Preferential oxidation of pyrite as a function of morphology and relict texture

Abstract

Environmental contamination from mines producing acid rock drainage, which is caused by sulphide mineral oxidation, represents one of the most significant environmental problems facing the international mining industry. This work investigates the mineral morphological effects on the rate of pyrite oxidation and the influence of relict morphological features on rapid oxidation and thus acid generation rates. Laboratory-based kinetic tests were performed on potentially-acid forming rock by measuring changes in pyrite mineralogical compositions, metal release and acid generation over time. The rate of pyrite oxidation is strongly dependent on the reactivity of two pyrite morphological forms (euhedral and framboidal). After 210 days 70–100% of all framboidal pyrite had undergone complete oxidation, which contributed to an initial high acid generation rate (peak concentration of 2927 mg L−1 CaCO3 after 120 days); subsequent acid generation rates (1730 mgL−1 CaCO3 after 390 days) were substantially lower. Scanning Electron Microscopy (SEM) micrographs clearly show the persistence of larger euhedral pyrite grains as a contributing factor to this on-going acidity after 390 days. Samples collected from laboratory humidity cells after 390, 480 and 720 days showed evidence of preferential dissolution associated with these large pyritic overgrowth textures. Clearly evident are prior relict framboid networks within larger euhedral pyrite grains suggesting that oxidative dissolution may be related to internal crystallographic defects associated with the overgrowth textures in these samples.