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

Gunning and Spraying - differences in these two methods of tundish wear lining, refractory lining repair and maintenance

5-April-2009

In this article we will try to understand the basics of Gunning and Spraying, two frequently used methods for enhancing the refractory lining life of a furnace or refractory lining repair and maintenance. Also, differences between Gunning and Spraying, preferential situations for either of these two methods in EAF, BOF, melting furnaces, steel ladles, tundish lining, etc.

There are instances when steel plants are found switching over from one method to other for refractory lining repair and maintenance depending upon their perceived and actual benefits obtained. But well-documented published data of such experiences, which can be of immense help for others, are either scanty or sparsely available. In the article A Comparative Evaluation of Different Types of Tundish Lining Refractories based on actual experiences in Steel Plants it has been tried to put together some such experiences made by others and were presented in some recently held different seminars and conferences on Refractories. 

GUNNING or GUNNABLE REFRACTORIES - Principles & Procedure

The gunning repair is a well-proven procedure by which refractory material can be applied quickly and cheaply. Initially these were alumino-silicate based and later converted to basic type magnesite based to assist with metallurgical practice.

Gunning or gunnable refractories are used for the hot repair of ladles and melting furnaces as well as relining or cold repair of the back lining. There are two basic methods: dry and wet gunning. With dry gunning the material is discharged from the machine with a maximum of 5% moisture and then fed to the gunning nozzle by an air stream where the required water, typically 5–10%, is added. With wet gunning the gunning material is moistened with water in a mixer and then pumped through a hose by means of an eccentric screw or a piston pump. At the end of the line the material is dispersed with compressed air and, if necessary, an additional liquid bonding/hardening agent can be added (shotcreting). Wet gunning has the problem that the machine or the gunning hose may get clogged by already moistened refractory material, particularly when not in continuous use; hence the process has to be carefully controlled. Also, the equipment requires more intensive cleaning and is not considered an efficient operation for applications less than 400 kg. With dry gunning, blockages in the conveying hose can be blown free by compressed air only. For optimum gunning and refractory performance the gunning material is essentially the same as the original refractory (plus binder material), but with a size distribution of 4 mm maximum. The applied thickness is typically in the range 10–30 mm and a vessel can normally be used again after 3–5 minutes.

Mixing of Water and Gunnable Refractory Materials

Three essential requirements for a good gunning repair of refractory lining are optimal moistening, homogeneous mixing of the gunning material with water, and a high quality gunning machine that guarantees even conveying. With pre-moistening a share of the gunning water is added some meters away from the gunning nozzle and relies on the turbulence within the conveyance for premixing. This approach, however, is very susceptible to operational problems. For instance adding too much water in advance may lead to clogging, particularly with quick binding systems where hardening starts inside the hose. Too much water reduces refractory quality and hence refractory lifetime. Also, the water can dissociate in the liquid steel to hydrogen, which can be detrimental to some steel grades (micro-cracks). With standard mixing heads (for dry gunning) the water is jetted through radial borings and, in order to compensate for inadequate initial moistening, the operator often works with a surplus of water so that the dust is minimized. This, however, often leads to use of incorrect water/ cement ratio, resulting in reduced refractory durability. Improvements can be achieved by use of pressure increasing pumps so that the water jet becomes sharper and more readily reaches the centre of the nozzle cross-section. Optimum nozzle-vessel distance is quite varied; between 40 and 1,000 mm depending on application.

Standard gunning refractories are magnesia, alumina or silica based refractory materials normally a monolithic applied on the areas that encounter severe wear out such as trunnions, scrap impact area and the slag line. These materials differ in price so the optimum choice depends on many factors such as steel grade, slag composition and the vessel used. A shooter type of gun is used for the gunning process to encounter hostile environment of the process. Consumption of gunned refractory is also extremely varied, ranging from 0.2-2 kg per ton steel depending on conditions of the vessel and environment.

Also read: Nomenclatures and Terminologies of Monolithic Refractory Products based on their setting or installation methods  

DIFFERENCE BETWEEN GUNNING AND SPRAYING

The principles, procedure and differences between Gunning and Spraying can be understood by the flow diagram below:

Gunning materials or gunnable refractories, refractory castables are transported through flexible hoses to the installation position, where the materials are wetted and projected through a handheld nozzle at the target area. Dry gunning allows operating farther away from the feed station, though wet gunning or shotcreting offers a faster output rate.

Since homogeneous mixing is possible in spraying (before the product is applied), the incorporation of special chemical additives can help to improve thermal stability properties of the lining and also impart good flexibility.   

The most commonly faced problems of using a gunning machine or in the process of gunning are -

• Dust formation during gunning
• High rebound losses leading to wastage and high consumption of material
• Difficulties in applying variable thickness leading to metal penetration and insufficient permanent refractory lives, and
• Difficult deskulling.

Whereas when the material is applied through spraying it has the following benefit -

• No dust formation during application
• No rebound loss hence minimal loss of material
• The lining thickness was better controllable, thus increasing permanent refractory lives
• Deskulling was better.

Benefits of using Steel Fibers and Organic Fibers in Refractory Castables and Monolithics

One of the most effective ways of improving the mechanical and thermal properties of refractory castables and other monolithic refractories is adding in suitable proportions of stainless steel fibers (SS) and organic fibers to the castable respectively.

Steel Fibers

Steel fiber reinforced refractory castables are very resistant to the tendency of the material to fall apart on thermal cycling. Stainless steel fibers greatly improve the flexural strength of the castable. And this added increase in ductility contributes significantly to the thermal shock and spalling resistance of the material. The fibers generally used are in size varying between 0.1 to 0.4 mm2 in cross-section & 20-40 mm in length. For monolithic SS is used either high chrome or high chrome nickel steels available in the market with different grades. One reason commonly reported that the thermal shock resistance of castables is greatly increased through addition of SS fibers because these fibers act as crack arresters, preventing cracks propagating. This is also possible that the microcracks caused by a mismatch in thermal expansion coefficients of matrix and fibers dissipate energy from larger cracks propagating as a result of thermal stress. However percentage of these fibers added becomes important because of two reasons as it has a direct impact on the fluidity of the castable, then it may also cause mixing difficult due to fiber-balling when added beyond 3% by volume. Another critical factor will be the maximum application temperature for the castable that those fibers present in the castable can resist oxidation (since these fibers can not perform beyond their melting temperature).

Organic Fibers

An effective means for improving the explosive spalling resistance of a castable is to add organic fibers to the formulation. It has been reported that the composition & concentration of fibers are not as important as melting temperature of the fiber, since these fibers after melting increase permeability at certain temp. & thereby reducing the explosive spalling tendency of the castables. The fibers generally used for this purpose are Polypropylene fibers, Polyester staple fibers, etc.

Here is one reference (work) of benefits of using refractory composition containing both stainless steel fibers and organic fibers. Because of these different advantages it have been found that both organic and SS fiber reinforced refractory castables provide substantial increase in service life and therefore, a considerable reduction in refractory maintenance cost and furnace down-time.

Also Read: 

Advantages of using Gel Bond and Colloidal Silica in Monolithic Refractories