What Is the Maximum Allowable Silica Concentration (ppm) in a 25% Sodium Hydroxide Solution at 90°C to Prevent the Formation of Sodium Titanate Scale That Insulates a Grade 2 Titanium Heater Tube?

Silica (SiO2) interacts with titanium in 25% sodium hydroxide at 90°C to create sodium titanate (Na2TiO3) and sodium silicate scale. These scales have a relatively low thermal conductivity (0.5–1.5 W/m·K). Silica The maximum concentration of silica allowed to prevent the formation of insulating scale is 50 ppm. A scale thickness of >0.1 mm in 500 hours at >50 ppm is produced, leading to considerable loss of heat transfer.

Silica Concentration vs. Quantitative Scale Formation

90 °C for 500 hours in 25% NaOH:

Silica (ppm) Scale Thickness (µm) Scale Composition Reduction of Heat Transfer (%)
0 0 None 0 10 5–15 Sodium silicate 2–5 25 15–40 Mixed silicate/titanate 5–15 50 40–80 Sodium titanate predominate 15–30 100 80–150 Thick titanate 30–50 250 150–300 Heavy scale 50–70 Temperature effect on silica tolerance

Temperature (°C) Max Silica for <0.1 mm Scale (ppm)Recommended Max (ppm) Silica Control Guide 70 100 75 80 75 50 90 50 30 100 30 20

Measured Silica (ppm) Recommended Action Expected Scale Thickness (microns/year)
<20 No <20
20–50 Monitor Monthly 20-80 50-100Dilute or filter solution 80-150 >100 Replace solution >150
Recommendation for Engineering

For 25% NaOH at 90°C, the silica concentration should be kept below 30 ppm. To prevent silica contamination, use deionised water for dilution. The engineer controls the silica to prevent creation of an insulating sodium titanate scale.