SEM INVESTIGATIONS OF SULFIDE OXIDATION IN LEACHING EXPERIMENTS OF MINE-ROCK WASTES

The Minnesota Department of Natural Resources has been conducting experiments to investigate mitigation of acid mine drainage from sulfide-bearing rock. The rock is finely-ground (<0.149 mm) Duluth Complex rock containing approximately 4% pyrrhotite (Fe_1-xS) and lesser amounts of chalcopyrite and pentlandite. The rock (75 g) was mixed with different amounts of rotary kiln (RK) fines (<10 microns) or limestone (<2.0 mm), placed into small reactors, and then rinsed weekly with 200 mL of deionized water. The effluent from these rinses was analyzed to assess water-rock reactions within each reactor. The objective of the experiment is to determine the effectiveness of alkaline additions: (1) for neutralizing acid produced by sulfide-mineral oxidation, and (2) for decreasing the rate of iron-sulfide oxidation to a level at which dissolution of host-rock minerals neutralizes the acid produced. Although the experiment has been in progress for more than 18 years, some reactors have been terminated. The unleached solids (original Duluth Complex rock), and the leached solids from one RK-fines mixture and one limestone mixture have been examined using scanning electron microscopy and energy dispersive spectrometry (EDS).

After more than 700 weeks of rinsing, carbonate grains persist in the limestone-loaded reactor, whereas no alkaline grains were detected in the RK-fines reactor. The degree of sulfide oxidation was characterized by the relative intensities of sulfur and oxygen peaks determined by EDS; a nearly continuous compositional variation between pyrrhotite and iron oxide is also observed. In many cases, the degree of oxidation varies considerably within individual grains, and the oxidized regions of sulfide grains commonly contain significant amounts of silicon and aluminum. Sulfide grains from the reactor loaded with RK fines are typically more oxidized than those from the unleached control sample and from the limestone-loaded reactor. These observations agree with the higher sulfate release and lower final drainage pH of the RK-fines reactor. In samples that we examined, the limestone more effectively mitigated acid generation than the RK fines.