Microwave Heating for Efficient Preparation of α-Hemihydrate from Phosphogypsum: Alleviating the Inhibitory Effect of a Crystal Modifier

The preparation of α-hemihydrate (α-HH) from phosphogypsum (PG) is inevitably hindered by the addition of crystal modifiers, which usually inhibit the conversion of PG and drastically prolong the reaction duration. To overcome this drawback, microwave heating, a volumetric heating technique, was employed to enhance the hydrothermal conversion of PG into α-HH in the presence of Al 2 (SO 4 ) 3, citraconic acid (CTA), and their composite modifier. Meanwhile, the conventional contact heating was also conducted for comparison of the conversion rate. The results indicate that increasing the concentrations of Al 2 (SO 4 ) 3 and CTA enables effective regulation of the crystal morphology and reduces the aspect ratio of α-HH, but this is accompanied by an evident prolongation of reaction time. In the modifier-free system, microwave heating achieved a 50% reduction in reaction period compared with contact heating. For systems containing Al 2 (SO 4 ) 3, CTA, or their composite modifier, the reaction time was reduced by 43–78% relative to contact heating, and PG could be completely transformed into α-HH even at high modifier concentrations. These results verify that microwave heating exhibits outstanding superiority in mitigating the inhibition of crystal modifiers on PG conversion and drastically shortening the reaction time. This work provides a feasible and efficient strategy to address the inhibition issue induced by crystal modifiers during the preparation of short-column α-HH.