Adsorption and morphology of oxidized starches on graphite

Highlights

• Non-linear depression profile of graphite when functioned with a series of oxidized starches.

• A low starch concentration exhibits similar surface adsorption but different morphology.

• A high starch concentration illustrates deviations of surface adsorption and morphology.

• Diverse conformations and structural distributions determine surface hydrophobicity changes.

Abstract

Hydrophobic gangue minerals such as talc, graphite and mica cause selectivity problems during flotation separation because they can be floated together with valuable minerals. This issue becomes increasingly prominent as the higher quality resources are depleted quickly. Here, we demonstrate a promising and green approach which involves simple and oxidative modifications of starch structure features to inhibit graphite floatability. The primary focus was on comprehensively interrogating how systematically developed oxidized starch derivatives differentially and uniquely modify graphite surface hydrophobicity. Adsorption tests, AFM imaging and topographical characterisation were conducted and correlated with flotation responses in the absence (baseline case) and presence of the starch derivatives. At a low concentration (10 mg/L), the starch derivatives exhibited relatively similar surface adsorption in contrast to profound dissimilar flotation outcomes. However, attention to AFM morphological visualizations and resolving the adsorbed structural distributions at the nanoscale level revealed compelling evidence of disparate appearances on graphite surface. The oxidation-induced preferential conformations of starch structure features were judged as the overarching factors conferring surface hydrophobicity reductions, which is important if graphite flotation inhibition is to be successful.

Introduction

Graphite, a crystalline form of the element carbon (C), is a prevalent gangue mineral that causes separation problems during froth flotation in the minerals industry (Afenya, 1982, Pugh, 1989). Its inimical existence along with valuable metal sources such as copper and lead sulphides demands its inhibition during froth flotation (Gredelj et al., 2009, Subramanian and Laskowski, 1993). Froth flotation is a popular surface-based technique that utilizes natural and reagent-induced differences in surface hydrophobicity as a basis of minerals separation (Li et al., 2019, Zhao et al., 2019, Chen and Tang, 2020). Graphite is made up of hexagonally packed C atoms and exhibits hydrophobicity on the basal plane surfaces (Afenya, 1982, Pugh, 1989). These basal plane surfaces enhance its propensity to be elevated through surface interactions with rising air bubbles during flotation (Beaussart et al., 2009). Additionally, graphite can also cause a surficial coating of other gangue minerals, imparting surface hydrophobicity and their unwanted subsequent floatability (Gupta, 2017).

The previous reports have described the role of biopolymers such as native starch, carboxymethyl cellulose (CMC) and dextrin (starch derivative from acid hydrolysis) as environmentally benign and non-toxic reagents to inhibit the flotation of graphite and other hydrophobic substrates such as coal, talc, molybdenite and carbonaceous pyrite etc. (Beaussart et al., 2009, Kaggwa et al., 2005). The prominently reported mode of inhibition involves polymer binding via hydrophobic bonding, orienting to form a hydrophilic layer on the mineral surface (Beaussart et al., 2009, Bera et al., 2013). This, in turn, impedes mineral surface attachment to air bubbles and transfer to the concentrates through true flotation. However, these biopolymers exhibit low performance and poor selectivity attributed to non-discriminatory adsorption on various mineral surfaces (Zhao et al., 2019, Liu et al., 2020, Liu et al., 2000). For native starch, macromolecular architectural complexity and uniformly distributed multiple –OH groups not only limit the selective inhibitory action but also contribute to an unclear understanding of the surface adsorption specifics at the atomic and molecular structure level. Nevertheless, besides low cost and wide availability, starch possesses flexibility for molecular modifications to improve suitability for specific functions (Meng et al., 2018, Chimonyo et al., 2020).

A promising example is simple or mild starch oxidative modifications. These modifications result in diverse and hierarchical structure features induced in the starch derivatives. In brief, the phenomena alter starch polarity functionality through partial oxidation of the –OH groups, forming COOH and CO groups on the glycosyl residues. A simultaneous transformation of the starch macromolecular structure also occurs through breakages of the amylose and amylopectin homopolymers constituting the starch backbone matrix. This enhances the chains’ steric freedoms and mobility (Shrestha and Halley, 2014). Another possibility is the homolytic cleavage of the C2C3 bond on the ring structure, which is likely only at extreme oxidizing conditions. The generated starch derivatives possess improved surface binding properties, solvation and low viscosity etc. They are commonly investigated as surface sizing agents with better film properties, highly required in food and non-food industries (e.g. plastics, paper, etc.) (Shrestha and Halley, 2014).

Comparatively, the literature is deficient of detailed studies on oxidatively modified starches as surface modifiers during minerals flotation. Therefore, little is known on the effects of starch oxidation in flotation research. In fact, systematic alterations of the starch structure features, size and shape to different magnitudes can be instrumental in determining the polymers surface assembly mechanisms as well as the conformations on mineral surfaces. We previously established that an oxidized starch derivative (Ox 5/120) holds a profound capacity to function as an efficacious reagent of choice for selective surface activity between chalcopyrite and graphite when compared to the unmodified version (Chimonyo et al., 2020). This prompts further fundamental research questions: what is the influence of sequential starch-structure modifications on mineral surface interactive behaviour and morphology, and how is graphite surface hydrophobicity differentially modified? These questions framed the scope and focus of this study.

A series of derivatives of starch was developed using various NaClO concentrations to attain different oxidation levels i.e. (Ox 1/120) (low level), Ox 5/120 (median level) and Ox 10/120 (high level) and adopted for a comparative investigation. The study focusses on graphite and comprehensively articulates how the different oxidized structure features modify carbon surface property and influence its depression during flotation. This sought to recommend the outstanding starch structure for depressing graphite following varying levels of oxidative modifications.

The main objective here was to emphasize on nature of surface adsorption and evolution of structural configurations of the differently oxidized starches on the hydrophobic graphite surface. Adsorption tests, AFM topographical imaging and nanoscale resolutions were employed as a reliable toolkit to elucidate the underpinning molecular surface activities for correlation with the mechanisms of hydrophobicity modifications and graphite flotation inhibition.