High-hardness and high-purity cyanide-free electroforming solution, preparation method and application thereof

Abstract

This invention discloses a high-hardness, high-purity cyanide-free electroforming gold solution, its preparation method, and its application, belonging to the fields of electrochemical deposition and jewelry manufacturing technology. The high-hardness, high-purity cyanide-free electroforming gold solution comprises: sodium gold sulfite (calculated as gold) 10-14 g / L, sodium sulfite 100-130 g / L, ethylenediamine sulfate 22-33 g / L, potassium citrate 25-35 g / L, and trace rare earth element additives 0.5-2 mg / L, with a pH value of 7.2-7.6. This invention refines grain size by introducing pre-complexed trace rare earth element additives, utilizing the difference in atomic radius to generate a pinning effect at grain boundaries; it also utilizes the preferential adsorption of sodium 3-mercapto-1-propanesulfonate at microscopic peaks to generate local polarization differences to fill microscopic valleys; and it reduces the internal crystallization stress of the electroformed layer by constructing a composite complex system of sodium sulfite, ethylenediamine sulfate, and DTPA.

Description

Technical Field

[0001] This invention relates to the field of electrochemical deposition and jewelry manufacturing technology, specifically to a high-hardness, high-purity cyanide-free electroforming gold solution, its preparation method, and its application. Background Technology

[0002] Electroforming gold is a fundamental manufacturing technology that uses electrochemical deposition to deposit metallic gold onto the surface of a mold to create jewelry. With the increasing environmental standards in the jewelry manufacturing industry and the expanding market demand for high-hardness pure gold jewelry, cyanide-free electroforming gold solutions have become a standard research focus in the field of electroforming jewelry manufacturing.

[0003] In the existing jewelry electroforming manufacturing industry, to eliminate the safety hazards of traditional cyanide plating solutions, industrial production widely adopts cyanide-free electroforming gold bath systems with sulfites as the main complexing agent. In practical applications, existing cyanide-free systems typically introduce specific metal ions into the plating bath, allowing these ions to co-deposit with gold ions during the electrodeposition process. This alloying mechanism alters the microcrystalline state of the gold layer, thereby improving the physical hardness of electroformed jewelry pieces.

[0004] Current cyanide-free electroformed gold systems generally suffer from the technical drawback of being unable to simultaneously achieve both hardness and purity. To achieve Vickers hardness in electroformed gold layers, existing technologies typically rely on conventional alloying hardening by adding high concentrations of non-gold metals such as cobalt, nickel, and iron to the plating bath. This conventional alloying hardening mechanism inevitably leads to an increased proportion of non-gold elements within the deposited gold layer, causing the gold layer purity to fail to meet the material science standard of ≥99.9% pure gold. Furthermore, the introduction of high concentrations of non-gold metals can easily trigger contact dermatitis. Therefore, how to overcome the limitations of conventional alloying mechanisms while maintaining high gold layer purity, and improve the hardness of cyanide-free electroformed gold layers through micro-grain refinement, is a technical problem that needs to be solved in the current field of cyanide-free electroformed jewelry manufacturing. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a high-hardness, high-purity cyanide-free electroforming gold solution, its preparation method, and its application. This solves the problems of existing cyanide-free electroforming gold systems, such as difficulty in achieving both hardness and purity, easy precipitation of sponge gold under high current density, high internal stress in the plating layer causing high-temperature welding blistering, and surface roughness.

[0006] To achieve the above objectives, the present invention provides the following technical solution: The first aspect of the present invention provides a high-hardness, high-purity cyanide-free electroforming gold solution, comprising the following components: sodium gold sulfite 10-14 g / L, sodium sulfite 100-130 g / L, ethylenediamine sulfate 22-33 g / L, potassium citrate 25-35 g / L, and trace rare earth element additives 0.5-2 mg / L; and the pH value is 7.2-7.6.

[0007] In electroforming gold solutions, trace amounts of rare earth element additives are introduced as grain refiners. Utilizing the difference in atomic radii between rare earth ions and gold atoms, the rare earth elements deposit at the gold grain boundaries during the gold electrodeposition and crystallization process, creating a pinning effect that hinders outward migration of the grain boundaries and refines the gold grain size to the nanoscale. Simultaneously, sodium sulfite serves as the primary complexing agent and antioxidant, while ethylenediamine sulfate acts as an auxiliary complexing agent. Together, they construct a basic composite complex system, enhancing the thermodynamic stability of the gold sulfite complex ions and preventing spontaneous reduction and precipitation of gold ions in the solution.

[0008] Preferably, the trace rare earth element additive is selected from one or a combination of two of lanthanum and cerium.

[0009] Preferably, the trace rare earth element additive is added after pre-complexation treatment with citric acid or tartaric acid. The rare earth ions are complexed and coated by citrate or tartaric acid, preventing hydrolysis and precipitation reactions of rare earth ions in a weakly alkaline environment with a pH of 7.2-7.6.

[0010] Preferably, the high-hardness, high-purity, cyanide-free electroforming gold solution also contains a leveling agent, sodium 3-mercapto-1-propanesulfonate, at a concentration of 0.5-2 mg / L. During the electrochemical deposition process, sodium 3-mercapto-1-propanesulfonate molecules preferentially adsorb at the microscopic peaks on the cathode surface, increasing the local cathodic polarization resistance at these peaks and limiting the deposition rate of gold ions. This promotes the preferential deposition of gold ions at the microscopic valleys with lower polarization, filling the microscopic valleys through local electrochemical deposition rate differences and eliminating surface roughness defects caused by the underlying conductive silver paste.

[0011] Preferably, the high-hardness, high-purity, cyanide-free electroforming gold solution also contains 35-45 g / L potassium chloride, 4-6 g / L DTPA, 0.2-0.4 mL / L polyethyleneimine (0.2% by mass), and 4-6 g / L dipotassium hydrogen phosphate. The addition of DTPA to the composite complex system further stabilizes the gold sulfite complex ions and controls the internal stress of the electroformed layer.

[0012] The second aspect of this invention provides a method for preparing the high-hardness, high-purity, cyanide-free electroplating gold solution provided in the first aspect of this invention, comprising the following steps: dissolving rare earth salts in a solution containing citric acid to form a complex, and then adding the complex to a main plating bath containing sodium gold sulfite; and electrolyzing the high-hardness, high-purity, cyanide-free electroplating gold solution after volume adjustment using a current density of 0.5-1.0 A / dm² until a uniform and dense gold layer is deposited at the cathode. Pre-forming a stable cerium citrate complex with citric acid with the rare earth salts avoids hydrolysis of free rare earth ions in the weakly alkaline main plating bath, preventing the formation of hydroxide precipitates and maintaining the stable complexed dissolution state of trace rare earth element additives in the mixed solution. After adding all the formulation components and adjusting the volume, electrolysis is performed using a low current density to complete the aging process, pre-consuming the unstable impurity ions and electrochemical byproducts initially present in the electroplating gold solution.

[0013] Preferably, before adding the main plating solution containing sodium gold sulfite, the pH value of the main plating solution is adjusted to 7.2-7.6, and the working temperature is maintained at 38-42℃.

[0014] A third aspect of this invention provides a method for electroforming using the high-hardness, high-purity, cyanide-free electroforming gold solution provided in the first aspect of this invention. Deposition is performed using a pulsed power supply with a current density of 0.5-1.5 A / dm². Powered by the pulsed power supply, during the reverse current phase, anodic dissolution is used to eliminate the concentration polarization layer on the cathode surface, replenishing the gold ion concentration at the interface and reducing the internal crystallization stress of the electroformed layer.

[0015] Preferably, the forward current density of the pulse power supply is 0.8-1.2 A / dm², the reverse current is 0.05 A / dm², and the duty cycle is 30%.

[0016] Preferably, during the deposition process, the temperature of the electroforming gold solution is kept constant at 40±1℃, and the electroforming conditions are controlled by cathode movement or air stirring.

[0017] This invention provides a high-hardness, high-purity cyanide-free electroforming gold solution, its preparation method, and its application. It has the following beneficial effects:

[0018] 1. This invention employs citric acid or tartaric acid to pre-complex trace rare earth element additives, preventing hydrolysis and precipitation of these additives in a weakly alkaline environment. Furthermore, during the electrodeposition crystallization process, the difference in atomic radii between rare earth ions and gold atoms creates a pinning effect at the gold grain boundaries, hindering outward migration of the grain boundaries. This achieves nanoscale grain size refinement of the electroformed gold layer while maintaining extremely low amounts of trace rare earth elements in the formulation. Consequently, the Vickers hardness and wear resistance of the electroformed gold layer are improved while maintaining its high purity.

[0019] 2. This invention introduces the leveling agent sodium 3-mercapto-1-propanesulfonate into the cyanide-free electroforming gold solution formula. The sodium 3-mercapto-1-propanesulfonate molecules preferentially adsorb at the microscopic peaks on the cathode surface, increasing the local cathode polarization resistance at the microscopic peaks and limiting the deposition rate of gold ions at the microscopic peaks. This forces gold ions to preferentially deposit at the microscopic valleys with lower polarization, thus filling the surface roughness defects caused by the underlying conductive silver paste by utilizing the difference in local electrochemical deposition rates. This results in a bright electroformed layer and eliminates the need for subsequent polishing processes.

[0020] 3. This invention uses sodium sulfite as the main complexing agent and antioxidant, and ethylenediamine sulfate combined with DTPA as auxiliary complexing agents to jointly construct a basic composite complexing system. This improves the thermodynamic stability of gold sulfite complex ions in cyanide-free electroforming gold solution, prevents spontaneous reduction and precipitation of gold ions, and effectively reduces the internal crystallization stress of the electroformed layer during the growth process while improving the deep plating capability and anti-impurity interference capability of the electroforming gold solution system. This, in turn, eliminates blistering defects caused by stress release in subsequent high-temperature welding processes of electroformed parts. Attached Figure Description

[0021] Figure 1 This is a flowchart of the preparation method of the present invention. Detailed Implementation

[0022] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the 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.

[0023] Please see the appendix Figure 1 This invention provides a high-hardness, high-purity cyanide-free electroforming gold solution, its preparation method, and its application.

[0024] Example:

[0025] This embodiment provides the specific preparation and application of 180L electroforming gold solution. The formula includes: 12g / L sodium gold sulfite (approximately 2.16kg, calculated as Au), 120g / L sodium sulfite (21.6kg), 5.5kg ethylenediamine sulfate, 30g / L potassium citrate (5.4kg), 40g / L potassium chloride (7.2kg), 5g / L DTPA (0.9kg), 54mL polyethyleneimine (0.2%), 1mg / L sodium 3-mercapto-1-propanesulfonate (MPS) (0.18g), 1mg / L cerium nitrate (0.18g), and 5g / L dipotassium hydrogen phosphate (0.9kg). The process window is: pH 7.4, temperature 40℃.

[0026] Specific steps for preparing 180L electroforming gold solution:

[0027] S1. Preparation of solution: Add about 150L of deionized water to a cleaned 180L electroforming tank and heat to 45℃.

[0028] S2. Dissolve the main complexing agent: Add 21.6 kg sodium sulfite, 5.4 kg potassium citrate, 7.2 kg potassium chloride, 0.9 kg DTPA, and 0.9 kg dipotassium hydrogen phosphate in sequence, and stir until completely dissolved.

[0029] S3. Adjust pH: Use potassium hydroxide solution to adjust the pH of the plating solution to 7.4.

[0030] S4. Add gold salt: Dissolve sodium gold sulfite containing 2.16 kg of gold in deionized water, then slowly add it to the tank and stir until homogeneous.

[0031] S5. Add hardener: Add 54 mL of 0.2% polyethyleneimine solution.

[0032] S6. Rare Earth Pretreatment and Addition: Take 500mL of deionized water, add 10g of citric acid, and stir to dissolve; add 0.18g of cerium nitrate, and stir until the solution is clear and a stable cerium citrate complex is formed; slowly inject this clear solution into the main plating tank under strong stirring.

[0033] S7. Leveling agent addition: Take 100mL of deionized water, add 0.18g of MPS to prepare a 0.18% solution, and add it dropwise to the plating solution.

[0034] S8. Volume adjustment and aging: Add deionized water to 180L; electrolyze with a small current of 0.5-1.0A / dm² for 4-6 hours until a uniform and dense golden yellow coating is deposited on the cathode plate.

[0035] Electroforming process parameters: Power supply mode uses pulse power, forward current density 0.8-1.2A / dm², reverse current 0.05A / dm², duty cycle 30%. Temperature is controlled at 40±1℃. Stirring method uses cathode movement (20 times / minute) or gentle air stirring.

[0036] Results: Tests showed that the Vickers hardness of the electroformed gold layer obtained using the above preparation steps and parameters reached 145-165 HV, which is more than 100% higher than that of traditional pure gold (60-90 HV), and the wear resistance is significantly enhanced. XRF testing showed that the gold layer purity was ≥99.95%, which meets the standard for pure gold jewelry. The plating is fully glossy and does not require subsequent mechanical polishing, allowing for direct molding and welding. Because it does not contain heavy metal impurities that cause brittleness, and organic inclusions are reduced through process control, there is no blistering during welding of electroformed parts. The entire process is cyanide-free, wastewater treatment is simple, and it complies with RoHS standards.

[0037] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A high-hardness, high-purity, cyanide-free electroforming gold solution, characterized in that, The high-hardness, high-purity, cyanide-free electroforming gold solution contains the following components: Sodium gold sulfite, calculated as gold, 10-14 g / L; Sodium sulfite 100-130 g / L; 22-33 g / L of ethylenediamine sulfate; Potassium citrate 25-35 g / L; And trace rare earth element additives of 0.5-2 mg / L; Furthermore, the pH value of the high-hardness, high-purity cyanide-free electroforming gold solution is 7.2-7.

6.

2. The high-hardness, high-purity cyanide-free electroforming gold solution according to claim 1, characterized in that, The trace rare earth element additive is selected from one or a combination of two of lanthanum and cerium.

3. The high-hardness, high-purity cyanide-free electroforming gold solution according to claim 1, characterized in that, The high-hardness, high-purity, cyanide-free electroforming gold solution also contains a leveling agent, sodium 3-mercapto-1-propanesulfonate, at a concentration of 0.5-2 mg / L.

4. The high-hardness, high-purity cyanide-free electroforming gold solution according to claim 1, characterized in that, The trace rare earth element additives need to be added after pre-complexation treatment with citric acid or tartaric acid.

5. The high-hardness, high-purity cyanide-free electroforming gold solution according to claim 1, characterized in that, The high-hardness, high-purity cyanide-free electroforming gold solution also contains 35-45 g / L potassium chloride, 4-6 g / L DTPA, 0.2% polyethyleneimine (0.2-0.4 mL / L), and 4-6 g / L dipotassium hydrogen phosphate.

6. A method for preparing the high-hardness, high-purity cyanide-free electroforming gold solution as described in claim 1, characterized in that, The method includes the following steps: dissolving rare earth salts in a solution containing citric acid to form a complex, adding the complex to a main plating solution containing sodium gold sulfite, and electrolyzing the high-hardness, high-purity cyanide-free electroplating gold solution after it has been brought to a constant volume using a current density of 0.5-1.0 A / dm² until a uniform and dense gold layer is deposited at the cathode.

7. The method according to claim 6, characterized in that, Before adding the main plating solution containing sodium gold sulfite, the pH value of the main plating solution is adjusted to 7.2-7.6, and the operating temperature is maintained at 38-42℃.

8. A method for electroforming using the high-hardness, high-purity, cyanide-free electroforming gold solution as described in claim 1, characterized in that, The method uses a pulsed power supply for deposition with a current density of 0.5-1.5 A / dm².

9. The method according to claim 8, characterized in that, The method uses a pulse power supply with a forward current density of 0.8-1.2 A / dm², a reverse current of 0.05 A / dm², and a duty cycle of 30%.

10. The method according to claim 8, characterized in that, In the deposition process, the electroforming gold solution is kept at a constant temperature of 40±1℃, and the electroforming conditions are controlled by cathode movement or air stirring.

Images