It's possible to separate some minerals by their density, once we have a sieved collection of grains of the same size. The denser grains fall to the bottom. Pouring material into a liquid of intermediate density will quickly separate a desired grain by floatation, though it must be dried thereafter and the fluid recycled. Floatation can be fine tuned based on the theory discussed below. In orbital space, zero gravity helps in this process, as a centrifuge can provide different levels of artificial gravity as different minerals settle at different rates.
Different materials have different affinities to a selected liquid and the air bubbles introduced.
The surface tension of the liquid is essential to fully understanding the process. Frothing reduces the surface tension of the liquid. Sulphide minerals of all types and sizes are the most easily floatable. On Earth, floatation is often opted because of its simplicity, selectivity and flexibility. Frothing and separation processes may be fairly interesting in low gravity.