Matthew Otwinowski

A physico-chemical model can produce a realistic description of a large-scale thermo-chemical behaviour of Waste Rock piles. Having demonstrated that the simple model derived from fundamental physical and chemical principles gives reasonable results, we feel confident that after a further refinement it is possible to construct a realistic and useful waste rock model.

Scaling analysis of a nonlinear reaction-transport model has been performed. The acid generation rates can be characterized by a physico-chemical scaling parameter δ which depends on pile porosity, pile size, ambient temperature, effective reactive pyrite surface area, heat of oxidation and thermal conductivity. The scaling parameter δ can be used as a practical indicator of ARD. The scaling parameter provides quantitative information about the relative importance of the factors responsible for acidic drainage. In particular, the effectiveness of impermeable covers increases with the pyrite concentration. Sample calculations of temperature profiles and acid generation rates have been performed for realistic parameter values in the absence of bacteria and without neutralization. Alternative scenarios with and without impermeable covers are discussed.

The scaling analysis indicates that geochemical and transport processes operate at the meso-scale in a way fundamentally different from the full-scale. When oxygen supply is controlled by the diffusion process, the Waste Rock Piles can exhibit thermodynamic catastrophes. A thermodynamic catastrophe occurs when relatively small changes of parameters, such as pile size, pile porosity, ambient temperature or effective active surface area result in a dramatic increase of the acid generation rate. The scaling parameter δ provides quantitative information about waste rock storage conditions, such as pile porosity and pile size, which have to be satisfied in order to avoid a thermodynamic catastrophe leading to accelerated acid generation rates.

The scaling analysis indicates that in order to avoid a thermodynamic catastrophe leading to high acid generation rates, the waste rock piles should be designed so that the value of the physico-chemical scaling parameter δ is less than a critical value δ^*. Illustrative examples of the practical applications of the critical scaling criterion are presented.

In the next step, air convection and water transport should be included in the model and quantitative results for the expected water quality should be obtained.

In an additional study one should analyze the problem of optimum pile shape as the function of the cover permeability and the total amount of waste rock to be stored.

All the quantities involved in the scaling analysis can be measured in independent simple laboratory experiments. The accuracy of simple experiments sufficient for a predictive waste rock model has been analyzed. Because realistic results are obtained with no additional adjustable parameters, we believe that after experimental validation and additional refinement (involving nonsymmetric boundary conditions, water transport and convection of air) the results of the scaling analysis can be used as simple practical guidelines for design of waste rock sites.

Quantitative information about the oxygen consumption by bacteria and the spatial distribution of bacteria concentration in waste rock piles is necessary for extending the present scaling analysis to the case of bacterial oxidation by Thiobacillus ferrooxidans. When such data become available, the quantitative results for bacterial oxidation reviewed in our previous study [SyT] can be used to modify the present scaling analysis. 

One should rely on the results of simple laboratory tests. In addition to the usual acid/base accounting tests, thermokinetic tests should be performed for different values of temperature, pH and different oxygen partial pressures in order to define the scaling curves Y^p and αT^q which are expected to be different for rocks with different neutralization potential. Representative samples of waste rock should be used for such tests. For the needs of a predictive model it would be sufficient to perform oxidation tests under isothermic conditions at temperature intervals of 15°C in the temperature range between 5°C and 80°C and oxygen partial pressures varying between 0.21 atm and 0.01 atm at 0.05 atm intervals. The same experiments should also measure the cumulative heat of oxidation/neutralization reactions. Such isothermic tests should be performed separately for pH greater than four and pH less than four in the presence of bacteria, at water saturation values characteristic for waste rock piles. For small pH values bacteria concentration and oxygen consumption should be monitored for different values of oxygen partial pressure. In this way one can produce in a relatively inexpensive way the input data necessary for a predictive model which should help to optimize the storage of waste rock and to provide a reliable tool useful for environmental assessment. Usual acid^ase accounting tests do not provide data necessary for a predictive model which should generate quantitative information about the effluent.

The value of heat, h, generated during the pyrite oxidation process can be different for different values of the acid neutralization potential. We could not find reliable data on the heat generated by calcite dissolution or the neutralization reactions, which would modify the value of h used in this report. We do not recommend, however, to measure these quantities separately. The cumulative heat generated during the weathering process is sufficient for the predictive reaction- transport model. The heat generated can be measured in the same isothermal experiment (proposed above) as the value of cooling necessary to preserve constant temperature of the oxidation process. In this way we can keep the same number of measurable coefficients as in our scaling model in which only the effective heat of pyrite oxidation is analyzed.