| Title | MECHANISM AND KINETICS OF CASIL PRECIPITATION |
|---|---|
| Authors | H.P. Fraser, J.H. Johnston, M.J. Cairns, T. Borrmann and M. Schweig |
| Year | 2019 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | Calcium silica hydrate, silica formation, scaling prohibitor, Wairakei |
| Abstract | Silica formation is a problem in geothermal power generation from wet steam resources, especially in binary plants. Our proprietary method has been successfully proven in removing silica by forming nano-structured calcium silicate hydrate (CaSil) at pilot plant scale. CaSil particles do not adhere to pipes and can be removed prior to re-injection to provide a useful product. The CaSil process has shown to be effective in our pilot plant located at the MB Century site and sourcing brine from the Wairakei binary cycle power plant, Taupo, New Zealand. If this process is to be applied widely, the kinetics and mechanism of the CaSil precipitation need to be fully understood to optimise the process for different brine compositions and recovery procedures. The precipitation reaction of our proprietary CaSil is facile and difficult to monitor, as there is no consistent accompanying pH or colour change. Using batch methods and filtration, combined with AAS, UV-Vis spectrophotometry and SEM, we investigated the Ca to Si ratio in the filter cake product and residual filtrate generated from reaction mixtures using different SiO2 and Ca concentrations, different Ca to Si ratios and being produced at different temperatures. Our results so far showed that the CaSil forming reaction is robust and leads to useful and recoverable products under a variety of different synthesis parameters. CaSil precipitation is preferred over SiO2 formation in most typical plant operating ranges therefore it provides a proprietary way of capturing dissolved silica species before a problematic silica deposition can take place. |