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REVETT MINERALS the technicians to see tons, power draw on the mills, water consumption, pump speeds, box levels on the flotation cells, etc. Process Engineering Resources, Inc. developed an on-stream analyzer (OSA) for the PLC system to track the flotation circuit's performance. The OSA provides real time economics, which allows the mill to maximize con- centrate grade. "We are able to look at cause and effect with chemical additions," Lloyd said. "In addition to xanthate, we use an MIBC to stabilize the froth along with the sulphidizer and a glycol-based frother to bring the froth back after the sulphidizer. The technicians in the plant can see the cause and effects as they modify the amounts of reagents with the OSA. The people who run mills see assays for today's products tomorrow and that's too late. By having an OSA running on math- ematical models created from assays in the lab at the mill, it's almost gospel; it's that close. We can run the plant and learn the cause and effect and maximize our economics continuously. "We're trying to get our technicians up to speed with instrumentation," Lloyd said. "When the Rock Creek project comes online, we will build a new fully- instrumented mill." Tailings Impoundment Miller takes great pride in the mine's environmental achievements. "The water leaving the mine site meets drinking water standards, but it does not meet aquatic standards because of elevated copper levels," Miller said. "However, we have one of the best tailings storage facilities in the mining business, which acts as a natural water treatment plant. The tailings are ground quartz, which is essentially a benign beach sand. Within the first few inches of alluvium material, the copper is attenuated. A chemistry changes allows the copper to precipitate out of solution." Monitoring wells sur- rounding the property verify the perform- ance of this passive water treatment process, which upgrades the water to aquatic standards. In reality, the impoundment repre- sents a minimal amount of disturbance. "We have been monitoring this site for 25 years now," Miller said. "The impound- ment is located on 750 acres of privately owned land. The footprint currently occu- pies about 350 acres." Miller explained that the mine is fully funded for final clo- sure reclamation, which he estimates at $6.5 million to $7 million. A $6.5 million bond has been placed in trust fund with AIG. Revett also has a $13 million surety bond within the state of Montana. The tailings disposal system begins at the thickener. Site boxes have been installed 20-ft deep to keep the pipes full. "We monitor the levels in the boxes and use a homemade valve system to control the level," Lloyd said. "With pressure transmitters and alarms on the pipelines all the way to the impound- ment, we have telemetry throughout the system. We can immediately identify a leak. If there is an issue, we have sec- ondary containment. We also have auto- matic dump valves. We can shut the mill down and dump the line to limit envi- ronmental issues." The decanted water overflows into a pond, where it its reclaimed immediately for the mine. The water from the tailings dam can be pumped back up if necessary. The impoundment progresses in 10-ft lifts. A fleet of Cat 631 scrapers deposits www.e-mj.com JUNE 2012 • E&MJ; 69