Luo, Jiean (2022) Fractional Recovery of Minerals from Waste Brines by Utilizing Metastable Solution Kinetic Behaviours. PhD thesis, Victoria University.

Abstract

Capturing minerals from waste brines such as seawater reverse osmosis concentrate can sustainably provide compounds critical to modern society, such as K2SO4, Li2(CO3), and MgCl2. While the precipitation of these minerals from complex mixtures to capture them in pure form is challenging, new insights into their fractional separation may be possible by investigating the non-equilibrium phase behaviour and phase changing kinetics in multi-ion saline systems which cannot be predicted from widely reported phase boundaries. The overall objective of this thesis was therefore to further the knowledge of the linkage between precipitation system operating conditions and metastable phase behaviours, which can be used for the mineral fractionation in multi-ion electrolyte systems. A potash brine system containing K, Na, Mg, SO4 and Cl was chosen as the research focus, and reflects the typical ion set in the majority of waste brine resources. The research in this thesis was conducted by three experimental phases:  Kinetics and solubility in a simple 3-ion system of Na–K–Cl: In this experimental phase, the linkage between selectivity of the targeted K ion and operational condition in dissolution was quantitatively measured by dissolving a KCl–NaCl mixture in hot water with different temperature. While the precipitation kinetics was measured under different cooling rates and initial brine ion compositions.  Kinetics and solubility in the complex 4-ion Na-K–Mg–Cl and 5-ion Na–K–Mg–SO4–Cl systems: This experimental phase explored the impact of metastable phase behaviour on the phase changing process in these more complex 4-ion and 5-ion systems. The impact of different precipitation conditions (e.g. cooling rate and dewatering rate) on stability of non-equilibrium metastable phase was quantitatively evaluated by comparing solution and solid phase components with equilibrium phase diagram.  K products formation using synthesised intermediates: In the final test, K salts of commercial interest like SOP (K2SO4) and Glaserite (K3Na(SO4)2) were synthesised via phase reaction from product intermediates(e.g. KCl and Leonite (K2Mg(SO4)2.4H2O)) formed from the above two experimental investigations. The result can justify the feasibility of the proposed fraction method in production. The precipitation kinetics' impact on mineral fractionations was revealed by detecting the precipitated solid phase components using X-ray powder diffraction (XRD). To precisely measure kinetic behaviours, this study featured a novel analysis involving in-situ microscopy to reveal the crystals' morphology and assembly status within the mother liquor. The solution components were measured by an inductively coupled plasma optical emission spectroscopy detector (ICP-OES) and a chloride ion-selective electrode (ISE-Cl) probe. The experimental results found the rapidly changing temperature was found to be able to achieve mineral fractionation in KCl-NaCl rich systems, while the metastable phase behaviour could delay the formation of certain phases for hours and provided opportunities for mineral separations, moreover, the mineral formation kinetics can restrict the number of components in solid phase for a better product purity. These discoveries provided insights into fundamental chemistry on metastable phase behaviour and phase changing kinetics and led to the proposed practical recommendations for the fractional recovery of mineral from waste brines.

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