Minerals Beneficiation - Infrared Identification of Silica Adsorbed on Thoria Surfaces

Colloidal silica dissolved in aqueous suspensions of high surface area thoria was permitted to adsorb on the thoria surface. Silica in three forms was identified by means of infrared spectroscopy and characterized by Si-O vibrational modes in the 9.0 to 10.5 region. A band at 9. 1 was attributed to free unad-sorbed silica, while adsorbed silica resulted in bands of greatest absorbency at 9.4 or 10.3p, depending upon the character of the surface bonding. Size considerations, the amount of silica adsorbed, and the character of the infrared spectrum for strongly adsorbed silica has been interpreted as evidence of formation of chain or layer silica linkages on the thoria surface. Infrared spectroscopy has been used successfully in recent years for the identification of minerals and synthetic inorganic substances. Unfortunately, infrared studies of silicate minerals have not proceeded to the point that unambiguous assignments may be made for Si-O vibrational modes as affected by lattice substitutions and structures. Several investigators have presented infrared spectra of silicate minerals, 1,7 and their work has pointed to the potential value of infrared to identify minerals as well as to note the sensitivity of various Si-O modes to structural and substitutional environments. The strong infrared absorption of various Si-O vibrational modes makes the infrared method a very useful tool for the study of silica adsorbed on the surface of solids of high surface area. Spaepen et a18 studied the adsorption of silica on thoria surfaces from aqueous solutions of silicic acid up to 300°C. A general review of the application of the infrared method to the study of surface adsorption has been presented by Pliskin and Eischens.9 EXPERIMENTAL Du Pont colloidal silica, Ludox SM (16.36% SiO2) and Ludox HS (30.25% SiO2) in aqueous suspensions were used as the sources of silica for the adsorption studies. The thoria used for this study was supplied by Union Carbide Nuclear Co.* under the preparation *Chemical Technology Div., Union Carbide Nuclear Co., Oak Ridge, Tenn. . number D-16-650. It was fired from the oxalate at 650°C and had a surface area of 32.4 ± 0.6 m2 per g measured by nitrogen adsorption. The chemical analysis of the D-16 thoria in ppm was: CO3= (5500), NO3- (< 4), so4±PO4 (100), and Si (20). The Ludox silica was placed in contact with the thoria in a two-liter autoclave10 between room temperature and 250°C. Slurries were made by placing 1 g of thoria in one liter of distilled water containing various additions of the silica sol. Samples were removed from the autoclave by opening a small stainless steel valve. Extracted samples were centrifuged for 10 min and excess solution was decanted. The sample was then dried for several hours at 80°C. Infrared samples were prepared by thoroughly mixing 10 mg of the thoria with 400 mg KBr in polystyrene vials on a "Wig-L-Bug" amalgamator. Pellets were pressed under vacuum and had final dimensions of 1.2 cm diam and 1.2 mm thick. Infrared spectra were