Article

A pilot-scale study of cryolite precipitation from high fluoride-containing wastewater in a reaction-separation integrated reactor

School of Metallurgical Science and Engineering, Central South University, Changsha 410083, China.
Journal of Environmental Sciences (Impact Factor: 2). 07/2013; 25(7):1331-7. DOI: 10.1016/S1001-0742(12)60204-6
Source: PubMed

ABSTRACT

Fluoride removal by traditional precipitation generates huge amounts of a water-rich sludge with low quality, which has no commercial or industrial value. The present study evaluated the feasibility of recovering fluoride as low water content cryolite from industrial fluoride-containing wastewater. A novel pilot-scale reaction-separation integrated reactor was designed. The results showed that the seed retention time in the reactor was prolonged to strengthen the induced crystallization process. The particle size of cryolite increased with increasing seed retention time, which decreased the water content. The recovery rate of cryolite was above 75% under an influent fluoride concentration of 3500 mg/L, a reaction temperature of 500C, and an influent flow of 40 L/hr. The cryolite products that precipitated from the reactor were small in volume, large in particle size, low in water content, high in crystal purity, and recyclable.

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    • "In the literature, many methods have been investigated to remove the excess of fluoride from water (Melidis 2015), such as adsorption (Loganathan et al. 2013), ion exchange (Kodama and Kabay 2001), chemical precipitation (Jiang et al. 2013), electrochemical techniques (Tezcan et al. 2013) and membrane processes like, nanofiltration (Chakrabortty et al. 2013), Donnan Dialysis (Boubakri et al. 2013), and electrodialysis (ED) (Menkouchi Sahli et al. 2007; Amor et al. 2001; Ben Sik Ali et al. 2010). Most of them suffer from one of the following drawbacks: high initial cost, lack of selectivity, low capacity and complicated or expensive regeneration. "
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    ABSTRACT: The water resources in Tunisia are very limited and not fairly distributed: 80 % of them are located in the north and only 20 % in the south. In addition, 50 % of them are characterized by a bad quality since they have salinity greater than 1.5 g L−1. The bad quality is not only due to the high salinity, but also to the presence of natural pollutants such as fluoride. In fact, according to physicochemical analyses of samples of tap waters collected by the Regional Service of Environmental Health from different monitoring sites in mining area (south of Tunisia), in February 2014, it was shown that waters were contaminated by fluoride and contained high concentrations of sulphate and chloride anions. Fluoride concentrations varied between 0.8 and 4 mg L−1, and then, greatly exceeded the World Health Organization (WHO) standards in some monitoring sites. In this study, the electrodialysis (ED) technique was applied to remove fluoride from these waters. Experiments were carried out using a pilot unit as a conventional ED in batch recirculation mode. It was shown that ED is an efficient technique to remove fluoride and to reduce salinity of water. Removal rate of fluoride tended to 92 % and the concentrations of different species in the treated water were below the amounts recommended by WHO for drinking water. Considering the effect of chloride and sulphate on fluoride removal through the anion exchange membrane, the performance of two types of ion exchange membrane (PC Cell and Neosepta) was compared. Experiments were carried out with synthetic solutions containing 0.0357 mol L−1 of NaHCO3 doped with 15 mg L−1 of fluoride. Sodium hydrogenocarbonate was replaced by sodium chloride or sodium sulphate at different values of molar ratio in order to maintain the initial ionic strength of the solution constant at 0.0357 mol L−1. It was shown that fluoride removal was influenced by chloride ions but not by sulphate ones.
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    • "Substantial research on fluoride removal has been conducted on the areas encompassing coagulation–precipitation, adsorption , ion exchange and membrane separation (Choi et al. 2001; Hu and Dickson 2006; Tahaikt et al. 2007; Hou et al. 2010; Chakrabortty et al. 2013; Jiang et al. 2013; Sasaki et al. 2013). Still millions continue to drink fluoride-contaminated unsafe water in a world where around one billion people do not have access to safe drinking water. "
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    ABSTRACT: For purifying fluoride-contaminated water, a new forward osmosis scheme in horizontal flat-sheet cross flow module was designed and investigated. Effects of pressure, cross flow rate, draw solution and alignment of membrane module on separation and flux were studied. Concentration polarization and reverse salt diffusion got significantly reduced in the new hydrodynamic regime. This resulted in less membrane fouling, better solute separation and higher pure water flux than in a conventional module. The entire scheme was completed in two stages-an upstream forward osmosis for separating pure water from contaminated water and a downstream nanofiltration operation for continuous recovery and recycle of draw solute. Synchronization of these two stages of operation resulted in a continuous, steady-state process. From a set of commercial membranes, two polyamide composite membranes were screened out for the upstream and downstream filtrations. A 0.3-M NaCl solution was found to be the best one for forward osmosis draw solution. Potable water with less than 1 % residual fluoride could be produced at a high flux of 60-62 L m(-2) h(-1) whereas more than 99 % draw solute could be recovered and recycled in the downstream nanofiltration stage from where flux was 62-65 L m(-2) h(-1).
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    ABSTRACT: Fluoride removal by precipitation of calcium fluoride is the most common practice in Taiwan to treat fluoride-containing wastewater in semiconductor or optoelectronic industries. Due to very fine CaF2 precipitates (∼0.1 μm), coagulants/flocculants are needed to facilitate sedimentation of CaF2. In turn, large amount of sludge is produced by CaF2 precipitation/sedimentation process. In this study, removal of fluoride from spent fluoride etching solution by cryolite crystallization was investigated. Experimental and chemical equilibrium modeling results show that a good control of reaction pH and Al:F molar ratio is the key to form cryolite successfully. Formation of cryolite is exothermic reaction and formation of cryolite is less favor under elevated temperature. The highest residual fluoride concentration is found at reaction temperature of 90 °C while those at 20 and 55 °C are similar. Under Al/F molar ratios of 1:3 and 1:6 and pH of 5.5, the precipitates produced at room temperature are very similar to the commercial cryolite according to XRD, SEM, and EDX analysis. Particle size of cryolite precipitates ranges from 3 μm to 15 μm and is much larger than CaF2 precipitates of 0.1 μm, resulting in rapid sedimentation. Cryolite crystallization process produces much less sludge volume than does by CaF2 precipitation.
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