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Crystallization is an important process that affects the properties of final products and is essential in nearly all chemical processing industries. In recent years, ultrasonic technology has received widespread attention due to its ability to enhance crystallization yield, improve crystal morphology and shape, and regulate the particle size and distribution of crystal products. It holds promising prospects for industrial crystallization. In this work, the ultrasonic cavitation effect and ultrasonic crystallization mechanism are described, and the influence of ultrasound on the crystallization effect of products is analysed and discussed. In addition, the application status of ultrasonic reactors and ultrasonic crystallization processes is introduced in detail, and the change trend from laboratory to industrialization is analyzed. Finally, the challenges and opportunities facing the industrialization of ultrasonic crystallization in future developments are discussed. The purpose of this work is to make the selective promotion or inhibition of ultrasound more helpful for industrial crystallization.

Ultrasound enhanced evaporating crystallization has been proposed to solve the problems of low crystallization yield and uneven particle size in the evaporating crystallization process of ammonium sulfate solution at atmospheric pressure. The effects of key operating parameters, including the ultrasound power, stirring speed, pH value, and ultrasound time, on the yield of ammonium sulfate product and the duration of solid–liquid transformation time are studied. The results show that the ultrasound crystallization can increase the ammonium sulfate yield by 52.9 %, reduce the solid–liquid transformation time of ammonium sulfate by 10 %, and obtain ammonium sulfate products with higher crystallinity and more uniform particle size. Ultrasound promotes the crystallization of ammonium sulfate by enhancing the transfer of heat in the solution and reducing the supersolubility of the ammonium sulfate solution from 937.5 g/L to 833.33 g/L. This study provides experimental justification for the use of ultrasound in atmospheric evaporative crystallization.

When organic matter, especially sodium oxalate (Na2C2O4), accumulates to a certain extent, it will seriously affect the alumina production process in the refinery and therefore urgently needs to be removed. This work attempts to illuminate the benefits of ultrasonic intensification of the crystallization process of Na2C2O4, taking the alumina refinery waste liquor, i.e., flat plate washing liquor, as a case study. The effects of different operating parameters (seed crystal addition amount, caustic soda concentration, reaction time, ultrasonic power) on the crystallization behavior and yield are discussed, and it is found that ultrasonic can increase the Na2C2O4 removal rate to 70.4%. The addition of ultrasonic promotes the morphological evolution of Na2C2O4 and is of great significance to the optimization of the components of the precipitated Na2C2O4. Specifically, the proportion of Na2C2O4 in the crystallized product reaches 64% with conventional conditions, while it reaches 77% with ultrasonic conditions. Therefore, ultrasonic can greatly reduce the alkali loss caused by the crystallization process of Na2C2O4 in flat plate washing liquor, which has great economic benefits.

Iron slag, a byproduct of the wet zinc refining process, contains a substantial amount of valuable metals such as iron and zinc, making it highly valuable for comprehensive recovery and reuse. However, before recovery, the iron slag requires dehydration pretreatment. The water content in iron slag remains relatively high and difficult to remove even after pressure filtration, leading to extended drying times, reduced drying efficiency, and increased energy consumption. This study explores a novel ultrasonic pretreatment process for iron slag. Using the response surface methodology, we investigated the effects of ultrasonic power, ultrasonic time, liquid-to-solid ratio, and their interactions on the water content, capillary suction time (CST), and filtration resistance of the slag. Regression equations were established to predict the relationships between the water content, CST, filtration resistance, and the various factors. The optimal process parameters were determined as an ultrasonic power of 60 W, ultrasonic time of 22 s, and a liquid-to-solid ratio of 4:1. Under these conditions, the dehydration performance of the iron slag was optimal. The measured values closely matched the predicted values, demonstrating the reliability of the model and the feasibility of the optimized process. Our study of the mechanism of ultrasonic action on iron slag found that under the influence of ultrasonic waves, the particle size of the slag significantly decreased, and the particle morphology changed. Compared to conventional drying, the drying rate of the iron slag after ultrasonic pretreatment was accelerated, and the drying time was reduced.

Replacing cadmium ions in cadmium-containing solutions with aluminum powder is beneficial for cadmium resource recycling and environmental protection. However, the conventional aluminum powder replacement method requires harsh temperatures and prolonged conditions. In this study, the effect and mechanism of ultrasound on the replacement of cadmium with aluminum powder were investigated at low temperatures. Ultrasound has been proven to promote the etching of alumina films through the use of TEM and XPS, providing mechanistic support for the superiority of the new process. A degree of Cd replacement as high as 95.08 % is achieved at a low temperature (60 ℃) and in a short time (20 min) when using ultrasonicated aluminum powder replacement, which is 42.17 % higher than that of conventional aluminum powder. Compared with conventional aluminum powder replacement conditions with the same effect, the introduction of ultrasound can reduce the temperature by 30℃ and shorten the replacement time by 2/3, which has significant advantages in reaction efficiency and safety. The strengthening mechanism of ultrasound on the replacement effect of aluminum powder at low temperatures is revealed through detailed discussions on the corrosion of alumina films, agglomeration of aluminum powder, and adhesion of replacement products to the surface of aluminum powder, dissolved oxygen in the solution, and redissolution of cadmium. Therefore, a new approach for replacing aluminum powder in solutions with high Cd2+ concentrations at low temperatures is proposed in this work, which is expected to solve the existing harsh and dangerous problems of industrial aluminum powder replacement.

Currently, removing arsenic (As) from ZnSO4 solution using lime presents several drawbacks, including high wet precipitate content, long reaction time, and the introduction of new impurities. In this study, we propose a novel ultrasonic (US) ozone one-pot method for effectively removing As from a high-arsenic ZnSO4 solution. In this method, as in ZnSO4 solution was removed by ultrasound enhanced ozone oxidation combined with zinc roasting dust (ZRD). No secondary pollution will occur with the addition of ZRD and ozone, as neither introduces new impurities. The experimental results show that under the conditions of initial As and Fe concentrations of 1640 mg/L and 2963 mg/L, US power of 480 W, frequency of 20 kHz, reaction temperature of 60 °C, reaction time of 1 h, ZRD dose of 12 g/L and gas flow rate of 900 mL/min, the removal rate of As can reach 99.4%. The introduction of US can further enhance the oxidation effect of ozone on As(III) and Fe2+ by increasing the solubility of ozone and promoting the production of ·OH radicals. Additionally, US cavitation and mechanical action increase the probability of contact between various reactants in the solution, facilitating the occurrence of reactions. US also reduces the aggregation of arsenic-containing precipitates and the encapsulation of ZRD by arsenic containing precipitates, thereby decreasing the amount of arsenic-containing precipitates. In comparison to the traditional lime method, this approach results in a significant reduction in the amount of arsenic-containing precipitate by 54.5% and a 60% decrease in the total reaction time. The As removal mechanism of our method encompasses ZRD neutralization, US-enhanced ozone mass transfer and decomposition, oxidation of As(III) and Fe2+, and adsorption and coprecipitation. Consequently, the proposed method provides a cost-effective, fast, safe and environmentally friendly alternative for treating arsenic-contaminated ZnSO4 solutions.

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.

Silicon-based sulfonic solid acids have the advantages of high catalytic activity and selectivity, easy separation from products, low equipment corrosion, and environmental protection, and sulfuric acid loading is the key to their preparation. To overcome the shortcomings of low acid loading and uneven distribution in the existing preparation methods of micron-sized silicon-based sulfonic solid acids, a method was proposed to prepare micron-sized silicon-based sulfonic solid acids using ultrasonic enhanced technology. The effect of different reaction parameters, such as time, power, and temperature of ultrasonication, sulfonation temperature and time, and sulfuric acid concentration, on acid loading in solid acid was investigated in this work. The results showed that a micron-sized mesoporous silica-based solid acid was successfully synthesized with a high acid content of 0.8633 mmol/g, uniform acid distribution, high specific surface area of 269.332 m2/g, and large average particle size of 172.142 μm in this work. The introduction of ultrasound was found to expand the carrier's pore volume and increase the carrier's specific surface area and the number of hydroxyl groups, thereby increasing the acid loading capacity and the specific surface area of the solid acid sample by 66.6 % and 10.97 % respectively, compared with the case without ultrasound.

A promising and sustainable approach for producing hydrogen peroxide is the two-electron oxygen reduction reaction (2e– ORR), which uses very stable graphitic carbon nitride (g-C3N4). However, the catalytic performance of pristine g-C3N4 is still far from satisfactory. Here, we demonstrate for the first time the controlled fabrication of carbon quantum dots (CQDs)-modified graphitic carbon nitride carbon (g-C3N4/CQDs-X) by ultrasonic stripping for efficient 2e– ORR electrocatalysis. HRTEM, UV–vis, EPR and EIS analyses are in good consistent which prove the in-situ generation of CQDs. The effect of sonication time on the physical properties and ORR activity of g-C3N4 is discussed for the first time. The g-C3N4/CQDs-12 catalyst shows a selectivity of up to 95% at a potential of 0.35 V vs. RHE, which is much higher than that of the original g-C3N4 catalyst (88%). Additionally, the H2O2 yield is up to 1466.6 mmol g−1 in 12 h, which is twice as high as the original g-C3N4 catalyst. It is discovered that the addition of CQDs through ultrasonic improves the g-C3N4 catalyst's electrical conductivity and electron transfer capability in addition to its high specific surface area and distinctive porous structure, speeding up the reaction rate. This research offers a green method for enhancing g-C3N4 activity.

In this work, the cause of abnormal color in ammonium sulfate products formed by flue gas desulfurization is revealed by investigating the conversion relationship between different sulfur-containing ions and their behavior in a sulfuric acid medium. Both thiosulfate (S2O32−) and sulfite (SO32− ＆ HSO3−) impurities affect the quality of ammonium sulfate. The S2O32− is the main reason for the yellowing of the product due to the formation of sulfur impurities in concentrated sulfuric acid. To address the yellowing of ammonium sulfate products, a unified technology (US/O3), using ozone (O3) and ultrasonic waves (US) simultaneously, is exploited to remove both thiosulfate and sulfite impurities from the mother liquor. The effect of different reaction parameters on the degree of removal of thiosulfate and sulfite is investigated. The synergistic effect of ultrasound and ozone on ion oxidation is further explored and demonstrated by the comparative experiments with O3 and US/O3. Under the optimized conditions, the thiosulfate and sulfite concentration in the solution is 2.07 and 5.93 g/L, respectively, and the degree of removal is 91.39 and 90.83%, respectively. The product obtained after evaporation and crystallization is pure white and meets the national standard requirements for ammonium sulfate products. Under the same conditions, the US/O3 process has apparent advantages, such as saving reaction time compared with the O3 process alone. Introducing an ultrasonically intensified field improves the generation of oxidation radicals ·OH, 1O2, and ·O2– in the solution. Furthermore, the effectiveness of different oxidation components in the decolorization process is studied by adding other radical shielding agents using the US/O3 process supplemented with EPR analysis. The order of the different oxidation components is O3(86.04%) > 1O2(6.53%) > •OH(4.45%) > •O2–(2.97%) for the oxidation of thiosulfate, and it is O3(86.28%) > •OH(7.49%) > 1O2(4.99%) > •O2–(1.25%) for the oxidation of sulfite.

In this work, the crystallization process of selenium was accelerated by ultrasonic wave. The effects of ultrasonic waves and conventional conditions of selenium crystallization were compared to understand the effects of different conditions on crystallization, including ultrasonic time, ultrasonic power, reduction temperature, and H2SeO3 concentration. The mechanism of ultrasound affecting selenium crystallization was also investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The experimental results showed that ultrasonic time, ultrasonic power, and reduction temperature significantly influenced the crystallization process and morphology of selenium. Ultrasonic time had a large effect on the completeness (all products have been crystallized) and integrity of the crystallization of the products. Meanwhile, ultrasonic power and reduction temperature had no effect on the completeness of crystallization. However, it had a significant effect on the morphology and integrity of the crystallized products, and different morphologies of the nano-selenium materials could be obtained by changing the ultrasonic parameters. Both primary and secondary nucleation are important in the process of ultrasound-accelerated selenium crystallization. The cavitation effect and mechanical fluctuant effects generated by ultrasound could reduce the crystallization induction time and accelerate the primary nucleation rate. The high-speed micro-jet formed in the rupture of the cavitation bubble generated is the most important reason to influence the secondary nucleation of the system.

The presence of organic compounds in the waste liquor is of serious environmental concern that has plagued the development of alumina industry (Bayer Process). The present work attempts to develop a green and efficient process for removal of organics utilizing combined effect of sonolysis and ozonation (US/O3). The effects of reaction duration, ozone concentration and ultrasonic power are assessed for sonolysis (US), ozonation (O3) and combination of sonolysis and ozonation (US/O3). The optimal conditions for US/O3 treatment system is identified to be a reaction duration of 7 h, ozone concentration of 7.65 g/h, and ultrasonic power of 600 W. The total organic carbon (TOC) removal and decolorization are 60.13% and 87.1%, respectively. The process can be scaled-up to industrial scale, which could potentially serve to be a convenient, safe and sustainable alternative to the exisiting treatment technologies. Additionally, the treated waste water can be reused contributing to an improvement in the overall economics.

In this work, a combination of ozone (O3) and ultrasound (US) has been firstly used to decolorize black concentrated sulfuric acid with high organic content. The effect of different reaction factors on the transparency, extent of decolorization, H2SO4 mass fraction, and organic pollutants removal is studied. In addition, the systematic interaction between ultrasound and ozone on the decolorization process is reviewed through comparative experiments of O3, US and US/O3. A sulfuric acid product that meets the requirements for first-class products in national standards, with an extent of decolorization of 74.07%, transparency of 70 mm, and a mass fraction of 98.04%, is obtained under the optimized conditions. Under the same conditions, it has been established that the treatment time can be saved by 25% using the US/O3 process compared to using O3. Further, the production of oxidative free radicals (•OH) in a concentrated sulfuric acid system is enhanced using the US/O3 process compared with O3. In addition, the degree of effectiveness of different oxidizing components on the decolorization process is revealed by adding different free radical shielding agents when the US/O3 process is used.