For the past decade, Rio Tinto has had a risk review program at their mining facilities, which is reviewed by the highest level of operations and site management. A major element of this risk review addresses the potential of Acid Rock Drainage (ARD) formation. Twenty-four mines and advanced projects have been vetted in this program. Elements of this risk review program have been integrated into the GARD Guide, a web-base international best practice guide for the prevention of ARD (www.gardguide.com) prepared under the direction of INAP (the International Network for Acid Prevention), a network of international mining companies dedicated to the prevention of ARD.
The closed Barneys Canyon gold mine located 40 km southeast of Salt Lake City, Utah is an example of how this risk review program and the techniques described in the GARD Guide are used to manage potentially acid-forming waste. One of the first steps in this management plan is to characterize the mine waste via mineralogical analysis and chemical testing, and determine the potential of the mine waste to form acid. About 5% of the mine overburden at the Barney’s Canyon mine contained elevated levels of sulfide and was potentially acid forming.
This sulfide-bearing mine waste, which contained from 1 to 1.5% sulfur, was separated and managed in a way to minimize its potential to be oxidized and form ARD. Six million tons of sulfide-bearing rock was placed in two specially designed waste repositories. The sulfide repositories had a footprint of less than 7% of the total 240 hectare waste rock dump area. The sulfide repositories were covered with a 4-m thick layer of benign waste rock, and the surface was covered with topsoil and planted with native vegetation.
Mining and milling at Barneys Canyon mine ended in 2001. Ten years after mining was concluded, there has been no indication of ARD from the sulfide repositories or the much larger low-sulfur, oxide waste-rock dumps. Today the vegetation on the waste dumps is dense and healthy as can be seen from the photo. There is no evidence of acidification, saline soils or stressed vegetation on the surface of the waste rock repositories or dumps.
The proactive ARD management approach necessitates forward looking characterization and prediction activities early in the mine design and operation. For this proactive ARD prevention strategy to be successful, it must be compatible with the life-of-mine plan and the day-to-day operations, and it requires “buy in” from the entire mine operation staff including senior management.