Experiences with filtered tailings
Bryan Ulrich of Knight Piésold and Co in Denver, Colorado and
Jeff Coffin of Knight Piésold and Co in Accra, Ghana, look at
filtered tailings terminology and practical application options
he use of the filtering process in tailings
management has been growing rapidly
over the past several years; however,
there are an oddly limited number of
publications regarding this topic. Davies et al
(2010) indicates that there are approximately
triple the number of filtered tailings facilities
as there are surface paste tailings facilities, but
the number of publications for surface paste
facilities far outweigh the number of
publications for filtered tailings facilities. This
disparity is chiefly due to the perceived relative
ease of the design and operation of a filtered
tailings facility compared to a surface paste
facility, but also the uniqueness, complexity
and idiosyncrasy of surface paste facilities.
In the tailings thickening continuum, filtered
tailings are typically seen to be at the opposite
end of the spectrum from conventional slurry
tailings. Filtered tailings, ie ‘dewatered’ tailings,
cannot be transported by pipeline due to their
low moisture content and therefore filtered
tailings are normally transported by conveyor or
truck, deposited, spread and compacted to form
the desired deposit. As discussed by Ulrich
(2003), there are no universal remedies for
tailings storage and every tailings facility should
be designed based on site-specific
requirements. The designer should realise the
wide variety of applications available for tailings
storage; filtered tailings technology is not a
panacea for tailings management, but rather a
viable option for the tailings designer under
certain circumstances (Davies and Rice, 2001).
T
Terminology
Filtered tailings facilities are often referred to as
‘dry stacks’ (eg Davies and Rice, 2001; USFS,
2010; Davies et al, 2010; Lupo and Hall, 2010),
while contradictory terms are also used for
certain portions or aspects of the deposit such
as ‘wet cake’ (Davies and Rice, 2001; Davies et
al, 2010; Newman et al, 2010), and zones of the
dry stack that may be ‘overly wet’ (Lupo and
Hall, 2010). Davies and Rice (2001) indicate that
the term dry stack has been adopted by
designers and regulatory authorities for filtered
tailings facilities, admitting that the facilities are
not truly dry, but that if practitioners bear in
mind that this is a somewhat misused term, it is
acceptable to continue using this terminology.
The term dry stack is rather misleading and
generally not a good engineering term since the
target moisture content coming from the filter
plant is typically desired to be somewhere
around the optimum moisture content based on
the Proctor compaction procedure (either the
standard or modified test, as determined by the
design engineer). Geotechnical engineers
associate the optimum moisture content with
moisture levels just below full saturation after
compaction, thus terming such a facility as a dry
stack is a misnomer.
The present authors would encourage
practitioners to abandon the use of the term dry
stack in favour of the more straightforward term,
‘filtered tailings’.
The basics
Rosemont Copper Company (2008) provided
information suggesting that experience shows
the most applicable projects for considering the
use of filtered tailings are those that have one or
more of the following attributes:
n The project is in an arid region where water
conservation is crucial;
n The project is in a cold region where water
handling is very difficult in winter;
n The project is in an area where seismicity
prohibits some forms of conventional tailings
impoundments;
n Topographic considerations exclude
conventional dam construction;
n Storage efficiencies of embankment dams are
not economical;
n The flow sheets show economic recovery is
enhanced by tailings filtration;
n The operating and/or closure liability of a
conventional tailings impoundment exceed
the incremental increase to develop a filtered
tailings facility.
The concept of a filtered tailings facility, its
design and operation is straightforward. The
tailings are first thickened to a given state,
generally using conventional thickeners. The
thickened tailings are then delivered to the filter
plant. There is a large variety of filters that may
be used, including vacuum types (drum, disc,
and belt) and pressure types (plate and frame,
and belt press).
As with other mine waste facilities, filtered
tailings facilities may be free-standing, side hill
or cross-valley structures, and may be
constructed in conjunction with other facilities,
such as waste rock facilities. If the filtered
tailings have a high moisture content and/or are
moderately plastic, it may be necessary to
construct a significant engineered embankment
to impound the filtered tailings.
In locations where precipitation occurs
seasonally, it is often beneficial to have separate
areas within the tailings facility to contain rainy
season tailings and dry season tailings. The dry
season tailings are often placed in the
outermost extents of the facility, where they can
be compacted to a degree where they can
provide containment for the rainy season
tailings (which may receive little to no
compaction and placed remotely from the
outermost compacted tailings which provide
their confinement). It is obviously important that
the designer of the facility considers how much
of each material is placed over time so that the
outer structural zone is constructed in time to
provide containment of the rainy season
tailings. This staged design development is
similar to ore loading plans that are developed
for heap leach facilities. Filtered tailings
facilities may be constructed with rocky covers
on their outer slopes for protection against erosion.
Many approaches may be taken to improve
the stability of a facility, for example, many
filtered tailings facilities are constructed with at
least a limited amount of drains at their base to
intercept and convey free water that may
accumulated within the deposit. It is important
that engineers designing these facilities
understand how much water may accumulate in
the tailings mass, whether a phreatic surface
may develop, and to what extent a pore
pressure regime may develop. Such pore fluids
may include water from external sources
(groundwater inflow or infiltration from
precipitation events) and/or an increase in the
degree of saturation due to self-weight
consolidation. An additional consideration for
slope stability is that some facilities must be
designed with a significant toe buttress to
improve slope stability conditions.
Material properties
Various authors have commented on the
importance of understanding material
properties of filtered tailings in advance of the
design process, and on the necessity of
obtaining and testing samples during the
operational life of a facility.
Little has been published regarding in-situ
testing of filtered tailings themselves, and in
many cases such activities would be only of
minimal benefit. However, when filtered tailings
facilities begin to approach heights exceeding
100 m, it seems warranted to conduct in-situ
testing, especially to check on the accuracies of
JUNE 2017 Supplement | International Mining P1