Insight on the mechanism of hexagonal pyrrhotite depression by starch during flotation

Abstract

Polysaccharides such as gelatinized starch have been largely employed to depress specific minerals during froth flotation. In this study, two samples of caustic gelatinized starch (paste starches) with different molecular weights (MW) were used as depressants for hexagonal pyrrhotite. There is an increasing interest in finding more environmentally friendly strategies to depress hexagonal pyrrhotite since this mineral is one major gangue minerals in nickeferrous ore deposits, for instance. The mechanism of adsorption of paste starches on hexagonal pyrrhotite was evaluated by different techniques. Equilibrium adsorption tests suggested that the adsorption is dependent on the depressant molecular weight if it is used in high concentrations. The sequence of conditioning also showed to be relevant. Pre-adsorbed collector highly influenced the adsorption of higher MW paste starch. Langmuir and Brunauer–Emmett–Teller (BET) model fittings of adsorption data in combination with Quartz Crystal Microbalance with Dissipation (QCM-D) results demonstrated a looser packing density of the higher MW paste starch. This conformation provides more room for water molecules within this depressant’s chains. Since trapped water molecules may generate a repulsive hydration force to air bubbles near the mineral surface, pyrrhotite conditioned with the high MW paste starch showed a lower flotation recovery. Higher hydration levels of the higher MW paste starch were also proven by Atomic Force Microscopy (AFM) tests, and indirectly quantified in the contact angle measurements (Washburn method). Although the higher MW paste starch adsorbs less, this depressant still provides a higher surface wettability. This study has reinforced the importance in considering the molecular size of a polysaccharide for sulfide mineral depression.