The Control of Acid Mine Drainage
Pollution by Biochemical Oxidation and
Limestone Neutralization Treatment
H. GORDON GLOVER, Scientist in Charge
Mine Water Investigation
National Coal Board, South Yorkshire Area
Golden Smithies Lane, Wath-upon-Dearne
Rotherham, Yorkshire
England
INTRODUCTION
Pyritic minerals are normally chemically stable in the alkaline and reducing
environment of the undisturbed strata, but oxidize slowly to form water soluble
sulphates when exposed to the atmosphere. This effect is a normal feature of
geological denudation and the iron oxide stains produced by the natural oxidation
of iron pyrite were used as an indication of outcropping coal measures in the early
search for coal. The rate of exposure of pyrite has been increased many fold by
intensive mining operations in some countries during the last few centuries and the
resulting acidic oxidation products have seriously contaminated the surface drain -
age systems and led to the sterilization of many thousands of miles of inland
watercourses. It is not surprising to find that the drainage from mines yielding
pyritic ores is acidic, but it is less obvious that the small amount of pyrite encountered in coal mines should lead to a major acid drainage problem. Pyrite is
and undesirable contaminant of coal and is separated as far as possible at the mine,
but unfortunately the pyrite is insufficiently pure to justify recovery and is left
behind in the mine workings and in spoil heaps on the surface. Acid mine drainage is generated when the oxidation products of the pyrite contaminate the surface drainage and the ground waters drained from the mine.
The term acid mine drainage is applied not only to drainages which have an
abnormally low pH value, but also to drainages containing apparently neutral substances such as ferrous sulphate which could react with the alkalinity of a natural
watercourse. Acid drainage from coal mining operations may contain sulphuric
acid and ferrous, ferric, aluminum and manganese salts in concentrations up to
several thousand mg/1 and also calcium, magnesium and sodium salts, carbon
dioxide and silicic acid. Acid coal mine drainages are usually encountered in
volumes of a few hundred thousand gals/day, but individual rates may reach several tens of millions of gals/day.
The acid coal mine drainage problem in the U.S.A. has been described in
many publications (1, 2, 3) and will not be considered in detail except in so far as
is necessary to justify the detailed consideration of treatment processes which is to
be presented. An authoritative estimate of the magnitude of the problem in the
U.S.A. (4) was that some 3, 500, 000 tons of sulphuric acid equivalent per year
was being discharged to inland watercourses and that some 10, 000 miles of rivers
and  streams  were  probably  contaminated  with   acid.    The source of the acid
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