GYPSUM
The chemical composition of gypsum is CaSO4. 2H2O it has three varieties, namely, selenite, alabaster and satinspar. Selenite is transparent and crystalline; alabaster is massive and satinspar is fibrous and silky. The principal uses of gypsum are in the manufacture of surgical plaster, ammonium sulfate fertilizer, pottery and cement soil reclamation and as an extender in paint. Besides, it is also used in chemical, cosmetics and textile industries. In India the use of gypsum for making ammonium sulfate fertilizer has been dispensed with since 1977 and it has been replaced by - product gypsum obtained during the course of production of phosphoric acid.

The principal uses of gypsum are discussed below: -

(A) Surgical Plaster: - A very high degree of purity is required for this use. Free water, CO2, SiO2 and other insoluble matter, iron and aluminum oxides, MgO and NaCl are considered as the deleterious constituents. The BIS has standardized the specifications for use of gypsum in this industry (IS: 1290 - 19), which is as follows: -

1.

Free water

1.0% (max.)

2.

CO2    

1.0% (max.)

3.

SiO2 and other insoluble matter

0.70% (max.)

4.

Iron and aluminium oxides

0.1% (max.)

5.

MgO   

0.5% (max)

6.

CaSO4. 2H2O

96.00% (min.)

7.

NaCl               

0.01% (max.)

 (B) Pottery: - The objectionable impurity are free water, CO2 Sio2 and other insoluble matter, iron and aluminum oxide, MgO BIS has standardized the specification (IS: 1290 - 1973) as follows: -

1.

Free water

1.0% (max.)

2.

CO2    

3.0% (max.)

3.

SiO2 and other insoluble matter

6.0% (max.)

4.

Iron and aluminium oxides

1.0% (max.)

5.

MgO   

1.5% (max)

6.

CaSO4. 2H2O

85.00% (min.)

7.

NaCl               

0.10% (max.)

(C) Ammonium sulfate fertilizer: The deleterious constituents are SiO2 and other matter, iron aluminium oxides, MgO and NaCl. The BIS has standardized the specification (IS: 1290 - 1973) as follows:

1.

SiO2 and other insoluble matter

6.0% (max.)

2.

Iron and aluminium oxides

1.50% (max.)

3.

MgO   

1.0% (max)

4.

CaSO4. 2H2O

85.00% (min.)

5.

NaCl               

0.603% (max.)

(D) Cement: The deleterious constituents are MgO and NaCl. The BIS has standardizes the specification (IS: 1920 - 1973) for use of GYPSUM in the manufacture of export quality cement, the former requiring a higher content of CaSO4.2H2O than the latter. The specification is as follows:

1.

MgO   

3.0% (max)

2.

CaSO4. 2H2O

70-75% (80-85 for export quality cement)

3.

NaCl               

0.603% (max.)

(E) Soil Reclamation: Fine size and a low content of Na2O are the deciding criteria. The BIS has standardized the specification (IS: 6046 - 1971) as follows: -

1.

CaSO4. 2H2O

50% (min.)

2.

Na2O  

Fieriness: Residue on micron sieve nil and on 150 micron sieve50% (max) 0.603% (max.)

(F) Extender in Paints: In this case, besides free water and CaSO4. 2H2O; some physical properties like size, from etc., are important. Lead is considered as highly objectionable constituent. The BIS has standardized the specification (IS: 69 - 1950) which is as follows:

1.

Free water

0.5% (max.) when heated 2 hrs. at 113OC

2.

CaSO4.2H2O

75% (min.)

3.

Fineness

Residue on 240 mesh B. S. test sieve 0.5%

4.

Oil absorption

With in 55 of approved sample.

5.

Color  

Close match to approved sample.

6.

Lead and its

0.5% (max.) when lead free gypsum compounds(Calculated is required as metallic lead)

7.

Physical form   

In the form of dry powder.

8.

Microscopic form

Material should consist of entirely the characteristic Crystals of gypsum.     

 (G) Chemical: There is no BIS specification. Gypsum containing at least 94% CaSO4. 2H2O and a maximum of 3.6% NaCl is generally used.

(H) Cosmetics: There is no BIS specification. Selenite variety of gypsum containing 28.75% CaO, 46.5% SO#, up to 3% SiO2 up to 0.25% Fe2O3 and up to 18% moisture is generally preferred.

(I) Textile: There is no BIS specification. Gypsum containing 82% CaO 2H2O and up to 6% moisture is generally preferred.

Date on all the deleterious constituents were lacking in the inventory as on 1.1.85. In all uses except surgical plaster, only key constituents CaSO4 2H2O has been considered as per the BIS specification. For classifying the reserves into surgical plaster grade of gypsum either the reserves specifically identified as of this grade has been considered or in cases where date on chemical analysis were variable some selected key constituents like CaSO4 - 2H2O, NaCl, MgO and free water were considered. The basis of classification of reserves is as follows:

1. Surgical Plaster
Either the reserve containing CaSO4. 2H2O 96% (min) NaCl 0.01% (max.) MgO 0.5% CO2
1.0% (max.) Free H2O 1.0% (max.) of the reserve otherwise identified as of this grade.
2. Fertilizer / Pottery 85 to 96% CaSO4. 2H2O
3. Cement / Paint              70 to 85% CaSO4. 2H2O
4. Soil reclamation 50 to 70% CaSO4. 2H2O

 

After examining the specification standardized by BIS and the current practice classification of reserves estimation for purpose of reserve estimation keeping in view the broad limitations of the reserves.

1.

Surgical Plaster grade

CaSO4. 2H2O 96% (min) MgO 0.5% (max.)
NaCl 0.01% (max.)

2.

Fertilizer grade

CaSO4. 2H2O 85% (min) MgO 1.0% (max.)
NaCl 0.003% (max.) Fe2O3 + Al2O3 1.5% (max.)

3.

Cement                       

CaSO4. 2H2O 85% (min) MgO 0.5%
NaCl 0.003% (max.)

4.

Soil reclamation grade

CaSO4. 2H2O 50% (min) Na2O up to 1%

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