Dewatering of coal plant tailings: Flocculation followed by filtration

A sustainable alternative to tailings dam disposal of coal refuse is mechanical dewatering of tailings, which provides fast production of dry solids and water reuse. In this study, flocculation followed by filtration of coal plant tailings, a new concept in tailings dewatering is investigated in detail. This paper focuses on the effect of preconditioning tailings with varying flocculants and dosages on filtration kinetics and the resultant moisture content of the filter cake. The results show that the cationic flocculant, MAGNAFLOC LT 425, requires a high dosage to produce a low moisture content filter cake and clear filtrate. Optimal sized flocs were produced with the anionic flocculant, MAGNAFLOC 5250, even though the particles are negatively charged. The kinetics of the filtration was dependent on the composition of process water as indicated by supporting sedimentation tests. The concentration of divalent alkali earth metals such as Ca²⁺ and Mg²⁺ allow for large floc growth by a bridging mechanism, which involves binding of the polymer and the negativity charged particle. Filtration and settling curves at this dosage were also supported by filter cake analysis using Darcy plots. It was found that the large floc size significantly increases the permeability of the filter cake. Floc size measurements and fractal dimension showed that while the large flocs were produced with anionic flocculant, the flocs produced with the cationic flocculant were small and weak. The results indicate that the optimum dosage and flocculant type for effective and efficient filtration of coal plant tailings is approximately 350 g/t of anionic flocculant at a 35% solids content and 40 kPa filtration pressure.     

Statistical evaluation of hyperbaric filtration for fine coal dewatering

A statistical design of parametric study of pressure filtration for fine coal dewatering is presented. The effects of five major parameters of the dewatering, i.e. applied pressure, filtration time, cake thickness, solids concentration and slurry pH, on cake moisture reduction and air consumption were investigated. The study was conducted starting with two level factorial experiments to identify the most significant parameters, and concluding with response surface methodologies to establish an optimum operating condition for the dewatering of fine coal. It was observed that applied pressure, cake thickness and filtration time were identified to be the key operating variables for reduction of filter cake moisture as well as air consumption. With the key parameters, an optimum condition for the dewatering was determined to be an applied pressure of 93 psi with a cake thickness of 2.5 cm and a filtration time of 4.8 minutes for the laboratory filtration system. At these optimum conditions the filter cake containing about 22 percent moisture by weight and consuming air by 4.1 m³/(m²·min·kg) on dry solid basis was obtained.     

Effect of Hydrodynamics During Crystallization on Mechanical Dewatering of Salicylic Acid

Mechanical cake dewatering is always desired to reduce the load on thermal dewatering (drying). Any change in the upstream process such as crystallization can have a significant influence on the filtration as well as cake dewatering characteristics. The present study deals with the effect of hydrodynamics (mixing intensity) during salicylic acid crystallization on the air dewatering characteristics in the subsequent pressure filtration. The mixing conditions during crystallization were varied by using three different types of agitators (anchor impeller [AI], curved blade turbine [CBT], and bar turbine [BT]) and by varying the speed of agitation. The effect of operating pressure and dewatering time on the final moisture content of the cake was also studied.

The crystal properties (crystal size and size distribution) were found to vary with the mixing intensity, which further influenced the cake dewatering kinetics as well as the residual moisture content. An AI, which is a laminar flow impeller, produced crystals with a wide size distribution and higher mean particle size, which resulted in cake with high porosity and hence higher moisture content. The high porosity (as well as high cake permeability) caused early air breakthrough, which resulted in ineffective dewatering of cake. Therefore, in this case the residual moisture in cake was found to be higher (27%) even at higher dewatering pressure (1.5 bar gauge) and longer dewatering time (90 s). A BT creates high turbulence during mixing and produced crystals with a relatively narrow size distribution and lower mean particle size, which provided low-porosity cakes. Such cakes could be efficiently dewatered and the final cake moisture content was found to decrease to about 15%, a significant improvement in the filterability of the cake. The dewatering data were modeled according to the correlation between irreducible cake saturation and capillary number for predicting the cake dewatering characteristics (residual moisture as well as dewatering kinetics) and the results were compared with the experimental data.     

Improving the dewetability characteristics of hydrophobic fine particles by air bubble entrapments

The present study deals with the role of surface hydrophobicity and air bubble entrapment on filtration / dewatering of fine coal particles (− 0.5 mm). The experimental results showed that hydrophobizing reagents increased the contact angle (or hydrophobicity) and decreased the filtrate surface tension, cake formation time and moisture content of the fine particles. In addition to these, when the air bubbles were introduced to the hydrophobic fine coal particles in water, the cake formation time and moisture content of the filter cake were further reduced, which may be attributed to the fact that micro/nano size air bubbles were entrapped on the surface of the fine particles in a slurry and increased the dewetting characteristics of the fine particles. As a result, this process may lower the amount of moisture trapped during the dewatering fine particles and increase the efficiency of the filters.     

The Effect of Flocculants and Surfactants on the Filtration Dewatering of Iron Ore Fines

Abstract

The filtration and separation characteristics of iron ore fines have been investigated. The experimental work included: characterization, evaluation of suitable flocculants to enhance settling and dewatering rate, determination of optimum dosages of flocculants, and influence of surfactants on nitration dewatering of both flocculated and unflocculated iron ore fines by vacuum filtration. The results showed that 1) the settling rate can be enhanced many-fold (from 2.52 to 90 m/h) by a suitable flocculant, 2) the residual filter cake moisture content can be reduced from 18.2% without reagents to 12.6% with suitable surfactant dewatering aids while the filtration rate can be enhanced from 4.8 to 97.2 L/h with suitable dewatering filter aids, and 3) the specific cake resistance to filtration can be brought down from 8.6 × 10⁹ to 1.0 × 10⁹ m/kg by using a surfactant sodium petroleum sulfonate at a concentration of 1.47 × 10^?3 kg/t. The economics of using flocculant filter aids and surfactant dewatering aids before thermal drying is described and discussed.     

表面活性剂用于细粉煤脱水的研究

精粉煤脱水是炼焦精煤洗选后的一个极其困难的课题 ,采用助滤剂促进滤饼水分的降低是一条经济而有效的途径 ,作者阐述了利用表面活性剂作为助滤剂 ,强化细粉煤的脱水 ,取得了显著效果。实验室两种煤样采用磺酸盐型表面活性剂 ,脱水后最佳降水值分别达到 4 9%和5 96 % ,大屯煤矿洗煤厂连续工业实验采用硫酸酯盐型表面活性剂 ,最佳降水值达到 6 0 8%。     

以颗粒煤为助滤剂对城市污泥进行二段过滤脱水的研究

以颗粒煤为助滤剂,采用二段过滤方式来强化城市污泥的脱水,并对滤饼进行热值分析,探讨城市污泥资源化利用前景.在研究中,具体分析探讨了煤的类型、煤的添加比、煤的粒径、搅拌强度、过滤压力以及煤的添加方式对过滤过程中相关参数的影响.得到以下结论:在污泥和煤的添加比为1:2,颗粒煤粒度为0.15～0.18 mm,低强度搅拌,常温...     

降低过滤机滤饼水分和细粒煤脱水浅析

根据对选煤厂细粒煤样的实验室优化脱水试验、工业试验及分析结果 ,对真空系统、气水分离器和分配头实施了技术改造 ,细粒煤脱水系统的运行状况得到显著改善 :圆盘真空过滤机滤饼水分降到 2 0 %左右 ,脱饼率在 95 %以上 ,每年的直接经济效益可达 30 0多万元     

Removal of Moisture from the Ultra Fine Particles Using Both High Centrifugal Force and Air Pressure

Abstract

A newly developed centrifugal batch dewatering unit was developed and used to remove the moisture contents of ultra fine particles (e.g., kaolin clay, silica, talc, and precipitated calcium carbonate). In the present method, air pressure was applied into the vessel of a conventional centrifuge to improve the efficiency of the dewatering. The experimental results showed that the moisture reductions of the samples could be enhanced more than 50% depending on dewatering conditions (i.e., G-force, spin time, applied pressure, cake thickness, particle size, surface hydrophobicity, and particle agglomeration). As a result, when the new centrifuge modification is used for the fine particle dewatering, cost and environmental concerns of thermal dryers could be considerably reduced and the handleability of the filter cakes could be significantly increased.     

我国细粒煤脱水技术与设备研究现状

搞好细粒煤分选是生产洁净煤的重要途径,选煤厂出厂产品水分的高低直接受细粒煤水分高低的影响。本文就加强细粒煤脱水的重要性进行论述,并介绍了近期研制成功的应用于工业生产的细粒煤脱水技术与设备     

Optimization Study of Municipal Sludge Conditioning,Filtering, and Expressing Dewatering by Partial Least Squares Regression

The effect of different conditioner doses and different applied pressures on the dewaterability of municipal sludge during the filtration and expression stage was assessed using a series of experiments referred to as a uniform design. The relative importance of conditioner dose and applied pressure on the dewatering characteristics of municipal sludge was evaluated with a quadratic model using partial least squares (PLS) regression analysis. It was shown that the developed quadratic model was able to accurately predict the dewatering characteristics during the whole process. It was further shown that lime as a physical conditioner had the most significant impact on filtration rate, expression rate, cake dry solids content, net sludge solids yield, and total cycle time. Aluminum polychloride as the inorganic flocculant had a quite complex effect although not as significant as lime. Filtration pressure had only a slight effect on dewaterability of the municipal sludge, though higher feeding pressure was needed in order to enhance the total feeding amount. Furthermore, expression pressure was only of benefit to cake dry solids content and expression rate and failed to improve other dewatering characteristics of the municipal sludge.     

煤气化细渣陶瓷膜真空脱水试验与数值模拟

气化细渣是煤炭气化过程的废弃物,高效脱水是其资源化利用、减量化处置的必要前提。本文采用陶瓷膜真空过滤系统开展了脱水实验并对脱水过程进行了数值模拟。气化细渣料浆浓度和液下吸附时间影响滤饼厚度且滤饼厚度增加导致水分运移路径增长,使得有效脱水时间增加;滤饼脱水过程的脱水速率值呈现非线性降低趋势且滤饼水分极限值为40%,这与气化细渣物化性质有关;真空度>0.08MPa时气化细渣滤饼中“通道水”能够在约24s有效脱除。Fluent数值模拟过程选用了欧拉模型并确定了陶瓷膜滤板和气化细渣滤饼的阻力系数,脱水过程的实验值与模拟结果误差小于5%,证实了模型可靠性。模拟过程分析了气化细渣脱水过程中压力场和水分含量分布云图的演变规律,结果表明,增加脱水系统真空度、降低滤饼厚度、提高“通道水”比例以及增大气化细渣颗粒等效当量直径能够提高气化细渣脱水效率。此外,陶瓷膜真空脱水过程所得滤液洁净度高且部分指标达到了工业用水的标准。     

Prediction of the air consumption when dewatering a filter cake obtained by pressure filtration

In this communication, we report some data for the dewatering of a coal flotation concentrate in which very fine particles occur Dewatering is carried out by pressure filtration (up to 6 bar) followed by blowing to partially dry the cake (the filtration and dewatering phase respectively). For the systems studied, a reasonable prediction of the air consumption in either a lab filter, or a one-element test filter, or a full-scale continuous filter is possible, based on measurement of the liquid permeability for saturated flow in a lab filter cake. Together with scaling-up characteristics of the filtration step (not reported here), this gives the possibility of a reliable prediction of the required size and energy consumption of a large-scale continuous-pressure filter based on bench-scale test data.     

KHYC型絮凝剂用于污泥脱水处理的研究

详细地论述了ＫＨＹＣ型高效絮凝剂在污泥脱水处理方面的应用研究。试验结果表明，该种絮剂的处理效果明显优于目前国内常用的ＰＡＭ，不仅操作简单，而且脱水效果好，滤饼含水率低，同时加药浓度很低，因而大大降低了处理费用。     

Dewatering of fine coal slurries by selective heating with microwaves

Comparative experimental studies on the dewatering of a fine coal slurry sample containing ≈52% moisture by microwave heating vis-à-vis thermal heating are reported. For thermal heating, thermogravimetric analysis (TGA) using different masses and different heating rates were employed. Similar experiments were carried out for dewatering by microwave heating using a conventional oven operating at 2.45 GHz and 800 W power. These quantitative experiments show that microwave heating is an order of magnitude more efficient than thermal heating, although about 10% of the residual moisture, perhaps trapped in the capillaries of the coal particles, is not removed even by microwave heating. Based on these results, a bench scale dewatering unit was designed and tested in which coal slurry in kg quantities was fed on a conveyor belt. The efficiency of this unit for dewatering fine coal slurries was estimated to be over 80% of the theoretical limit. A cost estimate of about $3/ton for the power consumed for 10% reduction of moisture is made.     

Effect of Cake Compression on the Performance of Centrifugal Dewatering

The effect of cake compression on the centrifugal dewatering is investigated under various rotating speeds and different kinds of cakes. A numerical method is proposed to estimate the capillary pressure and cake permeability under various cake saturations. The most dewatering in this study are operated at the funicular state of cake. The cake permeability decreases, while the capillary pressure increases rapidly, with the decrease of cake saturation during a centrifugal dewatering. The cake saturation can also be simulated once the relations among capillary pressure, cake permeability, and cake saturation are known. The deviations between simulated saturations and experimental data for compressible talc cakes are less than 2%. The centrifugal dewatering of compressible cake includes two mechanisms, the water squeeze due to cake compression and the centrifugal drainage due to pressure difference. The role of cake compression is dominant at the initial 500 s of the dewatering period, and these two factors are almost equal in magnitude as well as in importance when the cake approaches its equilibrium saturation. The porosity and compressibility of cake play the major roles in determining the equilibrium cake saturation. The equilibrium saturation of the compressible talc cake with a lower porosity is much higher than that of incompressible Al₂O₃ cake under a fixed rotating speed. An increase in rotating speed (centrifugal effect) results in a lower equilibrium cake saturation for either compressible or incompressible cakes; however, the increase in rotating speed is more efficient for an incompressible cake in the centrifugal dewatering.     

卧螺离心机在低浓度市政污泥脱水中的应用

介绍了卧螺离心机的主要特点以及在低浓度市政污泥脱水中的应用,探讨了差转速、污泥浓度对泥饼含固率,絮凝剂用量对固相回收率的影响。结果表明,低浓度市政污泥经离心脱水后平均泥饼含固率为23.28%,平均固相回收率为99.41%,处理量达到27m3/h。在卧螺离心机操作参数一定时,泥饼含固率随着差转速的减小而增大,随着污泥浓度的降低而降低。     

Involvement of Gypsum (CaSO4 · 2H2O) in Water Treatment Sludge Dewatering: A Potential Benefit in Disposal and Reuse

Abstract

This research assessed the use of gypsum (CaSO₄ · 2H₂O) as a skeleton builder for sludge dewatering since polymer conditioning of sludge affected only the rate of water release, not the extent of dewatering. The use of gypsum as a physical conditioner, in association with a polymer, could improve sludge filterability. More significantly, gypsum serves as a skeleton builder, forming a permeable and rigid lattice structure that can remain porous under high positive pressure during the compression step after the cake growth of the filtration, thereby maintaining the size of the micro‐passages through which water is expressed. Experiments using a high pressure cell apparatus showed that a further decrease of two to seven percent of the equilibrium moisture content of the sludge cake was achieved, for sludge thicknesses for dewatering of 1 to 10 cm, by the addition of gypsum with 60% of the original sludge solids when compared to the single polymer conditioning. The importance of the addition of gypsum in alum sludge dewatering is not only the improvement in the extent of dewatering, but also the potential application of transforming dewatered alum sludge from “waste” for landfill to useful “fertilizer” or to be used as a filter medium/adsorbent for wastewater treatment engineering.     

An Integrated Process of Olive Mill Wastewater Treatment

Abstract

Olive mill wastewater (OMWW) is considered as a challenge for environmental scientists. It is characterized by high values of total organics expressed by chemical oxygen demand (COD), biological oxygen demand (BOD₅), and phenols. In this paper, an integrated process of OMWW treatment, which includes lime precipitation, filtration using a novel technology of a membrane filter press (a pilot scale) and a post‐treatment using activated carbon adsorption for the filtrate, was studied. Lime precipitation has removed ～71% of phenols, ～39% of COD, and ～88% of BOD₅ with a rise in the total suspended solids (TSS) from 31 kg/m³ (before precipitation) to 69 kg/m³ (after precipitation).

The filtration and dewatering operation cycle is approximately 125–150 min. Filtration was examined at different feed pressure (3–5 bar), different slurry concentrations (23–69 kg/m³, dry weight), and filter aid pre‐coat with different additions of diatomite body feed. Specific cake resistance, α, was found to increase with the increase in feed pressure and to decrease with diatomite pre‐coating and slurry concentration increase. However, a further decrease in α values was obtained using diatomite body feed. Cake dewatering, via membrane squeezing, was applied using hot water (65°C) and cake moisture was dropped from ～64% before squeezing to 35% after squeezing. Twenty minutes were found enough for that stage to be accomplished. With vacuum application, for 30 min, over the hot cakes, cake moisture decreased to ～20% for cakes with an average thickness of 1 cm. The produced cakes lose more moisture by storing in open air (on the shelf), reaching an equilibrium value of 9% in 2–3 days. Calorific value of produced cake is 15.71 MJ/kg, suggesting strongly its use as an energy source. Activated carbon was used as an efficient sorbent for removing the remaining phenols and total organics from the filtrate as a post‐treatment showing promising results. Maximum removal of phenols and total organics, by lime precipitation and filtrate post‐treatment using activated carbon adsorption, reached ～99.7% and ～80%, respectively.

A process flowsheet and preliminary cost estimates are presented and compared with other processes. The thermal energy produced by the burning of the produced cakes would be enough for the thermal needs of both the olive mill plant and the proposed treatment process.     

A study of the improvement in dewatering behavior of wastewater sludge through the addition of fly ash

Wastewater sludge is classified as a difficult dewatering material (DDM) due to the high cake specific resistance (CSR). On the other hand, fly ash is classified as an easy dewatering material (EDM), which suggests that it might be able to improve the dewaterability of wastewater sludge. The water content and cake specific resistance of dewatered sludge without the addition of fly ash were 80% and 2.9×10¹⁴ kg/m, respectively. When 50% (by dry weight) fly ash was added to the sludge, the water content and cake specific resistance decreased to 29.4% and 2.9×10¹³ kg/m, respectively. The cake specific resistance and water content decreased with increasing fly ash additions. Therefore, the production of sludge cake can be reduced by adding fly ash, which can help minimize the social and environmental problems caused by the need to dispose of wastewater sludge.