Study of Stibnite Dissolution in Nitric Acid in the Presence of Organic Acids

Abstract

The nitric acid leaching of antimony from stibnite using organic (tartaric and citric) acids as complexing agents was investigated. Tartaric acid has been found to be a more effective complexing agent, providing up to 90% antimony recovery, while citric acid achieves 54% only. SEM and X-ray diffraction analysis showed tartaric acid to prevent antimony hydrolysis, preserving unreacted stibnite in the residue, while Sb₄O₄(OH)₂(NO₃)₂ particles were formed in the system with citric acid. Kinetic calculations have revealed that the nitric acid leaching of antimony with the addition of tartaric acid is limited by internal diffusion (R² > 0.94), the activation energy is 62.5 kJ/mol, and the empirical reaction orders for tartaric and nitric acids are 2.3 and 2.7, respectively. These data are confirmed by morphological and phase analyses, the mechanisms of action of organic acids have been substantiated, and a generalized kinetic equation describing the nitric acid leaching of antimony with the addition of tartaric acid is proposed.

1. Introduction

Antimony, a strategically important metal, is widely used in various industries, including the production of refractories, semiconductor materials, batteries, bearing alloys, and medical products [1,2]. The primary industrial raw material for antimony production is ore containing stibnite (Sb₂S₃), which accounts for the vast majority of global production [3].

Traditional pyrometallurgical methods for processing antimony concentrates, such as smelting in reverberatory or rotary furnaces, have a number of significant drawbacks [4,5,6,7]. These include high energy consumption, the formation of toxic gaseous emissions (SO₂, antimony oxide and arsenic vapors), and significant metal losses in slag and sublimates. These factors make pyrometallurgical processing environmentally problematic and economically less viable in today’s increasingly stringent environmental regulations. In this context, hydrometallurgical methods represent a promising and more sustainable alternative, enabling selective metal extraction, reducing environmental impact, and processing raw materials with more complex compositions.

Among the hydrometallurgical approaches, the most studied are leaching processes using various reagents, namely, hydrochloric acid [8,9], alkalis (NaOH and Na₂S) [4,5,10,11,12], bacteria [13,14], and iron salts [15]. However, each of these methods has its own limitations. Alkaline leaching, for example, effectively dissolves antimony sulfide to form thiosalts, but is characterized by the high cost of reagents and the need to dispose of spent solutions. Bacterial and hydrochloric acid leaching without oxidizing agents is slow and incomplete.

Nitric acid occupies a special place among the reagents for acidic leaching. As a strong oxidizing agent, it is capable of not only dissolving sulfide minerals but also oxidizing sulfur to sulfate ions [16,17]. The process of stibnite dissolution in nitric acid involves a complex set of redox reactions, which may lead to the formation of various compounds—the intermediate oxide (Sb₂O₃) and the final product, the sparingly soluble antimony (V) oxide (Sb₂O₅). The formation of these sparingly soluble compounds is a serious technological obstacle, since it leads to losses of the target metal in the precipitate and complicates its subsequent extraction.

To overcome this problem, the introduction of complexing agents, capable of stabilizing antimony ions in solution, into the leaching medium appears promising [18,19,20]. In this context, organic acids such as tartaric (C₄H₆O₆) and citric (C₆H₈O₇) ones demonstrate significant potential, as they are capable of forming strong, water-soluble complexes with the cations of many metals, including antimony (III and V), in acidic media. Studies have indeed confirmed the successful use of organic acids in hydrometallurgical processes, e.g., to improve the leaching of oxide and sulfide ores of nickel, copper, and zinc [21,22,23,24]. However, in the case of antimony, especially when combined with nitric acid, similar studies remain very limited. A critical point is that stibnite cannot be dissolved in organic acids alone, without the additional introduction of an oxidizing agent [20,25,26]. This is due to the fact that the stable sulfide matrix of the mineral must be destroyed. An oxidizing agent is necessary to oxidize sulfide ions, which would lead to the destruction of the stibnite crystal lattice and the release of antimony ions. Only then can the released antimony cations be effectively stabilized by organic acids through complexation, preventing their hydrolysis and precipitation.

Thus, the combined use of nitric acid as a solvent and organic acids as complexing additives represents a scientifically and practically significant task. Given the successful application of similar synergistic systems for other non-ferrous metals, a significant positive effect can be expected in stibnite processing as well. Exploring the synergistic effect of their combined action will not only deepen our understanding of the chemistry of sulfide mineral dissolution processes in complex redox systems but also develop the foundations of a new, more efficient, and environmentally friendly hydrometallurgical technology for processing antimony raw materials.

Therefore, the objective of this work was to study stibnite dissolution in nitric acid with the addition of tartaric and citric acids. The focus was on the process kinetics, the influence of organic acid and HNO₃ concentrations, and the role of the resulting complexes in increasing Sb recovery.

2. Materials and Methods

2.1. Analysis

Chemical analysis of stibnite, undissolved leaching residue, and leaching solutions was performed using inductively coupled plasma optical emission spectrometry (ICP-OES) on an EXPEC 6500 spectrometer (Focused Photonics Inc., Hangzhou, China), employing the 217.581 nm spectral line for antimony. Solid samples (0.15–0.2 g) were first completely dissolved using a PreeKem M3 microwave (PreeKem Scientific Instruments Co., Beijing, China) sample preparation system prior to analysis.

XRD analysis of the starting material was carried out on an XRD 7000 Maxima diffractometer (Shimadzu Corp., Tokyo, Japan), in the range of angles 5–80° 2θ with a step of 0.02° and an exposure time of 2 s/step. The obtained diffraction patterns were interpreted using Match! 3.0 software and the ICDD PDF-2 database.

Scanning electron microscopy (SEM) of undissolved stibnite leaching residue was performed using a JSM-6390LV microscope (JEOL Ltd., Tokyo, Japan) equipped with an energy-dispersive X-ray spectroscopy (EDX) module. The analysis was performed at an accelerating voltage of 20 kV, a probe current of 1 nA and a working distance of 15 mm.

2.2. Materials and Reagents

A natural stibnite sample from the Olimpiada deposit (Eastern Siberia, Russia) was used as the main material for the work. The chemical composition of the mineral, determined by ICP-OES, was 69.85% Sb and 28.88% S, corresponding to the stoichiometry of stibnite. The main impurity was quartz SiO₂ (1.5%), as confirmed by XRD (Figure 1). The mineral was ground in a PULVERISETTE 6 planetary mill (Fritsch GmbH & Co. KG, Welden Germany) under the following conditions: grinding time of 5 min, rotation speed of 250 rpm, and a grinding ball-to-material ratio of 10:1. After grinding, the sample was wet-screened on laboratory sieves to obtain a working fraction of −74 µm.

Nitric acid (HNO₃, 65%), tartaric acid (C₄H₆O₆, ≥99.5%), and citric acid (C₆H₈O₇, ≥99.5%) had analytical purity and were used without prior purification. Solutions for analysis were prepared using deionized water.

2.3. Experimental Procedure

Experiments on nitric acid leaching of antimony from stibnite were conducted in a Lenz Minni-60 laboratory reactor with a volume of 500 mL (Lenz Laborglas GmbH & Co. KG, Wertheim, Germany). They were started at a temperature 15–20 °C lower than required, as exothermic reactions ensured the pulp self-heated to the required values. Temperature accuracy was ensured by a high-precision Huber Kiss K6 thermostat (Huber Kältemaschinenbau AG, Offenburg, Germany), capable of maintaining set parameters with an error of no more than ±0.05 °C. Throughout the experiment, the actual temperature of the reaction pulp was continuously measured using a calibrated temperature probe immersed directly in the slurry. After reaching the required temperature, a stibnite sample was added to the reactor. To ensure pulp homogeneity, an IKA Eurostar 20 digital overhead stirrer (IKA®-Werke GmbH & Co. KG, Staufen, Germany) was used, providing intense and uniform stirring. The leaching experiments were designed to investigate the effects of temperature (range 60–80 °C), nitric acid concentration (range 4–7 M), and the type and concentration of organic acids (citric, tartaric; range 20–80 g/L). All leaching experiments were performed in triplicate. To study the leaching kinetics, 1 mL samples were taken using a Sartorius Proline automatic sample dispenser (Mine-beaIntecAachen GmbH & Co. KG, Aachen, Germany) at strictly defined time intervals (1, 2, 5, 10, 15, 30, 45, 60, 80, and 100 min) and diluted in 25 mL flasks. The dilution was made with 3% nitric acid + 20 g/dm³ tartaric acid. Each diluted sample was analyzed for antimony using inductively coupled plasma optical emission spectrometry. Sb recovery was calculated using Equation (1). Upon completion of the experiment, the pulp was filtered through a Buchner funnel. The cake, containing unreacted stibnite particles, was thoroughly washed with distilled water to remove residual solution. It was then dried in an oven at 80 °C to constant weight and subjected to X-ray diffraction and microscopic analysis. The obtained data were processed statistically to assess the accuracy and reproducibility of the results, as well as to plot kinetic curves for the process.

X = (m₁ × X₁ − m₂ × X₂)/(m₁ × X₁);

(1)

where m₁ is the initial sample weight (g), X₁ is the antimony content in the initial sample (%), m₂ is the weight of the undissolved residue after leaching (g), and X₂ is the antimony content in the undissolved residue (%).

Thermodynamic calculations were performed using HSC Chemistry Software v. 9.5 (Metso Outotec Finland Oy, Tampere, Finland).

3. Results

3.1. Thermodynamics of Nitric Acid Leaching of Stibnite

Thermodynamic calculations were performed using HSC Chemistry software v. 9.5 prior to the experimental work to theoretically justify the feasibility of the leaching process and to guide the selection of key experimental parameters.

The Gibbs free energy changes for reactions (2)–(6) were calculated to determine the feasibility of interactions between stibnite and nitric acid solution. The calculations were performed over a temperature range of 25 to 85 °C (

Table 1. Calculated Gibbs free energy changes for reactions (2)–(6).

Reaction No.	ΔG, kJ/mol
	25 °C	40 °C	55 °C	70 °C	85 °C
2	−344	−355	−367	−378	−389
3	−1630	−1683	−1735	−1787	−1839
4	−1411	−1455	−1498	−1541	−1583
5	−219	−228	−237	−246	−255
6	−168	−175	−182	−189	−196

).

Sb₂S₃ + 6HNO₃ = Sb₂O₃ + 6NO₂ + 3S + 3H₂O;

(2)

Sb₂S₃ + 28HNO₃ = Sb₂O₅ + 3H₂SO₄ + 28NO₂ + 11H₂O

(3)

Sb₂S₃ + 24HNO₃ = 2HSbO₂ + 3H₂SO₄ + 24NO₂ + 8H₂O;

(4)

2HSbO₂ + 4HNO₃ = Sb₂O₅ + 3H₂O + 4NO₂

(5)

Sb₂O₃ + 4HNO₃ = Sb₂O₅ + 4NO₂ + 2H₂O

(6)

Based on our calculations of the Gibbs free energy changes for reactions (2)–(6), it was established that stibnite dissolution in nitric acid could occur in various ways, namely, the formation of both poorly soluble antimony oxides (Sb₂O₃ and Sb₂O₅) and antimony acid (HSbO₂) is possible via reactions (2)–(4). Subsequently, upon the interaction of antimony acid and antimony (III) oxide with nitric acid, both compounds may transform into Sb₂O₅.

Pourbaix (Eh–pH) diagrams are an important tool for predicting the thermodynamic stability of various chemical compounds in aqueous solutions. To analyze the behavior of antimony compounds at several pH and Eh values, a Pourbaix diagram was plotted for the S–Sb–H₂O system (Figure 2a). The concentrations of antimony and sulfur were set according to the stoichiometry of stibnite (Sb₂S₃), i.e., the molar ratio of Sb:S was 2:3. The temperature was set to 80 °C. To more accurately predict the formation of products from nitric acid leaching of stibnite, an equilibrium distribution diagram (Figure 2b) was plotted, showing the equilibrium amounts of various antimony forms in the studied systems as a function of nitric acid consumption. The equilibrium distribution diagram (Figure 2b) was plotted using the ‘Equilibrium Compositions’ module, considering the stepwise addition of HNO₃ to a fixed amount of Sb₂S₃–1 Mol.

The Pourbaix diagram (Figure 2a) shows that in an acidic medium, at a potential of approx. −0.4 V, stibnite begins to dissolve to form antimony acid (HSbO₂). A further increase in the oxidation potential to −0.61 V leads to HSbO₂ conversion to antimony (V) oxide, indicating more complete oxidation of antimony. These calculated transitions are consistent with the thermodynamic driving forces suggested by reactions (4) and (5).

The equilibrium distribution diagram (Figure 2b), unlike the Pourbaix one, demonstrates the effect of nitric acid (HNO₃) consumption on the composition of the stibnite dissolution products. It is noted that antimony trioxide is also formed in addition to HSbO₂ during the initial dissolution. Increasing the nitric acid consumption up to >7 M leads to the predominant presence of antimony as Sb₂O₅. This is due to the fact that the oxidation potential of the medium increases significantly at high HNO₃ concentrations, ensuring further oxidation of Sb (III) to Sb (V). At nitric acid concentrations >8 M, almost complete oxidation of Sb₂O₃ and HSbO₂ to Sb₂O₅ occurs.

3.2. Nitric Acid Leaching of Stibnite with the Addition of Tartaric and Citric Acids

3.2.1. Effect of Temperature

The effect of temperature on the antimony extraction degree was studied at a nitric acid concentration of 5 mol/dm³, duration of 100 min, and tartaric and citric acid concentrations of 40 g/dm³, with a liquid-to-solid ratio of 20:1. The results are presented in Figure 3.

According to the graphs presented, nitric acid leaching in the presence of tartaric acid (Figure 3a) is characterized by a high rate at the initial stage (the first 1–15 min), after which the extraction rate decreases significantly. The maximum extraction rate at a temperature of 80 °C reaches 60% in 100 min. The curve shape indicates that the process rate becomes limited over time, either by diffusion processes through the resulting layer of reaction products or by a decrease in the reagent concentration. In the presence of citric acid, the general patterns are preserved in Figure 3b; however, the antimony extraction efficiency is significantly lower overall. Even at the maximum temperature of 80 °C, the dissolution degree does not exceed 47%. Moreover, the curves are flatter throughout the experiment, indicating an initially lower process rate compared to the system containing tartaric acid. This allows us to conclude that citric acid is a less effective complexing agent for antimony ions under these conditions or has an inhibitory effect on the oxidation kinetics of the sulfide matrix.

3.2.2. Effect of Organic Acid Concentration

The effect of organic acid concentrations on the antimony extraction degree was studied using a nitric acid concentration of 5 mol/dm³, duration of 100 min, and a temperature of 60 °C, with a liquid-to-solid ratio of 20:1. The results are presented in Figure 4.

Figure 4a shows a pronounced dependence of the extraction degree on the reagent concentration. Increasing the tartaric acid concentration from 40 to 80 g/dm³ significantly increases both the initial dissolution rate and the final degree of antimony extraction in 100 min. The curve for the concentration of 80 g/dm³ demonstrates rapid achievement of high extraction rates (78% in 15 min). In contrast, at the lowest concentration (20 g/dm³), the curve is flat, and the final extraction rate does not exceed 26%. This indicates that it is the lack of the complexing agent that may be the limiting factor preventing the complete transfer of antimony ions into solution.

Based on the results presented in Figure 4b, we conclude that citric acid has a less significant effect on antimony dissolution. Although a slight increase in the rate and degree of extraction is observed with an increase in the citric acid concentration from 20 to 80 g/dm³, the overall process efficiency remains rather low. Even at the highest studied concentration of 80 g/dm³, the extraction degree does not exceed 54%. All curves for citric acid exhibit a smooth rise with no sharp initial section, which may indicate a slower complexation mechanism or a different interaction mechanism with the mineral surface compared to tartaric acid.

3.2.3. Effect of Nitric Acid Concentration

The effect of nitric acid concentration on the antimony extraction degree was studied at an organic acid concentration of 40 g/dm³, duration of 100 min, and a temperature of 60 °C, with a liquid-to-solid ratio of 20:1. The results are presented in Figure 5.

Figure 5a, which depicts the process in the system with tartaric acid, shows a pronounced dependence of the degree and rate of antimony extraction on the HNO₃ concentration. The antimony extraction curves for concentrations of 6 and 7 mol/dm³ are characterized by a steep initial section, indicating a high rate of oxidation of the sulfide matrix. After 15 min, the rate slows down, reaching extraction values of approx. 57–70% in 100 min. Conversely, at low HNO₃ concentrations (4 and 5 mol/dm³), the process proceeds more slowly, and the final extraction does not exceed 40–46%.

Figure 5b also shows the effect of nitric acid concentration, but it is less significant, and the overall process efficiency remains rather low. However, increasing the HNO₃ concentration from 4 to 7 mol/dm³ leads to a noticeable increase in antimony extraction, from 17% to 52%.

3.3. Characteristics of the Resulting Undissolved Residues

3.3.1. Solid Residue from Nitric Acid Leaching of Stibnite with No Addition of Organic Acids

A micrograph and EDX mapping of the undissolved residue obtained at a nitric acid concentration of 5 mol/dm³, a 10-min run time, and a temperature of 60 °C are shown in Figure 6. Antimony recovery under these conditions was 6%.

The SEM results show particles with a plate-like shape. The EDX mapping results demonstrate the co-localization of antimony, oxygen, and nitrogen in plate-like formations, which provides direct evidence of the formation of antimony nitrate. Sulfur is distributed locally, indicating a residual Sb₂S₃ content.

The X-ray diffraction pattern shown in Figure 7 confirms the presence of antimony (III) oxide hydroxide nitrate (Sb₄O₄(OH)₂(NO₃)₂) in the residue, as well as residual stibnite, elemental sulfur, and quartz.

The chemical reaction between stibnite and nitric acid to form Sb₄O₄(OH)₂(NO₃)₂ is presumably multistage, first forming Sb₂O₃ (reaction (2)), then Sb₄O₄(OH)₂(NO₃)₂ (reaction (7)):

2Sb₂O₃ + 2HNO₃ = Sb₄O₄(OH)₂(NO₃)₂

(7)

3.3.2. Undissolved Residue from Nitric Acid Leaching of Stibnite with Tartaric Acid Added

A micrograph and EDX mapping of the undissolved residue obtained at a nitric acid concentration of 5 mol/dm³, a 10-min run time, a temperature of 60 °C, and a tartaric acid concentration of 40 g/dm³ are shown in Figure 8. Antimony recovery under these conditions was 24%.

The EDX mapping data indicate unreacted stibnite (Sb₂S₃) to be the main phase in the undissolved residue. This is significantly different from the system without organic acids, where Sb₄O₄(OH)₂(NO₃)₂ particles predominated. The EDX maps (Figure 8c,e) show a clear correlation in the distribution of antimony and sulfur, consistent with stibnite. The oxygen distribution map (Figure 8d) shows its weak and uneven localization, which correlates with the distribution of silicon (Figure 8f) and aluminum (Figure 8g), indicating its association with the silicate matrix of the gangue.

Also, the micrograph (Figure 8a) shows agglomerated particles with signs of the surface passivation onset. The area highlighted in red contains one agglomerated particle of interest. The EDX mapping for this particle at an enlarged scale (50 μm) is shown in Figure 9.

The EDX mapping data of agglomerated particles indicate the onset of particle agglomeration and stibnite passivation due to the formation of an elemental sulfur layer to block the access of reagents to the mineral surface.

The X-ray diffraction pattern of the undissolved residue from nitric acid leaching of stibnite in the presence of tartaric acid, shown in Figure 10, confirms the presence of elemental sulfur and quartz in the stibnite residue.

Thus, tartaric acid has been established to effectively bind antimony ions into soluble complexes, preventing their hydrolysis and precipitation. This allows the oxidation reaction to proceed more completely. The main factor limiting the process in the presence of tartaric acid is agglomeration of stibnite particles and blocking of their surface by the oxidation product (elemental sulfur). This result explains the high rate of antimony extraction initially, followed by a slowdown, as observed in Figure 3a, Figure 4a and Figure 5a.

The proposed chemical reaction of stibnite interaction with nitric and tartaric acids and the formation of the soluble complex Sb₂(C₄H₄O₆)₃ is multistage, first forming Sb₂O₃ (reaction (2)) and then Sb₂(C₄H₄O₆)₃ (reaction (8)):

Sb₂O₃ + 3C₄H₆O₆ = Sb₂(C₄H₄O₆)₃ + 3H₂O

(8)

3.3.3. Undissolved Residue from Nitric Acid Leaching of Stibnite with Citric Acid Added

A micrograph and EDX mapping of the undissolved residue obtained at a nitric acid concentration of 5 mol/dm³, a 10-min run time, a temperature of 60 °C, and a citric acid concentration of 40 g/dm³ are shown in Figure 11. Antimony recovery under these conditions was 21%.

The EDX mapping data (Figure 11c,e) demonstrate a correlation of antimony and sulfur, consistent with unreacted stibnite. The formation of needle-shaped particles is noted. Our EDX analysis of these particles reveals the co-localization of antimony, oxygen, and nitrogen, identified as Sb₄O₄(OH)₂(NO₃)₂. A closer look at one of these particles (Figure 12) reveals a morphology significantly different from the plate-shaped particles formed without organic acids (Figure 6), indicating a different crystallization mechanism for the reaction product in the presence of citric acid.

The X-ray diffraction pattern of the undissolved residue from nitric acid leaching of stibnite in the presence of citric acid, shown in Figure 13, confirms the presence of Sb₄O₄(OH)₂(NO₃)₂ therein.

These results explain the lower rate of nitric acid leaching of antimony with citric acid compared to tartaric acid, namely, citric acid forms less stable complexes with antimony ions (reaction (9)), which allows partial hydrolysis and precipitation of antimony (III) oxide nitrate hydroxide (Sb₄O₄(OH)₂(NO₃)₂) (reaction (7)).

3Sb₂O₃ + 2C₆H₈O₇ ↔ 2Sb(C₆H₅O₇)₂ + H₂O

(9)

3.4. Calculation of Kinetic Characteristics

Our analysis of the undissolved stibnite leaching residues has revealed fundamental differences in the mechanisms when using different organic acids. It has been found that with the addition of citric acid, antimony partially hydrolyzes to form Sb₄O₄(OH)₂(NO₃)₂. Therefore, calculating the kinetic characteristics and establishing the reaction regime using the shrinking core method (SCM) for nitric acid leaching of antimony is only valid for the system with tartaric acid added [27].

Table 2. Kinetic equations describing the rate-limiting SCM stages and R² values.

No.	Limiting Phase	Formula	R2
			50 °C	60 °C	70 °C	80 °C
7	Diffusion through the product layer (sp)	1 − 3(1 − X) 2/3 + 2(1 − X)	0.941	0.963	0.943	0.938
8	Diffusion through the product layer (pp)	X2	0.909	0.89	0.901	0.911
9	Diffusion through the product layer (cp)	X + (1 − X) ln(1 − X)	0.919	0.901	0.902	0.896
10	Diffusion through the liquid film (sp)	X	0.791	0.768	0.781	0.763
11	Surface chemical reaction (cp)	1 − (1 − X)1/2	0.797	0.782	0.804	0.78
12	Surface chemical reaction (sp)	1 − (1 − X)1/3	0.799	0.787	0.813	0.792

sp—spherical particles, pp—plate-like particles, cp—cylinder particles.

presents the main equations describing the SCM stages [27] and R² values for the temperature range of 50–80 °C.

The obtained correlation coefficients for the equations in Table 2 show that Equation (7) has the highest value over the entire temperature range under consideration, exceeding 0.94.

The apparent activation energy for antimony extraction was calculated using a plot of ln k_c vs. 1/T (Figure 14b), where k_c is the slope calculated in Figure 14a. The coefficient a found by plotting a straight line y = ax + b in these coordinates determines the slope of the line. According to Equation (13), derived from the Arrhenius law, the apparent activation energy for antimony extraction was evaluated to be 62.5 kJ/mol based on the straight line slope.

$$\ln k_c=\mathrm{l}\mathrm{n}A-{\displaystyle \frac{{\mathrm{E}}_{\mathrm{a}}}{\mathrm{R}\mathrm{T}}}$$

(13)

Using the ln k_c vs. ln(C₄H₆O₆)/ln(HNO₃) plots (Figure 15b,d), we also calculated the empirical concentration orders for tartaric (2.3) and nitric acid (2.7), where k_c is the slope found in a similar way as calculating the activation energy (Figure 15a,c) [27].

Such high reaction orders confirm the complex multistage mechanism of the process. The high order for nitric acid (2.7) emphasizes its key role as the oxidizing agent, while the significant order for tartaric acid (2.3) indicates its active role not only as a complexing agent but also as a reactant, influencing the process kinetics.

Thus, the main rate-limiting step of the process is internal diffusion, as confirmed by the highest R² value for the corresponding kinetic model. This is consistent with X-ray diffraction and EDX data, which showed the formation of passivating layers on the particle surface.

Based on the results obtained, graphs were plotted for all temperatures and nitric and tartaric acid concentrations to derive general kinetic equations for antimony leaching (Figure 16). The graph has allowed us to determine the slope a, which corresponds to k_o.

Based on the obtained results, a generalized kinetic equation was derived for the nitric acid extraction of antimony from stibnite with tartaric acid added:

1 − 3(1 − X) ^2/3 + 2(1 − X) = 17.62C_HNO3^2.7C_C4H6O6^2.31e^−62500/RTt

(14)

4. Discussion

The results of this study highlight the critical role of complexing agents in the nitric acid leaching of stibnite. The formation of an insoluble antimony (III) oxide nitrate hydroxide phase, Sb₄O₄(OH)₂(NO₃)₂, observed in our experiments without organic acids and with citric acid (Figure 6 and Figure 11) is consistent with the results of Ling et al. [28], who reported Sb₂O₃ conversion into insoluble Sb-OH-NO₃ compounds during nitric acid leaching for arsenic removal. This confirms that nitric acid itself acts primarily as an oxidizing agent, leading to the formation of insoluble antimony salts. The high efficiency of tartaric acid (over 90% recovery) compared to citric acid (54%) may be due to the higher stability of antimony complexes with tartaric acid in acidic media. As noted by Hu and He [20], tartaric acid forms strong complexes with Sb (III) that effectively prevent hydrolysis even at low pH. In contrast, citric acid, despite being a strong ligand for many metals, exhibits less efficiency in stabilizing antimony under nitric acid conditions, resulting in the partial precipitation of basic salts observed in our SEM analysis (Figure 12). From a kinetic viewpoint, the calculated activation energy of 62.5 kJ/mol is higher than typical ones for diffusion-controlled leaching (often cited at around 10–30 kJ/mol [27]), but is consistent with oxidative leaching mechanisms, where surface passivation by elemental sulfur plays a dominant role [29].

The next step is antimony extraction from the productive leach solution. Given the nitrate medium nature and the complexing properties of antimony, electrowinning appears to be a very promising method. Similar approaches were successfully applied to antimony electrowinning in another system [15]. Further research will focus on optimizing the electrowinning stage to produce metallic antimony or commercial antimony oxide, which will close the technological cycle of processing antimonite concentrates.

5. Conclusions

Based on our comprehensive study of nitric acid leaching of stibnite with organic acids added, we can conclude as follows:

• Tartaric acid proved to be a significantly more effective complexing agent than citric acid. Under optimal conditions, the maximum antimony recovery reached 78%–90% with tartaric acid, compared to only 45%–54% with citric acid. The concentration of organic acids was a critical parameter: increasing the tartaric acid concentration from 20 to 80 g/dm³ boosted antimony recovery from 26% to 76%, whereas the same increase for citric acid only improved recovery from 27% to 56%.
• Phase and morphological analyses (XRD, SEM-EDX) revealed fundamentally different mechanisms. In the absence of organic acids, passivating plate-like Sb₄O₄(OH)₂(NO₃)₂ particles formed. Tartaric acid effectively prevented the hydrolysis and precipitation of antimony, with the residue containing primarily unreacted stibnite and elemental sulfur, the latter causing passivation at later stages. In contrast, citric acid led to the formation of needle-like Sb₄O₄(OH)₂(NO₃)₂ particles, indicating insufficient complexation and partial hydrolysis.
• Kinetic studies for the system with tartaric acid established that the process is limited by internal diffusion through a product layer, with an activation energy of 62.5 kJ/mol. The empirical reaction orders were found to be 2.7 for HNO₃ and 2.3 for tartaric acid, confirming the complex multi-stage nature of the process where both oxidation and complexation play crucial kinetic roles. A generalized kinetic equation was derived to describe the process adequately.

In summary, the synergistic use of nitric acid and tartaric acid presents a highly effective hydrometallurgical method for processing stibnite, overcoming the key limitation of antimony hydrolysis and ensuring high metal extraction.