Sustainable valorization of syenite: Atmospheric-pressure leaching for selective aluminum and potassium recovery

An integrated atmospheric-pressure hydrometallurgical process was developed for the simultaneous selective recovery of aluminum and potassium from syenite without prior structural activation. A multivariable screening experimental design identified acid concentration and temperature as dominant leaching parameters; optimized conditions (12 M H 2 SO 4 , 80 °C, L/S = 7.5 mL g −1 , 0.5 h) yielded aluminum and potassium extraction efficiencies of 40.22 % and 34.33 %, respectively. Combined XRD, SEM/EDS, and TIMA characterization of leach residues revealed preferential dissolution of kalsilite over microcline, attributed to the lower polymerization degree of the feldspathoid Si–O–Al framework, alongside secondary Al/K-bearing sulfate phase formation and passive quartz enrichment, establishing a dissolution selectivity hierarchy of kalsilite > hematite/magnetite ≫ microcline > quartz. Among four systematically evaluated neutralization strategies, the first stage (pH 7.2) of a two-step ammoniacal precipitation strategy achieved near-complete Al (99.79 %) and Fe (99.95 %) removal with selective K + retention, markedly outperforming Ca(OH) 2 -based strategies penalized by gypsum co-precipitation and potassium occlusion. Thermogravimetric analysis guided controlled calcination at 492 °C, 617 °C, and 1000 °C; the latter yielded a mixed γ/α-Al 2 O 3 product (∼81 wt% Al 2 O 3 with residual Fe 2 O 3 , K 2 O, and SO 3 . This activation-free process provides a sustainable route for multi-element valorization of non-bauxitic aluminosilicate resources within a circular economy framework to produce γ/α-Al 2 O 3 , an N–S-dominant liquid fertilizer precursor with minor K enrichment, and mineral leach residue with slow-release agronomic potential.