Alkaline precipitation and aging of Cu(II) in the presence of sulfate

Abstract The effect of several operating parameters (reagents concentration, molar ratio, mixing protocol) on the composition and aging of the precipitates obtained by mixing cupric sulfate solutions with NaOH solutions was studied. The precipitate was characterized with chemical analysis (SO 4 /Cu molar ratio), scanning electron microscopy, and X-ray diffraction. pH, free cupric ion activity, and sulfate concentration were measured in the soluble phase. Precipitation of either basic cupric sulfates or cupric hydroxide/oxide may take place depending on the sequence of reagents addition. Provided that the base/CuSO 4 molar ratio does not exceed 1.5, the slow addition of base to dilute CuSO 4 solutions produces either pure posnjakite [CuSO 4 · 3Cu(OH) 2 · H 2 O] or posnjakite and brochantite [CuSO 4 · 3Cu(OH) 2 ] mixtures. Upon aging, posnjakite converts to the more stable brochantite, in agreement with previous works indicating posnjakite as a kinetic precursor of brochantite. The addition of excess base to the basic salt induces its slow conversion to tenorite (CuO), changing the morphology of the precipitate from platelet-like basic salt particles to feathery bundles of tenorite needles. Aged precipitate suspensions contain brochantite or tenorite, the predicted stable solids at lower and higher pH, respectively. However, the actual predominance region of brochantite seems to extend itself, by at least one pH unit, over the tenorite predominance region predicted by the geochemical model MINTEQA2. This discrepancy has been attributed to the small dimensions, and consequent higher solubility, of tenorite crystals. Potentiometric measurements on aged tenorite suspensions support this hypothesis, indicating a solubility product of about one order of magnitude higher than expected. The results of this work can help to understand and control the formation and conversion of basic cupric sulfates, that are of great importance in such fields as removal and recovery of copper from industrial wastewaters and sludges, and patina formation in copper corrosion.