Innovative Dual Flotation Process Using Algal Sodium Alginate as a Calcite Depressant for Enhancing Phosphate Recovery

Froth flotation is the most effective process for upgrading phosphate rock, but selective flotation of apatite from calcite is hindered by the physicochemical resemblance and limited selectivity of conventional fatty acid collectors. In this study, sodium alginate (SA), a naturally occurring and algal residual biomass-derived biopolymer, was evaluated as a green depressant for calcite in apatite direct flotation. SA-L and SA-F were extracted from two different varieties of algal biomass, Laminaria ochroleuca and Fucus spiralis, respectively, and characterized by Fourier Transform Infrared Spectroscopy (FTIR), X-ray Diffraction (XRD), Thermogravimetric Analysis (TGA), and Nuclear Magnetic Resonance (NMR). Microflotation tests confirmed that both SA extracts acted as highly selective depressants for calcite, achieving depression percentages of 90% for SA-L and 92% for SA-F with little impact on apatite recovery when using sodium oleate (NaOL) as an apatite collector. Dual direct flotation tests with very low-grade phosphate ore (39.77% Bone phosphate of lime (BPL) (P2O5 = 2.185 × BPL)) resulted in substantial enrichment of phosphate rock, reaching approximately 63% BPL. Mechanistic studies, such as zeta potential, surface wettability, and molecular dynamics simulations, demonstrated that SA binds with the calcite surface by chemical reactions to Ca2+ ions, thereby preventing collector (NaOL) adsorption and enhancing hydrophilicity. These findings highlight the flotation behavior of SA extracts, demonstrating their potential as a highly selective, eco-friendly alternative depressant for phosphate rock beneficiation.