Utilization of phosphogypsum for the preparation of α‐calcium sulfate hemihydrate in chloride‐free solution under atmospheric pressure

BACKGROUND This work aims to investigate the possibility of recycling by‐product phosphogypsum (PG) as raw material for preparing α‐calcium sulfate hemihydrate (α‐CSH) in chloride‐free salt solution under mild conditions. The dehydration of PG in a concentrated Ca(NO3)2 solution at 97 °C under atmospheric pressure was analyzed. The effect of Al2(SO4)3 on the phase‐transition kinetics as well as succinic acid concentration on the crystal morphology of α‐CSH were investigated in detail. RESULTS α‐CSH was successfully prepared from PG in 3.7 mol L‐1 Ca(NO3)2 solution at 97 °C under atmospheric pressure. Addition of Al2(SO4)3 accelerates dehydration rate of PG and decreases crystal size of α‐CSH. Moreover, succinic acid is an effective morphology modifier for α‐CSH. When the reaction is carried out with 5.84 mmol L‐1 Al2(SO4)3 and 0.15 wt% succinic acid, the α‐CSH obtained has aspect ratio of 1.2, and the corresponding paste shows enhanced 3D bending/compressive strengths of 11.3/41.7 MPa, which are comparable with those of commercial α‐CSH. CONCLUSIONS This chloride‐free system provides an improved alternative for preparing high quality α‐CSH, and helps to promote value‐added utilization of PG. © 2018 Society of Chemical Industry