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Aggregate densification in the thickening of flocculated suspensions in an un-networked bed
... This can happen as a result of the flocs being subjected to shear within a thickener, e.g. due to the action of rakes (Spehar et al., 2014; Gladman et al., 2010). Shear leads to the so-called aggregate densification, i.e. individual aggregates bind together more tightly (Farrow et al., 2000; Usher et al., 2009; van Deventer et al., 2011), which is highly beneficial for the dewatering process. ...
... Recently however a study has appeared (Grassia et al., 2014) suggesting how to incorporate aggregate densification into the framework of Kynch theory. This study would apply to situations where the degree of thickening required (measured by a target underflow solids fraction) is relatively modest so that either the underflow does not form a gel (Spehar et al., 2014) or else it is just barely gelled, with very significant amounts of ungelled material elsewhere in the thickener. Alternatively the study of Grassia et al. (2014) could be considered to correspond to a case where a particularly high solids flux is required (since there is known to be a trade-off between solids flux achieved and solids fraction attained during thickening (Usher and Scales, 2005)). ...
... In cases where aggregate densification is produced by raking the suspension, A depends on the rate of raking. Potentially aggregate densification could also be sensitive to solids concentration (Spehar et al., 2014), and since solids concentration varies with position in the thickener, and since position reached depends on residence time t res , potentially A could become a function of t res : such complications will however be neglected here. Following the explains how densification causes wider channels to be opened up between aggregates which results in lower frictional drag and faster aggregate settling. ...
... thickener. This is an important observation and indicates that the process is driven by inter-aggregate collisions as much as the effect of shear on an individual aggregate (Spehar et al. 2015) and forms the basis of the Outotec SET process (Loan & Arbuthnot 2010). ...
... shear rate (Spehar et al. 2015). Given that the process is not instantaneous and it is difficult to mix the entire thickener simultaneously, the residence time in the thickener and the amount of time a given aggregate is exposed to shear is important to the end result. ...
... The change in aggregate diameter as a result of densification is measured to be of order 15-25% 4. (Spehar et al. 2015). ...
... Indeed, the dewatering of cohesive sediment suspensions is of great interest more generally to minerals, wastewater processing and nuclear industries, where storage and packaging costs are high and so minimisation of waste volume is crucial. The ability to model these systems as a function of parameters such as rake speed, aggregate size and, of concern in this paper, bed density, has therefore been studied extensively through both semi-empirical settling models and CFD modelling of flocculation, aggregation and settling dynamics within thickener operations [6][7][8][9][10][11]. The application of various sludge blanket and particle concentration measurement techniques [12] such as X and -ray absorption [13] and conductivity and light based measurements [14] have thus been studied in previous reported literature. ...
... Due to the potential change in slope of the bed formed over time, it was not possible to eliminate the error reliably, although for most of the time, the off-set between the two measurement techniques was relatively constant (suggesting minimal changes to the slumping angle) before ratholing occured toward the end of the experiment. This indicates an increase in sediment concentration with bed height; an effect that has been widely reported and is due to both the increased residence time within the bed and the larger compressive forces exerted on the bed [6]- [8]. Figure 6 shows a colourmap of solids residence time vs bed height during the experiment calculated using the algorithm described in Equations 1 -5. ...
To aid in the transportation, dewatering and storage of radioactive UK legacy waste sludges at Sellafield Ltd., the application of an Acoustic Backscatter System (ABS) was investigated in this study, as a method for monitoring suspended solids concentration and consolidation of cohesive and aggregated sludges (that form the basis of many wastes at Sellafield). An ABS represents a particularly promising technology, as they can be used with minimal intrusion, to measure both particle size and concentration depending on prior system knowledge, through inversion of the return echo voltage response of pulse-echo signals in the MHz range. To assess the application for continuous, high concentration (~30 % v/v) thickened wastes, an ABS was utilised to characterise a pseudo-steady state laboratory thickener over hours, using flocculated calcite as a representative test material. Measurements were taken both horizontally through a consolidated bed using a 1 MHz transducer and vertically downwards with a 2 MHz transducer to measure sediment concentration above the settled bed and track bed height via the interface reflection. Characterisation of flocculated aggregate diameters was initially determined to be ~510 μm, using an inline Focussed Beam Reflectance Measurement (FBRM) device. The submerged 2 MHz probe indicated that there were only limited changes to the dispersion concentration or size over time. Changes in settled bed density, due to increasing bed height and changes in the underflow flow rate, were qualitatively measured via differences to the acoustic signal attenuation over time. An initial increase in attenuation with bed height was observed as the bed densified due to local compression. Once an equilibrium bed height was reached, the acoustic attenuation remained fairly constant with time, although the bulk underflow density was reduced with the increase in underflow rate. The solids residence time in the bed, determined from a transitive volume-balance model, was found to be ~3200 s for pseudo-steady state operation at a maintained bed height of ~0.175 m. Results of the study highlight the potential the ABS as a remote process monitoring tool for both relatively dilute suspensions and concentrated thickened mineral sludges, with potential applications across waste processing sites at Sellafield.
... where the aggregate volume is equivalent to the equilibrium suspension volume multiplied by the packing fraction , for which a nominal value of = 0.6 was assigned to account for random loose packing associated with BIOS [17,42,44]; in other words, the volume occupied by the hydrated aggregates is 60% of the total suspension volume, with the remaining 40% accounting for the open pore space between aggregates. Estimates for ℎ , obtained initially from Equation (8), were converted to corresponding final equilibrium volumes for each cycle. ...
... Additionally, during the process of settling under gravity in a vertical column, wall surface adhesion processes can affect the settling behaviour of the solids in the column. The fact that densification of suspensions can occur even in the absence of applied mechanical shear has been previously recognised by Spehar et al. [44] who identified inter-aggregate buffeting as a relevant mechanism. Benn et al. [45] note that wall effects are minor in the initial stages of solid settling but can become considerable during slower bed consolidation over periods of time much longer than the duration of experiments in the present study. ...
Bacterial-mineral aggregates are the products of a tight biogeochemical coupling between microbes and geological media and play an outsized role in governing the composition of natural waters through biogeochemical cycling and mineral formation and dissolution processes. The results of combined batch column settling experiments, volumetric analyses, and microscopic investigations demonstrate that composite bacteriogenic iron oxide aggregates are sensitive to densification in response to hydrodynamic shear, a physical fluid phenomenon that introduces significant alterations to aggregate size and structure, permeability, and settling and transport behaviour. After exposing aggregate suspensions to varying degrees of shear stress, final solids volume fractions decreased by as much as 75% from initial data, while aggregate bulk density saw increases from 999 kg•m-3 to as much as 1010 kg•m-3. Inverse modelling of time course data yielded estimates for settling rate constants and initial settling velocities that increased with shear stress application. As well as having implications for aqueous contaminant transport and potential bacterial bioenergetic strategies, these results suggest the preservation potential of microfossils formed from bacterial-mineral aggregates may be significantly reduced with shear-induced alterations, leading to a possible underrepresentation of these microfossils in the sedimentary record and a gap in our understanding of early life on Earth.
... This migration of fine particles or flocs can easily lead to the closure of dewatering channels, giving rise to more obvious flow with higher dewatering flow rate [113]. There are differences in the blocking effects of particles of different sizes, with small particles being more likely to change the original flow paths surrounded flocs; while the coarse particles cannot completely block the dewatering channel [114], making it easier for the liquid to bypass and continue to downward conduct [115]. As Fig. 8c shows, this inter-floc structure inside mud layer is disordered and complex, forming a liquid flow accumulation at the pore throat, which directly affects the blockage of the pore throat or formation of new dehydration pathways. ...
... The mud layer is too thick and the flow rate is too small, making it difficult to achieve the desired concentration adjustment effect. The height of the mud layer refers to the distance from the free settlement and interference settlement interface to the bottom of the container when the tailings particles or flocs settle [20]. The bottom flow concentration is mainly affected by the pressure formed by the height of the slurry, and the bottom flow concentration varies depending on the pressure on the slurry. ...
There are some problems in the application of slurry preparation technology, such as wide fluctuation range of underflow concentration, long settling time and low efficiency of solid–liquid separation. This is an important basis for researching the thick settling law of tailings slurry under the action of mechanical vibration and its influencing factors to solve these problems. To this end, a small vibration thickening testing machine and vibrating rod were designed and developed. Physical simulation experiments were conducted to analyze the settling characteristics of tailings slurry under different vibration duration, start time, vibration frequency, and vibration inertia single factors. The results show that: (1) Mechanical vibration can effectively accelerate the settling speed of tailings particles, but the relationship between them is a non-positive correlation, and mechanical vibration time control with in 5 mins is the best. With the delay of starting the vibration time, the final mass concentration first increases and then decreases. (2) As the vibration frequency increases, the final mass concentration of tailings settlement first increases and then decreases. When the eccentric vibrator speed is 6000 r/min, the best slurry settlement effect is achieved. (3) When the vibration inertia of the eccentric oscillator is 0.158 g·cm2 and the final mass concentration reaches 70.1%, the settling time only takes 210 min. (4) The lower the slurry concentration, the faster the settling speed. As the initial concentration increases, the final thickening time is also gradually prolonged. The research results provide some insights for the rapid thickening technology of rake-free paste thickeners.
... Some scholars have systematically investigated the flocculation, sedimentation and solidification characteristics of flocculants for tailing slurry to mitigate the dewatering of tailing slurry and thickened beds [14][15][16][17][18][19]. Although the new types of flocculants can form a compact floc structure and possess high strength by changing the bridging and flocculating environment between particles to improve the solidification efficiency of tailing beds, the tailing flocs exhibit strong porosity and contain a large quantity of water after flocculation; thus, the retained moisture can be released only through further compaction [20][21][22]. Xu et al. [23] studied the influences of solid content, adhesive proportion and cement reagent type of tailings on microstructural evolution. Deng et al. [24,25] analyzed the relationships of compressive strength with the proportion of pores with different radiuses and fractal dimensions according to the nuclear magnetic resonance porosity tests and the fractal characteristics of pore structure. ...
The tailing paste thickening technology was investigated to achieve goaf reduction treatment and tailing resource utilization of metal mines and reach the effect of controlling two hazards with one waste. However, superfine tailing particles could easily form suspended water-locking flocs in the thickening process, which seriously affected the increase in the underflow concentration in the thickener. Undisturbed compression-stage bed samples were extracted using an in situ sampling method through a continuous dynamic thickening experiment. Then, the morphologies and geometrical structures of micropores were analyzed through high-precision computed tomography scanning. Subsequently, the influences of the shear evolution of pore structure and seepage channel on the dewaterability of underflow slurry were explored by combining Avizo software and 3D reconstruction technology. The thickening and dewatering mechanism of underflow slurry was also revealed. Results showed that under the shear action, the flocs were deformed and compacted, forming a high-concentration underflow. On this basis, the original micropores were extruded, deformed and segmented. Moreover, many loose micropores were formed, the connectivity became poor and the total porosity declined. The diameter of the water-conducting channel in the sample was enlarged because of the shear force and the seepage effect improved. The maximum flow velocity inside the pores was 1.537 μm/s, which was 5.49% higher than that under the non-shear state.
... Owen et al. [8] showed that the rake may break the honeycomb structure of aggregates under low shear in the compression zone, and the intensity and duration of the rake action were critical factors. Many researchers [6,[9][10][11][12][13] concluded that the shear stress introduced by raking can dramatically improve the thickener performance in bed density, but it is still not enough to release an amount of water trapped in aggregates. However, at a business unit of Rio Tinto, a 2 wt. ...
Gravity thickening is an important aspect to solve numerous environmental and safety problems that were created by tailings discharging at low solid concentrations. Furthermore, in order to efficiently facilitate the separation of released water and solid sediments, a continuous thickening system with ultrasonic equipment has been used to investigate the thickening performance of copper-mine tailings under different ultrasonic frequencies (16 kHz, 20 kHz, 22 kHz, 25 kHz, and 28 kHz). After freeze-drying treatment, the underflow samples are imaged using the scanning electron microscope (SEM); then, the structure of floc or aggregates in the SEM images is quantifiably analyzed using the software of Image J. Results show that the underflow concentration increases as the ultrasonic frequency increases and decreases afterwards. A linear logarithmic function can explain the relationship between underflow concentration and run time at a certain ultrasonic. The underflow concentration is maximized at 64.47 wt. % when the ultrasonic frequency is 22 kHz. Based on the analysis on the microstructure of underflow samples, the minimum pore average size and pore average fraction are obtained when the ultrasonic frequency is 22 kHz, implying that 22 kHz is the optimum ultrasonic frequency combining the results of the underflow concentration.
... Settling in tailings pond under the influence of flocculants has been the most commonly adopted approach due to its higher convenience and feasibility rate when compared with traditional gravity settling which is slow and possesses low underflow concentration. Furthermore, by normal gravity settling, the settling of the ultrafine particles present in the overflow of the slurry is difficult [6,7]. There are three groups of flocculants: organic synthetic flocculants, such as polyethyleneimine and polyacrylamide derivatives; inorganic flocculants, such as polyaluminum chloride and aluminum sulfate; and naturally occurring flocculants, such as sodium alginate, chitosan, and microbial flocculants [8]. ...
The low-grade iron ore generally contains goethite as the major iron phase mineral and kaolinite and other minerals as gangue minerals. During the beneficiation of low-grade iron ore, a lot of fines were generated during different unit operations. These fines report to the tailing pond for the settling of the particles. The presence of kaolinite and goethite minerals reduces the settling rate. Therefore, different flocculants are used to improve the settling rate in the tailings pond. This study highlights the utilization of different flocculants in the settling of Iron Ore Tailings slurry (IOTs) in a tailings pond. Performances of different flocculants such as polyacrylamide, guar gum, and alstafloc are used to study the efficiency of the settling behaviour of IOTs. The tests were performed with fixed doses of flocculant consumption, slurry concentration, and flocculant concentration. Rheological studies were also performed on the IOTs samples to determine the viscoelastic behaviour for specific applications. From the study, it has been observed that the use of polyacrylamide as a flocculant in IOTs results in faster settling of iron ore, leaving a clear supernatant.
... New alternatives flocculants have been published to generate a denser flocs structure by modify the inter-particle bridging and flocculation circumstance, which for improving the tailings bed consolidation efficiency [20−22]. Furthermore, the flocs compaction behavior upon shear is also a key factor to improve the raking thickener performance [23,24]. The flocs shape became more spherical with the shear rates, in addition to the changes of flocs size and structure after shearing [25,26]. ...
Higher concentration is beneficial for the Paste and Thickened Tailings (PTT) operation in metal mine. Partial paste thickeners are produced lower density underflow. Flocculated tailings are intended to form a water entrapped network structure in thickener, which is detrimental to underflow concentration. In this study, the continuous thickening experiment was carried out for ultra-fine tungsten tailings to study the influence of rake shearing on underflow. The micro pores structure and seepage flow in tailings bed before and after shearing are studied by CT and simulation approach to reveal the shearing enhancement mechanism of thickening process. The results shown that, the underflow concentration is increased from 61.4 wt% to 69.6 wt% by rake shearing in a pilot scale thickener, the porosity decreased from 46.48% to 37.46%. The entrapped water discharged from sticks structure more than sphere spaces. In items of seepage, after shearing, the seepage flow channel of tailings underflow is becoming longer, which caused the decreasing average flow rate decreases and absolute permeability. The absolute permeability is negatively correlated with tortuosity. The rake shearing can destroy the flocs structure; change the effective stress to increase the concentration. Higher underflow concentration improves the waste recycling and water recovery rate, especially for arid areas.
... Thickener of brine Pinto et al. [47] Td8 Pilot scale column thickener Spehar et al. [48] Td9 Laboratory scale settler for biological sludge Spirandeli et al. [49] Td10 Pilot scale deep cone thickener Jiao et al. [50] Td11 Clarifiers for wastewater from treatment plant 3 m depth and 33 m diameter Weiss et al. [51] Td12 Desing and construction for thickener Spartalis [52] Td13 Thickener for tailings of copper from copper mine 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ...
The present work aims to represent the sedimentation curve of different industrial pulps through the Weibull model PDF, which employs two parameters k and G, dispensing the graphical methods to design thickeners and clarifiers, using exclusively mathematical equations. This provides greater accuracy and increases significantly the speed in sizing calculations. To ratify the use of the model, sedimentation curves of various types of pulps were generated in the laboratory, several types of curves were obtained through tests given in the literature and exhaustive comparison tests between the data obtained in the tests and those predicted by the model. The proposed model presented good data adjustment in relation to those obtained in several bibliographic references, presenting maximum error of 12%.
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... The calcite particles were flocculated with a high molecular weight, medium charge density anionic polymer 'Flowpam AN934SH' (SNF Ltd., U.K). Similar flocculated calcite systems have been investigated in studies of model mineral solidliquid separations (Benn et al., 2018;Gladman et al., 2010;Heath et al., 2006a;Hunter et al., 2015;Spehar et al., 2015;van Deventer et al., 2011). ...
The performance of a pilot-scale horizontal sedimentation tank was characterised utilising computational fluid dynamics (CFD) and a bespoke ultrasonic backscatter array, for both spherical glass and flocculated calcite separation. The CFD simulation was developed in OpenFOAM, using algebraic slip and hindered settling models, in order to solve the transport of multiple particle size classes, enabled through a population balance approach. Simulations of concentration compared closely to samples for the glass dispersions, but under-predicted concentration with flocculated calcite (likely due to complexities from modelling floc break-up in the mixer). In comparison, the acoustic array measured the calcite concentration with a high degree of resolution, where in particular, evidence suspension mobilisation near the outlet was observed, due to recirculation. Overall, we demonstrate the performance and current limitations of large-scale CFD for complex floc systems, as well as the use of ultrasonics to significantly aid process understanding, through online monitoring of solid-liquid separations.
... Mechanical shear within beds was identified by Spehar et al. [117] as enhancing the rate and extent of aggregate densification in both a laboratory-based fluidisation rig and a pilot-scale thickener. From this, Scales et al. [118] developed thickener models incorporating the shear effect on aggregate structures through a shear rate dependent densification rate and final extent of densification. ...
The presence of clays can make dewatering of tailings from mineral processing challenging. This review focuses on the interaction of clays in tailings under different conditions, including the impact of high salinity. The surface chemistry and aggregation behaviour of clays in suspension are first discussed, and this is then extended to clay aggregation responses during polymer-induced flocculation, with a view to developing an understanding of how flocculant properties and applied shear can be optimised to enhance dewatering processes. There is increasing interest in polymer dosing to high solids tailings streams to enhance dewatering on deposition, and the demands for this process are contrasted against those for conventional flocculation at lower feed solid concentrations.
The review concludes by highlighting a number of research challenges, which include that: (i) aggregate structures from polymer -assisted flocculation that are optimal for settling cannot also be assumed to be so for sediment consolidation; (ii) there is scope for reducing longer-term tailings impacts by choosing novel processing routes in the flowsheet design phase, but this requires insights on how tailings may behave after such processing at a much earlier stage; and (iii) most flocculation studies suffer from inconsistent, ill-defined or inappropriate conditions during the preparative stages, which prevent definitive conclusions being drawn on how high salinity impacts upon tailings flocculation, despite evidence that it can change aggregate density.
... Thus, the high concentration of suspended solids of the source rock remaining in the recycled water makes it difficult to reuse it during enrichment. Therefore, it is advisable to increase the deposition rate of fine particles by adding flocculants to the water, which are widely used in such cases (Spehar et al. 2015, Van Deventer et al. 2011and Artemiev 2017. With increasing particle deposition rate, the frequency of water turnover cycles increases, which creates favorable economic conditions for existing industries and is of considerable interest for the design of local high-speed thickeners and settling tanks, since it allows to abandon bulky sludge collectors and settling ponds that cause significant harm to the environment. ...
The aim of the work was to select effective flocculants for clarification of recycled water in the process of wet magnetic separation of iron ore. Types and brands of reagents have been selected, which allowed radically increasing the rate of deposition of impurities and reducing the turbidity of recycling water.
... where significant effective stresses are present in the soil. It is noted that significant studies of the effects of flocculants in dewatering behaviour (i.e. in thickeners) have also been carried out (Farrow et al. 2000;Spehar et al. 2015), including studies of the effects of shearing on the flocculated structure (Gladman et al. 2005(Gladman et al. , 2010. It is further noted that the majority the studies referenced herein involved preparation of PT material in a laboratory setting with minimal shearing to the specimens (beyond that required to mix the PT). ...
... is the measurement of how easily water flows through mud bed during thickening, indicating the dewatering extent of tailings [27]. Kozeny introduced the relation between permeability and the properties of the porous medium and it was later modified by Carman, and it is commonly known as the Kozeny-Carman equation [28]. ...
Thickening performance is greatly influenced by aggregate densification and channelling development; however, the microstructures of aggregates and channelling are unable to directly observe because of high turbidity mud bed during thickening. Computed tomography scan imaging technique has been introduced to investigate the effects of aggregate densification and channelling in the mud bed samples during unclassified tailings thickening. The samples were prepared through deep-freezing and freeze-drying techniques, immediately after sampling from the operation of a pilot thickener. Based on the information of void ratio, pore-size distribution, and permeability, obtained from 3D reconstruction images of the aggregates structure, aggregate densification and channelling development have been characterized. Channelling patterns have been classified by the size and shape of the connection throat of the pore between aggregates. As the aggregate structure broke and densified, the void ratio and pore size of the mud bed were decreased. Thickening performance enhancement was predicted based on solid flux and permeability of mud bed, and the result indicated raking on aggregate densification and channelling apparently improves the unclassified tailings dewatering extent.
... where V agg is the partial volume of the aggregate phase, and V t is the total sediment bed volume. If it is assumed that all of the solids in a well-flocculated suspension are contained in the aggregate phase, the solids volume fraction of the aggregate phase, ϕ agg , can be written as follows [6], ...
This paper reports the use ofmicro-computed tomography (CT) to measure changes in aggregate density in sed- iments offlocculated silica as a function of applied pressure over the course of a step-wise compression experi- ment. A micro-CT calibration method was developed that uses polymer-stabilized silica suspensions of varying solids concentration to permit the calculation of aggregate density and solids volume fraction directly from the micro-CT images. The results show that low-compression dewatering causes the removal of inter-aggregate water, while higher pressures are required to expel the intra-aggregate fluid. The solids volume fraction values calculated from micro-CT images were found to be in good agreement with those measured gravimetrically, by drying andweighing the samples. This work represents the first step in a description ofthe micro-scalemech- anisms of dewatering of flocculated porous media and is part of a larger research program that studies the dewatering ofdifficult-to-treat mineral tailings.
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... Moreover, the suspended solids concentration in the overflow water is high, which is unable to meet the environmental requirements. It is convenient and feasible to increase the settling velocity of tailings by adding flocculants; therefore, flocculating settling technology has been widely used in the area of tailings settling [14,15]. ...
Unclassified tailings are the main backfilling aggregates in mines and their settling is the first step in the utilization of tailings; thus, it is very important to determine their settling behavior. The aim of this study was to understand the flocculating settling behavior of unclassified tailings with different factors. The combination of property detection, laboratory experiments and industrial tests were used to assess the flocculating settling behavior of unclassified tailings; the orthogonal experimental design and the control variate method were used for an experimental design. The results show that the flocculating settling velocity of unclassified tailings decreases with the increase of slurry concentration and that this settling velocity increases first and then decreases with the increase of flocculant unit consumption. The underflow concentration is positively correlated with the slurry concentration and negatively correlated with the flocculant unit consumption and flocculant concentration. Slower feed velocity could produce higher concentration underflow but lower clarity overflow water. The greater the mud height, the higher the underflow concentration and the suspended solids concentration in the overflow water. The underflow concentration has a maximum at the rake speed of 0.3 r/min, and the rake speed has little effect on the suspended solids concentration in the overflow water. By analyzing the settling velocity, the underflow concentration, the suspended solids concentration in the overflow water and the solid flux, the following parameters of the flocculating settling experiments were determined: the flocculant type is APAM with a molecular weight of 12 million, the flocculant unit consumption is 30 g/t, the slurry concentration is 6 vol.%, the flocculant concentration is 0.1 wt.%, the rake speed is 0.3 r/min, and the feed velocity is 0.4 L/min (its solid flux is 0.523 t/(m²·h)). The industrial tests were carried out based on the laboratory settling data, and the appropriate selection parameters of the industrial tests were estimated.
... The hindered settling function R ϕ ( ) can be determined from the equation below ( Spehar et al., 2015). ...
... There could be also endogenous syneresis processes within the flocs or shear densification mechanisms, so that the flocs contraction originated by the expulsion of water [27] favors the rupture of bonds between primary flocs. The densification process can be caused by the shear due to the buffeting of flocs in sedimentation [28] and/or to the external and internal water flow through the flocs. The physical and chemical mechanisms described above can cause the fragmentation of the sludge structure and trigger a flocculation process leading to the acceleration of sludge. ...
The sedimentability of the activated sludge can be affected by the presence of a large variety of coagulants and polymers from a previous physical-chemical process. In this paper, the activated sludge settling process in industrial wastewater treatment plants where the sludge does not settle in a conventional way is studied. The two observed constant hindered settling velocity stages and the instant the intermediate sludge acceleration period occurs are described. A variation of the Richardson and Zaki model is used to characterize the two stages of constant settling velocity. The concentration of suspended solids, where a sudden decrease of hindered settling velocity was observed, is calculated. Finally, a new hypothesis to explain the processes triggering the collapse of the initial homogeneous sludge structure and the existence of an acceleration period is formulated.
... Ideally, this would also account for rake-induced dewatering, although most likely through the incorporation of sub-models developed in parallel, e.g. refer to aggregate densification studies by Usher et al. (2009) andSpehar et al. (2015). ...
There are numerous models of sedimentation in fine particle suspensions, derived from or validated with physical measurements. Such models could be applied to optimise and control gravity thickeners used for solid-liquid separation in mineral processing. However, these applications rely upon particle bridging by high molecular weight polymers to form large, low density and fragile aggregates. The evolution and refinement of sedimentation modelling for flocculated systems is restricted by a lack of meaningful control of flocculation conditions and inadequate detail in the experimental sedimentation data used for validation.
To address this problem, an experimental system was built to give high fidelity sedimentation data, its use demonstrated for flocculated calcium carbonate suspensions. Turbulent pipe flow flocculation offers tight control of dosage, mean shear rate and reaction time, with aggregate size monitored in-line. The pipe discharges into the base of a wide (190 mm ID) transparent column for which the contents can be isolated, with mudlines then determined from image capture and bed profiles by γ-attenuation. The use of wide columns minimised wall effects that can limit consolidation, while simultaneous efforts to maximise measurement reproducibility and sensitivity led to a greater ability to distinguish subtle impacts from variations in flocculation. Duplicate columns with detachable lower sections enable direct vane yield stress measurement at known bed depths and thereby plots of yield stress vs. solids fraction.
Modelling of such data will be described in subsequent publications, with the present study focusing on key experimental requirements and limitations, the form of data outputs and insights into flocculation impacts. For the latter, off-line determination of individual aggregate settling rate vs. size after bed sampling confirmed that higher solids volume fractions low within beds occurred through removal of both inter- and intra-aggregate liquor, i.e. there is a contribution from aggregate densification. Bed profiles and yield stress responses also indicate flocculant functional chemistry can alter how aggregation limits the solids volume fractions attained.
... Rheological behaviour of flocculated mineral suspensions, even in the absence of salts, can be a very complex issue [12,13] where several mechanisms are triggered jointly, including collisions between particles, polymer molecules and flocs; polymer adsorption on particle surfaces; reorientation of polymer chains adsorbed onto particles; particle and floc aggregation; floc fragmentation; re-aggregation of broken flocs; and restructuring and consolidation [14]. Flocs continuously change their size, structure, and permeability as the process progresses [15][16][17]. In the presence of salts, whether saline water or sea water, complexity increases. ...
The effect of salinity on the yielding properties and viscoelastic behaviour of flocculated kaolinite sediments was studied through stress growth and creep-recovery tests. Turbulent flocculation was performed on a detachable cylinder using the plunger method to promote efficient and controlled mixing. The suspensions were prepared at natural pH (between 5.8–6.0) with salinities in the range 0-0.5 M NaCl. Once a suspension was prepared, it was allowed to settle for 2.5 h, during which time the variation in sediment height was small. The sediment was removed with minimal disturbance and subjected to stress growth or creep-recovery tests on a rheometer operated in control rate and control stress modes, respectively. Kaolinite suspensions in the presence of flocculant and salt in concentrations larger than a small although critical concentration (NaCl 0.001 M) exhibit an inverse relationship between yield stress and settling rate with salt concentration, and direct relationship between shear strain and salt concentration. It is concluded that the particle network becomes denser and the crosslinking weaker with increasing salt. At the critical concentration of 0.001 M NaCl, flocculation is best, the initial settling rate and yield stress are the highest, and shear strain the lowest. The response of the floc network to salinity changes is the result of two mechanisms competition: (i) charge screening of both anionic particles and flocculant by salt counterions, and (ii) shielding of the active sites on the flocculant by salt cations that causes the polymer to fold into compact structures adopting balled-up conformations. The first mechanism promotes flocculation while the second supresses flocculation.
... where agg is the solids volume fraction in the aggregates and V agg /V total is the aggregate volume fraction, where V agg is the volume occupied by aggregates, and V total is the total volume [23]. ...
This paper describes the calculation and interpretation of optical backscattering (OBS) signal values for the measurement of aggregation state within settled particulate suspensions. An OBS height scan method is applied to suspensions of kaolin prepared at two different solid concentrations and flocculated by varying dosages of a non-ionic polyacrylamide. It was found that the root-mean-square scattering, Frms, and mean scattering, Fmean, signal values correlate very well with changes in relative aggregation state as indicated by changes in the final sediment bed density and calculated index of aggregation values. Application of the measured signal value versus concentration data allows the calculation of the solid volume fraction versus height profile within the settled bed.
... They analyzed the impact of solid concentration and deposition fluxes on settling velocity using settling curve, and obtained the industrial indicators, such as underflow concentration and thickener area. SPEHAR et al [6] gave a macro-mechanics description on the influence of gel concentration, compressive yield stress, shear yield stress, as well as sedimentation coefficient on the structure strength of floccules. ESWARAIAH et al [7] analyzed the settling characteristics of ultrafine iron ore slimes under various pH values, as well as different kinds of flocculants in different dosages. ...
Rapid dewatering and thickening of whole-tailings with ultrafine particles is one of the most important processes for the whole-tailings paste preparation. Deep-cone thickener, a kind of such process for the flocculation and settling of whole-tailings, is particularly necessary to study. However, there exist many problems in observing the flocculation and settling process of whole-tailings, as well as the particle size distribution (PSD) of whole-tailings floccules in deep-cone thickener. Population balance model (PBM) is applied to predict the PSD in deep-cone thickener, and LUO model and GHADIRI model are employed to study the aggregation and fragmentation mechanism of the whole-tailings particles, respectively. Through three-dimensional numerical simulation on the whole-tailings flocculation and settling in deep-cone thickener using computational fluid dynamics (CFD)-PBM, the distribution of density and turbulent kinetic energy in deep-cone thickener were obtained, at the same time the spatio-temporal changes of whole-tailings floccules particle size distribution are analyzed. Finally, the major flocculation position in deep-cone thickener is found and the flocculation settling rules of whole-tailings are achieved.
... However if the aggregate is sheared (e.g. under the action of a rake, Farrow et al., 2000, and/or by being buffeted by neighbouring aggregates, Spehar et al., 2014), it can undergo a process of densification, i.e. the solids in the aggregate bind together more tightly by expelling liquid (Farrow et al., 2000). Shear can therefore have a very significant effect on suspension dewatering (Gladman, 2004;Gladman et al., 2005Gladman et al., , 2010. ...
This study investigated the performance of an acoustic backscatter system (ABS) for the in situ particle characterization of complex wastes. Two sediments were used; a fine, milled calcite that was flocculated with anionic polyacrylamide, and naturally flocculated pond sludge. Particles were initially measured independently by light-based techniques to gain size, coefficient of variation (COV) and fractal dimensions. For acoustic experiments, a bespoke, high-fidelity ABS was employed with 1, 2.25 and 5 MHz probes and a recirculating mixing tank. Initially, the concentration independent attenuation and backscatter coefficients were measured for each system, using a robust calibration procedure at multiple concentrations. Comparisons of the total scattering cross-section (χ) and form function (f) were made between the experimental data and two semi-empirical models; a Solid Scattering model and a Hybrid model (where the effects of bound fluid are incorporated). Experimental data compared more closely to the Solid Scattering model, as it was assumed scattering was dominated by small, bound ‘flocculi’ rather than the macroscopic structure. However, if COV was used as a fit parameter, the hybrid model could give equally accurate fits for a range of input aggregate sizes, highlighting that important size and structure information can be gained from the acoustic models, if there is some a priori system data. Additionally, dual-frequency inversions were undertaken to measure concentration profiles for various frequency pairs. Here, the lowest frequency pair gave the best performance (with accurate measurements in the range of 20–35 g.L-1) as interparticle scattering was lowest.
In this study, the macro-structure of quartz flocs was assessed from X-ray computed tomography images. An image segmentation workflow was established to identify the floc sub-units that comprised each macro-floc, which permitted the macro-structure to be defined in terms of the number, size, and connectivity of the sub-units. A simplified structural model of non-penetrating spheres was used to understand the scaling relationships of the macro-floc properties. It was found that although sub-unit size varied widely, there was a limiting volume (2.8 × 10⁶μm³), above which floc growth occurs only by sub-unit aggregation. This approach permits the quantification of macro-floc structure and composition in 3D on the scale of tens of μm to the mm scale. Knowledge of macro-floc structure allows a deeper understanding of the role of macro-floc breakage and re-arrangement in subsequent dewatering treatments such as thickening or filtration.
This paper reports the use of micro-computed tomography (CT) to measure changes in aggregate density in sediments of flocculated silica as a function of applied pressure over the course of a step-wise compression experiment. A micro-CT calibration method was developed that uses polymer-stabilized silica suspensions of varying solids concentration to permit the calculation of aggregate density and solids volume fraction directly from the micro-CT images. The results show that low-compression dewatering causes the removal of inter-aggregate water, while higher pressures are required to expel the intra-aggregate fluid. The solids volume fraction values calculated from micro-CT images were found to be in good agreement with those measured gravimetrically, by drying and weighing the samples. This work represents the first step in a description of the micro-scale mechanisms of dewatering of flocculated porous media and is part of a larger research program that studies the dewatering of difficult-to-treat mineral tailings.
The dewatering of flocculated suspensions presents a significant challenge to water and waste water processing operations world-wide. Traditionally the dewatering process is enhanced through the use of flocculants or a surface chemical modifier to draw together fine particles and increase settling rates and sediment permeabilities, however, present methods of gauging chemical performance are somewhat empirical. Recently, Landman and White developed a theory of suspension filtration which incorporates the compressibility and permeability of a sediment into a filtration diffusivity, D(φ). D(φ) is a material property used to calculate the time scale of a dewatering process from first principles for a given set of process conditions i.e. final desired solids concentration, applied pressure, initial sediment height. A model alumina system and an alum water treatment sludge have been used to show the effect of flocculation conditions on compressibility, permeability (determined from filtration experiments) and the diffusivity, D(φ). Keywords Compressibility; dewatering; filtration; flocculation; permeability; sludge
An experimental investigation of the sedimentation of monodisperse colloidal silica spheres with grafted octadecyl chains with three different interaction potentials is presented. Small particles (0.27 μm) behaved as hard spheres in cyclohexane, but larger ones (0.60 and 0.94 μm) are weakly flocculated by van der Waals attractions. The smallest particles (0.08 μm) in hexadecane are strongly flocculated by attractions between the octadecyl layers. A medical computer tomography (CT) scanner provided an accurate and absolute density measurement without disrupting the process. For the hard spheres and the weakly flocculated systems, the kinetics of sedimentation for the dispersed phase could readily be predicted utilizing the flux curve. For flocculated networks, we found a power-law relationship between compressive yield stresses and solids fractions comparable with other experimental systems.
The phenomenological theory of sedimentation–consolidation processes of flocculated suspensions is extended to pressure filtration processes. The local mass and linear momentum balances for the solid and liquid component together with appropriate constitutive assumptions lead to a strongly degenerate (mixed hyperbolic–parabolic) nonlinear partial differential equation for the local solids fraction, which together with initial and boundary conditions determines a dynamic cake filtration process. In the case of a prescribed applied pressure function, we obtain a free boundary problem, in which the piston height has to be determined simultaneously with the solids concentration. A numerical algorithm approximating the physically correct solution, with possible discontinuities such as the cake/suspension interface, is presented and employed to simulate various cake filtration processes.
Dry disposal represents a paradigm shift in mining engineering. The tailings are engineered to suit the disposal requirements, as opposed to engineering a facility to accommodate the tailings. It is, therefore, important to understand how the material properties and operational parameters can be modified to produce the desired transport and deposition characteristics of the tailings. For thickened tailings disposal, dry stacking and paste fill, the rheology must be well understood to ensure maximum efficiency of the entire disposal operation. The paper presents the disposal of bauxite residue (red mud) as a case study and outlines alternative and simplified methods of rheological characterisation. The effect of both shear and compression history on pipeline transport is examined in order to identify favourable processing schemes. Prediction of the slope formed by the deposition of de-watered tailings is necessary to ensure maximum storage efficiency and stability. The slope is found to be dependent on rheological properties such as yield stress and viscosity, operational parameters such as depositional flowrate, and the topography or slope of the underlying base.
: A numerical model of gravity sedimentation and thickening was developed from the governing two-phase flow equations for the liquid and solid phases. The inertial and gravity terms in the solid and liquid momentum equations were retained in the gravity sedimentation and thickening model. An implicit, spacestaggered finite-difference algorithm was developed for the resulting coupled partial differential equations. Constitutive relationships describing the physical properties of the slurry were required to solve the numerical model. These constitutive properties describing the relationship between effective stress and porosity and between permeability and porosity were determined experimentally and by model calibration. The model was calibrated and verified using the data of dynamic porosity profiles of gravity sedimentation and thickening of kaolin suspensions in distilled water. INTRODUCTION A large fraction of the current cost of wastewater treatment is from the treatment and dispos...
After two years of internal research, Outotec has determined that applying uniform cumulativeshear to hindered settling flocculated aggregates results in a substantial change in the sedimentation process. A new technology, called Shear Enhanced Thickening (SET), has been developed as aresult. The function of a thickener is to optimise and accelerate the process of gravity sedimentation. The desired end product (be that maximised concentrated solids or clarified liquid) determines how the thickener is designed and operated. Laboratory results have shown that applying uniform cumulative shear to the hindered settling zone increases both the through put of a thickener and its resultant underflow density. Such a change in the sedimentation process has considerable design implications for full-scale thickeners, including a substantial reduction in required foot print and or major increases in underflow density. The theory and development of Shear Enhanced Thickening combined with lab and pilot scale verification will be discussed. The paper also describes the potential implications and benefits in full scale implementation of this technology. © (2010) by the Australasian Institute for Mining and Metallurgy (AusIMM).
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.
Channelling, which occurs during the batch thickening of concentrated slurries, allows water to escape the settling slurry at a high rate, thus increasing the interface settling velocity. This research shows that the upper layers of the settling column are progressively diluted and that the channel zone occurs at the interface between the diluted slurry and the slurry in compression. As the solids in the lower sections of the column compress and expel water, this water escapes by means of the channels.
A number of control systems for deep cone thickeners are discussed. The relationship between feed rate and underflow concentration for flotation tailing is described. Cones of different effective size have been used to investigate the relationship between size and throughput for a coal effluent and residence time measurements have been made for a series of radial positions in a deep cone thickener. Some data on the variation of solids concentration with depth are presented. (A)
Synopsis
The equation governing the one-dimensional consolidation of a fully saturated clay layer are derived here on the basis of assumptions more general than those usually adopted. The limitation of small strains has not been imposed and the variation of soil compressibility and permeability during consolidation has been taken into account. Furthermore, although Darcy's law is assumed to be valid, it is recast in a form in which it is the relative velocity of the soil skeleton and the pore fluid that is related to the excess pore fluid pressure gradient.
The consolidation of a thin clay layer, the self-weight stresses of which are negligible compared with those applied, is examined in detail. Non-homogeneity, time-effects intrinsic to the soil skeleton and compressibility of the pore fluid are incorporated in the theory, but a detailed consideration of their importance is deferred to later papers in which the case of the thick clay layer will also be presented.
Les équation régissant la consolidation à une dimension d'unne couche d'argile pleinement saturée sont déduites ici en se basant sur des suppositions plus générales que celles adoptées habituellement. La restriction des petites déformations n'as pas été de la perméabilité du sol pendant la consolidation a été prise en considération. En outre, quoique la loi de Darcy soit supposée être valable elle est énoncée sous une forme squelette du sol et l'eau interstitielle sont exprimées en fonction de la pente excédentaire de la pression de l'eau interstitielle.
La consolidation d'unne couche d'urgile mince, dont les constraintes de son poids propre sont négligeables en comparaintes de celles qui lui sont appliquées, est examinée détail. La non homogénéité, les effets du temps intrinsèques au squelette du sol et à la compressibilité de l'eau interstitielle sont incorporés à la théorie, mais une considération détailée de leur importance est remise à des exposés ultérieurs dans lesquels sera aussi présenté le cas de la couche épaisse d'argile.
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.
In this study both shear (G) and mixing time (t) effects on the performance of sludge conditioners were evaluated. It was found that the mixing requirements for optimal conditioning of a sludge can be represented by an equation relating a constant associated with a particular chemical dose to mixing time and shear raised to a power, x. The value of x seems to be a reasonable characterization parameter for the sensitivity or physical resistance of a sludge to shear. Unconditioned sludges had x values near 2.8 and were easily disaggregated. Conditioning of sludges reduced the exponent of G to values near 1.0, rendering the sludges relatively resistant to disaggregation associated with mixing.
A continuous laboratory column system has been used to investigate the consolidation behaviour of kaolin slurries flocculated by a nonionic flocculant under different conditions. Measurements of the steady-state bed density profile showed that higher agitation intensities during flocculation resulted in lower bed densities. The relative importance of compression versus shear for kaolin dewatering was determined by operating the steady-state column system at different bed heights above a mechanical rake. It was found that rake action, not compression, was the dominant dewatering mechanism. Samples taken from the consolidating bed before and after the rake were analysed by microscopy to determine the density–size relationship of the individual aggregates. This showed that the dewatering induced by the rake action occurred not only by removal of inter-aggregate liquor but also by densification of the aggregates (i.e. removal of intra-aggregate liquor). Measurements made on full-scale thickeners are also presented which demonstrate similar behaviour.
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.
We present an initial—boundary value problem of a quasilinear degenerate parabolic equation for the settling and consolidation of a flocculated suspension. The corresponding definition of generalized solu- tions is formulated. It is based on an entropy integral inequality in the sense of Kruz\ kov. From this definition, jump and entropy conditions that have to be satisfied at discontinuities, and an entropy condition valid on one boundary of the computational domain are derived. The latter implies a set-valued reformulation of the original boundary condition. It is interpreted geometrically and characterized by the solution of an auxiliary hyperbolic Riemann problem. ( 1998 B. G. Teubner Stuttgart—John Wiley & Sons, Ltd.
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.
An authoritative critique of the established and recently proposed methods of thickener design.
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.
The role of flocculants in changing the dewatering properties of aggregated particulate suspensions has been examined. The hindered settling and compressibility characteristics of suspensions have been measured for a range of aggregate formation conditions. The effect of post formation shear on the dewatering has also been quantified for a range of shear rates. A critical shear rate transition has been observed between beneficial and detrimental effects in dewatering.
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.
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.
Solid–liquid separation by the process of continuous sedimentation in a clarifier–thickener unit, or settler, is difficult to model. Simplified assumptions on the behaviour of the solids, the flows, the physical design of the settler, etc. still leave the fundamental process highly non-linear. A fairly simple model consists of a one-dimensional settler, with a constant or varying cross-sectional area, in which an ideal suspension of solids behaves according to the Kynch assumption (the settling velocity is a function of the local concentration only) and the conservation of mass. At the bottom of the settler the concentration increases with depth as a result of, among other things, compression and a converging cross-sectional area. It is important to understand fully the mathematical implications of the simplified assumptions before investigating more complex models. In this paper it is demonstrated what impact a converging cross-sectional area has on the increase in concentration at the bottom for incompressible suspensions (a consequence of Kynch’s assumption). This analysis leads to a natural boundary condition at the bottom, which is a special case of a generalized entropy condition for the type of partial differential equation under consideration. The mathematical problems concerning the boundary conditions at the top, bottom and inlet are resolved uniquely by this generalized entropy condition. One aim of the paper is to describe and elucidate this condition by examples leaving out some technical mathematical details. The construction of a unique solution, including the prediction of the outlet concentrations, is described by examples in the case of a constant cross-sectional area. Comparisons with numerical solutions are also presented.
The concentration or consolidation of suspensions of fine particles under the influence of a gravitational field has been analysed. The rate and extent of consolidation depends upon a balance of three forces, the gravitational driving force, the viscous drag force associated with flow of liquid in the sediment and a particle or network stress developed as a result of direct particle–particle interactions. In the case of colloidally stable suspensions, this particle stress is the osmotic pressure of the particles; in the case of flocculated or coagulated suspensions, it is the elastic stress developed in the network of particles. A constitutive equation is suggested for irreversibly flocculated suspensions undergoing consolidation which embodies the concept of a concentration-dependent yield stress Py(). This is then used to analyse the sedimentation behaviour of flocculated sediments and to derive expressions for the initial sedimentation rate. The initial rate of change of sediment height with time in a uniform gravitational or centrifugal field is given approximately by: [graphic ommitted] where B=Δρg0H0/Py(0), u0 is the sedimentation rate of an isolated particle, 0 is the initial (uniform) volume fraction of solids, r(0) is a dimensionless hydrodynamic interaction parameter, Δρ is the difference in density between solid and liquid, g is the gravitational or centrifugal acceleration and H0 is the initial sediment height. The theory accounts correctly for the equilibrium consolidation behaviour of strongly flocculated suspensions, and preliminary experimental data suggest that it is not inconsistent with their dynamic behaviour. The estimation of the yield stress Py() from a batch centrifuge experiment is also described.
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.
This study investigated the settling rates and sediment volumes of aqueous, flocculated kaolin suspensions as functions of kaolin concentration, container dimensions, and chemical composition of the aqueous phase. Equations which correlated the data were derived, based on a structural model which assumed that in a flocculated suspension, the basic flow units are small clusters of particles (plus enclosed water) called flocs. These flocs retain their identity under the mild forces experienced in gravity settling. At low shear rates, the flocs group into clusters of flocs, called aggregates. The aggregates may form networks which extend to the walls of the container and give the suspension its plastic and structural properties.
From the development of a compressional rheology of a flocculated suspension, a number of separation processes have been modeled. With respect to the two-stage pressure filtration process of compact formation and then consolidation, the rheological formulation appears at first examination to eb significantly at variance with the conventional engineering approach. The present model may be reconciled to the early engineering model. Moreover, the filtration parameters extracted from experiments can be related to the more fundamental rheological parameters, r(ϕ), the hindered settling factor, which is a function of solids volume fraction ϕ — it takes into account hydrodynamic interactions between local particles, which increase the drag forces. Useful estimates of the compact bed formation resistance and the formation time are provided as a function of applied pressure. The initial time dependence of the consolidation ratio on the elapsed consolidation time is not found to be a square root behaviour as suggested by the conventional modeling.
Characterization of the compressibility and permeability of flocculated suspensions is a time-consuming experimental process that often takes days to perform. A new pressure filtration method developed characterizes a sample only in hours. Using stepped pressure filtration, the compressive yield stress Py (ϕ) and a hindered settling function R(ϕ) of flocculated suspensions are determined as a function of the solids volume fraction ϕ. Traditional pressure filtration experiments involve the pressure filtration of a particulate suspension in which the time of filtration t is monitored as a function of piston height from which the specific volume of filtrate V is determined. Py (ϕ) is determined from the equilibrium solids volume fraction. The gradient d(t/V)/dV is traditionally used to determine R(ϕ) using a suspension filtration theory developed by Landman and White. The new method uses only one stepped pressure compressibility filtration test and one stepped pressure permeability filtration test to determine Py (ϕ) and R(ϕ) for multiple solids volume fractions and substantially decreases the time required for sample characterization. To eliminate the influence of a stepped pressure on calculated permeabilities, the analysis of experimental results was modified using the gradient dt/dV2. Results are presented for a zirconia suspension comparing both single and stepped pressure filtration.
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
The consolidation behavior of flocculated alumina suspensions has been analyzed as a function of the interparticle energy. Consolidation was performed by a centrifugal force field or by gravity, and both the time-dependent and equilibrium density profiles were measured by a gamma-ray absorption technique. The interparicle energy at contact was controlled by adsorbing fatty acids of varying molecular weight at the alumina/decalin interface. We found that strongly attractive interactions result in a particle network which resists consolidation and shows compressible behavior over a large stress range. The most weakly flocculated suspension showed an essentially incompressible, homogeneous density profile after consolidation at different centrifugal speeds. We also found a significant variation in the maximum volume fraction, φm, obtained, with φm∼ 0.54 for the most strongly flocculated suspension to φm∼ 0.63 for the most weakly flocculated suspension. The compresive yield stresses show a behavior which can be fitted to a modified power law. In this paper, we discuss possible correlations between the fitting parameters and physical properties of the flocculated suspensions.
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.
Mathematical models are potentially a valuable tool for the prediction of continuous gravity thickener operation. However, experience shows that existing mathematical models underestimate dewatering in thickeners for flocculated feed materials when predictions are made of either the underflow solids concentration for a given solids feed flux density or the maximum solids feed flux density achievable for a minimum underflow solids concentration set point. One reason postulated for this discrepancy is shear enhancement of sedimentation and bed dewatering as a result of aggregate densification. This process is not taken into account in conventional 1-D thickener models. A pilot scale column, operated at low bed heights without the addition of mechanical shear, produced results that compared well with 1-D model predictions. The effect of mechanical shear and/or greater bed height was to significantly enhance thickener performance relative to model predictions (as measured by underflow density or maximum solids flux density achievable for a nominated underflow density). An experimental method was developed that enabled shear to be incorporated into the suspension dewatering characterisation. The results suggest an order of magnitude increase in solid flux density can be expected under controlled shear conditions with polymer flocculated aggregates. The results also indicate that mechanical shear is not the only factor that can enhance dewatering, since higher beds, and hence longer residence times, also improve the achievable solids flux density. This is despite the fact that the thickener is operating in a regime that is predicted to be limited by the sediment permeability and not its compressibility. This suggests an additional mechanism must be at play in full scale operation and points a direction for further experimentation.
Predictive modelling of solid–liquid separation can greatly assist the design and operation of thickening and filtration equipment, improving water recovery and reducing costs. A phenomenological model describing continuous thickening has been previously developed with primary inputs being the material properties, (compressive yield stress and hindered settling function) derived from routine laboratory batch settling and filtration tests. This work aimed to validate the model by operating a pilot column continuously and measuring the underflow solids. The column was operated at two different solid fluxes and several bed heights. Additionally, the influence of flocculation conditions (polymer dosage and residence time) on thickening performance were studied. The model predicted the experimental underflow solids concentration at a given flux. For the observed underflow solids concentration, the ratio of the actual to predicted flux was observed to be between a factor of 1 (accurate) and 10. The model was most accurate for the lowest bed heights. This work confirmed the model was able to correctly predict trends for the case where minimal bed height and shear forces are present. Deviation from the model is postulated to be due to changes in the dewatering properties of flocculated aggregates over time that are not adequately captured using conventional batch sedimentation tests. The data from these tests are traditionally used as a key input to thickening models.
Thickener rakes are essential in the transport of sediment bed material to the underflow in conventional thickeners, however very few studies of bed transport have been published. In this paper, results from pilot-scale thickener experiments with tailor-made yield stress slurries are presented and compared to companion Computational Fluid Dynamics (CFD) simulations. Rake torque is a key issue in thickener operation and it was found that the yield stress of the suspension is the major factor in determining rake torque. Over a range of rake speeds, the measured torque was an almost linear function of yield stress. CFD simulations of the experiments allowed torque to be estimated, and results are shown to be within 20% of the measured values in all cases except the lowest (zero) yield stress suspension. Residence time distributions of solids in the bed were also measured and unusual results were found in which the relationship between residence time and distance from the underflow is not linear (or even monotonic). CFD results clearly show that for uniform sized rake blades, the over-delivery of an outer blade (compared to the next inner blade) sets up recirculation in the bed, especially in the outer regions of the tank, and this can result in long material pathways and hence long residence times. This picture is further complicated by the relative contributions of rake delivery and underflow rate, and indicates that a simple picture of plug flow in the bed is far from reality. The study illustrates the value that can be obtained from validated CFD modelling of thickener rakes.
To characterize and optimize the compressive dewatering of suspensions, rapidly determinable material parameters are required. A mathematical framework exists for characterization of dewatering using three parameters: the compressive yield stress Py (ϕ), the hindered settling function r(ϕ), and a solids diffusivity D(ϕ). A new variation on traditional constant pressure, batch filtration tests is described along with a computer-controlled filtration apparatus that enables complete characterization of a suspension with respect to dewatering in a matter of hours. The testing methods are validated experimentally on flocculated zirconia suspensions along with calculation of the material dewatering parameters. Application of the calculated parameters to modeling of a real filtration test shows excellent agreement. Practical implications of the modeling are also discussed.
The effects of microstructure on the compressive properties of aggregated alumina suspensions are determined by intentionally introducing heterogeneities into the suspension. Suspensions are prepared at a high volume fraction and diluted with low shear hand mixing to a series of initial concentrations. As the initial concentration is increased, larger heterogeneities are introduced, and the suspension becomes more compressible relative to the compressive yield stress of the uniform suspension. A simple model is proposed in which the heterogeneous suspensions compress by rearrangement of the dense aggregates until a critical concentration (ϕc, which coincides with the volume fraction prior to dilution) is reached. Above ϕc, the suspensions consolidate identically to the uniform suspension. With a single fitting parameter (the size of the heterogeneities), the model shows semiquantitative agreement with the experimental data.
Rheological and microscopical studies have been made to elucidate the effects of shear fields on the morphology of concentrated, aggregated model colloids. The models employed are well-characterised, predominantly chargestabilised polymer latices, coagulated by the addition of excess electrolyte. Continuous shear rheological and viscoelastic measurements indicate a very significant decrease in shear yield stress, apparent viscosity and shear modulus following prolonged shearing.
Electron microscopy reveals the source of these changes. Freshly coagulated suspensions form networks that are porous, strong and qualitatively similar to simulated structures for diffusion limited aggregation. Following protracted shearing, the network structure is rearranged to yield discrete, tightly packed aggregates with a characteristic size, which is principally a function of the primary particle size.
The present model for the sedimentation of colloidal systems includes a diffusion term in the governing equation which, in the regions above the sediment, acts as small perturbation to the Kynch (1952) theory. Within the sediment, diffusion is comparable to convection due to the the high solid volume fraction. An application of the method of matched asymptotic expansions to the conservation equation allows a complete description of the settling process to be formulated, with specific attention to volume-fraction evolution in the sediment.
Sedimentation and Thickening
- M C Bustos
- F Concha
- R Burger
- E M Tory
Bustos, M.C., Concha, F., Burger, R., Tory, E.M., 1999. Sedimentation and Thickening.
Kluwer Academic Publishers, Dordrecht, The Netherlands.