Why do gangue particles float in heavy-media separation?

In heavy-media separation, also known as sink-and-float separation, gangue particles float because they possess a lower density than the medium used in the process.1

This method utilizes a suspension of finely ground heavy minerals—such as magnetite or ferrosilicon—in water to simulate a fluid with a density higher than that of water. When ground ore is introduced into this suspension, the gangue—the valueless rock and minerals associated with the ore—naturally floats due to its lower density and is subsequently removed as tailings. Conversely, the valuable minerals, which have a higher density, sink to the bottom and are collected separately.12