A pilot-scale study of cryolite precipitation from high fluoride-containing wastewater in a reaction-separation integrated reactor
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.
- SourceAvailable from: Sankha Chakrabortty
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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. "
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).Environmental Science and Pollution Research 03/2015; 22(15). DOI:10.1007/s11356-015-4333-x · 2.83 Impact Factor
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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.Separation and Purification Technology 11/2014; 137:53–58. DOI:10.1016/j.seppur.2014.09.019 · 3.09 Impact Factor