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handbook of extractive metallurgy IV Part 5

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Tham khảo tài liệu 'handbook of extractive metallurgy iv part 5', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 1952 Handbook of Extractive Metallurgy Table 48.23 Typical properties of various silica gels. Property Regular density Intermediate density Slow dried Slow dried Fast dried Total volatiles 5 6 6 Na o dry basis 0.1 0.1 0.1 SiÓ4 dry basis 0.1 0.1 0.1 Al o dry basis 0.05 0.05 0.05 pl 4.0 7.5 4.0 Specific surface area m2 g 750 325 390 Pore volume cm3 g 0.40 1.1 1.8 Average pore diameter run 2.1 13.5 18.5 Water adsorption 10 R.H. 7.5 2.0 80 R.H. 35 17.0c Weight loss after heating in aứ at 954 C. b pH of an aqueous slurry of the gel at 5 solids. 90 water adsorption at 100 R.H. Several methods of forming hydrogel typically into spherical shapes have been developed. Some methods employ a rapid gelation after mixing in a nozzle followed by spray setting 648-651 while others form beads in an oil phase immiscible with the hydrosol 652 In some bead processes recycled dried and powdered gel is mixed with the fresh sol to improve the physical integrity of the dried gels toward disintegration after contact with liquid water 653 . The supercritical preparation of aerogels has undergone a renaissance since earlier commercial production 654 because of renewed interest in the insulation and lighttransmittance properties of these materials. The shift in applications toward large panels has. created the need for what are described as aerogel monoliths 631 . One such process 635 forms a silica alcogel then exchanges the alcohol for liquid CO2 before supercritical drying. The resulting monoliths show good light-transmittance properties. The monoliths typically 500 X 500 X 2.5 cm are used to fill the space in the middle of double-pane window s. Environmental Protection. In the sodium sil-icate H2SO4 processes the main envhonmen-tal concerns are the fate of by-product process chemicals normally aqueous Na2SO4 and small amounts of aqueous NH3 and pH control of this stream. Quality Specifications. Typical quality criteria for silica gel are chemical purity moisture content and porosity. .