Review on K-Feldspar Mineral Processing for Extracting Metallic Potassium as a Fertilizer Resource

Abstract

The K-feldspar mineral is an insoluble potassium resource with a high potassium content and the most extensive and abundant reserves. To address the insufficient supply of soluble potassium fertilizers in China, the application of appropriate processing methods to extract potassium from K-feldspar and transform it into a soluble potassium fertilizer is of great significance. To date, various techniques have been developed to extract potassium from K-feldspar and produce a soluble potassium fertilizer. This review summarizes the main methods, i.e., the hydrothermal, high-temperature pyrolysis, microbial decomposition, and low-temperature methods, for potassium extraction from K-feldspar. The mechanisms, efficiencies, impact parameters, and research progress of each potassium extraction method are comprehensively discussed. This study also compares the merits and drawbacks of the individual methods in terms of potassium extraction efficiency and practical operating conditions. The species of additives, reaction temperature, reaction time, particle size of K-feldspar, and dosage of additives significantly affected the potassium extraction efficiency. Moreover, the combination of different methods was very effective in improving the potassium extraction efficiency. This review elaborates the research prospects and potential strategies for the efficient utilization of the K-feldspar mineral as a fertilizer resource.

1. Introduction

With the world’s population growing rapidly, the challenge of increasing food production on limited arable land has become a critical issue for global agriculture [1]. Fertilizers are fundamental and vital inputs in modern agricultural production [2]. Among the essential fertilizers for crop growth, potassium is one of the three key nutrients, and its application to the soil can significantly increase crop yield by approximately 30% [3].

Potassium resources in the world are categorized into two types: soluble potassium resources and insoluble potassium resources [4]. Soluble potassium resources mainly include water-soluble potash such as seawater and brine from salt lakes, as well as soluble potassium minerals like potassium rock salt and halloysite [5]. On the other hand, insoluble potassium resources mainly consist of various types of potassium-rich rocks such as K-feldspar, white mica, and hydromica-like clay minerals [6]. Among them, soluble potassium resources have been the chief sourcing needed by the potash industry due to their low mining cost [7]. However, China’s soluble potassium resources are few, accounting for only 2.20% of the world’s reserves [8], which has resulted in the potash industry in China being constrained by the international potash market [3]. Fortunately, insoluble potassium resources have huge reserves, estimated to be more than 20 billion tons [9]. Therefore, insoluble potassium resources can serve as a viable alternative source for potassium fertilizers and there is an urgent need to accelerate the technological research on the development and utilization of insoluble potassium resources for the manufacture of potassium fertilizers.

Potassium feldspar (K-feldspar), as a type of insoluble potassium resource, is abundant in China, with reserves of over 10 billion tons [10]. K-feldspar is characterized by a higher potassium content, a wide distribution, and the large reserves [11]. However, compared to the recovery and processing of potash from soluble mineral raw resources, extracting potash from the insoluble mineral K-feldspar is more costly. This is because it requires extra processing procedures to convert insoluble potassium into soluble potassium, which necessitates considerable production efforts such as elevated temperatures, pressures, and the addition of additives (chemical, biological, etc.). Therefore, in order to solve the problem of the imbalance between supply and demand of potassium fertilizer in China, the efficient and cost-effective utilization of K-feldspar resources to produce soluble potassium fertilizers is of great significance [12]. Additionally, because the soluble potash deposits are abundant in Western countries, there has been relatively less research on the utilization of insoluble potash ores for the production of potassium fertilizers [6]. Hence, it is crucial to explore efficient technologies for extracting potassium from insoluble K-feldspar. In light of this background, this review analyzes and compares the efficiency of potassium extraction from K-feldspar using four different methods, i.e., the hydrothermal method, high-temperature pyrolysis method, microbial decomposition method, and low-temperature method, providing a comprehensive theoretical reference and research direction for future technological research on K-feldspar to extract potassium.

2. Introduction of Potassium Extraction from K-Feldspar

The chemical formula of K-feldspar is KAlSi₃O₈. Theoretically, pure K-feldspar is composed of 16.9% K₂O, 18.4% A1₂O₃, and 64.7% SiO₂ [13]. The crystal structure of K-feldspar is a three-dimensional mesh framework, as shown in Figure 1. Its basic unit cell consists of tetrahedra [SiO₄]⁴⁻ or [AlO₄]⁵⁻, which is formed by one Si or Al atom surrounded by four oxygen O atoms, as shown in Figure 1. Two tetrahedra units ([SiO₄]⁴⁻ or [AlO₄]⁵⁻) share one O atom to form a three-dimensional mesh framework through valence bond linkage, with K⁺ located in pores and channels within the skeleton, neutralizing the electrical properties [14]. The stable tetrahedral mesh framework of K-feldspar causes its physical and chemical stability to be very high; hence, under room temperature and atmospheric pressure, it has the ability to endure intense acids or alkalis. As a result, the potassium (K) in K-feldspar is not readily available to plants. Therefore, the development of technologies to convert the insoluble potassium in K-feldspar into soluble potassium is crucial for efficient utilization [15].

It is necessary to provide suitable additives and sufficient energy to break the crystal structure of K-feldspar to release soluble K+ with the purpose of both extraction of potassium and conversion of insoluble K-feldspar to a soluble phase [6,17]. Once the soluble potassium has been extracted from K-feldspar by using a variety of techniques, the extracted potassium is collected using a leaching solvent, such as water or acid. The potassium extraction efficiency of K-feldspar can be calculated by Equation (1):

$${\eta }_{K^{+}}=\frac{m_2}{m_1}\times 100\%$$

(1)

where η_K⁺ represents the potassium extraction efficiency of K-feldspar, m₁ represents the total mass of potassium (in the form of K₂O) in the original K-feldspar, and m₂ represents the mass of soluble K⁺ (in the form of K₂O) in the leaching solvent.

3. Progress of Potassium Extraction from K-Feldspar

Potassium extraction from K-feldspar has been reported recently utilizing a variety of techniques, including the hydrothermal method, high-temperature pyrolysis method, microbial decomposition method, and low-temperature method. In the following sections, the methodology, mechanisms, effect parameters, potassium extraction efficiency, as well as the advantages and disadvantages of each technique, will be discussed in detail.

3.1. Hydrothermal Method

The hydrothermal method involves potassium extraction in a solution containing alkaline additives (e.g., Ca(OH)₂, NaOH, and KOH) from K-feldspar in a closed system at 150–300 °C. Under these conditions, the pressure reaches 0.3–6.1 MPa depending on the temperature. After treatment with the hydrothermal method, the corrosion effect of the alkaline additives destroys the structure of K-feldspar, which leads to insoluble potassium solubilization in K-feldspar dissolving, leaching, and then converting into soluble potassium. Simultaneously, the other components of K-feldspar are transformed into other mineral phases [10,16]. The transition state theory can be used to described this process [18]. Firstly, there is an exchange between K⁺ in K-feldspar and cations in alkaline additives, such as Ca²⁺ and Na⁺, and as a result, the soluble K⁺ is leached out from the basic unit of K-feldspar as shown in Equation (2). Secondly, the corrosion by hot alkaline additives leads to the formation of surface silica-rich and aluminum-poor precursor aggregates (SiO₂-nH₂O), causing the breakage of the Al-O-Si bond as well as the aluminum leaching, as shown in Equation (3). Thirdly, the Si-O-Si group in the (SiO₂-nH₂O) undergoes irreversible hydrolysis, eventually resulting in the formation of other mineral phases, such as tobermorite, as described by Equation (4) [19,20].

$$\mathrm{C}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{s}\left({\mathrm{C}\mathrm{a}}^{2+},{\mathrm{N}\mathrm{a}}^{+}\right)+{\mathrm{K}\mathrm{A}\mathrm{l}\mathrm{S}\mathrm{i}}_3{\mathrm{O}}_8\to \mathrm{C}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{s}-{\mathrm{A}\mathrm{l}\mathrm{S}\mathrm{i}}_3{\mathrm{O}}_8+{\mathrm{K}}^{+}$$

(2)

$$\mathrm{C}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{s}-\mathrm{A}\mathrm{l}{\mathrm{S}\mathrm{i}}_3{\mathrm{O}}_8+{\mathrm{O}\mathrm{H}}^{-}+{\mathrm{H}}_2\mathrm{O}\to \left({\mathrm{S}\mathrm{i}\mathrm{O}}_2·{\mathrm{n}\mathrm{H}}_2\mathrm{O}\right)+\mathrm{A}\mathrm{l}{\left(\mathrm{O}\mathrm{H}\right)}_4^{-}$$

(3)

$$\left({\mathrm{S}\mathrm{i}\mathrm{O}}_2·{\mathrm{n}\mathrm{H}}_2\mathrm{O}\right)\to \mathrm{t}\mathrm{o}\mathrm{b}\mathrm{e}\mathrm{r}\mathrm{m}\mathrm{o}\mathrm{r}\mathrm{i}\mathrm{t}\mathrm{e}$$

(4)

3.1.1. Ca(OH)₂ Alkali Additive

In the hydrothermal method, alkali additives usually include three alkalis, i.e., Ca(OH)₂, NaOH, and KOH. The process using Ca(OH)₂ as the alkali additive is simple and relatively clean. Lan et al. [21] found that the potassium extraction efficiency η_K⁺ exceeded 90% when using Ca(OH)₂ as an alkaline additive. In addition, some studies have explored the use of low-cost CaO as an alternative to Ca(OH), considering economic considerations. For instance, Liu et al. [22] utilized CaO as the alkaline additive and achieved a η_K⁺ higher than 95% at the hydrothermal temperature of 250 °C. Previous studies revealed that the reaction factors such as reaction time, reaction temperature, water-to-solid ratio, and Ca/Si molar ratio significantly influence the η_K⁺, while the impact of K-feldspar origin is relatively minor. Liu et al. [22] found that, as the temperature or reaction time increased, the η_K⁺ showed an initial increase followed by a leveling off. Specifically, the η_K⁺ increased from 62% to 72% in the case of an increase in reaction time to 4 h from 1 h. However, with a further increase in reaction time to 5 h, a slight decrease in η_K⁺ to 71% was observed. With respect to this phenomenon, there is a predominant explanation that the reaction was incomplete at a short reaction time; thus, the potassium extraction was facilitated by prolonging the reaction time, but an excessive reaction time caused the product layer on the surface of the reactants to gradually thicken, obstructing mutual contact among different reactants. Among the studies by Liu et al. [22], they discovered that the extraction efficiency of potassium could be remarkably increased from 70% to 96% following an increase in the reaction temperature from 170 °C to 250 °C. This can be attributed to the higher temperature promoting more intense ionic activity and facilitating a more complete reaction. However, with a further increase to 270 °C, the η_K⁺ did not change significantly. Zhang et al. [23] optimized the potassium extraction efficiency by changing the water-to-solid ratio and found that the optimum η_K⁺ of 85% was achieved at a water-to-solid ratio of 20, due to its improved reaction homogeneity, increased dissolution of K-feldspar, and enhanced K⁺ migration rate. Lower water-to-solid ratios led to high slurry viscosity, while higher ratios reduced the contact area between Ca(OH)₂ and K-feldspar, leading to decreased potassium extraction efficiency. Liu et al. [22] investigated how the molar ratio of Ca/Si impacted the potassium extraction efficiency when using CaO as the additive and revealed that the η_K⁺ went up from 53% to 64% as the molar ratio of Ca/Si increased from 1.3 to 2.0. Nevertheless, a further increment of the molar ratio of Ca/Si up toward 2.1 induced a slight decrease regarding the efficiency of potassium extraction as low as 63%. This decrease was attributed to the formation of excessive Ca(OH)₂, which was generated from the chemical reaction between calcium oxide and water. The increased amount of calcium oxide resulted in excessive microsoluble calcium hydroxide adhering to the surface of K-feldspar, hindering its dissolution and slowing down the hydrothermal reaction process. Zhang et al. [23], Liu et al. [24], and Qiu et al. [25] studied the potassium extraction in different regions using CaO as the alkaline additive. It was demonstrated that there was little effect on the potassium extraction efficiency as the regions varied. In the case of K-feldspar from Henan Province [23], Shanxi Province [25], and Henan Province [24], the extraction efficiency of potassium was comparable, reaching 87%, 84%, and 85%, respectively.

3.1.2. NaOH Alkali Additive

In the NaOH system, changing the reaction parameters shows significant effects on the extraction efficiency of potassium. The reaction parameters of the reaction temperature, reaction time, reaction temperature, and dosage of NaOH have been investigated in-depth in previous studies. For instance, Ma et al. [26] achieved a high potassium extraction efficiency of 93.2% by using NaOH as the alkaline additive when conducting the reaction at 260 °C for 120 min. Hou et al. [27] discovered that the extraction efficiency of potassium was remarkably increased from 69.5% to 91.1% as the reaction temperature increased from 180 °C to 200 °C, attributed to the higher dissolution rate of potassium at higher temperatures. Nevertheless, a further increment of the temperature to 210 °C induced a slight increase regarding the η_K⁺ of only 91.4%. They also studied the influence of reaction time on η_K⁺ in the NaOH system. The findings revealed that by increasing the reaction time from 1.5 h to 2.5 h, the η_K⁺ showed a substantial improvement from 67.5% to 91.1%. This was attributed to the prolonged reaction time allowing for a more thorough reaction. Further increasing the reaction time to 3 h resulted in only a slight increase in η_K⁺ to 92.1%, indicating that there was a minor impact as the reaction time exceeded 2.5 h. Wang et al. [28] investigated the impact of the NaOH dosage and revealed that the efficiency of potassium extraction went up from 9.93% to 73.91% when the NaOH dosage increased from 0.4 g to 1.2 g. This increase in efficiency was attributed to the higher concentration of NaOH, which led to a more complete destruction of the lattice structure of K-feldspar. At an adequate amount of NaOH (1.2 g), a complete destruction of the K-feldspar structure was achieved. Further increasing the NaOH dosage to 1.4 g resulted in only a slight improvement in η_K⁺ to 75.39%. However, the NaOH system still presents challenges, such as the separation of Na⁺ from K⁺ in the products due to the presence of NaOH as the reactant, which poses difficulties in industrial production [29].

3.1.3. KOH Alkali Additive

When KOH is used as an additive, a significant advantage is that there is no need for further separation steps, unlike when NaOH is used. Su et al. [30] employed KOH as an additive to extract potassium, where the insoluble potassium in K-feldspar was converted to soluble potassium K⁺ and a high η_K⁺ of 95.73% was achieved. This process also allowed for the comprehensive utilization technology of silicon and aluminum. It was observed that when the hydrothermal reaction was carried out at 280 °C, the SiO₂ content in K-feldspar was leached out and transformed to K₂SiO₃, where the potassium in K₂SiO₃ could be recycled to KOH through causticizing with lime milk, and the resulting component was converted to kalsilite, a solid product with high solubility in acidic media.

3.1.4. Comparison of Ca(OH)₂, NaOH, and KOH Systems

Ma et al. [31] compared the performance of different additives in the hydrothermal method using Ca(OH)₂, NaOH, and KOH as additives separately. The potassium extraction efficiency was found to be comparable among the three systems, with optimal conditions achieving extraction efficiencies of 91%, 90%, and 95% in the Ca(OH)₂, NaOH, and KOH additives, respectively. This suggests that the choice of alkaline additive does not significantly impact the potassium extraction efficiency.

In practical operation, the three additives (Ca(OH)₂, NaOH, and KOH) have their own advantages and disadvantages when extracting potassium from K-feldspar. When choosing the appropriate method by which to extract potassium out of K-feldspar, it is important to consider the advantages and disadvantages of each system and make a reasonable choice based on the specific requirements and limitations of the application.

In the Ca(OH)₂ system, the filtrate obtained is a KOH solution, which could theoretically be used directly to yield potassium fertilizer. However, this system’s operating conditions are usually harsh, requiring large water-to-solid ratios (15–30), resulting in a low potassium salt concentration in the filtrate (K₂O, 5 g/L), which in turn requires higher energy consumption for concentration and evaporation to obtain high-concentration potassium salt products.

In the NaOH system, the obtained potassium salt concentration in the filtrate is relatively higher (around 50 g/L in K₂O), reducing the energy consumption for subsequent potassium salt production by evaporation. However, the solution produced in this system contains a mixed solution of Na⁺ and K⁺, requiring additional steps to separate Na⁺ and K⁺ to obtain pure potassium salt products, thereby increasing the complexity of practical operation.

In the KOH system, no complex Na⁺ and K⁺ separation process is required, and the KOH required for decomposing K-feldspar can be recycled, making it a potential efficient and clean production technology.

3.1.5. Combined Use of Several Alkaline Additives

In addition to individual use, combinations of additives can also be used to improve the potassium extraction efficiency from K-feldspar. For example, Liu et al. [32] used the combination of NaOH and Ca(OH)₂ additives in a hydrothermal reaction, which showed a higher η_K⁺ of 88.99% after undergoing a hydrothermally reaction at 240 °C for 6 h. It was much superior to the sole Ca(OH)₂ additive (η_K⁺ of 32.36%). The efficiency increase in the combined additives was attributed to the higher OH⁻ ions, where the water-soluble NaOH provided higher OH⁻ ions than the poorly soluble Ca(OH)₂. In addition, Ca²⁺ interacted with K-feldspar to form aggregates of tobermorite crystals on the K-feldspar surface, which hindered the extraction of potassium ions. Similarly, Zheng et al. [33] used the KOH and CaO additives together, and the hydrothermal reaction occurred at 220 °C for 10 h. The results revealed that the efficiency of potassium extraction went up from 40% to 90% by increasing the KOH:CaO mass ratio from 0.050 to 0.067. This increase in efficiency was attributed to the increased strength of the OH⁻ ions in the reaction solution.

3.1.6. Pretreatment before Hydrothermal Method

To further improve the potassium extraction efficiency in K-feldspar, pretreatment steps such as microwave and roasting have been explored prior to the hydrothermal reaction. Zhao et al. [34] subjected K-feldspar to microwave pretreatment at 400 W for 3 min, followed by the hydrothermal method using a combination of additives including NaOH and CaO. The η_K⁺ increased from 74% to 95% with microwave pretreatment. Similarly, another study by Zhao et al. [13] reported that the microwave pretreatment at 600 W for 15 min increased the η_K⁺ from 72.64% to 92%. These improvements are attributed to the uneven heating induced by microwave pretreatment, which creates micro-pores on the K-feldspar surface that facilitate contact with the hydrothermal solution, thereby accelerating the reaction and promoting potassium dissolution.

Roasting K-feldspar prior to a hydrothermal reaction has also been found to effectively improve the potassium extraction efficiency. For example, Liu et al. [35] conducted a study where they first calcined the mixture of CaCO₃ and K-feldspar for 1 h under 1050 °C, with the purpose of adding CaCO₃ to transform it into an alkaline additive calcium oxide (CaO) after calcination. Then, the calcined product was subjected to a hydrothermal reaction for 20 h under 190 °C. As shown, the potassium extraction efficiency increased by 35% compared to the uncalcined pretreatment. This improvement can be explained by the breakage of the skeletal structure that occurred in K-feldspar during calcination, which increased the solubility of K-feldspar and subsequently promoted the release of potassium in the hydrothermal reaction.

3.2. High-Temperature Pyrolysis Method

High-temperature pyrolysis refers to calcining in temperatures ranging from 800 °C to 1500 °C by adding additives, converting K-feldspar into water-soluble potassium salt, and extracting potassium with solvents (hydrochloric acid, nitric acid, etc.). Despite its high energy consumption, this method offers several advantages, such as simple operation process, easy adjustment of operation parameters, easy reaction control, no strong acid or alkali corrosion, and no wastewater yield [36,37]. The additives used in high-temperature pyrolysis can include carbonates, sulfates, and chlorides.

3.2.1. Carbonate

Carbonate additives (CO₃²⁻) mainly include sodium carbonate (Na₂CO₃), calcium carbonate (CaCO₃), and potassium carbonate (K₂CO₃). Liu et al. calcined K-feldspar at 875 °C for 90 min by using Na₂CO₃ as an additive. The potassium in K-feldspar was converted to soluble potassium by Equation (5) [38].

$$2\mathrm{K}\mathrm{A}\mathrm{l}{\mathrm{S}\mathrm{i}}_3{\mathrm{O}}_8+{\mathrm{Na}}_2{\mathrm{CO}}_3\to {\mathrm{Na}}_2\mathrm{O}·{\mathrm{Al}}_2{\mathrm{O}}_3·6{\mathrm{SiO}}_2+{\mathrm{K}}_2{\mathrm{CO}}_3$$

(5)

A previous study [39] investigated how the calcination temperature impacted the extraction efficiency of potassium (η_K⁺) from K-feldspar, using limestone and dolomite as additives, which are mainly composed of CaCO₃. The results showed that higher calcination temperatures resulted in higher η_K⁺. In detail, when the temperature increased from 1100 to 1250 °C, η_K⁺ increased from 20% to 80% and further increased to 100% at 1350 °C. This is attributed to the formation of sickle bauxite at higher temperatures, which has a less-ordered crystal structure compared to microcline, making potassium more easily dissolved. However, a higher calcination temperature brings the problem of higher energy consumption. In order to reduce the calcination temperature, Wu et al. [40] introduced an additional additive of sodium bicarbonate Na(HCO₃)₂ into the high-temperature pyrolysis process. When 4% Na(HCO₃)₂ was added, the η_K⁺ reached 80% at a lower calcination temperature of 1200 °C, which was 50 degrees lower than the temperature without the additive (1250 °C). This is because the smaller radius of Na⁺ ions compared to K⁺ ions reduced the mineral liquefaction temperature and benefited the potassium extraction. Moreover, the dosage of Na(HCO₃)₂ showed a significant influence on the η_K⁺; that is, the higher dose of Na(HCO₃)₂ resulted in a higher η_K⁺. For instance, by gradually increasing the mass ratio of Na(HCO₃)₂ from 0 to 2%, 4%, and 6%, the η_K⁺ increased from 9.3% to 52.6%, 80.0%, and 89.3%, respectively.

3.2.2. Sulfate

Another effective additive for increasing the η_K⁺ is sulfate. For example, by using CaSO₄ as an additive, K⁺ ion leaching occurs through an ion exchange process with Ca²⁺ ions resulting in the release of soluble K⁺ through the yield of K₂Ca₂(SO₄)₃. Meanwhile, the resulting SiO₂ byproduct further undergoes a reaction with CaSO₄ at high temperatures, such as 1200 °C, resulting in the formation of CaO∙nSiO₂ and the release of SO₂ gas. In this process, the K-feldspar particle size, calcination temperature, as well as the CaSO₄/K-feldspar mass ratio all showed a significant effect on the η_K⁺. Lü et al. [41] found that the CaSO₄ additive gave a 64% η_K⁺ when calcinating at 1200 °C for 2 h and the CaSO₄/K-feldspar mass ratio was 3:1. Jena et al. [42] discovered that the extraction efficiency of potassium could be remarkably increased from 60% to 85% following a decrease in the particle size down from 500 μm to 75 μm. Jena et al. [42] investigated the effect of calcination temperature on the η_K⁺ when using CaSO₄ as the additive. The extraction efficiency of potassium went up from 0% to 50% when the calcination temperature was increased from 900 °C to 1100 °C and attributed to the formation of an eutectic melt phase between CaSO₄ and K-feldspar at higher temperatures, promoting the transfer of mass and the rate of reaction. Nevertheless, a further increment of the calcination temperature to 1200 °C induced the η_K⁺ to decrease to 15%, because the increased formation of CaO raised the eutectic temperature. Moreover, η_K⁺ was also impacted by the CaSO₄/K-feldspar mass ratio. Lü et al. [41] revealed that the η_K⁺ changed when the mass ratio increased from 1:1 to 3:1, and then to 5:1, when the η_K⁺ first increased from the initial 57% to 71%, and then decreased to 32%. The ratio of 3:1 was identified as the optimum CaSO₄/K-feldspar mass ratio. Thus, there is an appropriate mass ratio for CaSO₄/K-feldspar, and the excess CaSO₄ in a high-mass ratio will generate SO₂, resulting in a smaller contact area and a negative effect on the η_K⁺.

3.2.3. Chloride

Indeed, chloride (Cl⁻) has also been identified as an efficient additive for increasing the potassium extraction efficiency. According to Jena et al. [42], the η_K⁺ was obtained as 92.5% under 900 °C through the addition of sodium chloride (NaCl). This was attributed to the NaCl additive melted to form chloride (Cl⁻) and sodium (Na⁺) ions, facilitating the replacement of K⁺ ions by Na⁺ ions via a solid chemical reaction, while Cl⁻ played a crucial role in stabilizing the released K⁺ ions to form KCl. However, the use of a chloride additive for potassium extraction from K-feldspar showed minimal impact on the performance with respect to calcination temperature. For example, Tülay Türk et al. [43] used a CaCl₂ additive to extract potassium at calcination temperatures of 800–950 °C for 1 h, where the η_K⁺ varied between 97.1% and 98.6%. The smaller particle size of K-feldspar resulted in better potassium extraction efficiency as demonstrated by Zhang et al. [44], where the particle size at 20, 80, 100, 120, 160, and 220 mesh showed a η_K⁺ of 43.74%, 70.09%, 80.11%, 86.32%, 89.07%, and 96.35%, respectively. This is attributed to the increased number of reaction sites on the K-feldspar surface with smaller particle sizes, allowing for more complete infiltration and reaction with additives. Similarly, Himanshu T et al. [45] used the planetary ball milling to reduce the K-feldspar particle size, and they subsequently observed an increase in η_K⁺ from 63% to 79% at the 800 °C calculated temperature. The chloride additive CaCl₂ worked better than NaCl, as reported by a comparative study from Jena S K et al. [46]. They found the CaCl₂ showed better performance than NaCl under the same operation conditions, in view of the fact that the η_K⁺ of CaCl₂ and NaCl was 86% and 44%, respectively. This result might be ascribed to the following reasons: First, Ca²⁺ presents better replacement for K⁺ due to its relatively smaller ionic radius than Na⁺ (0.1 nm for Ca²⁺ and 0.102 nm for Na⁺). Second, two K⁺ ions are released by one Ca²⁺ to reach the charge balance during the extraction process, which is more than the Na⁺ additive. Third, the presence of NaCl enabled the formation of water-soluble sodium silicate, which increases reactant viscosity and thereby obstructs the destruction of the K-feldspar crystal.

3.2.4. Comparison of the Carbonate, Sulfate, and Chloride Additives

Although carbonate, sulfate, and chloride have shown good performance as additives in high-temperature pyrolysis, their differences result in differences in the potassium extraction efficiency. This paragraph compares the differences among these types of additives. The potassium extraction efficiency η_K⁺ in the high-temperature pyrolysis method using different additives is shown in

Table 1. The potassium extraction efficiency η_K⁺ in high-temperature pyrolysis method using different additives.

Additives	Calcination Temperature (°C)	Calcination Time (h)	ηK+	Reference
CaCl2	850	1	99%	[48]
CaCl2	950	1	95%	[47]
CaSO4	1200	2	64%	[41]
CaCO3	1250	1	83%	[39]
NaCl + CaSO4	900	1	92%	[42]
CaCl2 + CaCO3	850	2	82%	[49]

. The species of anions present in the additives have been found to have a significant impact on the potassium extraction efficiency. Based on [47], when using Ca²⁺ as a cation, the chloride additive (CaCl₂) required the lowest calcination temperature to reach equivalent (or even higher) η_K⁺; for instance, at 850 °C, the η_K⁺ reaches 99% [48]. In contrast, sulfate additives (CaSO₄) only resulted in 64% η_K⁺ at 1200 °C [41], and carbonate additives resulted in 83% η_K⁺ at 1250 °C [39]. This difference in efficiency is attributed to the lower melting point of the chloride additive compared to the other two additives.

3.2.5. The Mixture Additives

In addition to using a single additive, the use of a mixture of additives has been shown to be an efficient approach for improving potassium extraction efficiency during high-temperature pyrolysis. Various additive mixtures, such as CaCl₂-NaCl, CaSO₄-CaCO₃, phosphogypsum-NaCl, and phosphogypsum-CaCO₃, have been reported often in the literature as being effective in enhancing the η_K⁺ from K-feldspar. The mixture of the CaCl₂-NaCl additives showed higher η_K⁺ compared to sole CaCl₂ and sole NaCl, as reported by Hu et al. [50] Specifically, the η_K⁺ with the CaCl₂-NaCl additives reached 93.65% after calcination at 800 °C for 1 h, which was 22.57% and 39.56% higher than sole CaCl₂ and sole NaCl, respectively. This improvement was attributed to the lower melting point of albite formed in the presence of the CaCl₂-NaCl additives, which resulted in enhanced potassium extraction. Zhang et al. [49] employed CaCl₂-CaCO₃ additives for the potassium extraction from K-feldspar, resulting in a remarkable η_K⁺ of 96% after calcination at 850 °C for 2 h. In comparison, the η_K⁺ was only 60% and 54% for the sole CaCl₂ and sole CaCO₃ additives, respectively. Similarly, Zhang et al. [10] obtained 92.02% extraction efficiency of potassium from K-feldspar using the mixed CaSO₄-CaCO₃ additives at 1500 °C for 2 h, which outperformed the use of pure CaSO₄ and CaCO₃ as additives. From an industry application perspective, the solid residue phosphogypsum, which mainly consists of calcium sulfate (CaSO₄), has been utilized as an additive in potassium extraction processes [51]. Jena et al. [42] found that the η_K⁺ with phosphogypsum-NaCl additives was 30% higher than the sole NaCl additive. This observation was in agreement with the earlier statement that the Ca²⁺ ions are more effective than Na⁺ ions for leaching potassium.

Moreover, different findings were observed by varying the mass ratio of additives. For instance, increasing the mass ratio of NaCl:phosphogypsum resulted in a higher η_K⁺. Specifically, when increasing the NaCl and phosphogypsum mass ratio from 0 to 1:1, the η_K⁺ increased from 61% to 92.8% [10]. On the other hand, in the case of CaCl₂-NaCl additives, there was no remarkable improvement in the η_K⁺ by altering the CaCl₂:NaCl mass ratio, because once the amount of additives is sufficient, a further increase will not bring more reaction opportunity for K-feldspar, limiting the potential for further efficiency gains with a change in the CaCl₂ and NaCl mass ratio [44].

In addition to improving the potassium extraction efficiency, the use of mixture additives can effectively reduce energy consumption. Taking advantage of the low calcination temperature requirement of chloride additives, the addition of chloride additives in combination with other additives has been shown to decrease the overall calcination temperature. For instance, a mixture of NaCl and CaSO₄ additives resulted in a higher η_K⁺ of 92% at a lower calcination temperature of 900 °C [42], compared to the CaSO₄ additive alone, which resulted in a lower η_K⁺ of 64% at a higher temperature of 1200 °C [41]. This can be attributed to the lower melting point of NaCl (801 °C), which allows for a lower optimum calcination temperature and improved η_K⁺ [44]. Similarly, a combination of the CaCl₂ and CaCO₃ additives [49] resulted in a significant decrease in calcination temperature compared to the sole CaCO₃ additive [39]; i.e., when reaching a η_K⁺ of 82%–83%, the required calcination temperature for the CaCl₂ and CaCO₃ additives was 850 °C, while it was 1250 °C for the CaCO₃ additive.

3.3. Microbial Decomposition Method

The microbial decomposition method is used to extract potassium through biochemical reactions between microorganisms and K-feldspar [52]. The most-studied microorganisms are bacteria and fungi. There are two different mechanisms to describe potassium extraction by using microorganisms. Firstly, the dissolution of K-feldspar by using microorganisms leads to deformation or disintegration of the K-feldspar lattice and subsequent leaching of K⁺ ions [53]. Secondly, microorganisms produce glucose organic acid and exopolysaccharide, which has the ability to release K⁺ [54]. The advantages of the microbial decomposition method include simple operation, no pollution, low cost, and environmentally friendliness. However, this method has not been widely adopted for industrial use due to its very slow decomposition rate, short lifespan, and low potassium extraction efficiency [10]. Therefore, future development of this technology should focus on optimizing it for a highly efficient, stable, fertile, and viable performance.

Silicate-solubilizing bacteria are commonly used for potassium dissolution from K-feldspar. Studies have reported that silicate-solubilizing bacteria can lead to a η_K⁺ of 23%, resulting in a 16.85% increase in soybean yield [55]. Several parameters, such as bacteria species and water content, can affect the η_K⁺ in the microbial decomposition method using silicate-solubilizing bacteria. For instance, Chi et al. [56] found that different species of silicate-solubilizing bacteria, namely GK and BK (isolated from the vegetable soil in Liaoning province), exhibited varying potassium extraction efficiencies of 39.7% and 32.3%, respectively. Water content also played a significant role in the η_K⁺. Bacillus glia, a silicate-solubilizing bacteria, was found to be ineffective in dry conditions. However, humid and waterlogged conditions were favorable for K-feldspar decomposition, resulting in an increment of 226% and 243% after incubation at 28 °C for 10 d, respectively [57]. Moreover, there was no significant difference between humid and waterlogged conditions because Bacillus glia is a facultative aerobic bacteria, which can promote the potassium release of K-feldspar under both humid and waterlogged conditions [58].

The fungi can also be used as the microorganism to extract potassium from the K-feldspar microbial decomposition method. At present, many fungal strains have been cultivated that degrade potassium-containing minerals, which provides a basis for the research and utilization of fungal resources for potassium solutions. Lian et al. [59] extracted potassium by the use of one thermophilic fungus strain, namely Aspergillus fumigatus, and the result showed that after 30 d of cultivation, the concentration of K⁺ reached 323 ppm. As a comparison, the K⁺ was only 10 ppm in the control group without using Aspergillus fumigatus. The potassium extraction was found to be affected by the K-feldspar particle size. Song et al. [60] revealed that the smaller K-feldspar particle size contributed to a better solubility of potassium; that is, when the particle size of K-feldspar decreased from 200 mesh to 400 mesh, the potassium release increased from 47.75 mg/L to 93.13 mg/L. The dissolution of potassium is also impacted by the dosage of K-feldspar. The results showed that in the dosage of 10–90 g/L, there was a linear relationship between the potassium dissolution concentration and the K-feldspar dosage. This is attributed to the excellent inclusion and shearing between mycelium and K-feldspar in the range that promoted the leaching of potassium.

3.4. Low-Temperature Method

The low-temperature method is usually performed on K-feldspar below 300 °C under atmospheric pressure by etching with acid and fluoride, which results in the release of potassium [61,62]. Unlike the high-temperature pyrolysis method, the low-temperature method operates at lower reaction temperatures. Unlike the hydrothermal method, it operates under atmospheric pressure rather the high pressure. The mechanism of the low-temperature method is presented in Figure 2, and it uses fluoride and inorganic acid to destroy the structure of oxy-silica tetrahedron and oxy-alumina tetrahedron consisting of Al-O and Si-O in K-feldspar to form SiF₄, AlF₃, and other substances. At the same time, potassium is released [62]. However, the acid added to the low-temperature method and the small amount of HF produced can cause corrosion to the equipment. And the toxic gas SiF₄ produced in the reaction can cause serious environmental pollution.

Peng [63] investigated the extraction of potassium by the H₂SO₄-HF system. When the reaction temperature of this low-temperature method was set at 90 °C, and the reaction time was 7 h, the η_K⁺ reached 85.4%. Ma et al. [64] adopted an H₂SO₄-CaF₂ ultrasound method to extract potassium from K-feldspar because ultrasonic waves can accelerate solution diffusion toward solids [65,66]. After ultrasound treatment, the η_K⁺ increased from 68.3% to 79.1% when the reaction temperature was 90 °C. Additionally, the CaF₂ dosage showed effects on the η_K⁺, and the η_K⁺ increased rapidly from 12.1% to 76.1% as the CaF₂ dosage increased from 0 g to 2 g, because the higher fluoride dosage resulted in higher amounts of HF, where the HF is the efficient reagent for K-feldspar decomposition. However, after a further increase in the CaF₂ dosage to 3 g, the η_K⁺ decreased to 59.7%, attributing to the consumption of effective components of H₂SO₄ by the excess CaF₂. The extraction efficiency of potassium using the low-temperature approach was impacted by K-feldspar’s particle size. For example, in Li Liang’s study [61], when the grinding time increased from 6, 8, and 10 to 12 min, the grinding products with less than 74 mm fractions of different grain sizes were produced, and the η_K⁺ increased from 77.13%, 85.6%, and 92.97% to 95.64%, respectively. That is because the longer the grinding time, the smaller the size of the K-feldspar, which increases the reaction contact area, resulting in a higher η_K⁺. The acid concentration affected the potassium extraction efficiency. Zhou et al. [67] found that the HCl concentration impacted the potassium extraction efficiency in the HCl-CaF₂ system. It was found that the η_K⁺ increased rapidly from 54% to 89.5% as the HCl concentration increased from 10% to 22.5%. One explanation for this could be that using greater amounts of acid results in higher amounts of HF, which encourages the disintegration of K-feldspar.

3.5. Comparison of Potassium Extraction Methods from K-Feldspar

Each method has its own advantages and disadvantages. In terms of potassium extraction efficiency, energy consumption, and ease of operational conditions, the properties of the four methods are compared and discussed. The hydrothermal method has the advantages of high potassium extraction efficiency and lower energy consumption and of being a clean product. But the concentration of the obtained soluble K⁺ is low, requiring additional evaporation and crystallization processes. The high-temperature pyrolysis method has the advantages of a simple operation process, a high potassium utilization rate, and no requirement of strong acid or alkali. However, it has a high energy consumption, large dosage, and high production cost. The microbial decomposition method is economically feasible and environmentally friendly. However, the η_K⁺ is much smaller than other methods and the decomposition speed is too slow. The low-temperature method is safe and has low energy consumption. However, the use of acid and HF cause equipment corrosion.

(1) The hydrothermal method operates at low temperatures (150–300 °C), mild conditions, which yield high-value products and allow for the full utilization of resources. However, the hydrothermal method requires high pressure, so the equipment requirements are high, resulting in a large investment in production. So far, it has not yet achieved industrialized production. This method requires a large material flow and high liquid-solid ratio, and the procedure for waste liquid treatment is complicated. The output of solid waste residue is relatively high. Although it can be made into bricks and cement ingredients, it is not economically feasible.

(2) The high-temperature pyrolysis method has the advantages of simple operation process, high potassium utilization efficiency, convenient adjustment of operation parameters, easy control of reaction, and absence of strong acid, alkali, and wastewater. It is the only method that underwent small-scale trials in the industry. However, it has high energy consumption and high production cost (temperature at 800–1500 °C). The atmospheric pollution emission causes environmental pollution, e.g., the generation of CO₂ and SO₂ when using CaCO₃ and CaSO₄ as additives, respectively.

(3) The microbial decomposition method is a simple, non-polluting, low-cost, and environmentally friendly process. However, due to the poor viability and reproduction ability of bacteria, it is difficult for them to survive in a natural environment. Moreover, the rate of decomposing potassium is too slow, so it has not been able to realize large-scale industrialization so far.

(4) The low-temperature method not only avoids the high-pressure operation in the hydrothermal method, but also avoids the high-temperature conditions in the high-temperature pyrolysis method. It is relatively safe and has low-energy consumption in actual operation. However, the additives of acid and fluoride are highly corrosive, increasing the investment cost of equipment. The emission of toxic SiF₄ gas causes environmental pollution. Therefore, the process prospects of low-temperature acid digestion are not very optimistic.

4. Conclusions and Prospects

This review summarizes the mechanisms, effect parameters and efficacy of four methods for potassium extraction from K-feldspar: the hydrothermal method, high-temperature pyrolysis, microbial decomposition, and low-temperature method. The efficiency is greatly affected by the operation parameters, such as the reaction temperature, the reaction time, the species of additives or microorganisms, the particle size of K-feldspar, and the mass ratio between the additives and K-feldspar.

Each method has its own advantages and disadvantages in terms of extraction efficiency, energy consumption, and ease of operational conditions. Based on these, the research prospects concern the potassium extraction from K-feldspar including:

(1) In the application field, the high-temperature pyrolysis method is a promising method, because it is the only method that underwent small-scale trials in the industry.

(2) In order to reduce costs, inexpensive alternatives can be used as reagents. For example, the Ca(OH)₂ additive can be replaced by relatively inexpensive CaO in the hydrothermal method. The phosphogypsum (a kind of solid waste) can be used as the substitute for the CaSO₄ additive in the high-temperature pyrolysis method.

(3) The combination technology should be adopted because it shows a far superior performance than single technology. For example, the mixture of additives presents better potassium extraction efficiency than the single one. The pretreatment by using a microwave or roasting on the hydrothermal method improves the potassium extraction efficiency.

(4) The residues after potassium extraction should be comprehensively utilized. The elements (e.g., Al and Si) in residues should be reasonably utilized to prepare high value-added products.

(5) The gas emitted, e.g., CO₂, SO₂, and SiF₄, might result in air pollution. It should be taken into consideration in further study.