Acid Rock Drainage - Mining and the Environment: From Ore to Metal - page 705

Acid Mine
Drainage

Sludge / Lime
Mixed Tank

Neutralization
Tank

Neutralization
Tank

HDS
Thickener

Polishing
Pond

To
Environment

Underflow

Solution
Recycle

Sludge Recycle

Flocculant Solution

Alternative
Fresh Water

Lime Slurry

Lime Make-up
and Storage

Flocculant Make-up
and Storage

Sludge Storage
Pond

FIGURE 17.9
High Density Sludge Treatment Process

Lime Flocculant

Source:
MEND 2001

For example:

→

M 2 2(OH)_ M(OH) 2 (17.6)

where M represents a metallic cation such as Cu 2 . Most metals precipitate from solution
as pH increases from 7 to 10.
Three types of lime-based treatment are commonly used:
●

Batch treatment by the addition of lime to ponded water;

●

Conventional treatment which involves continuous addition of lime to acidic streams; and

●

High Density sludge (HDS) process which uses a treatment plant for continuous
treatment.

Which ever treatment is used, storage and disposal of sludge is an important issue. HDS
plants produce much lower volumes of sludge than conventional treatment plants. The
HDS process is depicted schematically in Figure 17.9 . These treatments are 'active' treat-
ments in the sense that they require ongoing attention including replacement of reagents,
maintenance, and process adjustments.
A variety of 'passive' treatment technologies have been developed, which are intended
to be self-maintaining, requiring little if any maintenance or intervention. These processes
actually imitate processes that occur in nature, such as those that have led to the formation
of 'bog iron ore' and the formation of pyrite in coal beds. Passive treatments include both
aerobic systems where lime is used, and anaerobic systems including 'wetland treatment'
where sulphur is removed, forming sulphides in an organic substrate.
Three passive treatment systems are briel y described below with much more information
available from the technical literature, in particular in MEND (2001).

Which ever treatment is used,
storage and disposal of sludge is
an important issue.

(1) Anoxic limestone drains (ALD)
These drains are composed of crushed limestone, through which acidic waters l ow by
gravity, with progressive increase in alkalinity, as shown schematically in Figure 17.10 .
Metal removal takes place downstream of the ALD. Accordingly, ALDs are used in con-
junction with oxidative ponds or wetlands in which metal deposition takes place.

(2) Aerobic wetland treatment systems
A variety of complex biotic and abiotic processes take place in aerobic wetlands including
oxidation, adsorption, complexation, precipitation, and i ltration. As these systems are only
effective at treating net-alkaline waters, they are commonly used in conjunction with ALDs.