Copper Mining  Elmhurst College
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Ore Crushing
Asarco Mining Operations in Arizona
Mission Mine, Tucson
Text edited by Rob Vugteveen, Director, Asarco Mineral Discovery Center

 

Ore Crushing Operations

Outside of the Mission South Mill, trucks dump the ore into the primary crusher which reduces the rocks to 8 inches or smaller. Inside the mill, the ore is mixed with water in two rotating SAG mills which use the larger rocks and 8-inch steel balls to reduce the ore to about 10 mm or smaller. Then two ball mills grind the ore to about 0.2 mm with 3-inch steel balls. This copper ore slurry is finally pumped to the flotation deck.

Asarco milling link -rotary mills

Ore Crushing in Detail:
 
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!