*Article* **Extraction of Potassium from Feldspar by Roasting with CaCl<sup>2</sup> Obtained from the Acidic Leaching of Wollastonite-Calcite Ore**

**Tülay Türk , Zeynep Üçerler , Fırat Burat, Gülay Bulut and Murat Olgaç Kangal \***

Mineral Processing Engineering Department, Faculty of Mines, Istanbul Technical University, Maslak, Istanbul 34469, Turkey; turktu@itu.edu.tr (T.T.); ucerler@itu.edu.tr (Z.Ü.); buratf@itu.edu.tr (F.B.); gbulut@itu.edu.tr (G.B.)

**\*** Correspondence: kangal@itu.edu.tr

**Abstract:** Potassium, which is included in certain contents in the structure of K-feldspar minerals, has a very important function in the growth of plants. Turkey hosts the largest feldspar reserves in the world and is by far the leader in feldspar mining. The production of potassium salts from local natural sources can provide great contributions both socially and economically in the agriculture industry along with glass production, cleaning materials, paint, bleaching powders, and general laboratory purposes. In this study, potassium extraction from K-feldspar ore with an 8.42% K2O content was studied using chloridizing (CaCl<sup>2</sup> ) roasting followed by water leaching. Initially, to produce wollastonite and calcite concentrates, froth flotation tests were conducted on wollastonite-calcite ore after comminution. Thus, wollastonite and calcite concentrates with purities of 99.4% and 91.96% were successfully produced. Then, a calcite concentrate was combined with hydrochloric acid (HCl) under optimal conditions of a 1 mol/L HCl acid concentration, a 60 ◦C leaching temperature, and a 10 min leaching time to produce CaCl<sup>2</sup> . To bring out the importance of roasting before the dissolution process, different parameters such as roasting temperature, duration, and feldspar—CaCl<sup>2</sup> ratios were tested. Under optimal conditions (a 900 ◦C roasting temperature, a 60 min duration, and a 1:1.5 feldspar—CaCl<sup>2</sup> ratio), 98.6% of the potassium was successfully extracted by the water leaching process described in this article.

**Keywords:** potassium; calcium chloride; K-feldspar; wollastonite; roasting–leaching

#### **1. Introduction**

Potassium is an element that occupies an important place in human life and is widely used in different industries as a component of potash. Potassium fertilizer is an essential nutrient, particularly for fruit formation and development. It accelerates plant growth and increases crop yields. In addition to the fertilizer industry, large amounts of potassium salts are used annually to produce glass, cleaning materials, paint, and bleaching powders and for general laboratory purposes [1]. The consumption and cost of potassium salts vary annually and are between 25 and 30 million tons and USD 300 and 500, respectively. In Turkey, potassium salts (potassium chloride (KCl) and potassium sulfate (K2SO4)), which are needed by the chemical and fertilizer industries are provided by imports.

Water-soluble potash is used to produce potassium salts; however, the increase in the need for potassium salts and the decrease in water-soluble potash sources have led the producers to find an alternative source such as K-feldspars. Feldspars constitute more than 60% of the earth's crust and contain microcline, albite, and aluminum silicate with varying concentrations (5–12%) of potassium oxide (K2O) [2–4]. Turkey has approximately 14% of the world's high-quality feldspar reserves. While the most valuable sodium feldspar deposits are located in Western Anatolia (Çine, Milas, Yata˘gan, and Bozdo˘gan regions), the majority of K-feldspar deposits are found in the Kır¸sehir massive [5,6]. In addition to large K-feldspar reserves, there is also a significant amount of wollastonite-calcite deposits in the Kır¸sehir–Buzlukda˘gı region. About 4 million tonnes of potash are imported for agricultural

**Citation:** Türk, T.; Üçerler, Z.; Burat, F.; Bulut, G.; Kangal, M.O. Extraction of Potassium from Feldspar by Roasting with CaCl<sup>2</sup> Obtained from the Acidic Leaching of Wollastonite-Calcite Ore. *Minerals* **2021**, *11*, 1369. https://doi.org/10.3390/min11121369

Academic Editors: Xingjie Wang, Jia Yang and Shuai Wang

Received: 26 October 2021 Accepted: 29 November 2021 Published: 3 December 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

and industrial activities each year in Turkey. KCl and CaCl<sup>2</sup> salts, which are the main raw materials of the fertilizer industry, can be obtained from feldspar and wollastonite minerals, respectively. Obtaining these high value-added products from different industrial raw materials in the same geographical region significantly increases resource use and efficiency while reducing costs.

Wollastonite and calcite minerals, which are used in ceramics, plastics, metallurgy, paint, and other industries are usually found in the same deposits. The most common method for separating these two industrial minerals is flotation. As wollastonite has a needle-like structure, the flotation process can be performed in the presence of collectors such as amines or fatty acids. In a study by Kangal et al., the best result was obtained at pH 6 and 1500 g/t K-oleate dosage. Under these conditions, a calcite concentrate assaying 55.89% CaO, 0.35% SiO2, 0.03% Fe2O3, and 42.30% loss on ignition (LOI) was produced. At the same time, a wollastonite concentrate containing 52.71% SiO2, 44.65% CaO, 0.44% Fe2O3, and 0.60% LOI was obtained [7]. Several investigations have been carried out to separate wollastonite from calcite-rich wollastonite ore [8–10]. Çinku and co-authors indicated that a marketable wollastonite concentrate could be produced by flotation following magnetic separation from wollastonite ore containing 40% CaO and 48% SiO2. As a result of the flotation tests at pH 9, the optimal flotation conditions were determined as 1000 g/t of collector dosage, 1.7 kg/t of sodium silicate, and 500 g/t of caustic soda. Using oleic acid, a concentrate containing 54.43% SiO2, 41.44% CaO, 0.59% Fe2O3, and 0.97% loss on ignition was obtained. In addition, when oleic acid is used, a concentrate assaying 57.10% SiO2, 40.88% CaO, 0.30% Fe2O3, and 0.13% loss on ignition was produced [9]. In another important study by Ravi et al., a 97% purity wollastonite concentrate was obtained from low-content wollastonite ore through the flotation and magnetic separation methods. In the flotation experiments, a particle size of 100 µm and a sodium oleate concentration of 1 kg/t were found to be optimal, and a wollastonite concentrate (about 95% purity) with 2.64% iron content and 0.4% ignition loss was produced [9]. In the study by Bulut and co-authors, a wollastonite (about 87.9% purity) concentrate containing 0.44% Fe2O<sup>3</sup> and 52.71% SiO<sup>2</sup> with 0.60% loss on ignition was obtained using 1500 g/t of potassium oleate. In addition, a calcite concentrate containing 99.8% CaCO<sup>3</sup> was recovered as a by-product with 85.4% recovery [10].

CaCl<sup>2</sup> is an essential component for obtaining potassium salts from feldspar. Although it is not conventional to treat wollastonite with acid to produce CaCl2, acidic digestion of wollastonite can be used as a step in the carbonation pathway for CO<sup>2</sup> sequestration. Zhang et al. [11] studied the leaching of wollastonite at various hydrochloric acid (HCl) concentrations and temperatures. It was reported that 81.9% and 96.1% of calcium were dissolved using 1 and 4 mol/L HCl, respectively. By maintaining a 4 mol/L HCl concentration, constant leaching tests were performed at different temperatures (40, 60, 80, and 90 ◦C) and the calcium dissolutions values were achieved as 91%, 96.9%, 97.1%, and 97.5%, respectively.

In the literature, many studies have been carried out to produce KCl from run-of-themine feldspar ore using pure CaCl<sup>2</sup> salt. Using K-feldspar ore and pure CaCl2, roasting tests were carried out in a high-temperature furnace at various temperatures for 1 h. After the roasting was completed, the leaching process was performed under the constant conditions of a 10% solid-in-pulp ratio, a temperature of 60 ◦C, and a 500 rpm mixing speed [12]. As a result, a 98.3% potassium extraction was obtained with the dissolution process after roasting at 850 ◦C. In another study by Kangal et al. [13], a 90% dissolution efficiency was achieved at a 15% solid-in-pulp ratio and at a 40 ◦C temperature.

Yuan et al. [14] reported a 91% potassium dissolution (under the leaching condition of 70 ◦C, 30 min, and a solid–liquid ratio of 1:50) from a 50–75 µm potassium feldspar fraction with a 13.25% K2O content by roasting at 900 ◦C for 40 min with a 1:1.15 CaCl2– potassium feldspar ratio. Serdengeçti et al. [15] investigated the production of KCl from a potassium feldspar ore containing 9.69% K2O and succeeded in dissolving 99.8% of potassium through water leaching (a 60 ◦C temperature and a 120 min leaching time)

following the roasting process (1:1.5 potassium feldspar—CaCl<sup>2</sup> ratio, 850 ◦C of temperature, and 60 min of roasting time). Samantray et al. [16] investigated the effects of the CaCl<sup>2</sup> amount, roasting temperature, leaching time, and temperature on KCl production from feldspar ore containing 11.64% K2O. After roasting at 900 ◦C and with a ratio of 1:1 CaCl2—feldspar, more than 80% of potassium was dissolved after 30 min. Tanvar and Dhawan [17] investigated the effects of additives, roasting temperature, and time on the recovery of potash from feldspar containing 9.67% K2O. Roasting experiments were carried out at 950 ◦C for 60 min, and CaCl<sup>2</sup> was used as the slag material. At the end of citric acid leaching at room temperature, a 1:10 solid–liquid ratio, and 60 min, a 95% potassium dissolution efficiency was achieved. Samantray et al. [18] investigated the extraction of potassium from feldspar ore containing 11.64% K2O using eggshell powder as a calcium source. Feldspar and eggshells were ground together below 45 µm and roasted at 900 ◦C for 30 min with a 1:1.8 eggshell—feldspar ratio. Then, this mixture was leached with HCl for 30 min at room temperature and a 99% potassium recovery was obtained.

Today, the main approach of beneficiation and production is based on a circular economy and environment. In this study, potassium recovery from K-feldspar ore was achieved using CaCl<sup>2</sup> produced from wollastonite-calcite ore in the same region. In the first part of this study, wollastonite-calcite ore was subjected to flotation tests to produce wollastonite and calcite concentrates. High-grade CaCl<sup>2</sup> was produced through a HCl acid treatment using a calcite concentrate obtained as a result of flotation. Second, roasting followed by the water leaching process was applied to K-feldspar with CaCl2. With this original and innovative work, the extraction of potassium using wollastonite—calcite and K-feldspar ores located in a similar mineralization zone will be possible, and these local natural resources can be brought into the economy.

## **2. Materials and Methods**

#### *2.1. Material and Characterization*

The wollastonite-calcite and potassium feldspar ores were obtained from the Buzlukdagi Region, Kır¸sehir, Turkey. The representative ore samples were crushed below 2 mm using jaw, cone, and roller crushers, and wet sieving was performed to determine the particle size distribution. According to the results, the d<sup>80</sup> size of wollastonite and feldspar samples were found to be 1.2 and 0.5 mm, respectively. The BSE (Back-Scattered Electron) images of samples are given in Figure 1. The chemical contents of the samples (Table 1) were determined by inductively coupled plasma (ICP) at Activation Laboratories Ltd., Hamilton, ON, Canada. To determine the loss on ignition of the samples, the samples were placed in crucibles and weighed. After, they were kept in an oven at 100 ◦C overnight to remove moisture. Then, the temperature was raised to 550 ◦C, and the organic materials were completely removed after 4 h. The carbonate structure turned into oxides at 1000 ◦C after 2 h, and the ratio of the initial and final weight difference to the feed represented the ignition loss. *Minerals* **2021**, *11*, x FOR PEER REVIEW 4 of 13

SiO2 28.00 62.30 Al2O3 1.91 18.70 Fe2O3 0.45 1.89 TiO2 0.04 0.20 Na2O 0.47 4.32 K2O 0.40 8.40 CaO 48.20 2.16 MgO 1.15 0.26 P2O5 0.03 0.09 SrO 0.04 0.14 MnO 0.03 0.06 LOI 20.80 1.83

**Wollastonite-Calcite K-Feldspar** 

W - Wollastonite C - Calcite

**Figure 1.** BSE images of wollastonite-calcite and feldspar samples. **Figure 1.** BSE images of wollastonite-calcite and feldspar samples.

**Figure 2.** X-ray diffraction patterns of the wollastonite-calcite sample.

W

0 10 20 30 40 50 60 70 80

C W

C W W

C W

Position [°2Theta] (Copper(Cu))

**Table 1.** Chemical analyses of the representative samples.

LOI: Loss on ignition.

W

C W

WW

C W

0

500

1000

1500

2000

Counts

2500

3000

3500


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**Table 1.** Chemical analyses of the representative samples.

Wollastonite

Garnet

LOI: Loss on ignition.

An X-ray diffraction (XRD) analysis was conducted with a copper X-ray-sourced Panalytical X'Pert Pro diffractometer (Malvern Panalytical Ltd., Malvern, UK). PDF-4/Minerals International Centre of Diffraction Data (ICDD) software (PDF-4/Minerals, ICDD, Delaware County, PA, USA) was used for mineral characterization. The crystals' mineral phase ratio was determined through the Rietveld method. XRD patterns of wollastonite-calcite and feldspar samples are shown in Figures 2 and 3, respectively. According to the results, the secondary mineral was calcite in the wollastonite-calcite sample and the feldspar sample mostly consisted of potassium feldspar. CaO 48.20 2.16 MgO 1.15 0.26 P2O5 0.03 0.09 SrO 0.04 0.14 MnO 0.03 0.06 LOI 20.80 1.83 LOI: Loss on ignition.

K2O 0.40 8.40

Rutile

**Figure 2.** X-ray diffraction patterns of the wollastonite-calcite sample. **Figure 2.** X-ray diffraction patterns of the wollastonite-calcite sample.

The chemical and mineralogical properties of samples were defined with a differential thermal analysis (DTA) and the thermogravimetric analysis (TGA) equipment's STA 449 F3 Jupiter® thermal analyzer (NETZSCH, Selb, Germany). DTA and TGA curves of wollastonite-calcite, calcite concentrate (from flotation), and feldspar samples are illustrated in Figures 4–6, respectively.

0

200

400

600

800

Counts

1000

1200

1400

A - Albite

F - Feldspar (K-component)

**Figure 3.** X-ray diffraction patterns of the potassium feldspar sample. **Figure 3.** X-ray diffraction patterns of the potassium feldspar sample. samples are illustrated in Figures 4, 5, and 6, respectively.

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A F

**Figure 4.** DTA-TGA curves of the wollastonite-calcite sample. **Figure 4.** DTA-TGA curves of the wollastonite-calcite sample.

**Figure 4.** DTA-TGA curves of the wollastonite-calcite sample. As can be seen in Figure 4, a loss of 21% of weight occurs in the wollastonite ore as the temperature rises from 700 ◦C to 950 ◦C. The main reason for this loss is calcite, which constitutes more than 50% of the structure of the ore. The DTA and TGA results shown in Figure 5 indicate that a significant amount of structural deterioration occurred at 900 ◦C. Only 0.6% of weight loss was determined in the feldspar ore sample between 400 and 800 ◦C (Figure 6). Partial structural deterioration was encountered at 700–750 ◦C. This small loss might be explained by probable carbonate components in the ore.

**Figure 6.** DTA–TGA curves of the potassium feldspar sample. **Figure 6.** DTA–TGA curves of the potassium feldspar sample. **Figure 6.** DTA–TGA curves of the potassium feldspar sample.

#### As can be seen in Figure 4, a loss of 21% of weight occurs in the wollastonite ore as *2.2. Methods*

*2.2. Methods* 

*2.2. Methods* 

As can be seen in Figure 4, a loss of 21% of weight occurs in the wollastonite ore as the temperature rises from 700 °C to 950 °C. The main reason for this loss is calcite, which constitutes more than 50% of the structure of the ore. The DTA and TGA results shown in the temperature rises from 700 °C to 950 °C. The main reason for this loss is calcite, which constitutes more than 50% of the structure of the ore. The DTA and TGA results shown in Calcite must be treated with hydrochloric acid to produce CaCl2. Calcite reacts with HCl to form CaCl<sup>2</sup> and releases CO<sup>2</sup> and H2O as by-products (Equation (1)) [19].

$$\text{CaCO}\_3 + 2\text{HCl} \rightarrow \text{CaCl}\_2 + \text{CO}\_2 + \text{H}\_2\text{O} \tag{1}$$

Only 0.6% of weight loss was determined in the feldspar ore sample between 400 and 800 °C (Figure 6). Partial structural deterioration was encountered at 700–750 °C. This small loss might be explained by probable carbonate components in the ore. °C (Figure 6). Partial structural deterioration was encountered at 700–750 °C. This small loss might be explained by probable carbonate components in the ore. The potassium dissolution process can be finished by mixing the CaCl<sup>2</sup> obtained in the first equation with K-feldspar ore in certain proportions and by subjecting them to

HCl to form CaCl2 and releases CO2 and H2O as by-products (Equation (1)) [19].

HCl to form CaCl2 and releases CO2 and H2O as by-products (Equation (1)) [19].

Calcite must be treated with hydrochloric acid to produce CaCl2. Calcite reacts with

Calcite must be treated with hydrochloric acid to produce CaCl2. Calcite reacts with

roasting and leaching processes. The mixture of feldspar and CaCl2, which is exposed to a temperature higher than the decomposition temperature during the roasting process, reacts to form KCl salt. On the other hand, quartz and anorthite minerals are produced as by-products (Equation (2)) [14].

Since the KCl salt obtained as a result of this process is easily soluble, it can be taken into a solution by leaching with water.

$$\rm CaCl\_2 + 2KAlSi\_3O\_8 \to CaAl\_2Si\_2O\_8 + 4SiO\_2 + 2KCl \tag{2}$$

In the first step, wollastonite-calcite ore was subjected to comminution (below 75 µm) and then froth flotation was used to separate calcite and wollastonite. An amount of 300 gr of wollastonite-calcite ore was subjected to self-aerated Denver flotation equipment with a 1.5 L cell volume and a 1200 rpm mixing rate. The conditions of the flotation tests are given in Table 2. While calcite particles were floated with the help of a collector, wollastonite particles were not activated and taken as the sinking product. Afterward, the calcite concentrate was subjected to HCl acid leaching (Figure 7). Different HCl concentrations (0.25, 0.5, 2, 3, 4, and 6 mol/L), temperatures (25, 40, 60, and 80 ◦C), and leaching times (5, 10, 15, 30, 60, 90, and 120 min) were investigated to determine the optimal treatment conditions with a constant 1:2 solid–liquid ratio. After obtaining the pregnant solution, CaCl<sup>2</sup> was precipitated by evaporation. Second, the CaCl<sup>2</sup> sample obtained from the calcite concentrate was ground and mixed in an agate mortar with potassium feldspar in a 1:1.5 feldspar–CaCl<sup>2</sup> ratio until it had a homogeneous appearance. Afterward, 7.5 g of the mixture was spread in a thin layer on four porcelain crucibles (diameter: 8 cm) and the roasting tests were performed in a Protherm brand PLF 130/6 furnace at elevated temperatures of 800, 850, 900, and 950 ◦C for 1 h. After roasting, water leaching was performed for 2 h under constant conditions, namely a 10% solid-in-pulp ratio, a 60 ◦C temperature, and a 500 rpm mixing speed.

**Table 2.** The conditions of the flotation experiment.


After acid leaching, titration was used to determine HCl consumption. A 0.01 mol/L NaOH solution was prepared and transferred into a 10 mL burette. A beaker containing 30 mL of the test solution was placed under the burette. Four drops of phenolphthalein (a color-change indicator) were added to the solution, and the burette faucet was opened enough to allow a single droplet to pass. A 0.01 mol/L NaOH solution was mixed into the solution until it turned pink, at which point the faucet was closed, and the spent NaOH was detected. Lab-scale vacuum equipment with fine filter paper (Sartorius brand 391 code, blue label) was used for liquid–solid separation and the filtered cake was dried at 70 ◦C for approximately 24 h. The elemental analysis of the liquid was provided by the atomic absorption spectrometer (Varian brand AA240FS, Palo Alto, CA, USA).

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**Figure 7.** General flowchart of the chemical processing following flotation.

#### **Figure 7.** General flowchart of the chemical processing following flotation. **3. Results and Discussion**

#### After acid leaching, titration was used to determine HCl consumption. A 0.01 mol/L *3.1. Flotation of Wollastonite-Calcite Ore*

NaOH solution was prepared and transferred into a 10 mL burette. A beaker containing 30 mL of the test solution was placed under the burette. Four drops of phenolphthalein (a color-change indicator) were added to the solution, and the burette faucet was opened enough to allow a single droplet to pass. A 0.01 mol/L NaOH solution was mixed into the solution until it turned pink, at which point the faucet was closed, and the spent NaOH was detected. Lab-scale vacuum equipment with fine filter paper (Sartorius brand 391 code, blue label) was used for liquid–solid separation and the filtered cake was dried at 70 °C for approximately 24 h. The elemental analysis of the liquid was provided by the atomic absorption spectrometer (Varian brand AA240FS, Palo Alto, CA, USA). To produce CaCl<sup>2</sup> for the roasting and leaching experiments, previously, wollastonitecalcite ore was subjected to flotation experiments after comminution, and a calcite concentrate with low iron content was obtained in the first two flotation stages. In the following steps, the augite mineral was activated and started to float. On the other hand, wollastonite was not activated and remained in the cell despite the increasing potassium oleate concentration. As seen in Table 3, Float-1 and Float-2 products contained high grades of CaCO3, 91.55% and 92.77%, respectively. Float-3, Float-4, and Float-5 were characterized as tailings, since their Fe2O<sup>3</sup> contents were high. A marketable wollastonite concentrate having a 99.4% purity was produced with 0.44% Fe2O3, 0.95% CaCO3, and 49.70% SiO<sup>2</sup> contents using a 1500 g/t potassium oleate. Calcite concentrates obtained in the first two flotation stages were combined and a final calcite concentrate (91.96% CaCO3) with 0.11% Fe2O<sup>3</sup> and 2.14% SiO<sup>2</sup> was produced.


**Table 3.** Results of flotation tests.

C: Content; D: Distribution; P: Purity. \* Combined flotation products.
