3.2.2. Effect of Temperature

3.2.2. Effect of Temperature

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. 40 60 Ca Dissolution, %

The results in Figure 9 indicated that the leaching temperature had a considerable effect on the dissolution of calcium. Especially, no significant change was observed after the 2 mol/L HCl concentration at 40 ◦C. On the other hand, calcium dissolution yields increased continuously with increasing temperature while the dissolution of calcium started to stabilized after the 1 mol/L HCl concentration at 60 ◦C. Considering the gelation problem caused by the use of a vacuum after the experiment using a 2 mol/L HCl concentration, a 1 mol/L HCl concentration, and a 60 ◦C temperature were found to be more suitable for the dissolution process. Zhang et al. [11] indicated the importance of temperature on leaching in their study. After 20 min of dissolution at a 4 mol/L HCl concentration, 91.0%, 96.9%, 97.1%, and 97.5% calcium dissolution efficiencies were achieved at 40, 60, 80, and 90 ◦C, respectively. **Figure 8.** Calcium dissolution curve as a function of acid concentration. 0 20 01234567 HCl Concentration, M

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

#### 3.2.3. Effect of Leaching Time

depending on temperature.

The effect of leaching time on the calcium dissolution was investigated using various durations (5, 10, 15, 30, 60, 90, and 120 min). The HCl-Calcite ratio was kept constant at 2:1, and tests were performed at a 1 mol/L HCl concentration, a 60 ◦C temperature, and a mixing speed of 350 rpm. The results are shown in Figure 10. As seen in Figure 10, although there is not much change in recovery, the recovery decreases as the leaching time increases. A 5 min leaching time is not considered sufficient for the system to balance; therefore, a 10 min leaching time was preferred.

**Figure 10.** Calcium dissolution curve at different leaching times. **Figure 10.** Calcium dissolution curve at different leaching times.

#### *3.3. The Roasting Followed by Leaching*

achieved at 40, 60, 80, and 90 °C, respectively.

therefore, a 10 min leaching time was preferred.

3.2.3. Effect of Leaching Time

*3.3. The Roasting Followed by Leaching*  Roasting followed by water leaching was investigated by adopting Equation (2). CaCl2 produced in the previous process was treated with K-feldspar. The aim was to replace calcium ions with potassium at temperatures above the melting point of the feldspar sample. The feldspar—CaCl2 ratio was kept constant at 1:1.5, and the mixture was roasted at different roasting temperatures (800, 850, 900, and 950 °C) for an hour. Water was used as a solvent to dissolve the potassium from the roasted sample. Under constant conditions, a pregnant solution containing potassium ions was obtained. Table 4 Roasting followed by water leaching was investigated by adopting Equation (2). CaCl<sup>2</sup> produced in the previous process was treated with K-feldspar. The aim was to replace calcium ions with potassium at temperatures above the melting point of the feldspar sample. The feldspar—CaCl<sup>2</sup> ratio was kept constant at 1:1.5, and the mixture was roasted at different roasting temperatures (800, 850, 900, and 950 ◦C) for an hour. Water was used as a solvent to dissolve the potassium from the roasted sample. Under constant conditions, a pregnant solution containing potassium ions was obtained. Table 4 shows the results of the roasting process followed by leaching.

The results in Figure 9 indicated that the leaching temperature had a considerable effect on the dissolution of calcium. Especially, no significant change was observed after the 2 mol/L HCl concentration at 40 °C. On the other hand, calcium dissolution yields increased continuously with increasing temperature while the dissolution of calcium started to stabilized after the 1 mol/L HCl concentration at 60 °C. Considering the gelation problem caused by the use of a vacuum after the experiment using a 2 mol/L HCl concentration, a 1 mol/L HCl concentration, and a 60 °C temperature were found to be more suitable for the dissolution process. Zhang et al. [11] indicated the importance of temperature on leaching in their study. After 20 min of dissolution at a 4 mol/L HCl concentration, 91.0%, 96.9%, 97.1%, and 97.5% calcium dissolution efficiencies were

The effect of leaching time on the calcium dissolution was investigated using various durations (5, 10, 15, 30, 60, 90, and 120 min). The HCl-Calcite ratio was kept constant at 2:1, and tests were performed at a 1 mol/L HCl concentration, a 60 °C temperature, and a mixing speed of 350 rpm. The results are shown in Figure 10. As seen in Figure 10, although there is not much change in recovery, the recovery decreases as the leaching time increases. A 5 min leaching time is not considered sufficient for the system to balance;

**Table 4.** Potassium dissolution recoveries after roasting with calcium chloride.

shows the results of the roasting process followed by leaching.


According to the results, the highest potassium extraction was achieved at a 900 ◦C roasting time. Since CaCl<sup>2</sup> is an expensive additive, producing CaCl<sup>2</sup> from local reserves can be helpful in improving both the usage of natural resources and in producing highvalue-added products. In light of the data obtained in this study, it is possible to design a process that allows for the recovery of potassium from wollastonite-calcite and K-feldspar ores in the same region.

#### **4. Conclusions**

As it is known, mineral processing is very significant in terms of both technological and economic evaluation of raw materials before chemical beneficiation. In this study, using K-feldspar and CaCl<sup>2</sup> roasting followed by leaching tests were performed to extract potassium from the natural resources that were located in the same district. For that purpose, the comminuted wollastonite-calcite ore was subjected to flotation tests, and a marketable wollastonite concentrate and a high purity CaCO<sup>3</sup> were produced. Calcite concentrate from the flotation test was treated with HCl to produce CaCl2. Under the

optimal conditions of a 1 mol/L HCl acid concentration, a 60 ◦C leaching temperature, and a 10 min leaching time, 92.7% of calcium was extracted. The CaCl<sup>2</sup> produced by the evaporation method was mixed with K-feldspar in certain proportions and then subjected to roasting experiments followed by dissolving experiments. The optimal feldspar–CaCl<sup>2</sup> ratio was found to be 1:1.5, and 98.6% of the potassium was successfully extracted from the potassium feldspar ore. It is inevitable for economic development to produce KCl, which is very important for the fertilizer industry, from wollastonite-calcite and K-feldspar ores in the same region. Obtaining high-value-added products from different industrial raw materials increases the added value of the raw material considerably and will enable Turkey to use its natural resources more efficiently.

**Author Contributions:** T.T., Z.Ü., F.B., G.B. and M.O.K. conceived and designed the experiments; T.T., Z.Ü. and M.O.K. performed the experiments and analyzed the data; T.T., Z.Ü., F.B., G.B. and M.O.K. contributed to writing the paper. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Istanbul Technical University Circulating Capital Enterprise R&D, Project ID: 42017.

**Data Availability Statement:** No applicable.

**Acknowledgments:** The authors express their sincere thanks and appreciation to Istanbul Technical University Circulating Capital Enterprise R&D and BS Invest Co. for financial support and for kindly providing the feldspar samples.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

