Rational design of functionalized ionic liquids for silica gel suppression during leaching of coal fly ash: COSMO-RS guided screening

Abstract

Ionic liquids (ILs) have attracted considerable attention as green, tunable solvents for hydrometallurgical processes. Coal fly ash (CFA) has emerged as a promising secondary resource of rare earth elements (REEs). However, during conventional solvent leaching of rich-silica CFA, dissolved silica polymerizes into a dense gel. This silica gelation entraps liberated REEs, increases viscosity, and causes filtration obstructions, presenting a major bottleneck for efficient REE recovery. To overcome this challenge, specifically designed ILs can hinder gelation by interacting with monomeric silicic acid. Because testing millions of possible cation-anion combinations by trial and error is practically unfeasible and resource-intensive, a predictive computational framework is required. In this study, the conductor-like screening model for real solvents (COSMO-RS) was applied to computationally pre-screen 504 IL combinations, derived from 12 cations and 42 anions, for their thermodynamic affinity toward silicic acid. The potential efficacy of each IL design was evaluated based on activity coefficients, selectivity, capacity, and performance index. Specifically, tetrabutylammonium citrate (CA3, AN39), tetrabutylammonium oxalate (CA3,AN38), 1-butyl-1-methylpyrrolidinium acetate (CA10, AN20) emerged as the most efficient formulations among the 504 investigated ILs for suppressing silica gelation. This framework provides a rational, molecular-level blueprint for selecting ILs that suppress silica gelation, thereby enabling efficient hydrometallurgical recycling of industrial waste.