Technology reduces sulphur compounds
Protection of water quality has become one of the most important environmental challenges facing the mining industry. Although new mining projects can be designed to minimize impacts to the environment, many existing and abandoned mining operations have water quality problems.
The principal cause of water contamination is acid mine drainage (AMD), which is generated when residual sulphide minerals in waste rock, tailings and other mine components are exposed to air and water. These reactions can produce acidity and elevated concentrations of metals in drainage and seepage that can adversely affect surface and groundwater resources. To meet regulatory criteria for water quality in the receiving environment, many mine operators must consider treatment of mine water and other effluents prior to discharge.
The Canadian company BioteQ Environ- mental Technologies Inc. and the firm Paques Biosystems BV of the Netherlands have joined forces to market their respective patented BioSulphide Process and Thiopaq water treatment/metal recovery technologies under the name BioSulphide-Thiopaq. Applications for the technology include the treatment of, and metal recovery from acid mine drainage, lime plant feed, metallurgical bleed streams, and solutions in smelters, refineries and metals processing plants.
The first North American application of the technology is at the Caribou mine of Breakwater Resources Ltd. in New Brunswick, where acid mine drainage from the underground workings is being treated to remove all heavy metals as sulphides, including zinc, copper, cadmium and lead, upstream of an existing lime treatment plant. There is sufficient zinc in the mine water to allow the recovery of a zinc sulphide concentrate for sale. Outside of North America there are 14 industrial plants utilizing Thiopaq technology for the reduction of sulphur compounds.
The heart of the process is an anaerobic bioreactor, which is used to generate hydrogen sulphide from elemental sulphur at a cost significantly less than chemical sulphide reagents. The biogenic sulphide is passed to a liquid-gas contactor in which metals in the mine water are precipitated as sulphides and recovered in a conventional dewatering circuit. Water discharged from the plant, which contains very low concentrations of heavy metals, passes to the lime plant for final pH adjustment and the removal of aluminum and iron prior to final discharge to the environment. The zinc and other metals removed from the water, including copper, cadmium and lead, are recovered in a sulphide concentrate containing >52% zinc, which will be sold to Noranda Inc.’s Brunswick mine under contract.
Revenues from recovered metals will be used to partially offset water treatment operating costs. The plant also reduces lime costs as well as the sludge volume produced in the lime circuit. Furthermore, the heavy metal content of the lime sludge product is virtually eliminated, significantly reducing the potential long-term liabilities associated with it. BioteQ is currently evaluating an expansion of the plant at Caribou, to allow the retreatment of contaminated tailings concurrently with acidic drainage to recover additional copper and zinc and render the tailings inert for disposal in the exiting tailings impoundment. The contaminated tailings were produced by previous operators at Caribou, and contain significant values of soluble metal.
BioteQ is also pursuing other applications in North America and elsewhere. Recently, the company agreed to joint venture terms with Phelps Dodge Corp. to construct and operate a BioSulphide-Thiopaq plant for selective recovery of copper from acidic groundwater and other streams at a Phelps Dodge operation in the southwestern United States. Copper concentrates produced will be refined at Phelps Dodge’s Miami smelter. BioteQ and Phelps Dodge are currently evaluating several alternatives for the plant location.
BioteQ is working with Falconbridge Ltd. on selective nickel recovery from mine waste water, and with the U.S. Environmental Protection Agency on selective metal removal from mine drainage to augment or replace lime plants, as well as evaluating other copper opportunities in North and South America. BioteQ is also exploring potential applications in the metallurgical and other industries for solution control. For example, the BioSulphide-Thiopaq technology could be integrated with a copper heap leach operation to recover metals selectively from raffinate. In addition to reducing the inventory of economic metals circulating in the leach solution, there is potential for improved leach kinetics and metal recovery in heap leaching applications. The biological reduction process is also effective in the treatment of total dissolved solids (TDS as sulphate or SOx species), acidity, cyanide, and selenium.
Rick Lawrence is executive vice-president of BioteQ Environmental Technologies Inc., Vancouver, B.C.