ISSN 1070-4272, Russian Journal of Applied Chemistry, 2011, Vol. 84, No. 3, pp. 345−350. © Pleiades Publishing, Ltd., 2011. Original Russian Text © V.S. Rimkevich, A.A. Pushkin, Yu.N. Malovitskii, I.V. Girenko, 2011, published in Zhurnal Prikladnoi Khimii, 2011, Vol. 84, No. 3, pp. 353−358.
INORGANIC SYNTHESIS AND INDUSTRIAL INORGANIC CHEMISTRY
Study of Fluoride Treatment of Silica-Containing Raw Material V. S. Rimkevich, A. A. Pushkin, Yu. N. Malovitskii, and I. V. Girenko Institute of Geology and Nature Management, Far Eastern Branch, Russian Academy of Sciences, Blagoveshchensk, Amur oblast, Russia Received June 3, 2010
Abstract—An effect of ammonium hydrofluoride treatment of silica-containing raw material was examined. We described thermodynamics and kinetics of caking raw materials and their crushed materials, ammonium hexafluorosilicate sublimation, and amorphous silica formation. Constants of rate and activation energy of the chemical reactions were calculated. DOI: 10.1134/S1070427211030013
Nowadays known types of silica-containing raw material are applied to produce ammonium hexafluorosilicate and amorphous silica which are widely used in various branches of manufactures: chemical, paint-and-lacquer, medicine, and cosmetics, and have a great demand in the domestic and foreign markets. The most common silica-containing raw materials are quartz sands, which contain small amounts of contaminants. Developed well-known methods of production of ammonium hexafluorosilicate and amorphous silica associated with the use of complex multi-step processes require application of expensive reagents, specific source raw materials and equipment [1–3]. A goal of the study is to examine fluorination of the silica-containing raw materials and develop environmentally safe method of extraction of chemically pure ammonium hexafluorosilicate and amorphous silica from accessible and cheap raw materials: quartz sands.
than –0.001 cm. Quartz is a main rock-forming component present in form of β-SiO2 trigonal that is stable at normal temperature and pressure. For processing quartz sands we used ammonium hydrogen difluoride (NH4HF2), pure for analysis. Under normal conditions NH4HF2 is a stable crystalline solid of orthorhombic structure that is environmentally safe comparing with fluorine, hydrogen fluoride, and hydrofluoric acid and becomes effective fluorinating agent when heating. Fluorination reactions are typical of NH4HF2, molten hydrogen difluoride is the more vigorous fluorinating agent than fluorine gas [4]. Melting point of ammonium hydrogen difluoride is 126.8, and decomposition temperature, 238°С, solubility in water, 370 g cm–3 at 70°С. The experiments were carried out in an electric furnace of special design, where in a gradient-free working area was a nickel or nickel-lined steel reactor. Initial components, taken in specified proportions, were thoroughly mixed and placed in Teflon, glassy carbon or platinum containers: cups, or pots, which were put into the working area of the electric furnace and kept at 100– 500°C for 0.25–4.5 h. The value of weighted portions was 5–40 g. For trapping and collecting the volatile products we used dual-zone condenser, the absorption of gaseous ammonia occurred in a vessel with water. Synthesis of the amorphous silica was carried out in the hydrolysis apparatus, the regeneration of ammonium hydrogen difluoride, in a laboratory evaporator–crystallizer.
EXPERIMENTAL Objects under study were quartz molding sands of the following composition, wt %: SiO2 95.80, Al2O3 2.26, Fe2O3 0.17, TiO2 0.23, Na2O 0.09, K2O 0.97, other losses during calcination (o.l.c.) 0.40 (Chalgan deposit of feldspar kaolin-containing sands, Amur region). In experiments we used main fraction +0.01–0.04 cm of the quartz molding sand and its material crushed to size less 345
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To determine compositions of initial samples, intermediate phases and final products we used methods of chemical, spectral, thermal analysis, and XPA, which were employed in Analytical Center of Mineralogical and Geochemical Studies of FEB RAS. Chemical analysis of silicon content in the samples was carried out by SiO2 precipitation and by photometry. Fluoride and ammonia content was determined after H2SiF6 and NH3 distillation by titration of the obtained solutions by thorium nitrate and sulfuric acid, respectively. For X-ray phase analysis we used a DRONUM1 diffractometer (CuK-radiation, scanning speed 1 deg min–1) and an X-ray automatic minidiffractometer MD-10 EFA with a database for phase analysis ICDD PDF 2008. Emission spectral analysis of microimpurities was performed on a microspectrograph STE-1 with cross-dispersion techniques of blowing in a three-phase arc and sample evaporation from a channel of carbon electrode. Thermogravimetric studies were performed on an STA 449C Jupiter instrument in platinum crucibles with a covers at a heating rate of 2–5 deg min–1 (initial weighted portions 0.10–0.15 g). Based on the experimental data on the weight loss of samples for preset periods of time and temperatures and based on data of chemical analysis of the reaction products we computed kinetic parameters of the processes: constants of rate and activation energy. Degree of formation of reaction products required for further calculations was determined by equation: a = m / mcalc,
where m was the weight of the formed product, mcalc, a theoretically possible amount of the product. Calculation of the rate constants was conducted by a logarithmic technique [5], of constants of activation energy, by linearization according to Arrhenius equation [6] using software Microsoft Excel 2003. Experimental examinations of the fluoride processing were carried out by two steps: caking of the origin raw material with ammonium hydrogen difluoride at 100– 200°С and thermal treatment of the obtained precipitate in a temperature range of 300–500°С. An interaction mechanism of the quartz sand caking with ammonium hydrogen difluoride should be considered as multistage. In the start of the interaction formation of ammonium heptafluorosilicate [(NH4)3SiF7] occurs. 2SiO2 + 7NH4HF2 = 2(NH4)3SiF7 + 4H2O↑ + NH3↑.
(1)
Further at temperatures >100°С formation of ammonium hexafluorosilicate [(NH4)2SiF6] occurs according to the following reaction: 2SiO2 + 7NH4HF2 = (NH4)3SiF7 + (NH4)2SiF6 + 4H2O↑ + 2NH3↑ + HF↑,
(2)
and at 200°С (NH4)2SiF6 is formed: SiO2 + 3NH4HF2 = (NH4)2SiF6 + 2H2O↑ + NH3↑.
(3)
Thermodynamic calculations were conducted using data of [7, 8]. These calculations exhibit that a change in Gibbs energy ΔG for reactions (1), (2) under normal conditions are –39 and –36 kJ, respectively, and decrease to –306.1 and –354.5 at 200°С, respectively. For reaction (3) ΔG at 200°С is –49.9 kJ and decreases as temperature increases: ΔG300 = –1464.7 kJ. Results of examinations of an effect of the fluorinating agent excess (15% relative to stoichiometry) on an ammonium hexafluorosilicate yield demonstrated that the calculated stoichiometrical ratio of 1 : 2.85 is optimal. If this ratio is smaller than, e.g., 1 : 2.4 then the target yield diminishes to 65 wt %; in the case of the larger ratio (1 : 3.3) the target yield attains 99 wt %, however in this case an increased amount of ammonia is evolved and consumption of the fluorinating agent is not complete. Figures 1a and 1b show kinetic curves of a dependence of the amount of the evolved ammonia at caking the quartz molding sand of fraction +0.01–0.04 cm and its crushed material on time at various temperatures. By values of the constants of rate and activation energy (see the table) we can conclude that the processes proceed in the kinetic region. For the crushed material fluorination occurs more vigorously, and this fact is confirmed by larger values of the rate constants and smaller values of activation energy (see the table). This is explained by activation of a crushed grain surface and better interaction with the fluorinating agent. The method of improving the florination is temperature increase. In melt of NH4HF2 the interaction proceeds with maximum rate, and the optimal conditions for quartz sand of fraction +0.01–0.04 cm is reached at 200°C for 4.5 h and for the crushed material, at 200°C for 3.5 h. In this case ammonia evolving is attained 99% relative to the theoretically possible value. According to data of XPA the solid precipitates obtained after caking under equilibrium conditions
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The rate constant kс and activation energy Еа of caking the quartz sand with (NH4)2SiF6 and sublimation of volatile ammonium hexafluorosilicate Caking the quartz sand of +0.01–0.04 cm fraction
Caking the crushed quartz sand
Sublimation of volatile (NH4)2SiF6
T, °С
kс, min–1
T, °С
kс, min–1
T, °С
kс, min–1
100 150 175 200
0. 000928 0.002941 0.004485 0.006552
100 150 175 200
0.002202 0.005031 0.006382 0.008230
300 350 450 500
0.002268 0.010620 0.044069 0.061729
Еа, kJ mol–1
28.0
18.3
(Figs. 1a, 1b) consist of (NH4)3SiF7 (100°С), mixture of two complex salts (NH4)3SiF7 and (NH4)2SiF6 (150 and 175°С), and ammonium hexafluorosilicate at 200°С. In the course of caking [reactions (1), (3)] ammonia gas and water vapor are trapped in aqueous solution forming ammonia water (NH4ОН) according to reaction (4) NH3 + H2O = NH4ОН,
(4)
that feeds to a hydrolysis stage of ammonium hexafluorosilicate. Reaction (4) occurs at room temperature: ΔG25 = –9.2 kJ, and the change values of Gibbs energy increase with a growth in temperature: ΔG100 = –1.2 kJ. Evolving vapors of NH3 and HF in caking [reac-
10.8
tion (2)] interact into the second zone of the condenser forming ammonium fluoride: NH3 + HF = NH4F,
(5)
which is delivered to evaporator–crystallizer. For reaction (5) the change in Gibbs energy under the normal conditions is –39.1 kJ, and its value increase with temperature growth: ΔG100 = –23.3 kJ. According to the chemical analysis in a powder sinter, produced under the recovery or inert conditions at 200°C and exposure time 3.5–4.5 h, the impurity compound Al, Fe, Na, and K form simple fluorides, which remain in non-volatile residue. Purification of the product (NH4)2SiF6 from impurities was carried out by thermal
(a)
(b)
Fig. 1. Dependence of the amount of ammonia (%) evolved in caking (a) quartz sand of fraction +0.01–0.04 cm and (b) crushed quartz sand with ammonium hydrogen difluoride с on time τ (h) at various temperatures. Temperature, °С: (1) 100, (2) 150, (3) 175, (4) 200. A hatch denotes a theoretically possible ammonia amount. RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 84 No. 3 2011
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Fig. 2. Dependence of the degree of ammonium hexafluorosilicate formation α (fractions) on the process time τ (min). Temperature, °С: (1) 300, (2) 350, (3) 450, (4) 500.
treatment, which results in the sublimation of ammonium hexafluorosilicate at temperatures above 300°C, that is consistent with previously established data [9, 10]. Volatile (NH4)2SiF6 was trapped and collected in the first zone of the condenser. Figure 2 shows the kinetic dependences computed according to data of the weight loss of the non-volatile residue under the recovery conditions with formation of volatile ammonium hexafluorosilicate at the different temperatures and exposure time. The process is characterized by high values of the rate constants and small values of activation energy (see the table). The experimental data (Fig. 2) demonstrate that temperature of 500°С is the most favorable for formation of volatile (NH4)2SiF6 that occurs for short periods of time and after 30 min a computed theoretical amount of this product is attained. Results of the thermal analysis of the interaction of original qurtz sand of fraction +0.01–0.04 cm and its crushed material with ammonium hydrogen difluoride is shown on Fig. 3 in a temperature range of 25–350°С. Fluorination of the crushed material occurs at lower temperatures due to the high specific surface and better interaction with the fluorinating agent comparing with the original quartz sand. An endothermic effect at 107.5, 159.1, 204.3, and 295.1°C for the crushed material was recorded on the DSC
curve (Fig. 3). An effect at 295.1°C is the most deep and broad due to the superposition of the endothermic effect of (NH4)2SiF6 sublimation proceeding at this temperature. At the beginning of the interaction the formation of phase of (NH4)3SiF7 (107.5°C) occurs, that is confirmed by X-ray phase analysis of the samples, then with increasing the temperature, the formation of a mixture of phases (NH4)3SiF7 and (NH4)2SiF6 occurs until obtaining pure ammonium hexafluorosilicate (204.3°C) . In the thermogravimetric (TG) curve we noted endothermic effect at 159.1°С. The weight loss was 16.98%, that was consistent with the calculated data (17.16%) relative to water and ammonia losses [reaction (1)]. Then we removed HF and H2O and ammonia residues and in this case at 204.3°С the losses according to TG curve were 11.44%, by calculation, 10.93% [reactions (2), (3)]. For reaction (3) the maximum computed amount of evolved ammonia was established equal to 7.17 wt%. The formation of volatile product (NH4)2SiF6 started at temperatures above 204.3°С, the sublimation dominated at temperatures above 300.2°С. According to thermal analysis data the residual mass of fluoride of Al, Fe, Na and K in the non-volatile residue at 350°C was 9.07%. According to experimental data at 500°C the weight of non-volatile residue was equal to 5.25%, which was consistent with the calculated data: 5.15% [reaction (1)–(3)]. Fluoride of aluminum, iron and alkali metals can be extracted from the non-volatile residue. For the original quartz sand the weight of the non-volatile residue at 350°C was 19.84%, according to the experimental data at 500°C, 6.3%. This fact was explained by the presence in the precipitate of a small amount of unreacted quartz. According to data of the spectral analysis the volatile ammonium hexafluorosilicate possesses high purity: an amount of metallic impurities does not exceed 10–3– 10–5 wt% (Al, Fe, Mn, Mg, Cu). A microfime silica can be electrolytically extracted from solution of (NH4)2SiF6 [11]. Ammonium hexafluorosilicate is dissolved in water to a 5–25 wt% concentration and at temperatures of 25–90°С is reacted with ammonia water (25 wt% of NH3) to form a suspension at pH 8–9. In addition, the suspension is maintained at this temperature in the course of 1 h. Ammonium hexafluorosilicate is hydrolyzed in an aqueous alkaline solution according to reaction (6): (NH4)2SiF6 + 4 NH4ОН = SiO2↓ + 6 NH4F + 2H2O. (6)
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(b)
Fig. 3. DSC (a) and TG (b) curves of an interaction of the quartz sand with ammonium hydrogen difluoride. (Q) heat flow (μW mg–1), (Δm) a change of the sample weight, %), (Т) temperature, °С. The sample: (1) crushed, (2) original.
Reaction (6) occurs at room temperature: ΔG25 = –48.7 kJ, upon the temperature growth a change in the value of Gibbs energy increases: ΔG100 = –19.1 kJ. Use of ammonium hexafluorosilicate with concentration less than 5 wt% is not impractical since it promotes formation of a hard filtered SiO2 gel. An increase in am-
monium hexafluorosilicate concentration above 25 wt% leads to a decrease in the yield of the target product and worsening its quality. An applied temperature range of 25–90°С is caused by a fact that temperature growth over 90°C leads to an intense evaporation of the solution, and at the temperature
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below 25°C it is not possible to prepare the product of a required quality. Maintaining the obtained suspension for 1 h at the temperature of 25–90°С helps to stabilize the gel, and significantly improve its filterability. The formed suspension is filtered and washed with distilled water with mechanically agitating then the precipitate is dried on a filter to constant weight. The obtained amorphous silica is a chemically pure ultrafine powder (“white soot”) of the high specific surface (at least 90 m2 g–1) and good filterability [12], sizes of nanoparticles are 15–40 nm, an average value of size of pores is about 2 nm, an amount of impurities less than 10–4 wt %. The extracted amorphous silica corresponds to GOST 14922–77. However, according to economical calculations a cost of amorphous silica produced by the fluoride method is almost two times lower with no worsening the qualitative technical characteristics. In evaporation of aqueous solution of NH4F ammonium hydrogen difluoride is crystallized according to reaction (7) 2NH4F = NH4 НF2 + NH3↑.
(7)
Reaction (7) occurs at 100°С (ΔG100 = –12.0 kJ), and the values of the change in Gibbs energy decrease with a temperature growth: ΔG200 = –28.7 kJ. Formed NH4НF2 is fed to a start of the original raw material processing. CONCLUSIONS (1) As a result of the study of fluoride treatment of the silica-containing raw material we developed a rational method for extracting ammonium hexaflurosilicate and amorphous silica from available and cheap quartz sands with almost complete regeneration of ammonium hydrogen difluoride. (2) The developed method can be realized on standard test-industrial and industrial equipment produced by
domestic manufacturer of chemical devices. ACKNOWLEDGMENTS This work was supported by RFBR–FEB RAS “Far East” (grant no. 06-05-96041) and FEB RAS (grant no. 09-3A-02041). REFERENCES 1. Vol’fkovich, S.I., Boguslavskii, I.M., Kazakova, S.B., and Bogdanova, N.S., Khim. Prom-st’., 1971, no. 12, pp. 902–904. 2. Rakov, E.G., Khimiya i tekhnologiya neorganicheskikh ftoridov (Chemistry and Technology of Inorganic Fluorides), Moscow: MKhTI, 1990. 3. RF Patent 2061656. 4. Khimicheskaya tekhnologiya neorganicheskikh veshchestv (Chemical Technology of Inorganic Compounds), Akhmetova, T.G., Ed., Moscow: Vysshaya Shkola, 2002. 5. Stromberg, A.G. and Semchenko, D.P., Fizicheskaya khimiya (Physical Chemistry), Moscow: Khimiya, 1999. 6. Kireev, V.A., Kratkii kurs fizicheskoi khimii (Brief Course of Physcal Chemistry), Moscow: Khimiya, 1969. 7. Khimicheskaya entsiklopediya (Chemical Encyclopedia), Prokhorova, A.M., Moscow: Sov. Entsiklopediya, 1988, vol. 1. 8. Lidin, R.A., Andreeva, L.P., and Molochko, V.A., Spravochnik po neorganicheskoi khimii (Handbook on Inorganic Chemistry), Moscow: Khimiya, 1987. 9. Kurilenko, L.N., Laptash, N.M., Merkulov, E.B., and Glushchenko, V.Yu., Elektr. Zhurnal “Issledovano v Rossii,” 2002. 10. Mel’nichenko, E.I., Krysenko, G.F., Epov, D.G., and Marusova, E.Yu., Zh. Neorg. Khim., 2004, vol. 49, no. 12, pp. 1943–1947. 11. Marakushev, A.A., Zubenko, I.A., Malovitskii, Yu.N. et al., Byul. MOIP. Otd. Geol., 2005, vol. 80, no. 5, pp. 47–51. 12. RF Patent 2286947.
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