Optimization of Calcium Fluoride Crystallization Process for Treatment of High-Concentration Fluoride-Containing Semiconductor Industry Wastewater

Abstract

This study utilized a fluidized bed reactor (FBR) for fluoride removal from high-concentration fluoride-ion-containing simulated semiconductor industry wastewater and recovered high-purity CaF2 crystals. The effects of hydraulic retention time (HRT), pH, Ca2+ to F− ratio, upflow velocity, seed size and seed bed height were investigated by performing lab-scale batch experiments. Considering fluoride removal and CaF2 crystallization efficiency, 5 h HRT, pH 6, seed height of 50 cm and [Ca2+]/[F−] ratio of 0.55 (mol/mol) were found to be optimum. The effect of the interaction between the important process parameters on fluoride removal was further analyzed using response surface methodology (RSM) experimental design. The results showed that all the individual parameters have a significant impact (p = 0.0001) on fluoride removal. SEM-EDX and FTIR analysis showed the composition of the crystals formed inside FBR. HR-XRD analysis confirmed that the crystalline structure of samples was mainly CaF2. The results clearly demonstrated the feasibility of silica seed material containing FBR for efficient removal and recovery of fluoride as high-purity calcium fluoride crystals.

Process Intensification for Enhanced Fluoride Removal and Recovery as Calcium Fluoride Using a Fluidized Bed Reactor

“…The respective effluent fluoride concentration varied between 81 and 204 mg/L (Figure 1a). During this initial operation, the Ca 2+ -to-F − ratio was 0.55, which was based on the optimum ratio obtained during an earlier experiment [14]. However, as the desired low effluent fluoride concentration could not be achieved at this ratio and the calcium utilization was very high, the inlet calcium concentration was increased to make the Ca 2+ -to-F − ratio 0.6 from 11th day onwards.…”

“…The purity of the CaF 2 crystals was also calculated in this study by converting the elemental percentages to molar fractions and subtracting the impurities from the CaF 2 crystals (details shown in the Supplementary Material). By this method, the purity of the CaF 2 crystals obtained after 70 days of FBR operation is 91.12%, which is relatively high compared to many other previous studies [14]. In order to remove the additional fluoride present in the FBR effluent, an excess amount of calcium was injected into the reactor to reach a [Ca 2+ ]/[F − ] ratio of 0.65, 0.7, 0.75 and 0.8.…”

“…Some of the fluoride ions are captured in this calcium scaling, which does not precipitate onto the silica seed but rather comes out of the reactor as calcium fluoride fines [19]. The other reason is the breakdown of CaF 2 scales from the deposited crystals formed over the silica seeds due to shearing and tearing by upflow fluidization during continuous reactor operation over an extended time period [14]. Therefore, proper care must be taken regarding the operating time of the FBR so that optimum fluoride removal along with ideal CaF 2 crystal recovery can be achieved.…”

“…For the experimental conditions with a low HRT in the first stage (2 h and 3 h), the overall fluoride removal was not as good as the other two, despite having a high HRT of 2 h in the second stage. This could be attributed to the fact that this decrease in the fluoride removal efficiency was due to an insufficient reaction time between the reactants and the seeds in the reactor [14].…”

“…In this study, a laboratory-scale fluidized bed reactor (FBR) of 3.44 L volume was manufactured using acrylic material and used for the fluoride treatment and calcium fluoride crystallization, as depicted in Figure 12. A detailed description of the reactor setup is provided in a previous work [14]. The second-stage reactor was similar in size and shape to the first reactor, including all the inlet and outlet ports (Figure 12).…”