TemperatureEffects onRheologicalPropertiesof FreshThickened Copper Tailings that Contain Cement

Cemented paste backfill (CPB) is an economic and environmental friendly technique applied in underground mining for supporting surrounding rock and replacing the pillar. However, little is known about the temperature effects on CPB in mines having a large temperature fluctuation. +e main purpose of this research was to investigate the effect of temperature change on the rheological properties (e.g., shear stress and apparent viscosity) of CPB with copper mine tailings. Specifically, a series of rheological tests were conducted on 6 CPB samples using a Brookfield R/S+ Rheometer under various temperatures (2°C, 10°C, 20°C, 30°C, 40°C, and 60°C). Our results showed that both shear stress and apparent viscosity of these tailing samples increased with temperature rising from 2 to 60°C. Likewise, temperature has a significant impact on the Bingham yield stress of thickened tailings. +e yield stress decreased from 122 Pa (2°C) to 112 Pa (20°C) and then increased to 152 Pa (60°C). Moreover, the pipeline transport pressure drop of CPB at various temperatures was calculated, illustrating an obvious effect on the paste pipeline transport. Compared with 20°C, the pressure drop under 2°C and 60°C increased by 11% and 22%, respectively. +e results of this study indicate that the temperature plays an essential role in determining rheological properties of CPB and its engineering application in mines particularly with naturally fluctuating temperatures.


Introduction
Cemented paste backfill (CPB) is an economic and environmental friendly technique applied in underground mining for supporting surrounding rock and replacing the pillar [1][2][3].Over the years, the CPB technology has been widely adopted in the mining industry around the world [4][5][6].CPB is usually composed of thickened or filtered tailings from concentrating mills, binding materials, and mixing water.e solid (tailings and binder) weight concentration of CPB generally ranges from 70 to 85% depending on the tailings fineness.e additional proportion of binder is commonly between 2 and 7% of the whole paste weight.But it is reported that up to 10% binder is used for the improvement of CPB strength [7].
With the development of environmental technologies, one main challenge faced by CPB technology is the paste transport which is affected by the paste concentrations, solid compositions, stowing gradients, and temperature [8][9][10].However, temperature as a key factor has been given less attention.Indeed, various temperatures of CPB are universal existence in mines since mines are located in different geographical and altitude positions.Other parameters can also influence the temperature of CPB, such as mine depth and heat due to the binder hydration [3,11].When transported through the pipe network, the backfill increases the heat during the paths towards the production area [12].All of these factors can lead to different temperatures once CPB is in the pipeline for transportation.
However, studies on the e ect of temperature on rheological properties of CPB are less frequently investigated [13][14][15].Zhang et al. [16] investigated the rheological properties of fresh cement asphalt (CA) paste and discussed the in uencing factors such as type of asphalt emulsion, temperature, and time.e result showed that high temperature led to an increase in the initial yield stress of the CA paste samples.Nehdi and Martini [17] studied the e ects of the mixing time and temperature on the yield stress of cemented pastes incorporating various super plasticizers, where the yield stress was characterized by the oscillatory shear test.e result showed that the yield stress of cemented paste increases with the increase of temperature within the range of 22∼45 °C.Li [18] analyzed the problems in CPB by a loop-pipe pumping experiment.He pointed out that temperature rise causes an increase of paste viscosity and pipeline transport resistance which is disadvantageous for the paste pumping process.e increase of temperature was mainly due to the fact of binder hydration and pipe inner surface friction.Crowder [19] mentioned that the variation in yield stress and viscosity with temperature was quite signi cant.Yet, there is no speci c and systematic study on the paste rheological properties with temperature.
In China, most areas have four distinct seasons.Jiashi copper mine (copper grade greater than 1.176%), located in the west of Xinjiang autonomous region of China, was commissioned in 2004.Due to surrounding rock crushing and wet deposition weathering, Jiashi was proposed to introduce CPB to control these problems.e experimental results indicated that the CPB strength should reach up to 4 MPa and cement/tailings ratio should be about 1 : 5.With the mining depth increasing in the future, the paste transport distance will be increased as well.In addition, the highest and lowest temperature in this mine can be reached to 45 °C and −22 °C, respectively.Considering the abovementioned facts, the main objective of this research was to experimentally study the e ect of temperature on apparent viscosity, shear stress, yield stress, and pipeline transport of paste.

Materials Used.
Total tailings, binder, and tap water of the same mix proportion were used to prepare the paste samples under di erent temperature conditions.
2.1.1.Tailings.Unclassi ed tailings samples were collected from Jiashi copper mine in Xinjiang autonomous region, China.e ore body and surrounding rock of this copper mine are subject to signi cant argillization, turning into a clay-like material when in contact with water due to the fact of sandstone rock soil type.
is characteristic is harmful to the CPB transportation, and the temperature has a more sensitive e ect on its rheology parameter.
erefore, the consideration of the temperature e ect became more important for this type tailings CPB transportation assessment.e particle size distribution (PSD) of tailing samples was measured with Malvern Laser Mastersizer 2000 (Malvern, UK), which can measure particle size in the range of 0.02 to 2000 μm with an accuracy of ±1%.As shown in Figure 1, particles which are smaller than 74 μm account for 64.32 wt.% of the tailings, and ones smaller than 20 μm account for 29.8 wt.%.
Table 1 shows the physicochemical properties of the tailings.On the basis of the result of speci c gravity [20], the true solid density of the tailings was calculated by (1).With the pycnometer method as per standard ASTM D854, the speci c gravity (G s ) of the tailings was obtained: where ρ s is true solid density of dry tailing, G s is speci c gravity of dry tailing, and ρ w is the water density at 20 °C.It should be noted that G s is a dimensionless parameter.e tailings bulk density was measured on the basis of standard GB/T 14684-2001 (6.14.2.3).A known mass of dry tailings was lled into a measuring cylinder, and then, a rubber was covered on the at surface of the cylinder.e cylinder was tapped until the volume of tailings became  constant.e bulk density of the tailings was the ratio of the tailings mass to its volume in the cylinder [21].e dry tailings porosity (n) can be estimated as follows: where n is the porosity and ρ is the bulk density.
To ensure reproducibility of the results, the true solid density and bulk density of three tailing samples were analyzed in triplicate, and an average value of these three measurements are given in the Table 1.Prior to characterization of these physical properties, the tailing samples were dried in an oven at 105 ± 5 °C for 24 hours as per the standard ASTM D2216.
e mineralogical analysis was performed on micronized tailings by X-ray di raction (XRD).
e specimen pieces were ground as powders to ensure all particles size were ner than 80 μm. e apparatus used for XRD analysis was equipment that with an incident radiation of Cu Kα and an acceleration voltage of 35 kV and 30 mA.Data collection was done at angle 2θ increasing from 10 °to 80 °. e step size of this analysis is 0.005 °, and a counting time of each step is 0.5 s. e main chemical composition of the tailing sample is SiO 2 accounting for 64% in content.

Binder.
e binding agent used for paste production was Portland cement type I (PCI) from Xinjiang Tianshan Cement Co., Ltd. in China.

Mixing Water.
Tap water was used to prepare all the paste samples.Constant water amount (i.e., 221 g) was ensured to obtain the same paste consistency.

Mixing Procedure and Mix
Proportions.Since slurry temperature was below 2 °C, it starts to freeze, thus losing rheological properties.
erefore, the temperatures investigated in this study were 2, 10, 20, 30, 40, and 60 °C, ranging from the lowest owable paste temperature to the highest possible temperature in pipeline.
A total of 6 CPB samples with a constant binder content of 12.3%, water/cement (w/c) ratio of 2.11, tailings/cement (t/c) ratio of 5, and the same tailings type were prepared.Each paste sample was prepared in a 500 mL beaker.Total weight of each sample was 850 g (524.2 g tailings, 104.8 g cement, and 221 g water).e ratio of solid mass (tailings mass plus cement mass) to total mass (solid mass plus liquid mass) was 74%.Composition of di erent paste samples is given in Table 2.It should be noted that the only di erence among these samples was temperature.e tailing materials, binder, and water were mixed and homogenized for about 5 min to produce the desired paste mixtures.e six samples were named CPB-A, CPB-B, CPB-C, CPB-D, CPB-E, and CPB-F, respectively.

Temperature Control.
Two schematic representations of the experimental setup developed to achieve the desired temperatures are illustrated in Figures 2 and 3.One

CPB-A CPB-B CPB-C
Cooling system experimental setup was a small refrigerator with the lowest temperature of −4 °C.Since the experimental ambient temperature was about 28 °C, the temperature for the samples CPB-A, CPB-B, and CPB-C needed to be controlled in the refrigerator.CPB-A, CPB-B, and CPB-C were cooled at 2, 10, and 20 °C, respectively.e other experimental setup was kept in a constant temperature water bath [22][23][24].e experimental setup consists of an electronic unit controlling the water temperature.e heat is transferred to the paste samples via water.e water bath temperature was controlled with a precision of 0.1 °C.It was connected to a temperature sensor that continually measures the temperature in the water bath and automatically sends the data to the controller (3).e temperature control range is from +5 °C to +99 °C with a uctuation of ±0.5 °C.Temperature control for CPB-D, CPB-E, and CPB-F was achieved in this water bath.

Testing Methods.
For evaluating the rheological performance of the prepared paste samples, a Brook eld R/S+ Rheometer was employed to test the shear stress and apparent viscosity at various temperatures.
e Brook eld R/S+ Rheometer is given in Figure 4. e sample holders for rheological testing were cylindrical glass beakers with a diameter of 75 mm and a height of 115 mm. e height of the thickened tailings was about 100 mm to ensure immerse of the mixing rotor of the Brook eld R/S+ Rheometer into the prepared tailing samples.e samples were tamped 18 times using a glass rod to remove air bubbles and level the thickened tailings.After xing the beaker, the test mode was then set on the controlled shear rate (CSR), and the shear rate (s −1 ) and shear time (s) were set as 1 s −1 to 120 s −1 and 120 s, respectively [25].All measurements were in duplicate.Students' t-test was done on the two batch results to ensure the test repeatability.Finally, the average of the two test results was adopted for this study.

E ect of Temperature on Shear Stress and Apparent
Viscosity.Figure 5 shows the coupled e ects of shear time and temperature on apparent viscosity.With an increase of shear time, the structure of the paste was rapidly destroyed.Apparent viscosity decreased quickly within 10 seconds and was kept relatively stable.e apparent viscosity increased as a function of temperature with the same shear time.Figure 6 illustrates the e ect of temperature on initial apparent viscosity, IAV.
e regression equation of IAV as the function of temperature can be obtained: where y is the initial apparent viscosity (Pa•s), and x is the temperature ( °C).According to (3), the IAV exponentially increases with the increasing temperature.

E ect of Temperature on Shear
Stress. Figure 7 indicates the shear stress change under di erent paste temperatures.From Figure 7, the shear rate and temperature signi cantly a ect the paste shear stress.e slurry shear stress increased with the rise of temperature at low shear rate (i.e., < 2 s −1 ).Note that it is the relatively higher paste concentration brings a sudden rise when the rheometer starts.When the shear rate is above 2 s −1 , the shear stress decreased and then Rheometer increased gradually with the growth of the shear rate at the early stage (<10 s −1 ).It should be noted that the signi cance values of students' t-test for the duplicate tests are 0.72, 0.76, 0.93, 0.89, 0.88, and 0.86 at the temperature 2 °C, 10 °C, 20 °C, 30 °C, 40 °C, and 60 °C, respectively.ese signi cance values are much larger than 0.05, which indicates that the duplicate measurements have no signi cant di erence.
At higher temperature, the change in the amplitude of CPB shear stress is smaller within the tested shear rate range.At the lower temperature of 2 to 30 °C, smaller initial shear stress (ISS) was observed.However, the shear stress increased rapidly when the shear rate was increased to 2 s −1 .By comparison, the ISS at the temperature of 40 °C and 60 °C is higher than the low temperature, and the shear stress comes down rst and then goes up with the increase of shear rate.
According to the shear stress pro le, the temperature range of the slurry transport is divided into three areas: low temperature range (2-20 °C), medium temperature range (20-40 °C), and high temperature range (40-60 °C).e change of shear stress is the greatest with the increasing shear rate at the low temperature area; at medium temperature, the area is second; and at high temperature, the area is smallest.

Bingham Model.
e Bingham and Herschel-Bulkley models are commonly applied in paste rheology.When the critical stress at which the cement paste transforms from elastic to viscous state, the Bingham model is suitable for the paste research in this study [26].e Bingham model is as follows: where τ is the shear stress (Pa), τ 0 is the yield stress (Pa), μ B is the plastic viscosity (Pa•s), and c is the shear rate (s −1 ).e parameters obtained from the tting equation according to the Bingham model and test results are given in Table 3. e correlation coe cient (R 2 ) means a measure of the correlation between a dependent variable and a set of independent variables (two or more).As the correlation coe cient is more nearly to 1, it indicates that the dependent variable and independent variables are more relevant.ese coe cients were calculated by the regression of test results using the Bingham model.e lowest correlation coe cient (R 2 ) result by the Bingham model for di erent solid concentration levels is 0.9237, indicating that the Bingham model is suitable to describe the ow curves of these CPB samples.

E ect of Temperature on Regression Yield Stress.
e aim of this section was to explain the mechanism how temperature a ects the paste yield stress.According to Table 3, the relationship between Bingham yield stress and paste temperature is plotted in Figure 8.As shown in Figure 8, the paste yield stress rises along with temperature after an initial decrease until 20 °C.ey can be generalized in the following three aspects: (1) e paste has thaw at the temperature of 2 °C, which leads to high yield stress.As the temperature increases, the ow performance of slurry becomes better.Hence, the yield stress decreases gradually until it reaches 20 °C.(2) When the temperature of paste increases, the volume expansion coe cient of slurry exponentially increases (0.63 to 5.11 × 10 −4 K −1 ) [27]. is led to the tailings being pressed, and the interaction force of the particles increasing, which causes the yield stress to rise after 20 °C.(3) e cement was added in the experimental process.e hydration reaction should be considered in this study because slight hydration reaction occurred during the sample preparation and curing.e increase of the hydration reaction rate became more pronounced at higher temperature, re ecting the acceleration of hydration reactions of cement at high temperature [28].

E ect of Temperature on Pipeline Transport of CPB.
Pipeline transport is a key technology of CPB, and the pressure drop re ects the paste owing capacity.Pressure drop of the pipeline transport refers to the paste pressure di erence per unit owing length.Hence, in order to evaluate the e ect of temperature on the CPB pipeline transport, the pressure drop at di erent temperatures needs to be calculated.e following equation is an expression of pressure drop based on the Bingham model: where i m is the pressure drop (Pa•m −1 ), U is the ow velocity (m/s), and D is the pipe diameter (m).According to previous back lling experiences, the parameters of the paste pipeline transport are assumed as follows: pipe diameter is 150 mm, and paste ow velocity is 1 m•s −1 .e parameters in Table 3, together with the pipe diameter and ow velocity, were plugged into (5).e calculated pressure drop of paste and ascensional range at di erent temperatures are given in the Table 4.
Figure 9 illustrates the e ect of temperature on pressure drop, demonstrating the transport di culty of paste at di erent temperatures.From Figure 9, it is obvious that the temperature has an impact on the paste pipeline transport.
e paste pressure drop at 20 °C decreased by 11.2% and 22.2% compared with 2 °C and 60 °C, respectively.High temperature area is more adverse than low temperature range to paste transport.Hence, the temperature of paste in the pipeline should be controlled between 10 and 30 °C when back lling because the pressure drop at this temperature range is relatively lower.For example, when the temperature is over 40 °C, the Ar was 18.6%, which was greater than that at 20 °C. is temperature should be considered as a critical temperature for the paste transport.

Conclusions
is study attempted to advance our understanding on the in uence of temperature on the rheological properties (e.g., shear stress and apparent viscosity) and pipeline transport of CPB through a series of tests.e main conclusions derived from this study are summarized as follows: (1) e temperature played an essential role in determining rheological properties of CPB.Elevated temperature led to an increase of the paste apparent viscosity for many reasons (e.g., hydration reaction of binder and tailings) that in turn caused the shear stress to increase.Further, within a certain temperature, the shear stress increased with the rising of shear rate.
, where i int is the pressure drop of the temperature (Pa/m), and i int ′ is the lowest pressure drop of all the temperatures (Pa•m −1 ).yield stress and pressure drop of the paste pipeline transport.e yield stress decreased from 122.1 Pa to 112.2 Pa while the paste temperature rose from 2 °C to 20 °C.en, the yield stress increased from 112.2 Pa to 151.6 Pa while the paste temperature rose from 20 °C to 60 °C.Furthermore, the pressure drop of the paste pipeline transport with different temperatures was calculated.e pressure drop at 20 °C decreased by 11.2% and 22.2% compared with that of 2 °C and 60 °C, respectively.
(3) e most appropriate temperature range for the paste transport was determined to be from 10 to 30 °C.
Temperatures higher than 40 °C should be considered disadvantageous to paste transport.erefore, it is suggested in mine backfill operations that the effect of temperature on the paste rheological properties should be taken into account in the CPB pipeline transport.If the paste temperature is too low or too high, temperature control measures (e.g., reducing concentration, improving pumping pressure, and cooling pipeline artificially) should be taken in order to make the paste transport easy and reliable.
Despite the results obtained in this study, further studies are necessary to provide a better understanding of the effect of temperature on the paste transport.e following topics are recommended for further work: (i) effect of temperature on microscopic structure change of fresh paste, (ii) industrial experiments to testify the effect of temperature on the pipeline transport, and (iii) a numerical model allowing the prediction of the rheological properties with the composition, concentration, pipe diameter, and flow velocity, subjected to different temperatures.

Figure 1 :
Figure 1: Particle size distribution of the tailings used in the experiments.

Figure 2 :Figure 3 :
Figure 2: Schematic diagram of the refrigerator for the samples CPB-A, CPB-B, and CPB-C.

Figure 7 :
Figure 7: E ect of shear rate on shear stress of di erent paste temperatures.

Figure 8 :
Figure 8: E ect of temperature on paste Bingham yield stress.

Figure 9 :
Figure 9: E ect of temperature on the pressure drop of paste ow.

Table 1 :
Physicochemical characterization of the tailings.

Table 2 :
Composition of di erent paste samples used in this study.
Note: SST denotes sandstone sulphide tailings, and PCI denotes Portland cement type I.

Table 3 :
e tting parameters for di erent samples based on the Bingham model.

Table 4 :
Pressure drop of the pipeline transport at di erent temperatures.