A Tour of Asarco Mission Mine - Open Pit Copper Mine, Sahuarita AZ

Mrs. Dr. V's father is a professor of mining engineering, and one of the world's leading experts on optimizing mining.  He pioneered the use of computers for optimization - to get the most ore out of a mine using the least resources.  He has consulted around the world in many different countries with just about every type mine and mineral imaginable.  So, its no surprise that some of Karin's favorite memories as a kid are when he would take her to visit one of the mines in Germany.  She has a soft spot for mines and mining, and I like big equipment, so we wanted to go visit one of the big western mines while we're out west.

Karin was able to find an open-pit mine that gives tours just an hour away from where we were camped at Kartchner Caverns State Park in AZ, so we wandered over to learn more about copper mining.  Since Karin had visited an open pit mine before, I got to go on the guided tour while she took care of the dogs and worked on her math research.

The Mission Mine is owned by Asarco Grupo Mexico.  The information in this blog post is from the tour I took, their web site, and brochure.  

If you don't know, an open-pit mine is just that, a big old hole in the ground.  That's in contrast to having shafts and tunnels.  The benefit of open-pit mining is that you can use big equipment - really big equipment - to make a mine profitable, even when the desired ore is not super concentrated.  This picture shows me standing by one of the old, smaller haul trucks that's retired and on display at their museum.  The new ones are bigger (if you can imagine that) and more economical.  





 Apparently, there used to be 5 shaft mines in the same area, but they opened the open pit mine in 1961 and slowly gobbled those shaft mines up.  Here's a picture of the overburden (the horizontal mountain in the middle of the picture).  The over burden is the earth that has no ore or value, so you have to essentially get it out of the way to get to the ore (the good stuff).  Kinda like eating your vegetables, so you get dessert.

There is a herd of about 40-50 wild mustangs that call the area home.  We were lucky enough to see some on the bus trip up to the top of the mine.  They were smallish, compared to most horses I've seen.

Here's the view from the top.  It was so big I had to paste three pictures together to capture it all.  The mine is 2 1/2 miles long x 1 1/2 mile wide x 1500 feet deep.  That's over a quarter mile down.  To the left of this picture you can see how they cut the walls down using terraces.  Each of these is 40 feet high.  This picture doesn't really give you any of the feeling of just how big this place is.

 So, the way it works is that they drill a grid using this big drill.  You can tell the size by looking at the large work trucks next to the drill.  You can also see the grid of holes they've already drilled.  They take rock samples back to an on-site laboratory for analysis to determine how much ore is in the rock.  If there's none, it will be overburden and hauled to one area.  If it has copper, they take the ore for processing.  If it has other ores that have value (but are not their target, which is copper) they carefully store it away for possible future use.  Everything is carefully mapped out (using a GIS, I'd imagine) and our guide pointed out how the entire operation is optimized using computers.  If you look in the overall picture above, you can see this drill rig about 1/4 of the way in from the left of the picture.  After drilling, the fill the holes with explosives and explode the area, so they can get the rock out.  Apparently, they just exploded the day before our visit, so we didn't get to see that.

This is one of the small shovels, just moving over burden around (I think).  You can tell the size by the size of the service truck nearby.  In the overall picture (above), this is about 1/3 of the way from the left.

Meanwhile, the big shovel is loading the big trucks.  Here are three trucks and the big shovel.  The shovel is actually electric and power runs to it through a cable about as big around as my lower leg.  The trucks are diesel electric, like a locomotive.  Our guide said that the mine workers work a 12 hour shift, and once they get in the truck, there's no break unless their waiting to get loaded or waiting to unload.  Apparently making time to pee is pretty difficult.  You can see this area at the far right of the mine, about 1/6 of the way in, on the overall picture.

It takes about two scoops to load a truck, and off they go.  The hill is about an 8% grade, and they go 6 mph up it, trip after trip, 24 hours a day, 365 days a year.  The mine never shuts down.  Our guide said they have 25 haul trucks, and 20 are in use at any time.  The other five are in the shop for service, or new tires.  The tires are 12 feet in diameter (they're Michelins), and they have to replace them every six months at a cost of $50,000 per tire.  She also said that the trucks are so big, they have to take them apart to deliver them to the site.  Apparently, it takes 19 semi-trucks to haul all the pieces of one fo these big dump trucks to the mine site, and several weeks to assemble the dump truck.  She said that even though the driver sits way high in the cab, there's an 80 foot blind spot to the left of the truck where the driver can't see the ground.

Here's a dump truck moving along on a haul road.

Here's a picture of useless overburden and rock with ore that might be valuable some time that's carefully separated (but that doesn't have copper).

One of the largest issues they have is all the dust, so there are constantly huge trucks going around spraying water to keep the dust down.

The water trucks make an on-off-on pattern to keep from just making a muddy mess.  Here, two are going down the hill.  This is just right of middle in that overall picture.

Here's the schematic of how the ore is processed.  There's a primary crusher, that takes it down to about soccer ball sized rocks, then a SAG Mill, which rotates with 6" iron balls smashing the ore until it is stone sized.  Next it goes to ball mills, where 3" iron balls pound it literally into dust, about the consistency of talcum powder.  It's mixed with chemicals in water, and the copper separated out.

Here's the processing facility.

The raw ore...

The big wheel drum in the background is the SAG Mill, the drum in the foreground is the ball mill.  That SAG mill is about 20 feet in diameter, the ball mill is maybe 10 feet in diameter.  Each has many many shelves, about a foot or so wide, and either the 6" or 3" iron balls fall off the shelves as the drums rotate, crushing the ore to the consistency of powder.

The ore is mixed with lime, oil of pine, and a surfactant using giant mixers (kinda like a big kitchenaid).  Apparently, the oil of pine helps get the copper out of the ore mix (like the detergent Pinesol), and the surfactant makes bubbles which the copper sticks to.  I believe the lime helps glom together the part of the ore that's not copper (not sure about that though).  So, the surface of these bubbles you see are covered with copper - hence, the coppery sheen.

All that liquid along with the coppery bubbly stuff goes out here, where the copper bubbles are slothed off the top, and the leftover bits fall to the bottom.  The copper is de-watered, put on trucks, and hauled away to another processing plant in California.

The left over bits are pumped into this giant sediment retention basin...

 via this long plastic pipe.  The water is recycled and used again.  One sign I read said about 80% of water used in the mine is recycled.  The leftover ore leavings are simply left in the lagoon, which is built up over time to be thicker and thicker.

The trucks haul the copper to facilities where it is melted and formed into these plates, which are called anodes.  The anode is 99% pure copper and weighs about 750 pounds.  This is not pure enough to use in products like wire or plumbing pipes, so it must be further purified.  They ship these to a third facility in Texas.

At this facility, the anodes are put into a giant tank with pure sheets of copper called cathodes.  The tank is full of sulfuric acid, and electricity is passed between the anode and the cathode.  This causes the copper ions to pass from the anode onto the cathode, and in time, the anode is dissolved and the cathode becomes a thick plate of copper that's 99.99+ % pure.  Each cathode weighs about 350 pounds - they're pretty.

As it turns out, the sludge that's left over after you get the pure copper out of that 99% pure copper - i.e., the 1% of impurity - is extremely valuable.  It is primarily made up of gold, platinum, silver, and other valuable metals.  Our guide said that they sell the sludge to other companies for further refining.

In their Discovery Center, they point out that only one nation in the world produces more copper than the state of Arizona does alone, at 1.3 million tons per year.  (They didn't say what that nation was :-)  Also, they pointed out that all mines in AZ use less than 1/4 of 1% of the land in Arizona.

There was also a bit of information on reclamation (returning the land to a natural state after the mine closes), but it was not too in depth.

I really enjoyed this tour.  Like all "company tours," everything was sunshiney and wonderful with no environmental or societal issues.  However, our society does need copper - lots of it - to have the lifestyle we enjoy.  For instance, all the wind turbines we have seen in California and across the country are loaded with copper, as is the truck we pull our camper with.  They estimated that the average house has over 400 pounds of copper in it.  This all has to come from somewhere.  Sahuarita, Arizona is "somewhere."