JP2014010931A - Plasma processing method and plasma processing unit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a processing method which allows for efficient plasma processing for a processed liquid without being affected by the properties thereof, and to provide an economical processing unit.SOLUTION: A plurality of electrodes are placed in the air above the liquid level, and plasma is generated between the electrode and the liquid level by applying a voltage between the electrodes. Since the electrode does not come into contact with the processed liquid, plasma processing can be carried out while preventing contamination of the electrode metal component due to dissolution, corrosion, decomposition, consumption, or the like, of the electrode. Furthermore, since all electrodes are in the air, the number of electrodes generating plasma can be increased when compared with a plasma generation method where one electrode is placed in the liquid, resulting in efficient plasma processing.

Description

  The present invention relates to a plasma processing method and a processing apparatus related to a liquid such as water, and is applied to a plasma processing of a substance contained in the liquid or a substance constituting the liquid.

  In recent years, plasma technology involving liquids has been studied for application in a wide range of fields such as water treatment, oxidative decomposition treatment of organic substances, synthesis of metal fine particles, and dispersion treatment of nanoparticles. In developing an apparatus to be used for these purposes, it is desired to enable an efficient process while being simple and economical.

  As a method of generating plasma, as described in detail in Non-Patent Document 1, a method of applying a high voltage to a pair of electrodes is well known. The pair of electrodes can be broadly classified into three types: (a) both arranged in the liquid, (b) one arranged in the air, the other arranged in the liquid, and (c) both arranged in the air.

  As an example of (a), there is a method in which the tips of a pair of needle-shaped electrodes are arranged in the liquid to be treated so as to face each other with a gap of several mm, and a voltage is applied to cause breakdown between the electrodes and discharge. Using this method, Patent Document 1 synthesizes metal nanoparticles, and Patent Document 2 performs surface modification of ceramic particles. As an example of (b), a needle electrode is placed in the air above the liquid surface, a plate electrode is immersed in the liquid, a voltage is applied between the electrodes, and the tip between the needle electrode and the liquid surface is applied. There is a method of generating plasma, and in Patent Document 3, a hydrophobic carbon material is hydrophilized by this method. Examples of (c) include a method in which a discharge is generated between the electrodes in the air, a gas is flowed to the discharge site, and active species generated by the discharge are supplied onto the liquid surface of the liquid to be processed. A method of supplying a liquid to be processed to a place is mentioned.

  In the methods (a) and (b), at least one of the electrodes is in contact with the liquid to be treated. For this reason, it is unsuitable for the process of the liquid containing the component which causes melt | dissolution, corrosion, and alteration of an electrode. In particular, in the method (a), it is not only necessary to add salt to give electrical conductivity in order to discharge, but also the metal component of the electrode is mixed into the liquid to be treated because the electrode is consumed by the discharge treatment. There are known problems. Further, in the method of supplying the active species generated by the discharge by flowing a gas to the discharge location of (c) onto the liquid surface of the liquid to be processed, the processing efficiency is improved because the plasma is not directly irradiated to the liquid to be processed. There is a problem that decreases. Furthermore, in the method of supplying the liquid to be processed to the discharge location of (c), since the liquid is supplied as a continuous or intermittent liquid or a mist-like droplet, the contact time between the liquid to be processed and the plasma is long. There is a problem that the processing time is shortened or the processing capacity is reduced.

  As another method, a metal electrode coated with a thin dielectric layer and a plate-like metal electrode are immersed in an aqueous solution, and an alternating voltage is applied between these electrodes to generate plasma, so that the dye in the aqueous solution is removed. There is a method of decomposing (Patent Document 4). However, as described above, this method is also unsuitable for the treatment of liquids containing components that cause dissolution, corrosion, or alteration of electrodes and dielectric layers, and particularly in the treatment of liquids containing suspended substances. Discharge is hindered by adhesion of the suspended material to the surface, and the suspended material may physically wear and damage the dielectric layer.

JP 2008-013810 A JP 2010-222189 A International Publication Number WO2011 / 010620 A1 JP 2010-137212 A

B. R. Locke et al., Ind. Eng. Chem. Res. , Vol. 45, p. 882 (2006)

  The problem to be solved by the present invention is a processing method and an economy for the same, which enables plasma processing to be efficiently performed on a liquid to be processed while preventing electrode metal components from being mixed due to dissolution, corrosion, alteration, wear and the like of the electrode. Is to provide a typical processing apparatus.

  In the present invention, at least two electrodes are arranged in the air above the liquid surface, and a voltage is applied between the electrodes to generate plasma between the electrode and the liquid surface. FIG. 1 shows the basic configuration of the plasma processing apparatus of the present invention. Since the electrode does not come into contact with the liquid to be treated, plasma treatment can be performed while preventing the electrode metal component from being mixed due to dissolution, corrosion, alteration, wear and the like of the electrode. Further, since all the electrodes are in the air, the number of electrodes that generate plasma can be increased as compared with the plasma generation method in which one of the electrodes is disposed in the liquid, and efficient plasma processing is possible.

  The plasma generation method in the present invention is such that at least two electrodes are arranged in the air on the liquid surface, and the voltage between the electrodes is controlled by controlling the electrode-electrode distance and the electrode-liquid surface distance. It is characterized in that discharge is caused between an electrode disposed in the air and the liquid surface.

  Examples of the shape of the electrode used in the present invention include a needle shape, a hollow needle shape, a linear shape, and a flat plate shape. Among them, a needle-shaped one that generates a non-uniform electric field and lowers a dielectric breakdown voltage and easily generates plasma even at a low voltage is preferable. The material of the electrode is not particularly limited as long as it can maintain a stable discharge state, and includes copper, copper tungsten, silver, graphite, tungsten, titanium, stainless steel, molybdenum, aluminum, iron, and the like. . For the purpose of improving the durability of the electrodes, the surfaces of these electrodes may be coated with a different material.

  In the plasma generation method of the present invention, various systems such as a DC power supply, a pulse power supply, a low-frequency / high-frequency AC power supply, and a microwave power supply can be used as a power supply used for generating plasma. Among them, a 50 Hz or 60 Hz AC power source that can easily obtain a high voltage at a low cost and does not require a matching circuit is preferable. Specifically, an inverter neon transformer or a wound neon transformer is preferably used.

  In the plasma generation method of the present invention, a resistor, a capacitor, a coil, or the like may be introduced in the basic configuration shown in FIG. 1 so that stable plasma discharge can be performed. For example, by introducing a resistance between the air electrode and the power source, it is possible to stably generate plasma between the electrode and the liquid surface regardless of the conductivity level of the liquid to be processed. In addition, when a circuit is connected in parallel from a single power source and plasma is generated simultaneously in two reservoirs, a stable discharge can be obtained by introducing a capacitor between the air electrode and the power source.

  In the plasma generation method of the present invention, the distance between the electrode disposed in the air above the liquid surface and the liquid surface, the electrode-to-electrode distance of the electrode disposed in the air, and the applied voltage are discharged between the electrode and the liquid surface. There are no particular limitations as long as the conditions occur. It is desirable that the discharge does not occur between the electrodes in the air. Regarding the relationship between the electrode-electrode distance L and the electrode-liquid surface distance D that can generate a discharge between the electrode and the liquid surface, preferably L> 3D, more preferably L ≧ 5D, and even more preferably L ≧ 10D. is there. When the number of electrodes is three or more, the distance between the shortest different polarity electrodes is L, and the longest electrode-liquid level distance is D. D may be in a state where the electrode is slightly separated from the liquid surface, and is preferably larger than 0 mm and not larger than 50 mm. More preferably, D is 1 to 10 mm in order to maintain a stable discharge state. Further, it is preferable that the distance between each electrode having the same polarity arranged in the air and the liquid surface is equal in order to obtain a stable discharge. The applied voltage is influenced by the electrode arrangement, the electrode material, and the like, but is preferably greater than 0 kV and less than or equal to 30 kV in consideration of the economics and safety of the power source, electrode consumption, and the like. The applied voltage is most preferably 1 to 10 kV because of the ease of voltage application.

  Examples of the liquid that can be treated by the plasma generation method of the present invention include an aqueous solution, an organic solvent, a mixed solution of an aqueous solution and an organic material, and a solution in which a solid content is dispersed, but is not particularly limited.

  The plasma processing apparatus of the present invention may be in a state in which the upper part of the liquid level is opened as shown in FIG. 1 or in a state of being sealed so as to cover the electrode in the upper part of the liquid level. In a closed system, it is possible to discharge while introducing an arbitrary gas. Examples of the gas to be introduced include gases such as hydrogen, nitrogen, oxygen, argon, and carbon dioxide, mixtures thereof, and gasified compounds. The gas may be introduced either on the liquid surface or in the liquid.

  The plasma generation method of the present invention includes a decomposition treatment of a hardly decomposable substance in a liquid to be treated, a surface modification treatment such as an inorganic substance, an organic substance, a composite material of an inorganic substance and an organic substance contained in the liquid to be treated, and a raw material thereof. It can be used for producing the prepared material. Further, all the plasma generating electrodes are disposed in the air, and plasma processing is performed without bringing the electrodes into contact with the liquid to be processed. For this reason, it is possible to perform plasma treatment even if a substance that causes corrosion, wear, or the like is present in the liquid to be treated without accelerating the deterioration and consumption of the electrode.

FIG. 1 is a schematic diagram of a basic configuration of a plasma processing apparatus. FIG. 2 shows the result of the decomposition treatment of methylene blue in Example 1 and Comparative Example 1. FIG. 3 is a schematic view of a plasma processing apparatus for processing a liquid to be processed in two sets of processing tanks with a power source having a single output. FIG. 4 is a schematic view of a plasma processing apparatus for connecting two processing tanks in the apparatus shown in FIG. 3 to communicate the liquid to be processed and processing the liquid to be processed in a single power source. is there.

  Next, the plasma generation method and the plasma processing apparatus of the present invention will be described in detail with reference to examples, but the present invention is not limited thereto.

  The organic substance was decomposed using the plasma processing apparatus shown in FIG. A tungsten wire (φ1 mm) was used for the air electrode, a 10 mg / L methylene blue aqueous solution was used for the liquid to be treated, and a glass container was used for the storage tank. The amount of liquid to be treated was 100 mL, the electrode-electrode distance was 30 mm, and the electrode-liquid surface distance was 2 mm for both electrodes. An inverter type neon transformer (60 Hz) was used as a power source, and a voltage of 3 kV was applied for discharging. The plasma treatment was performed in an air atmosphere, and the liquid to be treated was stirred with a magnetic stirrer during the treatment.

(Comparative Example 1)
Plasma treatment was performed in the same manner as in Example 1 except that one of the air electrodes was immersed in the liquid to be treated.

  The concentration of methylene blue was determined by measuring the absorption spectrum of a solution prepared by adding a phosphate buffer to a methylene blue solution to make the pH constant. FIG. 2 shows the result of the decomposition treatment of methylene blue. In Comparative Example 1 in which one electrode was immersed in the liquid, it took about 19 minutes until half of the methylene blue was decomposed, whereas in Example 1 in which both electrodes were placed in the air, the time was as short as about 6 minutes. It became clear that it decomposed efficiently in time.

  Plasma discharge was performed using the plasma processing apparatus shown in FIG. 1, and the mixing of electrode metal components into the liquid to be processed was examined. The liquid to be treated was a 1 mol / L aqueous sodium hydroxide solution, and plasma was generated for 1 hour under the same conditions as in Example 1. The presence or absence of the electrode metal component was confirmed by ICP emission spectrometry.

(Comparative Example 2)
Plasma treatment was performed in the same manner as in Example 2 except that one of the air electrodes was immersed in the liquid to be treated.

  In Example 2, the tungsten concentration in the liquid to be treated was below the detection limit. On the other hand, in Comparative Example 2, about 60 ppm of tungsten was detected.

  Dispersion in water by surface modification of carbon was performed using the plasma processing apparatus shown in FIG. The liquid to be treated was a mixture obtained by adding 10 mg of carbon powder to 100 mL of water, and plasma treatment was performed for 15 minutes under the same conditions as in Example 1.

  Before the plasma treatment, most of the carbon powder was floated on the liquid surface or precipitated at the bottom of the container and was not dispersed in water. In Example 3, the carbon powder was satisfactorily immersed in water. Distributed.

  Using the plasma processing apparatus shown in FIG. 1, fine titanium oxide was dispersed in water by surface modification. The liquid to be treated is a mixture of 100 mL of water and 10 mg of fine particle titanium oxide (MT-500B manufactured by Teika Co., Ltd., average primary particle size 35 nm (catalog value)), and the other conditions are the same as in Example 1 and plasma for 2 hours. Processed.

  When the liquid to be treated before the plasma treatment was allowed to stand, the fine particle titanium oxide aggregated and settled, but in Example 4, the fine particle titanium oxide was well dispersed in water.

  Using the basic plasma processing apparatus shown in FIG. 1, the two poles are set to the same D, the tungsten electrode (φ1 mm) for the air electrode, 1 L of methylene blue aqueous solution of 10 mg / L for the liquid to be processed, storage A glass container was used for the tank. An inverter type neon transformer (60 Hz) was used as a power source, and a voltage of 3 kV was applied for discharging. The plasma treatment was performed in an air atmosphere, and the liquid to be treated was stirred with a magnetic stirrer during the treatment.

<Result>
Table 1 shows the plasma generation state when the plasma treatment is performed by changing the electrode-liquid level distance, the applied voltage, the electrode-electrode distance, and the power source.

  From Table 1, it can be seen that as the electrode-liquid level distance D increases, the applied voltage V required to generate plasma increases. It is understood that D is 25 mm or less in consideration of work safety and electrode consumption. Even when D is larger than 25 mm, it is possible to discharge by applying a higher voltage. However, in this case, it is preferable that V is 30 kV or less because inconveniences such as the electrode becoming red hot by the continuous treatment and the plasma generation state becoming unstable. It is more preferable that D is 1 to 10 mm and V is 10 kV or less so that an inexpensive and common neon transformer can be used and stable plasma discharge can be performed.

  Using the basic plasma processing apparatus shown in FIG. 1, plasma processing was performed while changing the electrode-electrode distance L, the applied voltage V, the electrode-liquid level distance D, and the power source. D and V were made constant for each type of power supply, and the others were the same as in Example 5.

<Result>
Table 2 shows the plasma generation state when the plasma treatment was performed by changing the electrode-liquid level distance, the applied voltage, the electrode-electrode distance, and the power source.

  When the electrode-electrode distance L was 2 to 3 times the electrode-liquid level distance D, discharge occurred between the electrodes. In the case of L> 3D and L <5D, a phenomenon was observed in which a discharge was generated between the electrode and the electrode, followed by a discharge between the electrode and the liquid surface. Therefore, L> 3D, more preferably L ≧ 5D, for discharging between the electrode and the liquid surface. Considering that some variation may occur in this relationship depending on the properties of the liquid to be processed, the processing atmosphere (the type of gas in the air), and the like, L ≧ 10D is more preferable.

  As shown in FIG. 3, the liquid to be processed placed in the two storage tanks was subjected to a decomposition process of organic substances by plasma with a power source having a single output. The volume of the liquid to be treated was 200 mL, the distance between the electrode and the liquid surface was set to 2 mm for all four, and the plasma treatment was performed under the same conditions as in Example 1.

  In all four electrodes, plasma was simultaneously generated between the electrode and the liquid surface. Moreover, the methylene blue density | concentration fell with time similarly to Example 1 using the plasma processing apparatus of FIG.

  As shown in FIG. 4, methylene blue was decomposed in a plasma processing apparatus in which two storage tanks were connected and the liquid to be processed was communicated. An insulating tube having an inner diameter of 6 mm was used for the connection part. The plasma treatment was performed under the same conditions as in Example 7 except for the above.

  In all four electrodes, plasma was generated between the electrode and the liquid surface. Further, the methylene blue concentration in the storage tanks decreased with time in the same manner as in Example 1 using the plasma processing apparatus of FIG.

  By using the processing apparatus according to the present invention, a needle electrode is disposed in the air on the liquid surface, which is a conventional technique, a plate electrode is immersed in the liquid, a high voltage is applied between the electrodes, and the needle electrode is Compared with the method of generating plasma between the tip and the liquid surface, plasma treatment can be performed more efficiently. Furthermore, since the electrode does not come into contact with the liquid to be processed, plasma processing of the liquid to be processed is possible without mixing of electrode metal components due to dissolution, corrosion, alteration, wear, or the like of the electrode. The plasma treatment method of the present invention can be used for water treatment such as removal of harmful substances from a liquid to be treated, wastewater treatment, material preparation using a chemical reaction of a substance contained in the liquid to be treated and a surface modification treatment of a solid. .

DESCRIPTION OF SYMBOLS 1 Storage tank which puts a to-be-processed liquid 2 To-be-processed liquid 3 Power supply 4 The electrode installed in the air above the liquid level 5 Ceramics tube 6 Plasma

Claims (3)

  A plasma generation method comprising: arranging a plurality of electrodes in the air above a liquid surface, and generating a plasma between the electrodes and the liquid surface by applying a voltage between the electrodes.   2. The plasma generation method according to claim 1, wherein a voltage is applied between the electrodes with a single power source to simultaneously generate plasma between the plurality of electrodes and the liquid surface. A plurality of electrodes are disposed in the air above the liquid to be treated, and a voltage is applied between the electrodes to generate plasma between the electrodes disposed in the air and the liquid surface of the liquid to be treated. A plasma processing apparatus.