Feldspar (Felspar, UK spelling)
Feldspar is the most abundant silicate with Potassium, sodium, and calcium or mixtures of these elements. The common feldspar is potassium feldspar, namely orthoclase K2O, Al2O36 SiO2, sodium feldspar is albite Na2O, Al2O36 SiO2, and calcium feldspar is anorthite CaO, Al2O36 SiO2, They occur in various colors but the pink brown and grey ones are common. 

Feldspar is mainly utilized in the manufacture of glass, pottery, ceramics, vitrified enamels, special electrical porcelain, wind plates and opalescent glass as well as glass ware In the ceramic industry feldspar is used both in the body and an a glaze for china wares, It is also used an a binding agent in the manufacture of abrasives.

There is no BIS specification for feldspar specifications of feldspar in various industries as prescribed by them is as below:  

Abrasives: -

 Feldspar in used in the form of powder as also lumps which have SiO2 - 60% min., Al2O3 18% min., and K2O / Na2O - 10% minimum. While CaO, Fe2>O3should be low 0.6% and 0.45% respectively, LOI should not be more than 2%.

 Ceramics: -

 It is mainly used as a fluxing agent and white wares contain good proportion of feldspar. There are two grades is used, Grade - I should have SiO2 - 68% maximum, Fe2O3 should be below 0.1% while ratio of K/Na should be 2:1. In Grade - II SiO2 is 72% maximum and Fe2O3 is same as that of Grade-I i.e. 0.1% max CaO should be in traces while MgO should not be present.

Glass Industry:

In glass industry potash feldspar is preferred as it introduces potash for clarity and alumina for durability and also controlling viscosity of molten glass. It should contain 10 to 13% of Na2O and K2O combined, 15 to 18% of Al2O3, 65 to 68% of SiO2 and Fe2O3 below 0.3%, while the size should be 30 to 80mesh.

Refractory:

 As already mentioned earlier feldspar in added as one of the batch constituent for refractory. Here also 11 to 12% of combined Na2O and K2O are preferred while SiO2 should be within 60 and 65%, A12O3 should be 18 to 19%, Fe2O3 should be below 1.5%, PCE = 4 orton cone and LOI should be below 0.8%. Size preferred is 2.5 10 cm.

Electrodes:

Feldspar finds use in electrode manufacture it acts as a flux and forms slag, which protects molten metal from oxidation. For this, Potash feldspar having 13 to 14% of K2O, 1 to 3% of Na2O, 63 to 65% SiO2, 17 to 20% of Al 2O3 and Fe2O3 up to 0.3% is preferred. LOI should be maximum up to 1%. Size 0.2 to 0.06.

 For the inventory of feldspar as on 01.01.1985 some data on actual uses in industries like glass, ceramic, pottery and abrasive has been reported by lessees from there lease holds and accordingly the reserves in these areas have been classified. For rest of the reserves in other areas where there is no information available the reserves have been classified as "unclassified".

The specification for ceramic and glass grades are given as below:

i

Ceramic:
SiO2

Fe2O3

K/Na
68% (min.)
0.1% (max.)
2:1

ii

Glass (Potash feldspar): SiO2
Fe2O3

K/Na

65% (min.)

below 0.3% (max.)
15% (min.)
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