Two-Stage Hydrophobic and Hydrophilic Aggregation for the Flotation Separation of Fine and Ultrafine Minerals
Creators: Wang, Daowei
Publisher: name: University of Alberta Library
Year: 2023

<p>In mineral processing froth flotation is the most important technique to recover and separate different minerals. However, the flotation process deteriorates significantly with low efficiency and selectivity when mineral particles are smaller than 20 µm. Low flotation kinetics of fine hydrophobic particles and high mechanical entrainment of fine gangue hydrophilic particles are two major problems associated with fine and ultrafine mineral flotation. <br>
In view of the two problems, the strategy of a two-stage hydrophobic and hydrophilic aggregation/flocculation process was proposed, aiming to separately aggregate hydrophobic particles and hydrophilic particles with appropriate approaches and ideally develop both hydrophobic and hydrophilic aggregates in the slurry simultaneously or sequentially. The two types of aggregates are then separated using conventional froth flotation procedures. Since the extensive study of hydrophobic particles aggregation in literature, significant efforts in this dissertation were made to investigate the aggregation, dispersion, and flocculation property of ultrafine hydrophilic particles and their entrainment reduction when different polymers were used in flotation. <br>
In this dissertation, hematite and quartz of high purity were used as a model mineral system. Flotation tests were conducted using a Denver D-12 laboratory flotation machine and a custom-built flotation column. Natural polymers including corn starch, corn dextrin, tannic acid and synthetic polymer poly(ethylene oxide) were used as depressants, dispersants, and flocculants for the study.<br>
It was found that the aggregation of hydrophilic particles benefited their entrainment reduction while the dispersion state aggravated their entrainment. In the reverse cationic flotation of quartz from hematite in the mechanical flotation cell, the polymer flocculation of hematite by corn starch and the hydrophobic flocculation of quartz by octadecyalmine acetate could significantly improve the flotation separation. When the corn starch was replaced by corn dextrin which had a much lower molecular weight, severe hematite entrainment to the froth product was observed. Test work using in-situ particle size analysis by focused beam reflectance measurement, coarse hematite particles, and particle surface hydrophilicity measurement, confirmed that the different separation results were solely caused by the different degrees of aggregation/dispersion of the hematite.<br>
As a typical gangue mineral, flocculation behaviors of ultrafine hydrophilic quartz by poly(ethylene oxide) (PEO, 8M) were investigated with and without tannic acid (TAN). It was found that using PEO alone as a flocculant, the floc size and flocculation effectiveness of quartz decreased in the order of pH 3.1 &gt; pH 6.3 &gt;&gt; pH 9.4. However, the prior addition of tannic acid reversed this trend so that floc size decreased in the order of pH 9.4 &gt; pH 6.3 &gt;&gt; pH 3.1. Tannic acid was also found to change the shear resistance and reversibility properties of quartz flocs formed by PEO in turbulence, depending on the TAN/PEO ratios and medium pH. Measurements including zeta potential, adsorption density, confocal scanning microscope and freeze-drying-SEM imaging indicated that the pathway of the dual polymer flocculation was via the initial formation of TAN-PEO associative complexes in solution and then bridging of quartz by the associative complexes. The TAN-PEO associative complexes showed different structures and configurations at different pH, in line with a change in hydrogen bonding, which in turn affected floc structures and properties under turbulent conditions.<br>
The TAN-PEO dual polymer combination was tested in column flotation to flocculate quartz and reduce its entrainment, with hematite floated directly using n-octyl hydroxamic acid (OHA) as a collector. It was confirmed that flocculating quartz by TAN-PEO polymer system to form hydrophilic flocs was effective to reduce its mechanical entrainment, however, the reduced degree of quartz entrainment was dependent on the agitation intensity applied for slurry conditioning. A linear correlation was established between mean floc size and the reduced entrainment degree when the average floc size was less than 68 µm. Moreover, dual TAN-PEO polymers enhanced both flotation kinetics and recovery for hematite by OHA, benefiting from a synergistic effect between OHA and PEO. Overall, a dual function of tannic acid was revealed in that it could not only improve the flocculation of quartz and helped to obtain more shear-resistant flocs by PEO but also could weaken the surface activity and foaming stability of PEO, both of which will be beneficial in reducing the entrainment of ultrafine quartz in flotation.</p>