effect of tailings properties on paste backfill performance

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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)

22

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.

55

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,

3737

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

4545

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,

6363

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 :