SMART DOG MINING^TM
It takes a smart dog to find hidden treasures

An Introduction to the Principles of Jigging

This being an introduction, the following is a general overview description of the principles behind jigging.  For a more detailed discussion of the principles see Introduction to Gravity Concentrating.   Jigs operate on the principles of particle motion under pulsed-flow hindered-settling conditions. The separating mechanisms are typically: hindered settling, differential acceleration, and consolidation trickling.

The liquid pulse in the jig, in its ideal form, is essentially a modulated sine wave (figure 2).  This wave is recommended by a number of authors from research or experience, but basically for the following reasons:

1)    The settling considerations are those described by hindered settling, with the difference that if the pulse frequency is high enough only the largest and heaviest particles reach their terminal velocities at any part of the jig stroke.

2)    The separation of minerals according to specific gravities is brought about by the combined acceleration of gravity and liquid pulsation, upward and downward, according to the formula 1, where p" is the apparent density (g/cc) of the medium, and p is the specific gravity of the particle.

                                                                   [1]

3)    In a true sine wave pulse, separation during the downward pulse could be cancelled by that during the upward pulse.  Thus, the wave form must be modified to minimize separation on the upward stroke by using a rapid upward pulse to lift the bed as a dense mass so relative particle motion is kept as small as possible.

4)    The downward pulse wave form must be modified to take full advantage of the downward acceleration due to gravity, followed by the acceleration caused by the downward liquid pulse.

 Early work in coal by G. A. Vissac presented the positions during one stroke, of two particles with equivalent settling velocities but with different specific gravities.  These are plotted in Figure 3.  From this only a few jig strokes would be needed to make an efficient separation in such a jig. How practical it is to modulate the normal sine curve stroke to this form depends on the requirements of the ore, the jig design, and costs of power and maintenance.

Figure 3:  Relative positions of coal (black square) and rock (open circle) in water during one jig cycle of ~1.2 seconds (after Vissac).

 From Figure 3, for larger particles, there is a part of the stroke where their downward travel is stopped by the supporting screen, and relative movement is not possible except for the percolation or trickling of the fine particles downward through the spaces. This effect is used to separate fine metallic particles, such as gold, which are drawn down through the bed of coarse gangue particles by the descending water.  Also, it helps to modulate the wave form by resistance to water flow during the downward stroke.

 From Figure 3 an upward flow of water will help the lifting of the bed on the upstroke and also reduce the time that the particles will be at rest on the screen. It will also help to separate the particles that respond to hindered settling separation. Too much upflow water will destroy the true jigging action, particularly during the downstroke, and equivalent settling velocity particles will not be separated.

 In any jig application the size distribution and the specific gravity of the particles of the ore will result in a unique situation requiring optimization of each of the jig actions.  It can be shown that for concentration of heavy fine particles the suction phase must be augmented, while for coarse heavy particles the pulsion phase is the most important.  These opposed considerations give rise to much of the divergence of opinion in optimizing stoke speed, amplitude and modulation and in jig design in general.

The basic design features are:

1)    A screen to support the mineral bed.

2)    A hutch or tank containing the liquid beneath the screen.

3)    A means of creating a jig stroke or relative motion between the liquid and the bed.

4)    A method of modulating the jig-stroke wave form.

5)    A method of regulating water up-flow.

6)    A method of supplying feed to the bed.

7)    A method of removing products from above the screen and from the hutch.