§ 4.2.4 Separating Minerals by Floatation and Vibration

It's possible to separate some minerals by their density, once we have a sieved collection of grains of the same size.

Just vibrating a bed of same sized grains will separate mineral grains into layers fairly well based on their "weight" in a centrifuge or in lunar gravity. 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.

The quick & dirty theory on floatation

Floatation is based primarily on surface phenomenon, not the specific gravity of the mineral.

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. Of interest is minerals' "wettability" or repellant properties, relative degree of readiness to adhere to bubbles, and affinity for certain types of chemical compounds or reagents.

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. The downside in space is the need to stringently recycle the liquid used for the floatation, as long as volatiles remain in short supply in earth orbit.

The quick & dirty theory on "gravity concentration"

There are two processes at work:

1. Small particles tend to settle at the bottom, and large particles at the top.
2. Given particles of the same size, heavier particles (i.e., higher specific gravity) will settle down further.