Method for recovering precious metals from chlorine bypass dust

Abstract

To provide a method for efficiently recovering a noble metal from chlorine bypass dust.SOLUTION: A method for recovering a noble metal from chlorine bypass dust is provided, the method including a first step for heating and agitating a liquid mixture comprising chlorine bypass dust and an aprotic polar organic solvent, a second step for filtering the liquid mixture after the first step, and a third step for mixing the filtrate separated in the second step with an inorganic acid aqueous solution, and filtering the liquid mixture to recover a precipitate.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for recovering precious metals from chlorine bypass dust. [Background technology]

[0002] When cement clinker is burned in a cement kiln, volatile components such as chlorine, alkali, and sulfur carried over from the cement raw materials and fuel are gradually concentrated as they circulate within the preheater system. Chlorine, among other volatile components, can cause problems such as preheater blockage. Therefore, a chlorine bypass system is employed in which a portion of the combustion gas is extracted from the kiln exhaust gas flow path from the end of the cement kiln to the lowest cyclone to remove chlorine. Since chlorine is unevenly distributed in the fine powder portion of the dust contained in the exhaust gas, it has been proposed to separate the dust into coarse and fine powders using a classifier, return the coarse powder to the cement kiln, and add the separated fine powder (chlorine bypass dust) containing potassium chloride and other elements to the cement grinding process within a range that does not exceed a specified chlorine concentration value, thereby reusing it as a cement raw material (Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 97 / 21638 Summary of the Invention [Problem to be solved by the invention]

[0004] In recent years, recycling of waste materials into cement raw materials and fuel has been promoted, and the waste materials processed at cement plants are diverse, including municipal waste incineration bottom ash, automobile shredder residue (ASR), and sewage sludge incineration ash. As the amount of waste processed increases, the amounts of volatile components such as chlorine, sulfur, and alkali carried into the cement kiln also increase, resulting in an increase in the amount of chlorine bypass dust generated. As a result, it is difficult to utilize all of the generated chlorine bypass dust in the cement manufacturing process, and since the amount of chlorine bypass dust generated is expected to increase further in the future, there is an urgent need to develop new methods for its effective utilization. An object of the present invention is to provide a method for efficiently recovering precious metals from chlorine bypass dust. [Means for solving the problem]

[0005] Chlorine bypass dust contains precious metals, and the present inventors have found that when chlorine bypass dust is mixed with a specific organic solvent, the precious metals dissolve in the mixed solution and become more easily extracted, and that by treating the resulting solution with an inorganic acid aqueous solution or a reducing agent, the precious metals can be efficiently and selectively recovered.

[0006] That is, the present invention provides the following [1] to

[13] . [1] A first step of heating and stirring a mixed liquid containing chlorine bypass dust and an aprotic polar organic solvent; a second step of filtering the mixture obtained after the first step; A third step of mixing the filtrate separated in the second step with an aqueous solution of an inorganic acid, and filtering the mixture to recover the precipitate. A method for recovering precious metals from chlorine bypass dust, comprising: [2] The method for recovering precious metals according to [1], comprising a fourth step, after the third step, of mixing the filtrate separated in the third step with a reducing agent, and filtering the mixture to recover a precipitate. [3] The method for recovering precious metals according to [1] or [2] above, wherein the aprotic polar organic solvent is one or more selected from the group consisting of dimethyl sulfoxide, propylene carbonate, N,N-dimethylformamide, and N-methylpyrrolidone. [4] The method for recovering precious metals according to any one of [1] to [3] above, wherein in the first step, the chlorine bypass dust is mixed with an aprotic polar organic solvent in an amount of 1.0 times or more by mass relative to the mass of the chlorine bypass dust. [5] The method for recovering precious metals according to any one of [1] to [4] above, wherein in the first step, the mixed liquid is stirred at a temperature of 60 to 140°C. [6] The method for recovering precious metals according to any one of [1] to [5] above, wherein in the second step, the filtrate is mixed with an aqueous inorganic acid solution in an amount such that the pH of the filtrate becomes 4 or less. [7] The method for recovering precious metals according to any one of [1] to [6] above, wherein the aqueous inorganic acid solution is at least one selected from an aqueous hydrochloric acid solution and an aqueous sulfuric acid solution. [8] The method for recovering precious metals according to any one of [2] to [7] above, wherein in the fourth step, a reducing agent is mixed in an amount of 0.01 times or more by mass relative to the filtrate. [9] The method for recovering precious metals according to any one of [2] to [8] above, wherein the reducing agent is one or more selected from ascorbic acid, sodium sulfite, and ferrous sulfate.

[10] The method for recovering precious metals according to any one of [1] to [9] above, wherein the mixed solution in the first step contains one or more metal halides selected from copper halide and alkali metal halides.

[11] The method for recovering precious metals according to any one of [1] to

[10] above, wherein the content of the metal halide in the mixed solution in the first step is 0.1 to 0.6 mol / L.

[12] The method for recovering precious metals according to any one of [1] to

[11] above, wherein the chlorine bypass dust is a washed product.

[13] The precious metal recovery method according to any one of [1] to

[12] above, wherein the precious metal includes gold and / or silver. [Effects of the Invention]

[0007] According to the present invention, precious metals can be efficiently and selectively recovered from chlorine bypass dust. Therefore, the present invention is useful as a new method for effectively utilizing chlorine bypass dust. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is a diagram showing an example of a precious metal recovery method of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0009] The method for recovering precious metals from chlorine bypass dust of the present invention essentially comprises steps 1, 2, and 3, and optionally includes step 4. Each step will be described in detail below.

[0010] [First step] The first step involves heating and stirring a mixture containing chlorine bypass dust and an aprotic polar organic solvent. This promotes the dissolution of precious metals in the chlorine bypass dust, enabling the precious metals to be extracted efficiently from the chlorine bypass dust. While the reason why the aprotic polar organic solvent promotes the dissolution of precious metals in the chlorine bypass dust is not entirely clear, it is speculated as follows.

[0011] Chlorine bypass dust contains not only halogen atoms such as chlorine and bromine, but also precious metals such as gold and silver, as well as metals such as copper, sodium, and potassium. In a mixture containing chlorine bypass dust and an aprotic polar organic solvent, copper ions (Cu 2+ ) undergoes the reaction shown in the following formula (1), resulting in the formation of monovalent Cu + It exists stably as

[0012] Cu 2+ + e - → Cu + (1)

[0013] Thus, in the mixed solution, Cu 2+ oxidizes other substances to form Cu +Because it is reduced to Cu 2+ acts as a strong oxidizing agent. This oxidation-reduction potential is higher than the corrosion potential of precious metals such as gold and silver, which promotes the dissolution of the precious metals into the mixed solution, allowing the precious metals to be efficiently extracted from the chlorine bypass dust. Examples of the dissolution formulas for gold and silver are shown in the following formulas (2) and (3), respectively.

[0014] TIFF0007747265000001.tif16170

[0015] In this specification, the term "chlorine bypass dust" refers to fine powder that is separated by extracting a portion of the combustion gas from a kiln exhaust gas flow path extending from the end of a cement kiln to the lowest cyclone in a cement manufacturing facility, cooling the extracted gas, and then classifying and separating the gas into coarse powder and fine powder. The chlorine bypass dust used in the present invention is not particularly limited to any particular type of cement raw material, as long as it is generated in the cement production process. Furthermore, the cement raw materials used in the cement production process may include ash obtained by incinerating municipal solid waste or industrial waste, such as sludge, waste plastics, scrap metal, scrap glass, scrap concrete, scrap ceramics, slag, and rubble, as well as shredder dust generated by crushing scrapped automobiles and discarded home appliances, and ash obtained by incinerating general waste.

[0016] In this step, the washed product can be used as the chlorine bypass dust. The method for washing the chlorine bypass dust with water is not particularly limited as long as the chlorine bypass dust is brought into contact with water. Examples of the method include a method in which the chlorine bypass dust is placed in a water tank and stirred, a method in which the chlorine bypass dust is immersed in water, and a method in which water is sprayed onto the chlorine bypass dust. A commercially available device such as a drum washer can also be used. Examples of water include tap water as defined in JIS A 5303 Appendix C, and water other than tap water (for example, river water, lake water, well water, groundwater, and industrial water). The amount of water used is preferably 2 to 15 times by mass, more preferably 3 to 12 times by mass, even more preferably 4 to 10 times by mass, and even more preferably 5 to 8 times by mass, relative to the amount of chlorine bypass dust. The temperature of the water can be selected as appropriate, but is preferably 5 to 70°C, more preferably 10 to 65°C, even more preferably 15 to 60°C, and even more preferably 18 to 55°C. After washing with water, the washed material may be separated into a supernatant and a precipitate using, for example, a dehydrator, and the precipitate may be collected. Any common device may be used, such as a separator plate-type, cylindrical-type, or decanter-type centrifuge, or a dehydrator such as a filter press or belt filter. The conditions for centrifugation may be appropriately selected.

[0017] The aprotic polar organic solvent is not particularly limited as long as it is nonionic, polar, and strongly solvating, and examples thereof include dimethyl sulfoxide, propylene carbonate, N,N-dimethylformamide, acetonitrile, ethylene carbonate, diethyl carbonate, and N-methylpyrrolidone. One or more aprotic polar organic solvents can be used. Among these, from the viewpoint of promoting the dissolution of the precious metal and improving the recovery rate of the precious metal, one or more selected from dimethyl sulfoxide, propylene carbonate, N,N-dimethylformamide, and N-methylpyrrolidone are preferred, and one or more selected from dimethyl sulfoxide and propylene carbonate are more preferred.

[0018] From the viewpoints of promoting dissolution of the precious metal and improving the extraction rate of the precious metal, the amount of the aprotic polar organic solvent used is preferably 1.0 times by mass or more, more preferably 1.1 times by mass or more, and even more preferably 1.2 times by mass or more relative to the mass of the chlorine bypass dust. Note that, since an excessive amount of the aprotic polar organic solvent used reduces production efficiency, the amount of the aprotic polar organic solvent used is preferably 1.5 times by mass or less, more preferably 1.4 times by mass or less, and even more preferably 1.3 times by mass or less relative to the mass of the chlorine bypass dust.

[0019] The mixed solution may contain one or more metal halides selected from copper halides and alkali metal halides in order to enhance the reduction reaction of copper in the chlorine bypass dust and promote the oxidation reaction of precious metals. The copper halides and alkali metal halides may be used alone or in combination, but it is preferable to use them in combination. Examples of copper halides include copper chloride (CuCl2) and copper bromide (CuBr2). One or more copper halides can be used. Examples of alkali metal halides include potassium chloride (KCl), potassium bromide (KBr), sodium chloride (NaCl), and sodium bromide (NaBr). One or more alkali metal halides can be used.

[0020] The content of the metal halide in the mixed solution is preferably 0.1 to 0.6 mol / L, more preferably 0.15 to 0.5 mol / L, and even more preferably 0.2 to 0.4 mol / L, from the viewpoint of enhancing the reduction reaction of copper and promoting the oxidation reaction of the noble metal.

[0021] When copper halide and alkali metal halide are used in combination as the metal halide, the molar ratio of copper halide to alkali metal halide (copper halide / alkali metal halide) is preferably 1 / 5 to 1 / 1, more preferably 1 / 3 to 5 / 6, and even more preferably 1 / 2 to 3 / 4, from the viewpoint of enhancing the reduction reaction of copper and promoting the oxidation reaction of noble metals. The total content of copper halide and alkali metal halide is as explained above.

[0022] The mixed liquid can be stirred using known mixing devices. For example, devices that can continuously charge and discharge, such as a device with a structure that rotates a horizontal cylindrical drum (a container with a scraping bar attached inside, a container with a structure that holds rods or balls to promote sludge breakup or separation of fine particles from coarse particles, or a shaft with protrusions or blades), as well as single- or double-shaft batch mixers used for mixing concrete may also be used. The chlorine bypass dust and the aprotic polar organic solvent may be fed into the apparatus in any order, or both may be fed into the apparatus simultaneously, and the order of mixing is not particularly limited. The raw materials may be fed into the apparatus intermittently or continuously.

[0023] The temperature of the mixed liquid is preferably 60 to 140°C, more preferably 85 to 130°C, and even more preferably 90 to 110°C, from the viewpoint of promoting dissolution of the precious metal and improving the extraction rate of the precious metal. The mixing time is preferably more than 0 hours and not more than 6 hours, more preferably 1 to 5 hours, and even more preferably 2 to 4 hours, from the viewpoint of promoting dissolution of the precious metal and improving the extraction rate of the precious metal.

[0024] [Second step] The second step is to filter the mixture obtained after the first step, thereby separating the filtrate rich in precious metals from the extraction residue. Examples of filtration include natural filtration, pressure filtration, reduced pressure filtration, and centrifugal filtration. Filtration can be performed alone or in combination of two or more methods. A single filtration method may be performed multiple times. Examples of filter media include filter cloth and screen.

[0025] Centrifugal filtration can be carried out using a centrifugal filter, and may be either continuous or batchwise. The centrifugal force used in centrifugal filtration can be selected appropriately, but is usually 200 to 2000 G, and from the viewpoint of production efficiency, 300 to 1500 G is preferred.

[0026] [Third step] The third step is to mix the filtrate separated in the second step with an aqueous inorganic acid solution, and then filter the mixture to recover the precipitate, which is enriched with precious metals, especially silver. Examples of inorganic acid aqueous solutions include aqueous solutions of hydrochloric acid (HCl), nitric acid (HNO), hydrofluoric acid (HF), hydrobromic acid (HBr), hydroiodic acid (HI), perchloric acid (HClO), iodic acid (HIO), and sulfuric acid (HSO). One or more of the inorganic acid aqueous solutions can be used. Among these, from the viewpoint of promoting the deposition of the precious metal, one or more selected from an aqueous hydrochloric acid solution and an aqueous sulfuric acid solution are preferred, and an aqueous hydrochloric acid solution is more preferred.

[0027] The amount of aqueous inorganic acid solution used can be selected appropriately depending on the type of acid, etc., but from the viewpoint of promoting the precipitation of precious metals, it is preferably an amount that results in a pH of the filtrate of 4 or less, more preferably an amount that results in a pH of 3.5 or less, and even more preferably an amount that results in a pH of 2 or less.

[0028] The filtrate and the aqueous inorganic acid solution can be mixed using, for example, a stirring device. The stirring device is not particularly limited as long as it is equipped with a stirring tank and stirring blades, and any known device can be used. For example, a single-shaft mixer or a twin-shaft mixer may be attached to the stirring tank. The size of the stirring tank can be selected appropriately depending on the production scale. The stirring blades are usually fixed to a rotating drive shaft. The shape of the stirring blades is not particularly limited, and can be selected appropriately from, for example, turbine blades, paddle blades, anchor blades, propeller blades, screw blades, helical ribbon blades, etc.

[0029] The time for mixing the filtrate and the aqueous inorganic acid solution can be selected appropriately. The mixing temperature may be room temperature (20°C ± 15°C) or may be heated.

[0030] After mixing, the mixture may be allowed to stand to promote the deposition of the precious metal. The standing may be performed, for example, by stopping the stirring and maintaining the mixture at room temperature. The standing time is preferably 3 to 48 hours, more preferably 12 to 36 hours, from the viewpoint of promoting the deposition of the precious metal and improving the recovery rate of the precious metal.

[0031] In this step, the mixed solution after stirring or the mixed solution after standing is filtered to recover the precipitate. The specific embodiment of the filtration is as described above. The collected material may be dried. The drying method is not particularly limited, and known heat drying methods can be used. The drying temperature is, for example, 80 to 300°C.

[0032] [Fourth step] The fourth step is to mix the filtrate separated in the third step with a reducing agent, and then filter the mixture to recover the precipitate, which is concentrated with precious metals, especially gold. The reducing agent is not particularly limited as long as it can reduce and precipitate the precious metal, and examples thereof include organic carboxylic acids such as citric acid, adipic acid, ascorbic acid, erythroascorbic acid, isoascorbic acid, erysorbic acid, and gallic acid, polyhydric phenols such as pyrogallol, catechol, and hydroquinone, organic amines such as aminohexanoic acid and hydrazine, sulfur-containing compounds such as cysteine ​​and thiourea, as well as sodium sulfite and ferrous sulfate. One or more reducing agents can be used. Among these, one or more selected from organic carboxylic acids, sodium sulfite, and ferrous sulfate are preferred because they have stable reactivity and can rapidly reduce precious metals, and one or more selected from ascorbic acid, sodium sulfite, and ferrous sulfate are more preferred.

[0033] From the viewpoint of promoting the deposition of precious metals and improving the recovery rate of precious metals, the amount of the reducing agent used is preferably 0.01 times by mass or more relative to the filtrate, more preferably 0.013 times by mass or more, and even more preferably 0.015 times by mass or more. The upper limit of the amount of the reducing agent used can be appropriately selected depending on the type of reducing agent, but is preferably 0.03 times by mass or less relative to the filtrate, and more preferably 0.02 times by mass or less.

[0034] The filtrate and the reducing agent can be mixed using a stirrer from the viewpoint of promoting the deposition of the precious metal and improving the recovery rate of the precious metal. Specific embodiments of the stirrer are as described above, but stirring may be performed in a sealed container or by placing the sealed container in a thermostatic shaker.

[0035] The time for mixing the filtrate and the reducing agent is preferably 0.1 to 3.0 hours, more preferably 0.5 to 2.5 hours, and even more preferably 1.0 to 2.0 hours, from the viewpoint of promoting the deposition of the precious metal and improving the recovery rate of the precious metal. The mixing temperature may be room temperature (20°C ± 15°C), but heating is preferred from the viewpoint of promoting the deposition of precious metals. The heating temperature is preferably 50 to 90°C, more preferably 60 to 80°C, from the viewpoint of promoting the deposition of precious metals and improving the recovery rate of precious metals.

[0036] After stirring, the mixture is preferably allowed to stand to promote the deposition of the precious metal. The standing may be performed by, for example, keeping the mixture in a state where stirring is stopped. The temperature for leaving the mixture at rest may be room temperature (20°C ± 15°C), but heating is preferred from the viewpoint of promoting the deposition of precious metals. The heating temperature is preferably 35 to 60°C, more preferably 40 to 50°C, from the viewpoint of promoting the deposition of precious metals and improving the recovery rate of precious metals. The standing time is preferably 50 to 100 hours, more preferably 60 to 90 hours, and even more preferably 70 to 80 hours, from the viewpoint of promoting the deposition of the precious metal and improving the recovery rate of the precious metal.

[0037] Next, the mixed solution after stirring or the mixed solution after standing is filtered to recover the precipitate. The specific embodiment of the filtration is as described above. The collected material may be dried. The drying method is not particularly limited, and known heat drying methods can be used. The drying temperature is, for example, 80 to 300°C.

[0038] The present invention has been described in detail above based on its embodiments. However, the present invention is not limited to the above embodiments. Various modifications of the present invention are possible without departing from the spirit and scope of the present invention. For example, since the extraction residue separated in the second step contains trace amounts of precious metals, the extraction residue may be mixed with chlorine bypass dust and reused as a raw material, as shown in Figure 1. Furthermore, the precipitate recovered in the third and fourth steps may be subjected to one or more physical separation processes selected from, for example, a classification process, an air separation process, a gravity separation process, and a magnetic separation process to further improve the purity of the precious metals. Each physical separation process can use equipment commonly used in the relevant technical field. [Example]

[0039] The following examples will explain the present invention in more detail, but the present invention is not limited to the examples below.

[0040] 1. Analysis of precious metals Aqua regia was added to the sample, which was then decomposed using a microwave sample pretreatment device (ETHOS EASY, Milestone) and used as the analytical sample. Quantitative analysis was performed using an ICP mass spectrometer (ICP-MS7700X, Agilent).

[0041] Example 1 Precious metals were recovered from chlorine bypass dust according to the flow shown in Figure 1. (Preparation of raw materials) Chlorine bypass dust and water at 20°C were placed in a centrifuge tube (volume 50 mL) in a mass ratio of 1:6 and manually stirred for 1 minute to prepare a slurry. Next, the slurry was centrifuged at 180 G for 180 seconds using a centrifuge (CN-2060, manufactured by AS ONE Corporation). After centrifugation, the precipitate and supernatant were separated, and the precipitate was collected as a water-washed chlorine bypass dust. (First step) Copper chloride (CuCl2) and sodium chloride were dissolved in 200 mL of dimethyl sulfoxide to a concentration of 0.2 mol / L each, and 200 g of washed chlorine bypass dust was added thereto and mixed with stirring at 120°C for 2 hours. (Second step) The mixture was filtered under reduced pressure to remove dissolved residue, and the filtrate was collected. (Third step) 200 mL of 1 M aqueous hydrochloric acid solution was added to the filtrate and mixed in. The mixture was then allowed to stand at 25° C. for 24 hours, and the precipitate was collected by filtration under reduced pressure. (Fourth step) To the filtrate separated in the third step, 3 g of ascorbic acid was added, and after stirring and mixing, the mixture was left to stand at 40°C for 72 hours, and the precipitate was collected by filtration under reduced pressure. The residue from the second step, the precipitate from the third step, and the precipitate from the fourth step were analyzed. The results are shown in Table 1.

[0042] Example 2 The precipitate was recovered in the same manner as in Example 1, except that in the first step, the mixture was stirred and mixed at 120°C for 4 hours. The residue from the second step, the precipitate from the third step, and the precipitate from the fourth step were analyzed. The results are shown in Table 1.

[0043] Example 3 The precipitate was recovered in the same manner as in Example 1, except that in the first step, the mixture was stirred and mixed at 100°C for 2 hours. The residue from the second step, the precipitate from the third step, and the precipitate from the fourth step were analyzed. The results are shown in Table 1.

[0044] [Table 1]

[0045] Table 1 shows that silver can be selectively recovered by heating and stirring a mixture containing chlorine bypass dust and an aprotic polar organic solvent, filtering the mixture, mixing the filtrate with an aqueous inorganic acid solution, and filtering again to recover the precipitate. Furthermore, gold can be selectively recovered by mixing the filtrate from which silver has been recovered with a reducing agent, filtering the mixture, and recovering the precipitate.

Claims

1. A first step of heating and stirring a mixed solution containing chlorine bypass dust containing gold, silver, and copper and dimethyl sulfoxide; a second step of filtering the mixture obtained after the first step; A third step of mixing the filtrate separated in the second step with an aqueous hydrochloric acid solution and filtering the mixture to recover a precipitate.

1. A method for recovering gold and silver from chlorine bypass dust, comprising:

2. 2. The method for recovering gold and silver according to claim 1, further comprising a fourth step, after the third step, of mixing the filtrate separated in the third step with a reducing agent and filtering the mixture to recover a precipitate.

3. 3. The method for recovering gold and silver according to claim 1, wherein in the first step, dimethyl sulfoxide is mixed in an amount of 1.0 times or more by mass relative to the mass of the chlorine bypass dust.

4. 4. The method for recovering gold and silver according to claim 1, wherein in the first step, the mixed solution is stirred at a temperature of 60 to 140°C.

5. The method for recovering gold and silver according to any one of claims 2 to 4, wherein in the fourth step, the filtrate is mixed with a reducing agent in an amount of 0.01 times or more by mass.

6. 6. The method for recovering gold and silver according to claim 2, wherein the reducing agent is one or more selected from the group consisting of ascorbic acid, sodium sulfite, and ferrous sulfate.

7. 7. The method for recovering gold and silver according to claim 1, wherein the mixed solution in the first step contains one or more metal halides selected from copper halide and alkali metal halides.

8. 8. The method for recovering gold and silver according to claim 7, wherein the content of the metal halide in the mixed solution in the first step is 0.1 to 0.6 mol / L.

9. The method for recovering gold and silver according to any one of claims 1 to 8, wherein the chlorine bypass dust is a water wash.

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