Limestone (calcium carbonate)
Calcium content of limestone should be as high as possible. (Dolomitic
limestones are less reactive and generally ineffective in treating Acid
Mine Drainage.) Advantages of using limestone include low cost, ease
of use, and formation of a dense, easily handled, sludge. Disadvantages
include slow reaction time, loss in efficiency of the system because
of coating of the limestone particles with iron precipitates, difficulty
in treating Acid Mine Drainage with a high ferrous-ferric
ratio, and ineffectiveness in removing manganese. Limestone treatment
is generally not effective for acidities exceeding 50 mg/l.
Hydrated Lime (calcium hydroxide)
Hydrated lime is normally the neutralizing agent of choice by the coal
mining industry because it is easy and safe to use, effective, and relatively
inexpensive. The major disadvantages are the voluminous sludge that
is produced (when compared to limestone) and high initial costs that
are incurred because of the size of the treatment plant.
Soda Ash (sodium carbonate)
Soda ash briquettes are especially effective for treating small Acid
Mine Drainage flows in remote areas. Major disadvantages are higher
reagent cost (relative to limestone) and poor settling properties of
the sludge.
Caustic Soda (sodium hydroxide)
Caustic soda is especially effective for treating low flows in remote
locations and for treating Acid Mine Drainage having a high manganese
content. Major disadvantages are its high cost, the dangers involved
with handling the chemical, poor sludge properties, and freezing problems
in cold weather.
Ammonia
Anhydrous ammonia is effective in treating Acid Mine Drainage having
a high ferrous iron and/or manganese content. Ammonia costs less than
caustic soda and has many of the same advantages. However, ammonia is
difficult and dangerous to use and can affect biological conditions
downstream from the mining operation. The possible off-site impacts
are toxicity to fish and other aquatic life forms, eutrophication
and nitrification. Fish species generally
exhibit low tolerance to unionized ammonia and toxicity levels can be
affected by pH, temperature, dissolved oxygen and other factors. Ammonia
use is not allowed in all States and, where permitted, additional monitoring
is required.
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