Surface Properties of New York Talc as a Function of pH, Polymer Adsorption and Electrolyte Concentration

A Thesis Submitted for the Degree of DOCTOR OF PHILOSOPHY

By:

Elizaveta Burdukova

Faculty of Engineering and Built Environment University of Cape Town

February 2007

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SYNOPSIS Talc is a common siliceous gangue mineral found in ore bodies beneficiated by flotation. Talc is a layered anisotropic mineral, with a mineral structure comprising of basal planes and edges. The basal planes of talc are considered completely neutral, while the edges of talc are thought to undergo a change from positive to negative at a highly acidic pH.

Talc is highly floatable, requiring polymeric depressants to be added to flotation pulps in order to prevent it from entering the concentrate and reducing its grade. Various polymers such as non-ionic guar gums and anionic Carboxymethyl cellulose (CMC) depressants are employed for this purpose. However, the industrial use of these reagents is still inefficient. One of the factors affecting the performance of these polymers as depressants is the water quality of the flotation medium, specifically the concentration and type of electrolyte present.

The objective of this thesis was to investigate of the surface properties and structure talc, and its effect on the adsorption mechanisms of different types of polymeric depressants in the presence of varying types of metal cations in solution in the context of mineral flotation.

The surface potential of talc was investigated using rheological, titration and electron microprobe analyses. It was found that the rheological behaviour of talc is inconsistent with the premise that the talc basal planes carry no electrical charge. Furthermore, potentiometric titration measurements showed that the point of zero charge of talc does not correspond to the electrophoretic isoelectric point of talc. The discrepancy was attributed to the unsuitability of zeta potential measurements for evaluation of anisotropic plate-like minerals.

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An alternative surface charge distribution of talc was proposed, whereby the basal planes of talc carry a negative charge as a result of substitution of silicon ions in the tetrahedral layers of talc with aluminium ions of a lesser charge. This was supported by both ToF - SIMS and electron microprobe analyses. It was also proposed that the edges of talc undergo a change from positive to negative in an alkaline pH range.

The effect of talc surface charge distribution on the adsorption characteristics of polymeric depressants was investigated using rheological measurements. The effect of the presence of calcium and potassium cations on the surface charge distribution of talc and hence polymer adsorption was studied using surface specific ToF-SIMS measurements.

The adsorption of guar gum onto talc was found to take place by means of hydrogen/hydrophobic bonding and was independent of the surface charge of the mineral particles. The adsorption of guar gum was found to take place primarily on the basal planes of talc.

The adsorption of CMC onto talc was found to take place by means of electrostatic attraction to the positively charged talc edges. The adsorption onto the basal planes of talc was found to take place by means of electrostatic attraction as well as acid/base interaction with metal hydroxide species present on the talc surface.

The effect of calcium and potassium ions on the adsorption of CMC as a result of solution interaction between polymer molecules and ions was studied by examining the using ToF - SIMS measurements. It was found that the adsorption of CMC molecules onto talc is further promoted by the chemical interaction between the polymer and Ca+2 ions in the solution medium, which causes an increased amount of polymer coiling, thus promoting denser adsorption.