Selective Cementation of Gold Using an Iron Oxide and Zero-Valent Aluminum Galvanic System from Gold–Copper Ammoniacal Thiosulfate Solutions

Abstract

Ammonium thiosulfate leaching is a promising alternative to the conventional cyanide method for extracting gold from ores. However, strategies for recovering gold from the leachate are less commercially used due to its low affinity to gold. The present study investigated the recovery of gold from the leachate using iron oxides (hematite, Fe2O3 or magnetite, Fe3O4). Cementation experiments were conducted by mixing 0.15 g of aluminum powder as an electron donor and 0.15 g of an electron mediator (activated carbon, hematite, or magnetite) in 10 mL of ammonium thiosulfate leachate containing 100 mg/L gold ions and 10 mM cupric ions for 24 h at 25 °C. The results of the solution analysis showed that when activated carbon (AC) was used, the gold was recovered together with copper (recoveries were 99.99% for gold and copper). However, selective gold recovery was observed when iron oxides were used, where the gold and copper recoveries were 89.7% and 21% for hematite and 85.9% and 15.4% for magnetite, respectively. An electrochemical experiment was also conducted to determine the galvanic interaction between the electron donor and electron mediator in a conventional electrochemical setup (hematite/magnetite–Al as the working electrode, Pt as the counter electrode, Ag/AgCl as the reference electrode) in a gold–thiosulfate medium. Cyclic voltammetry showed a gold reduction “shoulder-like” peak at −1.0 V using hematite/Al and magnetite/Al electrodes. Chronoamperometry was conducted and operated at a constant voltage (−1.0 V) determined during cyclic voltammetry and further analyzed using SEM-EDX. The results of the SEM-EDX analysis for the cementation products and electrochemical experiments confirmed that the gold was selectively deposited on the iron oxide surface as an electron mediator.

Role of Semiconductive Property on Selective Cementation Mechanism of Iron Oxides to Gold in Galvanic Interaction with Zero-Valent Aluminum from Gold–Copper Ammoniacal Thiosulfate Solutions

“…Gold and the lucrative value it bears have caused and continue to cause an increase in demand, which spiked its price up to USD 2000 per ounce, as this precious and novel metal has been used in several applications, such as the use of gold in official coins, in electronic devices, and in other applications, such as the use of gold in medical applications [1][2][3].…”

“…However, it is not currently in use because of its low recovery in pregnant solution; its higher selectivity and leaching rate for gold warrants it being investigated concerning recovery techniques. To address this issue, previous research has proven the possibility of increasing gold recovery via galvanic interaction with zero-valent aluminum, which accounts for up to 99.99% of gold recovery via reductive deposition [1,15].…”

“…Previous research has been employed to investigate the feasibility of iron oxides (the main product of roasting pyrite) as electron mediators in galvanic interaction with zero-valent aluminum in gold-copper ammoniacal thiosulfate media to resolve the issue of refractory gold pyritic ores [1]. Also, the previous study reported that iron oxide can be used as an electron mediator and can cement gold up to 90% on its surface [1].…”

“…Previous research has been employed to investigate the feasibility of iron oxides (the main product of roasting pyrite) as electron mediators in galvanic interaction with zero-valent aluminum in gold-copper ammoniacal thiosulfate media to resolve the issue of refractory gold pyritic ores [1]. Also, the previous study reported that iron oxide can be used as an electron mediator and can cement gold up to 90% on its surface [1]. Also, the difference between activated carbon and iron oxide has been highlighted and is the main interest of this study; activated carbon was able to recover both gold and copper at 99.99%, while the use of iron oxides was able to recover gold and copper around 86-90% and 15-21%, respectively.…”

“…Also, the difference between activated carbon and iron oxide has been highlighted and is the main interest of this study; activated carbon was able to recover both gold and copper at 99.99%, while the use of iron oxides was able to recover gold and copper around 86-90% and 15-21%, respectively. Thus, using iron oxides as electron mediators has selectively cemented gold in a copper ammoniacal thiosulfate medium [1]. The previous findings are advantageous because the copper in a barren solution with ammoniacal thiosulfate can be re-used for gold dissolution as copper ions are the main component in catalyzing gold dissolution.…”