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Effect of Tailings Properties Effect of Tailings Properties on Paste backfill on Paste backfill performanceperformance
M. fall, M. Benzaazoua, S. Quellet M. fall, M. Benzaazoua, S. Quellet
( University of Qucbec in Abitibi- ( University of Qucbec in Abitibi- Temiscamingus, Canada)Temiscamingus, Canada)
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Abstract :
The increasing use of paste backfill
in underground mining makes it
necessary to quantify the effect of
tailings properties ,such as physical
and chemical properties, on the
performance properties of cemented
paste backfill (CPB) .
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Hence, this paper presents the results
of an experimental investigation
carried out to evaluate the influence
of physical (tailings particle size and
density) and chemical properties
(tailings sulphide content ) of tailings
on the performance properties of the
paste backfill.
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The studied performance properties included: mechanical and economical performance ,water demand, pulp density and transportability. The gained results have shown that the tailings fineness and density influence significantly the performance properties of paste backfill.
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The sulphide content of the tailings
has also significant effect on the
performance of paste backfill. The
results of this study will contribute
to better understand the behaviors
of the paste backfill and optimize its
mixtures.
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1 Introduction 1 Introduction
The use of paste technology for
underground backfill has been
accepted as a cost-effective alternative
to rock and hydraulic backfill
worldwide in the mining industry. Its
using is extensive in Canadian
underground hard rock mines and
follows an increasing trend as well as
in many parts of the world.
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The application of paste backfill
leads to a significant reduction of
cyclical nature of mining, improves
ground conditions, ensures the
stability of the underground
excavations, speed up production
and greatly reduces environmental
costs .Additionally, paste backfill
applications , known to
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allow enhanced disposal of large
fractions of fine tailings that have
traditionally required permanent
surface disposal and management,
offer other significant environmental
and cost benefits for mines . However,
despite extensive use of this relatively
new technology, all effects of tailings
properties on paste backfill are not
fully known.
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Only a few works have shown that
the tailings particle size can
influence the strength of the
hardened paste backfill. But these
works only briefly described the
influence of tailings particle size on
CPB uniaxial compressive strength
(UCS).
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They had no information on the effect of
tailings particle size on all of the most
important quality criteria for backfill. They
also did not account for the influence of
tailings density and sulphide contents. Hence, cooperative research studies were
conducted by the above authors with
several Canadian underground hard rock
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mines currently using cemented paste backfill to investigate the effect of the physical and chemical properties of tailings on paste backfill as well as to optimize the desliming of mill tailings. This paper presents the results of the experimental part of these research studies.
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Thus, the main purpose of this
experimental study was to evaluate the
influence of tailings fineness and
density on the performance properties
of the paste backfill, and to study the
effect of the amount of sulphide
minerals contained in the tailing on
the quality of the paste backfill.
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The studied performance properties
included the strength, cost and
microstructure of the hardened
paste backfill and the water demand
for the fresh paste backfill. The
reactivity of the sulphide minerals
present in the tailings was also
evaluated.
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2 Results and Discussions2 Results and Discussions
2.1 Effect of physical properties of tailings on paste backfill performance
2.1.1 Effect of tailings fineness on the microstructure of paste backfill
The main results of the effect of tailings fineness on the microstructure of paste backfill are presented in Figure 2 to 3.
From this figure, it is clear that the proportion of fine tailings (<20um), i.e.,
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the fineness of the tailings materials used
in the sample mixture strongly influence
both the overall porosity of the cemented
backfill material( Figure 2)and the pore
size distribution within it (Figure2 and 3). The finer the tailings materials used, i.e.,
the greater the proportion of fine tailings
particles , the greater the overall porosity
of the backfill becomes .
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Figure2 shows that the total porosity of
all of the paste backfill samples
decreases as the proportion of fine
particles (<20um) in the tailings
material decrease. However, the extent
of porosity decrease is variable ,i.e.,
depends on fines content . The decrease in porosity with the
decrease in the fines content is greater
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for paste backfill made of fine
(fines<60%) or medium (fines:
60%~35%) tailings than those made
from coarse tailings . Figure 2 shows also the microstructure
(total porosity and void ratio) of the
paste material is strongly influenced by
the drainage ability of the fresh backfill .
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The drained paste backfill samples show
both less porosity and smaller void
ratios, As shown in Benzaazoua et al.
(2003), paste backfill mixes made from
coarse tailings loose more water (by
drainage) than those made from fine
tailings material . This water loss leads
to the settling of the paste backfill
(increasing
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of the packing density) and the
consequent reduction of total
porosity and void ratio of the backfill
material. These observations are in
substantial agreement with earlier
experimental investigations about
CPB drainage ability performed by
Belem et al.(2002).
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Figure3 also shows that the paste backfill specimens are characterized by two distinct pore size distribution, notably pore diameters between 0.05~1um and pore diameters between 1~10um . The contribution of pore diameters smaller than 0.05um or greater than 10um to overall porosity is low.
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The distribution of the two main pore diameters is significantly influenced by the proportions of fine tailings particles .
While the pore diameters between 0.05 and 1 um are relatively well represented in the samples made from fine and medium tailings , their volume is small in backfill samples made from coarse tailings (Figure3).
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Indeed, it can be observed in Figure 3,
that decreasing of proportion of fines in
the tailings is associated with a
decreasing of the volumes of pores with
diameter smaller than 1 um (Figure 3). The volume of macropores >1um is
greatest in paste backfill samples made
from tailings containing only 25% of
fines
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particles (coarse tailings). This may have
effect on paste backfill strength gain. 2.1.2 Effect of tailings fineness on paste
backfill strength development Figure4 show the compressive strength
development of the paste backfill related
to the size of the tailings fineness. It
points out that the proportion of fines
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(<20um) in the tailings materials has a
strong influence on the strength gain of
the CPB .It can also be noted that
coarse and medium tailings are more
favorable for paste backfill strength
gain. A CPB made of fine tailings
generates lower strength .Figure4 also
indicates that, for the undrained paste
backfill samples ,
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the UCS increases as the grain fineness decrease until approximately 35~55% of the fines content was reached, at which point it remains constant (Figure4a) or begins to slowly decrease (Figure4b) with decreasing grain fineness.
These observations can be attributed to the influence of the tailings fineness on the overall porosity of the paste backfill
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as well as to the effect on the pore
size distribution within it (Figure2
and 3),and to the influence of the
tailings particle size on the specific
surface of the tailings material. Indeed , from a fines content of
60~90 wt.% to coarse tailings with a
fines content of 25 and 35 wt.%,
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there is a grading improvement of the
tailings particle size
distribution( Table 2 and Fifgure1). This leads to a decrease of the void
spaces between the tailings particles
and consequently, to lower porosities
or void spaces within paste backfill
as shown in Figure2.
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This decreasing porosities or void
spaces thus causes an increase in paste
backfill strength. However, decreasing the porosity of the
backfill with a decrease in the
proportions of the fines tailings
particles (<20um) is not the only
parameter responsible for the variation
in backfill strength.
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Indeed, it can not explain the slight
decrease of backfill strength for a fines
proportion of 30%~25%. Analysis of Figure 3 shows that the pore
size distribution within the paste backfill,
particularly the proportions macropores
with diameter between 1~10um, seems
to play a significant role in the strength
gain
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of paste backfill. As observed in figure
3 and 4, at 25% fines content, the
proportions of macropores 1~10um
within the paste backfill increases
drastically. This higher proportion of
macropores (1~10um) may have
caused the small decrease of the
strength for paste backfill made from
coarse tailings
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(25%fines) compared to backfill
specimens made from tailings containing
50% to 40% fines particles. Thus, it can be
concluded that not only does the overall
porosity influence the strength of the
paste backfill, but the pore size
distribution, which is largely governed by
the proportions of fine particles present
in
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the tailings material, also plays a decisive role in the strength development of the cemented backfill.
In addition to the effect of overall porosity and pore size distribution on the backfill strength gain, the finer particles increase the specific surface of the tailings materials and thus increase the surface area that must be cemented,
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since the cement coats the surface of
the tailings particles. This also
contributes to strength decrease of the
paste backfill (Figure4) with increased
tailings fineness. From Figure 4a, it can
be observed that the drained paste
backfill samples show higher strength
than the undrained samples .
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This maybe attributed to the fact that
drainage of the excess water in the
fresh paste backfill leads to the settling
of and higher packing density of the
backfill . This causes a reduction in the total
porosity and void ratio of the backfill
material and consequently, to higher
strength.
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Additionally, the drainage of the excess
water affects positively the cement
hydration (Benzaazoua et al. 2003b) From Figure4, it can also be clearly seen
that the fineness of the tailings strongly
influences the rate of backfill strength
gain at early stage (up to 28 days
curing). Paste backfill mixes made from
coarse
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tailings (low proportion of fines) gain
strength faster than those made from
fine tailings (high proportion of fines).
This is caused by the fact ,that for a
given W/C, the volume of void spaces
between the tailings particles to be
filled by the cement hydration product is
smaller in the paste backfill specimens
made from
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coarser tailings (lower porosity) than
those made from finer tailings material. 2.1.3 Effect of tailings fineness on
water requirement of the fresh paste
backfill Figure5 shows the effect of the tailings
fineness on the water/cement ratio and
pulp density of cemented paste backfill.
It can be noted that W/C of the paste
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backfill increase with the grain fineness of the CPB . For a given slump, the pulp density of the CPB decreases as the fineness of the tailings increases .
This means fine tailings require more water for a given consistency than medium and coarse tailings.
The influence of tailings particle size on water demand comes from the fact
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that the cement paste in its function as an adhesive, coats the surface of all tailings particles.
Finer particles mean that there is more surface area to be wetted, which in turn yields both higher moisture levels and lower densities for a given consistency. Additionally, since the overall porosity and
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void ratio of the paste backfill decreases
from fine to coarse tailings , for a given total
weight of tailings and cement ,coarse tailings
require less water than medium or fine
tailings to reach the same consistency or pulp
density . The comparison of Figure4 and 5 indicates
that the optimal W/C ratio to produce paste
backfill with high strength is dependent on
the fineness of the used tailings materials.
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2.1.4 Effect of tailings density on strength
and binder consumption of the paste backfill Figure 6 shows that, for a given curing time,
there is a relationship between the strength
and the density of the tailings materials
used. It can be observed that increasing of
the tailings density (from Gs=3.2) gives the
paste backfill a higher strength for the same
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binder proportion (4.5% in weight) .This
increasing in U with the tailings density
is due to higher binder consumption in
volume, as shown in Figure 7. The latter
puts into relief that , the higher the
density of the tailings, the higher the
binder consumption (in volume), i.e. the
more expensive the backfill becomes.
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These results clearly demonstrate that
the evaluation of the binder content of
the CPB by weight percent, as commonly
used in the mining industry does not
show the real binder consumption of the
paste backfill ,in cases where the density
of the mill tailings strongly vary. They
also demonstrate the economic
significance of
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using tailings with low sulphide
content (tailings sulphide content
can be reduced by desulphurization)
for paste backfill design, since high
sulphide contents lead to higher
tailings density and consequently, to
more expensive paste backfill.
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2.2 Effect of chemical properties 2.2 Effect of chemical properties of tailings on paste backfill of tailings on paste backfill performanceperformance
The results of the effect of tailings
sulphur content on paste backfill
performance properties have shown that
the sulphur has double effect (physical
and chemical effect) on paste backfill
properties . The first effect is physical. Indeed,
increasing of the sulphur content in the
tailings give the latter higher density.
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This leads to paste backfill with higher
strength ( Figure 8) due to the higher
binder consumption in volume. The second effect is chemical. As shown
in Figure7, high sulphur content ( 39%
sulphur) in the paste backfill leads after
120 days to strength loss of the paste
backfill. This is caused by sulphate
attack.
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This raised the following question: Is
the sulphides oxidation within the
cemented paste backfill possible? The
results of oxygen consumption tests
(Figure9 and 10)bring some elements
of response to this question. Indeed,
Figure9 ,which represents the oxygen
consumption (ie reactivity of the
sulphide
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minerals in CPB) of cemented paste
backfill , shows that the amount of
consumed oxygen depends on the
sulphur content, and the reactivity of
the paste backfill specimens (except
CPB made from tailings with
39%sulphur) does not significantly
increase after 28 days of curing time.
The reactivity of the sulphide
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minerals in the cemented paste backfill
are low compared to this in the non
cemented tailings( Figure10). This low
reactivity can be attributed to the fact,
that the cemented paste backfill is
water saturated. This high water
saturation limits the diffusion of oxygen
through the cement matrix and
consequently its
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availability for oxidation reactions. Figure 10 puts in relief the relationship
between the degree of water saturation
and the sulphide reactivity in non
cemented tailings. It can be observed,
that at high water saturation (Sr.>85%),
the reactivity of the sulphides mineral
in the non cemented tailings become as
low as in cemented paste backfill.
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3 Conclusions3 Conclusions
The objective of this study was to study
the effects of the physical properties
(particle size and density) and chemical
properties (sulfur content) of tailings on
the performance properties of cemented
paste backfill. The presented results have shown that
the tailings particle size and density has
a
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considerable effect on the properties
( strength, cost, water demand,
microstructure) of the paste backfill. It
was demonstrated that the tailings
particle size, particularly the proportions
of fine tailings particles (<20um)
significantly affect the porosity of the
paste backfill and the pore size
distribution within it,
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its water drainage ability and
consequently, its strength development
and the water requirement for a given
consistency. It was also shown that not
only does the overall porosity influence the
strength of the paste backfill, but the pore
size distribution plays a decisive role in the
strength development of the cemented
backfill.
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The paste backfill water demand
increases with the fineness of the
tailings material used. Increasing the
tailings density is associated with
volumetrically higher binder
consumption, i.e. more expensive
backfill. In general, higher binder
consumption gives the paste backfill
higher strength.
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It has been demonstrated that higher
sulfur contents in the tailings increases
tailings density and consequently give
the cemented paste backfill higher
strength. This increased strength is caused by a
higher binder consumption. However,
high sulfur content can lead to strength
loss of cemented paste backfill through
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sulphate attack . This sulphate comes
principally from the oxidation of the
sulphide minerals in the non cemented
tailings. This work brings new light on the effect
of tailings on CPB properties that can
contribute to better optimization of
paste backfill mixtures and to a better
underst-
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anding of the behavior of cemented
paste backfill. Additionally, these experimental
results agree well with those of
modeling of the effect of paste backfill
components on its properties (Fall and
Benzaazoua 2003a,Fall and Benzaazoua
2003b,Fall and Benzaazoua 2004a).
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Acknowledgements References :