*3.2. CaCl<sup>2</sup> Production by Acid Leaching*

The final calcite concentrate obtained after the flotation process was treated with HCl to produce CaCl2. The effects of the concentration, temperature, and time on acid leaching were investigated. After the determination of the most suitable condition, CaCl<sup>2</sup> was mixed with potassium feldspar in certain proportions, the roasting and dissolution processes were completed, and finally, potassium was successfully extracted. *Minerals* **2021**, *11*, x FOR PEER REVIEW 10 of 13

#### 3.2.1. Effect of HCl Concentration 3.2.2. Effect of Temperature

Based on the formula in Equation (1), the acid treatment was applied to the calcite concentrate. HCl was used as a solvent to investigate the effect of various acid concentrations (0.25, 0.5, 1, 2, 3, 4, and 6 mol/L) on CaCl<sup>2</sup> production. The leaching temperature and time were chosen as 60 ◦C and 10 min, respectively. The HCl-Calcite ratio was adjusted to 2:1 by weight. Calcium dissolution efficiencies versus acid concentration are illustrated in Figure 8. After a suitable acid concentration was obtained, the effect of temperature on calcium dissolution efficiency was investigated at different temperatures (25, 40, 60, and 80 °C). The HCl–Calcite ratio was kept constant at 2:1, and tests were performed with a 10 min leaching time, a mixing speed of 350 rpm, and 2 different HCl concentrations (1 and 2 mol/L). The calcium dissolution efficiencies are presented in Figure 9.

**Figure 8.** Calcium dissolution curve as a function of acid concentration.

**Figure 8.** Calcium dissolution curve as a function of acid concentration.

100 Figure 8 clearly shows that 2 mol/L HCl provides an optimal calcium extraction rate of 93.7%. The 2 mol/L HCl concentration was kept constant in the following experiments, as there was no more than a 3% increase in dissolution efficiency at higher acid concentra-

**Figure 9.** Calcium dissolution efficiency curve in different hydrochloric acid concentrations

1 mol/L HCl

2 mol/L HCl

0 20 40 60 80 100

Temperature, °C

depending on temperature.

50

60

70

80

Ca Dissolution, %

90

tions. Similar results were obtained in the study of Zhang et al. [11], who performed the dissolution experiments to produce CaCl2. The highest calcium dissolution rate (96.1%) was obtained at a 4 mol/L HCl concentration, while 1 mol/L HCl led to only 81.9% calcium dissolution. To determine the amount of HCl consumption for our case, the titration method was chosen and the acid consumption was found to be 165 kg/ton for this study. mol/L). The calcium dissolution efficiencies are presented in Figure 9. 80 100

After a suitable acid concentration was obtained, the effect of temperature on calcium dissolution efficiency was investigated at different temperatures (25, 40, 60, and 80 °C). The HCl–Calcite ratio was kept constant at 2:1, and tests were performed with a 10 min leaching time, a mixing speed of 350 rpm, and 2 different HCl concentrations (1 and 2

*Minerals* **2021**, *11*, x FOR PEER REVIEW 10 of 13
