WO2024127271A1

WO2024127271A1 - Process for the production of chlorates

Info

Publication number: WO2024127271A1
Application number: PCT/IB2023/062619
Authority: WO
Inventors: Monica Santamaria; Francesco DI FRANCO; Giada TRANCHIDA
Original assignee: Società Chimica Bussi S.p.A.
Priority date: 2022-12-14
Filing date: 2023-12-13
Publication date: 2024-06-20

Abstract

The invention relates to a process for the production of alkali metal chlorates, in particular sodium chlorate, in an undivided electrolytic cell as well as to the use of a composition comprising hexavalent molybdenum and at least one buffering agent based on phthalates and/or phosphates to increase the selectivity of the electrodes, in particular the cathode, of said electrolytic cell.

Description

PROCESS FOR THE PRODUCTION OF CHLORATES

DESCRIPTION

Field of the invention

State of the art

On an industrial level, the production of chlorates is among the most important processes in electrochemistry. Chlorates, in particular sodium chlorate, are used as reagents for the preparation of perchlorate and sodium hypochlorite and chlorine dioxide (CIO2), a compound used, for example, in water treatment and pulp bleaching in the paper industry. Using chlorine dioxide rather than chlorine directly avoids the formation of unwanted chlorinated organic substances, which explains the growing demand for this product over the past 30 years.

Sodium chlorate is produced industrially through the electrolysis of an aqueous solution of sodium chloride (brine) at a controlled pH, in electrolytic cells of the undivided type with anodic production of hypochlorite and hypochlorous acid which, at process temperatures (70-90°C), disproportionate to form chlorate. As described in more detail below, the electrolyte initially consisting of a sodium chloride brine is progressively enriched in chlorate up to the required concentration.

In more detail, during the process, a concentrated brine is electrolysed at a temperature between 70°C and 90°C in an undivided cell with the formation of H2 at the cathode and chlorine at the anode, as described by the following reactions^1-13: cathodic reaction: 2 H2O + 2 e' — > H2 + 2 OH' (1) anodic reaction: 2 Cl' — > CI2 + 2 e' (2)

In particular, the chlorine produced at the anode hydrolyses to hypochlorite (CIO'), which in turn reacts in the bulk of the electrolyte solution to form chlorate (CIO's) according to the following reactions ¹'²'¹¹-¹³'³-¹⁰:

Cl₂ + H₂O Cl' + HCIO + H⁺ (3)

HCIO CIO' + H⁺ (4)

2 HCIO + CIO' CIO's + 2 Cl' + 2 H⁺ (5) One of the main problems in managing the chlorate production process is related to the onset of competitive processes at both cathode and anode, which negatively affect the production efficiency of sodium chlorate (NaCIO₃).

The process at the anode that is competitive with the development of chlorine is the oxidation of water with the formation of oxygen, a process that is thermodynamically favoured as it has a lower equilibrium potential than chlorine but kinetically requires much higher overvoltages. The use of DSA® (Dimensionally Stable Anodes) anodes in the plant reduces the probability of the oxygen development process (O2) becoming competitive with the chlorine development process (CI2) .

However, there are other parasitic reactions that can induce an increase in the oxygen content in the cell, such as the oxidation of hypochlorite ions, as shown below:

12 CIO’ + 6 H₂O 4 CIO-3 + 12 H⁺ + 8 Cl’ + 3 O₂ + 12 e’ (6)

HCIO + H₂O 3 H⁺ + Cl’ + O₂ + 2 e- (7) and the decomposition of hypochlorite ions in the bulk of the solution, according to the following reactions¹’⁴’⁶’¹⁰’¹⁴’¹⁵:

2 HCIO O₂ + 2 Ch + 2 H⁺ (8)

2 CIO’ O₂ + 2 Cl’ (9) with negative consequences not only in terms of process efficiency, but also in terms of plant safety since the process is carried out in undivided cells.

With regard to the cathode, on the other hand, working with undivided cells means that the reduction of both the chlorate and the reaction intermediates forming it can be competitive with the development of hydrogen, with negative effects on the efficiency of the process, in accordance with the following reactions:

CIOT + 6 H⁺ + 6 e- Ch + 3 H₂O (10)

CIO’ + 2 H⁺ + 2 e- Ch + H₂O (11)

Current technology involves the use of carbon steel cathodes on which the selectivity towards the hydrogen development process is ensured by a gelatinous and not very soluble chromium hydroxide Cr(OH)₃ coating, which is formed under operating conditions due to the reduction of the Cr(VI) dichromate ion to Cr(lll). The in-situ formation of the coating contributes to increasing the corrosion resistance of the steel as it also performs a protective function against the electrode, which is not cathodically protected during shut-down and is therefore easily attacked by hot concentrated solutions of chloride ions. Dichromate also performs the task of buffering the pH of the solution around the optimum value (between 6 and 7) for the sodium chlorate process^1- However, in order to maintain these optimum pH values, which help to control these competitive processes and thus to maintain a high yield, it is necessary to add substantial quantities of dichromate ions, which entails a number of disadvantages. The presence of dichromate (and the chromate in equilibrium with it) is, for example, undesirable in the subsequent production process of chlorine dioxide, and its separation from the chlorate by crystallisation is hindered by its very similar solubility.

In addition, hexavalent chlorine (Cr(VI)) is highly toxic, so much so that legislation requires its use only after authorisation by the European Commission and the treatment of process wastewaters.

In the literature, there are already a number of publications concerning possible alternatives to the use of sodium dichromate in electrolytic processes. Most of these concern the use of polyvalent alternative species to be added directly to the brine without intervening in the existing technology. Others concern the functionalisation of electrodes through the formation of ex situ coatings based on molybdenum ions^19-23 and/or other metals^524-28 on steel or titanium electrodes. Other possible solutions to increase the selectivity of the electrodes in the cell is to use split cells through the use of ion exchange membrane ^{1 29}.

The publications concerning coatings that are grown in situ during brine electrolysis can be further divided into those dealing with Mo(VI) as a possible alternative to sodium dichromate^16-18 and those dealing with the use of other polyvalent species that can be reduced in situ at the cathode during the brine electrolysis process (Mn, Ce, V, Zr, rare earth metals)^30-34. The latter are more expensive and/or less suitable in terms of environmental impact. With respect to the literature on molybdenum coatings grown in situ, it must be said that the experimental approach adopted does not correspond to the actual operating conditions of the electrolysis unit, since selectivity tests and the study of the efficiencies of the anodic and cathodic processes are carried out using split cells (to study the redox processes separately at the electrode/electrolyte interface)¹⁷ or undivided cells, as in the plant, but in semi-batch mode¹⁶. In particular, Li et al., (J Appl Electrochem. 2007;37(4):499-504) do not explain how efficiencies are estimated for HER, of which they only report a qualitative estimate since they do not discriminate the composition of the gas leaving the cell, and conclude by stating that the use of molybdate ions in the cell can catalyse the development of oxygen in the cell by not providing any alternatives.

On the other hand, Cornel et al. (J Electrochem Soc. 1993;140(11):3123-3129) suggests the possibility of reducing the concentration of molybdate ions in the cell by compensating for the buffering action of Mo(VI) through the addition of a phosphate ion bath, using either titanium or carbon steel as cathodes for the electrolysis process¹⁷. However, the operating conditions described in Cornell et al. (cell divided under batch conditions and in the presence of hypochlorite but not chlorate) are dramatically different from those of an industrial plant for the production of sodium chlorate, which instead involve working in an undivided electrolytic cell, in continuous flow and in the presence of chlorate as well. As will be evident to a technician in the field, also on the basis of the attached examples, applying the teachings of Cornell et al. to an industrial plant for the production of chlorinated sodium, the selectivity of the electrodes is unacceptable and leads to a collapse in efficiency.

In general, to the best of the inventors' knowledge, there is no accurate study of the effect of the proposed additives on the cell voltage and pH of the solution^{16-18 30-34} which is a key parameter in ensuring high production efficiencies of sodium chlorate.

There is therefore still a need to find alternative technical solutions to the use of chromium compounds in electrolytic processes, in particular in electrolytic processes for the production of sodium chlorate that do not have the drawbacks of those already known in the art.

SUMMARY OF THE INVENTION

With the aim of finding a solution to the aforementioned technical problem, and thus to find alternative species to the dichromate ion not subject to REACH authorisation (Regulation (EC) No 1907/2006), which are environmentally friendly and economically viable, and which are also capable of performing the same functions as Cr(VI) while guaranteeing comparable current efficiencies, the inventors of the present invention have carried out extensive experimental investigations of the use of carbon steel and titanium electrodes in a chlorate production process by electrolysis of concentrated brine with varying selected additives, monitoring over time the evolution of the efficiencies for the processes of hydrogen and oxygen development in the cell, the pH of the solution and the cell voltage. As will be apparent to a person skilled in the art, the proposed experimental approach also allows the study of electrode selectivity under actual plant operating conditions by simulating, on a laboratory scale, the electrolysis process through the use of a suitably designed and constructed undivided cell that allows continuous operation.

The inventors therefore intervened in the composition of the bath by selecting and adding a polyvalent precursor that can be reduced near the cathode surface during electrolysis analogous to the reduction process of Cr(VI) to Cr(lll) with the in-situ formation of a coating on the electrode surface. It is therefore an object of the present invention to provide a process for producing alkali metal chlorates in an undivided electrolytic cell characterised in that the electrolyte comprises an alkali metal chloride, molybdenum, preferably hexavalent molybdenum (VI) or salts thereof, and at least one buffering agent based on phthalate and/or phosphate ions, said electrolyte being free of chromium (i.e. chromate or dichromate ions), as defined in the first of the appended claims. Other features of the process of the invention are described in the respective dependent claims.

Another object of the present invention is a system for the production of alkali metal chlorates, in particular sodium chlorate, comprising at least one undivided electrolytic cell equipped with a plurality of cathodes and a plurality of anodes, said cell being fed with a chromium-free sodium chloride brine and enriched with molybdenum, preferably hexavalent molybdenum (Mo(VI)) or salts thereof, and at least one buffering agent chosen from phosphate and/or phthalate ions.

The electrolyte additive identified, which comprises molybdenum, preferably hexavalent molybdenum (Mo(VI)) or its salts, and at least one buffering agent based on phthalate and/or phosphate ions, is capable of

- ensure high electrode selectivity by preventing the competitive reduction of hypochlorite and chlorate ions at the cathode and the consequent decrease in NaCIOs production efficiency ;

- maintain the pH of the solution in a range between 5 and 7 to shift hydrolysis balances and maximise chlorate production;

- protect cathodes from corrosion during plant shutdown.

Thus, further object of the present invention is the use of molybdenum in any form, preferably hexavalent molybdenum (Mo(VI)), and phthalate and/or phosphate ions for the preparation of an electrolyte for the purpose of increasing the selectivity of electrodes, in particular the cathode, and/or inhibiting cathode corrosion phenomena in an undivided cell electrolytic cell, in particular for the continuous production of alkali metal chlorates.

These and further advantages will become apparent from the following detailed description of the invention as well as from the examples.

Glossary

Unless otherwise specified, the terms in this description are to be understood as commonly understood by a person skilled in the art.

In the context of this description, the term 'undivided cell' refers to an electrolytic cell that has no physical barriers (such as, for example, membrane or diaphragm) between the anode and cathode to separate the electrolyte. Therefore, in an undivided cell, or specifically, a chlorate undivided electrolytic cell, the cathode and anode are present in the same compartment.

In the context of the present invention, "in situ activation" means activation of the cathode, e.g. by coating or electrodeposition, performed during the electrolytic process of sodium chlorate production. In situ activation does not require disassembling the cell.

In the context of the present invention, HER means Hydrogen Evolution Reaction, relating to the process of developing hydrogen at the cathode during electrolysis of brine.

In the context of the present invention, OER means Oxygen Evolution Reaction, relating to the process of developing oxygen at the anode and/or in the bulk of the solution during electrolysis of brine.

In the context of the present invention, 'strong chlorate' is understood to mean the current leaving the electrolysis unit and sent to the chlorine dioxide reactor consisting of NaCI and NaCIOs produced after electrolysis of the brine and any additives.

In the context of the present invention, 'weak chlorate' refers to the chlorine dioxide reactor output current recirculated to the electrolysis unit consisting of unreacted NaCI, NaCIOs, HCI used to acidify the solution for the production of CIO2 and any additives.

In the context of the present invention, 'DSA' means Dimensionally Stable Anode, in particular an electrode consisting of a metal substrate (generally Ti) coated with a metal-conductive mixed oxide (generally RUO2) capable of ensuring high efficiencies for the process of developing chlorine at the anode.

In the context of the present invention, the term 'buffering agent or 'protective buffering agent refers to a chemical compound capable of minimising pH variations during electrolysis of brine so as to keep it in the range of values considered optimal for the target reaction and advantageously capable of inhibiting the cathode corrosion process, especially in the shut-down phases of the plant, when it is not cathodically protected.

Brief Description of Figures

Figure 1. Schematisation of the experimental set-up used for flow tests.

Figure 2. Magnification of the flow cell used for selectivity measurements.

Figure 3. Development efficiency of a) hydrogen and b) oxygen for carbon steel electrodes in 2.5 M NaCI containing 4 g/L Cr(VI).

Figure 4. Trend of a) pH of the solution and b) cell voltage during the electrolysis process in the presence of 4 g/L Cr(VI).

Figure 5. Trend of a) pH of the solution and b) cell voltage during the electrolysis process in 2.5 M NaCI containing 40 mM Mo(VI). Figure 6. Cyclovoltammetry on carbon steel recorded at 10 mV/s in 5 M NaCI containing 15 mM NaCIO, 40 mM Na2MoO4 and 50 mM KHF at 80°C for 10 cycles.

Figure 7. Overlapping cycles a) I and b) X of the CVs recorded in the different solutions. Figure 8. Overlapping cycles a) I and b) III of the CVs recorded in the anodic direction at 10 mV/s at 80°C in 1 M Na2SO4 with and without KHF using platinum as the working electrode.

Figure 9. Cyclovoltammetry on platinum recorded in the anodic direction at 10 mV/s at 80°C in the presence of 50 mM KHF with and without chlorides.

Figure 10. Trend of a) solution pH and b) cell voltage during the electrolysis process in 2.5 M NaCI containing 40 mM Mo(VI) and 50 mM KHF.

Figure 11. a) Experimental set-up for batch tests, b) enlargement of the cell and c) detail of the cell in operation.

Figure 12. Selectivity measurements for carbon steel electrodes as a function of added additives.

Figure 13. Trend of a) pH of the solution and b) cell voltage during the electrolysis process in 2.5 M NaCI containing 40 mM Mo(VI) and 32 mM Na^PC .

Figure 14. Efficiency values for a) HER and b) OER at varying added additives.

Figure 15. Efficiency values for a) HER and b) OER varying the additives added with and without chlorinated species in the brine.

Figure 16. Trend of a) pH of the solution and b) cell voltage during the electrolysis process in 2.5 M NaCI containing KHF with and without chlorinated species in the brine. Figure 17. Development of a) solution pH and b) cell voltage during the electrolysis process in 2.5 M NaCI containing NaH2PO4 with and without chlorinated species in the brine.

Figure 18. Efficiency values for a) HER and b) OER varying the additives added and the electrodes used.

Figure 19. Trend of a) pH of the solution and b) cell voltage during the electrolysis process in 2.5 M NaCI containing KHF as the electrodes used change.

Figure 20. Trend of a) pH of the solution and b) cell voltage during the electrolysis process in 2.5 M NaCI containing NaH2PO4 as the electrodes used change.

DETAILED DESCRIPTION OF THE INVENTION

As already mentioned, the aim of the present invention is to provide a system and process for the production of alkali metal chlorates, preferably sodium chlorate, in high yield and with no, or extremely low, use of chromium compounds.

The replacement of dichromate in implants can be carried out according to two different strategies: - find a species that has the same properties as dichromate and is therefore capable of performing the same functions as it (ensuring electrode selectivity, buffering the pH in the electrolyte, preventing electrode corrosion);

- decouple the various functions of dichromate and identify several different species that separately and efficiently perform at least one of the three functions of dichromate, overall providing similar performance to it.

After extensive experimentation, the inventors selected hexavalent molybdenum (Mo(VI)) as an alternative species to sodium dichromate. In particular, the solution proposed here involves the replacement of sodium dichromate (Na₂Cr₂O?) with sodium molybdate (Na₂MoO4), in which Mo(VI) can be reduced during the production of chlorate by inducing the formation of a coating at the cathode, similarly to dichromate.

The advantage of this solution is that the additive (hexavalent molybdenum salts plus phthalate and/or phosphate ions) is added directly to the brine under the same operating conditions as the plant in terms of bath composition, temperature and pH. The process of the invention, therefore, does not involve a coating step or formation thereof under operating conditions other than those of the chlorate production process itself. This process can therefore be perfectly integrated into existing production lines without having to modify the process itself.

The use of molybdate ions is also not subject to any particular restrictions. According to Regulation (EC) No. 1272/2008, in fact, molybdate ions are not considered hazardous, do not contain components that are considered persistent, bioaccumulative and toxic (PBT), or very persistent and very bioaccumulative (vPvB), and can therefore be used safely in plants. Finally, similarly to dichromate, sodium molybdate, for example, performs the task of buffering the pH of the bath during electrolysis, albeit in a narrower range than for dichromate (between 5 and 6 for molybdate ions and between 5 and 6.5 for dichromate ions)^{1 16}.

However, as reported in the literature^{1 16-18}, high amounts of molybdate ions in solution can catalyse the development of oxygen at the anode with negative consequences not only in terms of process efficiency, but also plant safety. For this reason, the experimental investigation was organised in such a way as to take into account a possible effect of Mo(VI) on the kinetics of oxygen development, concluding that it is preferable to contain the molybdenum concentration within 40 mM, also because higher molybdenum concentrations do not induce significant effects on the efficiency of the HER and the buffering effect of the additive itself.

By lowering the molybdenum concentration, the buffering effect of molybdenum is lost, and a buffering agent must be introduced. The invention therefore relates to a process for preparing alkali metal chlorates, in particular sodium chlorate, comprising the step of electrolysing an electrolyte comprising an alkali metal chloride in an electrolytic cell preferably an undivided electrolytic cell, and wherein said electrolyte comprises molybdenum in any form, preferably hexavalent molybdenum (VI), and in an amount not exceeding 40 mM and a buffering agent based on phthalate and/or phosphate ions, said electrolyte being free of chromium.

The electrolyte can be prepared from deionised water and adding an alkaline metal chloride, e.g. sodium chloride, to obtain a brine. The brine may contain e.g. 2.5 M NaCI.

Molybdenum may be added to the electrolyte in any form, for example, in elemental, ionic and/or compound form, typically in the form of hexavalent molybdenum (Mo(VI)), preferably molybdenum (VI) salts or compounds of alkali metals or alkaline earth metals. More preferably, molybdenum is added to the electrolyte in the form of molybdenum (VI) salts of alkali or alkaline earth metals provided that they are soluble in the brine to be electrolysed. By way of example, but not limitation, molybdenum is added to the electrolyte in the form of sodium molybdate (Na2MoO4), sodium molybdate dihydrate (Na2MoC>4 2 H2O) or mixtures thereof, preferably in the form of sodium molybdate.

The electrolyte comprises molybdenum, or rather hexavalent molybdenum, in an amount of 30 to 50 mM, e.g. 35 to 45 mM, preferably 40 mM. If more than one source of molybdenum is used, it is however preferable that the total quantity remains within the specified ranges to prevent the development of oxygen.

As mentioned, the electrolyte also includes a buffering agent, e.g. a buffering agent based on phthalate ions, a buffering agent based on phosphate ions or mixtures thereof.

Thus, the present invention relates to a process for the preparation of sodium chlorate comprising the step of electrolyzing an electrolyte in an undivided electrolytic cell, wherein said electrolyte comprises a brine of sodium chloride (NaCI), molybdenum in any form, preferably in the form of hexavalent molybdenum (VI) in an amount not exceeding 40 mM, and at least one buffering agent, said at least one buffering agent being chosen from a phthalate ion based buffering agent, a phosphate ion based buffering agent or mixtures thereof, said electrolyte being also substantially free or free of chromium.

The buffering agent based on phthalate ions is added to the electrolyte, e.g. in the form of potassium hydrogen phthalate (KCsHsC , KHF), preferably in an amount between 40 and 60 mM.

The buffering agent based on phosphate ions is added to the electrolyte, e.g. in the form of sodium hydrogen phosphate (NaH2PC ) or mixtures thereof, preferably in an amount between 20 and 40 mM. In other words, the sodium chloride brine is buffered by one or more buffering agents comprising phthalate or phosphate ions in concentrations of not less than 50 mM and 32 mM respectively, where phthalate ion concentration means the sum of the concentrations of all ionic species arising from phthalic acid in equilibrium with each other in aqueous solutions e.g, CsHsC ' and Cs^C ²', while phosphate ion concentration means the sum of the concentrations of all ionic species arising from phosphoric acid in equilibrium with each other in aqueous solutions e.g., H2PO4' , HPO4²; PC>4³'.

According to a preferred embodiment of the invention, the electrolyte solution comprises about 2.5 M NaCI, about 40 mM Na2MoO4 and about 50 mM KHF or about 2.5 M NaCI, about 40 mM Na2MoC>4 and about 32 mM NaH2PO4.

Preferably, the buffering agent is based on phthalate ions.

In fact, the experimental data of Example 6 indicate a deterioration in the performance of the steel electrode in the presence of the phosphate ion buffer (in the presence of 40 mM Mo(VI)) after the addition of NaCIO and NaCIOs, as suggested by the efficiency values for the hydrogen development process shown in Figure 15a.

In contrast, the r|H2 obtained using KHF as a buffer (in the presence of 40 mM Mo(VI)) is substantially the same as that obtained in the absence of the ions CIO' and CIO's (see Figure 15a and Example 5), thus demonstrating that even under more severe operating conditions, such as those in the plant, KHF is able to give the electrode high selectivity. Comparative data show that, under more severe operating conditions (i.e. in the presence of hypochlorite and chlorate ions) and similar to plant conditions, the efficiency of the HER decreases dramatically in the case of phosphate ions while it remains practically the same in the case of the phthalate buffer. This result therefore suggests better electrode behaviour in terms of selectivity and electrocatalytic properties when 50 mM KHF is added to the brine containing 40 mM Mo(VI).

An advantage of the present invention is that the electrolyte is free or substantially free of chromium (chromate and/or dichromate ions) whereby "substantially free" means that the amount of chromium in the electrolyte is less than 25 mM, preferably less than 20 mM, more preferably less than 10 mM, even more preferably less than 5 mM.

According to a preferred embodiment of the present invention, the electrolyte is free of chromium compounds.

The brine additive referred to in the present invention, and in particular including molybdate and at least one buffering agent based on phthalate ions, succeeds in maintaining the pH of the brine around 4 - 7, a pH that guarantees good process performance. The procedure according to any of the embodiments described here is particularly suitable for the preparation of sodium chlorate but can easily be adapted for the preparation of alkali metal chlorates in general, as will be immediately apparent to a technician in the field.

The process for producing alkali metal chlorates comprises introducing an electrolyte according to any one of the embodiments of the invention into an electrolytic cell, electrolysing the electrolyte to obtain an electrolysed chlorate solution, transferring the electrolysed solution to a chlorate reactor for further reaction and thus obtaining a more concentrated alkali metal chlorate solution. The chlorate formed is separated by crystallisation while the mother liquor is recycled and enriched in chloride for further electrolysis to form hypochlorite, or the chlorate-containing electrolyte is transferred to a separate reactor to be converted to chlorine dioxide, which is separated as a gas stream. The chlorate-depleted electrolyte is transferred back to the chlorate unit and enriched with chloride for further hydrolysis to give hypochlorite. The present invention also relates to a system for producing sodium chlorate comprising at least one undivided electrolytic cell equipped with a plurality of cathodes and a plurality of anodes, said cell being fed with an electrolyte solution according to any one of the embodiments described herein.

The electrolysis system of the invention preferably comprises carbon steel or titanium electrodes of a typically lamellar structure suitably spaced by means of separators made of an insulating material (e.g. Teflon, which is resistant up to about 260 °C and is chemically inert).

As will be evident from the experimental part below, using phthalates as a buffering agent, the procedure is reproducible and efficient whether titanium or carbon steel electrodes are used, the latter being preferred in any case.

The system according to any of the described forms of realisation preferably comprises a multiplicity of undivided electrolytic cells arranged in series.

The invention therefore also relates to the system described above as well as to the use of an electrolyte solution comprising a brine of sodium chloride (NaCI), hexavalent molybdenum in any form, and preferably in an amount not exceeding 40 mM, and at least one buffering agent, said at least one buffering agent being chosen from a buffering agent based on phthalate ion, a buffering agent based on phosphate ions or mixtures thereof, said electrolyte also being free or substantially free of chromium to increase the selectivity of electrodes, in particular of the cathode, and/or to decrease (or inhibit) corrosion of the cathode in an undivided cell electrolytic process, in particular in an electrolytic process for producing chlorates. More generally, the invention also relates to the association of molybdenum, preferably hexavalent molybdenum (VI), more preferably hexavalent molybdenum salts of alkali or alkaline earth metals, even more preferably sodium molybdate and phosphate and/or phthalate ions as an additive for electrolytic solutions, in order to increase the selectivity of electrodes, especially the cathode, in an undivided cell electrolytic process for the production of chlorates.

In an attempt to study the role of sodium molybdate during the chlorate production process and to evaluate the selectivity and electrocatalytic properties of the electrodes, baths with different compositions were used.

In particular, the effectiveness of the additive was studied by monitoring over time:

- the selectivity of the electrodes as the composition of the brine changes;

- the efficiency (r|H2 ) of the Hydrogen Evolution Reaction (HER) process at the cathode;

- the pH of the solution, which must be less than 6 (pH ~ 5.8) to ensure high development efficiencies of sodium chlorate;

- the cell voltage (AV);

- the efficiency (002) of the Oxygen Evolution Reaction (OER) due to the onset of competitive processes at the anode (oxidation of water) or dissociation of chlorinated species in the bulk of the solution.

Without wishing to be bound by any theory, on the basis of the experimental evidence gathered and reported in the experimental section below, it can be hypothesised that the ionic species added to the brine in accordance with any of the forms of realisation described here not only act by buffering the pH, but also by adsorbing to the cathodic surfaces forming films that inhibit the reduction and/or decomposition in the bulk of the solution of the chlorinated species.

The examples below show that brine additivated with buffering agents according to any of the embodiments of the invention improves electrochemical performance and process efficiency, allows the addition of chromium compounds to be eliminated, and most importantly, allows the selectivity of electrodes in an undivided electrochemical cell to be significantly increased.

The following examples are illustrative and are not intended to limit the scope of the invention in any way, which is instead as defined in the appended claims. Experimental part

Materials and Methods

Sodium molybdate dihydrate (Na2MoO4 2 H2 O, CAS 10102-40-6) RPE grade (Reagent Grade or Analytical Purity Grade) was used, purchased from Sigma Aldrich, which is however interchangeable with industrial grade reagents.

Carbon steel electrodes, the composition of which is given in Table 1 , and titanium electrodes (assay 99.99%) were used to carry out the experiments.

Table 1. Composition of carbon steel used as electrode for the development of H2.

The steel electrodes were treated with abrasive papers (P320, P800 and P1200) in such a way as to remove the native film or any impurities from the electrode surface before each experiment.

To assess the stability of the additives added to the brine, cyclo voltammetries were recorded under the operating conditions of the plant (i.e. at 80°C and 400 rpm and imposing a current density of 300 mA cm^-2). The experiments were carried out in a cell with a three-electrode configuration in which steel was used as the working electrode, platinum as the counter electrode and a silver/silver chloride electrode (Ag/AgCI, SSC, Sat. KCI, Uo = 0.197 V vs SHE) was used as the reference electrode.

The selectivity tests, on the other hand, made it possible to determine the inhibition capacity of the electrodes against competitive processes at the cathode (reduction of hypochlorite and chlorate ions) as the composition of the solution varied by comparing their performance on the basis of the efficiency of the hydrogen development process at the cathode during electrolysis of the brine by gas chromatograph analysis.

The measurements were carried out using the experimental set-up schematised in Figure 1 , consisting of:

• section (1) a flow-through cell where the electrolysis process takes place;

• section (2) a reactor with a volume of ~ 3 litres filled with 1 litre of solution and used as a feed tank for the cell and as a flash stage for the liquid-gas mixture leaving the electrolysis unit;

• section (3) a peristaltic pump for withdrawing the solution from the tank and for pumping the liquid-gas mixture out of the cell; • section (4) a trap with 0.1 M NaOH for the absorption of CI2 produced at the anode during electrolysis;

• section (5) a flowmeter for calculating the volumetric flow rate of the gases produced in the cell.

A 500 pl gas-tight syringe was used to collect and analyse the gas produced in the GC- TCD cell.

The selectivity tests were carried out in a closed, undivided flow cell, the image of which is shown in Figure 2. The cell was designed with reference to the elementary module that constitutes the electrolysis unit currently used at an industrial level in such a way as to reproduce the real operating conditions of the plant in the laboratory. In particular, these units consist of several elementary cells arranged in series and comprising electrodes of lamellar structure spaced (~ 5 mm) by means of separators made of insulating material.

Figure 2 also shows the construction details of the cell. In particular, the assembly of the electrolysis unit first involves the insertion of a cylindrical Teflon support and the DSA® anode mounted between two gaskets to ensure the sealing of the anode compartment. Subsequently, the 5 mm thick Teflon separator was inserted in such a way as to guarantee the electrical insulation of the electrodes, while at the same time ensuring a distance between the anode and cathode such as to minimise overvoltages due to the inter-electrode gap (in accordance, moreover, with the structure of the modules used in the system). After inserting the separator, the carbon steel electrode was mounted between two gaskets to seal the cathode compartment. The installation of the Teflon spacer septum made it possible to expose an electrode area for the reduction processes (development of hydrogen and possible reduction of hypochlorite and chlorate ions) of ~ 11 cm². Finally, the upper support was added, into which the threaded PVC quick couplings were mounted, allowing the cell to be fed with the feed coming from the tank (brine containing the additives, section (2) of Figure 1) by means of the peristaltic pump (section (3) of Figure 1 ) and to send the unreacted brine and the gases produced in the cell (H2, O2, non-hydrolysed CI2 and He) back to the tank (section (2) of Figure 1). The cell was finally anchored to a steel structure, which gave the electrolysis unit stability and robustness (see Figure 2). The electrodes were also mounted in such a way as to be easily accessible from the outside by means of a current generator, thanks to which it was possible to impose the current and thus induce the electrolysis process (see Figure 2). For these experiments, a two-electrode configuration was used in which carbon steel and titanium were used as cathodes and DSA® as a counter-electrode (site of the anodic chlorine development process).

The diagram shown in Figure 1 also shows the piping and all the connections between the different parts of the experimental set-up involving the reaction section, the separation of the liquid-gas mixture leaving the cell and the removal of the produced and non-hydrolysed chlorine.

In particular, the reactor (see section (2) of Figure 1) was used as both the cell feed and the flash separation stage of the liquid-gas mixture exiting the cell (see section (1) of Figure 1). Prior to the start of each test, the system was degassed for about 30 minutes with helium gas bubbled into the solution and used as an inert to eliminate the oxygen present in the system and to generate enough overpressure inside the reactor to ensure that the gases (He, H2, O2 and non-hydrolysed CI2) could be released. This also made it possible to monitor any oxygen production due to parasitic reactions during the electrolysis process.

After degassing the system, the cell was connected to the current generator, the circuit was closed and finally the pump was operated to move the solution from the tank (2) to the cell (1) in Figure 1.

During the experiments, the pH of the solution was also monitored to verify the effectiveness of the additive as a buffering agent. Specifically, the liquid was periodically withdrawn via a T-piece fitting mounted at the tank outlet and subsequently injected back into the system via an equal fitting mounted between the cell outlet and the inlet of unreacted brine recirculated in the reactor.

As shown in Figure 1 , the gas developed in the cell, consisting of He, H2, CI2 and O2 possibly produced by the secondary reactions at the anode or in the bulk of the solution, was sent, after the flash stage, to a trap containing 0.1 M NaOH (see section (4) of Figure 1) in order to preserve the gas chromatograph column from any chlorine present in the gas stream. The gas exiting the trap was withdrawn by gas-tight syringe via a T- connector with silicon septum and analysed by gas chromatography (GC) with a thermal conductivity detector (TCD) to estimate the amount of hydrogen produced in the cell and thus make relative comparisons between the electrodes as the composition of the brine changed.

In order to estimate the amount of hydrogen produced at the cathode, it was also necessary to determine the volumetric flow rate of the gas produced in the cell. For this reason, a flow meter was also inserted downstream of the trap in the experimental setup (see section (5) of Figure 1).

Specifically, during electrolysis, the gas exiting the trap (see section (4) of Figure 1) creates a soap bubble near the flowmeter inlet (see section (5) of Figure 1) which travels the length of the flowmeter at an unknown speed depending on the flow rate of the gas leaving the cell. To derive the speed at which the bubble travels, the time it takes the bubble to travel a volume of 10 ml (V) was measured with a stopwatch. Having noted the volume (V) and measured the time travelled by the bubble (f), the total flow rate F_tot (L min^-1) was derived as follows:

F_tot = V / t (12)

After measuring the total flow rate at each gas withdrawal, it was possible to derive the amount of hydrogen produced over time at the cathode (mole min^-1) as follows:

0H2 = Ftot ■ CH2 (13) where CH2 is the concentration of H2 (mol L^-1) obtained at the gas chromatograph after each sampling.

The gas was finally discharged continuously from the flowmeter outlet (see section (5) of Figure 1).

Knowing the amount of hydrogen developed at the cathode, it was also possible to estimate the efficiency of the process as follows: r|H2 = nH2 I nH2,teo (14) where nH2,teo is the maximum amount of hydrogen that can be developed during electrolysis of the brine in the absence of competitive processes.

In order to be able to determine the maximum amount of hydrogen (nH2,teo) produced under the same operating conditions as in industrial systems (80°C and 300 mA cm^-2), electrolysis of a 1 M NaOH solution was first carried out with the experimental set-up described above using carbon steel and DSA® electrodes as the site of the reduction and oxidation processes in the cell, respectively. In this system, in fact, the only possible cathodic process is the development of hydrogen since there are no chlorinated species resulting from the hydrolysis of the chlorine produced at the anode as is the case with brine electrolysis. Since there are no competitive species that can contribute to the development of H2, this system has been taken as the ideal and as a reference for estimating the cathode selectivity as the additives added to the brine used in the plant for the production of sodium chlorate vary. The molar rate of hydrogen produced in the cell following electrolysis of a 1 M NaOH solution is 2 mM min^-1.

In the same way, the molar flow rate of oxygen developed in the cell was derived as follows: no2 = Ftot ■ C02 (15) where C02 is the concentration of oxygen developed in the cell by oxidation of the water at the anode or through the decomposition reactions of the hypochlorite and chlorate ions in the bulk of the solution. The oxygen development efficiency was then derived as follows:

002 = no2 I no2,teo (16) where nO2,_teo is the molar flow rate of oxygen produced by the electrolysis of a 1 M NaOH solution. In this case, the O2 developed in the cell results exclusively from the oxidation of water at the anode (no parasitic reactions resulting from the presence of chlorinated species in solution in accordance with equilibria (6), (7), (8) and (9)).

All tests were carried out under agitated conditions (400 rpm) at 80°C by applying a current of 300 mA cm^-2, trying to simulate the operating conditions under which the electrolysis unit works in the plant. To carry out the experiments on a laboratory scale, 3.2 A was applied through the Voltcraft DPPS-32-15 current generator. The cell was kept in operation for 2 h while gas sampling for HER efficiency estimation was carried out every 15 min. At each sampling, the pH and cell voltage value were also measured.

The tests were initially carried out in a brine containing 2.5 M NaCI, a composition similar to that used in the plant at industrial level (~ 150 g/L), to which the selected additives were added. In this case, the presence of the competitive species (CIO- and CIO's) is ensured by the hydrolysis of the chlorine produced at the anode according to equilibria (3), (4) and (5). Subsequently, after optimising the composition of the brine, the tests were repeated by adding 15 mM NaCIO and 0.5 M NaCIOs to simulate the composition of the electrolysis solution entering the cell ("weak chlorate" solution).

The following are examples of electrolyte solutions according to any of the embodiments of the invention (solutions 2 - 8) compared to the electrolyte solution comprising dichromate (solution 1).

The solutions used are outlined below:

1. 4 g/L Na2CrC>27 (reference);

2. 40 mM (~ 8.24 g/L) Na₂MoO₄;

3. 40 mM (~ 8.24 g/L) Na₂MoO₄ + 50 mM (10.21 g/L) KHF;

4. 40 mM (~ 8.24 g/L) Na₂ MoO₄ + 32 mM (4.42 g/L) NaH₂ PO₄ ;

5. 15 mM NaCIO + 0.5 M NaCIO₃ + 40 mM (~ 8.24 g/L) Na₂MoO₄ + 50 mM (10.2 g/L) KHF;

6. 15 mM NaCIO + 0.5 M NaCIO₃ + 40 mM (~ 8.24 g/L) Na₂MoO₄ + 32 mM (4.42 g/L) NaH₂PO₄ ; 7. 15 mM NaCIO + 0.5 M NaCIO₃ + 40 mM (~ 8.24 g/L) Na₂MoO₄ + 50 mM (10.21 g/L) KHF using titanium as a cathode instead of carbon steel;

8. 15 mM NaCIO + 0.5 M NaCIO₃ + 40 mM (~ 8.24 g/L) Na₂MoO₄ + 32 mM (4.42 g/L) NaH₂PO₄ using titanium as a cathode instead of carbon steel.

Example 1

2.5 M NaCI + 4 g/L Na₂Cr₂O₇

In this example, we report the experimental results of selectivity tests carried out in brine containing 2.5 M NaCI to which 4 g/L of Na₂Cr₂O₇ (pHm ~ 3.9) was added, using carbon steel as the cathode and DSA® as the anode, as in the plant. In this case, the process of hydrogen development on the steel is influenced by the competitive processes at the cathode (reduction of hypochlorite and chlorate ions) due to the hydrolysis of the chlorine produced at the anode. The addition of the additive is in fact necessary to guarantee the selectivity of the electrode and thus high production efficiencies of the chlorate, according to current technology.

This experiment was reproduced in order to have a reference to compare the performance of electrodes in the solutions according to the invention containing the selected additives with that of carbon steel electrodes in the presence of sodium dichromate. The experimental set-up described in the materials and methods section was used for the tests (see Figure 1).

The selectivity measurements shown in Figure 3a confirm that the addition of Cr(VI) enables efficiencies (estimated by means of reaction (14)) of ~ 100 % to be obtained almost 15 minutes after the start of the test, thus confirming that dichromate is capable of efficiently and almost instantaneously breaking down competitive processes at the cathode due to the reduction of hypochlorite and chlorate ions produced by the hydrolysis of the chlorine developed at the anode. Efficiencies above 100 % are to be understood as values conditioned by the sensitivity of the measurement.

Figure 3b shows the efficiency values for the oxygen development process obtained through the reaction (16). After a slight increase, the r|o2 reaches a stationary value of around 14% after about 40 minutes of cell operation.

The pH values measured after each sampling suggest that after an initial increase, the additive is able to buffer the pH of the brine to a constant value of ~ 6 (see Figure 4a), very close to that at which sodium chlorate production should occur (pH ~ 5.8). Finally, the cell voltage does not change significantly during the process, remaining constant at ~ 4.4 V during the two hours of operation, as shown in Figure 4b. Finally, as shown by the photo of the electrode in the insert in Figure 4b, after two hours of operation, the electrode surface does not appear particularly damaged or affected by obvious corrosion.

This experiment is a necessary benchmark to determine the effectiveness of selected additives to replace Cr(VI), which is currently the technology used in sodium chlorate production plants.

Example 2

2.5 M NaCI + 40 mM Na₂MoO₄

According to an embodiment of the invention Na₂Cr₂O? is substituted with Na2MoO4 , in which Mo(VI) can be reduced at the cathode inducing the formation of a Mo/MoO_xfilm which increases the selectivity of the electrode. The selectivity of carbon steel was therefore investigated using a brine consisting of 2.5 M NaCI to which 40 mM Na₂MoO4 was added as a replacement for Cr(VI) using the experimental set-up described in Figure 1. The initial pH of the solution was corrected to ~ 5.8 with 6 M HCI so that the cell operated at the pH value considered optimum in the plant for sodium chlorate production.

In agreement with the efficiency values for the hydrogen development process estimated by means of reaction (14), the value of r|_H2 in the presence of 40 mM Mo(VI) after steadystate is on average ~ 77 % (see Figure 14a). The efficiencies obtained are lower than those estimated in the case of using Cr(VI), but still sufficiently high, thus suggesting that sodium molybdate is capable of inhibiting competitive reduction processes at the cathode (reduction of hypochlorite and chlorate ions produced by the hydrolysis of the chlorine developed at the anode) although less effectively than sodium dichromate. This result therefore suggests that the selected additive is capable of increasing the selectivity of carbon steel electrodes.

The efficiency for the oxygen development process estimated through reaction (16) in the presence of 40 mM Mo(VI) averages ~ 20% at steady state, which is slightly higher than that obtained in the case of Cr(VI) (r|o2,cr ~ 14%). This result indicates that the presence of the molybdate ions has a slight catalytic effect on the oxygen development process in the cell during brine electrolysis, as also reported in the literature^{1 16-18}. However, the increase in OER efficiency recorded compared to the Cr(VI) case is not such as to rule out the use of Na₂MoC>4 , considering the high electrode selectivity (see Figure 14b). Figure 5a shows the pH values measured after each sampling (~ 15 minutes) to check the buffering capacity of Na2MoO4 during cell operation. The pH trend for the solution containing Cr(VI) is also shown for reference.

The addition of the molybdate ion alone is not able to buffer the pH as in the case of sodium dichromate, as shown by the values in Figure 5a. In fact, after a sudden initial increase (approximately within twenty minutes from the start of the experiment), the pH becomes constant (pH ~ 8) but significantly higher than that measured in the presence of the sodium dichromate already in the first moments of cell operation, thus suggesting a lower buffering power of the additive than that of Cr(VI).

Figure 5b shows the cell voltage measured during operation of the electrolysis unit in the presence of molybdate ions. The AV measured in the presence of Cr(VI) is also shown in order to have a reference for assessing the influence of the selected additive on the ohmic drop in the cell.

The values shown in Figure 5b indicate that the addition of the additive has no negative effect on the cell voltage, which is even slightly lower (AVMO(vi) ~ 3.9) than that recorded in the presence of the dichromate ions (AVCr<vi) ~ 4.4). This result therefore suggests that the addition of Mo(VI) to the brine in place of Cr(VI) does not introduce any additional ohmic drop.

Finally, a photo of the electrode after two hours of operation of the cell continuously supplied with a 2.5 M NaCI solution containing 40 mM Na2MoO4 is shown in Figure 5b. The electrode surface appears partially covered by a layer of corrosion products, suggesting a weaker protective action of the additive against the steel electrode than against Cr(VI) (see Figure 4b).

Selectivity measurements therefore suggest that the addition of Mo(VI) is capable of imparting high electrocatalytic properties for the hydrogen development process and inhibiting competitive reduction processes at the cathode, albeit with slightly lower efficiencies than those obtained in the case of Cr(VI), as demonstrated by the efficiency values obtained for HER.

Baths containing 80 mM and 120 mM Na2MoO4 were also tested to see if an increase in the concentration of Mo(VI) could induce an improvement in electrode performance, but no beneficial effects were found. Increasing the amount of molybdate ions also had no significant effect on the pH of the solution. Therefore, using more concentrated solutions of Na2MoC>4 would represent an unjustified loss of reagent since there is no benefit on selectivity either. Since the additive cannot perform an effective buffering action, regardless of the Mo(VI) concentration, it was necessary to select buffers to be added to the brine containing Na2MoC>4 in such a way as to support the pH buffering action. Our proposed alternatives therefore involve the use of phthalate and phosphate ions to try to maintain the pH at values considered optimal for the production of NaCIOs during electrolysis of the brine. The following examples will therefore report the experimental results of using a phthalate ion buffer and a phosphate ion buffer added to support the molybdenum ions to fulfil the functions performed by Cr(VI) as a whole.

Example 3

2.5 M NaCI + 40 mM Na₂ MoO₄ + 50 mM KHF

The first strategy we propose involves the use of phthalate ions by adding potassium hydrogen phthalate (KCsHsC , KHF) in solution, a potassium salt of phthalic acid that dissociates in water according to the following equilibria³⁵:

C₈H₆O₄ C₈H₅O₄- + H⁺ pK_ai = 2.943 (17)

C₈H₅O₄- C₈H₄O₄ ²- + H⁺ pK_a2 = 5.432 (18)

In particular, equilibrium (18) is the one that maintains the pH in the optimal range for the process (pH ~ 5.8).

The reagent used in the laboratory as a precursor for the phthalate ion buffer was purchased from Sigma Aldrich (CAS 877-24-7). KHF is not on the list of substances considered hazardous according to Annex IV of REACH. It is also not a health hazard according to Regulation (EC) No 1272/2008. Finally, it does not contain any components considered to be either persistent, bioaccumulative and toxic (PBT), or very persistent and very bioaccumulative (vPvB) at concentrations of 0.1% or higher.

Before proceeding to study the selectivity of the electrode in the presence of phthalate ions, the redox processes at the metal/electrolyte interface were studied by cyclovoltammetry (CV). As shown in Figure 6, potential sweeps were recorded from open circuit potential (OCP) down to -1 .5 V vs SSC at a temperature of 80 °C and a rate of 10 mV/s for 10 cycles in 2.5 M NaCI containing 15 mM NaCIO, 40 mM Na2MoO₄ and 50 mM KHF (pH_in ~ 3.94).

As can be seen from the inset shown in Figure 6, the current density is very low and constant in the potential range between OCP (~ - 0.6 V vs SSC) and ~ - 1 V vs SSC, beyond which it increases exponentially due to the development of hydrogen.

Finally, Figure 7 shows the I- and X-cycles for the cyclic voltammetry recorded in the presence of molybdate and KHF ions and in the absence of additives (2.5 M NaCI) so as to have a reference. As can be seen from the CVs shown in Figure 7, there is no substantial difference in the current density trend, and thus in the processes occurring at the metal/electrolyte interface as the number of cycles changes.

Comparison of the cyclovoltammetries recorded in solutions in the absence and presence of phthalate ions suggests that the redox processes at the metal/electrolyte interface are essentially attributable to the reduction of molybdate ions, and subsequent formation of coatings, and the development of hydrogen. The experimental results thus suggest that the additive is stable, does not take part in and/or has no catalytic effect on competitive processes at the cathode hydrogen development.

Since existing technology involves the use of undivided cells, the stability of KHF at the anode half-element interface was also studied. The aim was to verify the existence of a possible catalytic effect on the kinetics of oxygen development in the cell by KHF. The tests also made it possible to rule out the possibility that the additive could take part in competitive oxidation processes that induce oxidation, and thus consumption, of the phthalate ion itself.

First of all, the potential sweeps were recorded in a chloride-free solution (1 M Na2SO4) in order to have a reference. In this case, in fact, the only process that can take place at the anode is the development of oxygen as there are no chloride ions in solution that can oxidise to gaseous Ch. Subsequently, CVs were also recorded in the presence of 50 mM KHF. Both CVs were recorded at a rate of 10 mV/s in the anodic direction from open circuit potential (~ 0.5 V vs SSC) to ~ 4 V vs SSC at 80°C using platinum as both anode and counter electrode.

As shown in the potential sweeps in Figure 8 showing the overlaps of cycles I and III in the presence and absence of KHF, there are no appreciable differences in the current density trend, which in both can be attributed to the oxygen development process. This experimental result therefore suggests that the phthalate ion does not catalyse the OER. The CV trend also shows that the additive does not oxidise at the anode, confirming its stability over time and over a wide potential range.

Subsequently, CVs were also recorded in 2.5 M NaCI in the presence of 50 mM KHF to monitor the behaviour of the phthalate ion during anodic polarisation in the solution simulating the brine used in the plant.

As shown by the CVs in Figure 9, the exponential increase in current density occurs at a lower potential (~ 1.1 V vs SSC) than the CVs recorded in the absence of chlorides (~ 1 .6 V vs SSC). This experimental result can be explained by the fact that in the case of brine, the development of CI2, a reaction that is thermodynamically and kinetically favoured over the development of O2, also occurs. Regardless of the value of the current density recorded, no appreciable difference exists in this case due to the addition of KHF in the brine.

The current density recorded in both cases is practically the same, confirming the stability of the additive under the operating conditions of the cell.

The HER efficiency values obtained through reaction (14) for selectivity tests carried out in 2.5 M NaCI containing 40 mM Mo(VI) and 50 mM KHF (pHm ~ 4.20) with the experimental set-up shown in Figure 1 , average about 71 % (see Figure 14a) after steady-state (~ 45 min). The estimated value is slightly lower but still comparable to that obtained in the solution containing 40 mM Mo(VI) (r|H2 ~ 77 %), thus suggesting that the addition of KHF in solution does not adversely affect the selectivity of the electrode and its electrocatalytic properties.

The efficiency value for OER estimated in the presence of phthalate ions by reaction (16) is higher than that estimated when Na2Cr2O? is added to the brine, especially during the first hour of cell operation. The value of r|o2 at steady state is ~ 18%, lower than that obtained in the absence of buffer (2.5 M NaCI containing 40 mM Na2MoC ), suggesting a lower catalytic effect for the OER when phthalate ions are added to the solution (see Figure 14b). This result therefore confirms that the addition of KHF to the brine containing 40 mM Mo(VI) has no catalytic effect on the oxygen development reaction, as already demonstrated by CVs (see Figure 8).

Figure 10 shows the pH trend in the presence of the selected additives (40 mM Mo(VI) and 50 mM KHF) during cell operation. For comparison, the values in the presence of Cr(VI) (reference for our experiments) and in the absence of phthalate ions (2.5 M NaCI containing 40 mM Na2MoC>4) are also shown.

The pH measured during the brine electrolysis process is initially lower than that measured in the presence of Cr(VI) (pHm ,cr<vi) ~ 6) and gradually increases over time to a final value of ~ 6.92. The value measured after the experiment is higher than that obtained after two hours of cell operation in the presence of dichromate ions, but is significantly lower than that obtained at the end of the experiment in the presence of Mo(VI) alone (i.e. in the absence of the KHF buffer). This experimental result therefore confirms the ability of the selected buffer to compensate for the buffering action of Na2MoC>4, which alone is unable to maintain the pH at a value lower than 6. However, the buffering action of the additive is very effective during the first hour of operation of the cell. After about 60 minutes, the pH changes to a value higher than that at which the cell works in the presence of the sodium dichromate (pH ~ 6). It is important to remember, however, that the operating conditions in the plant involve the recirculation in the cell of a weak chlorate solution previously acidified with HCI upstream of the chlorine dioxide production reactor in order to guarantee high efficiencies for the production of CIO2. The action of the buffer is therefore assisted by the acidification of the feed, thus enabling the pH of the solution to be maintained at around the value considered optimal for the production of NaCIOs despite the considerable amount of hydrogen produced in the cell, in agreement with the values of the efficiencies for HER obtained from the selectivity tests (see Figure 14a).

Finally, as shown in Figure 10b, the cell voltage trend is constant over time and equal to about 4.0 V. The AV value measured over time is comparable to that obtained in the absence of KHF (2.5 M NaCI containing 40 mM Mo(VI)) and much lower than that recorded in the presence of dichromate ions ( Cr(vi) ~ 4.4V).

This result is very important as it indicates that the addition of the bufferto the brine does not adversely affect the cell voltage value.

The image shown in Figure 10b shows the surface of the carbon steel electrode after two hours in 2.5 M NaCI containing 40 mM Na2MoC>4 and 50 mM KHF.

In particular, the electrode surface appears to be substantially free of corrosion products, and very similar to that of the steel electrode used as a cathode in the presence of Cr(VI) (see inset of Figure 4b), thus also suggesting a better protective action against the electrode during plant shut-down than in the case where only 40 mM molybdate ions are added to the brine.

Preliminary study of the effect of NaHaPC on the selectivity and electrocatalytic properties of the electrode

Again, electrode selectivity was determined on the basis of the amount of hydrogen developed at the cathode during electrolysis of the brine. The system used is simpler than the one described above, but still allowed the electrode selectivity to be qualitatively estimated using a batch system. The experimental results also made it possible to highlight the criticalities associated with the method of analysis, which subsequently led to the optimisation of the experimental set-up and the use of a system (such as the one described here) that would allow continuous operation by simulating the operating conditions of the plant using the flow cell.

The experimental set-up used for the experiment described below is shown in Figure 11a. The system consisted of a closed, undivided and thermostated cell (solution volume of 300 ml, see Figure 11 b) and a chlorine removal section produced at the anode downstream of the cell to preserve the chromatograph gas columns. The gas leaving the trap was then taken and injected into the TCD gas chromatograph by means of a gastight syringe, making it possible to estimate the amount of hydrogen produced in the cell and to make relative comparisons between the electrodes.

The test was carried out under stirred conditions (400 rpm) at 80°C by applying a current value of 300 mA cm^-2 (current value currently used in the plant) through the Voltcraft DPPS-32-15 current generator. The cell was kept in operation for 2 h, and gas sampling for estimating the H2 produced was carried out approximately every 15 minutes. The experiments were carried out with a two-electrode configuration in which steel was used as the cathode and DSA® as the anode, as shown in Figure 11c.

Figure 12 shows the experimental results of selectivity tests performed in 2.5 M NaCI containing 40 mM Mo(VI) and 32 mM NaH2PO4 for carbon steel electrodes. Also shown are the hydrogen moles developed at the cathode in the absence of buffer (2.5 M NaCI containing 15 mM NaCIO and 40 mM Mo(VI)), in order to assess the selectivity of the electrode with and without phosphate ions, and in the presence of Cr(VI), as this is our reference.

As shown by the values reported in Figure 12, the number of moles of H2 produced during electrolysis in the presence of NaH2PO4 is comparable to that estimated in brine without additives (2.5 M NaCI), thus suggesting the worsening of the selectivity of the electrode in the presence of the buffer used. This experimental result can be justified considering the high passivating power of phosphate ions, which adsorb on the electrode surface, limiting the number of active sites available for hydrogen development. A solution containing a lower concentration of phosphate ions (10 mM Nab^PC ) was also tested in an attempt to limit the likelihood of adsorption on the electrode surface, but no benefit was found on the selectivity and the amount of hydrogen developed at the cathode, which was identical to that obtained in the case of 32 mM NaH2PO4.

A photo of the electrode surface after electrolysis in the presence of phosphate ions is shown in Figure 12.

In this case, the electrode surface is totally free of corrosion products, despite the presence of hypochlorite ions in the solution. This result therefore suggests effective protection of the electrode when immersed in the solution containing molybdate and phosphate ions. However, the surface appears to be covered by a film probably formed by the adsorption of phosphate ions on the electrode, thus explaining the worsening of the electrode's selectivity (see inset of Figure 12).

Despite the drastic decrease in efficiency, the pH value measured after 2 h of electrolysis is about 6.5, a value lower than that obtained in the absence of phosphate ions (pH ~ 8.3) and comparable to that measured after the experiment in the presence of dichromate (pH ~ 6.4). This result therefore confirms the high buffering power of the phosphate ion buffer. The pH values measured at the end of each experiment are shown in Table 2.

Table . pH values measured at the end of each experiment as a function of additives added

In accordance with the experimental results described above, the experiment was then repeated with the experimental set-up used in Examples 1 - 3, using the flow cell as the electrolysis unit to test the selectivity of the electrode under the actual operating conditions of the plant. The fluid-dynamic conditions in the second case should in fact hinder the adsorption of phosphate ions on the electrode surface, thus increasing the selectivity and electrocatalytic properties of the electrode for HER.

Example 4

2.5 M NaCI + 40 mM Na₂ MoO₄ + 32 mM NaH₂PO₄

As an alternative to potassium dihydrogen phthalate, the use of a phosphate buffer in brine containing 40 mM Na₂MoO₄ was also proposed as an alternative to KHF.

The dissociation equilibrium of interest by which the pH can be maintained at ~ 5.8 under plant operating conditions is shown below³⁵:

H₂PO-₄ HPO²’₄ + H pK⁺ _a2 = 7.21

Therefore, sodium dihydrogen phosphate (NaH₂PO₄) was used as a precursor for the phosphate ion buffer for the experiments.

Although a publication exists on the addition of phosphate ions to brine containing Mo(VI)¹⁷, it was decided to repeat the experiment using the experimental set-up shown in Figure 1 in an attempt to study the behaviour of the buffer under real operating conditions using the flow cell. A lower concentration of molybdate ions was also used than that reported in the article.

Selectivity tests for carbon-steel electrodes used as cathodes during the electrolysis of a 2.5 M NaCI brine containing 40 mM Na₂MoO₄ and 32 mM NaH₂PO₄ (pHm ~ 5.46) resulted in higher efficiencies for the hydrogen development process than those obtained in the presence of 50 mM KHF (_nH2,NaH2Po₄ ~ 89%, see Figure 14a). This experimental result suggests that even in this case, the selected buffer has no negative effects on the electrode properties, thus ensuring high selectivity (comparable to that of Cr(VI)) and high efficiencies for the HER (and thus for the chlorate production process).

The efficiency value for the OER estimated by means of reaction (16) decreases, as in the case of the phthalate ions (see example 3) after about 60 minutes from the start of the experiment, resulting in an average value of ~ 19 % at steady state (see Figure 14b). The estimated r|o2 is therefore slightly higher than that of Cr(VI), but still lower than that obtained in the same solution without buffer (2.5 M NaCI containing 40 mM Mo(VI), o2 ~ 20%).

Figure 13a instead shows the pH trend of the solution measured every 15 minutes at each sampling time. In this case too, the values obtained in the absence of buffer (brine containing 40 mM Mo(VI)) and in the presence of phthalate ions are shown in order to compare the buffering power of the two additives. The pH values for the experiment carried out in the presence of Cr(VI) are also reported here.

In accordance with the values obtained, the pH of the solution during electrolysis in the presence of phosphate ions increases slightly from the initial value in the first few minutes of cell operation, remaining constant throughout the test. The value measured at steadystate is about 6.04, very similar to that recorded in the presence of Cr(VI) and lower than that obtained in the presence of 50 mM KHF, although the initial value in the latter case is higher (pHin, KHF ~ 4). This experimental result therefore suggests a better buffering power on the part of the phosphate ion buffer than that of the phthalate ion buffer.

The addition of phosphate ions to the brine containing 40 mM Mo does not lead to an increase in cell voltage compared to the case of KHF, as shown in Figure 13b. The AV values measured during the experiment are even lower than those obtained in the presence of Cr(VI).

Figure 13b shows the image of the electrode surface after two hours of cell operation using the solution containing 40 mM Na2MoO4 and 32 mM Na^PC .

In this case, the electrode surface appears free of corrosion products, suggesting a better protective action against the electrode than that exerted by the Mo(VI) ion alone (see Figure 5b) and that exerted in the presence of phthalate ions (see Figure 10b).

Table 3 summarised the values of the efficiencies for HER and OER when varying the additives added to the brine. The cell voltage and pH values measured before and after the electrolysis process were also reported.

Table 3. Efficiency values for HER and OER, cell voltage and pH before and after the electrolysis process as a function of the added additives.

The results obtained in the examples described so far suggest that the addition of Na2MoC>4 to the brine as a replacement for the Cr(VI) currently used in the plant to produce sodium chlorate is able to inhibit competitive reduction processes at the cathode as evidenced by selectivity tests. The experiments also suggest that a concentration of 40 mM molybdate ions is sufficient to ensure high efficiency for the hydrogen development reaction at the cathode, as shown in Figure 14a. Nevertheless, molybdate ions alone are not able to maintain the pH in the optimal range for the process.

For this reason, additives were selected to be added to the Mo(VI)-containing brine to compensate for the pH-buffering capacity of the molybdate ions.

In particular, the use of potassium hydrogen phthalate (KHF) as a buffering agent is able to ensure a high efficiency for the development of hydrogen at the cathode, even higher than that in the same solution without buffer (brine containing 40 mM Mo(VI)). Furthermore, the buffering capacity of the additive is able to maintain the pH at a lower average value than that obtained in the same solution without phthalate ions (see Table 3).

The use of sodium hydrogen phosphate as a buffering agent, on the other hand, guarantees an efficiency for the development of hydrogen at the cathode that is also in this case better than that in the same solution without buffer (brine containing 40 mm Mo(VI)) and that in the presence of phthalate ions, obtaining efficiency values for HER during cell operation that are comparable to those obtained in the case of Cr(VI) (see Figure 14a). In addition, the buffering power was also found to be betterthan that exerted by KHF, guaranteeing performance very similar to that recorded for Cr(VI) (see Table 3). Both additives added to the brine containing 40 mM Na2MoC>4 do not introduce any additional overvoltage as shown by the cell voltage values reported in Table 3. Moreover, they have no catalytic effect on the kinetics of the O2 development processes that may occur at the anode and/or in the bulk of the solution, obtaining slightly lower efficiencies for the OER than those estimated in the absence of the buffers (2.5 M NaCI containing 40 mM Mo(VI)). The r|o2 are in both cases (addition of KHF and Na^PC ) comparable to those obtained in the case of Cr(VI) (see Figure 14b). Buffers of phthalate ions and potassium ions are able to exert a better protective action against the electrode than brine containing only Na2MoO4, which is essential during plant shutdowns.

The following examples will describe the results of selectivity tests carried out in a brine solution (2.5 M NaCI) containing 40 mM Na2MoO4 varying the buffer added to support the Mo(VI) to which hypochlorite and chlorate ions were added to simulate the real operating conditions of the cell (weak chlorate solution).

Example 5

2.5 M NaCI + 15 mM NaCIO + 0.5 M NaCIO₃ + 40 mM Na₂ MoO₄ + 50 mM KHF

The objective of the experiment to be described below is to test the selectivity of the steel electrodes under more severe operating conditions, by adding hypochlorite ions and chlorate present in large quantities in the supply current of the plant's electrolysis unit from the CIO synthesis section (a solution called 'weak chlorate' rich in sodium chloride and unreacted chlorate, to be distinguished from the 'strong chlorate' solution representative of the current leaving the cell and thus rich in NaCIOs produced by electrolysis).

For this reason, the flow cell was fed with a brine containing molybdate ions (necessary to ensure high efficiency for the hydrogen development process) and the buffers selected for pH control to which NaCIO and NaCIOs were added to simulate the weak chlorate current.

The values of the efficiencies for the hydrogen development reaction shown in Figure 15a and obtained from selectivity measurements carried out in 2.5 M NaCI containing 15 mM NaCIO, 0.5 M NaCIOs , 40 mM Na2MoO4 and 50 mM KHF (pHm ~ 4.14) reach an average value of 75 % after steady-state, indicating a lower selectivity for the carbon steel electrode than that obtained in the presence of the dichromate ions, but still comparable to that obtained in Example 3 (brine containing 40 mM Na2MoO4 and 50 mM KHF, r|H2 ~ 71 %). This result therefore suggests that KHF is able to guarantee high electrode selectivity even under the most severe operating conditions, such as those of an electrolysis cell feed in a plant (current rich in hypochlorite and chlorate ions).

Figure 15b shows the estimated efficiencies for OER in the same solution. Also shown are the efficiencies for the oxygen development process obtained in the same solution in the absence of hypochlorite and chlorate ions (2.5 M NaCI containing 40 mM Mo(VI) and 50 mM KHF).

The values for the oxygen development efficiencies shown in Figure 15b indicate an increase in the oxygen molar flow rate (nO2,h_yPo ~ 28 %) that is greater than that produced in the absence of the competitive species CIO' and CIO'3 ( 02 ~ 18 %). The increase in OER efficiency is in this case attributable to the O2 development processes described in equilibria (6), (7), (8) and (9).

Similarly to Example 3 (brine containing molybdate and phthalate ions), the pH value of the solution measured during cell operation is higher than that measured when Cr(VI) is added to the brine, reaching a steady-state value of ~ 6.85, as shown in Figure 16a.

The addition of the hypochlorite and chlorate ions therefore does not affect the buffer's ability to buffer the pH of the solution.

The cell voltage value is also unaffected by the presence of NaCIO and NaCIOs, as shown in Figure 16b.

In fact, the AV of the cell in the presence of hypochlorite and chlorate ions is constant over time and equal to 3.9 V, which is lower than that recorded during cell operation in the presence of dichromate ions (AV ~ 4.4) and slightly lower than that recorded in the same solution in the absence of NaCIO and NaCIOs (AV ~ 4.0 V).

Finally, an image of the electrode surface after the electrolysis process in the presence of hypochlorite and chlorate ions in the phosphate ion buffer in the presence of Na2 MoO4 is shown in the insert of Figure 16b.

The electrode appears clean and free of corrosion products on the surface despite the presence of hypochlorite and chlorate ions (see insert of Figure 16b). This result therefore suggests that the electrode is sufficiently protected even when working in the presence of chloride species, which can lead to a shift in the corrosion potential of the steel towards more anodic values and thus more easily induce the onset of corrosion processes that can damage the electrode, especially in the shut-down phase of the system.

Table 4 lists the values of r|_H2, 002, cell voltage and pH measured at both the beginning and end of the experiments for carbon steel electrodes with varying additives added. The values of the efficiencies, pH and AV obtained in the presence of dichromate ions are also shown for comparison.

Table 4. Efficiency values for HER and OER, cell voltage and pH before and after the electrolysis process as a function of the added additives.

The values reported in Table 4 therefore suggest that the phthalate ion buffer can guarantee high electrode selectivity, in agreement with the efficiency values reported for the hydrogen development process, even in the presence of the hypochlorite and chlorate ions. The addition of KHF in the Mo(VI)-containing solution succeeds in containing the pH increase during electrolysis of the brine without adversely affecting the cell voltage, even under more severe conditions such as plant conditions (presence of NaCIO and NaCIO₃).

Example 6

2.5 M NaCI + 15 mM NaCIO + 0.5 M NaCIO₃ + 40 mM Na₂MoO₄ + 32 mM NaH₂PO₄

The experiment described in Example 4 (electrolysis of a brine solution containing 40 mM Mo and 32 mM NaH₂PO₄) was repeated this time in the presence of hypochlorite and chlorate ions, similarly to what was done for the phthalate ion buffer (see Example 5). Again, the aim is to test the selectivity of the electrode in the presence of CIO' and CIO'3. As in the case of the phthalate ion buffer, the addition of hypochlorite and sodium chlorate to the brine containing molybdate and phosphate ions made it possible to simulate the weak chlorate current used in the plant. This experiment allowed the additive to be tested under more severe conditions than those reported in the literature¹⁷ by also adding sodium chlorate.

In this case, the values of the efficiencies for HER shown in Figure 15a and estimated following the electrolysis of the brine of composition 2.5 M NaCI containing 15 mM NaCIO, 0.5 M NaCIOs , 40 mM Na₂MoO₄ and 32 mM NaH₂PO₄ (pHm ~ 5.24), reach a steady-state value of 63.5 %, indicating a significantly lower efficiency than that obtained in the case of Cr(VI) and that estimated using the brine containing 40 mM Mo(VI) and 32 mM NaH₂PO₄ in the absence of hypochlorite and chlorate ions, as described in Example 4 (see Table 3). This result therefore suggests a worsening of electrode selectivity when using weak chlorate solution as electrolysis unit feed in the presence of phosphate ions. Figure 15b shows the efficiency values for the OER estimated during the electrolysis of the solution containing hypochlorite and chlorate ions following the addition of the phosphate ion buffer supporting Mo(VI) to control the pH of the solution.

The efficiency for the oxygen development process in the cell following electrolysis of the brine in the presence of phosphate ions this time was approximately 25%, indicating (as in the case of the phthalate ion buffer, see example 5) a greater catalytic effect on the O₂ development processes in the cell than in the case described in example 4, i.e. in the absence of chlorinated species (2.5 M NaCI containing 40 mM Mo(VI) and 32 mM NaH₂PO₄). The pH values measured at each sampling (every 15 minutes) also increase slightly compared to those obtained in the presence of hypochlorite and chlorate ions, although after - 100 minutes, the final pH value is approximately 6, as shown in Figure 17a.

However, the pH trend during the experiment is comparable with that obtained in the presence of Cr(VI), thus suggesting that the additive is able to guarantee a buffering power comparable to that of sodium dichromate even in the presence of the hypochlorite and chlorate ions.

The cell voltage values shown in Figure 17b, on the other hand, indicate a slight increase in AV in the presence of CIO' and CIO'3 to approximately 4.1 V, which is slightly higher than that recorded in the absence of NaCIO and NaCIOs.

However, the AV value measured during the entire operating period of the cell is lower than that obtained in the presence of Na2Cr2O? (AVCr(vi) equal to 4.4V).

The inset in Figure 17b shows a photo of the electrode after the electrolysis process in 2.5 M NaCI containing 15 mM NaCIO, 0.5 M NaCIOs, 40 mM Mo(VI) and 32 mM NaH₂PO₄.

Despite the presence of hypochlorite and chlorate ions, the electrode surface is again clean and free of corrosion products. This result confirms the protective action of NaH2PO4 against the electrode, a fundamental property exerted by sodium dichromate to protect steel in the presence of chlorinated species, especially during plant shutdowns where the electrodes are not cathodically protected. The protective action appears to be better than that offered by phthalo ions under the same operating conditions, i.e. in the presence of hypochlorite and chlorate ions (see Figure 16b).

Finally, Table 5 summarises the values of the efficiencies for HER and OER, AV and solution pH for carbon steel electrodes used as cathodes in the presence of a phosphate ion buffer, with and without hypochlorite and chlorate ions. The values measured in the presence of Cr(VI) are also shown for comparison.

Table 5 . Efficiency values for HER and OER, cell voltage and pH before and after the electrolysis process as a function of the added additives.

The experimental results described in Example 6 indicate a deterioration in the performance of the steel electrode in the presence of the phosphate ion buffer (in the presence of 40 mM Mo(VI)) after the addition of NaCIO and NaCIOs , as suggested by the efficiency values for the hydrogen development process shown in Figure 15a.

On the contrary, the r|_H2 obtained using KHF as a buffer (in the presence of 40 mM Mo(VI)) is substantially the same as that obtained in the absence of the ions CIO' and CIO'3 (see Figure 15a and Example 5), thus demonstrating that even under more severe operating conditions, such as those in the plant, KHF is able to confer high selectivity to the electrode. As shown in Figure 15b, for both buffers there is an increase in OER efficiency corresponding to an increase in the amount of oxygen produced in the cell during brine electrolysis probably due to reactions (6), (7), (8) and (9).

The addition of chlorinated species to the KHF-containing solution, however, does not adversely affect the cell voltage and pH values compared to those obtained in the absence of hypochlorite and chlorate ions. In contrast, the cell AV and the pH of the solution after two hours of cell operation increase slightly in the case of the phosphate ion buffer following the addition of NaCIO and NaCIOs. However, the values measured are lower than those obtained in the solution containing sodium dichromate.

The comparison of the performance of the carbon steel in the two selected buffers thus indicates that under more severe operating conditions (i.e. in the presence of hypochlorite and chlorate ions) the efficiency of the HER decreases drastically in the case of the phosphate ions while it remains practically the same in the case of the phthalate buffer. This result therefore suggests a better behaviour of the electrode in terms of selectivity and electrocatalytic properties when 50 mM KHF is added to the brine containing 40 mM Mo(VI).

Example 7 - 2.5 M NaCI + 15 mM NaCIO + 0.5 M NaCIO₃ + 40 mM Na₂MoO₄ + 50 mM KHF, titanium electrode

As reported in the literature, the electrodes most commonly used as cathodes for the sodium chlorate production process are carbon steel and titanium^7-9’^17-19.

Therefore, after optimising the brine composition based on the experimental results described in Examples 5 and 6, the selectivity tests were repeated using titanium electrodes as cathodes for the hydrogen development process during electrolysis of the brine. Again, the experiment was carried out by keeping the cell running for 2 h by feeding a hot solution and monitoring the efficiencies of the HER and OER over time, as well as the effect of the addition of the additives on the solution pH and cell voltage. The tests were carried out at 80 °C and 400 rpm with the experimental set-up shown in Figure 1.

The values of the efficiencies for the HER in the case of titanium electrodes used as cathodes for the electrolysis of the brine with a composition of 2.5 M NaCI in the presence of hypochlorite and chlorate ions (15 mM NaCIO and 0.5 M NaCIOs) to which 40 mM Na2MoC>4 and 50 mM KHF were added (pHm equal to 4.12) reach a constant value of 69.8 % after a transient of about 45 minutes. The q values obtained are again lower than those obtained when using Cr(VI) in the plant (see Table 6). The efficiencies obtained are also slightly lower, but still comparable to those obtained in the case of carbon steel (~ 75 %) in the same solution (see Figure 18a).

Figure 18b shows the estimated efficiencies for the oxygen development process. Also shown are the OER efficiencies for the steel electrodes used as cathodes during electrolysis of the same solution.

The molar flow rate of oxygen developed during the electrolysis of brine to which 40 mM Mo(VI) and 50 mM KHF were added in the presence of the competitive species CIO' and CIO'3 is significantly lower (14.2 %) than that obtained for carbon steel, in agreement with the experimental results shown in Figure 18b. The OER efficiency is also comparable to that obtained when Cr(VI) is used as an additive (see Table 6). The result obtained thus suggests that the use of titanium electrodes used as cathodes for the electrolysis of brine containing Mo(VI) and KHF as a buffer in the presence of CIO' and CIO'3 has no significant catalytic effects on the kinetics of O2 development produced in the cell.

Figure 19a shows the pH trend during operation of the cell. Again, the pH values measured for the carbon steel electrode used in the same solution and those for Cr(VI) are shown in order to have a reference and assess the buffering capacity of the buffer selected as the electrode changes.

The pH measured at the end of the test is ~ 6.83, which is higher than that measured in the presence of Cr(VI) but substantially the same as that obtained in the case of carbon steel used as a cathode in the same solution (pHnn ~ 6.85, see Example 5). The result obtained therefore confirms the buffering capacity of the phthalate ion buffer, even when using titanium as an electrode instead of carbon steel.

Finally, Figure 19b shows the cell voltage trend using titanium as a cathode in the presence of hypochlorite and chlorate ions when Mo(VI) and phthalate ion buffer are added to the brine.

The value of the cell voltage during the electrolysis of the brine is also in this case constant and equal to ~ 4.4 V, a value very similar to that obtained in the case of Cr(VI) but significantly higher than that obtained in the case of using steel as the cathode during the electrolysis of the solution of the same composition (~ 3.9 V). The increase in cell AV of about 500 mV in the case of titanium can be explained by considering that the electrode is effectively passivated by exposure to air, so it is inevitable that part of the applied voltage is lost as a potential drop within the oxide. Finally, the image of the electrode surface after electrolysis of the brine containing KHF in the presence of 40 mM Mo(VI) and the hypochlorite and chlorate ions using titanium as the cathode is shown (see inset of Figure 19b).

As can be seen from the image in Figure 19b, the electrode surface is totally free of corrosion products, suggesting a better corrosion resistance than that of carbon steel, which is crucial during plant shut down. Furthermore, the use of titanium implies the reduction of the release of iron ions from the electrode into solution, which can induce contamination of the brine and thus a decrease in chlorate production efficiency due to the onset of competitive processes (redox reactions of the Fe²⁺/Fe³⁺ pair in solution). The use of titanium in the plant could also reduce the frequency of shutdowns and thus reduce the maintenance costs of filters downstream of the cell.

The experimental results therefore suggest that even for titanium electrodes, the addition of sodium molybdate to the brine can guarantee high efficiencies for the hydrogen development process. The addition of potassium hydrogen phthalate, on the other hand, ensures effective pH control during the process by compensating for the buffering action of Mo(VI). Selectivity tests have therefore shown that even with titanium, it is possible to effectively abate competitive hypochlorite and chlorate ion reduction processes using our proposed solution.

Example 8 - 2.5 M NaCI + 15 mM NaCIO + 0.5 M NaCIO₃ + 40 mM Na₂ MoO₄ + 32 mM NaH₂PO4 , titanium electrode

The experiment described in Example 7 was finally repeated using a brine containing molybdate and phosphate ions to which NaCIO and NaCIOs were added to study the selectivity of the titanium electrode in the solution simulating the weak chlorate feed sent into the cell.

Selectivity tests recorded in the 2.5 M NaCI composition brine containing 15 mM NaCIO, 0.5 M NaCIOs , 40 mM Na₂MoO4 and 32 mM NaH₂PO4 (pHm ~ 5.14) indicate that with the titanium electrode, values of r|_H2 of about 81 % are obtained at steady state (see Figure 18a), which is lower than that obtained in the case of Cr(VI), but higher than that estimated in the case of carbon steel used as electrode in the same solution suggesting a better selectivity for titanium in the bath containing phosphate, hypochlorite and chlorate ions (see Table 6).

Figure 18b shows the efficiencies for the OER. The efficiency for the oxygen development process for the carbon steel electrode used as a cathode in the same solution is also shown in order to compare the performance of both electrodes. The value of r|o2 estimated through reaction (16) averages 12 %, which is slightly lower than that obtained in the case of Cr(VI) (see Table 6). The estimated efficiencies are also lower than those obtained in the case of carbon steel in the same solution (see Figure 18b). The pH values shown in Figure 20a again confirm the high buffering power of the phosphate buffer. In particular, after a slight initial increase, the pH stabilises after about 60 minutes at an average value of 5.8, which is lower than that measured using carbon steel as the cathode in the same solution, and that obtained after two hours of cell operation in the presence of Cr(VI) (pHnn ,cr<vi) ~ 6, see example 1).

Finally, Figure 20b shows the cell AV values measured after each sampling.

The reported values indicate that the cell voltage in the presence of phosphate ions using titanium as a cathode for the chlorate production process is constant and equal to approximately 4.1 V during the 2 h of cell operation. The value obtained is also comparable to that measured using carbon steel as the electrode in the same solution. The cell voltage in both cases is slightly lower than that measured in the presence of dichromate ions, thus suggesting that the buffer selected does not adversely affect the AV of the cell during the electrolysis process even for titanium electrodes.

Finally, the inset of Figure 20b shows the image of the titanium electrode surface after electrolysis of brine containing molybdate and phosphate ions in the presence of NaCIO and NaCIOs.

The electrode surface appears clean and free of corrosion products as in the case of example 7 where the experimental results were described for the titanium electrode used as a cathode in the electrolysis process in the presence of the phthalate ion buffer. This result confirms the better corrosion resistance offered by titanium electrodes compared to that of carbon steel in the presence of chlorinated species (Cl; CIO' and CIO's) present in the cell feed brine.

Finally, Table 6 summarises the values of the hydrogen and oxygen development efficiencies, pH and cell voltage for titanium electrodes with varying additives added. Again, the values obtained in the presence of Cr(VI) are shown for reference, and those obtained in the same solutions for carbon steel in order to compare the performance of both electrodes.

Table 6. Efficiency values for HER and OER, cell voltage and pH before and after the electrolysis process as a function of added additives.

Comparing the performance of the titanium in the varying buffer used, it can be seen that following the addition of the phosphate ion buffer a higher efficiency is obtained for HER than that estimated in the presence of phthalate ions. In contrast, the efficiency for HER in the presence of NaH2 PO4 is lower than that estimated following the addition of KHF. The results obtained therefore suggest that titanium is able to effectively suppress the competitive processes at the cathode (reduction of CIO' and CIO'3 and formation of Cl’ ), at the anode (development of oxygen) and in the bulk of the solution (dissociation of CIO and CIO--3 ions and development of Cl’ and O2 ) when phosphate ion buffer is added to the brine containing 40 mM Mo(VI).

The pH of the solution also varies as the buffer added varies. In particular, as shown by the values in Table 6, the pH value in the case of the phosphate buffer is lower than that obtained in the presence of KHF and comparable with that measured in the presence of dichromate ions. This result again confirms the better buffering power of NaH2PO4 compared to potassium hydrogen phthalate, regardless of the electrode used.

The cell voltage value is also lower in the case of the phosphate ion buffer, as shown by the values in Table 6.

The experimental results described in Examples 7 and 8 therefore suggest that even for titanium electrodes, the addition of sodium molybdate to the brine is able to guarantee high hydrogen development efficiencies, even in the presence of hypochlorite and chlorate ions, which can induce the onset of competitive reduction processes at the cathode, and thus the decrease in efficiency for the HER and the NaCIO₃ production process. Again, as with carbon steel, buffers had to be selected to compensate for the buffering action of Mo(VI).

In particular, the efficiency values obtained for HER indicate a higher selectivity for titanium in the presence of the phosphate ion buffer. However, both selected buffers do not induce a significant increase in the amount of oxygen in the cell (oxidation of water at the anode and/or dissociation of hypochlorite and chlorate ions in the bulk of the solution) compared to the case of Cr(VI), as confirmed by the efficiency values for HER, thus suggesting better performance of titanium in the presence of hypochlorite and chlorate ions.

The pH measured during cell operation suggests a better buffering power of Na^PC than that of KHF. The cell voltage is also lower in the case of phosphate buffer, thus suggesting lower ohmic drops in the interelectrode gap.

The experimental results therefore suggest that it is possible to use titanium in the cell as an alternative to carbon steel as confirmed by selectivity tests. The electrode also exhibits the best performance when a phosphate ion buffer is used in addition to Mo(VI) to support Na2MoC>4 . However, as reported in the literature²⁶²⁷, these electrodes are subject to hydrogen embrittlement by the formation of TiH2 , so they could degrade much faster than carbon steel. Only a comparison of maintenance costs, linked in the case of carbon steel to filter cleaning operations following electrode corrosion, will make it possible to replace carbon steel in plant electrolysis cells with titanium.

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Claims

CLAIMS A process for the preparation of sodium chlorate comprising the step of electrolysing an electrolyte in an undivided electrolysis cell, wherein said electrolyte comprises a brine of sodium chloride (NaCI), hexavalent molybdenum (Mo(VI)) in an amount not exceeding 40 mM, and at least one buffering agent based on phthalate ions, said electrolyte also being substantially free of chromium. Process according to claim 1 , wherein hexavalent molybdenum is added to the electrolyte in the form of salts of Mo(VI) of alkali or alkaline earth metals, preferably sodium molybdate (Na2MoO4), sodium molybdate dihydrate (Na2MoO4 2 H2O) or mixtures thereof. Process according to claim 1 or 2, wherein the buffering agent based on phthalate ions is added to the electrolyte in the form of potassium hydrogen phthalate (KC8H5O4, KHF), preferably in an amount between 40 and 60 mM. Process according to any one of claims 1 to 3, wherein the electrolyte further comprises a phosphate ion buffering agent, preferably wherein the phosphate ion buffering agent is added to the electrolyte in the form of sodium hydrogen phosphate (NaH2PO4), preferably in an amount of between 20 and 40 mM. Process according to any one of claims 1 to 4, wherein said electrolyte comprises: 2.5 M NaCI, 40 mM Na₂MoO₄ and 50 mM KHF. Process according to any of the preceding claims, which takes place under controlled pH conditions between 5 and 7, preferably between 5 and 6. A system for the production of sodium chlorate comprising at least one undivided electrolytic cell equipped with a plurality of cathodes and a plurality of anodes, said cell being supplied with an electrolyte comprising a brine of sodium chloride (NaCI), hexavalent molybdenum (Mo(VI)) and in an amount not exceeding 40 mM, and at least one buffering agent based on phthalate ions, said electrolyte being also substantially free of chromium. System according to the preceding claim, wherein said cathodes comprise carbon steel or titanium and said anodes are dimensionally stable anodes (DSA) made of titanium, said cathodes and anodes being preferably of lamellar structure and spaced by separators made of insulating material. System according to claim 7 or claim 8 comprising a multiplicity of electrolytic cells arranged in series. Use of an electrolyte solution comprising hexavalent molybdenum (Mo(VI)) in an amount not exceeding 40 mM and at least one buffering agent based on phthalate ions, said electrolyte also being substantially chromium-free, to increase the selectivity of the electrodes, preferably of the cathode, and/or to decrease or inhibit corrosion of the cathode, in an undivided cell electrolytic cell, preferably in an undivided cell electrolytic cell for the continuous production of sodium chlorate. Use according to the previous claim, wherein this electrolyte solution a sodium chloride (NaCI) brine.