| Abstract |
Silica scale formation nearly always involves homogeneous nucleation of particles in the brine followed by transfer of the particles to the surface. Monomeric silicic acid may also deposit at a rate determined by surface reaction kinetics. The mass transfer rates of silica particles in boundary-layer flows and the monomer deposition rates have been estimated using a simple theoretical model. This has been used to rationalise the observed scaling rates in various field tests. It is concluded that in most cases very fine particles ( radius) are involved in deposition. Ohaaki brine is an exception as larger particles may form radius). As these field results can be explained in terms of polymerisation and particle deposition, other factors such as aeration do not appear to be involved. The model used does not include prediction of nucleation rates, ripening and early growth of the colloidal silica particles. This would be required for prediction of scaling rates, but further experimental data on nucleation rates would be needed to calibrate such a model. It is noted that additional field data on silica scaling would be useful, but it is essential that the size and number of particles involved be determined and correlated with the scaling rates. An experimental model was used to investigate particle transfer, but this work was not successful as the very fine particles involved could not be adequately simulated. |