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1
CEMENTED MILL TAILING AS BACKFILL MATERIAL FOR UNDERGROUND
MINES
SANTOSH KUMAR
Assistant Mines Manager, FACOR, Orissa, India -758078,
E-mail: [email protected]
BHANWAR SINGH CHOUDHARY
Assistant Professor, Department of Mining, Indian school of Mines Dhanbad, India-
826004,
E-mail: [email protected]
The tailings produced in milling process have traditionally been disposed of in
tailing ponds creating a waste disposal and environmental problems in terms of
land degradation, air and water pollution. The problem of storage and disposal
of the mill tailings create a considerable pressure on land availability. This
disposal practice is more acute in the metal milling industry where the fine
grinding, required for value liberation, results in the production of very fine
tailings in large percentage. Mill tailing is a fine sandy silt size non cohesive,
non-plastic material and it often considered a natural pozzolanic material due
to the presence of silica and calcium oxide and therefore its engineering
behavior can be improved by addition of cement, fly ash, waste glass or lime.
This paper includes discussions on the effectiveness of different paste mixes
with varying cement contents in a paste backfilling operation. The chemical
composition, mineralogy, specific gravity, particle size distribution, slump test,
flow ability test, setting test, direct shear test, and the uniaxial compressive
tests have been presented. A number of paste samples were prepared from mill
tailings and tested at 3, 7, 14, 21, and 28 days for uniaxial compressive strength
(UCS) at different cement percent. 18 samples prepared from mill tailings were
tested at 28 days for uniaxial compressive strength (UCS) at different pulp
density. And 24 samples were tested for uniaxial compressive strength with
varying composition of binder with superplasticizer for 28 and 90 days curing
for enhancement of strength.
Key words: Mill tailing, slum test, flow ability test, Superplasticizer.
1. Introduction
Filling of the mine voids has multiple reasons such as, a simple method of tailings disposal, or
as a void filler, in a few cases it is followed as an economic method for supporting the weak
wall rocks, permit maximum ore recovery, safe and selection extraction of ore deposits
without loss of ore and encountering dilution problems and lastly, for creating a working
platform in a few stoping operations. Based on the specific purpose of backfilling, the
composition of backfill material has been varied. According to Barrett et al., 1 the purpose of
the backfill is not to transmit the rock stresses, but to reduce the relaxation of the rock mass so
the rock itself will retain a load carrying capacity and will improve load shedding to crown
pillars and abutments. This leads to less deterioration in ground conditions in the mine,
improving operations and safety.
2
Cemented backfill became popular when it is taken as a means to support the weak wall rock.
However, the high price of Portland cement has thrown open the challenge of economic
viability. The consequence is that the researchers have tried to look for binder alternatives
which have eventually resulted in the application of high density slurry and paste backfill
materials that have improved backfill mechanical strength response, reduced cement
consumption and water disposal.
The placement of backfill underground directly reduces the quantity to be disposed on
surface. This has direct operating and capital cost benefits and reductions in future
rehabilitation costs.
There are two main types of Cemented Mill Tailing as Backfill: Hydraulic Fill, and Paste
Backfill. An adequate uniaxial compressive strength for a backfill in a typical mine is 0.7–2
MPa (100–300 Psi), and common strength specification is 1 MPa after 28 days, 2
. Hydraulic
fills are slurry fills having a pulp density in the range of 55–75% solids weight for weight,
Amaratunga et at. 3 and Viles et al.
4 state that as much as 30% of the total initial fills volume
is lost by dewatering. Hydraulic fills consist of classified coarse tailings along with a binder.
The fine tailings are usually excluded from the fill because their removal improves flow
characteristics provides better fill consolidation and subsequent water drainage, the high water
content allows the slurry to be transported by gravity or pumping at relatively high placement
rates through boreholes and pipelines. Level preparation and clean-up can be very time-
consuming with this type of fill. The high binder dosage needed to create a hydraulic fill with
good strength properties can be expensive.
Paste fill, on the other hand, has high solids content, usually with a pulp density in the range
of 75–88% solids weight for weight, 3. Paste backfill is cheap as comparison to rock fill or
hydraulic fill,5. This type of filling usually contains fine material. According to Archibald et
al.,6 and Slater,
7 as the concentration of fine particle (below 20μm) increases, viscous stresses
also increases, and paste become non-Newtonian in nature. And it promote just like Bingham
flow conditions. This viscous character is a dynamic property of paste. When the paste is
stationary, the attractive forces between particles or agglomerates form a three- dimensional
structure, which extends to wall of the pipe. The shear stress required to rupture this structure
and initiate flow, is called the yield stress. Below this stress the material behaves like an
elastic solid. As shear stresses and shear rates increases, the agglomerates gradually
reorientate and disintegrate, resulting in a decrease in the viscosity of the backfill material.
This process is known as shear thinning. At very high shear stresses and shear rates, the
reorientation and disintegration process reaches equilibrium, and the viscosity becomes
constant. Paste fills have gained popularity in the past few years due to several operational and
environmental benefits due to following achievement.
The water solid ratio for the paste fill is low, producing greater strength gain per unit
volume of cement added to consolidate the fill. So strengths approaching rock fill
can be achieved, while using less cement than hydraulic fills;
Facilitates a rapid mining sequence because strength is achieved earlier compared
with hydraulic fill;
Allows the use of waste rock and slag as well as the fine fraction of tailings, thereby
reducing surface tailings impoundment requirements; so reduced the environmental
costs of the mine;
A uniform graded backfill is capable of generating greater compressive strength due
to fewer voids;
Paste fill has higher stiffness than hydraulic sand fill because of reduced porosity;
Decant water from the fill is virtually eliminated, reducing costs and problems
associated with barricade set-up/level clean-up and wear on mine dewatering pumps;
The present bore-hold delivery systems of slurry fills can be used;
3
In saturated condition within the paste backfill the ingress of oxygen, so limiting the
potential for generation of acid mine drainage.
Due to these advantages the use of paste technology has been accepted worldwide in the
modern mining industry of today.
A superplasticizer is one of type of admixtures called water reducers that are used to reduction
in water requirement of mill tailings. Water reduction results in undesirable effect on setting,
bleeding, segregation and hardening characteristics. Superplasticizer is chemically different
from normal water-reducers, and is capable of reducing water contents by about 30%.
The transportation of cemented mill tailings in the form of paste through pipelines is one of
the main stages of paste backfill operations. One of the data-sets used for pipeline design
purposes are those correlating the yield stress of fluid material changes with changes in
friction loss and the diameter of pipes ,8, 9
and its used in the design of pumping energy
requirements for the transportation of paste backfills through pipelines,8, 10
.
2. Characteristics of Cemented Mill Tailing Backfill
The addition of cement to cohesionless mill tailings backfill results in material which provides
high strength and elasticity with time, 4, 11, 12
.
The presence of sulpher in mill tailings reduces the strength of backfill after certain time due
to the production of hydrogen ions causing an sulphate attack that dissolves the calcium
hydroxide found in hydrating cement and the precipitation of gypsum, ettringite and
monoaliminate sulphate by the reaction of aluminates in the cement the mineral species have
low molar densities compared to the cement components they replace, 6, 5, 13, 14,
and thus, cause
expansion in cements.
The addition of cement to tailings also decreases the permeability of tailings with finer
materials experiencing a greater percentage decrease, 13
. The effect of cementing reactions is
to reduce the porosity of the fill and block drainage paths.
Pulp density is vital role play in cemented mill tailings backfill for strength and Flowability
purpose. For strength purpose a high pulp density is ideal, 3, 14
.
Researchers 15, 16
shows that the solubilities of silica and alumina are greatly decreased at
reduce the pH levels, which can lead to a decrease in pozzolanic reactions as well as
decreased cation exchange capacity. They suggested that the high pH causes silica from the
clay minerals to dissolve and, in combination with Ca++, to form calcium silicate. This
reaction will continue as long as Ca (OH)2 exists in the mill tailing and there is available
silica. A very small amount of Ca (OH)2 was required to raise the pH to the target value.
Better strength and fluidity is achieved by the addition of superplasticizer in cemented mill
tailings, 17
.
3. Laboratory Testing of Cemented Mill Tailings
A number of laboratory tests were carried out to study the effect of material composition on
the strength of cemented mill tailings backfills, including tests for specific gravity, particle
size distribution, porosity, pH, Atterberg limits, permeability, uniaxial compressive strength,
rheological test.
The main objectives of developing the backfill laboratory testing were , first to identify a cost
– effective backfill mixture which will fulfill the desired strength and deformation behavior of
cemented mill tailing backfill as a function of binder content and cure time in uniaxial, So that
the mix characteristics will be adjusted in such a way that when underground opening is filled
with this mixture, the filled structure will safely withstand strata loading, and will limit
underground and surface movements; and second, to develop an understanding of the
performance of cemented paste backfill when exposed to superplasticizer.
4
(i) Specimens
The materials used in this paper are one of the Indian mines mill tailings, Portland cement, tap
water, and superplasticizer.
The basic properties of tailings are summarized in table-1. Figure-1 shows the particle size
distribution of tailings, determined by sieve analysis.
Table1: The basic properties of mill tailings
Parameters Value
Specific gravity 2.67
PH in water 7.89
Liquid limit (%) ----
Plastic limit (%) ----
Particle size distribution
Silt (%) 12.22
Fine sand (%) 86.82
Medium sand (%) 0.96
D10 71
D30 125
D50 140
D60 150
Cu 2.11
Cc 1.47
Permeability (cm/sec) 4х10-3
Fig. 1: Grain size distribution of tailings
On the basis of basic physical properties, the tailings were nonplastic. A qualitative
assessment of tailings mineralogy using X-Ray diffraction (XRD) indicated the mine tailings
consists mainly of quartz (SiO2), followed by Albite (NaAlSi3O8), Calcium Peroxide (CaO2),
Cordierite(Mg2Al4Si5O18), Potassium sulfate oxide(K2S2O5), sulfur (S7)and Sodium
Manganese Silicate (Na2Mn6Si7O21). Chemical composition was determined by scanning
electron microscope method given in Table 2.
0.1
1
10
100
0.01 0.1 1 10
Grain Size D(mm)
% P
assin
g
5
Table 2: Chemical Composition of mill tailing (determined by SEM method)
Chemical component % by weight Element % by weight
Na2O 5.88 Na 6.72
MgO 6.23 Mg 6.20
Al2O3 9.83 Al 8.97
SiO2 42.31 Si 35.55
SO3 4.89 S 3.79
K2O 1.1 K 1.67
CaO 9.08 Ca 11.76
TiO2 1.46 Ti 1.58
MnO2 .34 Mn .47
Fe2O3 17.43 Fe 21.29
NiO .1 Ni .13
ZnO 1.35 Zn 1.89
(ii) Unconfined Compression Test
The purpose of the uniaxial compression tests was to obtain unconfined compressive strengths
(UCS) and moduli as a function of binder content and cure time. The different percentage of
cement was sampled for each type of test: 3%, 6%, 10% and 20% by dry weight (Cement:
mill tailing), in all, 60 samples were cured on laboratory at pulp density 80 % for 3, 7, 14, 21
and 28 days at temperature 300C. Other 24 samples were cured on laboratory for 28 days for
different pulp density at 200C. Again 24 samples were cured on laboratory for 28 and 90 days
for different composition of superplasticizer at 200C at pulp density 77%.
The samples of 54 110 mm diameter by length were cast in wooden cylindrical molds
(Fig.2). After allowing them to set for 48 hours, al1 of the sample were removed from the
wooden molds and were waxed at both ends to prevent moisture loss due to evaporation and
possible oxidation of the samples.
Fig.2 Sample preparation in the wood mold
Immediately before the testing, both ends of the samples initially were done parallel by polish.
Samples length, diameter and weight of samples were measured. The sample was placed in
the testing frame its stroke control rate was 0.315 mm/min and brought into contact with the
load cell by adjusting the hydraulic ram. When the sample was failed load and deformation
was noted. UCS was calculated with the Secant value of Young’s modulus at 50% peak stress.
4. Results and Discussion
a. Effect of cement content and curing time on UCS and Young’s Modulus
The unconfined Uniaxial Compressive Strength (UCS) is calculated as the mean value of the
maximum stresses obtained during the testing of three samples of the same mill tailing and
cement mixture. The secant values of Young’s Modulus are calculated at the point
6
corresponding to 50% of the compressive strength value. Different Uniaxial Compressive
Strength (kPa) and Young Moduli (MPa) were obtained for different cement proportions and
the curing time.
Figure-3 shows the relationship between UCS and cement % and Fig.4 shows Young’s
modulus and cement % respectively for different curing period. These figures clearly show
the compressive strength and Young’s modulus of the fill increases with cement content and
curing time as expected. Compressive strength and elasticity are relatively low for 3% cement
in mill tailing, with a notable increase starting to occur for some mixes with a cement dosage
higher than 6%. All samples gradually gained strength and elastic modulus up to 28 days of
curing. These results agree with reports by Belem et al., 18
. UCS increases nonlinearly with the
cement dosage for the all cement mill tailing composition. Modulus values also follow the
same trend, increasing nonlinearly with binder dosage for all cement mill tailing composition.
The rate of increase in UCS and modulus values is higher in the initial 21 days compared to
its increment after 21 days.
High strength values were obtained in the samples containing high amount of cement. It can
also be seen from figure 3 that cement has high strength gain in the corresponding sample at
curing 28 days. Therefore, it can be concluded that longer curing period also plays an
important role for increasing the strength and moduli of paste backfill.
Fig.3: Effect of Cement and Curing time on Uniaxial Compressive Strength
0
100
200
300
400
500
600
700
800
900
3% 6% 10% 20%
Cement %
Yo
un
g's
Mo
du
lus
(MP
a)
for 3 days
for 7 days
for 14 days
for 21 days
for 28 days
0.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
3% 6% 10% 20%
Cement %
UC
S (
MP
a)
for 3 days
for 7 days
for 14 days
for 21 days
for 28 days
7
Fig.4: Effect of Cement and curing time on Young’s Modulus
b. Effect of Pulp Density on UCS
Figure-5 shows the relationship between UCS and different pulp densities for 28 days curing
period at 6% cement dry weight composition at 200C. Figure-6 shows the relationship
between moduli and pulp density.
When pulp density 83.3% is used then result of UCS of this sample after 28 days curing in
laboratory was 566 kPa, which is 135% more compared with UCS of sample with pulp density
66.7%. Similarly, Young’s Modulus of pulp density 83.3% sample is 86% more as compared to
that of the sample with 66% pulp density. It can therefore be inferred that the compressive
strength and Young’s moduli of the backfill samples are related to the pulp density. It is also
noticed from fig.5 that there is no more difference in strength values for the samples with pulp
densities between 83.3% and 80 %. This may be due to the required amount of water to react
with cement and develop bonds between tailing materials. The strength of the backfill decreases
as the pulp density decreases mainly because of the subsequent increase in overall porosity
caused by the water-filled voids. On drying these samples air voids are created which are likely
to decrease the strength of samples. On the contrary, the higher the pulp density ratio the
stronger due to greater cement particle interlocking with mill tailing and less air voids creation.
Fig.5: Effect of Pulp density on Uniaxial Compressive Strength
Fig.6: Effect of Pulp density on Young’s Modulus
0
100
200
300
400
500
600
83.3 80 77 74 71.4 66.7
Pulp density (%)
UC
S (
kP
a)
157.1
127.9 128.5121
86.9 84.3
0
20
40
60
80
100
120
140
160
180
83.3 80 77 74 71.4 66.7
Pulp density (%)
Yo
un
g's
Mo
du
lus (
MP
a)
8
c. Effect of Porosity
The effect of cement addition on porosity is given in table 3 and effect of porosity on uniaxial
compressive strength is given in figure-7.
Porosity has decreased with addition of cement in mill tailing due to fineness of cement. So
when we mix cement in mill tailing then void ratio of mill tailing decreased. So higher uniaxial
compressive strength has found in lower porosity due to greater particle interlocking and the
presence of more cement is available per unit volume of backfill.
Table 3 Effect of Cement addition on Porosity and UCS
Cement content in
Mill tailing (%)
Porosity (n %) UCS(kPa)
3 60 173.44
6 59.18367 635.25
10 58.69565 1702.8
20 55.5556 5159.3
Fig.7: Effect of Porosity on UCS for curing 3 days
d. Effect of Super Plasticizer
The results of all the UCS tests due to variation of superplasticizer are summarized in table 4
and Fig.8 and 9 for the different percentages of composition after 28 and 90 days of curing.
Fig.8 and 9 show the variation in the UCS and Young’s Moduli with the variation of
composition of paste backfill with superplasticizer for 28 and 90 days curing. Graph 7 shows
the maximum compressive strength 654.26 kPa (just double) of the composition MT: C: SP
containing 94:6:.2 ratios as compression to compressive strength 313.64 kPa of composition
MT: C: SP containing 94:6:0 ratios (control binder) respectively for 28 days curing.
Compressive strength of another binder in which MT: C: SP containing 96:4:.2 ratios are also
74% more strength as compression to that control binder. But effect of superplasticizer is not
good in binder which contains MT: C: SP containing 97:3:.3 ratios. Compressive strength
130.16 kPa of this binder is less than half value of compressive strength of control binder.
Graph 8 shows Young’s modulus 214.19 MPa of binder which contains MT: C: SP containing
94:6:.2 ratios are also 70 % more than that of the Young’s modulus of control binder. This type
of increment in compressive strength and stiffness has happened due to renders a lower porosity
hardened material and increased the rate of cement hydration in well dispersed cement so that
between cement –mill tailing better particle packing and denser structure upon hardening in
pastes contains admixture superplasticizer.
173.44635.25
1702.8
5159.3
0
1000
2000
3000
4000
5000
6000
55 56 57 58 59 60 61
Porosity (%)
UC
S (
kP
a)
9
Fig.8 and 9 clearly show the variation of curing time on its strength and moduli. Increment on
strength due to curing varies from 50- 100% for different composition. This has happened may
be due to long term hydration between cement and mill tailing.
The cement paste backfill mixture MT: C: SP containing 94:6:.2 developed the highest
unconfined compressive strength over a 90 days curing period and showed the maximum
stiffness development as compared to with other those of paste backfill specimens without
admixture.
But the cement paste backfill mixture MT: C: SP containing 96:4:.2 also developed the
required unconfined compressive strength over a 90 days curing period and showed the
maximum stiffness development as compared to with other those of paste backfill specimens
without admixture. So for economical purpose this composition is also best.
Table 4: Effect of SP on Compressive strength and Young’s Modulus for 28 and
90 days curing
28 days curing time
Composition UCS(kPa) Young's Modulus(MPa)
94:6:.2 654.26 214.2
96:4:.2 545.91 165.36
94:6:0 313.64 124.84
97:3:.3 130.16 150.90
90 days curing time
94:6:.2 938.9 234.0
96:4:.2 892.1 198.6
94:6:0 586.1 170.2
97:3:.3 331.87 97.3
Fig. 8: Effect of Super plasticizer on UCS for different curing days
654.3
545.9
313.6
130.2
938.9892.1
586.1
331.9
0
100
200
300
400
500
600
700
800
900
1000
94:6:.2 96:4:.2 94:6:.0 97:3:.3
Composition (MT:C:SP)
UC
S (
kP
a)
for 28 days
for 90 days
10
Fig. 9: Effect of super plasticizer on Young’s Modulus for different curing days
(i). Rheological test
Experimental Procedure
Cylindrical mould was used for determination of slump value due to many advantages over
the cone slump test, 19
. There is no required standard for the cylinder test. Cylinder was made
by PVS with the length 115 mm and diameter 102 mm. The both side of the cylinder was
opened so that slumped material is 100% consistent during lifting. And filling with sample
time used one strong smooth steal plate on top of cylinder. The cylinder was filled with
sample, and the cylinder lifted slowly and evenly. The change in height between the cylinder
and deformed material was measured (fig.10). The midpoint of the slumped material was
taken as the representative height. Heights were measured with a scale. Density and
concentration were measured at the time of testing. Average value was finding of three tests
for each. Cylindrical slump test (a) with superplasticizer (b) without superplasticizer
Fig. 10: Slump test of backfill with superplasticizer
Results obtained from the test are:
Yield stress of backfills without superplasticizer = 493 Pa
Slump height = 25.6 mm
Yield stress of backfills with superplasticizer =338 Pa
Slump height = 41.51 mm
The results of the slump tests performed with 6% (dry weight %) cements with 0.2%
superplasticizer and without superplasticizer (Fig.11). It has seen that there is very much
214.2
165.36
124.84
150.9
234
198.6
170.2
97.3
0
50
100
150
200
250
94:6:.2 96:4:.2 94:6:.0 97:3:.3
Composition (MT:C:SP)
Yo
un
g's
Mo
du
lus (
MP
a)
for 28 days
for 90 days
11
difference (155 Pa) on the yield value and also much difference between slump height
(15.91mm) between both while water 23% in solution present in both condition. So here
fluidity increased with superplasticizer.
Fig. 11: Effect of superplasticizer on Yield Stress
Setting Time Procedure
Setting time was determined by Vicat needle test (penetration test). Specimen for the Vicat
needle test were cylindrical cup 70 mm in diameter and 40 mm high. After being filling with
paste (pulp density 80%). The standard test method, the Vicat needle test, was used to
determine the initial and final setting times hydraulic cement. The initial setting is the
determined for the needle to reach a penetration depth 5mm in standard Vicat apparatus. The
final setting takes place when the needle does not visibly penetrate into the paste i.e., the
specimen has a solid structure.
Table 5: Vicat needle test result
Binder
content
Additive
Water(%) of total
weight
Setting time(min)
Initial Final
94:6
(MT:C) none 23 65 125
94:6
(MT:C)
SP.2% of dry
weight 23% of total weight 45 120
Table 5 shows the initial and final times of setting for paste in which one is without
superplasticizer and other is with superplasticizer. The data indicates that initial time setting
of paste with superplasticizer is less as compared to without superplasticizer paste. And final
time setting of both pastes is about same while superplasticizer paste is wet as comparison to
without paste. So for same slump value, time setting will be reduced in superplasticizer paste
than without superplasticizer paste.
(ii). Flow ability Test
For flow behavior test, one galvanized iron sheet 120 cm length was used at inclination 200
degree as shown in figures 12-15. Figures 12-15 show the flow characteristics of backfill
material. The result of the test performed with 4 different compositions. In first experiment for
flow test, 0.2 % superplasticizer was used in MT: C contains 94:6 ratios binder. In 2nd
experiment no superplasticizer was used for same combination. In 3rd
experiment, 0.2%
493
338
0
100
200
300
400
500
600
without SP with SP(.2%)
Yie
ld S
tress (
Pa)
12
superplasticizer was used in MT:C contains 96:4 ratios respectively. In 4th
experiment, 0.3%
superplasticizer was used in MT: C contains 97:3 ratios respectively. It has seen that there is
significant difference on the fluidity of different composition. At 0.2% superplasticizer in Mill
Tailing –Cement (94:6 ratios) binder, the fluidity increased compared to the other
composition. And in other composition some part of paste has flowed and some part has not
flowed. Higher the fluidity in first case was observed due to electrostatic repulsion between
particles, causing dispersion. In 3rd
and 4th
experiment an insufficient amount of cement may
be available to react with main hydration (i.e. calcium silicate hydrates or C-S-H) to produce
effective dispersion at later stage. Fine particle is also important role played with
superplasticizer for fluidity purpose.
Fig.12: Flow characteristics of mill tailing when 0.2 % superplasticizer mixed with
MT: Cement (94: 6)
Fig.13: Flow characteristics of mill tailing when no superplasticizer used in MT: Cement (94:
6)
13
Fig.14: Flow characteristics of mill tailing when 0.2 % superplasticizer mixed with
MT: Cement (96: 4)
Fig.15: Flow characteristics of mill tailing when 0.3 % Superplasticizer mixed with
MT: Cement (97: 3)
The rheological behaviour of two paste backfills characterized in this study was yield-pseudo
plastic. The superplasticizer controls not only the rheological behaviour of paste backfill, but
also their yield stress. Yield stress measurements in slump test method show reliable results
for superplasticizer as comparison to non superplasticizer paste backfills. So Based on the
results of this research, we can conclude that the use of superplasticizer in backfill material
will be economical because this will not increase the strength but also aids in the rheological
characteristics of paste backfill material.
5. Conclusions
An effort is made in the present study to generate value on physical, chemical, mechanical and
rheological characteristics of mill tailing samples. Mechanical and rheological characteristics
of mill tailings were found with different percentage of cement, water and superplasticizer.
Ninety different cemented backfill materials were tested to determine the influence of their
composition on the unconfined compressive strength in laboratory.
The studies lead to the following conclusions which may be of significance for the further
research and for the practice of mine backfill using cemented paste backfill method.
14
Predominant oxides found in the mill tailing samples are SiO2, Fe2O3, Al2O3, CaO, Na2O,
MgO, SO3, and TiO2. The sums of these oxides were above 90%. The presence of CaO at
9 % in the mill tailing samples indicates the good pozzolanic characteristic of mill tailings.
Generally mill tailings contain sulfate concentration. pH was found to be at 7.89 in this
sample and by SEM very less sulfur concentration has been observed and XRD reports
was not indicate of ferrous sulphide. So it is also good for strength purpose. Because
sulfate concentration decreased strength after 90 days. Due to this cause dilution problem
will be decreased.
Particle size distribution show that the percent of fine sand is 86.82%, for paste backfill
purpose and minimum15% of below size 20μm mill tailing will be required.
Coefficient of permeability of mill tailing is 4.08×10-3
cm/sec, which is very less and after
cement, addition its value again will be decreased. So it is not good for drainage in
hydraulic backfill purpose without any flocculent. This is good for paste backfill purpose.
After addition of cement and fine particle permeability will be decreased.
The material composition strongly influenced the strength of cemented backfill. Pulp
density is a critical determining factor in the strength of cemented backfill. Increase in its
value significantly increased the backfill strength.
Increased cement content increased the backfill strength.
Superplasticizer also play good impact for increment on its strength with cement,
Slump heights obtained from the slump cylinder experiments is observed to increase and
decrease yield stress with mixing of superplasticizer in binder. So its flow ability
increased with mixing of superplasticizer. Setting time has also not increased with
superplasticizer. Due to these good properties of superplasticizer, it may play a vital role
in paste backfill.
So the use of paste backfill in place of hydraulic backfills will be correct choice for backfill
operation. This shall not only enhance the performance of the backfill as a ground support
system but also likely to reduce the dilution of muck, and thus may result in the full recovery
of ore.
6. Reference
1 Barrett, J R., Coulthard, M A., and Dight, P. M., 1978. “Determination of Fill Stability”,
Mining with Backfill, 12th Canadian Rock Mechanics Symposium, CIM Special Volume
19, Sudbury, Ontario, May, pp. 23-25.
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