Precipitate- ̄otation of ̄uoride-containing wastewater from a semiconductor manufacturer was investigated. The process involves the addition of calcium chloride to generate precipitate and the subsequent removal of calcium ̄uoride (CaF ). Results from dispersed air ̄otation (DiAF) experiments2indicate that through the adjustment of molar ratio of calcium and ̄uoride ([Ca ]/[F ]) and pHvalues, the residual ̄uoride concentration of lower than 10 mg/L in the euent could be obtained. Theconcentration of sodium dodecyl sulfate (SDS) signi®cantly aected the removal eciency of CaF . 2Flotation reaction was not aected by pH. Flotation eciency decreased with increasing ionic strength and in the presence of sulfate. However, the depressed ̄otation could be improved by increasing SDS concentration. 5 1999 Elsevier Science Ltd. All rights reserved
It has been indicated that point of zero charge (PZC) of uorite is in the neutral to alkaline range (Miller and Hiskey, 1972; Popping et al., 1992; Hicyilmaz et al., 1997). We found that surface of CaF2 was positively charged under pH of 2.012.0 (Fig. 7(A)). When at pH of 7.0 2 0.1, z potential of CaF2 decreased with increasing concentration of SDS and a charge reversal was found when concentration of SDS was at 70 mg/L (Fig. 7(B)). It has been shown that
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Three types of surfactant, namely, SDS, sodium oleate (SOl) and n-dodecylammonium chloride (DAC), were chosen as frother and collector in this work as they have been used in the otation of uorite (Perea-Carpio et al., 1988; Micheal and Miller, 1996). Results show (Fig. 5) that SDS was the best, with 98% CaF2 removal, followed by SOl (64%) and DAC (11%). When SDS concentration was at 30 mg/L, 80% of CaF2 was removed in 10 min (Fig. 6). When SDS concentration was increased to 50 mg/L, 97% of CaF2 was removed. increased with The removal e(cid:129)ciency of CaF2 further increase in SDS concentration and over 99% removal was obtained. This is in conformity with literature that otation e(cid:129)ciency increases with increasing collector concentration (Sanciolo et al., 1992; Koutlemani et al., 1994; Zouboulis and Matis, 1995). Because of its importance in separ-
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, 1988). SDS is an anionic surfactant that has been widely used as frother and collector in many otation processes. Previous research indicates that SDS could reduce surface tension of the solution and make gas bubbles ner and more stable, so that otation reaction is facilitated. The interactions between SDS and CaF2 play a critical role in precipitate otation. Based on z potential results, it is proposed that SDS is adsorbed onto the uorite (CaF2) through electrostatic interaction, with the the collector electrical interactions between Fig. 4. Chemical speciation of uoride in the solution as predicted by GEOCHEM. Precipitation in uoride wastewater
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Fig. 5. E(cid:128)ect of surfactant type on the removal of CaF2. nonpolar end pointing toward the solution (Lin and Huang, 1994; Huang et al., 1995; Somasundaran and Krishnakumar, 1997). The hydrophobicity of CaF2 surface is enhanced and renders otation e(cid:128)ective. Bubble size also plays an important role in otation processes. It was observed in our experFig. 6. CaF2 removal as a function of collector (SDS) concentration. 3407 3408 C. Jane Huang and J. C. Liu Fig. 7. (a) Zeta potential of CaF2 as a function of pH. (b) Zeta potential of CaF2 in the presence of SDS. Precipitation in uoride wastewater iments that gas bubbles generated by SDS were much ner than those by SOl. That may explain why SDS performed better than SOl. Types of calcium salts
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