In this study, the flotation behavior of malachite was investigated using carboxyl-ftuictionalized carbon black nanoparticles (CB-NPs) as collectors in a hydrodynamically simplified Hallimond tube. We found that malachite floatability increased with increasing CB-NP concentration in the low range (0.5-2 kg/ton). To understand the mechanism of this higher floatability for low CB-NP concentrations, we examined the amounts of deposited CB-NPs on the malachite surface and hydrophobicity of the surface with deposited CB-NPs according to changes in the initial CB-NP concentration. The increase in malachite floatability was likely associated with increases in the amounts of deposited CB-NPs on the malachite surface, which enhanced malachite floatability by altering the hydrophobicity of its surface. Meanwhile, malachite floatability sharply decreased at CB-NP concentrations exceeding 2 kg/ton. To understand this unusual flotation behavior for high CB-NP concentrations, we also examined the amounts of CB-NPs deposited on the malachite surface and contact angles. However, the decreased malachite floatability could not be fully explained by these experiments, implying that at least one additional mechanism is involved. Therefore, additional flotation experiments including filtration processes and different flotation times were conducted, revealing that the unusual flotation behavior was likely due to the presence of suspended CB-NP aggregates, which reduced the kinetic rates of attachment between malachite and bubbles.