Hydrated lime is a commonly-used chemical for treating
AMD. It is sold as a powder that tends to be hydrophobic,
and extensive mechanical mixing is required to disperse it in water.
Hydrated lime is particularly useful and cost effective in large flow,
high acidity situations where a lime treatment plant with a mixer/aerator
is constructed to help dispense and mix the chemical with the water
(Skousen and Ziemkiewicz 1995). However, due to the kinetics of lime
dissolution and its neutralization efficiency, increasing the lime rate
above that required for acid neutralization increases the volume of
unreacted lime that enters the metal floc settling pond. Hydrated lime
can be purchased in 50-pound bags or in bulk. Bulk lime is preferred
by mine operators due to cost and handling advantages. It can be delivered
by barge, truck, or train to many sites and handled pneumatically. Proper
storage of hydrated lime is important in order to maintain its flow
characteristics and thus ensure efficient use. The appropriate silo
volume depends on the daily lime requirement, but should be large enough
to hold the amount of hydrate needed to last between scheduled deliveries
with a safety margin to cover periodic unexpected delivery delays. The
length of time that the system will be in operation is a critical factor
in determining the annual cost of a lime treatment system due to the
large initial capital expenditure that can be amortized over time. The
topography of the site is also an important cost factor with design
and structural costs increasing as the slope of the site increases.
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