The problem of pollution of water by mine drainage is at least as old as the mining industry itself. However, research on the formation, composition, treatment, and abatement of mine water is a relatively recent historical event.
At Bituminous Coal Research, Inc., acid mine drainage control has been an important area of work since 1944, at which time BCR began sponsorship of research at West Virginia University. The work at West Virginia University involved a detailed study of the acid mine drainage problem and led to the identification of iron oxidizing bacteria as important factors in the formation of acid mine water.
Chemical and Physical Properties of Mine Water
In order to appreciate the intent of this work area, a brief review of the formation of mine water is necessary.
Acid mine drainage results from the dissolution of oxidation products of pyrite in normally alkaline ground water together with the subsequent dissolution of other minerals in the resulting acidic solution.
The mechanism of the chemical reactions involved in pyrite oxidation is complex. It has generally been represented by the following overall chemical reaction.
(a) FeS2 + 7O + H2O = FeSO4 + H2SO4
(b) 2 FeSO4 + O + H2SO4 ↔ Fe2(SO4)3 + H2O
(c) 7 Fe2(SO4)3 + 8 H2O ↔ 15 FeSO4 + 8 H2SO4
(d) Fe2(SO4)3 + FeS2 + H2O ↔ 3 FeSO4 + 2S
(e) S + 3O + H2O ↔ H2SO4
Ferrous sulfate is the principal soluble oxidation product of pyrite. However, ferrous sulfate in solution is unstable, depending on pH and oxygen content of the water. At low pH <6, solutions of ferrous sulfate can be kept for long periods of time without change; above pH 6, in the presence of air, sunlight, and/or certain bacteria, ferrous iron oxidizes to ferric iron, the rate being dependent on the pH of solution. Mine water containing ferrous iron may be a clear solution, however, subsequent aeration, dilution with alkaline water, and/or bacterial action initiate the chain of reactions which lead to the formation of yellow to red precipitates which cause “red water.”
For the most part, the successful treatment of mine drainage concerns the efficiency of removal of iron from solution. In acid mine drainage, iron can occur both in the Fe+² and Fe+³ forms. At pH above 3, but less than 9, Fe+² is the stable form. Fe+³ hydrolyzes to completion at pH 3. The oxidation of Fe+² to Fe+³ is rapid and complete at pH levels above 7; however, at a lower pH, or in the presence of complexing agent, Fe+² is stable. Stumm, et al, Barthauer, Glover, and others have reviewed the problems associated with Fe+² oxidation.
One of the most talked about, but perhaps least understood, characteristics of mine water is acidity. The purpose of this paper is not to argue for or against any of the many analytical methods for measuring acidity; rather, an attempt is made to give an insight into the significance of acidity as it influences treatment of mine water.
The most widely used measure of acidity is pH. A workable definition of pH is the negative logarithm of the [H+] or
-log [H+] = pH.
Why measure [H+]? For one reason, it is a simple measurement made without calculations and the equipment is amenable to field use. However, the [H+], or pH of a mine water, is a useful measurement only when used with other pertinent data. The following table relates pH to equivalent H2SO4 content.
Process Development Studies
While considerable work has been done in this country and abroad on the treatment of mine drainage by lime and limestone results achieved thus far indicate that many gaps exist in the data. Because of this lack of information, diverse opinions of the applicability of lime and limestone neutralization have been held by workers in the field. Little systematic research has been conducted to resolve the differences in results achieved. The problems encountered in lime and limestone neutralization processes are both economic and technological; however, both types of problems are considered amenable to solution through research.
Most experimental work previously reported by others has involved large-scale exploratory research in which a particular mine water is treated with a particular reagent. In describing the results, little attention has been paid to the chemistry of the system being treated. Furthermore, only meager information is available on the effects of composition, chemical equilibria, temperature, oxidation potential, and on the treatability of a particular mine water by a specific process.
The reaction of lime with acidic mine water can be represented, by the following reactions:
Ca(OH)2 + 2H+ → Ca+² + H2O
Ca(OH)2 + Fe+² → Ca+² + Fe(OH)2
3 Ca(OH)2 + 2 Fe+³ → 3 Ca+² + 2 Fe(OH)3
3 Ca(OH)2 + 2 Al+³ → 3 Ca+² + 2 Al(OH)3
Ca(OH)2 + Mn+² → Ca+² + Mn(OH)2
These are overall reactions and do not depict the mechanism of the formation of the insoluble hydroxides nor the interactions and complex compounds which might be formed during the neutralization of mine water with lime.
The reaction of mine water with limestone includes the following equilibria:
CaCO3 + H+ ↔ Ca+² + HCO3-
HCO3- + H+ ↔ H2CO3
H2CO3 ↔ H2O + CO2