Dissolution Kinetics of Carbonates in Low-Grade Microgranular Phosphate Ore Using Organic Acids as Leaching Agents

Abstract

The present study investigates the process of selective leaching of low-grade phosphate ore of the Karatau basin using organic acids such as formic and citric acids. Chemical and instrumental analyses of the investigated phosphate ore were carried out, including X-ray diffraction (XRD), scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX), and Fourier-transform infrared spectroscopy (FTIR). Based on experimental data, reaction rate constants were calculated, and the obtained activation energies for each of the reagents were used. The reaction rate constants indicate that formic acid led to a more gradual increase in P₂O₅ concentration over time, while citric acid demonstrates a more significant increase in phosphorus concentration at all temperatures, especially at 70 °C. The activation energy for formic acid is 14.69 kJ/mol, indicating a diffusion-controlled reaction mechanism, whereas the activation energy for citric acid is higher, i.e., 35.78 kJ/mol, suggesting a more complex mechanism involving both diffusion and chemical processes. The present study highlights the importance of selecting appropriate reaction conditions to achieve maximum efficiency for the leaching of phosphate ore, depending on both temperature and reagent used.

1. Introduction

To support the further development of global agriculture, the industry has significantly increased the production of mineral fertilizers in recent years and continues to actively expand the volumes of this essential product [1]. The rapid growth in the production of phosphorus-containing mineral fertilizers requires an expansion of the phosphate raw material base. Rich phosphate deposits are the key source of phosphates for the production of phosphoric acid and complex fertilizers. However, the current reserves of these deposits are not fully able to meet the growing demand for phosphorus-containing fertilizers, making it necessary to expand the raw material base and introduce new methods for processing phosphate raw materials. In global phosphate fertilizer production practices, there is a focus on the processing new and lower-quality phosphate ores, driven by the significant depletion of high-quality phosphate deposits. In the production of phosphate fertilizers, there is a trend toward increasing their concentration and the active utilization of phosphate resources, particularly low-grade phosphates [2].

There are several approaches to enrich low-grade phosphorites, including flotation, separation, and mechanical technologies. Additionally, phosphate ore enrichment methods such as calcination methods, magnetic separation, and acid treatment are also used. Flotation enrichment methods are widely applied in many existing operations, as they are considered effective [3,4]. However, for the elimination of carbonates from low-grade phosphate raw materials, they prove to be ineffective. The primary reason is that carbonate compounds exhibit flotation properties similar to those of phosphate minerals, making their separation difficult. Furthermore, the presence of iron- and aluminum-containing minerals in the raw material, the high dispersion of phosphate grains, the high cost of flotation reagents, low product yield, and the formation of large amounts of slimes also limit the use of flotation [5,6].

The use of organic acids for selective leaching in phosphate enrichment has gained attention due to their ability to dissolve carbonate compounds without affecting phosphate minerals [7,8,9,10,11,12]. Studies have demonstrated the effectiveness of acids like formic, acetic, citric, and lactic in reducing carbonate content and increasing P₂O₅ levels in low-grade phosphate ores [13,14,15]. Research shows that acetic acid, in particular, is highly effective at lower temperatures and shorter reaction times. Optimal conditions, such as specific acid concentrations, temperatures, and reaction times, have been identified, significantly improving phosphate content while minimizing environmental impact [15].

Kazakhstan is also renowned for its rich phosphate reserves. The Karatau phosphorite basin, one of the largest phosphate deposits in the world, has been continuously providing phosphate ores since the 1940s for the production of yellow phosphorus and phosphoric acid, for export to the European Union, CIS countries, and Southeast Asia. However, the depletion of commercially significant deposits forces the industry and the scientific community to search for new methods to upgrade low-grade phosphate ore deposits. The main objective of the present study is to enrich low-grade microgranular phosphorites from the Karatau phosphorite basin utilizing the selective leaching of carbonates using citric and formic acids. Additionally, kinetic patterns are investigated to determine kinetic parameters and the period during which the process rate can be controlled.

2. Materials and Methods

2.1. Materials

Low-grade micro-grained phosphorite ores from the Karatau phosphorite basin (Southern Kazakhstan) were used as the research object. The raw material was ground using a laboratory mill (MShL-1, Moscow, Russia). Sample preparation was carried out in accordance with GOST 14180-80 [16], which regulates the methods for sampling and preparation for chemical analysis and moisture determination. Citric and formic acids produced by Duchefa Farma BV (Haarlem, The Netherlands) were the used organic acids.

2.2. Experimental Methods

A thermostatic bath with an integrated magnetic stirrer (MbWCL, Guangzhou, China) and a pH meter (I-160MI, Moscow, Russia) (Figure 1) were used for the selective leaching process. The enriched ore and diluted organic acid were placed into a flask, and the leaching process was conducted for up to 60 min at temperatures ranging from 25 to 70 °C.

2.3. Analytical Techniques

The chemical analysis of the original and obtained samples was carried out using known methods. The total concentration of phosphorus pentoxide (P₂O₅) was determined by the photometric method (λ = 440 nm) using a Trilon B solution. The phosphate determination method is based on dissolving the sample in a mixture of nitric and hydrochloric acids at its boiling temperature. A 2 g sample is placed into a 250 mL beaker, and 15 mL of nitric acid and 5 mL of hydrochloric acid are added. The mixture is heated till it starts boiling and kept boiling under a watch glass until the sample is completely dissolved. The volume is then increased to 50 mL with water, and the solution is boiled for an additional 5 min. After cooling, the solution is transferred to a 250 mL volumetric flask, diluted with water to the mark, thoroughly mixed, and filtered, discarding the first portion of the filtrate.

The carbonate content was determined by the gravimetric method: a 0.2 g sample of the rock was placed in a 100–150 mL conical flask and moistened with distilled water. Then, 25 mL of 0.2 N HCl solution was added using a burette or calibrated pipette. The flask was sealed with a stopper containing a 3–5 mm diameter glass tube and heated until gas bubbles (CO₂) stopped forming, indicating the end of decomposition. The solution was brought to a boil, cooled, and washed. Three drops of 0.1% methyl red solution were added, and the excess acid was titrated with 0.2 N NaOH until the pink color turned yellow. Then, calcium and magnesium contents were determined using potassium permanganate, ammonia, and ammonium oxalate, followed by filtration and dilution to 100 mL for analysis.

Instrumental methods of analysis included SEM with EDX (JEOL JSM-6490 LV, Tokyo, Japan), as well as FTIR performed using the IR-Prestige 21 instrument (Shimadzu, Japan). X-ray diffraction (XRD) was performed on a Bruker D8 equipment (Germany) using the PDF-2 database of the International Centre for Diffraction Data. Statistical data analysis was performed using the StatSoft software (v.2.3).

3. Results and Discussion

3.1. Characterization of the Raw Material

The chemical composition of the raw material is a key indicator determining its suitability for subsequent technological processes, such as beneficiation and processing.

Table 1. Chemical composition of the investigated low-grade phosphate ore, % (n = 8).

P2O5	CaO	MgO	K2O	Al2O3	Fe2O3	SiO2
13.36	24.12	2.87	1.04	6.01	2.39	36.14

shows the content of the main compounds in the sample selected for the study.

According to the data in this Table, the low content of phosphorus pentoxide indicates that this raw material does not meet the technological process requirements established by the technical standards of Kazphosphate LLP [17,18], a leading producer of phosphorus products in Kazakhstan and Central Asia. The high content of calcium oxide suggests a significant occurrence of calcite or other carbonate minerals, which may require removal or processing during beneficiation.

In addition to chemical analysis, SEM with EDX was used, and the results are shown in Figure 2.

In the presented SEM images with EDX spectra of low-grade phosphorite from the Karatau basin in Figure 2, a fine-grained structure of the material can be observed. The EDX spectrum shows the presence of major elements such as calcium, silicon, aluminum, phosphorus, iron, magnesium, potassium, and titanium. The detected calcium could potentially be associated with carbonate minerals such as calcite. However, since carbon was not measured in the EDX analysis, this conclusion is based on additional analyses, such as XRD, which can confirm the presence of carbonate phases like calcite in the material. A high silicon peak indicates a significant content of silicate minerals, for example, quartz. The presence of phosphorus indicates the presence of phosphate compounds; however, their content is relatively low compared to other elements. This confirms the fine-grained nature of the sample and the presence of a significant amount of impurities. A more detailed mineralogical structure can be described using X-ray diffraction patterns, and its results are presented in Figure 3.

X-ray diffraction (XRD) of low-grade phosphorite from the Karatau basin shows that the main mineral composition of the sample includes quartz (43.9%), fluorapatite (33.4%), dolomite (18.1%), and calcite (4.6%). The high quartz content is confirmed by the chemical analysis and SEM with EDX data, which indicate a significant presence of silica compounds corresponding to quartz. The presence of fluorapatite, as the primary phosphate mineral, explains the phosphorus content in the chemical composition; however, its proportion accounts for only one-third of the total volume. The detected inorganic compounds are also represented on the FTIR spectrum in Figure 4.

The FTIR spectrum of low-grade phosphorite in Figure 4 shows peaks corresponding to various functional groups associated with the main components of the sample. Peaks at 551.64 cm⁻¹ and 567.07 cm⁻¹ may be related to the bending vibrations of Si-O in quartz, which is confirmed by the high quartz content [19]. Peaks in the range of 694.37 cm⁻¹ and 729.09 cm⁻¹ may correspond to phosphate groups (PO₄³⁻) in fluorapatite [19]. The peak at 1454.33 cm⁻¹ may be associated with the vibrations of carbonate groups (CO₃²⁻) in dolomite and calcite [20], identified in both the XRD and chemical analyses (CaO and MgO contents).

3.2. Acid Leaching Experiments

The efficiency of the leaching process was evaluated by the level of carbonate (CO₂) removal from the phosphorites, changes in pH, and the increase in phosphorus pentoxide (P₂O₅) content. The results of the acid leaching experiments are presented below (Figure 5).

Figure 5a,b demonstrate changes in CO₂ concentration (in percentage) during the interaction of low-grade phosphorites with formic and citric acids at different temperatures and time intervals. An increase in temperature leads to a more rapid reduction in CO₂ concentration, indicating more efficient carbonate dissolution and the subsequent release of CO₂ at higher temperatures. Over time, for all temperature regimes, a decrease in CO₂ concentration is observed, which indicates the continuous dissolution of carbonates in the acidic medium. In the case of Figure 5a, at 25 °C, the CO₂ concentration decreases from 16.96% to 7.6% over 60 min, while at 70 °C, it decreases much faster—from 12.18% to 1.04% in the same period. Similar results were obtained in the study by Seitnazarov et al. [21], where increasing the temperature and duration of the process also enhanced the degree of decarbonization. Citric acid has a stronger effect on carbonate dissolution, especially at higher temperatures. As noted by Fei et al. [13], dolomite is more hydrophilic than apatite and dissolves well in citric acid. CO₂ concentration decreases faster compared to formic acid, particularly at 70 °C, where after just 60 min, the concentration drops to 0.2%. At 25 °C, the concentration decreases from 15.72% to 11.12% over 60 min, while at 70 °C, it decreases from 8.92% to 0.2% over the same period. These data indicate that citric acid is more effective for the dissolution of carbonates in phosphorites compared to formic acid, especially at higher temperatures. Elevated temperatures significantly accelerate the chemical interaction of acids with carbonates, leading to more intense CO₂ release.

The presented data in Figure 5c,d show changes in pulp pH depending on time and temperature. The difference in the effects of the two acids on pulp pH can be explained by their chemical properties and acid strength. Citric acid leads to a more significant increase in pulp pH. Increasing the temperature enhances the acid’s reaction with carbonates, causing greater CO₂ release and resulting in a more pronounced rise in pH. For example, at 70 °C, the pH increases from 2.32 to 3.25 over 60 min. As the process continues, the pH keeps rising, indicating the ongoing removal of carbonates and the formation of acidic reaction products. The temperature’s effect on pH when using formic acid is also noticeable, but the changes are less pronounced compared to citric acid. At 70 °C, the pH increases from 3.35 to 3.46 over 60 min. The influence of time is also evident, but the pH rise occurs more gradually and does not reach levels as high as those with citric acid. According to Zafar et al. [22], this may indicate that, at higher temperatures, formate solubility decreases, which is also accompanied by the contamination of the CO₂ gas stream with water and acid vapors.

The effect of acids on the solubility of carbonates significantly impacts the increase in P₂O₅ concentration. When using formic acid, regardless of temperature, the P₂O₅ concentration gradually increases over time, indicating the continued dissolution of carbonates and the release of phosphorus. Increasing the temperature accelerates this process, leading to a more rapid rise in P₂O₅ concentration. For example, in Figure 5e, at 70 °C, the P₂O₅ concentration increases from 16.19% to 19.14% over 60 min. In another study, where formic acid was used to treat phosphorites from the Dalir deposit in Iran, the phosphorus pentoxide content increased from 11% to 28% over 50 min [23]. However, the liquid-to-solid ratio in that case was 7:1, which required an additional step to dispose of the resulting formate solution. Compared to formic acid, citric acid shows a more significant increase in P₂O₅ concentration at all temperatures, likely due to the more effective dissolution of the carbonate components (Figure 5f). Similar to formic acid, increasing the temperature enhances the reaction efficiency. At 70 °C, the P₂O₅ concentration rises from 17.44% to 22.15% over 60 min.

3.3. Kinetic Patterns

The process of selective leaching involves the targeted dissolution of carbonate-containing minerals from phosphate raw materials using organic acids. When calcium carbonate (CaCO₃) and magnesium carbonate (MgCO₃) interact with citric acid (C₆H₈O₇) and formic acid (HCOOH), acid–base reactions (1)–(4) occur, leading to the formation of soluble calcium and magnesium citrates and formates:

3CaCO₃ + 2H₃C₆H₅O₇ → Ca3(C₆H₅O₇)₂ + 3H₂O + 3CO₂↑

(1)

3MgCO₃ + 2H₃C₆H₅O₇ → Mg3(C₆H₅O₇)₂ + 3H₂O + 3CO₂↑

(2)

CaCO₃ + 2HCOOH → Ca(HCOO)2 + H₂O + CO₂↑

(3)

MgCO₃ + 2HCOOH → Mg(HCOO)2 + H₂O + CO₂↑

(4)

At the same time, organic acids do not interact with the phosphate component. To determine the kinetic patterns, the Jander–Zeldovich model was applied, which is used to describe the kinetics of solid-state reactions, particularly those where the reaction rate is limited by diffusion through the product layer (5):

$${1-(1-\alpha )}^{1/3}=k·\sqrt{\tau }$$

(5)

where α is the degree of transformation (the proportion of carbonates dissolved in the process), k is the constant of the reaction rate, depending on the temperature, and t is the reaction time.

This model assumes that the process is controlled by diffusion and takes into account the change in the volume of the solid as reaction products form a layer around the unreacted core [24]. To describe the kinetic characteristics of the process, graphs were constructed based on experimental data (Figure 6). The value of α was calculated based on the degree of carbonate removal.

As shown in

Table 2. Calculated reaction rate constants.

Temperature, °C	Reaction Rate Constants, min−1
	For the Use of HCOOH	For the Use of C6H8O7
25	0.0049	0.0020
40	0.0060	0.0124
55	0.0076	0.0160
70	0.0108	0.0142

, the increase in reaction rate constants with rising temperatures for both acids indicates that diffusion through the product layer becomes faster at higher temperatures, which aligns with the main principles of the Jander model. In the case of formic acid, the rate constant increases more gradually, which may suggest a more uniform diffusion process through the product layer. Interestingly, for citric acid, the maximum rate constant is reached at 55 °C, after which it decreases at 70 °C. This could mean that, at higher temperatures, the product layer becomes denser or undergoes structural changes, making further diffusion of the reactant more difficult and reducing the overall reaction rate. Such behavior is also consistent with the Jander model, which assumes diffusion-limited reactions [24].

Based on the obtained reaction rate constant data, activation energy values were calculated using the Arrhenius equation [25] for both cases. According to the calculations, the activation energy for the use of HCOOH was 14.69 kJ/mol, while for citric acid, it was 35.78 kJ/mol. The low activation energy (14.69 kJ/mol) indicates that the leaching process of carbonates with formic acid occurs more easily and quickly. This means that less energy is required to initiate and sustain the reaction. In the context of the Jander model, this could suggest that diffusion through the product layer or the process of forming this layer is less restrictive when using formic acid. It is possible that formic acid penetrates the product layer more rapidly, or the reaction product layer forms less densely, which facilitates the continuation of the reaction.

The use of citric acid requires more energy to overcome the activation barrier. This could mean that citric acid either penetrates the product layer more slowly or that the layer becomes denser and less permeable. Within the framework of the unreacted core model, this may indicate that the diffusion of citric acid through the product layer is more difficult, requiring more energy to sustain the reaction [26]. The higher activation energy may also suggest that the product layer formed when using citric acid is more stable or denser, which hinders further reaction progress. Interpreting the obtained data according to [15,27], an activation energy of up to 25 kJ/mol corresponds to a diffusion-controlled mechanism. At the same time, for citric acid, the activation energy is close to 40 kJ/mol, indicating a possible mixed or chemically controlled behavior, where diffusion may play a lesser role, and the chemical interaction becomes more significant at certain temperature ranges.

4. Conclusions

• Temperature Impact: formic acid’s reaction rate increased with temperature, while citric acid showed higher rate constants at 55 °C but higher rate constants at 70 °C, indicating a complex mechanism.
• Activation Energy: the activation energy for formic acid was 14.69 kJ/mol, suggesting a diffusion-controlled process, while citric acid had a higher activation energy of 35.78 kJ/mol, indicating a mixed mechanism.
• Efficiency of Acids: citric acid was more effective in dissolving carbonates, particularly at elevated temperatures, compared to formic acid.
• Phosphate Enrichment: the P₂O₅ content increased to 22.15% with citric acid and to 19.14% with formic acid, demonstrating the effectiveness of both acids in phosphate enrichment.
• Optimal Conditions: formic acid was more efficient at lower temperatures, while citric acid required careful temperature control to maintain reaction efficiency.