Role of magnesium ions/salts in boiler scale generation?

Water analysis, treatment, and mineral recipes for optimum taste and equipment health.
Pressino

#1: Post by Pressino »

homeburrero wrote:* Limescale, the usual scale problem, is calcium carbonate. It, and similar magnesium precipitates are nicely removed by descaling products. But since your water has zero calcium and zero magnesium neither of those can build up. Another scale-like concern, very hard to descale, would be calcium sulfate. No worries there because R Pavlis has neither calcium nor sulfate. The worst scale-like deposit would be silica, but without silica in the water you won't get a problem there (and descaling is ineffective against silica anyway.)
I'm curious about the role of magnesium ions/salts in the generation of boiler scale in espresso machines. I know that most of the common scale consists of calcium compounds. The reason I'm curious about the role of magnesium is that BWT specifically mentions that some of their filters designed for espresso machines substitute magnesium for calcium supposedly to benefit taste but also claim that this prevents scale buildup. WLL has some video presentations that make the same point. Any comments on BWT/WLL's position on this matter? :?



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homeburrero
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#2: Post by homeburrero »

Pressino wrote:The reason I'm curious about the role of magnesium is that BWT specifically mentions that some of their filters designed for espresso machines substitute magnesium for calcium supposedly to benefit taste but also claim that this prevents scale buildup. WLL has some video presentations that make the same point. Any comments on BWT/WLL's position on this matter?
Magnesium carbonate is similar to calcium carbonate in that it dissolves in water as the bicarbonate, and when the water is heated and CO2 is driven out of solution the bicarbonate shifts to less soluble carbonate. But magnesium carbonate is more soluble than calcium carbonate by a factor of around 10, and far less prone to produce scale. At high concentrations and high pH I think it could precipitate, more likely as magnesium hydroxide or some mix of carbonate and hydroxide perhaps along with calcium carbonate.

Unfortunately there's no easy formula like the Langelier Saturation Index (which is just for calcium carbonate) for calculating scale from magnesium carbonates. It's complicated -- I've seen technical reports that a little magnesium ion in the water may even suppress the formation of limescale. Based on user reports on this site it appears that people using Matt Perger's Epsom salt + bicarbonate recipes with a magnesium hardness as high as 100 mg/L CaCO3 equivalent and bicarbonate at 50 mg/L CaCO3 equivalent are not seeing scale.

There's no question in my mind that replacing calcium with magnesium, as done in some BWT decarbonizing filters, would tend to reduce scale risk. Whether or not it's worthwhile for taste reasons is debatable.

As to the suitability of the Perger and similar Epsom salt recipes I agree that they would be far less scale prone than water with a similar level of calcium hardness. I think the issue with using Epsom salts for espresso machine use would be the downside of the sulfate. As long as the recipe has good alkalinity and the sulfate is not really high that is probably not a serious concern.


P.S.
If you were to use an Epsom + potassium bicarb recipe with no more than 130 mg/L Epsom and no less than 100 mg/L potassium bicarbonate, then you would have less than 51 mg/L sulfate (as ion), less than 53 mg/L magnesium hardness (CaCO3 equivalent), and more than 50 mg/L alkalinity (CaCO3 equivalent) , which seems prudent to me with respect to corrosion and should not produce scale.
Pat
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Pressino (original poster)

#3: Post by Pressino (original poster) »

Thanks for the explanation!