The solitary settling of a single mineral particle in a vast medium is called free settling, while the settling of individual mineral particles within a particle swarm is called interference settling. In free settling, the mineral particle is only affected by its own weight, the buoyancy of the medium, and drag, and is unaffected by other factors. However, in interference settling, individual mineral particles, in addition to being affected by their own weight, the buoyancy of the medium, and drag, are also affected by friction and collisions with other mineral particles and the effects of the suspended body, thus altering the settling velocity and trajectory of the individual particles. Interference settling is much more complex than free settling. In gravity separation practice, all settling is interference settling, but when the distance between particles in a particle swarm is large, that is, when the volume concentration (the ratio of the total volume occupied by the mineral particles to the total volume of the suspension) is very small (generally less than 3%), the interference between mineral particles becomes very weak, and this can be considered free settling.
In free settling, the terminal settling velocity of a mineral particle is affected by the particle density, particle size, and shape. Particles of different densities, sizes, and shapes can have the same final settling velocity when settling in the same medium under specific conditions. These different particles with the same final settling velocity are called isostatic particles. Clearly, isostatic particles include large particles with low density (let’s say their size is d₁) and small particles with high density (let’s say their size is d₂). The ratio of d₁ to d₂ is called the isostatic ratio.