Zero-waste valorization of K-rich igneous rocks for cleaner potassic fertilizers

Abstract

Potassium (K) deficiency in agricultural soils is a growing global concern, particularly in Africa, where soil nutrient depletion outpaces replenishment. This study investigates the potential of high-K igneous rocks with ultrapotassic syenite affinity as a sustainable source of K for agriculture through alkaline hydrothermal treatment. The objective was to optimize K release from these silicate rocks using a systematic experimental approach. A composite sample of ultrapotassic syenite rocks (∼15 wt% K 2 O) underwent alkaline hydrothermal treatment in an autoclave with varying time, temperature, particle size, CaO addition, and liquid (water) to solid ratio. A Fractional Factorial Design (FFD) was used to screen key variables, followed by a Central Composite Design (CCD) for K release optimization. Mineralogical characterization was performed using XRD, SEM-EDS, and XANES, while leaching tests assessed elemental release. Results showed that temperature, Solid (CaO)/Solid (syenite) ratio, and Liquid (water)/Solid (feed) ratio were the most significant factors affecting K release. The maximum K release of ∼70.6 % was achieved under conditions of 200 °C, 7 h, Solid (CaO)/Solid (syenite) of 0.9, and Liquid (water)/Solid (feed) of 0.43. XRD and SEM analyses revealed the formation of secondary phases such as portlandite and calcium-aluminum-silicate-hydrate (CASH). XANES analysis indicated the formation of K 2 SiO 3 under high treatment conditions. Leaching tests demonstrated rapid initial K release (up to 2096 mg/l within 15 min) followed by sustained slower release. This optimization study explores treated syenite ore as an innovative K fertilizer, demonstrating its potential to release essential nutrients like Ca and Si. Unlike conventional KCl fertilizers, this approach may improve soil properties and effective plant growth.

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