2025 Theses Doctoral

Complexity in Mineral-Ligand Interactions: Coordination Chemistry of Allyl and Alkyl Thionocarbamate Ligands and Their Interactions with Energy-Relevant Chalcophiles and Their Minerals

Guo, Shiqi

Several families of bifunctional short-chain organic S-based ligands are used widely in the commercial flotation separation and extraction of strategic and energy-critical base and precious metal sulfide minerals of chalcophiles such as Cu, Pt, Pd, Au, Ag, Ni, and Co, from both primary (ores) and secondary (tailings, recycled streams) sources [1, 2]. These ligands selectively adsorb on the desired minerals and make their surfaces sufficiently hydrophobic to facilitate their attachment to air bubbles in a flotation process. The functional groups in the traditional and widely used ligands (or flotation collectors) are xanthates, dithiophosphates, and a specific thionocarbamate (Figure 0.1). The R represents a hydrocarbon group that imparts hydrophobicity to the region of the mineral surface where the ligand adsorbs.

The use of a combination of collectors at different dosages and different sequences of collector addition is often employed to recover multiple mineral phases containing the target metals. The combined collectors can be classified as primary and secondary collectors. Some of the collector molecules investigated in this thesis function as secondary collectors, playing a crucial role in recovering valuable minerals that cannot be effectively floated by conventional primary collectors, while also minimizing the recovery of non-valuable sulfide minerals. The term secondary does not imply lesser importance; rather, both primary and secondary collectors are essential for achieving higher recoveries and grades in flotation.

With the global decline in grades and quality of ores, the mineral industry is experiencing reduced efficacy of these traditional ligands in the recovery of value minerals, especially in the coarse (>200 µm) and fine (<25 µm) size range. In the processing of Cu sulfide ores, for example, Cu values lost in the coarse and fine size ranges amount to ~$14 billion. Several alternative S-based ligands have been developed to overcome these deficiencies of traditional ligands, often serving as secondary collectors. One such alternative ligand is N-Allyl O-alkyl thionocarbamates (ATC, Figure 0.2b), which differ from their closest and traditionally used analog N-Alkyl O-alkyl thionocarbamate (DATC, Figure 0.2a), by the presence of the allylic double bond [2, 3]. Examples of alkyl and allyl thionocarbamates are shown in the figure below.

Empirical data from practical ore systems indicates that this seemingly small change in the structure (sub-nm scale) results in large changes in their interaction with certain chalcophiles (PGM, Cu, Au, Ag) and their minerals at the nm scale, translating into large changes in flotation outcomes at the macro level, with significant improvement in some systems, viz. enhancing flotation of partially (or poorly) liberated 1-300 µm complex particles (Figure 0.3) from low-grade mined ores.

In some ore flotation systems, the allyl thionocarbamate has exhibited superior performance attributes (e.g., enhanced hydrophobicity of the adsorbed layer, enhanced bubble attachment and flotation rates) relative to the alkyl analog in several mineral systems, e.g., platinum group minerals, Cu sulfides, and Zn sulfide [4]. Since the only structural difference between ATC and DATC is the allylic double bond, the observed changes in macro-level flotation outcomes and other factors such as solubility, adsorption kinetics, adsorbed layer conformation and distribution (patchiness), bubble attachment/detachment probabilities, etc., are expected to be related to the double bond. Yet, surprisingly, an understanding of exactly how the allylic double bond causes these changes is sorely missing in published literature.

Although there are 17 families of S-based ligands currently used in plant practice, over the past 80 years or so, ~95% of the studies of interactions of S-based ligands with chalcophile minerals have focused on the anionic, short-chain water-soluble xanthates (Structure a). The proposed (and widely subscribed to) mechanism of xanthate-mineral interaction has been shown to be an electrochemical process involving oxidative chemisorption (appearing as an anodic pre-wave in a voltammogram suggested to be metal xanthate formation) and/or oxidation of the xanthate ligand to dixanthogen (appearing as the second anodic peak in a voltammogram. These anodic processes are coupled with reduction of oxidized species (cathodic currents) and O₂ reduction. This electrochemical mechanism is often (blindly) applied to rationalize the behavior of all other (non-xanthate) ligands.

There is also a strong tendency to force experimental conditions to observe anodic currents and then propose pathways to indicate oxidation of the ligand to rationalize anodic currents under these (forced) unrealistic conditions. However, such a mechanism is not applicable for most of the other ligand classes, especially charge-neutral, non-oxidizable ligands such as allyl alkyl thionocarbamate (ATC) or its saturated analogue dialkyl thionocarbamate (DATC). Other, and more plausible, mechanistic pathways have not been considered in past studies; for example, Lewis Acid-Base (LAB, aka electron acceptor-donor) interactions and Hard-Soft Acid-Base (HSAB) concepts (used extensively in many other fields) are invariably ignored.

There have been very few studies in the past on allyl thionocarbamates, and these studies do not shed light on many unanswered and important questions:

a) How exactly does ATC or DATC with a single S donor in the thione group (C=S) adsorb and attach to a chalcophile site on the mineral surface?
b) What is the bonding mode?
c) What is the role of the allylic double bond in interaction with the chalcophiles?
d) What is the origin of the selectivity of ATC (or DATC) vis-à-vis xanthates?
e) Why is ATC (or DATC) more selective for certain chalcophiles (and minerals) than xanthate?

The major goals of the proposed research are to:

1. Answer fundamental questions concerning the interfacial and bulk coordination chemistry of thionocarbamates and several analogous ligands, specifically ATC and DATC, by incorporating LAB and HSAB concepts,
2. Enhance the currently-inadequate knowledge base on ligand-mineral interactions, and
3. Integrate investigations at different length scales (missing in past studies), which is of great importance for practical applications. These length scales can be conveniently grouped into: micro-scale (molecular-scale donor-acceptor interactions at mineral interfaces), meso-scale (bubble-particle attachment), and macro-scale (ore flotation outcomes).

Various techniques are used, as appropriate, to test hypotheses and answer specific critical questions at different length scales.

At the micro-scale, electrochemical and spectroscopic techniques (XPS, ToF-SIMS, AFM, FTIR, Raman, NMR) will be employed to analyze ligand adsorption on mineral surfaces and the binding states of surface or bulk metal-ligand complexes. Solution property assessments, including surface tension, solubility, pKa, and stability constant measurements, will be conducted to determine fundamental solution properties of the ligands. Additionally, DFT calculations will be used to answer specific questions about ligand conformations, electron densities on donor atoms, and electron delocalization in ligands and metal-ligand complexes.

At the meso-scale, adsorption measurements via TOC and contact angles quantify ligand adsorption density on mineral surfaces and the resulting hydrophobicity.

At the macro level, flotation tests on real ores will be used to evaluate the separation efficiency and selectivity of the studied ligands in complex mineral processing systems.

This study systematically explored the positive role of allylic C=C in thionocarbamate adsorption in terms of Lewis acid-base interactions and the adsorption capacity (interfacial activity). It also investigated the interactions of other S-based ligands with chalcophile minerals, as well as the modification of mineral surface chemistry using simple inorganic salts. This study made substantial additions and revisions to the surface chemistry, coordination chemistry, and flotation science knowledge bases.