A method for recovering silver from electroplating silver spent solution

Abstract

This invention provides a method for recovering silver from waste silver plating solution, belonging to the field of heavy metal industrial wastewater treatment technology. The invention involves adding sodium hypophosphite to waste silver plating solution containing complexed silver. After the sodium hypophosphite dissolves, the pH of the system is adjusted to alkaline. A reduction reaction is then carried out under heating conditions to generate elemental silver particles. This invention utilizes the in-situ generation of hydrogen free radicals from sodium hypophosphite under alkaline heating conditions to achieve direct reduction and complex breaking of stable silver nicotinic acid complexes and elemental silver conversion. This process is completed in one step, simplifying the traditional complex multi-step process of "oxidation-complex breaking-ion release-chemical precipitation" into a single reduction step, significantly shortening the treatment process and improving the reaction rate and treatment efficiency.

Description

Technical Field

[0001] This invention relates to the field of heavy metal industrial wastewater treatment technology, and in particular to a method for recovering silver from waste silver plating solution. Background Technology

[0002] In advanced manufacturing fields such as precision electronics, high-end electroplating, and optical coating, silver is widely used due to its excellent conductivity, reflectivity, and chemical stability. To maintain the homogeneity and reactivity of the process solution, ammonia, nicotinic acid, etc., are often used in the production process to form highly stable complexes with silver ions (such as [Ag(NH3)2)). + Ag(C6H4NO2)2 - This directly results in high concentrations of chemically inert complexed silver in the wastewater of related industries. Such pollutants are highly mobile and biotoxic in the environment, and traditional physical precipitation and biochemical processes have extremely low removal efficiency.

[0003] Currently, the mainstream approach to treating such wastewater still relies on advanced oxidation technologies coupled with chemical precipitation. This process aims to attack organic ligands with strong oxidizing free radicals (such as hydroxyl radicals), destroying the complex structure to release free silver ions, and then adding NaCl to form a silver precipitate. However, this technical route has significant limitations in terms of resource recovery efficiency and economics: First, the oxidation and complex-breaking process, targeting stable ligands such as nicotinic acid, often has slow reaction kinetics, high energy and reagent consumption, and incomplete complex breaking can easily lead to low metal recovery rates. Second, this process ultimately produces silver oxide or silver-containing sludge, with silver existing in the form of low-value compounds. Subsequent metallurgical recovery processes are complex and costly, while also generating a large amount of hazardous waste, contradicting the goal of source resource recovery.

[0004] Therefore, developing a green recycling technology that can directly target silver centers and efficiently convert complexed silver into high-value elemental silver in one step has become an urgent need for the industry. Summary of the Invention

[0005] To address the problems of complex processes, low resource recovery efficiency, and high risk of secondary pollution associated with existing advanced oxidation technologies for treating electroplating wastewater containing stable silver nicotinate complexes, a more direct, efficient, and high-value-added resource recovery method is proposed.

[0006] Specifically, existing technologies that indirectly attack organic ligands with strong oxidizing free radicals to break the complex have the following drawbacks: First, for stable ligands such as nicotinic acid, the oxidation-induced complex-breaking reaction is slow and energy-intensive, resulting in incomplete release of silver ions and a long recovery cycle; second, this process ultimately converts silver into low-grade compound sludge, leading to high subsequent refining costs and failing to achieve high-value direct recovery of precious metals; third, the oxidation process may generate complex intermediate products, posing environmental risks. Therefore, this invention provides a method that bypasses the inefficient indirect oxidation pathway and directly and rapidly converts complexed silver into high-purity elemental silver.

[0007] To achieve the above objectives, the present invention provides the following technical solution: This invention provides a method for recovering silver from waste silver plating solution. Sodium hypophosphite is added to the waste silver plating solution containing complexed silver. After the sodium hypophosphite dissolves, the pH of the system is adjusted to alkaline. Then, a reduction reaction is carried out under heating conditions to generate elemental silver particles.

[0008] In a preferred embodiment of the present invention, the complexed silver is a complex formed by silver ions and nicotinic acid and / or a complex formed by silver ions and ammonium ions. The waste liquid from silver plating also contains small amounts of other forms of silver, such as free silver ions and complexed silver from other systems.

[0009] In a preferred embodiment of the present invention, the concentration of silver ions in the waste silver plating solution is 1~11 g / L.

[0010] In a preferred embodiment of the present invention, the amount of sodium hypophosphite added is 0.5 to 2.5 times the total molar amount of silver in the waste silver plating solution.

[0011] More preferably, the amount of sodium hypophosphite added is 0.5 times, 1.0 times, 1.5 times, 2.0 times, or 2.5 times the total molar amount of silver in the waste silver plating solution.

[0012] In a preferred embodiment of the present invention, the pH is 11.0~12.5. The pH is adjusted by adding sodium hydroxide, potassium hydroxide, or ammonia to the reaction system.

[0013] More preferably, the pH is 11.0, 11.5, 12.0, or 12.5.

[0014] In a preferred embodiment of the present invention, the heating temperature is 60~90℃.

[0015] More preferably, the heating temperature is 60°C, 70°C, 80°C, or 60°C.

[0016] In a preferred embodiment of the present invention, the reduction reaction takes 30 to 120 minutes.

[0017] More preferably, the reduction reaction time is 30 min, 40 min, 50 min, 60 min, 70 min, 80 min, 90 min, 100 min, 110 min, or 120 min.

[0018] In a preferred embodiment of the present invention, after the reduction reaction is completed, the method further includes collecting elemental silver particles and washing and drying the elemental silver particles.

[0019] The present invention discloses the following technical effects: (1) This invention utilizes sodium hypophosphite to generate hydrogen radicals (·H) in situ under alkaline heating conditions, thereby achieving direct reduction and complex breaking of stable silver nicotinate complexes and elementalization of silver. This process is completed in one step, simplifying the traditional complex process of "oxidation-complex breaking-ion release-chemical precipitation" into a single reduction step, significantly shortening the processing flow and improving the reaction rate and processing efficiency.

[0020] (2) This invention directly converts complexed silver into a high-value-added elemental product. The silver generated in the reaction is recovered in the form of high-purity (≥99.9%), ultrafine (particle size usually below 1 micrometer) silver powder, and the product has high economic value. This completely avoids the disposal problems and resource waste caused by the low-grade silver-containing hazardous waste sludge generated by traditional methods, realizes the efficient resource recovery of precious metals, and conforms to the concept of circular economy. Attached Figure Description

[0021] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0022] Figure 1 The graph shows the change of silver ion concentration in the silver plating waste solution and the total Ag concentration in the NaH2PO2 system over time at different reaction temperatures (t = 40℃, 50℃, 60℃, 70℃, 80℃) when the initial pH = 11.0 and the NaH2PO2 dosage was 8.696 g in Example 1. Figure 2 The graph shows the change of silver ion concentration in the silver plating waste solution and the total Ag concentration in the NaH2PO2 system over time under different pH conditions (pH = 7, 8, 9, 10, 11, 12) when the NaH2PO2 dosage is 8.696 g and the reaction temperature is 70℃. Figure 3The graph shows the change of silver ion concentration in the silver plating waste solution over time under different NaH2PO2 dosages (1.87, 2.74, 4.35, 8.70, and 17.40 g / L) in the NaH2PO2 system when the reaction temperature is 70°C and pH=11.0 in Example 1. Figure 4 The X-ray diffraction pattern of the precipitate obtained in Example 1; Figure 5 The image shows the microstructure of the precipitate obtained in Example 1. Figure 6 This is a diagram showing the average particle size distribution of the precipitate particles obtained in Example 1. Detailed Implementation

[0023] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0024] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Any stated value or intermediate value within a stated range, as well as each smaller range between any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0025] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0026] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be apparent to those skilled in the art. This specification and embodiments are merely exemplary.

[0027] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0028] In the research on technologies for the one-step efficient conversion of complexed silver into elemental silver, in-situ reduction techniques based on active hydrogen species (such as hydrogen radicals, ·H) have shown unique potential. Hydrogen radicals possess extremely strong reduction potentials, and their high reducing power is sufficient to directly break stable Ag-coordinate bonds. More importantly, hydrogen radicals have atomic size and high reactivity, exhibiting excellent penetrability. They can effectively overcome the steric hindrance and electron cloud barriers formed by ligands such as nicotinic acid, directly interacting with the silver ions at the core of the complex, thereby achieving precise and efficient dissociation of the complex structure and the elemental reduction of silver.

[0029] The core of this invention lies in utilizing the strong reducing power and high penetrability of hydrogen radicals (·H) to achieve the direct reduction and high-value recovery of stable silver nicotinic acid complexes in electroplating wastewater. Hydrogen radicals have extremely high reduction potentials, which can effectively break silver-coordinate bonds. Moreover, their small atomic size allows them to penetrate the steric hindrance of organic ligands and directly target the silver ions at the center of the complex. To achieve this process, this invention adds sodium hypophosphite to the wastewater and heats it in a strongly alkaline environment with pH ≥ 11.0. Through a chain-initiated reaction, the pH bonds of hypophosphite are homolytically cleaved, continuously generating hydrogen radicals in situ (Formula (1)). Subsequently, hydrogen radicals mediate the decomposition of hypophosphite to generate hydrogen radicals. This is a chain-transfer process and the core step of autocatalysis (Formulas (2)-(3)). This active hydrogen species can rapidly reduce the complexed silver ions to zero-valent silver atoms, and the silver atoms aggregate and grow to form ultrafine silver particles. After the reaction, solid-liquid separation, washing, and drying yield high-purity (≥99.9%) elemental silver powder with a particle size typically below 1 micrometer. This method completes complex breaking, reduction, and resource recovery in one step, making the process simple and efficient. It avoids the complex processes and low-value sludge generation of traditional oxidation processes, achieving direct conversion and high-value recovery of precious metals from wastewater.

[0030] H2PO2 - → ·H + ·PO2H (1) ·H + H2PO2 - → H2+ ·HPO2 - (2) HPO2 - + H2O → ·H + H3PO3 (3) ·H + ·H → H2 (4) ·H + ·HPO2 - + H2O → H3PO3+ H + (5) 2·HPO2 - → H2P2O6 (6) Unless otherwise specified, the technical solutions described in this invention are all conventional solutions in the field, and the reagents or raw materials used are all purchased from commercial channels or are publicly available unless otherwise specified.

[0031] The formula for calculating the silver removal rate in waste silver plating solution is: Removal rate = (C-C0) / C0, where C is the concentration of silver ions in the system after the reaction, and C0 is the original concentration of anions in the system.

[0032] To better understand the present invention, the following embodiments further illustrate the content of the present invention, but the content of the present invention is not limited to the following embodiments.

[0033] Example 1 1000 mL of actual silver plating waste solution was transferred into the reactor using a graduated cylinder. The reactor was then placed on a magnetic stirrer for stirring. The silver ion concentration in the waste solution was 10.21 g / L, and the nicotinic acid concentration was 77 g / L. The initial pH of the reaction solution was adjusted using a 10 mol / L NaOH solution, followed by the addition of NaH₂PO₂ to the reactor. After the NaH₂PO₂ was completely dissolved and mixed, the solution was heated to the specified reaction temperature. At specified reaction time points (10, 20, 30, 40, 50, 60, 70, 80, and 90 min), 1 mL of reaction sample was transferred from the reaction system and filtered through a 22 μm microporous membrane. The total silver concentration in the supernatant was then analyzed. All experiments were repeated at least three times, and the mean and standard error of the experimental data were calculated.

[0034] (1) Investigate the effect of reaction temperature on the reduction of silver complex by sodium hypophosphite.

[0035] When the initial pH was 11.0, the dosage of NaH2PO2 was 8.696 g, and the reaction temperatures were 40℃, 50℃, 60℃, 70℃, and 80℃, the total silver concentration in the supernatant was measured, and the results are as follows: Figure 1 As shown.

[0036] Depend on Figure 1 It can be seen that, under the conditions of an initial pH of 11.0 and a NaH₂PO₂ dosage of 8.696 g, the reaction temperature has a significant impact on the reduction of silver-containing complexes by sodium hypophosphite. When the reaction temperature is 40℃ and 50℃, no significant reaction occurs; after the temperature is raised to 60℃, the total silver removal rate reaches 40% after 90 min; and when the reaction temperature is increased to 70℃ or higher, complete removal of total silver can be achieved after 90 min. This indicates that under alkaline conditions, temperature is the key factor driving the completion of this reduction reaction.

[0037] (2) Investigate the effect of initial pH value on the reduction of silver complex by sodium hypophosphite.

[0038] When the dosage of NaH2PO2 was 8.696 g and the reaction temperature was 70℃, the total silver concentration in the supernatant was measured at initial pH values ​​of 8.0, 9.0, 10.0, 11.0, and 12.0, respectively. The results are as follows: Figure 2 As shown.

[0039] Depend on Figure 2 It can be seen that, under the conditions of a reaction temperature of 70℃ and a NaH2PO2 dosage of 8.696 g, the initial pH value of the system has a decisive influence on the reduction and removal efficiency of silver. When the initial pH is 8.0 and 9.0, the total silver removal rate does not change significantly after 30 min of reaction, indicating that the reaction is difficult to proceed effectively in a weakly alkaline range. When the initial pH is increased to 10.0, the total silver removal rate reaches 23% after 90 min of reaction; and when the initial pH is further increased to 11.0 and 12.0, the total silver can be completely removed after the same reaction time. This proves that under heating conditions, a strongly alkaline environment (pH ≥ 10.0) is a necessary condition for driving the efficient generation of hydrogen free radicals and achieving complete reduction and breakdown of the silver complex.

[0040] (3) Investigate the effect of the amount of NaH2PO2 added on the reduction of silver complex by sodium hypophosphite.

[0041] When the initial pH was 11.0, the reaction temperature was 70℃, and the dosage of NaH2PO2 was 1.087, 2.714, 4.348, 8.696, and 10 g / L, the total silver concentration in the supernatant was measured, and the results are as follows: Figure 3 As shown.

[0042] Depend on Figure 3 It can be seen that the total silver removal rate significantly increases with the increase of NaH2PO2 dosage: when the dosage is 1.87 g / L, the total silver removal rate is 32% after 90 min of reaction; when the dosage is increased to 2.74 g / L, the total silver removal rate increases to 62% after 90 min of reaction; when the dosage is further increased to 4.35 g / L, 8.70 g / L, and 17.40 g / L, the total silver removal rate all reaches over 99%. The results indicate that, under the experimental conditions, the dosage of NaH2PO2 is the key factor affecting the completeness of the reduction reaction. Considering that the removal efficiency does not significantly improve when the dosage exceeds 8.696 g / L, from the perspective of balancing economy and efficiency, 8.696 g / L can be considered the optimal dosage for this system.

[0043] In addition, we recovered and characterized the precipitate obtained after solid-liquid separation following the reaction. After filtration and vacuum drying, X-ray diffraction (XRD) analysis showed that the precipitate exhibited only distinct diffraction peaks characteristic of elemental silver, demonstrating the high purity of the recovered product. Figure 4 Further observation of its microstructure using scanning electron microscopy (SEM) revealed that the silver powder consisted of nearly spherical particles. Figure 5 Particle size analysis showed that the silver powder particles were all less than 1 micrometer in size. Figure 6 This result directly confirms that this method can directly recover high-purity, ultrafine elemental silver powder from waste liquid.

[0044] Example 2 To verify the universality of this process, it was applied to ammonium ions ([Ag(NH3)2]). + The silver plating waste solution used as the ligand was a solution with a silver ion concentration of 12 g / L. The reaction was carried out at an initial temperature of 70℃, an initial pH of 12.0, and a NaH₂PO₂ dosage of 8.696 g / L to achieve a concentration of 8.696 g / L in the reaction system, for 90 minutes. Testing showed that the total silver removal rate exceeded 99.9%, achieving complete removal. This result confirms that this process is equally efficient for silver complex systems with ammonium ions as ligands and has good applicability.

[0045] In summary, this invention utilizes the technical principle of generating highly reactive hydrogen radicals (·H) in situ through the cracking of sodium hypophosphite under strongly alkaline and heating conditions. This enables the efficient reduction and breaking of stable silver nicotinic acid and silver ammonia complexes in electroplating wastewater, directly converting the complexed silver into high-purity elemental silver powder for recovery. The method can rapidly complete the reaction under mild conditions (50-90℃), and the total silver content in the treated wastewater is significantly lower than the limits specified in the "Electroplating Pollutant Discharge Standard". This process integrates wastewater treatment and resource recovery, featuring a simple flow, eliminating the need for complex oxidation steps, and preventing the generation of silver-containing hazardous sludge. It simultaneously achieves pollution control and the green preparation of high-value silver powder, offering significant advantages such as high efficiency, low cost, and environmental friendliness.

[0046] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for recovering silver from waste silver plating solution, characterized in that, Sodium hypophosphite was added to the waste silver plating solution containing complexed silver. After the sodium hypophosphite dissolved, the pH of the system was adjusted to alkaline. Then, a reduction reaction was carried out under heating conditions to generate elemental silver particles.

2. The method for recovering silver from waste silver plating solution according to claim 1, characterized in that, The complexed silver is a complex formed by silver ions and nicotinic acid and / or a complex formed by silver ions and ammonium ions.

3. The method for recovering silver from waste silver plating solution according to claim 1, characterized in that, The concentration of silver ions in the waste silver plating solution is 1~11 g / L.

4. The method for recovering silver from waste silver plating solution according to claim 1, characterized in that, The amount of sodium hypophosphite added is 0.5 to 2.5 times the total molar amount of silver in the waste silver plating solution.

5. The method for recovering silver from waste silver plating solution according to claim 1, characterized in that, The pH is 11.0~12.

5.

6. The method for recovering silver from waste silver plating solution according to claim 1, characterized in that, The heating temperature is 60~90℃.

7. The method for recovering silver from waste silver plating solution according to claim 1, characterized in that, The reduction reaction takes 30 to 120 minutes.

8. The method for recovering silver from waste silver plating solution according to claim 1, characterized in that, After the reduction reaction is completed, the process also includes collecting elemental silver particles and washing and drying them.

Images