Showing posts with label Crucible. Show all posts
Showing posts with label Crucible. Show all posts

June 25, 2009

Slag Corrosion (Slag Attack) Test of Refractories

Slag attack is particularly important. The structural strength of the refractory may be critically reduced by the solvent action of liquid slags. The slag attack on the refractories in contact may be in the two ways:

Corrosion - It is the wear and tear of refractories caused by static chemical attack of slag.

Erosion - It is wear caused my mechanical action i.e. the process of breaking and washing away of refractory materials by molten slag.

The conditions of operation are variable and complex. Hence the standardization of this test is difficult. However, there are various test methods, viz. Crucible test, Solid cube test, Suspended rod test, Model wall test, Cone test, Powder impact test, but none is exact simulative test. It is done in various ways to suit the working conditions. The following are an outline of different methods of Slag Corrosion Test :

=> This method is called ‘Pill test’ is used when the quantity of slag is less as compared to the quantity of refractories. The slag, more often in the form of a pill, is placed on the refractory body or in a cavity made in it and heated. The depth of penetration of the slag inside the refractory, the spread of the molten mass and also the corrosion or bloating is observed. Theses factors form the measures of the attack.

=> This is another method known as immersion method and is used when the quantity of the slag is far in excess of refractory. Here the refractory is subjected to attack of the slag by immersing a small piece of refractory in the molten slag. The depth of penetration of the slag inside the refractory is the measures of the attack. => Another test also known as Impingement method or Powder impact test consists of letting the slag fall on the refractory bricks at high temperature. Many a time a spray of solid powdered slag is directed against the hot refractory brick at an angle of 45O for a certain period and at a certain temperature. The extent of corrosion under gone by the refractory is the measure of the slag attack. Several types of furnaces have been designed for this test.

=> This is Fusion test and consists of making a mixture of different quantities of powdered slag and refractory material and studying the fusion material of the mixture. The interval between softening and flattening of the cone is supposed to indicate the critical range of deformation of refractories in contact with slag.

The extent of penetration of slag is to be carefully studied. The bore diameter, depth in the refractory test specimen, the overall specimen size, fineness, as also the quantity of slag to be tested should be equal in every case for obtaining comparative test results. Overall slag corrosion / erosion will depend on so many factors such as porosity of the refractory brick, the composition, nature of the brick and of the slag, the temperature and duration of the attack, load on the brick at during slag attack, the products of the reaction formed and the rapidity with which they are removed, etc. Thus various refractories are affected variously and therefore it is difficult to simulate the exact conditions encountered in service. Still one can get an approximate an approximate idea by doing the chemical analysis and studying the various phases developed at the slag-refractory interface through Microstructural and XRD (X-ray diffraction) analysis. Out of all the slag corrosion tests described above, most of which give a qualitative and comparative result only, there is one method which has been somehow accepted as standard is the German DIN 1069 based on crucible test.

Related Articles

=> Standard Methods for Testing of Refractories

=> Sampling of the Refractory Specimen for Inspection

=> Iron Making in Mini Blast Furnace (MBF)

=> ‘Black Core’ in Refractory Bricks


Classification of Kilns and Furnaces

The term ‘Kiln’ is normally used when referring to high temperature treatment of non-metallic materials in various industries like in the ceramic, cement (cement rotary kiln), lime (lime kiln) etc. whereas, when melting at high temperature is involved the term ‘Furnace’ is used as in steel manufacture (Blast Furnace, Basic Oxygen Furnace, Ladle Furnace), glass industries (Glass Melting Tank Furnace) etc. More about the meaning and function of a furnace and kiln has been dealt in one of our previous article: The Function of a Furnace (Kiln) used in Ceramic Industries [Read]. Here we shall discuss about the different criteria used for classifying a kiln or furnace.

Furnaces may be classified broadly:
(a) According to the nature of heating medium -
(1) Fuel fired furnace, (2) Electrically heated furnace.
(b) According to the form and shape of the space to be heated -
(1) Shaft kiln, (2) Crucible furnace, (3) Rotary kiln, (4) Hearth furnace, (5) Tunnel kiln,
(6) Muffle furnace, (7)Tank furnace etc.
(c) According to the process taking place -
(1) Ceramic (2) Metallurgical (3) Non-metallurgical etc.
(d) According to the products obtained -
(1) Lime kiln (2) Cement kiln (3) Copper smelter (4) Glass tank furnace (5) Gold furnace etc.
(e) According to the method of handling and firing the materials in its passage through the furnace -
(1) Direct fired (2) Indirect fired (3) Batch type firing (Intermediate, Periodic)
(4) Continuous type firing.

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October 26, 2008

Refractory Lining for Induction Furnace

The bottom structural part of Induction Furnace (Figure with details published in a separate article on Induction Furnace) on which main crucible lies is generally lined with bricks and do not require to be changed frequently. However, working face of the main crucible is lined with a suitable ramming mass. Selection of the ramming mass (R/M) entirely depends on the scrap melted (as given in the Table below) and operating parameters. The roof is lined with a suitable castable generally of High Alumina base and Low Cement Castable (LCC). The launder or spout of the furnace either is rammed or lined with refractory bricks. For side wall lining a suitable cylindrical former is essential. The former can be made either removable or consumable type. In case of consumable former the quality of the former should be compatible with the quality of the product (melt) to be produced. Generally bottom ramming is done first followed by the side wall ramming. Ramming is generally done layer-wise with the help of pneumatic rammers to ensure compaction and packing density. At the same time to avoid lamination between the layers each layer is to be scratched before putting fresh material for further ramming. The collar rim of the induction furnace crucible is made of the same refractory paste as used for constructing the crucible but with a greater addition of bonding materials & water. Then furnace heating schedule is to be followed carefully which will depend upon the furnace lining thickness, nature of ramming mass etc.


Selection of Refractories for Lining
Operation and furnace area wise chart of refractories with their Standard Specifications are given below for ready reference:
(Methods of Installation, furnace Heating Schedule etc. may be provided by the refractory vendor however, in the coming posts we shall discuss on the same also).


Furnace Operation / Area of Application
Refractory Specifications
(Std. Specfn)
Melting Mild Steel, Stainless
Steel, Manganese Steel
& Alloy Steels.
Type= Mag-Chrome R/M, MgO%= 70-85, Cr2O3%= 8-10, Sintering Temp (ST)= 800OC,Application Temp (AT)= 1750OC, Grading= 0-5 mm
Melting Cast Iron.
Type= Silica R/M, SiO2%= 97 (min),
AT= 1650OC, Grading= 0-6 mm
In the areas of Cover, Grout
of Ind Fur melting Aluminium
& its alloys.
Type= High Alumina R/M, Al2O3%= 78-80,
Fe2O3%= 1.5 (max), ST= 1100OC,
AT= 1750OC, Grading= 0-6 mm
In the areas of Cover, Top Cap, Spout/Receiver
Type= High Alumina R/M or LCC, Al2O3%= 90-92, Fe2O3%= 0.5 (max), ST= 1100OC,
AT= 1750OC, Grading= 0-6 mm
Melting Copper, Brass & Bronze
Type= Mullite base R/M, Al2O3%= 70 (min),
Fe2O3%= 0.5 (max), ST= 1100OC,
AT= 1750OC, Grading= 0-6 mm
Melting Lead, Zinc & Tin
Type= Fire clay R/M, Al2O3%= 40-45, ST= 1100OC, AT= 1650OC, Grading= 0-5 mm
Melting Cupro-Nickel alloys
Type= Spinel bonded R/M, MgO%= 70-72,
Al2O3%= 15 (min), ST= 1000OC,
AT= 1750OC, Grading= 0-5 mm
Patching between the campaigns
Type= Patching Mass, MgO%= 70-75,
Cr2O3%= 8-10, ST= 800OC,
Grading= 0-2 mm
Repairing Cover, Spout areas between the campaigns
Type= R/M or P/M, Al2O3%= 80-90,
Fe2O3%= 1 (max), ST= 1100OC, AT= 1700OC
Hot & Cold repairing of Uppercase, Inductor Lining & some Structural components
Type= LCC, Al2O3%= 60-80,
Fe2O3%= 1.5 (max)

Recent Articles

October 24, 2008

INDUCTION FURNACE

An Induction Furnace uses induction to heat a metal to its melting point which is based on the theory of Electro Magnetic Induction. Depending on their frequency (50 Hz - 250 kHz) these can be divided to three types:

  1. High Frequency
  2. Medium Frequency
  3. Low Frequency

Their capacities range from less than 1kg to 100MT, which are used for re-melting of iron & steel (steel scrap), copper, aluminium, precious metals and alloys. Even most modern foundries use this type of furnaces and now more iron foundries are replacing Cupolas with Induction Furnace to melt cast iron as the former emit lots of dust & other pollutants. The Steel making via Induction Furnace route has certain advantages & disadvantages:

Advantages of Induction Furnace


  1. It has no electrodes and electric arcs which allow productions of steel & alloys low in carbon and occluded gases without any quality problem.
  2. Low melting losses & alloying elements.
  3. High power efficiency, therefore, cost-effective.
  4. Precise control of the operating parameters.

Disadvantages of Induction Furnace


  1. Refining in Induction Furnace is not as intensive or effective as in Electric Arc Furnace (EAF).
  2. Life of Refractory lining is low as compared to EAF.
  3. Removal of S & P is limited, so selection of charges with less impurity is required.


Induction Furnaces are classified generally into two categories:

  1. Channel type induction furnace
  2. Coreless type induction furnace
Channel Type Induction Furnace design

Fig.- CHANNEL TYPE INDUCTION FURNACE

1. Channel Type Induction Furnace

These furnaces basically consist of a vessel to which one or more inductors are attached. The inductor is actually a transformer whereby the secondary winding is formed with the help of a loop of liquid metal confined in a closed refractory channel. In the furnace the energy is transformed from the power system at line frequency through a power supply to the inductor and converted into heat. One advantage of this type of furnace is that the vessel or upper case can be built in any practical size & shape to suit the application, but the disadvantage are like -
a. Power input limitation per inductor.
b. Necessity to maintain a liquid heel in the furnace always to avoid problems related to operational parameters and refractories.
For the above reasons Channel type Induction furnaces (Fig.) are treated as a receiver or holding vessel for homogenization of liquid metal with limited capability of melting.


Coreless Type Induction Furnace
Fig.- CORELESS TYPE INDUCTION FURNACE


2. Coreless Type Induction Furnace
These furnaces (Fig.) are designed like a cylindrical crucible surrounded by a power coil in which energy is supplied either directly from the network (line frequency) or through a frequency converter. The magnetic field generated by the coil carries the energy to the charge.
Related Articles
=> Refractory Lining of Induction Furnaces [Read]

=> The Function of a Furnace and Kiln used in Ceramic and other Industries [Read]
=> Refractories for Reheating Furnaces [Read]
=> Refractory Resistance to Carbon Monoxide (CO) Attack [Read] 
=> Blast Furnace - Refractory Lining Pattern [Read]
=> MIDREX - The Most Widely accepted Direct Reduction (DR) Process of Ironmaking [Read]
=> COREX® : An Innovative Ironmaking Metallurgical Process [Read]
=> Iron Making in Mini Blast Furnace (MBF) [Read]
=> Use of Kyanite as Refractory Raw Material [Read]
=> HYL III and SL/RN - The two widely accepted Direct Reduction (DR) Processes of Ironmaking [Read]