Synthesis of K2O–MgO–SiO2 compounds as slow-release fertilisers from acid-leached biotite residues

For the utilisation of mineral residues and improving the nutrient use efficiency in fertilisers, the K2O–MgO–SiO2 compounds prepared as slow-release K fertilisers by using the acid-leached biotite residues, K2CO3, and Mg(OH)2 as the starting materials were systematically studied and evaluated in this work. Sintering experiments were performed at 700–1000 °C and solubility experiments were conducted by dissolving the sintered samples in 0.50 M HCl solution, 0.10 M citric acid solution, and water. The results showed that K2MgSiO4 (KMS), KMS-K1.14Mg0.57Si1.43O4(K2M2S5)–K2MgSi3O8(KMS3) mixtures, and K2MgSi5O12 (KMS5) were synthesised at 800 °C, 850–900 °C, and 950 °C, respectively. These compounds are citrate-soluble, and the solubilities followed the order KMS > K2M2S5 ≫ KMS3 > KMS5. The release properties of the K2O accumulated in water for all the K2O–MgO–SiO2 compounds fit the release kinetics Elovich equations, which suggest that KMS and KMS-K2M2S5-KMS3 can be used as slow-release K fertilisers and that KMS5 with a much slower release of K2O may be more suitable as a soil conditioner. Evaluation of the raw materials, synthesis approach, product properties, and environmental benefits shows that K2O–MgO–SiO2 slow-release fertilisers are eco-friendly and offer plenty of advantages with regard to soil fertility, environmental protection, resource utilisation, and energy consumption.