Thickening of Mature Fine Oil Sands Tailings

The mature fine oil sands tailings (MFT) remain suspended in tailings disposal pond for decades because of the low sedimentation/consolidation rate. This study investigates the application of coagulation and electrokinetics to thicken, i.e., to increase the solid content of the MFT suspension. At first, the optimal ferric chloride concentration is identified as 350 mg/l in jar tests, under which the best thickening result is only 6.48% from an initial of 5% after 4 hours in cylinder coagulation tests. The electrokinetic thickening under an applied voltage gradient of 219 V/m reaches the final solid content of 18.75% after 7 hours. The combined coagulation (350 mg/l ferric chloride) and electrokinetic thickening under a continuous applied voltage gradient (219 V/m) is the most effective approach, which reaches the final solid content of 23.74%. The sedimentation theories in the free settling and hindered settling stages are validated using the experimental results, which is one of the major contributions of this research. The turbidity of the supernatant ranges between 4.8 NTU and 31 NTU in all treatments. Two regression models for the electrokinetic and the combined application of coagulation and electrokinetic tests are developed to relate the final solid content and the applied voltage gradient. The models and the independent variables are statically significant at 95% confidence level based on F-test and t-test results, respectively

An experimental investigation on utilization of ladle refined furnace (LRF) slag in stabilizing clayey soil

The contemporaneous world of development and urbanization generates extreme stresses on building infrastructures, and the use of soil as a load-bearing material is a prerequisite for such activity. However, if the foundation soil lacks enough bearing capacity and shear strength and shows excessive settlements, unescapable failures will occur in constructed infrastructures. As such, improvement or stabilization of soft clay soil is crucial before initiating any construction project to limit the potentiality of post-construction faults and damages. In this research, the efficiency of ladle refined furnace (LRF) slag has been investigated to stabilize the geotechnical properties of poor conditioned-soft clayey soil, which will also lessen the environmental degradation due to the unplanned dumping of LRF slag. Various experimental programs such as the Atterberg limit, consolidation, compaction, unconfined compressive strength, and California Bearing Ratio tests were carried out in this study. Experimental results revealed that adding LRF slag in different percentages augmented the strength and dry density of the original soil to a considerable extent, and the optimum percentage of LRF slag was 20%. 20% LRF content in the soil increased the UCS value of the soil by 219%, CBR value by 47%, and reduced consolidation settlement by 30%. Additionally, the test results were further evaluated by the SEM-EDS test

Coagulation enhanced electrokinetic settling of mature fine oil sands tailings

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. Keywords: Electrokinetics, Electrophoresis, Coagulation, Settling, Voltage gradient, MF

Electrokinetic thickening of mature fine oil sands tailings

Mature fine oil sands tailings (MFT) are the fine parts of oil sands tailings that remain suspended in tailings disposal pond for decades because of the low sedimentation/consolidation rate. This study applies electrokinetics to thicken, that is, to increase the solid content, of the MFT suspension. The geotechnical properties of MFT solids and chemical analysis of tailings pore water are measured, followed by electrokinetic (EK) tests. The results of this study indicate that EK thickening is very effective for the settling of MFT suspensions under an applied voltage of 219 V/m, the final solid content of the sediment reached 18·75% from an initial 5% after 7 h. The EK treatment is also very effective in clarifying the supernatant. The final turbidity of the supernatant is 30·6 NTU under 219 V/m applied voltage gradient. Regression models are developed using statistical software MINITAB 15 in coded and uncoded units to relate the final solid content of the sediment with the initial solid content and applied voltage gradient. The models and the independent variables are statistically significant at 95% confidence level based on F test and t test results, respectively

Electrophoresis and its applications in oil sand tailings management

Oil sands mines generate tailings during the extraction process, which are a mixture of water, clay, sand and residual bitumen. When the tailings are released to tailings ponds, the coarse solids settle quickly, whereas fine solids containing clay minerals, namely mature fine tailings (MFT), remain suspended for years, even decades. A study is carried out to assess the suitability of electrokinetic (EK) sedimentation to accelerate sedimentation of MFT. A series of laboratory-scale column experiments are carried out to examine the effects of electrophoresis during settling processes. The investigation focuses on the effects of EK sedimentation as related to the initial solid content of the tailings suspension, applied electric field intensity, water pH, and the use of an optimized coagulant FeCl. Based on the experimental data, an electric field intensity of 150 V/m along with an initial tailings solid content of no > 5% are the optimum condition for EK sedimentation of MFT, in terms of reducing the overall sedimentation time and increasing the final solid content. The results show that the current density of EK sedimentation for MFT should not be > 20 A/m to control the bubble effect and reduce power consumption

Strength development in clay soil stabilized with fly ash

The presence of optimum fly ash content in soil to increase the unconfined compressive strength (UCS) of clayey soil has been studied in the present work. Two types of fly ash, collected from coal combustion in an electric power plant, are used; one is Bangladeshi and the other one is Indian. The soil, collected from paddy field of Shahjalal University of Science and Technology (SUST), is classified as a mixture of inorganic silts and organic clays of medium to high plasticity. UCS tests have been conducted on moulds prepared with soil only and with soil containing fly ash with optimum moisture content (OMC) of 19%~24% at a curing time up to 3, 7, 14, 28 and 90 days. Observations showed that maximum dry density (MDD) and OMC of pure soil are 1.615 g/cm and 20.30%, respectively. The addition of fly ash content decreases MDD and increases OMC of the soil. With the increase in fly ash content, two zones of strength development have been observed: the active zone and the deterioration zone. The optimum fly ash content is found as 5% for both types of fly ash. 5% Bangladeshi fly ash content in the soil-fly ash mix produces 430.96 kPa and 474.53 kPa UCS at 28 and 90 days, respectively, while Indian fly ash generates 424.63 kPa and 434.01 kPa UCS, respectively

Experimental and numerical analysis of a slurry consolidation test on tailings

The deposition of fine-grained tailings as a somewhat thickened slurry remains the norm, and presents challenges in the assessment of their geotechnical parameters and the analysis of their settling and consolidation behaviour. The conventional oedometer and Rowe cell consolidometer are not suitable for testing tailings from a slurry state. In this paper, silty sand-sized gold tailings of low plasticity are tested in an instrumented slurry consolidometer and their consolidation is simulated numerically. The purpose-built slurry consolidometer measures 150 mm in diameter and 410 mm in height, and is instrumented with top and base load cells, seven pore water pressure (PWP) transducers starting from the base and spaced at 40 mm intervals spirally up the side of the consolidometer, and a displacement transducer. Up to 560 kPa stress can be applied to the top of the sample, equivalent to about 60 m depth of tailings. The tailings were placed in the slurry consolidometer in a series of layers at an initial 25% solids concentration by mass, and each layer was allowed to settle and the surpernatant water removed before the next layer was placed. Stepwise loading was then applied, since this is the conventional means of loading in a consolidation test, and is the basis of the available numerical analysis software. The difference with the slurry consolidometer over conventional consolidation testing is that the test can commence from a slurry state and follow the largestrain consolidation process. The numerical simulation of a large-strain consolidation problem is challenging due to extreme mesh distortions during numerical calculations. The emerging and innovative material point method (MPM) was used to numerically simulate the consolidation of the tailings from a slurry state. The analysis was carried out using the code and software developed by the MPM Research Community, Anura3D, focussing on PWP dissipation and the resulting consolidation. This numerical approach captured the consolidation behaviour well and provided a reasonable match with the experimental results

Settling, consolidation and shear strength behaviour of coal tailings slurry

The volume change and shear strength behaviour of tailings slurry with the changes in gravimetric moisture content is important to effectively utilise the storage volume and analysis of dam failure potential. Consolidation testing of tailings from a slurry-like to soil-like state is a critical task, and conventional consolidation apparatus does not have the capability of doing such testing, as the tailings slurries contain high water content. Settling tests conducted on slurries having a 25% solids concentration showed very low efficiency; final sediment was still slurry-like. An intrinsic point was identified based on pore water pressure dissipation during consolidation tests in a slurry consolidometer that can define two states of the tailings i.e. slurry-like and soil-like. In the slurry-like state, the volume change was greater for Slurry 2 than Slurry 1, while the inverse was observed in the soil-like state. The evolution of peak vane shear strength with the changes in moisture content was almost similar for both samples and obeyed the power function. Regression models have been developed and validated to predict the shear strength of materials at any gravimetric moisture content or void ratio

Application of time domain reflectometry method in monitoring state parameters of subgrade soil in pavement

Moisture content has a direct influence on structural performance of pavement. Every year in Australia, heavy vehicles damage roads when the moisture content of the pavement layers reaches a critical value because of heavy rainfall or flooding. When the moisture content changes, increased moisture contents often lead to premature pavement damage under cyclic loading. Time domain reflectometry (TDR) is a noninvasive technique for measuring moisture content. The existing and newly developed TDR sensors are calibrated in the laboratory with materials of subgrade layer from the pavement. These calibrated sensors are installed in both the shoulder and wheel paths of a newly constructed pavement to monitor the dielectric characteristics of the materials. The dielectric characteristics are further utilized to calibrate in situ moisture of the pavement materials as well as transverse moisture distribution across the pavement. Moreover, the electrical conductivity and density of the materials are estimated based on the information of dielectric properties. Finally, the state parameters, namely moisture content, electrical conductivity, and density are obtained from the road instrumentation that would improve the pavement management in terms of damage, like surface deformation or rutting

Strength development in fine-grained paddy field soil by lime addition

Due to high population density, the people of Bangladesh are building houses and the government is establishing infrastructure (such as roads, flyovers, bridges) on agricultural land that pose significant settlement issues. Consequently, such development works require prior improvement of the soft soil or improvement during construction to restrict post-construction failures. Soil stabilization, a conventionally used ground improvement method, is often utilized to alter engineering properties of fine-grained soil such as strength, stability, permeability, weathering resistance, etc. to maintain the required construction criteria. Moreover, the addition of lime in the soil is demonstrated to markedly reduce the hydraulic conductivity, which will prevent the vertical movement of pore water. With this in mind, in the current study an attempt has been undertaken to increase the strength of fine-grained soil of agricultural land by adding commercially available hydrated lime, which may in the future be used as foundation material. For this purpose, lime was added at various percentages (0–12%) by weight to soil collected from a paddy field. The study noted the collected soils as being inorganic silts of high plasticity or organic clays of medium to high plasticity. Unconfined compression tests were conducted on cylindrical mold prepared specimens with clayey soil in the presence of lime, and all specimens were cured for 3–90 days for strength development. The study revealed 7% added lime as the optimum content, with the corresponding unconfined compressive strength (UCS) observed to be 344 kPa and 356 kPa at the end of 28 days and 90 days, respectively. The evolution of unconfined compressive strength was higher in hotter and more alkaline environments