Method for producing purified carbon black

Abstract

To provide a method for producing purified carbon black with effectively reduced ash content.SOLUTION: A method for producing purified carbon black includes the following step (1) and step (2). Step (1): loading particles including carbon black and an oxidizer into a solvent, bringing them into contact with each other. Step (2): loading particles including carbon black and a base into a solvent, bringing them into contact with each other.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a method for producing purified carbon black. [Background technology]

[0002] In recent years, greater sustainability has been required within the tire industry in order to realize a circular society. One example of a method for utilizing used tires is a method in which used tires are pyrolyzed, the oil-converted components are used as petrochemical raw materials, and the carbon black recovered as a solid component is reused. For example, Patent Document 1 describes a method for producing a reinforcing filler from waste tires, which is characterized in that waste tires are pyrolyzed in a high-temperature non-oxidizing atmosphere to decompose heavy and light oils, gases and charcoal to produce a reinforcing filler, the pyrolysis being carried out for a sufficient period of time so that the acetone extractable content of the charcoal produced is 1% by weight or less, and the resulting charcoal is quenched in an atmosphere that blocks the intrusion of air, with the aim of providing a method for not only decomposing and recovering heavy and light oils and gases from waste tires but also producing a reinforcing filler having performance comparable to that of commercially available carbon black. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 52-100394 Summary of the Invention [Problem to be solved by the invention]

[0004] However, carbon black recovered from such waste tires contains a large amount of ash as impurities, such as silica and metal components used in tire manufacturing, and when the recovered carbon black is used in various products, it may have a negative effect on the physical properties of the products. For example, when it is reused as a tire material, there is a problem that a high ash content has a negative effect on the rubber physical properties such as tensile properties. Therefore, it is necessary to reduce the ash content of the recovered carbon black. The present invention relates to a method for producing purified carbon black having an effectively reduced ash content. [Means for solving the problem]

[0005] The present invention relates to the following [1]. [1] A method for producing purified carbon black, comprising the following steps (1) and (2): Step (1) A step of placing particles containing carbon black and an oxidizing agent in a solvent and contacting them Step (2) A step of placing particles containing carbon black and a base in a solvent and contacting them. Effect of the Invention

[0006] According to the present invention, there is provided a method for producing purified carbon black having an effectively reduced ash content. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0007] [Method of manufacturing refined carbon black] The method for producing purified carbon black of the present invention includes the following steps (1) and (2). Step (1) A step of placing particles containing carbon black (hereinafter also referred to as "recovered carbon black") and an oxidizing agent in a solvent and contacting them (hereinafter also referred to as "oxidizing agent contact step"). Step (2) A step of contacting particles containing carbon black and a base in a solvent (hereinafter also referred to as the "base contact step"). According to the method for producing purified carbon black of the present invention, purified carbon black having an effectively reduced ash content is provided.

[0008] Although the detailed reasons why the above effects are obtained are unclear, some of the reasons are thought to be as follows. Generally, particles containing carbon black recovered by thermal decomposition of rubber products such as tires contain ash containing metal compounds and silicon compounds as impurities, as described below. By treating such recovered carbon black with an oxidizing agent, the metal compounds are converted into chemical substances that are soluble in a solvent, and by treating it with a base, the silicon compounds are converted into chemical substances that are soluble in a solvent, making it possible to dissolve the ash in the recovered carbon black in the solvent, and it is believed that the ash can be removed by removing the solvent together with the dissolved ash. However, the mechanism by which the effects of the present invention can be obtained is not limited to this. Each step will be described below.

[0009] <Particles containing carbon black> In the present invention, the particles containing carbon black are not particularly limited as long as they are particles (particle group) containing carbon black, and each particle may be an aggregate of carbon black and other components described below, the particle group may contain particles consisting of only carbon black and particles consisting of only other components, or these particles may be mixed. The average particle size of the particles containing carbon black (recovered carbon black as a raw material) is preferably 10 nm or more and 1000 nm or less from the viewpoints of ease of handling, ease of solvent penetration, etc. The average particle size of the recovered carbon black can be measured, for example, by a transmission electron microscope, a specific surface area measurement method, an X-ray scattering method, a dynamic light scattering method, etc.

[0010] Examples of such recovered carbon black include those derived from rubber products such as tires, rubber crawlers, anti-vibration rubber, and conveyor belts, and those derived from scrap rubber generated during the manufacture and processing of rubber products. Among these, the recovered carbon black is preferably derived from used rubber products (particularly used tires), and more preferably recovered from used rubber products (particularly used tires) by pyrolysis. Since rubber products such as tires are discarded in large quantities annually, recycling is desired from the standpoint of environmental protection and sustainability, and such rubber products often contain carbon black as a reinforcing material.

[0011] An example of a component other than carbon black contained in recovered carbon black is ash. Here, "ash" refers to inorganic substances remaining after recovered carbon black or purified carbon black is thermally decomposed. The amount of ash is measured by the method described in the Examples below. Examples of such ash include metal compounds such as ZnO, ZnS, Al2O3, and CaO, silicon compounds such as SiO2, and mixtures thereof. In particular, when used tires are used as raw materials, it is believed that the ash contains a large amount of ZnO, ZnS, and SiO2 derived from the tire components. The carbon black content in the recovered carbon black is, for example, 60% by mass or more and 95% by mass or less based on the total amount of recovered carbon black (total amount of particle groups including carbon black). The ash content (content of inorganic substances other than carbon black) is, for example, 5% by mass or more and 40% by mass or less based on the total amount of recovered carbon black. Among these, an ash content of 10% by mass or more is preferable because the effect of reducing the ash content by the production method of the present invention can be significantly obtained. In the present invention, the contact-treated particles containing carbon black refer to particles containing carbon black that have been subjected to at least one of the contact steps (1) and (2).

[0012] <Oxidizing agent contact step (step (1))> Step (1) is a step of placing particles containing carbon black and an oxidizing agent in a solvent and contacting them. The oxidizing agent used in the step (1) is preferably one or more selected from the group consisting of hydrogen peroxide and organic peracids, from the viewpoints of availability and ease of handling. Examples of organic peracids include lower aliphatic percarboxylic acids (e.g., having 1 to 4 carbon atoms) such as performic acid, peracetic acid, perpropionic acid, trifluoroperacetic acid, and perbutyric acid, and aromatic percarboxylic acids such as perbenzoic acid and perphthalic acid. When an organic peracid is used as an oxidizing agent, the organic peracid may be added directly, or, as described below, an oxidizing agent such as hydrogen peroxide and an organic peracid precursor such as an organic acid that can react with the oxidizing agent to form an organic peracid may be added to the system to generate the organic peracid in the system. The combination of an oxidizing agent and an acid that can generate an organic peracid will also be described below.

[0013] The amount of such oxidizing agent added varies depending on the type of oxidizing agent, the amount of ash in the recovered carbon black, etc., and is not particularly limited, but is preferably 0.25 parts by mass or more, more preferably 1 part by mass or more, and even more preferably 5 parts by mass or more, relative to 100 parts by mass of recovered carbon black, and is preferably 10,000 parts by mass or less, more preferably 2,000 parts by mass or less, and even more preferably 500 parts by mass or less. When the amount added is within such a range, more effective removal of ash is possible. In this case, when an organic peracid precursor such as an organic acid is added in addition to the oxidizing agent to generate an organic peracid in the system, the amount of the oxidizing agent added does not refer to the amount of the organic peracid generated, but refers to the amount of the oxidizing agent added to react with the organic peracid precursor.

[0014] When the oxidizing agent is dissolved in a solvent and used, the amount of the solvent containing the oxidizing agent (hereinafter also referred to as "treatment liquid 1") added in a batchwise manner is, from the viewpoints of ease of handling, ash solubility, etc., preferably 0.5 parts by mass or more, more preferably 1 part by mass or more, even more preferably 2 parts by mass or more, and preferably 100 parts by mass or less, more preferably 50 parts by mass or less, even more preferably 20 parts by mass or less, relative to 1 part by mass of the recovered carbon black. The batchwise manner will be described later in the section on the washing step (4). From the same viewpoint, the total amount of the solvent used in step (1) is also preferably within the above range.

[0015] In addition, from the viewpoints of ease of handling, reactivity between the ash in the recovered carbon black and the oxidizing agent, and the like, the concentration of the oxidizing agent in the treatment liquid 1 is preferably 0.01 mass % or more, more preferably 0.1 mass % or more, even more preferably 1.0 mass % or more, and is preferably 70 mass % or less, more preferably 40 mass % or less, even more preferably 20 mass % or less.

[0016] In addition, the step (1) is preferably a step of further charging an organic acid into the solvent and contacting the organic acid with the organic peracid by the oxidizing agent, as described above, and the recovered carbon black comes into contact with the organic peracid as the oxidizing agent in the solvent. Therefore, the organic acid is preferably one that reacts with an oxidizing agent to generate an organic peracid. Examples of the organic acid include aliphatic carboxylic acids such as formic acid, acetic acid, propionic acid, butyric acid, and halogenated acetic acid (e.g., trifluoroacetic acid), and aromatic carboxylic acids such as benzoic acid and phthalic acid. Among these, from the viewpoint of not increasing ash content, being highly water-soluble and soluble in an aqueous solvent, and therefore being easy to remove excess organic acid by solid-liquid separation or washing with an aqueous solvent, lower aliphatic carboxylic acids (e.g., carbon number 1 to 4) such as formic acid, acetic acid, propionic acid, and butyric acid are preferred, more preferably formic acid and acetic acid, and even more preferably formic acid, from the viewpoint of ease of handling. The pKa of the organic acid is preferably 0.1 or more, more preferably 1.0 or more, even more preferably 2.0 or more, even more preferably 3.5 or more, and is preferably 6.0 or less, more preferably 5.0 or less. When the pKa is in such a range, the organic acid is easy to handle and can remove ash.

[0017] In addition, as a combination of an oxidizing agent and an organic acid, for example, a combination of hydrogen peroxide and an organic acid that reacts with hydrogen peroxide to generate an organic peracid can be mentioned. Among these, a combination of hydrogen peroxide and a lower aliphatic carboxylic acid is preferable, a combination of hydrogen peroxide and formic acid or a combination of hydrogen peroxide and acetic acid is more preferable, and a combination of hydrogen peroxide and formic acid is even more preferable. According to such a combination, an organic peracid is generated, so that it is considered that it is possible to remove ash more effectively than hydrogen peroxide alone. Furthermore, according to such an oxidizing agent or organic acid, the ash content is not increased, and further, since it is highly water-soluble and dissolves in an aqueous solvent, it is possible to easily remove the used chemicals (oxidizing agent, organic acid) by solid-liquid separation or washing treatment with an aqueous solvent, and handling is also easy.

[0018] The purpose of adding an organic acid is not limited to the generation of an organic peracid, and may be, for example, the purpose of adding an organic peracid to improve the stability of a system containing the organic peracid or to increase the solubility in a solvent of a product produced by the reaction of a metal compound in the ash with an oxidizing agent, and is not particularly limited. The molar ratio of the organic acid added to the oxidizing agent (hydrogen peroxide / organic acid (molar ratio)) is preferably 0.1 or more, more preferably 0.3 or more, even more preferably 0.5 or more, and is preferably 10 or less, more preferably 2 or less, even more preferably 1.3 or less, even more preferably less than 1.0, even more preferably 0.8 or less.

[0019] Furthermore, when the oxidizing agent and the organic acid are dissolved in a solvent and used as treatment liquid 1, the concentration of the organic acid in treatment liquid 1 immediately after preparation is, from the viewpoints of ease of handling and ease of production of the organic peracid, preferably 0.05 mass % or more, more preferably 0.2 mass % or more, and even more preferably 1.0 mass % or more, and is preferably 70 mass % or less, more preferably 40 mass % or less, and even more preferably 20 mass % or less.

[0020] The order and method of charging the recovered carbon black, the oxidizing agent, and the organic acid as required in the solvent and contacting them are not particularly limited. For example, the recovered carbon black, the oxidizing agent, and the organic acid as required may be simultaneously added to the solvent and mixed to be contacted, or the oxidizing agent and the organic acid as required may be dissolved (or suspended) in the solvent in advance to prepare a treatment liquid 1, and then the treatment liquid 1 and the recovered carbon black may be mixed to be contacted. Alternatively, the recovered carbon black may be suspended in the solvent in advance and then the oxidizing agent and the organic acid as required may be mixed to be contacted (Note that in this case, the oxidizing agent and the organic acid as required may also be prepared in advance as the treatment liquid 1). Alternatively, the organic acid and the recovered carbon black may be mixed in the solvent and then the oxidizing agent is added, and then the organic acid is added, and then the oxidizing agent and the recovered carbon black are mixed and contacted. Alternatively, the recovered carbon black may be mixed in the solvent and then the organic acid is added, and then the organic acid is mixed and contacted. Alternatively, the recovered carbon black may be contacted by passing (passing) the treatment liquid 1 in which the oxidizing agent and the organic acid as required are dissolved through a container (e.g., a column, etc.) that holds (fills) the recovered carbon black.

[0021] Among these, from the viewpoints of ease of production and effective reduction of metal compounds, a method of contacting recovered carbon black by mixing the treatment liquid 1 with the recovered carbon black after preparing the treatment liquid 1 is preferred. In addition to the above, from the viewpoint of further increasing the contactability, a method of contacting recovered carbon black by suspending recovered carbon black in a solvent in advance and then mixing the treatment liquid 1 prepared separately is preferred. From the viewpoint of effectively utilizing the nascent organic peracid, a method of adding an organic acid after mixing an oxidizing agent with recovered carbon black, mixing, and contacting the two is preferred. Furthermore, from the viewpoints of continuous performance of the solid-liquid separation step (3), washing step (4), and base contact step (2) described below and good treatment efficiency, a method of filling recovered carbon black into a packed container such as a column and passing treatment liquid 1 containing an oxidizing agent and, if necessary, an organic acid through the mixed solution is preferred.

[0022] During contact, it is preferable to appropriately stir the mixture as necessary. As the stirring method, a conventionally known method can be used, for example, stirring using a stirring blade or a rotor, stirring by ultrasonic waves or electromagnetic waves, shaking, bubbling, etc. From the viewpoint of stirring efficiency, industrially, stirring using a stirring blade is preferable.

[0023] The solvent used in the present invention is not particularly limited as long as it is a commonly used solvent as long as it does not impair the effects of the present invention, and may be an organic solvent or an aqueous solvent. From the viewpoints of environmental friendliness and ease of handling, however, an aqueous solvent is preferable. Such an aqueous solvent is preferably water 100% by mass, but may be a mixed solvent of water and an organic solvent having hydrophilicity, such as alcohols, e.g., methanol, ethanol, propanol, etc., or ketones, e.g., acetone, etc. From the viewpoint of environmental friendliness and ease of handling, the water content is preferably 50% by mass or more, more preferably 70% by mass or more, even more preferably 90% by mass or more, and even more preferably 95% by mass or more, based on the total amount of the aqueous solvent, and may be 100% by mass, and even more preferably 100% by mass.

[0024] The water used in the present invention is not particularly limited, and examples thereof include distilled water, deionized water, deaerated water, tap water, well water, etc., but from the viewpoint of the stability of the oxidizing agent during treatment, deionized water and distilled water are preferred, and deionized water is more preferred. The aqueous solvent may contain other additives such as a dispersant, as long as they do not impede the effects of the present invention. The present invention can achieve effective effects without containing a nitrogen hydride (a compound having an NH bond) as a dispersant.

[0025] The contact temperature (reaction temperature) in step (1) is, from the viewpoints of handleability and workability, preferably 10° C. or higher, more preferably 20° C. or higher, even more preferably 35° C. or higher, still more preferably 50° C. or higher, and is preferably less than 100° C., more preferably 90° C. or lower, even more preferably 80° C. or lower, and still more preferably 70° C. or lower. In addition, the pressure during contact is preferably normal pressure or increased pressure from the viewpoint of efficiently reducing the ash content. More specifically, from the above viewpoint, the pressure is preferably 0.1 MPa or more and preferably 0.3 MPa or less. In addition, from the viewpoint of ease of production, it is more preferable to contact under normal pressure (0.1 MPa). In addition, the contact time (reaction time) is preferably 1 minute or more, more preferably 10 minutes or more, even more preferably 30 minutes or more, and preferably 24 hours or less, more preferably 12 hours or less, even more preferably 3 hours or less.

[0026] <Base contact step (step (2))> Step (2) is a step of placing carbon black-containing particles and a base in a solvent and contacting them. The base used in step (2) is not particularly limited as long as it is a base that is usually used, and examples thereof include alkali metal hydroxides such as sodium hydroxide, potassium hydroxide, and lithium hydroxide; alkaline earth metal hydroxides such as calcium hydroxide; alkali metal carbonates such as sodium carbonate and potassium carbonate; aliphatic amines such as tetramethylamine; aromatic amines; hydroxides of ammine complexes such as diamine silver hydroxide and tetraamine copper hydroxide; and guanidines. These bases may be used alone or in combination of two or more. Among these, sodium hydroxide and potassium hydroxide are preferred, and sodium hydroxide is more preferred, from the viewpoints of being water-soluble, allowing the excess base to be easily removed by solid-liquid separation or washing with an aqueous solvent, and allowing the ash to be effectively removed.

[0027] The amount of base added varies depending on the type of base, the amount of ash in the recovered carbon black, etc., and is not particularly limited, but is preferably 0.4 parts by mass or more, more preferably 1 part by mass or more, and even more preferably 10 parts by mass or more, and is preferably 4000 parts by mass or less, more preferably 800 parts by mass or less, and even more preferably 200 parts by mass or less, relative to 100 parts by mass of recovered carbon black. When the amount added is within such a range, more effective removal of ash is possible.

[0028] In addition, when the base is dissolved in a solvent and used, the amount of the solvent containing the base (hereinafter also referred to as "treatment liquid 2") added, in a batchwise manner, from the viewpoints of ease of handling, solubility of the ash in the treatment liquid 2, and the like, is preferably 0.5 parts by mass or more, more preferably 1 part by mass or more, and even more preferably 2 parts by mass or more, relative to 1 part by mass of the recovered carbon black, and is preferably 100 parts by mass or less, more preferably 50 parts by mass or less, and even more preferably 20 parts by mass or less. From the same viewpoint, the total amount of the solvent used in step (2) is also preferably within the above range. In addition, the concentration of the base in the aqueous solution in this case is preferably 0.1 M or more, more preferably 0.5 M or more, even more preferably 0.8 M or more, and is preferably 20 M or less, more preferably 5 M or less, even more preferably 2 M or less, from the viewpoints of ease of handling, reactivity between the ash and the base, and the like. The pKa of the base (conjugate acid) is preferably 9 or more, more preferably 10 or more, and preferably 20 or less, more preferably 15 or less. When the pKa is within such a range, the product is easy to handle and easy to remove ash.

[0029] The solvent used in step (2) is the same as that used in the above-mentioned step (1). The order and method of charging the recovered carbon black and the base into the solvent and contacting them are not particularly limited. For example, the recovered carbon black and the base may be simultaneously added to the solvent and mixed to contact them. Alternatively, the recovered carbon black may be contacted by preparing a treatment liquid 2 in which the base is dissolved (or suspended) in the solvent, and then mixing the treatment liquid 2 with the recovered carbon black. The recovered carbon black may be contacted by suspending the recovered carbon black in advance in a solvent and then mixing the base (note that the base may also be prepared in advance as the treatment liquid 2). Furthermore, the recovered carbon black may be contacted by passing the treatment liquid 2 in which the base is dissolved through a container (e.g., a column or the like) that holds (fills) the recovered carbon black.

[0030] Among these contact methods, from the viewpoints of ease of production and effective reduction of ash content, it is preferable to prepare treatment liquid 2 and then mix the treatment liquid 2 with the recovered carbon black to effect contact. In addition to the above, from the viewpoint of further improving contactability, it is preferable to contact the recovered carbon black by a method in which the recovered carbon black is suspended in a solvent in advance and then a separately prepared treatment liquid 2 is mixed. Furthermore, from the viewpoints of being able to continuously carry out the above-mentioned oxidizing agent contact step (1), and the below-mentioned solid-liquid separation step (3) and washing step (4) and of good treatment efficiency, it is preferable to fill a packed container such as a column with the recovered carbon black and pass the treatment liquid 2 containing a base through the packed container. During the contact, it is preferable to appropriately stir the mixture as necessary. Examples of the stirring method include the same known methods as those described in the oxidizing agent contact step (1).

[0031] The contact temperature (reaction temperature) in step (2) varies depending on the type of base used and is not particularly limited. From the viewpoints of handleability and workability, the contact temperature is preferably 10° C. or higher, more preferably 20° C. or higher, even more preferably 35° C. or higher, still more preferably 50° C. or higher, and is preferably less than 100° C., more preferably 90° C. or lower, even more preferably 80° C. or lower, and still more preferably 70° C. or lower. In addition, the pressure during contact is preferably normal pressure or increased pressure from the viewpoint of efficiently reducing the ash content. More specifically, from the above viewpoint, the pressure is preferably 0.1 MPa or more and preferably 0.3 MPa or less. In addition, from the viewpoint of ease of production, it is more preferable to contact under normal pressure (0.1 MPa). In addition, the contact time (reaction time) is preferably 1 minute or more, more preferably 10 minutes or more, even more preferably 30 minutes or more, and is preferably 24 hours or less, more preferably 12 hours or less, even more preferably 3 hours or less.

[0032] The order of steps (1) and (2) is not particularly limited, and step (2) may be performed after step (1), or step (1) may be performed after step (2). From the viewpoint of effectively removing ash, it is preferable to perform step (2) after step (1). Furthermore, steps (1) and (2) may each be repeated. Alternatively, steps (1) to (2) may be carried out once and then the same series of steps may be repeated again. Similarly, steps (2) to (1) may be carried out once and then the same series of steps may be repeated again.

[0033] <Solid-liquid separation process (step (3))> It is preferable to have a step (3) of performing solid-liquid separation of the solvent after the contact treatment to obtain contact-treated particles containing carbon black after each of steps (1) and (2), or after the completion of all of steps (1) and (2). According to this step, the ash content dissolved in the solvent by the contact treatment, the surplus oxidizing agent after the contact treatment, and optionally chemicals such as organic acids and bases can be easily removed by removing the solvent by solid-liquid separation, and it is possible to obtain contact-treated particles containing carbon black with reduced ash content, i.e., purified carbon black. Step (3) may be performed after each of steps (1) and (2), i.e., step (3) may be performed after step (1) and step (3) may be performed after step (2). Alternatively, step (3) may be performed after completion of all steps (1) and (2), i.e., after step (1), step (2) may be performed without performing step (3) and then step (3) may be performed, or after step (2), step (1) may be performed without performing step (3) and then step (3) may be performed. From the viewpoint of further reducing impurities, it is preferable to carry out step (3) after each of step (1) and step (2), i.e., to carry out step (3) after step (1) and step (3) after step (2), or to carry out step (3) after step (2) and step (3) after step (1).

[0034] The solid-liquid separation in step (3) can be carried out by known solid-liquid separation means, such as settling, filtration, centrifugation, membrane separation, etc. By using such solid-liquid separation means, the excess oxidizing agent dissolved in the solvent, the organic acid added as needed, the excess base, and the ash are removed together with the solvent, making it possible to easily remove impurities such as the oxidizing agent, the organic acid, the base, and the ash from the particles containing carbon black. Among these, from the viewpoint of efficient and simple solid-liquid separation, reduced pressure filtration and pressure filtration using a filter are preferred. The pore size of the filter used in this case is not particularly limited and is appropriately selected depending on the particle size of the recovered carbon black used as a raw material. From the viewpoint of simplifying the work process, a method using a column is also preferred. According to the treatment using a column, a container is filled with recovered carbon black as a solid phase of the column, and treatment liquid 1 or treatment liquid 2 is passed through the column, so that solid-liquid separation (step (3)) can be performed simultaneously with the contact treatment in step (1) or step (2).

[0035] <Cleaning process (process (4))> It is preferable to have a step (4) of washing the particles to be contacted after the step (3). By washing the particles to be contacted, purified carbon black with a higher purity can be obtained. The washing method includes washing the contact-treated particles with a new solvent (hereinafter, also referred to as a "washing liquid"), and subjecting the washing liquid after washing to solid-liquid separation. Such a washing step may be carried out once or may be repeated two or more times. In addition, the removal of the washing liquid by solid-liquid separation in the washing step (4) may be carried out by the same solid-liquid separation means as that shown in the above step (3). The washing liquid used in the washing step is not particularly limited as long as it is a solvent capable of removing impurities from the particles to be contact-treated, but is preferably a solvent capable of dissolving the ash after the contact treatment, the oxidizing agent used in step (1), the organic acid used as required, and / or the base used in step (2), and more preferably a solvent capable of dissolving the ash after the contact treatment, the oxidizing agent used in step (1), the organic acid used as required, and the base used in step (2). Such a solvent makes it possible to remove impurities adhering to the particles to be contact-treated after solid-liquid separation.

[0036] Specifically, the solvents described in step (1) can be used as such a cleaning solution. The solvents used in steps (1), (2) and (4) may be the same or different. From the viewpoint of workability and efficiency, it is preferable to use the same solvent. In addition, as the same solvent, an aqueous solvent, more preferably water, is preferably used from the viewpoint of ease of handling. In this way, by performing steps (1), (2) and (4) using the same aqueous solvent (preferably water), the equipment and work can be simplified and costs can be reduced, and the chemicals such as the oxidizing agent used in the treatments of steps (1) and (2) and ash can be efficiently removed. In particular, when steps (1) to (4) are performed in a continuous manner such as a column described later, it is preferable to use the same aqueous solvent because additional washing operations, etc., caused by passing different solvents through the column are not required.

[0037] The process from step (1) to step (4) may be carried out in a batch manner or a continuous manner. Here, the batch method is, for example, a method in which a required amount of solvent (or processing liquid 1 or 2) is added to a container during the treatment in step (1) or step (2). More specifically, for example, the recovered carbon black, the oxidizing agent, and, if necessary, an organic acid are stirred and mixed in a solvent in a container (step (1)), followed by a solid-liquid separation treatment (step (3)) and, if necessary, a washing treatment (step (4)); and then, the recovered carbon black obtained after these treatments (note that only one of step (1) or step (2) is performed, and the recovered carbon black that has been subjected to the treatments in step (3) and, if necessary, step (4), is hereinafter also referred to as "crudely refined carbon black") and a base are stirred and mixed in a solvent in a container (step (2)), followed by a solid-liquid separation treatment (step (3)) and, if necessary, a washing treatment (step (4)). The continuous method herein refers to, for example, a method in which the treatment liquid used in steps (1) and (2) and, if necessary, the washing liquid used in step (4) are continuously delivered and discharged using a column or the like. More specifically, for example, a column is filled with recovered carbon black as a solid phase, and a treatment liquid 1 prepared by dissolving an oxidizing agent and, if necessary, an organic acid in a solvent is passed through the column (steps (1) and (3)), then, if necessary, a washing liquid is passed through (step (4)), then a treatment liquid 2 prepared by dissolving a base in a solvent is passed through (steps (2) and (3)), and then, if necessary, a washing liquid is passed through (step (4)).

[0038] <Drying process (process (5))> It is preferable to have a step (step (5)) of drying the contact treated particles after the above-mentioned step (3) or step (4). In particular, it is preferable that the drying step is carried out after the completion of steps (1) to (4). By carrying out sufficient drying, the particles can be suitably used in various products. The drying method is not particularly limited and may be a conventionally known method, such as natural drying, vacuum drying, hot air drying, freeze drying, spray drying, etc. Drying conditions such as temperature, pressure, and time are also not particularly limited and are appropriately determined depending on the drying method. For example, when the residue obtained by vacuum filtration after washing is left to dry, the drying conditions may be 80° C. and 120 minutes or more.

[0039] The method for producing purified carbon black of the present invention may further include a step of recovering particles containing carbon black from used tires. The particles containing carbon black can be recovered from used tires by a conventionally known method such as pyrolysis. The recovered carbon black used in the present invention may be commercially available.

[0040] The purified carbon black obtained by the production method of the present invention can be suitably used, for example, as a black pigment for paints, inks, etc., or as a reinforcing material for rubber products such as tires. EXAMPLES

[0041] Unless otherwise specified, "%" below indicates "% by mass." The oxidizing agent, acid, and base used in step (1) and step (2) were each prepared in advance using deionized water to give aqueous solutions with the concentrations shown in Table 1. Hereinafter, the aqueous solution of acid and the aqueous solution of acid and oxidizing agent used in step (1) will be referred to as "treatment liquid 1," and the aqueous solution of base used in step (2) will be referred to as "treatment liquid 2."

[0042] Examples 1 to 4 Experimental procedures As the particles containing carbon black, recovered carbon black (manufactured by Bolder Industries, product name "BolderBlack", ash content 17.7%) recovered from used tires was used. Step (1): 5 g of recovered carbon black and 50 g of treatment solution 1 (prepared so that the concentration of each component immediately after preparation was the value shown in Table 1) were placed in a 200 mL vial and stirred with a magnetic stirrer at 60° C. for 1 hour. The obtained slurry was filtered under reduced pressure using a 0.1 μm PTFE filter to separate the solid matter (particles to be contacted and treated) (step (3)). The solid matter was then washed with 100 mL of deionized water and filtered under reduced pressure (step (4)) to obtain crude carbon black.

[0043] Step (2): The entire amount of the crudely refined carbon black obtained above and 50 g of treatment solution 2 (1 M NaOH) were placed in a 200 mL vial and stirred with a magnetic stirrer at 60° C. for 1 hour. The slurry was filtered under reduced pressure through a 0.1 μm PTFE filter (step (3)), and the solid matter obtained was left on the PTFE filter. While maintaining the reduced pressure, 100 mL of deionized water was passed through the PTFE filter to wash it (step (4)). Thereafter, while maintaining the reduced pressure and leaving the solid matter on the PTFE filter, the filter was left to stand at room temperature for 60 minutes and dried (step (5)), to obtain purified carbon black.

[0044] Evaluation method The purified carbon black obtained by the above treatment was dried under reduced pressure at 100° C. for 12 hours. The dried refined carbon black was placed in an alumina crucible and heated for 4 hours in an electric furnace at 550° C. The ash content in the refined carbon black was calculated using the following formula. Ash percentage in refined carbon black (mass%) = (mass of residue / mass of refined carbon black after drying) x 100

[0045] Reference example 1 The ash content in the untreated recovered carbon black that had not been subjected to the treatments in steps (1) and (2) was calculated by the evaluation method in Example 1.

[0046] Comparative Example 1 The same procedure as in Example 1 was carried out, except that the treatment in step (1) was not carried out and the treatment in step (2) was carried out using treatment solution 2 (1 M NaOH) shown in Table 1.

[0047] Comparative Example 2 The same procedure as in Example 1 was carried out, except that the treatment in step (1) was carried out using treatment solution 1 (10% H2O2) shown in Table 1, and the treatment in step (2) was not carried out. The results of Examples 1 to 4, Reference Example 1, and Comparative Examples 1 and 2 are shown in Table 1.

[0048] [Table 1]

[0049] From the above results, it was found that the ash content was not sufficiently reduced by treatment with an oxidizing agent or treatment with a base alone, but that the ash content was significantly reduced by treatment with both an oxidizing agent and a base. Moreover, in Example 1 in which the treatment was carried out using an organic acid and an oxidizing agent in step (1), a more remarkable effect of reducing the ash content was observed.

Claims

1. A method for producing purified carbon black, comprising the following steps (1) and (2): Step (1) A step of placing particles containing carbon black and an oxidizing agent in a solvent and bringing them into contact with each other Step (2) A step of placing particles containing carbon black and a base in a solvent and contacting them

2. 2. The method for producing purified carbon black according to claim 1, wherein the step (1) further comprises charging an organic acid into the solvent and contacting the organic acid with the solvent.

3. 2. The method for producing purified carbon black according to claim 1, further comprising a step (3) of performing solid-liquid separation of the solvent after the contact treatment after each of steps (1) and (2), or after completion of all of steps (1) and (2), to obtain contact-treated particles containing carbon black.

4. 4. The method for producing purified carbon black according to claim 3, further comprising, after the step (3), a step (4) of washing the contacted treated particles.

5. 5. The method for producing purified carbon black according to claim 3, further comprising, after step (3) or step (4), a step (5) of drying the contacted particles.

6. The method for producing purified carbon black according to claim 1 , wherein the solvent is an aqueous solvent.

7. 2. The method for producing purified carbon black according to claim 1, wherein the particles containing carbon black are derived from used tires.

8. A tire containing purified carbon black obtained by the method for producing purified carbon black described in claim 1.