The gold electrolysis process flow, industrially known as the Wohlwill process, involves using crude gold (alloy gold ingots) with a purity of 90%–99% as the anode. Through an electrochemical reaction in a specific acidic electrolyte, high-purity gold (up to 99.99% (4N) or even 99.999% (5N)) is deposited on the cathode.
Harvesting & Washing of Gold Powder/Panels: Harvesting: After the electrolysis cycle (usually 24 to 48 hours), the cathode plate is removed. If a titanium cathode is used, the attached gold powder can be directly peeled off mechanically or manually. Precision Washing: The precipitated gold powder carries a high concentration of electrolyte (rich in gold ions and impurities such as copper and palladium). Multi-stage countercurrent washing with deionized water is necessary, and even repeated rinsing with dilute hydrochloric acid and ammonia is required until the conductivity of the washing water is close to that of deionized water. Then, it is sent to a vacuum drying oven for drying. 5. Smelting & Pouring of High-Purity Gold Ingots: The dried high-purity gold powder is placed in a dedicated graphite crucible and melted using a high-frequency induction furnace. Protective Measures: High-purity argon or nitrogen gas must be introduced during melting to provide an atmosphere protection against trace amounts of gaseous elements contaminating the gold. Final product: The molten gold is cast into standard investment gold ingots of 1 kg or 12.5 kg and stamped with a purity mark of 4N (99.99%) or 5N (99.999%).
Anode Casting Process: The crude gold is melted in a medium-frequency induction furnace. If the silver content in the crude gold is too high (exceeding 5%–10%), high-purity gold is usually added for dilution; otherwise, a dense silver chloride (AgCl) shell will rapidly form on the anode surface during electrolysis, hindering further electrolysis. Product: The molten gold is cast into a uniformly thick crude gold anode plate with hanging lugs.
Electrolytic Cell Assembly and Electrolyte Preparation: In a polypropylene (PP) or Teflon electrolytic cell, the coarse gold anode and cathode are suspended vertically in an alternating pattern. Cathode Material: Typically, extremely thin high-purity gold foil (Gold Starter Sheets) or specially passivated titanium/stainless steel plates (for easy removal of gold powder later). Electrolyte Composition: The core electrolyte is a mixed aqueous solution of chloroauric acid (HAuCl₄) and hydrochloric acid (HCl). Gold Ion Concentration: Typically maintained at 80-120 g/L. Free Hydrochloric Acid Concentration: 50-100 g/L.
DC Electrolysis Reaction: A high-precision rectified power supply is connected, controlling the voltage at 1.0V – 1.5V, maintaining the current density at approximately 400-800 A/m², and the electrolyte temperature is maintained at 55°C – 65°C via a heating system. Anodic reaction (dissolution): Gold atoms in the crude gold plate lose electrons and combine to form chloroaurate ions, which enter the solution. Au – 3e⁻ + 4Cl⁻ → AuCl₄⁻ Cathode reaction (deposition): Gold complex ions in the solution gain electrons on the cathode surface, reducing to pure gold crystals and adhering to the cathode. (\text{AuCl}_4^- + 3\text{e}^- \rightarrow \text{Au}\downarrow + 4\text{Cl}^-) Impurity fate (core purification mechanism): Silver (Ag): Silver in the anode combines with chloride ions in the solution, immediately forming insoluble silver chloride (AgCl) precipitate, which falls to the bottom of the tank to form anode mud. Platinum, palladium (Pt, Pd): Dissolve at the anode along with gold and enter the solution. However, because their electrode potential is lower than gold, under precisely controlled low voltage, they cannot be deposited at the cathode by discharge, but remain in the electrolyte and continuously accumulate. Copper, iron, and nickel: These also dissolve in the electrolyte and will not contaminate the gold at the cathode.
