Tuesday, March 28, 2017

ASARCO - Tucson Misson Mine Tour

March 17, 2017

ASARCO Mineral Discovery Center

The Mission open-pit copper mine was at one time five separate mining properties, but over the years, Asarco has combined them into one integrated mining operation. The mine occupies around 20,000 acres of private, State leased, and Indian land.  There are over 600 employees working two shifts, 24 hours a day, seven days a week.

Each year the mine has the capacity to process about 260,000 tons of copper concentrates which yields 132,700,000 pounds of copper and 1,234,000 ounces of silver. Annually, the Mission mine pays royalties and taxes to the State of Arizona and royalties to the Tohono O’odham Indian nation.

There is some old equipment near the building housing the gift shop and a small museum.  The large truck on the left was too small to be run economically.  It was replaced by a truck much larger that cost about $5,000,000.  They currently have 24 such trucks.  The six tires on the trucks cost $50,000 each  and last about 6 months before having to be replaced.  At one time the old tires were recycled, however they had too many of them and the market dried up.  Now the tires are dumped into a played out pit and covered up.

 Structure from an old mine that is near the visitors center.

The different colored rocks from the tailings pile come from different layers in the mine pit.  These pilings can be seen from I-19 for miles and the view from google earth show how large these mines are.  They are nothing more than ground up rocks with the copper removed.

 An 11-foot-diameter tire from a 240-ton haul truck provided a
great photo opportunity

The first stop on our tour was the open-pit viewpoint on the south rim of the Mission Mine. The viewpoint was securely fenced for safety, and it has four binocular telescopes to provide a close-up view of the trucks and shovels working in the mine.

The Mission Mine is a quarter-mile deep, two miles from north to south, and a mile-and-three-quarters from east to west. About six times the amount of earth moved to dig the Panama Canal has been mined here.

The haul trucks carry the ore out of the pit along a haulage road with a slope of no more than about nine percent. They dump the ore into a gyratory crusher which reduces the ore to eight inches or less — about the size of soccer balls.  The primary crusher may send the crushed ore on to a secondary crusher or pile it directly onto the coarse ore stockpile.

 The ore is ground into a fine powder by large rotating mills. The two types used in the Mission South Mill are called SAG (semi-autogenous grinding) mills and ball mills. SAG mills use larger pieces of ore to break up the smaller pieces (autogenous — does it by itself). The larger pieces break down as well. To help the process along, eight-inch-diameter steel balls are added to the rocks as they tumble inside the rotating mill (semi-autogenous — gets some help from the steel balls). The two SAG mills in the Mission South Mill each have two 3,000 horsepower electric motors. They can rotate in either direction which helps even out the wear on the steel liners inside the mill.
When the rocks are about 3/8-inch or smaller, they are fed as a slurry into the two ball mills. Each ball mill is turned by a single 3,000 horsepower electric motor.   These mills contain literally hundreds of thousands of three-inch diameter steel balls that pulverize the ore until it is like fine sand or face powder. Only then are the copper minerals broken free of the rest of the rock to be separated by flotation.
 The slurry of water and pulverized ore is mixed with milk of lime to raise the pH and small amounts of special reagents: a frother to make bubbles, and a collector chemical that causes the copper minerals to stick to those bubbles. Air is blown into the tank and the mixture is vigorously agitated like a high-speed blender.  Rising bubbles carry the copper minerals up and over the edge of the flotation tank. The bubbles break soon after they flow over the edge. The copper minerals are then ground up even finer and  purified by another flotation process.
CuFeS2The dried copper concentrate of about 28 percent copper is shipped to the smelter. It represents less than one percent of the material removed from the mine. Concentrate is just a fine powder of the mineral chalcopyrite which is a naturally occurring compound of copper, iron, and sulfur.
The material that sinks in the first flotation cell goes on to two more flotation cells to recover as much copper as possible. What doesn’t float is called tailings because it goes out the “tail end” of the flotation circuit.  About 80 percent of the water used

The South Mill
At the Mill, the copper ore is ground into a powder so the copper minerals can be separated by the froth flotation process.  

There was more to the tour but my old memory isn't what it used to be, so you will have to take the tour yourself to hear it all.  

I was chatting with the tour guide and asked if they backfilled the pits like they do with strip mining in Illinois where Gerry and I grew up.  She said no and then asked where in Illinois we lived and when I said 50 miles south of Springfield she then asked what town.  It was becoming apparent to me that she knew the area.  When I said Gillespie, IL she said she went to middle school there, high school in Mt. Olive and Litchfield, IL  What a small world we live in when a perfect stranger connects with you over where you lived.  

BTW:  She did a fine job on the tour and hopefully if you take it, she will be your guide.

That was our day, how was yours?

1 comment:

  1. We took that tour a couple of years ago and it was fun to read your post. It's so easy to forget all we've done and so a refresher from someone else brings back good memories.