Goal: Build one of the world’s deepest, most sophisticated underground laboratories: The Sanford Underground Research Facility (Sanford Laboratory).
Site: The former Homestake gold mine in Lead, South Dakota, once the largest producer of gold in America. Closed since 2003.
Problem: How to dewater the mine while filtering mine water containing ammonia and iron particulates before discharge into a cold-water fishery.
Design Team: The Environmental and Operational personnel at the Sanford Lab and Yardney Water Management Systems.
Solution: A custom-designed, custom-built Yardney reverse-stacked and stratified multi-media filter system (24 filters) with media method for extracting micronized iron.
The Sanford Laboratory has opened a research campus 4,850 feet underground where two important physics experiments will be protected from disruption by cosmic rays. One experiment will search for a mysterious substance called “dark matter.” The other will search for a rare form of radioactive decay. Those experiments will begin taking data in 2013.
The Homestake gold mine produced more than 40 million ounces of gold, but the mine was also a physics landmark. Nuclear chemist Ray Davis installed a solar neutrino detector on the 4,850 foot level in the mid-1960s. Davis went on to win a share of the 2002 Nobel Prize for Physics for that work. The Homestake mine was subsequently sealed shut in 2003 due to the cost of mining gold exceeding the market price.
Barrick Gold Corporation acquired the Homestake Mining Co. in 2002. In 2006 Barrick donated the mine in Lead, South Dakota to the state for use as a science laboratory. The South Dakota Science and Technology Authority (SDSTA) became the managing agency of the venture. The donation included Homestake’s prior existing water treatment plant. For SDSTA’s Ken Noren, lead treatment plant operator, and Duane Ennis, lead maintenance facility tech for the plant, the switch from mining to science meant their jobs were going to get very busy, challenging and demanding. John Scheetz, Environmental Manager for SDSTA, tells the story.
When the mine was decommissioned, the pumps were turned off which ultimately caused the water levels to rise. Sheetz says, “The mine is very deep. It drops to about 8,000 feet below the collar (top) of the shaft. The water level rose to 4,530 feet below this collar. But, the experiments were planned for the 4,850 foot depth level, so we had to dewater the mine past that depth.”
“The project team at SDSTA understood we needed to pump the water out, treat it and discharge it. But when the water sat in the mine for such a continued period of time, it reached quasi-equilibrium with the wall rock. The Homestake mine is an iron-hosted deposit despite a lot of gold being brought out of it. When the water came to quasi-equilibrium with the wall rock, the iron was exposed to water and dissolved. While that doesn’t sound like a problem, when we pump that water up to the surface, the iron precipitates into 1 micron-sized iron particles as the water becomes oxygenated,” Scheetz says. “The iron precipitate would then interfere with other processes and discharge downstream.”
Scheetz states, “Step one was to take an older water treatment plant, although still a very good one, and modify it so the plant could remove the iron before it entered the rest of the treatment system being developed.”
“We looked at a lot of different technologies, including processes that used chemicals and millina filters, where we would dose the water and enhance the precipitation and coagulation of the iron. That involved a lot of chemicals, and it didn’t work very well,” Scheetz said. “The SDSTA team looked at other technologies as well and one seemed promising—Media Filtration. Initially they rented filters from a number of vendors to test them as a pilot unit. Ultimately finding a solution with Yardney Water Management Systems.”
Scheetz says, “The people at Yardney were “Excellent” partners. They are very knowledgeable and helpful to explain how their equipment operates and how to achieve proper filtration. They had a good appreciation for some of the uniqueness of our problem once it was explained to them.”
Ron Gamble, Yardney’s Industrial Field Sales Manager says, “The mine water was characterized by a pH of 7 to 8, iron concentrations of 20 to 30 mg/L and ammonia of 3 to 4 mg/L. Equilibrium with the iron-bearing host rock under reducing conditions and the breakdown of thiocyanate (SCN) to ammonia contributed to the water chemistry. Several challenges also needed to be addressed. These included the high water treatment flow rate of 2,000 gpm, the desire for effluent water quality below 1 NTU for discharge into the environment, and the need for reducing the amount of backwash water created in the filtration process.”
In a brief four-week period, the teams designed and piloted a reverse-stacked and stratified Multi-Media filtration system. Gamble states, “The system significantly outperformed the temporary sand filter systems that were currently in use at the time, providing better effluent water quality and running more efficiently due to less frequent backwash events.”
Scheetz says, “Upon purchase of the system, Yardney fabricated the filtration systems for us and did a great job and within our outlined time line. In November 2009, the full-scale, multi tank, multi-skid system was installed and put on-line. Yardney has a good reputation for building high quality equipment for a good value.”
The teams also devised a proper method for extracting iron. Sheetz states “We used the Yardney pilot system to hone in on the proper media bed that would be most effective to filter out this very small iron-sized particle. After testing for several months we came up with a conclusion of how to make the system work as effectively as possible.”
“The new system required filter beds consisting of anthracite, granite and gravel. Very fortuitously for us, iron wants to stick to that media. It’s kind of strange in some ways that a very fine particle would stick to a bed of not-very-fine particles. You’d expect it to blow right through. But, if you operate the filter at the correct pressure and have the correct volume of water flowing through the filters, you don’t put too much energy into the flow. At the proper flow rate and pressure, the particles will get trapped within the media. But if you increase the flow rate too fast, the iron will continue right through the media and into your effluent water. So there’s a range at which we operate the filters to have them work effectively and as designed. And that’s very important!” Scheetz says.
The filters are backwashed every certain number of hours or on pressure deferential. Depending upon the amount of iron in the water that they are filtering at the time will determine how often the filters are backwashing. Water in some locations of the mine contains more iron than others thus creating more backwash events to retain the filter bed from getting compromised.
Backwashing the filter purges the media bed of contaminants. The backwash water is routed to a tank where polymers are added to enhance the size of the iron particles before sending it to a filter bag. “It’s a geotextile bag that traps the iron and then lets clean water flow out of the bag. That way we don’t have to use dewatering filter presses and belt filters, which are associated with a lot of labor, mess and cost,” he says.
After clean, filtered water exits the filters, it is mixed with other water that is being treated on site. It is then run through rotating biological contactors to remove the ammonia before being discharged into Gold Run Creek and on to Whitewood Creek, a cold-water fishery. “We’ve been very successful,” Scheetz says. “The water quality and water biology of the stream has been very good. We test it once a year for complete biological health, fish, macro invertebrates and algae.
“Ken, Duane and Yardney are the real heroes who made this project work. They deserve to be credited with the success of the filters, because they did so much work to make the system operate properly and as designed. Ken and Duane worked with Yardney very closely to lay all of the challenges out on the front end. The people at SDSTA worked with Yardney almost every day about desires, specifications, etc. for specific items on the filters so we could run them properly and effectively. Without Ken, Duane and Yardney equipment, the program would not have been successful.”
Scheetz said SDSTA is a satisfied customer. “Yardney was just great to work with in terms of a company; in terms of knowledge, of course; and also in terms of price and experience. We would certainly work with Yardney again if we had an opportunity to do this on a bigger scale.”