Alkali-Hydrothermal Treatment of K-Rich Igneous Rocks for Potassic Fertilizers production in Morocco

Potash is one of three important fertilizers (i.e., N, P and K) needed for plants and provides K as essential nutrient worldwide. The largest sources of raw materials for potash production are evaporites sedimentary rocks, mainly sylvite (potassium chloride -KCl) in the fertilizer industry. However, the potash resource’s is principally located in the northern hemisphere and their needs are highly important in southern countries, particularly in Africa due to crop demands . Furthermore, the low use of potash is exacerbated by the high market prices beyond the reach of farmers, demand inflations, and causing rampant K deficiency in several Africa soils. It is therefore imperative to search for alternative K sources, potentially using the locally available silicate minerals such as K-feldspars and feldspathoids, and other important K-concentrated minerals such as kalsilite or orthoclase present in many African countries. These minerals represent promising sources for the development of new and ecological fertilizers, particularly adapted to tropical soils. However, silicate minerals such as K-feldspar (KAlSi₃O₈) are characterized by a low dissolution rate that is not only extremely low compared to evaporitic potash but also does not allow having enough bioavailable potassium. Indeed, the silicon-oxygen tetrahedron (SiO₄)⁴⁻ and aluminum-oxygen tetrahedron (AlO₄)⁵⁻ configuration leads to a solid network structure that inhibits potassium availability. Consequently, to enhance potassium release, the crystal matrix of feldspar must be destroyed or at least altered. The present study was undertaken to investigate potential deposit of K-mineral sources present in Morocco. Six potassic igneous rocks (syenites and trachytes) from the Tamazeght, Jbel Boho, Ait Saoun, and El Glo’a regions (Morocco) were sampled and characterized. Then they were hydrothermally treated to enhance their K release for potential use as potassic fertilizers. The raw materials are mainly formed by microcline (up to 74%), orthoclase (20–68%), albite (36–57%), biotite muscovite (15–23%), and titanite, calcite, hematite, and apatite as accessory minerals. These samples were crushed and milled to reach a particle size <150 μm and mixed with 4 N NaOH solution in an autoclave. The powders were allowed to react with the solution at 170°C for 7h. X-ray diffraction (XRD), thermal gravimetric analysis (TGA), infrared spectroscopy (IRTF), and scanning electron microscopy (SEM-EDS) were carried out on treated samples to characterize the mineralogical and structural changes due to the alkali-hydrothermal treatment.The treated material was leached and the elements released were measured using inductively coupled plasma–atomic emission spectroscopy (ICP-AES). The hydrothermal process showed a strong effect on structure breakdown as well as on the release of K and other nutrients such as P, Fe, Si, Mg, and Ca. Therefore, the alkali-hydrothermal treatment allowed the release of 50.5 wt% K. Moreover, the release of Mg, Ca, Fe, P, K, and Si were also significantly increased. The ultimate project goal is to develop novel approach for locally production of K-based fertilizer from K-alkaline rocks and these encouraging results need to be examined further.