Mbissik, and 5 co-authors, reported in 2021 that due to the increasing demand for conventional sources of potassium (K) and their inaccessibility by African countries, K-rich igneous rocks are increasingly studied as potential alternative sources. In this study, 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 liquid/solid (L/S) ratio was about 44 mL/50 g. The powders were allowed to react with the solution at 170 °C for 7 h. For all tests, NaOH reacted completely with the powders and no liquid was observed after the treatment. 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 samples revealed the presence of sodic neoformed phases such as thermonatrite, sodalite, analcime, and cancrinite. The treated material was leached for a week using deionized water 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 significantly increased. Mg, Ca, Fe, P, K, and Si release within raw materials was about (0.5–3.6), (3.5–31.4), (0.01–0.4), (0.01–0.3), (20–55), and (4.6–8) mg/kg, respectively, whereas treated samples showed a higher release of these elements.
Quantitatively, Mg, Ca, Fe, P, K, and Si releases were about (10–11.8), (60–70), (7–20), (1.2–15), (218–1278), and (1119–2759) mg/kg, respectively. Consequently, the treated igneous rocks (syenite and trachyte) could be directly used as potassic fertilizers that would also be a source of other nutrients.
The authors concluded that Potassium-rich silicate rock could serve as an alternative to conventional potash resources. These rocks are advantageous because they are more locally available and contain macronutrients, micronutrients, and beneficial elements required for plant growth. In addition to K, they also provide the plant and soil with other elements such as Ca, Fe, Mg, S, P, and Si.
In this study, six samples coming from four regions in Morocco (Tamazeght, Jbel Boho, Ait Saoun, and El Glo’a) were investigated. Samples were first crushed and milled at D90 < 130 µm, then mixed with NaOH and treated at T = 170°C for 7 hrs. The final product was leached for one week using water as a solvent. The comparison between raw and hydrothermal material was evaluated using various techniques, such as XRD, TGA, FTIR, and SEM-EDS. ICP-MS allowed for the quantification of the potassium released in the solution. The following conclusions can be drawn from these experimental conditions:
- These results confirm again the importance of petrography and alterability for predicting the leaching behavior of K-bearing rock;
- K-rich rock needs to be hydrothermally treated before any application as fertilizer because hydrothermal treatment significantly enhances the release of potassium and other beneficial elements through the substitution ion mechanism;
- Temperature, the initial potassium content of the rock, alterability, and particle size are found to be important factors controlling the K release; and
- The approach used in this study provides a significant amount of sodium through the neoformed Na mineral, so it would be interesting to explore additives other than NaOH for further application in the fertilizer industry.
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