PASTE SUPPLEMENT 2019
Paste thickeners in backfill
Jerold Johnson at WesTech Engineering discusses the ins
and outs of thickener strategies and equipment in mine
backfill projects
he use of slurry tailings ponds is
associated with the higher risk of failures.
Mine backfill is gaining more focus in the
industry. It can be a means to dispose of sludge
and/or tailings which may contain hazardous
materials and to reduce surface environmental
impacts by storing tailings underground.
Backfill operations place the material used to fill
the stopes and to establish and retain safe-
working conditions. Backfilling, with binder, can
provide structural strength. The benefits of
using backfill include the following:
n Reduces surface environmental impacts
n Raises extraction ratio of ore
n Reduces dilution of ore
n Aids in pillar recovery
n Improves stability in the mine
n Reduces risk of rock bursting
T
Types of backfill
There are four general types of mine backfill;
dry fill, rockfill, sandfill, and paste backfill.
Unlike the other backfill types, the paste backfill
can use the full tailings. High densities are
produced (>70% solids) depending on the
solids density and particle size distribution
(PSD). Producing stable suspension without
water bleeding or segregation requires a
component of fines (<20 micron fraction a
minimum of 15-20% by weight). The backfill
suspension commonly has a slump in the 7-10 in
range for a 12 in cone slump test. Piston-type
pumps are generally used to pump paste
backfill underground. The benefits of paste
backfill over other types of backfill include:
n Greater strengths with less cement
n Not necessary to decant water from stopes
being filled
n Usually total tailings can be used
n Surface tailings impoundment requirements
are less due to surface stacking
n Mining cycle time is less because backfill
strength is achieved early
n Stopes can be filled continuously
n Ore dilution is reduced
n Allows under-cut-and-fill mining
n Reduced acid mine generation for sulfide
tailings
n Less expensive transportation than that for
rockfill
The financial benefit of these advantages
generally makes a paste backfill plant economic
despite requiring higher capital investment.
First, the paste backfill flowsheet dewaters the
total tailings in a thickener/filter combination.
The water content targeted for the cake exceeds
the optimum water content for the backfill
recipe. This allows back dilution to be used to
accurately meet the moisture design. Second,
the filter cake, water, and binder are blended.
Finally, the paste backfill suspension is
transported underground. The selection
between equipment options is based on the
ability to produce the optimum backfill recipe,
while optimising CAPEX and OPEX.
High-rate versus paste-type
Laboratory tests are used to select either a
high-rate thickener or a paste-type thickener to
feed the filter. The paste-type thickener produces
greater underflow density than high-rate
thickeners (10 to 15 percentage points higher),
giving two major advantages for backfill plants.
First, the higher weight percentage (wt%)
solids feed will reduce the hydraulic load on the
filters. High-density thickeners produce a
homogeneous low yield stress underflow which
will fill filter chambers and filter with disc or
drum filters. The amount of water can be
reduced as much as 50%. For vacuum filters, the
percent reduction of water is a good indication
of the reduction of the filtration form time. Table
1 shows the form cycle time reduction for three
difficult-to-filter tailings. The water in the feed
was reduced over 60%, and the form time was
reduced 50-60%.
Filters are often the bottleneck as a backfill
process is optimised. By replacing the high-rate
thickener with a paste-type thickener, the
increased filter capacity could de-bottleneck the
system.
Second, most backfill operations will not be
able to place all their tailings underground.
Therefore, there will be times that the tailings
will need to be placed on the surface. The
higher-density underflow is non-Newtonian,
suitable for surface stacks, giving the following
benefits over conventional slurry ponds:
n Increased water recovery, reducing the water
sent to the pond as much as 50%
n Reduced total system cost (CAPEX and OPEX
cost for the dewatering, transportation, and
impoundment costs); though the CAPEX cost
for the dewatering and transportation is
generally greater, the impoundment costs are
significantly reduced (+30% reduction have
been reported)
n Reduced failure risk due to increased deposit
strength; the final density of the tailings is
greater (increases of 10% or more in density
reported), and with no surface water, the risk
of failure is greatly reduced
n Smaller impoundment footprint with
increased density and steeper angle of
repose
n Decreased seepage due to strong wicking
forces that draw water up to the surface
where evaporation takes place; even drying
occurs two meters below the surface and with
layering the full stack in uniform; reduced
risk of liquefaction
n Reduced or no free water on the surface,
greatly reducing failure risk compared to
ponding water, which has been identified as a
major cause of extreme damage from
n Improved and early access for reclamation
upon closure
Paste-type thickeners have been in the
minerals market since the mid-1990s and have
been supplied in most ore types, for counter
current decantation (CCD) circuits, surface
stacking systems, concentrate dewatering, and
backfill operations. The following examples
show how paste thickeners have been
incorporated into a mine backfill flowsheet.
A China greenfield copper mine backfill system
includes a 14 m diameter WesTech Deep
Bed™ paste thickener as the first stage. The
thickener underflow feeds vacuum disc filters. The
cake is then used to produce the target moisture
and binder mixer for the backfill strength.
A Mexican gold mine was retrofitted with a 15
m diameter WesTech Deep Bed™ paste
thickener. The thickener treats a split of the
tailings stream. The other portion is dewatered
in pressure filters. The underflow and the filter
cake are blended for the backfill mixture.
Paste-type thickeners provide benefits for
brownfield retrofits or greenfield plants to
optimise dewatering circuits for paste backfill;
they reduce the hydraulic load on the filters and
capable of surface stacking the tailing when the
mine is not receiving backfill. IM
Table 1 Examples of reduced vacuum filter form time with increase in feed wt% solids