Friday, November 18, 2011

Asarco Copper Mine Tour

Copper and cactus are part of the landscaping outside of the Asarco Mineral Discovery Center where we began our tour of the mine. The mine is south of Tucson, on the San Xavier Reservation. When Asarco discovered the copper located here in 1955, it was required that they receive mining rights from the Tohono O'Odham Indians. Royalties have since then been paid to the Indians for their portion of the monthly throughput. Asarco also pays rental on lands used for waste dumping, tailings disposal and water wells. The mining of copper involves a series of steps from drilling and blasting to milling, smelting and refining. For our tour of the mine we first boarded a bus which took us to the open mine pit which covers approximately 2,000 acres of land. Piles of tailings surround the pit. The company has a policy of  vegetating the tailings dams.
It is quite a large hole. Our guide informed us that it will take about 23 years to retrieve all the copper out of this pit. Blasting of parts of the mine are done daily at 4PM. From the pit we rode over to the south mill. Here we saw large haul trucks empty rock into chutes and down into the mill where the first process of crushing the ore begins. Processing of the ore to extract the copper content is through a milling and concentrating operation. Inside this mill we saw the large containers where the ore is put through gyrating crushers and sag mills. In the sag mills ore is processed further using high carbon steel grinding balls. There is also a double deck vibrating screen which sizes down the ore to less than 2". I am probably simplifying the process way too much, it is very involved. Just before leaving the mill we saw large tanks where a chemical reaction is used to leach the copper out of the rock. Pine oil, lime and water is added and the resulting mix is a bubbling slurry, as pictured below.
 After that we walked outside of the mill where there are three large settling ponds. The first pond, pictured below, contains the copper which has been extracted through the milling process. After the water is removed it is transported to a smelter in Hayden, Arizona and then to a refinery in Texas. The mill processes 24,000 tons of ore daily.

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