Selective Separation of Antimony and Preparation of Sodium Antimonate by Sodium Salt Leaching-Synergistic Oxidation from High Arsenic Antimony Residue

Abstract

In this study, the catalytic air oxidation method was used to selectively form sodium antimonate from an antimony residue Na₂S-NaOH leaching solution of a high arsenic copper anode slime. In the first stage, the leaching process with Na₂S and NaOH media resulted in more than 98% leaching of antimony. The synergistic oxidation method was used to selectively separate antimony in the second stage. In this study, the oxidation rate of antimony was greater than 98% at the NaOH concentration of 50 g·L⁻¹ and a combined oxidation concentration of 0.75 g·L⁻¹ catechol + 0.75 g·L⁻¹ KMnO₄, under the air flow rates of 1.415 m³·min⁻¹ at 75 °C for 8 h. The pH of the crude sodium antimonate product was adjusted; subsequently, it was redissolved and precipitated to prepare refined sodium antimonate that meets the secondary product standard of China’s non-ferrous metal industry, which recommends an antimony recovery rate of >95.60%. After neutralisation, the liquid contains [As] < 0.10 g·L⁻¹, [Sb] = 0.16–0.38 g·L⁻¹, which can be reused in the composite leaching process. The apparent activation energy (Ea) of the catalytic oxidation reaction was 6.47 kJ·mol⁻¹; the results suggested that the reaction process was diffusion controlled. $-\frac{d\left[Sb\right]}{dt}=8.86\times {10}^{-5}\times e^{\frac{-778.44}{T}}\times {\left[Sb\right]}^{0.4906}\times {\left[NaOH\right]}^{1.190}$.

1. Introduction

Sodium antimonate is widely used in medicine, chemical industry, electronics, and other industries as a clarifying agent, decolorising agent, and flame retardant owing to its special physical and chemical properties [1]. Sodium antimonate can be prepared using pyrometallurgical and hydrometallurgical processes. The pyrometallurgical process [2] mainly uses metal antimony or antimony oxide as raw materials. Sodium antimonate products have been prepared by reacting sodium nitrate with antimony oxide under alkaline and high-temperature conditions, followed by washing, filtering, drying the product. Although the process is simple, it has certain disadvantages including high raw material requirements, low product purity, high energy consumption, and production of harmful gas such as NOx (x = 1 or 2) during the process. Thus, this process has not been extensively used. Sodium antimonate has been prepared by using antimony minerals or smelting intermediate materials as raw materials under acidic or alkaline conditions, followed by the addition of oxidants for leaching, purification, filtration, and other processes [3].

Oxidative leaching has been carried out in an acidic system, using HCl as a leaching agent, and Cl₂ and H₂O₂ as oxidants [4,5]. The leaching solution uses ethylenediaminetetraacetic acid (EDTA) or potassium ferrocyanide [6] to remove impurities and adjust pH to hydrolyse antimony and obtain HSbO₃ or SbOCl. Moreover, sodium antimonate has been prepared via alkali treatment or through the direct addition of NaOH, which hydrolyses Sb⁵⁺ [7,8]. Despite the advantages of this process such as strong adaptability to raw materials and high product purity, there are certain disadvantages including requirement of harsh operation conditions, considerable equipment corrosion, and significant wastewater generation, making it less susceptible. The alkaline system is leached in NaOH [7,9], KOH [10,11], and other systems, and uses air, H₂O₂, and other oxidants [12,13] for oxidation in order to prepare sodium antimonate. The process is short, and the operation is simple; however, the NaOH and KOH systems must be subjected to precipitation reconstitution owing to the high cost involved in the use of KOH and H₂O₂. Na₂S and the Na₂S-NaOH system [14,15] leach antimony to form sodium thioantimonite; subsequently, sodium antimonate is prepared via atmospheric air oxidation [12,16], high pressure oxidation [8], or catalytic oxidation [15,16]. After oxidation, S exists in the form of sodium thiosulfate in the solution, and the sodium thiosulfate by-product is obtained through neutralisation, impurity removal, concentration, and crystallisation. This simple process is widely used because of its high product purity, high yield, and low cost. Although many studies have reported the preparation of sodium antimonate using the Na₂S-NaOH system, the research mainly focuses on the optimisation of leaching or oxidation process. The kinetics of the process, especially the kinetics of the oxidation process, are not extensively studied, and the key control parameters are not clear [17].

In the present study, the high arsenic antimony residue is achieved by composite leaching of a copper anode slime (sulfuric acid + hydrochloric acid + sodium chloride), followed by adjustment of the pH of the leaching solution and precipitation of antimony. For this material, the research group used the Na₂S-NaOH system to conduct leaching research in the early stage, and antimony was successfully leached into the solution [18]. The selective and efficient recovery of antimony allows the preparation of sodium antimonate products that meet the standards of the non-ferrous metal industry. Additionally, the kinetics of the catalytic oxidation process is studied to clarify the reaction mechanism underlying the selective separation of antimony, and obtain the restrictive link, which is significant for industrial production. The process flow chart is shown in Figure 1.

2. Material and Methods

2.1. Materials

The high arsenic antimony residue was obtained from Jiangxi Copper Corporation Guixi Smelter (Yingtan, China) in southern China and was used in the experiments.

2.2. Experimental Principle

The high arsenic antimony residue mainly exists in the form of SbOCl. The first compound leaching process using Na₂S + NaOH is employed, where antimony is leached into the solution in the form of SbS₃³⁻. Copper/silver and other precious metals are retained in the leach residue. The reaction equation is

Sb₄O₅Cl₂ + 12Na₂S + 5H₂O = 4Na₃SbS₃ + 10NaOH + 2NaCl.

(1)

Antimony and arsenic exist as SbS₃³⁻ and AsS₃³⁻ ions in the leaching solution, respectively, under alkaline conditions. Their oxidation results in the formation of SbO₄³⁻, AsO₄³⁻, and S₂O₃²⁻ species under alkaline conditions. The reaction is as follows:

2Na₃SbS₃ + 2NaOH + 7O₂ + 5H₂O = 2NaSb(OH)₆↓ + 3Na₂S₂O₃,

(2)

2Na₃AsS₃ + 6NaOH + 7O₂ = 2Na₃AsO₄ + 3Na₂S₂O₃ + 3H₂O,

(3)

2Na₂S + 2O₂ + H₂O = Na₂S₂O₃ + 2NaOH.

(4)

On oxidising the leaching solution, NaSb(OH)₆ precipitates, while Na₃AsO₄ dissolves in water to separate and remove the impurities. The arsenic-containing solution forms calcium arsenate precipitate after liming precipitation, and the treated wastewater is discharged after meeting the standard.

2.3. Experimental Method

2.3.1. First Compound Leaching

The leaching experiments of the antimony-enriched high arsenic copper anode slime were conducted in 250 mL round-bottom flasks. The reaction solution was prepared according to a certain ratio of Na₂S and NaOH, and the antimony-enriched high arsenic copper anode slime was added to the above reaction mixture at a certain liquid-to-solid (L/S) ratio. The temperature was set and controlled using a water bath (DF-101S, Gongyi Yuhua Instrument Co., Gongyi, China).

First, the leaching experiments were conducted to study the influence of the leaching temperature (45–95 °C), Na₂S concentration (90–180 g/L), NaOH concentration (15–70 g/L), leaching time (0.5–3 h), and L/S ratios (3–7 mL/g). After a predetermined duration, the resultant slurry was filtered. The filtrate and washing solutions were subsequently subjected to chemical analysis to calculate the leaching efficiency.

2.3.2. Synergistic Oxidation

After the temperature rises to a certain value, add a certain amount of oxidation to the leachate solution; oxygen is flowed at a certain flow rate over this solution to carry out the reaction. The concentration of antimony in the filtrate, obtained after the completion of the reaction, was determined by the cerium sulphate titration method [19]. The oxidation efficiency of antimony ω (%), was calculated using Equation (5):

$$\omega ={\displaystyle \frac{C_0-C_1}{C_0}}\times 100\%,$$

(5)

where C₀ is the antimony concentration in the leaching solution (g·L⁻¹), and C₁ is the antimony concentration in the oxidised liquid (g·L⁻¹).

2.3.3. Kinetics Analysis

The oxidation efficiency of antimony was obtained by controlling different reaction temperatures (55–85 °C) and times (0–3 h) under the reaction conditions of a certain oxidation concentration (KMnO₄ concentration of 0.75 g·L⁻¹, catechol concentration of 0.75 g·L⁻¹), air flow rate (1.415 m³·min⁻¹), and NaOH concentration (250 and 300 g·L⁻¹). The kinetic results and apparent activation energy of the catalytic oxidation process were obtained using the half-life and isolated method.

2.4. Analytical Methods

2.4.1. Solid Material Analysis

The chemical composition of each element in high arsenic antimony residue, leaching residues, and precipitation products was analysed by X-ray fluorescence spectrometer (Magix pw2424, PANalytical B.V., Almelo, The Netherlands) determine the phases in the slag and chemical analysis determine the content of relevant elements. The main mineral phases present within the samples were identified by a Rigaku-TTRIII X-ray diffractometer (Cu target, Kα1, λ = 0.154 nm) (Tokyo, Japan), while the microstructure and phase compositions were ascertained via scanning electron microscopy (SEM, MIRA 3 LMH type from TESCAN Brno, S.r.o, Brno, Czech Republic).

2.4.2. Solution Analysis

The high arsenic antimony residue was dissolved by chemical dissolution method and low concentration solution. Its composition was determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES, Thermo Electron IRIS Intrepid II XSP, Waltham, MA, USA) quantitative analysis. High-concentration antimony solution using cerium sulphate titration method to determine the concentration of antimony in solution.

The cerium sulphate titration method conducted to measure the concentration of antimony [20] was as follows: 1~5 mL of leaching solution was measured in a 250 mL conical flask; potassium sulphate (2 g) and hydrazine sulphate (0.2 g) were added to this solution, followed by the addition of a small amount of water. Next, 20 mL sulfuric acid was added to the above reaction mixture, which was heated for 0.5 h to obtain a clear solution. After complete dissolution, the reaction mixture was cooled to room temperature followed by the addition of 100 mL of water. The resultant reaction mixture was boiled for 5 min, followed by the addition of 20 mL of hydrochloric acid and 2–3 drops of 0.1% methyl orange indicator. This mixture was immediately titrated with Ce(SO₄)₂ standard solution until the red colour disappeared at the end point. At the same time, a blank test was carried out.

The concentration of antimony can be calculated using Equation (6):

$$C_{Sb}={\displaystyle \frac{C\left(V_1-V_0\right)\times 121.76}{V}},$$

(6)

where C is the concentration of the standard solution of cerium sulphate (mol·L⁻¹), V is the volume of the liquid to be tested (mL), V₁ is the volume of the standard solution of cerium sulphate consumed by the sample solution during titration (mL), V₀ is the volume of standard solution of cerium sulphate consumed by the blank solution during titration (mL).

3. Results and Discussion

3.1. Characterisation of High Arsenic Antimony Residue

Table 1. Elemental composition of the high arsenic antimony residue.

Element	Sb	Cu	Bi	As	Pb	Au	Ag	Cl
Content (%)	34.7	8.99	0.703	7.47	1.61	0.006	0.36	9.38

shows elemental composition of the slime; the content of antimony was the highest, with a value of 34.7%, indicating its high recovery value. However, the content of arsenic, copper, lead was found to be 7.47%, 8.99%, and 1.61%, respectively(wt.%). Therefore, the material was difficult to handle.

Figure 2 shows the X-ray diffraction (XRD) pattern of the residue, which clearly indicates that the elements present in the slime material were relatively complex. Among the compounds detected, antimony mainly exists in the form of SbOCl and Sb₄O₅Cl₂, while Cu mainly exists in the form of CuCl₂ and Cu₃(AsO₄)₂. Other elements were also found in the form of PbCl, PbSO₄, BiOCl, AgCl, and other phases.

3.2. Selective Separation of Antimony with Compound Leaching

Antimony was selectively separated through compound leaching under optimal reaction conditions at a reaction temperature of 85 °C, a Na₂S concentration of 165 g·L⁻¹, a NaOH concentration of 30 g·L⁻¹, an L/S ratio of 5, and a leaching time of 1 h [18], while the remaining metal elements were left in the slag for the formation of sodium antimonate. The composition of the residues before and after compound acid leaching and the leaching efficiencies of the metals are summarised in

Table 2. Composition of the compound leaching slag and raw materials.

Element	Sb	Cu	As	Pb	Ag	Bi	Slag Rate
Raw materials (%)	34.7	7.64	7.47	1.39	0.31	0.703
Compound leaching slage (%)	5.39	22.73	1.04	4.09	0.87	1.907	31.24%

. The leaching efficiency of antimony was 94%, the antimony concentration of the leachate was 47.18 g·L⁻¹, the arsenic concentration was 9.32 g·L⁻¹, and the OH⁻ and S²⁻ concentrations were 11.40 and 2.67 g·L⁻¹, respectively. Both copper, lead, silver, and bismuth less than 3%, respectively. Suggesting copper, lead, silver and bismuth enrichment of approximately 290%, 290%, 280%, and 270%, respectively. The compound leaching slag can be returned to the copper anode slime smelting system for further recovery.

3.3. Preparation of Sodium Antimonate by Catalytic Oxidation

3.3.1. Effect of Type of Oxidation

Under conditions of sodium hydroxide concentration at 40 g·L⁻¹, reaction temperature at 55 °C, and air flow rate at 0.708 m³·min⁻¹, the reaction was conducted for 10 h to investigate the effect of different catalysts on the oxidation rate of antimony. The results are shown in

Table 3. Oxidation efficiency of antimony at the NaOH concentration of 40 g·L⁻¹ and under the air flow rate of 0.708 m³·min⁻¹ at 55 °C for 10 h in the presence of different types of oxidations.

Type and Dosage of Oxidation (g·L−1)	Blank	KMnO4 0.5	Phenol 0.5	Catechol 0.5	Catechol 0.5 + KMnO4 0.5 + Phenol 0.5	Catechol 0.5 + KMnO4 0.5
Antimony oxidation efficiency (%)	17.50	29.61	32.24	33.47	53.92	54.71

.

The catalytic effect of phenol and catechol as a single oxidation was similar, but the oxidation efficiency was low. However, the effect of the composite oxidation was stronger than that of a single oxidation. The combined oxidation composed of catechol and KMnO₄ exhibited the highest catalytic activity, while the effect of the combined oxidation composed of diphenol, KMnO₄, and phenol was weaker than that composed of catechol and KMnO₄. Therefore, the combination of catechol and KMnO₄ was selected as the oxidation, and not phenol because it showed a negative effect and further reduced the catalytic effect.

Catechol is oxidised by potassium permanganate to form catechol quinone (C₆H₄O₂), while potassium permanganate is reduced to manganese dioxide (MnO₂), generating potassium hydroxide (KOH) and water (H₂O). Additionally, hydroxyl radicals are released during the reaction. These hydroxyl radicals combine with oxygen, ultimately oxidising antimony from the Sb(III) to the Sb(V). The specific reaction equation is as follows:

3C₆H₄(OH)₂ + 2KMnO₄ = 2MnO₂ + 2KOH + 2H₂O + 3C₆H₄O_2,

(7)

C₆H₄O₂ + O₂ = C₆H₄O₂·O_2,

(8)

2Na₃SbS₃ + 2NaOH + 7C₆H₄O₂·O₂ + 5H₂O = 2NaSb(OH)₆↓ + 3Na₂S₂O₃ + 7C₆H₄O₂.

(9)

While MnO₂ absorbs Sb(III)in a basic system, oxidising it to Sb(V), it simultaneously generates Mn(OH)₃. Mn(OH)₃ then reacts with oxygen to regenerate MnO₂, maintaining the catalytic cycle. The specific reaction equation is as follows:

4Na₃SbS₃ + 4NaOH + 4MnO₂ + 13O₂ + 16H₂O = 4NaSb(OH)₆↓ + 6Na₂S₂O₃ + 4Mn(OH)₃,

(10)

4Mn(OH)₃ + O₂ = 4MnO₂ + 6H₂O.

(11)

3.3.2. Effect of Catalysts Concentration

Effect of KMnO₄ Concentration

The effect of KMnO₄ concentration on the oxidation efficiency of antimony at the NaOH concentration of 40 g·L⁻¹ and catechol concentration of 0.5 g·L⁻¹, and under air flow rate of 0.708 m³·min⁻¹ at 55 °C for 10 h, was investigated; the results are shown in Figure 3. As the concentration of KMnO₄ increases, the oxidation of antimony first increases and then decreases slowly. When the concentration increases to 0.75 g·L⁻¹, the oxidation efficiency of antimony reaches 72.52%. However, as soon as the concentration of KMnO₄ exceeds 1.0 g·L⁻¹, a slight colour change to light grey is observed, further affecting the product purity. The results indicate that when the concentration of KMnO₄ was 0.75 g·L⁻¹, a significant effect was observed on the oxidation efficiency of antimony.

Effect of Catechol Concentration

The effect of catechol concentrations on the oxidation efficiency of antimony at the NaOH concentration of 40 g·L⁻¹ and KMnO₄ concentration of 0.75 g·L⁻¹, under the air flow rate of 0.708 m³·min⁻¹, at 55 °C for 10 h was investigated; the results are shown in Figure 4. The oxidation of antimony first increases and subsequently becomes stable to 97% with an increase in the catechol concentration. However, as soon as the catechol concentration exceeds 0.75 g·L⁻¹, the colour of the precipitate deepens from white to grey-black, affecting product purity. The results indicate that when the catechol concentration was 0.75 g·L⁻¹, a significant effect was observed on the oxidation efficiency of antimony.

3.3.3. Effect of Air Flow Rate

The effect of the air flow rates on the oxidation efficiency of antimony at the NaOH concentration of 40 g·L⁻¹, KMnO₄ concentration of 0.75 g·L⁻¹, catechol concentration of 0.75 g·L⁻¹ and at 55 °C for 10 h, was investigated; the results are shown in Figure 5. The increase in the air flow rate increases the oxidation efficiency of antimony to 98%, which remained unaffected at 1.415 m³·min⁻¹. At this rate, the solubility of O₂ in the solution had reached a saturated state. An increase in the air flow rate had a negligible effect on antimony oxidation efficiency, but such an increase allows considerable heat loss and increases energy consumption. The results indicate that an air flow rate of 1.415 m³·min⁻¹ had a significant effect on the oxidation efficiency of antimony.

3.3.4. Effect of Reaction Temperature

The effect of reaction temperatures on antimony oxidation efficiency at the NaOH concentration of 40 g·L⁻¹, KMnO₄ concentration of 0.75 g·L⁻¹, and catechol concentration of 0.75 g·L⁻¹, under the air flow rates of 1.415 m³·min⁻¹ for 10 h, was investigated; the results are shown in Figure 6. The oxidation efficiency of antimony initially increases with increasing reaction temperature, but with further increase, no changes were observed. When the temperature exceeds 75 °C, the oxidation efficiency of antimony was maintained at ~99%. Increasing the reaction temperature increases the reaction efficiency while decreasing the solubility of O₂ in the solution; this results in a decrease in the O₂ concentration and a decrease in the reaction rate [21]. The results indicate that the reaction temperature was selected to be 75 °C.

3.3.5. Effect of NaOH Concentration

The effect of the NaOH concentration on the oxidation efficiency of antimony at the KMnO₄ concentration of 0.75 g·L⁻¹ and catechol concentration of 0.75 g·L⁻¹, under the air flow rates of 1.415 m³·min⁻¹ at 75 °C for 10 h, was investigated; the results are shown in Figure 7. Increasing the NaOH concentration increases the oxidation efficiency of antimony. However, when the NaOH concentration exceeds 50 g·L⁻¹, the oxidation efficiency of antimony does not change much and remains at ~99%. The results indicate that at the NaOH concentration 50 g·L⁻¹, a significant effect on the oxidation efficiency of antimony was observed.

3.3.6. Effect of Reaction Time

The effect of the reaction time on the oxidation efficiency of antimony at the NaOH concentration of 50 g·L⁻¹, KMnO₄ concentration of 0.75 g·L⁻¹, catechol concentration of 0.75 g·L⁻¹, and under the air flow rates of 1.415 m³·min⁻¹ at 75 °C, was investigated; the results are shown in Figure 8. With the increase in reaction time, the oxidation efficiency of antimony increases rapidly at first and then remains unchanged. However, the reaction reached the equilibrium when the reaction time was ~8 h, at which the oxidation efficiency of antimony reached >98%. The results indicate that the optimal reaction time was selected to be 8 h.

The solution contains arsenic and other impurities after the precipitation of antimony, which can be removed by precipitation using calcium salt.

3.4. Crude Sodium Antimonate Purification

A crude sodium antimonate product was formed by the reaction at the NaOH concentration of 50 g·L⁻¹, KMnO₄ concentration of 0.75 g·L⁻¹, catechol concentration of 0.75 g·L⁻¹ and under the air flow rates of air flow rate of 1.415 m³·min⁻¹ at 75 °C for 8 h. Its composition and industry standard values [22] are summarised in

Table 4. Composition of crude sodium antimonate product and industry standard.

Composition (%)	Sb2O5	Na2O≤	Fe2O3≤	CuO≤	As2O3≤	PbO≤	Cr2O3≤	V2O5≤
Standard	63.5–65.5	12–13.5	0.05	0.05	0.005	0.005	0.010	0.080
Product	62.39	12.57	0.023	0.015	0.013	0.006	<0.005	<0.005

.

It can be seen from Table 4 that the content of Sb₂O₅ of the product was lower than that of the standard. Moreover, the content of impurity elements arsenic and lead exceeded that of the standard, while the rest of the elements had equivalent values. In order to prepare products that meet the industry standards, crude sodium antimonate products need to be redissolved in hydrochloric acid to remove the impurities and recrystallised to prepare high-quality products.

Concentrated hydrochloric acid was used to dissolve crude sodium antimonate and heated to 70 °C, the pH was adjusted between 1 and 2 after filtration, and the filtrate was neutralised with the NaOH solution by adjusting the pH between 12 and 14 at room temperature. The resultant filtrate was reacted for 30 min to prepare antimonic acid after filtering the sodium products. The chemical composition was analysed post drying; the results are summarised in

Table 5. Composition of the refined sodium antimonate product with the industry standard.

Composition (%)	Sb2O5	Na2O≤	Fe2O3≤	CuO≤	As2O3≤	PbO≤	Cr2O3≤	V2O3≤
Standard	63.5–65.5	12–13.5	0.05	0.05	0.005	0.005	0.010	0.080
Product	64.9	12.57	0.023	0.003	<0.005	<0.005	<0.005	<0.005

. The results indicate that the antimony recovery rate can reach more than 95.60%.

It can be seen from Table 5 that the refined sodium antimonate product meets the secondary product standards of China’s non-ferrous metal industry (YS 22-2010). The SEM image of sodium antimonate in Figure 9 shows that the product particles were granular, and had irregular crystals with uniform particle size distribution and good dispersion; however, no obvious agglomeration was observed.

3.5. Kinetic Analysis

Equation (2) shows that the oxidation process is a multiphase process involving a gas–liquid phase, and the reaction includes the following steps:

(1)

The gaseous reactant, oxygen, diffuses from the gas phase to the gas–liquid phase interface.

(2)

Oxygen diffuses into the liquid phase from the gas–liquid phase interface and reacts in the liquid phase.

(3)

The reactants diffuse from the liquid phase to the gas–liquid reaction interface; and react with dissolved oxygen during the diffusion process.

(4)

The resulting product diffuses into the bulk of the liquid phase.

The kinetic equation for reaction (2) is expressed as

$$V=-{\displaystyle \frac{d\left[Sb\right]}{dt}}=k\times {\left[Sb\right]}^{\alpha }\times {\left[NaOH\right]}^{\beta }\times {\left[O_2\right]}^{\gamma }\times {\left[H_{2}O\right]}^{\delta },$$

(12)

where k is the reaction rate constant, and α, β, γ, and δ represent the order of the corresponding reactants 1, 2, 3, and 4, respectively. Among them, the concentration of H₂O in the solution was constant, and the solubility of O₂ in the solution system obeys Henry’s law. When the pressure of the reaction system is the standard atmospheric pressure, the Henry coefficient of air in the solution at different temperatures is summarised in

Table 6. Henry coefficient of air in water.

T (°C)	50	60	70	80	90
E × 10−3 (MPa)	9.58	10.1	10.5	10.7	10.8

[20].

According to Henry’s law, the O₂ concentration in the solution can be regarded as a constant, as seen in Table 2, within the temperature range of 55–85 °C, provided the air flow rate is held constant, and the Henry’s coefficient does not change considerably. Therefore, the oxidation efficiency of antimony can be considered to be only related to the [Sb] and [NaOH] concentrations in the solution; Equation (12) can be simplified as

$$V=-{\displaystyle \frac{d\left[Sb\right]}{dt}}=k_1\times {\left[Sb\right]}^{\alpha }\times {\left[NaOH\right]}^{\beta }.$$

(13)

In Equation (13), if the reactant concentrations are equal, the n-order reaction rate can be simplified as

$$V=-{\displaystyle \frac{d{C}_A}{dt}}=k_A{C}_A^n \mathrm{w}\mathrm{h}\mathrm{e}\mathrm{r}\mathrm{e} n\ne 1.$$

(14)

Equation (14) can be obtained after integrating Equation (13)

$${\displaystyle \frac{1}{n-1}}\left({\displaystyle \frac{1}{C_A^{n-1}}}-{\displaystyle \frac{1}{C_{A0}^{n-1}}}\right)=k_{A}t,$$

(15)

where ${\displaystyle \frac{1}{{\mathrm{C}}_{\mathrm{A}}^{\mathrm{n}-1}}}$is plotted against t, and $C_A={\displaystyle \frac{1}{2}}{C}_{A0}$ is substituted into Equation (15). The half-life of the reaction can be calculated as t_1/2 using Equation (16):

$$t_{1/2}={\displaystyle \frac{2^{n-1}-1}{k_A(n-1)C_{A0}^{n-1}}}.$$

(16)

Equation (16) can be obtained by taking the logarithm of Equation (17):

$$\mathrm{l}\mathrm{g}{t}_{1/2}=-\left(n-1\right)\mathrm{l}\mathrm{g}{C}_{A0}+\mathrm{l}\mathrm{g}B,$$

(17)

where $B={\displaystyle \frac{2^{n-1}-1}{K_A(n-1)}}$.

According to Equation (16), reaction coefficient n and rate coefficient k_A can be calculated from the slope and intercept of the straight line.

Since it was difficult to measure t_1/2 during the experiment, C_A was plotted against t, and different initial concentrations of C_A0 were selected from the curve to obtain the corresponding half-life; subsequently, reaction coefficient n and rate coefficient k_A were obtained by plotting the pair.

Since the oxidation reaction rate of antimony was related to the concentrations of antimony and NaOH, the isolation method was used, and the reaction rate was calculated using Equation (18):

$$V=K{C}_A^{\alpha }{C}_B^{\beta }.$$

(18)

The isolation method can be used to obtain the values of reaction orders α and β.

First, the initial reaction concentration was set to C_B ≫ C_A; next, C_B was considered approximately constant because the value remained unchanged during the reaction process. Equation (18) can be expressed as

$$V=K{C}_A^{\alpha }{C}_B^{\beta }=K^{\prime }{C}_A^{\alpha }.$$

(19)

The reaction rate was related to C_A, and $K^{\prime }$ and α were obtained using the half-life method. Under the condition that C_A0 remains unchanged, C_B0 was changed to make $C_{B0}^{\prime }=n{C}_{B0}$, and substituted in Equation (18):

$${\displaystyle \frac{V_2}{V_1}}={\left({\displaystyle \frac{C_{B0}^{\prime }}{C_{B0}}}\right)}^{\beta }=n^{\beta }.$$

(20)

After taking the logarithm, we obtain

$$\beta ={\displaystyle \frac{lg\frac{V_2}{V_1}}{lgn}},$$

(21)

where V₁ and V₂ are the instantaneous reaction rates at the corresponding time points of the two experiments.

Therefore, by changing the conditions of different C_B0 to conduct experiments, the β value was obtained using Equation (19). Finally, reaction rate k was obtained according to the formula, $K^{\prime }=K{C}_B^{\beta }$.

According to the Arrhenius formula, $lnK=\mathrm{l}\mathrm{n}A-{\displaystyle \frac{E_a}{T}}$ was plotted to obtain reaction activation energy $E_a$.

3.5.1. Half-Life Method

Different reaction temperatures were controlled and intermittent sampling was performed to analyse the concentration of antimony in the solution at the NaOH concentration of 250 g·L⁻¹, KMnO₄ concentration of 0.75 g·L⁻¹, catechol concentration of 0.75 g·L⁻¹, and under the air flow rates of 1.415 m³·min⁻¹; the results are shown in Figure 10.

The results in Figure 10 indicate half-life t_1/2 at the initial concentrations of antimony in reactants [Sb]₀ with 0.16, 0.14, 0.12, 0.10, 0.08, and 0.06 mol·L⁻¹. The plot of lg t_1/2 - lg [Sb]₀ was fitted; the result is shown in Figure 11.

According to the slopes and intercepts of the curves at different temperatures in Figure 11, n and k can be calculated; the results are summarised in

Table 7. Antimony oxidation reaction rates at antimony concentration series n and k at different temperatures.

T (°C)	55	65	75	85	Average Value
n	0.515	0.487	0.489	0.471	0.491
k	7.56 × 10−5	7.44 × 10−5	8.43 × 10−5	9.06 × 10−5	8.12 × 10−5

.

3.5.2. Isolation Method

The concentration of antimony in the solution was analysed by intermittent sampling at the NaOH concentration of 300 g·L⁻¹, KMnO₄ concentration of 0.75 g·L⁻¹, catechol concentration of 0.75 g·L⁻¹, under the air flow rates of 1.415 m³·min⁻¹ at 85 °C; the results are shown in Figure 12.

The reaction times required to achieve different [Sb] were calculated from the curve of equation in Figure 11; the values are summarised in

Table 8. Reaction schedule for reaching different [Sb] at different NaOH concentrations.

		[Sb] (mol/L)	0.16	0.14	0.12	0.10	0.08	0.06	0.04
	t/s
[NaOH] (g/L)
250			540.8	1085.8	1669.2	2300.4	2993.6	3771.5	4676.0
300			444.0	877.1	1342.4	1848.6	2408.9	3045.3	3801.3

.

Subsequently, an equation was derived to obtain the reaction efficiency at different concentrations. Additionally, β at different concentrations can be obtained using Equation (18) (where n = 300/250 = 1.2), as shown in

Table 9. Reaction rates and reaction orders at different [Sb].

[Sb] (mol/L)	0.16	0.14	0.12	0.10	0.08	0.06
V1 (mol/L/S)	3.79 × 10−5	3.55 × 10−5	3.30 × 10−5	3.03 × 10−5	2.74 × 10−5	2.40 × 10−5
V2 (mol/L/S)	4.77 × 10−5	4.46 × 10−5	4.13 × 10−5	3.77 × 10−5	3.37 × 10−5	2.920 × 10−5
$\beta$	1.271	1.252	1.227	1.192	1.141	1.057

.

The results in Table 8 indicate that the average value of β was 1.190. The oxidation reaction rate of antimony at different reaction temperatures can be calculated from reaction rate k, calculated by the half-life method (shown in Table 7), that is, k = $k/{\left[NaOH\right]}^{\beta }$. The results are shown in

Table 10. Oxidation reaction rate coefficient k of antimony at different temperatures.

T (°C)	55	65	75	85
k (mol·L−1)−1·s−1	8.54 × 10−6	8.40 × 10−6	9.52 E× 10−6	1.02 × 10−5

.

Therefore, the kinetic equation for the reaction rate of antimony oxidation is

$$-{\displaystyle \frac{d\left[Sb\right]}{dt}}=k\times {\left[Sb\right]}^{0.4906}\times {\left[NaOH\right]}^{1.190}.$$

(22)

3.5.3. Reaction Activation Energy

According to the values of the reaction rate coefficient at each temperature given in Table 9, ln k was plotted with ${\displaystyle \frac{1}{T}}$; the result is shown in Figure 13.

Figure 13 shows that the slope of the straight line was −606.54964; therefore, apparent activation energy Ea of the oxidation reaction can be calculated using the following formula:

$$-{\displaystyle \frac{E_a}{R}}=-778.43707.$$

The value of Ea of the oxidation reaction was calculated to be 6.47 kJ·mol⁻¹, indicating that the reaction process was controlled by diffusion. The reaction rate equation was $k=8.86\times {10}^{-5}\times e^{\frac{-778.44}{T}}$, and the reaction kinetics equation was $-\frac{d\left[Sb\right]}{dt}=8.86\times {10}^{-5}\times e^{\frac{-778.44}{T}}\times {\left[Sb\right]}^{0.4906}\times {\left[NaOH\right]}^{1.190}$.

4. Conclusions

(1)

In the first compound leaching stage, a leaching efficiency of 94% was achieved for antimony, while elements such as copper, lead, silver, and bismuth had leaching efficiencies of <3%. The results indicated that the enrichment of copper, lead, silver, and bismuth was approximately 290%, 290%, 280%, and 270%, respectively. The compound leaching slag can be returned to the copper anode slime smelting system for further recovery.

(2)

The oxidation rate of antimony reached more than 98% at the NaOH concentration of 50 g·L⁻¹, KMnO₄ concentration of 0.75 g·L⁻¹, catechol concentration of 0.75 g·L⁻¹ and under the air flow rates of 1.415 m³·min⁻¹ at 75 °C for 8 h, and crude sodium antimonate products were formed.

(3)

After the crude sodium antimonate product was redissolved and adjusted to pH 12–14, refined sodium antimonate with uniform particle size distribution, good dispersibility, which met the second-class standard of the non-ferrous metal industry. The recovery rate of antimony was found to be >95.60%, and the liquid after neutralisation contained [As] < 0.10 g·L⁻¹, [Sb] = 0.16-0.38 g·L⁻¹, which can be reused in the composite leaching process.

(4)

The apparent activation energy (Ea) of the oxidation reaction was 6.47 kJ·mol⁻¹, and the reaction process was diffusion controlled. The reaction rate equation was $k=8.86\times {10}^{-5}\times e^{\frac{-778.44}{T}}$, and the reaction kinetics equation was $-\frac{d\left[Sb\right]}{dt}=8.86\times {10}^{-5}\times e^{\frac{-778.44}{T}}\times {\left[Sb\right]}^{0.4906}\times {\left[NaOH\right]}^{1.190}$.