Buffering mining waste

The acid generated through sulfide oxidation reacts with the nonsulfide gangue minerals within the mine wastes. The most significant pH-buffering reactions in mine settings are the dissolution of carbonate minerals, aluminum hydroxide and ferric oxyhydroxide minerals, and aluminosilicate minerals. 

During long-term leaching in field situations, phases that initially buffer the pH/ A of the leachate can become exhausted with important consequences for the release rate of contaminants. For example, Baverman et al. (1997) showed that copper release from sulphidic mine waste became much more rapid after exhaustion of a pH-buffering calcite phase. In order to investigate such behaviours, serial batch tests and column leaching tests have been devised. 

Office of Surface Mining Reclamation and Enforcement, Excess Spoil, Coal Mine Waste, and Buffers for Perennial and Intermittent Streams, 73 Fed. Reg. 75,814 (2008) Duffy, Valley, 145,177 Interior Department Says Final Rule Clarifies Disposal of Coal Mining Waste Near Streams, 39 BNA Env. Rept. 2451 (2008) (mining industry delighted) Elizabeth Shogren, Mining Damage in Appalachia Extensive, U.S. Finds, EAT, May 30, 2003, at A32. 

Evangelou, V. P. 1996a. Pyrite oxidation inhibition in coal waste by P04 and H202 pH-buffered pretreatment. Int. J. Surf Mining, Reclam, and Environ. 10 135-142. 

Acidity is perhaps the most serious longterm threat from metal-bearing wastes. Water seeping from mine refuse has been passing increased metal concentrations into receiving waters for decades. The threat is especially great in waters with little buffer capacity - that is, in carbonate-poor areas, where dissolved-metal pollution can be spread over great distances. 

The acidity production can develop many years after disposal, when the neutralizing or buffering capacity in a pyrite-containing waste is exceeded. High concentration factors have been determined in inland waters affected by acidic mine effluents. 

In the closure of an open pit, confinement of the environmentally unstable material, such as pyrite or radioactive materials can be achieved by storage below the water table (Fig. 6). For example at the Wismut uranium mine in Germany radioactive and sulphide-rich waste is stored below the water table preventing extensive further breakdown. Above this is a buffer zone and low-level waste and a clay barrier which acts to reduce oxygen and water permeability and also absorb any radium emission (Chapman Hockley 1996 Hockley et al. 1997). 

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