Why is magnesium removed first and then calcium in the brine purification process for chlor-alkali production?

In the chlor-alkali process, the purpose of brine purification is to remove impurities, particularly hardness-forming ions such as calcium (Ca2+) and magnesium (Mg2+). These impurities can have a severe impact on the performance of the electrolyzer, causing issues such as scaling on the anode and cathode as well as membrane contamination, which in turn reduces electrolysis efficiency and product quality. There are several reasons why magnesium is usually removed before calcium in the purification of brine: 1. Difference in solubility: Under normal processing conditions, compounds of magnesium (such as magnesium hydroxide Mg(OH)2) have lower solubility than compounds of calcium (such as calcium hydroxide Ca(OH)2), making it easier to precipitate and remove them first. 2. Selectivity of precipitation reaction: Magnesium precipitates more easily than calcium under alkaline conditions. When a base (usually sodium hydroxide, NaOH) is added to saltwater, magnesium hydroxide precipitates at lower pH values, while calcium hydroxide requires a higher pH value. 3. Process efficiency: Precipitating magnesium first can reduce interference with the subsequent calcium removal process, as the precipitation of calcium salts usually requires stricter control conditions (such as a higher pH); removing calcium first might lower the efficiency of magnesium removal. 4. Stability under environmental conditions: Calcium hydroxide precipitates at higher pH values, while magnesium hydroxide precipitates at lower pH values; this order of treatment helps to avoid problems related to unstable precipitation and redissolution. Therefore, sodium hydroxide or other alkaline chemicals are usually used first to convert the magnesium ions dissolved in saline into magnesium hydroxide precipitate and remove them, after which the pH value is gradually increased to precipitate and remove the calcium ions. Such a sequence can efficiently remove calcium and magnesium ions, ensuring the smooth progress of the electrolysis process. .

Magnesium is removed first, followed by calcium; this is the sequence for refining brine in soda ash plants； In industry, lime milk is used to remove magnesium; the solubility product of magnesium hydroxide is 1.8×10^-11, while that of calcium hydroxide is 5.5×10^-6. When magnesium hydroxide is removed, calcium ions are not removed as well. After sedimentation, carbon dioxide is introduced into the solution to remove calcium ； In the laboratory, you can first remove magnesium using calcium hydroxide, and then remove calcium using calcium carbonate ； In chemical experiments, since the allowable error range is ±0.2%, a certain ion in the solution is considered to have been completely removed when its concentration is below 1×10∧-6.

The preprocessor we use is a floating clarification tank, along with a sodium carbonate solution for magnesium and calcium removal. Your explanation doesn’t quite make sense.