Elsevier

Hydrometallurgy

Volume 195, August 2020, 105342
Hydrometallurgy

Cryogenic-crosslink synthesis of Ba(OH)2@gelatin microcapsules for the removal of sulfate in lithium refinery

https://doi.org/10.1016/j.hydromet.2020.105342Get rights and content

Highlights

  • Synthesized Ba(OH)2@gelatin microcapsules by cryogenic-crosslink method

  • The microcapsules can remove SO42− to below 10 ppm to produce battery grade LiOH.

  • The SO42− loaded microcapsules can be easily separated from treated LiOH solution.

  • Secondary contamination of overdosed Ba2+ can be avoided.

Abstract

Sulfate (SO42−) is a major impurity in the production of battery-grade LiOH from acid leaching of ores using H2SO4. The conventional process to remove SO42− by precipitation with Ba2+, however, is difficult to control stoichiometrically in industry, and overdosed Ba2+ frequently causes secondary contaminations. Moreover, fine particles of BaSO4 precipitate require extra steps to remove, which significantly increases the cost of separation. To overcome these limitations, we developed a novel adsorbent, the Ba(OH)2@gelatin microcapsule, by embedding Ba(OH)2 inside the matrix of gelatin through a special cryogenic-crosslink method using liquid nitrogen to snap-freeze the gelatin-Ba(OH)2-water droplets followed by glutaraldehyde solutions to crosslink. The physical properties and SO42− adsorption performance were investigated to testify the microcapsules' potential in industrial removal of SO42− to below the 20 ppm threshold. The microcapsules can withstand a pressing force of 4 N and a maximum deformation of 73% on average, showing considerable physical strength and elasticity. It served as an effective domain for SO42− adsorption, in the best scenario reducing SO42− from 200 to 10 ppm, following a pseudo-second order kinetic model. The SO42− loaded Ba(OH)2@gelatin microcapsules can be easily removed from the treated solution and the potential contaminations of BaSO4 precipitate and overdosed Ba2+ can be avoided. The spent gelatin microcapsules can be regenerated by reloading with Ba2+ and recover the SO42− removal capacity for multiple cycles.

Introduction

The lithium battery has major applications in cars, computers, telephones, and other automatic equipments, with a demand growth of approximately 10% annually (Seddon and Economics: growth in lithium demand, 2018). The sulfuric acid leaching process is a classic method to extract lithium from spodumene and other ores (Meshram et al., 2014). In a typical industrial process, β-spodumene is treated with concentrated sulfuric acid to produce lithium sulfate (Li2SO4), which is then reacted with sodium carbonate or calcium hydroxide to produce lithium carbonate or lithium hydroxide (LiOH) (Tam Tran, 2015; Kuang et al., 2018). Therefore, sulfate (SO42−) is the major impurity in the production of battery-grade lithium compounds and it needs to be removed to less than 0.01 wt% in the solid product, equivalent to about 20 ppm in the solution (Boryta et al., 2005).

Sulfate ions can be removed by a wide variety of methods, such as chemical precipitation, ion exchange, membrane filtration, and electrodialysis. Anion exchange resin recovers SO42− from wastewater by elution of alkaline solution (Guimarães and Leão, 2014), while nanofiltration and electrodialysis separate SO42− using membrane systems (Galiana-Aleixandre et al., 2005; Pulkka et al., 2014). However, both ion exchange resin and membrane are susceptible to fouling, difficult to reach the 20 ppm limit, and their industrial applications are usually expensive. Comparatively, chemical precipitation is more straightforward, removing SO42− based on the low solubility of CaSO4 or BaSO4 (Sohr et al., 2017). Precipitation by excessive BaCl2 could achieve nearly 100% removal ratio for high concentration SO42− from pigment industry effluent (Navamani Kartic et al., 2018). The SO42− in wet flue gas desulfurization wastewater can be removed by the mixture of Ca(OH)2 and NaAlO2, reducing the SO42− from nearly 5000 ppm to less than 1000 ppm (Yu et al., 2018). Grageda et al. applied the precipitation process to remove major impurities from lithium brine, in which SO42− was precipitated by BaCl2, followed by ion exchange to further remove the remaining Ca2+ and Mg2+ cations (Grágeda et al., 2018). Although soluble barium salt/alkaline is commonly used as precipitant, it suffers from the limitation of secondary contaminations of overdosed Ba2+ and resultant BaSO4 precipitates in operation. The BaSO4 precipitates in the form of fine flocculent particles need to be removed by further separation processes such as nanofiltration, greatly increasing the capital and operation cost of the plant.

To overcome the limitation, we designed a novel adsorbent material with immobilized barium cation by using gelatin as the domain to encapsulate barium hydroxide (Ba(OH)2). Gelatin is a water-soluble biopolymer derived from irreversible hydrolysis of collagen, composed of many-sized polypeptides (Baier and Zisman, 1975). It can be blended with other chemicals to produce gelatin-based particles or membranes for adsorption or reaction. For instance, Chen et al. synthesized magnetic gelatin with iron chloride, which was highly magnetized and exhibited good performance in Cr (VI) adsorption from water (Chen et al., 2014). Cui et al. prepared gold nanocomposites coated with gelatin for bioreactions, in which the gold precursor was reduced to gold nanoparticles in situ (Cui et al., 2014). Through blending with gelatin, polymer films achieved better tensile strength, higher surface porosity, and reduced crystallinity, making it more appropriate for biomedical applications (Wang et al., 2005). Gelatin was also served as the base material in some multilayer scaffolds due to its good biocompatibility and capability for encapsulation of bioagents (Samal et al., 2015). In brief, gelatin is a cheap and easy material to be used as the domain to carry or encapsulate chemicals.

In this study, we encapsulated Ba(OH)2 into gelatin-based microcapsules to perform SO42− adsorption. The Ba(OH)2@gelatin microcapsules were prepared by a novel cryogenic-crosslink method first developed in this work, which may find applications in the preparation of many other encapsulated materials. The retention percentage of Ba2+ was kept as high as approximately 90% during the whole preparation process. The microcapsules showed a strong mechanical strength for packing and high efficiency to reduce SO42− concentration in aqueous solution to below 20 ppm. Importantly, the gelatin-based capsules can keep the Ba2+ precipitant and the BaSO4 precipitate immobilized, effectively preventing secondary contamination of the product solution. The microcapsules were recycled and reused multiple times.

Section snippets

Chemicals

The Ba(OH)2·8H2O (99.99 wt%) and Li2SO4·H2O (99.99 wt%) were purchased from Sigma-Aldrich. The glutaraldehyde (GA, 25 wt%) and gelatin powder (analytical reagent) were purchased from Thermo Fisher Scientific.

Synthesis of Ba(OH)2@gelatin microcapsule

The Ba(OH)2@gelatin microcapsules were prepared through the process in Fig. 1. Gelatin was dissolved into 10 mL water at 80 °C for 8 min, followed by stirring at the third minute to remove the air bubbles. Ba(OH)2 was dissolved into the gelatin-water mixture at 80 °C for 1 min to get the

Cryogenic-crosslink process for microcapsule synthesis

The motivation of the microcapsule preparation step was to find a way to let the droplets of the gelation solution set while holding its spherical morphology and not losing the encapsulated Ba(OH)2. However, dripping the gelatin solution directly into the GA solution formed sludge rather than microcapsules because the crosslink reaction was not fast enough to stop the mixing of both solutions in an aqueous system. Therefore, a novel cryogenic-crosslink method was developed in this work to

Conclusion

We have prepared Ba(OH)2@gelatin microcapsules with high physical strength and flexibility by a novel cryogenic-crosslink pathway to remove SO42− for refinery of battery-grade LiOH. In these microcapsules, barium was encapsulated into gelatin matrix and effectively reduced the SO42− concentration to below 20 ppm without introducing Ba2+ into the solution, satisfying the requirement of battery-grade LiOH refinement. The optimal weight ratio of Ba/gelatin for SO42− adsorption was 0.24/2.5. The

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.

Acknowledgement

This work was performed in part at the Materials Characterization and Fabrication Platform (MCFP) at the University of Melbourne and the Victorian Node of the Australian National Fabrication Facility (ANFF).

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