Magnesite is the carbonate of magnesium, MgCO3, theoretically containing 47.6% MgO
and 52.4% CO2. In nature, magnesite often transgresses into siderite (FeCO3), and an
intermediate mineral called breunnerite or brown spar containing up to 30% FeCO3 is
generally included in commercial magnesite. Breunnerite is mined in Austria, but FeCO3 is a
common impurity in many other magnesite deposits. Magnesite occurs in nature as secondary
deposits formed due to: (i) alteration of serpentines and dunites, (ii) replacement of dolomite
beds by magnesium-bearing solution associated with intrusive rocks, and (iii) vein fillings. In
the first type, the magnesite is crypto-crystalline, in the second type it is coarse crystalline and
layered (spathic), and in the third type it occurs as crystals. In India, the famous deposits
belong to the first and second types. The deposits of Chalk Hills of Salem area, Tamil Nadu
and of Dodhkanya of Karnataka are alteration products of dunite occurring as networks of
veins while those of Almora-Pithoragarh area, Uttarakhand were formed by replacement of
Raw magnesite is used directly for making some of the end products like chemicals and
synthetic rubber. In one use, namely fused magnesite refractory, it is used in fused form. But
in most of the end-uses, magnesite is used in one of the two main intermediate products
namely, caustic magnesia and dead-burnt magnesite or DBM. In all these three intermediate
products, magnesite is the raw material, and its specifications are discussed as follows.
- Caustic magnesia: Caustic Magnesia (low-calcined magnesite) is obtained by
calcining raw magnesite in a shaft or rotary kiln between 800-10000C. However, after
manufacturing, the caustic magnesia is generally not subjected to high temperatures during its
Caustic magnesia has a number of industrial uses, and consequently, various grades of
caustic magnesia with varying porosity (indicated by specific gravity) are manufactured to
suit the needs of the consuming industries by varying the temperature of calcination and also
the quality of raw magnesite. However, the minimum grade of magnesite specified by the
industries is: 40% (min.) MgO, 50% (max.) LOI (includes mainly CO2), 3.5% (max.) SiO2,
0.5% (max.) Fe2O3, 0.10% (max.) Al2O3, 3% (max.) CaO and 2.92% specific gravity 2.92.
At high temperatures, silica reacts with CaO to form beta di-calcium silicate which may
undergo rapid inversion to gamma form with considerable expansion resulting in crumbling
of the magnesite. If, in addition, alumina is also present, then low-melting calcium aluminosilicate
is formed. Hence both silica and alumina are objectionable in both DBM and fused
Fe2O3 combines with MgO to form magnesium ferrite (2MgO.Fe2O3) which cements the
pores thus lowering the porosity of the caustic magnesia.
Lower the specific gravity in the magnesite is, more it is likely to be porous and more
will be porosity and reactivity of the resultant caustic magnesia.
- DBM: Unlike caustic magnesia, DBM is used only in one class of products namely,
refractory. DBM as a refractory is subjected to not only high temperature of over 15500C, but
also repeated heating and cooling (thermal shocks).
Specifications of DBM: Quartz changes to beta-quartz at 5740C, then to beta-tridymite
(8700C) and finally to cristobalite (14700C), these changes being accompanied by volume
expansion. Each of these forms have their low-temperature forms, and on cooling, changes to
the low-temperature forms take place again accompanied by further volume changes. Thus
repeated heating and cooling of the refractories containing silica result in cracks. Moreover, at
the temperatures to which the DBM-based refractories are subjected, silica reacts with CaO to
form beta di-calcium silicate which may undergo rapid inversion to gamma form with
considerable expansion and thus reduce the DBM to dust. If, in addition, alumina is also
present, then low-melting calcium alumino-silicate is formed below 11000C temperature
itself. Besides, alumina forms silicates (kyanite, andalusite or sillimanite) with increase in
specific gravity (from 2.7 of silica to 3.2-3.7 of the silicates) and then mullite at temperatures
1200-16000C, with disturbance of the eutectics and resultant deformation of texture and
weakening of the refractory. Hence both silica and alumina are objectionable in DBM.
Fe2O3 is generally considered deleterious because it melts at a relatively lower
temperature of 11000C. But sometimes, a little of it is desirable in DBM refractories where it
is crushed and mould-pressed with the help of some binder. Fe2O3 combines with MgO to
form magnesium ferrite (2MgO.Fe2O3) which is a binding agent.
Keeping these factors in view, a set of specifications for DBM is stipulated. Commercial
DBM has been classified into 7 grades depending on the grade of the raw magnesite. The
highest grade DBM (super-grade) contains over 99% MgO. But the lowest quality accepted
by the industries contains MgO 85% (min), SiO2 6.5 % (max); Fe2O3 + Al2O3 5% (max); CaO
up to 2.5 per cent. By and large this specification is in conformity with that stipulated by the
Bureau of Indian Standards (BIS) in 1972 and 1977.
Specifications of magnesite for making DBM: For achieving the above grade in the DBM,
raw magnesite has to have some specific values for the chemical constituents. Moreover, for
making DBM, raw magnesite is sintered at a high temperature of 17000C. Therefore, the same
constituents namely, SiO2, Al2O3, Fe2O3 and CaO are considered deleterious for the same
reasons applicable to DBM. The Indian industries specify quality of magnesite for DBM as:
MgO 42.5% (min), up to SiO2 4 % (but most preferably up to 2.5%); Fe2O3 up to 5% (but
most preferably up to 2%); Al2O3 up to 2%; CaO up to 1.5% (but most preferably up to
- Fused magnesia: Fused magnesia is very costly and used in very special cases. The
industries specify practically pure magnesite containing above 99% MgO and only nil to trace
impurities. In this case, no binder is required and hence there is no need of even a minimum
amount of Fe2O3 unlike in the case of DBM.