We all met at the Avra Valley Airport around 8am. Several people offered rides to others since parking at the mine was limited and we set off for the mine about 8:15am, arriving at the main office just before 9am. Quentin Browne, mine geologist, was our tour guide and he provided us all with hard hats. After outfitting the vehicles with lights or flags and a few instructions, he led us into the mine area.
Our first stop was an overview of the rubble leaching at the Oxide pit. Quentin explained that the colors we observed in the mined areas were caused by supergene enrichment in the oxidized zone of the porphyry copper deposit. With the current price of copper, the ore grade at the mine is not high enough to justify normal milling and smelting of the ore. However a dump leaching process is an economic means of copper recovery. For this leaching operation the cash costs are approximately 50 cents per pound.
For the leaching process, explosive charges are carefully placed so that the rock is pulverized enough to be placed directly on the leach pad without further crushing. After placement on the leach pad, a series of tubing is laid out on top of the layer. A weak sulfuric acid solution is pumped through the tubing which is sprayed onto the surface of the heap. This solution percolates down through the pile of ore and dissolves the acid-soluble copper. The average grade is 0.4% copper and they recover about 60% of the copper in the leach ore. The solution collects in a pond at the bottom of the pit. The surrounding bedrock acts as a boundary to keep the leached solution within the pit. The leach solution grade is 1.47grams per liter of copper. This copper-bearing solution is then pumped to the solvent extraction / electrowinning (SX/EW) plant. At the SX/EW plant, organic chemicals remove the copper from the solution. The SX plant operates at 8000 gallons per minute of solution, nearly all of it is recycled except for evaporation. It runs 24 hours per day. The resulting electrolyte solution is transferred to the electrowinning process where copper is plated out as a 99.99% pure copper cathode. The anodes are made of lead, and the cathodes are stainless steel. There are 58 cells in the SX plant each containing many cathodes. Copper plates onto the cathodes and is then peeled off as sheets which are strapped into bundles weighing 1200-1500 lbs. The mine processes about 30,000 tons of ore a day, with 40,000 tpd being the max rate, resulting in about 50 tons of copper per day.
After leaving the Oxide pit we had a short second stop at the El Tiro pit with copper oxides. Then it was on to the North Silver Bell pit with its pyrite and chalcocite minerals. First Quentin explained more about the geology. The important copper minerals are chalcocite, malachite, chysocolla, and azurite. They do not recover any copper from chalcopyrite. Then members were allowed to take some time and collect rock samples of azurite, malachite and chrysocolla. Everyone managed to get some nice specimens to take home. After this we headed to the solvent extraction / electrowinning (SX/EW) plant. The group was then split into two smaller groups with a plant operator taking each group on a tour of the plant. The solvent extraction portion was all on the outside. This is where the solution from the pond at the leaching area is pumped. The copper in solution is concentrated over 30 times before being pumped to the tankhouse. In the tankhouse, the copper is recovered from solution using the electrowinning process and plated as high-purity cathodes.
All of the known deposits at Silver Bell are confined to an arcuate, pyritic zone of alteration that is 12 km long and 0.35 to 3 km wide. The geologists are still working on outlining the productive portions.
Quentin was a gracious host and we all enjoyed the tour and learned many new things. Pictures courtesy of Bob Boor.
 First pit that we visited. Colors caused by supergene enrichment in the oxidized zone of the deposit. Also note the tubing in the lower left which is on the leach pad. |
 Quentin discussing the geology at the Silver Bell North pit. Note the 28 yard shovel in the background. |
 Members looking for interesting rocks at the Silver Bell North pit. |
 Sandy Thomas in front of pure copper sheets at the SX/EW plant. |
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This page was last updated on 6 February 2002.