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The effects of cement on some physical and chemical behavior for surface paste disposal method
Journal of Environmental Management 231 (2019) 33–40
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Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman
Research article
The effects of cement on some physical and chemical behavior for surface paste disposal method
T
Serkan Tuylu∗, Atac Bascetin, Deniz Adiguzel Istanbul University, Engineering Faculty, Mining Engineering Department, 34320, Istanbul, Turkey
A R T I C LE I N FO
A B S T R A C T
Keywords: Tailings AMD Heavy metal mobilization Surface paste disposal method
Environmental impacts resulting from conventional tailings disposal such as tailings dam accidents are a common problem for base metal mines around the world. In this context, laboratory-scale studies have been carried out on the Surface Paste Disposal (SPD) method, which is one of the alternative surface storage methods. In this study, three different SPD designs were tested; and volumetric water content, oxygen consumption, and matric suction sensors in first, fifth and 10th paste layers plus pH- electric conductivity (EC) values were all measured. Specifically, it was determined that the amount of oxygen in the environment required for the oxidation of sulfur minerals is reduced in the cemented layers in Design 3. In addition, the cement additive keeps the pH values of the seepage in an alkaline environment (over 7) so that it minimizes the risks of Acid Mine Drainage (AMD) and heavy metals mobilization at low pH values. Also, the EC values started a downward trend and ion dissolution decreased in designs with cemented layers. As a result, it was understood from the sensor measurements that the cemented layers act like a barrier.
1. Introduction Failure in tailings dams, which are the most widely used conventional disposal method in the world, can result in an uncontrolled spill, a dangerous flow-slide, and/or the release of poisonous chemicals, leading to a major environmental disaster. Surface paste disposal (SPD), which is a storage method that has become increasingly important in recent years, can be considered an effective alternative technique for mine tailings management, and the topic of multiple studies. These studies have shown that SPD tailings storage can minimize the amount of free water, which is one of the biggest problems of conventional tailings dams. Additionally, SPD can allow the recycling of water; limit the risks related to the failure of dikes, and favor progressive mine sites rehabilitation (ICOLD and UNEP, 2001; Fourie, 2003; Welch, 2003; Liu et al., 2016). At the same time, cement binder can be added to the surface paste material when necessary to increase durability and stability as well as reducing the risk of heavy metals mobilization with Acid Mine Drainage (AMD). If cement or another binder is going to be used in the paste material mixture, it is desired that the amount of binder by solid weight should be ≤ 2% in terms of cost and able to store more tailings (Benzaazoua et al., 2004; Deschamps et al., 2011; Yilmaz et al., 2014; Bascetin and Tuylu, 2018; Bascetin et al., 2017, 2018). However, it is
∗
not economically feasible to add cement or other binders to the tailings when considering the amount of tailings produced. For this reason, it is first necessary to try different designs and to reveal the effects of these designs. The uncertainties surrounding this issue have not yet been fully resolved in terms of the SPD method. In this study, different designs were made by forming cemented and uncemented paste layers. The studies conducted in industrial areas throughout the world in terms of SPD have mostly taken the form of experiments, but have not yet been put into practice as an alternative method. These studies applied SPD placed in arid and extremely cold climates as Tanzania and Canada (Theriault et al., 2003; Yilmaz et al., 2014). Also, it was observed in the literature that potential environmental impacts have generally been analyzed geochemically (AMD, etc.) in terms of the column, humidity cell, and small-scale cabin tests (Verburg, 2001, 2002; McGregor and Blowes, 2002; Deschamps et al., 2007, 2008). As the experiments regarding the subject are new, an efficient model for the optimum storage design has not yet been developed. Our goal here was to find the optimum cemented design according to the measurement (volumetric water content, matric suction, oxygen consumption sensors, and pH-EC of the seepage) results in laboratory scale cabins with three different designs started simultaneously. The main aim of this study was to determine the optimal design that allows for successive layers of storage in a laboratory environment,
Corresponding author. Istanbul University, Faculty of Engineering, Department of Mining Engineering, 34320, Avcilar, Istanbul, Turkey. E-mail address: [email protected] (S. Tuylu).
https://doi.org/10.1016/j.jenvman.2018.10.007 Received 24 May 2018; Received in revised form 20 September 2018; Accepted 2 October 2018 0301-4797/ © 2018 Elsevier Ltd. All rights reserved.
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taking into account the physical and geochemical durability of mine process tailings. One of the other aims of the study was to enable the paste material to achieve physical durability so that it could remain stable under its own weight, or under static or dynamic loads, depending on the subsequent use of the storage area. In addition, the prevention of water and oxygen diffusion within the stored material after the storage is completed is another important issue. A storage method that meets these conditions will be able to minimize environmental risks and will be a suitable method for reclamation and reutilization conditions later on. 2. Materials and methods
Fig. 1. (a) General view of test cabins. (b) Test cabin designs with different configurations.
The paste material used in this study was obtained from lead-zinc process tailings from an underground mine in western Turkey. The region has a climate between temperate and cold. In winter times it is rainy, while in summer times it is hot and dry. The cut-and-fill underground production method was used in the mine studied. The mineral processing stages of the run-of-mine (ROM) ore extracted from underground include crushing, grinding, floatation, and filtering processes. An average of 400 tonnes of concentrate is obtained from an average 4800 tonnes of ROM ore supplied on a daily basis to the mineral processing plant. The remaining 4400 tonnes of solid waste is discharged to the tailings dam at a solid concentration of 15–20 wt%. The mine has four tailings dams: one closed, one fully filled, one currently filling, and one under construction. The tailings obtained from the processing plant contain risk in terms of AMD formation and heavy metals mobilization. Therefore, it is very important to understand the mineralogical components and elemental contents of tailings. To this end, the tailings were examined using ICPMS and XRD analysis. The mineralogical composition of tailings is mainly composed of Calcite (CaCO3), Quartz (SiO2), Feldspar minerals (Albite = Na(AlSi3O8)), and Pyrite (FeS2). The values of Cu, Pb, Zn, and Cd were found to be 217.2 mg/kg, 1500.1 mg/kg, 1548 mg/kg, 601.8 mg/kg, and 8.2 mg/kg respectively. The values exceeded the maximum values specified in EPA by 110 mg/kg, 110 mg/kg, 270 mg/ kg, 33 mg/kg and 4 mg/kg respectively. It is expected that the metals in paste tailings will generate considerable toxicity in soil and water in terms of environment. This heavy metals content can significantly generate toxicity in soil and water from an environmental standpoint. It was understood from the Modified ABA method and ICP-MS elemental analysis of paste tailings that there is a risk of heavy metal mobilization. However, the XRD and chemical analysis also revealed the presence of buffer minerals. It was envisaged that this risk could be minimized using the SPD method (Bascetin and Tuylu, 2018; EPA, 2002). In this context, after of the physical and chemical characteristics of Pb-Zn tailings were determined, mixtures of between 65 and 75% solid content (SC) by weight were prepared for paste material. For this mixture to be pumped for long distances, slump tests were conducted in order to achieve a slump value of 250 mm (acceptable for SPD), and the optimal SC value was determined accordingly. In order to fully understand the behavior of the paste tailings material during deposition, the test cabins in Fig. 1a were used to run simulations of the paste tailings material in surface storage with three different designs. While preparing the paste material, CEM I 42.5 Portland cement without additives (PC) produced in accordance with the EN 197-1 (2011) standard was used as a binder in certain configurations in order to investigate the effect of the binding material on paste technology. The solid-water ratio of the paste layers was determined according to a slump value of 250 mm to obtain suitable paste material for paste technology. Additionally, cement binder in the amount of ∼2% by solid material weight was added to some cemented layers depending on designed paste disposal. In this study, a cabin configuration with dimensions of 200 cm (length) * 70 cm (width) * 130 cm (height) were used to store the paste tailings. The cabin was also equipped with Decagon® Em50 five channel
Fig. 1. (continued)
voltage data logger for the VWC (Decagon® 5TM), matric suction (Decagon® MPS-1 and 2), and oxygen consumption (Apogee® SO-110 oxygen sensor) data collection. Surface behavior of paste material was simulated with all layers being uncemented in Design 1 (reference cabin), followed by some layers being cemented in Design 2, then Design 3 which contained cement binder (≤%2 solid content weight) storage cabins. The different sensors, such as VWC, matric suction potential and oxygen consumption, were placed on the first, fifth and 10th layers in lab-scale test cabin (Fig. 1b). Additionally, seepage water collected from the bottom of the cabins was subjected to pH and EC analysis. Thus, the optimal storage design for Pb-Zn tailings prepared for SPD was identified. Design 1 consists of uncemented paste layers. This test cabin was a control sample for the other test cabins where different configurations were utilized. Therefore, this cabin can also be called the “reference cabin”. Design 2 consists of only the bottom layer (first layer) with cemented paste tailings material, with all the subsequent layers above it having no cement. By creating an alkali medium with the cement structure in the first layer, the aim was to keep the high pH value of the pore water and prevent acid generation and/or heavy metals mobilization. In Design 3, layers 1 and 11 were deposited with cemented paste material with all the other layers in-between being stored as uncemented paste material. Here, it was considered that increasing the stability of the topmost layer of the storage area could be beneficial for reclamation works. It was also considered that the high pH value of the topmost layer, which would come in contact with free surface waters, would reduce the risk of AMD. The paste material was placed in layers in these test cabins. The layer height of the paste material stored into the cabins was determined to be 40 mm for each layer taking into consideration the spread width of the paste and the studies conducted in the literature (Benzaazoua et al., 2004; Deschamps et al., 2011). Paste storage comprised a total of 11 layers placed within the cabin. Additional layers were placed only once the previous layer had completed its curing time and until there was no volumetric water content loss – a time period of eight days. Data collection from the installed sensors was completed over 88 days with 34
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these sensors measuring the volumetric water content, matric suction, and oxygen consumption of the layers. This approach let the previous layer achieve stability. Three different sensors were placed in layers 1, 5 and 10 of each cabin in order to determine the values for volumetric water content, oxygen consumption, and matric suction (Bascetin and Tuylu, 2018; Bascetin et al., 2017, 2018). 3. Results and discussion After the completion of preparations for the three storage cabins, data collection from the installed sensors was completed over 88 days in the laboratory. Paste material storage for these three different designs then began. During the experiment period, all data were automatically recorded every 1 min. Approximately 126,000 items of data for the first layer, 80,000 for the fifth layer, and 23,000 items of data for the 10th layer were obtained in each design. These data were then analyzed. In this context, after the placing of the layers of paste material, the sensor measurements for the three different designs were given according to the sensor values in the following order: first, fifth, and 10th layers. While the paste material was being placed in layers in the cabins, sensors were placed in the first, fifth, and 10th layers. These sensors measure the volumetric water content, matric suction, and oxygen consumption of the layers.
Fig. 3. VWC and SC values of Layer 1 in Design 2 (cement in Layer 1 only).
3.1. Volumetric water content (VWC) The volumetric water content for Layer 1 and the correspondingly calculated values in the solid content by weight (SC = Solid Content/ Total Mass of Pulp) value are given in Figs. 2–4 for Design 1, Design 2, and Design 3 respectively. While the SC of Layer 1 in Design 1 shown in Fig. 2 (this layer does not contain cement) was ∼75% during initial storing, it was observed to increase to ∼82% after 88 days considering there was an opportunity for drainage. The upper layers that were deposited after 40 days were determined to have little effect on the bottom layer of the reference (non-binder) paste storage system and remained stable in the range of ∼80–82% of VWC (Fig. 2). The volumetric water content behavior of the first layers in Design 2 and Design 3, consisting of binded (cemented) paste material, is shown in Figs. 3 and 4. It is clearly seen that the first layers of both designs depict a similar trend and reach a stable range after day 32 with each subsequent layer storing based on VWC. As seen in Figs. 3 and 4, after the first layer is stored, an increase in volumetric water content of ∼35% is observed. It was determined that Design 2 remained stable between at 83%–86% of SC, and that Design 3 remained stable between and at 81%–84% of SC after the storing of the fifth layers. These results can be expressed as the paste material being consolidated with the loads upon it and reaching hydrostatic balance after a certain period of time (Figs. 2–4). This situation was observed
Fig. 4. VWC and SC values of Layer 1 in Design 3 (cement in Layers 1 and 11).
particularly after half of the total layer storing time had passed. It is thought that one of the most important causes of increase in VWC immediately after the pouring of the first layers in Design 2 and Design 3 was the volumetric increase in C-S-H gels formed in the hydration of cement. In addition, a thaumasite salt containing 15 molecules of water can be formed as a result of the sulfates in the medium acting on the calcium silicates (C-S-H) and can cause the softening of the material by expanding. Also, the mixtures formed with Portland cement turn into ettringite by combining with calcium sulfate (CaSO4) when they come in contact with sulfated waters, tricalcium aluminate (C3A). This material, also called Candlot salt, can crack the material and cause it to break due to the vast volume it occupies (Tuylu, 2016). However, as Design 2 and Design 3 remained stable in higher SC values when compared to Design 1, it is believed that C-S-H gels formed instead of thaumasite and ettringite salts, due to the comparatively low amount of cement added, and formed a solid bond between the grains. Judging by the VWC values obtained from the sensor measurements of Layer 5 of each design shown in Figs. 5–7, it is therefore understood that the capillary cracks are formed within its own structure thus preventing larger cracks from forming and enabling them to remain more saturated in comparison to Design 1. It can be seen in Fig. 5 that the fifth layer in Design 1 appears to be stable at ∼87–89% of SC after the initial storing. It can be seen from Figs. 6 and 7 that the fifth layer in both Design 2 and Design 3 exhibit similar behavior as their lowest layers are both cemented. The fact that these two designs have a SC value approximately 4% less than that of Design 1 means that the fifth layer and the other underlying layers are more saturated. In Figs. 6 and 7, it was determined that the VWC varied over a range of about 25% throughout each storing and drying period after the storing of the fifth layer. This value is five times greater in comparison to Design 1. It is therefore understood that water seepage after each formation of paste layer in Design 2 and Design 3 penetrates
Fig. 2. VWC and SC values of Layer 1 in Design 1 (no cement). 35
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Fig. 5. VWC and SC values of Layer 5 in Design 1 (no cement). Fig. 8. VWC and SC values of Layer 10 in Design 1 (no cement).
Fig. 6. VWC and SC values of Layer 5 in Design 2 (cement in Layer 1 only). Fig. 9. VWC and SC values of Layer 10 in Design 2 (cement in Layer 1 only).
Fig. 7. VWC and SC values of Layer 5 in Design 3 (cement in Layers 1 and 11). Fig. 10. VWC and SC values of Layer 10 in Design 3 (cement in Layers 1 and 11).
the lower layers much more. In a study by Hadimi et al. (2016), the cemented layer deposited on the top of the physical model seems to play a role in protecting underlying layers against evaporation. Within the scope of the study, no significant difference in the volumetric water contents of the 10th layers in the cabin experiments of the three different designs was measured (Figs. 8–10). It was determined that the SC values gradually approach 90% in a short time depending on the optimum water content of the 10th layers in these three cabins. It was determined that the SC value of the 10th layer in Design 1 remained within ∼87–88% for 16 days (Fig. 8). At the same time, the SC value of the 10th layer in Design 2 was measured as changing in the range of ∼86–88%, and the SC value of the 10th layer in Design 3 changed in the range of ∼87–88% (Figs. 9 and 10). It is seen in Figs. 8
and 9 that the volumetric water contents of the 10th layers increased by 4.5% in Design 1, and by 6.5% in Design 2 within about two hours of the 11th layer being stored. However, the volumetric water content in Design 3 increased by ∼5.5% within about nine hours of the 11th layer being stored (Fig. 10). It can be stated that the passage of seepage water from the 11th layer occurs over a long-time period because of the addition of cement to Layer 11 in Design 3. The changes in volumetric water content in the first, fifth and 10th layers for the duration of the storing of the 11 layers are given in Table 1 and in Fig. 11. According to these values, it is understood from
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Table 1 Changes in VWC during layer storing process. VWC (%) 1st Layer
VWC (%)
Design 1 Design 2 Design 3
5th Layer
10th Layer
0th Day
88th Day
Water Loss
0th Day
88th Day
Water Loss
0th Day
88th Day
Water Loss
∼100 ∼105 ∼110
∼66 ∼52 ∼60
34 53 50
∼110 ∼105 ∼110
∼41 ∼60 ∼61
69 45 49
∼110 ∼110 ∼110
∼44 ∼46 ∼44
66 64 66
1st Layer
5th Layer
10th Layer
80 70 60 50 40 30 20 10 0 Design 1
Design 2
Design 3
Fig. 11. Loss of volumetric water content during storing in three different designs.
Fig. 12. Matric suction values in Design 1 (no cement).
Table 1 that the loss of volumetric water content from the first layer in Design 1 is about twice that of the loss of volumetric water content from the fifth layer, and this value does not change significantly afterward. In Design 2 and Design 3, there is ∼16% more volumetric loss of water than the first layer in Design 1 as a result of the hydration caused by the cement addition of the first layers. However, because these cemented layers form a tighter structure than the other layers, they resulted in less water loss in their fifth layers. It was observed that in the 10th layers where the sensors are located, the 10th layers in the cemented designs and uncemented designs had similar water content values due to the decrease in saturation from the lower layers to the uppermost layers, and the acceleration of water seepage due to gravity towards the lower layers depending on height. When the volumetric water contents of the first, fifth and 10th layers of three different designs are examined from the geotechnical point of view, it is seen in Figs. 2–4 that the first layers having the greatest risk of liquefaction and shear stress when the loads of the layers stored atop the first layers are rapidly consolidated and drained. In Design 1 in particular, the water content of the lowest layer, which was exposed to maximum seepage water, remains stable after ∼4 weeks, and its shear strength was above 20 kPa (Bascetin and Tuylu, 2018). The water contents of the first layers remain constant at 18% (SC ∼ 82%) in Design 1, and about 14% (SC ∼ 86%) in Designs 2 and 3; and the water content value of %16.6 (SC ∼ 83.4%) of the total volume was approached in Design 1, whereas it fell below this value in Design 2 and Design 3. At the same time, it was observed that the binder material increased the stability of the paste in the face of excessive loads on the lowest layer by strengthening the bonds between grains. Self-weight consolidation and desiccation of the flowing paste were inhibited to prevent creep, increasing height, and deformation under loading of the other layers (Theriault et al., 2003; Tariq and Yanful, 2013).
Fig. 13. Matric suction of Design 2 (cement in Layer 1 only).
3.2. Matric suction pressure
Fig. 14. Matric suction of Design 3 (cement in Layers 1 and 11).
Also, as the number of layers increased, the effect of matric suction between the layers of the paste material was observed and it was found that the 10th layers in all three different designs stabilized at ∼88% SC 37
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by the end of the casting period. The results of matric suction pressure on paste material layers are given for Design 1 in Fig. 12, for Design 2 in Fig. 13, and for Design 3 in Fig. 14. It can be seen in Fig. 12 that the matric suction value of the first layer (−10 kPa) in Design 1 does not change during the storing period of 11 layers. Although the volumetric water contents in the fifth and 10th layers are the same, a slight increase in matric suction is measured. It can be said that this increase is due to the fact that matric suction is directly proportional to capillary height. Also Theriault et al. (2003) reported in their study that the matric suction values measured near the surface of the stored paste material layer varied between about 4 and 20 kPa within a few days. This corresponds to matric suction values in the layer pouring process of the 10th layer close to the surface in Design 1, which is stored without a binder. It can be seen in Fig. 13 that the pressure in the first layer in Design 2 increases from −10 kPa to −87 kPa due to the hydration-induced matric suction requirement formed during the initial storing because the first layer is cemented. However, with the second storing, it was discovered that the first layer remained at −10 kPa and did not change. In contrast to Design 1, the fifth layer appears to have no matric suction. Therefore, Layers 1 and 5 in Design 2 were measured as not falling below the ∼50% volumetric water content in the storing process, which is the 100% saturation limit, thus preventing the formation of suction stresses in these layers. Conversely, the matric suction of the 10th layer started to increase six days after the 11th layer was deposited, and reached −14 kPa at the end of storing. Here it is thought that the fall in volumetric water content to ∼46% is effective. Fig. 14 shows that matric suction values in Design 3 behave almost like the layers in Design 2. However, it is thought that the matric suction value of the 10th layer in Design 3 is higher than the 10th layers of other designs because the 11th layer of Design 3 is cemented and the capillary cracked structures that are formed after a while on the 10th layer increase the grain surface tensions after drying. Additionally, the matric suction of the 10th layer started to increase three days after the 11th layer was deposited upon it, and reached a value of −14 kPa at the end of storing. In addition to these results, the results regarding layer oxygen consumption, which are an important parameter for oxidation, are given in Fig. 15 for Design 1, Fig. 16 for Design 2, and Fig. 17 for Design 3 respectively.
Fig. 16. Oxygen consumption of Design 2 (cement in Layer 1 only).
Fig. 17. Oxygen consumption of Design 3 (cement in Layers 1 and 11).
at a level of 19.8%. The reason for this is thought to be that the seepage water remains in the layer for a while and cuts the air contact with the water-saturated paste material. It was found that the oxygen content in the fifth layer tended to decrease until the storing of the final layer, and that the lowest value fallen to was in the level of ∼17%. Given this trend, it is understood that the contact with oxygen in the air is cut off by being between the four underlying layers and the six overlying layers. In the storing of the 10th and 11th layers, the oxygen content of the 10th layer was measured as changing between very close values. However, Fig. 15 shows that the oxygen content, which falls to ∼18%, approached the oxygen value of 20.95% in the air after 16 days. This increase also occurs in a similar way in the first and fifth layers. This increase is thought to be caused by the cracks forming on the surface of the paste material moving inside due to the increase in water absorption potentials of the layers as well as the surface stresses of the particles. As shown in Figs. 16 and 17, the oxygen content of the first layers of Design 2 and 3 show a sudden decrease in levels to around 14% after the initial storing since they are cementitious. In all three designs, the volumetric water content of the first, fifth and 10th layers tried to stabilize after the first paste layer was stored. Also, the effect of the matric suction between the layers was observed, and it was strong for first cemented layers in Design 2 and 3. At the end of the storing process, as the oxygen consumption values remain below 20.95% from the measurements of the first, fifth and 10th layers of these two designs, it is understood that contact with air does not occur. Therefore, as stated in the study by Yilmaz et al. (2014), it can be explicitly expressed with this study that the cemented layer takes on a barrier role due to its lesser and slower permeability, keeping the upper layers saturated. Since the diffusion of oxygen to the layers with a higher degree of saturation occurs at a minimum level, the AMD risk of
3.3. Oxygen consumption According to Fig. 15, it is understood that the bottom layer in Design 1 comes into contact with oxygen in the air due to the cracking that forms shortly after the initial storing. However, measurements showed that after each layer of storing followed by the storing of the 3rd layer, the oxygen content within the first layer decreased to 17.5–16.5% and it increased again during the eight days' drying process, and remained
Fig. 15. Oxygen consumption for Design 1 (no cement). 38
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Design-1
Design-2
Design-3
14 ALKALINE
12 10 pH
8 6 4 2
ACID
0 0
1
2
3
4
5
6 7 Layers
8
9
10
11
12
Fig. 18. pH of seepage in three different designs.
said that this theory supports the decrease in the amount of dissolved ions in designs with cemented layers.
these layers is reduced (Figs. 16 and 17). 3.4. pH- electrical conductivity (EC)
4. Conclusion The pH and EC values of seepage water collected after the depositing of each layer were measured. The pH changes are shown in Fig. 18. The EC values are also given in Fig. 19. As seen in Fig. 18, the pH values in Design 1 are often lower than 7. Jarosikova et al. (2017) reported in their study that metals (Cd, Cu, Pb, Zn) exhibit the highest leaching at low pH or under acidic conditions (pH 3–6). On the other hand, it is understood that the cemented layers in Designs 2 and 3 do not allow the pH values to fall below 10 because they generate alkaline silica reactions in these layers after solving the alkali minerals that act as a barrier. The sulfide oxidation on seepage quality was reduced through the presence of minerals and cement that buffered pH (Verburg and Oliveira, 2016). In Fig. 19, it can be seen that the EC values of Design 1 are greater than 3 after the deposition of first layer, and this state continues in each subsequent deposition of other layers. In Designs 2 and 3, it was determined that the EC values started a downward trend and that ion dissolution decreased according to Design 1 after deposition of the first layer. As a result, the cement additive keeps the pH values of the seepage in an alkaline environment so that it minimizes the risks of AMD and heavy metals mobilization at low pH values. Furthermore, it can be
Design-1
In this study, three different designs were attempted for SPD; and the values of volumetric water content, matric suction, and oxygen consumption were measured. In this context, measurements showed that the first layers remained above the saturation limit of 53% volumetric water content in the three different designs. When the fifth layers were examined, it was determined that the volumetric water content value fell below 50% in Design 1, whereas it was well above the saturation limit in Design 2 and Design 3. Also, as the number of layers increased, the effect of the matric suction between the layers of the paste material was observed and it was found that the volumetric water content of the 10th layers of three different designs stabilized at 40–50% by the end of the pouring period. It was observed that the volumetric water content value decreased rapidly immediately after the pouring of the paste layers, remained constant for a while and then somehow slowly started to decrease again. This basis of this change can be expressed as time-dependent evaporation and the diffusion of the pore waters away from the medium found between the particles of paste material that is consolidated under its own load. In this study, the bottom and top cemented layers in
Design-2
Design-3
5
EC (mS/cm)
4 3 2 1 0 0
1
2
3
4
5
6 Layers
7
8
9
Fig. 19. EC values of seepage in three different designs. 39
10
11
12
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Design 3 and only the bottom cemented layer in Design 2 in particular made a positive contribution to pH and EC values. These designs are more prominent in terms of preventing AMD and heavy metals mobilization. As can be seen from this study, the bottom and top cemented paste layers ensure that the stored materials are more reliable in terms of physical and chemical properties. It was determined that the lower layers of the stored material are stable within a certain water content range under their own weight. It was also shown that the topmost cemented paste layer could prevent surface water and oxygen diffusion. Design 3 comes to the forefront according to all these evaluations. This design will reduce the environmental risks the most and will then be a suitable method for improvement conditions. It is understood in this study that cement has resistance against the physical and chemical effects that can and do occur particularly in atmospheric conditions. However, cement is a material that increases cost. For this reason, it may be possible to concentrate on different additive materials that can be used as a substitute. In subsequent studies that can be done in this area, pozzolanic materials (such as fly ash) in particular can be investigated by mixing at different rates instead of cement.
Mining with Backfill, Beijing, China, September 19–21, pp. 180–192. Deschamps, T., Benzaazoua, M., Bussière, B., Belem, T., Aubertin, M., 2007. The effect of disposal configuration on the environmental behavior of paste tailings. In: MINEFILL2007 in Montreal, Quebec, April 29 – May 3, 2007, pp. 2491. Deschamps, T., Benzaazoua, M., Bussière, B., Aubertin, M., Belem, T., 2008. Microstructural and geochemical evolution of paste tailings in surface disposal conditions. Miner. Eng. 21, 341–353. Deschamps, T., Benzaazoua, M., Bussière, B., Aubertin, M., 2011. Laboratory study of surface paste disposal for sulfidic tailings: physical model testing. Miner. Eng. 24, 794–806. EN 197-1, 2011. Cement – Part 1: Composition, Specification and Conformity Criteria for Common Cements. European Standard. EPA, 2002. A Guidance Manuel to Support the Assessment of Contaminated Sediments in the Freshwater Ecosystem. United States Environmental Protection Agency, Chicago, IL. Fourie, A.B., 2003. In Search of the Sustainable Tailings Dam: Do High-density Thickened Tailings Provide the Solution. School of Civil and Environmental Engineering, University of the Witwatersrand, South Africa, pp. 12. Hadimi, I., Benzaazoua, M., Maqsoud, A., Bussie`re, B., 2016. Effect of cementitious amendment on the hydrogeological behavior of a surface paste tailings' disposal. Innov. Infrastruct. Solut. 1 (1), 1–15. ICOLD, UNEP, 2001. Bulletin 121: tailings dams - risk of dangerous occurrences. In: Lessons Learnt from Practical Experiences, Paris,144. Jarosikova, A., Ettler, V., Mihaljevic, M., Kribek, B., Mapani, B., 2017. The pH-dependent leaching behavior of slags from various stages of a copper smelting process: environmental implications. J. Environ. Manag. 187, 178–186. Liu, Q., Liu, D., Liu, X., Gao, F., Li, S., 2016. Research and application of surface paste disposal for clay-sized tailings in tropical rainy climate. Int. J. Miner. Process. 157, 227–235. McGregor, R.G., Blowes, D.W., 2002. The physical, chemical and mineralogical properties of three cemented layers within sulfide-bearing mine tailings. J. Geochem. Explor. 76, 195–207. Tariq, A., Yanful, E.K., 2013. A review of binders used in cemented paste tailings for underground and surface disposal practices. J. Environ. Manag. 131, 138–149. Theriault, J., Frostiak, J., Welch, D., 2003. Surface disposal of paste tailings at the Bulyanhulu gold mine. In: Proceedings of the 2nd Mining Environment Conference, Sudbury, Ontario, pp. 1–8. Tuylu, S., 2016. Determination of Optimum Design Specifications for Surface Paste Tailings Disposal. Ph.D. Thesis, Advisor Prof. Dr. Atac Bascetin and Co-Advisor Prof. Dr. Mostafa Benzaazoua. Istanbul University Institute of Science. Verburg, R.B., 2001. Use of paste technology for tailings disposal: potential environmental benefits and requirements for geochemical characterization. In: IMWA Symposium, Proceedings of a Meeting Held 24-28 April 2001. Belo Horizonte, Brazil. Verburg, R.B., 2002. Paste technology for disposal of acid-generating tailings. In: Mining Environmental Management, July 2002, pp. 84. Verburg, R., Oliveira, M., 2016. Surface paste disposal of high-sulfide tailings at nevescorvo–evaluation of environmental stability and operational experience. In: Drebenstedt, Carsten, Paul, Michael (Eds.), Proceedings IMWA 2016, Freiberg/ Germany. Mining Meets Water – Conflicts and Solutions, pp. 361–367. Welch, D., 2003. Advantages of tailings thickening and paste technology. In: Responding to Change - Issues and Trends in Tailings Management, Golder Associates Report:5. Yilmaz, E., Benzaazoua, M., Bussière, B., Pouliot, S., 2014. Influence of disposal configurations on hydrogeological behaviour of sulphidic paste tailings: a field experimental study. Int. J. Miner. Process. 131, 12–25.
Acknowledgements This work was supported by the Scientific and Technological Research Council of Turkey (Project Numbers: 116M721) and also by the Scientific Research Projects Coordination Unit of Istanbul University (Project Nos: 19062, 40376, 27845, 21527, 20817). The authors are grateful to the Executive Secretariat of Scientific Research Projects of Istanbul University. References Bascetin, A., Tuylu, S., 2018. Application of Pb-Zn tailings for surface paste disposal: geotechnical and geochemical observations. Int. J. Min. Reclamat. Environ. 32 (5), 312–326. https://doi.org/10.1080/17480930.2017.1282411. Bascetin, A., Tuylu, S., Adiguzel, D., Ozdemir, O., 2017. Field properties and performance of surface paste disposal. In: Yilmaz, E., Fall, M. (Eds.), Paste Tailings Management. Springer, Cham. Bascetin, A., Tuylu, S., Ozdemir, O., Adiguzel, D., Benzaazoua, M., 2018. An investigation of crack formation in surface paste disposal method for pyritic Pb–Zn tailings. Int. J. Environ. Sci. Technol. 15 (2), 281–288. Benzaazoua, M., Perez, P., Belem, T., Fall, M., 2004. A laboratory study of the behaviour of surface paste disposal. In: Proceedings of the 8th International Symposium on
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Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman
Research article
The effects of cement on some physical and chemical behavior for surface paste disposal method
T
Serkan Tuylu∗, Atac Bascetin, Deniz Adiguzel Istanbul University, Engineering Faculty, Mining Engineering Department, 34320, Istanbul, Turkey
A R T I C LE I N FO
A B S T R A C T
Keywords: Tailings AMD Heavy metal mobilization Surface paste disposal method
Environmental impacts resulting from conventional tailings disposal such as tailings dam accidents are a common problem for base metal mines around the world. In this context, laboratory-scale studies have been carried out on the Surface Paste Disposal (SPD) method, which is one of the alternative surface storage methods. In this study, three different SPD designs were tested; and volumetric water content, oxygen consumption, and matric suction sensors in first, fifth and 10th paste layers plus pH- electric conductivity (EC) values were all measured. Specifically, it was determined that the amount of oxygen in the environment required for the oxidation of sulfur minerals is reduced in the cemented layers in Design 3. In addition, the cement additive keeps the pH values of the seepage in an alkaline environment (over 7) so that it minimizes the risks of Acid Mine Drainage (AMD) and heavy metals mobilization at low pH values. Also, the EC values started a downward trend and ion dissolution decreased in designs with cemented layers. As a result, it was understood from the sensor measurements that the cemented layers act like a barrier.
1. Introduction Failure in tailings dams, which are the most widely used conventional disposal method in the world, can result in an uncontrolled spill, a dangerous flow-slide, and/or the release of poisonous chemicals, leading to a major environmental disaster. Surface paste disposal (SPD), which is a storage method that has become increasingly important in recent years, can be considered an effective alternative technique for mine tailings management, and the topic of multiple studies. These studies have shown that SPD tailings storage can minimize the amount of free water, which is one of the biggest problems of conventional tailings dams. Additionally, SPD can allow the recycling of water; limit the risks related to the failure of dikes, and favor progressive mine sites rehabilitation (ICOLD and UNEP, 2001; Fourie, 2003; Welch, 2003; Liu et al., 2016). At the same time, cement binder can be added to the surface paste material when necessary to increase durability and stability as well as reducing the risk of heavy metals mobilization with Acid Mine Drainage (AMD). If cement or another binder is going to be used in the paste material mixture, it is desired that the amount of binder by solid weight should be ≤ 2% in terms of cost and able to store more tailings (Benzaazoua et al., 2004; Deschamps et al., 2011; Yilmaz et al., 2014; Bascetin and Tuylu, 2018; Bascetin et al., 2017, 2018). However, it is
∗
not economically feasible to add cement or other binders to the tailings when considering the amount of tailings produced. For this reason, it is first necessary to try different designs and to reveal the effects of these designs. The uncertainties surrounding this issue have not yet been fully resolved in terms of the SPD method. In this study, different designs were made by forming cemented and uncemented paste layers. The studies conducted in industrial areas throughout the world in terms of SPD have mostly taken the form of experiments, but have not yet been put into practice as an alternative method. These studies applied SPD placed in arid and extremely cold climates as Tanzania and Canada (Theriault et al., 2003; Yilmaz et al., 2014). Also, it was observed in the literature that potential environmental impacts have generally been analyzed geochemically (AMD, etc.) in terms of the column, humidity cell, and small-scale cabin tests (Verburg, 2001, 2002; McGregor and Blowes, 2002; Deschamps et al., 2007, 2008). As the experiments regarding the subject are new, an efficient model for the optimum storage design has not yet been developed. Our goal here was to find the optimum cemented design according to the measurement (volumetric water content, matric suction, oxygen consumption sensors, and pH-EC of the seepage) results in laboratory scale cabins with three different designs started simultaneously. The main aim of this study was to determine the optimal design that allows for successive layers of storage in a laboratory environment,
Corresponding author. Istanbul University, Faculty of Engineering, Department of Mining Engineering, 34320, Avcilar, Istanbul, Turkey. E-mail address: [email protected] (S. Tuylu).
https://doi.org/10.1016/j.jenvman.2018.10.007 Received 24 May 2018; Received in revised form 20 September 2018; Accepted 2 October 2018 0301-4797/ © 2018 Elsevier Ltd. All rights reserved.
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taking into account the physical and geochemical durability of mine process tailings. One of the other aims of the study was to enable the paste material to achieve physical durability so that it could remain stable under its own weight, or under static or dynamic loads, depending on the subsequent use of the storage area. In addition, the prevention of water and oxygen diffusion within the stored material after the storage is completed is another important issue. A storage method that meets these conditions will be able to minimize environmental risks and will be a suitable method for reclamation and reutilization conditions later on. 2. Materials and methods
Fig. 1. (a) General view of test cabins. (b) Test cabin designs with different configurations.
The paste material used in this study was obtained from lead-zinc process tailings from an underground mine in western Turkey. The region has a climate between temperate and cold. In winter times it is rainy, while in summer times it is hot and dry. The cut-and-fill underground production method was used in the mine studied. The mineral processing stages of the run-of-mine (ROM) ore extracted from underground include crushing, grinding, floatation, and filtering processes. An average of 400 tonnes of concentrate is obtained from an average 4800 tonnes of ROM ore supplied on a daily basis to the mineral processing plant. The remaining 4400 tonnes of solid waste is discharged to the tailings dam at a solid concentration of 15–20 wt%. The mine has four tailings dams: one closed, one fully filled, one currently filling, and one under construction. The tailings obtained from the processing plant contain risk in terms of AMD formation and heavy metals mobilization. Therefore, it is very important to understand the mineralogical components and elemental contents of tailings. To this end, the tailings were examined using ICPMS and XRD analysis. The mineralogical composition of tailings is mainly composed of Calcite (CaCO3), Quartz (SiO2), Feldspar minerals (Albite = Na(AlSi3O8)), and Pyrite (FeS2). The values of Cu, Pb, Zn, and Cd were found to be 217.2 mg/kg, 1500.1 mg/kg, 1548 mg/kg, 601.8 mg/kg, and 8.2 mg/kg respectively. The values exceeded the maximum values specified in EPA by 110 mg/kg, 110 mg/kg, 270 mg/ kg, 33 mg/kg and 4 mg/kg respectively. It is expected that the metals in paste tailings will generate considerable toxicity in soil and water in terms of environment. This heavy metals content can significantly generate toxicity in soil and water from an environmental standpoint. It was understood from the Modified ABA method and ICP-MS elemental analysis of paste tailings that there is a risk of heavy metal mobilization. However, the XRD and chemical analysis also revealed the presence of buffer minerals. It was envisaged that this risk could be minimized using the SPD method (Bascetin and Tuylu, 2018; EPA, 2002). In this context, after of the physical and chemical characteristics of Pb-Zn tailings were determined, mixtures of between 65 and 75% solid content (SC) by weight were prepared for paste material. For this mixture to be pumped for long distances, slump tests were conducted in order to achieve a slump value of 250 mm (acceptable for SPD), and the optimal SC value was determined accordingly. In order to fully understand the behavior of the paste tailings material during deposition, the test cabins in Fig. 1a were used to run simulations of the paste tailings material in surface storage with three different designs. While preparing the paste material, CEM I 42.5 Portland cement without additives (PC) produced in accordance with the EN 197-1 (2011) standard was used as a binder in certain configurations in order to investigate the effect of the binding material on paste technology. The solid-water ratio of the paste layers was determined according to a slump value of 250 mm to obtain suitable paste material for paste technology. Additionally, cement binder in the amount of ∼2% by solid material weight was added to some cemented layers depending on designed paste disposal. In this study, a cabin configuration with dimensions of 200 cm (length) * 70 cm (width) * 130 cm (height) were used to store the paste tailings. The cabin was also equipped with Decagon® Em50 five channel
Fig. 1. (continued)
voltage data logger for the VWC (Decagon® 5TM), matric suction (Decagon® MPS-1 and 2), and oxygen consumption (Apogee® SO-110 oxygen sensor) data collection. Surface behavior of paste material was simulated with all layers being uncemented in Design 1 (reference cabin), followed by some layers being cemented in Design 2, then Design 3 which contained cement binder (≤%2 solid content weight) storage cabins. The different sensors, such as VWC, matric suction potential and oxygen consumption, were placed on the first, fifth and 10th layers in lab-scale test cabin (Fig. 1b). Additionally, seepage water collected from the bottom of the cabins was subjected to pH and EC analysis. Thus, the optimal storage design for Pb-Zn tailings prepared for SPD was identified. Design 1 consists of uncemented paste layers. This test cabin was a control sample for the other test cabins where different configurations were utilized. Therefore, this cabin can also be called the “reference cabin”. Design 2 consists of only the bottom layer (first layer) with cemented paste tailings material, with all the subsequent layers above it having no cement. By creating an alkali medium with the cement structure in the first layer, the aim was to keep the high pH value of the pore water and prevent acid generation and/or heavy metals mobilization. In Design 3, layers 1 and 11 were deposited with cemented paste material with all the other layers in-between being stored as uncemented paste material. Here, it was considered that increasing the stability of the topmost layer of the storage area could be beneficial for reclamation works. It was also considered that the high pH value of the topmost layer, which would come in contact with free surface waters, would reduce the risk of AMD. The paste material was placed in layers in these test cabins. The layer height of the paste material stored into the cabins was determined to be 40 mm for each layer taking into consideration the spread width of the paste and the studies conducted in the literature (Benzaazoua et al., 2004; Deschamps et al., 2011). Paste storage comprised a total of 11 layers placed within the cabin. Additional layers were placed only once the previous layer had completed its curing time and until there was no volumetric water content loss – a time period of eight days. Data collection from the installed sensors was completed over 88 days with 34
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these sensors measuring the volumetric water content, matric suction, and oxygen consumption of the layers. This approach let the previous layer achieve stability. Three different sensors were placed in layers 1, 5 and 10 of each cabin in order to determine the values for volumetric water content, oxygen consumption, and matric suction (Bascetin and Tuylu, 2018; Bascetin et al., 2017, 2018). 3. Results and discussion After the completion of preparations for the three storage cabins, data collection from the installed sensors was completed over 88 days in the laboratory. Paste material storage for these three different designs then began. During the experiment period, all data were automatically recorded every 1 min. Approximately 126,000 items of data for the first layer, 80,000 for the fifth layer, and 23,000 items of data for the 10th layer were obtained in each design. These data were then analyzed. In this context, after the placing of the layers of paste material, the sensor measurements for the three different designs were given according to the sensor values in the following order: first, fifth, and 10th layers. While the paste material was being placed in layers in the cabins, sensors were placed in the first, fifth, and 10th layers. These sensors measure the volumetric water content, matric suction, and oxygen consumption of the layers.
Fig. 3. VWC and SC values of Layer 1 in Design 2 (cement in Layer 1 only).
3.1. Volumetric water content (VWC) The volumetric water content for Layer 1 and the correspondingly calculated values in the solid content by weight (SC = Solid Content/ Total Mass of Pulp) value are given in Figs. 2–4 for Design 1, Design 2, and Design 3 respectively. While the SC of Layer 1 in Design 1 shown in Fig. 2 (this layer does not contain cement) was ∼75% during initial storing, it was observed to increase to ∼82% after 88 days considering there was an opportunity for drainage. The upper layers that were deposited after 40 days were determined to have little effect on the bottom layer of the reference (non-binder) paste storage system and remained stable in the range of ∼80–82% of VWC (Fig. 2). The volumetric water content behavior of the first layers in Design 2 and Design 3, consisting of binded (cemented) paste material, is shown in Figs. 3 and 4. It is clearly seen that the first layers of both designs depict a similar trend and reach a stable range after day 32 with each subsequent layer storing based on VWC. As seen in Figs. 3 and 4, after the first layer is stored, an increase in volumetric water content of ∼35% is observed. It was determined that Design 2 remained stable between at 83%–86% of SC, and that Design 3 remained stable between and at 81%–84% of SC after the storing of the fifth layers. These results can be expressed as the paste material being consolidated with the loads upon it and reaching hydrostatic balance after a certain period of time (Figs. 2–4). This situation was observed
Fig. 4. VWC and SC values of Layer 1 in Design 3 (cement in Layers 1 and 11).
particularly after half of the total layer storing time had passed. It is thought that one of the most important causes of increase in VWC immediately after the pouring of the first layers in Design 2 and Design 3 was the volumetric increase in C-S-H gels formed in the hydration of cement. In addition, a thaumasite salt containing 15 molecules of water can be formed as a result of the sulfates in the medium acting on the calcium silicates (C-S-H) and can cause the softening of the material by expanding. Also, the mixtures formed with Portland cement turn into ettringite by combining with calcium sulfate (CaSO4) when they come in contact with sulfated waters, tricalcium aluminate (C3A). This material, also called Candlot salt, can crack the material and cause it to break due to the vast volume it occupies (Tuylu, 2016). However, as Design 2 and Design 3 remained stable in higher SC values when compared to Design 1, it is believed that C-S-H gels formed instead of thaumasite and ettringite salts, due to the comparatively low amount of cement added, and formed a solid bond between the grains. Judging by the VWC values obtained from the sensor measurements of Layer 5 of each design shown in Figs. 5–7, it is therefore understood that the capillary cracks are formed within its own structure thus preventing larger cracks from forming and enabling them to remain more saturated in comparison to Design 1. It can be seen in Fig. 5 that the fifth layer in Design 1 appears to be stable at ∼87–89% of SC after the initial storing. It can be seen from Figs. 6 and 7 that the fifth layer in both Design 2 and Design 3 exhibit similar behavior as their lowest layers are both cemented. The fact that these two designs have a SC value approximately 4% less than that of Design 1 means that the fifth layer and the other underlying layers are more saturated. In Figs. 6 and 7, it was determined that the VWC varied over a range of about 25% throughout each storing and drying period after the storing of the fifth layer. This value is five times greater in comparison to Design 1. It is therefore understood that water seepage after each formation of paste layer in Design 2 and Design 3 penetrates
Fig. 2. VWC and SC values of Layer 1 in Design 1 (no cement). 35
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Fig. 5. VWC and SC values of Layer 5 in Design 1 (no cement). Fig. 8. VWC and SC values of Layer 10 in Design 1 (no cement).
Fig. 6. VWC and SC values of Layer 5 in Design 2 (cement in Layer 1 only). Fig. 9. VWC and SC values of Layer 10 in Design 2 (cement in Layer 1 only).
Fig. 7. VWC and SC values of Layer 5 in Design 3 (cement in Layers 1 and 11). Fig. 10. VWC and SC values of Layer 10 in Design 3 (cement in Layers 1 and 11).
the lower layers much more. In a study by Hadimi et al. (2016), the cemented layer deposited on the top of the physical model seems to play a role in protecting underlying layers against evaporation. Within the scope of the study, no significant difference in the volumetric water contents of the 10th layers in the cabin experiments of the three different designs was measured (Figs. 8–10). It was determined that the SC values gradually approach 90% in a short time depending on the optimum water content of the 10th layers in these three cabins. It was determined that the SC value of the 10th layer in Design 1 remained within ∼87–88% for 16 days (Fig. 8). At the same time, the SC value of the 10th layer in Design 2 was measured as changing in the range of ∼86–88%, and the SC value of the 10th layer in Design 3 changed in the range of ∼87–88% (Figs. 9 and 10). It is seen in Figs. 8
and 9 that the volumetric water contents of the 10th layers increased by 4.5% in Design 1, and by 6.5% in Design 2 within about two hours of the 11th layer being stored. However, the volumetric water content in Design 3 increased by ∼5.5% within about nine hours of the 11th layer being stored (Fig. 10). It can be stated that the passage of seepage water from the 11th layer occurs over a long-time period because of the addition of cement to Layer 11 in Design 3. The changes in volumetric water content in the first, fifth and 10th layers for the duration of the storing of the 11 layers are given in Table 1 and in Fig. 11. According to these values, it is understood from
36
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Table 1 Changes in VWC during layer storing process. VWC (%) 1st Layer
VWC (%)
Design 1 Design 2 Design 3
5th Layer
10th Layer
0th Day
88th Day
Water Loss
0th Day
88th Day
Water Loss
0th Day
88th Day
Water Loss
∼100 ∼105 ∼110
∼66 ∼52 ∼60
34 53 50
∼110 ∼105 ∼110
∼41 ∼60 ∼61
69 45 49
∼110 ∼110 ∼110
∼44 ∼46 ∼44
66 64 66
1st Layer
5th Layer
10th Layer
80 70 60 50 40 30 20 10 0 Design 1
Design 2
Design 3
Fig. 11. Loss of volumetric water content during storing in three different designs.
Fig. 12. Matric suction values in Design 1 (no cement).
Table 1 that the loss of volumetric water content from the first layer in Design 1 is about twice that of the loss of volumetric water content from the fifth layer, and this value does not change significantly afterward. In Design 2 and Design 3, there is ∼16% more volumetric loss of water than the first layer in Design 1 as a result of the hydration caused by the cement addition of the first layers. However, because these cemented layers form a tighter structure than the other layers, they resulted in less water loss in their fifth layers. It was observed that in the 10th layers where the sensors are located, the 10th layers in the cemented designs and uncemented designs had similar water content values due to the decrease in saturation from the lower layers to the uppermost layers, and the acceleration of water seepage due to gravity towards the lower layers depending on height. When the volumetric water contents of the first, fifth and 10th layers of three different designs are examined from the geotechnical point of view, it is seen in Figs. 2–4 that the first layers having the greatest risk of liquefaction and shear stress when the loads of the layers stored atop the first layers are rapidly consolidated and drained. In Design 1 in particular, the water content of the lowest layer, which was exposed to maximum seepage water, remains stable after ∼4 weeks, and its shear strength was above 20 kPa (Bascetin and Tuylu, 2018). The water contents of the first layers remain constant at 18% (SC ∼ 82%) in Design 1, and about 14% (SC ∼ 86%) in Designs 2 and 3; and the water content value of %16.6 (SC ∼ 83.4%) of the total volume was approached in Design 1, whereas it fell below this value in Design 2 and Design 3. At the same time, it was observed that the binder material increased the stability of the paste in the face of excessive loads on the lowest layer by strengthening the bonds between grains. Self-weight consolidation and desiccation of the flowing paste were inhibited to prevent creep, increasing height, and deformation under loading of the other layers (Theriault et al., 2003; Tariq and Yanful, 2013).
Fig. 13. Matric suction of Design 2 (cement in Layer 1 only).
3.2. Matric suction pressure
Fig. 14. Matric suction of Design 3 (cement in Layers 1 and 11).
Also, as the number of layers increased, the effect of matric suction between the layers of the paste material was observed and it was found that the 10th layers in all three different designs stabilized at ∼88% SC 37
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by the end of the casting period. The results of matric suction pressure on paste material layers are given for Design 1 in Fig. 12, for Design 2 in Fig. 13, and for Design 3 in Fig. 14. It can be seen in Fig. 12 that the matric suction value of the first layer (−10 kPa) in Design 1 does not change during the storing period of 11 layers. Although the volumetric water contents in the fifth and 10th layers are the same, a slight increase in matric suction is measured. It can be said that this increase is due to the fact that matric suction is directly proportional to capillary height. Also Theriault et al. (2003) reported in their study that the matric suction values measured near the surface of the stored paste material layer varied between about 4 and 20 kPa within a few days. This corresponds to matric suction values in the layer pouring process of the 10th layer close to the surface in Design 1, which is stored without a binder. It can be seen in Fig. 13 that the pressure in the first layer in Design 2 increases from −10 kPa to −87 kPa due to the hydration-induced matric suction requirement formed during the initial storing because the first layer is cemented. However, with the second storing, it was discovered that the first layer remained at −10 kPa and did not change. In contrast to Design 1, the fifth layer appears to have no matric suction. Therefore, Layers 1 and 5 in Design 2 were measured as not falling below the ∼50% volumetric water content in the storing process, which is the 100% saturation limit, thus preventing the formation of suction stresses in these layers. Conversely, the matric suction of the 10th layer started to increase six days after the 11th layer was deposited, and reached −14 kPa at the end of storing. Here it is thought that the fall in volumetric water content to ∼46% is effective. Fig. 14 shows that matric suction values in Design 3 behave almost like the layers in Design 2. However, it is thought that the matric suction value of the 10th layer in Design 3 is higher than the 10th layers of other designs because the 11th layer of Design 3 is cemented and the capillary cracked structures that are formed after a while on the 10th layer increase the grain surface tensions after drying. Additionally, the matric suction of the 10th layer started to increase three days after the 11th layer was deposited upon it, and reached a value of −14 kPa at the end of storing. In addition to these results, the results regarding layer oxygen consumption, which are an important parameter for oxidation, are given in Fig. 15 for Design 1, Fig. 16 for Design 2, and Fig. 17 for Design 3 respectively.
Fig. 16. Oxygen consumption of Design 2 (cement in Layer 1 only).
Fig. 17. Oxygen consumption of Design 3 (cement in Layers 1 and 11).
at a level of 19.8%. The reason for this is thought to be that the seepage water remains in the layer for a while and cuts the air contact with the water-saturated paste material. It was found that the oxygen content in the fifth layer tended to decrease until the storing of the final layer, and that the lowest value fallen to was in the level of ∼17%. Given this trend, it is understood that the contact with oxygen in the air is cut off by being between the four underlying layers and the six overlying layers. In the storing of the 10th and 11th layers, the oxygen content of the 10th layer was measured as changing between very close values. However, Fig. 15 shows that the oxygen content, which falls to ∼18%, approached the oxygen value of 20.95% in the air after 16 days. This increase also occurs in a similar way in the first and fifth layers. This increase is thought to be caused by the cracks forming on the surface of the paste material moving inside due to the increase in water absorption potentials of the layers as well as the surface stresses of the particles. As shown in Figs. 16 and 17, the oxygen content of the first layers of Design 2 and 3 show a sudden decrease in levels to around 14% after the initial storing since they are cementitious. In all three designs, the volumetric water content of the first, fifth and 10th layers tried to stabilize after the first paste layer was stored. Also, the effect of the matric suction between the layers was observed, and it was strong for first cemented layers in Design 2 and 3. At the end of the storing process, as the oxygen consumption values remain below 20.95% from the measurements of the first, fifth and 10th layers of these two designs, it is understood that contact with air does not occur. Therefore, as stated in the study by Yilmaz et al. (2014), it can be explicitly expressed with this study that the cemented layer takes on a barrier role due to its lesser and slower permeability, keeping the upper layers saturated. Since the diffusion of oxygen to the layers with a higher degree of saturation occurs at a minimum level, the AMD risk of
3.3. Oxygen consumption According to Fig. 15, it is understood that the bottom layer in Design 1 comes into contact with oxygen in the air due to the cracking that forms shortly after the initial storing. However, measurements showed that after each layer of storing followed by the storing of the 3rd layer, the oxygen content within the first layer decreased to 17.5–16.5% and it increased again during the eight days' drying process, and remained
Fig. 15. Oxygen consumption for Design 1 (no cement). 38
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Design-1
Design-2
Design-3
14 ALKALINE
12 10 pH
8 6 4 2
ACID
0 0
1
2
3
4
5
6 7 Layers
8
9
10
11
12
Fig. 18. pH of seepage in three different designs.
said that this theory supports the decrease in the amount of dissolved ions in designs with cemented layers.
these layers is reduced (Figs. 16 and 17). 3.4. pH- electrical conductivity (EC)
4. Conclusion The pH and EC values of seepage water collected after the depositing of each layer were measured. The pH changes are shown in Fig. 18. The EC values are also given in Fig. 19. As seen in Fig. 18, the pH values in Design 1 are often lower than 7. Jarosikova et al. (2017) reported in their study that metals (Cd, Cu, Pb, Zn) exhibit the highest leaching at low pH or under acidic conditions (pH 3–6). On the other hand, it is understood that the cemented layers in Designs 2 and 3 do not allow the pH values to fall below 10 because they generate alkaline silica reactions in these layers after solving the alkali minerals that act as a barrier. The sulfide oxidation on seepage quality was reduced through the presence of minerals and cement that buffered pH (Verburg and Oliveira, 2016). In Fig. 19, it can be seen that the EC values of Design 1 are greater than 3 after the deposition of first layer, and this state continues in each subsequent deposition of other layers. In Designs 2 and 3, it was determined that the EC values started a downward trend and that ion dissolution decreased according to Design 1 after deposition of the first layer. As a result, the cement additive keeps the pH values of the seepage in an alkaline environment so that it minimizes the risks of AMD and heavy metals mobilization at low pH values. Furthermore, it can be
Design-1
In this study, three different designs were attempted for SPD; and the values of volumetric water content, matric suction, and oxygen consumption were measured. In this context, measurements showed that the first layers remained above the saturation limit of 53% volumetric water content in the three different designs. When the fifth layers were examined, it was determined that the volumetric water content value fell below 50% in Design 1, whereas it was well above the saturation limit in Design 2 and Design 3. Also, as the number of layers increased, the effect of the matric suction between the layers of the paste material was observed and it was found that the volumetric water content of the 10th layers of three different designs stabilized at 40–50% by the end of the pouring period. It was observed that the volumetric water content value decreased rapidly immediately after the pouring of the paste layers, remained constant for a while and then somehow slowly started to decrease again. This basis of this change can be expressed as time-dependent evaporation and the diffusion of the pore waters away from the medium found between the particles of paste material that is consolidated under its own load. In this study, the bottom and top cemented layers in
Design-2
Design-3
5
EC (mS/cm)
4 3 2 1 0 0
1
2
3
4
5
6 Layers
7
8
9
Fig. 19. EC values of seepage in three different designs. 39
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Design 3 and only the bottom cemented layer in Design 2 in particular made a positive contribution to pH and EC values. These designs are more prominent in terms of preventing AMD and heavy metals mobilization. As can be seen from this study, the bottom and top cemented paste layers ensure that the stored materials are more reliable in terms of physical and chemical properties. It was determined that the lower layers of the stored material are stable within a certain water content range under their own weight. It was also shown that the topmost cemented paste layer could prevent surface water and oxygen diffusion. Design 3 comes to the forefront according to all these evaluations. This design will reduce the environmental risks the most and will then be a suitable method for improvement conditions. It is understood in this study that cement has resistance against the physical and chemical effects that can and do occur particularly in atmospheric conditions. However, cement is a material that increases cost. For this reason, it may be possible to concentrate on different additive materials that can be used as a substitute. In subsequent studies that can be done in this area, pozzolanic materials (such as fly ash) in particular can be investigated by mixing at different rates instead of cement.
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Acknowledgements This work was supported by the Scientific and Technological Research Council of Turkey (Project Numbers: 116M721) and also by the Scientific Research Projects Coordination Unit of Istanbul University (Project Nos: 19062, 40376, 27845, 21527, 20817). The authors are grateful to the Executive Secretariat of Scientific Research Projects of Istanbul University. References Bascetin, A., Tuylu, S., 2018. Application of Pb-Zn tailings for surface paste disposal: geotechnical and geochemical observations. Int. J. Min. Reclamat. Environ. 32 (5), 312–326. https://doi.org/10.1080/17480930.2017.1282411. Bascetin, A., Tuylu, S., Adiguzel, D., Ozdemir, O., 2017. Field properties and performance of surface paste disposal. In: Yilmaz, E., Fall, M. (Eds.), Paste Tailings Management. Springer, Cham. Bascetin, A., Tuylu, S., Ozdemir, O., Adiguzel, D., Benzaazoua, M., 2018. An investigation of crack formation in surface paste disposal method for pyritic Pb–Zn tailings. Int. J. Environ. Sci. Technol. 15 (2), 281–288. Benzaazoua, M., Perez, P., Belem, T., Fall, M., 2004. A laboratory study of the behaviour of surface paste disposal. In: Proceedings of the 8th International Symposium on
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