Method for recycling silver-containing waste to prepare 4n silver powder

Abstract

This invention discloses a method for preparing 4N silver powder from silver-containing waste, specifically: (1) acid leaching of silver-containing waste; (2) filtering the acid-leached silver-containing waste to obtain leachate and leachate residue; (3) adjusting the pH of the leachate to 4-5 to precipitate impurity metal ions in the solution; (4) filtering the precipitated solution to obtain filtrate and filter residue; (5) adding a silver precipitation agent to the filtrate to generate white silver chloride precipitate; (6) filtering the silver precipitation solution to obtain filtrate and high-purity AgCl powder; (7) adding a reducing agent to the high-purity AgCl powder to generate 4N silver powder. This invention effectively solves the problem that silver chloride precipitation easily coats or carries impurity metal ions, resulting in low purity of the subsequently prepared silver powder, through the impurity removal step. At the same time, the process is simple, has few steps, low cost, and adopts a fully wet recovery method, which is conducive to the industrialization of the process.

Description

Technical Field

[0001] This invention relates to the field of precious metal recycling technology, specifically to a method for recycling silver-containing waste to prepare 4N silver powder. Background Technology

[0002] Due to its excellent physicochemical properties, silver is mainly used in industry as an electrical contact material, composite material, welding material, photosensitive material, catalyst, and electroplating agent. However, with the increasing depletion of silver mine resources and the substantial increase in industrial silver-containing waste, two-thirds of the silver in currently commercially available silver-containing products originates from this waste. Therefore, recycling industrial silver-containing waste not only enables the reuse of secondary resources but also reduces the environmental pollution caused by silver-containing waste.

[0003] Currently, the main recycling process for silver-containing waste in the precious metals recycling industry involves pretreatment, leaching, precipitation or reduction to crude silver, and refining. During nitric acid leaching, silver is extracted from the silver-containing waste, but a large amount of impurity metal ions are also leached out. While the silver precipitation process separates the silver from these impurity ions, the formation of silver chloride can easily coat or trap these ions, resulting in low purity silver chloride and consequently, low purity silver powder. To improve the purity of silver chloride, repeated washing with dilute nitric acid or redissolution and precipitation is often used to reduce the impurity content. However, repeated acid washing or redissolution and precipitation not only complicates the process but also generates large amounts of wastewater, increasing the burden on wastewater treatment.

[0004] In conclusion, how to reduce the influence of impurity metal ions and improve the purity of silver powder is an urgent problem to be solved. Summary of the Invention

[0005] The purpose of this invention is to provide a method for recycling silver-containing waste to prepare 4N silver powder. By adjusting the pH of the leachate obtained after acid leaching, impurity metal ions are precipitated out, thereby achieving the purpose of impurity removal, reducing the influence of impurity metal ions, and improving the purity of silver powder. In addition, pH adjusters and functional microspheres are used in the impurity removal step to further improve the impurity removal effect.

[0006] The objective of this invention is achieved as follows:

[0007] A method for preparing 4N silver powder from silver-containing waste includes the following steps:

[0008] (1) Acid leaching: Silver-containing waste (mass fraction greater than 75%) is placed in a nitric acid solution with a concentration of 3.5-5 mol / L for acid leaching. The mass ratio of nitric acid solution to silver-containing waste is (3-5):1. The mixture is heated and stirred at 75-85℃ for 4-5 hours.

[0009] The main chemical reactions in this step are as follows:

[0010] Ag+H+ →Ag + ;

[0011] M+H 2+ →M 2+ (M represents metals such as Cu, Zn, Fe, and Ni).

[0012] (2) Filtration: Filter the silver-containing waste after acid leaching in step (1) to obtain leachate and leach residue.

[0013] (3) Removal of impurities: Stir the leachate obtained in step (2) evenly and slowly adjust the pH value of the solution to 4-5 so that the impurity metal ions in the solution precipitate.

[0014] The main chemical reactions in this step are as follows:

[0015] M 2+ +OH - →M(OH)2↓ (M represents metals such as Cu, Zn, Fe, Ni).

[0016] (4) Filtration: Filter the solution after precipitation in step (3) to obtain filtrate and filter residue. The filter residue is used to recover impurity metals.

[0017] (5) Silver precipitation: Stir the filtrate obtained in step (4) evenly for 30-45 minutes, add silver precipitation agent to generate white silver chloride precipitate. The silver precipitation agent is hydrochloric acid or hypochlorous acid, and the amount of silver precipitation agent is 1.1-1.3 times the stoichiometric ratio.

[0018] The main chemical reactions in this step are as follows:

[0019] Ag + +Cl - →AgCl↓;

[0020] Or 2Ag + +2ClO - →AgCl↓+O2↑.

[0021] (6) Filtration: Filter the solution after silver precipitation in step (5) to obtain filtrate and AgCl powder. The filtrate is used to recover impurity metals.

[0022] (7) Reduction: Deionized water is added to the AgCl powder obtained in step (6) to form a slurry. The mixture is stirred evenly and a reducing agent is slowly added to reduce the powder to 4N silver powder. The reducing agent is hydrazine hydrate, and the amount of reducing agent is 1.1 to 1.2 times the stoichiometric ratio.

[0023] The main chemical reactions in this step are as follows:

[0024] 2AgCl + 2N2H4·H2O → 2Ag + 2NH4 + +2Cl- +N2↑+H2O.

[0025] In step (3), the pH value of the solution can be adjusted using ammonia water with a mass fraction of 5-10%. In this invention, an improvement has been made by using a pH adjuster to slowly adjust the pH value of the solution, thus avoiding the impact of excessively rapid precipitation of impurity metals.

[0026] The preparation method of the pH adjuster includes the following steps:

[0027] S1. Mix guar gum and pyridine evenly, heat and stir at 60-90℃ for 10-30 min, then add stearoyl chloride dropwise, react at 60-80℃ for 4-6 h, precipitate with 95% ethanol after the reaction, filter the precipitate and wash with (95% ethanol) to obtain guar gum stearate.

[0028] S2. Mix guar stearate with water until homogeneous, then add borax and stir at 70-90℃ for 3-5 hours to obtain cross-linked guar gum.

[0029] S3. Soak the obtained cross-linked guar gum in sodium hydroxide solution for 1-3 hours, then sonicate it in hot alcohol (40-50℃ ethanol) for 20-60 minutes, then remove it and evaporate it by rotary evaporation and freeze-drying to obtain the treated cross-linked guar gum.

[0030] S4. The treated cross-linked guar gum is placed in an alkaline solution to swell for more than 24 hours, and then rotary evaporated and freeze-dried to obtain the pH adjuster.

[0031] Further, in step S1, the mass ratio of guar gum, pyridine, and stearoyl chloride is 1:(2-5):(0.2-0.4); in step S2, the mass ratio of guar gum stearate to water and borax is 1:(8-20):(1-2).

[0032] Further, in step S3, the concentration of the sodium hydroxide solution is 10-30%, and the amount added is 5-10 times the mass of the cross-linked guar gum; in step S4, the alkaline solution is an aqueous solution containing 5-20% ammonia, an aqueous solution of sodium carbonate with a mass concentration of 10-30%, or a sodium hydroxide solution with a mass concentration of 5-20%, and the mass ratio of the treated cross-linked guar gum to the alkaline solution is 1:(0.6-1.2).

[0033] The present invention also provides a functional microsphere for the method of preparing 4N silver powder from silver-containing waste recycling, wherein in step (3), when the pH value of the solution is adjusted to the point where precipitation begins to occur, functional microspheres are added. The preparation method of the functional microsphere includes the following steps:

[0034] A. Mix β-cyclodextrin with potassium hydroxide solution (mass concentration 30-40%) until homogeneous, and add 2,3-epoxypropyltrimethylammonium chloride aqueous solution dropwise under stirring at 70-90℃, and react for 40-80 min;

[0035] B. Cool the reaction solution from step A to room temperature, and continue to add epichlorohydrin dropwise under stirring. After reacting for 1-2 hours, add kerosene containing emulsifier and stir for 8-10 minutes (800-1500 r / min). Then, heat to 55-65℃ and stir (100-300 r / min) for 6-10 hours. After the reaction is complete, filter the product and wash it with dilute hydrochloric acid, methanol, distilled water and acetone. The obtained solid is dried under vacuum to obtain the first microsphere.

[0036] C. Mix the first microsphere with (5-10 times the amount) dimethylformamide evenly, add the mixture of maleic anhydride and dimethylformamide under stirring at 30-40℃, and after the addition is complete, raise the temperature to 80-90℃ and continue the reaction for 10-16 hours. The product obtained is washed with acetone and water in sequence, and dried under vacuum to obtain the second microsphere.

[0037] D. Immerse the second microsphere in sodium alginate aqueous solution for more than 2 hours, then evaporate by rotary evaporation and dry to obtain the third microsphere;

[0038] E. Mix sodium caseinate, gelatin, and water to prepare a material liquid, spray the material liquid onto the surface of the third microsphere, and dry to obtain the functional microsphere.

[0039] Further, in step A, the mass ratio of β-cyclodextrin, potassium hydroxide, and 2,3-epoxypropyltrimethylammonium chloride is 1:(0.8-1.4):(1-3), and the mass concentration of the aqueous solution of 2,3-epoxypropyltrimethylammonium chloride is 5-10%; in step B, the amount of epichlorohydrin added is 1.5-2.5 times the mass of β-cyclodextrin, the emulsifier is Tween 20, the mass ratio of emulsifier to kerosene is (0.03-0.05):1, and the amount of kerosene added is 6-10 times the mass of β-cyclodextrin.

[0040] Further, in step C, the mass ratio of maleic anhydride to dimethylformamide is 1:(3-6), and the amount of maleic anhydride added is 1-2 times the mass of the first microsphere; in step D, the mass ratio of sodium alginate to the second microsphere is (0.06-0.16):1, and the mass concentration of the sodium alginate aqueous solution is 3-5%; in step E, the mass ratio of sodium caseinate, gelatin, and water is (0.08-0.16):(1-1.4):1, and the amount of material solution sprayed is 4-8% of the mass of the third microsphere.

[0041] Furthermore, the filtrate obtained from the filtration in step (4) is treated as follows before undergoing the silver precipitation step:

[0042] Under stirring conditions, add nitric acid solution to the filtrate to adjust the pH to 2-3, and continue stirring (30-60 min). After filtration, the treated solution can be used for the silver precipitation step (5).

[0043] When functional microspheres are used in the method for preparing 4N silver powder from silver-containing waste, in step S2 of the pH adjuster preparation method, cationicly treated pumice powder is added when guar stearate is mixed with water. The cationic treatment method is as follows: the pumice powder is immersed in an aqueous solution of a cationic surfactant for 30-60 minutes, and then the water is evaporated. The mass ratio of pumice powder, cationic surfactant, and guar stearate is (0.1-0.4):(0.003-0.006):1, and the cationic surfactant is a quaternary ammonium salt cationic surfactant.

[0044] The beneficial effects of this invention are:

[0045] 1. This invention adjusts the pH of the leached silver nitrate solution to precipitate impurity metal ions, thereby increasing the purity of the silver nitrate solution after impurity removal; it avoids the repeated dilute nitric acid washing or redissolution precipitation process of silver chloride, and shortens the cycle for recycling and preparing 4N silver powder.

[0046] 2. In the silver precipitation stage, the present invention uses hydrochloric acid or hypochlorous acid to precipitate silver, ensuring the acidity of the system, reducing the phenomenon of impurity metal ions being coated or entrained by the generated fine silver chloride, improving the purity of silver chloride, and the impurity metal content is less than 50 ppm.

[0047] 3. The entire process of this invention is a wet recycling method for silver-containing waste, which is conducive to the industrialization of the process.

[0048] 4. Because the pH value changes rapidly during the precipitation of impurity metals using ordinary alkaline solutions, excessively rapid precipitation can lead to incomplete reactions and the entrainment of silver ions, resulting in silver loss and reduced yield. Therefore, this invention employs a pH adjuster made by adsorbing alkaline solutions with cross-linked guar gum. By controlling the release of the alkaline solution, the pH value of the system is slowly adjusted, allowing the precipitation of impurity metal ions to proceed gradually, reducing the entrainment of silver ions and improving the yield. Due to this slow-release effect, the alkaline solution of this invention can be used with a wide range of weak or strong bases.

[0049] 5. In preparing the pH adjuster, in order to improve the adsorption effect of cross-linked guar gum, the present invention first esterifies the guar gum and then cross-links it. Then, the esterified part is hydrolyzed under alkaline conditions, and the free hydrolyzed compounds are washed away with hot alcohol. The cross-linked guar gum obtained by the treatment contains a large number of pores, which improves the adsorption performance.

[0050] 6. Since some of the impurity metal precipitates in this invention are flocculent, they are prone to aggregation, which affects the continuation of the precipitation reaction and can easily carry silver ions. Therefore, this invention also adds a functional microsphere when the impurity metal precipitate begins to form during the impurity removal step. The microsphere is made by combining cyclodextrin microspheres with sodium alginate and then coating the surface with a membrane material containing sodium caseinate. When used in the leaching process, the surface membrane material will slowly release sodium caseinate. Sodium caseinate can combine with the impurity metal precipitate in the leaching solution to promote sedimentation, increase the sedimentation rate, and reduce the impact of the impurity metal precipitate on the system. The cyclodextrin microspheres themselves can also promote the sedimentation of the metal precipitate.

[0051] 7. In the functional microspheres of the present invention, after the membrane material is consumed, sodium alginate inside can be slowly released. Sodium alginate can adsorb some impurity metals to promote the removal of the remaining impurity metals, thereby further improving the purity of silver.

[0052] 8. The functional microspheres of this invention use cyclodextrin microspheres as a carrier. A cationic compound is first introduced into the microspheres, and then cross-linked on the surface to form microspheres. This promotes the binding of the cyclodextrin microspheres with sodium alginate (whose solution contains free -COO-). Furthermore, the surface of the cyclodextrin microspheres is acylated with maleic anhydride, resulting in microspheres with a large number of carboxyl groups on their surface. This enhances the pH sensitivity of the cyclodextrin microspheres and regulates the release of sodium alginate.

[0053] 9. Since sodium alginate or sodium caseinate may remain in the filtrate after the functional microspheres are processed, and the present invention uses a strong acid for silver precipitation, sodium alginate or sodium caseinate is easily converted into precipitate during the silver precipitation stage and mixes with silver chloride, affecting the silver precipitation step. Therefore, the present invention adjusts the pH of the filtrate to 2-3 with nitric acid before silver precipitation, so that sodium alginate or sodium caseinate can be converted into insoluble matter and filtered out, thereby improving the purity of silver.

[0054] 10. In order to prevent substances such as sodium caseinate released from the functional microspheres from adsorbing and affecting the release of alkaline solution in the pH adjuster, the present invention also introduces cationic pumice powder into the cross-linked guar gum of the pH adjuster, so that the pH adjuster can be suspended above and reduce contact with the functional microspheres; at the same time, after the pumice powder is cationically treated, it also reduces the adsorption of metal ions by the pH adjuster. Attached Figure Description

[0055] Figure 1 This is a process flow diagram of the present invention for preparing 4N silver powder from silver-containing waste;

[0056] Figure 2 It refers to the percentage of impurity metal precipitation during the impurity removal process in the process of preparing 4N silver powder from silver-containing waste in this invention. Implementation

[0057] The present invention will be further described below with reference to the embodiments and accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. It should be noted that the accompanying drawings of the present invention are all simplified and non-precise scales, used only for convenient and clear illustration of the present invention.

[0058] Example 1

[0059] This embodiment provides a method for preparing 4N silver powder from silver-containing waste, including the following steps:

[0060] (1) Acid leaching: Take 10 kg of silver-containing waste (mass fraction greater than 75%) and place it in a reaction vessel with a nitric acid solution of 3.5 mol / L for acid leaching. The liquid-solid ratio is 5:1, and the mixture is stirred at 75℃ for 5 h.

[0061] (2) Filtration: The silver-containing waste after acid leaching in step (1) is filtered to obtain leachate and leachate residue. The leachate residue is used for the recovery of precious metals such as Au, Pd, and Pt.

[0062] (3) Removal of impurities: Stir the leachate obtained in step (2) evenly, and slowly add 10% ammonia water to adjust the pH value of the solution to 4, so that the impurity metal ions in the solution precipitate.

[0063] (4) Filtration: Filter the solution after precipitation in step (3) to obtain filtrate and filter residue. The filter residue is used for the recovery of impurity metals such as Cu, Zn, Fe, and Ni.

[0064] (5) Silver precipitation: Stir the filtrate obtained in step (4) evenly for 30 minutes, add hydrochloric acid with a stoichiometric ratio of 1.1 times, and a white silver chloride precipitate will be generated.

[0065] (6) Filtration: Filter the solution after silver precipitation in step (5) to obtain filtrate and AgCl powder. The filtrate is used for the recovery of impurity metals such as Cu, Zn, Fe, and Ni.

[0066] (7) Reduction: Add deionized water to the AgCl powder obtained in step (6) to form a slurry, stir evenly and slowly add hydrazine hydrate with a stoichiometric ratio of 1.1 times while stirring, and reduce to generate 4N silver powder.

[0067] Example 2

[0068] Based on Example 1, this example provides a method for preparing 4N silver powder by recycling silver-containing waste. In step (3) during the impurity removal process, a pH adjuster is used to slowly adjust the pH value of the solution to avoid the impact of impurity metal precipitation being too fast.

[0069] The preparation method of the pH adjuster includes the following steps:

[0070] S1. Mix guar gum and pyridine evenly, heat and stir at 60°C for 30 min, then add stearoyl chloride dropwise, react at 60°C for 6 h, precipitate with 95% ethanol after the reaction, filter the precipitate and wash with (95% ethanol) to obtain guar gum stearate; the mass ratio of guar gum, pyridine and stearoyl chloride is 1:2:0.2.

[0071] S2. Mix guar stearate with water until homogeneous, then add borax and stir at 70°C for 5 hours to obtain cross-linked guar gum; the mass ratio of guar stearate to water and borax is 1:8:1.

[0072] S3. Soak the obtained cross-linked guar gum in 5 times the amount of 30% sodium hydroxide solution for 1 hour, then sonicate it in hot alcohol (40℃ ethanol) for 20 minutes, then remove it, evaporate it by rotary evaporation, and freeze dry it to obtain the treated cross-linked guar gum.

[0073] S4. The treated cross-linked guar gum is placed in 0.6 times the amount of alkaline solution (an aqueous solution containing 10% ammonia) to swell for more than 24 hours, and then rotary evaporated and freeze-dried to obtain the pH adjuster.

[0074] The rest is the same as in Example 1.

[0075] Example 3

[0076] This embodiment provides a method for preparing 4N silver powder from silver-containing waste. In step (3), when the pH of the solution is adjusted to the point where precipitation begins to occur, functional microspheres are added. The preparation method of the functional microspheres includes the following steps:

[0077] A. Mix β-cyclodextrin with potassium hydroxide solution (30% by mass) until homogeneous, and add 2,3-epoxypropyltrimethylammonium chloride aqueous solution (5% by mass) dropwise under stirring at 70°C. React for 80 min. The mass ratio of β-cyclodextrin, potassium hydroxide, and 2,3-epoxypropyltrimethylammonium chloride is 1:0.8:1.

[0078] B. Cool the reaction solution from step A to room temperature, and continue to add epichlorohydrin dropwise under stirring. After reacting for 1 hour, add kerosene containing Tween 20, stir at 800 rpm for 10 minutes, then heat to 55°C and stir (100 rpm) for 10 hours. After the reaction is complete, filter the product and wash it with dilute hydrochloric acid, methanol, distilled water and acetone. The obtained solid is dried under vacuum to obtain the first microsphere. The amount of epichlorohydrin added is 1.5 times the mass of β-cyclodextrin, the mass ratio of Tween 20 to kerosene is 0.03:1, and the amount of kerosene added is 6 times the mass of β-cyclodextrin.

[0079] C. Mix the first microsphere with (5 times the amount) dimethylformamide evenly, and add a mixture of maleic anhydride and dimethylformamide in a mass ratio of 1:3 under stirring at 30°C. After the mixture is added, raise the temperature to 80°C and continue the reaction for 16 hours. Wash the product with acetone and water in sequence, and dry it under vacuum to obtain the second microsphere. The amount of maleic anhydride added is 1 times the mass of the first microsphere.

[0080] D. Immerse the second microsphere in an aqueous solution of sodium alginate (mass concentration of 3%) for more than 2 hours, then evaporate and dry to obtain the third microsphere; the mass ratio of sodium alginate to the second microsphere is 0.06:1.

[0081] E. Sodium caseinate, gelatin, and water are mixed in a mass ratio of 0.08:1:1 to prepare a material liquid. The material liquid is sprayed onto the surface of the third microsphere, with the spraying amount being 4% of the mass of the third microsphere. After spraying, it is dried to obtain the functional microsphere.

[0082] In this embodiment, the filtrate obtained from step (4) of the method for preparing 4N silver powder from silver-containing waste is treated as follows before undergoing the silver precipitation step:

[0083] Under stirring conditions, add nitric acid solution (concentration of 3.5 mol / L) to the filtrate to adjust the pH to 3, and continue stirring for 30 min. After filtration, the treated solution can be used for the silver precipitation step (5).

[0084] The rest is the same as in Example 1.

[0085] Example 4

[0086] This embodiment is a combination of embodiments 2 and 3.

[0087] Example 5

[0088] Based on Example 4, this example provides a method for preparing 4N silver powder from silver-containing waste, wherein step S2 of the pH adjuster preparation method is as follows:

[0089] Guar stearate was mixed evenly with water and cationic pumice powder, and then borax was added. The mixture was stirred at 70°C for 5 hours to obtain cross-linked guar gum. The mass ratio of guar stearate to water and borax was 1:8:1.

[0090] The cationic treatment method for pumice powder is as follows: the pumice powder is immersed in the aqueous solution of the quaternary ammonium salt cationic surfactant for 30 minutes, and then the water is evaporated. The mass ratio of pumice powder, quaternary ammonium salt cationic surfactant and guar stearate is 0.1:0.003:1.

[0091] The rest is the same as in Example 4.

[0092] Example 6

[0093] This embodiment provides a method for preparing 4N silver powder from silver-containing waste, including the following steps:

[0094] (1) Acid leaching: Take 10 kg of silver-containing waste and place it in a reaction vessel with a nitric acid solution of 4 mol / L for acid leaching. The liquid-solid ratio is 4:1. Heat and stir at 80℃ for 4.5 h.

[0095] (2) Filtration: The silver-containing waste after acid leaching in step (1) is filtered to obtain leachate and leachate residue. The leachate residue is used for the recovery of precious metals such as Au, Pd, and Pt.

[0096] (3) Removal of impurities: Stir the leachate obtained in step (2) evenly, and slowly adjust the pH value of the solution to 4.5 using a pH adjuster to precipitate the impurity metal ions in the solution.

[0097] (4) Filtration: Filter the solution after precipitation in step (3) to obtain filtrate and filter residue. The filter residue is used for the recovery of impurity metals such as Cu, Zn, Fe, and Ni. Under stirring conditions, add nitric acid solution (concentration of 4 mol / L) to the filtrate to adjust the pH to 2.5, and continue stirring for 45 min. After filtration, the treated solution is obtained.

[0098] (5) Silver precipitation: Stir the treatment solution obtained in step (4) evenly for 35 minutes, add hydrochloric acid with a stoichiometric ratio of 1.2 times, and a white silver chloride precipitate is generated.

[0099] (6) Filtration: Filter the solution after silver precipitation in step (5) to obtain filtrate and AgCl powder. The filtrate is used for the recovery of impurity metals such as Cu, Zn, Fe, and Ni.

[0100] (7) Reduction: Add deionized water to the AgCl powder obtained in step (6) to form a slurry, stir evenly and slowly add hydrazine hydrate with a stoichiometric ratio of 1.15 times to reduce and generate 4N silver powder.

[0101] The preparation method of the pH adjuster includes the following steps:

[0102] S1. Mix guar gum and pyridine evenly, heat and stir at 75°C for 20 min, then add stearoyl chloride dropwise, react at 70°C for 5 h, precipitate with 95% ethanol after the reaction, filter the precipitate and wash with (95% ethanol) to obtain guar gum stearate; the mass ratio of guar gum, pyridine and stearoyl chloride is 1:3.5:0.3.

[0103] S2. Guar stearate is mixed evenly with water and cationic pumice powder, then borax is added, and the mixture is stirred at 80°C for 4 hours to obtain cross-linked guar gum; the mass ratio of guar stearate to water and borax is 1:14:1.5.

[0104] The cationic treatment method for pumice powder is as follows: immerse the pumice powder in an aqueous solution of quaternary ammonium salt cationic surfactant for 45 minutes, and then evaporate to remove moisture. The mass ratio of pumice powder, quaternary ammonium salt cationic surfactant, and guar stearate is 0.25:0.0045:1.

[0105] S3. Soak the obtained cross-linked guar gum in 7.5 times the amount of 20% sodium hydroxide solution for 2 hours, then sonicate it in hot alcohol (45℃ ethanol) for 40 minutes, then remove it, evaporate it by rotary evaporation, and freeze dry it to obtain the treated cross-linked guar gum.

[0106] S4. The treated cross-linked guar gum is placed in 0.9 times the amount of alkaline solution (20% sodium carbonate aqueous solution) to swell for more than 24 hours, and then rotary evaporated and freeze-dried to obtain the pH adjuster.

[0107] In step (3) of this embodiment, when the pH value of the solution is adjusted to the point where precipitation begins to occur, functional microspheres are also added. The preparation method of the functional microspheres includes the following steps:

[0108] A. Mix β-cyclodextrin with potassium hydroxide solution (35% by mass) until homogeneous, and add 2,3-epoxypropyltrimethylammonium chloride aqueous solution (7.5% by mass) dropwise under stirring at 80°C. React for 60 min. The mass ratio of β-cyclodextrin, potassium hydroxide, and 2,3-epoxypropyltrimethylammonium chloride is 1:1.1:2.

[0109] B. Cool the reaction solution from step A to room temperature, and continue to add epichlorohydrin dropwise under stirring. After reacting for 1.5 hours, add kerosene containing Tween 20, stir at 1200 rpm for 9 minutes, then heat to 60°C and stir at 200 rpm for 8 hours. After the reaction is complete, filter the product and wash it with dilute hydrochloric acid, methanol, distilled water and acetone. The obtained solid is dried under vacuum to obtain the first microsphere. The amount of epichlorohydrin added is twice the mass of β-cyclodextrin, the mass ratio of Tween 20 to kerosene is 0.04:1, and the amount of kerosene added is eight times the mass of β-cyclodextrin.

[0110] C. Mix the first microspheres with (8 times the amount) dimethylformamide evenly, and add a mixture of maleic anhydride and dimethylformamide in a mass ratio of 1:4.5 under stirring at 35°C. After the mixture is added, raise the temperature to 85°C and continue the reaction for 13 hours. Wash the product with acetone and water in sequence, and dry it under vacuum to obtain the second microspheres. The amount of maleic anhydride added is 1.5 times the mass of the first microspheres.

[0111] D. Immerse the second microsphere in an aqueous solution of sodium alginate (4% by mass) for more than 2 hours, then evaporate by rotary evaporation and dry to obtain the third microsphere; the mass ratio of sodium alginate to the second microsphere is 0.1:1.

[0112] E. Sodium caseinate, gelatin, and water are mixed in a mass ratio of 0.12:1.2:1 to prepare a material solution. The material solution is sprayed onto the surface of the third microsphere, with the amount sprayed being 6% of the mass of the third microsphere. After spraying, the microsphere is dried to obtain the functional microsphere.

[0113] The rest is the same as in Example 5.

[0114] Example 7

[0115] This embodiment provides a method for preparing 4N silver powder from silver-containing waste, including the following steps:

[0116] (1) Acid leaching: Take 10 kg of silver-containing waste and place it in a reaction vessel with a nitric acid solution of 5 mol / L for acid leaching. The solid-liquid ratio is 3:1, and the mixture is stirred at 85℃ for 4 hours.

[0117] (2) Filtration: The silver-containing waste after acid leaching in step (1) is filtered to obtain leachate and leachate residue. The leachate residue is used for the recovery of precious metals such as Au, Pd, and Pt.

[0118] (3) Removal of impurities: Stir the leachate obtained in step (2) evenly, and slowly adjust the pH value of the solution to 5 using a pH adjuster to precipitate the impurity metal ions in the solution.

[0119] (4) Filtration: Filter the solution after precipitation in step (3) to obtain filtrate and filter residue. The filter residue is used for the recovery of impurity metals such as Cu, Zn, Fe, and Ni. Under stirring conditions, add nitric acid solution (concentration of 5 mol / L) to the filtrate to adjust the pH to 2, and continue stirring for 60 min. After filtration, the treated solution is obtained.

[0120] (5) Silver precipitation: Stir the treatment solution obtained in step (4) evenly for 45 minutes, add hypochlorous acid with a stoichiometric ratio of 1.1 times, and a white silver chloride precipitate is generated.

[0121] (6) Filtration: Filter the solution after silver precipitation in step (5) to obtain filtrate and AgCl powder. The filtrate is used for the recovery of impurity metals such as Cu, Zn, Fe, and Ni.

[0122] (7) Reduction: Add deionized water to the AgCl powder obtained in step (6) to form a slurry, stir evenly and slowly add hydrazine hydrate with a stoichiometric ratio of 1.2 times to reduce and generate 4N silver powder.

[0123] The preparation method of the pH adjuster includes the following steps:

[0124] S1. Mix guar gum and pyridine evenly, heat and stir at 90°C for 10 min, then add stearoyl chloride dropwise, react at 80°C for 4 h, precipitate with 95% ethanol after the reaction, filter the precipitate and wash with (95% ethanol) to obtain guar gum stearate; the mass ratio of guar gum, pyridine and stearoyl chloride is 1:5:0.4.

[0125] S2. Guar stearate is mixed evenly with water and cationic pumice powder, then borax is added, and the mixture is stirred at 90°C for 3 hours to obtain cross-linked guar gum; the mass ratio of guar stearate to water and borax is 1:20:2.

[0126] The cationic treatment method for pumice powder is as follows: Pumice powder is immersed in an aqueous solution of quaternary ammonium salt cationic surfactant for 60 minutes, and then the water is evaporated. The mass ratio of pumice powder, quaternary ammonium salt cationic surfactant and guar stearate is 0.4:0.006:1.

[0127] S3. Soak the obtained cross-linked guar gum in 10 times the amount of 10% sodium hydroxide solution for 1-3 hours, then sonicate it in hot alcohol (50℃ ethanol) for 60 minutes, then remove it, evaporate it by rotary evaporation, and freeze dry it to obtain the treated cross-linked guar gum.

[0128] S4. The treated cross-linked guar gum is placed in 1.2 times the amount of alkaline solution (10% sodium hydroxide solution) to swell for more than 24 hours, and then rotary evaporated and freeze-dried to obtain the pH adjuster.

[0129] In step (3) of this embodiment, when the pH value of the solution is adjusted to the point where precipitation begins to occur, functional microspheres are also added. The preparation method of the functional microspheres includes the following steps:

[0130] A. Mix β-cyclodextrin with potassium hydroxide solution (40% by mass) until homogeneous, and add 2,3-epoxypropyltrimethylammonium chloride aqueous solution (10% by mass) dropwise under stirring at 90℃. React for 40 min. The mass ratio of β-cyclodextrin, potassium hydroxide and 2,3-epoxypropyltrimethylammonium chloride is 1:1.4:3.

[0131] B. Cool the reaction solution from step A to room temperature, and continue to add epichlorohydrin dropwise under stirring. After reacting for 2 hours, add kerosene containing Tween 20, stir at 1500 r / min for 8 minutes, then heat to 65℃ and stir (300 r / min) for 6 hours. After the reaction is complete, filter the product and wash it with dilute hydrochloric acid, methanol, distilled water and acetone. The obtained solid is dried under vacuum to obtain the first microsphere. The amount of epichlorohydrin added is 2.5 times the mass of β-cyclodextrin, the mass ratio of Tween 20 to kerosene is 0.05:1, and the amount of kerosene added is 10 times the mass of β-cyclodextrin.

[0132] C. Mix the first microsphere with (10 times the amount) dimethylformamide evenly, and add a mixture of maleic anhydride and dimethylformamide in a mass ratio of 1:6 under stirring at 40°C. After the mixture is added, raise the temperature to 90°C and continue the reaction for 10 hours. Wash the product with acetone and water in sequence, and dry it under vacuum to obtain the second microsphere. The amount of maleic anhydride added is twice the mass of the first microsphere.

[0133] D. Immerse the second microsphere in an aqueous solution of sodium alginate (mass concentration of 5%) for more than 2 hours, then evaporate and dry to obtain the third microsphere; the mass ratio of sodium alginate to the second microsphere is 0.16:1.

[0134] E. Sodium caseinate, gelatin, and water are mixed in a mass ratio of 0.16:1.4:1 to prepare a material solution. The material solution is sprayed onto the surface of the third microsphere, with the amount sprayed being 8% of the mass of the third microsphere. After spraying, the microsphere is dried to obtain the functional microsphere.

[0135] The rest is the same as in Example 5.

[0136] Comparative Example 1

[0137] The difference between this comparative example and Example 1 is that the method for preparing 4N silver from silver-containing waste does not include step (3) impurity removal and step (4) filtration.

[0138] Comparative Example 2

[0139] The difference between this comparative example and Example 1 is that the pH value of the solution is adjusted to 3.5 in step (3).

[0140] Comparative Example 3

[0141] The difference between this comparative example and Example 1 is that the pH value of the solution is adjusted to 5.5 in step (3).

[0142] Comparative Example 4

[0143] The difference between this comparative example and Example 5 is that the preparation method of the pH adjuster does not include steps S1 and S3, that is, the esterification of guar gum and subsequent alkaline hydrolysis are not performed. In this case, the starting material guar gum stearate in step S2 is replaced with guar gum.

[0144] Comparative Example 5

[0145] The difference between this comparative example and Example 5 is that the preparation method of the pH adjuster does not include step S3.

[0146] Comparative Example 6

[0147] The difference between this comparative example and Example 5 is that the pumice powder is not subjected to cation treatment in step S2 of the pH adjuster preparation method.

[0148] Comparative Example 7

[0149] The difference between this comparative example and Example 6 is that 2,3-epoxypropyltrimethylammonium chloride is not added to step A of the preparation method of the functional microspheres. In this case, step A is: β-cyclodextrin and potassium hydroxide solution are mixed evenly.

[0150] Comparative Example 8

[0151] The difference between this comparative example and Example 6 is that the preparation method of the functional microspheres does not include step D.

[0152] Comparative Example 9

[0153] The difference between this comparative example and Example 6 is that the preparation method of the functional microspheres does not include step E.

[0154] Comparative Example 10

[0155] The difference between this comparative example and Example 6 is that, in step E of the preparation method of the functional microspheres, the material solution does not contain sodium caseinate.

[0156] Comparative Example 11

[0157] The difference between this comparative example and Example 7 is that in the method of preparing 4N silver by recycling silver-containing waste, step (4) is not adjusted to pH 2 by adding nitric acid after filtration. At this time, step (4) is: filtering the solution after precipitation in step (3) to obtain filtrate and filter residue. The filter residue is used for the recovery of impurity metals such as Cu, Zn, Fe, and Ni.

[0158] I. Purity of silver chloride and silver powder obtained by this invention

[0159] Table 1

[0160]

[0161]

[0162] As can be seen from Table 1, after the impurity removal process of the present invention, the purity of the silver powder obtained in Examples 1 to 7 of the present invention is all above 99.990%, and the silver powder with a purity greater than 4N can be obtained in the end.

[0163] Building upon Example 1, Example 2 used a pH adjuster to slowly adjust the pH of the leachate, causing impurity metals to precipitate slowly, reducing the entrainment of silver ions, thus improving the purity and yield of the silver powder. Building upon Example 1, Example 3 used functional microspheres to promote the sedimentation of impurity metals, ultimately improving both the purity and yield of the silver powder. In Example 4, when the pH adjuster and functional microspheres were used in combination, both precipitation and sedimentation of impurity metals were promoted. However, because substances such as sodium caseinate released from the functional microspheres affected the release of alkali from the pH adjuster, the purity and yield of the silver powder were not significantly improved. Building upon Example 4, in Example 5, cationic pumice powder was introduced into the cross-linked guar gum during the preparation of the pH adjuster, ultimately resulting in a significant improvement in the purity and yield of the silver powder.

[0164] Compared with Example 1, Comparative Example 1 did not use the impurity removal process, while Comparative Examples 2 and 3 adjusted the pH value in the impurity removal step to be outside the range of the present invention. As a result, the purity of the silver powder obtained was significantly reduced and the yield also decreased.

[0165] Comparative Examples 4-6 changed the preparation steps of the pH adjuster, Comparative Examples 7-10 changed the preparation steps of the functional microspheres, and Comparative Example 11 changed the filtration step (4). As a result, the purity and yield of silver powder decreased to varying degrees.

[0166] II. Sedimentation effect of the functional microspheres of the present invention on impurity metal precipitates

[0167] In the impurity removal process of step (3) of the method for preparing 4N silver powder from silver-containing waste of the present invention, the height position of the impurity metal precipitate in the leachate is monitored, and the percentage of the maximum precipitate height is calculated according to the following formula: Precipitation height percentage = (maximum height of metal precipitate / total height of leachate) × 100%. The results are shown in the appendix. Figure 2 .

[0168] It can be seen that, compared with Example 1, the use of functional microspheres in Examples 3, 5, 6, and 7 of the present invention significantly improved the sedimentation effect of impurity metals. Compared with Example 6, Comparative Examples 7-10 changed the preparation steps of the functional microspheres, and the sedimentation effect also deteriorated, demonstrating the specific effect of the functional microspheres of the present invention.

[0169] It should be noted that the above description is only a specific embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for recovering silver from silver-containing scrap to produce 4N silver powder, characterized by: Includes the following steps: (1) Acid leaching: The silver-containing waste is placed in a nitric acid solution for acid leaching, and heated and stirred; (2) Filtration: Filter the silver-containing waste after acid leaching in step (1) to obtain leachate and leachate residue; (3) Removal of impurities: Stir the leachate obtained in step (2) evenly and slowly adjust the pH value of the solution to 4-5 so that the impurity metal ions in the solution precipitate. (4) Filtration: Filter the solution after precipitation in step (3) to obtain filtrate and filter residue; (5) Silver precipitation: Stir the filtrate obtained in step (4) evenly, add silver precipitation agent, and generate white silver chloride precipitate; (6) Filtration: Filter the solution after silver precipitation in step (5) to obtain filtrate and AgCl powder; (7) Reduction: Add deionized water to the AgCl powder obtained in step (6) to form a slurry, stir evenly and slowly add reducing agent to reduce to generate 4N silver powder; In step (3), a pH adjuster is used to slowly adjust the pH value of the solution. The preparation method of the pH adjuster includes the following steps: S1. Mix guar gum and pyridine evenly, heat and stir for 10-30 minutes, then add stearoyl chloride dropwise, react at 60-80℃ for 4-6 hours, precipitate with 95% ethanol after the reaction, filter and wash the precipitate to obtain guar gum stearate. S2. Mix guar stearate with water until homogeneous, then add borax and stir at 70-90℃ for 3-5 hours to obtain cross-linked guar gum. S3. Soak the obtained cross-linked guar gum in sodium hydroxide solution for 1-3 hours, then sonicate it in hot alcohol for 20-60 minutes, then remove it and evaporate it by rotary evaporation and freeze-drying to obtain the treated cross-linked guar gum. S4. The treated cross-linked guar gum is placed in an alkaline solution to swell for more than 24 hours, and then rotary evaporated and freeze-dried to obtain the pH adjuster.

2. The process for the recovery of silver values from silver containing materials to produce 4N silver powder as claimed in claim 1 wherein: In step (1), the concentration of nitric acid solution is 3.5-5 mol / L, the mass ratio of nitric acid solution to silver-containing waste is (3-5):1, the heating temperature is 75-85℃, and the stirring time is 4-5h; in step (5), the silver precipitation agent is hydrochloric acid or hypochlorous acid, the amount of silver precipitation agent is 1.1-1.3 times the stoichiometric ratio, and the stirring time is 30-45min; in step (7), the reducing agent is hydrazine hydrate, and the amount of reducing agent is 1.1-1.2 times the stoichiometric ratio.

3. The method for preparing 4N silver powder from silver-containing waste according to claim 1, characterized in that: In step S1, the mass ratio of guar gum, pyridine, and stearoyl chloride is 1:(2-5):(0.2-0.4); in step S2, the mass ratio of guar gum stearate to water and borax is 1:(8-20):(1-2).

4. The process for the recovery of silver values from silver containing materials to produce 4N silver powder as claimed in claim 1 wherein: In step S3, the concentration of sodium hydroxide solution is 10-30%, and the amount added is 5-10 times the mass of cross-linked guar gum; in step S4, the alkaline solution is an aqueous solution containing 5-20% ammonia, an aqueous solution of sodium carbonate with a mass concentration of 10-30%, or a sodium hydroxide solution with a mass concentration of 5-20%, and the mass ratio of the treated cross-linked guar gum to the alkaline solution is 1:(0.6-1.2).

5. The method for preparing 4N silver powder from silver-containing waste according to claim 1, characterized in that: Step (3) When the pH of the solution is adjusted to the point where precipitation begins to occur, functional microspheres are added. The preparation method of the functional microspheres includes the following steps: A. Mix β-cyclodextrin with potassium hydroxide solution until homogeneous, and add 2,3-epoxypropyltrimethylammonium chloride aqueous solution dropwise under stirring at 70-90℃, and react for 40-80 min; B. Cool the reaction solution from step A to room temperature, and continue to add epichlorohydrin dropwise under stirring. After reacting for 1-2 hours, add kerosene containing emulsifier, stir for 8-10 minutes, then heat to 55-65℃ and stir for 6-10 hours. After the reaction is complete, filter the product and vacuum dry the obtained solid to obtain the first microsphere. C. Mix the first microspheres with dimethylformamide evenly, and add the mixture of maleic anhydride and dimethylformamide under stirring at 30-40℃. After the addition is complete, raise the temperature to 80-90℃ and continue the reaction for 10-16 hours. The product is washed with acetone and water in sequence and dried under vacuum to obtain the second microspheres. D. Immerse the second microsphere in sodium alginate aqueous solution for more than 2 hours, then evaporate by rotary evaporation and dry to obtain the third microsphere; E. Mix sodium caseinate, gelatin, and water to prepare a material liquid, spray the material liquid onto the surface of the third microsphere, and dry to obtain the functional microsphere.

6. The process for the recovery of silver values from silver containing materials to produce 4N silver powder as claimed in claim 5 wherein: In step A, the mass ratio of β-cyclodextrin, potassium hydroxide, and 2,3-epoxypropyltrimethylammonium chloride is 1:(0.8-1.4):(1-3), and the mass concentration of the aqueous solution of 2,3-epoxypropyltrimethylammonium chloride is 5-10%. In step B, the amount of epichlorohydrin added is 1.5-2.5 times the mass of β-cyclodextrin, the emulsifier is Tween 20, the mass ratio of emulsifier to kerosene is (0.03-0.05):1, and the amount of kerosene added is 6-10 times the mass of β-cyclodextrin.

7. The process for the recovery of silver values from silver containing materials to produce 4N silver powder as claimed in claim 5 wherein: In step C, the mass ratio of maleic anhydride to dimethylformamide is 1:(3-6), and the amount of maleic anhydride added is 1-2 times the mass of the first microsphere; in step D, the mass ratio of sodium alginate to the second microsphere is (0.06-0.16):1, and the mass concentration of the sodium alginate aqueous solution is 3-5%; in step E, the mass ratio of sodium caseinate, gelatin, and water is (0.08-0.16):(1-1.4):1, and the amount of material solution sprayed is 4-8% of the mass of the third microsphere.

8. The method for preparing 4N silver powder from silver-containing waste according to any one of claims 1, 5 to 7, characterized in that: The filtrate obtained from step (4) is treated as follows before proceeding with the silver precipitation step: Under stirring conditions, add nitric acid solution to the filtrate to adjust the pH to 2-3, and continue stirring for 30-60 minutes. After filtration, the treated solution can be used for the silver precipitation step (5).

9. The method for preparing 4N silver powder from silver-containing waste according to any one of claims 1, 5 to 7, characterized in that: In step S2 of the preparation method of the pH adjuster, when guar stearate is mixed with water, cationic treated pumice powder is also added. The cationic treatment method is as follows: the pumice powder is immersed in an aqueous solution of cationic surfactant for 30-60 minutes, and then the water is evaporated. The mass ratio of pumice powder, cationic surfactant and guar stearate is (0.1-0.4):(0.003-0.006):

1. The cationic surfactant is a quaternary ammonium salt cationic surfactant.

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