Novel application of depletion attraction in malachite flotation: A significance of depletant molecular weights

Malachite, a copper carbonate mineral, exhibits ordinary flotation performance and thereby requires certain collectors and depletants to improve the floatability. Conventional collectors, particularly sulfur and phosphorus-based collectors, provide satisfactory performance but can present malignant environmental consequences with cost-intensive operations when employed in large-scale applications. In the present study, the effects of an eco-friendly depletant’s (polyethylene glycol, PEG) molecular weight (MW) on malachite floatability were experimentally and theoretically investigated along with the association of with solution ionic strength (IS). Flotation performance increased from ~22–65% when the MW was increased from 1 × 102 – 1 × 103 g/mol, respectively, whereas it was only ~20% without PEG. A further rise in the MW (< 5 ×103 g/mol) had a negligible effect on the floatability. The associated radius of gyration (Rg) values for the depletant MWs of 1 × 102 and 5 × 103 g/mol were 0.41 nm and 4.28 nm, respectively. The depletant’s MW also influenced the depletion force (VDEP) and a larger MW at a certain separation distance was found to be efficient in reducing the energy barrier (EB) between malachite and bubbles, and thus higher malachite floatability was observed with 5 × 103 g/mol PEG than 1 × 102 g/mol PEG. Floatability was also favored by IS: for the fixed depletant’s MW value of 1 × 103 g/mol, floatability increased from ~ 65% to > 96% when IS was increased from 1 to 20 mM, respectively. The IS can modify the Debye length (ld) and thereby can control the extent of repulsive electrostatic interactions. However, for ld > Rg or lower IS, the VDEP was unable to reduce the EB owing to the insignificant attachment interaction between malachite and bubbles. Therefore, PEG with a larger MW in combination with a higher IS is expected to hold notable potential for efficient separation of malachite from silica. The outcome of this research suggested that a new concept introduces the application of PEG as depletant in malachite flotation. [Display omitted] •Separation of malachite and silica by flotation with PEG as depletant.•The malachite floatability improves with increasing molecular weight of PEG.•Larger molecular weight PEG was more effective to reduce the energy barrier between malachite and bubble.•Ionic strength reduces the Debye length (ld), thereby enhancing the floatability.