Magnesium hydroxide, Mg(OH)₂, is made by hydrating caustic calcined magnesia or from seawater and brines. It is widely used to neutralise acidic wastewater as an alternative to caustic soda and lime.

Advantages

  • Built-in buffering: thanks to its low solubility it usually limits pH to around 9, without the sharp spikes seen with caustic soda when overdosed.
  • Higher neutralising capacity: per kilogram it neutralises about 37% more acid than caustic soda.
  • Safer handling: it is not a strong caustic like sodium hydroxide, so it is easier to transport and store.
  • It helps precipitate heavy metals as hydroxides and recover phosphate as struvite.

Limitations

Low solubility is a limitation as well as an advantage: the reaction is slower and needs enough contact time. Because it is used as a slurry, mixing and dosing equipment that prevents settling also matters.

Why the raw material matters

The performance of magnesium hydroxide depends on the purity and reactivity of the magnesia it is made from, and that starts with clean raw magnesite fired at the right temperature. We cover other uses of caustic calcined magnesite in our article on sintered and caustic calcined magnesite.