Control of acid mine drainage from tailings areas is widely recognized as one of the
most serious environmental issues facing many base metal, gold and uranium mine
operators today. While collection and treatment of acid mine drainage is commonly
practiced at active mine sites, it is generally accepted that continuation of treatment
practices for an indefinite period in the post operating phase is neither desirable nor
practical. Besides the obvious problems associated with maintaining an effective
treatment system after mining activities have ceased, the disposal of chemical
treatment plant sludge produced from the neutralization of acid mine drainage is a
major operational problem.

Originating from work initiated in the mid-1980’s by the Canada Centre for Mineral and
Energy Technology, the Reactive Acid Tailings Assessment Program for Base Metal
Tailings (RATAP.BMT) was developed as a predictive modelling tool to investigate the
factors and processes which control the oxidation of sulfide minerals, to simulate acid
generation in mine tailings, to estimate the long-term potential for acid generation in
tailings, and to evaluate the effects on acid generation of alternative closeout concepts.

This report describes the extension of the validation of the model by:

1. a discussion of quality assurance procedures followed during each stage of code
   development and documentation of recent code modifications;
2. a critical review of the concepts behind the program; and
3. a comparative evaluation of computer simulations with a more complete database
   for the Waite-Amulet zinc/copper mine tailings.

RATAP.BMT addresses questions more numerous and more complex than those
addressed by other models. It permits evaluation of the limitations of other modelling
work and of many additional questions which are beyond the scope of the other
models. However, RATAP.BMT requires a more knowledgeable user and, thus. is
more difficult for a novice user to program.

The model considers the following processes:

• sulfide mineral oxidation kinetics as a function of water temperature, oxygen concentration. mass of pyrite, pH. phosphorous concentration, carbon dioxide content, and moisture content.
• oxygen pore-gas diffusivity and its control upon the oxygen flux into the tailings.
• shrinkage of sulfide mineral grains as they oxidize.
• depth-dependent differences in the chemical characteristics of the tailings.
• temperature variation with depth due to oxidation of pyrrhotite, pyrite, chalcopyrite,
• sphalerite and arsenopyrite
• moisture variations with depth in the unsaturated zone.
• kinetic reactions between porewater and relevant minerals.
• porewater transport of metals including aluminum, iron. calcium, magnesium, potassium. silica, copper, zinc, and of anions including arsenic, sulfate, and carbonate.

Model validation was based on the comparison of model predictions for selected
parameters to data collected on the Waite-Amulet tailings during the Phase 2 and
Phase 3 field studies. Part of the data was used to calibrate the site dependent
parameter estimates. The second part of the data was used to verify the model both in
the temporal and spatial (i.e. with respect to depth) sense. The modelled constituents
included temperature, pH, oxygen concentration (variation with depth), and porewater
concentration of sulfate, ferrous and ferric iron, dissolved copper, and zinc.

The final test involved running the model both in a deterministic and probabilistic
manner. The probabilistic simulations were based on selecting parameter values from
established distribution functions. The geometric mean values of the probabilistic
outputs were used for comparison to the results of the deterministic runs using nominal
parameter values.