Engineering and Mining Journal - Whether the market is copper, gold, nickel, iron ore, lead/zinc, PGM, diamonds or other commodities, E&MJ takes the lead in projecting trends, following development and reporting on the most efficient operating pr
Issue link: https://emj.epubxp.com/i/70939
REVETT MINERALS luxury of running one train or the other. If the mine gets ahead of the mill, then we can fire up both trains. "The mills are running with a 40% charge of 2- and 3-in. balls," Lloyd said. "The undersize from the mills reports to a set of classifying cyclones and then a bulk sulphide flotation process begins." A potassium amyl xanthate (PAX) is used to promote flotation with the sul- phides. "The mill routinely processes copper sulphides, occasionally lead sul- phides, even small amounts of spha- lerite—all of them will float," Lloyd said. "We have a simple process that's compli- cated by non-sulphide copper ores. Some of the non-sulphides occur as oxides, such as cuprite, or copper carbonates, such as malachite and azurite. We do not do as well as with cuprite as we do with the carbonates." To float the non-sul- phide copper, a sulphidizing agent is added to place sulphur on the surface of the particles. The xanthate then makes the particles hydrophobic. The vast majority of the copper processed occurs as a sulphide (85%). "The recovery with sulphide ranges between 90% to 95%," Lloyd said. "The recovery for non-sulphide is considerably lower at 50%. Our composite copper recovery is about 85%-86%." As mentioned previously, the ratio is 100 tons of feed to make 1 ton of con- centrate. "We have to run a lot of tons to make money," Lloyd said. Lloyd refers to the three rows of roughers as the golden mile. "It takes about 20 minutes for the slurry to travel from one end to the other," Lloyd said. "At the end, the slurry reports to the tail- ings stream. It's really important that we recover all of the silver and copper that we can in those 20 minutes. Otherwise it's a lost opportunity." The rougher circuit makes a really low grade copper concentrate (5% to 7% copper). That reports to the cleaner cir- cuit, which is about 1/5 the size of the rougher circuit. "We take the rougher con and dewater it in a thickener, regrind it down to 45 microns (325 mesh), and then feed it to the cleaner circuit, which upgrades the 5% copper concentrate to 35% to 40% copper concentrate," Lloyd said. "Our final cons are thickened to 50% solids and then they report to a drum filter. We usually make a con of 12% moisture. We now have a saleable product." The concentrate is hauled to a rail siding in Libby, Montana. The Troy mill yields a really clean cop- per concentrate. It does not have the usual impurities associated with sul- phides, such as arsenic, antimony, bis- muth, mercury, etc. It is highly sought after. Revett recently shipped 5,000 mt to South America. In March, they shipped more concentrate to a smelter in Hayden, Arizona. Revett is currently considering a small roast, leach, electro-win plant, which would be the first in the North America. "Because our cons are so clean, we are considering a roast and weak acid leach process for the cons," Lloyd said. "Last year, Hazen did a Phase 1 study and we were impressed with the copper recovery [96%], but not so much with the silver recovery [60%]. "Now we are starting Phase II test work at Montana Tech in Butte. Researchers will take copper all the way through the electro-winning process and find the issues with the sil- ver. We hope to have a feasibility study completed later this year." The Revett Formation has a small amount of chloride in the form of mica. Lloyd suspects that during the roast, chlorides are fuming and tying up the sil- ver. Silver chloride is insoluble in weak acid. "We're looking for 90% on silver so it's important to get that recovery as high as possible," Lloyd said. Revett Minerals has invested in an aggressive exploration program. 68 E&MJ; • JUNE 2012 During the restart, the mill was also upgraded with PLC controls that allow www.e-mj.com