One-step synthesis of short columnar α-calcium sulfate hemihydrate from titanium white waste acid | Scientific Reports

Scientific Reports volume 14, Article number: 24809 (2024) Cite this article

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The waste sulfuric acid solution emerged as a main emission substance from titanium dioxide production is called titanium white waste acid (TWWA). The disposal of TWWA has been a concern due to its potential impact on the environment. A green way including one-step preparation and purification stages of α-calcium sulfate hemihydrate (α-HH) from TWWA and lime mud via a neutralization reaction in the hydrothermal pressure apparatus was developed. The preferred experimental conditions were obtained, i.e., lime mud/TWWA/sodium citrate ratio: 16/16/1, 140 ℃, 0.5 MPa, reaction time: 10 min. The recovery rate of α-HH in the whole process was 89.5%. The method presented high efficiency and selectivity in conversion to a short-columnar-shaped α-HH under addition of 3.40 × 10−2 mol/L sodium citrate as a crystal modifier. The structure and composition of the obtained α-HH products were confirmed by XRD, TGA, and SEM. Compared with titanium gypsum, the obtained short columnar α-HH with hexaprismatic morphology showed excellent mechanical properties. According to GB/T 9776 − 2008 “Calcined gypsum”, the compressive strength of 7 days of the cemented short-columnar α-HH was about 6.35 MPa, which it meets the strength requirements for building gypsum.

More than 90% amount of titanium dioxide (TiO2) in China is produced through the sulfuric acid method due to the mature processes, low cost, and low-quality requirements of raw material quality1,2. The titanium white waste acid (TWWA) is emerged as a main emission substance from titanium dioxide production. It is estimated that the annual production of TWWA in China exceeds 25 million tons, and the disposal of TWWA has been a significant concern due to the environmental impact and potential for secondary pollution. Less than 10% of the total TWWA is comprehensively utilized due to its high water content (30–50%), high viscosity, and presence of a variety of metal impurities3.

Currently, various studies have been conducted to explore innovative methods for the extraction and utilization of valuable components from these wastes. Direct concentration of sulfuric acid (H2SO4) from TWWA using ion exchange membranes has been done. Guo et al.4 designed semi-interpenetrating network membranes by grafting poly(dimethyl aminoethyl methacrylate) to PVDF, which exhibited remarkable acid block properties. Similarly, Feng et al.5 explored the use of hollow fiber membranes to directly concentrate H2SO4 from wastes acid. However, the substantial proton leakage of the anion exchange membrane seriously deteriorates its work performance. Pang1 used chemical dehydration coupled with multi-effect evaporation to treat waste sulfuric acid in the titanium dioxide production process. Additionally, TWWA also contained a large number of Fe and Al, as well as a certain amount of rare metal such as Ti, V, and Sc. Zhang et al.6 proposed a new technology for the step extraction and comprehensive utilization of TWWA, focusing on the preparation of doped iron phosphate through selective precipitation. This approach aims to address the challenges of secondary pollution associated with the extraction of acid resources and valuable components from TWWA. Li et al.7, and Chen et al.8 successfully separated and purified scandium from spent sulfuric acid solutions generated during titanium dioxide production. The studies highlighted the usage of a synergistic extraction system for efficient enrichment and purification of scandium from the waste solution. However, it is still a big challenge to apply them on a large-scale production due to high costs and technical bottlenecks. For low-concentration acid wastes with acidity only of 4–8% a small amount of membrane exchange concentration treatment is applied, which has high cost, low economic value, and cannot achieve full environmental and resource utilization.

Hence, finding innovative methods for recovering valuable materials from TWWA has become the goal of research in the last decades. Song et al.9 explored a novel approach for high-efficiency recovery of titanium dioxide, 21% hydrochloric acid, and organic solvents from titanium white waste acid. This study demonstrated the potential for integrated vacuum distillation and calcination to increase the concentration of metatitanic acid from TWWA thick slurry. Ma et al.10 conducted a study on the preparation of titanium-rich materials through pressure leaching of titanium concentrate from titanium dioxide waste acid. The titanium-rich materials in a TiO2 grade of above 80% could be obtained. Zhu et al.11 used TWWA to dissolve the representative elements, such as Na, Sc, and Al from red mud to achieve the goal of “treat waste with waste”.

Neutralization technology of TWWA has been firstly reacted with alkaline substances and then the filtered to obtain the high-density sludge, during which contained 70–80 wt% titanium gypsum (CaSO4∙2H2O)12,13. Romanovski et al.14 recycled the high-quality synthetic gypsum and anhydrite derived from the spent sulfuric acid and lime mud as usual conditions for synthesis of gypsum: 55 ℃ and atmospheric pressure. They further developed a green approach for the filtrate of synthetic gypsum production from water treatment coagulation sediments and spent sulfuric acid. And the concentrated filtrate, which synthesized by the solution combustion synthesis method, showed good coagulation results and facilitated its use as a precursor for synthesizing magnetic sorbents and photocatalysts15. In general, the microstructure and cementation behavior of titanium gypsum are quite deferent from the calcined gypsum (CaSO4∙1/2H2O), which leads to the relatively low compressive strength of CaSO4∙2H2O when utilized directly as a cementing material16. The rod-like gypsum crystals of calcium sulfate (CaSO4) were prepared from recycled red gypsum, then the as-prepared CaSO4 was neutralized acidic wastewater to produce high silica residue (HSR) after 6 cycles, which could be used as a cementing material after desulfurizationvia calcination at 1200 ℃ for 2 h17. Nevertheless, the use of expensive commercial sulfuric acid for its production does not allow obtaining a cost-effective product18,19.

The objectives of the work were: (1) the spent solution of TWWA and lime mud can be the used as raw materials for one-stage preparationof hexaprismatic-rod-shaped α-HH crystals via the neutralization reaction in the presence of sodium citrate as crystal modifier in 10 min reaction time, during which no CaSO4∙2H2O was generated as an intermediate product. (2) The direct control of length-to-diameter ratio of α-HH crystals can be performed by the concentration of sodium citrate. (3) Furthermore, an environmental and economic estimation of this proposed technology was also discussed.

To obtain short-columnar-shaped α-HH crystals, starting materials such as quick lime mud and TWWA were used directly, and the molar ratio of Ca2+ and SO42- were chosen to be equal as 0.16 mol. The brownish yellow sediments by neutralizing acidic wastewater with quick lime mud added at different concentrations of sodium citrate were shown in Fig. 1. According to Eq. (1), the highest yield of intended products of α-HH was calculated to be about 89.5% when added sodium citrate as 3.40 × 10−2 mol/L. XRD patterns of calcium sulfate dihydrate and the α-HH products prepared at different concentrations of sodium citrate were shown in Fig. 1. The CaSO4·2H2O using as a controller exhibited three characteristic diffraction peaks at approximately 2θ = 11.65° (020), 20.76° (021), 23.42° (040), corresponding to the crystal planes. However, for α-HH prepared under different conditions, the peak at 2θ (020) shifted to 25.78°, and three peaks at 2θ = 14.86° (200), 29.69° (400), and 31.86° (204) appeared. (200), (020), (400) crystal planes were parallel to the c-axis, and (204) crystal plane was parallel to the b-axis. Peak positions and parameters of crystals were in a good agreement with appropriate JCPDS (75–0250) data of α-CaSO4·1/2H2O and indicated a high degree of crystallinity. And no other crystalline phase of CaSO4·2H2O were observed (Fig. 1a-c). When the reaction time prolonged form 5 min to 90 min, no characteristic diffraction peaks of CaSO4·2H2O had been observed except those of α-CaSO4·1/2H2O (Fig. S1). Increasing sodium citrate concentrations and prolonging reaction time were favorable for formation of α-HH with higher crystallization. And in XRD spectra the intensity of diffraction peaks of α-CaSO4·1/2H2O continued to increase a little when the reaction time ranged in 10–60 min. This result may happen due to the higher crystallization of α-HH when the concentration of sodium citrate reached 3.40 × 10−2 mol/L20.

(A): The photo of the brownish yellow sediments by neutralizing acidic wastewater with lime mud. (B): XRD patterns of CaSO4·2H2O (d) and α-HH prepared at different sodium citrate concentrations: (a) 0; (b) 1.70 × 10−2 mol/L; (c) 3.40 × 10−2 mol/L.

To further identify the chemical composition of the obtained products α-HH by hydrothermal preparation, TG and DSC were employed to characterize CaSO4·2H2O and α-HH produced without addition of sodium citrate. In TGA curves of Fig. 2, the weightlessness of α-HH in the temperature range from 125 ℃ to 200 ℃ was approximately 6.38%, which was very close to the theoretical content of water of crystallization of α-HH. The inert residue of 19.87% was left in the TGA curve of α-HH, which may be related to the impurities of the used quick lime mud. More interesting, the purity of the obtained α-HH would increase form 72% to above 98% if using pure calcium oxide instead of quick lime mud.

TG (Above) and DSC (Below) curves of CaSO4·2H2O (b) and α-HH prepared without addition of sodium citrate (a).

In the DSC curves, CaSO4·2H2O exhibited two endothermic peaks at 147.6 ℃ and 167.7 ℃, correspond to the removal of 1.5 water molecules from CaSO4·2H2O to form β-calcium sulfate hemihydrate (β-HH) and continuedly dehydration of 0.5 water molecules to form soluble anhydrous gypsum (γ-CaSO4)21. The exothermic peak at 365.1 ℃ corresponded to the conversion of γ-CaSO4 to insoluble anhydrous gypsum (β-CaSO4). However, the aforementioned characteristic peaks of CaSO4·2H2O did not emerged in the DSC curve of the α-HH, indicating that there was no evidence of CaSO4·2H2O produced during the whole reaction process. There was only an endothermic peak at 162.2 ℃, and no exothermic peak at about 370 ℃, which could draw a conclusion that the main composition of the obtained α-HH products was CaSO4·1/2H2O based on Chen’s work21. The results of TG and DSC analysis were consistent with those of XRD, which confirmed that α-HH could be fast, efficiently and selectively prepared by neutralizing TWWA with quick lime mud in hydrothermal system at 0.5 MPa and 140℃ with additionsodium citrate as crystal modifier, during which no CaSO4∙2H2O was generated as an intermediate product.

Figure 3 showed the FT-IR spectra of α-HH prepared with and without addition of sodium citrate, respectively. As shown in Fig. 3(a), the peaks at 3616 cm-1, 3553 cm-1 and 1622 cm-1 should be assigned to the O − H vibration of the crystal water molecule in the α-HH crystals. The triple peaks near 1156 cm-1 could be due to υ3SO42- stretching. The peak at 1005 cm-1 should be due to stretching of υ1SO42-. The peaks at 659 and 600 cm-1 should be attributed to the stretching of υ4SO42- 22,23. As shown in Fig. 3(b), the peak at 3402 cm-1 was generated in the presence of sodium citrate by the stretching of O − H in sodium citrate. Two adsorption peaks were observed in the range of 2800 to 3000 cm-1, which should be attributed to the asymmetric (2974 cm-1) and symmetric (2924 cm-1) stretching vibrations of methylene (− CH2−). These results further confirmed the interaction between sodium citrate and α-HH crystals to preferentially adsorb on the surfaces of α-HH crystals.

FT-IR spectra of α-HH prepared with and without addition of sodium citrate: (a) 0; (b) 3.40 × 10−2 mol/L.

X-ray photoelectron spectroscopy (XPS) was performed to seek further evidence of whether and where sodium citrate molecules adsorbed on α-HH surfaces. The C1s peak at 284.8 eV, which is related to the surface adsorption of carbon during exposure of the sample to the ambient atmosphere, was used as a reference. Figure 4 depicted the C1s XPS spectra of α-HH formed with and without addition of sodium citrate. In the absence of sodium citrate, the C1s peak at 284.8 eV could be attributed to carbon contamination24. With the addition of sodium citrate, the intensity of C1s peak at 284.8 eV increased significantly, which originated from the carbon atoms in the hydrocarbon chain of sodium citrate. In addition, a new C1s peak appeared at 288.5 eV, which should be assigned to the carboxyl or carboxylate groups of sodium citrate molecules25,26.

C1s XPS spectra of the α-HH prepared with and without addition of sodium citrate: (a) 0; (b) 3.40 × 10−2 mol/L.

The FT-IR and XPS results demonstrated that sodium citrate authentically adsorbed on surfaces of α-HH crystal. But these results could not quantify the difference in the adsorption sites of sodium citrate on α-HH crystal planes. Here, the EDS data was used to analyze the elemental contents of the different α-HH crystal planes prepared in the presence of 3.40 × 10−2 mol/L sodium citrate. Results in Fig. 5 showed that in a well-developed α-HH crystal, area B in the terminal plane contained larger contents of carbon and oxygen compared with area A selected in the side plane. That is, sodium citrate molecules tend to absorb in the terminal planes than not in the side planes during the epitaxial growth process of α-HH crystals with sodium citrate as a crystal modifier.

SEM-EDS analysis of the α-HH crystals prepared in the presence of 3.40 × 10−2 mol/L sodium citrate.

SEM images of the obtained α-HH crystals in the presence of the different concentrations of sodium citrate were presented in Fig. 6. It could be seen from Fig. 3a that the length and diameter of α-HH crystals was both small, showing columnar or rod-like shapewithout addition of sodium citrate. As the concentration of sodium citrate used was extended from 0 to 3.40 × 10−2 mol/L, the lengthof α-HH samples gradually became shorter and the diameter increased slightly. The morphology of α-HH crystals gradually changed to short columnar shape with more uniform size (Fig. 3b-e). However, the length of the columnarα-HH crystals became shorter, and the diameterof the crystals remain uneven when the concentration of sodium citrate was more than 3.40 × 10−2 mol/L. The flaky crystals appeared, too. Short columnarα-HH crystals with rough surface, uniform size, diameter of 0.8 μm and length of 2.0 μm could be obtained when sodium citrate was added under 3.40 × 10−2 mol/L. And the average aspect ratio of the columnar α-HH crystals reached the minimum values of 2.36.

(Left): SEM images of α-HH crystals prepared under different sodium citrate concentrations, (a) 0; (b) 0.85 × 10−2 mol/L; (c) 1.70 × 10−2 mol/L; (d) 2.55 × 10−2 mol/L; (e) 3.40 × 10−2 mol/L; (f) 4.25 × 10−2 mol/L. (Right): Average length, diameters, and aspect ratio of the α-HH.

The preferential adsorption of sodium citrate molecules on the terminal planes of the α-HH crystals played a crucial role in the growth of α-HH with a short columnar morphology. However, the reason for why sodium citrate inclined to adsorb in the α-HH terminal planes remained to be explored. Sun et al.27 showed the crystal lattice of α-HH consists of a −Ca−SO4−Ca−SO4− chains synthesized by repeating ionic bonds between Ca2+ and SO42- atoms, in which each S atom is covalently bonded to four O atoms to form a tetrahedral angle. The chain structure may be able to explain the fact that α-CaSO4·0.5H2O crystals normally grow in the one-dimensionally along the c-axis direction (Fig. 7A). These chains are arranged hexagonally and form a scaffold parallel to the c-axis with continuous channels with a diameter of about 4.5 Å, in each of which one water molecule is bound to two calcium sulfate molecules28. The structure shows a denser distribution of SO42- ions on the side planes of (200) and (020) and a denser distribution of Ca2+ ions on the terminal planes of (204), with the parallel channels partially filled with water molecules. The crystals also have an electrostatic charge, which is transferred to water molecules by the dipole resulting from the preferential orientation29. Therefore, the (200) and (020) planes of the α-HH crystal are negatively charged and the (204) planes are positively charged, resulting in a stronger sodium citrate adsorption on the terminal planes than that on the side planes. As can be seen in Fig. 7(B), once sodium citrate took precedence to occupy the terminal planes of the growing α-HH crystal, the absorbed carboxylic acid ions from sodium citrate would complete with crystal elements for Ca2+ binding and further growth, which resulted that the growth rate of α-HH crystals along the c-axis was minimized. And the (204) planes adsorbed preferentially sodium citrate molecules had lower surface free energy compared to that of the (200) and (020) planes. The α-HH crystal elements then tend to grow along the (200) and (020) planes30, and lead to the formation of short columnar α-HH crystals finally.

(A): Crystal structure of α-HH lattice along c axis. (B): Schematic diagram of the preparation of short columnar α-HH.

As said above, the purity of the prepared α-calcium sulfate hemihydrate (α-HH) samples was affected by the purity of the used alkaline substances. The purity of α-HH samples was estimated to be 72–74% when using quick lime mud in which the active ingredient of CaO was about 62.94% that was lower than commercially available high-strength gypsum. But the setting time of the α-HH samples and high-strength gypsum was both 15 min, which indicating that the inert materials in the α-HH samples didn’t influence the setting process. The compressive strength of the test blocks mixed with α-HH samples and pure high-strength gypsum was tested accordingto GB/T 17669.3–1999 standard, and the results were 6.35 and 10.62 MPa, respectively. As we known, the standard of compressive strength for the first level of calcined gypsum was above 4.9 MPa. Therefore, the mechanical performance of the α-HH samples reached the requirements of first level in the GB/T 9776 − 2008“calcined gypsum”.

Production of high-strength gypsum (α-CaSO4∙1/2H2O) from commercial reagents like sulfuric acid and quick lime is obviously economically ineffective due to the high cost and low profit of gypsum. Therefore, the considered option of titanium white waste acid and lime mud is the most pragmatic choice. The proposed green technology for a one-stage synthesis of α-calcium sulfate hemihydrate using the spent TWWA and lime mud is shown in Fig. 8. And the cost of preparation of short columnar α-HH was estimated to be 380 RMB/ton, which was significantly lower 1000 RMB/ton than the fee of treating and storing TWWA and lime mud. In the technology of obtaining α-HH from industrial wastes, in addition to one ton of the target product, 17 tons of a liquid phase was also formed, which was proposed of residual sodium citrate and traces of metal ions. The payments of adsorption treating of the liquid phase were 25.5 RMB to meet the production water standards for titanium dioxide enterprises. Currently, the raw materials for building gypsum plaster include three types: CaSO4·2H2O, β-CaSO4·1/2H2O, and α-CaSO4·1/2H2O. The domestic market prices are 300–500 RMB/ton, 500–800 RMB/ton, and 1000–1800 RMB/ton respectively. Therefore, the proposed approach of one-stage synthesis of α-HH using the spent TWWA and lime mud exhibited notable competitive advantages in the target market of building gypsum plaster.

Schematic illustration for a one-stage synthesis of α-calcium sulfate hemihydrate using the spent TWWA and lime mud.

In this study, α-calcium sulfate hemihydrate crystals with short columnar shape were prepared using TWWA and lime mud as raw materials from the production of titanium dioxide. It was found that one-stage direct synthesis of α-HH can be performed in the presence of sodium citrate as a crystal modifier at 140 ℃ and 0.5 MPa pressure. The effect of the concentration of added sodium citrate effected the chemical composition and morphology of the α-HH crystals. And the average aspect ratio of α-HH crystals decreased to 2.4. According to GB/T 9776 − 2008 “Calcined gypsum”, the setting time and compressive strength of 7 days of the cemented short-columnar α-HH was estimated about 15 min and 6.35 MPa, which approached or even exceed the required standards of the first lever of calcined gypsum. A number of ecological and economic indicators showed that such an approach is advantageous from a technological, environmental, and economic point of view.

Sodium citrate tribasic dehydrate (C6H5Na3O7·2H2O, ≥ 99.0%, devoted as sodium citrate), calcium sulfate dehydrate (CaSO4·2H2O, ≥ 99.0%) and anhydrous ethanol(CH3CH2OH, AR, ≥ 99.7%) were purchased from Sinopharm Chemical Reagent Co. (Shanghai, China). Aluminium potassium sulfate dodecahydrate (KAl(SO4)2·12H2O, 99.5%) was purchased from MACKLIN Co. (Shanghai, China). A high-strength gypsum sample (industrial grade, α-CaSO4•1/2H2O) was received from Kebeisi Co. (Beijing, China). Quick lime mud (CaO, 62.94%) and titanium white waste acid were presented from Fangyuan Titanium Dioxide Co. (Wuhan, China) as raw materials. And the concentration of SO42- as the main ion in TWWA were determined to be 30.68% by titration. All reagents and raw materials were used as received.

The α-calcium sulfate hemihydrate (α-HH) was prepared via hydrothermal synthesis technique from the acid wastes. Briefly, 2 g KAl(SO4)2·12H2O (1.06 × 10−2 mol/L) and a certain amount of C6H5Na3O7·2H2O were added to the mixture solution of ground lime mud (14.2 g, containing 0.16 mol CaO) and deionized water (332.82 mL ) in a reaction kettle. Under continuous N2 flow to 0.5 MPa, the solution was heated to 140 ℃ with stirring of 600 rpm. Then 50.98 g TWWA (containing 0.16 mol SO42-) were added along the feeding port into the reaction kettle and reacted over 10 min.

The reaction solution was next transferred to a Buchner funnel, filtered by suction, pressed out and the solid product was collected. The crude productof α-calcium sulfate hemihydrate (α-HH) was cleaning with anhydrous ethanol until the pH values of clear liquid reached to 7, and then dried at 80 ℃ under vacuum. The yield of α-HH can be expressed as follows:

The purified product, α-HH, was dried under vacuum at least two hours before characterization. X-ray diffraction (XRD) analysis of CaSO4·2H2O and the synthesizedα-HH were measured using a Bruker D8 Advance diffractometer with Cu Kα radiation. The scanning rate was 8°·min-1and the diffraction angle of 2θ was from 10° to 40°. Thermogravimetric analysis (TGA) of products were carried out on a TGA 1 Thermogravimetric Analyzer (Mettler Toledo, Switzerland) from 50 to 400 °C at a rate of 10 °C min−1 under a nitrogen atmosphere. And differential scanning calorimetry (DSC, Q2000, TA Instrument, USA) were performed from 50 to 400 °C at a rate of 10 °C min−1.

Scanning electron microscopy (SEM-EDS, Sigma500, Germany) analysis of α-HH were performed to investigate morphology and elemental composition.The microstructure parameter of α-HH, such as average length, diameters and aspect ratios, were analyzed using Nanosizer software to collect around 100 complete α-HH particles in SEM images.The FT-IR spectra of α-HH samples were taken with a Nicolet iS50 (Thermo Fisher, USA) spectrometer in the wave number region of 400–4000 cm-1. X-ray photoelectron spectroscopy (XPS) of α-HH samples was performed with the ESCALAB 250XiXPS (Thermo Fisher, USA).

The high-strength gypsum powder as well as the obtainedα-HHproduct was mixed with water according to 5:4 mass ratios, respectively. The setting time was evaluated according to the standard GB/T 17669.4–1999 “Gypsum plaster–Determination of physical properties”. And after aging 7 days, the compressive strength of the test blocks was tested according tothe standard GB/T 17669.3–1999 “Gypsum plaster–Determination of mechanical properties”.

All data used and analyzed during the current study are available from the corresponding author on reasonable request.

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This study received the financial support from the Chinese National Program for High Technology Research and Development (Grant number 2013AA032302).

Ministry of Education Key Laboratory for the Green Preparation and Application of Functional Materials, School of Materials Science and Engineering, Hubei University, Wuhan, 430062, Hubei, China

Xiaoxue Hu, Xunzi Zhang, Yiwei Dong, Luwei Chen, Tingting Yang, ChangfengYi & Qing Gao

Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Wuhan, 430062, Hubei, China

ChangfengYi & Qing Gao

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X X Hu and X Z Zhang: Data curation, writing original draft and scheme drawing; Y W Dong: Conduction of samples synthesis; L W Chen: SEM and EDX Characterization; T T Yang: Collection and analysis of data, read and approved the final manuscript, as well as funding acquisition; F C Yi: Assistance the thermal analysis; Qing Gao: Experimental design and supervision, writing (review and editing).

Correspondence to Tingting Yang or Qing Gao.

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Hu, X., Zhang, X., Dong, Y. et al. One-step synthesis of short columnar α-calcium sulfate hemihydrate from titanium white waste acid. Sci Rep 14, 24809 (2024). https://doi.org/10.1038/s41598-024-75949-2

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Received: 18 April 2024

Accepted: 09 October 2024

Published: 22 October 2024

DOI: https://doi.org/10.1038/s41598-024-75949-2

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