Dissolution of the smithsonite-rhodochrosite (ZnCO3-MnCO3) solid solutions in aqueous solution at 25 °C

A series of smithsonite-rhodochrosite solid solutions [(Zn1-xMnx)CO3] (X-Mn = 0.00-1.00) were prepared in lab to study their dissolution process in aqueous solution. The molar ratios Mn/(Zn + Mn) of the synthetic solids (X-Mn) were very close to that of the starting mixtures. The X-ray diffraction (XRD) patterns of the synthetic solids showed continuously peak shifting to lower 2 theta value with the increasing of X-Mn. For the dissolution in the nitrogen-degassed and the air-saturated water, the Zn and Mn contents in aqueous phase went up quickly to a constant value; after dissolution, the weak (200) peak of Zn-5(CO3)(2)(OH)(6) (hydrozincite) emerged in the XRD patterns of the solids with X-Mn = 0.00-0.30. For the dissolution in the carbon-dioxide-saturated water, the Zn and Mn contents in water reached high values quickly after 48 h similar to 120 h, and then declined to a stable value. After 270 d of dissolution, the aqueous molar ratios Mn/(Zn + Mn) were greater than or close to X-Mn of the corresponding solids. For the dissolution in the nitrogen-degassed, the air-saturated and the carbon-dioxide-saturated waters at 25 degrees C, the mean log IAP values (ion activity product) at the equilibrium state (approximate to solubility product log K-sp) were determined to be -10.56, -10.54 and - 10.71 for smithsonite, and - 10.29, -10.26 and - 10.18 for rhodochrosite, respectively. With the increasing of X-Mn the log IAP values increased non-linearly from -10.54 similar to -10.71 to -10.18 similar to -10.29, while the Delta Gf degrees decreased linearly from -735.08 to -736.04 kJ/mol and - 813.99 to -814.63 kJ/mol. The constructed Lippmann diagrams showed that the (Zn1-xMnx)CO3 solid solutions dissolved incongruently, and the dissolution data points moved progressively up and exceeded the solutus curve, and then move along the (Zn1-xMnx)CO3 saturation curves from left to right or right to left for the solids with lower X-Mn (0.10-0.30) and higher X-Mn (0.40-0.90), respectively.