Mine

Started: 2022-04-18 18:50:56

Submitted: 2022-04-18 21:38:54

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Visiting a massive open-pit copper mine

We finished seeing the Titan Missile Museum just before noon. It seemed like we might want to eat lunch before proceeding along to our next adventure; Julian was getting cranky and I attributed it to the missile silo tour dragging on too long with its occasional allusions to the complete annihilation of human life in a nuclear holocaust delivered by the very missile silos we were touring, though he might have just been hangry. We ate lunch at a strip mall across the freeway in Green Valley, then drove north a couple of miles to the ASARCO Mineral Discovery Center visitor's center and tour.

The visitor's center was located at the ASARCO Mission open-pit copper mine and explained the process of mining ore and refining it into copper. I signed up for the next tour (departing in an hour from when we arrived at the visitor's center) and took the kids into the museum, where we watched a series of videos that explained the process and contextualized the parts of the mine we were about to see.

The tour kicked off with a tour guide giving another overview of the mining and refining process, then we climbed into a bus to drive into the mine. The bus drove down a normal two-lane paved road into the desert, between the Interstate highway to the east and a three-kilometer-long ridge of mine tailings, more than a hundred feet high, to the west. Some scattered vegetation grew on the ridge, looking like a half-hearted revegetation program; though this was the middle of the Sonoran Desert so the natural desert terrain itself looked only incompletely vegetated. At the end of the ridge the road turned to the west, between the extension of the original ridge and a new ridge, twice as high, on the other side of the road. The road gradually climbed between the twin ridges of mine tailings until we could look down into a flat tailings pond, stretching towards the horizon.

The bus continued to climb, winding its way between massive piles of mine tailings, until it reached the employee parking lot; and beyond the lot the gate to enter the mine itself. Inside the mine the bus driver switched to the left side of the road, because the massive haul trucks were built with their driving cabs on the right so they had better visibility when driving on the left side of the road. We drove past the processing mill and crossed one of the haul roads (requiring radio contact with the mine control center to make sure there were no haul trucks on the road), then we stopped at an overlook into the pit and stepped out to take a look.

As promised, this was a massive open-pit mine, more than two miles on a side and thousands of feet deep. Binoculars were set up on stands so we could look down into the mine to see the haul trucks running up and down the haul roads.

It was hard to get a real sense of scale; at a distance the haul trucks looked like normal sized dump trucks, though we were assured that they were really house-sized trucks, being filled by massive cranes with buckets large enough to eat whole a normal-sized dump truck. (It almost felt like an exercise in forced perspective: I couldn't completely convince myself that I wasn't just looking at a much smaller, much closer diorama set up with Tonka-sized toy dump trucks, except for the fact that I can go look at the ASARCO Mission South mine in the satellite view on Google Maps and it looks exactly like I remember it.)

The best frame of reference I had for the size of the haul trucks were the 12-foot tires, shown by example at the overlook above the mine.

The first step in mining the copper is drilling holes for core samples at the bottom of the pit. Then, if the core samples indicate there's enough copper in the ore, the holes are used to blast the rock, and cranes deliver the ore into haul trucks.

The open pit mine was covered with a network of haul roads, where haul trucks carried their massive loads of ore into the mills on the edge of the pit. (I was reliably assured that this is a massive haul truck; but there's almost nothing in this picture that really gives me any sense of scale.)

We left the mine overlook and drove back down to the mill, where the haul trucks dump the ore to be refined.

We visited the South Mill, a massive dusty industrial building surrounded by piles of ore in various states.

A series of conveyer belts on one side of the mill collected crushed ore from the haul trucks into piles, and the piles were then fed inside the mill.

The first stage of the ore processing in the mill was a series of crusher drums. The drums spun at a steady pace, rolling the ore inside filled with softball-sized steel balls that crushed the ore into a fine powder.

The next stage were the settling troughs, where the ore powder was mixed with chemicals and water. If I understand the chemistry right, the copper would attach itself to the chemicals (they told me what the chemicals were, but I forgot) and float to the top in a frothy foam, and the settling ponds were set up to separate the foam and dry it, while the remaining rock was filtered out of the muddy water and set aside in the tailings ponds.

Seeing the massive industrial building filled with dusty equipment running by itself made me think of the movie Aliens. I half-expected a xenomorph to sneak around the far side of one of the pumps as I watched.

Outside the mill were a series of larger settling ponds where the frothy copper compounds collected at the top of the ponds. Once dried, this compound was about 25% copper, and it was the output of this mine. Trucks carried the copper sludge to a refinery in Texas where a series of furnaces and electrolysis-assisted acid baths further refined the copper into proper industrial purity, at least 99.99% pure copper.

From the mill we returned to the bus and rode back to the visitor's center to wrap up the tour. I walked through the random assortment of old mining equipment outside the visitor's center, with a collection of much smaller retired haul trucks that were still the biggest dump trucks I'd ever seen. It was the middle of the afternoon and it was hot and dry, and I was glad to get back inside the air-conditioned car to head back to the hotel.

I took Julian to the hotel pool while Calvin stayed in the room, then we headed out to eat pizza for supper before retiring for the night.