Printed Circuit Board (PCB) manufacturing is one of the fast‑growing segments within the global electronics industry. Gold‑plating procedures widely adopt cyanide‑formulated plating solutions, where cyanide serves as a complexing agent for gold ions. Cyanide‑containing gold‑plating waste liquid, gold‑stripping solution and its rinse water contain toxic cyanide ions and dissolved gold. Direct discharge will trigger severe environmental pollution, yet such waste streams hold high resource recovery value, with typical gold concentration around 0.1 g/L. Conventional gold‑recovery technologies include zinc‑powder cementation, activated‑carbon adsorption and conventional electrolysis. Zinc‑powder replacement generates extra chemical by‑products and increases subsequent wastewater‑treatment loads. Activated‑carbon adsorption is applicable only for low‑gold dilute waste and suffers difficult regeneration. Traditional electrolysis using flat stainless‑steel or titanium cathodes suffers low electrolytic efficiency and difficult separation of deposited gold from cathode plates. A patented electrolytic process addresses these drawbacks, achieving high‑rate gold recovery and partial cyanide degradation for PCB cyanide‑bearing gold‑plating waste liquor.
Raw Waste Liquid Scope and Full‑Process Technical Steps
The target waste liquid features gold ion concentration between 10 mg/L and 1 000 mg/L. The complete procedure consists of three core steps: pretreatment, electrolytic gold deposition, and gold‑powder collection and purification.
Step 1: Pretreatment
Glucose is added into the cyanide‑bearing gold‑plating waste liquid to trigger redox reactions with oxidizing agents present in the industrial waste stream. The reaction produces non‑oxidative waste liquid ready for electrolysis. Oxidizing substances must be eliminated; otherwise, the cathode mesh will corrode during electrolysis. Operators can test residual oxidizing activity with potassium‑iodide‑starch test paper. A dark‑blue colour indicates remaining oxidants. An alternative practical verification method is immersing a pre‑weighed waste gold plate into treated waste liquid for 10 minutes. If the weight remains unchanged after drying, oxidizing activity has been fully removed.
Step 2: Electrolytic Gold Deposition
The pretreated non‑oxidative cyanide‑bearing waste liquid is pH‑adjusted to 3.5‑9.5 before entering the electrolytic cell. Electrode configuration: A cylindrical mesh cathode is sleeved outside a central rod‑shaped anode. The cathode mesh is copper or copper alloy with copper mass fraction ≥ 85 %. The anode is made of titanium or titanium alloy material. Electrical parameters: DC voltage 2.5 V‑4 V, operating current 10 A‑20 A. Conductive salts can be added into solution when necessary to improve electrical conductivity and boost electrolytic performance. A circulation pump is installed inside the electrolytic tank. Its outlet is arranged at one end of the electrode assembly. Circulating flow accelerates mass transfer of gold‑bearing complex ions toward cathode surfaces. The electrolysis operation shall be terminated when residual gold content in waste liquid drops below 5 ppm.
Electrochemical reactions take place on both electrodes. Gold exists as Au(CN)₂⁻ complex ions in cyanide solution. At the cathode, Au(CN)₂⁻ gains electrons and elemental gold deposits onto the mesh cathode. Anode reactions decompose cyanide ions into carbon dioxide, nitrogen and minor ammonia gas. Cyanide concentration of electrolyzed effluent decreases by 20 %‑30 %, reducing the burden of subsequent cyanide‑removal wastewater treatment. Since ammonia and other gaseous by‑products are generated, electrolysis must proceed under well‑ventilated conditions.
Step 3: Gold Recovery and Purification
After electrolysis completes, the gold‑deposited cylindrical cathode mesh is taken out and fragmented. The fragmented cathode material is dissolved in mixed acid (aqua regia). The mixed acid is prepared by mixing analytical‑grade hydrochloric acid and analytical‑grade nitric acid at volume ratio 3‑4 : 1. The mass‑to‑volume ratio of gold‑loaded cathode material to mixed acid is controlled at 1‑2 : 5.
After dissolving and solid‑liquid filtration, urea is added into filtrate to remove residual unreacted nitric acid. Then sulphur dioxide (SO₂) is introduced as reducing agent to fully reduce gold ions. Vacuum filtration is performed after reduction reaction finishes. Filter cake is washed with 80 °C‑90 °C hot hydrochloric acid and filtered again, followed by water rinsing. The obtained solid substance is crude gold powder.
After removing moisture from gold powder, users may carry out high‑temperature smelting: calcine gold powder at 1085 °C to obtain solid gold ingots.
Key chemical reactions in the gold‑refining stage:
- Au + 4HCl + HNO₃ → HAuCl₄ + NO↑ + 2H₂O
- 2HAuCl₄ + 3SO₂ + 6H₂O → 2Au↓ + 8HCl + 3H₂SO₄
Process Performance and Technical Advantages
The process achieves gold recovery efficiency higher than 95 %, and residual gold in electrolytic effluent falls below 5 ppm. Recovered gold purity reaches 99.95 %, which meets commercial gold‑trading standards.
Main technical merits are listed below:
- No new foreign chemical contaminants are introduced into the solution during electrolysis; gold separation features high selectivity for high‑purity gold output.
- In‑situ partial cyanide decomposition occurs in electrolysis, lowering cyanide load for downstream wastewater treatment, bringing both economic and environmental benefits.
- Cylindrical large‑surface‑area copper‑alloy mesh cathode replaces traditional flat‑plate cathodes, improving electrolytic efficiency. Fragmentation and acid‑dissolution treatment of spent cathode mesh solves the long‑standing difficulty of mechanically stripping gold from solid cathode plates.
Even though cyanide‑free gold‑plating technologies are being researched and promoted globally, cyanide‑based gold‑plating is still widely adopted in PCB production for stable bath performance and good plating quality. This electrolytic recovery method provides a practical industrial solution for treating cyanide‑containing gold‑plating waste liquid, helping electronics manufacturers realize circular‑economy precious‑metal recycling while complying with environmental‑protection regulatory requirements.
