Gold leaching by copper(II) in ammoniacal thiosulphate solutions in the presence of additives. Part II: Effect of residual Cu(II), pH and redox potentials on reactivity of colloidal gold

摘要

The effect of additives on the dissolution of gold colloids (20 nm) by copper(II) in ammoniacal thiosulphate media has been investigated by measuring the residual gold and Cu(II) concentrations in solution using UV-visible spectrophotometry, and the redox/mixed potentials (E H or E mix) using platinum and gold electrodes immersed in solution. Results show a beneficial effect of increasing free NH 3 on gold dissolution caused by an increase in pH from 8.9 to 11 which enhances residual Cu(II) and E H. The beneficial effect of additives follows the descending order: AgNO 3 > NaCl > Na 2SO 4 > no additives ∼ Na 2CO 3 > NaNO 3 > Pb(NO 3) 2 > Na 2S 4O 6 ∼ Na 2S 3O 6 > Na 2SO 3. The two reagents AgNO 3 and NaCl do not affect the residual Cu(II) concentration, E H or E mix but enhance gold dissolution (∼ 100% in 1-2 h) due to the involvement of Ag(I) and Cl - in the anodic reaction of gold. Both Na 2SO 4 and Na 2CO 3 have beneficial effects on residual Cu(II) which enhance E H and retard the reaction with thiosulphate due to ion-association and stabilisation of Cu(NH 3) x 2 +. Thus, the gold dissolution is slow and steady with Na 2CO 3 (95% in 5 h). Despite the faster initial gold dissolution in the presence of Na 2S 3O 6 due to high E H and E mix, gold dissolution reaches a plateau (∼ 85% after 2 h). The presence of Na 2S 4O 6 causes lower E H and E mix, lower residual Cu(II); and gold dissolution reaches a plateau (∼ 85% after 3 h), indicating surface blockage. Strong interaction between Cu(II) and Na 2SO 3 is evident from very low residual Cu(II), low E H and E mix, causing low gold dissolution reaching a plateau (∼ 20%) after 6 h.