JP7217872B2 - Noble metal separation and recovery method and precious metal fine particles recovered by the method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method of contacting a mixed solution in which noble metals are dissolved with a chitosan film or a chitosan derivative to separate and recover only noble metal ions as noble metal fine particles, and noble metal fine particles recovered by the same method.

Precious metals such as gold, platinum, palladium, and their alloys have various properties such as catalytic action, magnetism, and hydrogen storage. It has become an indispensable resource for the creation of discs and even new functional materials. In addition, rare metals such as indium and gallium are used in liquid crystal panels, photovoltaic panels, etc., and are closely related to our lives.

These precious metals and rare metals are usually obtained mainly by mining in mines, etc., but they are extremely scarce in terms of resources and are unevenly distributed in some regions of the earth. On the other hand, a considerable amount of precious metal is recovered from the waste of electronic devices such as used mobile phones, even compared to the amount of mined. For this reason, in recent years, used electronic devices have been reconsidered as so-called urban mines from the viewpoint of resource reuse and economy (Non-Patent Document 1).

At present, in addition to solvent extraction methods, adsorption methods using industrial chelate resins or ion exchange resins with sulfur or nitrogen as coordinating atoms are used to recover precious metals and rare metals. In the latter adsorption method, the precious metal ions adsorbed on the resin are recovered to the aqueous solution side with a desorption agent, but since ammonia, thiourea, and hydrochloric acid are used as desorption reagents, the subsequent treatment process becomes more complicated. I have to. An example is the predominant use of sulfur atom systems such as thiols, thioethers, thiouretes, etc. in the gold recovery processes discussed in this application.

In recent years, the inventors have developed a new recovery method using chitosan as means for recovering such noble metals or rare metals. Chitosan is obtained by deacetylating chitin, a polysaccharide that constitutes the shells of crustaceans such as crabs and shrimps. For example, Patent Document 1 discloses a method of recovering indium and gallium using a chitosan derivative into which phenylphosphinic acid is introduced.

Further, Patent Document 2 discloses a method for recovering molybdenum, tungsten and vanadium by using an adsorbent for metal ions, including crosslinked chitosan (CLAC).

Further, in Patent Document 3, by using an adsorbent obtained by reacting porous chitosan fine particles, hydroxyquinoline or its derivative, and aldehyde, and by changing the desorption conditions such as the composition and pH of the desorption agent solution, , palladium, gold, platinum, indium and gallium.

That is, the various metal ions adsorbed to these chitosans or chitosan derivatives are highly selectively or specifically converted into free metal ions depending on the liquid property by contacting with each aqueous solution of acid, alkali, thiourea, etc. It is desorbed and recovered in the solution as (Patent Documents 1 to 3).

JP 2010-179205 A Japanese Patent Application Laid-Open No. 2010-260028 JP 2014-4518 A

Junji Shibata, Akihiko Okuda, "Precious Metal Recycling Technology," Resource and Material, 2012, Vol. 118, pp. 1-8

As described above, in the process of recovering precious metals using industrial chelate resins, ion exchange resins, or conventional chitosan-based adsorbents, precious metal ions are converted into [AuCl ₄ ] ⁻ , [PdCl ₄ ] ²⁻ , [PtCl ₆ ] ^2- , [RhCl ₅ ] ^2- , etc., is brought into contact with an aqueous solution of acid, alkali, or thiourea, and the metal species dissolves into the solution and is separated and recovered as free ions. is characterized by In addition, the recovered liquid contains additives such as a desorbing agent in addition to the noble metal ions, and post-treatment is also required. Furthermore, as in the example described above, depending on the desorbent, there is a problem that the subsequent treatment process becomes considerably complicated.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method for separating and recovering precious metals, which can be recovered as fine particles of noble metals in a simple and inexpensive manner.

In order to solve the above problems, the precious metal separation and recovery method of the present invention comprises:
A method for recovering precious metals using chitosan or a chitosan derivative, comprising:
a step of forming chitosan or a chitosan derivative into a film to form a chitosan film;
adding the chitosan film to a solution containing noble metal ions to adsorb the noble metal ions to the chitosan film;
and a step of subjecting the noble metal ions adsorbed to the chitosan film to reduction treatment and desorbing them as fine noble metal particles.
According to this feature, since the precious metal ions captured by the adsorbent can be recovered as fine precious metal particles by reduction treatment, compared to the conventional method of desorbing the precious metal ions into the solution, the use of chemicals can be reduced, the cost can be reduced, and the apparatus can be corrosion can be reduced.
In addition, chitosan or a chitosan derivative used as an adsorbent is inexpensive and chemically very stable, so that it can be used industrially for a long period of time.

In the reduction treatment, the type of reducing agent, the pH of the reducing agent aqueous solution, the reduction time, or the concentration of the reducing agent are changed to control the morphology or particle size of the noble metal fine particles.
According to this feature, by changing the type of reducing agent in the reduction treatment, the pH of the reducing agent aqueous solution, the reduction time, or the concentration of the reducing agent, the morphology or particle diameter of the noble metal fine particles can be controlled, so that the Precious metal microparticles can be recovered.

The method for separating and recovering precious metals in the present invention comprises:
A method for recovering precious metals using chitosan or a chitosan derivative, comprising:
adding chitosan or a chitosan derivative having reducing ability to a solution containing noble metal ions;
and a step of solidifying the noble metal ions dissolved in the solution as fine noble metal particles.
According to this feature, only noble metal ions can be selectively adsorbed and recovered as solid noble metal fine particles without using a reducing agent, so the noble metal can be recovered by an extremely simple process, and the cost can be greatly reduced. .

The fine particles of precious metals in the present invention are precious metals recovered by the method for separating and recovering precious metals described above, wherein the shape of the noble metals is spherical, triangular or hexagonal, and the size is nano-sized or micro-sized. is characterized by
According to this feature, the precious metal itself separated and recovered by the above method can be used industrially, the form and size of the precious metal can be specified, and the functions and applications can be expanded in various electronic members, medical fields, and the like.

1 is an appearance photograph of a chitosan film prepared for use as an adsorbent. Fig. 2 shows SEM images of samples obtained by adsorbing gold ions on a chitosan film and then performing reduction treatment with various reducing agents. FIG. 10 is an SEM image of a sample obtained by using a chitosan film as an adsorbent, Na ₂ EDTA as a reducing agent, and changing the pH of the reducing solution. FIG. Fig. 4 shows SEM images of samples obtained by using a chitosan film as an adsorbent and Na ₂ EDTA as a reducing agent and changing the reduction time. It is an SEM image of a sample obtained by reduction for about 24 hours using a chitosan film as an adsorbent and sodium oxalate as a reducing agent. Fig. ₂ shows SEM images of samples obtained by using a chitosan film as an adsorbent, Na2EDTA as a reducing agent, and varying the concentration of the reducing solution. Fig. 2 shows SEM images of samples obtained by using a chitosan film as an adsorbent, sodium oxalate as a reducing agent, and varying the concentration of a reducing solution. 1 is a TEM image of a product obtained by a contact reaction between chitosan acetate and [AuCl ₄ ^] -ion aqueous solution.

A mode for carrying out the method for separating and recovering precious metals according to the present invention will be described below based on examples.

In the present invention, a method for separating and recovering gold as a representative example of precious metals is shown as an example. Silver, platinum, and palladium, which have similar properties to gold, have the same reduction treatment effect as that obtained for gold. The same treatment can also be applied to rhodium, iridium, ruthenium, and osmium, which are listed as other noble metals.

The size of the gold microparticles produced in the present invention is about 5 nm to 2.5 μm, but the range can be expanded by adjusting the reduction time or the like. It refers to fine particles with a size of 1 nm to about 100 μm. In addition, the morphology of the fine gold particles produced in the present invention is mainly spherical, triangular, or hexagonal solid matter, and refers to solid agglomerates regardless of the morphology of regular or amorphous.

In the present invention, gold ions (the chemical form of "gold ions" is [AuCl ₄ ] ^- , but hereinafter, unless otherwise required, they are simply referred to as "gold ions" or "Au(III)"). Chitosan and chitosan acetate, which is a derivative thereof, were used as the adsorbent, but methods for separating and recovering gold ions using these two kinds of adsorbents differ in the mechanism of desorption and atomization of adsorbed gold ions by reduction treatment. Therefore, the first is (A) a method for separating and recovering gold using chitosan as an adsorbent, and the second is (B) a method for separating and recovering gold using chitosan acetate as an adsorbent, which will be described below.

(A) Method for Separating and Recovering Gold Using Chitosan as Adsorbent A method for separating and recovering gold using chitosan as an adsorbent will be described. The steps of the gold recovery method of the present invention include a film forming step of forming the adsorbent, a contacting step of contacting the adsorbent in the form of a film with gold ions, and a process of treating the adsorbent with a reducing agent to remove gold ions. It is composed of a desorption step in which gold fine particles are desorbed from the adsorbent.

In the present invention, a method of forming a film using chitosan and using it as an adsorbent is described, but the present invention is not limited to this, and a method of forming a film using a chitosan derivative and using this as an adsorbent is also included. be

[Film forming process]
A chitosan film as an adsorbent is prepared by the method of Example A-1 described later. The chitosan film refers to a thin film, and the thickness of the film is desirably 0.01 mm to 2.0 mm from the viewpoint of film formation time and ease of handling.

[Contact process]
Gold ions can be adsorbed on the surface of the chitosan film by immersing the chitosan film formed in the film forming process in a sodium chloride solution in which gold ions are dissolved and shaking at a constant temperature. It is desirable to use 1 g or more of the adsorbent per 1 mmol of Au(III).

[Desorption step]
When a reducing agent aqueous solution is added to the chitosan film on which Au(III) has been adsorbed in the contacting step, the surface of the chitosan film forms mainly spherical, partially triangular or hexagonal shapes depending on the type of reducing agent and reduction conditions. Gold nanoparticles with a diameter of 50 nm to 400 nm are produced. This is because, in the conventional desorption process, the adsorbed gold ions are recovered as free ions in an acid or alkaline solution, whereas in the process of the present invention, the gold ions can be recovered as solid gold fine particles. This is to demonstrate.

Examples of reducing agents include sodium ascorbate, sodium oxalate, disodium ethylenediaminetetraacetate (hereinafter referred to as “Na ₂ EDTA”), sodium citrate and sodium borohydride, but sodium bisulfite and thiosulfate Sodium or the like can be applied.

EXAMPLES The present invention will be described in more detail with reference to examples below, but the scope of the present invention is not limited to these examples.

[Example A-1] Separation and recovery of gold and production of gold nanoparticles using chitosan as an adsorbent and various reducing agents 1) Method for preparing chitosan membrane Chitosan powder (manufactured by Kimika Chitosan) 1.0 g, acetic acid 0.4 g was dissolved in 8.6 g of distilled water to make the total amount 10 g, and stirred for about 3 hours to prepare a 10 wt % chitosan-4 wt % acetic acid aqueous solution. About 5 mL of the solution was spread on a petri dish and allowed to dry for two nights. After that, the dried membrane was taken out, and 10 mL of sodium hydroxide, ammonia water and sodium hydrogen carbonate were added in this order at 0.1N to carry out alkali treatment. After standing for 2 to 3 hours, washing with distilled water and drying at room temperature, a chitosan film was obtained. The resulting chitosan film was transparent and had a thickness of about 1.0 mm, and tended to be slightly thick at the edges due to drying in the petri dish.

2) Adsorption of gold ions Adsorption experiments were performed by a batch method. The chitosan film obtained by the method described above was immersed in 40 mL of an aqueous sodium chloride solution containing 1000 ppm of Au(III) and shaken at 30° C. and 120 rpm for 20 minutes. As shown in FIG. 1, the chitosan film after the treatment was colored from transparent to yellow, and hardening was also observed. Further, it was confirmed by atomic absorption spectrometry that all of the Au(III) in the aqueous phase after shaking had been adsorbed to the solid phase.

3) Reduction treatment Sodium ascorbate, sodium oxalate, _Na2EDTA , sodium citrate and sodium borohydride were used as reducing agents. After the chitosan film after adsorption of Au(III) was washed with distilled water, a 0.2 molL ⁻¹ aqueous solution of a reducing agent was added, and the film was shaken at 30° C. and 120 rpm for 24 hours. It was confirmed that when the reducing agent was added, the chitosan film rapidly changed color from yellow to black/dark purple.

The surface of the reduction-treated film obtained by the above operation was observed with a scanning electron microscope (SEM: S5500, Hitachi), and the results are shown in FIG. Reducing agents are indicated as a) sodium ascorbate, b) sodium citrate, c) sodium oxalate, d) Na ₂ EDTA, e) sodium borohydride.

As can be seen from a), c), and d) in FIG. 2, when a) sodium ascorbate, c) sodium oxalate, and d) Na ₂ EDTA were added, a large number of fine gold particles were generated. On the other hand, in b) the addition of sodium citrate, a very small amount of fine gold particles was confirmed. It was also confirmed that when e) sodium borohydride was used, irregular gold aggregates were produced.

In addition, when the size of the fine gold particles was measured based on the SEM image, it became clear that the size of the particles generated differs depending on the type of reducing agent. The measurement results are shown in Table 1 below.

[Example A-2] Effect of pH on reduction treatment In the case of using Na ₂ EDTA as the reducing agent in the above-described "Example A-1", a 0.2 mol L ^-1 aqueous solution of the reducing agent was added to an aqueous sodium hydroxide solution. was added to adjust the pH of the reducing solution to 6.40, 7.15 and 8.15. After that, the film surface obtained by processing under the same conditions was observed by SEM. The results are shown in FIG. The measured pH shows a) 6.40, b) 7.15, c) 8.15. As the pH during reduction increased, the particle size of the gold nanoparticles formed decreased, and tended to settle down to about 50 nm in the neutral to basic range. Table 2 below shows the measurement results of the particle size.

[Example A-3] Effect of reduction time in reduction treatment As in [Example A-2] described above, Na ₂ EDTA was used as a reducing agent, and a 0.2 molL ^-1 aqueous solution of the reducing agent and a chitosan film were prepared. The contact time was changed in the range of 0.5 hours to 24 hours, and the surface of the obtained film was observed by SEM. The results are shown in FIG. The measured reduction times are a) 0.5 h, b) 1.0 h, c) 2.0 h, and d) 24 h. The particle size of the produced gold microparticles was almost constant at 100 nm for all reduction times of 2 hours or less, but became 200 nm after 24 hours. From this, it was clarified that fine particles are produced in a very short time, but the particle size increases when the reduction time is lengthened. Table 3 below shows the measurement results of the particle size.

On the other hand, SEM observation of the surface of the film obtained by performing the reduction treatment for about 24 hours using a 0.2 molL ^-1 aqueous solution of sodium oxalate was carried out. The results are shown in FIG. On the surface of the chitosan film, gold single crystals having a triangular shape with a side of about 900 nm to about 2.5 μm or a hexagonal shape with a longest diameter of about 700 nm to 1.5 μm together with spherical nanoparticles with a particle size of about 200 nm to about 500 nm. The formation of similar particles was confirmed.

[Example A-4] Effect of reducing agent concentration on reduction treatment As in [Example A- ₂ ] described above, Na EDTA was used as the reducing agent, and the concentration of the reducing agent was changed from 0.2 molL ^-1 to 0. .1 molL ⁻¹ to 0.02 molL ⁻¹ , and the same experiment was performed. The results are shown in FIG. The measured reducing agent concentrations are a) 0.2, b) 0.1, c) 0.05, and d) 0.02 (unit: molL ⁻¹ ). As the reducing agent concentration decreased from 0.2 molL ⁻¹ to 0.02 molL ⁻¹ , there was a tendency for the particle size of the generated gold microparticles to decrease. Table 4 below shows the measurement results of the particle size.

FIG. 7 shows the results of examining the reducing agent concentration dependency in the same manner, using sodium oxalate instead of the reducing agent. The measured reducing agent concentrations are a) 0.5, b) 0.25, c) 0.125, and d) 0.05 (unit: molL ⁻¹ ). A similar trend was observed with Na ₂ EDTA as the reducing agent concentration was decreased. Table 5 below shows the measurement results of the particle size.

As described above, it has become possible to process chitosan into a film, to adsorb Au(III), and to form fine particles using each reducing agent. Adsorption of gold was very rapid, and at least 40 mg of gold could be adsorbed and recovered with a chitosan film prepared using 1 g of chitosan. When several types of reducing agents were used for reduction, the size of the produced particles tended to be strongly affected by the type of reducing agent. In particular, the reduction with Na oxalate and Na ₂ EDTA confirmed the production of spherical gold nanoparticles with a uniform particle size.

(B) Method for Separating and Recovering Gold Using Chitosan Acetate as Adsorbent Next, a method for separating and recovering gold using chitosan acetate as an adsorbent will be described. This method is a method for separating and recovering gold, characterized by adding a chitosan derivative having a substituent with reducing ability to a solution containing gold ions, thereby reducing and solidifying the gold ions as fine gold particles. .

_In the present invention, an example in which chitosan acetate is used as an adsorbent as chitosan to which reducing ability is imparted will be described. Also included are chitosan derivatives having functional groups such as phosphate groups in part.

In addition, the separation and recovery method of the present method (B) is a combination of the gold ion contact step and the desorption/solidification step in the method described in the separation and recovery method of (A) above in terms of treatment operation. , which apparently proceeds as a single process. As a result, the size of the gold microparticles produced is at the level of several nanometers, and the morphology of the gold particles is like a single crystal, which is a major feature of this method.

In the present invention, chitosan acetate having a chemical structure represented by the following formula (II) is prepared by using chitosan having a chemical structure represented by the following formula (I) as each polymerized unit. By using this chitosan acetate as an adsorbent, adsorption of gold ions from the solution and reduction treatment are performed stepwise or simultaneously, making it possible to recover the gold ions as fine gold particles. The present separation and recovery method will be described in detail below.

[Example B-1] Separation and Recovery of Gold and Production of Gold Nanoparticles Using Chitosan Acetate as Adsorption/Reducing Agent 1) Preparation of Chitosan Acetate A method for synthesizing chitosan acetate will be described. 2 g of chitosan (grade F) was added to a mixed solution of 20 mL of sodium hydroxide aqueous solution having a concentration of 10 molL ⁻¹ and 20 mL of isopropanol, and the mixture was stirred for 12 hours. Then, chloroacetic acid (5 equivalents) dissolved in 20 mL of isopropanol was added dropwise over 30 minutes, followed by reaction at 60° C. for 6 hours. Excess ethanol was added to the obtained reaction mixture to precipitate the chitosan component, and then the precipitate was washed with distilled water and ethanol. After the precipitate was dissolved in distilled water to remove insoluble components, ethanol was again used for precipitation to obtain the desired product, chitosan acetate.

2) Contact reaction between chitosan acetate and gold ion aqueous solution After adding 1 mL of acetic acid to 50 g of a 1 wt% aqueous solution of chitosan acetate, 2 mL of chloroauric acid HAuCl ₄ having a concentration of 25 mmolL ^-1 was added, and the mixture was stirred at 60°C for 12 hours. The resulting solution exhibited a wine red color suggesting the production of gold nanoparticles. Ethanol was added to this solution and the solid phase obtained by centrifugation was dried.

FIG. 8 shows the result of a transmission electron microscope (TEM) of the reaction product obtained by the above operation. Note that b) is an enlarged TEM image obtained. The particle size was 5 nm to 20 nm, and gold nanoparticles having spherical, triangular, or hexagonal morphology were confirmed.

Thus, in this method, since the precious metal can be recovered as solid noble metal particles by the reduction treatment, the subsequent handling steps are not only simplified, but the recovery operation can also be used to create precious metal particles having useful functions. will be connected.

INDUSTRIAL APPLICABILITY The present invention contributes to the creation of noble metal fine particles having new functionality, in addition to being applicable to the noble metal recycling business.

Claims (2)

A method for recovering precious metals using chitosan , comprising:
a step of forming chitosan into a film to form a chitosan film;
adding the chitosan film to a solution containing noble metal ions to adsorb the noble metal ions to the chitosan film;
and a step of subjecting the noble metal ions adsorbed to the chitosan film to a reduction treatment with sodium ascorbate, sodium oxalate or Na ₂ EDTA to desorb them as noble metal fine particles. 2. The method according to claim 1, wherein in the reduction treatment, the type of reducing agent, the pH of the reducing agent aqueous solution, the reduction time, or the concentration of the reducing agent is changed to control the morphology or particle size of the noble metal fine particles. A method for separating and recovering precious metals.

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