PYRITE OXIDATION IN TWO PHASE LIQUID+VAPOR WATER CONDITIONS

Owing to its significance in the formation of acid mine drainage, the rate of pyrite oxidation has received persistent attention for many years. The vast majority of experimental rate measurements have been made under water saturated (liquid) conditions, and a few have are available for the reaction in vapor-only conditions. However, in field settings, either naturally occurring or associated with mineral extraction activity, liquid+vapor conditions are the most common.

As part of environmental permitting programs, the mineral industry has come to rely on empirical measurements of pyrite (and other sulfide mineral) reaction rates using the Humidity Cell Test (HCT). This laboratory test exposes prepared mine rock samples to liquid+vapor water in the presence of atmospheric oxygen, provided in excess. The HCT is a standardized test method that has been evaluated for reproducibility, but limited effort has been made to assess geochemical processes (e.g. reaction rates) relative to existing geochemical literature.

A series of well constrained HCT experiments have been conducted using simple mineral assemblages, with known mineral grain sizes. Tests have been conducted using pyrite + quartz and pyrite + quartz + calcite. Additional testing has been conducted using two separate pyrite grain sizes. Though difficult to constrain, the proportion of liquid to vapor water in the system has also been investigated by altering the established procedure to increase the vapor relative to liquid phase (vapor-enhanced).

Results to date (8 of 20 weeks test duration) indicate that the rate of pyrite oxidation occurs faster than in vapor-only conditions, but substantially slower than liquid-only conditions. Further, the rate in HCT measurements for vapor-enhanced conditions is considerably slower than standard prescribed test conditions. Test results appear to support previous work that suggests that pyrite oxidation is enhanced with increased flushing of adsorbed water in vapor-only conditions. Data from tests conducted using two different pyrite grain sizes (reactive surface area) indicate the rate of pyrite oxidation in liquid+vapor phase conditions does not scale linearly with reactive surface area.