Combining ozone and ultrasound technology to remove S²⁻ in Bayer liquor

Highlights

• •A method of ultrasonic combine with ozone to remove S²⁻ is proposed.
• •Conversion law of active sulfur in oxidation process is studied.
• •The order of radical content in the oxidation system is •OH > ¹O₂ > O₂•⁻ > SO₄•⁻.
• •Reduces the hazard of S²⁻ in alumina production.
• •The removal rate of S²⁻ reached 93.83%.

Abstract

High content sulfur (S²⁻) in Bayer liquor can increase alkali consumption, accelerate equipment corrosion, especially seriously affect alumina production. The removal of S²⁻ in Bayer liquor is studied using ultrasonic enhanced ozone method, which significantly improves the removal efficiency. Results indicate that the best removal efficiency of 93.83 % is obtained with reaction duration of 20 min, oxygen flow rate of 80 L/h, ultrasonic power of 60 W and reaction temperature of 60 °C. The comparative analysis shows that the removal efficiency of S²⁻ is 25.34 % higher than that of ozone (O₃) system after introducing ultrasound (US), indicating that US accelerates the mass transfer process of O₃ and increases the hydroxyl radicals (•OH) content. For further explanation of the mechanism of US/O₃ system, EPR and XPS spectra are applied to analyze the content of free radical and the form of sulfur in Bayer liquor, indicating that the content of free radical in US/O₃ system is more than US and O₃ systems, and all sulfur is converted to SO₄²⁻ after full oxidation.

1. Introduction

Aluminum (Al) is one of the most abundant metallic elements in the Earth's crust [1], [2], which is widely used in many fields such as aviation, transportation, electronics, and architecture. 95 % of alumina is produced by Bayer process, which mainly comprises the following key stages: using high alkali to digest bauxite at high temperature and pressure, removing the red mud by solid–liquid separation, obtaining aluminum hydroxide precipitation by adding aluminum hydroxide seed, and calcining to obtain aluminum oxide [3], [4].

However, it faces the number of challenges with the development of the aluminum industry, especially considering the increase of alumina demand and insufficient bauxite reserves. The high-sulfur bauxite with high grade is rich reserves in China, but they cannot be effectively utilized due to the sulfur. The sulfur in bauxite mainly exists in the form of pyrite (FeS₂), and the others are marcasite, melnikovite and sulfate sulfur, etc [5]. It reacts with alkali liquor to form various forms of sulfur, mainly namely S²⁻ which accounts for 90 % − 94 %, and the rest are thiosulfate ion (S₂O₃²⁻), sulfite ion (SO₃²⁻), sulfate ion (SO₄²⁻), etc [6]. If the above sulfur compounds into the Bayer liquor with the digestion process, it will have a certain negative impact, increase the alkali consumption, decrease the decomposition rate of species, and accelerate equipment corrosion [7], [8]. When sulfur is transferred to Bayer liquor, sodium hydroxythioferrate (Na₂[FeS₂(OH)₂]•2H₂O), FeS₂, ferrous sulfide (FeS) and NaFeS₂ can be formed. In particular, the Na₂[FeS₂(OH)₂]•2H₂O has a higher solubility than iron compounds. As the Bayer liquor cools, the complex can precipitate out due to the reduction in the solubility of Na₂[FeS₂(OH)₂]•2H₂O, resulting in contaminating hydrogen alumina and increasing the iron content in alumina product [9], [10].

Therefore, reducing the content of S²⁻ in Bayer liquor and improving the quality of aluminum oxide are crucial to develop the aluminum industry. In order to eliminate the harm of S²⁻, there are many researches on the desulfurization methods, such as flotation desulfurization, roasting desulfurization, desulfurization by precipitation and wet oxidation desulfurization. However, flotation desulfurization process is unstable due to the large fluctuation of raw ore composition [11]. Roasting desulfurization faces the problems of cost and energy consumption [12], [13]. Desulfurization by precipitation faces the problems of introducing impurities and high costs [14]. Although the removal rate of wet oxidation desulfurization is high and no impurities are introduced, there are some problems such as harsh reaction conditions and weak oxygen oxidation ability [15], [16]. So, it is urgent to find a reasonable and effective desulfurization method to reduce the harm of high sulfur content to alumina products.

Referring to the method of wet oxidation desulfurization, O₃ is chosen instead of oxygen to make up for the low oxidation capacity and low potential of oxygen. O₃ is directly oxidized as an oxidant in the alkaline system of Bayer liquor, and it can also generate active groups including •OH for indirectly oxidation [17], [18], [19]. However, it is reported that the single O₃ system has low ozone dissolution and slow mass transfer, which causes high-energy consumption [20], [21]. Aiming at the defects of O₃, US is introduced into the system as an external field strengthening method [22]. US, usually refers to mechanical waves with a frequency above 20 kHz, with cavitation and mechanical effect as the main mechanism. Meanwhile, the cavitation effect by US in the solution can also decompose H₂O to produce •OH and hydrogen peroxide (H₂O₂), thus provides oxidation capacity for the reaction system, as shown in Eqs. (1), (2), (3), (4).

$$H_{2}O\stackrel{\mathit{cavitation}}{\to }H·+·OH$$ (1)

$$H^{·}+·OH\to H_{2}O$$ (2)

$$2·OH\to H_2{O}_2$$ (3)

$$2H^{·}\to H_2$$ (4)

When US is introduced into the single O₃ system, it has a remarkable effect in the following aspects: (i) the composition of the solution becomes more uniform due to the O₃ fully dispersed by mechanical effect; (ii) the propagation of US in solution, decreasing the thickness of the bubbles which promote mass transfer of O₃; (iii) after entering the cavitation bubble, ozone becomes unstable and more easily decomposed into free radicals with high redox potential under the action of local high temperature and pressure caused by its rupture [23]. Linada K has studied the mechanism of pentachlorophenol degradation by US and O₃. The results shows that US can enhance the decomposition of O₃ and form a large number of •OH [24]. Liu et al. has studied the oxidation of ammonia in different systems, and reported that less than 13 % of ammonium radical ion (NH₄⁺) is oxidized and the nitrate ion (NO₃⁻) yield is 11.2 % in the US/O₃ system, which are more than O₃ or US system [25]. Thus, the synergistic effect of US and O₃ makes up for the defects of ozone and extends the possibility of industrial application. However, US/O₃ has never been reported to remove S²⁻ in Bayer liquor so far.

In the present study, the process to removal of S²⁻ in Bayer liquor by US/O₃ is proposed. The effects of different reaction factors, i.e., reaction duration, oxygen flow rate, and ultrasonic power on the desulfurization are discussed. Among with analyzing the morphology of sulfur before and after oxidation, further, the transformation law of sulfur in the process of oxidation is explored.

2. Experimental

2.1. Materials and reagents

The solution used is obtained from aluminium hydroxide wash liquor in the Bayer process, which is supplied by a company in Yunnan province, China. Different forms of sulfur and other major components are shown in Table 1. It can be seen that the content of S²⁻ in the original feed liquid is low. In order to study the desulfurization effect in the system, sodium sulfide nonahydrate (Na₂S·9H₂O, ≥ 99 %) is added into the solution to improve the sulfur concentration as 1.46 g/L. Other reagents such as sodium thiosulfate (Na₂S₂O₃·5H₂O, ≥ 99 %), potassium iodide (KI, ≥ 99 %), potassium iodate (KIO₃, ≥ 98 %), formaldehyde solution (HCHO, ≥ 38 %), glacial acetic acid (H₂C₂O₄, ≥ 98 %), sodium hydroxide (NaOH, ≥ 96 %) and zinc acetate ((CH₃COO)₂Zn·2H₂O, ≥ 98 %) are purchased from Aladdin Ltd., China. All chemicals are analytical grade reagents, and can be used without further purification.

Table 1.

Composition and characteristics of sodium aluminate solution used for experiment.

Component(g/L)	Na2S	Na2SO3	Na2S2O3	Na2SO4	NT	Nk	Nc	Al2O3
Initial content	0.3	0.48	1.568	0.8	124	117	7	47.5
Test content	1.46	0.48	1.568	0.8	124	117	7	47.5

Note: Na₂S: the mass concentration of Na₂S in Bayer liquor, g/L; Na₂SO₃: the mass concentration of Na₂SO₃ in Bayer liquor, g/L; Na₂S₂O₃: the mass concentration of Na₂S₂O₃ in Bayer liquor, g/L; Na₂SO₄: the mass concentration of Na₂SO₄ in Bayer liquor, g/L; N_T: total alkali (Na₂O_T), represents the sum of alkali in the form of Na₂O_K and Na₂O_C in Bayer liquor, g/L; N_C: carbonate alkali (Na₂O_C), represents the alkali existing in the form of Na₂CO₃, calculated as Na₂O, g/L; N_K: caustic alkali (Na₂O_K), represents the alkali in the form of sodium aluminate and NaOH in Bayer liquor, calculated as Na₂O, g/L; Al₂O₃: the mass concentration of Al₂O₃ in Bayer liquor, g/L; α_k: 4.05, caustic ratio, represents the ratio of caustic to alumina molecular concentration in sodium aluminate solution.

2.2. Experimental apparatus and operation conditions

The experimental equipment of the laboratory-scale desulfurization system is shown in Fig. 1. The system includes ozonizer, US processor, constant temperature water bath, three-necked flask and exhaust treatment device. The ozonizer (OZ-30G, Guangzhou Tianshe Technology Co., Ltd, China) through high-voltage discharge to convert oxygen into O₃ (oxygen cylinder is used as the air source, and 68.8 mg O₃ is generated per 1 L oxygen). In order to facilitate data collation, the oxygen flow rate is used to express O₃ dosage in subsequent data). The US processor (2.0 kW, 25 kHz, WM-2000 T, Shanghai Weimi Technology Co., Ltd, China) and the ultrasound cycles (work time 9 s, stop time 1 s) transmits US of varying power into the solution with the help of an amplitude transformer of titanium alloy placed 2–3 cm below the liquid level. To match the production process conditions, the temperature of the Bayer liquor is kept at 60 °C by constant temperature water bath (HH-6, Guohua Electric Appliance Co., Ltd, China) during the experiment. The desulfurization is carried out in 300 mL Bayer liquor and under different ultrasonic powers (20, 40, 60, 80, 100 W) and oxygen flow rates (40, 60, 80, 100, 120 L/h). The samples are taken every 10 min and the content of sulfur is analyzed according to section 2.3. The exhaust gas (excess ozone) is absorbed in a container filled with 30 % sodium sulfite (Na₂S₂O₃) solution for preventing air pollution.

Fig. 1.

Schematic diagram of the equipment for treating sulfur in Bayer liquor by US/O3. (1) ozonizer, (2) ultrasonic processor, (3) gas flowmeter, (4) aerated stone, (5) ultrasonic amplitude transformer of titanium alloy, (6) Bayer liquor, (7) constant temperature water bath, (8) exhaust treatment device.

2.3. Analytical methods

The content of active sulfur (S²⁻, S₂O₃²⁻ and SO₃²⁻) in Bayer liquor is determined by indirect iodimetry. Firstly, an excess of iodine standard solution is added into Bayer liquor to oxidize the active sulfur as described in Eqs. (5), (6), (7), and the solution is titrated with sodium thiosulfate standard solution (0.01 mol/L), and the volume of sodium thiosulfate consumed is recorded as V₁. Secondly, the solution is titrated with sodium thiosulfate standard solution (0.01 mol/L) after removing S²⁻ by zinc acetate, and the volume of sodium thiosulfate consumed is recorded as V₂. Lastly, formaldehyde is added to the solution in which the S²⁻ is previously removed to mask the SO₃²⁻, and the S₂O₃²⁻ is titrated with iodine standard solution (0.01 mol/L). And then, the volume of iodine standard solution is recorded as V₃.

$$S^{2-}+I_2=2I^{-}+S$$ (5)

$$S{O}_3^{2-}+I_2=S_4{O}_6^{2-}+2I^{-}$$ (6)

$$S{O}_3^{2-}+I_2+H_{2}O=S{O}_4^{2-}+2I^{-}+2H^{+}$$ (7)

Eqs. (8), (9), (10) describe the calculation formula of active sulfur content:

$$\rho (S_2{O}_3^{2-})=c(1/2I_2)\times V_3\times 2\times 112=2.24\times V_3$$ (8)

$$\rho (S{O}_3^{2-})=[c(1/2I_2)\times 20-c(N{a}_2{S}_2{O}_3)\times V_2-c(1/2I_2)\times V_3]\times 80=0.8\times (20-V_2-V_3)$$ (9)

$$\rho (S^{2-})=\{c(1/2I_2)\times 20-c(N{a}_2{S}_2{O}_3)\times V_2-[c(1/2I_2)\times 20-c(N{a}_2{S}_2{O}_3)\times V_1]\}\times 32=0.32\times (V_2-V_1)$$ (10)

where, ρ(S₂O₃²⁻), ρ(SO₃²⁻) and ρ(S²⁻) correspond to the mass concentration of S₂O₃²⁻, SO₃²⁻ and S²⁻ in Bayer liquor (g/L), respectively. c(1/2I₂) is the concentration of iodine standard solution (mol/L), c(Na₂S₂O₃) is the concentration of sodium thiosulfate standard solution (mol/L), 112 is the molar mass of S₂O₃²⁻ (g/mol), 80 is the molar mass of SO₃²⁻ (g/mol), 32 is the molar mass of S²⁻ (g/mol).

Additionally, the content of sulfate (SO₄²⁻) in Bayer liquor is determined by gravimetric method. Firstly, the Bayer liquor is acidified with hydrochloric acid to remove the active sulfur and CO₃^2–. Followed by, the filtrate is heated to boiling after the liquor filtering by medium speed qualitative filter paper. And then barium chloride is added to precipitate sulfate, keeping the solution boiled until the precipitate is well-formed. After that, the precipitate is roasted in a crucible, cooled naturally to room temperature, weighed. Where the sulfate content is calculated as Eq. (11).

$$\rho (S{O}_4^{2-})=W\times 41.125$$ (11)

where W and 41.125 are the weight of barium sulfate after being burned and the coefficient of barium sulfate conversion to sulfate, respectively.

The change of binding energy before and after sulfur oxidation in Bayer liquor is analyzed by X-ray photoelectron spectroscopy (XPS, PHI-5000versaprobeIII, Ulvac-Phi, Japan). The active free radicals in the US/O₃ system are determined by Electron Paramagnetic Resonance (EPR, A300, Bruker, Germany), and its contents are obtained by integrating the EPR curve.

3. Results and discussion

3.1. Effects of reaction duration

The effect of reaction duration on removal of S²⁻ is evaluated under the operating conditions of oxygen flow rate of 60 L/h, temperature of 60 °C, ultrasonic power of 60 W, reaction duration from 10 to 60 min, and the results are shown in Fig. 2(a) and (b).

Fig. 2.

Effect of reaction duration on desulfurization results: (a) S²⁻; (b) S₂O₃²⁻, SO₃²⁻ and SO₄²⁻.

It can be seen in Fig. 2(a) that the concentration of S²⁻ decreases sharply with the increase of reaction duration and then curve gradually flattens. The content of S²⁻ decreases from 1.46 g/L to 0.25 g/L at 20 min with the removal rate of 82.87 %. However, when the time is extended to 60 min, the content of S²⁻ only decreases from 0.25 g/L to 0.03 g/L, with the desulfurization efficiency of 15.07 %. This implies that oxidation efficiency gradually decreases with the increase of reaction time.

S²⁻ is oxidized to S₂O₃²⁻ and SO₃²⁻ at this stage, as shown in Eqs. (12), (13):

$$S^{2-}\stackrel{\mathit{ozone}}{\to }{S}_2{O}_3^{2-}$$ (12)

$$S^{2-}\stackrel{\mathit{ozone}}{\to }S{O}_3^{2-}$$ (13)

That is because that most of O₃ is consumed by S²⁻ in the early stage of the reaction. Then, the reduction of O₃ concentration leads to worse oxidation effect with increasing the reaction time. Fig. 2(b) shows that the contents of S₂O₃²⁻ and SO₃²⁻ increase firstly, and then decrease as the reaction time progresses. Exceptly, the content of SO₄²⁻ shows a trend of increasing slightly and then rising rapidly with the increase of the reaction time. S₂O₃²⁻ and SO₃²⁻ are formed by the oxidation of S²⁻ at the early stage of the reaction, and the concentrations are increased from 1.57 g/L to 2.87 g/L and from 0.48 g/L to 1.45 g/L, respectively. The S₂O₃²⁻ and SO₃²⁻ with high concentrations are oxidized to SO₄²⁻ at the middle stage of the reaction when the concentration of S²⁻ is low, which results in the decrease of the content of S₂O₃²⁻ and SO₃²⁻ and the content of SO₄²⁻ increases rapidly.

In addition, the S²⁻ is converted into higher valence sulfur (S₂O₃²⁻, SO₃²⁻ and SO₄²⁻) by US/O₃, then the SO₄²⁻ are precipitated by evaporation concentration of the solution and removed from the red mud in practical production. The S₂O₃²⁻ is removed by sodium silica residue. Meanwhile, the downward trend of S²⁻ indicates that the oxidation effect cannot be significantly improved by further increasing the reaction time, so the reaction of 20 min is selected as the suitable condition.

3.2. Effect of oxygen flow rate

The advantage of O₃ oxidation over other chemical oxidation methods is that it provides two possible oxidation routes. Namely, O₃ as an oxidant, reacts with sulfur directly and rapidly with acidic conditions. While, O₃ is induced by the hydroxide ions in alkaline conditions to produce hydroxyl free radicals and other active radical [26], [27]. The decomposition of O₃ in the alkaline system produces a series of chain reactions and active radical formation reactions, as shown in Eqs. (14), (15), (16), (17), (18), (19), (20), (21), (22) [28]:

$$O_3+O{H}^{-}\to H{O}_2^{-}+O_2$$ (14)

$$O_3+H{O}_2^{-}\to H{O}_2^{·}+O_3^{·-}$$ (15)

$$O_2^{·-}+H^{+}\to H{O}_2^{·}$$ (16)

$$O_3+O_2^{·-}\to O_3^{·-}+O_2$$ (17)

$$O_3^{·-}+H^{+}\to H{O}_3^{·}$$ (18)

$$H{O}_3^{·}\to O_3^{·}+H^{+}$$ (19)

$$H{O}_3^{·}\to ·OH+O_2$$ (20)

$$O_3+·OH\to H{O}_4^{·}$$ (21)

$$H{O}_4^{·}\to H{O}_2^{·}+O_2$$ (22)

The effect of oxygen flow rate on desulfurization effect is evaluated by varying oxygen flow rate from 40 L/h to 120 L/h under the fixed conditions of ultrasonic power of 60 W, reaction duration of 20 min, temperature of 60 °C, and the results are shown in Fig. 3(a) and (b).

Fig. 3.

Effect of oxygen flow rate on desulfurization results: (a) S²⁻; (b) S₂O₃²⁻, SO₃²⁻ and SO₄²⁻.

It is obvious from Fig. 3(a) that the content of S²⁻ gradually decreased and then reaches a plateau with the increase of oxygen flow rate. When the oxygen flow rate increases to 120 L/h, the content of S²⁻ in Bayer liquor decreases from 1.46 g/L to 0.03 g/L, and the removal efficiency reaches 97.94 %. The content of O₃ and its decomposition products (free radicals) increases with the increase of oxygen flow rate. Meanwhile, the increase of O₃ concentration accelerates the mass transfer of O₃ from the gas phase to the cavitation bubble and enhances the removal efficiency of S²⁻ [29]. When the oxygen flow rate increases from 80 L/h to 120 L/h, the residual content of S²⁻ in the solution decreases from 0.09 g/L to 0.03 g/L, and the removal efficiency increases from 93.83 % to 97.94 % without obvious increase, which can be explained by the limited solubility of O₃. Then, the removal efficiency tends to be stable with the increase of O₃. The content of S₂O₃²⁻ and SO₃²⁻ increases from 1.57 g/L to 3.06 g/L and from 0.48 g/L to 0.78 g/L, respectively, as shown in Fig. 3(b). O₃ and its decomposition products preferentially reacts with S²⁻ to form S₂O₃²⁻ and SO₃²⁻. After that, the S₂O₃²⁻ and SO₃²⁻ are oxidized to the SO₄²⁻ and the concentration decreases accordingly, the content of SO₄²⁻ increases steadily.

In summary, the best oxygen flow rate is selected as 80 L/h considering the cost of oxygen and the removal effect of S²⁻.

3.3. Effect of ultrasonic power

During the ultrasonic power tests, the impact of various ultrasonic power on the remove of sulfur with an oxygen flow rate of 80 L/h, a reaction duration of 20 min, and temperature of 60 °C is illustrated in Fig. 4(a) and (b).

Fig. 4.

Effect of ultrasonic power on desulfurization results: (a) S²⁻; (b) S₂O₃²⁻, SO₃²⁻ and SO₄²⁻.

Fig. 4(a) shows the decreases in sulfur concentration with the increase of ultrasonic power. The S²⁻ concentration decreases from 1.46 g/L to 0.05 g/L with the ultrasonic power increases to 100 W. The cavitation effect is enhanced by the increase of ultrasonic power, meanwhile, more free radicals are released from the cavitation bubble collapse to provide an oxidizing atmosphere and conducive to the redox reaction in the system [30]. Moreover, US, as an external field strengthening method, can be used to enhances the turbulent of the solution by transient cavitation. The increase of ultrasonic power, thereby accelerates the mass transfer process of O₃ from the gas phase to the liquid phase in the solution. It is worth mentioning that further increase in US power (from 60 W to 100 W) does not result in further reduction of S²⁻ concentration. A higher power does not necessarily result in better removal efficiency. The intensity of cavitation effect increases with the enhancement of ultrasonic power, but the cavitation effect will trend to balance when power reaching a certain value. Moreover, increasing power can produce a large number of useless bubbles, which increases the scattering attenuation, weakens the US in the original direction and reduces the cavitation intensity [31].

In tests conditions, the content of SO₃²⁻ and S₂O₃²⁻ shows an upward trend and then gradually decreases versus ultrasonic power, while SO₄²⁻ increases. When a high-power is used, more free radicals are released and the content of SO₃²⁻ and S₂O₃²⁻ increases to 1.06 g/L and 3.06 g/L, respectively. Then, as continues oxidized by remaining oxidizing agent, the content of SO₃²⁻ and S₂O₃²⁻ decreases and the SO₄²⁻ concentration increases to 3.02 g/L. According to the test results and removal effect, the optimal ultrasonic power condition is 60 W.

According to our theoretical and experimental results, the optimal reaction parameters are as follows: ultrasonic power 60 W, oxygen flow rate 80 L/h, reaction duration 20 min, and temperature 60 °C.

3.4. Effect of different oxidation systems

In order to examine the enhance of US on O₃, the removal effect of O₃ system and US/O₃ system are compared under the condition of an oxygen flow rate of 80 L/h, reaction duration of 20 min, and temperature of 60 °C. The ultrasonic power is selected as 60 W to conduct the experiment and the results are shown in Fig. 5.

Fig. 5.

Desulfurization effect of single O₃ and US/O₃ system.

It can be seen that the relationship between sulfur content and oxygen flow rate in both systems is consistent with the description in section 3.2, the contents of S²⁻ are decreased firstly with the increase of oxygen flow rate and then tend to be stable. The removal effect of US/O₃ system is better than that in the O₃ system with the same oxygen flow rate. The content of S²⁻ is decreased from 1.46 g/L to 0.68 g/L in O₃ system with the oxygen flow rate of 40 L/h, while is dropped to 0.31 g/L in US/O₃ system, with an increased removal rate of 25.34 %. In the case of US/O₃ system, the mass transfer of O₃ at the gas–liquid interface is enhanced by the cavitation effect. On the other hand, collapse of cavitation bubbles cause local transient hot spots with high temperatures and pressures which enhances the decomposition processes of O₃ to yield more free radicals, as shown in Eqs. (23), (24), (25) [32], [33]:

$$O_3\to ·OH$$ (23)

$$O_3+US\to O_3+O({}^{3}P)$$ (24)

$$O({}^{3}P)+H_{2}O\to 2·OH$$ (25)

And compared with the literatures [34], [35], [36], this method obviously improves the efficiency of desulfurization and makes up for the blank of desulfurization process in sodium aluminate solution. Thus, more free radicals are formed for the same amount of O₃ by US/O₃ system than single O₃ system, and the O₃ oxidation rate is improved.

3.5. Active groups in different oxidation systems analysis

The EPR characterization is often used to identification or quantification of short-lifetime free radicals. And the operating conditions for EPR is as follows: 30 μL of Dimethyl pyridine N-oxide (DMPO) (200 mM deionized water as solvent) are mixed with the 30 μL of sample and use a capillary tube to draw a certain amount of mixed solution into the EPR sample chamber for hydroxyl radical testing. EPR spectra is recorded at room temperature using the following instrumental conditions: magnetic field: 3500 G; sweep width: 100 G; scan time: 30 s; number of scans: 3; modulation amplitude: 1.000 G. The DMPO and 2,2,6,6-Tetramethylpiperidin-1-oxyl (TEMPO) are as the spin-trapping agent to trap the free radicals, e.g., •OH, SO₄•⁻, O₂•⁻ and ¹O₂ [37].

In Fig. 6(a), the peak with following characteristics: the 4-fold characteristic peak with an intensity ratio of 1:2:2:1 and the peak with an intensity ratio of 1:1:1:1:1:1 are attributed to the hyperfine splitting peaks of DMPO-•OH and DMPO-SO₄•⁻ adducts, respectively. It can been seen according to the peak intensity and area, the contents of •OH and SO₄•⁻ in the three oxidation systems follows the sequence: US/O₃ system > O₃ system > US system, and the content of •OH in each oxidation systems is much higher than that of SO₄•⁻. The source of SO₄•⁻ may be converted from the original SO₄²⁻ in Bayer liquor (the content of SO₄²⁻ in Bayer liquor is about 0.8 g/L), and as shown in Eqs. (26), (27) [38], [39]:

$$H^{+}+S{O}_4^{2-}\to HS{O}_4^{-}$$ (26)

$$\mathit{HS}{O}_4^{-}+·OH\to S{O}_4^{·-}+H_{2}O$$ (27)

Fig. 6.

Free radicals in Bayer liquor treated with US/O₃; (a) EPR spectra of •OH and SO₄•⁻; (b) EPR spectra of O₂•⁻; (c) EPR spectra of ¹O₂; (d) Content of free radical. Conditions: Bayer liquor 300 mL, ultrasonic power 60 W, oxygen flow rate 80 L/h, reaction duration 20 min and temperature 60 °C.

The hyperfine splitting peaks of DMPO-O₂•⁻ and TEMPO-¹O₂ adducts are shown in Fig. 6(b) and (c). The DMPO-O₂•⁻ generally has the following characteristic of six peaks, and the third and fifth peaks with relatively low intensity. The spectra with three-line spectrum with equal intensities is characteristic of TEMPO-¹O₂ adducts. O₂•⁻ and ¹O₂ are produced as an intermediate product in the chain reaction of O₃-decomposition, and also have some content in US/O₃ system. Based on the above analysis, there are four free radicals includes •OH, SO₄•⁻, O₂•⁻ and ¹O₂ existing in these systems, with only different contents. Then, the quantitative analysis for the four free radicals are performed, and the results are shown in Fig. 6(d). The content of •OH is much higher than that of other three free radicals, and the order of content in the oxidation system is listed as •OH > ¹O₂ > O₂•⁻ > SO₄•⁻. So, the main oxidation capacity is provided by •OH throughout the reaction. The content of •OH is 16.710 × 10⁻⁵ mol/L in US/O₃ system is greater than the sum of US system (6.756 × 10⁻⁵ mol/L) and O₃ system (9.192 × 10⁻⁵ mol/L). The results prove that the oxidation capacity of the system is improved by US enhanced O₃.

3.6. X-ray photoelectron spectroscopy (XPS) analysis

XPS analysis is performed to further investigate the valence state of sulfur in Bayer liquor before and after oxidation (the liquor is treated with sufficient oxidation). The operating conditions for XPS are as follows: total acquisition time: 28.7 s, number of scans: 3, spot size: 400 mum, energy step size: 0.100 eV and number of Energy Steps: 191. And the results are shown in Fig. 7. High resolution spectra of S 2p in Fig. 7(a) shows one peak centered at 169.7 eV corresponding to the binding energy of SO₄²⁻ in the Bayer liquor [40]. A high intensity peak appeared at 162.9 eV is the characteristic of FeS₂ [23]. The other three peaks are located with binding energies of 168.7 eV (due to S₂O₃²⁻), 166.4 eV (due to SO₃²⁻), and 161.9 eV (due to S²⁻) [41].

Fig. 7.

S 2p XPS spectra of Bayer liquor (BE values are corrected with contamination carbon as reference at 284.80 eV); (a) before oxidation; (b) after oxidation.

Notably, the spectra of S 2p of the after oxidized Bayer liquor (Fig. 7 (b)) only reveals the characteristic peak at 169.7 eV, which belongs to SO₄²⁻. After oxidation treatment, the characteristic peaks of active sulfur (S₂O₃²⁻, SO₃²⁻ and S²⁻) are disappeared, which might be converted to SO₄²⁻ by active group with high oxidizing properties. Meanwhile, the peak intensity of SO₄²⁻ increases compared with that before oxidation, indicating that the content of SO₄²⁻ increases, and this phenomenon can corroborate the previous speculation related to the characteristic peaks of active sulfur are disappeared.

4. Conclusions

The effects of ultrasonic power, oxygen flow rate, reaction duration and different oxidation systems on desulfurization are investigated. The spin-trapping EPR analysis and XPS analysis are harnessed to the mechanism of US enhanced O₃ and the change of valence state of sulfur before and after oxidation in Bayer liquor. The following are the major conclusions:

• (1)The desulfurization efficiency of S²⁻ is up to 93.83 % is obtained under the optimal operation conditions: ultrasonic power of 60 W, oxygen flow rate of 80 L/h, reaction duration of 20 min, and temperature of 60 °C.
• (2)The order of free radical content in the oxidation system is •OH > ¹O₂ > O₂•⁻ > SO₄•⁻, and •OH contributes to the main oxidizing properties. The content of •OH in US/O₃ system is greater than the sum of US system and O₃ system, indicating that US enhance O₃ to produce more •OH.
• (3)S²⁻ is preferentially oxidized to form S₂O₃²⁻ and SO₃²⁻ at the early stage of the reaction. With the oxidation reaction proceeds and the content of S²⁻ decreases, S₂O₃²⁻ and SO₃²⁻ are subsequently oxidized to the major and stable product, i.e., SO₄²⁻.

The results show that the combined technology of O₃ with US can as a feasible combined technique to decrease the content of sulfur in Bayer liquor, and reduces the harm of S²⁻ to alumina products.

CRediT authorship contribution statement

Xuxu Wang: Methodology, Investigation, Writing – original draft. Jianfeng Ran: Formal analysis. Haisheng Duan: Resources. Ying Chen: Resources. Jiaping Zhao: Resources, Formal analysis. Shaohua Yin: Funding acquisition, Conceptualization, Supervision. Shiwei Li: Review & editing. Libo Zhang: Project administration.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.