Section Five
SOLIDIFICATION, FOUNDRY AND
COMBUSTION RESIDUES
36    THE USE OF FLY ASH AND PORTLAND CEMENT
TO CHEMICALLY FIX METAL MINE DRAINAGE
TREATMENT SLUDGES
Craig C. Hustwit, Civil Engineer
U. S. Bureau of Mines
Pittsburgh Research Center
Pittsburgh, Pennsylvania 15236
INTRODUCTION
The conventional treatment of metal mine drainages produces a sludge consisting principally
of metal hydroxides and oxides. The volume of these sludges can be substantial, commonly rang- ■
ing from 5% to 10% of the volume of mine drainage treated. Sludge volumes of 33% of the
treated water volume have been reported.' In addition, mine drainage treatment sludges are pH-
sensitive. Exposure to even mildly acidic environments can cause the solid-phase metal hydroxides and oxides to dissolve and mobilize metals. Figure 1 illustrates the pH-phase relationship of
several metals commonly found in metal mine drainages. High volumes and chemical instability
make mine drainage sludge disposal a challenging engineering problem and an expensive component of mine drainage treatment.
Sludge generated during the treatment of mine drainage typically has a high water content and
low density. Sludge density is normally expressed in % solids. Sludges separated from treated
water in settling ponds have 0.5% to 4.5% solids.1 Clarifiers produce sludges with 1% to 7%
solids. A dewatering operation is often used to decrease the sludge volume prior to disposal. Lagoons can serve as sludge settling units, dewatering units, and permanent disposal sites.1 They
are relatively inexpensive where land values are low. Sludges dewatered in lagoons have between 0.5% and 20.0% solids.1 This wide density range reflects differences in lagoon design parameters, initial mine drainage quality, and the neutralizing reagent used. Sludge disposal in lagoons is problematic since mine drainage treatment sludges possess thixotropic properties and
can liquify if subjected to vibrations.1 Sand filters with underdrains can produce a sludge cake
with densities ranging from 15% to 25% solids.' However, removing the sludge cake from sand
filters with earth moving equipment can be difficult. Vacuum filters produce sludge cakes with
solids between 8.8% and 30.9%.' All sludge dewatering methods add to the total treatment costs.
The high-density sludge (HDS) process was developed by Bethlehem Mines Corporation. In
this process, a split of sludge is recirculated, often by mixing it with the lime slurry prior to the
neutralization step. HDS treatment systems generate sludges having between 15% and 40%
solids.'-2
The probability that metals will leach from a mine drainage sludge in a landfill is principally
dependent on the ambient pH and the metals present in the sludge. If the environment becomes
acidic but the sludge is well-buffered, then the onset of leaching may be delayed or even prevented. There is evidence that some metal hydroxides and oxides become less pH-sensitive as
they age.3 Some reagents, such as hydrated lime, do not completely dissolve in water. When this
occurs, the undissolved fraction setties with the sludge and buffers the sludge at a high pH. There
are no commonly used treatment technologies or methods to convert the metal hydroxides and
50th Purdue Industrial Waste Conference Proceedings, 1995, Ann Arbor Press, Inc., Chelsea, Michigan 48118.
Printed in U.S.A.
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