Synergistic Interaction of N-(2-((2-Aminoethyl)amino)ethyl)dodecanamide and Sodium Oleate for Depressant-Free Flotation Separation of Lepidolite

Efficient lepidolite flotation at neutral pH and low temperatures without depressants remains challenging. This study introduces a novel amide–diamine surfactant, N-(2-((2-aminoethyl)amino)ethyl)dodecanamide (N-123), combined with sodium oleate (NaOL), for efficiently separating lepidolite from albite and quartz. Flotation experiments on ternary mixed minerals (mass ratio 1:1:1) demonstrated that at pH 7.0, N-123/NaOL (1:4) achieved Li2O recoveries of 84.25% at 25 °C and 73.17% at 5 °C, outperforming conventional dodecylamine (DDA)/NaOL at 54.93% at 25 °C and 32.77% at 5 °C. An amide–diamine structure provides good water solubility and a low Krafft point, enabling low-temperature flotation. Surface tension results showed that N-123 and N-123/NaOL exhibited higher surface activity than DDA and DDA/NaOL, respectively, and that both maintained relatively high surface activity at 5 °C. Wettability and froth stability tests confirmed synergistic co-assembly of N-123/NaOL at the interface, enhancing lepidolite hydrophobicity while reducing froth height and half-life to minimize gangue entrainment. ζ-potential, Fourier transform infrared (FT-IR) spectroscopy, in situ atomic force microscopy (AFM), and force measurements revealed that N-123/NaOL carried a net negative charge and co-adsorbed on lepidolite mainly via hydrophobic forces, while adsorption on albite and quartz was suppressed by electrostatic repulsion. Thus, the synergistic N-123/NaOL collector enables efficient, depressant-free lepidolite flotation.