Evolution of concentrated Fe2(SO4)3 solution, a process including both evaporation of the solution and post-evaporation aging of the precipitates, was studied at 2 and 50 °C under controlled relative humidity (RH). At 50 °C and 42–47% RH, ferricopiapite [Fe4.67(SO4)6(OH)2·20H2O] and rhomboclase [(H5O2)Fe(SO4)2·2H2O] first crystallized from the starting solution, and then combined to form kornelite [Fe2(SO4)3·7.5H2O] at 42% RH or to paracoquimbite [Fe2(SO4)3·9H2O] at 46–47% RH. At 2 °C and 34–43% RH, initially crystallized ferricopiapite and rhomboclase appeared to be stable and did not proceed to form a single ferric sulfate hydrate phase over 385 days. At both 2 and 50 °C and RH ≤ 31%, an amorphous ferric sulfate formed. The amorphous ferric sulfate was preserved longer at low RH conditions, e.g., RH ≤ 11%, than higher RH, at which it slowly transformed to crystalline phases of rhomboclase and ferricopiapite, as observed at 31% RH and 50 °C. Combining the results from this study and those from our previous study at 25 °C, the ferric sulfate phase evolution at 2, 25, and 50 °C were mapped and compared. Temperature shows a strong effect on the evolution kinetics; low T may inhibit the evolution from reaching an equilibrium state. Also, an RH and T-controlled in situ X-ray diffraction (RH-T-XRD) method was used to study phase transitions of ferric sulfate hydrates at temperatures from 25 to 80 °C. A dehydration of paracoquimbite to ferric sulfate pentahydrate [Fe2(SO4)3·5H2O] was identified at 80 °C. The results are discussed with a previously constructed ferric sulfate RH–T phase diagram by Ackermann et al. (2009).

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