Effect of Molecular Structure of Organic Acids on the Crystal Habit of α-CaSO4·0.5H2O from Phosphogypsum

Xianbo Li,Qin Zhang

doi:10.3390/cryst10010024

Abstract

The organic acid crystal modifiers play an important role in the control of the crystal habit of α-hemihydrate gypsum (α-CaSO4·0.5H2O) from phosphogypsum, but the molecular structure characteristics of crystal modifiers have not been clarified, which makes it difficult to judge whether an organic acid has the ability to regulate the crystal habit of α-CaSO4·0.5H2O directly. In this work, the effect of organic acids with different molecular structures on the crystal habit of α-CaSO4·0.5H2O and its adsorption differences onto the α-CaSO4·0.5H2O surface were explored. The results show that the molecular structure characteristics of crystal modifiers contain two or more carboxylic groups (COOH) that are separated by two methylene or methine groups. Furthermore, organic acids with the regulation ability can adsorb on the surface of α-CaSO4·0.5H2O and change its growth habit. With the increase in the crystal modifier concentration, the α-CaSO4·0.5H2O crystals are shortened in length and enlarged in width, resulting in the decrease in aspect ratio and the increase in compressive strength. Conversely, when the adsorption of ineffective organic acids on the surface of α-CaSO4·0.5H2O was not detected, the α-CaSO4·0.5H2O crystals remained long hexagonal prisms. These results have guiding significance for the screening of novel organic acid crystal modifiers.

Highlights

Phosphogypsum (PG) is an industrial solid waste produced during the production of phosphoric acid from phosphate ore [1,2], which is an important renewable gypsum resource
The preparation of α-hemihydrate gypsum (α-CaSO4 ·0.5H2 O) using PG as the raw material is an important direction for its high value-added utilization. α-CaSO4 ·0.5H2 O has excellent mechanical strength, work performance, and environmental performance, which has been widely used in precision casting, high-end ceramics, functional filler, decorative materials, and so on [8,9]
The results show that the compressive strength increases with an increase in the L-Asp concentration

Summary

Introduction

Phosphogypsum (PG) is an industrial solid waste produced during the production of phosphoric acid from phosphate ore [1,2], which is an important renewable gypsum resource. The main constituent of PG is calcium sulfate dihydrate (CaSO4 ·2H2 O), but it contains impurities such as soluble phosphate, co-crystallized phosphorus, soluble fluoride, and organic matters as well as heavy metals and radionuclides [3,4,5,6]. The preparation of α-hemihydrate gypsum (α-CaSO4 ·0.5H2 O) using PG as the raw material is an important direction for its high value-added utilization. The impurities have a detrimental effect on the preparation of α-CaSO4 ·0.5H2 O; the soluble phosphate and co-crystallized phosphorus can decrease the slurry pH and retard the hydration of α-CaSO4 ·0.5H2 O, and reduce the strength of hardened gypsum. The soluble fluoride can decrease the gypsum strength [10].

Methods

Results

Conclusion