Application
Electrowinning is widely adopted to recover gold from cyanide solutions desorbed by activated carbon. It delivers high-quality finished products and has little impact on the composition of desorption solution. Featuring a fully enclosed operation process, it minimizes precious metal loss and boasts higher safety performance than cementation method, serving as a core procedure in gold hydrometallurgy.
The stainless steel electrowinning cell is key equipment for gold hydrometallurgy. It is mainly applied in the cyanide leaching and activated carbon desorption process to conduct electrolysis on high-concentration gold-bearing desorption solution. By means of direct current electrolysis, gold is precipitated and gathered from pregnant solution so as to realize efficient gold recovery.—Gold adsorption by activated carbon is carried out in complex multi-component solutions containing large amounts of impurities such as copper, zinc, nickel, iron and sulfur. With the selective adsorption property of activated carbon, most impurities remain in tailing slurry and get separated from gold. The loaded carbon screened out from slurry is then desorbed to obtain pregnant solution with high concentration of gold and silver cyanide complex ions and low impurity content, which creates ideal solution conditions for gold and silver recovery via electrowinning or precipitation method.
Working Principle
Gold is extracted from gold-bearing pregnant solution through DC electrolysis. Gold ions are reduced and precipitated under electric field, and finally deposited and accumulated on cathode steel wool, realizing the separation and extraction of gold from liquid. When powered on, the electrolyte circulates inside the cell. Gold-cyanide complex ions continuously migrate to the cathode for reduction and precipitation, while hydrolysis and cyanide ion oxidation reactions occur at the anode to ensure stable overall operation.
Steel wool is commonly used as cathode for gold electrowinning in alkaline cyanide solution. It features excellent alkali resistance without corrosion or dissolution, and causes negligible impact on the purity of desorption solution. The electrowinning parameters are similar for gold recovery from zinc-cyanide complex solution, and a cell voltage of 2.4V can achieve favorable results.
The desorption solution is high-purity gold and silver cyanide solution with gold concentration of 300~600g/m³. Traditional plate cathode leads to extremely low current efficiency for such solution, while expanding cathode specific surface area can achieve ideal electrolysis effect.
Cathodes made of multi-layer stainless steel mesh are corrosion-resistant and convenient for gold recovery. High-pressure water flushing can strip off over 97% of deposited gold. Conventional stainless steel grades including 304 and 316 are fully applicable.
Process
Step 1: Preparation of Gold-bearing Electrolyte
After cyanide leaching of gold ore, gold in pregnant solution is preliminarily enriched by activated carbon adsorption. Gold-loaded carbon is desorbed under high temperature and high pressure to obtain high-concentration gold-rich electrolyte, which is fed into stainless steel electrowinning cells for electrolysis.
Stainless Steel Electrowinning Cell
It adopts rectangular cell body made of plastic. The anode is perforated stainless steel plate, and the cathode is stainless steel wool installed inside stainless steel cathode frames or plastic frames.
Cell body: Integrally formed of corrosion-resistant materials, suitable for working conditions of alkaline gold-bearing electrolyte.
Cathode: Equipped with stainless steel cathode frames filled with stainless steel wool. Its large specific surface area provides sufficient reaction sites for reduction and deposition of gold ions.
Anode: Made of stainless steel plate with stable electrical conductivity, good corrosion resistance and low wear loss, ensuring stable electrolysis operation.
Step 2: Electrodeposition
After power is connected, the electrolyte circulates inside the cell. Gold-cyanide complexes undergo reduction reaction on the surface of stainless steel cathode and are deposited into metallic gold.
Electrolysis Process
Cathode Reaction: Precipitation of gold, silver and evolution of hydrogen gas
Au(CN)₂⁻ + e⁻ === Au + 2CN⁻
Ag(CN)₂⁻ + e⁻ === Ag + 2CN⁻
2H₂O + 2e⁻ === H₂ + 2OH⁻
Anode Reaction: Generation of oxygen and oxidation products of cyanide ions
4OH⁻ – 4e⁻ === 2H₂O + O₂
CN⁻ + 2OH⁻ – 2e⁻ === CNO⁻ + H₂O
2CNO⁻ + 4OH⁻ – 6e⁻ === 2CO₂ + N₂ + 2H₂O
The desorption solution flows through multiple electrowinning cells equipped with pairs of anodes and cathodes. Under current density of 8~15A/m² and cell voltage of 2.5~3.5V, the gold deposition rate exceeds 99%. The spent electrolyte can be reused as desorption solution or sent back to leaching process after supplementing sodium cyanide and sodium hydroxide. During operation, the solution color gradually fades while gold continuously accumulates on steel wool until reaching the scheduled deposition cycle.
Step 3: Gold Refining
The maximum gold loading capacity of stainless steel wool is 20 times its own weight, which shall be taken out before reaching this limit. After electrowinning, take out the gold-loaded steel wool with gold adhered on the surface. The steel wool can be dissolved with hydrochloric acid to obtain gold powder, followed by high-temperature smelting. The steel wool is oxidized into slag, while gold gathers at the furnace bottom due to its high density and chemical inertness, and is finally cast into high-purity gold ingots with purity no less than 99.99%.
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