The industrial gold electrolysis process—primarily known as the Wolver process—is an electrochemical refining system that refines crude gold anode rods (typically 90% to 98% pure) into ultra-pure fine gold crystals at the cathode, achieving a purity of 99.99% (4N) or 99.999% (5N). Detailed Operational Steps:
Protective Atmosphere Smelting and Melting: The dried gold powder is placed in a high-purity graphite crucible and heated in a high-frequency induction furnace. Shielding: Argon or nitrogen gas is continuously pumped onto the surface of the molten metal to prevent trace amounts of oxygen or nitrogen in the air from altering the batch purity. Final form: Liquid gold is poured into a cast iron mold to create internationally certified 1-kilogram or 12.5-kilogram gold bars, stamped with the number “99.99”.
Anode Melting and Casting Operation: Unrefined gold ingots or jewelry fragments are melted in an induction furnace at temperatures exceeding 1100°C. Silver Content Threshold Limit: If the silver content in the raw gold exceeds 5% to 8%, it must be diluted with pure gold before casting. Excess silver will cause a thick, impermeable silver chloride (AgCl) crust to form on the anode surface during electrolysis, completely blocking the current. Finished Product: The molten metal is poured into a specially designed open mold to form a crude gold anode plate with integrated hooks.
Battery Assembly and Electrolyte Preparation: Electrode: Cast coarse gold anodes are vertically suspended in a heavy PVDF or polypropylene bath, alternating with the cathodes. Cathode Material: Ultra-thin pre-rolled 99.99% pure gold starting sheet, or passivated titanium/stainless steel plate for easy scraping. Electrolyte: The chemical bath consists of a precise mixture of tetrachloroauric acid (HAuCl₄) and hydrochloric acid (HCl) in deionized water. Gold ion concentration (Au³⁺): Maintained between 80 and 120 g/L. Free hydrochloric acid: Maintained between 50 and 100 g/L to prevent premature precipitation of gold ions from the solution.
Electrochemical Reaction (Refining Stage): The IGBT rectifier introduces highly controllable DC current into the system. The voltage is locked at a low level of 1.0V to 1.5V, the current density is set at 400 to 800 A/m², and an in-line heater maintains the solution temperature between 55°C and 65°C. Anodic reaction (dissolution): Solid crude gold loses electrons and dissolves into a soluble gold chloride complex: (\text{Au}-3\text{e}^{-}+4\text{Cl}^{-}\rightarrow \text{AuCl}{{4}^{-}) Cathode reaction (deposition): Gold ions migrate in the liquid, gain electrons at the cathode, and crystallize into high-purity gold metal: (\text{AuCl}{{4}^{-}+3\text{e}^{-}\rightarrow \text{Au}\downarrow +4\text{Cl}^{-}) Impurity behavior (purification mechanism): Silver (Ag): Reacts immediately with free chloride ions to form insoluble silver chloride (AgCl). It detaches from the anode surface and sinks to the bottom to form anode mud. Platinum and palladium (Pt, Pd): Dissolve together with gold. However, because their electrochemical potentials are lower than gold, they cannot be deposited on the cathode under low voltage conditions. They remain permanently suspended in the liquid matrix. Base metals (copper, iron, nickel): Dissolve in the acid bath and remain there, without contaminating the cathode.
Harvesting and Chemical Cleaning: Harvesting: Every 24 to 48 hours, the cathode plate is lifted by a bridge crane. Newly grown gold crystals are peeled from the reusable titanium substrate by mechanical scraping or stripping. Purification Cleaning: The collected gold crystals are immersed in a gold-rich, toxic electrolyte. To achieve 4N or 5N standards, the crystals must undergo multi-stage countercurrent rinsing using hot deionized water, dilute hydrochloric acid, and ammonia (to dissolve any residual trace silver chloride particles). The gold is then dried in a vacuum oven.
