Silica Scaling in Water Treatment: Mechanism and Mitigation

Scaling is a significant process and flow assurance issue for water treatment facilities. Normally, calcite and other sulphate and carbonate scale are attracted most of attention.  While the sulphate and carbonate based scale can be managed through ion exchange by removing alkali earth metal ions, silica will normally remain in the water and cause scaling issue downstream. The impact of silica scaling becomes more eminent in desalination process.  There are three types of silica scaling:

  1. mono silica scaling
  2. Polymerisation of mono silica
  3. Metal silicate scaling, e.g. Al- SiO4, Mg-SiO4, CaSiO4 and Fe-SiO4 etc

Understanding of silica scaling chemistry is critical to the mitigate the silica scaling issue in thermal process, e.g. multi-effect distillation(MED), as well as reverse osmosis process. Silica scaling is governed largely by amorphous silica deposition, polymerisation of monosilica and deposition of metal silicate. The mitigation of silica scaling in process shall consider all three types of silica scaling.It is well known that condition of scaling for these three types silica scale are very different. While amorphous silica  and polysilica scale are mostly mitigated with high pH condition, metallic silicate e.g. Al, Ca, Mg- SiO4 starts to precipitate from pH 9.5.The difference in silica scale behaviour in temperature also causes more challenge in silica mitigation in water treatment, particularly in thermal process. The mono silica and poly-silica showed a normal solubility, i.e. the solubility of the species increases with the temperature. Therefore, for amorphous silica, the highest scaling potential will happen at lower temperature process, e.g. flash chamber (~50oC) of multi-effect distillation (MED). However, the metallic silicate such as Ca- SiO4, Al- SiO4, Mg- SiO4 demonstrates the invisible solubility.  The potential of scaling will be more severe at high temperature process, e.g. effect -1 of MED.To better manage the silica scaling issue, brine species in different stage of process needs to be simulated to paint a better picture of scaling. This will give an indicator of scaling tendency ( thermodynamics)in the different stage of process.

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