1.Potential quality risks associated with concentration gradients in the machinery
If the silver electrolyte lacks both convective circulation and mechanical agitation—and if the diffusion-based mass transfer of ions is the rate-limiting step—ions will continuously deposit at the cathode. This causes the silver ion concentration in the layer adjacent to the cathode surface to drop steadily, while silver ions from the bulk solution fail to diffuse in quickly enough to replenish the supply. Consequently, a concentration difference—or concentration gradient—forms near the cathode; in addition to the silver ion gradient, concentration gradients for impurity metals and hydrogen ions also develop in the cathode zone. The presence of these gradients not only compromises product quality but can even prevent the production process from proceeding smoothly.
2.Mechanical Agitation: An Essential Component of the Electrolytic Cell
To dissipate concentration gradients, accelerate diffusion-based mass transfer, and ensure a continuous supply of silver ions to the cathode—thereby sustaining production—it is necessary to upgrade the electrolytic equipment by incorporating a mechanical agitation system. Such a system also facilitates the diffusion of hydrogen ions, slowing their depletion in the cathode zone and preventing metal hydrolysis and the formation of hydroxides.
3.Reciprocating Mechanical Agitation
Driven by a motor, a rotating cam causes agitation rods to move back and forth between the anodes and cathodes. With an agitation rod positioned between each anode-cathode pair, the system serves two purposes:
1) agitating the electrolyte to prevent concentration polarization and promote diffusion; and
2)performing a “branch-breaking” action to prevent the formation of silver crystals that could lead to short circuits between the electrodes.
4.Agitation Parameters
1. Electrolyte mechanical agitation speed: 20–22 cycles/minute;
2. Motor power: 1.5 kW is sufficient.
3 Control and Adjustment of Electrolyte Acidity
3.1 Control Range for Electrolyte Acidity
When the raw material for silver refining consists primarily of silver-copper alloys (such as 72AgCu scrap) or alloys containing other impurities (like 56AgCuZnSn), impurities such as copper, zinc, tin, lead, and iron—in addition to silver—may enter the electrolyte during the electrolytic refining process. Different metals have distinct hydrolysis conditions; for instance, tin dissolves slowly in dilute nitric acid, forming Sn(NO3)2 and NH4NO3. Tin is prone to hydrolysis—Sn(OH)x species begin to hydrolyze at pH 0, while Sn(OH)2 hydrolyzes at pH 0.9; additionally, Fe(OH)3 hydrolyzes at pH 1.5. Since the raw materials are essentially free of palladium and platinum, the electrolyte acidity is not constrained by the upper limit of 10–20 g/L. However, if the target acidity is set at pH 1–2, distinguishing between the two values using wide-range pH paper is difficult, as both pH 1 and pH 2 appear red on the paper. A pH of 2 corresponds to an [H+] concentration of 10⁻² mol/L, which converts to an HNO3 concentration of 0.63 g/L; therefore, the HNO3 concentration in the solution must be increased to prevent the hydrolysis of base metals
3.2 Method for Measuring Electrolyte Acidity
The control standard for electrolyte acidity has been revised to a more precise concentration unit (g/L), and the acidity level has been increased approximately tenfold compared to the previous standard; consequently, the former method of using pH test strips to monitor acidity is neither scientific nor suitable. Therefore, to strictly control the acidity/alkalinity of the electrolyte, it is essential to employ titration equipment—such as burettes and stands—and use the titration method to measure acidity. 3.3 Adjustment of Electrolyte Acidity
Electrolyte acidity, along with parameters such as electrolysis current density and electrolyte temperature, must be measured hourly and adjusted as necessary to ensure that all control indicators remain consistent throughout the 24-hour operation.
3.3 Adjustment of Electrolyte Acidity
Parameters such as electrolyte acidity, electrolysis current density, and electrolyte temperature must be measured hourly and adjusted as necessary to ensure that all control indicators remain consistent throughout the 24-hour period. Continuous 24-hour monitoring and control of each electrolytic cell—including temperature, acidity, and cell voltage—along with the stirring and circulation of the electrolyte, ensure the smooth operation of the electrolysis process.
4. Conclusion
The electrolytic waste treatment process was improved to meet the requirements of electrolyte circulation. Circulation equipment—such as an elevated electrolyte tank and acid pumps—was added to optimize the electrolytic production workflow. A mechanical stirring system was installed to overcome concentration polarization within the electrolyte. Electrolyte acidity control parameters and methods were adjusted, and operational protocols were established to ensure round-the-clock monitoring of the electrolytic cells; by maintaining strict quality control at every stage of the process, the production of high-quality deposited silver is achieved. Through mold improvements and “double-safeguard” measures—such as acid quenching and head-cropping of the electrolytic silver—the formation of hydroxides during production is prevented, and any hydroxides that do form are removed during post-processing, resulting in a high-quality finished electrolytic silver product.