Method for recovering precious metals from activated carbon

Abstract

To provide a method for safely recovering noble metals from an activated carbon with adsorbed noble metals represented by gold at a relatively low cost.SOLUTION: A method for recovering noble metals from an activated carbon includes the steps of: pulverizing activated carbon to which a noble metal represented by gold is adsorbed, preferably to a particle size of 0.1 mm or less; charging the pulverized activated carbon into a roasting furnace and roasting the activated carbon in the atmospheric atmosphere at a roasting temperature of 700 to 800°C for 12 hours or more; and concentrating the noble metal by subjecting the solid matter containing the noble metal remaining after the roasting to a wet treatment. The wet treatment includes the steps of: immersing the solid matter in a solution containing, preferably, a chemical agent chosen depending on the type of noble metal to be a recovery target as a main component to elute the noble metal of the recovery target; and collecting an eluate containing the obtained noble metal by an electrowinning method.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for recovering precious metals from activated carbon, and more particularly to a method for recovering precious metals such as gold, silver, and platinum by roasting and hydrotreating activated carbon to which the precious metals have been adsorbed. [Background technology]

[0002] Precious metals such as gold, silver, and platinum have excellent physical, chemical, and mechanical properties, and are therefore used not only in jewelry but also in a variety of other fields. Gold (Au), in particular, excels in electrical conductivity, environmental resistance, and workability, and is therefore used in the form of gold plating on the surfaces of connection terminals and connectors that connect devices together, which make up electronic devices such as smartphones, personal computers, and communication devices, to prevent oxidation of the contact points. In recent years, in order to make effective use of resources, efforts have been made to recover and reuse the highly rare gold from discarded electronic devices.

[0003] Known methods for recovering gold from the above-mentioned electronic device processing scraps, waste materials, etc. include a method using an iodine / iodide solution and a method using an alkaline cyanide solution. The former method involves crushing gold-containing materials such as gold-plated conductor wires and scrap circuit boards to an appropriate particle size, then immersing the materials in an aqueous solution containing iodine and iodide to elute the gold in the form of a gold-iodine complex, and passing the resulting eluate through an ion exchange resin to adsorb the gold-iodine complex. A known method for recovering gold from the ion exchange resin to which the gold-iodine complex has been adsorbed involves burning the resin to concentrate and recover the gold.

[0004] On the other hand, the latter method using an alkaline cyanide solution involves crushing the gold-containing material to an appropriate particle size, as described above, and then immersing it in a cyanide-based eluent such as an alkaline cyanide solution to convert the gold into a gold-cyanide ion complex [Au(CN)2]. -The gold is then eluted in the form of HCl, and the resulting eluate is brought into contact with activated carbon to adsorb the gold-cyanide ion complex onto the activated carbon. A known method for recovering gold from activated carbon to which the gold-cyanide ion complex has been described is to elute the gold using a solution containing a specific chemical from the activated carbon to which the gold-cyanide ion complex has been adsorbed, and then recover gold from the resulting eluate by electrowinning.

[0005] For example, Patent Document 1 discloses that a dilute cyanide solution of an alkali metal or alkaline earth metal and an alkali are added to finely ground gold ore to convert gold into a gold cyanide ion complex [Au(CN)2]. - The gold complex is dissolved from the ore in the form of a gold cyanide ion complex, and the resulting slurry containing the gold cyanide ion complex is brought into countercurrent contact with spherical activated carbon with a diameter of approximately 1 to 2 mm to adsorb the gold complex onto the activated carbon. The activated carbon with the adsorbed gold complex is then separated from the slurry and washed with hydrochloric acid, after which a hot cyanide solution is applied to desorb the gold complex from the activated carbon, and the resulting cyanide solution containing the gold complex is subjected to electrolysis to recover gold. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 03-030834 Summary of the Invention [Problem to be solved by the invention]

[0007] In the technology of Patent Document 1, the activated carbon after desorbing the gold complex is loaded into a rotary kiln, where it is regenerated by heat treatment in a steam stream at 650°C for 30 minutes, and then reused. As described above, the precious metal recovery method of Patent Document 1 is based on the premise that the activated carbon will be regenerated and reused. Therefore, four steps, namely, a washing step, a desorption step, an electrolysis step, and a regeneration step, are required, which can result in higher costs. The present invention has been made in view of the above-mentioned circumstances, and aims to provide a method for recovering precious metals, such as gold, from activated carbon to which the precious metals have been adsorbed, at relatively low cost. [Means for solving the problem]

[0008] In order to achieve the above object, the method for recovering precious metals from activated carbon according to the present invention comprises: a cyanide decomposition step in which sodium hypochlorite is added to a precious metal-containing eluate containing a cyanide ion complex of a precious metal to maintain the oxidation-reduction potential at a predetermined value or higher, and a mineral acid is added to adjust the pH to 6 to 8; a step in which the precious metal-containing eluate treated in the cyanide decomposition step is brought into contact with activated carbon to adsorb the cyanide ion complex of a precious metal onto the activated carbon; Precious metals Cyanide ion complex the activated carbon on which the precious metals have been adsorbed is crushed; the crushed activated carbon is placed in a roasting furnace and roasted at a roasting temperature of 700 to 800°C in an air atmosphere for 12 hours or more; and the precious metal-containing solid matter remaining after the roasting is subjected to a wet treatment to concentrate the precious metals. [Effects of the Invention]

[0009] According to the present invention, it is possible to recover precious metals, such as gold, at relatively low cost from activated carbon to which the precious metals have been adsorbed. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a block flow diagram of a method for recovering precious metals from activated carbon according to an embodiment of the present invention. BEST MODE FOR CARRYING OUT THE INVENTION

[0011] Hereinafter, an embodiment of the method for recovering precious metals from activated carbon according to the present invention will be described, taking the case where the precious metal is gold as an example. The activated carbon onto which gold, the precious metal targeted by the recovery method of this embodiment of the present invention, is adsorbed is preferably, but not limited to, activated carbon onto which a gold-cyanide ion complex is adsorbed, the activated carbon being obtained by eluting gold, the precious metal to be recovered, from waste electronic components using an eluent containing cyanide ions in the form of a gold-cyanide ion complex and contacting the activated carbon with the eluate containing the obtained gold-cyanide ion complex.

[0012] Specifically, as shown in Figure 1, gold-containing materials, such as waste electronic circuit boards, are crushed using a crusher in the crushing step. The resulting crushed material is then immersed in an eluent, such as a dilute cyanide solution of an alkali metal or alkaline earth metal, in the immersion step, to elute the gold in the gold-containing materials as gold-cyanide ion complexes. The resulting gold-containing eluent, containing the gold-cyanide ion complexes, is then introduced into a reaction vessel, preferably made of FRP or stainless steel, equipped with a pH meter, ORP meter, and a stirrer. Sodium hypochlorite is added while stirring to maintain the redox potential at or above +350 mV (against a silver / silver chloride reference electrode). A mineral acid, such as sulfuric acid, is then added to adjust the pH to 6-8. This decomposes the cyanide in the gold-containing eluent. Because the decomposition rate of cyanide is relatively fast, maintaining the redox potential and pH under the above conditions for approximately 30 minutes is sufficient.

[0013] After confirming that the pH of the gold-containing eluent in the reaction vessel is between 6 and 8 using a pH meter, a predetermined amount of activated carbon is introduced into the activated carbon inlet at the top of the reaction vessel during the activated carbon adsorption process. This allows the gold-cyanide ion complex in the gold-containing eluent to be adsorbed onto the activated carbon. To ensure this adsorption onto the activated carbon, it is preferable to maintain stirring for at least 60 minutes after introducing the activated carbon. The agitator is then stopped, and the post-adsorption eluent is withdrawn from the outlet of the reaction vessel together with the activated carbon in the form of a slurry, which is then introduced into a filter. The activated carbon can then be recovered by solid-liquid separation of the slurry. The adsorption of the gold-cyanide ion complex onto the activated carbon is not limited to the batch process using the reaction vessel described above. Adsorption can also be achieved by passing the gold-containing eluent withdrawn from the reaction vessel through a packed tower filled with activated carbon.

[0014] Next, the activated carbon with the gold cyanide ions adsorbed thereon is dried in an activated carbon drying process. Then, in an activated carbon pulverization process, the activated carbon is loaded into a pulverizer such as a disk-type vibrating mill and pulverized to a particle size of 0.1 mm or less, preferably 0.05 mm or less. The particle size is determined by sieving. For example, a particle size of 0.1 mm or less refers to particles under 0.1 mm that pass through a 0.1 mm mesh screen. Therefore, to pulverize to a particle size of 0.1 mm or less as described above, a 0.1 mm mesh screen can be installed downstream of the pulverizer. In this case, particles over 0.1 mm that do not pass through the screen are returned to the upstream pulverizer. The pulverized activated carbon that passes through the screen is then loaded into a roasting furnace in the roasting process and roasted in air at a roasting temperature of 700-800°C for 12 hours or more. The gold-containing solids remaining after the roasting process can be wet-processed to recover the target gold in a concentrated form.

[0015] The preferred wet treatment method for the solid material involves immersing the gold-containing solid material in a solution primarily composed of a chemical selected according to the type of precious metal to be recovered. For example, using aqua regia or cyanide as the chemical can effectively elute the precious metal. When the precious metal to be recovered is gold, it is particularly preferable to use aqua regia as the chemical. In this case, gold can be efficiently eluted by immersing 100 parts by mass of the gold-containing solid material in 1,000 milliliters of aqua regia. The gold-containing eluate thus obtained can be recovered at high quality by a conventional recovery method involving the addition of a reducing agent. [Example]

[0016] Scrap substrates were used as gold-bearing materials. Three 600g samples were taken from a gold-adsorbed activated carbon lot, which was obtained by processing the gold-bearing material from the crushing process to the activated carbon drying process shown in Figure 1. The gold content of each sample was measured using an ICP atomic emission spectrometer, and the average of these analysis results was used as the gold content of the activated carbon. Next, the gold-adsorbed activated carbon samples taken from the same lot were mixed and then loaded into a disk-type vibrating mill. The mixture was then crushed and sieved repeatedly to produce four samples with different particle sizes: Sample 1 (under 10 mm and over 2 mm), Sample 2 (under 2 mm and over 0.1 mm), Sample 3 (under 0.1 mm and over 0.05 mm), and Sample 4 (under 0.05 mm). Further crushing of the under 0.05 mm powder was discontinued because further crushing using a crusher increased the amount of dust dispersed.

[0017] Each of the samples 1 to 4 prepared above was divided into four trays of the same size, each containing 100 g. These were then placed in a roasting furnace and roasted at 700-800°C in an air atmosphere for 5, 8, 12, and 15 hours, respectively. Gold was extracted by immersing 100 parts by mass of the remaining solid material in 1000 ml of aqua regia. The resulting eluate was then recovered using a reducing agent. The gold recovery rate was calculated from the mass of gold extracted and the gold content of the activated carbon measured above. The results are shown in Table 1 below.

[0018] [Table 1]

[0019] The results in Table 1 above show that gold can be recovered at an extremely high recovery rate by roasting gold-adsorbed activated carbon for 12 hours or more after crushing it. In particular, it is clear that almost all of the gold can be recovered from the activated carbon by crushing it until the particle size is 0.1 mm or less.

Claims

1. A method for recovering precious metals from activated carbon containing precious metals, comprising: a cyanide decomposition step of adding sodium hypochlorite to a precious metal-containing eluent containing a cyanide ion complex of the precious metal to maintain the oxidation-reduction potential at a predetermined value or higher, and adding a mineral acid to adjust the pH to 6 to 8; a step of bringing the precious metal-containing eluent treated in the cyanide decomposition step into contact with activated carbon to adsorb the cyanide ion complex of the precious metal onto the activated carbon; a step of pulverizing the activated carbon to which the cyanide ion complex of the precious metal has been adsorbed; a step of loading the pulverized activated carbon into a roasting furnace and roasting it in an atmospheric atmosphere at a roasting temperature of 700 to 800°C for 12 hours or more; and a step of wet-treating the precious metal-containing solid matter remaining after the roasting to concentrate the precious metal.

2. 2. The method for recovering precious metals according to claim 1, wherein in the pulverizing step, the activated carbon on which the precious metals are adsorbed is pulverized to a particle size of 0.1 mm or less.

3. 3. The method for recovering precious metals according to claim 1, wherein the wet treatment comprises the steps of: immersing the solid material in a solution containing, as a main component, a chemical selected according to the type of precious metal to be recovered, thereby eluting the precious metal to be recovered; and recovering the precious metal contained in the obtained eluate by adding a reducing agent.

4. 4. The method for recovering precious metals according to claim 3, wherein the precious metal is gold and the chemical is aqua regia or cyanogen.

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