Morphology‐Controlled Preparation of α‐Calcium Sulfate Hemihydrate from Phosphogypsum by Semi‐Liquid Method

Abstract

Phosphogypsum (PG), a by-product of phosphoric acid industry, causes environmental issues. In this paper, PG was effectively transformed into alpha-calcium sulfate hemihydrate (ɑ-CSH) via a semi-liquid method. The phase evolution and the effects of different organic acids and pH values on crystal morphology of the α-CSH were investigated by SEM, XRD and EDS. The properties of the prepared α-CSH with different aspect ratio were also compared. The results indicate that PG can be successfully transformed into α-CSH by the semi-liquid method at 140°C for 3.0 h. The morphology of the produced α-CSH is significantly influenced by the concentrations of organic acid and pH value. α-CSH crystal with well-morphology is obtained under the condition of 0.1 wt.% maleic acid and pH = 7.0. The obtained α-CSH with uniform particle size has the aspect ratio of 1.2. The 2 h and 3 d bending/compressive strengths of the hardened paste prepared from the α-CSH reach 6.4/20.6 MPa and 10.5/43.7 MPa respectively, which are comparable to those of the commercial α-CSH. Moreover, the lower aspect ratio of ɑ-CSH results in more compact microstructure and higher mechanical strengths of the hardened paste.

Facile preparation of high-strength α-CaSO4·0.5H2O regulated by maleic acid from phosphogypsum: experimental and molecular dynamics simulation studies

“…Without crystal modifier, the 2 h and 3 d bending/compressive strengths of α-HH crystals were 1.79/4.51 MPa respectively. The mechanical strength of α-HH was excellent compared to bending/compressive strengths of α-HH prepared in the modifier-free system, and the performance was superior to that of commercial α-HH [33]. So this product could be widely applied in the field of special binder systems, building materials, ceramics industry, molding, medicine and so on.…”

“…α-HH crystals with a lower aspect ratio shows better mechanical properties than higher aspect ratio [32]. Solid-liquid ratio is one of the most important parameters that determines α-HH crystal morphology [33]. Thus, SEM morphology ( Fig.…”

Preparation of α-calcium sulfate hemihydrate from industrial by-product gypsum: A review

“…Generally, α-HH can be prepared from pretreated PG at a pressure of 0.12~0.70Mpa, a temperature of 105~150°C, and for 2~16h in an autoclave (Singh and Rai, 1988;Yang et al, 2009;He et al, 2014;Duan et al, 2017;Mi et al, 2017). The well-known commercial process for α-HH production from PG by the autoclave process is the Giulini method (Hignett, 1980;Roszczynialski et al, 1996b).…”

“…Meanwhile, in order to further reduce energy consumption and avoid secondary pollution, researchers proposed the semi-dry process (Duan et al, 2017) and the semi-liquid process (Mi et al, 2017), which adopted an autoclave-drying integrated process without washing steps to simplify the hydrothermal autoclave method.…”

The study on the effect of flotation purification on the performance of α-hemihydrate gypsum prepared from phosphogypsum

“…The preparation of α -hemihydrate gypsum by phosphogypsum has attracted the attention of scientists and technicians. Mi et al used the “semi-liquid method” to prepare α-HH from phosphogypsum, and the product obtained by mixing 0.1% Maleic acid at pH = 7 had the best morphology, with a 3d compressive strength of 43.7 MPa and a 3d flexural strength of 10.5 MPa 31 ; Lu et al used phosphogypsum as raw material and studied the effect of the number of cycles on crystal morphology and mechanical strength by using the atmospheric pressure CaCl 2 solution method, and found that high strength gypsum with 95% purity could be prepared after 6 cycles 32 . In the preparation of α-HH from phosphogypsum, the hemihydrate gypsum usually has a long columnar shape and incomplete crystals due to its growth characteristics, so it is necessary to add a crystal transfer agent to assist crystal growth during the conversion process to obtain more dense and regular crystals.…”