Jig Models: Bringing Operation and Theory Closer Together
The ASTRAD jig model is a mathematical model for predicting both
the rate and ultimate separation of ore by density (and size) in a
jig. It provides a convenient framework for interpreting, mass
balancing, and scaling jig test work data from small laboratory
batch jigs through to pilot plants and full scale industrial jigs.
An entire series of stratification tests for a known feed
washability can usually be summarized by two main parameters: a
mobility coefficient, and a diffusion coefficient. Once these
parameters have been determined, either by test work or from
historically similar washability data, it is possible to calculate
the following data:
-
Partition curves (split to product as a function of
density),
-
Grade/Recovery curves (provided the density grade
relationship is known),
-
Jig capacity as a function of the above curves, jig
dimensions, and feed rate,
-
What-if scenarios (what if the feed rate increases by 10%?;
What if the ore quality degrades by 5% etc.).
One of the main features of ASTRAD is the ability to determine
improvements in jig yield as a function of jig size and feed rate
which permits an economically optimum jig size to be selected. It
can also determine the range of yields achievable over an ore body
which has a variable but known washability (density distribution).
Theory
In a jig bed, whether it be batch or continuous, the particles
are subjected to a jigging action for a given residence time (the
batch and continuous residence times being related by the transport
velocity profile in the continuous jig).
The particles start in a mixed state and the denser particles
progressively drop to the bottom of the bed, and the less dense ones
to the top of the bed. The rate at which this happens depends on the
density of the particle relative to the surrounding material, and
the fluid dynamics of the jig pulse. Typically finer particles
separate more slowly and less well than coarser particles.
Eventually the material separates to an equilibrium state where
the material is in balance between the tendency to separate and the
tendency to remix due to fluid turbulence forces and stochastic
particle interactions. For this reason the final equilibrium only
approaches that achievable by a heavy liquid result.
The key factors in determining the jig capacity for an ore are
therefore:
- Residence time of particles in the jig (a function of feed
rate and jig dimensions),
- The density (and size) range of the product and reject
components of the feed,
- The separation and remixing rate determined by the jig
pulse.
The design size is chosen to best match the residence time in the
jig with that required for the material in the bed to approach
equilibrium (maximum separation). The operation point of the jig
pulse is set to maximize the separation rate while minimizing the
turbulent remixing effects.