Natron–lime causticization and hydrothermal feldspar dissolution under lid-set pressures: a PHREEQC-validated engineering protocol (ULCH)

This paper evaluates whether a plausible natron–lime alkaline recipe, operated at artifact-derived lid-set pressures, can generate industrially relevant feldspar dissolution rates over 5–60 minutes. The analysis combines (i) a canonical stoichiometric causticization reaction that converts sodium carbonate (natron proxy) to sodium hydroxide in the presence of lime, and (ii) geochemical reaction-path simulations using PHREEQC (USGS standard software) to compute aqueous speciation and feldspar dissolution kinetics. PHREEQC speciation confirms strongly alkaline conditions at the relevant temperatures: for a 3.67 mol/kgw NaOH-equivalent solution, pH = 12.29 at 112°C and pH = 12.40 at 105°C. Kinetic simulations using Palandri–Kharaka transition-state rate laws predict that total feldspar dissolved is 8.27 g at 30 min (112°C) and 5.50 g at 30 min (105°C), with a water-only control dissolving only 0.0045 g, indicating a ≥10³× acceleration from alkaline chemistry. The modeling is intended as a feasibility screen and a quantitative target for laboratory replication.