| Abstract |
he Olkaria geothermal project currently has an installed capacity of 799.3 MW and is operated by the Kenya Electricity Generating Company (KenGen) in Naivasha, Kenya. A comprehensive geochemical characterization of scale deposits was conducted using petrography, XRD, XRF, SEM-EDX, 27Al and 29Si MAS solid state NMR. The samples showed the presence of amorphous opal-A exemplified by a broad band showing moderate to intense scattering ranging from 19-30° 2θ, centered at 3.6 - 4.6 Å with distinct maxima around 21.5-24.5° 2θ (~3.9 Å). Strong quartz peaks were seen at 4.26 Å (~20.8° 2θ) in the (1,0,0) lattice and at 3.34 Å (~26.6° 2θ) in the (1,0,1) lattice, sometimes accompanied by other minor peaks. From the 27Al MAS NMR spectrums, sharp peaks were seen at 52.5 ppm, 52.2 ppm, and 56.3 ppm, as well as small minor peaks at 12.9 ppm, 3. 9 ppm, and 16.5 ppm relative to [Al(H2O)6]3+, suggesting that aluminum ions were substantially present as 4-coordinate species, but minor amounts of 6-coordinate species also existed. The mechanism for quartz precipitation is that monosilicic acid or monosilicate ion is adsorbed on the surface of a solid, and the critical nucleus of quartz is formed and grows to crystalline quartz by continuous adsorption of [Si(OH)_4 ] or [Si(OH)_3 O^- ] by homogenous nucleation. Amorphous aluminosilicates form as the fluid flashes to > pH 9, and a rapid reaction between [Si(OH)_4 ] or [Si(OH)_3 O^- ] and [Al(OH)4]- occurs to form opal-A. |