Consideration of enthalpic and entropic energy contributions to the relative rates of chalcopyrite dissolution in the presence of aqueous cationic impurities
To understand the effect of aqueous impurities on chalcopyrite dissolution during acid metalliferous drainage and hydrometallurgical processes, batch dissolution experiments were carried out at 650 and 750mV (SHE), pH1 and 35-75 o C in the presence or absence of aqueous cationic additives. Activation energies (E a ) for chalcopyrite dissolution in the presence of additives at 750mV, derived using a modified 'time to a given fraction' method, demonstrate that E a varies with reaction extent. The overall trend of evolution from interface- to transport-controlled mechanism was independent of additive type or addition. This suggests that it is not primarily variation in the enthalpy of dissolution that controls the significant changes in relative dissolution rates on addition of additives. Rather, the relative dissolution rates on addition of aqueous cations are found to relate to the M-O framework volume (M and O being the cationic additive and oxygen within H 2 O in the first sphere of hydration, respectively). There is good inverse linear correlation between this volume and the relative dissolution rate per unit ionic strength which varies as K + >Al 3+ >no additive>Mg 2+ >Na + >Ca 2+ . It is proposed that this effect is related to the relative strength of hydration of the additives with the commensurate entropic effect on solute hydration being advantageous for chalcopyrite dissolution on K + and Al 3+ addition and detrimental for the other additives. H 4 SiO 4 , the dominant aqueous species upon Si addition under the conditions examined, has low affinity for water, resulting in a detrimental entropic contribution to solute hydration and dissolution. The effect of Fe addition on relative dissolution rate is convoluted by the role of Fe 3+ as the primary oxidant for chalcopyrite dissolution. On addition of aqueous Fe to dissolution at 750mV, the effect is detrimental due to the detrimental effect on the entropy of hydration of the solute and the relative abundance of Fe 3+ . At 650mV the effect of the greater presence of Fe 3+ due to Fe addition dominates and the dissolution rate is enhanced.
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