Feasibility of fly ash-based composite coagulant for coal washing wastewater treatment
Highlights
► Coal washing wastewater was analyzed. ► Several fly ash-based composite coagulants were prepared to treat coal washing wastewater. ► The effluent with the high COD and SS removal was obtained after treatment.
Introduction
The production of large effluent volumes by coal washing is a significant problem in the preparation of coal for coal-fired power plants [1], [2]. The wastewater usually has a characteristic of dark black color and nauseating odor. Moreover, organic and inorganic matters, such as phenols, polycyclic aromatic hydrocarbons and humic substances, have also been detected in the effluent [3]. These contribute to chemical oxygen demand (COD) [4]. An aqueous suspension of coal and clay increases wastewater turbidity and color. Particles in the suspension do not undergo gravitational sedimentation at acceptable rates, owing to their surface charge and small particle size, which also cause the formation of a stable colloidal suspension [5]. The disposal of coal washing wastewater is therefore an expensive process. A potential method for reducing the volume of effluent discharged to the environment is to recycle and reuse the effluent after treatment.
The coagulation process has attracted considerable attention, as it can yield high removal efficiencies in wastewater treatment. Coagulation not only removes organic compounds together with suspended solids, but may also decolorize waste streams [6], [7]. Several coagulants have been used for wastewater treatment in laboratory- and pilot-scale experiments over recent years, predominantly aluminum and iron-based coagulants, including aluminum sulfate (AS), aluminum chloride (AC), poly-aluminum sulfate (PAS), poly-aluminum chloride (PAC), ferric chloride (FC), ferric sulfate (FS), poly-ferric sulfate (PFS) and poly-alumino-iron sulfate (PAFS) [8]. However, most coagulant manufacturers use ores or chemical salts of iron and aluminum as raw materials. These are not only costly, but also consume large quantities of the earth's limited mineral resources [9], [10]. Although coagulation/flocculation with PAC (dosage: 80–240 mg l−1, 5–10% solution) and anionic polyacrylamide (A-PAM) (molecular weight: more than 3,000,000, dosage: 1.0–4.0 mg l−1, 0.1–0.26% solution) can be favorable in reducing the suspended solids concentration and COD in coal washing wastewater, the coagulant dosage and concentration are proportional to the waste materials in the water [11], [12]. Furthermore, conventional coagulants including PAM and PAC are expensive and corrosive, and PAM monomers are strongly neurotoxic, thereby endangering the environment. Based on these problems, an effective, chemically non-invasive and low-cost coagulant should be developed to meet stringent environmental regulations placed on the quality of effluent discharged.
Fly ash (FA) is a solid waste residue produced from coal combustion in coal-fired power plants. It is estimated that the annual production of fly ash is more than 500 million tons throughout the world [13]. Traditionally, the majority of coal fly ash has been dumped into landfills, which could have harmful effects on the use of land and its maintenance [14]. In addition, as it is small and light, it is easily airborne by wind [15]. Untreated, fly ash may cause serious environmental problems by polluting the water, atmosphere, soil and landscapes through dust generation [16]. As a result, there has been an increased interest in the utilization of fly ash in recent years. Some fly ash is recycled in civil construction materials, but overall, the global recycling rate of coal ash is only approximately 15% [17]. Some researchers have tested the utilization of coal fly ash as a coagulant to treat sewage sludge, because fly ash is rich in iron oxides and aluminum, with the weight ratio of these substances appropriate for producing complex coagulants [18], [19], [20]. Fly ash could therefore be converted into a valuable coagulant for wastewater treatment, which could not only be helpful in solving the solid waste disposal problem, but could also reduce costs [21].
On the basis of the above discussion, in this study, several composite coagulants were prepared by leaching of fly ash in hydrochloric acid. The feasibility of using the resulting coagulants in coal washing wastewater treatment was assessed by evaluating the settling characteristics and reduction in COD and suspended solids (SS) after coagulation. The coal washing wastewater, synthesized coagulants and sludge slurry after treatment were examined by scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), X-ray fluorescence (XRF), X-ray diffraction (XRD), particle-size analysis and zeta potential determination, and the metal ion concentration in the supernatant was also analyzed. Coagulant dosage and pH were also investigated to optimize conditions for treatment of coal washing wastewater.
Section snippets
Materials
Coal washing wastewater and a sample of raw fly ash were collected separately from a coal washing plant and power plant in Yulin, Shaanxi province of China, which is the location of one of the world's seven largest coalfields. The chemical properties of the fly ash as determined by XRF by the fly ash supplier are listed in Table 1. Analytical reagent grade hydrochloric acid (HCl), calcium oxide (CaO) and sodium carbonate (Na2CO3) were purchased from Chemicals Ltd, Xi’an (China).
Experimental procedure
Three composite
Analysis of coal washing wastewater
The major characteristics of the coal washing wastewater are presented in Table 2. High COD and SS values were found to be characteristic of the wastewater, possibly owing to the argillization of coals in the washing process. This results in the production of many sludge particles, which contribute to the dark black color of the wastewater and long slurry settling times. Treatment of wastewater of this type depends significantly on sludge properties and particle size distributions [27].
Conclusions
Based on the above discussion, FA-HCl-Y3 is an inexpensive coagulant, and it is at least as feasible as the coagulants used conventionally in coal washing wastewater treatment plants. The effluent after treatment was clear with low pollutant concentration. The COD and SS removal was up to 96.48% and 99.61%, respectively. Utilization of the fly ash and production of a valuable chemical reagent for the treatment of wastewater could not only be helpful to the environment in solving solid waste
Acknowledgments
This work was supported in part by the Nature Scientific Research Foundation of Shaanxi Provincial Education Office of China (09JK843, 11JK0607), the Scientific Research Project of the Shaanxi Province of China (2009K06-26, 2011K17-07-04).
References (37)
- et al.
An emphasis on optimum fuel production for Indian coal preparation plants treating multiple coal sources
Fuel
(2010) - et al.
Mutagenicity of eluent by hot water extraction of various coals: effect of chlorination
Fuel
(2008) - et al.
Fluorescence study of the solubilization of benzo[a]pyrene: application to its detection in coal washing waters
Anal. Chim. Acta
(1991) - et al.
Characterization of eluent by hot water extraction of coals in terms of total organic carbon and environmental impacts
Fuel
(2005) - et al.
High-pH-magnesium coagulation-flocculation in wastewater treatment
Adv. Environ. Res.
(2003) - et al.
Improvement of paint effluents coagulation using natural and synthetic coagulant aids
J. Hazard. Mater.
(2006) - et al.
Production of a poly-alumino-iron sulphate coagulant by chemical precipitation of a coal mining acid drainage
Miner. Eng.
(2010) - et al.
A novel process for recovering clean coal and water from coal tailings
Miner. Eng.
(2000) A review on the utilization of fly ash
Prog. Energy Combust.
(2010)- et al.
Sewage sludge conditioning with coal fly ash modified by sulfuric acid
Chem. Eng. J.
(2010)
Mechanism and kinetics of aluminum and iron leaching from coal fly ash by sulfuric acid
Chem. Eng. Sci.
The kinetics of producing sulfate-based complex coagulant from fly ash
Chem. Eng. Process.
Removal of dyes from aqueous solution using fly ash and red mud
Water Res.
Production of a complex coagulant from fly ash
Chem. Eng. J.
Production of a new wastewater treatment coagulant from fly ash with concomitant flue gas scrubbing
J. Hazard. Mater.
Influence of pretreating activated sludge with acid and surfactant prior to conventional conditioning on filtration dewatering
Chem. Eng. J.
Aluminium sulfate as coagulant for highly polluted cork processing wastewater: evaluation of settleability parameters and design of a clarifier-thickener unit
J. Hazard. Mater.
Floc size estimation in iron induced electrocoagulation and coagulation using sedimentation data
Int. J. Miner. Process.
Cited by (61)
Novel method of poly aluminum chloride extraction from kaolin and its application for wastewater treatment
2024, Desalination and Water TreatmentResearch on the mechanism of nanofiltration membrane fouling in zero discharge process of high salty wastewater from coal chemical industry
2021, Chemical Engineering ScienceCitation Excerpt :As a water treatment additive with good water solubility and flocculation effect, it is widely used in the treatment of various industrial wastewater. Such as metallurgical wastewater (Zhou et al., 2016; Sun et al., 2016), mineral processing wastewater (Fijałkowska et al., 2020), coal washing wastewater (Yan et al., 2012; Shi et al., 2020; Zheng et al., 2015), electroplating wastewater (Desai and Murthy, 2012; Li et al., 2003), papermaking wastewater (Zhang et al., 2017; Lin et al., 2015), and drinking water. It can quickly adsorb suspended particles in wastewater by bridging and charge neutralization between particles to form larger flocs to accelerate sedimentation and clarification (Saha and Sarkar, 2012; Zhang et al., 2017; Sun et al., 2016).
Fly ash from coal combustion as improver of anaerobic digestion: A review
2021, Journal of Environmental Chemical EngineeringCitation Excerpt :The chemical composition of the FA does not only depend on the type of coal the ash comes but also on the type of combustion processes employed in each thermoelectric plant. In order to determine its composition, different analytical techniques have been applied, such as: X-ray fluorescence techniques [41–45], acid digestion using inductively coupled plasma mass spectrometry (ICP-MS) and inductively coupled plasma atomic emission spectrometry (ICP-AES) [46], X-ray diffraction [47], atomic absorption spectroscopy (AAS) [48,49], optical emission spectroscopy [50], and occasionally X-ray photoelectron spectroscopy (XPS), secondary ion mass spectroscopy (SIMS) or gas chromatography [35,51]. Table 2 shows the main compounds present in the FA by region.
Collaborative disposal of multisource solid waste: Influence of an admixture on the properties, pore structure and durability of foam concrete
2021, Journal of Materials Research and TechnologyCitation Excerpt :FA is thought to contain various heavy metals, such as Hg, As, Cr, Cd, V, and Pb, and it is a severe threat to human health, e.g., nervous system diseases and renal dysfunction. FA is thought to affect the chemical oxygen demand (COD), turbidity and color of water [6]. In addition, FA has been considered the cause of fog and haze in recent research due to its surface charge and small particle size [7].
Roles of a mixed hydrophilic/hydrophobic interface in the regulation of nanofiltration membrane fouling in oily produced wastewater treatment: Performance and interfacial thermodynamic mechanisms
2021, Separation and Purification TechnologyCitation Excerpt :Moreover, the interactions among hydrophilic and hydrophobic organic molecules (i.e., mixed hydrophilic/hydrophobic interface) should be investigated to identify the mitigating mechanisms of the mixed interface in NF membrane fouling by hydrophobic organics. Anionic polyacrylamide (APAM) is a water-soluble macromolecule polymer that is mainly employed for flocculation of various types of industrial wastewater, such as metallurgical wastewater [26], coal washing wastewater [27] and sludge dewatering [28]. APAM can also be employed for clarification and purification of drinking water [26,29] and as an additive to increase the viscosity of solutions used to enhance CO recovery during CO production [6,30].