Environmental Engineering Reference
In-Depth Information
19
Mine and industrial dams
19.1
GENERAL
This chapter is intended to give an overall appreciation of mine and industrial tailings dis-
posal practice. It is written for those who have not had extensive experience in tailings dis-
posal and serves as an introduction to the subject, rather than forming a complete basis
for design of a tailings disposal system.
The authors' experience is that the majority of mine tailings are disposed of into rea-
sonably conventional dams and the principles of dam engineering covered in the rest of
this topic apply. Some tailings are disposed of using the tailings themselves to construct
the embankment e.g. using upstream, centreline or downstream methods, and this requires
special design and construction features (see Section 19.5 ) . More recently, thickened dis-
charge, co-disposal and paste technology has been developed which is applicable in some
situations and these methods are described briefly. This chapter mostly concentrates on
the understanding of what tailings are and their properties, the methods of discharge, pre-
diction of deposited density, the methods of disposal, estimation of seepage from tailings
disposal areas and the particular aspects of slope stability, internal erosion and piping and
performance under earthquake which are important in tailings disposal.
19.2
TAILINGS AND THEIR PROPERTIES
19.2.1
What are mine tailings?
Mine tailings are the end product of mining and mineral processing, after the mineral has
been extracted, or the unwanted material separated, e.g. shale and clays from coal; clayey
fines from iron ore or bauxite. As shown in Figure 19.1 , the process will often involve
crushing and grinding, leaching or separation, followed by dewatering or thickening
before discharge to the tailings disposal area as a slurry. For coal and bauxite, the ore will
not be ground and the process is one of separation of the other rock particles from the
coal, or finer clays and silts from the bauxite gravel by washing and separation.
The thickening process may be assisted by addition of flocculants and/or other chemi-
cals, e.g. polyelectrolytes such as the Magnafloc range, and inorganic salts such as gyp-
sum. The polyelectrolytes are generally high molecular weight polyacrylamides, which
have positive (cationic) or negative (anionic) charges. These adsorb to the electrically
charged particles in the tailings and form large flocs which settle more quickly in the
thickener. Inorganic salts such as gypsum (CaSO 4 ) operate by cation exchange, with high
charge density Ca cations replacing low charge density Na cations, reducing disper-
sion effects and promoting flocculation as discussed in Chapter 7.
These chemicals affect the properties of the tailings discharged to the tailings disposal
area and must be included in trial processes when testing tailings properties.
Dewatering is usually carried out in a thickener, with the dewatered tailings being
recovered from the discharge cone as shown in Figure 19.2 .
 
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