SILICATE CHEMISTRY & SILICATE MANAGEMENT IN RO OPERATIONS

To reduce the speed of silicate scaling, strategies generally involve manipulating the water chemistry (such as pH and ion concentrations) and employing chemical additives (antiscalants/inhibitors) or physical pretreatment methods to prevent or retard the polymerization of silica.

Here’s how to reduce silicate scaling speed:

  • Adjusting pH Levels
    • The rate of silicic acid polymerization is strongly pH-dependent.
    • Polymerization is relatively fast between pH 6.5 and 8.5, but decreases to very low values at pH 5.5 and below, and also above 9.5.
    • Operating at acidic pH (e.g., pH 4-5) can retard silica polymerization, allowing for high RO recovery rates (e.g., 84-96%) even with high total silica concentrations (exceeding 1,000 mg/L) in the RO concentrate.
    • Similarly, operating at alkaline pH (e.g., pH 10-11) can increase silica solubility in the concentrate stream, which also helps manage scaling.
    • In acid solutions, the rate of polymerization of supersaturated solutions is “especially slow,” allowing non-equilibrium solutions to persist for months or years.
  • Controlling Temperature
    • Silica polymerization tendency increases with temperature, meaning higher temperatures lead to a decrease in soluble silica concentration over time.
    • At low-temperature ranges (below 50 °C), silicate scale inhibition efficiencies are generally very good and stable. This is because the adsorption process (of inhibitors) is faster than the polymerization process of silicate scale, effectively suppressing crystallite growth.
    • However, when the working temperature exceeds a certain threshold (e.g., above 50 °C), the polymerization process rate of silicate scale can surpass the adsorption rate, causing silicate scale particles to accelerate their growth and precipitation, leading to greatly reduced scale inhibition rates. Rates of both dissolution and polymerization are “enormously faster” at temperatures near the boiling point.
  • Utilizing Chemical Inhibitors (Antiscalants)
    • Chemical additives can be used to inhibit colloidal silica formation.
    • Inhibitory additives can only delay or retard silicate polymerization, thus enhancing its solubility and maintaining silicate in its soluble forms; ideally, colloidal silica does not form in such cases.
    • This approach helps prevent polymerization of soluble silicates, keeping silica soluble, and can also inhibit agglomeration and deposition onto membrane surfaces through electrostatic repulsion.
    • Cationic polymers have been studied as effective silica scale growth inhibitors in process waters. NOT FOR RO OPERATIONS. Cationic Polymers are not suitable for RO Applications.
      • Examples include polyethyleneimine (PEI) at 10 ppm, polyallylamine hydrochloride (PALAM) at 20 ppm, and poly(acrylamide-co-diallyl-dimethylammonium chloride) (PAMALAM) at 80-100 ppm, all showing efficacy in inhibiting silica polymerization.
      • For instance, after 24 hours, PEI at 10 ppm offered 55% inhibition, PALAM at 20 ppm showed 65% inhibitory activity, and PAMALAM at 80 ppm showed 60% inhibition.
    • It is important to note that inhibitory activity may drop on longer silicate polymerization times, such as after 48 or 72 hours. After 72 hours, all inhibitory activity can be lost, with soluble silicate levels becoming virtually identical to the control solutions.
    • A careful balance between the silicate level and the additive cationic charge is necessary for successful application, as excessive cationic charge can lead to agglomeration of negatively charged colloidal silica particles and deplete the “active” inhibitor.
    • However, many anti-scalants are not effective for preventing the formation of aluminum silicates.
  • Controlling Ion Concentrations
    • The presence of cations like calcium (Ca2+) and magnesium (Mg2+) favors and increases the rate of silica polymerization and the formation of silicate deposits. Magnesium hardness is noted to be more effective than calcium hardness in accelerating silica polymerization.
    • Therefore, reducing the concentrations of these ions can help slow scaling. Pretreatment methods like softening can reduce magnesium and calcium hardness, which can improve silica removal efficiency.
    • Controlling residual aluminum (Al) ions in the feed of Reverse Osmosis (RO) processes, potentially through alternative coagulants, can be an effective way to control the formation of aluminum-silicon (Al-Si) fouling.
  • Avoiding Supersaturation and Pretreatment
    • Silicate polymerization occurs only in supersaturated waters with respect to silica. The greater the original degree of supersaturation, the faster is polymerization.
    • Thus, avoiding supersaturation of make-up water with respect to silicate ion is a preferred preventive option.
    • Pretreatment methods can be employed to lower the silica concentration in the RO feed water, allowing for stable operation even under silica oversaturation.
      • Desilicizers can remove silica before water enters the system.
      • Seeded precipitation/aggregation is effective in reducing silica concentration from supersaturation to the saturation concentration.
      • Anion exchange (IX) resins, particularly strong base anion (SBA) exchange resins in hydroxide form, can be applied to remove dissolved silica.
      • Activated alumina adsorbents also show high silica selectivity.
    • The processes of silica dissolution and polymerization in dilute solutions are generally slow, allowing supersaturation to exist for considerable periods (days, weeks, or even months/years at low pH).

Created on June 4th, 2025, from 15 research and R&D articles on the silicate solubility subject. Reviewed by Dr. Lagref Jean-Jacques.

Share

Twitter Facebook LinkedIn