The Formation Process and Mechanism of Total Activated Potassium During the Preparation of Si–Ca–K–Mg Fertilizer from Molybdenum Tailings

Abstract

Silicon–calcium–potassium–magnesium fertilizer (Si–Ca–K–Mg fertilizer), a critical acidic soil conditioner for remediating polluted acidic soils, encounters a significant challenge: substantial potassium loss through flue gas during high–temperature calcination, which increases production costs. This study optimized the blending ratio of molybdenum tailings (MTs) with CaCO₃ and CaSO₄, systematically investigating the interplay between clinker–soluble potassium, volatile potassium loss, and total activated potassium content during calcination. Key findings include the large–scale utilization of molybdenum tailings; a mass ratio of mMTs:mCaCO₃:mCaSO₄ = 1:0.5:1.0 leads to a total activated K₂O content of 3.05 wt.%. Enhancing nutrient efficiency by increasing the proportion of additives (with a mass ratio of 1:0.7:0.4) results in a total activated K₂O content of 4.50 wt.%, which is 1.5 times the national standard. Mechanistically, calcination decomposes potassium feldspar (K–feldspar) in the tailings into leucite and SiO₂. CaO derived from CaCO₃ reacts with SiO₂ to form calcium silicate, facilitating the decomposition of leucite into water–soluble kaliophilite. Simultaneously, thermal diffusion promotes the ion exchange between Ca²⁺ of CaSO₄ and K⁺ of feldspar and leucite, thereby forming potassium sulfate. However, part of this potassium sulfate, along with some water–soluble kaliophilite, volatilizes at high temperatures, contributing to flue gas loss. Recycling the lost potassium back into fertilizers enables complete potassium utilization. This work establishes a robust framework for efficiently producing Si–Ca–K–Mg fertilizer from molybdenum tailings, addressing key challenges in potassium retention and resource recycling during industrial synthesis.

1. Introduction

Molybdenum metal finds extensive application in the production of construction–grade steel [1] and in power generation systems that operate without fossil fuels [2]. China possesses substantial molybdenum resources. As of 2022, the country’s proven molybdenum reserves were 5.9005 million tons, ranking first globally. Concurrently, China’s molybdenum metal production reached 112,800 tons in the same year [3]. Despite China’s abundant molybdenum resources, the ore grade is relatively low [4]. The beneficiation process for molybdenum results in an annual discharge of approximately 36 million tons of molybdenum tailings (MTs) [5]. Long–term stockpiling of MTs occupies vast land areas, destroys surface vegetation, and induces soil erosion and desertification, posing significant threats to ecological sustainability. Therefore, the comprehensive utilization of MTs holds significant importance. Previously, the focus of utilizing MTs was primarily on their conversion into various building materials. The heavy metals present in building materials derived from tailings may potentially pose a threat to the human living environment. In our prior research, our team proposed a high–value alternative utilization strategy, specifically converting MTs into a Si–Ca–K–Mg fertilizer that is citric acid soluble [6]. The Si–Ca–K–Mg fertilizer derived from MTs exhibits extremely low heavy metal content, thereby satisfying the current environmental and safety standards for agricultural applications.

The MTs contain a variety of minerals, including hematite, pyrite, calcite, quartz, mica, and K–feldspar [7,8,9,10,11]. Notably, K–feldspar, a member of the feldspar group, is a frame silicate mineral with the molecular formula KAlSi₃O₈ [12,13,14], capable of providing potassium. It is widely recognized that China’s potash fertilizer consumption has consistently exceeded its domestic production. In recent years, approximately 50% of potash fertilizer demand has been met through imports [15]. Consequently, the development of mineral–based potash fertilizers holds significant importance for China’s agriculture. However, the potassium in K–feldspar is insoluble and cannot be directly utilized. Given its exceptional chemical stability [16], effective extraction of potassium requires appropriate methods to convert insoluble K–feldspar into soluble potassium compounds. The methods for extracting potassium from K–feldspar include high–temperature calcination leaching [17,18], hydrothermal treatment [14,19,20], low–temperature decomposition [21], and microbial leaching [22]. Currently, researchers have successfully extracted potassium from K–feldspar using high–temperature calcination methods [23,24], achieving promising results. For instance, Lü et al. [25] utilized calcium sulfate to decompose K–feldspar for potassium extraction. In the process, K–feldspar underwent ion exchange with Ca²⁺, resulting in the formation of the soluble product K₂Ca₂(SO₄)₃. Zhao et al. [26] investigated the high–temperature products of K–feldspar by adjusting the calcination temperature and using equal amounts of additives (limestone and dolomite). They found that when the calcination temperature was below 1250 °C, potassium primarily existed in the forms of leucite and kaliophilite within the Si–Ca–K–Mg fertilizer. When the temperature exceeded 1300 °C, the crystallinity of the minerals gradually deteriorated, leading to an almost amorphous structure, which facilitated the dissolution of potassium. Zhong et al. [27] conducted thermodynamic calculations to investigate the effects of reaction temperature and reagent dosage on the recovery ratio of potassium from K–feldspar. They found that at a reaction temperature of 1100 °C, with a mass ratio of K–feldspar:CaSO₄:CaCO₃ of 1:1:3, and a reaction time of 40 min, the recovery ratio of potassium exceeded 90%. Additionally, XRD characterization of the calcined residue confirmed the formation of soluble K₂SO₄. Wang et al. [28] achieved a maximum potassium extraction ratio of 90% by using gypsum and calcium carbonate as additives at a calcination temperature of 1050 °C. Chao et al. [29] conducted X–ray diffraction (XRD) analysis on the products of K–feldspar calcined at 1200 °C and, in conjunction with phase diagrams, investigated the thermal decomposition process of the K–feldspar–CaSO₄–CaO system. The results showed that KAlSi₃O₈ initially decomposed into KAlSi₂O₆, releasing SiO₂. Subsequently, KAlSi₂O₆ reacted with CaO to form the intermediate product K₂SiO₃. Due to the instability of K₂SiO₃, it further reacted with calcium sulfate to generate K₂SO₄.

Previous studies have investigated the reactant compositions and reaction processes of K–feldspar–CaCO₃, K–feldspar–CaSO₄, and K–feldspar–CaCO₃–CaSO₄ systems, primarily focusing on pure K–feldspar. However, no reports exist on preparing soluble potassium fertilizers using tailings containing K–feldspar. In our early research, we successfully activated K–feldspar in molybdenum tailings (MTs) via calcination, producing qualified Si–Ca–K–Mg fertilizer with minimal additives [6]. Notably, previous studies did not consider potassium lost during calcination as flue gas. Our earlier work showed that this volatile potassium loss is significant. From economic and environmental perspectives, recovering this potassium is essential. Building on existing research, this study delves into the high–temperature behavior and reaction mechanisms of MTs–CaCO₃, MTs–CaSO₄, and MTs–CaSO₄–CaCO₃ systems. It elucidates the mineral phase reconstruction of K–feldspar in MTs and examines the composition and formation of total activated potassium in calcination products (i.e., the sum of soluble potassium in the clinker and volatile potassium in the flue gas). This research provides a theoretical and technical foundation for maximizing potassium utilization from MTs and producing high–quality Si–Ca–K–Mg fertilizer.

2. Experimental Section

2.1. Materials

The molybdenum tailings (MTs) used in this study are sourced from Hebei province, China, and appear as gray powder (Figure 1a). The particle size distribution of the MTs, as shown in Figure 1b, exhibits a relatively small overall particle size with a broad distribution. The particles primarily range from 1 to 100 μm, and 90% of them are smaller than 74 μm. XRD analysis (Figure 2) reveals that the primary mineral components in MTs are microcline (KAlSi₃O₈), orthoclase (KAlSi₃O₈), quartz (SiO₂), and dolomite (CaMg(CO₃)₂). Both microcline and orthoclase are varieties of K–feldspar. The chemical composition of the MTs, as shown in

Table 1. The elemental composition of molybdenum tailings (MTs) (wt.%).

Na2O	MgO	Al2O3	SiO2	K2O	CaO	TiO2	Fe2O3	PbO	P2O5
0.758	6.17	10.547	58.997	7.983	8.214	0.247	4.575	0.005	0.083
SO3	Cl	MnO	CuO	ZnO	Rb2O	SrO	Y2O3	CeO2	MoO3
0.386	0.042	0.191	0.008	0.059	0.016	0.014	0.002	0.091	0.006

, is characterized by the following: 7.983 wt.% of K₂O, 58.997 wt.% of SiO₂, 8.214 wt.% of CaO, 6.17 wt.% of MgO, and 0.006 wt.% of MoO₃. The levels of other heavy metals are very low, which makes the MTs suitable for use as a raw meal of mineral fertilizers.

The calcination additives, CaCO₃ and anhydrous CaSO₄, used in the experiment are of analytical grade. Deionized water is used for all experimental procedures.

2.2. Experimental Procedures and Corresponding Calculation Formulas

The process for preparing Si–Ca–K–Mg fertilizer from MTs was as follows: Firstly, MTs, CaCO₃, and anhydrous CaSO₄ were uniformly mixed in a predetermined mass ratio to form the raw meal mixture. Next, the mixture was placed in a muffle furnace and heated at a rate of 10 °C/min to the target temperature under an air atmosphere, followed by calcination for the specified duration. After cooling, the resulting solid product, referred to as clinker, was ground to produce the final Si–Ca–K–Mg fertilizer, which contained soluble potassium. During calcination, flue gas was directed out for treatment, and condensates from the flue gas were collected in a gas collection bottle containing volatile potassium.

The experimental conditions were designed as follows: Firstly, under experimental conditions with a constant additive proportion, an investigation was conducted on the calcination temperature and duration. The calcination temperature was set within the range of 800–1300 °C, while the calcination time was controlled between 30 and 150 min. Subsequently, the types and dosages of agents, including CaCO₃ and CaSO₄, were examined. Finally, further exploration was conducted on the optimal ratio and dosage of CaCO₃ to CaSO₄.

Each experimental group is repeated three times. The final data are averaged, and the standard deviation is computed using a formula to assess the degree of data dispersion. The formula for standard deviation is as follows:

$$\mathrm{S}\mathrm{D}=\sqrt{{\displaystyle \frac{{\sum }_{\mathrm{i}=1}^{\mathrm{n}}{\left(\left({\mathrm{x}}_{\mathrm{i}}\right.-\overline{\mathrm{x}}\right)}^2}{\mathrm{n}-1}}}.$$

Here, ${\mathrm{x}}_{\mathrm{i}}$ represents the data from a single experiment, $\overline{\mathrm{x}}$ is the average value, and $\mathrm{n}$ is the number of repetitions ($\mathrm{n}=3$).

According to the Chinese national standard “Calcium Magnesium Potassium Silicate Fertilizer” (GB/T 36207-2018) [30], the soluble potassium, silicon, calcium, and magnesium in the clinker (Si–Ca–K–Mg fertilizer) were extracted and their contents were determined. This standard specifies that the soluble K₂O content in the fertilizer shall be 3.0 wt.%. The extraction methods for silicon, calcium, potassium, and magnesium elements are as follows: Accurately weigh a sample of 0.2 g to 3 g (accurate to 0.0001 g) and transfer it into a 250 mL volumetric flask. Add 150 mL of hydrochloric acid solution (0.5 mol/L), preheated to 28–30 °C, seal the flask tightly, and shake the flask to disperse the sample in the solution. Maintain the solution temperature at 28–30 °C and subject it to oscillation at a frequency of (180 ± 20) r/min for 30 min. Subsequently, remove the volumetric flask, cool it to room temperature, dilute it with water to the calibration mark, mix thoroughly, and perform dry filtration. Discard the first few milliliters of filtrate, and retain the remaining filtrate for subsequent analysis. In this study, 0.5 mol/L hydrochloric acid was chosen as the leaching agent based on the industry standards NY/T 2273-2012 [31] and NY/T 2272–2012 [32]. The strong acidic conditions were employed to extract potentially soluble nutrients from soil conditioners and quantify their maximum release potential, rather than to simulate the acidic environment of natural soils. It is worth noting that the typical pH range of natural soils is 4.5–8.5, which differs substantially from the acidity level of the bleaching agent used.

The raw meal, clinker, and residue remaining after extraction of soluble nutrients were digested into solutions using a microwave digestion system. These solutions were then diluted and made up to volume for the subsequent determination of elemental concentrations.

During the preparation and leaching process of Si–Ca–K–Mg fertilizer, the material flow of potassium is illustrated in Figure 3. Let m₀ be the total potassium mass in the raw meal before calcination, m₁ be the total potassium mass in the clinker after calcination, m₂ be the volatile potassium mass lost to flue gas during calcination, m₃ be the soluble potassium mass dissolved in the leaching solution after leaching, and m₄ be the insoluble potassium mass remaining in the leaching residue. Experimental results show that volatile potassium in the flue gas is water–soluble and can be recovered for use as fertilizer. Therefore, it can be combined with soluble potassium to form the total activated potassium mass, denoted as m₅. The following relationships are described in

Table 2. Mass balance of potassium and calculation formula for potassium ratios.

Name	Calculation Formula
Total potassium mass balance	m0 = m1 + m2
Clinker potassium mass balance	m1 = m3 + m4
Total activated potassium mass balance	m5 = m2 + m3
Clinker potassium ratio (wt.%)	W1 = m1/m0 × 100
Volatile potassium ratio (wt.%)	W2 = m2/m0 × 100
Soluble potassium ratio (wt.%)	W3 = m3/m0 × 100
Insoluble potassium ratio (wt.%)	W4 = m4/m0 × 100
Total activated potassium ratio (wt.%)	W5 = m5/m0 × 100

.

2.3. Analytical Methods

In this study, the elemental composition of MTs, raw meal, fertilizer, leach residue, and leaching solution was determined using inductively coupled plasma optical emission spectrometry (ICP–OES) (Spectroblue, Spectro, Kleve, Germany). Solid samples were first digested into solutions prior to analysis. The mineral phases of MTs, fertilizer, leach residue, and flue gas condensate were characterized using X–ray diffraction (XRD) analysis performed on an Advance D8 instrument (Bruker, MA, USA). The microstructure of MTs, fertilizers, and leaching residues was characterized using scanning electron microscopy coupled with energy dispersive spectroscopy (SEM–EDS) (JSM–7900F, JEOL, Tokyo, Japan). The FactSage8.0 software was employed to calculate the Gibbs free energy and phase diagram for potential reactions in the MTs–additives system.

3. Results and Discussion

3.1. Research on the Content Regulation of Total Activated Potassium

To maximize the utilization of potassium in MTs, it is essential during the calcination process to convert as much potassium from K–feldspar into total activated potassium in the fertilizer (i.e., the sum of soluble potassium in the clinker and volatile potassium in the flue gas), thereby enhancing plant absorption and utilization. Based on the calcination temperature, calcination time, and batching ratio range established in our previous work [6], this study systematically examines the effects of these parameters on the formation of total activated potassium during the calcination of MTs.

3.1.1. The Influence of Calcination Temperature and Calcination Time

With a fixed mass ratio of MTs:CaCO₃:CaSO₄ = 1:0.7:0.4 and a calcination time of 120 min, the ratios of soluble potassium, volatile potassium, and total activated potassium increase as the calcination temperature rises from 800 to 1200 °C, reaching 55.31%, 32.79%, and 88.10%, respectively (Figure 4a). At 1300 °C, these ratios stabilize. As the temperature increases to 1200 °C, the soluble K₂O content in the clinker rapidly increases from 0.21 wt.% to 2.94 wt.%, approaching the standard requirement of 3.0 wt.% (Figure 4b). The total activated K₂O content, including volatile potassium in the flue gas, rises from 0.37 wt.% to 4.50 wt.%, exceeding the GB/T 36207-2018 requirement by approximately 50%. This is primarily due to the high temperature required for K–feldspar (KAlSi₃O₈) in MTs to react with CaCO₃ and CaSO₄, forming mineral phases that provide soluble K. Temperature plays a crucial role in reconstructing the K–feldspar mineral phase [17], which will be further analyzed in subsequent sections.

Under the experimental conditions of a calcination temperature of 1200 °C and a mass ratio of MTs:CaCO₃:CaSO₄ = 1:0.7:0.4, the influence of calcination time on potassium activation in MTs was investigated. As shown in Figure 4c, the total activated potassium ratio increases gradually with extended calcination time, reaching 88.23% at 150 min. Concurrently, the volatile potassium ratio rises steadily, while the soluble potassium ratio declines. These observations suggest that prolonged calcination enhances potassium activation. As shown in Figure 4d, the total activated K₂O content in the Si–Ca–K–Mg fertilizer increases with calcination time, while the soluble K₂O content in the clinker decreases due to enhanced potassium volatilization. Within the calcination time range of 30 to 150 min, the total activated K₂O content consistently meets national standards. Notably, at 120 min, the total activated K₂O content reaches 4.50 wt.%. However, when the calcination time exceeds 120 min, the soluble K₂O content fails to meet the standard requirements. Considering both optimal potassium activation and the highest total activated K₂O content, a calcination time of 120 min is deemed appropriate.

3.1.2. The Influence of Additives Ratio

After determining the calcination temperature and time, the appropriate raw meal ratio is critical for preparing high–quality Si–Ca–K–Mg fertilizer. Therefore, this study conducts an in–depth and comprehensive investigation into the effects of different additive dosages on potassium activation under three conditions: (1) using CaCO₃ alone as an additive, (2) using CaSO₄ alone as an additive, and (3) using a composite of CaCO₃ and CaSO₄ as an additive.

Under calcination conditions of 1200 °C for 120 min, the impact of the mass ratio of MTs:CaCO₃ on potassium activation in MTs was investigated. As illustrated in Figure 5a, when the mass ratio of MTs:CaCO₃ increases from 1:0.4 to 1:0.9, the total activated potassium ratio rises from 24.21% to 59.51%. The ratios of soluble potassium and volatile potassium in MTs exhibit a similar trend to the total activated potassium ratio, increasing from 16.54% and 7.67% to 43.34% and 16.17%, respectively. As illustrated in Figure 5b, under all ratios, the soluble K₂O content in the fertilizer fails to meet the requirements of GB/T 36207-2018, with the highest value reaching only 2.47 wt.%. The total activated K₂O content meets the standard only when the mass ratio of MTs:CaCO₃ is 1:0.9. Therefore, using CaCO₃ alone to prepare Si–Ca–K–Mg fertilizer is not economically viable due to the low utilization ratio of potassium.

Under the experimental conditions of a calcination temperature of 1200 °C and a calcination time of 120 min, the influence of the mass ratio of MTs:CaSO₄ on the activation effect of potassium in MTs was studied. As the dosage of CaSO₄ gradually increases, the ratios of total activated potassium, soluble potassium, and volatile potassium also increase correspondingly (Figure 5c). When the mass ratio of MTs:CaSO₄ reaches 1:0.7, these ratios approach their peaks, at 73.45%, 40.49%, and 32.96%, respectively. Further increasing CaSO₄ has little effect, and the ratios remain stable, possibly due to excessive CaSO₄ increasing SO₂ release, reducing reactant contact area, and affecting the reaction [25]. The ratios of volatile and soluble potassium are similar, likely from Ca²⁺ in CaSO₄ exchanging with K⁺ in K–feldspar (KAlSi₃O₈) in MTs to form K₂SO₄, and more CaSO₄ promotes K₂SO₄ generation. As shown in Figure 5d, under all tested ratios, the soluble K₂O content in the fertilizer does not meet GB/T 36207-2018, with a maximum of only 1.88 wt.%. The total activated K₂O content meets the standard only when the mass ratio of MTs:CaSO₄ is 1:0.7 to 1:0.9. Thus, using CaSO₄ alone as an additive can hardly produce qualified Si–Ca–K–Mg fertilizers.

The effect of the mass ratio of MTs:CaCO₃:CaSO₄ on the activation of potassium in MTs was investigated under the conditions of a calcination temperature of 1200 °C and a calcination time of 120 min (Figure 6a).

With a fixed MTs to additive mass ratio of 1:0.6, significant differences in potassium activation are observed when using CaCO₃ or CaSO₄ individually versus a blend of both (MTs:CaCO₃ = 1:0.6, Figure 5a; MTs:CaSO₄ = 1:0.6, Figure 5c) compared to MTs:CaCO₃:CaSO₄ = 1:0.5:0.1 (Figure 6a). Using CaCO₃ alone results in lower total, soluble, and volatile potassium activation ratios by 37.87%, 39.89%, and 43.11%, respectively, compared to the composite additives. This highlights the synergistic effect of the compound additives in enhancing potassium activation [6]. The total activated potassium ratio with composite additives is 52.62%, 12.20% lower than with CaSO₄ alone, but its soluble potassium ratio of 38.98% is significantly higher than the 25.14% achieved with CaSO₄ alone. Additionally, the volatile potassium ratio of 13.64% is much lower than the 34.79% seen with CaSO₄ alone, indicating that adding CaCO₃ reduces soluble potassium volatilization.

Additionally, in terms of the soluble K₂O content and total activated K₂O content of the products, when compound additives are used, the fertilizer achieves values of 2.21 wt.% and 3.05 wt.%, respectively. Notably, the total activated K₂O content meets the national standard (Figure 6b). In contrast, when CaSO₄ or CaCO₃ is used alone as additives, neither value meets the standard requirements, and their soluble K₂O contents are only 76.02% and 66.97% of that achieved with the compound additives, respectively (Figure 5b,d and Figure 6b). K–feldspar (KAlSi₃O₈) in MTs decomposes into leucite (KAlSi₂O₆) and liquid SiO₂. The CaO formed from the decomposition of CaCO₃ absorbs the liquid SiO₂. Excess CaO then reacts with CaSO₄ and leucite (KAlSi₂O₆) to form kaliophilite (KAlSiO₄) and potassium sulfate (K₂SO₄) [6]. Therefore, the performance of compound additives is superior to that of individual additives.

When the MTs to additive mass ratio is fixed at 1:0.7–0.9, potassium activation varies greatly based on using CaCO₃ or CaSO₄ as single additives (Figure 5a,c) or in different combined ratios (Figure 6a). For compound additives, the ratios of total activated potassium, soluble potassium, and volatile potassium are at least 37.74%, 35.81%, and 43.06% higher than using CaCO₃ alone. Using compound additives ensures the total activated K₂O content meets the 3.0 wt.% national standard. When the MTs:CaCO₃:CaSO₄ mass ratio is 1:0.7:0.2, the soluble K₂O content also meets the standard (Figure 6b). In contrast, using CaCO₃ alone only meets the total activated K₂O requirement at a 1:0.9 mass ratio, and the soluble K₂O fails to meet the standard in the 1:0.7 to 1:0.9 range (Figure 5b). Thus, compound additives are better than using CaCO₃ alone.

When the mass ratio of MTs:CaSO₄ is 1:0.7, its potassium activation effect is stronger than that of the compound additive with a mass ratio of MTs:CaCO₃:CaSO₄ = 1:0.5:0.2. The activated potassium and volatile potassium ratios are 20.52% and 43.15% higher, respectively, while the soluble potassium ratio is 5.60% lower. But at MTs:CaCO₃:CaSO₄ ratios of 1:0.6:0.2 or 1:0.7:0.2, the potassium activation is better than at MTs:CaSO₄ ratios of 1:0.8 to 1:0.9. When using CaSO₄ alone, the total activated potassium and soluble potassium ratios are at least 6.29% and 28.94% lower than those with compound additives (Figure 5c and Figure 6a). In contrast, the volatile potassium ratio with compound additives is at least 34.17% lower than with CaSO₄ alone. Whether using CaSO₄ alone or with CaCO₃, when the MTs–to–additive mass ratio is 1:0.7–0.9, the total activated K₂O content exceeds 3.00 wt.%. However, the soluble K₂O content with CaSO₄ alone is much lower, only 30.08% of that with compound additives (Figure 5d and Figure 6b). In terms of additive cost and potassium utilization efficiency, compound additives are better than CaSO₄ alone.

Upon setting the mass ratio of MTs to additives at 1:1 to 1:1.3, the ratios of total activated potassium, soluble potassium, and volatile potassium during compound batching are each greater than 84.00%, 54.00%, and 28.00% (Figure 6a). Additionally, the total activated K₂O content exceeds 4.20 wt.% in all cases, significantly surpassing the national standard, and the soluble K₂O content in the clinker consistently exceeds 2.70 wt.% (Figure 6b).

Therefore, the following conclusions can be drawn regarding the additives to prepare Si–Ca–K–Mg fertilizer: (1) the use of compound additives (CaCO₃ and CaSO₄) is essential, as their performance significantly surpasses that of individual additives; (2) the appropriate ratio range for compound additives is 1:0.6–1.3; (3) to ensure that the total activated K₂O content meets or exceeds the national standard, a mass ratio of CaCO₃ to CaSO₄ between 1.2:1 and 5:1 is most appropriate.

3.1.3. Determination of the Process Conditions for Preparing Si–Ca–K–Mg Fertilizer

Based on the experimental results, a calcination temperature of 1200 °C and a calcination time of 120 min are suitable conditions for preparing qualified Si–Ca–K–Mg fertilizer from MTs. However, the mass ratio of additives has a significant impact on the final product quality. Considering the following factors: firstly, the calcined clinker is directly used as fertilizer, necessitating that the soluble K₂O content in the clinker meet national standards; secondly, volatile potassium from flue gas can be recovered and added to the clinker, ensuring that the total activated K₂O content of the final fertilizer meets and exceeds national standards. Therefore, an optimized ingredient scheme, as presented in

Table 3. Process schemes for preparing Si–Ca–K–Mg fertilizer.

Schemes	Purpose	Mass Ratio of Mixture	Addition Ratio of MTs, wt.%	Total Activated Potassium Ratio, %	Soluble K2O Content, wt.%	Total Activated K2O Content, wt.%
1	Maximizing the addition ratio of MTs	MTs:CaCO3:CaSO4 =1:0.5:0.1	62.50	52.52	2.21	3.05
2	The highest ratio of total activated potassium	MTs:CaCO3:CaSO4 =1:0.7:0.6	43.48	92.36	2.76	4.33
3	The highest content of soluble K2O in clinker	MTs:CaCO3:CaSO4 =1:0.7:0.2	52.63	84.89	3.55	4.21
4	The highest content of total activated K2O in fertilizer	MTs:CaCO3:CaSO4 =1:0.7:0.4	47.62	88.1	2.91	4.50

, can be established:

In Table 3, Scheme 1 achieves the highest addition ratio of MTs in raw meals, reaching 62.50%. Under this scheme, the mass ratio of MTs:CaCO₃:CaSO₄ is 1:0.5:0.1, resulting in a total activated potassium content of 52.52%. Although the soluble K₂O content is only 2.21 wt.%, the inclusion of volatile K₂O increases the total activated K₂O content to 3.05 wt.%, thereby just meeting the national standard. Scheme 2 can achieve the maximum of 92.36% of the total activated potassium ratio in MTs, transforming nearly all the potassium in K–feldspar into soluble potassium. In this scheme, the mass ratio of MTs:CaCO₃:CaSO₄ is 1:0.7:0.6, with a MTs addition ratio of 43.48 wt.% in the raw meal. Although the soluble K₂O content is only 2.74 wt.%, the total activated K₂O content significantly increases to 4.33 wt.% when volatile K₂O content is accounted for. Scheme 3 can achieve the maximum soluble K₂O content of 3.55 wt.% in clinker (Si–Ca–K–Mg fertilizer) directly, without accounting for the reuse of volatile potassium. Under these conditions, the mass ratio of MTs:CaCO₃:CaSO₄ is 1:0.7:0.2, with an MTs addition ratio of 52.63 wt.% in the raw meal. Notably, the total activated potassium ratio reaches 84.89%, resulting in a total activated K₂O content of 4.21 wt.%. Scheme 4 can achieve the highest total activated K₂O content in the Si–Ca–K–Mg fertilizer. By integrating the soluble potassium from the clinker and the volatile potassium from the flue gas, the total activated K₂O content in the fertilizer reaches 4.50 wt.%, surpassing the national standard by 50%. In this scheme, the mass ratio of MTs:CaCO₃:CaSO₄ is 1:0.7:0.4, with an MTs addition ratio of 47.62 wt.% in the raw meal. Notably, the total activated potassium ratio reaches 88.10%, and the soluble K₂O content reaches 2.91 wt.%, closely approaching the national standard.

Therefore, these research findings provide a solid foundation for optimizing the ingredient formula in accordance with the actual requirements of the production process.

3.2. The Formation Process and Mechanism of Soluble Potassium in the Fertilizer

3.2.1. Thermodynamics of the Reactions Between MTs and Additives

The complex composition of MTs results in a correspondingly intricate reaction process with additives CaCO₃ and CaSO₄. To accurately analyze the high–temperature reactions between MTs and additives, FactSage software is used to investigate the thermodynamics of these reactions. First, the Phase module is utilized to calculate the binary phase diagram of KAlSi₃O₈–SiO₂ (Figure 7a), as well as the ternary phase diagrams of SiO₂–Al₂O₃–CaO (Figure 7b) and SiO₂–MgO–CaO (Figure 7c). Subsequently, the Equilib module is employed to determine the Gibbs free energy changes (ΔG) for potential reactions during the calcination process (Figure 7d–f).

As previously discussed, potassium ions are incorporated into the stable tetrahedral framework of K–feldspar (KAlSi₃O₈), rendering weak acid solutions ineffective for extracting potassium from this mineral [33]. Therefore, we investigate the mechanism of potassium release using the KAlSi₃O₈–SiO₂ binary phase diagram. As shown in Figure 7a, K–feldspar decomposes at 1150 ± 50 °C to form leucite (KAlSi₂O₆) and liquid silica (SiO₂). Upon cooling, K–feldspar regenerates from leucite and liquid silica when the temperature reaches the remelting point [34,35]. Thus, calcining K–feldspar alone is insufficient to produce soluble potassium. To address this, a fluxing agent, primarily CaO, is introduced. First, CaO reacts with liquid SiO₂ to form calcium silicate, preventing K–feldspar regeneration [36,37]. Second, CaO promotes the reaction of leucite with excess CaO to form kaliophilite (KAlSiO₄). If CaSO₄ is used instead, potassium sulfate (K₂SO₄) can be produced.

The ternary phase diagram of SiO₂–Al₂O₃–CaO (Figure 7b) shows key minerals such as gehlenite (Ca₂Al₂SiO₇), wollastonite (CaSiO₃), dicalcium silicate (Ca₂SiO₄), and anorthite (CaAl₂Si₂O₈), confirmed in previous studies [38]. The colored curves represent full liquid phase isotherms, with the lowest temperature (1100 °C) at the eutectic point where feldspar, SiO₂, and calcium silicate regions intersect. In preparing Si–Ca–K–Mg fertilizer from MTs, wollastonite provides essential silicon nutrients for plants, while gehlenite supplies calcium nutrients. To achieve these mineral phases, precise control of raw material composition and reaction conditions is crucial. Wollastonite can be synthesized when the molar ratio of CaO:SiO₂ is 1:1 and the temperature is between 1100 and 1350 °C. Gehlenite synthesis requires a molar ratio of CaO:SiO₂:Al₂O₃ = 1:1:2 under the same temperature range. To avoid forming insoluble dicalcium silicate, the temperature should not exceed 1350 °C. Additionally, the molar ratio of CaO to SiO₂ should deviate from the stoichiometric ratio of 2:1 to prevent dicalcium silicate formation.

Figure 7c presents the SiO₂–MgO–CaO ternary phase diagram. It is evident that the phase regions of akermanite (Ca₂MgSi₂O₇) and enstatite (Mg₂SiO₄) fall within the low melting temperature range of 1100 to 1350 °C, which aligns with previous findings [39]. To achieve the synthesis of akermanite, it is crucial to precisely control the raw meal ratio and calcination temperature. Specifically, when the molar ratio of CaO:MgO:SiO₂ is 2:1:2, the calcination temperature should be strictly maintained within the range of 1100 °C to 1350 °C to effectively synthesize akermanite. Additionally, to prevent the formation of insoluble diopside (CaMg(SiO₃)₂), the molar ratio of CaO:MgO:SiO₂ = 1:1:2 should be avoided. Through in–depth analysis of the SiO₂–Al₂O₃–CaO and SiO₂–MgO–CaO ternary phase diagrams, important theoretical guidance can be provided for determining the optimal addition amounts of CaCO₃ and CaSO₄ when preparing Si–Ca–K–Mg fertilizer from MTs. According to the insights revealed by these phase diagrams, the formation of insoluble dicalcium silicate (Ca₂SiO₄) and diopside can be effectively avoided, thereby promoting the reaction to proceed preferentially in the direction of forming wollastonite, gehlenite and akermanite.

The following sections will calculate the reaction thermodynamics separately using CaCO₃ and CaSO₄ as individual additives, as well as using both as composite additives. The relationship between ΔG and temperature for the main high–temperature reactions in each system is illustrated in Figure 7d–f. Since CaCO₃ decomposes into calcium oxide (CaO) around 900 °C, CaO is used for the calculations.

The possible reactions that may occur in the KAlSi₃O₈–CaO system are as follows:

$${\mathrm{K}\mathrm{A}\mathrm{l}\mathrm{S}\mathrm{i}}_3{\mathrm{O}}_8={\mathrm{K}\mathrm{A}\mathrm{l}\mathrm{S}\mathrm{i}}_2{\mathrm{O}}_6+{\mathrm{S}\mathrm{i}\mathrm{O}}_2,$$

(1)

$${\mathrm{K}\mathrm{A}\mathrm{l}\mathrm{S}\mathrm{i}}_3{\mathrm{O}}_8+\mathrm{C}\mathrm{a}\mathrm{O}={\mathrm{K}\mathrm{A}\mathrm{l}\mathrm{S}\mathrm{i}}_2{\mathrm{O}}_6+{\mathrm{C}\mathrm{a}\mathrm{S}\mathrm{i}\mathrm{O}}_3,$$

(2)

$${\mathrm{K}\mathrm{A}\mathrm{l}\mathrm{S}\mathrm{i}}_2{\mathrm{O}}_6+\mathrm{C}\mathrm{a}\mathrm{O}={\mathrm{K}\mathrm{A}\mathrm{l}\mathrm{S}\mathrm{i}\mathrm{O}}_4+{\mathrm{C}\mathrm{a}\mathrm{S}\mathrm{i}\mathrm{O}}_3,$$

(3)

$${\mathrm{K}\mathrm{A}\mathrm{l}\mathrm{S}\mathrm{i}}_3{\mathrm{O}}_8+2\mathrm{C}\mathrm{a}\mathrm{O}={\mathrm{K}\mathrm{A}\mathrm{l}\mathrm{S}\mathrm{i}\mathrm{O}}_4+{2\mathrm{C}\mathrm{a}\mathrm{S}\mathrm{i}\mathrm{O}}_3,$$

(4)

$${\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_3+{\mathrm{S}\mathrm{i}\mathrm{O}}_2+2\mathrm{C}\mathrm{a}\mathrm{O}={\mathrm{C}\mathrm{a}}_2{\mathrm{A}\mathrm{l}}_2\mathrm{S}\mathrm{i}{\mathrm{O}}_7,$$

(5)

$$\mathrm{M}\mathrm{g}\mathrm{O}+{2\mathrm{S}\mathrm{i}\mathrm{O}}_2+2\mathrm{C}\mathrm{a}\mathrm{O}={\mathrm{C}\mathrm{a}}_2{\mathrm{M}\mathrm{g}\mathrm{S}\mathrm{i}}_2{\mathrm{O}}_7.$$

(6)

As shown in Figure 7d, within the temperature range of 800 to 1300 °C, the Gibbs free energy (ΔG) of reaction 1 is negative, indicating that K–feldspar (KAlSi₃O₈) can decompose to form leucite (KAlSi₂O₆) and silica (SiO₂). The ΔG values for reactions 2 to 4 are also negative, suggesting that the addition of CaO facilitates the decomposition of the silicate framework in K–feldspar. Specifically, K–feldspar first reacts with CaO to form insoluble leucite and soluble wollastonite (CaSiO₃). Wollastonite primarily provides the soluble silicon component in the Si–Ca–K–Mg fertilizer. Subsequently, excess CaO further reacts with leucite to form soluble kaliophilite (KAlSiO₄), which mainly supplies the soluble potassium component in the fertilizer. As shown in Figure 7b–d, the reactions 5 and 6 indicate that gehlenite (Ca₂Al₂SiO₇) can be synthesized when the molar ratio of Al₂O₃:SiO₂:CaO is 1:1:2. In contrast, akermanite (Ca₂MgSi₂O₇) forms when the molar ratio of MgO:SiO₂:CaO is 1:2:2. Gehlenite and akermanite contribute soluble calcium and magnesium ions to the Si–Ca–K–Mg fertilizer, respectively.

The possible reactions that may occur in the KAlSi₃O₈–CaSO₄ system are as follows:

$$2{\mathrm{K}\mathrm{A}\mathrm{l}\mathrm{S}\mathrm{i}}_3{\mathrm{O}}_8+8\mathrm{C}\mathrm{a}{\mathrm{S}\mathrm{O}}_4={\mathrm{K}}_2{\mathrm{S}\mathrm{O}}_4+{\mathrm{C}\mathrm{a}}_2{\mathrm{A}\mathrm{l}}_2\mathrm{S}\mathrm{i}{\mathrm{O}}_7{+4\mathrm{C}\mathrm{a}\mathrm{S}\mathrm{i}\mathrm{O}}_3{+7\mathrm{S}\mathrm{O}}_2{+3.5\mathrm{O}}_2+{\mathrm{C}\mathrm{a}}_2\mathrm{S}\mathrm{i}{\mathrm{O}}_4,$$

(7)

$$2{\mathrm{K}\mathrm{A}\mathrm{l}\mathrm{S}\mathrm{i}}_2{\mathrm{O}}_6+6\mathrm{C}\mathrm{a}{\mathrm{S}\mathrm{O}}_4={\mathrm{K}}_2{\mathrm{S}\mathrm{O}}_4+{\mathrm{C}\mathrm{a}}_2{\mathrm{A}\mathrm{l}}_2\mathrm{S}\mathrm{i}{\mathrm{O}}_7{+2\mathrm{C}\mathrm{a}\mathrm{S}\mathrm{i}\mathrm{O}}_3{+5\mathrm{S}\mathrm{O}}_2{+2.5\mathrm{O}}_2+{\mathrm{C}\mathrm{a}}_2\mathrm{S}\mathrm{i}{\mathrm{O}}_4,$$

(8)

$$2\mathrm{K}\mathrm{A}\mathrm{l}\mathrm{S}\mathrm{i}{\mathrm{O}}_4+3\mathrm{C}\mathrm{a}{\mathrm{S}\mathrm{O}}_4={\mathrm{K}}_2{\mathrm{S}\mathrm{O}}_4+{\mathrm{C}\mathrm{a}}_2{\mathrm{A}\mathrm{l}}_2\mathrm{S}\mathrm{i}{\mathrm{O}}_7{+\mathrm{C}\mathrm{a}\mathrm{S}\mathrm{i}\mathrm{O}}_3{+2\mathrm{S}\mathrm{O}}_2{+\mathrm{O}}_2,$$

(9)

$$\mathrm{M}\mathrm{g}\mathrm{O}+{2\mathrm{S}\mathrm{i}\mathrm{O}}_2+\mathrm{C}\mathrm{a}\mathrm{O}=\mathrm{C}\mathrm{a}\mathrm{M}\mathrm{g}(\mathrm{S}\mathrm{i}{\mathrm{O}}_3{)}_2.$$

(10)

As shown by the reaction 7 in Figure 7e, at a calcination temperature of 1400 °C, the addition of CaSO₄ can cause K–feldspar (KAlSi₃O₈) to decompose into soluble potassium sulfate (K₂SO₄), gehlenite (Ca₂Al₂SiO₇), wollastonite (CaSiO₃), and insoluble dicalcium silicate (Ca₂SiO₄). As shown by the reactions 8 and 9 in Figure 7b,c,e, at a calcination temperature of 1350 °C, CaSO₄ can react with leucite (KAlSi₂O₆) and kaliophilite (KAlSiO₄) to form soluble potassium sulfate, gehlenite, and wollastonite. Additionally, insoluble dicalcium silicate is also generated. As shown by the reaction 10 in Figure 7c,e, when the molar ratio of MgO:SiO₂:CaO is 1:2:1, insoluble diopside (CaMg(SiO₃)₂) is formed. Diopside is insoluble in both water and acid, making it difficult to achieve soluble magnesium in the preparation of Si–Ca–K–Mg fertilizer using the MTs–CaSO₄ system. However, in actual experiments, the corresponding compounds can be synthesized at a calcination temperature of 1200 °C due to the presence of elements such as K, Na, and Fe in the MTs, which lower the sintering temperature.

The possible reactions that may occur in the KAlSi₃O₈–CaSO₄–CaO system are as follows:

$$2{\mathrm{K}\mathrm{A}\mathrm{l}\mathrm{S}\mathrm{i}}_3{\mathrm{O}}_8+8\mathrm{C}\mathrm{a}{\mathrm{S}\mathrm{O}}_4+2\mathrm{C}\mathrm{a}\mathrm{O}={{\mathrm{K}\mathrm{A}\mathrm{l}\mathrm{S}\mathrm{i}\mathrm{O}}_4+\mathrm{K}}_2{\mathrm{S}\mathrm{O}}_4+{\mathrm{C}\mathrm{a}}_2{\mathrm{A}\mathrm{l}}_2\mathrm{S}\mathrm{i}{\mathrm{O}}_7{+6\mathrm{C}\mathrm{a}\mathrm{S}\mathrm{i}\mathrm{O}}_3{+7\mathrm{S}\mathrm{O}}_2{+3.5\mathrm{O}}_2+{\mathrm{C}\mathrm{a}}_2\mathrm{S}\mathrm{i}{\mathrm{O}}_4,$$

(11)

$$2{\mathrm{K}\mathrm{A}\mathrm{l}\mathrm{S}\mathrm{i}}_3{\mathrm{O}}_8+6\mathrm{C}\mathrm{a}{\mathrm{S}\mathrm{O}}_4+2\mathrm{C}\mathrm{a}\mathrm{O}={\mathrm{K}}_2{\mathrm{S}\mathrm{O}}_4+{\mathrm{C}\mathrm{a}}_2{\mathrm{A}\mathrm{l}}_2\mathrm{S}\mathrm{i}{\mathrm{O}}_7{+4\mathrm{C}\mathrm{a}\mathrm{S}\mathrm{i}\mathrm{O}}_3{+5\mathrm{S}\mathrm{O}}_2{+2.5\mathrm{O}}_2+{\mathrm{C}\mathrm{a}}_2\mathrm{S}\mathrm{i}{\mathrm{O}}_4,$$

(12)

$$2{\mathrm{K}\mathrm{A}\mathrm{l}\mathrm{S}\mathrm{i}}_3{\mathrm{O}}_8+3\mathrm{C}\mathrm{a}{\mathrm{S}\mathrm{O}}_4+4\mathrm{C}\mathrm{a}\mathrm{O}={\mathrm{K}}_2{\mathrm{S}\mathrm{O}}_4+{\mathrm{C}\mathrm{a}}_2{\mathrm{A}\mathrm{l}}_2\mathrm{S}\mathrm{i}{\mathrm{O}}_7{+5\mathrm{C}\mathrm{a}\mathrm{S}\mathrm{i}\mathrm{O}}_3{+2\mathrm{S}\mathrm{O}}_2{+\mathrm{O}}_2$$

(13)

The formation of soluble potassium in the KAlSi₃O₈–CaSO₄–CaO system primarily stems from reactions 11 to 13 (Figure 7f). At a temperature of 1250 °C, the Gibbs free energy change (ΔG) for reaction R11 is negative. At a calcination temperature of 1200 °C, the ΔG values for reactions 12 and 13 are also negative. As shown by the reactions 2, 3, R8 and 11 in Figure 7d–f, K–feldspar (KAlSi₃O₈) first reacts with CaO to form leucite (KAlSi₂O₆). Leucite then further reacts with excess CaO to form kaliophilite (KAlSiO₄). Additionally, leucite can react with CaSO₄ to form potassium sulfate (K₂SO₄). In the above–mentioned experiments, it is observed that the total activated potassium ratio in the mass ratio of MTs:CaCO₃ = 1:0.8 is 40.36% lower than that in the mass ratio of MTs:CaCO₃:CaSO₄ = 1:0.6:0.2. This indicates that the synergistic effect of CaSO₄ is particularly critical for the potassium activation process. Therefore, incorporating an appropriate amount of CaSO₄ during the preparation of Si–Ca–K–Mg fertilizer is essential.

3.2.2. The Mechanism of Mineral Phase Reconstruction During the Calcination Process

To investigate the mineral composition of the fertilizer and the source of soluble nutrients, XRD analysis was performed on the clinker sintered at 1200 °C and the insoluble residue after leaching. The results are presented in Figure 8a,b. It can be observed that when MTs are calcined without additives, the primary phases in the clinker and leaching residue are K–feldspar (KAlSi₃O₈) and quartz (SiO₂). This indicates that K–feldspar is insoluble in the 0.5 mol/L dilute hydrochloric acid used for leaching. Consequently, the potassium within it cannot be absorbed and utilized by plants. These findings are consistent with the results presented in Figure 8a.

As shown in Figure 8a, when the mass ratio of MTs:CaCO₃ is 1:0.9, the primary mineral phases formed are leucite (KAlSi₂O₆), kaliophilite (KAlSiO₄), wollastonite (CaSiO₃), gehlenite (Ca₂Al₂SiO₇), and akermanite (Ca₂MgSi₂O₇). These findings align with the previously discussed thermodynamic analysis. The XRD results of the leaching residue (Figure 8b) show that the main mineral phases remaining are leucite, K–feldspar, quartz, and diopside (CaMg(SiO₃)₂). This indicates that under 0.5 mol/L dilute hydrochloric acid leaching conditions, kaliophilite, wollastonite, gehlenite, and akermanite dissolve, while leucite, K–feldspar, and diopside do not.

As shown in Figure 8a, when the mass ratio of MTs:CaSO₄ is 1:0.9, the main mineral phases generated include potassium sulfate (K₂SO₄), diopside (CaMg(SiO₃)₂), gehlenite (Ca₂Al₂SiO₇), wollastonite (CaSiO₃), and dicalcium silicate (Ca₂SiO₄). In contrast, Figure 8b indicates that the primary mineral phases in the leaching residue are wollastonite, diopside, and dicalcium silicate, with the peak intensity of wollastonite significantly lower than that in the fertilizer. This suggests that under these leaching conditions, potassium sulfate and gehlenite are soluble, wollastonite is partially dissolved, while diopside and dicalcium silicate remain insoluble.

As shown in Figure 8a,b, when the mass ratio of MTs:CaCO₃:CaSO₄ is 1:0.7:0.4, the main mineral phases formed include kaliophilite (KAlSiO₄), potassium sulfate (K₂SO₄), wollastonite (CaSiO₃), gehlenite (Ca₂Al₂SiO₇), and akermanite (Ca₂MgSi₂O₇). In contrast, the primary mineral phases in the leaching residue are quartz (SiO₂), leucite (KAlSi₂O₆), and diopside (CaMg(SiO₃)₂). This indicates that in the MTs–CaCO₃–CaSO₄ system, the formed kaliophilite, potassium sulfate (K₂SO₄), wollastonite, gehlenite, and akermanite are all soluble in the 0.5 mol/L dilute hydrochloric acid used for leaching. These minerals can provide soluble potassium, magnesium, silicon, and calcium elements for the preparation of Si–Ca–K–Mg fertilizer. In contrast, leucite and diopside are insoluble under these leaching conditions and therefore cannot provide soluble nutrients.

To investigate the phase reconstruction mechanism of main minerals in MTs during calcination, the effect of reaction temperature (800–1200 °C) on phase evolution was examined using a mass ratio of MTs:CaCO₃:CaSO₄ = 1:0.7:0.4. As shown in Figure 8c, with increasing calcination temperature, the characteristic peaks of K–feldspar (KAlSi₃O₈), anhydrite (CaSO₄), and quartz (SiO₂) diminish, while the diffraction peaks of wollastonite (CaSiO₃), gehlenite (Ca₂Al₂SiO₇), akermanite (Ca₂MgSi₂O₇), kaliophilite (KAlSiO₄), and potassium sulfate (K₂SO₄) become more prominent. This confirms that the thermodynamic predictions align well with earlier experimental results. Notably, leucite (KAlSi₂O₆) exhibits structural instability, reacting with CaO to form kaliophilite (Reaction 3) and with CaSO₄ to form potassium sulfate (Reaction 8). To investigate soluble potassium species, the calcination products were leached with 0.5 mol/L dilute hydrochloric acid. The XRD patterns of the leaching residues (Figure 8d) show that at 1200 °C, the peaks of wollastonite, gehlenite, akermanite, kaliophilite, and potassium sulfate disappear. The residual minerals are primarily leucite, quartz, and diopside (CaMg(SiO₃)₂). For clinker calcined at 800–1100 °C, the main minerals in the leaching residue are K–feldspar and quartz. These results indicate that potassium sulfate and kaliophilite are the primary sources of soluble potassium in the calcination products.

Figure 9 shows the SEM images and EDS mapping results of MTs, Si–Ca–K–Mg fertilizer, and the leaching residue of fertilizer.

Table 4. Element content of MTs, Si–Ca–K–Mg fertilizer, and leaching residue of fertilizer (wt.%).

Element	MTs	Si–Ca–K–Mg Fertilizer	Leaching Residue of Fertilizer
O	48.91	44.93	54.07
Mg	3.46	2.44	2.13
Al	5.66	3.13	1.43
Si	31.35	14.87	34.80
S	0.40	3.98	0.96
K	6.47	4.77	0.79
Ca	3.76	25.88	5.80

summarizes their elemental compositions.

As in Figure 9a, MTs mainly comprise particulate matter. While a few particles are about tens of micrometers in size, most are smaller than 10 μm, consistent with Figure 1b. From the EDS mapping, some particles with distinct edges and large grain sizes have only silicon and oxygen, probably quartz (denoted by 1). Aluminum and potassium elements are distributed similarly, uniformly and densely, indicating K–feldspar in MTs exists mainly as fine–grained particles (denoted by 2). Particles with high calcium and no other elements are likely calcium carbonate (denoted by 3). And particles with calcium and magnesium but lacking other elements may be dolomite (denoted by 4). This result highly agrees with the MTs phase analysis in Figure 2.

By analyzing the elemental composition of the Si–Ca–K–Mg fertilizer (Figure 9b and Table 4), two major changes are noted. Firstly, the particle size distribution changes significantly, with more fine particles, suggesting a possible solid–phase reaction between MTs and additives. Secondly, the elemental composition changes, with increased calcium content due to additives. Based on the EDS mapping results, further analysis reveals that the distribution of calcium and aluminum elements exhibits a relatively concentrated pattern, likely corresponding to gehlenite (denoted by 1). Additionally, the distribution of calcium and silicon elements is also relatively concentrated, suggesting the presence of wollastonite (denoted by 2). Furthermore, the concentration of calcium and magnesium elements indicates the possible presence of akermanite (denoted by 3). In addition, the sulfur content has significantly increased, which is clearly attributable to the introduction of CaSO₄. In addition, based on the element distribution patterns, some sulfur elements co–localize with potassium elements, indicating the presence of potassium sulfate (denoted by 4). Meanwhile, another portion of the potassium element exhibits a similar distribution to aluminum elements, suggesting that this potassium fraction primarily exists as kaliophilite (denoted by 5). This inferred result agrees with the XRD pattern analysis (Figure 8a, MTs:CaCO₃:CaSO₄ = 1:0.7:0.4) and reaction mechanisms of Equations (5), (6) and (11).

From the element content of the leaching residue of fertilizer presented in Figure 9c and Table 4, two significant changes are evident. First, the particle size has further reduced, and pores have formed in the block structure, indicating that the fertilizer has dissolved. Second, the distribution and concentration of elements like silicon, calcium, potassium, magnesium, aluminum, and sulfur have markedly decreased, reflecting the dissolution of soluble minerals and demonstrating the fertilizer’s good solubility. Notably, sulfur content has significantly decreased, and the remaining sulfur no longer correlates with potassium, suggesting that potassium sulfate has dissolved, leaving behind insoluble sulfates like calcium sulfate. The remaining potassium aligns closely with aluminum, indicating that insoluble potassium primarily exists as aluminosilicate minerals such as K–feldspar or leucite (denoted by 1). Increased silicon and oxygen content implies a higher presence of quartz (denoted by 2). Concentrated distributions of calcium and magnesium suggest the presence of diopside (denoted by 3). These findings agree with the XRD analysis of the leaching residue (Figure 8b, MTs:CaCO₃:CaSO₄ = 1:0.7:0.4).

The findings show that CaCO₃ and CaSO₄ react with MTs to form soluble compounds such as kaliophilite, potassium sulfate, wollastonite, gehlenite, and akermanite. These reaction products provide the essential elements for the Si–Ca–K–Mg fertilizer, consistent with the theoretical thermodynamic calculations.

3.3. The Formation Mechanism and Process of Volatile Potassium

The phase reconstruction mechanism of potassium–containing minerals during calcination has been clearly elucidated. Specifically, K–feldspar (KAlSi₃O₈) reacts with CaO (derived from the high–temperature decomposition of CaCO₃) and CaSO₄ to form leucite (KAlSi₂O₆), kaliophilite (KAlSiO₄), and potassium sulfate (K₂SO₄). However, leucite exhibits a highly stable structure that is resistant to decomposition at elevated temperatures [26], and therefore does not contribute to the formation of volatile potassium. Consequently, the volatile potassium released during calcination primarily originates from the decomposition of kaliophilite and potassium sulfate (K₂SO₄). To investigate the volatilization behavior of potassium, calcination experiments by mixing pure K–feldspar (with a K₂O content of 15.6 wt.%) and CaCO₃ in a mass ratio of 1:0.9 were initially conducted. According to previous thermodynamic calculations and characterization analyses, K–feldspar is expected to be fully converted into kaliophilite during this process. As shown in Figure 10a, the volatilization ratio of potassium from kaliophilite increases with rising calcination temperature. At 1200 °C, the volatilization of potassium reaches 9.01 wt.%, indicating that the kaliophilite formed during calcination partially decomposes at high temperatures, releasing volatile potassium. In addition, a separate calcination experiment on pure potassium sulfate was also conducted. As shown in Figure 10a, the volatilization ratio of potassium from potassium sulfate increases with rising calcination temperature, too. At 1200 °C, the volatilization amount reaches 13.49 wt.%. Therefore, these two sets of experimental data demonstrate that the volatile potassium in the flue gas during fertilizer preparation primarily originates from kaliophilite and potassium sulfate.

Under the experimental conditions of a mass ratio of MTs:CaCO₃:CaSO₄ = 1:0.7:0.4, a calcination time of 1200 min, and a calcination temperature of 1200 °C, the solid formed by condensing the flue gas was collected and characterized by using XRD and SEM–EDS. As shown in Figure 10b, the main components of the flue gas condensate product include amorphous SiO₂, K₂SO₄, K₂Al₂₄O₃₇, and (Ca, K)₄(Si, Al)₅O₁₁(SO₄, CO₃), indicating a complex composition. The K₂SO₄ directly originates from the volatilization of potassium sulfate formed at high temperatures, consistent with the previous findings. In contrast, the volatilization products of kaliophilite may have undergone complex reactions in the flue gas, leading to the formation of various potassium–containing complex salts. The SEM image in Figure 11 reveals that the amorphous silica in the condensate primarily exists in the form of nanofibers, an intriguing observation that warrants further investigation. The potassium–containing phases are distributed as particles and flakes within these fibers. The EDS results shown in the figure indicate that the condensate mainly consists of silicon, along with sulfur, potassium, calcium, and other elements, corroborating the XRD findings.

4. Conclusions

This paper proposes and refines a method for producing high–quality Si–Ca–K–Mg fertilizer enriched with total activated potassium from molybdenum tailings (MTs). Specifically, the volatile potassium that escapes with the flue gas during the calcination process is collected and integrated with the soluble potassium in the calcined clinker. This approach significantly increases the available potassium content in the fertilizer and enhances the utilization efficiency of potassium in MTs. By adjusting the raw meal formula, several optimized batching schemes can be achieved, resulting in different performance metrics. For instance, the highest activation ratio of potassium in MTs reaches 92.36%, the maximum content of total activated potassium in the fertilizer is 4.50 wt.%, and the highest soluble potassium content in the calcined clinker is 3.55 wt.%. These schemes can meet various production requirements. Subsequently, this paper investigates the intrinsic mechanisms underlying the preparation of high–quality Si–Ca–K–Mg fertilizer from MTs. Through a combination of thermodynamic calculations, experiments, and characterizations, it elucidates the mineral phase transformation process wherein the primary minerals in MTs (such as K–feldspar, quartz, and dolomite) are converted into water–soluble and citric–soluble minerals like potassium sulfate, kaliophilite, wollastonite, gehlenite, and akermanite under the synergistic effects of CaCO₃ and CaSO₄. This process reveals the source of soluble nutrients in the fertilizer. Additionally, this study is the first to identify the formation process and occurrence state of volatile potassium in flue gas and elucidates the mechanism and characteristics of the high–temperature decomposition of kaliophilite and potassium sulfate formed during calcination, leading to the generation of volatile potassium. This work holds significant importance for both the theoretical research and industrial practice of preparing Si–Ca–K–Mg fertilizer from MTs. Additionally, it provides a valuable reference for the utilization of other tailings resources containing K–feldspar in mineral fertilizers, thereby promoting the high–value and large–scale application of tailings resources.