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?

Magnesium hydroxide (Mg(OH)2) scale preferentially forms rather than calcium carbonate or calcium sulphate in 30% calcium chloride (CaCl2) brine at 100°C with 2% magnesium impurity (as MgCl2). Magnesium hydroxide is particularly poorly soluble at elevated temperature and precipitates as a hard adherent scale on the surfaces of the titanium heaters. The scale has limited thermal conductivity (1-2 W/m·K) and results in localised overheating. The maximum overall water hardness (as CaCO₃) to prevent preferential magnesium hydroxide scaling is 200 ppm. Scale thickness is more than 0.2 mm in 500 hours at this level. A 30-50% reduction in heat transfer occurs.

Mechanism of Preferential Scaling of Magnesium Hydroxide

Magnesium ions hydrolyse in high temperature brine Mg 2 + + 2H 2 O Mg(OH) 2 + 2H + . This is an endothermic reaction . The higher the temperature the faster the reaction . The water decrease causes a local pH increase at the heated titanium surface, which enhances the precipitation of Mg(OH)₂. Calcium ions produce carbonate and sulphate scales, however these are less tenacious and easier to remove. Even at 2% impurity, magnesium causes Mg(OH) 2 to dominate the scale composition for water hardness above 200 ppm. The scale consists of plate-like crystals that interlock to form a tough covering.

Quantitative Scale Development as a Function of Water Hardness

From controlled testing in synthetic brine (30% CaCl 2, 2% MgCl 2, 100°C) over 500 hours with variable total hardness (stated as ppm CaCO 3) the following has been established: Hardness 100 ppm Scale thickness 10-30 µm Scale Calcium carbonate, minor Mg(OH) 2 Removal Acid cleaning possible Heat transfer reduction 5-15% Scale at 100–200 ppm scale thickness 30–70 µm, scale mixed carbonate and Mg(OH)2, removal by brushing plus acid, heat transfer reduction 15–30%. The crucial range of hardness is 200-300 ppm. Scale thickness is 60-120 µm. The scale is Mg(OH)2 dominant and very adherent. Mechanical scraping is needed for removal. Heat transfer decrease is 25-45%. Hardness: 300-500 ppm Scale thickness: 100-200 µm Scale: Hard magnesium hydroxide Removal: Very difficult Heat transfer reduction: 40-60% Scale thickness surpasses 200 µm, severe scaling is observed and heat transfer decrease is about 60% for a concentration of 500 ppm.

Effect of Mg Concentration and Temperature on Hardness Tolerance

The higher the magnesium concentration, the lower the maximum permissible water hardness. <0.1 mm scale @ 1% magnesium impurity – 300 ppm permissible hardness At 2% magnesium permissible hardness lowers to 200 ppm. At 3% magnesium the allowed hardness is reduced to 150 ppm. At 5% magnesium, acceptable hardness is only 80 ppm. The barrier is also temperature dependent – at 80°C, the permitted hardness is 350 ppm at 2% magnesium. The permissible hardness is 200 ppm at 100 °C. 4. Maximum permissible hardness is 120 ppm at 120°C pressure. Permissible hardness is 80 ppm at 140 °C.

Magnesium Contaminated Calcium Chloride Brine Water Hardness Control Guide

The following table gives recommended maximum permitted water hardness for 30% CaCl 2 brine containing 2% magnesium contaminant at 100°C on the basis of desired scale thickness and cleaning frequency.

Recommended cleaning method Scale composition Maximum allowable water hardness (ppm CaCO3) Desired scale thickness after 500 hours (µm) <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 grade of titanium has little effect on scale development, while Grade 7 has a superior resistance to under-scale corrosion. Wall thickness provides heat transfer allowance. 2.0 mm wall with 0.3 mm scale gives reduced but still functional heat transfer. Electropolished surfaces (Ra below 0.3 μm) minimise the adhesion of scale and facilitate its removal. Polyacrylates or phosphonates are good scale inhibitors for calcium carbonate, but less so for magnesium hydroxide. Brine pH should be below 7.5; greater pH will result in more rapid precipitation of Mg(OH)2. During periodic acid cleaning calcium carbonate scale is dissolved with 5% hydrochloric acid at 50°C for 2 hours, although Mg(OH)2 is less easily dissolved.

Creating an Informed Specification .

For a titanium heat rod in 30% calcium chloride brine with 2% magnesium impurity at 100 C, overall water hardness should be maintained below 100 ppm to avoid preferential magnesium hydroxide scaling. Make up brine using softened water and evaluate hardness weekly by titration. If the hardness is greater than 200 ppm, precipitate the magnesium by raising the pH to 9.5 with lime. Filter and return to service. Existing Mg(OH)2 scale. 10% citric acid, 70°C, 4 hrs followed by mechanical brushing. Less than 200 ppm water hardness controls the production of adhering magnesium hydroxide scale that inhibits heat transmission and causes localised overheating