Experimental validation of a 1-D continuous thickening model using a pilot column

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.     

The working conditions in a continuous pilot thickener at optimal load

The meaning of a derived analytical expression giving the critical solids flux as a function of a well defined relationship between the underflow concentration and the solids load in a continuous thickener at optimal load is discussed. In order to test the derived expression, thickening of unflocculated CaCO₃ has been made in a continuous pilot thickener at optimal load. Two different feed concentrations were used. A marked influence of the feed concentration is found when thickening at critical conditions. Hence the underflow concentration is not a well defined function of the solids load meaning that the critical solids flux cannot be found from the continuous thickening data. Furthermore thickening capacity is found to be higher than for an ideal thickener. This phenomenon is explained by the existence of a non-uniform concentration and slurry flux in each thickening cross-section in a transition zone above the critical zone. The non-uniform character is assumed to be caused by the flow behaviour of the incoming feed.     

The solids concentration distribution in the deep cone thickener: A pilot scale test

Cemented backfill or surface deposition of paste tailings is increasingly being considered as a simple and effective means of reducing the hazards of conventional slurry deposition and recovering water for recycle. Although gravity thickening has been widely used in the mineral industry to increase the solids concentration of tailings, the accurate prediction of the concentration distribution in three-dimensions and discontinuous operational state has proven to be difficult. We investigated the axial and radial solids concentration distribution at discontinuous state in a pilot deep cone thickener as a function of bed height and residence time. The feed flux of lead/zinc tailings was 0.254 t·h^?1· m^?2 with a flocculant (high molecular weight anionic polyacrylamide) dose of 20 g/t. The thickened solids bed was sheared by a rotating rake at a rate of 0.2 rpm. The underflow was recirculated at a flux of 0.5 t·h^?1·m^?2, which can introduce additional shear stresses into the bed. The results of the bed density profile showed that, beside the clarification zone, the area below the feedwell could be divided into four zones: the dilution zone caused by free settling and diffusing action, the hindered settling zone in which the concentration was lower than the gel point, the unraked bed zone with a large concentration gradient and, finally, the raking zone with the highest slurry concentration and lower concentration gradient.     

Consolidation and aggregate densification during gravity thickening

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.     

Semi-batch test of sedimentation. Application to design

In a batch test of sedimentation with flocculated suspensions, from the evolution of the supernatant-suspension discontinuity height with time, it is possible to obtain the design parameters for continuous gravity thickeners. Nevertheless, the design is only approximate because normally the surface height of the sediment, growing on the bottom of the cylinder, is not visible. On the other hand, the design corresponds to sediment heights much smaller than the initial height of the batch tests, and consequently to very small sediment heights. In the method presented in this paper, the semi-batch test is carried out with periodical withdrawals of supernatant and additions of fresh suspension at the top of the liquid, obtaining a zig-zag variation. Considering the Kynch theorems, that are applicable to the suspension above the sediment, it is possible to obtain the variation of the sediment surface height versus time, and this sediment height can be close to the initial height of the suspension. In this way, it is possible to obtain the exact relationship between the settling flux density of solids in the hindered settling or non-compression zone, and consequently the design parameters for a continuous thickener can be obtained for an underflow solids concentration range and sediment height range wider than those obtained from a batch test.     

Thickener dynamic analysis,accounting for compression effects

The dynamic behaviour of the thickening zone of a continuous thickener has been analysed, and numerical solutions of the resulting differential equations obtained for a range of conditions. For high throughput, low underflow concentration operation, mechanical (compressive) support of the solids can be very small, providing that sludge funnelling is taken into account. Neglecting mechanical support in such cases gives results which are fairly accurate, but limited by the presence at the top of the sediment of a shallow layer in which mechanical support is significant. For low throughput, high underflow concentration operation hydrodynamic support of the solids can be small, and its neglect gives again fairly accurate results. In general, both hydrodynamic and mechanical support of solids have to be taken into account.     

Pilot-Scale Demonstration of Deep Cone™ Paste Thickening Process for Phosphatic Clay/Sand Disposal

A pilot-scale Deep Cone? thickener (DCT) from Dorr-Oliver EIMCO has been employed at a phosphate mine in central Florida to investigate the effects of key operating parameters, including feed rate, sand addition rate, flocculant dosage, and bed depth on waste clay thickening performance. The pilot-scale field testing successfully demonstrated the simultaneous production of an underflow paste product and a clear overflow water stream. Typical overflow water recovery and underflow solids recovery were more than 88% and 98%, respectively with a residence time of about 2 hours. The highest clay content and total solids content in the paste were higher than 25% and 35%, respectively, when the flotation sand tailing was added to the clay slurry at a clay/sand ratio of 2:1 by weight.     

Sedimentation and electro-osmotic dewatering of coal-washery slimes

Dewatering of three samples of fine-tailings from two Australian coal washeries, where the clay content of the tailings is predominantly either the swelling or the non-swelling type, is examined on the laboratory scale. Thixotropic slurries of ≈ 55 wt% solids are achieved by sedimentation of the crude or prethickened tailings. Electro-osmotic treatment of these slurries yields a moist but load-bearing material ≈ 67 wt% solids up to a relatively dry, hard, and compact cake ≈ 80 wt% solids, at a power consumption equivalent to 5–8 and 40–50 kWh, respectively, per tonne of the 55 wt% slurries. The corresponding cost per tonne of particulate matter present, at 2.2 cents kWh^?1, is 20–32 cents for 67 wt% solids and $1.60–$2.00 for 80 wt% solids. (To express these costs per tonne of moisture removed, multiply by 5.6 and 3.2 respectively). The only mechanical process currently used at Australian coal washeries is centrifugal dewatering of thickener underflow after conditioning with polyelectrolyte. A wet, non-load bearing product (37 wt% solids in the one sample tested) is obtained at a cost in the range $1–$2 per tonne of particulate matter present. If an efficiency similar to that for the laboratory electro-osmotic treatment can be realised on the industrial scale, then the comparison favours the electro-osmotic treatment, particularly considering that the drier product could be saleable as a fuel.     

Controlling thickener underflow rheology using a temperature responsive flocculant

Continuous solid–liquid separations with the temperature responsive flocculant poly (N‐isopropylacrylamide) (PNIPAM) were conducted in a pilot‐scale thickener for the first time, using fine quartz as the feed slurry. The performance in continuous operation was compared to batch sedimentation. The increase in sediment consolidation on cooling in batch sedimentation was less significant in the continuous operation due to kinetic limitations of the deeper sediment bed and shorter residence times in the pilot‐scale thickener. The reduction in underflow rheology which results when using the temperature responsive polymer as flocculant is significant. Paste‐like behavior results when underflow is discharged at 50°C, whereas low viscosity, near Newtonian behavior results when the underflow is discharged at 20°C. Compared to conventional polyacrylamide‐based flocculants, PNIPAM produces higher concentration underflow but lower clarity overflow and most importantly, significantly reduced underflow rheology (viscosity and yield stress). Temperature responsive flocculants have significant potential to reduce underflow pumping energy and cost for mineral tailings. © 2014 American Institute of Chemical Engineers AIChE J, 60: 2940–2948, 2014     

Modélisation Mathématique de la Décantation de la Boue Rouge

A good performance of the solid‐liquid unit operation is required for the economical exploitation of the Bayer process. A computational fluid dynamics (CFD) model simulating the operation of the last washing stage mud thickener of a large Canadian alumina plant is presented. The parametric study of the impact of changes in four parameters shows that the diameter of flocculated red mud particles, the feed flow rate and the radius of the feed well are critical parameters for the operation of the thickener.     

Heat transfer in a draft tube spouted bed with bottom solids feed

Detailed temperature measurements were obtained for a draft tube spouted bed of 2.6-mm glass beads with bottom solids feed at different air flow rates, air inlet temperatures, solids feed rates and bed heights. The temperature profiles showed that air temperature drops significantly with the axial position in the spout, while air and solids temperatures are uniform in the annulus. The drop in air temperature along the spout decreases with increasing air flow rate and increases with solids feed rate. A model was adapted for use with the spouted bed configuration studied here and its predictions were compared with the experimental data. In general, the model could accurately predict the temperature profiles in the bed, since the largest deviations were observed for the air temperature in the spout and were always less than 5%. Although the model provided for a low deviation in predictions, air temperature in the spout was systematically over-estimated.     

Distinctions between low density and high density circulating fluidized beds

While many fundamental studies have been carried out in relatively low density circulating fluidized beds (LDCFB), there has been little reported fundamental study of the high density (including high solids flux and/or high solids concentration) circulating fluidized bed (HDCFB), although many commercial CFB setups are operating under high density conditions. Catalytic gas-phase reactions tend to require higher gas velocity and higher solids flux and/or concentration than gas-solids reactions, and therefore it is necessary to make a distinction between the two types of operations. The study of hydrodynamics and other fundamentals of HDCFB will help in understanding the fundamentals and thus improving the design and operation of existing HDCFB reactors such as FCC risers, and may also lead to other applications requiring even higher solids/gas feed ratios and/or higher solids concentration in the riser. On the other hand, high density operation can only be achieved by properly choosing the gas blower, solids feeding system and CFB geometry to avoid the instabilities resulting from insufficient pressure head from the gas blower and downcomers.     

The characteristics of particle flow in the overflow and underflow standpipe of fluidized beds

Characteristics of particle flow in the standpipes of a 10 cm I.D.×120 cm high fluidized bed were investigated. The standpipes used in this experiment were vertical overflow and vertical underflow standpipes. Sand particles and polyethylene powders were employed as the bed materials. The effects of standpipe diameter, gas velocity and particle properties on the solid flow rate were determined. The experimental results showed that the flow behaviors of solids through the overflow and underflow standpipes are different with variations of operating conditions. For both standpipes, the mass flow rate of solids was strongly dependent on the standpipe diameter. For the overflow standpipe, the increase of gas velocity increased the solids flow rate. But for the underflow standpipe it decreased the solid flow rate. From the measured pressure drops, solid fractions in the standpipes were determined by the momentum balance. The obtained experimental data of solids mass flow rate were well correlated with the pertinent dimensionless groups for underflow as well as overflow standpipes.     

丁烷氧化制顺酐循环流化床反应系统的温度匹配(Ⅱ)性能模拟、分析与反应温度匹配

利用本文 (Ⅰ )报发展的模型和算法对整个CFB系统的性能和不同氧种的历时变化与操作条件的相互关系进行了分析 ,特别是揭示了提高流化床再生器反应温度 ,使两反应器的性能和反应速率更好匹配的显著效果 ,为进一步开发研究指明了方向     

Applications of the phenomenological theory to several published experimental cases of sedimentation processes

In one space dimension, the phenomenological theory of sedimentation predicts the sedimentation–consolidation behavior of a flocculated suspension in dependence of two constitutive functions describing its material behavior, the solids flux density (or hindered settling function) and the solid effective stress. These functions are assumed to depend only on the local volumetric solids concentration. In this contribution, we review several experimental and theoretical studies of sedimentation in settling columns. We first resume the theories that have been employed to interpret the experimental measurements and then apply the phenomenological model to the available data. The two constitutive functions involved are determined from the published concentration, permeability and effective stress data. The mathematical model is then solved numerically using these functions, and the resulting predictions of settling behavior are compared with the respective authors’ experimental findings and interpretations. In one case, the information obtained from a batch settling experiment is used to simulate continuous sedimentation.     

Solids Holdup of High Flux Circulating Fluidized Bed at Elevated Pressure

Experimental studies on the solids holdup of a high‐flux circulating fluidized bed (HFCFB) at an operating pressure up to 0.5 MPa were carried out. The effects of operating pressure, solids mass flux and superficial gas velocity on the solids holdup distribution were systematically tested. It was found that the solids holdup at elevated pressure increases with increasing solids mass flux but decreases with increasing superficial gas velocity, which is similar to the trends at atmospheric condition. As a result, the condition of a high‐density circulating fluidized bed (HDCFB), i.e., solids holdup in everywhere of the riser is larger than 10 %) is easier to be achieved at elevated pressure than in a HDCFB operated at atmospheric pressure. In the current work, the condition of a full HDCFB with Geldart group B particles has been achieved successfully at 0.5 MPa.     

循环型高通量输送床的建立与控制

密相输送床气化和双流化床气化是基于循环型流化床反应器发展起来的两种新型煤和生物质气化技术,根据这两种技术对流动的要求,提出了在循环流化床的下行床底部耦合一段移动床,为输送床内的流动提供足够高的驱动压力而提高颗粒循环量的技术思想。在根据该思想而建立的直径90 mm的输送床实验装置上的实验研究表明,利用所提出的床型构造可在表观气速9.6 m•s^-1下实现400 kg•m^-2•s^-1的颗粒循环量。输送床的一次风速和移动床松动风速是影响颗粒循环量和输送床内颗粒浓度的主要因素,但循环量随输送床一次风速的增大而增加的走势弱于普通循环流化床。移动床松动风速在小于颗粒最小流化速度的范围内轻微变动即可显著改变颗粒循环量和输送床内颗粒浓度。在保持输送床总气速不变的前提下,通过二次风可在40%的比例范围内调节颗粒循环量,且调节作用随二次风位置的增高而减弱。     

Data Analysis and Modeling of Constant Pressure Batch Dewatering of Fine Particle Suspensions

The problems of data analysis and modeling of experimental constant pressure batch dewatering of materials forming compressible cakes are considered. Dewatering in these materials is typically completed in two stages, viz. cake formation and cake consolidation. A data representation method especially useful for determining the transition point between the filtration and consolidation stages, as well as for comparing accuracy of model predictions, is illustrated. It is shown that dewatering occurs via one of three qualitatively different pathways. A simplified model for engineering analysis of the process is presented. A time-invariant spatially uniform volume fraction of solids approximation is invoked in the cake formation stage. A time-dependent spatially uniform volume fraction of solids assumption is made in the cake consolidation stage. The two models contain four model parameters and have a common physical basis in Darcy's law. Interrelationships between key process parameters are determined and employed to predict the temporal evolution of dewatering in the cake consolidation stage as well as the end point of dewatering.     

Experimental profiles of lateral mixing of feed particles in a three‐dimensional fluidized bed

Solids mixing data of high quality is one of the most crucial steps for quantitative studies, but it is a difficult task to obtain in a fluidized bed especially with a 3D configuration. Therefore a novel sampling technique is developed with bed collapse method, for measuring lateral mixing of feed particles in a 3D fluidized bed. The sampling tool is designed using a “bottom‐to‐top sampling” idea. Its development, configuration and measurement repetition are discussed in detail. The effects of mixing time, fluidizing gas velocity, and particle size of bed material on the tracer distribution are investigated. A quantitative comparison of lateral dispersion coefficient shows that our results agree fairly well with measurements and predictions of correlations for lab‐scale fluidized systems in previous studies. The presented 2D profiles of the lateral mixing can be used to validate fundamental solids mixing models or verifying convenient measurement techniques. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

平底沉降槽内固液分离与赤泥沉降的数值模拟

采用计算流体力学软件Fluent、简化的三维Eulerian两相流和Realizable k-e湍流模型,并结合SIMPLE算法对直径40 m的氧化铝赤泥沉降槽内的固液分离过程进行数值模拟,以得到槽内流场形态和赤泥浓度的分布规律. 通过对比计算结果和现场测试数据,并结合自定义分析评价指标,分析了有无溢流围堰和中心下料桶的不同进料方式下沉降槽的固液分离和赤泥沉降的流动规律. 结果表明,去掉沉降槽的溢流围堰后浆液的固相浓度降低84.3%;采用自吸式射流进料管,60~64 g/L固相浓度范围内的浆液在中心桶内所占体积由3.70%提高到14.36%,可有效促进中心桶内赤泥浓度分布的均匀性,提高平底沉降槽内赤泥的沉降分离效率.