The K-feldspar ore powder sample with certain particle sizes was
mixed uniformly with anhydrous calcium chloride powder according
to the specific mass ratios in an agate mortar. The mixture was shaped
into tablets with a diameter and thickness, respectively, of 10 mm and
5–7 mm using a tablet machine (HW-01, Tianjin, China) under 3 MPa
pressure and was then dried in a muffle furnace at 300 °C for 1 h to
remove the water adsorbed during the mixing. Three tablets (approximately
4 g) placed in a porcelain boat were weighed accurately and
calcined in a tube furnace at preset temperatures in a dry or damp nitrogen
atmosphere at gas flow of 3 × 10−4m3·min−1 for certain periods of
time. The calcined slag thus obtained was withdrawn, cooled naturally
to room temperature in a desiccator, pulverized and then leached. The dissolution was conducted in deionized water with the mass ratio of
liquid to solid of 50:1 at 70 °C for 30 min to extract potassium, then
the slurry was filtered (the process is shown in Fig. 2).
The contents of potassium and chlorine in the resulting filtrates
were measured. The extraction ratio of potassium and the thermal hydrolysis
ratio of calcium chloride were calculated. The calcined slag
and leach residue were characterized by XRD and SEM.