•Pure scheelite minerals were ground by rod and ball mills.•Rod mill particles have larger values of elongation and flatness.•Both mill particles have a similar expose intensity of abundant {112} surface.•Rod mill particles expose more {101} surface while more {001} surface for the ball mill ones.•Rod mill particles exhibit better hydrophobicity and floatability.Grinding for mineral liberation is a prerequisite for a successful flotation separation. Different grinding media produce mineral particles with different surface properties and flotability. In this study, the surface properties and flotation behavior of scheelite particles having a size of − 74 + 38 μm produced by ball and rod mills were studied through single mineral flotation experiment, scanning electron microscopy (SEM) observation, wettability measurement, and X-ray diffraction (XRD) test. The wettability and flotation results showed that, compared to the ball mill particles, the rod mill ones have a lower critical surface tension and thus a greater hydrophobicity when treated with the collector solution, and accordingly perform a better flotation recovery using oleate as the collector. In addition, the rod mill particles have a smaller specific surface area, so the full monolayer adsorption of the collector on their surfaces is achieved at a lower oleate concentration. The SEM analysis further confirmed that mineral grains obtained from the rod mill possess larger elongation and flatness values, which are essentially required for their attachment with air bubbles. The XRD observations revealed that mineral particles from both mills (i.e. ball and rod) have similar exposure intensity of abundant {112} surface. However, the rod mill particles have more {101} surface exposed, while the ball mill particles have more {001} surface exposed, leading to a stronger interaction of the collector with the rod mill particles. Keeping in view the stronger interaction with the collector and the easier attachment to air bubbles, the rod mill scheelite particles are deemed to be more hydrophobic and have a higher flotation recovery. These details studied will help establish the relation between the particle surface properties and the grinding media, and provide guidance for optimizing flotation separation.Download high-res image (215KB)Download full-size image

•A collector mixture of lead (II) ion/BHA exhibits stronger collecting ability for cassiterite compared with adding lead (II) ion and BHA in sequence at the same dosage.•An improved separation of cassiterite/calcite can be achieved using the collector mixture in the presence of CMC.Tin and its compounds have a variety of applications. It is recovered as cassiterite from ore deposit by flotation. Traditionally, the use of fatty acids and their derivatives with limited selectivity of cassiterite over calcite (the main gangue mineral) has been replaced by benzohydroxamic acid (BHA). BHA shows poor collecting ability to cassiterite, even when lead nitrate (LN) is added as the activator. In this work, the flotation results show that cassiterite can be separated efficiently from cassiterite-calcite binary mixed minerals using a mixture of LN/BHA as collector and carboxymethyl cellulose (CMC) as depressant. The separation performance of the mixture of LN/BHA is much better compared with adding LN and BHA in sequence at the same dosage. Zeta potential measurements indicate that the mixture of LN/BHA exhibits much stronger ability to adsorb on cassiterite compared with adding LN and BHA in sequence even in the presence of CMC.

Anisotropic surface broken bond densities and surface energies of six surfaces of scheelite were calculated with density functional theory (DFT). The calculation results show that its surface energy is directly proportional to its surface broken bond density. {1 1 2} and {0 0 1} surfaces are the two predominant cleavage or exposed planes. The prediction of morphology is well consistent with our experimental observations based on XRD. Anisotropic adsorption behaviors and wettability of these two surfaces after being immersed in sodium oleate and DDA solutions were studied by means of contact angle measurement, AFM and flotation test. The contact angle values at a given NaOl concentration decreased in the order {1 1 2} > {0 0 1}. As for DDA, similar wettability for the two surfaces was observed. The results were interpreted by AFM and interaction energies for the adsorption of surfactants on each surface calculated in accordance with molecular dynamics (MD) simulation.Graphical abstractHighlights► Anisotropic surface energies of scheelite were calculated by DFT calculations. ► Two predominantly cleavage planes, e.g. {1 1 2} and {0 0 1} surfaces were confirmed. ► Anisotropic adsorption behaviors oleate molecules at two surfaces were observed. ► Anisotropic adsorption behaviors were interpreted based on AFM and MD simulations.

•Mixed dodecylamine/oleate collectors used for scheelite flotation.•Single mineral and mixed minerals flotation tests conducted at pH 7.•Better collecting ability and selectivity for mixed collector observed.•Selective flotation separation of scheelite from calcite was achieved.In this study, flotation behavior of scheelite and calcite was investigated using mixed cationic/anionic collectors of dodecylamine (DDA) and sodium oleate (NaOL). Individual NaOL or DDA was also tested for comparison. The flotation results of single mineral and mixed binary minerals demonstrated a high selectivity and recovery for the flotation of scheelite from calcite at pH 7 using mixed DDA/NaOL collectors. The adsorption mechanism of the selective separation was analyzed through zeta potential measurement.

•RNH2 form of DDA adsorbs one each mineral via Ca–N bond and hydrogen bonding.•Different adsorption behavior of DDA on two minerals lies in cationic species RNH3+.•A monolayer structure of DDA species is formed on each mineral surface.•DDA species are much more compactly arranged on scheelite surface.In this work, the adsorption behavior of dodecylamine (DDA) on scheelite and calcite mineral surfaces were investigated by molecular dynamics (MD) simulation, zeta potential measurement, AFM observation, contact angle measurement, and flotation test. The results implied that, the different adsorption behavior of DDA on scheelite and calcite surfaces was mainly attributed to cationic species RNH3+ of DDA. The adsorption of neutral species RNH2 and complex precipitates produced by the reactions between RNH3+ and anionic species released by both mineral surfaces, also played important roles. In a DDA solution (1 × 10−4 mol/L, pH 7.5–8.0), a handful of RNH2 of DDA adsorbed on both scheelite and calcite surfaces through N–Ca bonding and hydrogen bonding between hydrogens of –NH2 group and mineral surface oxygens. On positively charged calcite surface, RNH3+ adsorbed on a moderate number of CO32− sites through electrostatic attraction and hydrogen bonding, which caused a moderate zeta potential increase for calcite. On negatively charged scheelite surface, a large number of cationic species RNH3+ could easily adsorb on abundant WO42− sites, which caused a significant increase of zeta potential for scheelite. The different adsorption behavior led to a heavier monolayer coverage of DDA species on scheelite surface, and thus a more hydrophobic surface and a better flotation recovery of scheelite. The present work helps propose a new adsorption model of DDA on scheelite and calcite.

•Selective flotation of scheelite using a collector mixture 733+MES.•Mixing mass ratio 4:1 of 733:MES is preferred.•Low effect by water hardness and water glass addition.Collector 733, a sodium soap (C12–16COONa) is widely used industrially for scheelite flotation. Low selectivity of 733 collector is always observed. In this study, a collector mixture of 733 and MES (sodium fatty acid methyl ester sulfonate) demonstrated a high selectivity for the flotation of scheelite from calcite and fluorite. An optimal mass ratio 4:1 of 733:MES was found, producing a 65.76% WO3 concentrate grade with a recovery of 66.04% from a feed material containing only 0.57% WO3. In addition, the effect of water hardness and water glass addition were studied. The results indicated that the presence of Ca2+ or Mg2+ had little effect on the adsorption of the collector mixture at the scheelite surface. Addition of water glass for depressing calcite and fluorite had no significant effect on the adsorption of the collector mixture on the scheelite surface. The advantages of this new collector mixture (733+MES) include lower cost, low dosage, high tolerance against water hardness and high selectivity, and this collector mixture has great potential for industrial application.733+MES collector mixture at 4:1 mass ratio achieves the best flotation separation performance with a scheelite recovery of 70%.Download full-size image