Preparation, properties and uses of magnesia

Preparation, properties and uses of magnesia

Magnesium oxide (magnesia, MgO) is a white powder broadly similar to calcium oxide (lime, CaO) and is rarely found in nature as such but more commonly as the carbonate form including the less common mineral complex with calcium carbonate (carnallite).
Magnesium oxide is traded commercially as light burned magnesia, dead-burned magnesia, calcined and electrofused forms (used as refractory lining). The amount of heating reduces the reactivity of magnesium oxide (including with carbon dioxide on storage) reflecting its surface area to mass (and density).

Uses

Magnesium oxide is the principal input for the magnesium compounds industry, refractories and for insulating elements in electric furnaces. In the USA, some 36 per cent is for animal feeds and fertilisers, 19 per cent for chemical processing, 18 per cent for metallurgical purposes (refractories, electrical, water treatment, gas scrubbing, 17 per cent of manufacturing aids. In Australia important uses are in the production of nickel and HBI DRI by BHP.

Process of manufacture

There are many processes for producing magnesium oxide all start with magnesium chloride from seawater, sometimes the mineral carnallite or a magnesium carbonate mineral. Most involve the thermal (not electric) decomposition of magnesium hydroxide in a rotary or shaft kiln.
One process reacts calcium oxide (lime) with magnesium chloride, such as from magnesium rich (bitterns) in water, to produce magnesium hydroxide and calcium chloride as waste. The magnesium hydroxide is readily converted to the oxide by heating.
MgCl₂ + CaO > Mg(OH)₂ and CaC_l2
About 1.4 tonnes of lime is required for each tonne of MgO produced.
The magnesium hydroxide is insoluble and is separated from the calcium chloride and sodium chloride under controlled conditions. The magnesium hydroxide is then heated in a furnace (shaft or rotary kiln similar to that used for lime production) to produce the desired form of magnesium oxide.
Mg(OH)₂ > MgO +H₂O
This process is also commonly, but not exclusively, used when producing magnesium metal based on using magnesium chloride.
Another process begins with the burning (calcination) of magnesite (a magnesium carbonate) which is more direct but often produces a lower grade of the oxide (90 to 98 per cent) though Qmag, with an usually high quality magnesite can produce a high purity form (see later).
Dead Seas Periclase in Israel uses a concentrated magnesium rich brine from the Dead Sea with is sprayed into a reactor at about 1700ºC to decompose the magnesium chloride to the oxide producing hydrochloric acid as a by-product. The oxide is converted to the hydroxide which is washed to purify and then calcined to yield the various forms of the oxide. A 99 per cent purity form is produced.
Worldwide about two-thirds of magnesium oxide (some 11 million tonnes per year) produced is from sea water (co-produced with salt production) and the rest from natural mineral deposits magnesite (magnesium carbonate) dolomite (calcium magnesium carbonate) and salt domes.
It is relevant to note that the production of magnesium metal commonly, but not exclusively, involves magnesium hydroxide as an intermediate (noting this is readily converted to the oxide). This could suggest the production of magnesium oxide from a chloride source is suited for integration with a magnesium metal producer as a side product.
The common process for production of magnesium reacts a solution of magnesium chloride (as found in sea water and bitterns) with lime (calcium oxide) to produce magnesium hydroxide and calcium chloride which is generally flushed to sea. Magnesium hydroxide is simply heated to drive off the water leaving magnesium oxide. The magnesium oxide is then chlorinated by reaction with hydrochloric acid (while the chlorine later recovered and converted to hydrochloric acid. (A Russian process for magnesium metal uses magnesium chloride in an impure form), while there are other technologies as well (including silicothermic and carbothermic processes). See end of report. These should be evaluated with the production of magnesium oxide.