The crystal growth kinetics of alpha calcium sulfate hemihydrate in concentrated CaCl2–HCl solutions

Abstract

The crystal growth kinetics of calcium sulfate α-hemihydrate (α-HH) in nearly constant supersaturated HCl-CaCl 2 solutions were investigated. Two types of solutions were used, the first had a low HCl (1.4 mol/L) and high CaCl 2 (2.8 mol/L) concentration and the second had a high HCl (5.6 mol/L) and low CaCl 2 (0.7 mol/L) concentration. These conditions were chosen to represent the first and last stage of a newly developed stage-wise HCl regeneration process. The seeded growth experiments were carried out in a stirred, temperature controlled semi-batch reactor in which supersaturation was kept constant by simultaneous addition of CaCl 2 and Na 2 SO 4 solutions. The influence of the following parameters on α-HH crystal growth was studied: temperature (70-95°C), specific power input of stirring (0.02-1.29 W/kg) and equimolar inflow rate of CaCl 2 and Na 2 SO 4 (0-0.6 mol/h). The crystal growth rate was derived from particle size distribution measurements made with the laser light diffraction technique. It was found that the surface area normalized crystal growth rate increased linearly with the molar inflow rate up to 0.3 mol/h, at higher inflow rates no further increase of the growth rate was observed. Temperature and specific power input, within the investigated ranges, did not show a marked effect on the growth rate, attributable to a diffusion/adsorption controlled growth process. An interesting finding of the present research is the establishment of a positive relationship between the narrowing of the width of the particle size distribution with increasing crystal growth rate. The results show that the resulting particle size distribution is positively related to the reagent inflow rate, a finding that can be applied to the industrial design and scale-up of the α-HH crystallization/HCl regeneration process.

Phase transformation kinetics of calcium sulfate phases in strong CaCl2HCl solutions

“…1 Upon differential scanning calorimetry analysis (DSC) the HH crystals were confirmed to be of the alpha variety as reported in previous studies (Feldmann and Demopoulos, 2012). The mechanism of transformation was probed via SEM examination.…”

“…The phase purity is further indicated with the aid of the SEM images (Fig. 3) of DH, α-HH and AH, which show distinct morphologies, ranging from plate-like, acicular to fibrous, respectively. These morphologies are typical for calcium sulfate phases formed under the conditions investigated (Al-Othman and Demopoulos, 2009;Feldmann and Demopoulos, 2012;Girgin and Demopoulos, 2004).…”

“…The experiments were conducted in a closed mechanically agitated batch reactor full description of which was provided in a previous publication (Feldmann and Demopoulos, 2012). To obtain a well-mixed solution the speed of a 45°pitched 3-blade glass stirrer was set to 300 rpm and baffles were mounted on the inside.…”

“…In each test the reactor was filled with 750 mL solution made of different CaCl 2 and HCl composition. The tested solution compositions reflect steady state conditions of a continuous reactor circuit (Feldmann and Demopoulos, 2012). In addition crystals of either dihydrate or hemihydrate were added to monitor their transformation kinetics.…”

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

“…[32] Initially, Ca 2+ , SO 4 2-and H 2 O are produced when CSD dissolves in the reaction solution. This process is slow and takes a long time to reach solution saturation.…”