Cellulose acetate flakes, and gold recovery method and concentration method using same

Abstract

These cellulose acetate flakes are composed of cellulose acetate having a degree of acetyl substitution of 0.80-2.90, the average space velocity SV derived through a prescribed method being more than 1.0 h−1 to 120 h−1. This gold recovery method includes filling a cylindrical container with the cellulose acetate flakes to produce a filled column, and passing a gold-ion-containing liquid through the filled column to adsorb the gold ions to the cellulose acetate flakes.

Description

Cellulose acetate flakes and gold recovery and concentration methods using the same

[0001] The present disclosure relates to cellulose acetate flakes. In particular, the present disclosure relates to cellulose acetate flakes for packing a gold recovery column, a gold recovery column packed with cellulose acetate flakes, and a gold recovery method using the column.

[0002] In recent years, many electrical and electronic devices, such as mobile phones, have been used and discarded. These electrical and electronic devices contain various metals, including precious metals and rare metals. Therefore, discarded electrical and electronic devices have been called "urban mines" for extracting precious metals and have attracted attention. Gold (Au), in particular, is a material used in bonding wires for connecting IC chip electrodes to lead frames, and is used in large quantities. Therefore, there is a demand for technology to efficiently recover gold from urban mines.

[0003] For example, a known technique involves crushing electronic components containing gold and then immersing them in aqua regia, a mixture of nitric acid and hydrochloric acid, to extract and dissolve the precious metal. After filtration as needed, a reducing agent is added to the aqua regia solution to separate low-purity crude gold. This crude gold contains impurities, including metals other than gold and insoluble materials that cannot be removed by filtration. To recover gold with a purity of 99.99% (four nines), the process of refining the separated crude gold must be repeated multiple times, which poses a problem of high energy costs.

[0004] Japanese Patent Laid-Open Publication No. 1-111453 (Patent Document 1) discloses a technique for separating rare metals such as samarium by passing a solution containing the rare metal through a filter-equipped column packed with small spherical ion exchange resins.

[0005] Japanese Patent Laid-Open Publication No. 2014-109064 (Patent Document 2) proposes a metal recovery method that uses a polysaccharide carboxylic acid ester as a metal capture agent to capture metals in a solution. In this patent document 2, powdered or fibrous cellulose acetate is added as a metal capture agent to a solution containing gold ions, and the resulting mixture is stirred, causing the gold ions to be adsorbed onto the cellulose acetate.

[0006] Japanese Patent Laid-Open Publication No. 2017-052961 (Patent Document 3) discloses a cellulose acetate powder that is made of cellulose acetate having an acetylation degree of 53-56% and a 6% viscosity of 30-200 mPa s, and has a cumulative pore volume of 0.2 ml / g or more, a ratio of particles having a particle size of 500 μm or more of 40% or less, and an angle of repose of 51° or less. This cellulose acetate powder is produced by pulverizing cellulose acetate flakes.

[0007] Japanese Patent Application Laid-Open No. 2020-204002 (Patent Document 4) describes a cellulose acetate cellulose having a molar xylose content of 0.5 mol% to 1.0 mol%, and a spectrophotometric color of 0.47 cm －１ The water content is less than 2% by mass and the specific surface area is less than 9 m ２ WO 2016 / 020952 (Patent Document 5) discloses cellulose acetate flakes having an average acetylation degree of 60.0 to 61.5%, a viscosity-average polymerization degree of 360 to 440, and a filtration rating of 100 or less.

[0008] Japanese Patent Application Laid-Open No. 1-111453 Japanese Patent Application Laid-Open No. 2014-109064 Japanese Patent Application Laid-Open No. 2017-052961 Japanese Patent Application Laid-Open No. 2020-204002 International Publication No. 2016 / 020952

[0009] Patent Document 1 does not describe a technology for selectively separating only gold. Furthermore, the technology of Patent Document 1 requires a polycondensation reaction of a phenolic compound having a carboxyl group, a compound having a phenolic hydroxyl group, and an aldehyde compound under alkaline conditions using a known method, which is complicated and has problems with low recovery efficiency. Patent Document 2 uses a batch-type adsorption operation, which has problems such as a decrease in adsorption efficiency as the gold ion concentration in the gold ion-containing solution decreases. Furthermore, considering industrial process costs, gold recovery technology using a packed column is advantageous. However, according to the findings of the present inventors, columns packed with powdered cellulose acetate experience excessive pressure loss during liquid passage, resulting in low liquid flow rates or even complete failure. Furthermore, it is difficult to uniformly pack cellulose acetate in a long-fiber bundle into a column, and partial overpacking can result in low liquid flow rates or complete failure. To efficiently recover gold from a gold-containing solution, a packing material that has excellent liquid permeability when packed into a column and can selectively adsorb gold is required. Patent Document 2 suggests the selective adsorption of gold by cellulose acetate, but as mentioned above, it is difficult to use powdered or long-fiber bundle-shaped cellulose acetate as a column packing material. Patent Documents 3 to 5 disclose flaked cellulose acetate as another embodiment. However, all of these techniques are proposed for the purpose of molding and processing cellulose acetate flakes, and do not specify the shape and size of the cellulose acetate flakes themselves.

[0010] Cellulose acetate flakes that can be uniformly packed in a column and exhibit excellent liquid permeability have not yet been proposed. An object of the present disclosure is to provide cellulose acetate flakes that can be used as a packing material for gold adsorption (recovery) columns. Another object of the present disclosure is to provide a method for selectively adsorbing and recovering gold from a gold-containing solution, particularly a gold-containing solution obtained by dissolving gold from electronic components, decorative product processing waste, waste catalysts, etc., using aqua regia.

[0011] The cellulose acetate flakes of the present disclosure are made of cellulose acetate having an acetyl substitution degree of 0.80 or more and 2.90 or less. The cellulose acetate flakes are spun at an average space velocity (SV) of 1.0 h －１ Over 120 hours －１ The average space velocity SV is determined by the amount of liquid (m3) flowing out from the bottom per unit time when a column (inner diameter 10 mm) packed with cellulose acetate flakes is placed vertically so that the height from the bottom is 2.0 cm, and a hydrochloric acid solution (temperature 20°C ± 5°C) with a concentration of 2.0 mol / L is introduced from the top of the column and allowed to flow down through the column under atmospheric pressure. ３ / h) is calculated by multiplying the volume of space occupied by the cellulose acetate flakes in the column (m ３ ) can be calculated by dividing by

[0012] The gold adsorption column of the present disclosure is packed with the above-mentioned cellulose acetate flakes as a packing material.

[0013] The gold recovery method of the present disclosure includes filling a cylindrical container with the above-mentioned cellulose acetate flakes to prepare a packed column, and passing a liquid containing gold ions through the packed column to adsorb the gold ions onto the cellulose acetate flakes.

[0014] The present disclosure may be a method for concentrating a gold solution, comprising: filling a cylindrical container with the above-described cellulose acetate flakes to prepare a packed column; passing a liquid containing gold ions as a stock solution through the packed column to adsorb the gold ions onto the cellulose acetate flakes; and passing an eluate through the packed column containing the cellulose acetate flakes to which the gold ions have been adsorbed, to obtain a liquid containing gold ions at a higher concentration than that of the stock solution.

[0015] The cellulose acetate flakes of the present disclosure can ensure high liquid permeability even when packed into a column. A column packed with the cellulose acetate flakes of the present disclosure can pass a large amount of gold-containing solution through the column in a short time without increasing the column internal pressure, allowing for efficient and selective separation and adsorption of gold. Furthermore, this column allows gold ions adsorbed on the cellulose acetate flakes to be easily desorbed by passing a eluent through the column. Furthermore, because the cellulose acetate filler is relatively inexpensive, gold can be easily recovered as a solid by incinerating or dissolving the gold-adsorbed cellulose acetate flakes.

[0016] An example of a preferred embodiment will be described in detail below. Each configuration and combination thereof in each embodiment is merely an example, and addition, omission, substitution, and other modifications of configurations are possible as appropriate within the scope of the present disclosure. The present disclosure is not limited by the embodiments, but only by the scope of the claims. Furthermore, each aspect disclosed in this specification can be combined with any other feature disclosed in this specification.

[0017] In this specification, the range "X to Y" means "X or more and Y or less," "ppm" means "ppm by mass," and "%" means "% by mass (wt.%)." Furthermore, "% by mass" means so-called weight percent, and does not mean mass concentration. Unless otherwise noted, all test temperatures in this specification are room temperature (20°C ± 5°C).

[0018] [Cellulose acetate flakes] The cellulose acetate flakes of the present disclosure are made of cellulose acetate having an acetyl substitution degree of 0.80 or more and 2.90 or less, and －１The mean space velocity (SV) is greater than 0.05. The cellulose acetate may contain a substituent other than an acetyl group as long as the effects of the present disclosure are obtained. Here, the mean space velocity (SV) is measured using a column with an inner diameter of 10 mm. Specifically, the column is arranged so that the column axis is vertical, and cellulose acetate flakes are packed into the column to a height of 2.0 cm. Thereafter, a hydrochloric acid solution (temperature: 20°C ± 5°C) with a concentration of 2.0 mol / L is introduced from the top of the column at a rate of 3.1 ml / min and allowed to flow down the column under atmospheric pressure. The amount of liquid (m) flowing out from the bottom per unit time is measured. ３ / h) is calculated by multiplying the volume of space occupied by the cellulose acetate flakes in the column (m ３ Specifically, this volume is calculated as V = 0.005 (m) × 0.005 (m) × π × 0.02 (m) = 1.57 × 10 ―６ (m ３ )

[0019] The cellulose acetate flakes of the present disclosure can selectively adsorb gold ions in solution. Conventional columns used for gold recovery utilize the phenomenon in which molecules with specific ion-exchange groups for chelating activity strongly bind to metal ions in aqueous solution. In contrast, the adsorption mechanism of the present disclosure is different from conventional methods. It is believed that gold is adsorbed through a specific physical interaction between a portion of the molecular structure of the cellulose acetate flakes of the present disclosure and gold. More specifically, it is believed that the size of the pockets (voids) specifically formed by multiple cellulose chains matches the size of the gold ions, resulting in the specific physical interaction. The inventors have discovered that cellulose acetate flakes capable of passing a specified hydrochloric acid solution through the flakes at a mean space velocity (SV) within a specific range improve the contact state with the liquid containing gold ions, resulting in more efficient selective gold adsorption through the physical interaction. Furthermore, in conventional gold recovery columns, when desorbing gold that is tightly bound to the packing material, it is necessary to use a special solution containing an acidic component as the extraction liquid. However, according to the present disclosure, in which gold is adsorbed through physical interactions, gold can be easily desorbed using water (pure water) as the extraction liquid.

[0020] For example, the surface structure and pocket size of cellulose acetate flakes vary depending on the production method and production conditions. Therefore, in order to obtain cellulose acetate flakes that undergo specific physical interactions with gold ions and selectively adsorb gold in solution, it is necessary to control multiple properties, such as surface area, pore structure, surface polarity, and particle size distribution, in addition to particle shape. The present inventors have found that these properties of cellulose acetate flakes are reflected in the average space velocity (SV) measured under specified conditions. They have also found that by setting this average space velocity (SV) within a specific range, along with the degree of acetyl substitution of cellulose acetate, cellulose acetate flakes that can selectively adsorb and recover gold can be obtained. In other words, the cellulose acetate flakes of the present disclosure have a specific external appearance, i.e., flakes, and have a microstructure and physical properties that are specified using the parameter average space velocity (SV) as an index.

[0021] Furthermore, the cellulose acetate flakes of the present disclosure have excellent liquid permeability when packed in a column. By passing a gold-containing solution through this column, gold can be selectively and efficiently recovered. In other words, the cellulose acetate flakes of the present disclosure can be used as a packing material for a gold adsorption (recovery) column. The gold recovery column of the present disclosure can recover only gold by adsorbing and desorbing it with extremely high selectivity from a solution in which gold-containing materials, known as urban mines, are dissolved with a metal dissolving solution (aqua regia) or the like. Furthermore, the cellulose acetate flakes of the present disclosure can also selectively recover only gold from a hydrochloric acid solution in which a wide variety of metal ions are dissolved. For example, the cellulose acetate flakes of the present disclosure can selectively recover gold (Au) even from a solution containing other metal elements, such as Li, Be, B, Na, Mg, Al, Si, P, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Rb, Sr, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Te, Cs, Ba, Hf, Ta, W, Re, Os, Ir, Pt, Ti, Pb, Bi, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Th. Furthermore, even when the solution contains as many as 67 different metal elements (Li, Be, B, Na, Mg, Al, Si, P, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Rb, Sr, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Te, Cs, Ba, Hf, Ta, W, Re, Os, Ir, Pt, Au, Ti, Pb, Bi, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Th), a superselective gold adsorption column that selectively adsorbs gold can be obtained.

[0022] Furthermore, the cellulose acetate flakes of the present disclosure can efficiently adsorb gold even in solutions with a gold concentration as low as 5 ppm. Furthermore, the cellulose acetate flakes of the present disclosure can preferentially adsorb gold even from solutions containing metals other than gold at concentrations 200 times or more higher than the gold concentration. Therefore, the cellulose acetate flakes of the present disclosure can be applied to the recovery of gold ions from diluted hydrochloric acid-nitric acid mixtures containing aqua regia, which is commonly used as a metal dissolving solution in urban mines. For example, exhaust gas catalysts and hard disks for electronic components contain large amounts of platinum along with gold. However, the technology of the present disclosure can selectively recover only low-content gold from these solutions.

[0023] In this specification, the term "flakes" is defined as particles having a flat shape such as thin plates, plates, scales, etc. The planar shape of the cellulose acetate flakes of the present disclosure can be various shapes such as circular, elliptical, polygonal, etc.

[0024] The width of the cellulose acetate flakes may be 0.1 mm or more and 10 mm or less, and the thickness may be 0.05 mm or more and 0.1 mm or less. Alternatively, the width of the cellulose acetate flakes may be 0.1 mm or more and 1 mm or less, and the thickness may be 0.05 mm or more and 0.1 mm or less. Alternatively, the width of the cellulose acetate flakes may be 0.1 mm or more and 0.17 mm or less, and the thickness may be 0.05 mm or more and 0.1 mm or less. The cellulose acetate flakes are long (longer) in shape, with a width greater than a thickness.

[0025] Here, the width of cellulose acetate flakes is the average diameter (average of the maximum and minimum diameters) of the flake particles in the plane having the maximum area, and the thickness of cellulose acetate flakes is the average diameter of the flake particles in the cross section perpendicular to the plane having the maximum area. The width and thickness of cellulose acetate flakes can be determined by microscopic observation. For example, the width and thickness of 100 flake particles randomly selected from an SEM image (magnification 10x) obtained by a scanning electron microscope can be read and averaged to determine the width and thickness of the cellulose acetate flakes.

[0026] The cellulose acetate constituting the cellulose acetate flakes has an acetyl substitution degree DS of 0.80 or more and 2.90 or less, preferably 0.90 or more, more preferably 1.00 or more, even more preferably 1.10 or more, and preferably 2.80 or less, more preferably 2.70 or less, and even more preferably 2.60 or less. The acetyl substitution degree DS is preferably 0.90 or more and 2.80 or less, more preferably 1.00 or more and 2.70 or less, and even more preferably 1.10 or more and 2.60 or less. Note that the cellulose acetate may contain other substituents besides acetyl groups, as long as the effects of the present disclosure are obtained. Other substituents include acyl groups such as a propionyl group, a butyryl group, a pentanoyl (valeryl) group, a hexanoyl group, a heptanoyl group, an octanoyl group, a nonanoyl group, an undecanoyl group, a dodecanoyl group, a tridecanoyl group, a tetradecanoyl (myristoyl) group, a pentadecanoyl group, a hexadecanoyl group, a heptadecanoyl group, and an octadecanoyl (stearoyl) group.

[0027] The degree of acetyl substitution DS of cellulose acetate can be determined by converting the acetylation degree AV (%), which means the amount of acetic acid bound per unit mass of cellulose, using the following formula: DS=162.14×AV×0.01 / (60.052−42.037×AV×0.01) Here, the acetylation degree AV of cellulose acetate can be measured in accordance with the description of ASTM: D-817-91 (Testing methods for cellulose acetate, etc.). The degree of acetyl substitution DS of cellulose acetate can be determined by the following formula: １It can also be determined by H-NMR measurement.

[0028] The cellulose acetate flakes of the present disclosure have an average space velocity (SV) of 1.0 h , as determined by the above-mentioned method. －１ From the viewpoint of improving gold recovery efficiency, －１ More than 10.0 h is preferable. －１ More preferably, 50.0 h －１ More preferably, 80.0 h －１ The average space velocity SV is particularly preferably 120 h －１ The following is preferred: 110h －１ More preferably, the average space velocity (SV) is 1.0 h －１ Over 120 hours －１ Preferably, 2.0 h or less －１ More than 110 hours －１ Less than 10.0 h is more preferable. －１ More than 110 hours －１ More preferably, 50.0 h －１ More than 110 hours －１ More preferably, 80.0 h or less －１ More than 108 hours －１ Even more preferably, 85.0 h －１ More than 108 hours －１ The following is particularly preferred: The average space velocity SV is 85.0 h －１ Over 92.0 hours －１ The following range is also preferred: The method and conditions for measuring the average space velocity SV will be described in detail in the Examples below.

[0029] Here, the term "space velocity (SV)" refers to the velocity of a liquid passing through a packing material in a packed column, for example, and is defined as the reciprocal of the time during which the liquid contacts the packing material. In this specification, the term "average space velocity SV" refers to the volume (m ) of the packing material (i.e., cellulose acetate flakes) packed in the column. ３ ) and the volumetric flow rate (m) of the liquid passing through this packing material per unit time under atmospheric pressure. ３ / h). That is, when the cellulose acetate flakes of the present disclosure are used as a column packing material, the average space velocity SV is an index of the liquid permeability of the column, and is an index showing the processing capacity (liquid processing amount) per unit time of this column (for example, a column for adsorption (recovery) of gold). Hereinafter, in this specification, the "average space velocity SV" determined under atmospheric pressure to specify the cellulose acetate of the present disclosure and the "space velocity SV" under pressure set in the adsorption test etc. described later will be referred to as the "average space velocity SV" ’ " is used in distinction from ".

[0030] As mentioned above, the cellulose acetate flakes of the present disclosure have a flat shape. This flat shape is thought to enable uniform packing in a column and to contribute to an improvement in the average space velocity (SV) by forming interparticle voids through which the treatment liquid can easily pass. Furthermore, as mentioned above, the average space velocity (SV) is also affected by the particle size distribution, specific surface area, total pore volume, surface polarity, etc. of the cellulose acetate flakes. In other words, the average space velocity (SV) in the present disclosure can be said to be an index that reflects multiple physical properties such as the particle size distribution, specific surface area, total pore volume, as well as the shape and size of the cellulose acetate flakes. As a result of extensive investigation, the present inventors have found that the average space velocity (SV) of the cellulose acetate flakes is 1.0 h at atmospheric pressure. －１ It has been found that the object of the present disclosure can be achieved by selecting the physical properties of the cellulose acetate flakes in a well-balanced manner within a range in which an average space velocity (SV) of more than 1000 kJ / cm 2 can be obtained.

[0031] The various physical properties that are desirable for achieving the average space velocity SV will be explained below one by one.

[0032] The particle size distribution of cellulose acetate flakes is measured in accordance with the general rules for sieving test methods described in JIS Z8815 using a test sieve specified in JIS Z8801-1:2006. That is, a 20-mesh (841 μm) sieve and a tray are attached to a rotary tap machine (manufactured by Iida Seisakusho Co., Ltd., tapping: 156 times / min, rolling: 290 times / min), and 100 g of sample is vibrated for 5 minutes. The ratio of the mass of the sample on the sieve to the total weight (100 g of sample) is then calculated, thereby determining the proportion of particles with a particle diameter of 850 μm or more. Details of the method and conditions for measuring particle size distribution will be described later in the Examples.

[0033] In the particle size distribution of the cellulose acetate flakes of the present disclosure, the proportion of particles having a particle diameter of 850 μm or more may be 10% or more, 50% or more, 60% or more, 70% or more, 80% or more, 85% or more, or 90% or more, with the upper limit being 100%. In the particle size distribution of the cellulose acetate flakes of the present disclosure, the proportion of particles having a particle diameter of 850 μm or more may be 10% or more and 100% or less, 50% or more and 100% or less, 60% or more and 100% or less, 70% or more and 100% or less, 80% or more and 100% or less, or 90% or more and 100% or less. If there are many particles with a particle diameter of less than 850 μm, the packing density in the column may become too high, which may increase the pressure loss during liquid passage. Furthermore, when the ratio of particles having a particle diameter of 850 μm or more is high, the average space velocity SV can be increased. In order to adjust the particle size distribution of the cellulose acetate flakes of the present disclosure, they may be classified using a sieve with appropriate openings.

[0034] From the viewpoint of suppressing an increase in pressure loss during column passage, the mass-based cumulative 50% particle size (median size: D50) determined from the particle size distribution of the cellulose acetate flakes of the present disclosure may be 40 mesh or more, 35 mesh or more, or 30 mesh or more. Furthermore, from the viewpoint of improving packing density, the mass-based cumulative 50% particle size (median size: D50) may be 10 mesh or less, 12 mesh or less, or 14 mesh or less. The mass-based cumulative 50% particle size in the present disclosure is defined as the mesh size of a sieve when the mass of particles passing through the mesh exceeds 50% of the total mass of the sample in a sieving test using a sieve specified in JIS Z8801-1:2006.

[0035] The cellulose acetate flakes of the present disclosure have a BET specific surface area of ​​1.8 m ２ / g or more, and ２ / g or more, ２ / g or more, and ２ / g or more, ２ / g or more, and ２ / g or less, and ２ / g or less, and ２ / g or less. The BET specific surface area may be 1.8 m ２ / g or more 25m ２ / g or less is preferable, and 2m ２ / g or more 20m ２ / g or less is more preferable, ２ / g or more 18m ２ / g or less is more preferable, and 5m ２ / g or more 18m ２ / g or less is more preferable, and 7m ２ / g or more 18m ２ / g or less is even more preferable. The BET specific surface area is measured by the BET method using nitrogen gas adsorption in accordance with the method described in JIS Z8831. Details of the measurement method and measurement conditions will be described later in the examples.

[0036] The specific surface area of ​​cellulose acetate flakes generally increases as the particle size decreases. As described above, the cellulose acetate flakes of the present disclosure adsorb gold ions through physical interactions, so a larger specific surface area is advantageous. However, a larger specific surface area may result in a smaller particle size, which may reduce liquid permeability.

[0037] The cellulose acetate flakes of the present disclosure have a total pore volume of 0.005 cm ３ / g or more, and ３ / g or more, and ３ / g or more, and ３ / g or more, and ３ / g or less, and ３ / g or less, and ３ / g or less. The total pore volume may be 0.005 cm ３ / g or more 150cm ３ / g or less is preferable, and 0.007 cm ３ / g or more 10cm ３ / g or less is more preferable, and 0.009 cm ３ / g or more 1.0cm ３ The total pore volume is measured by nitrogen gas adsorption. Details of the measurement method and conditions will be described later in the examples.

[0038] The total pore volume of cellulose acetate flakes, in relation to the specific surface area, indicates the state of voids on the flake surface. The total pore volume in the present disclosure is a physical property different from the cumulative pore volume determined by mercury intrusion porosimetry. The so-called cumulative pore volume is usually measured using a mercury porosimeter (Quantachrome's PoreMaster 60) or the like, and indicates the volume of relatively large pores. On the other hand, the total pore volume in the present disclosure is measured by gas adsorption, and indicates the volume including finer voids.

[0039] The cellulose acetate flakes of the present disclosure may have a load displacement of 10 mm or less when loaded with 10 N. This load displacement can be determined by filling a 10 ml cylindrical syringe (made of polypropylene, inner diameter 17 mm) with cellulose flakes to form a packed layer 50 mm in height, compressing this packed layer at a compression rate of 10 mm / min using a universal tensile tester (manufactured by A&D Co., Ltd. under the trade name "RTG1310"), measuring the height of the packed layer when loaded with 10 N, and calculating the difference from the height of the packed layer before compression.

[0040] In this specification, the load displacement is an index showing the hardness of the cellulose acetate flakes. Cellulose acetate flakes having a load displacement of 10 mm or less when loaded with 10 N are less likely to deform when packed into a column at high density or when pressure loss during liquid passage increases, thereby ensuring the desired liquid permeability. From the viewpoint of obtaining high liquid permeability, the load displacement at 10 N may be 8 mm or less, 6 mm or less, or 4 mm or less, with the lower limit being 0 mm. The load deformation of the cellulose acetate flakes when loaded with 10 N may be 0 mm or more and 10 mm or less, 0 mm or more and 8 mm or less, 0 mm or more and 6 mm or less, or 0 mm or more and 4 mm or less.

[0041] As long as the effects of the present disclosure are obtained, the molecular weight and molecular weight distribution of the cellulose acetate constituting the cellulose acetate flakes are not particularly limited. For example, the weight-average molecular weight Mw of the cellulose acetate may be 100,000 or more, 150,000 or more, or 170,000 or more, and may be 400,000 or less, 300,000 or less, or 250,000 or less. The weight-average molecular weight Mw is preferably 100,000 or more and 400,000 or less, more preferably 150,000 or more and 300,000 or less, and even more preferably 170,000 or more and 250,000 or less.

[0042] The molecular weight distribution Mw / Mn, defined by the weight-average molecular weight Mw and the number-average molecular weight Mn of cellulose acetate, is not particularly limited. For example, the molecular weight distribution Mw / Mn of cellulose acetate may be 2.0 or more, 2.5 or more, or 3.0 or more, and may be 6.0 or less, or 5.0 or less. The molecular weight distribution Mw / Mn is preferably 2.0 or more and 6.0 or less, more preferably 2.5 or more and 5.0 or less, and even more preferably 3.0 or more and 5.0 or less.

[0043] The weight-average molecular weight Mw, number-average molecular weight Mn, and molecular weight distribution Mw / Mn of cellulose acetate are determined by gel permeation chromatography (GPC) under the following measurement conditions: Guard column: PolyPore GUARD, size 50 x 7.5 mm (Agilent Technologies Inc.) Column: PolyPore, size 300 x 7.5 mm x 2 (Agilent Technologies Inc.) Elution solution: NMP + 0.1 M LiBr Sample concentration: 0.50% w / v Injection volume: 50 μL Column temperature: 55°C Flow rate: 0.5 mL / min Detector: RI (differential refractometer) Apparatus: HPLC Prominence + analysis program Lab Solutions Ver. 5.73 (Shimadzu Corporation) Standard sample: Polymethyl methacrylate (M-M-10 set) (Agilent Technologies, Inc.)

[0044] As long as the effects of the present disclosure can be obtained, the 6% viscosity of the cellulose acetate constituting the cellulose acetate flakes is not particularly limited. For example, the 6% viscosity of the cellulose acetate may be 10 mPa·s or more, 50 mPa·s or more, 70 mPa·s or more, or 90 mPa·s or more, or 300 mPa·s or less, 200 mPa·s or less, or 150 mPa·s or less. Furthermore, the 6% viscosity of the cellulose acetate constituting the cellulose acetate flakes is preferably 10 mPa·s or more and 300 mPa·s or less, more preferably 50 mPa·s or more and 200 mPa·s or less, and even more preferably 70 mPa·s or more and 150 mPa·s or less.

[0045] The 6% viscosity of cellulose acetate is measured using an Ostwald viscometer. Specifically, 3.00 g of a dry sample is dissolved in a solvent to prepare a solution with a cellulose acetate concentration of 6 wt / vol%. The time (s) for the solution to fall is measured at 25±1°C using an Ostwald viscometer, and the product of this time (s) and the viscometer coefficient is calculated as the 6% viscosity (mPa s). The viscometer coefficient is determined by a known method using a standard solution for viscometer calibration.

[0046] As described above, desirable physical properties for achieving a predetermined mean space velocity (SV) tend to facilitate the production of cellulose acetate flakes with a predetermined mean space velocity (SV) when the particle size is large, the particle size distribution is large (i.e., the particle size is non-uniform), the specific surface area is large, and the flakes have high hardness. Specifically, the liquid permeation rate tends to be high when the particle size is large, the particle size distribution is small (uniform), the pores are small or absent, and the total pore volume is small, the specific surface area is small, the substituents exposed on the particle surface are absent or few, or the hydrophobic portion (the cellulose skeleton) is exposed or has low polarity, or the hardness is high. On the other hand, the liquid permeation rate tends to be low when the particle size is small, the particle size distribution is large (non-uniform), the specific surface area is large, the pores are large or absent, or the total pore volume is large, the substituents exposed on the particle surface are numerous, or have high polarity, or the hardness is low.

[0047] However, multiple characteristics such as surface area, pore structure, surface polarity, particle size distribution, etc., vary in relation to one another and affect liquid permeability. For example, the specific surface area of ​​cellulose acetate flakes usually increases as the particle size decreases. However, even if the particle size is small, if the pores are not open or are small, the specific surface area may decrease as a result. Therefore, in the present disclosure, instead of determining the optimal numerical ranges for each individually, the cellulose acetate flakes are specified using the average space velocity SV, which reflects these characteristic values, as an index. That is, when the average space velocity SV obtained by a specific method is 1.0 h －１By making the cellulose acetate flakes exceed the above range, the fine shape and physical properties, such as particle size distribution, pore size and shape, surface hydrophilicity / hydrophobicity, and the amount of change during column packing (hardness, clogging tendency), which are characteristics that change simultaneously and in conjunction with each other, can be specified, and the cellulose acetate flakes of the present disclosure can be obtained.

[0048] [Method for Producing Cellulose Acetate Flakes] The method for producing cellulose acetate flakes according to the present disclosure includes a step of mixing a dope containing cellulose acetate having an acetyl substitution degree of 0.80 to 2.90 with a precipitant. Here, the dope refers to a reaction liquid obtained in the cellulose acetate synthesis process (hydrolysis process including the acetylation process) using acetic acid as a reaction solvent. In other words, the dope also refers to an acetic acid solution in which cellulose acetate is dissolved. Therefore, the dope according to the present disclosure may be obtained by dissolving existing cellulose acetate flakes in acetic acid. Furthermore, the cellulose acetate may contain substituents other than acetyl groups, as long as the effects of the present disclosure are achieved. Other substituents include acyl groups such as propionyl, butyryl, pentanoyl (valeryl), hexanoyl, heptanoyl, octanoyl, nonanoyl, undecanoyl, dodecanoyl, tridecanoyl, tetradecanoyl (myristoyl), pentadecanoyl, hexadecanoyl, heptadecanoyl, and octadecanoyl (stearoyl). When the dope is dried without mixing it with a precipitant, the entire dope becomes film-like, making it difficult to suitably obtain the cellulose acetate flakes of the present disclosure.

[0049] The cellulose acetate flakes of the present disclosure can be obtained by contacting the dope with a poor solvent (precipitant) for cellulose acetate to precipitate cellulose acetate. Alternatively, the cellulose acetate flakes of the present disclosure can be obtained by adjusting the precipitation conditions in a step of contacting the dope with a poor solvent to obtain a precipitate at the end of the cellulose acetate synthesis process. Contacting the dope with a poor solvent to obtain a precipitate at the end of the cellulose acetate synthesis process has been known as a method for producing cellulose acetate flakes. However, the precipitation process under commonly used conditions does not favorably produce the cellulose acetate flakes of the present disclosure, i.e., cellulose acetate flakes exhibiting a mean space velocity (SV). This is because the applications of conventionally produced cellulose acetate flakes, i.e., cellulose acetate, are cigarette filters, optical films, extrusion molded products, etc. Among these, the main applications of cigarette filters and optical films are produced by dissolving the cellulose acetate flakes in an organic solvent. Therefore, cellulose acetate flakes used for these applications are preferably uniform in order to maintain solubility in organic solvents. Meanwhile, in the present disclosure, cellulose acetate flakes exhibiting a predetermined mean space velocity (SV) are preferably those with a large particle size, a large particle size dispersion (i.e., non-uniform particle size), a large specific surface area, and high flake hardness. To obtain such flakes, the following precipitation conditions, which are different from conventional ones, are desirable. Specifically, water, for example, is used as a poor solvent (precipitation solution) for cellulose acetate. In the step of mixing a dope containing cellulose acetate with a precipitant, the concentration of cellulose acetate in the reaction solution (dope), the temperature of the dope, the temperature of the precipitant (poor solvent), the mixing or dropping rate of the dope and the precipitant, the stirring time, and the like are adjusted to obtain precipitated flakes with a large particle size dispersion and a large specific surface area. As a result, cellulose acetate having an acetyl substitution degree of 0.80 to 2.90 and precipitated for 1.0 h under predetermined conditions can be obtained. －１ It is possible to obtain cellulose acetate flakes exhibiting an average space velocity SV of more than

[0050] In the step of mixing the cellulose acetate-containing dope with the precipitant, the stirrer used may be a twin-screw kneader or a magnetic stirrer. From the viewpoint of facilitating the production of flakes having a predetermined average space velocity (SV), the temperatures of the dope and precipitant before mixing are preferably low, more preferably 40°C or lower, even more preferably 30°C or lower, and particularly preferably 25°C or lower. After mixing the dope and precipitant, the precipitation temperature (i.e., the temperature of the mixture of the dope and precipitant) is preferably maintained at 40°C or lower, more preferably 30°C or lower, and even more preferably 25°C or lower. The higher the temperatures of the dope and precipitant before mixing and the precipitation temperature, the lower the average space velocity (SV) of the resulting cellulose acetate flakes tends to be. For example, if either the temperature of the dope and precipitant before mixing or the precipitation temperature significantly exceeds 40°C, it is difficult to obtain the flakes of this embodiment.

[0051] The method for precipitating cellulose acetate in a dope is not particularly limited, as long as it can produce cellulose acetate flakes exhibiting the average space velocity (SV) obtained by the above-described method. For example, it is preferable to slow the settling rate as much as possible, from the viewpoint of facilitating the production of cellulose acetate flakes exhibiting the desired average space velocity (SV). One such method is to continuously add a cellulose acetate-containing dope dropwise to a precipitant being stirred. By slowing the settling rate, precipitates are formed not only on the droplet surfaces in contact with the precipitant, but also from the liquid interface between the precipitant and the dope to the interior of the dope droplets. This results in the formation of a continuous structure between the dope and the precipitant from the surface to the interior, resulting in flakes with a large specific surface area and high hardness.

[0052] When a dope containing cellulose acetate is continuously dropped into a stirring precipitant, the cellulose acetate concentration in the dope is preferably about 10% by mass. Furthermore, the stirring speed of the precipitant (poor solvent) is preferably as slow as possible. Stirring at a low speed favorably produces flakes with larger particle sizes. For example, a stirring speed of 1 to 110 rpm favorably produces the cellulose acetate flakes of this embodiment. The higher the stirring speed, the smaller the mean space velocity (SV) of the resulting cellulose acetate flakes tends to be. Vigorous stirring at speeds significantly exceeding 110 rpm makes it difficult to favorably produce the flakes of this embodiment. The temperature of the dope and precipitant may be 40°C or lower, preferably 25°C or lower. When high-temperature aging is performed at a very high dope temperature and accompanied by vigorous stirring, the resulting flakes have small particle sizes and are soft (have low hardness), making it difficult to obtain a suitable average space velocity (SV) for the cellulose acetate flakes of this embodiment. Larger droplets are preferable, and continuous dropping is also preferred. Acetic acid may be added to the dope before dropping it into the precipitant to reduce the cellulose acetate concentration in the dope. Dilute acetic acid may be used as the precipitant instead of water. The standing time after stopping stirring is preferably 0 minutes or more than 0 minutes to about 1 hour. When the standing time is about 0 to 1 hour, the cellulose acetate flakes of this embodiment can be preferably obtained. The longer the standing time, the higher the average space velocity (SV) of the resulting cellulose acetate flakes tends to be. Standing times significantly exceeding 1 hour make it difficult to obtain the flakes of this embodiment.

[0053] To obtain cellulose acetate flakes exhibiting a desired average space velocity (SV), it is necessary to minimize the rate at which cellulose acetate in the dope precipitates beyond its solubility limit and maximize the spatial spread of the dope droplets dropped into the precipitant (poor solvent). Cellulose acetate flakes obtained under these conditions have a large specific surface area and total pore volume, and a small load displacement (described later), despite their large particle size. Furthermore, by forming them using this process, the surface condition of the cellulose acetate flakes can be made more hydrophilic. As a result, a higher average space velocity (SV) can be obtained. Cellulose acetate flakes with these characteristics have a desired average space velocity (SV) and can selectively and efficiently recover gold ions. Industrially, the average space velocity (SV) of the resulting cellulose acetate flakes can be adjusted by appropriately changing the shape, capacity, etc. of the precipitation tank.

[0054] The type of precipitant is not particularly limited as long as the effects of the present disclosure are obtained. It can be appropriately selected depending on the desired degree of acetyl substitution. For example, the precipitant may be a poor solvent for cellulose acetate having the desired degree of acetyl substitution, such as water, dilute acetic acid, or a mixture thereof. A preferred precipitant is dilute acetic acid. The concentration of dilute acetic acid may be 5% by mass or more, or 10% by mass or more, preferably 15 to 35% by mass. The higher the concentration of dilute acetic acid, the higher the average space velocity (SV) of the resulting cellulose acetate. If the concentration of dilute acetic acid significantly exceeds 35% by mass, it becomes difficult to obtain the flakes of this embodiment. The precipitant may contain an organic solvent as long as the effects of the present disclosure are not impaired. However, if the precipitant contains an organic solvent, it is difficult to obtain the desired average space velocity (SV). Furthermore, low-substituted cellulose acetate tends to precipitate in a powder form unless the preferred precipitation method and precipitation conditions are used. Therefore, in order to obtain low-substituted cellulose acetate flakes, the precipitation method and precipitation conditions must be selected appropriately. To obtain the cellulose acetate flakes of the present disclosure, it is important to appropriately adjust the combination of (1) the temperature of the dope and precipitant before mixing, and the precipitation temperature, (2) the stirring speed, (3) the standing time after stopping the stirring, and (4) the concentration of the precipitant (especially dilute acetic acid, etc.).

[0055] As mentioned above, in order to obtain a desired average space velocity (SV), a method and conditions that result in a low sedimentation velocity are preferred. Specifically, a method and conditions that slowly mix the dope and precipitant are preferred so that a gold ion adsorption surface is formed on the outer surface of the flakes and so that irregularities are formed on the surface of the flakes. Furthermore, a slow stirring speed for the dope and precipitant is preferred. The slower the stirring speed for the dope and precipitant, the wider the particle size distribution of the resulting flakes. High-speed stirring tends to result in flakes with a uniform particle size, which tends to decrease the average space velocity (SV).

[0056] A dope containing cellulose acetate having a degree of acetyl substitution of 0.80 or more and 2.90 or less can be obtained, for example, by a step (a) of reacting cellulose with acetic anhydride in the presence of an acid catalyst and an acetic acid solvent to produce cellulose acetate, and a step (b) of hydrolyzing the produced cellulose acetate to adjust the degree of acetyl substitution to 0.80 or more and 2.90 or less.

[0057] In the step (a) of producing cellulose acetate, cellulose is acetylated by reacting it with acetic anhydride in the presence of an acid catalyst and an acetic acid solvent. The cellulose is preferably activated. Examples of the activation treatment include a method in which acetic acid or acetic acid containing 1 to 10% by mass of sulfuric acid (sulfur-containing acetic acid) is added to the raw cellulose in one or two stages.

[0058] As the raw cellulose, wood pulp (softwood pulp, hardwood pulp, etc.), cotton linter, etc. can be used. These raw celluloses can be used alone or in combination of two or more. For example, softwood pulp can be used in combination with cotton linter or hardwood pulp. When wood pulp is used, crushed pulp crushed into a cotton-like state using a disc refiner or the like can be used.

[0059] The α-cellulose content of the starting cellulose may be, for example, 90% by mass or more, 92% by mass or more, 95% by mass or more, or 97% by mass or more.

[0060] When acetic acid or acetic acid containing 1 to 10% by mass of sulfuric acid (sulfur-containing acetic acid) is added to raw cellulose for activation, acetic acid and / or sulfur-containing acetic acid can be added in an amount of 10 to 500 parts by mass per 100 parts by mass of raw cellulose. Acetic acid or sulfur-containing acetic acid may be added to the raw cellulose in a single step, or acetic acid may be added followed by sulfur-containing acetic acid after a certain time has elapsed, or acetic acid may be added after sulfur-containing acetic acid has been added. Furthermore, the activation treatment may involve adding acetic acid and / or sulfur-containing acetic acid to the raw cellulose, followed by allowing the mixture to stand at 17 to 40°C for 0.2 to 48 hours, or by sealing and stirring the mixture at 17 to 40°C for 0.1 to 24 hours.

[0061] An example of an acid catalyst used in the step of acetylating cellulose (preferably cellulose after activation treatment) to produce cellulose acetate is sulfuric acid. For example, acetylation may be performed by adding cellulose to a mixture of acetic acid, acetic anhydride, and sulfuric acid, or by adding a mixture of acetic acid and acetic anhydride and sulfuric acid to cellulose.

[0062] The ratio of acetic acid to acetic anhydride in acetylation may be 200 to 400 parts by mass of acetic anhydride relative to 300 to 600 parts by mass of acetic acid, or 240 to 280 parts by mass of acetic anhydride relative to 350 to 530 parts by mass of acetic acid. Furthermore, the ratio of cellulose to a mixture of acetic acid and acetic anhydride in acetylation may be 500 to 1,000 parts by mass of the mixture of acetic acid and acetic anhydride relative to 100 parts by mass of cellulose. The ratio of cellulose to sulfuric acid in acetylation may be 0.5 to 15 parts by mass, 5 to 14 parts by mass, or 7 to 11 parts by mass of concentrated sulfuric acid relative to 100 parts by mass of cellulose. The acetylation reaction of cellulose may be carried out by stirring at 20 to 55°C for 30 minutes to 36 hours from the start of the reaction.

[0063] In step (b), in which the produced cellulose acetate is hydrolyzed to adjust the degree of acetyl substitution to 0.80 or more and 2.90 or less, for example, a neutralizing agent containing water (including water vapor); dilute acetic acid; or a carbonate, acetate, hydroxide, or oxide of calcium, magnesium, iron, aluminum, zinc, or the like is added to terminate the acetylation reaction during the hydrolysis (saponification). Note that dilute acetic acid refers to a 1 to 50 mass % aqueous acetic acid solution. Magnesium acetate and calcium acetate are preferred neutralizing agents.

[0064] The time for the hydrolysis (saponification) reaction (hereinafter referred to as the aging time) is not particularly limited and can be adjusted appropriately depending on the desired degree of acetyl substitution. Here, the aging time refers to the time from the start of adding the neutralizing agent to the end of the hydrolysis (saponification) reaction. By increasing the aging time, the degree of acetyl substitution can be reduced.

[0065] The temperature of the hydrolysis (saponification) reaction (hereinafter referred to as the aging temperature) is preferably 50 to 100°C, more preferably 70 to 90°C, and is preferably maintained at that temperature for 20 to 120 minutes. Here, the aging temperature refers to the temperature in the reaction system during the aging time.

[0066] By adjusting the conditions of the hydrolysis reaction, a dope containing cellulose acetate having an acetyl substitution degree of 0.80 to 2.90 can be obtained. By mixing this dope with a precipitant under the above-mentioned conditions, cellulose acetate flakes having the properties of the present disclosure can be obtained. Here, the dope containing cellulose acetate refers to a solution in which the cellulose acetate or a mixture containing cellulose acetate is dissolved in a solvent. This mixture is a reaction mixture obtained by an acetylation reaction or a hydrolysis reaction.

[0067] A dope containing cellulose acetate having an acetyl substitution degree of 0.80 to 2.90 may be mixed with a precipitating agent, which is the aforementioned precipitation process for cellulose acetate flakes, and the precipitated cellulose acetate flakes may then be separated and dried. Methods for separating the cellulose acetate flakes include dehydration by filtration or centrifugation. Known methods, such as air drying at a predetermined temperature or drying under reduced pressure, can be used to dry the separated cellulose acetate flakes. The combined use of electromagnetic wave drying may result in a decrease in the aforementioned mean space velocity (SV).

[0068] [Gold Adsorption Column] The gold adsorption column of the present disclosure is packed with cellulose acetate flakes having an acetyl substitution degree of 0.80 to 2.90 as a packing material. By passing a gold-containing solution through this column, gold ions in the solution are selectively adsorbed onto the cellulose acetate flakes. The cellulose acetate flakes with adsorbed gold ions can be collected and incinerated or dissolved in a good solvent to separate and recover the gold ions. Furthermore, the cellulose acetate flakes with adsorbed gold ions can be collected and mixed with a desorption liquid (e.g., water) to desorb the gold ions. According to the definition of "column" in JIS K0214 (2013) "Terminology in Analytical Chemistry (Chromatography)," the term "column" refers to "a tube or capillary tube packed with a packing material in which sample components are primarily separated." A "gold adsorption column" refers to a cylindrical or cylindrical container having an inner diameter and length, filled with a packing material capable of adsorbing gold. For example, the gold adsorption column of the present disclosure may have a columnar or cylindrical container filled with a packing material, a liquid inlet port at one end, and a liquid outlet port at the other end, and these liquid inlet port and liquid outlet port may be sealed to prevent air from flowing in from outside the column. In this type of gold adsorption column, the gold-containing solution or the eluent can be passed through under pressure, which facilitates adjustment of the space velocity SV' described below.

[0069] From the viewpoint of improving gold recovery efficiency, the packing rate of the cellulose acetate flakes in the gold adsorption column may be 10% or more, 15% or more, or 18% or more. The packing rate may be 40% or less. The packing rate is the ratio of the volume of the space occupied by the packing material to the total capacity of the column.

[0070] From the viewpoint of improving the gold recovery efficiency, the average space velocity (SV) when passing a 2.0 mol / L hydrochloric acid solution (temperature 20°C ± 5°C) through this gold adsorption column was set to 1.0 h －１ It is enough if it exceeds 2.0h －１ It may be 10.0 h or more. －１ It may be 50h or more. －１ It may be 80h or more. －１In order to prevent an increase in pressure loss during column flow, the average space velocity (SV) may be 120 h －１ may be less than or equal to 110h －１ It may be the following:

[0071] The material and size of the column packed with cellulose acetate flakes are not particularly limited as long as the effects of the present disclosure can be obtained. For example, the column material can be appropriately selected from resin, glass, metal, etc. Since the gold-containing solution passed through the gold recovery column may contain a strong acid such as aqua regia, acid-resistant resin or glass is preferred as the column material.

[0072] The inner diameter of the column may be 2 mm or more, 5 mm or more, 100 mm or more, or 1,000 mm or more, taking into consideration the packing property of cellulose acetate flakes. The length of the column may be 2 mm or more, 20 mm or more, 200 mm or more, or 20,000 mm or more, taking into consideration the processing capacity and suppression of pressure loss during liquid passage.

[0073] [Gold Recovery Method] The gold recovery method of the present disclosure comprises passing a liquid containing gold ions through the above-mentioned gold adsorption column to adsorb the gold ions in the liquid onto cellulose acetate flakes. More specifically, the gold recovery method of the present disclosure may comprise filling a cylindrical container with cellulose acetate flakes having an acetyl substitution degree of 0.8 to 2.9 to prepare a packed column, and passing a liquid containing gold ions through the packed column to adsorb the gold ions onto the cellulose acetate flakes.

[0074] For example, in the case of a packed column installed so that the column axis is vertical, the liquid flow direction may be from above or below. For example, Japanese Patent Application Laid-Open No. 2006-192400 discloses a column regeneration method in which both the raw liquid (liquid to be treated) and the column regeneration chemical solution flow downward. However, this method requires that all gold ions adsorbed to the cellulose acetate flakes at the top of the column be swept downward, which may result in reduced efficiency. Therefore, the liquid flow direction may be changed during gold ion adsorption and desorption. As long as the effects of the present disclosure are obtained, the installation direction of the packed column is not limited to the vertical direction, and the packed column may also be installed horizontally.

[0075] From the viewpoint of improving the adsorption efficiency of gold ions by the cellulose acetate flakes in the packed column, the space velocity SV' when passing the liquid containing gold ions through the packed column is 1.0 h －１ It is enough if it exceeds 2.0h －１ It may be 10.0 h or more. －１ It may be 50h or more. －１ It may be 80h or more. －１ From the viewpoint of improving the adsorption amount and suppressing the pressure loss during liquid passage, the space velocity SV' is 120 h －１ may be less than or equal to 110h －１ may be less than or equal to 100h －１ In other words, in the gold recovery method of the present disclosure, the liquid containing gold ions is －１ It is preferable to pass the liquid through the packed column at a flow rate that provides a space velocity SV' exceeding 1.0 h. For example, a tube pump connected to the packed column can be used to send the liquid containing gold ions through the packed column. The space velocity SV' can be adjusted by adjusting the liquid sending speed of the tube pump. From the viewpoint of improving adsorption efficiency, the liquid may be passed through at a space velocity SV' that exceeds the average space velocity SV of the packed cellulose acetate flakes, but in this case, the pressure inside the column may become excessive. From the viewpoint of suppressing pressure loss, the space velocity SV' during the liquid passage is set to 1.0 h. －１ Over 120 hours－１ The following ranges are preferred:

[0076] Methods for separating and recovering gold ions adsorbed on cellulose acetate flakes as a solid include dissolving the cellulose acetate flakes with adsorbed gold ions in a good solvent, recovering the insoluble portion containing gold, and drying the recovered portion. Alternatively, the cellulose acetate flakes with adsorbed gold ions may be incinerated, and the incineration residue containing gold may be recovered. That is, this gold recovery method may further include dissolving or incinerating the cellulose acetate flakes with adsorbed gold ions. If necessary, the insoluble portion or the incineration residue may be purified by known purification means to recover high-purity gold.

[0077] If necessary, high-purity gold may be recovered by a desorption means. For example, gold ions can be desorbed from cellulose acetate flakes by introducing a desorption liquid such as water into the packed column after adsorption of gold ions and passing the liquid through. Alternatively, gold can be recovered as a solid by adding a reducing agent or the like to the desorption liquid containing the desorbed gold ions to cause precipitation. Furthermore, by using a small amount of desorption liquid, a gold-containing solution with a higher concentration than the original gold ion-containing liquid can be obtained. Therefore, gold can also be recovered using a process suitable for high-concentration solutions, such as electrolytic reduction.

[0078] In other words, the present disclosure relates to a method for concentrating a gold solution. This method involves passing a liquid containing gold ions (stock solution) through the aforementioned gold adsorption column to adsorb the gold ions in the liquid onto cellulose acetate flakes, and then passing a elution liquid through the column to desorb the gold ions adsorbed on the cellulose acetate flakes, thereby obtaining a liquid containing gold ions at a higher concentration than in the stock solution. Specifically, the method includes filling a cylindrical container with cellulose acetate flakes having an acetyl substitution degree of 0.8 to 2.9 to prepare a packed column, passing the gold ion-containing liquid as the stock solution through the packed column to adsorb the gold ions onto the cellulose acetate flakes, and passing the elution liquid through the packed column containing the gold-ion-adsorbed cellulose acetate flakes to obtain a liquid containing gold ions at a higher concentration than in the stock solution.

[0079] An example of a gold-containing liquid is a gold-containing solution obtained by collecting gold-containing materials from discarded electronic devices and dissolving them in aqua regia. That is, the gold recovery method of the present disclosure may further include preparing a liquid containing gold ions by dissolving or extracting gold from the gold-containing material in aqua regia. The gold-containing materials described above generally have a low gold content. The gold-containing solution obtained by dissolving the gold-containing material in aqua regia also has a low gold ion concentration. Known methods such as adding a reducing agent or electrolytic reduction cannot be applied to such dilute solutions because they are industrially cost-effective. In contrast, the gold recovery method and gold solution concentration method of the present disclosure can be suitably applied to liquids containing gold ions at a concentration of 1000 ppm or less, preferably 800 ppm or less, and more preferably 500 ppm or less. Furthermore, by applying the gold recovery method and gold solution concentration method of the present disclosure, gold can be efficiently adsorbed and recovered even at gold ion concentrations as low as about 5 ppm. Furthermore, gold can be preferentially adsorbed and recovered even from a liquid in which other metals than gold are present at concentrations 200 times or more higher. For example, with the cellulose acetate flakes of the present disclosure, gold (Au) can be selectively recovered even from a solution containing other metal elements, such as Li, Be, B, Na, Mg, Al, Si, P, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Rb, Sr, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Te, Cs, Ba, Hf, Ta, W, Re, Os, Ir, Pt, Ti, Pb, Bi, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Th.Furthermore, the gold recovery method and gold solution concentration method of the present disclosure can also be applied to liquids containing as many as 67 types of metal elements (Li, Be, B, Na, Mg, Al, Si, P, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Rb, Sr, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Te, Cs, Ba, Hf, Ta, W, Re, Os, Ir, Pt, Au, Ti, Pb, Bi, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Th). According to the present disclosure, gold can be efficiently recovered from discarded electronic components, decorative processing waste, waste catalysts, etc.

[0080] The present disclosure will be specifically described below using examples, but the technical scope of the present disclosure is not limited to these examples. Unless otherwise specified, the test temperature was room temperature.

[0081] The acetylation degree AV, the acetyl substitution degree DS, the weight average molecular weight Mw, and the 6% viscosity described in the examples and comparative examples described later were measured by the methods described above. Other physical properties were evaluated by the following methods.

[0082] [Morphological Observation] The external appearance of the examples and comparative examples was confirmed by observation with a scanning electron microscope (Hitachi Ion Sputter Model E-1030 manufactured by Hitachi).

[0083] [Particle size: 20 mesh on] A sieve separation test was conducted in accordance with the provisions of JIS Z8801-1:2006. That is, a sieve with a nominal mesh size of 850 μm, a maximum mesh size tolerance of 127 μm, an average mesh size tolerance of 29 μm, a maximum wire diameter of 580 μm, and a minimum wire diameter of 430 μm was used to determine the percentage of particles with a particle size of 850 μm or more. Specifically, JIS test sieves and trays with mesh sizes of 4,000 μm (5 mesh), 1,700 μm (10 mesh), 1,000 μm (16 mesh), 850 μm (20 mesh), and 500 μm (32 mesh) were prepared. First, a sieve with a mesh size of 4,000 μm was attached to a rotary tap machine (manufactured by Iida Seisakusho Co., Ltd., tapping: 156 times / min, rolling: 290 times / min), and 100 g of sample was placed on the sieve. After vibrating for 5 minutes, the sample on the tray was collected. The sample was then placed on a sieve with a mesh size of 1,700 μm and sieved under the same conditions. Similarly, the sample was sieved sequentially through sieves with mesh sizes of 1,000 μm, 850 μm, and 500 μm, and the mass of the sample on each sieve with a mesh size of 4,000 μm, 1,700 μm, 1,000 μm, 850 μm, and 500 μm was measured. A particle size distribution was created using the ratio of the mass of the sample on each sieve to the total mass (100 g) used in the sieving test and the mesh size of each sieve. In this particle size distribution, the total mass of the samples on the sieves with openings of 4,000 μm, 1,700 μm, 1,000 μm, and 850 μm was determined, and the ratio (%) to the total mass (100 g) was calculated to represent the ratio of particles with a particle size of 850 μm or more (20 mesh on). Similarly, a sieving test was conducted using a JIS test sieve with an opening of 106 μm (150 mesh), and the mass of the sample that passed through this sieve was measured, and the ratio (%) to the total mass (100 g) was calculated to represent the ratio of particles with a particle size of less than 106 μm (150 mesh pass).

[0084] [BET specific surface area and total pore volume] The BET specific surface area (m ２ / g) and total pore volume (cm ３The measurement was performed using a Belsorp Max (manufactured by BEL JAPAN INC.) measuring device. The samples were pretreated at 100°C for 60 minutes before measurement. The flake and powder samples were measured using 0.04 g of each sample, and the fibrous (cotton-like) sample was measured using 0.2 g of each sample.

[0085] [Load Displacement Amount] A 10 ml cylindrical syringe (made of polypropylene, inner diameter 17 mm) was placed so that its axial direction was vertical, and the sample was filled to a height of 50 mm from the bottom of the syringe to form a packed bed. This packed bed was compressed at a compression rate of 10 mm / min using a universal tensile tester (manufactured by A&D Co., Ltd., trade name "RTG1310"), and the height H1 (mm) of the packed bed at a load of 10 N was measured. The difference H2 - H1 from the height H2 (mm) of the packed bed before compression was calculated. The average of five measurements was taken as the load displacement amount (mm).

[0086] [Column Characteristics: Measurement of Mean Space Velocity (SV) by Flow Test] A glass column with a stopcock (manufactured by Asahi Seisakusho, inner diameter 10 mm) was installed with the stopcock fully open, with the column axis oriented vertically. Approximately 0.5 g of packing material (e.g., cellulose acetate flakes) was allowed to fall naturally until the column reached a height of 2.0 cm from the bottom of the column, creating a packed column. Specifically, the packing material was added to the column in approximately three batches, tapping the side of the column by hand after each addition, until the column reached a height of 2.0 cm from the bottom. A 2.0 mol / L hydrochloric acid solution was added to the top of the column using a tube pump ("Tubing Pump 1973" manufactured by AS ONE Corporation) at a flow rate of 3.1 g / min.

[0087] After the tube pump started, the effluent flowing out from the bottom of the column at 0-2 minutes, 2-4 minutes, and 4-6 minutes after the 2.0 mol / L hydrochloric acid solution reached the top of the column was collected and weighed. At a liquid feed rate of 3.1 g / min, no pressure was applied during the liquid flow, and the hydrochloric acid solution flowed down the column under atmospheric pressure. The flow rate per minute (g / min) was calculated from the average of the three collected amounts. Similarly, the liquid flow rate per hour (m ３ / h) was calculated to obtain the volume of the space occupied by the packing material packed in the column (m ３ ) to obtain the average space velocity SV (h －１ ) was calculated. Here, the volume V (m ３ ) is the column cross-sectional area (m ２ ) and the height (m) of the packing material. Specifically, V = (0.005 × 0.005 × π) × 0.02 = 1.57 × 10 ―６ (m ３ ) In addition, "1.57E-6" in Tables 1 to 4 below is the above 1.57 x 10 －６ means.

[0088] [Adsorption Test and Desorption Test] First, test solutions A to E were prepared by dissolving salts of each metal in a hydrochloric acid solution or a hydrochloric acid-nitric acid mixed solution having the concentration (mol / l) shown in Table 5 below to give the concentration (ppm) shown in Table 5 below. The metals mentioned above are gold (Au), platinum (Pt), iron (Fe), zinc (Zn), aluminum (Al), chromium (Cr), copper (Cu), titanium (Ti), lead (Pb), palladium (Pd), nickel (Ni), tin (Sn), and iridium (Ir).

[0089] Next, a packed column was prepared in the same manner as in the above-mentioned liquid flow test. The packed column was connected to the above-mentioned tube pump, and the space velocity SV' was 1.0 (h －１ The pump operating conditions were adjusted so that the space velocity SV' was 1.0 (h －１ The column was pressurized to maintain the pressure at 1000 kJ / min, and the test solution was passed through the column under pressure. The metal ion concentrations (ppm) in the collected effluent were then measured using an ICP atomic emission spectrometer (Agilent 5110, manufactured by Agilent Technologies, Inc.).

[0090] The metal ion concentration (ppm) in the effluent was designated C1, and the metal ion concentration (ppm) in the test solution before passing through the column was designated C2, and the adsorption rate A (%) of each metal was calculated using the following formula: A (%) = (C2 - C1) / C2 × 100. Furthermore, the amount of gold (Au) adsorbed (mg) onto the packed column was calculated from the concentration C2 (ppm) in the test solution before passing through the column and the adsorption rate A (%), with the volume of the collected effluent designated N1 (ml).

[0091] Thereafter, a desorption test was carried out by feeding distilled water as the desorption liquid instead of the test liquid. －１ The pump operating conditions were adjusted so that the eluate flowing out from the packed column was equal to or less than 100 ppm. The eluate was collected and the metal ion concentration (ppm) in the eluate was measured by the method described above.

[0092] The amount of the collected eluate was defined as N2 (ml), the metal ion concentration in this eluate was defined as C3 (ppm), and the amount of effluent collected in the adsorption test, N1 (ml), and the metal ion concentration in this effluent, C1 (ppm), were used to calculate the desorption rate B (%) of each metal according to the following formula: B (%) = (C3 × N2 × 0.001) / (C1 × N1 × 0.001) × 100

[0093] The results using test liquid A are shown in Tables 6-8 below. The results using test liquid B are shown in Tables 9-11 below. The results using test liquid C are shown in Tables 12-14 below. The results using test liquid D, in which the effluent flowing out of the packed column at 0-1 hour, 1-2 hours, and 2-3 hours after passing the liquid, are shown in Table 15 below. The results using test liquid E, in which the space velocity SV' during passing the liquid was 5.0 h －１ , 10.0h －１ , 20.0h －１ The results of collecting the effluent from the packed column 0 to 0.5 hours after the liquid was passed through are shown in Table 16 below.

[0094] Comparative Example 1 Comparative Example 1 used was the cellulose acetate (trade name "L-50" manufactured by Daicel Corporation) described in Example 1 of JP 2014-109064 A (Patent Document 2). This cellulose acetate was in powder form and had a degree of substitution of 2.5, a weight-average molecular weight (Mw) of 200,000, and a molecular weight distribution (Mw / Mn) of 5.0. When the cellulose acetate of Comparative Example 1 was subjected to a sieving test using a 150-mesh JIS sieve, all of the powder passed through the 150-mesh JIS sieve. Furthermore, the above-mentioned physical properties were measured, and a liquid flow test and an adsorption test were conducted. The results are shown in the table below. In the table, a load displacement of "<1" indicates a value of less than 1 mm. In the liquid flow test and adsorption test of Comparative Example 1, the test liquid overflowed from the column immediately after the tube pump started operating, making it impossible to pass the liquid through the column.

[0095] Example 1 Cellulose acetate of Comparative Example 1 (trade name "L-50" manufactured by Daicel Corporation, degree of substitution 2.5, weight average molecular weight (Mw): 200,000, molecular weight distribution (Mw / Mn): 5.0) was dissolved in acetic acid to prepare 40 ml of dope with a concentration of 20 wt %. Next, 120 ml of dilute acetic acid with a concentration of 20 wt % was prepared as a precipitation solution. The precipitation solution was stirred at 10 rpm using a low-speed stirrer and adjusted to a temperature of 20°C. 40 ml of the dope was slowly added and stirred for 10 minutes. The stirring was then stopped and the cellulose acetate flakes were left to stand for 40 minutes to harden the interior of the cellulose acetate flakes. The obtained flakes were washed and dried to obtain cellulose acetate flakes of Example 1. The obtained cellulose acetate was measured for the above-mentioned physical properties and subjected to a liquid passage test and an adsorption test. The results are shown in the table below. It was confirmed by microscopic observation that Example 1 was in the form of flakes with a width of 0.1-1 mm and a thickness of 0.05-0.1 mm.

[0096] [Example 2-4] Cellulose acetate with a substitution degree of 2.5, 6% and a viscosity of 70 (mPa s) was dissolved in acetic acid to prepare 40 ml of a 10 wt % dope. Next, a 10 wt % dilute acetic acid solution was prepared as a precipitation solution. The solution was stirred at 100 rpm using a magnetic stirrer and adjusted to 25°C. The entire dope was added dropwise from a 200 ml measuring pipette into the precipitation solution.

[0097] In Examples 2 and 4, stirring was stopped and the mixture was left for 30 minutes to harden the cellulose acetate flakes to the inside. The resulting flakes were washed and then dried to obtain the cellulose acetate flakes of Examples 2 and 4. Microscopic observation confirmed that the cellulose acetate flakes of Example 2 had a flake shape with a width of 0.1-0.17 mm and a thickness of 0.05-0.1 mm.

[0098] In Example 3, stirring was stopped and the mixture was left standing for 40 minutes to harden the cellulose acetate flakes to their interiors. The resulting flakes were washed and dried, and then sieved using a JIS sieve. Some of the flakes with a particle size of 20 mesh on were removed and mixed with flakes with a particle size of 20 mesh pass so that the ratio of the 20 mesh on particle size was 10 wt %, thereby obtaining the cellulose acetate flakes of Example 3. Note that the amount of cellulose acetate flakes passing through a 150 mesh was less than 1% in all of the cellulose acetate flakes of Examples 2 to 4. The above-mentioned physical properties were measured, and the results of a liquid passage test and an adsorption test are shown in the table below.

[0099] Comparative Example 2: The cellulose acetate fiber bundle of Example 12 of JP 2014-109064 A (Patent Document 2) was used as Comparative Example 2. Specifically, a dope containing dissolved cellulose acetate (trade name "L-50" manufactured by Daicel Corporation) was spun by a dry spinning method to obtain a long-fiber bundle of cellulose acetate with a single fineness of 0.9 denier and a total fineness of 20,000. Note that, since Comparative Example 2 is a long-fiber bundle, particle size measurement by sieving was not possible. The above-mentioned physical properties were measured, and a liquid flow test and an adsorption test were conducted. The results are shown in the table below. In Comparative Example 2, the liquid was passed through the tube pump for several minutes after starting operation, but continued operation thereafter caused the test liquid to overflow from the column, making it impossible to continue the test.

[0100] [Examples 5-7] The cellulose acetate of Comparative Example 1 (trade name "L-50" manufactured by Daicel Corporation) was dissolved in acetic acid to prepare 40 ml of a 20 wt % dope. Next, a 30 wt % dilute acetic acid was prepared as a precipitation solution. The solution was stirred at 100 rpm using a magnetic stirrer and adjusted to 25°C. The entire dope was added dropwise from a 200 ml measuring pipette into the resulting precipitation solution. After stopping the stirring, the solution was left for 40 minutes to allow the cellulose acetate flakes to harden to their interiors. The resulting flakes were washed and dried, and then sieved using a JIS sieve. Some of the flakes with a particle size of 20 mesh on were removed and mixed with flakes with a particle size of 20 mesh pass so that the ratio of the 20 mesh on flakes was as shown in Table 2 below, thereby obtaining the cellulose acetate flakes of Examples 5-7. The ratio of the 150 mesh pass flakes was less than 1% in all of these samples. The above-mentioned properties were measured, and a liquid flow test and an adsorption test were carried out. The results are shown in the table below.

[0101] Example 8 Cellulose acetate flakes were precipitated in the same manner as in Example 5, except that the standing time after stopping the stirring was changed to 20 minutes. The obtained flakes were washed and dried, and then pulverized using a Makino-type pulverizer (manufactured by Makino Sangyo Co., Ltd., model number: DD-2-3.7) to obtain the cellulose acetate flakes of Example 8. The pulverization conditions were a rotation speed of 2450 rpm and a screen diameter of φ5.0 mm. According to the JIS sieving test, the proportion of flakes with a particle size of 20 mesh or larger was 19 wt %, and the proportion of flakes with a particle size of 150 mesh or larger was 11%. The above-mentioned physical properties were measured, and a liquid passage test and an adsorption test were also performed. The results are shown in the table below.

[0102] Example 9 and Comparative Example 7 To 1 part by mass of cellulose acetate (manufactured by Daicel Corporation, trade name "L-50"), 5.1 parts by mass of acetic acid and 2.0 parts by mass of water were added, and the mixture was stirred for 3 hours to dissolve the cellulose acetate. After adding 0.13 parts by mass of sulfuric acid to this solution, hydrolysis was carried out by maintaining the solution temperature at 70°C in Example 9 and 95°C in Comparative Example 7. To prevent precipitation of cellulose acetate during hydrolysis, water was added to the system in two portions. That is, after 1 hour, 0.67 parts by mass of water was added to the system over 5 minutes. After an additional 3 hours, 1.33 parts by mass of water was added to the system over 10 minutes, and the reaction was continued for another 7 hours at 70°C in Example 9 and for 2.5 hours at 95°C in Comparative Example 7.

[0103] The reaction solution after hydrolysis was cooled to room temperature (approximately 25°C), and 15 parts by weight of a precipitation solvent (methanol) was added to the reaction mixture to form a precipitate. The precipitate was recovered as a wet cake with a solids content of 15% by weight, and washed by adding 8 parts by weight of methanol and draining until the solids content reached 15% by weight. This process was repeated three times. The washed precipitate was neutralized by washing twice more with 8 parts by weight of methanol containing 0.004% by weight of potassium acetate, and then dried to obtain low-substituted cellulose acetate. To obtain flakes from the resulting low-substituted cellulose, pyridine was added to a concentration of 20 wt% to prepare a concentrated solution. 100 mL of methanol was placed in an Erlenmeyer flask. The concentrated pyridine solution and methanol were cooled to a temperature sufficient to prevent condensation, and 5 mL of the cooled concentrated pyridine solution was gently added to the cooled methanol. After addition, the mixture was stirred with a chemical stirrer to precipitate flakes. The obtained flakes were washed with water and dried to obtain cellulose acetate flakes of Example 9 and Comparative Example 7. The above-mentioned physical properties were measured and a liquid flow test and an adsorption test were carried out, the results of which are shown in the table below. In the liquid flow test and adsorption test for Comparative Example 7 and Example 9, the flakes dissolved after the tube pump started operating, making it impossible to pass liquid through them.

[0104] Example 10 Cellulose acetate (degree of substitution 2.50, trade name "L-50" manufactured by Daicel Corporation) was dissolved in acetic acid to prepare 40 ml of a 20 wt % dope. Next, 120 ml of 20 wt % dilute acetic acid was prepared as a precipitation solution. The precipitation solution was stirred at 100 rpm using a stirrer and adjusted to a temperature of 20°C. 40 ml of the dope was slowly added and stirred for 10 minutes. The stirring was then stopped, and the precipitated cellulose acetate flakes were immediately washed with water, filtered, and dried to obtain cellulose acetate flakes of Example 10. The above-mentioned physical properties were measured, and a liquid flow test and an adsorption test were conducted. The results are shown in the table below.

[0105] Comparative Example 3 Cellulose acetate flakes were prepared according to Comparative Example 1 of JP 2017-52961 A. Specifically, hardwood prehydrolyzed kraft pulp with an α-cellulose content of 98.4 wt% was disintegrated into a flocculent state using a disc refiner. 26.8 parts by mass of acetic acid was sprayed onto 100 parts by mass of the disintegrated pulp (moisture content: 8%), and the mixture was thoroughly stirred and then allowed to stand for 60 hours as a pretreatment for activation (activation step).

[0106] The activated pulp was added to a mixture consisting of 323 parts by weight of acetic acid, 245 parts by weight of acetic anhydride, and 13.1 parts by weight of sulfuric acid. The mixture was pre-cooled to 5°C. The temperature was adjusted from 5°C to a maximum of 40°C over 40 minutes, and acetylation was carried out for 90 minutes from the time the pulp was added to the mixture. A neutralizer (24% aqueous magnesium acetate solution) was then added over 3 minutes to adjust the amount of sulfuric acid (amount of aging sulfuric acid) to 2.5 parts by weight. The reaction bath was then heated to 75°C, and water was added to adjust the water content of the reaction bath (amount of aging water) to 52 mol%. Aging was carried out at 85°C for 100 minutes, and the aging was stopped by neutralizing the sulfuric acid with magnesium acetate to obtain a reaction mixture containing cellulose acetate, i.e., a dope.

[0107] 1,200 parts by mass of dilute acetic acid (10 wt%) at 20°C and 400 parts by mass of a reaction mixture containing cellulose acetate (dope temperature: 85°C) were added to a commercial mixer (Panasonic, model number: MX-152SP-W) and stirred with the mixer for 4 seconds to cause precipitation. The temperature of the precipitation liquid at this time was approximately 37°C. The precipitated cellulose acetate was immediately washed with water, filtered, and dried to obtain cellulose acetate flakes of Comparative Example 3. The above-mentioned physical properties were measured, and a liquid flow test and an adsorption test were also conducted. The results are shown in the table below.

[0108] Comparative Examples 4 and 6 Cellulose acetate flakes were precipitated in the same manner as in Example 5, except that the temperature of the precipitation solution (precipitation liquid) was adjusted to 30°C and the standing time after stirring was changed to 20 minutes. The obtained flakes were washed and dried, and then pulverized using a Makino-type pulverizer (manufactured by Makino Sangyo Co., Ltd., model number: DD-2-3.7) to obtain cellulose acetate flakes of Comparative Examples 4 and 6. The pulverization conditions were a rotation speed of 2450 rpm and a screen diameter of φ5.0 mm. The pulverization time was adjusted so that the ratios of 20 mesh-on and 150 mesh-pass particles measured by the JIS sieving test were as shown in Table 3 below. As a result, in Comparative Example 6, the ratio of 20 mesh-on particles was 1.9 wt % and the ratio of 150 mesh-pass particles was 10%, while in Comparative Example 4, the ratio of 20 mesh-on particles was 0.5 wt % and the ratio of 150 mesh-pass particles was 15%. The above-mentioned properties were measured, and a liquid flow test and an adsorption test were carried out. The results are shown in the table below.

[0109] Comparative Example 5 In Comparative Example 5, similar to Comparative Example 2, the fiber bundle-like cellulose acetate of Example 12 of JP 2014-109064 A (Patent Document 2) was used. However, during the liquid flow test and adsorption test, the amount packed into the column was changed to 0.01 g. The above-mentioned physical properties were measured, and the results of the liquid flow test and adsorption test conducted with a packed amount of 0.01 g are shown in the table below.

[0110] In Comparative Example 8, powdered cellulose acetate propionate (acetyl substitution degree 0.07, propyl substitution degree 1.91, total substitution degree 1.98) was used instead of cellulose acetate. The above-mentioned properties were measured and a liquid flow test was conducted, and the results are shown in the table below.

[0111] Comparative Example 9 In Comparative Example 9, cellulose acetate flakes were produced according to the manufacturing method described in Comparative Example 5 of JP 2017-052961 A. The resulting cellulose acetate flakes had an acetylation degree AV of 53.9%, a 6% viscosity of 87 mPa·s, and a particle ratio of 500 μm or larger of 99.0 wt%. Because Comparative Example 5 of JP 2017-052961 A did not disclose the dope temperature (precipitation liquid temperature), a dope temperature of 60°C was adopted as the technical standard. The basis for 60°C is the description of a dope temperature of 60 to 80°C in "Cellulose Acetate Property and Application," a textbook in this field. Comparative Example 9 was used to replicate Comparative Example 5 of JP 2017-052961 A. The physical properties described above were measured, and a liquid flow test was conducted. The results are shown in the table below.

[0112] [Comparative Example 10] In Comparative Example 10, cellulose acetate flakes were produced according to the production method of Comparative Example 6 in JP 2017-052961 A. The dope temperature was 60°C, as in Comparative Example 9. The obtained cellulose acetate flakes had an acetylation degree AV of 55.4%, a 6% viscosity of 75 mPa·s, and a ratio of particles with a particle size of 500 μm or more of 98.0 wt%. This Comparative Example 10 was a reproduction of Comparative Example 6 in JP 2017-052961 A, and the above-mentioned physical properties were measured and a liquid flow test was conducted. The results are shown in the table below.

[0113] [Comparative Example 11] In Comparative Example 11, a precipitate was obtained by the same procedure as in Example 4 of WO 2014 / 142166. This precipitate was then subjected to the same procedure as in Example 4 of WO 2014 / 142166 to obtain a low-substituted cellulose acetate. The obtained precipitate and low-substituted cellulose acetate were in powder form. The obtained low-substituted cellulose acetate was measured for the above-mentioned properties and subjected to a liquid flow test, the results of which are shown in the table below.

[0114] [Comparative Example 12] Cellulose acetate flakes were produced in the same manner as in Example 1 of WO 2018 / 139319. For Comparative Example 12, cellulose acetate flakes were used that had undergone a pulverization process (paragraph

[0104] of WO 2018 / 139319) and a dipping process (paragraph

[0106] of WO 2018 / 139319). The obtained cellulose acetate flakes were quite fine for flakes and were essentially powder-like. The above-mentioned physical properties were measured, and the results of a liquid flow test are shown in the table below.

[0115] [Comparative Example 13] In Comparative Example 13, cellulose acetate was obtained by the same method as in Example 1 of JP 2021-161320 A. Specifically, during hydrolysis, the reaction mixture was transferred to a pressure-resistant container, and 98 parts by weight of hot water at about 90 ° C. was added and mixed. A microwave (Anton Paar, Monowave 100, frequency 2,455 MHz) was used as a heat source to irradiate the reaction mixture from the outside of the pressure-resistant container, and the temperature reached 148 ° C. in 90 minutes and was maintained at 148 ° C. for 12 minutes (deacylation step). The dope temperature after the deacylation step was about 100 ° C. in a state where atmospheric pressure was released. This 100 ° C. reaction product (dope) was added to a large amount of dilute acetic acid aqueous solution under vigorous stirring, and the flaky cellulose acetate was separated. After that, it was thoroughly washed with water and dried to obtain a product. The acetylation degree of the obtained cellulose acetate was 55.2% (total acetyl substitution degree: 2.43), and the 6% viscosity was 95 mPa s. The obtained flakes had a fine particle size. The above-mentioned physical properties were measured, and a liquid flow test was conducted. The results are shown in the table below.

[0116] [Comparative Example 14] In Comparative Example 14, cellulose acetate was precipitated in the same manner as in Comparative Example 1 of JP 2018-119052 A. The precipitated cellulose acetate was washed with water, immersed in a dilute aqueous calcium hydroxide solution (20 ppm), filtered, dried, and pulverized using a Makino pulverizer (manufactured by Makino Sangyo Co., Ltd., model number: DD-2-3.7). The pulverization conditions were a rotation speed of 2450 rpm and a screen diameter of φ5.0 mm. The obtained product was powdered cellulose acetate, with an acetylation degree AV of 55.8% and a 6% viscosity of 110 mPa s. The physical properties described above were measured, and a liquid flow test was conducted. The results are shown in the table below.

[0117]

[0118]

[0119]

[0120]

[0121]

[0122]

[0123]

[0124]

[0125]

[0126]

[0127]

[0128]

[0129]

[0130]

[0131]

[0132]

[0133] In Table 1-2, when compared with Example 5-7 in which only the particle size distribution was changed by sieving, there is a tendency that a smaller particle size (lower content of particle size (20 mesh On)) leads to a larger BET specific surface area, and as a result, the average space velocity SV tends to be slower (smaller SV value). Example 10, which was obtained using the same cellulose acetate as Example 5-7 as a raw material, had a larger BET specific surface area than Example 5-7, but also a larger average space velocity SV. This is thought to be because, although the BET specific surface area was large due to the large total pore volume, the large particle size (high content of particle size (20 mesh On)) and the high load displacement (hardness) increased the interparticle voids, resulting in a large average space velocity SV.

[0134] Comparing Example 5 with Example 8, Example 8, which was subjected to pulverization, had a smaller particle size (a lower content of particle size (20 mesh On) and a higher content of particle size (150 mesh pass)) and a larger BET specific surface area, but the average space velocity (SV) was also higher. This is thought to be because the hydrophobic portion (skeleton) of the cellulose was exposed to the particle surface due to pulverization.

[0135] Comparing Example 1 and Example 6, the particle size distributions (particle size (20 mesh On) and particle size (150 mesh pass)) were the same and the BET specific surface areas were similar, but the average space velocity (SV) was significantly different. The cellulose acetate flakes of Example 6 were obtained under different precipitation conditions from those of Example 1, and it is believed that this resulted in a decrease in the average space velocity (SV) due to the exposure of more hydrophilic functional groups on the particle surface than in Example 1.

[0136] As shown in Table 4, the desired average space velocity (SV) was not obtained in Comparative Examples 8-14. The reason for this is thought to be that Comparative Example 8 was in powder form, which reduced the interparticle space when packed into a column. In Comparative Examples 9 and 10, a twin-screw kneader was used to knead a precipitant into a reaction mixture containing cellulose acetate at a high temperature of 60°C, the technical standard, resulting in very soft cellulose acetate flakes. As a result, the flakes crumbled, reducing the interparticle space, and the cellulose acetate flakes of the present disclosure were not obtained. Similarly, Comparative Examples 12 and 14, which used a twin-screw kneader, produced a powdery product, presumably failing to produce the desired flake-shaped product. Comparative Example 11 used low-substituted cellulose acetate, which could not be precipitated into flakes in the first place.

[0137] In Comparative Example 13, a dilute aqueous acetic acid solution (precipitant) was added to the reaction product containing cellulose acetate under vigorous stirring to separate the cellulose acetate. This resulted in a high settling rate and the spatial spread of the dope (reaction product) droplets in the precipitant could not be increased, which is thought to be why the desired average space velocity SV could not be obtained.

[0138] As shown in Tables 1-4 and 6-16, the cellulose acetate flakes of the Examples have superior liquid permeability compared to the Comparative Examples. Furthermore, it was confirmed that gold can be selectively and efficiently recovered from a solution containing multiple metals using a column containing the cellulose acetate flakes of the Examples as a packing material. Furthermore, desorption with water enabled the production of a gold-containing solution with a higher concentration than the original (untreated) gold-containing solution. These evaluation results clearly demonstrate the superiority of the present disclosure.

[0139] Disclosed Items Each of the following items discloses a preferred embodiment.

[0140] [Item 1] The degree of acetyl substitution is 0.80 or more and 2.90 or less, and the average space velocity (SV) determined by the following method is 1.0 h －１ Over 120 hours －１(The average space velocity SV is determined by the amount of liquid (m) flowing out from the bottom per unit time when a column (inner diameter 10 mm) packed with cellulose acetate flakes to a height of 2.0 cm from the bottom is vertically arranged, and a hydrochloric acid solution (temperature 20°C ± 5°C) with a concentration of 2.0 mol / L is introduced from the top of the column and allowed to flow down through the column under atmospheric pressure.) ３ / h) is calculated by multiplying the volume of space occupied by the cellulose acetate flakes in the column (m ３ ) [Item 2] BET specific surface area is 1.8 m ２ / g or more 25m ２ Item 3: The cellulose acetate flakes according to item 1, wherein the total pore volume is 0.005 cm 3 / g or less. ３ / g or more 150cm ３ / g or less. [Item 4] The cellulose acetate flakes according to any one of Items 1 to 3, wherein in a particle size distribution determined using a JIS test sieve, the proportion of particles having a particle diameter of 850 μm or more is 50% or more. [Item 5] The cellulose acetate flakes according to any one of Items 1 to 4, wherein the load displacement measured by the following method is 10 mm or less. (The load displacement is determined by filling a 10 ml cylindrical syringe (made of polypropylene, inner diameter 17 mm) with cellulose acetate flakes to form a packed layer with a height of 50 mm, compressing this packed layer with a universal tensile tester (trade name "RTG1310" manufactured by A&D Co., Ltd.) at a compression rate of 10 mm / min, measuring the height of the packed layer at a load of 10 N, and calculating the difference from the height of the packed layer before compression.) [Item 6] The cellulose acetate flakes according to any one of Items 1 to 5, which are used as a packing material for a gold adsorption column. [Item 7] A gold adsorption column packed with the cellulose acetate flakes according to any one of Items 1 to 6 as a packing material. [Item 8] The gold adsorption column according to Item 7, in which the packing rate of the cellulose acetate flakes is 10% or more. [Item 9] A method for recovering gold, comprising passing a liquid containing gold ions through the gold adsorption column according to Item 7 or 8, and adsorbing the gold ions in the liquid onto the cellulose acetate flakes. [Item 10] A method for recovering gold, comprising: filling a cylindrical container with the cellulose acetate flakes according to any one of Items 1 to 6 to prepare a packed column; and passing a liquid containing gold ions through the packed column and adsorbing the gold ions onto the cellulose acetate flakes. [Item 11] A method for recovering gold, comprising passing the liquid containing gold ions through the packed column for 1.0 h. －１10. The method for recovering gold according to Item 10, wherein the liquid is passed through the packed column at a flow rate that provides a space velocity SV' exceeding 10. [Item 12] The method for recovering gold according to Item 10 or 11, further comprising dissolving or incinerating the cellulose acetate flakes to which gold ions have been adsorbed. [Item 13] The method for recovering gold according to any one of Items 10 to 12, further comprising preparing the liquid containing gold ions by dissolving or extracting gold from a gold-containing material in aqua regia. [Item 14] A method for concentrating a gold solution, comprising: filling a cylindrical container with the cellulose acetate flakes according to any one of Items 1 to 6 to prepare a packed column; passing a liquid containing gold ions as a stock solution through the packed column to adsorb the gold ions onto the cellulose acetate flakes; and passing an eluate through a packed column containing the cellulose acetate flakes to which gold ions have been adsorbed, to obtain a liquid containing gold ions at a higher concentration than in the stock solution. [Item 15] A method for producing cellulose acetate flakes according to any one of Items 1 to 6, comprising a step of mixing a dope containing cellulose acetate having an acetyl substitution degree of 0.80 to 2.90 with a precipitant, wherein the temperatures of the dope and the precipitant during the mixing are 40° C. or lower. [Item 16] Use of the cellulose acetate flakes according to any one of Items 1 to 6, in the production of a packed column packed with the cellulose acetate flakes, in which a liquid containing gold ions is passed through the packed column as a stock solution to adsorb gold ions to the cellulose acetate flakes. [Item 17] Use of the cellulose acetate flakes according to any one of Items 1 to 6, wherein the cellulose acetate flakes are used in the production of a packed column packed with the cellulose acetate flakes, and gold ions are selectively adsorbed onto the cellulose acetate flakes by passing a liquid containing a plurality of metal ions through the packed column as a stock solution.[Item 18] Use of cellulose acetate flakes in the production of a packed column according to Item 16 or 17, wherein an eluate containing gold ions at a higher concentration than that in the stock solution is obtained by passing the eluate through a packed column containing the cellulose acetate flakes having gold ions adsorbed thereon. [Item 19] An average space velocity (SV) of 2.0 h is used. －１ More than 110 hours －１ Item 20: The cellulose acetate flakes according to any one of items 1 to 6, wherein the average space velocity (SV) is 10.0 h or less. －１ More than 110 hours －１ Item 21: The cellulose acetate flakes according to any one of items 1 to 6 and 19, wherein the average space velocity (SV) is 50.0 h or less. －１ More than 110 hours －１ The cellulose acetate flakes according to any one of items 1 to 6 and items 19 to 20, wherein the average space velocity (SV) is 80.0 h or less. －１ More than 108 hours －１ The cellulose acetate flakes according to any one of items 1 to 6 and items 19 to 21, wherein the average space velocity (SV) is 85.0 h or less. －１ More than 108 hours －１ The cellulose acetate flakes according to any one of items 1 to 6 and items 19 to 22, wherein the average space velocity (SV) is 85.0 h or less. －１ Over 92.0 hours －１ The cellulose acetate flakes according to any one of items 1 to 6 and items 19 to 23, having a BET specific surface area of ​​7 m or less. ２ / g or more 18m ２ The cellulose acetate flakes according to any one of items 1 to 6 and items 19 to 24, wherein the total pore volume is 0.009 cm 3 / g or less. ３ / g or more 1.0cm ３ / g or less. [Item 27] ​​The cellulose acetate flakes according to any one of items 1 to 6 and items 19 to 26, having an Mw of 170,000 or more and 250,000 or less. [Item 28] The cellulose acetate flakes according to any one of items 1 to 6 and items 19 to 27, having a 6% viscosity of the cellulose acetate constituting the cellulose acetate flakes of 70 mPa·s or more and 150 mPa·s or less. [Item 29] The cellulose acetate flakes according to any one of items 1 to 6 and items 19 to 28, having an acetyl substitution degree DS of 1.10 or more and 2.60 or less. [Item 30] The cellulose acetate flakes according to any one of items 1 to 6 and items 19 to 29, having a width of 0.1 mm or more and 1 mm or less and a thickness of 0.05 mm or more and 0.1 mm or less. [Item 31] The cellulose acetate flakes according to any one of items 1 to 6 and items 19 to 30, having a shape in which the width is longer than the thickness. [Item 32] The cellulose acetate flakes according to any one of items 1 to 6 and items 19 to 31, having a particle size distribution in which the proportion of particles having a particle diameter of 850 μm or more is 10% or more and 100% or less. [Item 33] The cellulose acetate flakes according to any one of items 1 to 6 and items 19 to 32, having a load displacement measured by the following method of 0 mm or more and 6 mm or less. (The load displacement amount is determined by filling a 10 ml cylindrical syringe (made of polypropylene, inner diameter 17 mm) with cellulose acetate flakes to form a packed layer 50 mm high, compressing this packed layer at a compression speed of 10 mm / min using a universal tensile tester (manufactured by A&D Co., Ltd. under the trade name "RTG1310"), measuring the height of the packed layer at a load of 10 N, and calculating the difference from the height of the packed layer before compression.) [Item 34] The width is 0.1 mm or more and 1 mm or less, and the thickness is 0.05 mm or more and 0.1 mm or less, the width is longer than the thickness, in the particle size distribution of the cellulose acetate flakes, the proportion of particles having a particle diameter of 850 μm or more is 10% or more and 100% or less, and the BET specific surface area is 7 m２ / g or more 18m ２ / g or less, and the total pore volume is 0.009 cm ３ / g or more 1.0cm ３ / g or less, Mw is 170,000 or more and 250,000 or less, and the degree of acetyl substitution DS is 1.10 or more and 2.60 or less.

Claims

1. The degree of acetyl substitution is 0.80 or more and 2.90 or less, and the average space velocity (SV) determined by the following method is 1.0 h －１ Over 120 hours －１ The average space velocity SV is determined by the volume (m) of the space occupied by the cellulose acetate flakes, the height of which is 2.0 cm from the bottom surface of the flakes. ３ ) is 1.57 x 10 －６ (m ３ A column (inner diameter 10 mm) packed with cellulose acetate flakes was placed vertically, and a hydrochloric acid solution (temperature 20°C ± 5°C) with a concentration of 2.0 mol / L was poured into the top of the column. The solution was allowed to flow down the column under atmospheric pressure, and the amount of liquid (m) flowing out from the bottom per unit time was measured. ３ / h) is calculated based on the volume of the space occupied by the cellulose acetate flakes in the column, 1.57 × 10 －６ (m ３ )

2. BET specific surface area is 1.8m ２ / g or more 25m ２ 2. The cellulose acetate flakes according to claim 1, wherein the cellulose acetate flakes have a viscosity of 1 / g or less.

3. Total pore volume is 0.005 cm ３ / g or more 150cm ３ 3. The cellulose acetate flakes according to claim 1, wherein the cellulose acetate flakes have a viscosity of 1 / g or less.

4. 4. The cellulose acetate flakes according to claim 1, wherein the ratio of particles having a particle size of 850 μm or more is 50% or more in a particle size distribution determined using a JIS test sieve.

5. 5. The cellulose acetate flakes according to claim 1, wherein the load displacement measured by the following method is 10 mm or less. (The load displacement amount is determined by filling a 10 ml cylindrical syringe (made of polypropylene, inner diameter 17 mm) with cellulose acetate flakes to form a packed layer 50 mm in height, compressing this packed layer at a compression rate of 10 mm / min using a universal tensile tester (trade name "RTG1310" manufactured by A&D Co., Ltd.), measuring the height of the packed layer at a load of 10 N, and calculating the difference from the height of the packed layer before compression.)

6. The cellulose acetate flakes according to any one of claims 1 to 5, which are used as a packing material for a gold adsorption column.

7. A gold adsorption column packed with the cellulose acetate flakes according to any one of claims 1 to 6 as a packing material.

8. 8. The gold adsorption column according to claim 7, wherein the cellulose acetate flakes have a packing rate of 10% or more.

9. 9. A method for recovering gold, comprising passing a liquid containing gold ions through the gold adsorption column according to claim 7 or 8, and allowing the gold ions in the liquid to be adsorbed onto the cellulose acetate flakes.

10. 10. Packing the cellulose acetate flakes according to claim 1 into a cylindrical container to prepare a packed column. and A liquid containing gold ions is passed through the packed column to adsorb the gold ions onto the cellulose acetate flakes. Methods of recovering gold, including:

11. The liquid containing gold ions was left for 1.0 h. －１ 11. The method for recovering gold according to claim 10, wherein the liquid is passed through the packed column at a flow rate that gives a space velocity SV' of greater than 1000.

12. 12. The method for recovering gold according to claim 10 or 11, further comprising dissolving or incinerating the cellulose acetate flakes having the gold ions adsorbed thereon.

13. 13. A method for recovering gold according to any one of claims 10 to 12, further comprising preparing the liquid containing gold ions by dissolving or extracting gold from a gold-bearing material in aqua regia.

14. 10. Packing the cellulose acetate flakes according to claim 1 into a cylindrical container to prepare a packed column. A liquid containing gold ions is passed through the packed column as a stock solution to adsorb the gold ions onto the cellulose acetate flakes. and and passing the eluate through a packed column containing the cellulose acetate flakes having gold ions adsorbed thereon to obtain a liquid containing gold ions at a higher concentration than that of the original solution.