Magnesite
Magnesite is the carbonate of magnesium (MgCO3). The primary use of raw magnesite is in the calcination industry where it is calcined to form caustic magnesia (low calcined magnesite), dead burnt magnesite of fused magnesia. About 98% of raw magnesite is consumed in calcination industry. The other industries where raw magnesite is used are mosaic tiles, electrodes, chemicals and manufacture of magnesium metal. The dead burnt magnesite and fused magnesia are used in refractory industry to manufacture various refractory products. The caustic magnesia or low calcined magnesite is used as animal feed stuff and in the manufacture of exichloride cement etc.

The refractory industry is the most important consumer of magnesite. In India, about 98% of the total consumption is accounted for by the refractory industry. The refractory manufactures have indicated the following specifications of magnesite for use in the refractory industry.

Physical:

Specific gravity

Hardness

 

2.95 to 3.20

3 to 4.5 on Mhos scale.

Chemical:

MgO

SiO2

Fe2O3

Al2O3

CaO

L. O. I.

 

42.5 (min.)

2.5% (max.)

2% (max.)

2% (max.)

1% (max.)

47.5 to 49.5%

These are the general specifications. However, some manufactures allow silica up to 4% CaO content up to 1.5% and iron content up to 5%.

She sub-committee on refractory raw materials of the DGTD in their report dated August 1985 has examined the specifications of raw magnesite. It may be recalled that the former sub-committee of the DGTD panel had suggested two sets of specifications for raw magnesite and DBM obtained from Salem Magnesite and U. P. Magnesite. These are:

 

Salem Magnesite
 Raw                        DBM

U. P. Magnesite
Raw                       DBM

MgO

42.5% (min.)

85% (min.)

42.5% (min.)

85% (min.)

SiO2

3.25% (max.)

6.5% (max.)

--

--

CaO

--

3% (max.)

--

2.5% (max.)

R2O3

--

4% (max.)

--

5% (max.)

The present sub-committee in their report mentioned above has opined that it would be proper to prescribe specifications for all varieties of DBM utilized by the refractory maker. The committee is of the opinion that even though some of the better grade DBM thus prescribed is not possible to be made from Indian magnesite, presently, the production of beneficiated concentrates from the proposed plans is likely to make raw materials, of suitable grades available indigenously for the preparation of high purity DBM. Accordingly, the following seven grades of DBM have been prescribed:

 

Super

Grade-I

Grade-II

Grade-III

Grade-IV

Grade-V

Grade-VI

MgO

+99%

97-99%

94-97%

92-94%

92-94%

88-90%

85-88%

SiO2

0.5%

1% max.

1-2.5%

2.5-4%

4-5.5%

5.5-6%

06.58%

BD

3.4% min.

3.4%

3.35% min.

3.25%

3.15%

3.10%

3.05% min.

R2O3

0.05%

--

--

--

-

--

--

CaO (Salem)

--

--

--

--

1.5% max.

1.5% max.

1.5% max.

DBM of grade I, II & III can be obtained only from beneficiated magnesite, which is likely to materialize after the beneficiation plants of M/s Dalmia Magnesite Corporation and Tamil Nadu Magnesite Ltd., commence production. Grade-IV DBM is produced by M/s Dalmia Magnesite Corporation from Salem Magnesite after careful hand sorting. Grade-V DBM can be obtained from the magnesite of J & K and Salem. Grade-VI represents the bulk of Indian DBM produced from Salem and U. P.

 The BIS (IS: 3607 1979) has produced from the following specifications for magnesite for chemical industry.

S. No.

Characteristic         

Requirement

i

Loss on ignition, percent by mass min.

48.00

ii

Silica (as SiO2) percent by mass, max.

02.00

iii

Alumina (as Al2O3) percent by mass, max.       

00.30

iv

Iron Oxide (as Fe2O3) percent by mass, max.   

00.40

v

Magnesium (as MgO) percent by mass, min.

45.00

vi

Calcium (as CaO) percent by mass, max.

01.00

Future Trends: - Indian Magnesite is generally not high quality. The magnesite of Salem is characterized by high silica content and that of Utter Pradesh by high lime. Both silica and lime are deleterious constituents and render the mineral unsuitable for refractory purpose, if they exceed the stipulated limits. The problems of high silica and lime are vexing the refractory industry since long. Beneficiation of magnesite at economic cost to produce high-grade material seems to be the only answer and the future supplies may be of beneficiated concentrates of magnesite. It has already been mentioned above that two firms are setting up beneficiation plants. The other source of high purity magnesia will be the seawater. In India, M/s Periclase India Ltd., is setting up a sea water magnesia plant in Andhra Pradesh.

In the mineral inventory as on 01.01.1985 prepared by the Indian Bureau of Mines in collaboration with the Geological Survey of India and other exploration agencies, the end-use classification of reserves is as under:

a) High-grade:

MgO

SiO2

CaO

and CaO/SiO2

Directly useable for making high-grade DBM

42.5% (min.)

1.00% (max.)

1.5% (max.)

1 to 2 or below 0.5

b) Medium-grade:

MgO

SiO2

CaO

and CaO/SiO2

Directly useable for making ordinary DBM

42.5% (min.)

1 to 4%

1.5% (max.)

1 to 2 or below 0.5

c) Beneficiable / Low-grade:

MgO

SiO2

CaO

and CaO/SiO2

May be useable after dressing / beneficiation

(-) 42.5% (min.)

(+) 4%

(+) 1.5%

above 2 or below 0.5 and 1

The following end-use classification of reserves is prevalent:

a) High-grade:

MgO

SiO2

CaO

Directly useable for making high-grade DBM

42.5% (min.)

2.5% (max.)

1.5% (max.)

b) Medium-grade:

MgO

SiO2

CaO

Directly useable for making ordinary DBM

42.5% (min.)

2.5 to 4%

1.5% (max.)

c) Beneficiable / Low-grade:

(I) MgO

SiO2

(II) MgO

MgO

 

42.5% to 38% may be possible to

4 to 8% reduce SiO2 by simple dressing as sorting.

Less than 38% beneficiation by

More than 10% which or other process.

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