Abstract:

The complete redesign of a mine waste rock storage facility provides an opportunity to assess the effectiveness of leading practice waste rock management against earlier approaches. At this site, the economic mineralization is associated with finely laminated to thinly bedded dolomitic and pyritic siltstones, massive dolomitic breccias, and pyritic carbonaceous shales. The initial design of the main waste rock storage facility was based on a simple binary classification which only considered acid base properties of the rock. While broadly correct, the classification did not account for differing rates of sulfide reactivities or for the risk of neutral metalliferous drainage. As a result, the chosen construction methods (e.g. high lifts, insufficient capping) resulted in high rates of sulfide oxidation and development of localised high temperatures, especially within the carbonaceous shales. A comprehensive review of the site geochemistry resulted in a new classification of the waste according to reactivity and ARD potential, and the waste rock facility was completely redesigned based on multiphase (transient air/water/temperature modelling linked to geochemistry) reactive geochemical modelling. In-situ gas and temperature monitoring was installed, and radically different management strategies have been implemented. These include changes in geometry; increased segregation of the most reactive waste and placement in internal cells with low lifts and low air permeability barriers to reduce advective oxygen transport; use of interim wet season covers to reduce water infiltration; placement of lower reactivity and non-acid forming material around the core. The changes have yielded very positive results with the successful remediation of high temperatures and ongoing temperature and oxygen monitoring confirming that these measures are working.

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Mr. Johan Fourie