L-aspartic acid: A crystal modifier for preparation of hemihydrate from phosphogypsum in CaCl2 solution

Abstract

L-aspartic acid (L-Asp) was proposed as crystal modifier to regulate the morphology of hemihydrate (HH) prepared from phosphogypsum (PG) in CaCl2 solution. Effect of L-Asp on dehydration kinetics of PG, morphology and mechanical properties of HH as well as microstructure of hardened gypsum were investigated by crystal water content, strength and SEM measurements. The adsorption mechanism of L-Asp onto the HH crystals was explored by FTIR, XPS and zeta potential analyses. The results show that the L-Asp chemisorbs on the surfaces of HH by carboxyl (COO−) complexing with Ca atoms, and reduces the dehydration rate of PG to HH by extending the crystal growth period. The dehydration kinetics can be well described by the Boltzmann equation. With increasing L-Asp concentration from 0.00 to 2.50 mM, the HH crystals are shortened in length and enlarged in width with the aspect ratio decreasing from 5.74 to 1.21; meanwhile, the water requirement of normal consistency decreases from (41.5 ± 0.7)% to (31.2 ± 0.3)% and the corresponding bending/compressive strengths increase from (6.2 ± 0.6)/(13.1 ± 1.1) MPa to (10.7 ± 0.7)/(30.2 ± 1.4) MPa, respectively. Furthermore, the hardened gypsum presents a more compact microstructure with uniform in size and morphology in the presence of 2.50 mM L-Asp in comparison with blank test. Therefore, it can be concluded that the L-Asp is an effective crystal modifier for preparation of short columnar HH.