Ultrasound pretreatment improves separation efficiency of apatite and calcite: insights into surface adsorption and crystal structure

Ultrasonic treatment facilitates the flotation separation of calcite and apatite. • Ultrasonic treatment increases the specific surface area of calcite. • Ultrasonic treatment promotes the adsorption of NaOL on calcite surface. • Ultrasonic treatment enhances the exposure of high-energy crystal planes of calcite. • NaOL exhibits stronger adsorption energy on the high-energy crystal planes. Calcite is a widely distributed carbonate gangue mineral, which often coexists with calcium-bearing valuable minerals, making their separation difficult. This study aimed to clarify the enhancement mechanism of ultrasonic pretreatment on the flotation performance of calcite and its separation efficiency between calcite and apatite. Combined with density functional theory (DFT) calculations, multiple characterization techniques including specific surface area analysis, atomic force microscopy, microcalorimetry and X-ray photoelectron spectroscopy (XPS) were adopted to systematically explore the influences of ultrasonic pretreatment on the surface characteristics, crystal plane exposure of calcite, as well as the adsorption behavior of sodium oleate (NaOL) on calcite surfaces. The results indicate that at a NaOL concentration of 7.5 × 10⁻ 5 mol/L, ultrasonic pretreatment increases the calcite recovery from 59.46% to 92.54%, and improves the separation efficiency of apatite from calcite. Specifically, the cavitation and mechanical oscillation effects induced by ultrasound cause surface micro-etching of calcite, remarkably increasing the specific surface area and surface roughness while exposing more active sites. Meanwhile, ultrasound regulates the crystal plane exposure of calcite, significantly increasing the proportion of high-activity crystal planes. XPS and microcalorimetry results confirm that ultrasound facilitates the formation of a denser and more stable NaOL adsorption layer on the calcite. DFT calculations further verify that NaOL exhibits stronger adsorption energy on the high–activity crystal planes. Collectively, ultrasonic pretreatment improves calcite floatability through the synergistic effects of surface activation, crystal plane optimization, and adsorption enhancement. This study provides a novel method and theoretical support for the flotation separation of calcite from phosphate ore.