New perspective on the depassivation mechanism of chalcopyrite by ethylene thiourea

The catalytic effect of ethylene thiourea (ETU) during the acidic ferric sulfate leaching of chalcopyrite was studied through various approaches. Bioleaching experiments showed that the addition of ETU rapidly dissolved the Cu-rich passivation layer on chalcopyrite, bringing the dissolved Cu/Fe mole ratio back to 1. Continuation of the leaching experiment shows that ETU also enhances copper extraction from depassivated chalcopyrite. Bioleaching tests carried out on pure covellite showed a similar catalytic effect from ETU. Chemical leaching tests conducted on chalcopyrite in acidic ferric/ferrous sulfate solution confirmed that the catalytic effect of ETU does not originate from changes in solution potential. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) of the leached chalcopyrite surfaces showed that ETU can supress the formation of a copper polysulfide phase during leaching. Ex-situ I-V analysis proved that the product layer is a p-type semiconductor and forms a diode with the n-type bulk mineral. ETU helped dissolve the p-type surface product layer and therefore transitioned the passivated chalcopyrite from a diode back to its native resistor form. Electrochemical impedance spectroscopy (EIS) analysis further demonstrated that the catalytic effect of ETU is more pronounced for passivated chalcopyrite than for a fresh surface. Combined, the results prove that ETU is an excellent remedy for passivated chalcopyrite as it readily catalyzes the dissolution of the passive layer and then continues to catalyze the leaching of native chalcopyrite.