
==== Front
Ultrason Sonochem
Ultrason Sonochem
Ultrasonics Sonochemistry
1350-4177
1873-2828
Elsevier

S1350-4177(24)00294-3
10.1016/j.ultsonch.2024.107046
107046
Original Research Article
Mechanism and kinetics of ultrasound-enhanced CaCO3 precipitation for indium enrichment in zinc oxide dust leaching solution
Li Shiju 3098252236@qq.com
ab⁎
Wang Haibei whaibei_01@163.com
b⁎⁎
Wang Shengdong b
Xie Feng a
Sun Xudong b
a School of Metallurgy, Northeastern University, Shenyang 110819, China
b BGRIMM Technology Group, Beijing 100160, China
⁎ Corresponding author at: BGRIMM Technology Group, Beijing 100070, China. 3098252236@qq.com
⁎⁎ Corresponding author. whaibei_01@163.com
24 8 2024
11 2024
24 8 2024
110 10704613 5 2024
7 8 2024
23 8 2024
© 2024 The Author(s)
2024
https://creativecommons.org/licenses/by-nc/4.0/ This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).
In this study, ultrasound-enhanced calcium carbonate precipitation was used to enrich indium in zinc oxide dust leachate, and the effects of precipitation endpoint pH and ultrasound power on the indium precipitation behaviour were investigated, and the optimal conditions of ultrasound-enhanced precipitation were obtained to be the precipitation endpoint pH of 4.0 and the ultrasound power of 200 W. The precipitation rate of indium under these conditions was 99.79 %. At the same time, the effects of ultrasonication and conventional stirring on the indium precipitation kinetics were compared, which proved that ultrasound can shorten the time for precipitation to reach equilibrium and reduce the amount of calcium carbonate used, and the theory of ultrasonication activation energy was put forward. The activation energy of ultrasonication was Eu-a = 2.63 KJ/mol, and that of conventional precipitation was 9.78KJ/mol, which proved that ultrasonication could reduce the activation energy of the precipitation reaction, and promote the rapid precipitation reaction. The kinetic model of ultrasound-enhanced indium precipitation is lnC0-lnCt = exp(0.11339–318.54/W).t + A. In addition, the mechanism of ultrasound-enhanced calcium carbonate precipitation of indium was revealed by XRD, SEM-EDS, XPS and TEM analyses of the precipitated residue, it was demonstrated that ultrasound can inhibit the precipitation of zinc, and the ZnCO3 phase was found in the ultrasonically precipitated residue. This study provides a new idea for indium enrichment, and the future focus will be on the scale-up of the ultrasound-enhanced precipitation device.

Keywords

Ultrasound
Precipitation
CaCO3
Indium
Zinc oxide dust
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pmc1 Introduction

Indium is a key strategic metal that is an important raw material for the electronics industry [1], [2]. Almost 70 % of indium is used in the production of indium tin oxide film (ITO), and About 30 per cent of indium is consumed in areas such as semiconductors, solders and alloys[3].Indium belongs to one of the “three rare” metals, its content in the earth's crust is about 50–200 ppb, and there is no independent indium ore, usually mainly in the form of associated metal dispersed in sphalerite and tin ore, so indium is a by-product of zinc and tin smelting [4], [5]. Zinc oxide dust from the zinc smelting process contains approximately 0.1–0.3 per cent indium, and the recovery of indium from zinc oxide dust has a certain economic value [6].

The content of indium in the ore is very low, generally can not be directly extracted indium, indium needs to be enriched, common methods of indium enrichment are precipitation, extraction, adsorption, ion exchange and other methods [7], [8], [9], [10], [11], [12]. The advantages of the precipitation method are simplicity of operation and low pollutant emissions, while the disadvantage is that other metal ions will also co-precipitate with the target metal ion [13], [14]. The advantages of solvent extraction are easy to operate, simple equipment and the extractant can be recycled; the disadvantages are the volatility of the solvent and the emission of a large amount of waste extractant [15], [16]. The advantage of adsorption is that it is green, the disadvantage is that it is inefficient and the adsorbent is expensive [10], [17]. The advantages of ion exchange are large enrichment ratios and high efficiency, while the disadvantages are the need for high energy consumption and high production costs [18]. In order to solve the shortcomings of the above methods, some researchers have started to use precipitation to enrich indium from leachate. Common precipitants include sodium hydroxide, sodium carbonate and oxalic acid. However, the alkalinity of sodium hydroxide is too large, and it is inconvenient to control the termination PH of precipitation and oxalic acid is more expensive, which leads to an increase in the production cost of enterprises [4], [19], [20]. Calcium carbonate is considered to be a more ideal reagent for indium precipitation and enrichment due to its low cost and moderate alkalinity [21].

Ultrasound is a mechanical wave with a very short wavelength that can propagate in liquids, solids and air, and it has thermal, cavitation and mechanical effects [22]. Ultrasonic propagation in the medium will produce internal friction, part of the acoustic energy will be absorbed by the medium into thermal energy so that the temperature of the medium to increase, this heating method compared with other heating methods to achieve the same effect [23]. Ultrasonic propagation in the liquid will produce cavitation effects, including the emergence of gas nuclei, the growth of microbubbles and microbubble bursting 3 steps, which in turn release a large amount of energy [24], [25]. The mechanical action of ultrasound can lead to emulsification and dispersion of liquids and dispersive stirring of solids [26]. Therefore, it is feasible to use ultrasound for intensive precipitation enrichment of indium.

Jibo Jiang et al. used sodium tripolyphosphate to precipitate indium and showed that indium could be completely precipitated at pH 2.6, 25 °C and 1.5 h [13]. Kenneth N et al. used chemical precipitation to separate indium and tin and showed that indium precipitates when the pH of the solution is increased above 4.5, therefore, the pH of the solution can be controlled to achieve the separation of indium and tin [7]. Alguacil et al. used sodium borohydride to precipitate indium from an indium-containing solution after hydrochloric acid stripping and found this precipitate to be monomeric indium and indium oxide [27].

In this study, indium was enriched by calcium carbonate precipitation from zinc oxide dust leachate using ultrasound enhancement, and the effects of precipitation termination PH, ultrasound power on indium precipitation behaviour were investigated, and the effect of ultrasound power on indium precipitation kinetics was investigated, revealing the mechanism of indium precipitation. After literature search, ultrasound-enhanced precipitation of indium has not seen relevant reports. The innovation of this study is that the effect of ultrasonic power on the kinetics of indium precipitation has been investigated in detail, and the theory of ‘ultrasonic activation energy’ has been put forward to obtain a chemical kinetic model of ultrasound-enhanced precipitation.

2 Experimental

2.1 Reagents

Sulphuric acid is industrial grade, purity greater than 96 %; calcium carbonate is analytically pure, purity greater than 99 %; reduced iron powder is analytically pure, purity greater than 99 %; solution is prepared with deionised water.

2.2 Preparation of zinc oxide dust leaching solution

The zinc oxide dust leaching solution was prepared using an ultrasound-microwave synergistic leaching method. The detailed process of preparation is described in detail in the literature [28]. The process conditions for the preparation of the leaching solution were ultrasonic-microwave power of 200 W, liquid–solid ratio of 8:1, sulfuric acid concentration of 180 g/L, and leaching time of 30 min. Prior to the precipitation experiments, the leachate was reduced with iron powder for 20 min with ultrasound-assisted reduction. The purpose was to reduce Fe3+ to Fe2+ in the leachate, which facilitated the subsequent precipitation of indium. The ionic concentration of the leaching solution after reduction of iron powder is shown in Table 1.Table 1 Ion concentrations in the leaching solution after reduction of iron powder.

Element	Concentration (g/L)	
In	0.564	
Zn	68.150	
Fe2+	1.490	
Al	0.400	

2.3 Precipitation processes

Precipitation experiments were carried out in a 500 ml four-necked flask with a PH meter inserted to measure real-time pH while an ultrasound probe was inserted for ultrasound intensification. Calcium carbonate was added slowly and when the pH reached 4.0, the ultrasonic generator was immediately switched off to stop the experiment and the precipitate was quickly filtered. The flow of the experiment is shown in Fig. 1, the actual experimental procedure is shown in Fig. 2.Fig. 1 Schematic diagram of the experimental procedure.

Fig. 2 Actual experimental procedure.

2.4 Calculation and characterization

The phase structure of the precipitated residue was tested using an X-ray diffractometer (D8 ADVANCE) manufactured by Bruker, Germany. The surface morphology and energy spectra of the precipitated residue were tested using SEM (SU8020) and EDS (HORIBA) manufactured by Hitachi. The crystal structure of the precipitated residue was tested using a transmission electron microscope (JEM1200EX) manufactured by JEOL, Japan. XPS spectra of the precipitated residue were tested using an X-ray photoelectron spectroscopy tester (Thermo escalab 250XI) manufactured by Thermo Electron Corporation, USA. Ion concentrations were determined using inductively coupled plasma emission spectroscopy (ICP-OES). The precipitation rate was calculated using equation (1).(1) R=C1V1-C2V2/C1V1

where C1 is the ion concentration in the leachate before precipitation and V1 is the volume of the liquid before precipitation; C2 is the ion concentration in the precipitation supernatant and V2 is the volume of the precipitation supernatant.

2.5 Principle of indium precipitation by CaCO3

The pH values for hydrolysis of In3+, Zn2+, Fe2+, Al3+ at 25 °C are shown in Table 2, so the separation of indium, zinc, iron and aluminium can be achieved by controlling the precipitation termination pH. CaCO3 is a strong base and weak acid salt, and hydrolysis of CO32– produces OH–, which increases the pH of the solution. The hydrolysis equation of CO32– is as follows:(2) CO32-+H2O=HCO3-+OH-

(3) HCO3-+H2O=H2CO3+OH-

Table 2 The pH values for hydrolysis of In3+, Zn2+, Fe2+, Al3+.

In3+	Zn2+	Fe2+	Al3+	
3.0	5.65	6.645	3.3	

Meanwhile, the E-pH diagram of the Ca-S-H2O system was plotted using HSC 6.0 software, as shown in Fig. 3. From Fig. 3, it can be seen that CaSO4·2H2O is relatively stable.Fig. 3 E-pH diagram of Ca-S-H2O system.

3 Results and discussion

3.1 Temperature rise characteristics of ultrasound in the calcium carbonate-leaching solution system

The temperature rise characteristics of ultrasound in the calcium carbonate-leach solution system are shown in Fig. 4, and the ultrasound power was examined in the range of 100–250 W.Fig. 4 Temperature rise characteristic curve of ultrasound under calcium carbonate-leach solution system.

From Fig. 4, it can be seen that the higher the ultrasonic power, the faster the heating rate of the solution. The maximum temperature of the solution can reach 60.7 °C, 83.9 °C, 90.9 °C, and 91.3 °C when the ultrasonic power is 100 W, 150 W, 200 W, and 250 W. It shows that the ultrasonic waves have a thermal effect when propagating in the solution, and no additional heating is required. Meanwhile, the ultrasonic power of 200 W is the optimal power condition. Fitting Fig. 4, the temperature rise rate equation can be obtained, as shown in Table 3.Table 3 Temperature rise rate equation of ultrasound under calcium carbonate-leaching solution system.

Ultrasonic power/W	Temperature rise rate equation	R2	
100	T = 22.5 + 0.89 t-0.0052t2	0.998	
150	T = 22.7 + 1.85 t-0.014t2	0.994	
200	T = 45.0 + 1.84 t-0.016t2	0.784	
250	T = 49.0 + 1.74 t-0.015t2	0.717	

3.2 Effect of precipitation termination pH on indium precipitation rate

The effect of solution endpoint pH on indium precipitation is shown in Fig. 5, with an ultrasonic power of 200 W.Fig. 5 Effect of precipitation termination pH on indium precipitation rate.

From Fig. 5, it can be seen that the precipitation rates of indium, zinc and aluminium increased gradually with the increase of pH, and when the pH was 4.0, the precipitation rates of indium, zinc and aluminium were 99.78 %, 20.70 % and 96.75 %, respectively. After that, the precipitation rates of indium and aluminium entered into a plateau period with the increase of pH, and the precipitation rate of zinc gradually increased. Since indium and aluminium precipitated at similar pH, aluminium also precipitated almost completely at pH 4.0, our study is consistent with these findings [29], [30]. However, S.M. Javad Koleini et al. [20] used ammonia as precipitant for conventional precipitation of indium at pH=6 to achieve complete precipitation of indium. The precipitation rate of zinc reached 20.7 % at pH 4, which could be attributed to the formation of ZnCO3 precipitate under the action of ultrasound, this is because the ZnCO3 phase was found in the XRD analysis of the precipitated residue.

3.3 Effect of ultrasonic power on indium precipitation rate

The effect of ultrasonic power on indium precipitation rate is shown in Fig. 6, with a precipitation termination pH of 4.0.Fig. 6 Effect of ultrasonic power on indium precipitation rate.

From Fig. 6, it can be seen that the precipitation rate of indium, zinc and aluminium increases slowly with the increase of ultrasonic power, and when the ultrasonic power is 200 W, the precipitation rates of indium, zinc and aluminium are 99.79 %, 20.71 % and 96.76 %, respectively. Wei Zhou et al. found that the higher the ultrasound power, the greater the energy of ultrasound [31]. After that, the ultrasonic power increased and the precipitation rate of the metals remained almost constant. Also, it can be seen from Fig. 6 that the effect of ultrasonic power on the precipitation rate of indium was not significant under the same conditions of precipitation termination pH.

3.4 Effect of ultrasonic power on indium precipitation kinetics

It is assumed that the chemical reaction for ultrasound-enhanced precipitation of indium from calcium carbonate is either a primary reaction or a secondary reaction. The kinetic equations for the primary and secondary reactions are as follows [32]:(4) r=dCdt=k1C

Equation (5) is obtained by integrating equation (4):(5) lnC0-lnCt=k1t+A

(6) r=dCdt=k2C2

Equation (7) is obtained by integrating equation (6):(7) 1Ct-1C0=k2t+B

where k1 is the equilibrium constant for the primary reaction, k2 is the equilibrium constant for the secondary reaction, C0 is the concentration at equilibrium, Ct is the concentration at time t, and A and B are constants.

The kinetic curves for ultrasound and conventional precipitation are shown in Fig. 7. Linear fitting of Fig. 7 with Eqs. (5), (7) gives Fig. 8(a), (b), (c) and (d), respectively.Fig. 7 Kinetic curve of indium precipitation (a. Ultrasound; b. Conventional).

Fig. 8 Linear fit of the kinetic curve (a, b. Ultrasound; c, d. Conventional).

From Fig. 8, it can be seen that the linear correlation coefficient (R2) of Fig. 8(a) is better than that of Fig. 8(b), so the kinetic model of ultrasound-enhanced indium precipitation from calcium carbonate is consistent with a first-order reaction. Referring to the Arrhenius equation: lnK = −Ea/RT+C, we approximated the ultrasonic power equal to the thermodynamic temperature, and then obtained the equation: lnK = −Ea/RW+C. At the same time, the plot of lnK-1000/W was plotted as shown in Fig. 9(a). We named the activation energy obtained as ‘ultrasonic activation energy’ and calculated Eu-a = 2.63KJ/mol. It was demonstrated that the introduction of ultrasound can reduce the activation energy of the reaction and promote the rapid onset of the precipitation reaction. The kinetic model for ultrasonic precipitation was calculated as:lnC0-lnCt=exp0.11339-318.54W.t+A

Fig. 9 Fitting plot of lnK-1000/W,lnK-1000/T.

The fit of the conventional precipitation curves to Fig. 8(c) has a better R2 than Fig. 8(d), therefore, the kinetic model for conventional precipitation is also consistent with a first-order reaction. Based on the Arrhenius equation, lnK-1000/T was plotted as shown in Fig. 9(b). The activation energy Ea = 9.78 KJ/mol for the conventional precipitation was also calculated. The kinetic model for conventional precipitation is expressed as:lnC0-lnCt=exp1.66626-1176.19T.t+A

The activation energy of ultrasonic precipitation was 2.63 KJ/mol, while that of conventional precipitation was 9.78 KJ/mol, which proved that the introduction of ultrasound could reduce the activation energy of the precipitation reaction and promote the precipitation reaction.

3.5 Comparison of ultrasonic and conventional precipitation

The comparison between ultrasonic and conventional precipitation is shown in Table 4. As can be seen from Table 4, under the same precipitation termination pH conditions, the precipitation rates of indium by ultrasonic precipitation and conventional precipitation methods were almost the same, but the precipitation rate of zinc by ultrasonic precipitation was lower than that by conventional precipitation, which proved that ultrasonic precipitation could inhibit the precipitation of zinc. Meanwhile, the ultrasonic precipitation can significantly shorten the precipitation time by 22 min under the cavitation effect of ultrasound. In addition, ultrasonic precipitation can save the amount of calcium carbonate compared to conventional precipitation, the reason is that the introduction of ultrasound can promote the hydrolysis of CO2-3 to produce more OH–, so that the pH of the precipitation solution rises rapidly, which will reduce the consumption of calcium carbonate.Table 4 Comparison of ultrasound sedimentation and conventional precipitation.

Method	Time/min	Precipitation termination pH	Consumption of CaCO3	Ion precipitation rate/%	
In	Zn	Fe2+	Al	
Ultrasonic precipitation	23	4.0	6.34	99.79	20.70	0	96.75	
Conventional precipitation	45	4.0	9.4	99.61	29.36	0	99.98	

3.6 Mechanism analysis

XPS of indium, zinc, aluminium and calcium for conventional and ultrasonic precipitation residue is shown in Fig. 10.Fig. 10 XPS image of precipitation residue (Ultrasonic precipitation terminated at pH 4.0, with an ultrasonic power of 200 W; Conventional precipitation termination pH 4.0, temperature 80 °C).

As can be seen from Fig. 10, ultrasonic precipitation did not change the elemental valence states of indium, zinc, aluminium, and calcium compared to conventional precipitation, this is because the binding energies of indium, zinc, aluminium and calcium in ultrasonically precipitated slag and conventionally precipitated slag are the same. but ultrasonic precipitation resulted in the displacement of the diffraction peaks of the elements indium, zinc, aluminium, and calcium. In addition, the diffraction peak intensities of indium, aluminium and calcium in ultrasonically precipitated slag were all stronger than those of indium, aluminium and calcium in conventionally precipitated slag. However, the diffraction peak intensity of Zn in the ultrasonically precipitated slag is weaker than that of Zn in the conventional precipitated slag, which indicates that ZnCO3 precipitate was generated in the ultrasonically precipitated slag, while Zn(OH)2 precipitate was generated in the conventional precipitated slag, which is in agreement with the results of XRD analysis. The probable reason is that the cavitation effect of ultrasound accelerates the dissolution of calcium carbonate, and then the zinc ions combine with the carbonate to produce a ZnCO3 precipitate. In addition, the diffraction peaks of indium and aluminium in the ultrasonically precipitated residue were higher than those in the conventionally precipitated residue, proving that ultrasound can promote the precipitation of indium and aluminium. However, the diffraction peak of zinc in the ultrasonically precipitated residue was lower than that in the conventionally precipitated residue, proving that ultrasound can inhibit the precipitation of zinc. Ultrasonic cavitation can release strong energy locally in a short period of time, generating secondary effects such as microjets, shock waves, thermal effects, sonoluminescence, and hydroxyl radical release [33], [34], [35]. The mechanism of ultrasound-enhanced calcium carbonate precipitation of indium is shown in Fig. 11. As can be seen from Fig. 11, under the cavitation effect of ultrasound, the dissolution of calcium carbonate in solution is accelerated to Ca2+ and CO32–, and then CO32– undergoes a two-step hydrolysis to produce OH–, after which In3+ combines with OH– to produce In(OH)3 precipitation.Fig. 11 Mechanism of ultrasound enhanced precipitation of indium from calcium carbonate.

3.7 Characterisation

3.7.1 XRD analysis

The XRD patterns of calcium carbonate, conventional precipitated residue, and ultrasonically precipitated residue are shown in Fig. 12.Fig. 12 XRD pattern of calcium carbonate, conventional precipitation residue, ultrasonic precipitation residue (Ultrasonic precipitation terminated at pH 4.0, with an ultrasonic power of 200 W; Conventional precipitation termination pH 4.0, temperature 80 °C).

As can be seen from Fig. 12, the main phase of the conventional precipitation residue is CaSO4·2H2O, Zn(OH)2. The main phases of the ultrasonically precipitated slag were CaSO4·2H2O, ZnFe(SO4)2·7H2O, Zn(OH)2, ZnCO3; and no obvious diffraction peaks were observed due to the low content of indium and aluminium. Interestingly the ultrasonic precipitation produced a ZnCO3 precipitate, the possible reason being that the cavitation of ultrasound induced the reaction between Zn2+ and HCO-3 produced by the hydrolysis of CO2-3 to produce a ZnCO3 precipitate (Zn2+ + CO2-3 = ZnCO3).

3.7.2 Surface morphology analysis

The surface morphology of calcium carbonate, conventional precipitated slag, and ultrasonically precipitated slag is shown in Fig. 13.Fig. 13 Surface morphology of (a) calcium carbonate, (b) conventional precipitated residue, (c) ultrasonically precipitated residue (Ultrasonic precipitation terminated at pH 4.0, with an ultrasonic power of 200 W; Conventional precipitation termination pH 4.0, temperature 80 °C).

From Fig. 13, it can be seen that the calcium carbonate grains are distributed in the shape of squares, and these squares are stacked on top of each other. The grains of the regular precipitation residue are in the form of irregular flakes, and irregular tiny flakes are attached on top of the flake grains, and this flake grain should be CaSO4·2H2O, Dorozhkin S V et al. showed that the grains of CaSO4·0.5H2O are prismatic [36] and the larger particles attached to the flake grains may be Zn(OH)2, and the smaller particles may be In(OH)3. The grains of the ultrasonically precipitated slag showed prismatic distribution, the larger prisms should be CaSO4·2H2O, the smaller prisms should be ZnCO3, and the small particles attached to the prisms should be Zn(OH)2 or In(OH)3. Pérez-Maqueda L A et al. showed that In(OH)3 is distributed in granular form by SEM analysis [37]. It proves that the introduction of ultrasound destroys the phase structure of the precipitated residue, and makes the physical phase of the residue undergo a significant change, which is in agreement with the results of the XRD analyses.

3.7.3 EDS analysis

The energy spectral distributions of the linear and cross-sectional scans of the conventional and ultrasonically precipitated residues are shown in Fig. 14, Fig. 15. As can be seen from Fig. 14, the diffraction peaks of indium, zinc and aluminium appear in both the conventional and ultrasonically precipitated residue, indicating that indium and aluminium have been completely precipitated in the residue, and a small amount of zinc has been precipitated at the same time, which is consistent with the results of the ICP-OES analysis. Meanwhile, the weight percentage of carbon in the ultrasonically precipitated residue is higher than that in the conventional residue, which proves that the ultrasonically precipitated residue generates ZnCO3; this is in agreement with the results of XRD analysis. In addition, the weight percentage of zinc in the ultrasonically precipitated residue is lower than that in the conventional precipitated residue, which proves that ultrasound can inhibit the precipitation of zinc, which is consistent with the results of ICP-OES analysis.Fig. 14 Linear scanning energy spectrum distribution of conventional and ultrasonically precipitated residues (a. Conventional precipitated residue; b. Ultrasonic precipitated residue) (Ultrasonic precipitation terminated at pH 4.0, with an ultrasonic power of 200 W; Conventional precipitation termination pH 4.0, temperature 80 °C).

Fig. 15 Energetic elemental distribution of cross sectional scans of conventional and ultrasonically precipitated residue (a. Conventional precipitated residue; b. Ultrasonic precipitated residue) (Ultrasonic precipitation terminated at pH 4.0, with an ultrasonic power of 200 W; Conventional precipitation termination pH 4.0, temperature 80 °C).

As can be seen from Fig. 15, the agglomeration of the elements Ca, In and Al in the ultrasonically precipitated slag is more pronounced than in the conventionally precipitated slag, which proves that ultrasound can promote the precipitation of indium and aluminium. Libo Zhang et al. have been confirmed ultrasonic energy can shatter solid particle and hinder the growth of solid grain, which is beneficial for solid–liquid reaction [38]. However, the agglomeration of zinc elements in the ultrasonically precipitated slag is significantly weaker than that in the conventional precipitated slag, proving that ultrasound can inhibit the precipitation of zinc. The reason may be that the cavitation of ultrasound promotes the combination of zinc ions and carbonate ions, which leads to the weakening of hydrolysis of carbonate, and then inhibits the precipitation of zinc.

3.7.4 TEM analysis

Low voltage transmission electron microscopy (TEM) of conventional and ultrasonically precipitated residues are shown in Fig. 16, Fig. 17.Fig. 16 TEM plot of conventional precipitation residue (Conventional precipitation termination pH 4.0, temperature 80 °C).

Fig. 17 TEM plot of ultrasonic precipitation of residue (Ultrasonic precipitation terminated at pH 4.0).

As can be seen from Fig. 16, Fig. 17, the crystal structure of the two precipitated slags did not undergo any obvious change compared to the ultrasonic precipitation and conventional precipitation, and both of them showed a rod-like structure, and the crystals of this rod-like structure were supposed to be CaSO4·2H2O In(OH)3 precipitation was irregularly granularly attached to the CaSO4·2H2O It was demonstrated that the introduction of ultrasound only accelerated the precipitation reaction and did not significantly disrupt the structure of the precipitated residue. In addition, no significant lattice diffraction streaks were observed in the ultrasonically precipitated residue and the conventional precipitated residue.

4 Conclusion

In this study, ultrasound-enhanced calcium carbonate precipitation was used to enrich indium in zinc oxide dust leaching solution, and the main conclusions are as follows:

(1) The optimum conditions for ultrasound-enhanced indium precipitation and enrichment were end-point pH 4.0 and ultrasound power 200 W. Under these conditions, the indium precipitation rate was 99.79 %.

(2) The activation energy for conventional isothermal precipitation is 9.78 kJ/mol, and the kinetic model is lnC0-lnCt = exp(1.66626–1176.19/T).t + A. We have creatively developed the theory of ‘ultrasonic activation energy’ and calculated the ultrasonic activation energy to be 2.63 KJ/mol. the kinetic model is lnC0-lnCt = exp(0.11339–318.54/W).t + A. It proves that the introduction of ultrasound can reduce the activation energy of precipitation reaction. The kinetic models of conventional precipitation and ultrasonic precipitation both conform to first-order reactions.

(3) Compared with conventional precipitation of indium, ultrasonic precipitation can significantly shorten the time to reach equilibrium and reduce the amount of calcium carbonate used under the same precipitation termination pH conditions.

(4) Characterisation of the precipitated residue by XRD, XPS, SEM-EDS and TEM revealed the mechanism of ultrasound-enhanced precipitation of indium. In addition, ultrasound can inhibit the precipitation of zinc.

(5) The mechanism of ultrasound enhanced calcium carbonate precipitation of indium is that under ultrasound enhancement, carbonate ions hydrolyze to form OH–, and then In3+and OH– combine to form indium hydroxide precipitation.

In the future, researchers should focus on studying large-scale ultrasonic precipitation equipment to promote industrialization.

Funding

This study was financially supported by the efficient enrichment, separation and purification of rare metal gallium-germanium-indium of National Key R&D Program of China (No: 0101-2133-1).

CRediT authorship contribution statement

Shiju Li: Writing – original draft, Software, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization. Haibei Wang: Writing – review & editing, Project administration, Funding acquisition. Shengdong Wang: Software. Feng Xie: Supervision, Methodology. Xudong Sun: Software.

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.

Acknowledgements

This work is supported by efficient enrichment, separation and purification of rare metal gallium-germanium-indium of National Key R＆D Program of China (No: 0101-2133-1 ).
==== Refs
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