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Showing posts with label Refractories. Show all posts
Showing posts with label Refractories. Show all posts

Blast Furnace Taphole Clay Materials (Refractories)

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27-June-2020

With the prevailing trend of increasing capacities of Blast Furnaces the conditions for applications of Taphole Clay (refractory materials) have become more stringent as there has been a significant increase both in the pig iron temperatures and taping durations. These requirements are met by increasing Al2O percent and adding some SiC (around 15 to 20 percent) along with metals and nitrides as special additives in blast furnace taphole materials (refractories). Hence, depending on the capacity of the Blast Furnace the specifications of Taphole Clay to be used would vary. The binders used vary from purely coal tar based mixes to resin bonded ones or a dual bond.

The function of the taphole clay (taphole material) in a blast furnace is to enable smooth operation of the taphole, maintain constant taphole length and ensure separation of hot metal and slag. The following table depicts the various functions and property matrix for taphole clay.

Table: Function vs. Property Matrix of Blast Furnace Taphole Clay

Closing Hole

Easy plugging and drilling.

=> Plasticity of fireclay.

=> Good gas permeability.

=> Proper sintering.

Hearth Protection

Constant length.

=> Resistance to hot metal and slag.

=> High stickiness to the furnace wall.

Constant Delivery

Erosion resistance.

=> Expansive nature.

=> Resistance to hot metal and slag.

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Blast Furnace Trough Materials (Trough Mix)

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29-Aug-2009

The flow rate of molten metal and slag through the trough system increase many times in case of larger blast furnaces. To get good campaign life special attention must be given to both refractory lining and design i.e. trough geometry and cooling system. The trough design is based on fluid flow characteristics along with thermo-chemical reactions. The important parameters of the geometry would be - length, width, depth at drainage point, distance between iron and slag over-flow, skimmer opening dimensions, and side-wall angle. Each of the above parameters affects the campaign life of the furnace trough if not designed properly. Cooling helps to bring down the hot face temperature, and thus the wear by way of chemical attack, infiltration and thermal stresses.

Development of sophisticated materials and innovative installation and repair techniques now make it possible to hold hot metal in today’s deep pooling type iron troughs for a week or more without draining for maintenance. In this area the traditional graphitic high alumina ramming masses used in the past have been replaced by high quality, low moisture, metal or organic fiber containing  castables with  Al2O3 - SiC - C as the standard refractory base material (dry ramming masses / gunning compound / ULCC) for troughs. The important physical properties for this material are - thermal expansion, hot strength and thermal conductivity. The following shows the role of different constituents of trough mix (material).
Table: Role of different components in the
Blast Furnace Trough Mix (Refractory)
Material
Role
Alumina components
=> Volume stability
=> Wear resistance
Silicon Carbide (SiC)
=> Wear resistance
=> Oxidation resistance
=> Slag penetration resistance
Carbon
=> Spalling resistance
=> Slag penetration resistance
Anti-oxidants
=> Oxidation resistance
=> Hot MOR
Resin
=> Hot MOR
=> Binding strength

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

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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.
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