Temperature and humidity effects on ferric sulfate stability and phase transformation

Abstract

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 [Fe_4.67(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).