Martin Marietta Production of Magnesium Hydroxide and Magnesium Oxide
In a synthetic magnesia process, naturally occurring brine is mixed with both dolime and water to produce an aqueous suspension containing magnesium hydroxide and calcium chloride:
CaCl2 + MgCl2 + H2O + (CaO•MgO) + 2H2O —> 2Mg(OH)2 + 2CaCl2 + H2O
CaCl2 + MgCl2 + H2O = brine
(CaO•MgO) = dolime
2H2O = water
2Mg(OH)2 = magnesium hydroxide
2CaCl2 = calcium chloride
H2O = water
The magnesium hydroxide and calcium chloride produced from this reaction exist together but in two distinct physical states: magnesium hydroxide is formed as solid particles while the calcium chloride is dissolved in the aqueous phase. An aqueous suspension containing solid particles is also referred to as a “slurry”.
Gravity is used to separate the solids from the liquid in the aqueous suspension since magnesium hydroxide is heavier than water. “Milk of magnesia” is another name for magnesium hydroxide. In the photo at right, the freshly made magnesium hydroxide slurry (white) is pumped over to another holding tank and allowed to settle.
The blue color indicates the watery layer containing dissolved magnesium chloride that has formed above the settled magnesium hydroxide particles.
The bottom solids are then filtered through a drum filter to remove any remaining water and given a series of water washes to remove chlorides from the material (see photo at right).
The washed filter cake is finally directed to a source of heat, such as a rotary kiln shown in the photo to the right, where it is thermally decomposed (calcined) to produce magnesium oxide:
2Mg(OH)2 + heat –> 2MgO + 2H2O (steam)
Several types of kilns can be used in the calcination step. Calcination not only converts magnesium hydroxide to magnesium oxide, but is also the most important step for determining how the final product will be used.
Next: Magnesia Grades