Synthesis of alpha-hemihydrate gypsum from cleaner phosphogypsum

Phosphogypsum (PG), as a waste by-product with high content and complicated impurities, is difficult to be utilised effectively. Herein, cleaner PG is prepared with HCl-H2SO4 method, and thus is used to synthesise alpha-hemihydrate gypsum (alpha-HH) in CaCl2 solution under atmospheric pressure. Morphology and mechanical strength are controlled by introducing varying dosages of modifier (0-0.06 wt% of the PG). Results show that reaction time extends from 120 min to 390 min and average length/diameter (L/13) ratio of alpha-HH crystal is decreased from 10.3 to 0.6 with the increase in the modifier dosage from 0 to 0.045 wt%. When the modifier dosage is 0.04 wt%, the converted product shows the best mechanical performance and fine commercial application prospect. The impurity content of alpha-HH synthesised from cleaner PG significantly decreases to approximately 5% of that from traditional PG. The mechanical strength of alpha-HH prepared from cleaner PG is better than that from traditional PG. The work presents a new and practical method to utilise PG efficiently. (C) 2018 Elsevier Ltd. All rights reserved.