USAGE OF THE COMPONENTS ADDITIVE INTO SUPERCRITICAL WATER FLUID FOR CORUNDUM CRYSTALS PURIFICATION DURING SYNTHESIS

Transcription

1 USAGE OF THE COMPONENTS ADDITIVE INTO SUPERCRITICAL WATER FLUID FOR CORUNDUM CRYSTALS PURIFICATION DURING SYNTHESIS Danchevskaya M.N.*, Ivakin Yu.D., Torbin S.N. Chemistry Department, Moscow State University, Moscow , Leninskie Gory Russia Fax: In the present report the results of investigations of the influence of some fugitive compounds on a level of purification of corundum crystals obtained from aluminum hydroxide in supercritical water fluid (SCWF) are represented. As the components additives in a water fluid were utilized: urea (carbamide) and primary alcohol (ethanol). The addition of these compounds into supercritical water fluid leads in decrease of content of interstitial impurity into synthesized corundum in several times. INTRODUCTION The processes of solid-phase transformations of oxides, oxyhydroxides and hydroxides in supercritical water fluid (SCWF) proceed in conditions of dynamic quasi-equilibrium between a solid phase and fluid. In particular, the process of dehydration of aluminum hydroxide (hydrargillite) in an atmosphere of a water fluid with simultaneous hydration its structure is carried out. This process is accompanied by an increase of mobility of crystal lattice that results in facilitation of reorganization of its structure in a direction of formation thermodynamic more stable product. The processes of solid-phase transformations of oxides, oxyhydroxides and hydroxides in supercritical water fluid (SCWF) are occurring in conditions of dynamic quasi-equilibrium between a solid phase and fluid. The process of corundum formation from hydrargillite in water passes through the stages: Al(OH) 3 AlOOH + H 2 O α-al 2 O 3 + 2H 2 O. Hydrargillite Boehmite Corundum Earlier, the authors had established that the molecules of a fluid actively participate in process of reorganization of a solid phase both on first and at the second stages of transformation hydrargillite into corundum [1, 2]. The chemical composition of a fluid influences both on rate of process and on properties of obtained corundum: the size of crystals, habitus, optical and mechanical performances [3]. In supercritical water fluid under temperature 400ºC and pressure 23 MPa the crushed aluminum hydroxide was transforms into corundum with bipyramidal habitus. Corundum synthesized from aluminum hydroxide in supercritical fluid of primary alcohols in the same conditions has preferred hexagonal plate habitus. (Figure 1a and b). If raw material was Al 2 O 3 the formation of corundum in supercritical fluid of primary alcohols does not take place, but the ordering pattern of source aluminum oxide is watched. This fact testifies to necessity of involvement of water molecules in process of transformation hydrargillite. In case of transformation hydrargillite into corundum in supercritical fluid of primary alcohols the activators of the process were the molecules of structural water coming out of structure of hydrargillite. 1

2 Even the small admixture of some compounds in water fluid changed properties of synthesized corundum. In this connection the authors had made an attempt to use the components of some organic compounds in water fluid during corundum synthesis to purify of impurities. a Figure 1. a. Corundum synthesized in supercritical fluid of ethanol; b. Corundum synthesized in supercritical water fluid. b MATERIALS AND METHODS The synthesis of corundum from aluminum hydroxide (hydrargillite) was carried out in supercritical water fluid with various organic additives in autoclaves from rustless steel under 400 C and pressure 23 MPa. As precursor industrial hydrargillite of the mark GD-00 was utilized. To water fluid were added urea (carbamide) or primary alcohol (ethanol). The alcohol was added in amount % and urea % concerning mass of hydrargillite. The composition and contents of impurities in hydrargillite, boehmite and in corundum were determined by methods: chemical-spectral analysis, photoluminescence and electron-diffuse reflection. The chemical-spectral analysis of impurities in hydrargillite, boehmite and corundum was fulfilled by a method of an emissive spectrometry with excitation of a spectrum in an arc of a continuous current with usage of a spectrometer DFC-8. The products of carbamide and alcohols destruction in supercritical water in contact with Al 2 O 3 nh 2 O by the chromatographic (the chromato-mass-spectrometer Finnigan INCOS 50) and mass-spectrometry (the mass-spectrometer MI-1311) were studied. The solid-phase transformation products of aluminum hydroxide by methods X-Ray diffraction were investigated (the diffractometer DRON 3M). The electron-microscopic photos were carried out on the device Cam Scan Series 2. The spectra of photoluminescence were registered on the device SDL 2M at ambient temperature in interval nm under exiting irradiating by light 254 nm. The spectra of diffuse reflection of samples of products solid-phase transformation hydrargillite were studied using the spectrometer "Specord M40". RESULTS AND DISCUSSION In a supercritical water fluid during transformation of hydrargillite and boehmite into water-free corundum an intensive water removal of their structure occurs. Some impurities were partially deleted from precursor together with water. Besides due to quasi-equilibrium of a solid phase with a water fluid at adding into reactionary medium doping components they 2

3 easily enter into structure of a crystal during its transformation. Also some elements as a technological impurity from constructional materials into products of synthesis can be embedded. In the table 1 the results of a chemical-spectral analysis of impurities into raw material (hydrargillite) and of product its heat treatments in a supercritical water fluid (boehmite) are given. The prolongation of time of boehmite synthesis from hydrargillite results in increase of its contamination by elements transferred from the steel container and autoclave, mainly by transition elements. Table 1. Chemical composition (ppm) of impurities into hydrargillite and boehmite obtained under 23MPa; 400ºC Samples, Synthesis time Fe Cr Cu Ca Mg Ba Na K Hydrargillite GD Boehmite, 1 h < Boehmite, 2 h < Boehmite, 3 h < At the same time the content of impurities of alkaline and alkaline-earth metals was decreasing. During the stage of transformation boehmite into corundum the further diminution of the content of impurities of alkaline and alkaline-earth metals was occurred. The content of impurities of transition elements was remained at the same level. The impurity of chrome and manganese in corundum is well diagnosed by method electron diffuse reflection and by luminescent analysis. In figure 2 the spectra of diffuse reflection of hydrargillite and boehmite and corundum obtained from it in SCWF are demonstrated. Figure 2. The spectra of diffuse reflection of boehmite and corundum with impurity manganese (Mn ppm). The spectra of diffuse reflection of hydrargillite and corundum with impurity chromium ( Cr ppm). An impurity iron and other heavy metals cause the absorption band in the field of nm. In this field the bands caused by vacancies defects were situated also. The band at 370 nm is referred to an impurity Cr +6, and band at 410 nm and 555 nm to an impurity of ions 3

4 Cr +3. The impurity of ions Mn +3 (is more exact Mn +4 -Mn +2 ) is characterizing by bands in area nm. The increase of time of the synthesis and the treatment of boehmite in SCWF is exhibited in variation of intensity of absorption bands of impurity ions of metals (Figure 3). From a figure 3 it is shown that in some cases the prolongation of time of the hydrargillite hydrothermal treatment can give magnification of an impurity of chrome in 10 times. Figure 3. The spectra of diffuse reflection of hydrargillite and boehmite with impurity chromium (Cr +6 ). The study of the influence of composition additives in water fluid on properties of boehmite and corundum synthesized in SCWF has shown that some organic compounds safeguard products of syntheses in steel autoclaves from technologic impurity. The usage of these components in SCWF allows to obtain corundum with smaller impurity level, than in raw material (hydrargillite). In figure 4 the spectra of luminescence and composition of impurities in corundum synthesized in SCWF without the components and with the component of urea are shown. The specter of a luminescence of fine crystalline corundum synthesized from hydrargillite in SCWF, consists of two bands in area nm and nm referring to F - centers of different types (Figure 4a), and bands conditioned by impurity ions of manganese (678 nm) and chrome (693 nm) (Figure 4b). The specter of luminescence of corundum synthesized from hydrargillite in SCWF Figure 4 a. The bands of luminescence of Figure 4 b. The bands of luminescence of F centers in structure of corundum. Mn and Cr in structure of corundum. 4

5 The spectrum luminescence and chemical-spectral analysis demonstrate that the amount of impurities in corundum synthesized in SCWF with urea or alcohol is less, than without these components and is smaller than in a raw material - hydrargillite Figure 5. The spectra of diffuse reflection of corundum synthesized in SCWF without (1) and with (2) the addition of urea. The content of impurities (ppm) Ca Cr Cu Fe Mg Mn Figure 6. The spectra of diffuse reflection of corundum synthesized in SCWF with the addition of ethanol. The content of impurities (ppm) Ca Cr Cu Fe Mg Mn In a figure 5, 6 the spectra of diffuse reflection of corundum synthesized in SCWF without the components and with the addition of urea or ethanol are shown. It is visible that the intensity of diffuse reflection in all area of a spectrum is increased at usage of these components; that corresponds to a decrease of entrapped impurities. The earlier fulfilled investigations [4] have shown, that ethanol in supercritical conditions (T = 400 o C; P = 230 MPa) in water fluid at the presence of an aluminous is exposing to a catalytic pyrolysis according to the schema: R is C 1 -C 2 OH R is C 2 -C 6 CO, CO 2, H 2, H 2 O, C n H 2n, C n H 2n+2, n =1 5; The urea in SCWF is decomposed according to scheme: OH NH O 2 N N NH NH H N O + H O CO NH H 2 N O HO N OH Biuret Cyanuric acid Besides, it is known that the urea can form stable complexes with salts of metals [5]. 5

6 The additives and products of a catalytic pyrolysis of the used components penetrate into solids, are grafting to an aluminous matrix and influence on the process of crystals forming. It is shown up in variation of a habitus and size of forming crystals. During transforming of precursor (hydrargillite) into boehmite and into a finished product (corundum) at intensive restructuring of crystal the amount of vacancies is increasing. Due to this there is a possible permeation of molecules from fluid into a solid phase. In a figure 7 the variation of a luminescent emission of F-centers during synthesizing corundum is shown. It corresponds to variation of amount of vacancies in pattern of crystals. During transforming of hydrargillite into boehmite and boehmite into corundum the vacancies concentration is increased that easies diffusion of molecules of fluid in a crystal. Figure 7. Dependence of luminescent It is possible to suspect that alcohol or urea and emission of F-centers (band 440 nm, line the products of their decomposition in water 1), and content of obtained corundum (line fluid interact with impurity ions of metals, 2) on duration of treatment in SCWF of forming compounds, and it promote to an exit boehmite. The luminescence excitation was of impurities from a crystal. carried out by light 254 nm. CONCLUSION Thus, the components of some organic compounds in SCWF exhibit dual operating: they preclude a carrying from walls of autoclave to solid of the contaminating elements and promote the purifying of the forming crystals of corundum. Usage of the mentioned above components additives in a water fluid during synthesis of corundum allows to diminish impurity level in obtained corundum almost in several times. REFERENCES: [1] DANCHEVSKAYA M.N., IVAKIN YU.D., TORBIN S.N., PANASYUK G.P., BELAN V.N., VOROSHILOV I.L., High Pressure Research, Vol. 20, 2001, p [2] DANCHEVSKAYA M.N., IVAKIN YU.D., TORBIN S.N., KREISBERG V.A., MARTYNOVA L.F., Proc. 7-th Meeting on Supercritical Fluids, Antibes, France, 6-8 December, 2000, p [3] DANCHEVSKAYA M.N., TORBIN S.N., IVAKIN YU.D., OVCHINNIKOVA O.G., MURAVIEVA G.P., Proceeding of the 6 th International Symposium on Supercritical Fluids, Versailles (France), April 2003, tome.2, p th [4] DANCHEVSKAYA M.N., IVAKIN YU.D., Proceeding of the 6 International Symposium on Supercritical Fluids,Versailles (France), April 2003, tome.2, p [ 5] KUCHERAYAVII V.I., GORLOVSKII D.V., Russ.Chem.Journal, Vol. 4, 1983, p. 47 6