Green Valley 2: Asarco Mine Tour
Jan 20, 2015
|After our tour of the Titan Missile complex, we had a quick sandwich and headed over to the Mineral Discovery Museum from which the Asarco Mine Tour begins. The Mineral Discovery Museum is a free museum which tells the story of copper mining using video and real items.
Arizona is nicknamed “The Copper State” because of the great deposits of copper that God in nature has placed here. If Arizona was a country, it would be the second largest producer of copper in the entire world. Only Chile produces more copper than Arizona.The star on the Arizona state flag is copper-colored because the red metal is so important to the state’s economy. A miner is even depicted on the state seal. Asarco currently operates three copper mine properties in Arizona: We visited the Mission Mine south of Tucson.
From the Mineral Discovery Museum we loaded onto a bus to be driven up to the Mission Mine Complex. This is one of three mines owned by the Asarco Group. It is called the Mission Mine because it is next to and mines/leases land on the San Xavier Reservation for the Hohokom O'odham Natives. There is a national historic mission,Mission San Blac, from the same era as the California missions, on the reservation and which is open to the public. Much of the local area has the name Mission in it.
The mine is absolutely huge. Called generally the Pima Mine, it is composed of many sections. We visited only one section - one "mine" and crushing facility. If you want to see where this place is, and how large it is (miles and miles), here are the GPS coordinates for Google Earth. The Asarco Mineral Discovery Museum is to the west of the town of Sahuarita, north of Green Valley. It is at 31 59 53.01 -110 59 39.23 The rock crushing facility is at 31 59 52.78 -111 03 00.15 Be sure to back way out to see the entire set of open pit mines and the Duval Mine to the south next to Green Valley. But zoom in on the rock crushing facility because that is where many of the pictures were taken.
(Also, while out looking up these coordinates on Google Earth, I also looked up the Titan Missile location which is near the Duval Mine at the north part of Green Valley. It is 31 54 11.22 -80 59 55.31)
Since mining is not something I know much about, I have copied from the Asarco website a great deal about just the parts which happen on site here, the actual mining and rock crushing and liquifying of the copper. Smelting and refining are done in another Arizona location and in Texas. The rest of this post is from the website. If, after reading about what is done directly at the Mission Mine, you want to continue your education on the smelting and refining of copper, go to http://www.asarco.com/about-us/our-locations/asarco-mineral-discovery-center/making-copper/refining/
From the website:
Mining engineers rely on information collected by mine geologists to develop a computer model of the copper deposit. Sophisticated interactive graphical computer programs are used to carefully plan and track the progress of the mining operation on a daily, weekly, monthly, and annual basis.When the Mission Mine was being planned almost 50 years ago, it took several engineers working several months to develop a single mine plan. In that period of time, the price of copper could have changed significantly, thus making the mine plan inappropriate. With current computer technology, changes to the mine plan can now be made in a few days by one mining engineer. A number of mine plans based on different metal prices can be developed to help the mine better respond to changing market conditions. When the price of copper is high, lower-grade ore can be cost-effectively mined along with the better grade ore. When the price is low, the mining operations can focus on the higher grade ores.
Drilling and Blasting Crew
Blast holes are drilled in a pattern partially determined by the hardness of the rock. The goal is to fracture the rock enough that the large shovels can load it in the haul trucks without encountering big unbroken boulders that would be too large for the throat of the primary crusher.The holes are loaded with a mixture of ammonium nitrate and fuel oil (ANFO). Safety is the highest priority at the Asarco Mission Complex, so the blasting crew detonates the charge during shift change when no one is in the pit. Blasting technology has steadily improved, and mine blasts may not seem as dramatic as in the past. This is because now more energy goes into fracturing the rock in place, so not as much rock and dirt gets thrown into the air.
Big Trucks and Big Shovels
The large electric shovel can fill a 240-ton capacity haul truck in only three passes! The Mission mine has several big shovels and a whole fleet of large trucks — some with a carrying capacity of 320 tons each!The shovels are powered by 4,160 volts of electricity carried to each one by an “extension cord” more than two inches thick (and of course those are copper wires in that cable!). The mine electricians make sure the pit operations have safe and reliable electrical service.
Truck Dispatch System
A central high-speed, computer system keeps track of every truck and shovel. It “looks ahead” to determine when a shovel will need a truck to load and then which truck will be in that vicinity at that time. This system reduces the time a truck will wait in line at a shovel to be loaded, as well as the time a shovel is waiting for a truck to arrive.The Dispatch system was developed here in Tucson by Modular Mining Systems, Inc. and is used in mines all over the world. Dispatch also monitors truck vital signs like oil pressure and temperature, which helps identify equipment problems before they cause a breakdown.
Mine roads are kept damp by spraying them with 10,000-gallon water trucks that are almost as big as the haul trucks. This effectively reduces the amount of dust kicked up by the haul trucks and other traffic on the mine roads.Water jets in the crushers and at the ends of conveyor belts also serve to keep wind-blown dust to a minimum.
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.TOUR NOTE:
On your tour from the Mineral Discovery Center, you may get to see the haul trucks dumping ore into the crusher.
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 mi
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.
The 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 in the milling process is reclaimed and re-used. The rest is used to keep the tailings damp and to prevent wind-blown dust.
Smelting and Refiningg are done at separate plants, one about 70 miles north in Hayden (the smelter) and the material to be refined is taken away on trains to the refinery in Amarillo, Texas.