Use of Kyanite as Refractory Raw Material

21-July-2020

Kyanite is one of the Alumino-Silicate group of minerals comprising Sillimanite, Kyanite, Andalusite, Dulmortierite, Topaz and Mullite all with similar chemical composition but different physical behavior and hence, uses. Kyanite, raw as well as calcined, have separate applications as refractory raw materials because of their distinct characters. 

Raw Kyanite

Raw Kyanite (Refractory Grade)

To be suitable for refractory raw material, kyanite should have very negligible amount of impurities which include free silica, alkali, iron oxides, calcium and magnesium. Kyanite, Sillimanite and Andalusite all these three minerals convert to Mullite and silica when they are calcined to temperatures between 1250^OC and 1500^OC. The inversion kyanite to mullite and silica (glass) begins at the periphery of grains and this rate of conversion depends the following:

>> Particle size

>> Firing temperature

>> Soaking time

>> Impurities present.

The heating of raw kyanite is accompanied with its volume expansion and a decrease in its specific gravity from 3.6 to 3.06, which takes place over a small range of temperature around up to 1350^OC. 
Because of this property raw kyanite - 

• Raw kyanite is extensively used for making high alumina insulation bricks, insulating mortars and castables.
• Raw kyanite fines (pulverized) can be added to refractory mixes in different proportions as a measure to control or compensate the overall shrinkage due to other raw materials. 

Calcined Kyanite

Calcined Kyanite (lumps) to be used in making Refractories

In order to make it a volume stable refractory material, kyanite is pre-calcined at 1420^OC to mullite and Cristobalite before use. Sometimes the lumps are very hard to crush after calcination. To avoid this, after calcination the kyanite lumps are, sometimes, quenched in water to make them crumble easily. Thereafter, it is ground, graded into various fractions as per requirement. These grains of calcined kyanite being volume stable are used with other raw materials for making refractory bricks and castables. The various refractory properties are:

P.C.E - above 1785^OC or 35+ (Orton)

RUL (refractoriness under load) - 1750^OC

Porosity = 24 - 25%

Properly calcined kyanite is a very good refractory raw material because of its high alumina percentage and low iron contents. As compared to other refractory raw materials kyanite can be sometimes, very handy for boosting alumina content and other refractory properties of the product at the same time maintaining its cost effectiveness.

Refractories made from calcined kyanite possess:

• Higher modulus of rupture and creep resistance
• Lower co-efficient of expansion
• High thermal shock resistance
• Increased durability, about three times that of ordinary bricks
• Superior resistance to salt attacks and chemical corrosion
• Very low co-efficient of spalling

Related Article: Kyanite - Properties and Indian Occurrences