The second law of thermodynamics is the fundamental theoretical basis for the ability of different minerals to be stratified in a gravity field according to their different specific gravities and thus separated.

Jigger

　　According to the theory of the second law of thermodynamics, any system in the world has a tendency to spontaneously proceed in the direction of free energy reduction. In the gravity sorting bed, large specific gravity minerals, high potential energy; small specific gravity minerals, small specific gravity, low potential energy; therefore, in the appropriate loose conditions, high potential energy of heavy minerals can be spontaneous to the bottom of the movement of low potential energy. And then achieve the purpose of stratification and separation according to specific gravity.

　　The second law of thermodynamics also indicates that all changes in nature do not return to their original state and require external conditions, and that there is a great difference between the initial and final stages of a change, and this difference affects the direction of the change. Therefore, with the help of the second law of thermodynamics, it is possible to artificially create such differences, such as by jigging, so that minerals can be separated.

　　The size of the difference in the specific gravity of the minerals and the appropriate means of loosening them are intrinsic to the theory of jigging beneficiation.

　　Only there is a large enough difference in specific gravity, the minerals can be separated according to the specific gravity, so "the size of the difference in specific gravity of minerals" is the internal cause of jig separation.

　　If there is a large enough difference between the specific gravity of the two minerals, the separation conditions are not suitable, or the degree of looseness is not enough, it is also possible that the minerals have been separated according to the specific gravity of the minerals and then re-mixed. Therefore, "suitable means of loosening" is the external cause of jigging separation.

　　This also tells us why coarse particles should be sorted under turbulent conditions, while fine particles should be sorted under weak turbulent conditions or even near laminar conditions, and for fine-grained materials, it is necessary to use laminar flow, and it is necessary to use the theory of interlaminar shear.

　　In terms of the theory of re-selection, under gravity field conditions, as the size of the mineral sorting decreases, its effective gravity in water becomes smaller and smaller. In turn, the uplift force on the fine particles is increasing due to the tension and bonding effect of the water. At a certain point even exceeds the gravity effect, so that the micro-fine particles of minerals can not sink and go with the flow. Therefore diversified transformation of jigger, flexible control of water volume, stroke, stroke, bed layer, etc., is an effective way to reduce the cost of gravity separation and improve the efficiency of gravity separation.