The Bagdad copper mine, 167 km northwest of Phoenix, Ariz., may appear to be in the middle of nowhere, but in terms of technological advances and innovations it is right on the cutting edge.
Wholly owned and operated by Phelps Dodge Corp., Bagdad is one of the oldest solvent extraction electrowinning (SX-EW) operations in North America– cathode copper has been produced here continuously since 1970. The mine’s colourful history, however, predates the advent of SX-EW production by nearly 90 years.
Copper was first discovered here in 1882 and the story goes that during this time several families mined native copper from small adits. The grades were so rich and the copper so plentiful that the town reportedly got its name after a son, working in an adit, said to his father, “I need another Bag Dad”.
Despite the lore and early successes, underground mining in a conventional sense didn’t begin until the end of the Great Depression in 1936. From that time on, technological advances and expansions to infrastructure have been important factors to the development and longevity of the operation. Some of these developments have included: conversion to an open pit operation in 1945, several mill expansions taking copper output from 2,500 tons per day in 1941 to 6,000 tons per day in 1962, and a US$240-million investment in 1973 to purchase new equipment, 374 housing units and a 40,000-tons/day concentrator.
New technologies: Pressure leaching on trial
Oxide ore historically formed 10% of Bagdad’s total production, and was processed using SX-EW methods. Sulphide ore accounts for all the copper production today, and it is processed using conventional milling, smelting and refining methods. A new technology is now being investigated that is a hybrid of the two processes: Pressure leaching takes a sulphide-derived copper concentrate and subjects it to a higher pressure and temperature to form a pregnant leach solution suitable for SX-EW processing.
In March 2003, Phelps Dodge commissioned a commercial demonstration pressure leach plant that, if proven successful, could have far-reaching consequences beyond the confines of the Bagdad mine. The hope is that this technology will be able to be applied to other deposits with chalcopyrite ores, which account for approximately 70% of the world’s known copper reserves.
Previous attempts to leach copper from chalcopyrite ores have been disappointing. Past failures of materials and processes yielded recoveries in the 10% to 15% range. A new pressure leach process developed by Phelps Dodge and Placer Dome Inc. showed merit, and Phelps invested US$40 million to design and construct the plant. The Bagdad plant began operation in April 2003 and is meeting company expectations.
Sulphide ore at Bagdad averages 0.36% Cu and is separated from oxide ore and milled to produce a copper concentrate containing 34% copper. Part of the concentrate is sent to Phelps Dodge’s facility in Miami, Ariz., for smelting and refining. The remaining concentrate, which amounts to 85,000 pounds per day or 63,000 tons annually, is sent in a slurry to the pressure leaching plant.
The pressure leach vessel measures 11.5 feet by 53 feet and has five compartments each constructed of refractory brick-lined steel. The slurry is subjected to a temperature of 225C and a pressure of 475 psi for a 70-minute period. The pregnant leach solution from the vessel is then added to the lower grade, oxide-derived pregnant leach solution and sent to the electrowinning tank house to produce copper cathode. Copper recoveries from the concentrate leach plant are 98%. The plant produces approximately 35 million pounds of copper annually and accounts for 15% of Bagdad’s production.
The major benefits of this process are, depending on power costs, lowered processing costs when compared with traditional smelting and refining methods. In addition capital costs may be reduced especially if there is excess SX-EW capacity. Furthermore, because the technology generates sulphuric acid (more than 140 tons per day) which is needed for leaching, operating costs can be lowered.
The Phelps Dodge-Placer Dome technology is being developed under a technology development agreement, and is being evaluated on the basis of its reliability, cost of production, product quality and economic viability over an extended period of time. This evaluation phase should be completed before the second quarter of 2004. If successful, Phelps Dodge would begin testing concentrate from other locations and would study alternative configurations before installing a full-scale plant. The company thinks a full-scale operation could be up and running within two to three years.
Phelps Dodge is looking at other potential cases including its 51%-owned El Abra and 82%-owned Cerro Verde operations in South America, each of which have significant sulphide reserves. Placer Dome and Phelps Dodge also indicate the technology may have applications with precious and other metals.
Efficient mining on a grand scale
While new processes are pushing the metallurgical envelope, advances have also been made to traditional mining methods. Over its long history, Bagdad has grown in size. Once a small artisanal operation, the Bagdad porphyry copper-molybdenum open pit mine is now massive. The pit is kidney-shaped, measuring 2,000 m long, spanning 1,500 m wide, reaching a depth of almost 610 m and has bench heights of 15 m.
Phelps Dodge uses 17 million pounds of explosives annually to break this volume of muck. It moves 200,000 tons of material daily with a large fleet of equipment that includes 21 270-ton Cat 793 trucks (270-ton), two P&H 4100 shovels (56-58 yd3), two P&H 2300 shovels (25 yd3), four Cat rubber tired dozers, seven Cat crawler dozers, four Cat graders, and four Ingersoll-Rand drills.
Even after the 60-year production history, Bagdad still has millable sulphide reserves of 873.6 million tons averaging 0.36% copper and 0.02% molybdenum. At 100% capacity, the operation could produce 250 million pounds of copper, 9 million pounds of molybdenum, 750,000 ounces of silver and 2,000 ounces of gold annually. In 2002, Bagdad operated at 50% capacity to produce 136.8 million pounds of copper in concentrate and 31,157,000 pounds of heap leach SX-EW copper, almost 9.5 million pounds of molybdenum and 545,000 ounces of silver. Currently, Bagdad is operating at about 80% of capacity.