Synthesis of sulfomethylated depolymerized alkali lignin and its application in scheelite flotation

Scheelite serves as the main resource of tungsten. However, the recovery rate and efficiency of tungsten from scheelite ore is low due to the similar floatability of scheelite and calcium gangue. To improve the separation efficiency of scheelite from gangue, an bio-based and eco-friendly depressant, sulfomethylated depolymerized alkali lignin (SMDL), was designed in this work. It is synthesized by the oxidation, hydrolysis, electrocatalytic depolymerization, and sulfomethylation modification of alkali lignin (AL). FTIR, GPC, and NMR measurements was used to confirm the sulfonation degree and molecular weight in different depolymerization strategies. SMDL-2, which had the lowest molecular weight and the highest degree of sulfonation (2.2 mmol/g), presents stronger depressing effects on fluorite and calcite. The flotation recoveries of scheelite, calcite, and fluorite with SMDL-2 were 85.27 %, 13.34 %, and 12.76 %, respectively. The charge and electrostatic potential distribution results reveal that SMDL has abundant calcium-binding interaction sites, and these sites are positively correlated with the degree of sulfonation. Mechanistic analysis indicated that SMDL interact with the calcium sites of fluorite and calcite through hydrophilic –SO₃, thus realizing the selective depression of fluorite and calcite. The utilization of SMDL can facilitate the flotation separation of high-grade scheelite and the high-value utilization of lignin waste resources. © 2025 Elsevier B.V.