides the cathode compartment into two sub-cathode compartments 5, 6, one upstream, the other downstream of said cathode.

 The anode 4 is disposed in the immediate vicinity of the separating membrane 2 in the anode compartment 7.

 The feeding of the upstream cathodic sub-compartment 5 with reagents is ensured by the supply line of oxygen or oxygen-containing gas 8 and by the fine line of aqueous sodium hydroxide solution 9 via line 10.

 The oxygenated water produced in the electro-reduction reaction of oxygen in the presence of water and sodium hydroxide in contact with the cathode accumulates in the downstream cathode sub-compartment 6 and can be recycled in part, in view of its enrichment, via the pipes 12, 13, the pump 14 and the pipe 10, to the upstream cathodic sub-compartment 5, after a new supply of oxygen and sodium hydroxide solution, until the desired concentration of water is obtained oxygenated for the withdrawal. The oxygenated water at the desired concentration is withdrawn through line 11.

 Similarly, the excess of the aqueous oxygen-sodium emulsion can be withdrawn from the cathode sub-compartment 5 via line 15 and recycled via line 13, pump 14, line 10, in the upstream cathodic sub-comparator. After a new supply of oxygen and sodium hydroxide solution.

 The medium, contained in the anode compartment 7, and rich in gaseous oxygen forms in contact with the anode, is discharged from said compartment through the pipe 16, via a forced anode circuit on which are inserted a pump 17 and a separator 18; the liquid effluent is recycled by ducting 19 to said anode compartment 7 and the gaseous effluent via line 20 to the oxygen supply 8.

 In operation, the cathode and the anode being connected to the terminals of a direct current source, oxygen or a gas containing it, fed via line 8, is emulsified in the aqueous solution of sodium hydroxide (for example at a concentration of , 1 to 2.5 mol / l) fed through the pipe 9, and the emulsion obtained in the pipe 10 is introduced by the pump 14 into the cathode sub-compartment 5 upstream of the cathode 3, where a vortex created at this effect is intended to complete this emulsification and to impose on it a rapid circulation on the surface of the cathode 3. The greater part of the emulsion is forced to pass through the cathode, while the line 15 makes it possible to control the flow rate of the passage through the cathode and to eliminate excess gaseous by evacuating part of the emulsion which is recycled via line 13 to be returned by means of pump 14 to the same sub-compartment 5, after a new supply of oxygen in 8 and soda in 9.

 The emulsion which passes through the porous cathode 3 in the direction of the sub-cathode compartment 6 downstream undergoes, in contact with the cathode, an electrochemical reduction reaction of the oxygen it contains, with formation of hydrogen peroxide in the electrolytic medium .

 In the sub-cathode compartment 6 downstream, the medium is enriched in hydrogen peroxide during electrolysis; it is recycled to the upstream subcompartment 5 through line 12 until the desired concentration of oxygenated water is obtained in this circuit, for drawing off via line 11.

 The medium circulating in the anode compartment 7 and through the anode circuit 16 driven by the pump 17 is a sodium hydroxide solution of the same nature as that in which the emulsion of oxygen introduced into the cathodic subcompartment is carried out. 5, that is, at a concentration of about 0.1 to 2.5 moles / liter.

 In the anode compartment 7, in contact with the anode 4, the electrolysis reaction causes the formation of gaseous oxygen, which is separated in the anode circuit at 18, to be optionally returned to 8 on the oxygen supply. of the cell.

 The nonlimiting examples which follow are given to illustrate the invention.

Example 1

 An electrochemical cell, as described above and shown in the single figure of the drawing, is equipped with a cathode graphite felt (RGV 2000) of 3.5 mm thick and whose fibers have 9 pm in diameter , having undergone a hot-dip hydrophilic treatment in a 50% sulfochromic solution.

This felt is contained in two collectors constituted by perforated nickel plates.

 The separating membrane is a microporous polypropylene membrane (Celgard 5511). The anode is a perforated nickel sheet.

 The oxygen emulsion flow rate is 3 l / min / dm. The conditions of temperature and pressure are normal.

 The results obtained with various combinations of steps are given in the table below.

~~~~~ TABLE IV

<tb><SEP> Concentration <SEP> 20 <SEP>><SEP> 20 <SEP>"<SEP> -5 <SEP> 30
<tb> NaOH, <SEP> g / l
<tb> Concentration
<tb> R2 <SEP>, g / l <SEP> 10 <SEP>><SEP> 14 <SEP> - <SEP> 15 <SEP>>
<tb><SEP> (in <SEP> scheme)
<tb> Rapporr <SEP> 0.5- <SEP> 0, <SEP> 7 <SEP> ----)
<tb> Ratio <SEP> 0 <SEP> 5 <SEP> - <SEP> - <SEP>><SEP> 0.7 <SEP>><SEP> 0.5 <SEP> - <SEP>)
<tb> N202 / NaON
<tb><SEP> ..... <SEP> ~~ <SEP> ~ <SEP>. <SEP> ..
<Tb>

Density <SEP> of <SEP> 2 <SEP> 2 <SEP> 4 <SEP> 6 <SEP> 2 <SEP> 4 <SEP> 6 <SEP> 2 <SEP> 4 <SEP> 6
<tb> current <SEP> A2 <SEP>'4<SEP>' 6 <SEP>'2<SEP> A / dm
<tb> Quantity <SEP> d1elec- <SEP> ~ <SEP>. <SEP> ~
<tb> tritiy <SEP> kA / hkgH202 <SEP> 2.0
<tb><SEP> H202 <SEP> I
<tb> Tension <SEP> applied <SEP> 8 <SEP> 2.4 <SEP> 1.6 <SEP> 2.2 <SEP>2.83> 1
<tb><SEP> V <SEP>3><SEP> 1,82,43,11,62,22,8
<tb> Energy <SEP>> 6 <SEP> 4.8 <SEP> 6.2 <SEP> 4.1 <SEP> 5.45 <SEP> 7.0 <SEP> 3.6 <SEP> 5, 0 <SEP> 6.4
<tb> Energy <SEP> 3,6 <SEP> 4,8: <SEP> 6,2 <SEP>, 1 <SEP> 11202
<tb> Yield <SEP> Faradic <SEP> 80 <SEP> 70 <SEP> 70 <SEP> 70 <SEP> 70 <SEP> 70
<tb><SEP> w <SEP> 180
<Tb>
The H2O2 / NaOH ratios obtained are particularly high (0.5 and 0.7); faradic yields are particularly interesting (70 and 80%).

 These results also show that solutions with the same concentrations of H 2 O 2 and the same sodium concentrations can be obtained with different current densities provided that the potential is adjusted to the level required to effect the reduction of oxygen under optimal conditions. .

 The energy consumption decreases with the current density but, on the other hand, the electrode surface must then be increased to pass the amount of electricity required. The optimum is therefore determined for each particular case according to the short periods of electric energy and investment costs.

For example, to make oxygenated water at a concentration of 10 g / l with a H2O2 / NaOH ratio of 0.5, the previous table shows
- that the concentration of soda must be set at 20 g / l,
- that it is necessary to fix the density of current, according to the size
of the cell available, at a value of 2 or 4 or 6 A / dm
As soon as the current density has been fixed, for example at 4 A / dm, the other parameters are also fixed: voltage 2.4 V; amount of electricity 2 kA / h.kg Ho02; energy 4.8 kWh / kg H202; faradic yield 80%.

Example 2

 The hydrodynamic regime characterizing the flow and the injection of the fluids has a preponderant influence on the functioning of the cell as shown by the data below. This regime has the effect of forcing the oxygen emulsion to sweep at a high rate the surface of the cathode. Increasing the flow rate makes it possible to increase the current density that can be passed to a given potential and the higher the current density, the greater the electrolytic efficiency.

TABLE V 02 / NaOH: Emulsion of oxygen in sodium hydroxide up to 8
saturation
Cathode / anode / separating membrane: cf example 1

<tb> Flow rate <SEP> emulsion <SEP> Density <SEP> of <SEP> current
<tb><SEP> l / min / dm2 <SEP> allowable <SEP> to <SEP> -700 <SEP> mV / Hg.HgO
<tb><SEP> 1 <SEP> 1.25
<tb><SEP> 3 <SEP> 3
<Tb>
It is observed that the higher the flow rate, the higher the current density recorded at constant potential is high. Thus at -700 mV / Hg.HgO, going from 1.25 A / dm for a low flow (11 / min) to 3 A / dm for a flow rate of the order of 3 1 min.

Example 3

 The cell described in Example 1 is equipped with different samples of the same carbon felt, one of which is not hydrophilized and the other hydrophilized, according to two different methods. The hydrophilization treatment makes it possible to increase the current density that can be passed to the oxygen reduction potential in H2O2 (-700 mV) (with reference to the calomel electrode).

TABLE VI

<tb><SEP> Density <SEP> of <SEP> current
<tb><SEP> Hydrophilization <SEP> mA / cm2
<tb><SEP> nil <SEP> 15
<tb><SEP> Processing <SEP> with <SEP> a
<tb> sulfochromic <SEP> mixture <SEP> 40
<tb><SEP> (50%)
<tb> Treatment <SEP> to <SEP> methanol <SEP> 40
<Tb>
Example 4

The importance of the separating membrane and the various possible possibilities are illustrated by the results below.
TABLE VII
Oxygen: emulsion in soda 40 gil
Flow rate: 3 l / min / dm
Cell / electrodes: see example 1
Temperature: 20 C
Current density: 30 mA / cm

<tb><SEP> Concentration <SEP> H202 <SEP> (g / l)
<tb> Membrane <SEP> separator <SEP> (C <SEP> = <SEP> 10 <SEP> g / l <SEP> for <SEP> a <SEP> yield
<tb><SEP> of <SEP> 100%)
<tb> Nil <SEP> 0,2
<tb> Microporous Membrane <SEP>
<tb><SEP>.<SEP> hydrophobic <SEP> (copolymer <SEP> acrylo- <SEP> 6.3
<tb><SEP> Nitrile / PVC <SEP> - <SEP> Acropor <SEP> ANH) <SEP>
<tb><SEP> Hydrophilic <SEP> Polypropylene <SEP> 8.4 <SEP>
<tb><SEP> (Celg <SEP> a <SEP> rd <SEP> 3501)
<tb> Membrane <SEP> exchange
<tb> (Cationic <SEP> (IONAC <SEP> MC <SEP> 3470)) <SEP> 9.2
<Tb>
According to this table, the cation exchange membranes and the hydrophilic microporous membranes lead to much more interesting results than the hydrophobic microporous membranes.

Example 5

This example is intended to highlight the interest of hydrogen peroxide produced according to the invention and used from its manufacture without the addition of stabilizers, unlike commercial peroxides. The example of application chosen is the bleaching of paper pulp, according to a sequence C (chlorination) P (peroxide) D (chlorine dioxide). Peroxide is produced according to Example 1 with a ratio
H2O2 / NaOH = O, 533.

TABLE VIII Leg: Kraft pulp of various hardwoods (oak, beech, etc.) prd-
treated with oxygen and Kappa number = 10.1.

<Tb>

<SEP> Whitening
<tb><SEP> Stage <SEP> C <SEP> (Chlorination) <SEP> Z <SEP> Cl2 / Paste <SEP>: <SEP> 2.8 <SEP>
<tb><SEP> temperature <SEP>: <SEP> 400C <SEP>
<tb><SEP> time <SEP>: <SEP> 30 <SEP> min
<tb><SEP> protector <SEP> 0.1% <SEP> urea / pflte <SEP>
<tb><SEP> Stage <SEP> P <SEP> (peroxide) <SEP>% <SEP> NaOH / pSte <SEP>: <SEP> 1.5
<tb><SEP> temperature <SEP>: <SEP> 50 C <SEP>
<tb><SEP> time <SEP>: <SEP> 60 <SEP> min <SEP>
<tb><SEP>%<SEP> H2O2 <SEP>: <SEP> 0.8
<tb> Peroxide <SEP> of <SEP> Alkaline Peroxide <SEP>
<tb><SEP> trade <SEP> according to <SEP> the invention
<tb><SEP> Stage <SEP> D <SEP> (Dioxide)
<tb><SEP>%<SEP> ClO2 <SEP> 1.1 <SEP> 1.0
<tb><SEP> temperature
<tb><SEP> i <SEP> 3h <SEP> 3h <SEP>
<tb><SEP> whiteness <SEP> 89.3 <SEP> 90.2
<tb><SEP> degree <SEP> of <SEP> poly- <SEP> 800 <SEP> 800
<tb><SEP> Meritoring
<Tb>
According to the preceding table, the hydrogen peroxide in alkaline aqueous medium according to the invention makes it possible to obtain a paste of the same quality (degree of polymerization = 800) with even a slight gain in whiteness (90.2 against 89, 3) and a reagent economy (chlorine dioxide: 1.0% against 1.1%) probably due to the fact that the peroxide according to the invention, not stabilized, used from its production is more active than the stabilized peroxide of commerce .

Example 6: endurance tests
The cell described in Example 1, equipped with a membrane
Celgard K 511 and a RVG 2000 felt were put into operation for 3000 hours, the temperature being maintained at 21 ° C. Every 150 hours, the potential taken by the cathode was taken The concentration of H 2 O 2 is found to be substantially constant, depending on the time of use of the cell, the average value is 16 g / l at a current density of 5 A / dm 2.

Claims (13)

R E V E N D I C A T IO N S A process for the production of oxygenated water in aqueous alkaline solution by electroreduction of oxygen or an oxygen-containing gas in the presence of water and an alkaline hydroxide in an electrolytic cell comprising a cathode and an anode, characterized in that an oxygen emulsion or a gas containing it is passed through an electrolyte constituted by an aqueous solution of an alkaline hydroxide, through a porous cathode constituted by a felt or a fabric of carbon or graphite, inside said cell, for forming oxygenated water in the electrolyte by reducing the oxygen in the presence of water and an alkaline hydroxide in contact with the cathode, this cathode being further separated from the anode by a semipermeable separating membrane to prevent the contact of hydrogen peroxide formed with the anode. 2. Electrolysis cell for the electrochemical production of hydrogen peroxide in alkaline aqueous solution, by electroreduction of oxygen or an oxygen-containing gas, in the presence of water and an alkaline hydroxide, characterized in that it comprises - a porous cathode made of felt or fabric of carbon or graphite> - anode made of nickel or nickel-plated steel in the form of a perforated plate, - a cathode compartment for accommodating said cathode, - an anode compartment following said cathode compartment  for accommodating said anode, - a semipermeable separating membrane for separating the compartment  cathode of the anode compartment, and also - a device for supplying the cathode compartment with a  emulsion of oxygen or gas containing oxygen in a solution  aqueous solution of an alkaline hydroxide, - a device for withdrawing oxygenated water in aqueous solution alca  line formed of the cathode compartment, and - a device for supplying the anode compartment with a solution  aqueous solution of an alkaline hydroxide. 3. Electrolysis cell according to claim 2, characterized in that the cathode compartment is partitioned by the cathode in two sub-compartments, one upstream, the other downstream of this cathode, the emulsion feed. oxygen is formed in the upstream subcompartment and the withdrawal of oxygenated water formed in the subcompartment downstream. 4. Cell electrolysis according to claim 2 or 3, characterized in that the cathode is made of carbon or graphite felt or fabric previously subjected to a hydrophilization treatment. 5. Electrolysis cell according to claim 4, characterized in that said hydrophilization treatment consists of an immersion of the felt or fabric in a lower alcohol, in particular pure ethanol, at the boiling temperature of the latter, during about 2 hours, followed by washing and rinsing with water. 6. Electrolysis cell according to claim 4, characterized in that this hydrophilization treatment consists of an immersion in sulfochromic acid at about 50%, hot, for about 3G s. at 5 min, followed by washing and rinsing with water. 7. Electrolysis cell according to claim 2 or 3, characterized in that the separating membrane is constituted by a cation exchange membrane or a microporous membrane of the hydrophilic type. 8. Electrolysis cell according to claim 3, charac terized in that it comprises a circuit for recycling the contents of the sub-cathode compartment downstream to the supply of the sub-cathode compartment upstream in order to bring the water concentration oxygenated in said downstream subcompartment to the desired level. 9. Electrolysis cell according to claim 3, characterized in that it comprises a device for creating a vortex in the upstream cathodic sub-compartment, in order to constrain the emulsion to be swept at a high rate, the surface of the cathode. 10. Electrolysis cell according to claim 3 or 9, characterized in that a nozzle is provided to remove the excess gas from the emulsion of the upstream cathode sub-compartment and recycle to the feed of this sub-compartment cathodic upstream. 11. Electrolysis cell according to claim 3, charac terized in that it comprises a circuit for evacuating gaseous oxygen formed in the anode compartment, in a forced anode circulation in an open circuit, with recycling of the gaseous effluent. to feeding the upstream cathode sub-compartment and recycling the liquid effluent to said anode compartment. 12. Electrolysis cell according to claim 2 or 3, characterized in that the concentration of the aqueous alkali hydroxide solution is maintained between Q, 1 mole / liter and 2.5 moles / liter. 13. Cell electrolysis according to claim 2 or 3 or 4, characterized in that the cathode felt or carbon or graphite cloth is held between two collectors tssle perforated nickel or nickel-plated steel.