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Effect of Calcium Silicates on Immobilization of Fluorine in Aqueous Solution

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Effect of Calcium Silicates on Immobilization of Fluorine in Aqueous Solution ISIJ International, Vol. 41 (2001), No. 5, pp. 513–518 Effect of Calcium Silicates on Immobilization of Fluorine in Aqueous Solution Hongye HE and Hideaki SUITO Research Institute for Advanced Materials Processing, Tohoku University, Katahira, Aoba-ku, Sendai 980-8577 Japan. (Received on December 18, 2000; accepted in final form on February 20, 2001) Immobilization of fluorine in aqueous solution with 2CaO · SiO2 (C2S) and 3CaO · SiO2 (C3S), and the effect of C2S and C3S on the fluorine immobilization with 3CaO · Al2O3 (C3A) and 12CaO · 7Al2O3 (C12A7) have been studied. The hydration products are identified by X-ray diffraction method and the fluorine-substituted com- pounds are confirmed by electron microprobe analysis. It is found that C3S appears more effective to immo- Ϫ Ϫ bilization of fluorine than C2S and (OH ) site in calcium silicate hydrates is substituted with F ion. The be- havior of fluorine immobilization by calcium aluminates changes with the addition of calcium silicates be- cause calcium silicates affect the formation of Ca3Al2(OH)12ϪxFx, which is the most effective calcium alumi- Ϫ nate hydrate in view of the incorporation of F ion. The fluorine immobilization with C12A7 can be promoted in the presence of appropriate amount of C2S or C3S. KEY WORDS: immobilization; fluorine; calcium silicate; calcium aluminate; hydration. 2. Experimental 1. Introduction 2.1. Materials Calcium silicates, 3CaO·SiO2 and 2CaO·SiO2, exist in cementitious materials such as Portland cement, slag-based 2CaO·SiO2 (C2S) and 3CaO·SiO2 (C3S) were prepared cement and steelmaking slag. Calcium silicate hydrate by sintering the mixtures of analytical grade CaCO3 and formed by cementitious reaction of calcium silicates in the SiO2 in an appropriate proportion at 1 400 and 1 600°C, re- presence of water is a well known host for a variety of spectively, for 2 d. For the formation of 2CaO·SiO2, waste ions such as Co, Mo, and so on.1,2) Even though there 1 mass% P2O5 was added to prevent the dusting of C2S, are a great deal of coverage on the immobilization of which is caused by the transformation from b-C2S to g- harzadous elements by cement-base materials, the specific C2S. The methods of the preparation of 12CaO·7Al2O3 (C12A7) and 3CaO·Al2O3 (C3A) are described in detail else- attention has not been paid to the study on the fluorine re- 3) moval. where. All compounds were confirmed by X-ray diffrac- We have been working on a project to immobilize fluo- tion analysis (XRD). rine in hot metal dephosphorization slags by using sec- 2.2. Shaking Test ondary refining slags. The main constitutes of the latter are Shaking test was made at room temperature. The test calcium aluminates (12CaO·7Al2O3 and 3CaO·Al2O3) and procedure which is based on the Japanese standard test pro- calcium silicates (3CaO·SiO2 and 2CaO·SiO2), of which 3) cedure (codified as Environment Agency Notice 46) is al- 12CaO·7Al2O3 is the major phase. In our previous study ready explained in previous article.3) The particle size of 12CaO·7Al2O3 has been found less effective than 3CaO· calcium silicates and calcium aluminates is less than Al2O3 to the immobilization of fluorine in aqueous solution. 0.1 mm and the fluorine aqueous solution of 20 ppm was To stabilize fluorine-bearing wastes by using secondary re- made by diluting hydrofluoric acid (47%) with distilled fining slags, the knowledge on how calcium silicates affect water. The solid sample and fluorine aqueous solution were the hydration of calcium aluminate which is related to fluo- charged in 500 ml polyethylene bottle and were shaken hor- rine removel is of crucial importance. izontally at a speed of 200 times per min. For comparison, In this study, the immobilization of fluorine in aqueous Յ special grade CaF2 (99.9% purity, grain size 2 mm) was solution with 3CaO·SiO2 and 2CaO·SiO2 is first studied, also used. The concentration of F in aqueous solution was and then the effects of calcium silicates on the F immobi- determined by the selective ion-electrode method (JIS-K- lization by calcium aluminates and their mechanisms are 0101), and those of Ca, Al and Si were determined by in- discussed based on the microscopic observation, X-ray dif- ductively coupled plasma (ICP) atomic emission spectrom- fraction analysis and electron microprobe analysis. etry.3) 2.3. X-ray Analysis and Microscopic Observation X-ray diffraction analysis, microscopic observation, and 513 © 2001 ISIJ ISIJ International, Vol. 41 (2001), No. 5 Fig. 2. SEM images of hydrated C2S (a) and C3S (b) particle sur- face obtained at shaking time of 24 h. will be discribed in next section. 3.1.2. Microscopic Observation The X-ray diffraction analysis failed to prove the forma- Fig. 1. Immobilization of F by C2S and C3S with size less than tion of fluorine-containing calcium silicate hydrates be- 0.1 mm. cause the hydration products were not well crystallized, electron microprobe analysis with energy dispersive X-ray namely, almost amorphous. The Ca(OH)2 phase was identi- (EDX) and wave-length dispersive X-ray (WDX) spec- fied by XRD only in the case of C3S. troscopy are used for the identification of hydration prod- Calcium silicate hydrates are usually known as poorly ucts and fluorine-bearing compounds. crystallized phase or gel. In cement chemist’s notation, these phases are generally referred to as C–S–H, where C, S and H represent CaO, SiO2 and H2O, respectively. The 3. Results and Discussion composition of these calcium silicate hydrate gels changes 3.1. Immobilization of Fluorine with Calcium Silicates during the reaction period and it also varies with water/solid ratio and temperature. 3.1.1. Time Dependence of Immobilization of Fluorine The SEM image of the surface of hydrated C S and C S The variations of the concentrations of F, Si and Ca in 2 3 particles (0.84–1.0 mm) obtained by shaking for 24 h are aqueous solution with shaking time for C S, C S and CaF 2 3 2 shown in Fig. 2. In the case of C S particles, the fluorine- are shown in Fig. 1. It can be seen that when finely ground 2 containing calcium silicate hydrates, determined by EDX C S particles (Յ0.1 mm) are shaken in the fluorine aqueous 3 and WDX, have the CaO/SiO molar ratio of 1.5 to 2, with solution, high Ca and extremely low Si contents are ob- 2 the F uptake of 1.5 to 2 mass%. In the case of C S particles, served along with a rapid decrease of the F content in the 3 the fluorine-containing hydrates have the CaO/SiO molar initial period of 30 min. This immobilization mechanism 2 ratio of 1.5 to 2.5 with the F uptake of 2.6 to 4.3 mass%. A will be explained in Sec. 3.1.3. In the case of C S only a 2 small amount of CaF was also observed on the surface of slight decrease of the F content is observed, characterized 2 C S particles treated in F aqueous solution for 2 h. The C S by relatively high Si and low Ca contents, compared with 3 3 particles are almost completely covered by the C–S–H hy- the case of C S. The pH value in aqueous solution was 3 dration products, whereas the C S particles remain almost about 12 for C S and 13 for C S. 2 2 3 unattacked even after 24 h. This observation coincides with The results for CaF were obtained by shaking CaF 2 2 the degree of fluorine-immobilization by C S and C S powders (Յ2 mm) in distilled water. The initial pH was ad- 3 2 shown in Fig. 1. justed to 12.0 by the addition of a small amount of 20 w/v% NaOH solution. The dissolution of CaF2 sustains the F con- 3.1.3. Mechanism of Fluorine Immobilization tent in aqueous solution constant around 10 mass ppm, During the hydration of C3S, calcium hydroxide precipi- which is almost the same level as that obtained by C2S. The tates from aqueous solution and calcium silicate hydrate Ca content in aqueous solution is around 2.7 ppm. The sol- gels, C–S–H, are formed on the particle surface, of which 2 ubility product of CaF2, [Ca][F] , calculated from these F the C–S–H gel with the CaO/SiO2 molar ratio of 1.5 and and Ca contents is 10Ϫ10.49. This value is in good agreement 2.5 has high capacity of F uptake. The structure of C–S–H with that in previous report (10Ϫ10.41).4) is similar to that of tobermorite5,6) and jennite6) in many as- It can be concluded, therefore, that C3S is more effective pects. than C2S to the immobilization of fluorine in aqueous solu- The dissolution of C3S by reaction (1) continuously rais- tion. This is explained by the fact that the hydration of C3S es the concentration of Ca in aqueous solution, which leads proceeds much faster than C2S and thus a large amount of to the formation of CaF2 and calcium hydroxide in aqueous fluorine-containing calcium silicate hydrates are formed, as solution by reactions (2) and (3), respectively. © 2001 ISIJ 514 ISIJ International, Vol. 41 (2001), No. 5 ϩ ϭ 2ϩϩ Ϫϩ Ϫ 3CaO·SiO2 3H2O 3Ca HSiO3 5OH .....(1) 2ϩϩ Ϫϭ Ca 2F CaF2 ..........................(2) 2ϩϩ Ϫϭ Ca 2OH Ca(OH)2 .....................(3) Since the Ca content in aqueous solution is considerably high, it is expected that fluorine in aqueous solution is im- mobilized mainly through the formation of CaF2 in the early stage, as shown in Fig.
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