Selective flotation separation of scheelite from calcite using hexamethylenediamine tetramethylene phosphonic acid as a novel depressant

Scheelite, a strategic calcium-bearing mineral, usually coexists with calcite. Froth flotation is the most commonly used method of separating scheelite from calcite, but it is difficult to selectively separate them because of their similar Ca active sites. Sodium silicate with a high dosage and low selectivity is usually used as calcite depressant, which leads to the loss of scheelite and formation of stable colloidal dispersions of fine particles in the tailings slurry, making tailings treatment difficult. Therefore, this work developed a novel depressant, hexamethylenediamine tetramethylene phosphonic acid (HDTMP or H6L), which is a hexabasic acid. The flotation results of single mineral and binary mixed minerals showed that HDTMP at a concentration of 2 × 10−5 mol/L could selectively depress calcite with 4 × 10−5 mol/L NaOL at pH 10.0. Adsorption capacity, Ca2+ concentrate, and contact angle measurements showed that HDTMP was adsorbed more on calcite. Zeta potential measurement and solution chemistry calculations indicated that HL5− anion was the main active species of HDTMP at pH 10.0. Density functional theory (DFT) calculations further revealed that HDTMP had stronger interaction with calcite through chemically bonding with Ca sites and hydrogen bonds (P-O⋯Hwater and P-O H⋯Ocalcite) formed by the phosphate groups in HDTMP and H/O atoms on calcite surfaces. Therefore, HDTMP presents substantial potential for practical applications as a novel calcite depressant in scheelite flotation.