Occurrence and Uses
Magnesium carbonate occurs in nature in several minerals as hydrated, basic and double salts, as shown above. The two principal minerals are magnesite, MgCO3 and dolomite, a double salt, CaCO3•MgCO3. Both minerals are used as source materials in the production of magnesium metal. Also, they are calcined to produce basic refractory bricks. Other applications of magnesium carbonate are in flooring, fireproofing and fire-extinguishing compositions; as a filler material and smoke suppressant in plastics; as a reinforcing agent in neoprene rubber; as a drying agent and for color retention in foods; in cosmetics; in dusting powder; and in toothpaste. The high purity magnesium carbonate is used as an antacid in medicine; and as an additive to table salt. Another important application of magnesium carbonate is as a starting material in producing a number of magnesium compounds.
Magnesium carbonate is obtained mainly by mining its natural mineral magnesite. The trihydrate salt, MgCO3•3H2O, is prepared by mixing solutions of magnesium and carbonate ions in the presence of carbon dioxide. Alternatively, it may be produced by carbonation of a magnesium hydroxide slurry with carbon dioxide under pressure (3.5 to 5 atm) and at a temperature below 50°C which yields soluble magnesium bicarbonate:
Mg(OH)2 + 2CO2 → Mg(HCO3)2
The solution is filtered to remove impurities and the filtrate is subjected to vacuum or aeration to yield insoluble magnesium carbonate as a hydrated salt:
Mg2 + 2HCO3¯ → MgCO3 + CO2 + H2O
Under ordinary conditions, anhydrous magnesium carbonate cannot be prepared in aqueous systems. The anhydrous salt, however, can be made under very high partial pressures of carbon dioxide.
Basic magnesium carbonate occurs in nature as the mineral hydromagnesite. The basic salt is obtained by mining the ore followed by purification. The basic carbonates also can be made by drying the magnesium carbonate trihydrate at about 100°C. Alternatively it can be prepared by simply boiling a solution of magnesium bicarbonate. The bicarbonate is obtained by carbonation of a magnesium hydroxide slurry below 50°C and under a CO2 partial pressure of 3.5 to 5 atm. Composition of the basic carbonate produced by the above methods is 4MgCO3 •Mg(OH)2•4H2O.
Another basic salt, MgCO3•Mg(OH)3•3H2O is precipitated when magnesium salt solution is treated with sodium carbonate solution. The reactions probably are:
CO32– + H2O → HCO3¯ + OH¯
2Mg2+ + CO32– + 2OH¯ → MgCO3•Mg(OH)2
Magnesium carbonate, MgC03, is a light, bulky white powder, very slightly soluble in water,loses CO2 even on gentle heating. It is used extensively in the manufacture of food products, pharmaceuticals, cosmetics, and magnesium salts.