Technology to deal with mine wastewater improving
By its very nature, mining is disruptive to the environment. As orebodies are explored, blasting and core sampling disturb the earth. Open pit mining requires the removal of huge quantities of overburden that are stripped away to reveal the precious ores beneath. Underground mining, while more targeted in its extraction methods, is no less intrusive, as shafts are bored hundreds of metres into the ground and rock faces are cut open to excavate and hoist the ore to surface.
As the rock is disturbed, so is the natural flow of water. Problems emerge when metal sulphides become acidic after being exposed to air and water. This phenomenon, known as acid rock (or acid mine) drainage, is a common occurrence in metal mines, particularly after the mine has been shuttered and water floods the workings. Tailings ponds can also be a source of acid rock drainage, as can the liquid that drains from coal piles.
The water that seeps through often contains a toxic brew of heavy metals such as cadmium, copper and zinc. When that water flows into streams and large waterbodies, the effects on local human and wildlife populations can be devastating.
Britannia Mine Remediation Project
In British Columbia, Canada, an ambitious project to rectify the effects of acid rock drainage was undertaken in the mid-2000s at the Britannia mine.
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Located an hour’s drive north of Vancouver, the Britannia mine was the largest copper producer in the British Commonwealth during the 1920s and ‘30s. Over 50 million tons of ore were extracted between 1904 and 1974; at its peak, the mill processed 7,000 tons per day, mostly copper, but also zinc, lead, cadmium, silver and gold.
A thriving community emerged at theminesite on the shores of Howe Sound, where an estimated 60,000 people lived during the 70-year life of the mine, enduring fatalities, landslides, floods and fires — all documented in the recently opened Britannia Mine Museum.
While the mine was undoubtedly a success, it had a dark side: acid rock drainage.
“It was often cited as one of the largest sources of metals pollution discharged into a marine environment in North America, so it is one of the larger volumes of ARD,” says Gerry O’Hara, a consultant with Golder Associates. O’Hara manages the Britannia Mine Remediation Project where, on behalf of the B.C. government’s Ministry of Natural Resource Operations, he leads a team of consulting and contracting companies involved with investigation and remediation work at the mine, public consultation, permitting and regulatory liaison.
O’Hara describes the mine, with its 210 kilometres of underground tunnels, as “like a swiss cheese,” with water entering the mine mostly through open pits and glory holes at the top of the mountain where the mine workings begin. Before remediation, the water moved through the mine, became acidic and exited at two levels, one midway up the mountain, where it flowed into Britannia Creek, and another at the base near sea-level. The water was discharged directly into Howe Sound through a deepwater outfall.
The ARD problem was mitigated to a minor degree by timber and concrete copper launders used at the mine to extract copper from the water.
In the late ‘90s a number of factors converged to bring about the remediation of the site. According to O’Hara, these included growing environmental awareness, the Waste Management Act/1997 Contaminated Sites Regulation and a 2001 agreement whereby the mine’s former owners agreed to pay $30 million towards remediation in exchange for indemnity against additional environmental liabilities.
The province has committed $99 million to remediating the site.
Even after 70 years of mining, there are still enough metals in the mine wastewater to justify a lime treatment plant. O’Hara says that the mine discharges an average of about a tonne of copper and zinc every three days. Lime treatment works by bringing the pH of the contaminated water to a point where the heavy metals are insoluble. The metals precipitate into tiny particles, which are separated from the liquid to form a sludge, which is then dewatered and disposed of.
In 2004 EPCOR was awarded the contract to build a high-density sludge treatment plant. The company designed, built and financed the plant and is operating it on a 20-year contract with the provincial government.
One of the unique features of the system is the use of the mine workings as a hydraulic reservoir. At the base of the mountain, near sea level, a concrete plug holds back the stored water. A network of pipes and valves downstream of the plug allows the water to be brought to the treatment plant in a controlled manner where it is collected in two reactor vessels for the first stage of the treatment process. Lime is added to the water, mixed with a polymer, and the pH is raised from 4 to around 9. The water then proceeds to a clarifier, where the sludge settles and the clean water is discharged to the environment through a deepwater outfall. Some of the sludge is recycled into the first reactor, thus producing a higherdensity sludge and reducing the volume of waste.
What remains, a metal-laden sludge, is pressed into a filter cake and stored temporarily in a concrete-lined basin near the plant before final disposal to a permitted on-site facility.
Between 2,500 and 3,000 cubic metres of filter cake are collected every year.
O’Hara says that the quality of the discharged water is excellent.
“It’s much better than the permit requires. We often find that copper and other metals aren’t detected in the water. It’s very good quality.
”As for the marine environment, O’Hara says that prior to mine remediation the water quality in Howe Sound was poor, with fish unable to survive and little to no aquatic life on the shoreline. Now, five years after the treatment plant began operating, life is coming back. Fucus (rock weed), mussels, barnacles, and microrganisms eaten by fish “are all indications that the water quality is improving significantly,” says O’Hara.
Recovering Metals from Mine Wastewater
The success of the Britannia Mine Remediation Project shows that lime treatment plants can be effective in solving the problem of acid rock drainage. However, the plants do have their drawbacks. Lime treatment sometimes fails to meet tighter discharge limits imposed by regulators. It also comes with a by-product, lime sludge, which must be disposed of safely and constantly monitored, all of which, of course, comes at a cost.
Perhaps most important, traditional lime treatment plants are not able to recover the metals for re-sale — an idea seized upon by BioteQ, a Canadian environmental technology company.
The Vancouver-based firm has developed technology for recovering dissolved metals and removing sulphates from contaminated water. The technology can be applied to acid rock drainage, metallurgical bleed streams, lime plant effluent, tailings water and gold processing.
“The main driver for customers is environmental compliance. They need to meet specific discharge limits for metals concentration, so they’re looking to us to provide them with a solution to ensure that they’re always within compliance,” says Tanja McQueen, BioteQ’s VP, Corporate Development.
“What we’re seeing increasingly is that regulators are making those limits tighter and tighter. And so companies are being really challenged to find new ways of dealing with those tighter discharge limits.
” According to McQueen, BioteQ’s customers benefit from the technology because metals can be recovered and sold, thus generating a revenue stream to offset treatment costs. Moreover, lime sludge is removed that otherwise creates an environmental liability.
At the Dexing copper mine in China, BioteQ partnered with Jiangxi Copper Corporation to treat wastewater produced by mine drainage from waste dumps and stockpiles and to remove dissolved copper and ferric iron. Last year the process treated about 5.5 billion litres of water and recovered about 1.7 million pounds of copper.
BioteQ offers two main technologies for dealing with mine wastewater: metals recovery and sulphate removal.
Using biological and chemical processes, sulphides recover metals that can be shipped to smelters for refining. In mines with large flows of water but low concentrations of dissolved metals, BioteQ recovers the metals using an ion exchange process.
Examples of recoverable metals include copper, cobalt, cadmium, zinc, nickel, molybdenum, rhenium and vanadium.
BioteQ also uses ion exchange to remove sulphates in mine wastewater. The company’s Sulf-IX process is an alternative to traditional reverse osmosis (RO) membrane technology used to remove sulphates and other impurities from wastewater.
“It’s a bit of a game changer for the industry,” says McQueen, noting that the company’s first commercial-scale plant is currently being conditioned at a mine in the United States.
BioteQ’s SART technology, developed by SGS Lakefield and Teck, is used in gold orebodies containing base metals such as copper. The process removes the copper that interferes with the cyanide leaching operation and recycles the cyanide. The result is a reduction in cyanide use, a lighter environmental footprint and higher gold yields.
McQueen says that she expects the high gold price to entice more mines to use SART that previously would have found it too expensive. It is currently being used at a mine in Turkey and a mine in Mexico.
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