Dissolution Amplification by Resonance and Cavitational Stimulation at Ultrasonic and Megasonic Frequencies

Acoustic stimulation offers a green pathway for the extraction of valuable elements such as Si, Ca, and Mg via solubilization of minerals and industrial waste materials. Prior studies have focused on the use of ultrasonic frequencies (20-40 kHz) to stimulate dissolution, but mega sonic frequencies (≥1 MHz) offer benefits such as matching of the resonance frequencies of solute particles and an increased frequency of cavitation events. Here, based on dissolution tests of a series of minerals, it is found that dissolution under resonance conditions produced dissolution enhancements between 4x-to-6x in Si-rich materials (obsidian, albite, and quartz). Cavitational collapse induced by ultrasonic stimulation was more effective for Ca- and Mg-rich carbonate precursors (calcite and dolomite), exhibiting a significant increase in the dissolution rate as the particle size was reduced (i.e. available surface area was increased), resulting in up to a 70x increase in the dissolution rate of calcite when compared to unstimulated dissolution for particles with d50\u3c 100 μm. Cavitational collapse induced by mega sonic stimulation caused a greater dissolution enhancement than ultrasonic stimulation (1.5x vs 1.3x) for amorphous class F fly ash, despite its higher Si content because the hollow particle structure was susceptible to breakage by the rapid and high number of lower-energy mega sonic cavitation events. These results are consistent with the cavitational collapse energy following a normal distribution of energy release, with more cavitation events possessing sufficient energy to break Ca-O and Mg-O bonds than Si-O bonds, the latter of which has a bond energy approximately double the others. The effectiveness of ultrasonic dissolution enhancement increased exponentially with decreasing stacking fault energy (i.e., resistance to the creation of surface faults such as pits and dislocations), while, in turn, the effectiveness of mega sonic dissolution increased linearly with the stacking fault energy. These results give new insights into the use of acoustic frequency selections for accelerating elemental release from solutes by the use of acoustic perturbation

Cooling-Rate Effects in Sodium Silicate Glasses: Bridging the Gap between Molecular Dynamics Simulations and Experiments

Although molecular dynamics (MD) simulations are commonly used to predict the structure and properties of glasses, they are intrinsically limited to short time scales, necessitating the use of fast cooling rates. It is therefore challenging to compare results from MD simulations to experimental results for glasses cooled on typical laboratory time scales. Based on MD simulations of a sodium silicate glass with varying cooling rate (from 0.01 to 100 K/ps), here we show that thermal history primarily affects the medium-range order structure, while the short-range order is largely unaffected over the range of cooling rates simulated. This results in a decoupling between the enthalpy and volume relaxation functions, where the enthalpy quickly plateaus as the cooling rate decreases, whereas density exhibits a slower relaxation. Finally, we demonstrate that the outcomes of MD simulations can be meaningfully compared to experimental values if properly extrapolated to slower cooling rates

On the Application of Inertial Microfluidics for the Size-Based Separation of Polydisperse Cementitious Particulates

The early‐age performance of concrete is determined by the properties of the cementitious binder and the evolution of its chemical reactions. The chemical reactivity, and to some extent, the composition of cementitious particles can depend on particle size. Therefore, it is valuable to physically separate cementing minerals into well‐defined size classes so that the influences of both particle size and composition on reaction progress can be studied without the confounding effects of a broad particle size distribution. However, conventional particle separation methods (e.g., density fractionation, wet sieving, field-flow extraction, ultrasonification-sedimentation) are time-consuming and cumbersome and result in poor particle yields and size-selectivity, thus, making them unsuitable for processing larger volumes of cementitious powders (on the order of grams). This study applies a novel inertial microfluidics (IMF) based procedure to separate cementitious powders on the basis of their size. Special attention is paid to optimizing operating variables to ensure that particles in a fluid streamline achieve unique equilibrium positions within the device. From such positions, particles can be retrieved as per their size using symmetrical outlet configurations with tuned fluidic resistances. The approach is critically assessed in terms of: (1) its ability to separate cementitious powders into narrow size bins, and therefore its feasibility as a fractionation procedure, and (2) quantitatively relating the operating parameters to the particle yield and size selectivity. The study establishes metrics for assessing the ability of IMF methods to classify minerals and other polydisperse particles on the basis of their size

Enthalpy Landscape Dictates the Irradiation-Induced Disordering of Quartz

Under irradiation, minerals tend to experience an accumulation of structural defects, ultimately leading to a disordered atomic network. Despite the critical importance of understanding and predicting irradiation-induced damage, the physical origin of the initiation and saturation of defects remains poorly understood. Here, based on molecular dynamics simulations of α-quartz, we show that the topography of the enthalpy landscape governs irradiation-induced disordering. Specifically, we show that such disordering differs from that observed upon vitrification in that, prior to saturation, irradiated quartz accesses forbidden regions of the enthalpy landscape, i.e., those that are inaccessible by simply heating and cooling. Furthermore, we demonstrate that damage saturates when the system accesses a local region of the enthalpy landscape corresponding to the configuration of an allowable liquid. At this stage, a sudden decrease in the heights of the energy barriers enhances relaxation, thereby preventing any further accumulation of defects and resulting in a defect-saturated disordered state

How Brine Composition Affects Fly Ash Reactions: The Influence of (Cat-, An-)ion Type

Hypersaline brines can be solidified and stabilized via the hydraulic and pozzolanic reactions between fly ash(es) and calcium-based additives. Although recent work has examined fly ash reactivity in single-salt ("simple") hypersaline brines (ionic strength, Im > 1 mol/L), the effects of mixed-salt solutions on fly ash reactivity remain unclear. Herein, the reactivity of a Class C (calcium oxide [CaO]-rich) or Class F (CaO-poor) fly ash mixture with calcium hydroxide is reacted in solutions bearing sodium chloride (NaCl), calcium chloride (CaCl2), magnesium chloride (MgCl2), sodium sulfate (Na2SO4), or combinations thereof for 1.5 ≤ Im ≤ 2.25 mol/L, from 1 week until 24 weeks. Expectedly, sulfate anions promote the formation of sulfate phases (i.e., ettringite, monosulfoaluminate, U-phase), while chloride anions induce the formation of Cl-AFm compounds (i.e., Kuzel's and Friedel's salt). Although the Class C fly ash's reactivity is similar across different anions (for a fixed cation and Im), Class F fly ash shows a small change in reactivity depending on the anion present. NaCl suppresses (Class C and Class F) fly ash reactivity by up to 30 % as compared to neat CaCl2 and MgCl2-based brines. Thermodynamic modeling reveals that NaCl induces a considerable increase in pH-up to 13.7, where many hydrated phases of interest cease to be the major phase expected-as compared to CaCl2 and MgCl2 brines (pH < 13). In mixed-salt brines, anion immobilization is competitive: Sulfate achieves a greater level of incorporation into the hydrates, as compared to chloride. These results offer new understanding of how the brine composition affects solidification and stabilization and thereby yield new insight into improved approaches for wastewater disposal

Fly Ash–Ca(OH)2 Reactivity in Hypersaline NaCl and CaCl2 Brines

The disposal of highly concentrated brines from coal power generation can be effectively accomplished by physical solidification and chemical stabilization (S&S) processes that utilize fly ashes as a reactant. Herein, pozzolanic fly ashes are typically combined with calcium-based additives to achieve S&S. While the reactions of fly ash-(cement)-water systems have been extensively studied, the reactivity of fly ashes in hypersaline brines (ionic strength, Im > 1 mol/L) is comparatively less understood. Therefore, the interactions of a Class C (Ca-rich) fly ash and a Class F (Ca-poor) fly ash were examined in the presence of Ca(OH)2, and their thermodynamic phase equilibria were modeled on contact with NaCl or CaCl2 brines for 0 ≤ Im ≤ 7.5 mol/L. At low ionic strengths (<0.3 mol/L), reactivity and stable phase assemblages remain effectively unaltered. However, at high(er) ionic strengths (>0.5 mol/L), the phase assemblage shows a particular abundance of Cl-AFm compounds (i.e., Kuzel's and Friedel's salts). Although formation of Kuzel's and Friedel's salts enhances the Class F fly ash reactions in both NaCl and CaCl2 brines, NaCl brines compromise Class C fly ash reactivity substantially, while CaCl2 results in the reactivity remaining essentially unchanged. Thermodynamic modeling that accounts for the fractional and noncongruent dissolution of the fly ashes indicates that their differences in reaction behavior are provoked by differences in the prevalent pore solution pH, which affects phase stability. The outcomes offer new insights for matching fly ashes, Ca additives, and brines, and accounting for and controlling fly ash-brine interactions as relevant to optimizing physical solidification and chemical stabilization applications