Metastability of α-calcium Sulfate Hemihydrate during Preparation from FGD Gypsum in Salt Solution and Anti-fouling Research
|Keywords||α-calcium sulfate hemihydrate metastable lifetime citric acid rotatingcylinder electrode crystallization fouling|
As an important class of cementitious materials, a-calcium sulfate hemihydrate (a-HH) has diversified applications. The salt solution method exhibits distinctive competencies in terms of non-intensive energy consumption and enormous productivity, which has been granted as a potential alternative for a-HH preparation. However, α-HH prefers to hydrate to dihydrate (DH) or dehydrate to anhydrite (AH) in salt solution due to its metastability, leading to the decrease of product quality and formation of undesirable crystallization fouling. Therefore, α-HH metastable lifetime (MLT) expansion is of great importance for product quality control and scale prevention on pipe walls. In this paper, the MLT of α-HH was comprehensively investigated in Ca-Mg-K salt solutions. On the basis of experimental results in batch reactor, effects of temperature and fluid hydrodynamics on the scale deposition of calcium sulfate on pipe surface were established.For the first time, the phase transit from a-HH to DH or AH was carefully traced containing citric acid followed by a careful measurement of MLTs. α-HH transforms to DH and exhibits an increase in MLT from0.5h to48h along with temperature from25to65℃, and then the transformation bends to AH and shows a monotonous decrease in MLT to6h at95℃. A0.1-1.0mM of citric acid allows a redouble expansion of the MLT, particularly in a temperature range of45-75℃beyond54hours with1.0mM citric acid addition.The rotating cylinder electrode (RCE) apparatus under continuous flow was exploited to simulate calcium sulfate scale formation on PVC substrate. The influences of temperature, the fluid hydrodynamics, and citric acid were explored. The DH scale deposition rate decreases from5.36×10-4to2.06×10-5mg/(cm2·h) with temperature increases from25to55℃, whereas a further temperature increase from65to95℃results in an increasing AH scale deposition rate from2.66×10-4to6.01×10-4mg/(cm2·h). The scale deposition rate represents an increasing trend with Reynolds number increases. The formation of crystallization fouling is predominantly controlled by diffusion mechanism in the laminar flow. Citric acid shows strongly retarding effects especially for DH scale formation on the specimen.The dissertation reveals that trace amount of citric acid considerably expands MLT of a-HH in salt solution. The optimization and modification of the hydrodynamic conditions and citric acid concentrations should be taken into serious account for the mitigation of calcium sulfate crystallization fouling. This work provides a guideline for product quality control as well as scale prevention during industrial preparation of a-HH.