Does lime softening remove magnesium?

[catalanotte]

I have been trying to reduce alkalinity with lime and dropped from about 360 ppm to 50 ppm as CaCO3. I also have high magnesium (52 ppm) and am hopeful that the precipitate contains some Mg. Brewersfriend water calculator assumes all of the precipitate is CaCO3 but I have read other articles that indicate magnesium hydroxide can also drop out, but magnesium carbonate is highly soluble and likely won't. Does anyone know how to predict the ratio of CaCO3 to Mg(OH)2 I should expect? Is there anything I can do (pH or temperature) to get more Mg to drop?

I started with 9 gal of water
KH as CaCO3 - 360 ppm
PH 7.5.
Ca - 80 ppm
Mg - 52 ppm

Added
13 g Lime
10 g CaCl
5 g Gypsum

Ended with
KH as CaCO3 - 50 ppm
PH 10.3
Precipitated out 28 g (oven dried)

I did some math on the reactions and don't believe that I lost enough carbonate to make up the full 28 grams so I am assuming it is coming from magnesium hydroxide. Any thoughts.

I could just blend with RO/DI but find this process interesting and want to engineer the water to make it work.

 

[mabrungard]

The capability to precipitate magnesium from solution is dependent upon the degree of temporary hardness. If its high enough, raising the water pH to above 11 should result in magnesium hydroxide precipitation. It looks more like a gel, whereas calcium carbonate looks like....chalk. Targeting an even higher pH does speed the reaction, but then there is more hydroxyl to neutralize. Following the reaction, the water can be combined with raw water or aerated to bring the pH down. In many cases, a minor acid addition is needed to further reduce the pH into a 'normal' range.

 

[ajdelange]

I don't even know how to predict the amount of CaCO3 that will precipitate other that to say you can nominally get down to about 1 mVal hardness and alkalinity with lime treatment. That's why it is important to analyze the post treatment water to see what you actually did.

In situations where there is high magnesium you can get some of it using 'split treatment' in which you calculate the amount of lime needed to treat the whole volume of water and add it all to half or 2/3 the volume you wish to treat. This runs the pH way up to the point where Mg(OH)2 precipitates and you decant off this. Now, while monitoring pH, you titrate back to pH 8.3 with the remainder of the water to be treated (the bicarbonate is the acid). Clearly the amount of magnesium precipitated is 1/2 to 2/3 of the amount contained in the total volume treated.