The fluid mechanics of mineral extraction*

The ongoing energy transition revolves around the utilization of strategic minerals at unprecedented scales. Elements like lithium, predominantly extracted from natural brines, are increasingly demanded for the electrification of the economy. Current production methods require the pre-concentration of the mineral, which is achieved in multi-stage ponds where ambient evaporation extracts water from the brine, precipitating the most abundant salts (e.g. sodium chloride) while the target species (lithium) gets concentrated. However, this process has long operating times, requires extensive land use, and depletes surrounding aquifers due to uncontrolled evaporation. Here, we demonstrate how porous cellulose crystallizers can, through evaporation-driven capillary flow, increase the rate of lithium extraction per unit of surface area by an order of magnitude, thereby reducing land use and operation times by over 90% [Chen et al., Nature Water (2023)]. A simple transport model recapitulates the observed high lithium selectivity, which stems from the sharp salt gradients at the top of the evaporator that result from the interplay between advection and diffusion. Motivated by the strong influence of fluid flow in this industrially critical process, we also explore the wide range of underlying transport processes that are present in realistic scenarios. Through mathematical modeling and microfluidic experiments, we illustrate how physicochemical factors like the variable activity and viscosity of the brine solution, which change noticeably near saturation, are key to understand and quantify in order to advance the next generation of mineral extraction technology.