Technology enhances Red Dog processing

As production steadily increases, investment in mill upgrades is making a big difference in the quality of lead and zinc concentrates at Teck Cominco's Red Dog mine in northwest Alaska. Bringing in new equipment and enhancing systems is always a challenge because the facility is already a labyrinth of massive SAG mills, ball mills and flotation columns, mill engineer Curtiss Ehrsam said at a meeting of the Alaska Miners Association and Society of Mining Engineers in Anchorage April 11.

Recent improvements have included the installation of three Microcel columns at a capital cost of somewhere between $110,000 and $220,000, for a net present value of $4.1 million. "What we found during surveys of our plant was that our column cells weren't really doing the work they needed to be doing, and it was because the bubble size wasn't right," Ehrsam said. "We needed finer bubbles that were able to carry more material out. ... It was getting a lot more free zinc in it than it should be getting."

The Microcels take slurry from the bottom of the column and run it through a manifold, where air is injected into it. The slurry then goes through a static mixer that mixes the air and slurry together, injecting fine gas bubbles. "This has made a significant impact on the ability of the columns to recover material, where in the past we stuck with whatever grade they'd give us and we didn't really have a lot of opportunity to improve the recovery, because when you started adding more air, the bubbles would just get bigger and it wouldn't necessarily mean that they could attach as many particles on it," Ehrsam said.

'Expert system' control equipment introduced

Another improvement has been the introduction of "expert system" control equipment on the grinding and flotation circuits and cameras to control flotation reagent additions, at a capital cost of $550,000 for a net present value of $2.4 million. The cameras were installed in both the lead and the zinc rougher circuits. "They measure the froth velocity as the material is going over the edge of the cell, it also looks at the bubbles as well, and with that we added automatic air control so we can actually control these cells to optimize the pull rate on the cells," Ehrsam said.

Lead circuit modifications were also introduced at a capital cost of $1.5 million, for a net present value of $6.8 million. "This was needed in order to deal with the higher-grade material that we had," Ehrsam said. "We added another column into the circuit, we did a bunch of pumping improvements, and then with that we're planning additional Microcel installations in order to help it out." Before this work was done, too much zinc was getting into the lead concentrate, Ehrsam added.

Red Dog's third water treatment plant was built at a capital cost of $4 million with the goal of reducing the total dissolved solids in the tailings pond. A high-density sludge process takes water at 10,000-12,000 TDS, treats it with lime and aerates it. The sludge then goes to the tailings impoundment with water that is lower in total dissolved solids or TDS and combines with the other water in the impoundment. "Then feeding back to the other water treatment plants we have a lower overall TDS coming in there, so the plants will treat stuff that's around 3,900 TDS and prepare it for discharge, which we like to be under 2,900 TDS by the time it leaves our sand filter," Ehrsam said.

"We're changing and growing, we've got new ore bodies to move into, and so we're also doing geo-metallurgical mapping to examine the variability of the Aqqaluk deposit and improve prediction for the block model," Ehrsam continued.

"There's big variations in this ore body, so it's important to do testing for this." Aqqaluk contains very fine sphalerite (the mineral that hosts the zinc) and coarse sphalerite, and a good block model will enable Red Dog's processing team to blend it properly and optimize performance in the plant.

"We also want to look at the grain size for the ability to grind it, and then our ability to recover it as well, too, once we do liberate it," Ehrsam said.

Upgrade improves quality of lead concentrate

Current upgrade projects at Red Dog include using the carbohydrate dextrin to improve the quality of the lead concentrate. "Without dextrin on, as the TOC (total organic carbon) in the feed climbs, the silica in the lead concentrate climbs," Ehrsam said. "Once we put on the dextrin that changes. We find that even with the TOC going up in the feed we're quite a bit lower in our silica content in the lead. Staying below 4 percent is extremely important for us because quite a few lead smelters don't want anything above 4 percent."

A relocation of the mill's tertiary cyclones, which was under way in mid-April, will eliminate two process pumps and provide safer access for operators and mechanics when they need to climb up to them. "The current location gets really hot in the summer time and it's really hard to access the cyclones for maintenance," Ehrsam said. "We're also incorporating a better design for the cyclones as well, so that they work better for us," he added.

Red Dog has also installed a Jameson cell, which is a high-intensity, high-efficiency flotation device that recycles material at a rate of 3,000 or 4,000 gallons per minute. "Out of those six conventional cells we entrain a lot of zinc and lead minerals that really we'd like to return to the circuit in order to improve our overall recovery," Ehrsam said. "So we found that we could get better performance, less entrainment, if we did another stage of cleaning and used the Jameson cell for this purpose." The Jameson cell makes a finer bubble than a conventional flotation cell and the tailings that come out of it return to the rougher circuit so that more lead and zinc can be recovered.

The timing of future projects will depend on their economics and how well tests work out, Ehrsam said. Teck Cominco hopes to purchase Xstrata's IsaMill technology for finer regrinding, which again should improve grade and recovery of both lead and zinc. The company is also looking at options for improved air control in the zinc cleaning and at using soda ash to reduce the amount of lead in the zinc concentrate and vice versa. In the plant services area, improvements in lime handling are planned. In water treatment, Red Dog's management would like to increase the mine's capacity for water discharge.