For a Titanium Heat Rod Operating in a 30% Calcium Chloride Brine at 100°C with a Magnesium Impurity of 2%, What Is the Maximum Allowable Water Hardness (as CaCO₃) to Prevent Preferential Magnesium Hydroxide Scaling?

At 100°C in 30% calcium chloride (CaCl 2 ) brine with 2% magnesium impurity (as MgCl 2 ), magnesium hydroxide (Mg(OH) 2 ) scales preferentially precipitate over calcium carbonate or calcium sulfate. Magnesium hydroxide is particularly low in solubility at elevated temperatures and builds a hard, adhering scale on the titanium heater surfaces. The scale has low thermal conductivity (1-2 W/m.K) which results in local over-heating. The overall hardness (as CaCO3) permissible to prevent preferential scaling of magnesium hydroxide is 200 ppm. At this setting, the scale thickness is 0.2 mm in 500 hours and the heat transfer is reduced by 30–50 %.

Preferential Magnesium Hydroxide Scale Formation Mechanism

At high temperature brine Mg2+ hydrolysis: Mg2+ + 2H2O -> Mg(OH)2 + 2H+. This reaction is endothermic and the higher the temperature the faster it proceeds. The reduction of water promotes the local pH at the heated titanium surface, furthering the precipitation of Mg(OH)2. Calcium ions precipitate calcium carbonate and calcium sulfate scale but these are less adherent and can be more easily removed. If the water hardness is more than 200 ppm, the scale is mainly composed of Mg(OH)2 even with 2% magnesium impurity. as the scale expands, plate-like crystals interlock, creating a tough layer.

Quantitative scale of formation versus water hardness

Controlled testing in synthetic brine (30% CaCl2, 2% MgCl2, 100°C) over 500 hours with varied total hardness (expressed as ppm CaCO3) has established the following. Scale thickness 10-30 µm Scale calcium carbonate with minimal Mg(OH) 2 Removal achievable by acid washing Heat transfer reduction 5-15% Hardness < 100 ppm At 100–200 ppm hardness scale thickness is 30–70 µm, scale is mixed carbonate and Mg(OH)₂, removal is by brushing plus acid, and heat transfer reduction is 15–30%. 200-300 ppm Critical threshold Scale thickness 60-120 µm Scale type Mg(OH)2 dominant, high adherence Removal Mechanical scraping Heat transfer reduction 25-45% At hardness of 300–500 ppm the scale thickness is 100–200 µm, it is hard magnesium hydroxide, it is very difficult to remove, reduction of heat transmission is 40–60 %. If the concentration is above 500 ppm, the scale thickness is more than 200 μm, severe scaling occurs, and the reduction in heat transfer is more than 60%.

Effect of Magnesium Concentration and Temperature on Hardness Tolerance

The maximum allowable water hardness decreases with increasing magnesium concentration. For 0.1 mm scale and 1% magnesium impurity, acceptable hardness is 300 ppm. The allowed hardness goes down to 200 ppm at 2% magnesium. At 3 % magnesium the allowable hardness decreases to 150 ppm. At 5% magnesium the allowed hardness is only 80ppm. Temperature also influences the threshold with permissible hardness at 2% magnesium being 350 ppm at 80°C. Allowed hardness at 100 ° C is 200 ppm. 120 ppm is OK hardness at 120 Deg C under pressure. The permissible hardness at 140°C is 80 ppm.

Control of Water Hardness in Magnesium-Contaminated Calcium Chloride Brine: A Guide

The following table shows the recommended maximum acceptable water hardness for 30% CaCl2 brine with 2% magnesium impurity at 100°C. The recommendations are based on the desired scale thickness and cleaning frequency.

Desired Scale Thickness After 500 Hours (µm) Maximum Allowable Water Hardness (ppm CaCO₃) Scale Composition Recommended Cleaning Method <30 (low scaling) <100 Calcium carbonate Acid cleaning (5% HCl) 30–70 (moderate) 100–200 Mixed carbonate/Mg(OH)₂ Acid + brushing 70–120 (high) 200–250 Mg(OH)₂ dominant Mechanical scraping >120 (severe) >250 Hard Mg(OH)₂ Not recommended Engineering Beyond Water Hardness Control

The titanium grade has little influence on the scale development; nevertheless, Grade 7 exhibits a superior resistance to under-scale corrosion. Wall thickness offers a heat transfer allowance; 2.0 mm wall with 0.3 mm scale has lessened but still usable heat transmission. Electropolished surfaces (Ra < 0.3 µm) minimise scale adherence and facilitate removal. Polyacrylates and phosphonates are effective scale inhibitors for calcium carbonate, but not very effective for magnesium hydroxide. The pH of the brine should be kept below 7.5, as higher pH speeds up the precipitation of Mg(OH)₂. Periodic acid cleaning by 5% hydrochloric acid at 50°C for 2 hours can dissolve calcium carbonate scale, but Mg(OH) 2 is not easily dissolved.

Making a well-informed specification

The overall hardness of water should be less than 100 ppm in 30 % calcium chloride brine at 100 °C with 2 % magnesium impurity to avoid preferential scaling of magnesium hydroxide for a titanium heat rod. Use softened water for brine make-up. Check hardness weekly by titration. If the hardness is above 200 ppm, precipitate the magnesium by raising the pH to 9.5 with lime and filter before returning to service. For existing heaters with Mg(OH)2 scale, use 10% citric acid at 70°C for 4 hours followed by mechanical brushing. Adherent magnesium hydroxide scale is not formed to reduce heat transfer and cause localized overheating by controlling the water hardness to less than 200 ppm.