Magnesium is usually present in water or brine as bicarbonate, sulfate, or chloride. It may also be produced in water solutions, when dolomitic lime is used to neutralize waste acid. With the exception of magnesium hydroxide, magnesium compounds are very soluble. The solubility of magnesium hydroxide is about 8 mg/l at ambient water temperatures. However, when precipitated without an excess of hydrogen ion, solubility, including supersaturated mangesium hydroxide, rises to about 20 mg/l.
If precipitation is carried out in the presence of a high concentration—up to 5% by weight of previously precipitated hydroxide—supersaturation is reduced. Magnesium hydroxide usually precipitates as a flocculant material, which settles slowly and will only concentrate to about 1% by weight. However, when precipitated in the pres ence of previously precipitated solids, the settling rate and density of the settled sludge increase considerably.
Magnesium is not considered a contaminant in waste-water unless it is present in a brine (saltwater). However, concentrations in excess of 125 mg/l can exert a cathartic and diuretic effect. In addition, magnesium salts break down on heating to form boiler scale.
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