| Aluminum fluoride and cryolite have been used for aluminum industry as important auxiliary agents or additives, China is one of the most important countries in exporting aluminum fluoride and cryolite, accounts for a large share in the international market. There are many differences in the quality of aluminum fluoride and cryolite due to their different production processes, so many countries don’t make agreement on charging the two chemicals, and many companies doubt about it too, especially for the aluminum fluoride. Relative Safety Data Sheets show that the total fluorine and fluorine dissolving in aqueous of solid samples are the main hazard characteristics, and the total fluorine in sample is also an important indicator that can help decide the quality of aluminum fluoride and cryolite; However, it is difficult to quantify the fluorine on the condition of Al3+coexisting, it needs more delicate preparation. In this project,we compared the hazardous difference between hydrated aluminum fluoride and anhydrated aluminum and studied the risk of cryolite to provide specific datas as reference for production, storage and using of the two chemicals; As existing methods have the disadvantage of long time consuming, tedious operation, toxic and so on, a sample method of pre-preparation was proposed; The best conditions of ISE and titration of rest calcium for determination of fluorine in aluminum fluoride and cryolite were studied based on the pre-paration; in addition, the possibility of GC-derived method for determination of fluorine dissolving in aqueous was investigated. The principal contents and results of thiswork were as follows:(1) First, we compared the hazardous difference between hydrated aluminum fluoride and anhydrated aluminum, studied the possible risk of cryolite. Compared with anhydrated aluminum fluoride, hydrated aluminum fluoride (containing AIF3·3H2O) was poorer in thermal stability, when temperature was186℃, hydrolysis reaction happened and HF re leased.The fluorine content in aqueous solution of hydrated aluminum fluoride was up to300mg.L-1because of the crystal water existing, it was far higher than the limit(10mg.L-1) in Integrated Wastewater Discharge Standard, higher than that of anhydrated aluminum fluoride and cryolite, thus increasing the risk of reaction between anhydrated aluminum fluoride and sodium hydroxide too. The corrosion rates of A3steel in water-solution of aluminum fluoride or cryolite were0.078mm.a-1(hydrated aluminum fluoride),0.22mm-a-1(anhydrated aluminum fluoride) and0.40mm.a-1(cryolite) respectively, therefore aluminum fluoride or cryolite could not be classified to corrosives. After all,the risk order was anhydrated aluminum fluoride<cryolite<hydrated aluminum fluoride. (2) Second, effects of Al3+coexisting on forms of F and pre-preparation conditions on fluorine-determination of solid sample were studied. It found that the forms of Al-F complexes changing along with the pH and the ratio of F to Al. On the condition of pH<8, F:A1=3,forms as A1F2+, A1F3were in charge and AIF3was getting more as pH growing, it was incompatibie to A1F2; When the ratio of F to Al was6, only AIF3was showed in the19F-NMR spectrum. Oherwise, F-was the only form in aqueous when pH>8, it was irrelevant to the ratios of F to Al. Based on the fomer theory and the hazardous characteristics of aluminum fluoride and cryolite, NaOH was chosen to react with aluminum fluoride or cryolite for sample preparation. Aluminum fluoride was studied as representive, effects of the mount of NaOH, reacting temperature and time had been investigated; The best reacting conditions were showed as follows:1:6of A1F3:NaOH,200℃as the reacting temperature and lh as the reacting time. Forms of Al-F complexes of samples were studied by19F-NMR and27Al-NMR after preparation, only F-and [A1(OH)4]-existed.(3) Third, Methods of ISE and titration were developed to determine the total fluorine in aluminum fluoride and cryolite by investigating the effects of pH(for both), masking regents(only ISE), concentration of F" and Al3+(only ISE). The best conditions for ISE were:pH=5-6, sodium citrate as masking regent and diluting the concentration of F-to2mg.L-1; The relative error was<0.80%, the recovery was94.25～105.83%. For titration of the rest calcium, pH had an important impact on the determination of fluorine. The best condition was pH=8～9, the relative error was≤1.18%, the recovery was74.00～98.75%. Method of gas chromatography derivatization was developed by studying the effects of the way to add derivating agent, bath time and condition of shaking. The best conditions were adding derivatizating agent directly, shaking for1minute,10minutes as bath time. GC/FID determination of fluorine in water solution of aluminum fluoride was in agreement with that detected by ISE. The recovery was88.89～113.33%, the limits of detection was0.026mg.L-1(S/N=3), the linear range was2-9mg.L-1=0.9990, R2=0.9970, and the relative standard deviations(RSD%) was6.80%(n=5). |
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