Text edited by Rob Vugteveen, Director, Asarco
Mineral Discovery Center
The primary crusher reduces ore from the trucks to 8-inches
or less. That material goes to the primary stock pile directly
that you see crushed rock falling onto in your image. Two conveyors
(out of view to the right in your image) pull this material directly
into each SAG mill where it is combined with water and mill balls
(as needed).
Ore is constantly flowing in and out of the SAG mills. The
outflow passes through a set of vibrating screens and a water
spray. The last screen has 10 mm x 20 mm slots in it. Anything
that goes through this last screen or is washed off the bigger
chunks by the water spray goes onto the ball mills. The oversized
material is conveyed either directly back into the SAG mills,
or it travels on a conveyor (the one closest to you and rising
to the left in your image) to be stored in the "recycle
pile" right.
If you looked at that recycle pile, you would note that the
rocks are remarkably similar in size. This is called the "critical
size" and it is about the same size as the space created
between four mill balls that collide together in a tetrahedron.
This size varies from SAG mill to SAG mill, but is dependent
on the ball size and ore hardness. We may crush these rocks to
a smaller size before feeding them back into the SAG mills, and
you can see a small crushing plant between the front and back
conveyors of your image. Material is pulled off the bottom of
the recycle pile (out of view on the left) and rides up the conveyor
to the top of the cone crusher (I'll describe that some other
time). Dust is captured, and the crushed material rides up the
conveyor you can just see at the lower right of your image and
into the SAG mills.
The material in the recycle pile may also include broken
mill balls. Those behave badly in the cone crusher and such damage
must be avoided. The conveyor between the recycle pile and the
cone crusher passes under two magnets which pull the ball fragments
off and throw them to the side where they are collected for recycling.
I digress even further, but if you look at that pile of rusty
brown ball fragments, you see black blobs as well. The magnets
also pull off rocks that contain large amounts of magnetite.
Unfortunately for us, these magnetite-rich cobbles are also high
in chalcopyrite. Perhaps your students could devise a cost-effective
way to separate the mill ball fragments from the magnetite-rich
rocks!
