V.G. Lutsenko/ Materials Chemistry and Physics 115(2009)664-669 The bromoform(2.9 gcm-3) and mixtures of CHBra-CCl4(2. 15-2.65gcm-3 silicon dioxide phases(2.16, 2.33, and 2.65g cm-3 for a-tridymite, a-cristobalite, d a-quartz, respectively)[12 SiC (3. 21 gcm-3)[13]. Fesi (4.93 gcm-3)[14] .and l01 he concentrates were then dried at 150C(boiling b) mperature for CHBrg is 149.C) ating table of particle iC whiskers in aqueous solution(pH 9-10)was fed as a thin layer transverse to nclined riffled surface of the plat ibrating. Due to gravity. the coarse particles( Sioz. Sic and Fesi) deposited onto the (a) ate surface, while fine particles(sic whiskers )were removed by he suspension that flowed off different sections of as of silicon dioxide with 11 oz particle contents up to 60-80 mass% have been prepared, which were then ied at 110C. The Fesi particles were extracted from concentrates by magnetic separatIo phase analysis(DRON-2. 2 Theta(de -radiation). optical microscopy, scanning electron microscopy(SEM, Superprober 733, Jeol), thermogravimetry(Q-1500D, Hungary), and chemical analysis [15]-The Fig. 1. Difractograms of Sic whiskers with different content of impurity phase a- ties(fe, Al, cristobalite:(a)as-synthesized Sic whiskers with 6.7 mass% Sio2: (b)concentrate nethod (SiOz particles were decomposed by concentrated hF acid and by smelting №1(62mass%SiO2) les was determined by the method of heavy liquids [ 16] through immersing particles in a CHBr3-CCl4 solution with density varied with the step of 0.01 from 2.22 to 2.38 gcm-3 (a DT to magnetic separation(suspensions in CCl4)and examined for the presence of g ES. Jamp-10, Jeol) and high- The Sic whiskers were cleaned from surface film of silicon dioxide in an aque- us solution of hydrofluoric ersing crystals upon mixing for 20 mi 58 the crystals were filtered off, washed in distilled water, and dried. The drying erature was varied from 20 to 150C. The dispersing of Sic whiskers in aqueous I pH 1-12 was carried ou stirring(500rpm) of the whereas to enhance water alkalinity the nh4 oh was used. The sedimentation kinet ics of suspensions of whiskers in aqueous solutions was studied in glass cylinders nder conditions which prevented contact of suspensions with COz in air. TG TG Time(min Time(min 3. Results Fig. 2. Thermograms of a-cristobalite concentrates: (a)concentrate e2(0.374 g. 83 mass% Sioz): (b)concentrate No5(0.604g, 80.1 mass%Sio The concentrates of silicon dioxide contained 60-97 mass%Sio2 SiC whiskers and particles, and less than 1-1.5 mass% carbon phases nd FeSi. The X-ray phase analysis showed that the concentrates surface of macropores in a-cristobalite particles(fig. 2). The con- contained only one crystalline phase of SiO2, namely a-cristobalite tent of water in a-cristobalite particles that were extracted from (Fig. 1). The phases of a-quartz and a tridymite and also particle us sus on the particle of X-ray-amorphous phase Sio, were not detected (table 1) of 8-15 mass%. The thermograms for Sioz concentrates that were The heating curves for concentrates of SiO2 particles exhibited extracted in a bromoform did not show endothermic effect(at about endothermic effect at about 260c due to polymorphic transfor- 100C), and there was no decrease in mass in the temperature range mation of a-cristobalite to B-cristobalite [ 17(Fig. 2). When storing 40-240.C the dried Sio2 concentrates in air(relative humidity >50%)there is The a-cristobalite particles extracted from concentrates increase in the mass of only concentrates extracted from aqueous CHBr3-CCl4 solution exhibited substantial difference in size( from uspensions Heating of those concentrates at temperature in the few micrometers to few hundreds micrometers). The main portion range of 40-240oC(the temperature of maximum reaction devel- of a-cristobalite particles(45-70 mass%)had an effective diame- opment is 102C)resulted in a decrease in their mass due to release ter in the range 63-200 um. The density of a-cristobalite particles of water adsorbed in the micropores and mesopores, and also at the varied in relation to their microporosity from 2. 28 to 2.36g cm-3 Properties of particle concentrates and individual particles of Sioz Conditions for extraction Phase composition, particle size ontent of SiO, and other CHBr3, light fraction ncentrate№2 HBr3-CCl4(2.65 gcm-3). light fraction Cristobalite SiC. Concentrate e 3 CHBr3-CCl4(2.4gcm-3) light fraction a-Cristobalite Particles from concentrate N 1 Grinding, CHBra-CCl4(2.50gcm-3) heavy fraction Sic Particles from concentrate Ne 1 Grinding, CHBra-CCl4(250gcm-3), light fraction a-Cristobalite, SiC, C Particles from trate e 3 CHBr3-CCl4(2. 35 gcm-3), light fraction -Cristobalite; particle size: -5-700 97. Impurities. Fe-<0.1 0-35 Concentrate№4 Concentrating table a-Cristobalite, SiC, Fesi 58-77 Concentrate№5 a-Cristobalite sic 60-8V.G. Lutsenko / Materials Chemistry and Physics 115 (2009) 664–669 665 The bromoform (2.9 g cm−3) and mixtures of CHBr3–CCl4 (2.15–2.65 g cm−3) were used to extract concentrates of SiO2 on account of difference in densities of silicon dioxide phases (2.16, 2.33, and 2.65 g cm−3 for -tridymite, -cristobalite, and -quartz, respectively) [12], SiC (3.21 g cm−3) [13], FeSi (4.93 g cm−3) [14], and nanographite (<2.2 g cm−3). The concentrates were then dried at 150 ◦C (boiling temperature for CHBr3 is 149 ◦C). The second method for concentrating SiO2 particles is based on gravitational extraction on the concentrating table of particles of less than 1 mm in size from aqueous suspensions in relation to particle density and shape. The suspension of SiC whiskers in aqueous solution (pH 9–10) was fed as a thin layer transverse to inclined riffled surface of the plate of concentrating table, which was continuously vibrating. Due to gravity, the coarse particles (SiO2, SiC and FeSi) deposited onto the plate surface, while fine particles (SiC whiskers) were removed by a water flow. The vortical flows formed between riffles promoted separation of particles. As a result, the suspension that flowed off different sections of the plate contained different amounts of SiO2 particles. In such a way the concentrates of silicon dioxide with SiO2 particle contents up to 60–80 mass% have been prepared, which were then dried at 110 ◦C. The FeSi particles were extracted from concentrates by magnetic separation. The concentrates obtained were studied by X-ray phase analysis (DRON-2, Cu K-radiation), optical microscopy, scanning electron microscopy (SEM, Superprober 733, Jeol), thermogravimetry (Q-1500D, Hungary), and chemical analysis [15]. The concentrations of impurities (Fe, Al, and Ca) were determined by atomic-absorption method (SiO2 particles were decomposed by concentrated HF acid and by smelting with NaOH). The density of SiO2 particles was determined by the method of heavy liquids [16] through immersing particles in a CHBr3–CCl4 solution with density varied with the step of 0.01 from 2.22 to 2.38 g cm−3. The SiC whiskers separated from -cristobalite in a bromoform were subjected to magnetic separation (suspensions in CCl4) and examined for the presence of surface SiO2 film by auger-electron spectroscopy (AES, Jamp-10, Jeol) and high￾resolution transmission electron microscopy (HRTEM). The SiC whiskers were cleaned from surface film of silicon dioxide in an aque￾ous solution of hydrofluoric acid by dispersing crystals upon mixing for 20 min. Then the crystals were filtered off, washed in distilled water, and dried. The drying tem￾perature was varied from 20 to 150 ◦C. The dispersing of SiC whiskers in aqueous solutions with pH 1–12 was carried out via mechanical stirring (500 rpm) of the suspensions for 5 min. To enhance acidity of distilled water, the HCl acid was used, whereas to enhance water alkalinity the NH4OH was used. The sedimentation kinet￾ics of suspensions of whiskers in aqueous solutions was studied in glass cylinders under conditions which prevented contact of suspensions with CO2 in air. 3. Results The concentrates of silicon dioxide contained 60–97 mass% SiO2, SiC whiskers and particles, and less than 1–1.5 mass% carbon phases and FeSi. The X-ray phase analysis showed that the concentrates contained only one crystalline phase of SiO2, namely -cristobalite (Fig. 1). The phases of -quartz and -tridymite, and also particles of X-ray-amorphous phase SiO2 were not detected (Table 1). The heating curves for concentrates of SiO2 particles exhibited endothermic effect at about 260 ◦C due to polymorphic transfor￾mation of -cristobalite to -cristobalite [17] (Fig. 2). When storing the dried SiO2 concentrates in air (relative humidity >50%) there is increase in the mass of only concentrates extracted from aqueous suspensions. Heating of those concentrates at temperature in the range of 40–240 ◦C (the temperature of maximum reaction devel￾opment is 102 ◦C) resulted in a decrease in their mass due to release of water adsorbed in the micropores and mesopores, and also at the Fig. 1. Difractograms of SiC whiskers with different content of impurity phase - cristobalite: (a) as-synthesized SiC whiskers with 6.7 mass% SiO2; (b) concentrate №1 (62 mass% SiO2). Fig. 2. Thermograms of -cristobalite concentrates: (a) concentrate№2 (0.374 g, 83 mass% SiO2); (b) concentrate №5 (0.604 g, 80.1 mass% SiO2). surface of macropores in -cristobalite particles (Fig. 2). The con￾tent of water in -cristobalite particles that were extracted from aqueous suspensions depends on the particle porosity, and it is of 8–15 mass%. The thermograms for SiO2 concentrates that were extracted in a bromoform did not show endothermic effect (at about 100 ◦C), and there was no decrease in mass in the temperature range 40–240 ◦C. The -cristobalite particles extracted from concentrates in CHBr3–CCl4 solution exhibited substantial difference in size (from few micrometers to few hundreds micrometers). The main portion of -cristobalite particles (45–70 mass%) had an effective diame￾ter in the range 63–200 m. The density of -cristobalite particles varied in relation to their microporosity from 2.28 to 2.36 g cm−3 Table 1 Properties of particle concentrates and individual particles of SiO2. Specimen designation Conditions for extraction Phase composition, particle size Content of SiO2 and other impurities, mass% Concentrate № 1 CHBr3, light fraction -Cristobalite, SiC, C 62–78 Concentrate № 2 CHBr3–CCl4 (2.65 g cm−3), light fraction -Cristobalite, SiC, C 75–83 Concentrate № 3 CHBr3–CCl4 (2.4 g cm−3), light fraction -Cristobalite, SiC, C 89–95 Particles from concentrate № 1 Grinding, CHBr3–CCl4 (2.50 g cm−3), heavy fraction SiC <0.5 Particles from concentrate № 1 Grinding, CHBr3–CCl4 (2.50 g cm−3), light fraction -Cristobalite, SiC, C 95–97 Particles from concentrate № 3 CHBr3–CCl4 (2.35 g cm−3), light fraction -Cristobalite; particle size: ∼5–700 m (>200 m – 10–20 mass%; <63 m – 20–35 mass%) >97; Impurities: Fe – <0.1; Al – <0.05; Ca – <0.02 Concentrate № 4 Concentrating table -Cristobalite, SiC, FeSi 58–77 Concentrate № 5 Concentrating table, magnetic separation -Cristobalite, SiC 60–81