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... There are several thickener design methods applied in the mining industry including Coe-Clevenger (Coe, 1916) or Talmage-Fitch (Talmage and Fitch, 1955) approaches. Besides easy-to-determine parameters such as liquid-solid ratio within the thickener and at the discharge point, and the flowrate of dry solids fed to the thickener, the settling rate of suspended particles represented by the initial settling rate (ISR) of the mudline separating the liquid and solid phases is needed for calculating the thickener area (Parsapour et al., 2014;Wills, 2006). However, research on an accurate method of mudline formation and ISR determination has probably been underestimated and poorly reported in the literature. ...
... Despite its automation mode, this method seems to be incompatible to lab-scale experiments and difficult to record the speedy settlement occurring within some seconds, not to mention a high investment and complex operation. A number of other settling rate measurements has been applied in colloidal suspension studies, which track the mudline height against time manually (Parsapour et al., 2014;Xiong et al., 2020) or by capturing digital images of the settling column at regular intervals (Ofori et al., 2012). The ISR then was worked out as the initial slope of the settling curves. ...
... The rapid development of mining exploitation has resulted in a lot of mine tailings that urgently need to be dewatered to mitigate short-term and long-term detrimentally environmental impacts and the scarcity of underground water resources. Among many chemical and mechanical dewatering method, gravity sedimentation or thickening has been still the most popularly employed dewatering technique in the mining industry due to its low investment, quick water circulation, high capacity, and particularly very low shear forces which facilitate the agglomeration of fine solid particles as well as downstream mechanical processes such as filtration or centrifugation (Bürger et al., 2020;Parsapour et al., 2014;Wills, 2006). An accurate determination of the suspension settling rate would greatly contribute to a proper design of thickener that is critical to an effective separation, significantly bolsters the recycling of process water, and reduces the dependence on environmentally detrimental tailings dams (Alam et al., 2011). ...
Article
Thickening as the most widely applied dewatering technique in the coal and mineral processing industry involves the rapid settling of flocculated suspensions of fine tailings particles to produce a compact phase of solid at the bottom and a clear phase of liquid on the top. The mudline separating the two phases can be monitored and used to optimise the critical step of flocculation and design the thickeners. Its initial speed known as the initial settling rate (ISR) of flocculated suspensions has been used in the optimisation, yet the experimental determination of the speed is currently unreliable. Here, we have proposed and developed a novel method that can automatically detect the mudline formation and determine ISR accurately. Specifically, the settling of flocculated suspensions is filmed using a normal digital camera. The movie is digitised and analysed image by image using an in-house MATLAB code to detect the mudline and its relative position versus time. The relative position as a mathematical function of time is then approximately described by an exponential dependence and regression analysis. ISR is determined by the first derivative of the function with respect to time calculated at the mudline formation. This newly developed technique was experimentally applied to establish the optimal conditions of flocculating fine clay suspensions and clay-rich fine coal tailings from a coal washing plant. Our novel technique is fully automated and provides accurate data needed for optimising the thickening process that are free from potential errors caused by the experimentalist in laboratories.
... Their characteristics are highly dependent on the type of polymer-slurry interaction and the floc formation process (Dias et al., 2004;Jarvisa et al., 2005). Therefore, the yield stress is strongly dependent on the dosage and polymer structure as well as tailings characteristics, such as particle size and shape, solids concentration, surface charge density, solid density, bulk density and pH (Sofrá and Boger, 2011;Parsapour et al., 2014;Wu et al., 2015). This paper reports the development of a flocculant system for the mining industry that can simultaneously target several properties, such as the solid-liquid separation process, water recovery (qualitative and quantitative) and paste formation ...
... The increase in C 0 led to growth of the sediment τ 0 . This occurred because the difference in the C 0 influences the shape and structure of the flocs formed, and consequently the settling velocity and sediment cohesion (Jarvisa et al., 2005;Sofrá and Boger, 2011;Parsapour et al., 2014). Between the two polymers, R-10 reached higher values of static and dynamic τ 0 , reaching 1400 Pa and 83 Pa, respectively, for pulps with 23% (w/w) of C 0 . ...
Article
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Mining activities worldwide are subject to serious geotechnical accidents involving tailings dams. Therefore, many studies have focused on tailings dewatering before their deposition. This article studies the rheological properties of flocculated and thickened nickel tailings in order to determine the effect of the initial solid concentration and type of rheology modifying agents (two anionic polymeric flocculants) on their stability for disposal in dams. The sediment was submitted to rheological tests, with the vane technique, and slump tests, to obtain information about static and dynamic yield stress and slump height, respectively. The results showed a strong influence of the initial solid concentration and the polymeric flocculant on the particle aggregation. Both particle cohesion and yield stress of the sediment increased, reaching static yield stress of around 1,400 Pa. Both flocculants were efficient in thickening by batch sedimentation, allowing water recycling in the range of 70-90% with turbidity less than 100 NTU and the production of sediment with static yield stress much higher than specified for mineral paste.
... Settling velocity testing is typically applied in metallurgical studies on sludge settling for estimation of the thickening capacity of flocculated suspensions (57). This method has also been used in oceanography for estimating the porosity of large suspended particles (58). ...
Article
Flocculants provide an innovative solution for managing construction stormwater runoff with their efficiency in reducing turbidity in the effluent. With proper application and dosage, these chemicals improve the performance of sediment control practices in capturing fine-sized soil particles. Flocculants function by providing electromagnetic charges to attract soil particles into large molecular bridges of flocs. Following this process, suspended particle removal can be achieved through gravitational forces. Despite the benefits of using flocculants, their implementation may create a potential risk of polluting downstream waterbodies and harming aquatic life in case of a high residual concentration in discharge. Determining residual concentrations is challenging for practitioners because of a lack of field applicable testing methods that work across a variety of flocculant types. This study developed a field applicable methodology for detecting residual flocculant concentrations by using settling velocity as an identifier of flocculant concentration. In total, 14 products comprised of polyacrylamide, sodium montmorillonite, chitosan, agricultural gypsum, and alum-based products were evaluated. Known flocculant concentrations ranging from 0% to 30% of the manufacturer’s recommended dosage, were mixed with a fine soil passing through No. 200 sieve and allowed to settle in a graduated cylinder to record the settling velocity. Regression analysis was conducted on the experimental data to develop concentration versus settling velocity relationships which indicated that more than 90% of known concentration values are dependent on the settling velocity. The study provides a framework for practitioners to identify residual flocculant concentrations in field conditions by relating settling velocity with chemical concentrations.
... In this configuration, a removal of 95% was achieved after 100 bedvolumes (25 L of wastewater), corresponding to an absolute decrease of 166 NTU to 8 NTU. These results are similar to tests in which stormwater or road runoff is treated with sand filtration and during which a suspended solid removal between 80% and 90% was obtained [29,30] and can be attributed to physical retention of the pollutants. Similar to the hydrocyclone tests, it was also tried to determine the changes in suspended solids concentration and dry matter content. ...
Article
Scrap material recovery and recycling companies are producing wastewater in which common pollutants (such as COD, nutrients and suspended solids), toxic metals, polyaromatic hydrocarbons and poly chlorinated biphenyls frequently can exceed the discharge limits. Lab-scale optimisation of different possible physical-chemical treatment techniques was performed on the wastewater originating from three different companies in view of further testing at pilot-scale testing and implementation at full-scale. The lab-scale tests demonstrate that sedimentation or hydrocyclone treatment as stand-alone technique cannot be used for proper treatment of this type of wastewater. Dual bed filtration or coagulation/ flocculation proved to be more promising with removal efficiencies of about 71% to 95% (dual bed filtration), respectively 61% to 97% (coagulation/flocculation) for the above mentioned pollutants (metals, polyaromatic hydrocarbons and poly chlorinated biphenyls).
... Addition of flocculents (long-chain polymers) in a dilute stable suspension bridges particles together, leading to the formation of highly porous and irregularly shaped flocs that have complex shapes, and possess rapid initial settling velocity, trapping some liquid in their structures (Parsapour et al., 2014). The nature of flocs depends on factors such as the physical properties of the solids (surface chemistry, size, size distribution, shape, density), the nature of the liquid (viscosity, dielectric constant), the nature of the suspension (electric charge, pH, ionic strength, temperature), and the nature of the flocculent (chemical properties of the main part and chains, molecular weight, molecular weight distribution, electric charge, charge density). ...
Article
Full-text available
The re-use of waste water from coal preparation plant tailings is always desirable, but is generally problematic in terms of solid-liquid separation. Dewatering and disposal of coal slurry at a high percentage of moisture poses a major technological and environmental challenge to mining and mineral processing industries world-wide. Characterization and flocculation of these coal tailings reveals that the separation of fine coal tailings depends on factors such as size distribution, slurry viscosity, zeta potential, pulp density, circuit design, plant operating parameters, and the behaviour of solid particles in an aqueous medium. The present study was carried out on -600 mm coal tailings collected from a coking coal preparation plant of Jharia Coalfield, India, using anionic flocculent. Characterization studies revealed that the zeta potential was highest (-44 mV) at a pH of 9.0, the size range of the particles was 1.1-700 mu m, and the average ash content was 62.57%. Flocculation studies were performed at different solid concentrations, pH values, and polymer dosages. The results indicated an efficient settling rate of 178.15 mm/min at 8% solid (w/w) at a flocculent dosage of 32.50 g/t solid. The turbidity of the clear liquid obtained was 7.42 NTU.
... Addition of flocculant (long-chain polymers) in a dilute stable suspension bridges the particles together, leading to the formation of highly porous and irregularly shaped flocs that have complex shapes and possess a rapid initial settling velocity and some liquid trapped in their structures [10]. The nature of flocs depends on factors such as physical properties of solids (surface property, size, size distribution, shape, density), the nature of the liquid (viscosity, dielectric constant), the nature of the suspension (electric charge, pH, ion strength, temperature), and the nature of the flocculant (chemical properties of main part and chains, molecular weight, molecular weight distribution, electric charge, density of electric charge). ...
Article
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A parameter “Settling Index (SI)” has been proposed mathematically. The index was used to evaluate the performance of the flocculation studies of coal fines in the tailings collected from a coal preparation plant of India. Effect of the process parameters on the settling rate were investigated using anionic polyacrylamide as the flocculant at the following parameter levels: Pulp density: 6–12% solids (w/w); slurry pH: 4–10; and flocculant dosage: 6.5–26 g/t-solid. The optimum conditions for settling were obtained at the flocculant dosage of 19.5 gpt, pH 8, and pulp density of 8% solid (w/w) and showed a settling rate about 168 mm/min. At the optimum condition of settling rate, the density of settled slurry was 40.84% solid (w/w). The settling index was found to be a useful tool for analyzing the settling performance. An empirical model was developed using the experimental settling rate data with an R2 value of .929.
... The cylinder was filled with a predetermined amount of sample and water, and flocculants were also added when needed. The cylinder containing the suspension was then mixed by reversing the cylinder five times (Parsapour et al., 2014), and finally mudline height was recorded against time. ...
Article
Full-text available
Maximization of water recovery in thickeners is an essential step in the mineral processing industries. The Gol-E-Gohar iorn ore processing plant uses a center pier thickener with a diameter of 38 m and depth of 4 m. The feed rate is 50 t/h solids with a concentration of 5% (w/w), and it has been designed to provide an underflow containing 50% solids. The performance of the thickener was not as expected judged by the overflow stream containing 0.6% solids and water loss of 52 m3/h in the underflow. A criterion called the index of flocculation incorporating the perimeter-based fractal dimension of flocs, floc size, and settling velocities was proposed and used to determine the optimum flocculant dosage which was found to be 20 g/t. The fractal dimension of flocs in a pulp containing 5% solids was found to be between 1.16 (no flocculant) and 1.56 (40 g/t flocculant) which indicated highly branched and loosely bound structures for the flocs as flocculant dosage increased. At these conditions, the size of the flocs which was recorded by a digital microscope increased from 93 μm to 403 μm. A new feedwell with a special design with the aim of preventing flocs breakage and maximizing energy dissipation of pulp was constructed and installed. These modifications led to an increase in the thickener drivehead torque from 5% to 18% of the rated torque, indicating a higher bed depth. Finally, installation of a PID controller reduced the drivehead torque relative oscillation from 52.0% to 12.7%. These changes increased the solids concentration of the underflow from 49.0% to 56.9%, decreased the overflow solids concentration from 0.6 % to values smaller than 0.01% and resulted in a reduction of 27% in water loss in the thickener underflow.
... Chemical conditions, reagent selection and process operating conditions are evaluated based on the fundamental mechanisms involved in particle destabilization and flocs development [14]. The Nature of flocs depends on factors such as physical properties of solids (surface property, size, size distribution, shape, density), nature of liquid (viscosity, dielectric constant), nature of suspension (electric charge, pH, ion strength, temperature), and nature of flocculant (chemical properties of main part and chains, molecular weight, molecular weight distribution, electric charge, density of electric charge) [15]. Also, the properties of flocs, such as flocs size, density, structure and strength significantly influence the settling rate of flocs and hence the performance of solid-liquid separation. ...
Article
Full-text available
Full factorial design of experiment was employed to evaluate the effect of variables on the settling performance of coal fine. Influence of the variable parameters was discussed using 2D surface plots. Settling rate and turbidity were more sensitive to the flocculant dosage followed by the pH and the feed density. Highest settling rate of 190 mm/min was obtained with turbidity of supernatant liquid as 9 NTU at flocculant dosage of about 26 gpt and pH 9. Coefficient of correlation (R2) between the experimental and the predicted settling rate and the turbidity were found to be as 0.96 and 0.93 respectively.
... Finally, the shaker was turned on and the mud line height was recorded at different time points. In a well-accepted manner, settling graphs were plotted and slopes of their linear parts were calculated and defined as settling velocities of coal particles (Garmsiri & Haji Amin 2012;Parsapour et al. 2014). ...
Article
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This study aimed to experimentally investigate and statistically test changes in the settling velocity of particles falling in an aqueous solution due to an orbital shaking of the container. A series of settling experiments were conducted on coal samples under different combinations of shaking speed and initial concentration of the particles. First, a generalized version of the factorial design (a split-plot design) was utilized to obtain the acquired data. Then, an appropriate statistical model was fitted to experimental data using settling velocity as a response while shaking speed and initial coal concentration were defined as factors. Graphic patterns confirmed our hypotheses that settling velocity is affected by the shaking speed, initial coal concentration and the interaction of these two factors. Results of the analysis indicated that the main effects of both initial coal concentration (F-statistic=775.75) and shaking speed (F-statistic=11.96) on the settling velocity are strongly significant (both with a P-value near zero). Moreover, a strong interaction of the effects of these factors on the response variable was also observed (F-statistic is 10.57 with a P-value of near zero). Based on the results, combination of 4% coal concentration and 50 rpm shaking speed resulted in the highest settling velocity of 1.21 cm/s. In contract, a combination of 10% coal concentration and 100 rpm shaking speed led to a settling velocity of only 0.55 cm/s. Findings of this study may have invaluable benefits to the mineral and water treatment industries where design and construction of thickeners and sedimentation tanks equipped with orbital shakers can significantly accelerate the sedimentation of mineral particles as well as water contaminants.
... The addition of flocculants (long-chain polymers) in a dilute stable suspension bridges the particles together, leading to the formation of highly porous and irregularly shaped flocs. These flocs have complex shapes, and possess a rapid initial settling velocity and some liquid trapped in their structures (Parsapour et al., 2014). ...
Article
Full-text available
In the present investigation, flocculation and settling studies were carried out using a cationic flocculant. A new parameter “water recovery index” (WRI), i.e. the percentage of water recovered as the supernatant water, has been used to evaluate the performance of settling. This index was found to be a useful tool for analyzing the settling performance. The highest index value of 89.51% was obtained at 6% solid concentration, slurry pH 6.5, and at a flocculant dosage of 4.86 gpt. The empirical model developed for the settling rate indicated a coefficient of correlation (R²) of 0.99 between the experimental and predicted values.
... The cylinder was filled with a predetermined amount of sample and water, and flocculants were also added when needed. The cylinder containing the suspension was then mixed by reversing the cylinder five times (Parsapour et al., 2014), and finally mudline height was recorded against time. ...
Conference Paper
Maximization of water recovery in thickeners is an essential step in the mineral processing industries. The Gol-E-Gohar iorn ore processing plant uses a center pier thickener with a diameter of 38 m and depth of 4 m. The feed rate is 50 t/h solids with a concentration of 5% (w/w), and it has been designed to provide an underflow containing 50% solids. The performance of the thickener was not as expected judged by the overflow stream containing 0.6% solids and water loss of 52 m3/h in the underflow. A criterion called the index of flocculation incorporating the perimeter-based fractal dimension of flocs, floc size, and settling velocities was proposed and used to determine the optimum flocculant dosage which was found to be 20 g/t. The fractal dimension of flocs in a pulp containing 5% solids was found to be between 1.16 (no flocculant) and 1.56 (40 g/t flocculant) which indicated highly branched and loosely bound structures for the flocs as flocculant dosage increased. At these conditions, the size of the flocs which was recorded by a digital microscope increased from 93 µm to 403 µm. A new feedwell with a special design with the aim of preventing flocs breakage and maximizing energy dissipation of pulp was constructed and installed. These modifications led to an increase in the thickener drivehead torque from 5% to 18% of the rated torque, indicating a higher bed depth. Finally, installation of a PID controller reduced the drivehead torque relative oscillation from 52.0% to 12.7%. These changes increased the solids concentration of the underflow from 49.0% to 56.9%, decreased the overflow solids concentration from 0.6 % to values smaller than 0.01% and resulted in a reduction of 27% in water loss in the thickener underflow. 20
... The sedimentation curve has two distinct parts, respectively two lines the two straight segments of the curve, representing the tangents traced to the curve at the appropriate points 0 and the final time form an angle. The intersection of the bisector curve represents the point defines the critical height and critical time values to be computed short, from the upper side of the cylinder to the floor, solids concentration, gently grows and at the floor it reaches a maximum [8]. The water clarification curve corresponding to the height of 55 cm from the bottom of the cylinder in which the turbidity of water was measured was plotted in Figure 5. for sediment conveyance modelling near research will extend the application of the model to estimate the sedimentation rate of suspensions consisting of small and light particles, or heavier and larger par more complex, for further testing of different models. ...
Article
Full-text available
Our study proposes a new mathematical model for predicting the gravitational settling velocity of sediment particles, a measure to evaluate water clarity, to provide information to environmental engineers, first responders dealing with environmental emergencies or farmers, for the most rapid and efficient management responses. New and simplified simulation program are developed to the study sedimentation process. The mathematical model proposes a first-order differential equation who is solved by a numerical method algorithm. The proposed formula has the highest degree of predictive accuracy when compared with experimental data.
... Applying appropriate chemicals would ensure an effective flocculation, which significantly enhances the solid-liquid separation. The measurement of initial settling rates has been employed in various previous studies [47,[54][55][56][57] as a critical method to evaluate the effectiveness of chemical flocculation, yet its accuracy requires improvement due to the difficulty in detecting the solid-liquid interface between the flocculated tailings particles and the supernatant during the settling process [58]. In these studies, a settling profile presenting the position of the settling interface as a function of time were plotted and the slope of the initial linear portion was interpreted as the initial settling rate. ...
Article
Solid-liquid separation to dewater mine tailings is critical to transforming tailings dams that are hazardous to the environment and surrounding communities into safer and sustainable disposal storages by dry stacking and co-disposals. Current tailings dewatering by thickening and filtration has experienced big challenges due to the presence of fine clay minerals with complex surface properties and gelation caused by their swelling characteristics. They retain water in their structure, settle very slowly in thickeners, and pass through filter media during filtration. Here, we have systematically investigated the effect of clay mineral surface chemistry on settling, flocculation, and dewatering using pure clay samples and coal tailings samples. Our results indicate the critical role of clay surface charge in the successful dewatering of the clay samples. A two-step procedure including neutralizing the clay surface charge by salts and then flocculating by salt-resistant flocculants can be significantly effective in the dewatering. The newly-developed dewatering technique by combining chemical reagents and mechanical force of solid bowl centrifugation can deliver successful dewatering outcomes by achieving the required final moisture of dewatered tailings and providing improved water quality for recycling. This paper highlights the importance of clay surface chemistry in controlling clay interactions with chemical reagents, and solid-liquid separation to dewater the clay-rich tailings inherently present in the mining industry.
... [8] Addition of flocculant (long-chain polymers) in a dilute stable suspension bridges the particles together, leading to the formation of highly porous and irregularly shaped flocs which have complex shapes, and possess a rapid initial settling velocity with some liquid trapped in their structures. [9] The nature of flocs depends on factors such as physical properties of solids (surface property, size, size distribution, shape, density), nature of liquid (viscosity, dielectric constant), and nature of suspension (electric charge, pH, ion strength, temperature), along with the nature of flocculant that includes chemical properties of main part and chains, molecular weight, molecular-weight distribution, electric charge, density of electric charge, and so on. Also, the properties of flocs, such as flocs size, density, structure, and strength, significantly influence the settling rate of flocs and hence the performance of solid-liquid separation. ...
Article
Full-text available
In the present investigation, an attempt has been made to analyze the settling behavior of coal fine tailings using an anionic flocculant within the workable range of pH and pulp density. Box-Behnken design of experiments coupled with response surface methodology was employed to analyze the settling rate of particles and the turbidity of the supernatant liquid. Influence of the variables on the settling rate and the turbidity of the supernatant liquid has been presented and discussed through 3D surface plots. The settling rate was found to be more sensitive to the flocculant dose followed by pH and feed pulp density. Turbidity of the supernatant liquid was found to be more sensitive to the flocculant dosage followed by pulp density and the pH. Coefficient of correlation (R2) between the experimental and the predicted values of the settling rate and the turbidity of the supernatant liquid were found to be as 0.97 and 0.99 respectively.
Conference Paper
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In this research, the attempts made to standardize and modify the flocculantization unit of the magnetite concentration plant of Golgohar was investigated. Proper flocculation is the key to successful operation in any thickener. Flocculant preparation in the taillings thickener of Golgohar magnetite concentration plant is performed by two conditioning devices. During the process audit, the problems were identified and in order to solve them, construction and installation of static mixers, replacement of flowmeters of flocculant preparation devices, repair of dry flocculant suction device and adjustment of prepared flocculant concentration were carried out. As a result, the efficiency of the prepared and injected flocculant to the thickener increased
Article
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Relevance of the work is due to the possibility of creating a friction drive of rotating parts of units for ore mining and dressing plant. These are pulp thickeners based on a traction device with adjustable pressure of driving wheels against a rail ensuring linear dependence of the developed propulsive effort on external resistance. This eliminates the need for additional dead-weight, reduces specific quantity of metal and energy intensity of a drive. Purpose of the work. Substantiation of parameters of the friction drive based on a lever-type draw gear with adjustable resistance as function of pressure from the driving wheels that ensures reliable wheel-rail adhesion in a mode of elastic sliding; this eliminates tilt of the device on a ring-type rail, longitudinal slide and undercarriage wear. Research methodology. A systematic approach has been used, which includes analytical and computational methods using mathematical modeling, structural and kinetostatic analysis, as well as some conventional concepts of classical mechanics and mechanism and machine theory. Results and their application. The main dependences of parameters of the traction device are determined taking into account peculiarities of work on the ring-type rail and ensuring reliable wheel-rail adhesion in a mode of elastic sliding. The friction drive based on the traction device that is capable of developing a propulsive effort independent of adhesion weight of the traction unit can be used as a drive for pulp thickeners with a peripheral drive and as a drive for rotating parts of large units such as cement kilns, mills, cylinder dryers and mixers. Conclusions. The features of the operation of the friction drive for the ore mining and dressing plant using the ring-type rail are considered; methods for eliminating possible tilts of the device causing undercarriage wear are given; correcting factors for the ratio of driving wheel diameters and geometric parameters of the mechanism depending on the radii of the rail working surfaces are proposed.
Article
The floc morphology was investigated in a continuous-flow reactor, in order to understand the evolution of flocs in practical flocculation and sedimentation processes in water utilities. Kaolin-humic acid suspension was used as the test water, and polyaluminum chloride was chosen as the coagulant. An in-situ recognition system was applied to analyze the floc size, boundary fractal dimension, and eccentricity ratios. Particle numbers and turbidity were also determined in the sedimentation stage. At a coagulant dose of 1 mg/L as Al, the average floc size increased from 62 to 78 μm and the boundary fractal dimension was around 1.14, suggesting that flocs were compact and continuously grew during the entire flocculation process. However, with the dose increased to 5 mg/L, the average floc size decreased and stabilized at around 65 μm, with the fractal dimension of 1.20. It can be concluded that the excess coagulant doses resulted in the formation of chain-shaped, lower density, and more branched structure flocs, thereby restricting flocs' further growth in the subsequent flocculation. Floc morphology analysis suggested that charge neutralization dominated in the initial flocculation stage, then the bridge and sweep mechanisms were dominant in the subsequent flocculation. In addition, compared with the traditional inclined plate settler, a novel V-shaped plate settler introduced in this study had an advantage in small size floc (less than 5 μm) removal. The V-shaped region could promote aggregate restructuring and re-flocculation; therefore, the V-shaped plate settler provides an alternative method for sedimentation.
Article
Beneficiation of coal is a water intensive process, and the water is recovered in downstream operations in the high rate thickener (HRT) and vacuum belt filtration (VBF). Clean coal particles entering the HRT have an inherent tendency to enter the overflow because they are extremely fine and hydrophobic. Hence, flocculants (polymers) are used to floc the ultrafine particles so that they settle and enter the underflow. Ultrafine coal particles (−37 μ), which are highly hydrophobic, if present in the stream, will not be efficiently captured by using flocculants alone. This is observed in the normal plant operation at one of the beneficiation plants in the Jharia region, Jharkhand, India. Approximately 6–7% of the feed to the HRT enters the over flow (supernatant water). Hence, this amount of clean coal particles is not recovered and account for production losses. A comprehensive settling and dewatering test work was conducted with clean coal slurry using several commercially available reagents. Studies were also done on different combinations of reagents, including coagulants and surfactants along with flocculants, to improve the settling behavior of the coal particles. The reagents were assessed based on their performance with respect to settling rates, overflow water clarity, and compaction of the underflow solids. Emphasis was given to the ability of the reagents in compacting thickener underflow to higher densities so that maximum water could be recovered from the thickener. Pretreatment of highly hydrophobic coal slurry with a combination of coagulant and surfactant followed by flocculation showed very good settling characteristics with clear overflow.
Article
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The slurry settling characteristics are the most important to design a thickener in process industries. In this work, the iron ore slurry from the screw classifier overflow was used for the settling study. It was observed that the original slurry exhibited a low settling velocity and a turbid supernatant during the settling process. Commercial flocculating agents with anionic, cationic, and nonionic characters were used to improve the settling behavior of suspensions, which were added into the slurry at different ranges of slurry pH values, respectively. The settling results show that the use of flocculants increase the settling rate by several times. Compared with the cationic and nonionic flocculants, the anionic flocculant is more effective in enhancing the slurry settling rate. The small dose of the anionic flocculant is found to be more effective, but the other flocculants are less effective even at higher dosages. The simulation of an industrial thickener was carried out based on the laboratory settling data, and the appropriate design and selection parameters of the industrial thickener were estimated.
Book
Mineral Processing Design and Operationslt is expected to be of use to the design engineers engaged in the design and operation of mineral processing plants and including those process engineers who are engaged in flow-sheets development. Provides an orthodox statistical approach that helps in the understanding of the designing of unit processes. The subject of mineral processing has been treated on the basis of unit processes that are subsequently developed and integrated to form a complete strategy for mineral beneficiation. Unit processes of crushing, grinding, solidliquid separation, flotation are therefore described in some detail so that a student at graduate level and operators at plants will find this book useful. Mineral Processing Design and Operationslt describes the strategy of mathematical modeling as a tool for more effective controlling of operations, looking at both steady state and dynamic state models. * Containing 18 chapters that have several worked out examples to clarify process operations * Filling a gap in the market by providing up-to-date research on mineral processing * Describes alternative approaches to design calculation, using example calculations and problem exercises.
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The processing of fine particles inevitably produces slimes. These present a dewatering problem, for which gravity thickening provides a partial or preliminary solution. The paper shows the effects of practical factors in carrying out laboratory sizing tests for thickeners. Several methods of determining thickener unit area have been compared for a lightly coagulated phosphate slurry and a highly flocculated kaolin slurry. Practical techniques of mixing and flocculant addition have significant effects on batch settling data. Pulp temperature and cylinder dimensions are very important. Errors in sizing thickeners for highly flocculated slurries by the standard methods are discussed. Such methods are suitable for less compressible slurries, however. Rules of thumb for flocculent selection are given. (A)
Book
Important factors which influence the settling rate of a given metallurgical slurry include: feed dilution or solids-liquid weight ratio; size and shape of the particulate solids; specific gravity differential between the solids and liquid phases; presence of electrolytes and/or flocculants; pulp viscosity; and slurry temperature. In addition other parameters which may influence settling rate, thus thickener performance include: method of flocculant application, particle size distribution, particulate wetting characteristics, feeding arrangements, rake speed and type, existence of convection currents, wind disturbance, and evaporation. All of these factors should be examined, and the effect upon thickener performance brought about by manipulating the factor over the expected operating range for performance optimization. Refs.
Article
Sedimentation in highly flocculated suspensions of fine particles, such as clays, occurs through individual settling units (flocs) that have very open structures and correspondingly low densities (typically less than 5% solids by volume). Such settling processes rapidly lead to the buildup of a bed of flocs that are essentially close-packed, with an inter-floc porosity of perhaps 50%. However, the overall solids content is no more than about 2% to 3% by volume. Consequently, the thickening process is dominated by the bed consolidation rather than by the simple settling of the individual flocs. Experimental measurements of sediment consolidation in flocculated clay suspensions indicate, for a given system, that consolidation rates are essentially determined by the solids concentration and can be evaluated using the classical Kynch treatment. However, these rates are strongly dependent on the physical, as well as the chemical, conditions of the flocculation process. Furthermore, the extent to which gravity consolidation proceeds depends on the total weight of the sediment (i.e., on the bed height and the solids concentration). The implications with respect to thickener operation and the requirements for the laboratory testing for thickener design are discussed.
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The paper presents a development of thickening theory that provides a reliable method for determining the size of continuous thickeners from batch settling tests. It also offers a description of thickener operation performance based on the proposed method.
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Flocculation was found to improve the sedimentation process but its effectiveness depended on several factors such as the pulp density of feed, the type of dosage of flocculating reagent, the pH of pulp, the intensity of mixing flocculant with tailings sand and the use of stirring during compaction. It is shown that for a sedimentation process taking place under given conditions, underflow pulp density and sedimentation time are related by an expression.
Article
Engineers have long recognized the desirability of correlating the data obtained from small-settling tests with commercial work as carried on in large tanks. This need, though most apparent in designing new installations, frequently arises also in existent plants, since a large range of experimental work can be performed without interfering with regular operation.
Article
This paper aims at investigating the effects of densification of aggregates within a suspension on thickener dewatering performance. The comparisons of the maximum permitted underflow solids flux calculated from both an initial undensified thickener and a densified thickener were achieved. Large underflow solids fluxes were attained in densified thickeners. The effects of densification on the bed heights and on the solids residence Limes required to achieve a given underflow solids flux and a given underflow solids volume fraction were also computed and compared. Substantial reductions in the bed heights and the solids residence times are possible in densified cases. Previous studies have assumed the Functional form of the compressive yield stress in the suspension so as to give an exceedingly weak gel in the neighbourhood of the solids volume fraction at the top of the bed. The implications of considering a different gel rheology with a rather stronger gel were considered. The effects of this new rheology lead to a slightly less sharp spatial gradient in the solids volume fraction near the top of the bed. In addition, the effect of varying the underflow solids volume fraction was considered. The observations of substantial increases in underflow solids fluxes and substantial reductions in bed heights and solids residence times were only achieved when the underflow solids volume fraction was less than or comparable with the solids volume fraction within the aggregates. However, if the underflow solids volume fraction was considerably larger, aggregates were considered to be overlapping and interpenetrating. As a result, the improvements in thickener performance due to densification were insignificant.
Article
A one dimensional dewatering model for a thickener consistent with time-dependent densification of aggregates within a sludge suspension has been developed in this paper. The effects of different densification rate parameters and different underflow solids fluxes on the predictions of the sludge rheological properties and thickener performance have been explored. Pre-shearing of aggregates (which leads to the aggregates densifying to some extent before entering the thickener consolidating bed) must occur if a large underflow solids flux is required in a densified thickener. This increases the suspension gel point, and hence the solids volume fraction at the top of the bed. The solids volume fraction at the top of the bed will however be the initial undensified gel point in the case of a small underflow solids flux, where no pre-shearing of aggregates needs to occur. The effects of densification and/or pre-shearing on the sludge rheological properties and thickener performance have been predicted. For a densified thickener, an algorithm has been developed to determine the maximum permitted underflow solids flux, qmax, assuming any possible solids volume fraction at the top of the bed and the maximum underflow solids flux, qmu, constraining to the initial undensified gel point at the top of the bed. Regarding the aggregate densification rate parameter, as this parameter was increased, the sludge rheological properties evolved significantly during thickening and thickener performance was substantially enhanced. Moreover, the total solids residence time required to achieve the desired underflow solids volume fraction was significantly reduced both for a higher densification rate parameter and in the presence of pre-shearing in a densified thickener.
Article
The equations governing the settling and consolidation of flocculated fully networked suspensions under the influence of gravity, based on the assumption that the network possesses a compressive yield stress Py(oslash;) that is a function of local volume fraction (oslash;) only, are discussed. They are nonlinear partial differential equations with two moving boundaries, one at the top of the bed and the other marking the position of the consolidation region. A novel technique is used to solve these numerically. The time evolution of the volume fraction in the sedimenting column and the two moving boundaries are computed, and their dependence on the physical properties of the system are discussed. Analytic results for the steady state and the small time behavior are given.
Article
The sedimentation behaviour of particulate slurries in laboratory scale batch settling experiments is important to the prediction of full-scale gravity thickening behaviour. However, the material throughput predicted by modelling of thickener performance from material properties derived from conventional batch experiments has been shown to be up to 100 times less than observed (at full-scale). Such discrepancies have been attributed to various shear and compression-induced effects (prevalent at full-scale) that modify flocculated aggregate structure, including aggregate densification. Standard Kynch batch settling analysis has been modified to incorporate the effect of aggregate densification behaviour on material properties. Based on this modification, a semi-analytical method is presented, which provides improved real-time predictions of batch sedimentation behaviour. This method can produce a slurry–liquid interface height versus time profile up to the end of the fan region, based on a functional form of aggregate densification behaviour. Additionally, aggregate densification theory incorporated into the compressive yield stress is used to predict the final equilibrium bed height. A functional form is proposed that incorporates parameters which describe the rate and the extent of densification. Validations of this method are presented against theoretical numerical simulations and experimental data.
Article
A novel approach for relating suspension batch settling data (settling height vs time) to model settling flux functions is presented. The approach exploits a new closed-form solution relating settling height and time for a very simple functional form of the settling flux. The simple form in question employs a so-called hindered settling factor (a key material property in suspension rheology) that is taken to be a straight-line function of the solids fraction. The closed-form solution for settling height and time has a parametric dependence on the slope and intercept of the above-mentioned (straight line) hindered settling property: thus a functional relationship between batch settling height and suspension material property parameters is established. Moreover by adjusting the slope and intercept parameters, the closed-form solutions for settling height vs time can be matched to batch settling experimental data, and thereby the settling flux can be directly obtained. Unlike classical approaches for determining settling flux functions from batch settling data (i.e. Kynch theory), nowhere does the new approach require data for settling velocity: this gives it an in-built robustness to experimental noise compared to any approach that obtains experimental settling velocities via finite differences of settling heights, since such velocities tend to be far more noisy than the settling heights themselves. In a typical physical system, a straight-line relationship between hindered settling factor and solids volume fraction will only be a reasonable approximation over a very restricted domain of solids fraction. However, over a wider domain, it is possible to approximate the hindered settling factor vs solids fraction dependence via a sequence of straight-line relations, each taken over a narrow interval of solids fraction. A functional form for settling flux is thereby obtained, useful for suspension dewatering calculations and engineering equipment design for suspension/sludge processing. The novel approach for determining settling flux has been applied both to experimental and synthetic batch settling data, and has performed robustly.
Article
An authoritative critique of the established and recently proposed methods of thickener design.
Article
Thickening is the most widely applied dewatering technique in mineral processing. Thickeners are used to increase the concentration of suspensions by sedimentation, accompanied by the release of a clear liquid. As the particles get finer the thickening process encounters difficulty due to a significant change in the particles settling behavior. The batch settling tests of coal refuse of a coal washing plant that contained 91% particles smaller than 38 µm and 0.6% coarser than 75 µm showed that the optimum feed percent solids that provided highest flux (solids handling capacity) was 4%. The flux of the pulp with the plant solids concentration (i.e., 10% by weight) was 60% lower than that of the pulp with 4% solids. A thickener with a diameter of 22 m based on the dilution of feed from solids concentration of 10% to 4% was designed. Monitoring of the thickener performance for a period of one month in the plant indicated that an average feed rate of 25t/h (dry solids) with solids concentration of 10% could be thickened to an underflow concentration of 26.5% with a clear water overflow. It was found that the key component of the successful operation of the thickener is the dilution of the feed, without dilution the overflow loses its clarity and the system ceases to operate under predetermined conditions. Based on the results of established CFD (Computational Fluid Dynamics) studies, a feeding system that efficiently dissipated the energy of the incoming flow and a staged flocculant addition regime were utilized in the design and operation of the thickener.
Article
Batch flocculation tests in cylinders (jars) or stirred vessels suffer from poor control over the reaction time and involve broad shear rate distributions. The use of a linear pipe reactor provides continuous turbulent mixing of flocculant and slurry at well-defined mean shear rates. Combined with in-line, real-time monitoring of the aggregation state with a focused beam reflectance measurement (FBRM) probe, reaction times can be determined to within a fraction of a second. The resultant reaction profiles of aggregate growth and breakage obtained while varying different conditions (dosage, solids concentration, shear rate, split dosing) provide new insights into flocculation kinetics that are more relevant to mineral processing applications. In this study, the reaction profiles are examined in terms of the effects of flocculant adsorption, applied shear and solids dilution on the aggregate structures formed. Also presented is the first definitive evidence of “post-aggregation”, where surface adsorbed flocculant remains active at short reaction times (up to the peak in aggregate size in the measured profiles), allowing additional aggregate growth under subsequent reduced shear. At reaction times beyond the peak, little or no additional growth is observed. This behaviour explains the observed discrepancy between size and settling rate reaction profiles, and highlights the importance of optimising hydrodynamics in industrial flocculation applications to maximise settling flux.
Article
Solid–liquid separation of tailings slurries in gravity thickeners relies on the efficient mixing of slurry and dilute polymer flocculant solutions within the feedwell. Computational fluid dynamics (CFD) can provide predictions of solids distribution, liquor velocity and shear rate within a feedwell, and with the incorporation of an adsorption model, can also assess the effectiveness of flocculant mixing. This study presents the first use of feedwell CFD to examine the effect of the flocculant inlet direction and velocity on the subsequent distribution and adsorption of flocculant. When flocculant is injected inside the feed stream, a high inlet nozzle velocity will maximise adsorption, with injection preferably vertically upward or towards the feedwell walls. For injection inside the dilution stream (vertical upflow of liquor within the feedwell), the flocculant should be directed either upwards or inwards away from the strong downward flow of the feed stream, with the velocity not critical.At flocculant inlet velocities predicted by CFD to enhance mixing and adsorption, the shear rate experienced within the injection pipe exceeds that in the feedwell, and the duration under higher shear may be greater. Pipe flow studies for several flocculants have confirmed reductions in activity at a solution concentration of 0.025wt.%; this effect diminishes with greater dilution. Much of this lost activity is recovered on standing, indicating that the applied shear leads to a mixture of chain scission (irreversible) and entanglement (reversible). Minimising the duration of such shear effects on flocculant solution transport to the feedwell is essential, as the potential for increased flocculant demand and reduced flocculation efficiency can easily exceed any benefit from improved feedwell mixing.
Article
Laboratory batch cylinder tests are commonly used to assess the effectiveness of flocculants to enhance the settling rate of suspensions. In this paper, the performance obtained from batch cylinder tests is compared to that from a new flocculation system based upon vertically mounted, concentric rotating cylinders (Couette geometry). It has been shown that the results from batch cylinder tests are very dependent upon factors such as the number of inversions and the cylinder diameter. These limitations are overcome by conducting flocculation in a continuous mode under reproducible mixing conditions such as prevail within a mechanically driven Couette device. Results obtained with this equipment are found to be of much higher reproducibility than from cylinder tests conducted under set conditions.
Article
The aim of gravity thickening processes is to increase the solids concentration of particulate slurries. Gravity thickening depends on the difference in densities between the solid and liquid phases. The solids settle to a more concentrated slurry zone towards the underflow at the thickener base, while relatively solids free liquid rises to the overflow at the top. Predictive modelling of gravity thickener performance from experimentally determined material properties has been shown to under predict throughput by a factor of up to 100. One phenomenon proposed to account for some of this discrepancy is aggregate densification, whereby aggregates compact and become smaller when subjected to shear forces in the thickening process. As the aggregates decrease in size, through densification, the tortuosities around the aggregates will decrease, thus leading to a significant net decrease in the resistance to fluid flow. Dewatering theory has been applied to predict the impact of aggregate densification on the material properties that describe dewatering. The anticipated decrease in aggregate size and associated increase in the density of these aggregates is expected to increase the settling rate. It is further expected on this basis that the material will settle to higher solids concentrations. The impact of these changes on gravity thickening performance is modelled and predicted to be significant.
Article
An algorithm has been developed to predict steady state thickener operation from fundamental material properties, properly accounting for compression of the suspension network structure within the sediment bed. The material properties include the compressive yield stress, Py(ϕ), and the hindered settling function, R(ϕ). Py(ϕ) reflects the suspension network strength as a function of solids volume fraction ϕ, while R(ϕ) is inversely related to the permeability. The required inputs to the model include Py(ϕ) and R(ϕ) curve fits, thickener diameter as a function of height, solids density, liquid density and feed solids volume fraction. The model output is either solids throughput or solids flux as a function of underflow solids concentration, for a range of suspension bed heights. As a bonus, the solids residence time in the suspension bed can also be determined.
Article
A new experimental procedure for determining the solids flux curve is detailed. The procedure is based on measuring the concentration at various heights of a bed of settled solids formed during a semicontinuous sedimentation test. Unlike conventional thickener area calculations, the test procedure demonstrates the dependence of the flux curve on the system feed flux. At a given feed flux, there was good agreement between the solids flux curve determined by the method and continuous sedimentation results. The widely used method of Kynch was found to be restricted to relatively high effective feed fluxes and consequently there was poor agreement with continuous tests at low feed fluxes. The flux curve determined by the method of Coe and Clevenger was invalid for all feed fluxes. A case study shows how the required thickener area would vary for a number of fine coal processing options.
Article
Batch settling tests are considered in order to obtain dewatering material properties of suspensions/sludges towards the low end of the range of solids fractions. Plausible functional forms are considered for fitting batch settling test (height vs. time) data. In particular, power law and exponential decay functions are shown to be reasonable fits to simulated synthetic batch settling data. These forms are subsequently employed to reconstruct functional relationships between a settling flux function and suspension solids fraction. The functional relationships so obtained are found to be faithful representations of the flux function used to generate the simulated settling data, with improved agreement being achieved by restricting the interval of solids fraction across which the reconstruction is performed. The results suggest that general features only (and not fine details) of batch settling curves are required to reconstruct settling flux functions. In the particular case where power law fits are employed to describe the settling height data, an analytic formula can be derived linking the settling flux function explicitly and directly to the power law fitting parameters. This simplifies immensely the technique for extracting parameters for the settling flux. When applied to real experimental data, errors arising from using the power law fits tend to be small compared to those inherent in the experimental measurements themselves.
Article
The theory assumes that the speed of fall of particles in a dispersion is determined by the local particle density only. The relationship between the two can be deduced from observations on the fall of the top of the dispersion. It is shown that discontinuous changes in the particle density can occur under stated conditions.
Article
Flocs generated by various shear forces exhibit different characteristics of size, strength and structure. These properties were investigated by employing a continuous optical monitoring and a microscope with CCD camera to directly monitor aggregation under six different shear intensities. The floc structure was characterized by the fractal dimension. The results showed that the flocculation index (FI) decreased from 1.16 at 20 rpm to 0.25 at 250 rpm and the floc size decreased from 550 μm to 150 μm, meantime, the FI value showed a good correlation with floc size. In order to determine the floc strength, two methods were used. One was the strength factor, ranging from 18.3% to 62.5%, calculated from FI curve, and the other was a theoretical value between 0.005 N/m2 and 0.240 N/m2, estimated by calculation. The floc strength increased with the G value in both cases. Furthermore, the fractal dimension increased with G and its value was between 1.30 and 1.63. The relation between fractal dimension and strength was also obtained.
Book
Wills' Mineral Processing Technology provides practising engineers and students of mineral processing, metallurgy and mining with a review of all of the common ore-processing techniques utilized in modern processing installations. Now in its Seventh Edition, this renowned book is a standard reference for the mineral processing industry. Chapters deal with each of the major processing techniques, and coverage includes the latest technical developments in the processing of increasingly complex refractory ores, new equipment and process routes. This new edition has been prepared by the prestigious J K Minerals Research Centre of Australia, which contributes its world-class expertise and ensures that this will continue to be the book of choice for professionals and students in this field. This latest edition highlights the developments and the challenges facing the mineral processor, particularly with regard to the environmental problems posed in improving the efficiency of the existing processes and also in dealing with the waste created. The work is fully indexed and referenced. · The classic mineral processing text, revised and updated by a prestigious new team · Provides a clear exposition of the principles and practice of mineral processing, with examples taken from practice · Covers the latest technological developments and highlights the challenges facing the mineral processor · New sections on environmental problems, improving the efficiency of existing processes and dealing with waste.
Article
The hindered settling function R(ϕ) is a material function that quantifies the interphase drag of colloidal suspensions for all solids volume fractions ϕ. A method is presented to estimate R(ϕ) from batch-settling tests for solids volume fractions between the initial solids volume fraction, ϕ0, and the solids volume fraction at which the suspension forms a continuously networked structure, ϕg, known as the gel point. The method is based on an analytic solution of the associated inverse problem. Techniques are presented to address initialization mechanics observed in such tests as well as experimental noise and discrete data. Analysis of synthetic and experimental data suggests that accurate estimates of R(ϕ) are possible in most cases. These results provide scope for characterization of suspension dewaterability from batch-settling tests alone. © 2005 American Institute of Chemical Engineers AIChE J, 2005
Article
The equations governing consolidation in a continuous-flow gravity thickener are developed based on the assumption that a flocculated suspension possesses a compressive yield stress Py(ϕ) that is a function of local volume fraction only. These equations are used to model the steady state operation of a thickener. The bed height required to achieve a given underflow concentration is found to be a relatively sensitive function of the details of the Py(ϕ) function, particle flux through the thickener, and variations in the cross-sectional area of the thickener. The limiting values of the underflow concentration ϕu for a given flux or the limiting values of flux for a desired ϕu are studied and shown to exist only for cylindrical and converging thickeners.
Article
Polyacrylate flocculation of haematite in synthetic Bayer liquors has been studied to determine the effect of increasing caustic strength and carbonate concentration. The drop in flocculation efficiency observed with increasing caustic concentration (0.09 to 6.23 M) was shown not to be due to changes in the flocculant adsorption density or the flocculant molecular size in solution. The reduction in the settling rate could be only partially explained by the physical effect of the higher solution viscosity. Consequently, it is postulated that the increased viscosity also causes a shift in the balance between the rate of flocculant adsorption and the interparticle collision rate, which reduces the proportion of flocculant molecules forming bridges between particles. The drop in flocculation efficiency with increasing carbonate (0.03 to 0.63 M) in the synthetic Bayer liquor was attributed to the slow rate at which flocculant can displace carbonate adsorbed on the haematite surface.
Article
The prediction and optimisation of the performance of dewatering devices such as filters and thickeners requires the laboratory measurement of fundamental material characteristics. This work presents the application of recently developed methods for the extraction of meaningful phenomenological properties from filtration and sedimentation testing of wastewater sludges, which has previously been limited due to theoretical and experimental constraints. The results for a sample of digested wastewater treatment sludge described a material with weak network strength that was quite permeable at low solids concentrations and the gel point was about 1 v/v%. The sample compressed to high solids concentrations at moderate pressures (up to 40 v/v% at 400 kPa) but became extremely impermeable at these high concentrations. The solids concentration dependencies of the material characteristics had a form that was expected for highly compressible materials such as wastewater sludges, but had not been reported previously. The extracted material characteristics were then used to predict the test results to show that the characterisation was valid.
Article
The present work deals with the estimation of average floc diameter using sedimentation and optical data. This study was applied to a kaolin suspension destabilized by chemical dosing with iron salts (FeCl3 or FeSO4) or electrocoagulation (EC) with iron electrodes. Optimum pH and coagulant concentration were defined by turbidity measures on destabilized suspensions. The suspensions were subsequently settled in a vertical column. Critical heights of sedimentation delimiting hindered settling and consolidation ranges were estimated when plotting data of settling rate of the suspension–supernatant height as a function of time. Average floc diameter and density were assessed considering the Richardson and Zaki model in the hindered settling range. Floc images obtained by optical microscopy allows us to check the relevance of the floc sizing derived from sedimentation data. Eventually, the estimated size and density of flocs generated by EC and chemical dosing techniques were compared.
Article
The design and operation of flocculation processes are discussed in the context of the specific requirements of dewatering systems such as sedimentation and filtration. Chemical conditions, reagent selection and process operating conditions are evaluated based on the fundamental mechanisms involved in particle destabilization and floc development. Opportunities for control of floc characteristics through appropriate process design are described. Specific requirements for different dewatering processes are discussed.
Article
From their ancient beginnings to the present day, coagulants and flocculants, used to enhance solid–liquid separation by particle aggregation, are described; particular reference is given to developments in the mineral processing industry. The impact of polyacrylamide-based flocculants on technical effectiveness, disposal possibilities and environmental control, over the last 40 years, has had a profound effect on the efficiency and design of solid–liquid separation equipment. Recent developments, in terms of molecular architecture, offer new horizons for the future at a time when requirements for separating solid particles from liquid are becoming more demanding in the minerals industry.
Article
Modelling the sedimentation of suspensions with partial differential equations requires constitutive relations (material properties) to be known. Restricted to suspensions obeying Kynch's assumption (ideal suspensions), this paper deals with the inverse problem, which is to estimate the batch-settling flux function from experimental data. A new batch-settling test is suggested, from which it is theoretically possible to estimate a large part of the flux function for lower concentrations containing the extreme point. From a standard batch-settling test, a large part of the flux function for higher concentrations can be estimated with the famous method by Kynch. For these two parts, simple general explicit formulae are derived, which contain only the initial concentration and height variables, the interface height and its derivative as a function of time. The method is demonstrated on synthetic and experimental data. Further experimental development of the new test is required. The aim of the paper is to present a theoretical foundation for the method, including the explicit formulae as a solution of the inverse problem.
Article
A series of flocculation tests were performed to investigate the effect of low-shear rates (G = 3-16 s(-1)) on flocculation of kaolin suspension by polyaluminum chloride (PACl), with the goal of understanding floc growth mechanisms. Results were reported in terms of floc average size (d(p)) and boundary fractal dimension (D(pf)), derived from a non-intrusive optical sampling and digital image analysis technique. As expected, the rate of floc aggregation increased with increasing G, resulting in faster changes in aggregate size and structure in the initial stage of flocculation. Nevertheless, steady state was attained faster for D(pf) than for d(p) at the same shear rates, possibly due to the self-similarity of fractal aggregates. An interesting finding was that at G = 3 s(-1), an obvious plateau was observed for the average-size evolution at steady state; for shear rates of 6 and 7 s(-1), the flocs exhibited some decrease after reaching the peak of size, mainly as a result of floc settling at steady state; and for G = 11-16 s(-1), a decrease in floc size was possibly attributed to the irreversibility of PACl-floc breakage. The process of floc growth was described using a fractal growth model, which defined flocculation as the result of the combined processes of aggregation and restructuring. The conceptual model could effectively characterize temporal changes in floc size and structure, and found that fragmentation followed by reformation seemed to be more effective in forming larger and more compact aggregates than the restructuring process due to erosion and reformation, which may provide useful insights for the design of flocculation reactors.
Article
Flocs generated in various aluminum coagulants treatment exhibit difference in size, strength and structure. The formation, breakage and re-formation of flocs were investigated by using a laser diffraction instrument (Malvern Mastersizer 2000, Malvern, UK). Different Al coagulants--conventional Al salt (AlCl(3)), polyaluminum chloride (PAC-1) and the purified polyaluminum chloride (PAC-2)--were investigated for the coagulation of humic acid (HA). The results showed the properties of HA flocs depend on which species of aluminum used. The monomeric and dimeric aluminum species (denoted as Al(mono)) formed the largest and strongest HA flocs with longest growth time. The HA flocs formed by Al(13)O(4)(OH)(24)(7+) (Al(13) for short) is the smallest and weakest. The properties of HA flocs made it possible to investigate the relationship between the flocs growth process and aluminum species.
Article
The main focus of the paper is to review current understanding of floc structure and strength. This has been done by reviewing current theoretical understanding of floc growth and breakage and an analysis of different techniques used for measuring floc strength. An overview has also been made of the general trends seen in floc strength analysis. The rate of floc formation is a balance between breakage and aggregation with flocs eventually reaching a steady-state size for a given shear rate. The steady-state floc size for a particular shear rate can, therefore, be a good indicator of floc strength. This has resulted in the development of a range of techniques to measure floc size at different applied shear levels using a combination of one or more of the following tools: light scattering and transmission; microscopy; photography; video and image analysis software. Floc strength may be simply quantified using the initial floc size for a given shear rate and the floc strength factor. More complex techniques have used theoretical modelling to determine whether flocs break by large-scale fragmentation or smaller-scale surface erosion effects, although this interpretation is open to debate. Impeller-based mixing, ultrasound and vibrating columns have all been used to provide a uniform, accurate and controllable dissipation of energy onto a floc suspension to determine floc strength. Other more recent techniques have used sensitive micromanipulators to measure the force required to break or compress individual flocs, although these techniques have been limited to the measurement of only a few hundred flocs. General trends emerge showing that smaller flocs tend to have greater strength than larger flocs, whilst the use of polymer seems to give increased strength to only some types of floc. Finally, a comparison of the strength of different types of floc (activated sludge flocs, organic matter flocs, sweep flocs and charge neutralised flocs) has been made highlighting differences in relative floc strength.