JPS6376268A - Preparation of electrolyte for redox-flow cell - Google Patents

Abstract

PURPOSE:To simply and easily prepare the electrolyte of redox-flow cell by dissolving a chromium ore reduction product and/or ferrochrome in hydrochloric acid in an oxygen-free atmosphere and firtering the residue. CONSTITUTION:A chromium ore reduction product and/or ferrochrome are/is dissolved in hydrochloric acid in an oxygen-free atmosphere and the residue is filtered in an oxygen-free atmosphere. The chromium ore reduction product means a partial reduction preduct obtained in the reduction process of chromium ore to ferrochrome, and usually contains ten or more % metallic chromium and its reduction ratio is about 50%. Since ferrochrome and/or chromium ore reduction product are/is low cost material, the cost of on the electrolyte can remakably be reduced. Since the need of an electrolytic bath is eliminated and impurities are removed in a dissolving process, the production process is made simple, and a desired electrolyte can easily be prepared.

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention relates to a secondary battery for power storage, and more particularly to a method for preparing an electrolyte for a redox flow battery using chlorides of iron and chromium as active materials. .

[Prior art]

Japan's electricity demand has been increasing for the past nine years, but fluctuations in electricity demand have also tended to become more pronounced year by year, reflecting the sophistication of the industrial structure and improvement in the standard of living of the people. For example, if the daytime power demand in the summer is 100, the demand in the morning is 30 or less. On the other hand, from an electric power supply perspective, the proportion of nuclear power generation and large-scale coal-fired power generation, where fluctuations in output are undesirable, is on the rise, increasing the need for power storage equipment.

Current electricity storage is done through pumped hydropower generation,
Due to limited locations, new energy storage technologies,
Among them, secondary batteries, which are said to have a high possibility of being realized technically and economically, are being actively researched, and among them, redox flow batteries in which redox systems are brought into contact through a diaphragm are attracting particular attention.

This redox flow battery uses iron ions (Fe"/Fe") and chromium ions (Cr"/Cr") as shown in the drawing.
In this type of battery, an aqueous solution of ions whose valences change as shown in FIG.

When iron ions are used in the positive electrode liquid and chromium ions are used in the negative electrode liquid as the redox liquid, the charge/discharge reaction is expressed by the following equation.

Therefore, the power left over at night is converted into AC/DC through the inverter 1, and then charged into the redox flow battery. Fe"
/Cr”+.

Next, when there is a shortage of electricity during the day (1) (2)
It is discharged according to the reaction in the discharge direction of the equation, and after orthogonal conversion by an inverter, it is supplied to the power system. This is a power storage system using redox flow batteries.

As the electrolytic solution used in this redox flow battery, a chloride aqueous solution acidic with hydrochloric acid is used due to its characteristics such as solubility of iron and chromium and charging/discharging reaction rate. Among these, chromium solution in particular
In order to suppress hydrogen generation, which is a side reaction, high-purity electrolytic chromium is used, which is expensive and has been a major problem in putting this battery into practical use.

Therefore, chromium ore and reduced chromium ore are cheaper.

Although a method for preparing a chromium solution from ferrochrome etc. is desired, these methods contain many impurities such as heavy metals and therefore have drawbacks such as a large amount of hydrogen generation and a decrease in coulombic efficiency. Therefore, as a method for preparing an electrolyte for a redox flow battery from chromium ore, etc., Japanese Patent Application Laid-Open No. 60-115174
As shown in Figure 2, a method has been proposed in which after dissolving in hydrochloric acid, the solution is introduced into the cathode chamber of an electrolytic cell, and impurities are removed by electrodeposition on the electrodes of the cathode chamber by electrolysis.

However, this method requires electrolytic cell equipment.

Further, there is a drawback that the operation is complicated, such as requiring electrolysis.

[Purpose of the invention]

An object of the present invention is to provide a simple and easy method for preparing a redox flow battery electrolyte from reduced chromium ore or ferrochrome.

[Structure of the invention]

The present invention provides an electrolyte solution for a redox flow battery, which is characterized in that a reduced chromium ore or/and ferrochrome is dissolved in hydrochloric acid in an oxygen gas-blocked atmosphere, and then the residue is filtered in an oxygen gas-blocked atmosphere. This is a preparation method.

The chromium-containing raw material used in the present invention is a reduced chromium mineral and/or ferrochrome. Here, the term "reduced chromium ore" refers to a product that has undergone partial reduction when producing ferrochrome from chromium ore, and usually contains about 10% of metallic chromium and exhibits a reduction rate of about 50%.

In the present invention, one or more chromium raw materials are pulverized to an appropriate particle size as required, and then dissolved in hydrochloric acid in an atmosphere cut off from oxygen gas. Chromium and iron in the raw materials react and dissolve according to the formula below.

Cr+2HC2→CrCQz+ H, ↑ (3) Cr
+ 3HCQ n CrC et al. + 3/2H, ↑ (4)
CrO+ 2HCQ 4 CrCQ2H,O(5)Fe
+ 2HCQ −+ FeCQ2H, ↑ (6
)FeO+ 2HCQ →FeCQ2+ HzO(7)
To isolate the dissolution tank from oxygen gas, the top of the dissolution tank should be covered, the gas outlet duct should be separated from the dissolution tank, or it should be constructed to prevent outside air from entering by an appropriate method such as sealing it with water. It's fine. In this case, since hydrogen gas is generated during melting, even if oxygen gas initially exists in the melting tank, it will be purged by the generated hydrogen gas, so an atmosphere in which oxygen gas is blocked can be quickly achieved, but Preferably, before melting, the inside of the melting tank is purged with a gas that does not contain oxygen, such as nitrogen or argon.

Furthermore, in order to speed up the reaction during this dissolution, it is also effective to heat the solution or stir it with a stirrer or the like.

Through this dissolution operation with oxygen gas cut off, iron is dissolved in the form of divalent ions, and chromium is dissolved in the form of divalent and trivalent ions, and the potential of the solution becomes less base than -0.4V with respect to the saturated amber electrode. It will be done. At the potential of this solution, impurities such as heavy metals that promote hydrogen generation as a side reaction at the negative electrode of the redox flow battery are suppressed from dissolving to a concentration that does not have any effect.

If a conductive carbon material is present during the melting operation of the present invention, the effect of suppressing the dissolution of impurities such as heavy metals will be significant and will be even more effective. That is, during melting, due to the presence of the conductive carbon material, a very small amount of dissolved impurities such as heavy metals is deposited on the surface of the conductive carbon material using the electron transfer medium according to the following formula.

M” + nCr” −+ M↓+nCr”
(8) Here, M+ indicates impurities such as heavy metals, and the conductive carbon material used is a carbon material with a specific resistance value of 0.1Ω or less, including carbon powder, carbon fiber, graphite powder, Examples include graphite fiber and carbon black. The effective amount of coexistence during dissolution is 0.1 g or more, more preferably 0.5 g or more per 1 kg of dissolved ferrochrome or reduced chromium mineral.

After the dissolution operation as described above is completed, residues of carbon, silica, etc. that do not dissolve in hydrochloric acid remain, so they are filtered out in an atmosphere cut off from oxygen gas as in the case of dissolution. This is because impurities such as heavy metals whose dissolution is suppressed are present in this residue, and when the filtration operation is performed in the presence of oxygen gas, divalent chromium ions in the solution are oxidized and the potential of the solution increases. This is because impurities such as heavy metals are dissolved, and filtration must be performed in an atmosphere where oxygen gas is blocked. The method of blocking oxygen gas during filtration is to cover the top of the filter, as in the case of a dissolution tank, and provide a gas inlet duct and an outlet duct in the cover, and to remove oxygen gas such as nitrogen from the inlet duct. It is sufficient if the structure is such that it allows gas to flow and prevents the intrusion of outside air.

As a method for separating the residue by filtration, known methods such as vacuum filtration, pressure filtration, and natural filtration can be used.

The hydrochloric acid solution of chromium and iron obtained by filtering the residual liquid can be used as an electrolyte for redox flow batteries without any problems. After the separation, when the filtrate is brought into contact with the conductive carbon material in an atmosphere that blocks oxygen gas,
The reaction of formula (8) occurs in the same way as when they coexisted during dissolution,
The effect is that impurities such as heavy metals are further removed. The method of contacting the conductive carbon material may be as follows: filling a container with the conductive carbon material, purging the filled container with an oxygen-free gas such as nitrogen, and then passing the filtrate through the container.

In this case, filtration becomes easier than in a method in which a conductive carbon material is present at the time of melting. Also, since the conductive carbon material can be used repeatedly when the solution is prepared in several batches, there is an advantage that the amount used can be reduced by about half.

In the present invention, since there are no undissolved impurities such as heavy metals in the hydrochloric acid solution of chromium and iron obtained as described above, there is no problem in bringing it into contact with oxygen gas.

Since the solution to be prepared in the redox flow battery of the present invention is preferably in a discharged state containing trivalent chromium ions and divalent iron ions for handling purposes, the solution obtained by the present invention is once exposed to the atmosphere.

As a result, divalent chromium ions in the solution are oxidized to trivalent chromium ions by oxygen, and an electrolytic solution in the desired discharge state can be obtained.

〔Effect of the invention〕

The method for preparing an electrolytic solution according to the present invention uses inexpensive raw materials such as ferrochrome and/or reduced chromium ore compared to a method in which high-purity electrolytic chromium is dissolved in hydrochloric acid, so that the solution preparation cost can be significantly reduced. Also, compared to the method of dissolving chromium ore etc. in hydrochloric acid and then removing impurities by electrodeposition,
(1) No electrolytic bath equipment is required; (2) Impurities can be removed at the same time as melting, which simplifies the production process and makes it possible to prepare a desired electrolytic solution extremely easily.

[Embodiments of the invention]

Examples of the present invention will be shown below.

Example 1 Ferrochrome was placed in a closed type dissolution tank with an inlet and an outlet duct, nitrogen gas was flowed through the inlet duct to purge oxygen gas in the dissolution tank, 12N hydrochloric acid was introduced, and the ferrochrome was dissolved by heating. I let it happen.

Thereafter, the solution was introduced into a filter, filtered under reduced pressure under a nitrogen atmosphere, and the residue was filtered off. The chromium and iron concentrations of this filtrate were measured by atomic absorption spectrometry and were 1.50 mol/Q and 0.68 mol/Q, respectively.

After adding water to this solution and adjusting the hydrochloric acid concentration to 4N, it was used as a redox flow battery electrolyte with an electrode area of 1
It was charged and discharged using a small 5 cd liquid flow type cell. The structure of the electrolytic cell of this battery consists of a phenol resin-bound carbon plate as a current collector plate, carbon cloth as a liquid flow electrode, and a cation exchange membrane as a diaphragm, with the positive and negative electrodes having the same structure. . As a result of a charge/discharge experiment at a temperature of 40° C. and a current of 0.6 A, the charge/discharge coulomb efficiency was 97%, and the charge/discharge voltage efficiency was 86%.

Comparative Example 1 Ferrochrome and 12N hydrochloric acid were placed in a dissolution tank with an open top, and the ferrochrome was dissolved by heating. Thereafter, the solution was introduced into a filter, filtered under reduced pressure in air, and the residue was filtered off. The chromium and iron concentrations of this filtrate were determined by atomic absorption spectrometry to be 1.53 mol/Q and 0.70 mol/Q, respectively.
Met.

After adding water to this solution to adjust the hydrochloric acid concentration to 4N, charging and discharging it as a redox flow battery electrolyte under the same conditions as in Example 1 resulted in a charge-discharge coulombic efficiency of 83%. The voltage efficiency was 85 bu.

Example 2 Chromium ore reduced pellets were used in place of the ferrochrome in Example 1, and after pulverizing them, a solution was prepared under the same conditions as in Example 1. The chromium and iron concentrations in the filtrate after dissolution and filtration were 1.35 mol/Q and 1.04 mol/Q, respectively.

After adding water to this solution and adjusting the hydrochloric acid concentration to 4N, using the small liquid flow type cell shown in Example 1,
When charged and discharged under the same conditions, the charge/discharge coulomb efficiency was 96%, and the charge/discharge voltage efficiency was 86%.

Comparative Example 2 A reduced chromium ore pellet was used in place of the ferrochrome in Comparative Example 1, and after pulverizing it, a solution was prepared under the same conditions as in Comparative Example 1. The chromium and iron concentrations in the filtrate after dissolution and filtration were 1.37 mol/2 and 1.05 mol/Q, respectively. After adding water to this solution and adjusting the hydrochloric acid concentration to 4N, it was charged and discharged under the same conditions as Comparative Example 1.
The charge/discharge coulomb efficiency was 81% and the charge/discharge voltage efficiency was 85%.

Example 3 When melting in Example 1, 0.5 per unit of ferrochrome
A solution was prepared under the same conditions as in Example 1 by coexisting carbon cloth at a ratio of 1.5 g. The chromium and iron concentrations in the filtrate after dissolution and filtration were 1.51 mol/Ω and 0.67 mol/Ω, respectively.
It was 12. After water was added to this solution to adjust the hydrochloric acid concentration to 4N, it was charged and discharged under the same conditions as in Example 1, resulting in a charge/discharge coulombic efficiency of 98% and a charge/discharge voltage efficiency of 87%.

Example 4 Ferrochrome was dissolved under the same conditions as in Example 1, and the residue was filtered off. This filtrate was introduced into a packed container filled with carbon cloth and purged with nitrogen in advance, and allowed to flow.

The chromium and iron concentrations in the resulting solution were each 1.4
They were 8 mol/Ω and 0.70 mol/Q.

After water was added to this solution to adjust the hydrochloric acid concentration to 4N, it was charged and discharged under the same conditions as in Example 1, and the charge/discharge coulomb efficiency was 98% and the charge/discharge voltage efficiency was 87%.

[Brief explanation of the drawing]

The drawing is a conceptual diagram of a power storage system using redox flow batteries. 1... Inverter, 2... Flow type electrolytic cell, 3...
Positive electrode, 4... negative electrode, 5... ion exchange membrane. 6... Positive electrode liquid tank, 7... Negative electrode liquid tank, 8...
- Positive electrode liquid pump, 9... negative electrode liquid pump.

Claims (3)

[Claims] (1) Dissolve chromium ore reduced product or/and ferrochrome in hydrochloric acid in an atmosphere cut off from oxygen gas, and then
A method for preparing an electrolyte for a redox flow battery, which comprises filtering out residue in an atmosphere where oxygen gas is blocked. (2) The method for preparing an electrolytic solution according to claim 1, characterized in that a conductive carbon material is allowed to coexist during dissolution in hydrochloric acid. (3) The method for preparing an electrolytic solution according to claim 1, characterized in that after filtering off the residue, the solution is brought into contact with a conductive carbon material in an atmosphere cut off from oxygen gas.