Determination of the available acid-generating potential of waste rock, part II: Waste management involvement

Abstract

Open-pit mining operations often produce large amounts of waste rock that are characterized by large particle size distributions (PSD). Waste rock are normally deposited in surface piles that contain grains varying from a few microns to meters in size. Furthermore, the mineralogical and textural properties of the waste rock are influenced by their PSD. The diameter of physical locking of sulfides (DPLS) is a newly suggested parameter that was defined using an automated mineralogy system to separate waste rock according to their geochemical reactivity. Three lithologies (A, B, and C) were extracted from an open-pit gold mine and their geochemical behaviors, with respect to degree of sulfide liberation, were evaluated using column kinetic tests. The main results of this study showed that fine fractions of the studied waste rock were more sulfidic compared to coarse fractions. Moreover, sulfide liberation was negligible for fractions >2.4 mm. Consequently, 2.4 mm was defined as the critical diameter of sulfide reactivity for the three studied waste rock. Column kinetic tests were used to confirm this hypothesis and to assess the geochemical behavior of the three lithologies. Geochemical analyses of leachates from the column tests showed that pH values remained between 7 and 8 and the instantaneous concentrations of metals such as iron (Fe) and zinc (Zn) were below environmental limits over the entire test duration (543 days). Considering sample reactivity, the data showed that the fine fractions primarily control the geochemical behavior of total samples. Sulfide oxidation rates were high for fractions <2.4 mm, whereas they were negligible for fractions >2.4 mm; total samples showed intermediate rates. For example, in lithology B, pyrite oxidation rates were 12.46 μmol/kg/day, 2.43 μmol/kg/day, and 0.27 μmol/kg/day for the fine fraction (<2.4 mm), total sample (<5 cm), and coarse fraction (>2.4 mm) respectively. Sulfide and carbonate contents and their liberation were defined as key factors controlling the geochemical behavior of the studied waste rock, which was confirmed by the correlation factor between calcium leaching vs. carbonate liberation and between sulfur leaching vs. sulfide liberation. The coarse, unreactive fraction (>2.4 mm) comprised a high proportion of the total sample weight (up to 90 wt. %). Screening waste rock, after blasting, according to the critical diameter of sulfide reactivity could be an efficient technique for global waste rock management that will reduce the economic costs related to waste rock pile reclamation.