Ultrasonic Pulp Conditioning-Induced Nanoparticles: A Critical Driver for Sonication-Assisted Ultrafine Smithsonite Flotation

Extensive studies have established that ultrasonic micro-jets and acoustic cavitation selectively intensify interfacial interactions at multiphase boundaries, thereby enhancing the flotation of soluble salt minerals and oxide ores. Although a growing body of evidence shows that pulp-borne nanoparticles (i.e., nanosolids, colloids, and nanoscale gas nuclei) mediate these effects, their role in the flotation of ultrafine smithsonite after collector addition has not yet been systematically examined. To fill this gap, we compared the flotation response of ultrafine smithsonite under conventional stirring (SC) and ultrasonic conditioning (UC), using sodium oleate (NaOL) as the collector, and dissected the governing mechanisms across three pillars, mineral–NaOL interaction, particle aggregation, and frothability, with particular attention paid to how nanoparticles modulate each dimension. The flotation results show that flotation performance under UC is dictated by NaOL concentration. At low NaOL levels (i.e., below 4 × 10−4 M), UC depresses both recovery and kinetics relative to SC, while at high NaOL levels, the trend reverses and UC outperforms SC. Mechanistic analysis reveals that sonication erodes mineral surfaces and generates cavitation, flooding the pulp with various nanoparticles. When NaOL is scarce, zinc-containing components and zinc-rich nanosolids sequester the collector through non-selective adsorption and precipitation, leaving smithsonite poorly hydrophobized. Consequently, particle aggregation and pulp frothability are markedly inferior to those in the SC system, so the flotation recovery and kinetics remain lower. As the NaOL concentration rises, smithsonite becomes adequately hydrophobized, and the pulp fills with hydrophobic zinc-rich nanosolids, along with cavitation-induced gas nuclei or tiny bubbles. These nanoparticles now act as bridges, accelerating the aggregation of ultrafine smithsonite once sonication stops and agitation begins, while simultaneously improving frothability. Although the strong dispersive action of ultrasound still suppresses initial flotation kinetics, cumulative recovery ultimately surpasses that of SC. The findings delineate a nanoparticle-regulated flotation paradigm and establish a critical NaOL concentration window for effective UC in ultrafine smithsonite flotation. This framework is readily transferable to the beneficiation of other ultrafine, soluble oxidized minerals (rhodochrosite, dolomite, etc.).