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Showing posts with label Refractories Technical (Basics). Show all posts
Showing posts with label Refractories Technical (Basics). Show all posts

Refractory Installation Procedure and Heating Schedule to be followed after starting an Induction Furnace

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Installation of refractories in any furnace is a tricky process and after the lining is done the most important thing is the heating schedule. That means how the furnace after repairing or with new refractory lining should be started, what should be the rate of heating (rising temperature) and holding time at any particular temperature. You cannot start the furnace by raising its temperature to peak at one go, as otherwise the refractories will be damaged or even the refractory lining may fall apart. The furnace starting heating schedule depends on various aspects including, thermal conductivity of the refractories used.


Here is a step-by-step guide for installation of Refractories (Ramming Masses, etc.) in Induction Furnace also the heating schedule that should be followed after starting the furnace:


Read: Refractory Lining Installation of Pipes and Chutes in a Furnace


1. Scrub all loose materials and clean the furnace.

2. Before using heat the refractory material (Ramming mass) at about 100OC to make it free from moisture. Spread the material at the furnace bottom to about 50 - 60 mm thick layer at each time. Then ram the layer uniformly using a suitable rammer. The material has to be rammed layer by layer to get maximum compaction. Before ramming, little bit poking with a rod help to drive away the air-pockets trapped within the loose refractory material spread.

3. Place the steel former on the rammed bottom. Then fix the steel iron block at the center of the steel former to get uniform thickness throughout the furnace wall.

4. For ramming the upper portion of the side-wall just above the induction coil, mix the dry refractory material with 1-1.5% Sodium Silicate solution and 3-4% water.

5. Best results can be achieved by following the heating schedule for the furnace as mentioned hereunder -

Furnace Heating Schedule

Furnace Temperature

Rate of Heating with Holding Time

Ambient temp to 100OC

@ 30OC / hr.

Hold at 100OC.

4-6 hr depending upon the lining thickness.

100O - 800OC.

@ 50OC / hr.

Hold at 800OC.

2 - 3 hr.

800O - 1400OC.

@ 100OC / hr.

Hold at 1400OC.

4 - 6 hr.

1400OC to furnace operating temperature.

@ 100OC / hr.

The Functions of Silica Fume (Microsilica) and other Ultrafine or Microfine Additives in Refractory Castables

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The unshaped refractories, commonly known as Monolithic refractories are manufactured by suitably blending graded refractory aggregates, binders, fillers & (/or) special additives used for modification of ultimate properties. The refractory aggregates chosen for the formulation of refractory castables have a major contribution in determining their ultimate product quality. Along with this and many other input parameters, especially the high temperature properties of all type of Castables (Aluminous / Basic) depend a lot on the additive’s level and type used. All ingredients, binders, and additives of different chemical compositions and grading are chosen, blended to provide the proper characteristics for various applications of monolithics. 

Effect of Water Addition vs. Ultrafine Additives (Silica Fume) in Castable Refractories


For castable refractories the mobility of various particles is essential for proper placement of the castable. One method to achieve mobility is by adding more water, but this increases porosity & thus affects the performance both at normal & elevated temp. Therefore, addition of water should be minimized. The desired fluidity can be achieved by maintaining the coarser particles separated from one another by suspension of fines and ultrafine additives combined with the state of flocculation within the suspension. The role of the grains of these ‘Microfines or Ultrafine Additives’ can be compared with those of the balls in a ball-bearing.


What are the commonly used Ultrafine Additives in Refractory Castables?


Read: Benefits of using Steel Fibers and Organic Fibers in  Refractory Castables and other Monolithic refractories


The commonly used microfines or ultrafine additives are Silica Fume also known as Microsilica or Fumed Silica, microfine Alumina (Al2O3), ultrafine green Chrome Oxide (Cr2O3) etc. Silica fume is a by-product of producing silicon metal or ferrosilicon alloys. Silica fume consists primarily of amorphous (non-crystalline) silicon dioxide (SiO2). Because of its super fine particles, large surface area, and the high SiO2 content, silica fume is a very reactive.

The addition of Microsilica or Silica Fume has other advantages also, since these micro fine silica particles easily react with alumina present in the material to form Mullite which, in turn, helps in enhancing refractory properties of the product.

The addition of certain percentage of superfine green Chrome Oxide (Cr2O3) in Alumina castables increases the slag corrosion resistance & HMOR of the product significantly because of the formation of Alumina-Chrome (Corundum) solid-solution.

There are several manufacturers of fumed silica. Some well-known global brands of fumed silica are of Elkem Materials, AEROSIL of Evonik Industries, Norchem Concrete Products. 

 

Read: Advantages of using Gel Bond and Colloidal Silica in Monolithic Refractories