Molybdenum tailings calcination for phase composition of Si-Ca-K-Mg fertilizers: Modification of phase transformation and its effects on plant growth

•An innovative method of applying molybdenum tailings into fertilizers.•The complex salt of CaCO3 + CaSO4 synergistically activates nutrients in tailings.•The calcination mechanism involves a liquid–solid reaction and a diffusion reaction.•The optimal calcination ratio is 1 (molybdenum tailings): 0.7 (CaCO3): 0.2 (CaSO4).•The calcined product stimulates plant growth adjust soil pH. The high value and comprehensive utilization of molybdenum tailings for the production of fertilizer of calcium silicon magnesium potassium (Si-Ca-K-Mg fertilizer) was investigated, considering the abundant potassium feldspar (K-feldspar) content in the tailings. The effects of calcination temperatures, times, and additives (CaCO3 and CaSO4) on reaction degree, final product type, and phase transition process were studied. The results indicated that at 1200 °C, calcination time of 120 min and m(molybdenum tailings):m(CaCO3):m(CaSO4) = 1:0.7:0.2, the final product met Type I product requirements according to Chinese national standards GB/T36207-2018 as well as extremely low heavy metal contents. In the calcination process, CaO resulted from CaCO3 absorbs the liquid phase produced during K-feldspar decomposition to form CaSiO3, facilitating the K-feldspar decomposes into KAlSiO4. Simultaneously, the Ca2+ in CaSO4 replaces undecomposed K-feldspar’s K+. Additionally, CaO reacts with Al2O3, MgO, and SiO2 to produce gehlenite (Ca2Al2SiO7) and akermanite (Ca2MgSi2O7), providing plants with Ca, Mg, and Si elements. The nutrients of the fertilizer were readily assimilated by plants, improving acidic soil without causing any harm to the soil or vegetation. This work offers new ideas for large-scale and high-value utilization of molybdenum tailings as well as addressing the shortage of soluble potassium resource in China.