Patent Description:
In an electrochemical reaction, an oxidation reaction in an anode chamber of an electrolytic bath and a reduction reaction in a cathode chamber of the electrolytic bath occur simultaneously.

In most electrochemical processes, a target product is manufactured through an anodic or cathodic reaction. On the opposite counter electrode, an acid or a base is produced through the electrochemical reaction of water, which is a solvent in a solution. These reactants are generated as byproducts, and are treated as a waste solution or used for another purpose.

For example, <CIT> discloses "a conductive diamond electrode, and a sulfuric-acid electrolysis method and a sulfuric-acid electrolytic apparatus using the same" as a technology for producing an oxidant using an oxidation reaction occurring in an anode chamber of an electrolytic bath. As shown in <FIG>, an anode chamber <NUM>, in which a conductive diamond anode <NUM> is received and which is filled with an electrolyte solution including sulfuric acid ions, and a cathode chamber <NUM>, in which a conductive diamond cathode <NUM> is received and which is filled with sulfuric acid at the same concentration as that in the anode chamber <NUM>, are partitioned by a porous PTFE diaphragm <NUM>. An anolyte supply port <NUM> is connected to the anode chamber <NUM>, and sulfuric acid, which is an anolyte, is supplied to the anode chamber <NUM> through the anolyte supply port <NUM>. Further, a catholyte supply port <NUM> is connected to the cathode chamber <NUM>, and a catholyte is supplied to the cathode chamber <NUM> through the catholyte supply port <NUM>. The oxidizing material solution generated in the anode chamber <NUM> is discharged through an anolyte outlet <NUM>. Further, the hydrogen and the residual sulfuric acid solution generated in the cathode chamber <NUM> are discharged through a catholyte outlet <NUM>. When sulfuric acid, which is an anode electrolyte, is injected into the anode chamber, peroxodisulfuric acid is generated through an anodic reaction. In the cathode chamber, hydrogen (H<NUM>) and hydroxide ions (OH-) are generated through the electrolytic reaction of H<NUM>O, which is a solvent in the cathode solution. The hydroxide ions (OH-) meet hydrogen ions (H+) moved after the reaction in the anode or hydrogen ions (H+) in the sulfuric acid (H<NUM>SO<NUM>) supplied to the cathode chamber, so that the concentration of the cathode solution is controlled through a neutralization reaction. Generated H<NUM> is mainly diluted and then discarded. As described above, the oxidizing material, which is a peroxodisulfur oxide generated using an anodic reaction, is produced as a target product, and the hydroxide ions generated using a cathodic reaction are discarded through a neutralization reaction.

As another example of the prior art, <CIT> discloses "an electrochemical reduction method of carbon dioxide using a solution including potassium sulfate", in which chemical products are produced using a reduction reaction occurring in a cathode chamber of an electrolytic bath. <FIG> is an exemplary view showing a conventional electrochemical conversion process of carbon dioxide under a solution condition including potassium sulfate. In a first step, on an oxidation electrode portion, oxygen is generated according to the oxidation reaction of water, which is a solvent, and hydrogen ions (H+) are generated. K+ and H+ in a solution phase pass through a cation membrane to a reduction electrode unit. On the reduction electrode unit, carbon dioxide is converted into formate (HCOOK) using an electrode reaction. In this process, KOH is continuously supplied to an oxidation reaction unit in order to balance the ions. In a second step, one equivalent of potassium sulfate is produced when two equivalents of formic acid are produced using sulfuric acid (H<NUM>SO<NUM>). In the third step, formic acid is distilled and potassium sulfate is precipitated, so that the formic acid and the potassium sulfate are separated.

The <CIT> discloses methods and systems for electrochemical conversion of carbon dioxide to organic products including formate and formic acid. The system includes an electrochemical cell including a cathode compartment containing a high surface area cathode and a bicarbonate-based catholyte saturated with carbon dioxide. The high surface area cathode includes an indium coating and having a void volume of between about <NUM> % to <NUM> %. The system also includes an anode compartment containing an anode and an acidic anolyte. The electrochemical cell is configured to produce a product stream upon application of an electrical potential between the anode and the cathode.

The <CIT> discloses a preparing of an alkali metal hydroxide by a water splitting and hydrolysis. A two-compartment water splitter having alternating cation and bipolar membranes is used to remove alkali metal cations from an aqueous alkali metal chloride solution so as to produce an acidified salt solution. The coproduct is a base such as NaOH or KOH. The acidified salt solution is then fed to a membrane or diaphragm electrolytic cell for production of chlorine and alkali metal hydroxide.

The <CIT> discloses an electrolytic cell and a method for producing chemical products by means of an electrolytic cell. The electrolytic cell comprises an anode having an anode-side electrolyte and a cathode having a cathode-side electrolyte, wherein the anode-side electrolyte comprises a sulfate which is oxidized to a peroxodisulfate, and wherein the cathode-side electrolyte includes carbon dioxide which is reduced at the cathode, which is reduced at the cathode. The method for producing chemical products by means of an electrolytic cell oxidizes a sulphate to a peroxodisulphate at the anode of the electrolytic cell and reduces carbon dioxide at the cathode.

Further prior art is discussed by <NPL>.

Carbon dioxide, which is a reactant, is injected into the cathode chamber in a gas phase or in the state of being dissolved in an electrolyte, and formic acid ions are generated through a cathodic reaction. In the anode chamber, O<NUM> is generated through a decomposition reaction of H<NUM>O, which is a solvent, and generated O<NUM> is mainly diluted and then discarded.

The problems with the conventional technologies are as follows.

First, among the electrolytic reactions of an anode and a cathode, only the reaction of one side is used to generate a necessary chemical agent, and the product produced at the other electrode is obtained as a byproduct.

Second, a chemical material such as hydrochloric acid or sulfuric acid needs to be injected in order to convert formate into formic acid by acidifying the formate, which generates byproducts.

An object of the present invention is to provide an electrolysis method using an electrolysis system capable of simultaneously producing two high-value-added chemical products.

Another object of the present invention is to provide an electrolysis method using an electrolysis system capable of reducing the amount of electricity used in processes and generation of byproducts.

Another object of the present invention is to provide an electrolysis method using an electrolysis system in which formic acid is produced so that the injection of a chemical material such as hydrochloric acid or sulfuric acid when formate is converted into the formic acid is minimized.

The problem of the present invention is solved by the appended set of claims.

In order to accomplish the above objects, the present invention provides an electrolysis system that includes a diaphragm electrolytic bath, including an anode and a cathode disposed to face each other, and a diaphragm for partitioning an anode chamber in which the anode is positioned and a cathode chamber in which the cathode is positioned, so that an electrolyte solution containing sulfuric acid ions is supplied to the anode chamber to thus generate peroxodisulfate through an anodic reaction and a carbon dioxide gas or an electrolyte solution containing the gas dissolved therein is supplied to the cathode chamber to thus generate formate, and a concentration-acidification unit for receiving the formate, generated in the cathode chamber of the diaphragm electrolytic bath, so that the formate is reacted with hydrogen ions supplied from the outside or supplied using self-generation to thus be converted into formic acid.

According to an embodiment of the present invention, the concentration-acidification unit of the electrolysis system may include an acidification unit, for receiving the hydrogen ions necessary for the reaction using sulfuric acid from an external supply source to thus convert the formate supplied from the diaphragm electrolytic bath into the formic acid, and a separation unit, for separating and concentrating the formic acid and sulfate produced in the acidification unit to thus produce the formic acid.

More preferably, the sulfate separated using the separation unit may be supplied as a raw material of the anode chamber of the diaphragm electrolytic bath.

According to another embodiment of the present invention, the concentration-acidification unit of the electrolysis system may include a second electrolytic bath including a second anode chamber for receiving the formate generated in the cathode chamber of the diaphragm electrolytic bath and generating the hydrogen ions necessary for the reaction through an electrolytic reaction of a second anode to thus convert the formate into the formic acid, a second diaphragm through which cations, generated by converting the formate into the formic acid in the second anode chamber, pass, and a second cathode chamber for generating compounds including the cations moved from the second anode chamber through the second diaphragm.

Hydroxide ions, which are generated through an electrolytic reaction of a second cathode positioned in the second cathode chamber by supplying water to the second cathode chamber of the second electrolytic bath, may react with the cations, which are moved from the second anode chamber through the second diaphragm, to thus generate a hydroxide compound.

In another embodiment, hydroxide ions, which are generated through an electrolytic reaction of a second cathode positioned in the second cathode chamber by supplying sulfuric acid to the second cathode chamber of the second electrolytic bath, may react with the hydrogen ions of the sulfuric acid to thus be neutralized, sulfuric acid ions remaining after neutralization may react with the cations, which are moved from the second anode chamber through the second diaphragm, to thus generate sulfate, and the generated sulfate may be supplied as a raw material of an anode chamber of the diaphragm electrolytic bath.

According to another embodiment of the present invention, the concentration-acidification unit of the electrolysis system may include an electrodialysis stack in which the anode and the cathode are provided to face each other at both ends thereof, a plurality of cation-exchange membranes and a plurality of anion-exchange membranes are alternately arranged between the anode and the cathode to thus form compartments in spaces therebetween, and the formate generated in the cathode chamber of the diaphragm electrolytic bath and sulfuric acid as a supply source of the hydrogen ions from the outside are supplied to the compartments so that the formic acid and sulfate are separately generated in the different compartments using selective ion-passing characteristics of ion-exchange membranes.

The electrodialysis stack may include, as a repeating unit, four compartments of: a first compartment between a first cation-exchange membrane as a basis and a first anion-exchange membrane disposed closest to the first cation-exchange membrane in a cathode direction, a second compartment defined by the first anion-exchange membrane and a second cation-exchange membrane, disposed closest to the first anion-exchange membrane in the cathode direction, a third compartment defined by the second cation-exchange membrane and a second anion-exchange membrane, disposed closest to the second cation-exchange membrane in the cathode direction, and a fourth compartment defined by the second anion-exchange membrane and another first cation-exchange membrane, disposed closest to the second anion-exchange membrane in the cathode direction, the same compartments may communicate with each other and the different compartments may be isolated from each other in the electrodialysis stack, the formic acid may be generated and concentrated in any one compartment of the first compartment and the third compartment, the sulfate may be generated and concentrated in the remaining compartment, and the formate and the sulfuric acid may be separately supplied to the second compartment and the fourth compartment and then diluted.

According to another embodiment of the present invention, the concentration-acidification unit of the electrolysis system may include a water-splitting electrodialysis stack in which the anode and the cathode are provided to face each other at both ends thereof, and a plurality of amphoteric ion-exchange membranes and a plurality of ion-exchange membranes are included between the anode and the cathode, so that the hydrogen ions and hydroxide ions, generated due to a water decomposition characteristic of the amphoteric ion-exchange membranes, and the formate, supplied from the diaphragm electrolytic bath using the selective ion-passing characteristic of the ion-exchange membranes, are converted into the formic acid.

In the water-splitting electrodialysis stack, between the anode and the cathode at both ends thereof, a cation-exchange membrane C, the amphoteric ion-exchange membrane BPM, and an anion-exchange membrane A may be arranged in order of position from an anode side, an anion-exchange layer BA of the amphoteric ion membrane BPM may be provided to face the cation-exchange membrane C to thus form a compartment, and a cation-exchange layer BC may be provided to face the anion-exchange membrane A to thus form another compartment, the compartment between the cation-exchange membrane C and the anion-exchange layer BA of the amphoteric ion-exchange membrane BPM may constitute a base/salt-generation chamber, the compartment between the cation-exchange layer BC of the amphoteric ion-exchange membrane BPM and the anion-exchange membrane A may constitute a formic-acid generation chamber, and the compartment between the anion-exchange membrane A and the cation-exchange membrane C may constitute a formate injection chamber.

The formate generated in the cathode chamber of the diaphragm electrolytic bath may be supplied to the formate injection chamber, so that formic acid ions are passed through the anion-exchange membrane A and cations are passed through the cation-exchange membrane C to thus perform dilution, the hydroxide ions generated in the anion-exchange layer BA of the amphoteric ion-exchange membrane BPM may be bonded to the cations passing through the cation-exchange membrane C in the base/salt-generation chamber to thus generate hydroxide, followed by concentration, and the hydrogen ions generated in the cation-exchange layer BC of the amphoteric ion-exchange membrane BPM may be bonded to the formic acid ions passing through the anion-exchange membrane A in the formic-acid generation chamber to thus generate the formic acid, followed by concentration.

The anion-exchange membrane may include a monovalent-selective-anion-exchange membrane.

According to another constitution, in the water-splitting electrodialysis stack, between the anode and the cathode at both ends thereof, a cation-exchange membrane C and the amphoteric ion-exchange membrane BPM may be arranged in order of position from an anode side and an anion-exchange layer BA of the amphoteric ion-exchange membrane BPM may be provided in an anode direction, the water-splitting electrodialysis stack may include a base/salt-generation chamber, positioned between a cation-exchange membrane C1 and the anion-exchange layer BA of the amphoteric ion-exchange membrane BPM, and a formic-acid generation chamber, positioned between a cation-exchange layer BC of the amphoteric ion-exchange membrane BPM and a cation-exchange membrane C2. In the formic-acid generation chamber, the formate supplied from a cathode chamber of the diaphragm electrolytic bath may react with the hydrogen ions generated through the cation-exchange layer BC of the amphoteric ion-exchange membrane BPM to thus generate the formic acid, and remaining cations may pass through the cation-exchange membrane to the base/salt-generation chamber, and in the base/salt-generation chamber, the cations passing from the formic-acid generation chamber may meet the hydroxide ions generated through the anion-exchange layer BA of the amphoteric ion-exchange membrane BPM to thus generate hydroxide.

In the water-splitting electrodialysis stack, sulfuric acid may be supplied to the base/salt-generation chamber, the hydroxide ions generated in the anion-exchange layer of the amphoteric ion-exchange membrane BPM may be subjected to a neutralization reaction with the hydrogen ions of the supplied sulfuric acid to thus generate sulfuric acid ions, the sulfuric acid ions may be bonded to the cations passing through the cation-exchange membrane to thus generate and discharge salts of sulfate, and the discharged sulfate may be supplied as a raw material of an anode chamber of the diaphragm electrolytic bath.

An electrolysis method according to the present invention includes an electrolysis process of supplying an electrolyte solution containing sulfuric acid ions to an anode chamber of a diaphragm electrolytic bath that includes an anode and a cathode, disposed to face each other, and a diaphragm for partitioning the anode chamber in which the anode is positioned, and a cathode chamber in which the cathode is positioned to thus generate peroxodisulfate through an anodic reaction, while a carbon dioxide gas or an electrolyte solution containing the gas dissolved therein is supplied to the cathode chamber of the diaphragm electrolytic bath to thus generate formate, and a concentration-acidification process of supplying the formate generated in the cathode chamber of the diaphragm electrolytic bath, so that the formate is reacted with hydrogen ions supplied from the outside or supplied using self-generation to thus be converted into formic acid.

In the electrolysis method according to the present invention, the concentration-acidification process may include an acidification step of supplying the hydrogen ions necessary for the reaction using sulfuric acid from an external supply source so that the formate supplied from the diaphragm electrolytic bath is converted into the formic acid, and a separation step of separating and concentrating the formic acid and sulfate produced during the acidification step to thus produce the formic acid.

The electrolysis method according to another embodiment of the present invention may further include supplying the sulfate separated during the separation step as a raw material of an anode chamber of the diaphragm electrolytic bath.

In the electrolysis method according to another embodiment of the present invention, the concentration-acidification process may include supplying the formate generated in the cathode chamber of the diaphragm electrolytic bath and generating the hydrogen ions necessary for the reaction through an electrolytic reaction of a second anode disposed in a second anode chamber to thus perform conversion into the formic acid, passing cations, which are generated by converting the formate into the formic acid in the second anode chamber, through a second diaphragm, and generating a compound including the cations moved from the second anode chamber through the second diaphragm.

In the electrolysis method according to another embodiment of the present invention, hydroxide ions, which are generated through a water decomposition electrolytic reaction of a second cathode positioned in a second cathode chamber, may be reacted with the cations moved from the second anode chamber through the second diaphragm to thus generate a hydroxide compound.

The electrolysis method according to another embodiment of the present invention may further include supplying sulfuric acid to a second cathode chamber, so that hydroxide ions, which are generated through a water decomposition electrolytic reaction of a second cathode positioned in the second cathode chamber, are reacted with the hydrogen ions of the sulfuric acid to thus perform neutralization, sulfuric acid ions remaining after the neutralization are reacted with the cations moved from the second anode chamber through the second diaphragm to thus generate sulfate, and the generated sulfate is supplied as a raw material of an anode chamber of the diaphragm electrolytic bath.

In the electrolysis method according to another embodiment of the present invention, in the concentration-acidification process, an electrodialysis stack, in which an anode and a cathode are provided to face each other at both ends thereof and a plurality of cation-exchange membranes and a plurality of anion-exchange membranes are alternately arranged between the anode and the cathode to thus form compartments in spaces therebetween, may be used to thus supply the formate generated in the cathode chamber of the diaphragm electrolytic bath and sulfuric acid as a supply source of the hydrogen ions from the outside to the compartments, so that the formic acid and sulfate are separately generated in the different compartments using selective ion-passing characteristics of ion-exchange membranes.

In the electrolysis method according to another embodiment of the present invention, in the concentration-acidification process, the hydrogen ions and hydroxide ions, generated through a water decomposition characteristic of an amphoteric ion-exchange membrane, and the formate, supplied from the diaphragm electrolytic bath using a selective ion-passing characteristic of an ion-exchange membrane, may be converted into the formic acid using a water-splitting electrodialysis stack that includes the amphoteric ion-exchange membrane and the ion-exchange membrane.

The electrolysis method according to another embodiment of the present invention may further include injecting sulfuric acid to a base/salt-generation chamber of the water-splitting electrodialysis stack, so that sulfate is generated through a water-splitting electrodialysis process and the generated sulfate is supplied as a raw material of an anode chamber of the diaphragm electrolytic bath.

The electrolysis system and electrolysis method according to the present invention have the following effects.

First, a single electrolytic bath can be used to simultaneously produce two high-value-added chemical products and minimize the generation of byproducts through an electrolytic reaction, thereby minimizing subsequent treatment processes.

Second, it is possible to reduce the amount of electricity used in the processes and the generation of byproducts by integrating the two processes.

Third, an effect of reducing greenhouse gases can be expected due to the use of carbon dioxide.

Fourth, it is possible to minimize the additional supply of chemical materials to convert formate into formic acid.

Fifth, it is possible to reuse byproducts formed when the formate is converted into the formic acid.

Hereinafter, an electrolysis system and an electrolysis method according to the present invention will be described in detail with reference to the accompanying drawings.

<FIG> is an exemplary view showing the constitution of an electrolysis system according to an embodiment of the present invention. The constitution includes a diaphragm electrolytic bath <NUM> and a concentration-acidification unit <NUM>. The diaphragm electrolytic bath <NUM> includes a diaphragm <NUM>, and an anode chamber <NUM> and a cathode chamber <NUM> disposed to face each other with the diaphragm <NUM> interposed therebetween. The anode chamber <NUM> having an anode <NUM> and the cathode chamber <NUM> having a cathode <NUM> are disposed to face each other.

An electrolyte solution including sulfuric acid ions (SO<NUM><NUM>-) is supplied to the anode chamber <NUM>. In the present embodiment, a sodium sulfate (Na<NUM>SO<NUM>) aqueous solution is described as an example of an electrolyte including the sulfuric acid ions, but the example is not limited thereto, and sulfate such as potassium sulfate (K<NUM>SO<NUM>) and ammonium sulfate (NH<NUM>SO<NUM>) may be used.

In the anode chamber <NUM>, the sulfuric acid ions (SO<NUM><NUM>-) are converted into persulfuric acid ions (S<NUM>O<NUM><NUM>-) through an oxidation reaction of the anode <NUM> to thus generate sodium persulfate (Na<NUM>S<NUM>O<NUM>) as shown in the reaction equation of the following Equation (<NUM>).

2Na<NUM>SO<NUM> , Na<NUM>S<NUM>O<NUM> + 2Na+ (diaphragm passage) + 2e-. Equation (<NUM>).

The sodium ions (Na+) remaining after the reaction are moved from the anode chamber <NUM> to the cathode chamber <NUM> through the diaphragm <NUM>.

Meanwhile, carbon dioxide (CO<NUM>) is supplied to the cathode chamber <NUM> in a gas phase or in a state in which carbon dioxide is dissolved in a cathode electrolyte solution, thus generating formic acid ions (HCOO-) through the electrolytic reaction of the cathode <NUM> shown in Equation (<NUM>). The generated formic acid ions (HCOO-) are bonded to cations (Na+) moved from the anode chamber <NUM> through the diaphragm <NUM> to thus generate sodium formate (HCOONa), which is one of the formates (Equation (<NUM>)). Sodium hydroxide (NaOH), generated together with the sodium formate, is shown in order to describe a balance of the ions. The types of products may depend on the components of the electrolyte solution supplied to the cathode chamber <NUM>, such as carbon dioxide (CO<NUM>).

CO<NUM> + H<NUM>O + 2e- , HCOO- + OH-. Equation (<NUM>).

2Na+ + HCOO- + OH-□→ HCOONa + NaOH. Equation (<NUM>).

As described above, a reaction, in which peroxodisulfate (Na<NUM>S<NUM>O<NUM>) is generated from sulfate, in the anode chamber and a reaction, in which formate (HCOONa) is generated from carbon dioxide, in the cathode chamber may be simultaneously performed, thus simultaneously producing useful chemical agents in the anode chamber and in the cathode chamber.

The produced peroxodisulfate, which is the anode product, is a strong oxidant and is used for a synthetic resin polymerization catalyst, a de-sizing agent for fibers, a metal-surface-treating agent, an analytical reagent, and a treating agent for decomposing chemical materials. The chemical materials decomposed by the peroxodisulfate are chemical materials that cause pollution of soil, ground water, drainage, and waste, and are materials that are regulated by the Soil Contamination Measures Act, such as volatile organic compounds, cyanides, and metal cyano complexes, or are crude oil derivatives subject to guidelines for oil films and oil odors. Target materials containing the chemical materials decomposed by the peroxodisulfate may have the form of solid, liquid, or slurry.

Meanwhile, the anode <NUM> of the diaphragm electrolytic bath <NUM> may include an electrode manufactured using one or more materials selected from the group consisting of BDD (boron-doped diamond), DLC (diamond-like carbon), platinum (Pt), platinum plating, and DSA. The cathode <NUM> may include an electrode manufactured using one or more materials selected from the group consisting of BDD (boron-doped diamond), DLC (diamond-like carbon), lead (Pb), mercury (Hg), titanium (Ti), indium (In), tin (Sn), gold (Au), silver (Ag), zinc (Zn), nickel (Ni), iron (Fe), platinum (Pt), and amalgam.

The diaphragm <NUM> of the diaphragm electrolytic bath <NUM> is preferably a cation-exchange membrane. In particular, the diaphragm is more preferably a fluorine-based cation-exchange membrane.

Sodium formate (HCOONa), which is the formate generated in the cathode chamber of the diaphragm electrolytic bath <NUM>, is provided to a concentration-acidification unit <NUM>. In the concentration-acidification unit <NUM>, a reaction with the hydrogen ions (H+) supplied from the outside or supplied using self-generation occurs to thus convert the formate (HCOONa) into formic acid (HCOOH).

<FIG> is an exemplary view showing the constitution of the supply of sulfuric acid (H<NUM>SO<NUM>) from the outside as a source of supply of hydrogen ions (H+). The hydrogen ions (H+) of the supplied sulfuric acid (H<NUM>SO<NUM>) are used to convert the formate (HCOONa) into the formic acid (HCOOH) in the concentration-acidification unit <NUM>, and the remaining cations (Na+) meet the sulfuric acid ions (SO<NUM><NUM>-) to thus perform the reaction of Equation (<NUM>), in which sulfate (Na<NUM>SO<NUM>) is generated. The generated sulfate (Na<NUM>SO<NUM>) may be re-supplied as a raw material of the anode chamber <NUM> of the diaphragm electrolytic bath <NUM> to thus reduce the usage of sulfuric acid ion electrolytes supplied to the anode.

The generated formic acid (HCOOH) is used for a leather treatment agent, a rubber coagulant, a dyeing aid, a mordant, leather tanning, a medicine, an epoxy plasticizer, plating, a disinfectant, flavoring, and an organic synthetic raw material. Currently, the formic acid may be used for various purposes, such as fuel of a fuel cell.

HCOONa + NaOH + H<NUM>SO<NUM> → HCOOH + Na<NUM>SO<NUM> + H<NUM>O. Equation (<NUM>).

<FIG> is an exemplary view showing the specific constitution of <FIG>.

The concentration-acidification unit <NUM> includes an acidification unit <NUM>, to which the formate (HCOONa) is supplied and in which the sulfuric acid (H<NUM>SO<NUM>) is added as a supply source of hydrogen ions (H+), as shown in <FIG>, to thus produce the formic acid (HCOOH) using the chemical reaction shown in Equation (<NUM>), and a separation unit <NUM>, in which the generated formic acid (HCOOH) and sulfate (Na<NUM>SO<NUM>) are separated and concentrated to thus generate the target formic acid (HCOOH) and in which the sulfate (Na<NUM>SO<NUM>) is re-supplied as a raw material of the anode chamber <NUM> of the diaphragm electrolytic bath <NUM>.

<FIG> is an exemplary view showing the specific constitution according to the embodiment of <FIG>.

The concentration-acidification unit <NUM> according to <FIG> may include a second electrolytic bath <NUM> having a structure similar to that of the diaphragm electrolytic bath <NUM>. As shown in the drawing, a separate second electrolytic bath <NUM> includes a second anode chamber <NUM> and a second cathode chamber <NUM> disposed to face each other with a second diaphragm <NUM> interposed therebetween. A second anode <NUM> is disposed in the second anode chamber <NUM> and a second cathode <NUM> is disposed in the second cathode chamber <NUM> so as to face each other.

Sodium formate (HCOONa), which is the formate generated in the cathode chamber <NUM> of the diaphragm electrolytic bath <NUM>, is supplied to the second anode chamber <NUM>. In the second anode chamber <NUM>, the hydrogen ions (H+) necessary for the reaction may be produced through the water decomposition electrolytic reaction of the second anode <NUM> as shown in Equation (<NUM>) to thus produce a target material using a reaction (Equation (<NUM>)) for conversion into the formic acid (HCOOH).

H<NUM>O → <NUM>+ + ½O<NUM>(↑) + 2e-     Equation (<NUM>).

HCOONa + NaOH + <NUM>+ → HCOOH + H<NUM>O + 2Na+□     Equation (<NUM>).

Sodium ions (Na+), which are the cations remaining after the conversion into the formic acid (HCOOH), are moved to the second cathode chamber <NUM> through the second diaphragm <NUM>, and hydroxide ions (OH-) are generated through the electrolytic reaction of water at the second cathode <NUM>, so that the hydroxide ions meet the sodium ions (Na+) moved through the second diaphragm <NUM> to thus generate sodium hydroxide (NaOH) in the second cathode chamber <NUM>.

<FIG> is an exemplary view showing the specific constitution according to the embodiment of <FIG>. In another aspect, as shown in <FIG>, when additional sulfuric acid (H<NUM>SO<NUM>) is supplied to the second cathode chamber <NUM>, the hydroxide ions (OH-) generated at the second cathode <NUM> are bonded to the hydrogen ions (H+) of the sulfuric acid (H<NUM>SO<NUM>), thus being neutralized to water (H<NUM>O). The remaining sulfuric acid ions (SO<NUM><NUM>-) meet the sodium cations (Na+) moved through the second diaphragm <NUM> to thus generate sodium sulfate (Na<NUM>SO<NUM>), and the sodium sulfate is re-supplied as a raw material of the anode chamber <NUM> of the diaphragm electrolytic bath <NUM>, whereby the usage of raw materials for the anode may be reduced.

A concentration-acidification unit <NUM> according to another embodiment of the present invention may be implemented by an electrodialysis stack in which an anode and a cathode are provided to face each other at both ends thereof and in which a plurality of cation-exchange membranes and a plurality of anion-exchange membranes are alternately arranged between the anode and the cathode to thus form compartments in spaces therebetween.

The formate (HCOONa) generated in the cathode chamber <NUM> of the diaphragm electrolytic bath <NUM> and the sulfuric acid (H<NUM>SO<NUM>) as a supply source of the hydrogen ions (H+) from the outside are supplied to the compartments of the electrodialysis stack. When a direct-current power source is supplied to each of the anode and the cathode, electrodialysis occurs due to the selective ion-passing characteristics of the ion-exchange membranes, so that the formic acid and the sulfate are separately generated and concentrated in the different compartments.

The constitution of the electrodialysis stack in an electrodialysis process is shown in <FIG>. The electrodialysis process according to <FIG> will be described in more detail hereinafter. When a cation-exchange membrane is taken as a basis and is referred to as a first cation-exchange membrane C1, a first anion-exchange membrane A1, a second cation-exchange membrane C2, a second anion-exchange membrane A2, and another first cation-exchange membrane C1' may be defined to be arranged and disposed in sequence starting from the first cation-exchange membrane C1 and moving in a cathode-side direction.

The first cation-exchange membrane C1 and the first anion-exchange membrane A1 form a first compartment <NUM>, the first anion-exchange membrane A1 and the second cation-exchange membrane C2 form a second compartment <NUM>, the second cation-exchange membrane C2 and the second anion-exchange membrane A2 form a third compartment <NUM>, and the second anion-exchange membrane A2 and another first cation-exchange membrane C1' form a fourth compartment <NUM>. The four compartments are repeatedly formed using the ion-exchange membranes provided between the anode and the cathode so that multiple compartments are layered. Same compartments communicate with each other, but different compartments are isolated from each other. That is, the first compartment communicates with another first compartment, but the first compartment and the second compartment, the third compartment, or the fourth compartment are isolated from each other, so that the mass transfer is not performed without passing through the ion-exchange membrane.

In the electrodialysis stack, the formic acid (HCOOH) and the sulfate (Na<NUM>SO<NUM>) are generated and concentrated in the first compartment <NUM> and the third compartment <NUM>, respectively, and a sulfuric acid (H<NUM>SO<NUM>) solution and the formate (HCOONa) generated in the cathode chamber <NUM> of the diaphragm electrolytic bath <NUM> are supplied to the second compartment <NUM> and the fourth compartment <NUM>, respectively, and then diluted.

Hereinafter, a detailed description will be given with reference to the drawings. First, when the sulfuric acid (H<NUM>SO<NUM>) is supplied to the second compartment <NUM>, when the formate (HCOONa) is supplied to the fourth compartment <NUM>, and when a direct-current power source is supplied to electrodes at both ends thereof to thus perform electrodialysis, hydrogen ions (H+), which are cations of the sulfuric acid (H<NUM>SO<NUM>) supplied to the second compartment <NUM>, are passed through the second cation-exchange membrane C2 to the third compartment <NUM> in a cathode direction, and the sulfuric acid ions (SO<NUM><NUM>-), which are anions, are passed through the first anion-exchange membrane A1 to the first compartment <NUM> in an anode direction. Further, sodium ions (Na+), which are the cations of sodium formate (HCOONa), which is the formate supplied to the fourth compartment <NUM>, are passed through another first cation-exchange membrane C1' positioned in a cathode direction to the first compartment <NUM> adjacent thereto in another cathode-side direction. The formic acid ions (HCOO-), which are anions, are passed through the second anion-exchange membrane A2 positioned in an anode direction to the third compartment <NUM>. Through the above-described process, the sulfuric acid (H<NUM>SO<NUM>) of the second compartment and the formate (HCOONa) of the fourth compartment are separately passed and then diluted.

In the first compartment <NUM>, the sulfuric acid ions (SO<NUM><NUM>-) moved from the second compartment <NUM> meet the sodium ions (Na+) moved from the fourth compartment <NUM> adjacent thereto in another anode side direction to thus generate sodium sulfate (Na<NUM>SO<NUM>). In the third compartment <NUM>, the hydrogen ions (H+) moved from the second compartment <NUM> meet the formic acid ions (HCOO-) moved from the fourth compartment <NUM> to thus generate formic acid (HCOOH).

Through this series of processes, the generation and concentration of the sodium sulfate (Na<NUM>SO<NUM>) in the first compartment <NUM>, the dilution of the sulfuric acid (H<NUM>SO<NUM>) in the second compartment <NUM>, the generation and concentration of the formic acid (HCOOH) in the third compartment <NUM>, and the precipitation of sodium formate (HCOONa) in the fourth compartment <NUM> occur simultaneously.

The sodium formate (HCOONa) may be supplied to the second compartment <NUM> and the sulfuric acid (H<NUM>SO<NUM>) may be supplied to the fourth compartment <NUM>. In this case, the formic acid (HCOOH) is generated in the first compartment <NUM> and the sodium sulfate (Na<NUM>SO<NUM>) is generated in the third compartment <NUM>. Further, the anode is positioned at a left end and the cathode is positioned at a right end in the drawing, but they may be reversely positioned, and even in this case, the movement of the ions is achieved in the same manner.

In the electrodialysis, the solution supplied to each compartment is circulated and supplied in a semi-batch manner, so that generation water or dilution water may be obtained at a desired concentration. Further, preferably, separate electrode water is circulated and supplied to electrode spaces in which the anode and the cathode are positioned.

A concentration-acidification unit <NUM> according to another embodiment of the present invention may be implemented as a water-splitting electrodialysis stack in which an anode and a cathode are provided to face each other at both ends thereof and a plurality of amphoteric ion-exchange membranes and a plurality of cation/anion-exchange membranes are included between the anode and the cathode, so that hydrogen ions (H+) and hydroxide ions (OH-), generated due to the water decomposition characteristic of the amphoteric ion-exchange membranes, and the formate (HCOONa), supplied from the cathode chamber <NUM> of the diaphragm electrolytic bath <NUM> using the selective ion-passing characteristic of the ion-exchange membranes, are converted into formic acid (HCOOH).

The constitution of the water-splitting electrodialysis stack is shown in <FIG> and <FIG>.

<FIG> is an exemplary view showing the constitution of the water-splitting electrodialysis stack as the concentration-acidification unit according to another embodiment of <FIG>. The constitution of the water-splitting electrodialysis stack according to <FIG> will be described in more detail as follows. As shown in <FIG>, in the water-splitting electrodialysis stack, the anode and the cathode are provided to face each other at both ends thereof. Between the anode and the cathode, a cation-exchange membrane C, an amphoteric ion-exchange membrane BPM, and an anion-exchange membrane A form a single unit cell, and a plurality of unit cells is layered from the anode in a cathode direction. The layering is performed so that the anion-exchange layer BA of the amphoteric ion-exchange membrane BPM is adjacent to the cation-exchange membrane C and the cation-exchange layer BC of the amphoteric ion-exchange membrane BPM is adjacent to the anion-exchange membrane A.

The compartment defined by the cation-exchange membrane C and the anion-exchange layer BA of the amphoteric ion-exchange membrane BPM constitutes a base/salt-generation chamber <NUM>, the compartment defined by the cation-exchange layer BC of the amphoteric ion-exchange membrane BPM and the anion-exchange membrane A constitutes a formic-acid generation chamber <NUM>, and the compartment defined by the anion-exchange membrane A and the cation-exchange membrane C constitutes a formate injection chamber <NUM>.

Hereinafter, the operation of the invention according to the drawings will be described in detail. First, when sodium formate (HCOONa), which is the formate, is injected into the formic-acid-injection chamber <NUM> and when a direct-current power source is supplied to each of the anode and the cathode, the sodium ions (Na+) of the sodium formate (HCOONa) are passed through the cation-exchange membrane C from the formic-acid-injection chamber <NUM> to the base/salt-generation chamber <NUM>, and the formic acid ions (HCOO-) are passed through the anion-exchange membrane A to the formic-acid generation chamber <NUM>. Further, hydrogen ions (H+) are generated in the cation-exchange layer BC and hydroxide ions (OH-) are generated in the anion-exchange layer through a water decomposition reaction in the amphoteric ion-exchange membrane BPM. The generated hydroxide ions (OH-) meet the sodium ions (Na+) passing through the cation-exchange membrane C in the base/salt-generation chamber <NUM> to thus generate sodium hydroxide (NaOH), and the hydrogen ions (H+) meet the formic acid ions (HCOO-) moving through the anion-exchange membrane A in the formic-acid generation chamber <NUM> to thus generate formic acid (HCOOH).

Through the above-described process, the sodium formate (HCOONa) is diluted in the formate injection chamber <NUM>, the sodium hydroxide (NaOH) is generated and concentrated in the base/salt-generation chamber <NUM>, and the formic acid (HCOOH) is generated and concentrated in the formic-acid generation chamber <NUM>.

<FIG> is an exemplary view showing the constitution of a water-splitting electrodialysis stack as a concentration-acidification unit according to the embodiment in which sulfuric acid (H<NUM>SO<NUM>) is supplied as a hydrogen ion (H+) supply source. Referring to the constitution of <FIG>, the sulfuric acid (H<NUM>SO<NUM>) may be supplied to the base/salt-generation chamber <NUM> of the water-splitting electrodialysis stack, and the hydrogen ions (H+) of the supplied sulfuric acid (H<NUM>SO<NUM>) may meet the hydroxide ions (OH-) generated in the anion-exchange layer BA of the amphoteric ion-exchange membrane BPM during the electrodialysis process to thus be neutralized, thereby generating water. The remaining sulfuric acid ions (SO<NUM><NUM>-) may meet the sodium ions (Na+) moving through the cation-exchange membrane C to thus generate sodium sulfate (Na<NUM>SO<NUM>), and the generated sodium sulfate (Na<NUM>SO<NUM>) may be supplied as a raw material of the anode chamber <NUM> of the diaphragm electrolytic bath <NUM>, thereby reducing the amount of chemicals that are used.

Preferably, all of the ion-exchange membranes closest to the anode and the cathode are cation-exchange membranes. Further, separate electrode water may be circulated and supplied to the compartment partitioned into the anode and the cathode. Further, preferably, the solution supplied to each compartment is circulated in a semi-batch manner so that the production water and dilution water are obtained at desired target concentration values.

Further, preferably, the anion-exchange membrane A of the water-splitting electrodialysis stack includes a monovalent-selective-anion-exchange membrane. Through this constitution, the passage of other anionic components included in the formate, which is generated from the cathode chamber <NUM> of the diaphragm electrolytic bath <NUM> and then supplied to a formate injection chamber <NUM>, may be excluded, so that formic acid (HCOOH) having higher purity is produced.

<FIG> is an exemplary view showing the constitution of another water-splitting electrodialysis stack as a concentration-acidification unit according to another embodiment of <FIG>. <FIG> is an exemplary view showing the constitution of another water-splitting electrodialysis stack as a concentration-acidification unit according to another embodiment of <FIG>. The water-splitting electrodialysis stack according to <FIG> and <FIG> has a structure in which the anion-exchange membrane A is excluded from the water-splitting electrodialysis stack of <FIG> and <FIG>. That is, according to the constitution of <FIG>, the anode and the cathode are positioned to face each other at both ends thereof, and a plurality of cation-exchange membranes C and a plurality of amphoteric ion-exchange membranes BPM are alternately arranged between the anode and the cathode. The amphoteric ion-exchange membrane BPM is provided so that the anion-exchange layer BA faces a side at which the anode is positioned and the cation-exchange layer BC faces a side at which the cathode is positioned.

The compartment defined by the cation-exchange membrane C and the anion-exchange layer BA of the amphoteric ion-exchange membrane BPM constitutes the base/salt-generation chamber <NUM>, and the compartment defined by the cation-exchange layer BC of the amphoteric ion-exchange membrane BPM and the cation-exchange membrane C constitutes the formic-acid generation chamber <NUM>. In this constitution, when sodium formate (HCOONa), which is the formate generated in the cathode chamber <NUM> of the diaphragm electrolytic bath <NUM>, is supplied to the formic-acid generation chamber <NUM>, and when a direct-current power source is supplied to the anode and the cathode, the sodium ions (Na+) of the sodium formate (HCOONa) supplied to the formic-acid generation chamber <NUM> are passed through the cation-exchange membrane C, which is positioned in a cathode direction to the base/salt-generation chamber <NUM>. Further, hydrogen ions (H+) are generated in the cation-exchange layer BC and hydroxide ions (OH-) are generated in the anion-exchange layer BA through a water decomposition reaction in the amphoteric ion-exchange membrane BPM. Accordingly, in the base/salt-generation chamber <NUM>, the hydroxide ions (OH-) generated in the anion-exchange layer BA of the amphoteric ion-exchange membrane BPM meet the sodium ions (Na+) moved through the cation-exchange membrane C to thus generate sodium hydroxide (NaOH). In the formic-acid generation chamber <NUM>, the hydrogen ions (H+) generated in the cation-exchange layer C of the amphoteric ion-exchange membrane BPM meet the formic acid ions (HCOO-) that do not pass through the cation-exchange membrane C to thus generate formic acid (HCOOH). The purity of the formic acid produced in the two-compartment-type water-splitting electrodialysis stack may be lower than that in the three-compartment-type water-splitting electrodialysis stack of <FIG> and <FIG>. However, since the anion-exchange membrane is removed, there are merits in that the constitution of the process is further simplified and an inter-membrane voltage is lowered, thus reducing power consumption, and in that a lifespan is increased.

In another two-compartment-type water-splitting electrodialysis stack according to <FIG>, the sulfuric acid (H<NUM>SO<NUM>) is injected into the base/salt-generation chamber <NUM> as in <FIG>. The injected sulfuric acid (H<NUM>SO<NUM>) is subjected to a neutralization reaction to thus generate sodium sulfate (Na<NUM>SO<NUM>), as in the embodiment of <FIG>. The sodium sulfate may be re-supplied as a raw material of the anode chamber <NUM> of the diaphragm electrolytic bath <NUM> to thus reduce the amount of raw materials that are used.

Further, as shown in <FIG>, the cation-exchange membrane may be used as the membrane adjacent to the electrode, an electrode solution may be supplied separately, and the operation may be performed using a semi-batch process, thereby further increasing operational efficiency.

<FIG> is a flowchart showing the progress of an electrolysis method according to the embodiment of the present invention. As shown in the drawing, electrolysis operations are simultaneously performed in the anode chamber and the cathode chamber of the electrolytic bath. The sulfate is supplied to the anode chamber of the electrolytic bath to thus generate sodium persulfate (Na<NUM>S<NUM>O<NUM>), which is peroxodisulfate (S11A), and carbon dioxide is supplied to the cathode chamber of the electrolytic bath to thus generate formate (S11B).

The formate generated in the cathode chamber of the electrolytic bath is converted into formic acid through a concentration-acidification process (S12).

<FIG> is a flowchart showing the progress of an electrolysis method according to another embodiment of the present invention. Sulfate is supplied to the anode chamber of the electrolytic bath to thus generate sodium persulfate (Na<NUM>S<NUM>O<NUM>), which is peroxodisulfate (S21A), and carbon dioxide is supplied to the cathode chamber of the electrolytic bath to thus generate formate (S21B).

The formate generated in the cathode chamber of the electrolytic bath is supplied to a concentration-acidification unit, and sulfuric acid (H<NUM>SO<NUM>) is supplied as a supply source of hydrogen ions (H+), necessary for converting the formate into the formic acid, to the concentration-acidification unit (S22).

The formic acid (HCOOH) and the sulfate (Na<NUM>SO<NUM>) generated through a chemical reaction are separated and concentrated to thus generate a target formic acid (HCOOH) in the concentration-acidification unit (S23).

Claim 1:
An electrolysis method comprising:
an electrolysis process of (S21A) supplying an electrolyte solution containing sulfuric acid ions to an anode chamber (<NUM>) of a diaphragm electrolytic bath (<NUM>) that includes an anode and a cathode disposed to face each other, and a diaphragm (<NUM>) for partitioning the anode chamber (<NUM>) in which the anode is positioned and a cathode chamber in which the cathode is positioned to thus generate peroxydisulfate through an anodic reaction, while a carbon dioxide gas or an electrolyte solution containing the gas dissolved therein is supplied (S21B) to the cathode chamber of the diaphragm electrolytic bath (<NUM>) to thus generate formate; and
a concentration-acidification process of supplying the formate generated in the cathode chamber of the diaphragm electrolytic bath (<NUM>), so that the formate is reacted with hydrogen ions supplied from an outside or supplied using self-generation to thus be converted into formic acid;
wherein the concentration-acidification process includes:
(S22) an acidification step of supplying the hydrogen ions necessary for the reaction using sulfuric acid from an external supply source so that the formate supplied from the diaphragm electrolytic bath (<NUM>) is converted into the formic acid; and
(S23) a separation step of separating and concentrating the formic acid and sulfate produced during the acidification step to thus produce the formic acid, and further comprising:
(S24) supplying the sulfate separated during the separation step as a raw material of the anode chamber (<NUM>) of the diaphragm electrolytic bath (<NUM>).