Theme images by Storman. Powered by Blogger.
Best viewed in Google Chrome




Popular Posts


Showing posts with label Refractory Raw Materials. Show all posts
Showing posts with label Refractory Raw Materials. Show all posts

How to Prevent Formation of Iron Spots (Crater) in Refractory Bricks as a measure of Quality Control

- 1 comment
21-Jan-2023 (Updated)

What makes a refractory product sell? The factors determining the sales success are many and varied. They include market conditions, the nature of the product, and the image of the manufacturing organization in the market as well as of the product created by advertising, the socio-cultural background of customers, credit facilities, Customer Delight and so on.

But one major factor that appears certainly in all conditions (excluding monopoly or extreme scarcity) is product quality as perceived by the customer. The Production Department must accept its fundamental responsibility for the manufactured quality of its refractory products. The workforce with effective training and adequate equipment must be capable of -
1. consistently producing to specification and 

2. recognizing and reacting when an operation goes out of control.

Adequate systems must be established to ensure that corrective action is taken. The quality control for Refractory Bricks refers to the following aspects:

=> Manufacturing defects.

=> Dimensional tolerances.
=> Physical & Chemical material properties.
=> Quantities, Marking, Labelling and Packing.

While Sampling for visual and dimensional inspection, iron spot (crater) is always an important criteria of inspection. It is not an uncommon site or if one had a chance of visiting brick yards of a few Refractory manufacturing units then he must have noticed that in many such plants refractory bricks worth crores (millions) or thousands of tons are lying rejected because of developed iron spots (grey fusion spots) in them

Before going into the steps as how to control the formation of these iron spots or iron craters in Refractory Bricks let us discuss a few more things in detail.

Iron Craters or similar Black Spots are actually melt phenomena, caused by mainly iron oxide or lime or some other low temperature fusion and melting elements mostly, alkalies. An Iron spot on the surface of a refractory brick can be tested with a hammer (I assume Refractory persons would be acquainted with this tool :-) ) to determine the possible presence of crater which can be dangerous for the brick and so, for the furnace after lining. Some common reasons/sources of these iron spots and preventive actions required to be taken regularly to stop formation of such black spots and maintain the quality of refractory bricks are outlined below:

Refractory Raw Materials
Preventive Measure: Checking the Raw Material & removing free iron from it before grinding / using. Sorting of Slag / Iron patch especially in calcined fire clay and other calcined raw materials at the kiln yard itself.
Responsibility: Mill house, Calcination Kiln, Checking & RM yard.

Own Rejection (Refractory Grog)
Preventive Measure: Iron containing (contaminated) grog to be kept in strict isolation.
Responsibility: Checking & RM yard.

Outside Grog
Preventive Measure: Visual inspection in the Truck itself by breaking up the lumps into pieces before taking into the raw material yard. Briquette test in the Laboratory.
Responsibility: Checking & RM yard. Laboratory (Quality Assurance).

Maintenance Waste
Preventive Measure: Proper cleaning of waste & scraps after Maintenance of any machine in the shop-floor e.g. iron chips, welding tips etc. before handover to the Production Deptt (there should be a proper Handover – Takeover system). Cleaning before & after maintenance of each machine (Mill House, Mixer, Press etc).
Responsibility: Concerned person of the Maintenance Deptt. Shift in charge of the concerned area (Mill House, Press, and Production).

Free iron in Mill House Silo Material
Actions Required: Due to Beater => regular cleaning of Magnetic drum, Plate & Roller Magnets. Due to Rusted plates => Silo cleaning once in a fortnight.
Responsibility: Mill House. Maintenance. Production departments.

Free iron from Press Hoppers / Mill Hoppers
Actions Required: Due to Rusted plates => Hopper cleaning once in a fortnight.
Responsibility: Production.

These are the certain necessary steps which, if taken properly, can arrest the formation of grey craters and iron spots which become visible on the surface of refractory bricks only after the firing is over that means everything is over!

Sillimanite - Properties, Mineralogy and Occurrence in India

- No comments

Sillimanite belongs to the rich Alumino - Silicate group of minerals and is a polymorph of Kyanite and Andalusite represented by the chemical formula Al2SiO5. According to Wikipedia to this mineral has been named after American chemist Benjamin Silliman (1779 - 1864). Sillimanite is a very important refractory raw material especially, for high alumina refractories which are extensively used in Iron and Steel, Petrochemical, Electrical, Cement, Zinc and Glass industries.

Mineralogy of Sillimanite

Sillimanite is found as brown, grayish, pale green or white orthorhombic mineral with long slender, needle shaped crystals. Optically, it is subhedral, elongated, colourless and non-pleochroic (or colourless to pale brownish yellow) in plane polarized light. Sillimanite has low r.i., one set cleavage, and straight extinction in grains which are parallel to elongation. In pure form it has specific gravity 3.23 and Moh’s hardness 6 - 7.5.

Kyanite - Properties, Mineralogy and Indian Occurrences

Indian Occurrence of Sillimanite

Sillimanite - Properties, Mineralogy and Occurrence in India
Sillimanite occurs in fibrous masses mainly in the form of bundles and is characteristic of the innermost zone of contact metamorphosed rocks. Because of this nature of its occurrence this mineral is also known as Fibrolite.

Massive Sillimanite deposits of the Nongstoin state in the Khasi Hills of Assam (Meghalaya) in India commonly known as Assam Sillimanite, was the world famous for its quality and quantity. The host rocks were Cordierite - Biotite - Quartz - Muscovite - Gneiss or a Sillimanite - Quartz - Schist with intrusions of Granite. Now it is not available since the reserves have been depleted. The only other important occurrence is around Pipra in Madhya Pradesh (India) popularly known as Rewa Sillimanite and in Bhandara district of Maharashtra (India). The disadvantage of Rewa sillimanite is that it slightly sensitive to thermal shocks. An authorized supplier of Rewa / Bhandara sillimanite is M/s Pavri Kyanite Mines, Nagpur, Maharashtra. An alternate source is available today from Beach Sand (a by-product of Indian Rare Earths Limited, a Government of India Undertaking under its Department of Atomic Energy). One has to fulfill certain formalities with Indian Rare Earths Limited in order to procure sillimanite sand from them. The beach sand deposits are the potential sources of sillimanite along with rutile, ilmanite, monazite, garnet, zircon and quartz. The Chavara (Kerala, India), Manavalakurichi (Tamil Nadu, India) and Chatrapur (Orissa, India) are among the richest deposits in the world. Indian beach sand deposits contain about 9% rutile, 6% garnet, 3% sillimanite and 1% monazite and zircon. After magnetic removal of Monazite, sillimanite, garnet, zircon and rutile are separated from one another by gravity tabling, electrostatic and flotation techniques. 

The following table shows the Chemical Composition of Sillimanite of two different sources:

Constituents (%)

Rewa / Bhandara

Indian Rare Earths

(Beach Sand)









TiO2 / ZrO2









2.16 (TiO2)









0.60 (ZrO2)

  Also Read: 

Use of Sillimanite as Raw Material in Refractories

- No comments

Sillimanite as a natural and untreated mineral is a very important raw material for high alumina refractories which are extensively used in Iron and steel, Petrochemical, Electrical, Cement, Zinc and Glass industries. - Sillimanite Rocks image
Sillimanite when heated above 1545­OC converts to Mullite and the excess silica as glass, crystoballite or tridyamite. The formation of the glassy phase can be reduced by addition of a small percentage of technical or calcined pure alumina fines (like - HGRM 30 etc) which reacts with this excess silica to form mullite, which in turn help in enhancing the quality of the product.

Due to the very low expansion or contraction on heating, sillimanite need not be calcined before use. Unlike sillimanite from most of the sources in the World which are used as it is, the Rewa sillimanite (found in Madhya Pradesh, India), because of its impurities, should not be used as such. It is always better once to wash these lumps in the raw material yard itself and then after shifting to the Mill House and crushing, grinding pass through magnetic separator to eliminate the free iron impurities.

The ideal firing temperature of green refractory bricks made of sillimanite grains as a major raw material is 1450 - 1500OC, to be fired either in a batch type or a tunnel kiln. The soaking time will vary depending upon the volume, shape, setting and other constituents of the bricks (particularly raw clay used and sintering aid, if any).

Sillimanite Refractories

Sillimanite refractories are characterized of high refractoriness, very low coefficient of thermal expansion, high refractoriness under load (RUL) and mechanical strength with great resistance to thermal shock (spalling resistance), abrasion and slag corrosion. Due to their exceptionally high resistance to spalling and corrosive actions of molten glass, chemical attacks of soda, borax and other frits, they are most suitable for Glass Melting Furnace (GMT), Oil fired Furnace, Cement Rotary Kiln, Blast Furnace, Electric Arc Furnace (EAF) roofs, Hot Metal Mixer, Combustion Chambers and Metallurgical operations done in Zinc Furnace, Gold Refining Furnace etc. - Sillimanite Bricks image

Particularly in Glass industry sillimanite refractories have got many applications, such as in glass melting tank furnaces (to be discussed in detail in a separate article) in all parts open to the products of combustion like combustion chambers, flues, door pillars which may have to support heavy load at high temperatures, recuperators and such other parts which are liable to be subjected to fluctuations of temperature. Assam Sillimanite once available in good quality and quantity, even for export, were used to be cut into blocks of various sizes from solid rock at the site of deposits itself which were then sent to the user’s site for their direct use in construction of Glass Melting tank Furnace bottom. But now it is a forgotten past! However, there are quite a few suppliers in India who manufacture these GT blocks, mostly using certain percentage of sillimanite sand or even sillimanite lumps after crushing and grinding. To name a few are Maithan Ceramics Limited (MCL), Tata Refractories Limited (TRL), OCL India Limited, ORIND (quality ?) etc. So far the properties like density (BD), porosity, mechanical strength, slag corrosion resistance and consistency of performance etc. are concerned, high capacity machine pressed bricks are far too superior to those made by pneumatic ramming. Although it is a general practice to give some ‘patching/finishing’ manually before inspection - dispatch to particularly big and complicated shape refractory bricks but from the customers’ point of view it is most important that during inspection it must be ensured that except for the ‘look’ only, the refractory brick (or block) does not depend much on the ‘finishing’, if at all, done on it. Patching is a wrong practice as it is done to camouflage the flaws which could be detected by seeing the brick.

For GT blocks the machine finish of the surface is very important. It must be evenly polished or ground to ensure that the warpage is negligible. One of the main criteria for acceptance of these blocks should be that in the assembly there should not be any open joint (the specification could be from 0.2mm - 0.3mm filler gauge up to a maximum 20 - 25mm depth from the top). This is a must to avoid the penetration and subsequent crystallization of glass and alkali vapours in these joints. To meet this criterion the manufacturer should have facilities for grinding of these blocks minimum in 4 faces and in some bricks up to 6 faces and then marked accordingly.