Morphological characteristics of starch sol-gel and its influences on flocculation of fine particles

Abstract

Pasting starch into sol-gels by alkali is one of the most common methods for its use as flocculants in flotation. Any variation in its gelatinization process, however, could lead to different starch solutions with certain charges, sizes, and configurations of the segments, influencing their flocculating properties onto mineral surfaces. The purpose of this study is to identify the morphological characteristics of the starch sol–gel digested with different concentrations of alkali and their flocculation mechanisms on hematite through a series of tests, such as viscosity, turbidity, adsorption, pasting titration, settling, zeta potential All results pointed out that the gel-sol transition of starch was possibly governed by alkali concentrations or starch to alkali weight ratios. As the sodium hydroxide concentration increases, an increase in viscosity but a decrease in turbidity of the starch solution was obtained, inducing an upwards trend in the settling rates of the particle flocs. A suitable sol–gel by 1.25% sodium hydroxide (a 1.6 wt ratio of starch to NaOH) at turbidity of near 2 NTU and a viscosity of 258 mL/g harvested a maximum adsorption density of 11.5 mg/g hematite at a settling rate of 7.1 × 10−5 m/s. A certain amount of acidic contents on the segment chains resulting from alkali gelatinization play an important role in determining the morphological characteristics of the starch sol–gel at different pH values affecting adsorption density on iron oxides. It is worthy to be noted that the starch remnants at very small sizes produced by high concentrations of NaOH by over 1.50% or at a starch to alkali weight ratio of less than 1.3 were hard to achieve high efficiency on flocculating the particles but occasionally dispersing them instead. It could attribute to the less possibility of “bridging interactions” between the particles and these small segments with negative charges, which may limit the size and compactness of the particle flocs, and, consequently, reduce their settling rates.