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Coagulation enhanced electrokinetic settling of mature fine oil sands tailings
International Journal of Mining Science and Technology xxx (2018) xxx–xxx
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International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst
Coagulation enhanced electrokinetic settling of mature fine oil sands tailings Shriful Islam a,b,⇑, Julie Q. Shang a a b
Department of Civil and Environmental Engineering, The University of Western Ontario, Ontario N6A 3K7, Canada Department of Civil and Environmental Engineering, Shahjalal University of Science and Technology, Sylhet 3114, Bangladesh
a r t i c l e
i n f o
Article history: Received 10 August 2017 Received in revised form 6 November 2017 Accepted 18 April 2018 Available online xxxx Keywords: Electrokinetics Electrophoresis Coagulation Settling Voltage gradient MFT
a b s t r a c t The mature fine oil sand tailings (MFT) remain as slurry in the tailings pond for long time. The dewatering and consolidation of MFT for sustainable management is an important task for the mining industry. The objective of this study is to accelerate electrokinetic settling of MFT solids in suspensions in presence of optimal coagulant. In the first phase, optimal coagulant and coagulant dosage for settling of suspension are identified, i.e., ferric chloride at 350 mg/L. It is found that the chemical treatment is not much effective; the final solid content of the sediment is only 6.48% from an initial of 5%. In the second phase, combined coagulation and electrokinetic treatment is carried out to enhance the settling effect. The direct electric current is applied in continuous and intermittent modes on MFT suspensions placed in electrokinetic cell. The results show the final solid content reaches 23.74% under the combined application of 350 mg/L ferric chloride and 218.75 V/m applied voltage gradient in the continuous mode. The intermittent current mode with 40% save in power consumption produces a settled sediment having 20.84% final solid content. Ó 2018 Published by Elsevier B.V. on behalf of China University of Mining & Technology. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction The mature fine tailings (MFT), the fine fractions of solids in oil sands tailings, take years to reach solid contents of 30–35% by gravity [1,2] and complete consolidation of untreated MFT may take thousands of years [3,4]. The oil sand industries use large quantity of water to extract bitumen from oil sands. The slow settling of fine particles in the tailings is challenging to the management of containment facilities for a long time beyond the mine closure [5]. The aim of settling of tailings is to generate an underflow with high solids concentration, which will ease further dewatering. In this study, laboratory experiments are conducted to optimize the operating conditions for MFT settling using combined coagulant and electrokinetic treatment. The Athabasca oil sands reserve in the northern Alberta, Canada, is the largest in the world, which contains on average 12% bitumen, 85% minerals (mainly quartz, silts and clay) and 5% water. The major clay minerals are kaolinite (40–70%), illite (28–45%) and trace montmorillonite [6]. The mature fine oil sands tailings
⇑ Corresponding author at: Department of Civil and Environmental Engineering, Shahjalal University of Science and Technology, Sylhet 3114, Bangladesh. E-mail address: [email protected] (S. Islam).
usually consist of about 33% solid content with the average liquid limit and plastic limit being 40–75% and 10–20%, respectively [3,7]. Coagulation is a common technique in water and wastewater treatment to enhance sedimentation of fine solids. Aluminum sulphate, ferrous sulphate and ferric chloride are commonly used as coagulants [8,9]. Ferric chloride has been used as an effective coagulant for the sedimentation of contaminated Welland River sediment [10]. Pourrezaei and El-Din have used alum (Al2(SO4)318H2O) as coagulant to treat oil sands processing water [11]. Various types of coagulants and flocculants are used in settling and dewatering of mine tailings, and among them Al3+ and Fe3+ salts are most commonly used since trivalent ions are more effective than monovalent or divalent ions [12]. Jar tests are universally used to assess the optimum dose for coagulation in a given medium [13]. In this research the common inorganic salts, i.e. ferric chloride, aluminum sulphate and aluminum chloride are examined as coagulant to settle the MFT suspension. The cations of coagulant attract the negatively charged particles and make larger flocs of particles in the suspensions, and enhance gravity sedimentation. Electrophoresis is the movement of ions or other charged particles towards electrodes under the influence of a direct electric current (DC). The charged particles move towards the oppositely charged electrodes. Settling of fine solids can be accelerated with properly designed electrode layout. Chemical reactions,
https://doi.org/10.1016/j.ijmst.2018.04.012 2095-2686/Ó 2018 Published by Elsevier B.V. on behalf of China University of Mining & Technology. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Please cite this article in press as: Islam S, Shang JQ. Coagulation enhanced electrokinetic settling of mature fine oil sands tailings. Int J Min Sci Technol (2018), https://doi.org/10.1016/j.ijmst.2018.04.012
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i.e., oxidation and reduction, take place at the anode and cathode, respectively. The United of States of Bureau of Mines conducted research on electrokinetic dewatering of coal mine tailings in the 1960s [14]. The CSIRO of Australia performed research in the 1970s on electrokinetic dewatering of tailings [15]. Shang discussed the principle of electrokinetic sedimentation of clay suspension [16]. Settling of particles are occurred in two stages: free settling (solids settle at a constant rate when the solid concentration are low in the suspension) and hindered settling (when solid concentrations are high but consolidation of solids is not started). Following Shang [16] and Islam and Shang [17], the equations for free settling and hindered settling stages under a direct current can be expressed as:
vf
¼ bf u
ð1Þ
v h ¼ bh unr
ð2Þ
2. Materials and methods
where vf is the suspension’s settling velocity due to DC electric current; u is the individual particle’s settling velocity; b is the factor relating particle’s selling velocity and suspension’s settling velocity; n represents the porosity in the suspension and r stands for sedimentation coefficient. According to Russel et al. [18], particle velocity, u (m/s) induced by electric current is a functions of zeta potential which depend on the surface charge of particles and can be expressed as:
u¼
fEew
ð3Þ
l
where f stands for zeta potential, V; E represents voltage gradient, V/m; ew is the permittivity of the fluid, F/m; and m is the viscosity of the fluid, Ns/m2. Replacing Eq. (3) into Eqs. (1) and (2),
vf
¼ bf
v h ¼ bh
fEew
ð4Þ
l
fEew
l
neutralized with application of coagulants. The concept of combined application of electric current is to use self-weight of large flocculated particles (high gravitational force) and activity of electrophoresis together. The efficiency of the combined application will be compared with the lone application of electrophoresis that is done in first part of this research [17]. The objective of this research is to accelerate the electrokinetic settling of MFT suspensions in EK cell with the addition of optimal coagulant. The coagulant-enhanced electrokinetic settling is expected to increase the solid concentrations of MFT in limited time, which will reduce the volume of suspension a lot and the supernatant water can be re-circulated to the oil sands processing system.
ð5Þ
nr
The electrokinetic settling velocity is the function of applied voltage gradient and properties of fluid (permittivity, viscosity) and solid particles (zeta potential, a function of surface charge of particles). These equations discussed above have been validated in first part of this research where only electric current has been used to accelerate gravitational settling and so the interested readers are requested to go to Ref. [17]. When a direct current is passing from anode (+Ve) to cathode (Ve) in water, oxidation and reduction happens at anode and cathode, respectively. The electrochemical reactions in anode and cathode release oxygen and hydrogen gas, respectively, from electrolysis of water, which are:
2H2 O 4e ! O2 +4Hþ
The first phase of this study includes two steps: jar tests to decide the coagulant and optimum coagulant dosage, and cylinder coagulation tests using the results found in jar tests. The second phase of the study consists of combined applications of coagulant and DC electric current to thicken MFT suspensions. The electric current is applied in continuous and intermittent modes. The properties of mature fine oil sands tailings (collected from the disposal pond in Fort McMurray, Alberta, Canada.) used in this study are presented in Table 1 [17]. The particle size analysis shows that the MFT contains 82.5% silt and 17.5% clay-sized solids (Fig. 1), and specific gravity of particles is 2.58. The MFT solids contain quartz, illite, and kaolinite [17,21]. The physical characteristics of current MFT are similar to that of the raw MFT studied by Roshani et al. [22]. The MFT suspensions for this study are prepared in three large containers by mixing MFT and deionized water to solid concentrations of 5%, 10%, and 15% (mass/mass). The suspensions are sealed and stored in room temperature (20–22 °C). The suspensions are mixed thoroughly by a mechanical mixer before testing, and the sample’s initial solid content is checked randomly to ensure the predetermined initial solid content.
(At anode)
H2 O+4e ! 2H2 +4OH
(At cathode)
ð6Þ ð7Þ
The above electrochemical reactions can corrode the electrodes if not properly selected. Electrophoresis has been introduced to accelerate settling of river sediments [10], cohesive soils [19] and oil sand tailings [17,20]. The combined application of electrophoresis and coagulation has been done for the sedimentation of river sediments by Buckland el al. [10]. The combined application of EK and coagulation for very fine tailings like MFT has not been found in the open literature. As discussed before, the efficiency of electrophoresis enhances with the increase in surface charge of the particles. On the other hand, surface charge of particles is reduced and partially
2.1. Experimental approach 2.1.1. Jar tests procedure Jar tests were carried out to select the most effective coagulant and its optimal dosage for settling MFT suspensions. Alum (Al2(SO4)318H2O), aluminum chloride (AlCl3) and ferric chloride (FeCl3) were tested. The conditions for jar tests are summarized in Table 2. The mixing paddles were placed in six 500 mL beakers filled with MFT suspensions. The prepared stock coagulant solutions (10 g/L concentration) were added by titration to the
Table 1 Properties of mature fine oil sands tailings [17]. Properties
MFT composition
Water content w (%) Water pH Viscosity of MFT liquids at 20 °C (cP) EC of MFT (mS/cm) Oil and grease (kg/m3) Specific gravity Gs Zeta potential (mV) Atterberg limits
158 8.19 14.3
Organic matter (%) Grain size
Plastic limit, PL (%) Liquid limit, LL (%) Plasticity index, PI (%) 17.9 Sand (%) Silt (%) Clay (%)
3.51 1.67 2.58 52.6 36.0 54.4 18.4 0.0 82.5 17.5
Please cite this article in press as: Islam S, Shang JQ. Coagulation enhanced electrokinetic settling of mature fine oil sands tailings. Int J Min Sci Technol (2018), https://doi.org/10.1016/j.ijmst.2018.04.012
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Table 3 Conditions of coagulation tests at cylinder. Conditions
Specific information/values
No. of cylinders Size of cylinders
7 1L 35 cm height 6.03 cm diameter Borosilicate glass 4 5 Ferric chloride (FeCl3) Varies between 0 and 450
Cylinder making material Thickening period (h) Initial solid content (%) Type of coagulant Coagulant dosage in the suspension (mg/L) Recorded room temperature during tests (°C) Fig. 1. Grain size distribution of mature fine oil sands tailings.
Table 2 Conditions of jar experiments. Conditions
Specific information/values
No. of beakers Size of beakers
6 500 mL 9 cm height Borosilicate glass 6 100 (rapid mixing) 20 (slow mixing) 2 (rapid mixing) 20 (slow mixing) 60 Initial solid content Coagulants Coagulant dosage 5, 10, 15 Alum (Al2(SO4)318H2O) Aluminum chloride (AlCl3) Ferric chloride (FeCl3) Varies between 0 and 700
Beaker making material No. of mixing blades Revolution of mixing blades (r/min) Mixing periods (min) Settlement period (min) Variables in tests
Initial solid content (%) Types of coagulants
Coagulant dosage in the suspension (mg/L) Recorded room temperature during tests (°C) Test identification: 5Al = Jar test with alum for 5% MFT 5AC = Jar test with AlCl3 for 5% MFT 5FC = Jar test with FeCl3 for 5% MFT 10Al = Jar test with alum for 10% MFT 10AC = Jar test with AlCl3 for 10% MFT
20–22
10FC = Jar test with FeCl3 for 10% MFT 15Al = Jar test with alum for 15% MFT 15AC = Jar test with AlCl3 for 15% MFT 15FC = Jar test with FeCl3 for 15% MFT
beakers to reach predetermined coagulant dosages, as summarized in Table 2. A blank sample was kept in a beaker as the control test, in which no coagulant was added. All jar tests were performed in 3 steps, i.e., 2 min mixing at 100 r/min (revolution per minute),
Test identification: 1CC = Control cylinder coagulation test (0 FeCl3) 3CC = Cylinder coagulation test with 200 m/L FeCl3 5CC = Cylinder coagulation test with 350 m/L FeCl3 7CC = Cylinder coagulation test with 450 m/L FeCl3
20–22
2CC = Cylinder coagulation test with 100 m/L FeCl3 4CC = Cylinder coagulation test with 300 m/L FeCl3 6CC = Cylinder coagulation test with 400 m/L FeCl3
20 min mixing at 20 r/min, and 60 min settling. The rapid mixing stage (100 r/min) disperse alum solution and mixes with the particles in MFT suspensions. In the slow mixing stage (20 r/min) the destabilized particles were mixed slowly and collided to each other, forming larger agglomerates (floc). Finally, the flocs settled by gravity to form sediment at the bottom of the jar in the settling phase. All jar tests were conducted at room temperature (20–22 °C). 2.1.2. Cylinder coagulation test Based on the results of jar tests, the cylinder coagulation tests to enhance settling were conducted using ferric chloride in the MFT suspension of 5% initial solid content (ISC). Seven graduated cylinders of 1 L volume (35 cm height, 6.03 cm diameter, made of borosilicate glass) were used in the tests, with conditions summarized in Table 3. The cylinders were filled up to the mark with the MFT suspension and the stock solutions of ferric chloride were added by titration into the cylinders in concentrations shown in Table 3. All the tests were conducted at room temperature (20–22 °C). 2.1.3. Combined coagulation and electrokinetic settling tests 2.1.3.1. Continuous application of electric current. A column for combined coagulant and electrokinetic settling tests was designed and constructed (Fig. 2), which was used in electrokinetic thickening of
Fig. 2. Electrokinetic settling column set-up [17].
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simultaneously, with the ferric chloride dosage 350 mg/L, for 7 h. The applied voltage gradients varied between 0 V/m and 281 V/ m. The test conditions for combined coagulation and electrokinetic settling are summarized in Tables 4 and 5.
Table 4 Conditions of combined coagulation and electrokinetic settling tests. Conditions
Specific information/values
Dimensions of the column
1 L volume 16.8 cm height 8.9 cm inside diameter Plexiglass
Column material Type of electrodes: Anode Cathode Vertical distance between anode and cathode (cm) Power source Initial solid content (%) Applied voltage (V) Coagulant dosage (mg/L) Thickening period (h) Recorded room temperature during tests (°C)
Titanium coated with iridium oxide Stainless steel mesh 16 DC power supply 5 Varies between 0 and 45 350 ferric chloride 7 16.5–18
mature fine oil sands tailings, Part 1 of this research [17,23]. Strong and transparent plexiglass was used in preparing the confining cell of the column, which is 16.8 cm in height with inside and outside diameter of 8.9 cm and 9.02 cm, respectively. The electrodes were placed in the column, in parallel at 16 cm spacing, with the cathode at the top and anode at the bottom. The anode was made of titanium mesh iridium oxide coating, and the cathode was made of woven wire stainless steel mesh, which were successfully used in electrophoresis and electroosmotic dewatering of oil and sands tailings with no noticed corrosion [17,20,21]. Both electrodes had the wire thickness of 0.16 cm and they were connected to a DC power supply to generate the electron movement into the suspension. The openings in the electrodes allowed the generated gases to escape from the suspension and removal of sediment from base of the column. The MFT suspensions of 5% solids were used in all the tests as the higher initial solid concentration reduced the efficiency of settling. The chemical coagulant and DC current were applied
2.1.3.2. Intermittent application of electric current. The effect of intermittent current and coagulant was studied in the same column described in Section 2.1.3.1. The 5% MFT suspension was poured in the column and tested for 7 h. The intermittent current was applied on the suspension along with 350 mg/L ferric chloride dosage. The applied voltage gradient was 219 V/m under varying power consumption coefficients, v. However, some experiments were executed with applied voltage gradient of 94 V/m and 156 V/m for the regression model development. Table 6 shows the details of test conditions. Seven series of tests were carried out to observe the effect of intermittent current, all with 350 mg/L ferric chloride dosage, i.e., the control (v = 0), five tests with v = 50%, 60%, 70%, 80%, and 90%, and continuous DC current (v = 100%). In five intermittent current tests the power on time was kept as 10 min (constant) and power off time are 10 min, 6.67 min, 4.29 min, 2.5 min, and 1.11 min to maintain the predetermined power consumption coefficient of 50%, 60%, 70%, 80%, and 90%, respectively. The reason for using intermittent electric current is to reduce the effect of gas generated due to electrochemical reactions on settling of particles and to observe how the settling efficiency changes with power discontinuity. 3. Results and discussion 3.1. Jar tests for selection of coagulant and optimum dosage Fig. 3 shows the variation of height of mudline and final solid content with varying coagulant dosage for the MFT suspension of 5% initial solid content with different coagulants. Without coagulant no settling was observed within testing periods of 82 min
Table 5 Duration of free settling, free settling velocity, effective treatment time and electrical resistivity under combined electrokinetics and coagulation for MFT suspension. Test ID number
Voltage gradient (V/m)
Ferric chloride dose (mg/L)
Duration of free settling (min)
Free settling velocity (cm/min)
Start of consolidation/ Effective treatment time (min)
Electrical Resistivity (X-m)
1ECC 1EC 2EC 3EC 4EC 5EC 6EC 7EC 8EC 9EC
0 0 62.5 94 125 156 188 219 250 281
0 350 350 350 350 350 350 350 350 350
225 285 270 240 210 180 120 105 105
0.02 0.023 0.03 0.033 0.04 0.047 0.06 0.067 0.073
390 345 345 375 330 285 255 240 225
13.5 12.2 12.4 13 13.6 13.6 13.2 12.6
Note: EC = Combined electrokinetics and coagulant, ECC = Control test in combined electrokinetics and coagulant application.
Table 6 Duration of free settling, free settling velocity and effective treatment time under the combined intermittent applied voltage gradient and 350 mg/L ferric chloride for MFT. Test ID number
Power consumption coefficient v (%)
Ferric chloride dose (mg/L)
Duration of free settling (min)
Free settling velocity (cm/min)
Start of consolidation/Effective treatment time (min)
1ICC 1IC 2IC 3IC 4IC 5IC 6IC 7IC
0 0 50 60 70 80 90 100
0 350 350 350 350 350 350 350
225 135 120 105 90 90 120
0.02 0.04 0.044 0.051 0.055 0.058 0.06
390 390 390 375 345 285 255
Note: IC = Combined intermittent electrokinetics and coagulant, ICC = Control test in combined intermittent electrokinetics and coagulant application.
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Fig. 3. Height of mudline and final solid content against coagulant dosage with different coagulants for 5% MFT.
and the solid content of suspensions remained constant. The final solid content reached maximum 6.06% from an initial of 5% at the end of the test with 350 mg/L ferric chloride as coagulant (test ID 5FC). The results show that ferric chloride is the best among three coagulants (i.e., alum, aluminum chloride, and ferric chloride) tested in terms of accelerating the sedimentation of MFT suspensions. It is believed that this is attributed to the higher electro-negativity and atomic mass of iron (Fe). The electronegativity of Fe3+ and Al3+ is 1.83 and 1.61, respectively, according to The Pauling scale, which means Fe ion attracts negatively charged clay particles more strongly than that of Al. In addition the flocs of ferric chloride compound are heavier than those of aluminum salts because the atomic mass of Fe and Al is 55.8 amu and 27 amu, respectively; hence the former settles more quickly than the later. Amokrane et al. also reported that iron salts seem to be more efficient than that of aluminum as coagulant on landfill leachate in removing turbidity [8]. The result is also consistent with a previous study on the river sediment [10]. Therefore, ferric chloride is selected as the coagulant for settling the MFT suspension. Fig. 4 shows the trends of height of mudline and final solid content with ferric chloride dosage to MFT suspensions of 10% (10FC) and 15% (15FC) initial solid contents (ISC), respectively. It is evident from the observations that the effectiveness of chemical coagulation reduces with increasing suspension solid content as the density of solids in the suspension grows.
Fig. 5. Relationship between height of mudline and elapsed time under varying concentrations of ferric chloride.
consistent with a previous study on MFT [23,24]. The trend lines are similar, i.e. initially a steep straight line, followed by a parabolic shaped line and finally a flat straight line marks the start of consolidation. As shown in Fig. 5, 350 mg/L ferric chloride dosage, which is test 5CC, produces the best result in the test series. The average settling velocity is the first derivative of the mudline vs. time plot and can be expressed as:
v¼
dH dt
ð8Þ
Fig. 5 shows the relationship between height of mudline and elapsed time under different ferric chloride dosage at pH = 7.25 (suspension pH) found in the study. No settling was observed in the control test (1CC) during the test duration of 4 h, which is
where v is the suspension’s settling velocity; H is the height of mudline; and t is the time. Since there is no gravitational settling registered without coagulant added over the testing period, as shown in Fig. 5, the measured settling velocity is solely attributed to coagulation. Fig. 6 shows the plot of settling velocity against time for different ferric chloride concentrations. The initial flat lines are free settling followed by non-linear hindered settling stage, and finally reached to a plateau (very low velocity), mark the start of consolidation zone. The free settling velocity under 350 mg/L ferric chloride (test 5CC) is 0.067 cm/min (1.17 105 m/s). The plots of the final solid content and turbidity of the supernatant after 4 h settling tests against ferric chloride concentration are shown in Fig. 7. With the most effective ferric chloride dosage of 350 mg/L (test 5CC), the final solid content is 6.48% after 4 h, which indicates an increase of 29.6% compared to the initial solid content of 5%. Turbidity of the supernatants also shows that sample with 350 mg/L of ferric chloride concentration (5CC) yields the best results. The lower dosage could not completely destabilize the suspended solids whereas the higher dosage destabilize colloidal particles [25,26], and form extra clouds due to the presence of excess ions from the coagulant.
Fig. 4. Trends of height of mudline and final solid content with varying ferric chloride dosage for tests 10FC and 15FC.
Fig. 6. Relationship between settling velocity and elapsed time under varying concentrations of ferric chloride.
3.2. Coagulation test in cylinder
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Fig. 7. Plot of final solid content and turbidity of supernatant against ferric chloride dosage.
The electrical conductivity increases linearly with the increase in the ferric chloride dosage in MFT suspensions (Fig. 8), which is consistent with a study on water treatment using ferric chloride [27]. This is attributed to the addition of ferric chloride that increases the ionic concentration in the suspension. On the other hand, the magnitude of zeta potential of the particles decreases proportionally with the increase in the ferric chloride dosage (Fig. 8), which is attributed to the charge neutralization. The effect of suspension pH is observed in cylinder coagulation tests and the pH of the suspensions is adjusted using 0.1 mol/L HCl (hydrochloric acid) and 0.1 mol/L NaOH (sodium hydroxide). Fig. 9 shows the trends of mudline under the optimum ferric chloride dose of 350 mg/L (5CC). The final height of mudline decreases with the increase in the initial suspensions pH (4–6). This is attributed to the presence of hydrogen ions that neutralizes the particle charge and enhances sedimentation. Then the height of mudline increases with the rise in pH at the suspension pH above 6, which may be interpreted as the increased negative surface charge of particles. The results are consistent with Baghvand et al. [28], who reported the optimum water pH (5–6) in removing turbidity. The electric conductivity decreases with increasing pH of the MFT suspensions (Fig. 10) probably due to decrease in surface activity of the clay minerals. On the other hand, the absolute value of zeta potential increases with increasing pH of the MFT suspensions (Fig. 10) because of presence in more ions in the suspension. The results are consistent with a previous study of Guo and Shang [21].
3.3. Application of continuous current combined with optimum coagulant dosage 3.3.1. Effect of applied voltage gradient The lone application of electrophoresis on mature fine oil sands tailings were described in Islam and Shang [17] and Alam et al. [20]. The deposition of mudlines with elapsed time in the
Fig. 8. Variation of electric conductivity and zeta potential with varying dosage of ferric chloride.
Fig. 9. Variation of height of mudline and elapsed time under varying pH for 5CC.
Fig. 10. Variation of electric conductivity and zeta potential with suspension pH for 5CC.
combined application of electrophoresis and coagulant, as shown in Fig. 11, are linear initially, become nonlinear with time, and finally reach a plateau. The initial linear decrease in height of the mudline with time is the free settling stage when the particles interactions are very low. The nonlinear part of the trend lines represent the hindered settling stage due to increased interaction between particles. The plateau of the trend line marks the end of settling and start of self-weight consolidation, which is called compression phase in settling. The increase in the applied voltage gradient decreases the final height of mudline, i.e., increase of the sediment density. When the applied voltage gradient exceeded 219 V/m, the height of mudline remained the same after 7 h testing, i.e., at 3.7 cm. The final solid content in test 7EC after 7 h is 23.74% under the applied voltage gradient of 219 V/m and 350 mg/L ferric chloride, showing an increase of 375% in the solid content (Fig. 12). This represents much higher sediment density compared to that of the
Fig. 11. Deposition of mudline with time under the combined application of applied voltage gradient and ferric chloride.
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Fig. 12. Variation of solid content and increase in solid content in test 7EC.
chemical treatment with ferric chloride in test 5CC (final solid content 6.48%) as discussed before and compared to the lone application of electrophoresis [17,20]. The average settling velocity is calculated using Eq. (7). Fig. 13 shows the settling velocity vs. time in all tests, with results summarized in Table 5. The free settling velocity increases with the increase in the applied voltage gradient. On the other hand, an increase in the applied voltage gradient reduces the time of free settling, leading to the earlier start of the self-weight consolidation. The average free settling velocity of the suspension under the ferric chloride only (control test 1EC) is 0.02 cm/min, while with the applied voltage gradient of 219 V/m (7EC), the free settling velocity is 0.06 cm/min. As a result, the effective settling time decreases with the application of DC current along with ferric chloride. 3.3.2. Power consumption The effective voltage gradient increases linearly with the applied voltage gradient (Fig. 14). As shown the voltage efficiency increases with the increasing applied voltage gradient (up to 219 V/m), ranging from 92.4% to 97%. The results show that the Ti/IrOx-SS316 anode-cathode combination performs well in the presence of ferric chloride. It is also noticed from Fig. 14 that the voltage efficiency is almost constant when the applied voltage gradient is 219 V/m or higher. The current density varies linearly with the change in the applied voltage gradient and the results are plotted in Fig. 15. The electrical resistivity, q of the suspensions is determined from the Ohm’s law,
q¼
E J
UI V
Fig. 14. Variation of effective voltage gradient and voltage efficiency between anode and cathode with applied voltage gradient.
ð9Þ
where q is the electrical resistivity, X-m; E is the applied voltage gradient, V/m; and J is the current density, A/m2. Table 5 shows the suspensions’ electrical resistivity values. The values of electrical resistivity are almost constant as the applied voltage gradient increases the current density linearly and initial solid content of the suspension is same, 5%. The power consumption per unit volume of the suspension for the combined coagulation and electrokinetic settling tests is calculated using the following equation:
P¼
Fig. 13. Evolution of settling velocity with time under the combined applied voltage gradient and ferric chloride.
ð10Þ
where P is the power consumption per unit volume, W/m3; U is the applied voltage, V; I is the electric current, A; and V is the volume of the MFT suspension, m3. The results are presented in Fig. 15, which shows the relationship between power consumption and applied voltage gradient is almost linear until 219 V/m and then increases exponentially. So it is critical to identify the optimal applied voltage gradient to minimize the loss of power. The similar result is visible in lone application of electric current [17]. Under the optimal voltage gradient of 219 V/m (test 7EC), the energy consumption would be 3.51 KW/m3.
Fig. 15. Variation of current density and power consumption with applied voltage gradient combined with ferric chloride.
3.3.3. Zeta potential and turbidity The absolute values of particle zeta potentials increase linearly with the increase in the applied voltage gradient, shown in Fig. 16, because the higher electric current accelerates the flow of electrons. The zeta potential of tailings solids is 14.7 mV under the applied voltage gradient of 219 V/m, test 7EC, in combination with ferric chloride dose of 350 mg/L at the end of 7 h tests. The final turbidity of supernatant water at different applied voltage gradients is shown in Fig. 16. The values of turbidity increases almost linearly with the increase in applied voltage gradient, because generation of gas bubbles is high in high electric current. With the applied voltage gradient of 219 V/m (test 7EC) the water turbidity is 203 NTU at the end of the test. It is noticed that the turbidity of water decreases to 22.7 NTU after 1 h from the end of the test, attributed to dissipation of gas bubbles.
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Fig. 16. Evolution of final zeta potential of particles and turbidity of supernatant with applied voltage gradient under the combined application of electrokinetics and coagulation.
Fig. 18. Evolution of settling velocity with time under the combined intermittent applied voltage gradient and ferric chloride.
3.4. Application of intermittent current 3.4.1. Effect of power consumption coefficient The mudline vs. time plot for seven tests, i.e. the control and six tests under the applied voltage gradient of 219 V/m with various consumption coefficients, are presented in Fig. 17. Within the tests subjected to intermittent current (v = 50% to 90%) and 350 mg/L ferric chloride, the height of mudline decreases with the increase in the power consumption coefficient. The continuous applied voltage gradient (v = 100%, test 7IC) generates the best settling effect. The results are consistent with a previous study on the river sediment [10]. The plot between settling velocity and elapsed time under the combined intermittent current and 350 mg/L ferric chloride is shown in Fig. 18. The details of free settling velocity are summarized in Table 6. The free settling velocity increases with the increase in power consumption coefficient under the same voltage gradient, which reduces the effective treatment time. The final solid content of MFT increases with the increase in power consumption coefficient, as shown in Fig. 19. The percentage increase in the final solid content varies between 289% and 375% for the power consumption coefficient of 50–100%, under the applied voltage of 219 V/m combined with 350 mg/L ferric chloride. The continuous applied voltage gradient (test 7IC) shows the best settling effect. From the economic perspective test 3IC, intermittent current with 60% power consumption coefficient, produces a good result with sediments of 20.8% final solid content.
Fig. 19. Variation of final solid content and turbidity of supernatant with power consumption coefficient under the combined intermittent applied voltage gradient and 350 mg/L ferric chloride.
3.4.2. Turbidity of supernatant The turbidity of water at varying power consumption coefficient under the applied voltage gradient of 218.75 V/m combined with 350 mg/L ferric chloride is shown in Fig. 19. The turbidities of water range between 113 NTU and 174 NTU for the power consumption coefficient of 50–90%, which shows with the turbidity of water increases with the increase in power consumption coefficient. The reason is long time application of applied electric current accelerates the generation of gas bubbles. It is noticed that the turbidity of water varies from 20.5 NTU to 28.6 NTU one hour from the end of the test, indicating the majority of turbidity is caused by gas bubbles generated by electrochemical reactions in electrokinetic process. 4. Comparison of settling efficiency among coagulation, electrokinetics and combined tests
Fig. 17. Variation of height of mudline with time under the combined application of intermittent applied voltage gradient and ferric chloride.
The comparison of settling effects by coagulation, electrokinetics and the combined application under continuous and intermittent current are tabulated in Table 7. It is observed that settling effect of ferric chloride coagulant is minimal (test 5CC). On the other hand, settling by lone application of electrokinetics significantly increased the solid content from an initial 5% to final 18.75% [17]. After 7 h combined EK and chemical coagulant (350 mg/L ferric chloride) tests, the final solid content was 23.75% and 20.84% under continuous (v = 100%, 7EC/7IC) and intermittent (v = 60%, 3IC) current, respectively. It is evident from this study that application of 350 mg/L ferric chloride as coagulant increases the final solid content of MFT within reasonable testing period of 7 h. The reason of increasing settling efficiency in
Please cite this article in press as: Islam S, Shang JQ. Coagulation enhanced electrokinetic settling of mature fine oil sands tailings. Int J Min Sci Technol (2018), https://doi.org/10.1016/j.ijmst.2018.04.012
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S. Islam, J.Q. Shang / International Journal of Mining Science and Technology xxx (2018) xxx–xxx Table 7 Comparison of thickening results by coagulation, electrokinetics and their combined application. Properties to be compared
Coagulation (350 mg/L CC)
EK (219 V/m) [17]
Combined application (cont., 7EC)
Combined application (int., 3IC)
Initial solid content (%) Final solid content (%) Total treatment time (h) Free settling velocity (cm/min) Duration of free settling (h) Zeta potential of particles (mV) Turbidity of supernatant (NTU)
5 6.48 4 0.067 0.75 22.67 14.4
5 18.75 7 0.047 2.00 14.70 30.6
5 23.75 7 0.060 2.00 16.30 22.7
5 21.57 7 0.050 1.50 22.0
Note: EK = electrokinetics, cont. = continuous electric current, int. = intermittent electric current with v = 60%.
combined application is the enhancement of gravitational force due to high self-weight of flocs of particles that combines with electrophoretic effect. Though the free settling velocity is the highest (0.067 cm/min) for coagulation (5CC), however the durations of free settling under coagulation and electrokinetic settling were 0.75 h and 2 h, respectively, indicating the free settling velocity contributed more with electrokinetics (test 7EC). The final turbidities of supernatant are 14.4 NTU, 30.6 NTU and 22.7 NTU under settling by coagulation, electrokinetics, and combined application, respectively, while the corresponding zeta potentials of particles were 22.6 mV, 14.7 mV, and 16.3 mV. 5. Conclusions The key goal of this study was to increase the solid content of the MFT suspension, which will significantly reduce the volume of tailings, and will reduce the time and cost of tailings management and rehabilitation. Three inorganic chemical coagulants (varying dosage) and mode of electrokinetics (continuous and intermittent current) with varying voltage gradient were experimented. Laboratory testing results of present study show that the application of coagulant is technically feasible in accelerating the electrokinetic settling of MFT suspension, which provides a significant improvement of solid concentration from 5% to 23.75%. The main results and conclusions are summarized as follows: (1) In the series of jar experiments, ferric chloride at 350 mg/L, test 5FC, generated the best coagulation effect in terms of the final solid content of MFT among different coagulants studied at 350 mg/L concentration. However, the effectiveness is judged minimal as the final MFT solid content was 6.48% after 4 h, compared to 5% initial solid content after 4 h in cylinder coagulation tests. (2) The final solid content of MFT was 23.74% from an initial of 5% under the combined continuous applied voltage gradient of 218.75 V/m and 350 mg/L (test 7EC) for 7 h, an increase of 374.5%. The zeta potential of the MFT solids reduced to 8.4 mV after 7 h treatment, which generated instability of suspended solids and promoted settling. The turbidity of water was 22.7 NTU after 1 h from the end of the test when the generated gas dissipated. (3) The constant electric current combined with ferric chloride (test 7EC) generated best settling effect (produced sediments of 23.75% final solid content) because of high selfweight of flocs of particles. Intermittent electric current with 60% power consumption coefficient (test 3IC) reached the final solid content to 20.84%. This optimal condition of intermittent current (test 3IC) is better than that of continuous mode in economic perspective, which saves the power consumption by 40%.
Acknowledgment The authors acknowledge the support of The University of Western Ontario and the Natural Science and Engineering Research Council of Canada (NSERC). References [1] Beier N, Sego D. The oil sands tailings research facility. Geotech News 2008;62 (2):72–7. [2] Jeeravipoolvarn S, Scott JD, Chalaturnyk RJ. 10 M standpipe tests on oil sands tailings: long-term experimental results and prediction. Can Geotech J 2009;46 (8):875–88. [3] Mikula RJ, Kasperski KL, Burns RD, Mackinnon MD. Nature and fate of oil sands fine tailings. In: Schramm LL, editor. Suspensions, fundamentals and applications in the petroleum industry. Washington: American Chemical Society; 1996. p. 677–723. [4] Beier N, Wilson W, Dunmola A, Sego D. Impact of flocculation-based dewatering on the shear strength of oil sands fine tailings. Can Geotech J 2013;50(9):1001–7. [5] Krizek JR. Geotechnics of high water content materials. West Conshohocken: ASTM International; 2000. p. 3–28. [6] Chalaturnyk RJ, Scott JD, Ozum B. Management of oil sands tailings. Petrol Sci Technol 2002;20(9–10):1025–46. [7] FTFC-Fine tailings fundamentals consortium. Advances in oil sands tailings research. Alberta: Alberta department of energy, oil sands and research division; 1995. [8] Amokrane A, Comel C, Veron J. Landfill leachates pre-treatment by coagulation flocculation. Water Res 1997;31(11):2775–82. [9] Uygur A, Kargi F. Biological nutrient removal from pre-treated landfill leachate in a sequencing batch reactor. J Environ Manage 2004;71(1):9–14. [10] Buckland DG, Shang JQ, Mohamedelhassan E. Electrokinetic sedimentation of contaminated well and river sediment. Can Geotech J 2000;37(4):735–47. [11] Pourrezaei P, El-Din MG. Coagulation-flocculation pre-treatment of oil sands process affected water. In: Proceedings of 1st international oil sands tailings conference (IOSTC), Edmonton. Alberta: University of Alberta; 2008. [12] Jewell RJ, Fourie AB. Paste and thickened tailings- a guide. 2nd ed. Australian Centre for Geomechanics: The University of Western Australia, Western Australia; 2006. [13] Reckhow DA, Singer PC, Trussell RR. Ozone as coagulant aid. In: Proceedings of AWWA national conference on ozonation, recent advances and research needs. Denver: AWWARF Denver; 1986. [14] Stanczyk MH, Feld IL. Electro-dewatering tests of Florida phosphate rock slime. vol. 6451 of Report of Investigations: department of the Interior, US Bureau of Mines; 1964. p. 1–19. [15] Lockhart NC. Advances in solid-liquid separation, Columbus. Ohio: Battelle Press; 1986. p. 241–74. [16] Shang JQ. Electrokinetic sedimentation: a theoretical and experimental study. Can Geotech J 1997;34(2):305–14. [17] Islam S, Shang JQ. Electrokinetic thickening of mature fine oil sands tailings. Envion Geotech 2017;4(1):40–55. [18] Russel WB, Saville DA, Schowalter WR. Colloidal dispersions. Cambridge, UK: Cambridge University Press; 1989. [19] Kim SS, Lee M, Kim DH. Electrokinetic settling and sedimentation behavior of cohesive soils in dilute suspension. J Environ Sci Health 2008;43(8):975–84. [20] Alam R, Shang JQ, Islam S. Electrophoresis and its applications in oil sands tailings management. Int J Miner Process 2017;161:41–9. [21] Guo Y, Shang JQ. A study on electrokinetic dewatering of oil sands tailings. Environ Geotech 2014;1(2):121–34. [22] Roshani A, Fall M, Kennedy K. A column study of the hydro-mechanical behavior of mature fine tailings under atmospheric drying. Int J Min Sci Tech 2017;27(2):203–10. [23] Islam S. Thickening of mature fine oil sands tailing. London, Ontario: University of Western Ontario; 2014.
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Contents lists available at ScienceDirect
International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst
Coagulation enhanced electrokinetic settling of mature fine oil sands tailings Shriful Islam a,b,⇑, Julie Q. Shang a a b
Department of Civil and Environmental Engineering, The University of Western Ontario, Ontario N6A 3K7, Canada Department of Civil and Environmental Engineering, Shahjalal University of Science and Technology, Sylhet 3114, Bangladesh
a r t i c l e
i n f o
Article history: Received 10 August 2017 Received in revised form 6 November 2017 Accepted 18 April 2018 Available online xxxx Keywords: Electrokinetics Electrophoresis Coagulation Settling Voltage gradient MFT
a b s t r a c t The mature fine oil sand tailings (MFT) remain as slurry in the tailings pond for long time. The dewatering and consolidation of MFT for sustainable management is an important task for the mining industry. The objective of this study is to accelerate electrokinetic settling of MFT solids in suspensions in presence of optimal coagulant. In the first phase, optimal coagulant and coagulant dosage for settling of suspension are identified, i.e., ferric chloride at 350 mg/L. It is found that the chemical treatment is not much effective; the final solid content of the sediment is only 6.48% from an initial of 5%. In the second phase, combined coagulation and electrokinetic treatment is carried out to enhance the settling effect. The direct electric current is applied in continuous and intermittent modes on MFT suspensions placed in electrokinetic cell. The results show the final solid content reaches 23.74% under the combined application of 350 mg/L ferric chloride and 218.75 V/m applied voltage gradient in the continuous mode. The intermittent current mode with 40% save in power consumption produces a settled sediment having 20.84% final solid content. Ó 2018 Published by Elsevier B.V. on behalf of China University of Mining & Technology. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction The mature fine tailings (MFT), the fine fractions of solids in oil sands tailings, take years to reach solid contents of 30–35% by gravity [1,2] and complete consolidation of untreated MFT may take thousands of years [3,4]. The oil sand industries use large quantity of water to extract bitumen from oil sands. The slow settling of fine particles in the tailings is challenging to the management of containment facilities for a long time beyond the mine closure [5]. The aim of settling of tailings is to generate an underflow with high solids concentration, which will ease further dewatering. In this study, laboratory experiments are conducted to optimize the operating conditions for MFT settling using combined coagulant and electrokinetic treatment. The Athabasca oil sands reserve in the northern Alberta, Canada, is the largest in the world, which contains on average 12% bitumen, 85% minerals (mainly quartz, silts and clay) and 5% water. The major clay minerals are kaolinite (40–70%), illite (28–45%) and trace montmorillonite [6]. The mature fine oil sands tailings
⇑ Corresponding author at: Department of Civil and Environmental Engineering, Shahjalal University of Science and Technology, Sylhet 3114, Bangladesh. E-mail address: [email protected] (S. Islam).
usually consist of about 33% solid content with the average liquid limit and plastic limit being 40–75% and 10–20%, respectively [3,7]. Coagulation is a common technique in water and wastewater treatment to enhance sedimentation of fine solids. Aluminum sulphate, ferrous sulphate and ferric chloride are commonly used as coagulants [8,9]. Ferric chloride has been used as an effective coagulant for the sedimentation of contaminated Welland River sediment [10]. Pourrezaei and El-Din have used alum (Al2(SO4)318H2O) as coagulant to treat oil sands processing water [11]. Various types of coagulants and flocculants are used in settling and dewatering of mine tailings, and among them Al3+ and Fe3+ salts are most commonly used since trivalent ions are more effective than monovalent or divalent ions [12]. Jar tests are universally used to assess the optimum dose for coagulation in a given medium [13]. In this research the common inorganic salts, i.e. ferric chloride, aluminum sulphate and aluminum chloride are examined as coagulant to settle the MFT suspension. The cations of coagulant attract the negatively charged particles and make larger flocs of particles in the suspensions, and enhance gravity sedimentation. Electrophoresis is the movement of ions or other charged particles towards electrodes under the influence of a direct electric current (DC). The charged particles move towards the oppositely charged electrodes. Settling of fine solids can be accelerated with properly designed electrode layout. Chemical reactions,
https://doi.org/10.1016/j.ijmst.2018.04.012 2095-2686/Ó 2018 Published by Elsevier B.V. on behalf of China University of Mining & Technology. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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i.e., oxidation and reduction, take place at the anode and cathode, respectively. The United of States of Bureau of Mines conducted research on electrokinetic dewatering of coal mine tailings in the 1960s [14]. The CSIRO of Australia performed research in the 1970s on electrokinetic dewatering of tailings [15]. Shang discussed the principle of electrokinetic sedimentation of clay suspension [16]. Settling of particles are occurred in two stages: free settling (solids settle at a constant rate when the solid concentration are low in the suspension) and hindered settling (when solid concentrations are high but consolidation of solids is not started). Following Shang [16] and Islam and Shang [17], the equations for free settling and hindered settling stages under a direct current can be expressed as:
vf
¼ bf u
ð1Þ
v h ¼ bh unr
ð2Þ
2. Materials and methods
where vf is the suspension’s settling velocity due to DC electric current; u is the individual particle’s settling velocity; b is the factor relating particle’s selling velocity and suspension’s settling velocity; n represents the porosity in the suspension and r stands for sedimentation coefficient. According to Russel et al. [18], particle velocity, u (m/s) induced by electric current is a functions of zeta potential which depend on the surface charge of particles and can be expressed as:
u¼
fEew
ð3Þ
l
where f stands for zeta potential, V; E represents voltage gradient, V/m; ew is the permittivity of the fluid, F/m; and m is the viscosity of the fluid, Ns/m2. Replacing Eq. (3) into Eqs. (1) and (2),
vf
¼ bf
v h ¼ bh
fEew
ð4Þ
l
fEew
l
neutralized with application of coagulants. The concept of combined application of electric current is to use self-weight of large flocculated particles (high gravitational force) and activity of electrophoresis together. The efficiency of the combined application will be compared with the lone application of electrophoresis that is done in first part of this research [17]. The objective of this research is to accelerate the electrokinetic settling of MFT suspensions in EK cell with the addition of optimal coagulant. The coagulant-enhanced electrokinetic settling is expected to increase the solid concentrations of MFT in limited time, which will reduce the volume of suspension a lot and the supernatant water can be re-circulated to the oil sands processing system.
ð5Þ
nr
The electrokinetic settling velocity is the function of applied voltage gradient and properties of fluid (permittivity, viscosity) and solid particles (zeta potential, a function of surface charge of particles). These equations discussed above have been validated in first part of this research where only electric current has been used to accelerate gravitational settling and so the interested readers are requested to go to Ref. [17]. When a direct current is passing from anode (+Ve) to cathode (Ve) in water, oxidation and reduction happens at anode and cathode, respectively. The electrochemical reactions in anode and cathode release oxygen and hydrogen gas, respectively, from electrolysis of water, which are:
2H2 O 4e ! O2 +4Hþ
The first phase of this study includes two steps: jar tests to decide the coagulant and optimum coagulant dosage, and cylinder coagulation tests using the results found in jar tests. The second phase of the study consists of combined applications of coagulant and DC electric current to thicken MFT suspensions. The electric current is applied in continuous and intermittent modes. The properties of mature fine oil sands tailings (collected from the disposal pond in Fort McMurray, Alberta, Canada.) used in this study are presented in Table 1 [17]. The particle size analysis shows that the MFT contains 82.5% silt and 17.5% clay-sized solids (Fig. 1), and specific gravity of particles is 2.58. The MFT solids contain quartz, illite, and kaolinite [17,21]. The physical characteristics of current MFT are similar to that of the raw MFT studied by Roshani et al. [22]. The MFT suspensions for this study are prepared in three large containers by mixing MFT and deionized water to solid concentrations of 5%, 10%, and 15% (mass/mass). The suspensions are sealed and stored in room temperature (20–22 °C). The suspensions are mixed thoroughly by a mechanical mixer before testing, and the sample’s initial solid content is checked randomly to ensure the predetermined initial solid content.
(At anode)
H2 O+4e ! 2H2 +4OH
(At cathode)
ð6Þ ð7Þ
The above electrochemical reactions can corrode the electrodes if not properly selected. Electrophoresis has been introduced to accelerate settling of river sediments [10], cohesive soils [19] and oil sand tailings [17,20]. The combined application of electrophoresis and coagulation has been done for the sedimentation of river sediments by Buckland el al. [10]. The combined application of EK and coagulation for very fine tailings like MFT has not been found in the open literature. As discussed before, the efficiency of electrophoresis enhances with the increase in surface charge of the particles. On the other hand, surface charge of particles is reduced and partially
2.1. Experimental approach 2.1.1. Jar tests procedure Jar tests were carried out to select the most effective coagulant and its optimal dosage for settling MFT suspensions. Alum (Al2(SO4)318H2O), aluminum chloride (AlCl3) and ferric chloride (FeCl3) were tested. The conditions for jar tests are summarized in Table 2. The mixing paddles were placed in six 500 mL beakers filled with MFT suspensions. The prepared stock coagulant solutions (10 g/L concentration) were added by titration to the
Table 1 Properties of mature fine oil sands tailings [17]. Properties
MFT composition
Water content w (%) Water pH Viscosity of MFT liquids at 20 °C (cP) EC of MFT (mS/cm) Oil and grease (kg/m3) Specific gravity Gs Zeta potential (mV) Atterberg limits
158 8.19 14.3
Organic matter (%) Grain size
Plastic limit, PL (%) Liquid limit, LL (%) Plasticity index, PI (%) 17.9 Sand (%) Silt (%) Clay (%)
3.51 1.67 2.58 52.6 36.0 54.4 18.4 0.0 82.5 17.5
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Table 3 Conditions of coagulation tests at cylinder. Conditions
Specific information/values
No. of cylinders Size of cylinders
7 1L 35 cm height 6.03 cm diameter Borosilicate glass 4 5 Ferric chloride (FeCl3) Varies between 0 and 450
Cylinder making material Thickening period (h) Initial solid content (%) Type of coagulant Coagulant dosage in the suspension (mg/L) Recorded room temperature during tests (°C) Fig. 1. Grain size distribution of mature fine oil sands tailings.
Table 2 Conditions of jar experiments. Conditions
Specific information/values
No. of beakers Size of beakers
6 500 mL 9 cm height Borosilicate glass 6 100 (rapid mixing) 20 (slow mixing) 2 (rapid mixing) 20 (slow mixing) 60 Initial solid content Coagulants Coagulant dosage 5, 10, 15 Alum (Al2(SO4)318H2O) Aluminum chloride (AlCl3) Ferric chloride (FeCl3) Varies between 0 and 700
Beaker making material No. of mixing blades Revolution of mixing blades (r/min) Mixing periods (min) Settlement period (min) Variables in tests
Initial solid content (%) Types of coagulants
Coagulant dosage in the suspension (mg/L) Recorded room temperature during tests (°C) Test identification: 5Al = Jar test with alum for 5% MFT 5AC = Jar test with AlCl3 for 5% MFT 5FC = Jar test with FeCl3 for 5% MFT 10Al = Jar test with alum for 10% MFT 10AC = Jar test with AlCl3 for 10% MFT
20–22
10FC = Jar test with FeCl3 for 10% MFT 15Al = Jar test with alum for 15% MFT 15AC = Jar test with AlCl3 for 15% MFT 15FC = Jar test with FeCl3 for 15% MFT
beakers to reach predetermined coagulant dosages, as summarized in Table 2. A blank sample was kept in a beaker as the control test, in which no coagulant was added. All jar tests were performed in 3 steps, i.e., 2 min mixing at 100 r/min (revolution per minute),
Test identification: 1CC = Control cylinder coagulation test (0 FeCl3) 3CC = Cylinder coagulation test with 200 m/L FeCl3 5CC = Cylinder coagulation test with 350 m/L FeCl3 7CC = Cylinder coagulation test with 450 m/L FeCl3
20–22
2CC = Cylinder coagulation test with 100 m/L FeCl3 4CC = Cylinder coagulation test with 300 m/L FeCl3 6CC = Cylinder coagulation test with 400 m/L FeCl3
20 min mixing at 20 r/min, and 60 min settling. The rapid mixing stage (100 r/min) disperse alum solution and mixes with the particles in MFT suspensions. In the slow mixing stage (20 r/min) the destabilized particles were mixed slowly and collided to each other, forming larger agglomerates (floc). Finally, the flocs settled by gravity to form sediment at the bottom of the jar in the settling phase. All jar tests were conducted at room temperature (20–22 °C). 2.1.2. Cylinder coagulation test Based on the results of jar tests, the cylinder coagulation tests to enhance settling were conducted using ferric chloride in the MFT suspension of 5% initial solid content (ISC). Seven graduated cylinders of 1 L volume (35 cm height, 6.03 cm diameter, made of borosilicate glass) were used in the tests, with conditions summarized in Table 3. The cylinders were filled up to the mark with the MFT suspension and the stock solutions of ferric chloride were added by titration into the cylinders in concentrations shown in Table 3. All the tests were conducted at room temperature (20–22 °C). 2.1.3. Combined coagulation and electrokinetic settling tests 2.1.3.1. Continuous application of electric current. A column for combined coagulant and electrokinetic settling tests was designed and constructed (Fig. 2), which was used in electrokinetic thickening of
Fig. 2. Electrokinetic settling column set-up [17].
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simultaneously, with the ferric chloride dosage 350 mg/L, for 7 h. The applied voltage gradients varied between 0 V/m and 281 V/ m. The test conditions for combined coagulation and electrokinetic settling are summarized in Tables 4 and 5.
Table 4 Conditions of combined coagulation and electrokinetic settling tests. Conditions
Specific information/values
Dimensions of the column
1 L volume 16.8 cm height 8.9 cm inside diameter Plexiglass
Column material Type of electrodes: Anode Cathode Vertical distance between anode and cathode (cm) Power source Initial solid content (%) Applied voltage (V) Coagulant dosage (mg/L) Thickening period (h) Recorded room temperature during tests (°C)
Titanium coated with iridium oxide Stainless steel mesh 16 DC power supply 5 Varies between 0 and 45 350 ferric chloride 7 16.5–18
mature fine oil sands tailings, Part 1 of this research [17,23]. Strong and transparent plexiglass was used in preparing the confining cell of the column, which is 16.8 cm in height with inside and outside diameter of 8.9 cm and 9.02 cm, respectively. The electrodes were placed in the column, in parallel at 16 cm spacing, with the cathode at the top and anode at the bottom. The anode was made of titanium mesh iridium oxide coating, and the cathode was made of woven wire stainless steel mesh, which were successfully used in electrophoresis and electroosmotic dewatering of oil and sands tailings with no noticed corrosion [17,20,21]. Both electrodes had the wire thickness of 0.16 cm and they were connected to a DC power supply to generate the electron movement into the suspension. The openings in the electrodes allowed the generated gases to escape from the suspension and removal of sediment from base of the column. The MFT suspensions of 5% solids were used in all the tests as the higher initial solid concentration reduced the efficiency of settling. The chemical coagulant and DC current were applied
2.1.3.2. Intermittent application of electric current. The effect of intermittent current and coagulant was studied in the same column described in Section 2.1.3.1. The 5% MFT suspension was poured in the column and tested for 7 h. The intermittent current was applied on the suspension along with 350 mg/L ferric chloride dosage. The applied voltage gradient was 219 V/m under varying power consumption coefficients, v. However, some experiments were executed with applied voltage gradient of 94 V/m and 156 V/m for the regression model development. Table 6 shows the details of test conditions. Seven series of tests were carried out to observe the effect of intermittent current, all with 350 mg/L ferric chloride dosage, i.e., the control (v = 0), five tests with v = 50%, 60%, 70%, 80%, and 90%, and continuous DC current (v = 100%). In five intermittent current tests the power on time was kept as 10 min (constant) and power off time are 10 min, 6.67 min, 4.29 min, 2.5 min, and 1.11 min to maintain the predetermined power consumption coefficient of 50%, 60%, 70%, 80%, and 90%, respectively. The reason for using intermittent electric current is to reduce the effect of gas generated due to electrochemical reactions on settling of particles and to observe how the settling efficiency changes with power discontinuity. 3. Results and discussion 3.1. Jar tests for selection of coagulant and optimum dosage Fig. 3 shows the variation of height of mudline and final solid content with varying coagulant dosage for the MFT suspension of 5% initial solid content with different coagulants. Without coagulant no settling was observed within testing periods of 82 min
Table 5 Duration of free settling, free settling velocity, effective treatment time and electrical resistivity under combined electrokinetics and coagulation for MFT suspension. Test ID number
Voltage gradient (V/m)
Ferric chloride dose (mg/L)
Duration of free settling (min)
Free settling velocity (cm/min)
Start of consolidation/ Effective treatment time (min)
Electrical Resistivity (X-m)
1ECC 1EC 2EC 3EC 4EC 5EC 6EC 7EC 8EC 9EC
0 0 62.5 94 125 156 188 219 250 281
0 350 350 350 350 350 350 350 350 350
225 285 270 240 210 180 120 105 105
0.02 0.023 0.03 0.033 0.04 0.047 0.06 0.067 0.073
390 345 345 375 330 285 255 240 225
13.5 12.2 12.4 13 13.6 13.6 13.2 12.6
Note: EC = Combined electrokinetics and coagulant, ECC = Control test in combined electrokinetics and coagulant application.
Table 6 Duration of free settling, free settling velocity and effective treatment time under the combined intermittent applied voltage gradient and 350 mg/L ferric chloride for MFT. Test ID number
Power consumption coefficient v (%)
Ferric chloride dose (mg/L)
Duration of free settling (min)
Free settling velocity (cm/min)
Start of consolidation/Effective treatment time (min)
1ICC 1IC 2IC 3IC 4IC 5IC 6IC 7IC
0 0 50 60 70 80 90 100
0 350 350 350 350 350 350 350
225 135 120 105 90 90 120
0.02 0.04 0.044 0.051 0.055 0.058 0.06
390 390 390 375 345 285 255
Note: IC = Combined intermittent electrokinetics and coagulant, ICC = Control test in combined intermittent electrokinetics and coagulant application.
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Fig. 3. Height of mudline and final solid content against coagulant dosage with different coagulants for 5% MFT.
and the solid content of suspensions remained constant. The final solid content reached maximum 6.06% from an initial of 5% at the end of the test with 350 mg/L ferric chloride as coagulant (test ID 5FC). The results show that ferric chloride is the best among three coagulants (i.e., alum, aluminum chloride, and ferric chloride) tested in terms of accelerating the sedimentation of MFT suspensions. It is believed that this is attributed to the higher electro-negativity and atomic mass of iron (Fe). The electronegativity of Fe3+ and Al3+ is 1.83 and 1.61, respectively, according to The Pauling scale, which means Fe ion attracts negatively charged clay particles more strongly than that of Al. In addition the flocs of ferric chloride compound are heavier than those of aluminum salts because the atomic mass of Fe and Al is 55.8 amu and 27 amu, respectively; hence the former settles more quickly than the later. Amokrane et al. also reported that iron salts seem to be more efficient than that of aluminum as coagulant on landfill leachate in removing turbidity [8]. The result is also consistent with a previous study on the river sediment [10]. Therefore, ferric chloride is selected as the coagulant for settling the MFT suspension. Fig. 4 shows the trends of height of mudline and final solid content with ferric chloride dosage to MFT suspensions of 10% (10FC) and 15% (15FC) initial solid contents (ISC), respectively. It is evident from the observations that the effectiveness of chemical coagulation reduces with increasing suspension solid content as the density of solids in the suspension grows.
Fig. 5. Relationship between height of mudline and elapsed time under varying concentrations of ferric chloride.
consistent with a previous study on MFT [23,24]. The trend lines are similar, i.e. initially a steep straight line, followed by a parabolic shaped line and finally a flat straight line marks the start of consolidation. As shown in Fig. 5, 350 mg/L ferric chloride dosage, which is test 5CC, produces the best result in the test series. The average settling velocity is the first derivative of the mudline vs. time plot and can be expressed as:
v¼
dH dt
ð8Þ
Fig. 5 shows the relationship between height of mudline and elapsed time under different ferric chloride dosage at pH = 7.25 (suspension pH) found in the study. No settling was observed in the control test (1CC) during the test duration of 4 h, which is
where v is the suspension’s settling velocity; H is the height of mudline; and t is the time. Since there is no gravitational settling registered without coagulant added over the testing period, as shown in Fig. 5, the measured settling velocity is solely attributed to coagulation. Fig. 6 shows the plot of settling velocity against time for different ferric chloride concentrations. The initial flat lines are free settling followed by non-linear hindered settling stage, and finally reached to a plateau (very low velocity), mark the start of consolidation zone. The free settling velocity under 350 mg/L ferric chloride (test 5CC) is 0.067 cm/min (1.17 105 m/s). The plots of the final solid content and turbidity of the supernatant after 4 h settling tests against ferric chloride concentration are shown in Fig. 7. With the most effective ferric chloride dosage of 350 mg/L (test 5CC), the final solid content is 6.48% after 4 h, which indicates an increase of 29.6% compared to the initial solid content of 5%. Turbidity of the supernatants also shows that sample with 350 mg/L of ferric chloride concentration (5CC) yields the best results. The lower dosage could not completely destabilize the suspended solids whereas the higher dosage destabilize colloidal particles [25,26], and form extra clouds due to the presence of excess ions from the coagulant.
Fig. 4. Trends of height of mudline and final solid content with varying ferric chloride dosage for tests 10FC and 15FC.
Fig. 6. Relationship between settling velocity and elapsed time under varying concentrations of ferric chloride.
3.2. Coagulation test in cylinder
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Fig. 7. Plot of final solid content and turbidity of supernatant against ferric chloride dosage.
The electrical conductivity increases linearly with the increase in the ferric chloride dosage in MFT suspensions (Fig. 8), which is consistent with a study on water treatment using ferric chloride [27]. This is attributed to the addition of ferric chloride that increases the ionic concentration in the suspension. On the other hand, the magnitude of zeta potential of the particles decreases proportionally with the increase in the ferric chloride dosage (Fig. 8), which is attributed to the charge neutralization. The effect of suspension pH is observed in cylinder coagulation tests and the pH of the suspensions is adjusted using 0.1 mol/L HCl (hydrochloric acid) and 0.1 mol/L NaOH (sodium hydroxide). Fig. 9 shows the trends of mudline under the optimum ferric chloride dose of 350 mg/L (5CC). The final height of mudline decreases with the increase in the initial suspensions pH (4–6). This is attributed to the presence of hydrogen ions that neutralizes the particle charge and enhances sedimentation. Then the height of mudline increases with the rise in pH at the suspension pH above 6, which may be interpreted as the increased negative surface charge of particles. The results are consistent with Baghvand et al. [28], who reported the optimum water pH (5–6) in removing turbidity. The electric conductivity decreases with increasing pH of the MFT suspensions (Fig. 10) probably due to decrease in surface activity of the clay minerals. On the other hand, the absolute value of zeta potential increases with increasing pH of the MFT suspensions (Fig. 10) because of presence in more ions in the suspension. The results are consistent with a previous study of Guo and Shang [21].
3.3. Application of continuous current combined with optimum coagulant dosage 3.3.1. Effect of applied voltage gradient The lone application of electrophoresis on mature fine oil sands tailings were described in Islam and Shang [17] and Alam et al. [20]. The deposition of mudlines with elapsed time in the
Fig. 8. Variation of electric conductivity and zeta potential with varying dosage of ferric chloride.
Fig. 9. Variation of height of mudline and elapsed time under varying pH for 5CC.
Fig. 10. Variation of electric conductivity and zeta potential with suspension pH for 5CC.
combined application of electrophoresis and coagulant, as shown in Fig. 11, are linear initially, become nonlinear with time, and finally reach a plateau. The initial linear decrease in height of the mudline with time is the free settling stage when the particles interactions are very low. The nonlinear part of the trend lines represent the hindered settling stage due to increased interaction between particles. The plateau of the trend line marks the end of settling and start of self-weight consolidation, which is called compression phase in settling. The increase in the applied voltage gradient decreases the final height of mudline, i.e., increase of the sediment density. When the applied voltage gradient exceeded 219 V/m, the height of mudline remained the same after 7 h testing, i.e., at 3.7 cm. The final solid content in test 7EC after 7 h is 23.74% under the applied voltage gradient of 219 V/m and 350 mg/L ferric chloride, showing an increase of 375% in the solid content (Fig. 12). This represents much higher sediment density compared to that of the
Fig. 11. Deposition of mudline with time under the combined application of applied voltage gradient and ferric chloride.
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Fig. 12. Variation of solid content and increase in solid content in test 7EC.
chemical treatment with ferric chloride in test 5CC (final solid content 6.48%) as discussed before and compared to the lone application of electrophoresis [17,20]. The average settling velocity is calculated using Eq. (7). Fig. 13 shows the settling velocity vs. time in all tests, with results summarized in Table 5. The free settling velocity increases with the increase in the applied voltage gradient. On the other hand, an increase in the applied voltage gradient reduces the time of free settling, leading to the earlier start of the self-weight consolidation. The average free settling velocity of the suspension under the ferric chloride only (control test 1EC) is 0.02 cm/min, while with the applied voltage gradient of 219 V/m (7EC), the free settling velocity is 0.06 cm/min. As a result, the effective settling time decreases with the application of DC current along with ferric chloride. 3.3.2. Power consumption The effective voltage gradient increases linearly with the applied voltage gradient (Fig. 14). As shown the voltage efficiency increases with the increasing applied voltage gradient (up to 219 V/m), ranging from 92.4% to 97%. The results show that the Ti/IrOx-SS316 anode-cathode combination performs well in the presence of ferric chloride. It is also noticed from Fig. 14 that the voltage efficiency is almost constant when the applied voltage gradient is 219 V/m or higher. The current density varies linearly with the change in the applied voltage gradient and the results are plotted in Fig. 15. The electrical resistivity, q of the suspensions is determined from the Ohm’s law,
q¼
E J
UI V
Fig. 14. Variation of effective voltage gradient and voltage efficiency between anode and cathode with applied voltage gradient.
ð9Þ
where q is the electrical resistivity, X-m; E is the applied voltage gradient, V/m; and J is the current density, A/m2. Table 5 shows the suspensions’ electrical resistivity values. The values of electrical resistivity are almost constant as the applied voltage gradient increases the current density linearly and initial solid content of the suspension is same, 5%. The power consumption per unit volume of the suspension for the combined coagulation and electrokinetic settling tests is calculated using the following equation:
P¼
Fig. 13. Evolution of settling velocity with time under the combined applied voltage gradient and ferric chloride.
ð10Þ
where P is the power consumption per unit volume, W/m3; U is the applied voltage, V; I is the electric current, A; and V is the volume of the MFT suspension, m3. The results are presented in Fig. 15, which shows the relationship between power consumption and applied voltage gradient is almost linear until 219 V/m and then increases exponentially. So it is critical to identify the optimal applied voltage gradient to minimize the loss of power. The similar result is visible in lone application of electric current [17]. Under the optimal voltage gradient of 219 V/m (test 7EC), the energy consumption would be 3.51 KW/m3.
Fig. 15. Variation of current density and power consumption with applied voltage gradient combined with ferric chloride.
3.3.3. Zeta potential and turbidity The absolute values of particle zeta potentials increase linearly with the increase in the applied voltage gradient, shown in Fig. 16, because the higher electric current accelerates the flow of electrons. The zeta potential of tailings solids is 14.7 mV under the applied voltage gradient of 219 V/m, test 7EC, in combination with ferric chloride dose of 350 mg/L at the end of 7 h tests. The final turbidity of supernatant water at different applied voltage gradients is shown in Fig. 16. The values of turbidity increases almost linearly with the increase in applied voltage gradient, because generation of gas bubbles is high in high electric current. With the applied voltage gradient of 219 V/m (test 7EC) the water turbidity is 203 NTU at the end of the test. It is noticed that the turbidity of water decreases to 22.7 NTU after 1 h from the end of the test, attributed to dissipation of gas bubbles.
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Fig. 16. Evolution of final zeta potential of particles and turbidity of supernatant with applied voltage gradient under the combined application of electrokinetics and coagulation.
Fig. 18. Evolution of settling velocity with time under the combined intermittent applied voltage gradient and ferric chloride.
3.4. Application of intermittent current 3.4.1. Effect of power consumption coefficient The mudline vs. time plot for seven tests, i.e. the control and six tests under the applied voltage gradient of 219 V/m with various consumption coefficients, are presented in Fig. 17. Within the tests subjected to intermittent current (v = 50% to 90%) and 350 mg/L ferric chloride, the height of mudline decreases with the increase in the power consumption coefficient. The continuous applied voltage gradient (v = 100%, test 7IC) generates the best settling effect. The results are consistent with a previous study on the river sediment [10]. The plot between settling velocity and elapsed time under the combined intermittent current and 350 mg/L ferric chloride is shown in Fig. 18. The details of free settling velocity are summarized in Table 6. The free settling velocity increases with the increase in power consumption coefficient under the same voltage gradient, which reduces the effective treatment time. The final solid content of MFT increases with the increase in power consumption coefficient, as shown in Fig. 19. The percentage increase in the final solid content varies between 289% and 375% for the power consumption coefficient of 50–100%, under the applied voltage of 219 V/m combined with 350 mg/L ferric chloride. The continuous applied voltage gradient (test 7IC) shows the best settling effect. From the economic perspective test 3IC, intermittent current with 60% power consumption coefficient, produces a good result with sediments of 20.8% final solid content.
Fig. 19. Variation of final solid content and turbidity of supernatant with power consumption coefficient under the combined intermittent applied voltage gradient and 350 mg/L ferric chloride.
3.4.2. Turbidity of supernatant The turbidity of water at varying power consumption coefficient under the applied voltage gradient of 218.75 V/m combined with 350 mg/L ferric chloride is shown in Fig. 19. The turbidities of water range between 113 NTU and 174 NTU for the power consumption coefficient of 50–90%, which shows with the turbidity of water increases with the increase in power consumption coefficient. The reason is long time application of applied electric current accelerates the generation of gas bubbles. It is noticed that the turbidity of water varies from 20.5 NTU to 28.6 NTU one hour from the end of the test, indicating the majority of turbidity is caused by gas bubbles generated by electrochemical reactions in electrokinetic process. 4. Comparison of settling efficiency among coagulation, electrokinetics and combined tests
Fig. 17. Variation of height of mudline with time under the combined application of intermittent applied voltage gradient and ferric chloride.
The comparison of settling effects by coagulation, electrokinetics and the combined application under continuous and intermittent current are tabulated in Table 7. It is observed that settling effect of ferric chloride coagulant is minimal (test 5CC). On the other hand, settling by lone application of electrokinetics significantly increased the solid content from an initial 5% to final 18.75% [17]. After 7 h combined EK and chemical coagulant (350 mg/L ferric chloride) tests, the final solid content was 23.75% and 20.84% under continuous (v = 100%, 7EC/7IC) and intermittent (v = 60%, 3IC) current, respectively. It is evident from this study that application of 350 mg/L ferric chloride as coagulant increases the final solid content of MFT within reasonable testing period of 7 h. The reason of increasing settling efficiency in
Please cite this article in press as: Islam S, Shang JQ. Coagulation enhanced electrokinetic settling of mature fine oil sands tailings. Int J Min Sci Technol (2018), https://doi.org/10.1016/j.ijmst.2018.04.012
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S. Islam, J.Q. Shang / International Journal of Mining Science and Technology xxx (2018) xxx–xxx Table 7 Comparison of thickening results by coagulation, electrokinetics and their combined application. Properties to be compared
Coagulation (350 mg/L CC)
EK (219 V/m) [17]
Combined application (cont., 7EC)
Combined application (int., 3IC)
Initial solid content (%) Final solid content (%) Total treatment time (h) Free settling velocity (cm/min) Duration of free settling (h) Zeta potential of particles (mV) Turbidity of supernatant (NTU)
5 6.48 4 0.067 0.75 22.67 14.4
5 18.75 7 0.047 2.00 14.70 30.6
5 23.75 7 0.060 2.00 16.30 22.7
5 21.57 7 0.050 1.50 22.0
Note: EK = electrokinetics, cont. = continuous electric current, int. = intermittent electric current with v = 60%.
combined application is the enhancement of gravitational force due to high self-weight of flocs of particles that combines with electrophoretic effect. Though the free settling velocity is the highest (0.067 cm/min) for coagulation (5CC), however the durations of free settling under coagulation and electrokinetic settling were 0.75 h and 2 h, respectively, indicating the free settling velocity contributed more with electrokinetics (test 7EC). The final turbidities of supernatant are 14.4 NTU, 30.6 NTU and 22.7 NTU under settling by coagulation, electrokinetics, and combined application, respectively, while the corresponding zeta potentials of particles were 22.6 mV, 14.7 mV, and 16.3 mV. 5. Conclusions The key goal of this study was to increase the solid content of the MFT suspension, which will significantly reduce the volume of tailings, and will reduce the time and cost of tailings management and rehabilitation. Three inorganic chemical coagulants (varying dosage) and mode of electrokinetics (continuous and intermittent current) with varying voltage gradient were experimented. Laboratory testing results of present study show that the application of coagulant is technically feasible in accelerating the electrokinetic settling of MFT suspension, which provides a significant improvement of solid concentration from 5% to 23.75%. The main results and conclusions are summarized as follows: (1) In the series of jar experiments, ferric chloride at 350 mg/L, test 5FC, generated the best coagulation effect in terms of the final solid content of MFT among different coagulants studied at 350 mg/L concentration. However, the effectiveness is judged minimal as the final MFT solid content was 6.48% after 4 h, compared to 5% initial solid content after 4 h in cylinder coagulation tests. (2) The final solid content of MFT was 23.74% from an initial of 5% under the combined continuous applied voltage gradient of 218.75 V/m and 350 mg/L (test 7EC) for 7 h, an increase of 374.5%. The zeta potential of the MFT solids reduced to 8.4 mV after 7 h treatment, which generated instability of suspended solids and promoted settling. The turbidity of water was 22.7 NTU after 1 h from the end of the test when the generated gas dissipated. (3) The constant electric current combined with ferric chloride (test 7EC) generated best settling effect (produced sediments of 23.75% final solid content) because of high selfweight of flocs of particles. Intermittent electric current with 60% power consumption coefficient (test 3IC) reached the final solid content to 20.84%. This optimal condition of intermittent current (test 3IC) is better than that of continuous mode in economic perspective, which saves the power consumption by 40%.
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