In a previous article we looked at how some South American miners and mines have tackled the thorny issue of water supply and mine water management. This article takes a look at similar projects elsewhere around the world – Africa, Japan, and the US.
Thermal coal mining and the eMalahleni Water Reclamation Plant In South Africa (Anglo American / BHP)
eMalahleni is located in a very water stressed region of South Africa. With an average rainfall of just 1mm per annum, the local water authority struggles to meet the water demands of the town's residents. Currently the major water supply for the town is Witbank Dam but the quality is poor.
In order to avoid stressing the water situation further Anglo American spent a considerable amount of time and resources researching and developing a mine water treatment plant to treat the mine water from their thermal coal operations in the district. As a result the eMalahleni Water Reclamation Plant was commissioned in 2007. BHP have a 'right of use' agreement to use the plant to treat the waste mine water from their nearby South Witbank Colliery. Anglo also came to an arrangement with the local water authority to boost the town's drinking water supply with treated water from the plant.
During its first years of operation the original plant processed 30ML of water each day, the majority of which was pumped into the local water supply. By the end of 2011 the plant had processed 30 billion litres, and was supplying 12% of the town's daily water requirements. The plant was expanded in 2013 to treat 50ML a day. A third phase intended to treat waste mine water from other coalmines in the area is planned.
The Trekkopje Uranium Mine and Erongo Desalination Plant in Namibia (Areva)
The Trekkopje Uranium Mine is also located in a region where ensuring an adequate local water supply prior to commencement of operations was problematic, both in terms of availability and access restrictions. There is groundwater but it's salty and the chlorides in it interfere with the heap leach processing of the ore. Therefore, its use is restricted to dust control. The chlorides also rule out seawater so the only feasible solution was to install a desalination plant, which duly happened in 2010.
The Erongo Desalination Plant was the first of its kind in Namibia and is located on the coast about 50 kms from the mine site. The plant uses extreme filtration and reverse osmosis to treat seawater, turning it into clean water suitable for use in the heap leaching process. Once treated, the water is pumped to the mine site. However, the site only needs 70% of the volume of water produced by the plant so the excess is diverted to the local water authority for use as required. This 'excess' is enough to supply almost half the domestic and commercial water needs of the Eronga Region. Furthermore, the life of the plant will be at least twice that of the mine so when mining operations wind down, the plant will be taken over by Namibia's water authority Namwater.
Forecasted water usage on the mine site itself was also significantly reduced (by around 1/3rd) through the installation of an on-site water treatment facility and by installing infrastructure that allows the water to be reused.
Treating Wastewater From The Closed Toyoha Mine In Japan (JX Nippon Mining & Metals)
The Toyoha Mine in Hokkaido was until its closure in 2016 the largest global producer of indium. It also produced lead, silver, and zinc. Much of the ore came from underground shafts and ore residues were pumped into large tailings dams. With the mine's closure came the requirement to do something about the water in the underground openings and the tailings dams before they could get into local river systems and ground water and contaminate them with heavy metals and acidic mine water.
During the life of the mine, wastewater was treated to remove contaminants before being pumped into the Shirai River. The Shirai River joins up with the Toyohira River further downstream, and that river supplies water, including (treated) drinking water, for the city of Sapporo. When the mine closed, 2 wastewater treatment plants were built at the site. One treats the wastewater from the tailings dam, and began operations in 2008. The second plant treats water from the underground operations, and started operating in 2011. The treatment plants use a high-density sludge that is continually recycled. The sludge neutralises the lime that makes the water acidic, and the treated water is then discharged into the Shirai River. Continual tests are run on the water before it's discharged and so far it meets all government regulations with respect to both acid and heavy metal levels.
This part of Japan is prone to heavy snows over winter so all the plant facilities are indoors to allow for year round operation. Both plants have backup power supplies in the event of power failures and a second treatment line that can be brought into operation if there is too much water coming in for a single line to handle. The second line can also be used if the other line malfunctions. Pipes taking the water from the tailings dam to the treatment plant are laid inside a cement culvert to prevent any leakage into the surrounding soils. There are also emergency underground water storage tanks that can take incoming water should the plant malfunction or shut down for any reason.
Dealing With The Closure Of Historic Homestake Mine In South Dakota, USA (Barrick Mines)
The historic Homestake gold mine near Lead in South Dakota first started operating in 1876 and closed in 2001 when operator Homestake Mining Company was bought out by Barrick. As you can imagine of a mine with a 125-year operational history, it had grown a bit like topsy with bits and pieces all over the place. There were 300 to 400 miles of underground workings alone. Additionally, over the mine's long life a number of different ore processing methods had been used including cyanidation, amalgamation, and gravity concentration. This had led to the establishment of several processing plants spread across the site along with associated infrastructure. All up, Barrick acquired a gold refinery, a mill site, a tailings disposal facility, 3 waste rock facilities, 300 – 400 miles of underground workings, and a massive open pit plus a large assortment of historical and other mine facilities. All of which had to be reclaimed, rehabilitated, shut down, pulled down, filled in etc as required.
The massive reclamation project started with the mine's closure and was still going over a decade later. Water is a major issue in this part of South Dakota. The region is semi arid and high quality water not particularly plentiful. During the early years of the mine's operation, Homestake had developed an extensive system for collecting fresh water from the natural springs in the area. This water supplied both the mine and the surrounding communities. Upon the mine's closure, the entire system and all the water it supplied was handed over to local authorities.
One of the first reclamation projects was pulling down the old mill site and removing all contaminant materials. This allowed natural water flow from the site (ie rain) to run directly into the surrounding water catchment without having to be diverted for treatment first. Storm water diversions were built on the waste rock facilities to prevent excess water flushing sediments from the sites into the river system. At the same time, the sites were graded to create stream systems and replanted with native vegetation to further reduce sediment flow. The result is a network of new streams that have attracted wild life back into the ecosystem.
Elsewhere, capture systems have been installed to collect seepage and contaminated surface water for diversion to the water treatment plants. In the plant it's treated via reverse osmosis before being relayed back into the clean water dispersal system. Constant monitoring and testing of the water in nearby streams is ongoing to ensure water quality remains within specified guidelines. Any selenium in the seepage water has also been reduced to a less soluble state via biological reduction. So far water testing indicates the reclamation project has been successful.