Method for concentrating inorganic pigments

Abstract

To provide a method for concentrating inorganic pigments that consumes little energy and efficiently extracts inorganic pigments such as titanium dioxide from residual ink without using coagulants. [Solution] The present invention is a method for concentrating inorganic pigments contained in waste ink for reuse. The concentration method includes a gelling step of mixing a gelling solvent with the waste ink containing the inorganic pigment, resin, and solvent to gel the resin and allow it to settle together with the inorganic pigment, and a gel separation step of separating the settled gelled material and supernatant liquid after the gelling step.

Description

[Technical Field]

[0001] The present invention relates to a method for concentrating inorganic pigments for reusing inorganic pigments contained in waste ink. [Background Art]

[0002] In recent years, reusing used ink as ink has been studied in order to reduce environmental load. As one reuse method, there is a method that uses residual ink (waste ink) used in gravure printing and the like as a raw material, and a relatively large amount of the residual ink is expected to be available for disposal.

[0003] Since residual ink contains a large amount of solvent, it is necessary to separate and remove a large amount of solvent in order to extract and reuse inorganic pigments such as titanium oxide from the residual ink. As methods for removing the solvent, there are methods of removing the solvent by distillation or combustion. However, these methods require a large amount of energy, and combustion generates a large amount of CO2, which is not preferable.

[0004] Accordingly, for example, Patent Document 1 discloses a method for recovering pigments from waste ink by adding a mixed solvent to the waste ink and allowing the pigments in the waste ink to settle. [Prior Art Documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. Sho 63-34763 [Summary of the Invention] [Problem to be Solved by the Invention]

[0006] However, there has been room for further improvement in recovery efficiency (concentration efficiency) particularly in methods for recovering inorganic pigments from waste ink.

[0007] While it is possible to extract inorganic pigments by filtration, the particle size of inorganic pigments such as titanium dioxide used in printing is extremely small, around 0.20 to 0.35 μm. Filtering them with filter paper or mesh would quickly become clogged, making filtration difficult. Another method involves adding a coagulant to the remaining ink to precipitate the inorganic pigment and separate it from the solvent. However, this method is undesirable because it requires a large amount of expensive precipitating agent, and the added precipitating agent must be removed in a later process. While sedimentation by centrifugal separation is a possible method, it is generally very difficult to separate the material, making it economically impractical.

[0008] The object of the present invention is to provide a method for concentrating inorganic pigments that consumes little energy and efficiently extracts inorganic pigments such as titanium dioxide from leftover ink without using flocculants or the like. [Means for solving the problem]

[0009] The present invention relates to a method for concentrating inorganic pigments contained in waste ink for reuse. The concentration method is A gelling step is performed in which a gelling solvent is mixed with the waste ink containing the inorganic pigment, resin, and solvent, thereby gelling the resin and causing it to settle together with the inorganic pigment. The method includes a gel separation step for separating the settled gelled material and the supernatant liquid after the gelation step. [Effects of the Invention]

[0010] According to the present invention, it is possible to provide a method for concentrating inorganic pigments that consumes little energy and efficiently extracts inorganic pigments such as titanium dioxide from leftover ink without using flocculants or the like. [Modes for carrying out the invention]

[0011] Embodiments of the present invention will be described below.

[0012] <Method for concentrating inorganic pigments> The concentration method of this embodiment is a method for concentrating inorganic pigments for reuse of inorganic pigments contained in waste ink. The concentration method includes a gelation step and a gel separation step in that order.

[0013] (Gelation process) In the gelling process, a gelling solvent is mixed with waste ink containing inorganic pigments, resin, and solvent, causing the resin to gel and settle together with the inorganic pigments.

[0014] As for the specific method of the gelation process, waste ink may be added to the gelling solvent, or the gelling solvent may be added to the waste ink. Furthermore, when adding a gelling solvent to waste ink, it is preferable to stir the mixture while adding the gelling solvent. This is because, for example, if the gelling solvent is added to waste ink that has accumulated at the bottom of the container, the waste ink will not dissolve in the gelling solvent. Therefore, adding waste ink to the gelling solvent is preferable because it allows for efficient gelation of the resin in the waste ink with a simple operation that does not require stirring or other procedures.

[0015] In this regard, we investigated the differences that arise from differences in the gelation process when white ink (waste ink) is added to water (Verification 1) and when water is added to white ink (Verification 2). The white ink used in the verification was "Finewrap NTV PET High Concentration White" (manufactured by DIC Graphics Co., Ltd.). This white ink is an oil-based ink for gravure printing that contains titanium dioxide as an inorganic pigment, urethane resin as a resin, and methyl ethyl ketone (MEK) and isopropyl alcohol (IPA) as solvents. As a result, in Verification 1, when white ink was added little by little to a sufficiently large amount of water in the container, a precipitate formed at the bottom of the container. Upon examining the bottom of the container with a spatula, the precipitate was found to be a sticky gel-like substance, which is thought to be a gelled resin. On the other hand, in Verification 2, when water was added while stirring the white ink in the container, precipitation formed at the bottom of the container when a certain amount of water had been added. When checking the bottom of the container with a spatula, the precipitate was found to be a sticky gel, which is considered to be a gelled resin. It should be noted that even if water is added when white ink has accumulated at the bottom of the container, the white ink does not dissolve, so stirring was required while adding water.

[0016] It should be noted that when a mixed solvent is added to waste ink as disclosed in Patent Document 1, the distillation process becomes complicated when attempting to recover and reuse the solvent or gelling solvent by distillation or other methods. Therefore, when reusing solvents and gelling solvents, it is preferable to use fewer types of solvents.

[0017] (Gel Separation Step) In the gel separation step, the precipitated gelated product after the gelling step is separated from the supernatant.

[0018] As a specific method for the gel separation step, the precipitated gelated product after the gelling step may be drained through a drain pipe provided near the bottom of the container in which the gelling step was performed, or the supernatant may be removed from the waste ink after the gelling step, and the remaining gelated residue may be recovered.

[0019] According to the present embodiment, inorganic pigments such as titanium oxide can be efficiently extracted with low energy consumption by removing the resin and solvent via combustion or other methods from the residue (gelled product) containing the inorganic pigment, resin, and reduced amount of solvent after the supernatant removal step. This enables reduction of environmental load.

[0020] It should be noted that although the residue containing the gelled product after the supernatant removal step still contains part of the solvent that was contained in the waste ink, the amount of solvent that needs to be removed from the residue is, for example, about 1 / 10 of that in the case without gelling.

[0021] Furthermore, a step of removing the solvent and the like from the gelled product to reduce the amount of the solvent and the like before combustion or the like may be performed. This step can be performed by, for example, filtration or a method of placing the gelled product on a mesh and allowing the solvent to fall naturally. This step can further reduce the energy consumption required for removing the resin and the solvent by a method such as combustion, and can further reduce the environmental load.

[0022] In the concentration method of the present embodiment, there is no need to add a third component such as a flocculant, so an operation of separating the flocculant after collection is not required, and contamination of the inorganic pigment with the flocculant can also be prevented, so high-quality inorganic pigment can be recovered. In addition, it is economically preferable because an expensive flocculant is not required. The separated solvent can be recovered again by distillation or the like, so it is also environmentally favorable. Since titanium oxide can be recovered as a gelled product with a high active ingredient concentration instead of a liquid, the subsequent transportation efficiency is also good.

[0023] (Waste ink) The waste ink contains an inorganic pigment, a resin, and a solvent.

[0024] The inorganic pigment in the waste ink that is the target of the recycling method of the present embodiment is preferably an expensive inorganic pigment such as titanium oxide contained in white ink. Examples of other inorganic pigments include barium sulfate contained in matte varnish, pearl pigments, mica, aluminum pigments, silica, calcium carbonate, and the like.

[0025] The resin has, for example, a function of constituting a coating film after application and a function of forming an emulsion for maintaining the dispersed state of the inorganic pigment in the solvent.

[0026] The solvent may be an oily solvent or an aqueous solvent. That is, the waste ink may be an oil-based ink or a water-based ink.

[0027] Waste ink refers to waste ink from printing processes such as gravure printing inks and flexographic printing inks. These inks are used, for example, for printing shrink labels and wrap labels.

[0028] Printing inks used for printing shrink labels, wrap labels, and the like require a thinner printing layer to display colors such as white, compared to other inks (for example, inks for building materials). Therefore, they have a higher concentration of inorganic pigments such as titanium dioxide. For this reason, the concentration method of this embodiment allows for the efficient recovery of inorganic pigments such as titanium dioxide.

[0029] The transmittance of titanium dioxide (white pigment) contained in gravure printing inks or flexographic printing white inks is approximately 0.2 to 0.35. The transmittance is expressed by the following formula: Transmission density D(OD(λ))=Log10[T(λ) / I(λ)]=Log10T(λ)-Log10I(λ) λ: Wavelength [T(λ) / I(λ)]: Transmittance T(λ): Amount of transmitted light in the wavelength band I(λ): Amount of incident light in the wavelength band

[0030] Waste ink is, for example, ink that contains a hardening agent. Typically, printing inks are used while stored in an ink pan. With inks that do not contain a hardener, any remaining ink after use can be reused if stored in a drum or 18-liter can in a way that prevents the solvent from evaporating easily, resulting in a relatively small amount of waste ink. On the other hand, inks that contain a hardener harden over time, so any remaining residue after use must be discarded, resulting in a large amount of waste ink. For this reason, the concentration method of this embodiment is particularly useful when recovering and reusing inorganic pigments from waste ink containing a hardener.

[0031] Examples of inks containing a curing agent include two-component curing inks. Examples of two-component curing inks include two-component curing white inks that are used by adding an isocyanate or aziridine-based curing agent to an oil-based ink containing titanium dioxide (white pigment) and an acrylic resin immediately before use. Since two-component curing inks harden over time after the two components are mixed, even unused ink residue cannot be used further and must be discarded.

[0032] Furthermore, the waste ink may contain other additives such as pH adjusters, chelating agents, surfactants, antibacterial and antifungal agents, conductivity imparters, plasticizers, antioxidants, and ultraviolet absorbers.

[0033] The concentration method of this embodiment will be described in more detail below, with reference to the case where the waste ink is water-based ink (Embodiment 1) and the case where it is oil-based ink (Embodiment 2).

[0034] <Embodiment 1: Water-based ink> In Embodiment 1, the waste ink is water-based ink.

[0035] The solvent in water-based inks is an aqueous solvent. As the aqueous solvent, at least one solvent selected from, for example, alcohol and water is preferably used.

[0036] Examples of resins contained in water-based inks include acrylic resins and urethane resins. Water-based inks may also contain cellulose resins.

[0037] The gelling solvent in Embodiment 1 is a good solvent with stronger dissolving power for the resin than the aqueous solvent mentioned above.

[0038] As such a good solvent, at least one solvent selected from alcohols, ester solvents, and ketone solvents can be suitably used. Examples of alcohols include methanol, ethanol, and isopropyl alcohol (IPA). Examples of ester solvents include ethyl acetate and n-propyl acetate. Examples of ketone solvents include methyl ethyl ketone (MEK) and acetone. Other good solvents include benzene-based solvents such as toluene.

[0039] By adding the above-mentioned good solvent to waste water-based ink, the resin, which was dispersed as an emulsion in the water-based solvent, precipitates and gels due to the aggregation of emulsion particles by the good solvent, and settles in the waste ink. When the gelled material settles, it settles together with the inorganic pigment. Therefore, by separating the settled gelled material from the supernatant liquid and recovering the inorganic pigment from the gelled material containing the concentrated inorganic pigment, the inorganic pigment can be efficiently extracted.

[0040] Furthermore, the resin contained in the water-based ink may be a resin having carboxyl groups in its side chains. In this case, for example, if the aqueous solvent contains amines or ammonia, the carboxyl groups will ionize, causing the resin to dissolve in the aqueous solvent, thereby dispersing the inorganic pigment encapsulated in the resin within the ink. In this case, by using an acidic solvent as the gelling solvent, the resin can be precipitated and gelled, causing it to settle, and the inorganic pigment can also settle together with it.

[0041] <Embodiment 2: Oil-based ink> In Embodiment 2, the waste ink is oil-based ink.

[0042] The solvent in oil-based inks is an oil-based solvent (organic solvent). As the oil-based solvent, at least one solvent selected from alcohols, ester-based solvents, and ketone-based solvents, similar to the good solvents mentioned above, can be suitably used, and benzene-based solvents can also be used. For example, in an oil-based ink containing a urethane resin, an ester-based solvent or a ketone-based solvent is preferably used as the oil-based solvent. Also, for example, in an oil-based ink containing an acrylic resin, a mixed solvent of ethyl acetate and isopropyl alcohol (IPA) is preferably used as the solvent.

[0043] Examples of resins contained in oil-based inks include acrylic resins and urethane resins. Oil-based inks may also contain cellulose resins.

[0044] The gelling solvent in Embodiment 2 is a poor solvent with weaker dissolving power for the resin than the oily solvent described above.

[0045] As such a poor solvent, at least one solvent selected from water and alcohol can be suitably used. Examples of alcohols include methanol, ethanol, and isopropyl alcohol (IPA). For example, waste ink from oil-based inks containing urethane resins can be efficiently gelled, especially by using alcohol as the gelling solvent (poor solvent).

[0046] By adding the poor solvent to the waste ink, the resin that was dissolved in the oily solvent precipitates and gels due to the reduced solubility of the poor solvent, and settles within the waste ink. When the gel settles, it settles together with the inorganic pigment. Therefore, by separating the settled gel from the supernatant liquid and recovering the inorganic pigment from the gel containing the concentrated inorganic pigment, the inorganic pigment can be efficiently extracted.

[0047] Furthermore, when water is used as the gelling solvent, the amount of organic solvent used to dry the gel is reduced, allowing for safe drying. Water is also inexpensive, making it economically advantageous. Additionally, because it does not mix with and separates from the organic solvent in waste ink, it facilitates not only the reuse of inorganic pigments from the gel but also the efficient recovery and reuse of the organic solvent.

[0048] Furthermore, when the gelling solvent is an alcohol, the solvent and gelling solvent remaining in the gelled product separated in the gel separation step can be removed by volatilization with less energy. Suitable alcohols that require less energy for volatilization and are inexpensive include monohydric alcohols with 3 or fewer carbon atoms, such as methanol, ethanol, propanol, and isopropyl alcohol.

[0049] The present invention is not limited to the embodiments described above and can be modified in various ways. Two or more embodiments selected from the various embodiments described above may be combined as appropriate, or at least one configuration (part of a configuration) selected from the various embodiments described above may be replaced with a part of a configuration of another embodiment.

Claims

1. A method for concentrating inorganic pigments contained in waste ink for reuse, A gelling step is performed in which a gelling solvent is mixed with the waste ink containing the inorganic pigment, resin, and solvent, thereby gelling the resin and causing it to settle together with the inorganic pigment. A concentration method comprising a gel separation step for separating the precipitated gelled material and the supernatant liquid after the gelation step.

2. The aforementioned solvent is an aqueous solvent, The concentration method according to claim 1, wherein the gelling solvent is a good solvent that has a stronger dissolving power for the resin than the aqueous solvent.

3. The aforementioned resin includes an acrylic resin or a urethane resin. The aqueous solvent is at least one solvent selected from alcohol and water. The good solvent is at least one solvent selected from alcohols, ester solvents, and ketone solvents. The concentration method according to claim 2.

4. The aforementioned solvent is an oily solvent. The concentration method according to claim 1, wherein the gelling solvent is a poor solvent having weaker dissolving power for the resin than the oily solvent.

5. The aforementioned resin includes an acrylic resin or a urethane resin. The oily solvent comprises at least one solvent selected from alcohols, ester solvents, and ketone solvents. The poor solvent is at least one solvent selected from water and alcohol. The concentration method according to claim 4.