Study on Pore-Scale Reactive Transport Model for Acid In-Situ Leaching of Uranium Based on the Lattice Boltzmann Method

[Background]: Reactive transport processes involved in acid in-situ leaching of sandstone uranium deposits lead to changes in pore structure and reactive transport parameters, potentially causing pore blockage. [Purpose]: This study aims to conduct pore-scale simulations of multi-component reactive transport, providing a methodological reference for investigating the mechanisms of pore structure evolution and changes in reactive transport parameters during acid in-situ leaching. A pore-scale reactive transport model based on the lattice Boltzmann method (LBM) is used to directly simulate these evolutions. [Methods]: Fluid and solute transport were modeled using the D2Q9 and D2Q5 LBM models, respectively. Chemical reactions were coupled to simulate the dissolution of calcium carbonate minerals and the formation of gypsum precipitation in porous media. The pore structure was updated based on mineral concentration thresholds, simulating the evolution of solute concentration, pore structure, and flow fields as reactive transport progressed. Benchmark tests for calcium carbonate dissolution and gypsum formation were conducted to validate the model's accuracy. Image analysis was applied to capture the evolution of key reactive transport parameters in the porous media. [Results]: The results demonstrate that the LBM effectively simulates the evolution of pore structure, flow fields, solute concentration distributions, and reactive transport parameters over leaching time in heterogeneous porous media. Benchmark tests show that the dissolution rate of calcium carbonate under standard conditions is 4.39×10-8 mol·cm-2·s-1, consistent with the reported range of (4.18∼4.59) ×10-8 mol·cm-2·s-1. The simulation of gypsum formation shows an error of less than 5% compared to theoretical calculations. Additionally, the evolution of reactive transport parameters reveals that porosity increases monotonically, while reactive surface area decreases monotonically. Tortuosity and permeability fluctuate within ranges of 97.2%∼100% and 99.9%∼112.3%, respectively. However, the specific evolution patterns of reactive transport parameters are influenced by the initial pore structure and mineral distribution. This method provides a quantitative approach for studying these evolutionary processes.  [Conclusions]: Pore-scale reactive transport simulations offer deeper insights into the mechanisms behind pore structure evolution and blockage formation during acid in-situ leaching. Furthermore, this approach provides dynamic reactive transport parameters that serve as valuable references for simulating leaching processes in well fields.