HCl with or without additive was firstly added into the reactor. The amount of additives added was expressed as a percentage of the
total moles of calcium ions in the solution. Under stirring
(200 rpm), the solution was then heated to 102 C, the boiling point
of the solution at the ambient pressure, in an oil bath. Next, the preheated,
80 mL, 1 mol/L CaCl2 and H2SO4 were added into the reactor
using a peristaltic pump at the same flow rate as described in a previous
publication [18]. The resulting reaction mixture was refluxed
for 2 h. Finally, the samples of the reaction mixture were withdrawn,
rapidly filtered, washed with boiling deionized water and
anhydrous ethanol successively, and dried at 60.0 C for 5 h.