Probing the interfacial role of hydrolysed polymaleic anhydride for efficient target adsorption in scheelite–calcite flotation system: Experimental application and atomic-scale mechanisms

Abstract

Tungsten, a strategic non-ferrous metal critical for advanced industrial applications, predominantly exists as underutilized scheelite resources characterized by fine-grained intergrowths with calcite that are challenging to separate. This study deciphers the atomic-scale mechanism underlying the selective flotation separation of scheelite from calcite mediated by hydrolyzed polymaleic anhydride (HPMA), a novel environmentally benign reagent, through integrated experimental characterization and computational simulations. Micro-flotation assays quantitatively demonstrated HPMA’s exceptional selectivity, suppressing calcite recovery from 91.64% to 18.61% at pH 7 (10 mg·L−1 dosage) while preserving scheelite floatability. Fourier transform infrared spectroscopy revealed HPMA preferentially adsorbs on calcite, efficiently hindering sodium oleate (NaOL) attachment, whereas NaOL selectively binds to scheelite. X-ray photoelectron spectroscopy analysis confirmed carboxyl (–COO−) group chemisorption at calcite’s Ca sites, evidenced by a 0.26 eV negative shift in Ca 2p3/2 binding energy and new Ca–O bond formation. Density functional theory (DFT) simulations quantified adsorption energetics: HPMA exhibited stronger affinity for calcite (104) surfaces (−1166.441 kJ·mol−1) versus scheelite (112) (335.180 kJ·mol−1). Mulliken bond population analysis quantified interfacial bonding nature. The calcite–HPMA interface formed polar covalent bonds (populations 0.23–0.28), contrasting with NaOL’s ionic interactions (population 0.13) on scheelite. This covalent advantage enables HPMA to preferentially passivate calcite surfaces, suppressing NaOL co-adsorption and facilitating selective scheelite recovery through differential surface reactivity modulation.

1. Introduction

Tungsten, a strategic non-ferrous metal, finds critical applications in advanced technologies ranging from ballistic components and rocket propulsion systems to precision cutting tools and aerospace alloys [1–2]. China dominates global tungsten reserves with 5.2 million metric tons, exceeding the cumulative reserves of 30 tungsten-producing nations by over 300%, while maintaining dominant global production and export volumes [3–4].

Tungsten primarily occurs as scheelite (CaWO4) and wolframite (FeWO4) in nature [5]. With commercially viable wolframite deposits largely depleted, efficient exploitation of scheelite resources has become imperative. However, skarn-type scheelite deposits exhibit low-grade, fine-grained characteristics and complex impurity associations, posing substantial challenges for beneficiation [6]. Scheelite commonly coexists with calcite in fine-grained intergrowth patterns, further complicated by their near-identical surface charge characteristics and shared calcium cation (Ca2+) centers in crystal structures [7]. This structural similarity induces competitive chemisorption of reagents during flotation, leading to elevated reagent consumption and reduced concentrate grades. Additionally, as semi-soluble salt minerals, scheelite and calcite demonstrate heightened solubility in aqueous solutions, promoting interfacial ion exchange that enhances surface property convergence [8–9]. Such physicochemical synergism significantly impedes selective separation efficiency.

The efficacy of flotation separation hinges on the selective adsorption of reagents governed by mineral surface properties and molecular structure compatibility [10]. Initial scheelite beneficiation employed conventional collectors including fatty acids, phosphoric acid derivatives, and chelating agents [7,11–12]. Contemporary studies reveal that sole collector application yields suboptimal scheelite recovery, necessitating synergistic inhibitor integration to minimize reagent consumption and enhance system stability [13–14]. Phosphate-based depressants exhibit unique calcium mineral selectivity through surface adsorption and secondary product formation. Chen et al. [15] demonstrated sodium phytate (SP)-mediated scheelite–calcite separation using sodium oleate (NaOL) collector, where SP preferentially depressed calcite via zeta potential modulation (−Δζ = 12.3 mV, Δζ represents the potential difference between the sliding surface of the particle and the bulk solution of the dispersing medium.) through chelated Ca2+ complexation, while maintaining scheelite surface activity (Δζ < 2 mV), achieving 91.2% WO3 recovery. Among the organic inhibitors, carboxymethyl cellulose (CMC) belongs to a class of common organic polymers, which originates from negatively charged polysaccharides converted from plant cellulose. Wang et al. [16] reported negligible inhibition of NaOL adsorption on scheelite (<5% coverage reduction) versus 78% suppression on calcite.