Refractory Lining Installation of Pipes and Chutes in a Furnace

7-July-2020

‘Piping’ and ‘Chutes’ are used in furnaces for the purpose of transporting hot air, gases or solid usually accompanied with fine, hot dust particles. Depending on the existing stress, the piping is lined with single layer or multilayer insulating refractory materials. 

Examples for piping with diameters up to 2000 mm are gas piping for Reformers and metal slides in Preheaters of Cement Plants. Examples for piping with diameters above 2000 mm are recirculation shafts in Lignite Power Plants and Hot Blast Mains in Blast Furnace of Steel Plants.

Below given are certain points (rules) must be taken care of while doing refractory lining of pipes, chutes and circular or curved structures in a furnace:

Also Read - Refractory Installation of Ceramic Fibers in Kiln and Furnace Linings

=> Material and personal transport must be coordinated with the piping designer during the planning phase. Manholes, equipment for scaffolds and material transport must be determined.

=> If the refractory lining consists of several layers, each layer should be installed separately or section by section in order to prevent material mix-up.

=> Expansion joints may not be smaller or larger than what is indicated in the drawings. Mortar residue or other contamination (dust etc.) must not get into the expansion joints.

=> Expansion joint bricks, closed bricks and bricks for bevel areas must be measured, finished and installed precisely. Hollow spaces must be avoided due to danger of background currents.

=> If piping is lined on the ground, a trial installation of the closer piece to be fitted into each stand of pipes must be undertaken or it must be lined on site.

=> The lining on the ground is only possible if both piping and refractory designer have already determined length and weight of the pipe chutes. This will help prevent undesired deformations of the lined pipes.

=> In order to prevent transportation damages the lining of pipe chutes having larger diameters should be protected by braces. These braces must be of such a design and make that they can be securely transported and can be easily removed without damaging the lining.

=> The refractory lining dimensions, especially at the ends of pipe chutes, must be precisely observed. Crowding and damage to the refractory lining during assembly should be prevented.                

=> Below expansion joints must be approved and cleared by the piping designer before refractory lining work starts. Special attention must be given to observance of the cold dimensions (initial tension / stress) and flow direction.

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Ideal Grain Size Distribution of Refractory Raw Materials Mixture for making Magnesia Carbon Bricks

6-July-2020

Mag Carbon Bricks

Magnesia carbon refractory bricks (MgO-C) or Carbon containing Magnesite refractories have been extensively used by steel makers for the secondary treatment of steel in basic oxygen furnaces, electric arc furnaces, and ladle furnaces. Mag Carbon refractory bricks are widely used in slag lines of BOF (Basic Oxygen Furnace) because of their superior wear resistance. The service life of Magnesia Carbon refractories used in BOFs have been pushed quite significantly (largely due to slag splashing and gunning improvements) even as the service conditions have become more severe due to the increased operating temperature required for continuous casting and the need to produce cleaner steel.

Mag Carbon bricks are made of high purity magnesia, high quality graphite, antioxidants and some additives with a suitable binder (bonding agent). Selection of raw materials, their grading and grain size distribution (Granulometry) and composition together have ultimate role in the development of various physical properties, microstructure and ultimately thermo-mechanical properties of Mag Carbon refractory bricks (MgO-C). Various different types of MgO (Magnesite) grains provide different levels of corrosion resistance.

From the literature and plant applications it has been established that Magnesia-carbon bricks having 3 mm particle size show better wear resistance as well as a few other characteristics as compared to the bricks with 5 mm size grains.

Graphite, Anti-Oxidants (Additives) and Binders used in the composition of Magnesia Carbon Bricks

The graphite flakes used in these bricks impart -

=> High thermal conductivity

=> Good thermal shock resistance

=> Low thermal expansion

=> Non-wettability by liquid slag

=> Low corrosion rates by slags

Graphite contents of typical bricks range from 4 - 35% natural flake graphite. Since oxygen affinity of carbon is very high so different kinds of antioxidant minerals are used (in fines or superfines) in order to protect refractory material against chemical corrosion. The REDOX reactions in magnesia carbon can be reduced by selection of high purity magnesite, large crystal size and use of graphite with low impurities. Slag corrosion resistance of MgO-C refractories can be improved by use of magnesite grains with less reactivity i.e. fused magnesite grains of high Bulk Density (BD) and high purity.

The above are some of the reasons which explain how selection of various raw materials can affect the performance of magnesia-carbon bricks. More on this aspect and the compositions of Magnesia-carbon refractory bricks will be discussed in a separate post:

Refractory composition, production of Magnesia Carbon Refractory Bricks and their properties

Here, our topic is Granulometry i.e. overall grading and the grain size distribution, suitable for the best performance of MgO-C bricks. Grading and the grain size distribution are important as these are directly related with the following properties of Magnesia-carbon bricks:

=> Porosity

=> Mechanical strength

=> Spalling resistance

=> Microstructure and phase development

=> Wear resistance    

From the experience of various trials and performances it has been found that 0 - 4 mm grading is the best for MgO-C refractory bricks for all general applications and also for different shapes like Tap Hole Blocks, Sleeves, etc. (except Slide Gate refractories which will be different).

Bonding agents or binders used in Mag Carbon bricks and other carbon refractory products are immiscible with graphite and other refractory raw materials. At room temperature, they rely on binder to cure. Generally these binders or bonding agents are resin, asphalt or an organic matter and can be divided into three types: phenolic resin, modified asphalt, petroleum cracking by-product category.

The grain size distribution (granulometry) of the press mixture (powder) for MgO-C bricks with different Graphite percentages as they should be are given in the following table:   

Apparent Porosity and True Porosity of Refractory Samples

Pores in Refractory Bricks (Porosity)

Porosity is the percentage relationship between the volume of the pore space and the total volume of the refractory sample. Apparent Porosity does not include the volume of the sealed pores. The True Porosity includes the volume of the sealed pores also. The usual difference between the apparent porosity and the true porosity is of the order of 1 to 2 percent unless the proportion of the sealed pores is high. The true porosity figure is the higher than the apparent porosity figure as the true porosity includes the volume of the sealed or closed pores also. The difference between the two values represents the percent volume of closed pores. Porosity can be controlled by the following:

=> By controlling the texture of the brick i.e. by controlling the size distribution of the particles.

=> By the methods of green manufacturing and composition.

=> By controlling the firing temperature, soaking time etc.

=> Quality of raw materials i.e. the inherent grain porosity of the raw materials used.

Higher the porosity, lower will be the strength of the brick. Bricks with lower porosity will have greater resistance to slag attack and more sensitiveness to fluctuations in temperature. Their thermal conductivity will be more.

Suggested Article: Bulk Density of Refractory Samples 

Hence, apparent porosity is the percentage ratio of the void space in the refractory specimen to the total bulk volume of the same. There are two methods used for the determination of apparent porosity of refractory materials which are:

=> Boiling point method, and

=> Evacuation method.

Refractory Samples (Refractory Specimen) measuring 6.5 cm x 6.5 cm x 4 cm is cut from burnt refractory bricks by a cut off wheel from within its core and cleaned any dust or loose particles adhering to its surface and are dried in an oven at 110^OC to a constant weight. For graded materials take 3 to 5 mm size grains and dry at 110^OC. It can be determined by following the steps given in any of the Standard Methods for Refractory Testing like - ASTM, Indian Standards (IS), Ghost, DIN etc.

True Porosity (%) = {1 - (Apparent Sp. Gr. ÷ True Sp. Gr.)} x 100

Apparent Porosity (%) = {(Soaked Wt - Dry Wt) ÷ (Soaked Wt - Suspended Wt)} x 100

 Related Articles 

• How to manipulate test results of Apparent Porosity (AP) during Testing of Refractory Bricks (Refractory Inspection)
• Permeability of Refractory Bricks and Monolithics
• How to check Alkali Resistance (AR) of Refractory Lining - Bricks and Castables

Permeability of Refractory Bricks and Monolithics

5-July-2020

Permeability of any refractory material is defined as the volume of a gas or air which will pass through a cubic centimeter of the material under a pressure of 1 cm of water per second. 

Fig - Refractory Bricks being stacked in stock yard

Permeability is calculated by the following formula:

Permeability = (Vol. of gas/air x thickness) ÷ (Area x Time of flow x Pressure difference)

It is determined by forcing a known volume air or gas through a cube. Time of flow, pressure difference and dimensions of specimens are noted. However, there are ready-made apparatuses and systems available in the market for testing permeability of refractory bricks and monolithics. There is no direct dependence permeability has on porosity however, permeability depends upon the existence of closed pores or channel pores and is a measure of these, whereas porosity measures the total pore volume including closed pores.

Suggested Article: Pore Size Distribution in Refractories 

However, this property permeability for refractory bricks or monolithics become important wherever molten liquid like metal, slag, glass etc. come in contact with the refractory lining i.e. wall of the furnace or container especially, when gas under pressure is present. Due to the anisotropic nature of refractories, the result will depend upon on factors such as the direction of flow and presence or absence of the original skin on the test specimen.

Low permeability is more important than low porosity from the point of view of slag resistance of the refractory. Uniform permeability is an indication of absence of cracks in the refractory.

Related Articles:

• Different tests and procedures to check slag corrosion resistance of refractory products
• How to check Alkali Resistance (AR) of Refractory Lining - Bricks and Castables

Refractory Saggers (Saggars) and Muffles

Saggers or Saggars are boxes or cases made of refractory materials in which pottery ware, glazed or unglazed articles are heated to higher temperature. In other words saggar is a refractory container for protecting ware during heating. Because they are kept in the sagger they do not come in contact with the flames and objectionable gases. In pottery works they form an important item and are used in large numbers. Saggars are usually made of fireclay and grog but for special use they can be made of other refractory materials.

Muffle is an enclosure in a furnace protecting the ware from the flame and products of combustion. These are also made of the same type of refractory raw materials as saggers. Muffles are generally used in small furnaces for firing small quantities of wares.

The most important property of these saggers and muffles is that they should have sufficient durability. They should also have good load bearing capacity, high mechanical strength (CCS and MOR) and also high thermal conductivity. Usually a highly refractory clay with good binding power is used so that it can take a large amount of grog. Very highly plastic clay is never suitable due to its high shrinkage. Usually two clays are used one being a refractory fireclay as base material and the other a plastic clay as a bond material. Grog is used in various sizes so that the saggers (saggars) and muffles are porous enough to withstand the thermal shocks. The grog must be made from a highly refractory fireclay.

Since the proportion of grog is larger than the clay so there should be a through mixing of the two. The shaping (green manufacturing) can be done by either of the following processes:

1. Hand moulding or Pneumatic Ramming

2. Casting

3. Power pressing.

The articles are kept in the saggers and these saggers are piled one on the top of the other in the furnace. One pile may have 15 saggers depending on the size. These piles are commonly called “Bungs”. The space between the two is closed by a wad of clay so that flames etc. do not enter the saggers (saggars) and the articles inside are heated by the clean heat alone.

To fire enameled ware or for firing pottery decoration etc. muffles are used. These are open at one end and are kept over a small fire box in such a way that the flames from the fuel envelope the muffle from all sides. One end remains open and this acts as a door to place things or to take them out. This is closed by a plug or door before firing.

The bottom of the saggar as well as muffle is usually made thicker than the sides and in some cases the bottom is made from a mixture containing higher percentage of refractory grog. After green manufacturing a saggar or a muffle is dried slowly and than fired to a temperature of around 1350^OC.

Saggers and Muffles should have a high mechanical strength in unburned stage and also at high temperature. They should be refractory enough to withstand the high temperature and should be able to resist the thermal shocks and should have a good thermal conductivity. Their porosity should be 20 - 25% and should have a stable volume. It is important that the walls of the saggars and muffles have a uniform thickness as otherwise strains are set due to unequal distribution of heat. These saggers (saggars) or muffles are also made of sillimanite, kyanite, silicon carbide etc. using these in the form of grog with a binding agent.