Measurements and analysis of xanthate chain length effect on bubble attachment to galena surfaces

Highlights

• Measurement of bubble-galena attachment time using a developed apparatus.

• Attachment time is greatly affected by xanthate chain length and bubble size.

• Attachment time was modeled by the film drainage with multilayer dispersion forces.

• This study sheds light on the principle for the bubble-surface attachment in flotation.

Abstract

Although available flotation experiments show that sulphide flotation recovery increases with increasing xanthate chain length, quantitative analysis to reveal the underlying mechanisms is missing. Herein, we investigate the chain length effect by using a recently developed attachment apparatus and an advanced theory to determine and validate the attachment time of air bubbles to galena surfaces in solutions of ethyl, propyl, butyl and amyl xanthate collectors. Specifically, the attachment time was accurately determined from either transient force curves or transient film thickness of the bubble-surface attachment. The theory was based on the film drainage theory and colloidal forces, which include the advanced multilayer dispersion (van der Waals) interactions and the electrical double-layer interaction. The multilayer dispersion theory has allowed us to describe the effect of galena hydrophobicity induced by the xanthate chain length in terms of interfacial gas enrichment (IGE) on the bubble-surface attachment. The experimental results show that the attachment time is greatly affected by both the hydrophobicity and the bubble size, and agree with the theoretical results. Critically, it was possible to accurately quantify the attachment time of the hydrophobised galena surfaces to bubbles using the measured transient force curves and the advanced multilayer dispersion theory incorporating the IGE layer. Finally, this study made it possible to explain the effect of the hydrophobicity and bubble size on attachment time and provides the principle for the attachment mechanism of hydrophobic minerals to air bubbles.

Introduction

The attachment of air bubbles to mineral surfaces in water is a major phenomenon in froth flotation (Nguyen and Schulze, 2004). Usually, to represent objectively, this attachment is described in terms of attachment time (Xing et al., 2017b). Because the attachment time, also known as the induction time, is a key parameter that directly represents flotation kinetics (Albijanic et al., 2010), it was previously used to model flotation recovery. Typically, particles with a short attachment time exhibit fast flotation kinetics (Xing et al., 2017b). Many studies employed attachment time as a metric to analyse flotation (Albijanic et al., 2010).

Meanwhile, the bubble-mineral attachment is determined by various surface chemical properties, and the hydrophobicity of the particle surface dominantly determines the attachment (Albijanic et al., 2010). As the hydrophobicity increases, the attachment interaction between the bubble and the particle becomes stronger (You et al., 2020). Thus, to improve the flotation efficiency, the hydrophobicity of the mineral surfaces is increased by adsorbing collectors. (Wills & Finch, 2016). Flotation experiments showed an increase in flotation recovery and rate with the increasing chain length of collectors (Ackerman et al., 1987, Han et al., 2019).

Although this chain length effect on attachment interaction between the bubble and the mineral surface was proved experimentally, quantitative analysis to reveal the underlying mechanisms is missing (Albijanic et al., 2010). The attachment process can be divided into three components – drainage, rupture, and three-phase contact line (TPCL) expansion (Albijanic et al., 2010). The drainage, which can be defined as the process of thinning until the intervening liquid film thickness becomes the critical film thickness (h_c) (or critical rupture thickness, h_cr), is the most dominant component among the three described components (Manor et al., 2008). Therefore, most theoretical studies on attachment adopted the drainage theory. Drainage is determined by the Laplace pressure, which corresponds to bubble deformation and surface forces (Nguyen and Schulze, 2004), which include the van der Waals force and the electrical double-layer force (Nguyen and Schulze, 2004). Thus, it is necessary to analyse the attachment using thin-film drainage theory and surface forces. However, because hydrophobic surfaces cannot properly account for be described by the celebrated Derjaguin–Landau–Verwey–Overbeek (DLVO) theory of surface forces, accurate model predictions remain a challenge. (Albijanic et al., 2010, Xing et al., 2017a).

In recent studies, the multilayer dispersion theory has received attention since the nanobubbles have been observed at water-hydrophobic interfaces by neutron reflectivity experiments, x-ray reflectivity or AFM (Azadi et al., 2015, Doshi et al., 2005, Hampton and Nguyen, 2010). It was discovered that these nano-bubbles were generated by gases dissolved in water (Peng et al., 2013). Several studies suggested that the nano-micro bubbles can attribute to the rupture of the liquid film and the hydrophobic interaction is a consequence of bubble-bubble interaction involving capillary forces (Kosior et al., 2015, Hampton and Nguyen, 2010). These nanobubbles form an interfacial gas enrichment (IGE) layer on the surface in laminar flow (Azadi et al., 2015, Peng et al., 2013). Therefore, it has been suggested that the surface force of a hydrophobic surface can be calculated using the advanced multilayer dispersion (van der Waals) force after taking the IGE layer thickness into account (Han et al., 2020).

In this study, we investigate the effect of chain length of collectors on the bubble-mineral surface attachment by measuring the attachment time and using a multilayer dispersion theory to determine and validate the attachment time of air bubbles to mineral surfaces in solutions of collectors with different chain length. The attachment time was determined by a recently developed proprietary apparatus from transient force vs. time and transient film thickness vs. time curves, which, in turn, were obtained using a tensiometer and high-speed camera, respectively. We measured the time taken by air bubbles to attach to mineral surfaces as a function of surface hydrophobicity and bubble size.

Galena, a sulphide mineral recovered by flotation (Wills & Finch, 2016), was used as the representative mineral surface in this study. Galena is an important mineral in the lead-zinc flotation systems critical to the automobile industry and is relatively stable in water, relatively hydrophobic and has a good crystal structure (cubic) with planar faces (Xie et al., 2016). Thus, its hydrophobicity can be controlled by conditioning with xanthates (thiol-type surfactants) of different carbon-chain lengths (Özün and Ergen, 2019) and it can be reduced in roughness more easily than other minerals. Further, experiments were conducted in conditions similar to those encountered in the actual flotation process of galena.

The measured results were quantified by a drainage model developed using the multilayer dispersion theory that incorporates the IGE of dissolved gases. The advanced van der Waals pressure was calculated using the Hamaker function, which considers the thickness of the IGE layer on the galena surface. The multilayer dispersion theory has allowed us to describe the effect of galena hydrophobicity induced by the xanthate chain length in terms of IGE on the bubble-surface attachment. Finally, this study made it possible to explain the effect of the chain length of the collector on attachment time and provides the principle for the attachment mechanism of hydrophobic minerals to air bubbles.