Catalytic role of LiCl in the partial decomposition of phosphate tailings: experimental and theoretical calculations

Regarding the crucial issues in the resource utilization of phosphorus tailings, like the high energy consumption of the traditional pyrolysis process and the low separation efficiency of calcium and magnesium, we came up with a medium-temperature semi-decomposition approach using LiCl as the catalyst. This can selectively pyrolyze CaMg(CO 3 ) 2 in phosphorus tailings. Characterization tools such as TG-DTG, SEM and XRD, combined with molecular dynamics theory calculations, were used to reveal the catalytic mechanism of LiCl and to establish a pyrolysis kinetic model. The results show that LiCl enters the dolomite interlayer by short-range diffusion, and utilizes the synergistic effect of the electron delocalization of Li + and the high electronegativity of Cl − to promote the embedding of Li + in the interlayer and perturbing the crystal lattice structure, which reduces the pyrolysis energy barrier and accelerates the C O bond breaking of the carbonate, and at the same time, changes the Ca 2+ /Mg 2+ coordination environment, which promotes the semi-decomposition of dolomite and realizes the efficient dissociation of MgO. Under the optimized process (1.5% LiCl, calcined at 550 °C for 90 min), the decomposition rate of elemental magnesium in phosphorus tailings reached 93.68%, the apparent activation energy was reduced from 205 kJ/mol to 122.47 kJ/mol. The pyrolysis process exhibits a typical three-stage feature: the initial stage is controlled by a three-dimensional interfacial reaction; the intermediate stage is controlled by a two-mechanism transition from a power-law-type kinetic model to a one-dimensional interfacial reaction model; and the final stage is dominated by a diffusion-controlled process.