Fluidized bed reactor for fluoride removal

doi:10.1016/j.cej.2004.12.017

Chemical Engineering Journal

Volume 107, Issues 1–3, 15 March 2005, Pages 113-117

https://doi.org/10.1016/j.cej.2004.12.017 Get rights and content

Abstract

The common methods for fluoride removal from industrial wastewater involve chemical precipitation. This process generates large amounts of a water rich sludge requiring disposal with increasing costs. Due to the high water content and the low quality of the sludge, reuse of fluoride is not an economical option. The crystallization in a fluidized bed reactor offers an alternative to the conventional precipitation.

The influence of the supersaturation and recycling conditions in order to control the efficiency of the process was studied. A high supersaturation level decreases the fluoride removal efficiency due to the formation of small particles (fines). The main mechanism concerning the precipitation of calcium fluoride in a fluidized bed was established. Fines were produced by nucleation in liquid phase (homogeneous and secondary nucleation) in the reactor and the dilute fluoride tank, in order to decrease the amount of fines it is very important the control of the supersaturation.

Introduction

The objective and principles of the environmental policy in the European Union (EU) consists of preventing, reducing and as far as possible eliminating pollution by giving priority to intervention at source and ensuring prudent management of natural resources, in compliance with the principle of pollution prevention. The goal is an integrated pollution control so that it is possible to reduce the emissions in order to promote the sustainable development. This concept aims to harmonize the economical, social and environmental dimensions of development strategies and it is now a key feature of policy making in the EU.

The Council Directive 96/61/EC set up EU legislation on Integrated Pollution Prevention and Control (IPPC) [1]. This was enacted in Spain in July 2002 under the Pollution Prevention and Control Law 16/2002 [2]. Installations detailed in the directive will be phased into the new regime on a sectoral basis up to the year 2007.

Furthermore, an important new feature to be remarked is the supplementing of permitting with voluntary environmental management systems, particularly ISO 14001 and EMAS, and with energy conservation agreements.

In accordance with IPPC Directive, the installations are operated in such a way that waste production is avoided. However, in case it is technically and economically impossible it should be disposed off avoiding or reducing any impact on the environment.

In this sense, fluoride wastewater is a typical effluent which neutralization is very important due to the hazardous effects. Fluoride is considered as a pollutant under regulation, therefore, it is necessary some treatment to reduce its concentration.

The method for fluoride removal from industrial wastewater generally involves a chemical precipitation process [3], [4], [5]. Several treatment methods to remove fluoride have been employed. Precipitation is the most common treatment technology. Fluoride is removed by adding an alkali compound such as lime or hydrated lime to adjust the wastewater pH to the point where the fluoride exhibits minimum solubility. Then the precipitated fluoride is removed by a proper solid–liquid separation technique such as sedimentation and filtration. The conventional fluoride removal process generates huge amounts of a water rich sludge, which has to be disposed off with increasing costs. Due to the high water content and the low quality of the sludge, reuse of fluoride is not an economical option.

Recently, several processes for fluoride removal from wastewaters have been developed. The fixed bed packed with granular calcite allows high efficiencies of fluoride removal without sludge generation [6], [7]. However low effective conversion rates of calcite and formation of lumps in the fixed bed are obtained. Precipitate flotation [8] and other processes that involve recycle to reduce sludge and costs [9] are also used, but they are not able to recover the product.

An alternative option is to apply controlled crystallization in a fluidized bed reactor instead of conventional precipitation. The operating cost in the crystallization process are similar to the conventional precipitation. Nevertheless the derived benefits are important (reuse, reduction of chemical waste and reduction of discharge fee). Table 1 shows an estimate costs evaluation [14].

Continuous experiments in a fluidized bed reactor were carried out in order to study the influence of the supersaturation conditions and the recycle conditions.

Section snippets

Process and system description

The process is based on the crystallization of calcium fluoride upon seed grains in a fluidized bed using seed grains of silica sand.

The chemistry of the process is similar to the conventional precipitation. By dosing a calcium salt to the wastewater (e.g. lime, calcium chloride), the solubility of CaF₂ is exceeded and fluoride is converted from the aqueous solution into solid crystal. Process includes the following reactions:Ca(OH)₂ → Ca²⁺ + 2OH⁻Ca²⁺ + 2F⁻ → CaF₂

The reactor consists of a vessel

Materials and methods

The reactor was filled with sand with a diameter of 0.15–0.30 mm and density of 2.56 g cm⁻³. The static height of the packed sand in the reactor was 0.20 ± 0.01 m.

The synthetic fluoride wastewater used in the feed was obtained by diluting a more concentrated fluoride solution. Fluoride wastewaters of 300–2000 mg l⁻¹ were treated.

Chemical grade reactants and demineralized water were used. The temperature in the fluidized bed was 20 ± 2 °C.

The calcium reagent and fluoride solutions were injected into

Results and discussion

The supersaturation has an important effect on the process related to the nucleation mechanism, to the fines formation and the efficiency of the process. The degree of saturation can be defined as a ratio of the ion activity product to the solubility product as $β = \frac{(C a^{2 +}) {(F^{-})}^{2}}{K_{sp (Ca F_{2})}}$

At a given pH and overdose of calcium, the supersaturation depends only on the fluoride concentration of the wastewater. Fig. 4 reveals the effect of the inlet fluoride concentration referred to the supersaturation

Conclusions

A high supersaturation level decreases the fluoride removal efficiency due to the formation of fines.

The main processes that involve the precipitation of calcium fluoride in a fluidized bed in viable technically conditions were established. Fines were produced by nucleation in liquid phase (homogeneous and secondary nucleation) in the reactor and dilute fluoride tank.

It is not possible to obtain efficiencies higher than 40% in viable technically conditions when it is necessary to dilute the

Acknowledgements

This research has been financially supported by Ministry of Science and Technology of Spain (Project No. PPQ2000-0251). We would like to thank Head Office of Research of Spain (Grant No. FPI2000-4858).

References (14)

D.F. Lawler et al.
Equalization/neutralization modelling: an application to fluoride removal
Water Res.
(1984)
J. Garside
Industrial crystallization from solution
Chem. Eng. Sci.
(1985)
OJ (1996) Council Directive 96/61/EC of 24 September 1996 concerning Integrated Pollution Prevention and Control,...
LEY 16/2002, de 1 de Julio, de prevención y control integrados de la contaminación, BOE No 157.2.7.2002, 2002, p....
S. Saha
Treatment of aqueous effluent for fluoride removal
Water Res.
(1993)
Z. Yang et al.
Reaction of CaO, Ca(OH)₂ and CaCO₃ whit dilute fluoride streams
Light Met.
(1990)
M. Yang et al.
Fluoride removal in a fixed packed with granular calcite
Water Res.
(1984)

There are more references available in the full text version of this article.

Cited by (82)

Pilot test on acidic fluoride-containing wastewater treatment in the photovoltaic industry through induced crystallization with a focus on calcium fluoride recovery
2024, Separation and Purification Technology
The utilization of hydrofluoric acid in the photovoltaic (PV) industry results in the generation of substantial volumes of acidic fluoride-containing wastewater, emphasizing the significance of wastewater treatment and the reclamation of fluorine resources. In this study, a pilot test was conducted on acidic fluoride-containing wastewater from a PV cell manufacturer in Yangzhou City, Jiangsu Province, China, employing a combined process system integrating chemical crystallization circulating pellet fluidized bed (CrystPFB) and high-efficiency solid-liquid separation pellet fluidized bed (SepPFB). The pilot test outcomes indicate that utilizing CaF₂ as a seed crystal and employing a joint addition of Ca(OH)₂ and CaCl₂ at a combined dosage of 2,300–3,700 mg/L, under conditions of influent fluoride concentration ranging from 800–3000 mg/L and a pH range of 2.23–3.20, reduces fluoride levels in the effluent to 20–30 mg/L. The combined process system achieves a turbidity in the effluent below 5 NTU, a fluoride recovery efficiency of 45–63 %, and an operational cost ranging from 1.68 to 2.46 RMB/m³. The physical analysis of the reclaimed crystalline calcium fluoride particles reveals a dominance of CaF₂ on the particle surface, with a purity exceeding 90 %. This study provides essential data supporting the application of the CrystPFB + SepPFB combined process system for treating acidic fluoride-containing wastewater and recovering calcium fluoride in the PV industry, establishing a groundwork for subsequent projects.
Study on the effect of supplementary cementitious materials on the volume stability of mortar mixed with laboratory-made high-content fluorine-type accelerator
2023, Journal of Building Engineering
The high-content fluorine-type accelerator (AF) can be prepared from fluorine-containing wastewater due to its low cost and is widely used in many projects. In this study, the fluorine-type alkali-free liquid accelerator was synthesized by simulating fluorine-containing waste liquid in the experiment, and its performance was tested and shown to meet the standard requirements. Subsequently, three supplementary cementitious materials (granulated blast furnace slag-GBFS, fly ash-FA, and silica fume-FS) were used to study the effect of supplementary cementitious materials on the setting time, mechanical properties and volume stability of mortar mixed with accelerators. The results show that the addition of supplementary cementitious materials shortens the setting time of the cement paste, while maintaining the requirements on compressive strength. Among these materials, the incorporation of silica fume makes the best effect of shortening the setting time, and yields the highest increase in compressive strength. In the 180-day test, both FA and GBFS can reduce the autogenous shrinkage and drying shrinkage of mortar, and the shrinkage reduction effect of FA is more prominent. FS significantly increases the autogenous and drying shrinkage of the mortar. By observing the changes to mass and humidity under the drying conditions of mortar, it is found that FS can effectively reduce mass loss and humidity change. This is because the early hydration reaction of FS occurs earlier than that of FA and GBFS, and it can play a role in refining the pore structure. However, the addition of FS aggravated the shrinkage of the mortar by increasing the pore size of 5–50 nm, resulting in higher capillary tensile stress and greater shrinkage driving force. FA increased the mass loss and internal humidity reduction of the mortar, but was less driven by capillary tensile stress due to coarsening of the pore structure. This study provides theoretical guidance for practical construction application.
New nonporous fillers-based hybrid membranes for gas separations and water treatment process
2023, Current Trends and Future Developments on (Bio-) Membranes: Modern Approaches in Membrane Technology for Gas Separation and Water Treatment
Considering the superiority of membrane applications over traditional processes in answering the world’s call for energy saving and emission reduction, the chapter will only shrink the horizon and lay focus on nonporous fillers-based hybrid membranes fabrication strategies with lab-scale or in compliance with industry standards, its current trends and future possible applications in gas separation and wastewater treatment. We aim to provide in-depth reviews discussing new materials, characterization, and computational simulation, campaigning vigorously for opening innovative insights into the generation of zero-emission power technologies with high economic merits by examining the key points in these membranes.
The recovery of sulfur as ZnS particles from sulfide-contained wastewater using fluidized bed homogeneous crystallization technology
2022, Chemical Engineering Journal
Sulfide wastewater that is anthropogenically generated from industrial activities is highly corrosive, hazardous, and harms the natural ecosystem. This study uses a novel fluidized-bed homogeneous crystallization (FBHC) method to remove sulfide ions from an aqueous solution. Zinc is used as a precipitant to crystallize ZnS homogeneously in the FBHC reactor to reduce the sludge, which is commonly produced in a conventional chemical precipitation process. The optimal pH value, [Zn²⁺]₀/[S^2-]_0 M ratio, sulfide cross-sectional surface loading (L, kg-S/m².hr), and hydraulic retention time (HRT) for the system are established, to optimize the sulfur removal efficiency. The maximum crystallization ratio and the total removal efficiency for sulfur are 97.7% and 98.8%, respectively, at pH = 5.4, a [Zn²⁺]₀/[S^2-]_0 M ratio of 1, a cross-sectional surface loading of 2.2 kg-S/m².hr, and an HRT number of 6 with an initial sulfur concentration of 320 mg/L. The solid products are collected and identified as zinc sulfide (wurtzite) using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS).
Liquid solid fluidized bed crystallization granulation technology: Development, applications, properties, and prospects
2022, Journal of Water Process Engineering
Fluidized bed crystallization granulation technology was first applied to water softening and gradually achieved improved results from initial exploration to large-scale chemical plants. It has received wide attention and development because of its unique and excellent characteristics. For example, the process is operable under normal temperature conditions and on a large scale. It features high mass transfer rate, low water content and high purity of particle crystals, and easy solid–liquid separation. The application direction has gradually expanded to recover fluoride, remove phosphate, recover heavy metals, and synthesize crystals. This article reviews the application of fluidized bed crystallization in water softening, fluoride recovery, phosphate removal, recover heavy metals, and synthetic crystals in recent decades. The structure of a fluidized bed reactor, carrier material, fluid flow rate, feed concentration, dosage, and other important operating parameters were discussed. This work lays a theoretical foundation for the development and application of high efficiency and industrialized fluidized bed crystallization granulation. This review concludes with an overview of the challenges faced by this research focus and the authors' personal views.
Optimizing the recovery process of ceramic grade calcium fluoride from hydrofluoric/hexafluorosilicic acid wastewater
2022, Journal of Cleaner Production
Citation Excerpt :
Due to its simplicity, flexibility, and low cost, the addition of calcium salts to form calcium fluoride (CaF2) is the most common way to remove fluoride from wastewater in Taiwan. However, this process produces huge amounts of water-rich and low-purity sludge (40–60% of CaF2) (Aldaco et al., 2005) which is non-reusable and must be disposed of with increasing costs (Aldaco et al., 2008; Jiang and Zhou, 2017). Meanwhile, the fluorite mineral resources are forecasted suffering a shortage of 600,000 to 800,000 tons in the global markets by 2026 (Kutepatil, 2018).
Chemical precipitation using calcium salts has been widely applied in hydrofluoric acid wastewater treatment due to its simplicity, flexibility and economy. Coagulants/flocculants are used to improve solid-liquid separation in a fine precipitate of calcium fluoride (CaF₂). As a result, a large amount of sludge is generated by CaF₂ precipitation process. In order to move towards a circular economy, the current study developed an environment-friendly and cost-effective approach to treat concentrated hydrofluoric/hexafluorosilicic acid wastewater (∼117 g F⁻/L) obtained from an optoelectronic industry in Taiwan and to recover fluoride as the high purity CaF₂, as well as to reduce the amount of final waste disposal. The influence of three independent variables, including calcium-to-fluoride molar ratio ([Ca²⁺/F⁻], silicon concentration ([Si⁴⁺]), and polymer dosage applied, on the purity of recovered CaF₂ products, was investigated and optimized by combining response surface methodology (RSM) with central composite design (CCD). The ceramic grade CaF₂ of 85.6–87.9% was achieved under the optimum conditions as [Ca²⁺/F⁻] of 0.618–0.645, [Si⁴⁺] of 1.48 g/L, and polymer concentration of 17.2–17.8 mg/L. The final turbidity, residual fluoride concentration in the supernatants and fluoride removal efficiency were 14.8–16.2 NTU, 62.2–67.6 mg/L and greater than 99%, respectively. The value of R² = 0.9498 and adjusted R² = 0.9319 indicated an adequate correlation between experimental results and predictions by the quadratic model. In addition, a new insight into the CaF₂ purification method has been proposed based on the idea of pH control.

View all citing articles on Scopus

View full text

Fluidized bed reactor for fluoride removal

Abstract

Introduction

Section snippets

Process and system description

Materials and methods

Results and discussion

Conclusions

Acknowledgements

Water Res.

Chem. Eng. Sci.

Treatment of aqueous effluent for fluoride removal

Water Res.

Reaction of CaO, Ca(OH)2 and CaCO3 whit dilute fluoride streams

Light Met.

Fluoride removal in a fixed packed with granular calcite

Water Res.

Reaction of CaO, Ca(OH)₂ and CaCO₃ whit dilute fluoride streams