Wat. Res. Vol. 33, No. 16, pp. 3403±3412, 1999 # 1999 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/99/$ - see front matter
PRECIPITATE FLOTATION OF FLUORIDE-CONTAINING WASTEWATER FROM A SEMICONDUCTOR MANUFACTURER C. JANE HUANG and J. C. LIU* Department of Chemical Engineering, National Taiwan University of Science and Technology, 43 Keelung Road, Section 4, Taipei 10672, Taiwan (First received 1 July 1998; accepted in revised form 1 January 1999) AbstractÐPrecipitate-¯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 (CaF2). Results from dispersed air ¯otation (DiAF) experiments indicate that through the adjustment of molar ratio of calcium and ¯uoride ([Ca2+]/[Fÿ]) and pH values, the residual ¯uoride concentration of lower than 10 mg/L in the euent could be obtained. The concentration of sodium dodecyl sulfate (SDS) signi®cantly aected the removal eciency of CaF2. Flotation 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. # 1999 Elsevier Science Ltd. All rights reserved Key wordsÐcalcium ¯uoride, ¯otation, ¯uoride, ¯uorite, pH, precipitate, semiconductor, wastewater
The semiconductor industry is one of the most important part of the manufacturing sector in Taiwan. Taiwan is currently the 4th largest semiconductor manufacturer in the world. The manufacturing process, which requires extremely high precision, generates both conventional and hazardous wastes (Vagliasindi and Poulsom, 1994). The management of waste has become an important issue in the industry as a result of stringent environmental regulation and possible liability. Among varieties of pollutants, hydro¯uoric acid (HF) is a major concern. It is used extensively in semiconductor manufacturing for wafer etching and quartz cleaning operations. Fluoride concentration of 1,000±3,500 mg/L is found in typical wastewater of local semiconductor industry (Chou et al., 1994). In Taiwan the maximum permissible limit of ¯uoride is 15 mg/L in industrial euent. Fluoride contamination in certain aquatic systems worldwide has caused health concern (Altinas et al., 1987; Singh et al., 1987; Nell and Livanos, 1988). Previous studies show that ¯uoride in water and wastewater can be treated to a desirable limit by adsorption (Hao and Huang, 1986; Wasay et al., 1994; Lai and Liu, 1996), or precipitation. Lime and calcium salt precipitation of ¯uoride can practically reduce the residual *Author to whom all correspondence should be addressed. [Tel.: +886-2-27376627; fax: +886-2-27376644; e-mail: [email protected]
¯uoride concentration to 10±15 mg/L or even lower (Parthasarathy et al., 1986; Saha, 1993). Wastewater treatment in semiconductor plants involves acid neutralization and ¯uoride precipitation. Calcium salts are used to form CaF2 precipitate. Polymeric ¯occulent is also utilized to improve sludge settling. When dealing with dilute wastewater, ¯otation technique possesses some distinctive advantages: rapid operation, low space requirements, ¯exibility of application and moderate cost (Lin and Huang, 1994). With one of the highest population density in the world, land is limited and very costly in Taiwan. Land acquisition has always been dicult in the rapid growth and expansion of the industry. Flotation processes appear to be more favorable compared with precipitation ones. The major objective of the study is to assess the application of precipitate ¯otation technique in the treatment of ¯uoride-containing wastewater from semiconductor manufacturers. Generally, precipitate ¯otation is a process that involves concentration of ionic species by initially forming precipitate and removal of the precipitate from the dilute aqueous solution by transfer to surface by gas bubbles (Matis and Mavros, 1991). The technique has been applied to the separation of Cd in the form of hydroxide (Zouboulis and Matis, 1995), heavy metals in the form of sul®de precipitate (Lazaridis et al., 1992) and phosphate in the form of calcium phosphate and hydroxyapatite (Kato et al., 1993). The major advantage of precipi-
C. Jane Huang and J. C. Liu
tate ¯otation over ion ¯otation is the lower surfactant requirement (Matis and Mavros, 1991). In fact, natural ¯uorite (CaF2) is of great industrial signi®cance and has been widely used in manufacturing of glass, iron and steel and aluminum. The separation and puri®cation of ¯uorite are almost all by ¯otation processes and much work has been done on the understanding of surface properties of ¯uorite (Perea-Carpio et al., 1988; Wu and Forsling, 1995; Micheal and Miller, 1996). There are therefore experiences that can be introduced in the precipitate ¯otation of ¯uoride-containing wastewater. In addition, another potential advantage of the current study is the reuse of the precipitate as a valuable resource. Fig. 1. The apparatus for precipitate ¯otation.
MATERIALS AND METHODS
Wastewater sample was obtained from a semiconductor manufacturing plant in Hsinchu, Taiwan. The plant, one of the international leading integrated circuits (IC) manufacturers, is representative of semiconductor industry in Taiwan. The plant treats ¯uoride-containing wastewater with units of equalization, rapid mixing with NaOH and Ca(OH)2, slow mixing in the presence of ¯occulent and gravity settling. Wastewater was taken from the equalization tank in October 1997. The pH of the wastewater was 3.5. Total suspended solid content was very low (1.5 mg/ L). Major composition of the wastewater (Table 1) was analyzed using an ion chromatograph (Dionex, DX-100) and an atomic absorption spectrophotometer (GBC 904AA). The ¯otation system utilized is similar to that described earlier (Lin and Liu, 1996). Fig. 1 depicts the apparatus used. An acrylic ¯otation column of 55 cm in length with an inside diameter of 4.0 cm was used. A lipped side arm at 5 cm from the top of the column serves as the foam discharge port. There is a gas sparger (pore size 10±16 mm, Merck) at the bottom of the column and a side arm with stopcock for sampling. Nitrogen gas passes through a pressure regulator (Norgren), a ¯owmeter (J&W) and a humidi®er (Merck) before ¯ows into the column. Sodium dodecyl sulfate (SDS), sodium oleate (SOl) and n-dodecylammonium chloride (DAC) were used as frother and collector. Measured amounts of CaCl2, NaNO3, or Na2SO4 and wastewater were added to a 500 mL volumetric ¯ask, placed on a stirrer (Corning) and ®xed amount of stock solution of surfactant was then added. The pH was adjusted with 0.5 N NaOH and 0.5 N HNO3. The solution was stirred for 10 min to allow the precipitation reaction to proceed to completion before treatment by ¯otation. Steady ¯ow rate of nitrogen gas was adjusted before the wastewater suspension was transferred to the ¯otation column. The duration of ¯otation was 10 min for all runs. Sample was taken at certain time intervals. Measurement of suspended solid (Standard Methods, 1985) was modi®ed by using a membrane ®lter with smaller pore size (0.2 mm, MFS) to prevent the very ®ne CaF2 particles from breaking through the ®lter. Also lower drying temperature (808C) was chosen for the protection of the membrane ®lter. Fluoride concentration in the ®ltrate was measured by a speci®c ion electrode (ASI). Zeta poTable 1. Major chemical composition of wastewater (mg/L) ÿ
[SO2ÿ 4 ]
[PO3ÿ 4 ]
tential was analyzed by a zeta meter (Photo ELS-600). Experimental procedures were the same as described in our previous work (Lai and Liu, 1996; Lin and Liu, 1996).
RESULTS AND DISCUSSION
Eect of molar ratio Eects of [Ca2+]/[Fÿ] on the removals of ¯uoride and CaF2 are illustrated in Fig. 2. The concentration of SDS was at 50 mg/L, nitrogen gas ¯ow rate at 50 ml/min, pH at 7.0 2 0.1 and ¯otation time of 10 min. When [Ca2+/[Fÿ] was equal to 0.5, residual ¯uoride concentration of 12 mg/L in the aqueous solution was found. When [Ca2+]/[Fÿ] was further increased to 1.0, 1.5 and 2.0, residual ¯uoride concentrations of 3.4, 2.9 and 2.8 were found, respectively. This indicates that eective removal of ¯uoride can be achieved so long as [Ca2+]/[Fÿ] was greater than 1.0. As for CaF2, eective removal (>97%) was found regardless of variations in the value of [Ca2+]/[Fÿ]. The molar concentration ratio was thus kept at 1.0 for the following experiments. Eect of pH It has been pointed out that pH is one of the most important parameters in the ¯otation processes. Depending on pH dierent interfacial properties and reaction routes may be found (Matis and Mavros, 1991; Lin and Liu, 1996). Eects of pH on ¯otation are shown in Fig. 3. Removal of CaF2 was all very eective and was not aected by pH. Yet, residual ¯uoride concentration of 91.5 mg/L was found at pH of 2.0. When pH was equal or higher than 3.0, residual ¯uoride concentration of lower than 9.7 mg/L was observed. The slight increase in solubility of CaF2 at pH <5 is due to incomplete precipitation and the hydrolysis of Fÿ to form HF; and at pH >12 due to the formation of calcium hydroxyl species (Wu and Forsling, 1995). To assess equilibrium chemical speciations of ¯uoride, a computer software GEOCHEM (Sposito and Mattigod,
Precipitation in ¯uoride wastewater
Fig. 2. Removal of Fÿ and CaF2 as aected by [Ca2+]/[Fÿ].
Fig. 3. Removal of Fÿ and CaF2 as a function of pH.
C. Jane Huang and J. C. Liu
1979) was utilized. Results are shown in Fig. 4. Soluble HF(aq) constitutes the larger portion only when pH is lower than 1.0. Precipitate of CaF2 starts to form and becomes dominant when pH is equal to, or higher than 2.0, while other soluble species constitute only negligible fraction. Though some deviation exists, both thermodynamic prediction and actual experimental results are in good agreement. Eect of surfactant 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. The removal eciency of CaF2 increased with further increase in SDS concentration and over 99% removal was obtained. This is in conformity with literature that ¯otation eciency increases with increasing collector concentration (Sanciolo et al., 1992; Koutlemani et al., 1994; Zouboulis and Matis, 1995). Because of its importance in separ-
ation processes, characteristics of hydrous ¯uorite surface have been studied extensively. 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 2.0±12.0 (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 the collector ions adsorbed at the air±liquid interface can enhance the resistance of the bubble to rupture, may migrate to the solid surface and thereby increase the hydrophobicity of the solid particle and may provide an electrical potential to the bubble, thus leading to long-range electrical interactions between gas bubbles and solid particles (Perea-Carpio et al., 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
Fig. 4. Chemical speciation of ¯uoride in the solution as predicted by GEOCHEM.
Precipitation in ¯uoride wastewater
Fig. 5. Eect 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 eective. Bubble size also plays an important role in ¯otation processes. It was observed in our exper-
Fig. 6. CaF2 removal as a function of collector (SDS) concentration.
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 To examine the treatment eciency, three types of calcium salts, CaCl2 Ca(OH)2 and Ca(NO3)2, were used, respectively. Experimental conditions were the same, except that SDS concentration was at 30 mg/L. It is found in Fig. 8 that the best removal eciency (88%) of CaF2 was achieved when CaCl2 was used, followed by Ca(NO3)2 (84%) and Ca(OH)2 (64%). Moreover, lowest residual ¯uoride concentration (3.84 mg/L) was obtained when CaCl2 was used, followed by Ca(NO3)2 (4.59 mg/L) and Ca(OH)2 (5.44 mg/L). As for sludge, measured as suspended solid before subject to ¯otation removal, Ca(OH)2 generated highest amount (1,325 mg/L), followed by that of Ca(NO3)2 (1,230 mg/L) and of CaCl2 (1,175 mg/L). Though the majority of semiconductor plants in Taiwan still utilizes Ca(OH)2 in treating ¯uoride-containing wastewater, it appears that CaCl2 is the favorite choice. It is found in the current study that not only best removal eciencies of ¯uoride and CaF2 can be obtained, but also least amount of sludge is generated when CaCl2 was used. Other potential advantages of CaCl2 are its acidity that prevents clogging in pipes and the ease of handling.
Eects of ionic strength At pH of 5.5 2 0.1, eects of ionic strength on ¯otation were investigated (Fig. 9). It was found that ¯otation was signi®cantly aected by ionic strength. Residual ¯uoride concentration increased from 3.4 to 8.5 mg/L as ionic strength increased from 0.05 M to 0.5 M NaNO3. It is probably due to the fact that the activity was lowered as ionic strength was increased. Meanwhile, the removal of CaF2 decreased as ionic strength increased; notably when concentration of NaNO3 was higher than 0.3 M. Only 76.8% of CaF2 was removed when concentration of NaNO3 was at 0.5 M. It has been indicated that the separation eciency decreases with increasing concentration of inert salt in solution (Matis and Mavros, 1991; Lin and Huang, 1994; Huang et al., 1995). From z potential data (Table 2), the z potential of CaF2 in the solution at pH of 5.5 2 0.1 decreased with increasing concentration of NaNO3 in the absence of SDS. When SDS was added, similar eect of NaNO3 on z potential of CaF2 was found. This implies that the electrostatic interaction between collector and solid surfaces was weakened and SDS could not be eectively adsorbed onto the CaF2 surfaces (Lin and Huang, 1994). This could explain the decreased separation eciency under high ionic strength in the current study. We also observed during experiments that gas bubbles were larger and tended to rupture more easily when ¯owed upward to the air±liquid
Fig. 8. Eect of types of calcium salt on the removal of CaF2.
C. Jane Huang and J. C. Liu
Fig. 9. Removal of Fÿ and CaF2 as aected by ionic strength.
surfaces. The ¯otation eciency could be improved from 76.8% to 95.9% by increasing SDS concentration to 100 mg/L (Fig. 10). This has been illustrated before (Matis and Mavros, 1991; Lin and Huang, 1994; Huang et al., 1995). However, the addition of 20 mg/L of sodium oleate did not improve the separation eciency. Interference of anions Since various types of acids are widely used in etching and cleansing in semiconductor manufacturing, it is common to ®nd sulfate, nitrate and phosphate ions in ¯uoride-containing wastewater (Chou et al., 1994). Though only trace amount of sulfate was found in our wastewater sample, the eect of sulfate on ¯otation was studied through the spiking of Na2SO4. Results are shown in Fig. 11. The residual ¯uoride concentrations increased from 4.01 Table 2. Eect of NaNO3 on z potential of suspension systems at pH of 5.5 2 0.1 NaNO3 (M)
z potential (mV)
0.05 0.10 0.30 0.50 0.05 0.10 0.30 0.50
0 0 0 0 50 50 50 50
37.5 35.0 20.0 2.0 22.0 19.5 7.5 0.1
to 14.42 and 19.87 mg/L when sulfate concentration was increased to 100 and 500 mg/L, respectively. The depressed removal of ¯uoride was presumably resulted from the competition for calcium ion between ¯uoride and sulfate. Similar eect was found on the removal of CaF2. The removal eciency decreased from 98.0% to less than 60% as sulfate concentration was increased to higher than 100 mg/L. Similar eects have been observed in ¯otation reactions (Matis and Mavros, 1991; Lin and Huang, 1994). Since sulfate ions are found to adsorb onto ¯uorite surfaces (Popping et al., 1992), it is proposed that the direct competition between sulfate and SDS for adsorption sites on CaF2 surfaces causes the depressed ¯otation. Another explanation is the weakened electrostatic interactions between SDS and CaF2 as a result of the ion exchange reactions between sulfate ions and the ¯uoride ions at the surfaces (Wu and Forsling, 1995). Again, it can be improved by increasing SDS concentration and eective (>90%) removal of CaF2 could be achieved.
This study demonstrates that ¯uoride-containing wastewater from the semiconductor manufacturer can be eectively treated by precipitate ¯otation technique. Values of [Ca2+]/[Fÿ] and pH are critical
Precipitation in ¯uoride wastewater
Fig. 10. Removal of CaF2 under dierent ionic strength.
Fig. 11. Removal of Fÿ and CaF2 as aected by sulfate ion.
C. Jane Huang and J. C. Liu
in determining the residual ¯uoride concentration, as evidenced by both experimental results and thermodynamic prediction. The combined use of CaCl2 and SDS can produce the best treatment eciency. The removal of CaF2 is completed in 10 min and is controlled by SDS concentration. SDS is adsorbed onto the CaF2 surface through electrostatic interaction and renders the surface more hydrophobic; ¯otation reaction is thus facilitated. It is found that the separation eciency decreases with increasing ionic strength and sulfate concentration, but can be improved by increasing SDS concentration. REFERENCES
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