Executive Summary

This report describes the results of a research project which has been initiated with the purpose to provide a quantitative analysis of the interrelated elementary chemical and biological processes which are responsible for pyrite oxidation and acid rock drainage (ARD). The highly nonlinear nature of the kinetic equations describing coupled chemical and microbial reactions involved in pyrite oxidation raised serious questions about the predictability of the environmental impact of acid rock drainage.

The main objective of this project was to determine whether the coupled chemical reactions involved in a multistage oxidation of pyrite lead to irregular or chaotic in time changes of products of the chemical and microbial reactions responsible for acid rock drainage. The main conclusion of the model analysis described in this report is the absence of such an irregular temporal behaviour. The set of nonlinear kinetic equations for the chemical reactions involved in pyrite oxidation does not produce a chaotic behaviour or other types of chemical oscillations. The nonlinear nature of the elementary nonequilibrium processes is responsible for the presence of the quasi-equilibrium values of the concentrations of ferrous and ferric iron. This property is a key to understanding the complexity of acidic drainage and should be helpful in designing efficient ways of minimizing acidic drainage. The presence of the quasi-equilibrium states increases our chances to formulate predictive ARD models. This study does not exclude, however, physico-chemical oscillations when processes of water and oxygen transport are included in a future model. (The analysis of transport processes was outside the scope of the present project designed as a low-budget preliminary analysis of the nonlinear chemical and biological kinetics.)

Several experimental results are reevaluated and, in some cases, values of rate constants different than those previously published in the literature are determined. A kinetic model in the form of coupled nonlinear ordinary differential equations is constructed for the coupled chemical reactions responsible for acid rock drainage. The equations describe the time dependence of the concentrations of the hydronium ions, ferrous and ferric iron, sulphate and oxygen dissolved in water.

In our analysis a clear distinction is made between the chemical and bacterial reactions which require the presence of dissolved oxygen and the chemical and bacterial processes which do not require oxygen.

At pH values greater than four, the process of pyrite oxidation is mainly due to the pyrite oxidation by oxygen dissolved in water:

FeS₂(s) + 7/2 O₂ + H₂O = Fe²⁺ + 2SO₄^2- + 2H⁺ (R1)

Ferrous iron is released to the water solution where it is oxidized to ferric iron:

Fe²⁺ + 1/40₂ + H⁺ = Fe³⁺ + 1/2 H₂0 (R2)

At pH values less than four the ferric iron reacts with pyrite

FeS₂(s) + 14Fe³⁺ + 8H₂0 = 15Fe²⁺ + 2SO₄^2- + 16H⁺ (R3)

At high pH values the ferric iron reacts further with oxygen and water, and forms ferric hydroxide which precipitates:

Fe³⁺ + 3H₂0 = Fe(OH)₃(s) + 3H⁺ (R4)

Reactions (Rl), (R3) and (R4) produce acid, which, if not neutralized, mobilizes the metal ions, contained in the waste rock. High pH values can be maintained by neutralizing minerals which often are present in the waste rock or by minerals (like calcite) added to the waste rock. The process of neutralization by calcite is described by two reactions

CaCO₃ + 2 H⁺ + S0₄^2- + 2H₂0 = CaSO_4· 2H₂0+ H₂CO₃^o (R5.1)

CaCO₃ + H⁺ + S0₄^2- + H₂0 = CaSO_4· 2H₂0+ HC0₃^- (R5.2)

The relative rates of the reactions (R5. 1) and (R5.2) depend on pH values and are responsible for the efficiency of the neutralization process. The rate of oxidation of ferrous iron increases with increasing pH values and the neutralizing potential of the reactions (R5. 1) and (R5.2) decreases when pH increases. This leads to a stoichiometric incompatibility between acid-generating and acid-neutralizing reactions. Minimizing the stoichiometric incompatibility during the neutralization process should reduce the amount of sludge generated and lower the cost of neutralization. Since the analysis of the neutralization process is limited to equilibrium conditions for the neutralizing species (pH is a control parameter), further analysis is required.

At pH less than four, ferric hydroxide is soluble and the reaction of pyrite oxidation by ferric iron contributes to acidic drainage. The source of ferric iron may be reaction (R2) (oxidation of ferrous iron) or the ferric hydroxide formed higher in the pile and washed down to a region where pH is low. The reaction of pyrite oxidation by ferrous iron may also be initiated if an insufficient amount of neutralizing minerals is used.

The kinetic model is analyzed for different regimes corresponding to possible different situations at various sites. The rates of pyrite oxidation and oxygen depletion are analyzed at different temperatures between 273K and 333K, and at concentrations of dissolved oxygen corresponding to the concentration of oxygen in the gaseous phase ranging from 21 % to 2 %. The ratio between the active surface area, S and the water volume, V, varies between 0.1 m²/l and 100 m²/l. The nonlinear nature of the elementary chemical processes involved is responsible for a dramatic increase in iron concentration by increasing acidity. The competition between increasing temperature and decreasing concentration of oxygen dissolved in water is analyzed in detail. The increasing temperature, while accompanied by a lower concentration of dissolved oxygen, leads to the oxidation rates increasing about ten times per a 20 K increase in temperature. Computer simulations for the concentrations of hydronium ions, ferrous iron, ferric iron and sulphate generated during time intervals ranging from a few hours to several months have been performed for different values of the chemical and physical parameters which control the process of acidic drainage. In some cases, the nonlinear kinetic equations have been solved analytically and several useful closed-form mathematical formulae have been obtained.

At low pH values and at temperatures about 303 K, Thiobacillus ferrooxidans at concentrations on order of one gram of wet cells per litre, can accelerate the process of pyrite dissolution by about a thousand times. Kinetic equations for the bacterial processes of pyrite oxidation by dissolved oxygen and by ferric iron are proposed for the first time. Bacterial processes accelerate each of the reactions (Rl), (R2) and (R4) in a different way. The reaction (RI) is accelerated about three hundred times. The reaction (R2) becomes about a million times faster. The reaction (R4) is accelerated by bacteria about three times.

The reactions of the bacterial oxidation of ferrous iron to ferric iron and the pyrite oxidation by ferric iron provide a nonlinear negative feedback mechanism which is responsible for a smaller than desired effect of slowing-down pyrite oxidation by reducing oxygen concentration. When oxygen partial pressure decreases from 0.21atm to 0.04atm (i.e. by 75 %), the rate of pyrite oxidation by Thiobacillus ferrooxidans decreases by only 30 %. This negative feedback mechanism is also responsible for a chemistatic bacterial action and prolonged bacterial activity in an acidic environment.

Several problems which merit further experimental and modelling studies are identified.

Quantitative results presented in this study should be confronted with field data and, after calibration, the kinetic model presented here can be used as a part of a comprehensive physical waste rock model and an underwater disposal model.

Last Modified: 2003-11-26		Important Notices

Français | Contact Us | Help | Search | Canada Site
Home | What's New | CANMET-MMSL | MMS Site | NRCan Site