Chemical and mineral analyses of Mwea clays

Abstract

Raw clays from Mwea area, Kirinyaga County, in the Republic of Kenya have been analyzed for their major and minor chemical elements. The clays were analyzed after subjection to different heating temperatures, of between 100 and 550°C, cooling and washing them with mineral acids of different concentrations. The results of the study show that the major components of the raw clays, when expressed as the oxides are: silica (SiO2), 43.5 to 52%; alumina (Al2O3), 17 to 22%; iron (III) oxide (Fe2O3), 12 to 16%; titanium (IV) oxide (TiO2), 3.5 to 5.3%; Lime (CaO), 1 to 3%; magnesium oxide (MgO), 0.5 to 1.25%. Elements occurring in very small quantities in the range of 0.1 to 0.9%, again when expressed as oxides are, Na2O, 0.30 to 0.40%; K2O, 0.20 to 0.30%; MnO, 0,14 to 0.46%. It was found that when the raw clays were boiled with either H2SO4 or HCl of different concentrations, the level of iron was reduced to 7 to 5% Fe2O3, depending on the concentration of the mineral acid used. Thus, both 18 M H2SO4 and 11 M HCl were found to be the most effective in iron removal. Furthermore, iron-removal was more effective when clay had been heated to a temperature of 400 to 550°C with the effectiveness being higher, the higher the temperature to which clay had been heated. Thus, when clay was heated to 550°C, cooled and washed with 11 M HCl, it contained only 0.08% Fe2O3. Very similar results were obtained by using 18MH2SO4 instead of11 M HCl. Unfortunately; such a residue contained very little clay if any because, it had nearly 85% SiO2, and a mere 2.66% Al2O3. Mineral determination of the raw clays has shown that Mwea clays are predominantly montmorrillonites. In some sites such as Kandongu, the clays are highly contaminated with quartz. In addition, those elements which could be quantified, there were also elements which occur in only trace amounts and their presence was only shown on the X-ray diffraction spectra. These include copper, zinc, manganese, cobalt, nickel, zirconium, gallliumthallium, selenium, terullium, tantalum, thorium, silver and even gold. The acid-soluble aluminium compound in the raw clay was present as the mineral gibbsite, AlO.OH. There were two sites with relatively high levels of titanium (3.5-5% TiO2).The titanium in the two sites was present as the mineral ilmenite, FeTiO3 (Cumbiri) and the mineral anatase, TiO2 (Kiandegwa). Elemental analysis was carried out using atomic absorption spectrophotometry, X-ray fluorescence spectrophotometry and ethylenediaminetetraacetic acid (EDTA) titrations. Within experimental error, the three methods gave very similar results. Key words: Clay, X-ray fluorescence, atomic absorption spectrometry, X-ray diffraction, ethylenediaminetetraacetic acid (EDTA) titrations.