Environmental Engineering Reference
In-Depth Information
and grinding in laboratory processing studies often are not truly representative of those
produced by the operational process plant. This can lead to the perception that laboratory
tests do not accurately predict field behaviour. The authors' experience is that laboratory
tests on representative samples can reasonably predict field behaviour.
19.2.3.2
Particle size
Figure 19.4
gives some examples of particle size distributions from washery, oxidized and
hard rock tailings.
The particle size distribution for oxidised tailings and tailings from washeries (e.g. coal,
iron ore, bauxite) depends on the test method used and particularly on whether disper-
sants are added.
The standard soil mechanics test for determination of particle size of fine materials is to
use a hydrometer analysis with a dispersant (calgon, i.e. sodium hexametaphosphate)
added to break the particles to their constituent size. In the thickener, and as transported
to the tailings disposal area, the particles may remain flocculated.
Figure 19.5
shows par-
ticle size distributions for tailings from two coal mines (Wambo, Hunter Valley; Riverside,
Central Queensland); a bauxite mine (Weipa, North Queensland) and iron ore mines
(Newman and Hamersley, Western Australia). Tests with and without dispersant show
that the “true” behaviour is to act as a silt-sand mix, whereas with dispersant added the
tailings have a high clay size fraction.
19.2.3.3
Mineralogy
The tailings' behaviour can often be related to the mineralogy of the constituent particles.
To illustrate this,
Tables 19.2
and
19.3
list mineralogy, chemistry and soil mechanics
properties of several tailings. The following should be noted:
- The bauxite tailings from Weipa have a high proportion of amorphous clay minerals
which, while fine grained, lead to a relatively low liquid limit and plasticity index and
give favourable (for fine tailings) sedimentation characteristics;
- The coal washery tailings from Wambo have a high proportion of sodium montmoril-
lonite, which leads to a very low settled density;
- The coal washery tailings from Riverside, and gold tailings from North Kalgurli, contain
clays which have intermediate activity between Weipa and Wambo and behave accord-
ingly. The coal content of Wambo and Riverside tailings give a low specific gravity;
- The iron ore tailings from Newman and Hamersley have a significant haematite con-
tent, giving a high specific gravity. The clay minerals are not very active and, although
fine grained, the tailings tend to sediment favourably;
- The Broken Hill tailings have been produced from high strength unweathered rock and
yield a nonplastic silt-sand mix which settles rapidly to a relatively high density.
19.2.3.4
Dry density and void ratio
Table 19.4
gives some typical in-place dry densities and void ratios. The lower void
ratios/higher densities correspond to greater depths within a deposit and would be typical
of tailings which have been placed into a water pond, i.e. sub-aqueous placement. If des-
iccation of the tailings occurs (i.e. drying in the sun), dry densities would usually be 20%
to 50% higher than those shown.
The in-place density also depends on the thickness of tailings (the thicker the tailings
the higher the average density) and whether the tailings drain vertically towards the foun-
dation, with higher effective stresses and higher densities when consolidation is complete.
General trends are better shown by the void ratio because the dry density is significantly
affected by the specific gravity of the particles. Vick (1983) indicates that most high
