Understanding Surface Forces and Interaction Mechanisms in Mineral Flotation by Atomic Force Microscopy

• Author / Creator

  Liyuan Feng

• Bubble-particle interactions are ubiquitous and crucial to mineral fotation. The bubble-particle attachment process is mainly governed by three surface forces: van der Waals (vdW), electrical double-layer (EDL), and hydrophobic (HB) forces. Atomic Force Microscopy (AFM) has become the most prominent technique to measure the surface forces due to its versatility and pico-Newton resolution. By anchoring a micron-sized oil droplet or bubble onto a tipless AFM cantilever, the so-called AFM bubble probe allows for accurate force measurement involving deformable bodies. In this study, a number of force measurements were performed with the advantage of the AFM bubble probe technique to understand the surface forces in mineral fotation. The interaction mechanisms were underpinned by the well-established Stokes-Reynolds-Young-Laplace (SRYL) model.
  Molybdenite (MoS2) is a mineral that has drawn great interest because of its potential application in various felds. To facilitate the fotation of molybdenite, the mineral pulp is commonly treated with nonpolar oil additives to promote hydrophobicity and to form an oil bridge between ultrafne molybdenite particles for agglomeration. In this study, dodecane was chosen as a model oil to investigate the fotation mechanisms of molybdenite with nonpolar oil. The interaction forces between a micrometer-sized dodecane droplet and the molybdenite basal plane in various electrolyte solutions were directly measured by the atomic force microscope droplet probe
  technique. The effects of added salts, ionic strength, and solution pH on interaction forces were evaluated by considering van der Waals, electrical double-layer (EDL), and hydrophobic forces. The experimentally measured force curves were found to agree well with the Reynolds lubrication model and the augmented Young-Laplace equation. The results show that the competition between repulsive EDL forces and attractive hydrophobic forces was directly responsible for oil-molybdenite attachment behavior. High pH and low salinity (<24 mM NaCl) led to strong repulsive EDL forces, which stabilized the interaction and prevented the attachment of oil to molybdenite. Both low pH and high salinity facilitated the attachment of oil to molybdenite through the depression of
  EDL force, allowing attractive hydrophobic force to dominate. The hydrophobic attraction wasquantifed with an exponential decay length of 1.0 ± 0.1 nm. Furthermore, calcium ions decreased the magnitude of the surface potentials of both oil and molybdenite more than that seen with the same ionic strength of sodium ions, suggesting the suppressed EDL repulsion. This study provides quantitative information about the surface forces between oil and the molybdenite basal plane and an improved understanding of the fundamental interaction mechanisms governing molybdenite recovery by mineral fotation.
  The wettability and surface potential of millerite (NiS) under pH 4 and pH 12 were investigated. Experimental data suggested that elemental sulfur and/or polysulfde (S2 n−) formed under acidic condition (pH 4) rendered the millerite surface hydrophobic and favored the bubble-millerite attachment, where HB force dominated the interaction. In contrast, hydrophilic Ni(OH)2 formed under pH 12, acted as a passivation layer, and caused the millerite surface to become less hydrophobic. Thus repulsive EDL force inhibited the bubble-millerite attachment.
  The infuence of oxidation using H2O2 solution on pentlandite was studied in borax buffer solution. The additive of H2O2 did not alter the hydrophilicity but increased the surface roughness
  by introducing nanoscale asperities onto pentlandite. Combined with PIBX treatment, pentlandite showed chemical heterogeneity and partial hydrophobicity but still allowed for bubble attachment. When pentlandite underwent direct xanthate treatment, it showed less chemical heterogeneity
  and stronger hydrophobicity, and micron-sized bubble attachment could readily be induced as desired. The characteristic lengths of HB forces were found to be 0.9 ± 0.1 nm and 1.2 ± 0.1 nm for H2O2-PIBX treated pentlandite, and PIBX treated pentlandite, respectively. In addition, the chemical properties of pentlandite was well-characterized at nanoscale by adhesion force map.
  This study provides quantitative information about the surface forces between oil/bubble and mineral surfaces and an improved understanding of the fundamental interaction mechanisms in mineral fotation.

• Subjects / Keywords

  • Atomic Force Microscopy
  • Molybdenite
  • Millerite
  • Pentlandite
  • Bubble

• Graduation date

  Fall 2019

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/r3-0y3k-5t61

• License

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