The Research On Treating Industrial Fluorine Wastewater By Two-stage Chemical Precipitation And Adsorption Process

The original process to treat fluorine waste water derived from the solar cellindustry in Shanxi province was chemical precipitation with two stages of "alkalinecalcium salt precipitation+aluminum salt flocculation precipitation", but the fluorineconcentration of outflow could not reach the standard limit according to the originalprocess parameters. In order to meet the limit concentration, the process parameters wasoptimized in lab, and the optimized parameters was applied within reaction tanks in site. Itwould cost more reagent and bring about environmental risk to treat the fluorinewastewater to the standard limit by the method before, then a process of two stages of"neutral calcium salt precipitation+aluminium salt flocculation precipitation" plusadsorption（adsorbent of γ-Al₂O₃） was suggested, and the process parameters also wasoptimized in lab to study the feasibility of the modified process. The fluorine adsorptioncapacity of γ-Al₂O₃was limited and it needed to regenerate frequently, then the Fe/γ-Al₂O₃was suggested for a better adsorption capacity. It’s research on the preparation,characterization, defluoridation ability, regeneration method and application of Fe/γ-Al₂O₃was carried out. The project provided technical support and a new material to treatfluorine wastewater derived from solar cell industry.The optimal process parameters of chemical precipitation with two-stage wasachieved in lab, calcium hydroxide was used in the first stage, and the pH was set as7-9,aluminium sulfate was used to flocculate the fluorine in the second stage, the pH was set as7approximately, the aluminium sulfate dose was above2000mg/L, and the result in siteindicated that the fluorine concentration of outflow was about15mg/L, which was close tothe emission standard; The optimal process parameters of "two stage chemicalprecipitation and adsorption process" was achieved in lab, the pH in the first stage was7-9, and the pH in second stage was7approximately, aluminium sulfate dose was 500mg/L, in the adsorption process, the dose of γ-Al₂O₃was240g/L, agitated for8h,after treating, the concentration of fluoride could decrease from30mg/L to lower than10mg/L, the effluent could reach the emission standard.The result of preparation, characterization, defluoridation ability and the regenerationmethod of Fe/γ-Al₂O₃was as below:①the optimal preparation method of Fe/γ-Al₂O₃was: Adding γ-Al₂O₃into solution with0.072mol/L FeSO4·7H₂O, the mess ratio of of γ-Al₂O₃to FeSO4·7H₂O was10:1, then the fixed solution was agitated continuously for15min by sonication with ultrasonic wave power of250W, and the solution temperature of20℃, filtered and dried, then the modified adsorbent was achieved;②Fe/γ-Al₂O₃wascharacterized by SEM, TEM, XRD, BET and XPS, the result showed that iron was existedin the form of amorphous iron oxide, the mass percent of iron the modified adsorbent was0.54%, the BET specific surface area and pore volume of Fe/γ-Al₂O₃were304.3m2/g and0.341g/cm3, lower than γ-Al₂O₃of （320.0m2/g and0.356g/cm3）;③When solutiontemperature was25℃, pH was7, Fe/γ-Al₂O₃could fit the Langmuir isotherm, thefluorine adsorption capacity of Fe/γ-Al₂O₃was1.65mg/g, while γ-Al₂O₃was1.15mg/g.Fe/γ-Al₂O₃was analyzed to fit the pseudo-second-order model, and both surfaceadsorption and intra-particle diffusion contribute to the rate-determining step of theadsorbent;④Fe/γ-Al₂O₃adsorbed the fluorine was put in distilled water after beensoaked in solution with1%NaOH, the pH in distilled water was adjusted to5-8, then thesolid was filtered and used to adsorb fluorine again, the adsorption ability was the same asthe new prepared Fe/γ-Al₂O₃.The Fe/γ-Al₂O₃was used to treat fluorine wastewater derived from the solar cellindustry in bench scale. The result showed that the fluorine concentration could decreasedfrom about30mg/L to10mg/L when the initial pH of the wastewater was3-9, Fe/γ-Al₂O₃dose of40g/L, agitated time of24h for static adsorption; At the condition of volumetricspace velocity of1.7h-1, the breakthrough time could be3.5h for dynamic adsorption.Comparing to γ-Al₂O₃, the Fe/γ-Al₂O₃could adopt to wider pH range, had longerbreakthrough time（5time of the γ-Al₂O₃）, and could take more environmental benefit.