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

Advantages of using Gel Bond and Colloidal Silica in Monolithic Refractories

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5-Jan-2009

What is Gel Bond?

The principle behind this bonding is the formation of a ‘gel’ from a ‘sol’ which surrounds the refractory Aggregates through a network skeleton which, with further heating, develops strength & ultimately goes through sintering to form ceramic bonding. Actually the mechanism is thixotropy, which lies in the fact that some substances, when agitated (under mechanical force), pass from the state of a ‘gel’ to that of a colloidal dispersion ‘sol’ and goes back to the state of a ‘gel’ again when the mechanical forces stop. The phenomenon of thixotropy is based on the theory of dispersion & subsequent flocculation of ultrafine powders. Various sols used in the bonding process e.g. Silica, Alumina, Zircon, and Titania. The incorporation of gel bond in place of conventional binders (High Alumina Refractory Cement) has made it possible to improve the high temperature properties of castable refractories considerably mainly because of the absence of low-melting phases (CA, CA2, C12A7, C2AS, C4AF) and impurities.


Advantages of Gel Bond

Several advantages of the gel-bond compositions compared to LCC & ULCC as have been reported are:

  • Less mixing time since gel bond formulations do not require other minor additives or deflocculants like the cement containing castables.
  • Shorter drying time and so reduced drying flaws. This is because water is not added or required for mixing.
  • Better refractoriness because of the absence low melting phases like- anorthite or gehlinite.
  • Colloidal silica being more viscous than the water, help to maintain more separation of refractory particles which, in turn, provide better thermal shock resistance.
  • Better chemical resistance.
  • Because of the various superior properties of gel bond castables / pumpables as described above, they yield longer campaign life, less downtime and so reduce cost of furnace operation.
  • Longer shelf life since there is no hydratable phase as in LCC, ULCC.


Applications of Gel Bond Castables / Pumpables (Gel Bond Monolithic Refractories)


Because of Gel bond Castables / Pumpables (Gel bond monolithic refractories) have been found to give better results in terms of both conveniences of applications as well as properties in almost all type of industries:

  • in cement industries - high temperature rotary kiln burning zone, rotary kiln incinerators lining
  • in glass industries - outside the Glass Melting Tank furnace and sidewalls and roofs
  • in Blast furnace trough - because of the better flowability these can be more conveniently installed by a pump with reduced installation time
  • in Torpedo and other transfer Ladles then Tundish back-up lining, Electric furnace Deltas and Runners
  • in secondary operations like - Reheating furnace hearth, roof. The installation of colloidal silica bonded castables / pumpables has shown significant improvement especially in reheating furnace roof areas, both during installation and drying (which has been found to take about 60% less time than the conventional ramming mixes & plastics)

Colloidal Silica / Silica Sol

Colloidal Silica or Sol or Silica Sol are the different names, consists of a stable dispersion amorphous silica particles. To achieve this, the silica particles must be small enough such that they are largely unaffected by gravity. Therefore, silica particle sizes are usually of the order of less than 100 nanometers. Initially colloidal silica was used in refractories for the purpose of coating in various applications like ingot casting, investment casting etc. It was during late 80’s when for the first time colloidal silica started to be used as bonding agent in monolithic refractories. During late 80’s refractories based on colloidal silica became available in the market in ramming, gunning and castable formulations. The development of gel bond refractories with colloidal silica as the bonding agent has been a major breakthrough in refractory technology. Since the type of colloidal silica used in refractories is available commercially, it became easy for many to take advantage of this technology. In lieu of conventional binders, colloidal silica can be used as bonding agent in all type of monolithic refractories such as castables, ramming and gunning mixes. Its use in castables has given rise to the convenience of refractory applications by pumping, thus providing a considerable advantage over conventional binders. Another big advantage is that unlike calcium aluminate cement bonded refractories; these refractories do not require following specific temperature parameters for drying and hence reduce drying flaws, installation time. Colloidal silica bonded castables / pumpables not only perform better and reduce costs of furnace operation, but also eliminate work place hazards for workers. The nano sized particles of colloidal silica, due to their higher viscosity consistency, maintain uniform inter-particulate distances resulting in increase of the permeability of the mix and hence provide smooth and speedy drying as well as improved reactivity also increasing the castable sinterability, promoting mullite formation. Colloidal silica bonded castables / pumpables can be extensively used in blast furnace cast house refractories (Alumina - Silicon Carbide - Graphite formulations) as well as for all other applications as mentioned above.

Continuous Casting Tundish Lining Refractories: Practices, Advantages and Disadvantages

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 25-Sept-2020

TUNDISH (BACKGROUND)

Continuous casting has been a landmark achievement in the area of steel making. Continuous casting refractories directly control the molten steel in the last stage of liquid steel processing and these are therefore, required to have high stability and special properties. In any continuous casting shop, tundish acts as a buffer vessel between steel ladle and mould. It serves the purpose of reservoir as well as fulfills certain metallurgical functions like floatation of inclusion, control of flow to the moulds, thermal and chemical homogenization etc. Small wonder that over the years, there have been a continuous change in the practices of refractory lining of tundish around the world.


Steel Tundish complete diagram - www.industry.guru

From a mere reservoir and distribution vessel, the tundish started to be viewed as a steel refining vessel and a totally new field in the process of steel making technology emerged known as Tundish Technology. In this article we have briefly mentioned about the developments and practices of tundish lining refractories in their chronological order and also the main features, merits and demerits of different types of refractories used in tundish lining to enable an individual steel maker decide what could be the best for his plant.


TYPES OF TUNDISH LINING REFRACTORIES - Advantages and Disadvantages

There is a host of different tundish lining refractories which can be categorized into 5 major types (also in a roughly chronological order):     

  1. Brick Lining
  2. Gunnable Tundish Lining
  3. Tundish Board Lining
  4. Sprayable Tundish Lining
  5. Tundish Dry Lining (in-situ formed)

Also Read: Importance of Tundish Design and Flow Modifier Refractories in Steel Making


Brick Lining

With the first commercial introduction of continuous casting in around 1960’s initially the same concept of refractory brick lining technology as used in other metal containing vessels was applied to continuous casting tundish lining. Tundish bricked lining refractories were of high alumina type used in direct contact with liquid steel, after pre-heating. It was essentially an extension of ladle refractory practices to the tundish and continuous casting.


Merits:

  • low risk of H picking by molten steel
  • no sand
  • low inventory
  • no investment in equipments
  • low washout risk

 Demerits:

·        intensive curing required

·        highly labour intensive

·        poor insulation

·        late stage temperature drop in casting operations due to high thermal conductivity of the brick lining resulting into metal heat loss affecting the metallurgical parameters

·        “Cold Start” not possible

·        large tundish fleet required

·        difficult deskulling (stripping)

·        joints

·        long tundish preparation time

Too many difficulties led some people to opt for a trowellable, and subsequently gunnable, over-lining at some added costs.


Gunnable Tundish Lining

Gunnable refractory lining in tundish is said to have commercially started in Japan to overcome some of the problems of bricked linings. Initially such tundish lining refractories were alumino-silicate based and later converted to magnesite based or basic type to assist with metallurgical practice. In this method the dry refractory powder of the right composition after fluidization is transferred and installed on the tundish wall by using a gunning machine to obtain a monolithic lining. Though it provided a monolithic joint-free structure and relatively improved deskulling but little was gained in the way of preheat times or heat losses due to the relatively high density of the gunned linings. There was still a tendency for the linings to crack and spall during rapid preheat and this also precluded the use of gunnable refractory lining for cold start practices. 


Merits:

  • low risk of H picking by molten steel
  • no sand
  • low inventory
  • no joints
  • less labour intensive
  • relatively easy installation in lesser time
  • relatively less difficult to deskull

 Demerits:

·        intensive curing required

·        high wastage because of rebound losses

·        poor insulation

·        “Cold Start” not possible

·        high washout risk

·        low thermal stability

·        high shrinkage causes high stress, subsequent crack formations during operation whereas a low shrinkage can be a barrier for easy deskulling

·        dust problems

·        energy intensive

·        long T/D cycle

·        high costs

·        investment in equipment


Tundish Board Lining

The mid 1970’s saw the introduction of a new type of tundish wear lining; which were board systems comprising low density, highly insulating, disposable, pre-formed, and pre-cured refractory boards. Easy deskull, no equipment investment and the low cost of silica variety also contributed to its run-away popularity among many steel makers. FOSECO’s GARNEX became a household name in Indian continuous casting circles during this time. Initially silica based boards were used which allowed only “cold start” practice. Magnesite based boards were introduced in mid 1980’s to fulfill the requirement of pre-heatability, i.e., a “hot start” practice for low hydrogen considerations in the manufacture of high alloy quality steels. However, the labour intensiveness, presence of joints and sand backing, and breakages etc remained as inherent handicaps of board system.


Merits:

  • low risk of H picking (when hot)
  • uniform liner shape
  • no need to cure
  • good insulation
  • cold start possible
  • easy deskull
  • low energy requirement
  • short T/D cycle
  • no investment in equipments
  • low washout risk
  • low cost (silica-based board)

 Demerits:

·        joints

·        sand backing

·        hydrogen picking risk (when cold)

·        labour intensive

·        high inventory

·        handling/breakage problem

·        high cost (magnesite-based boards)         

In response to increasing trends for making cleaner steel and also with the advent of Continuous Casting technology a variety of refractories for tundish lining have into the market. However, board system is still popular in countries where labour costs are low and application technologies are not readily available. Tundish board, in particular, developed for energy-saving are finding wide acceptance. 


Sprayable Tundish Lining

Because of some of the above difficulties there was already a push towards automation of the tundish lining system. Meanwhile, advances in machine design and chemical formulation technology in advanced countries led to the development of a “Spray” system, in which a thick slurry could be transported after thorough mixing, and finally deposited onto the tundish after “atomizing” with compressed air. The first robotic application system was commissioned in 1982 which from the later half of the 1980’s started to be widely used in developed countries due to the significant benefits of lower placed density and better control of the lining thickness than gunned linings.

The sprayable tundish lining refractories are mainly MgO and SiO2. The MgO content is usually in the range of 70% to 90% with balance percentage of SiO2. For longer duration of sequence casting higher amount of MgO along with higher thickness of the refractory lining is needed. It was no longer required to transfer the dry powder after fluidization (as required in gunning). This enabled the addition of fibers and other chemicals to the mass and homogeneous mixing and deposition became a reality. The lining could be preheated and the cast taken in a “hot start” fashion, or allowed to cool to room temperature and taken as a “cold start” tundish. While curing, it needs to be controlled to ensure lining integrity and this demands that the tundish permanent lining is ideally below 100 degrees C for satisfactory placement. Wet processes such as sprayable lining with up to 30% water addition by weight and the presence of hoses and spills may create OH and S issues in the steel plant. Even then this spray lining system was able to successfully combine many of the advantages of board and gunning, while eliminating the disadvantages like - joints, sand backing, rebound losses, dust problems, poor insulation etc. 

Merits:

  • low risk of H picking
  • no joints
  • no sand
  • low inventory
  • less labour intensive
  • easy deskull
  • good insulation
  • “cold start” also possible
  • controllable lining thickness
  • robotic application for big size tundish (involve large investment)

Demerits:

·        investment in equipments

·        intensive curing required

·        moderate washout risk

·        relatively longer T/D cycle (than boards)

Also Read: Gunning and Spraying - differences in these two methods of tundish wear lining, refractory lining repair and maintenance


Tundish Dry Lining

Tundish Dry Lining Curing System - schematic diagram (www.industry.guru)
Dry tundish linings were introduced in Europe probably in 1986. The system differ from all previous processes in the sense that it is applied in a dry powder form and do not require the addition of water to form the tundish working lining. Generally it utilizes a resinous bond (Binder / Catalyst reaction) which is activated by relatively low amounts of heat (around 160OC). Vibration may or may not be required, depending upon the product being used, but it is essential to use a former and the dry powder is fed in the gap between the tundish permanent lining and the former. The hot air is introduced at approximately 400OC and the heating cycle takes around 45 minutes with further 30minutes for cooling. Thus a lot time can be saved while on the negative side; the dry system still has lower insulation (due to higher density) and is dependant on crainage in the tundish bay for installation. But one major advantage of dry lining is that because of the absence of water in this system there is no direct adhesion to the permanent tundish lining which ensures good deskulling and prolongs life of tundish lining. Besides, the smooth finish on a dry tundish lining and ability to consistently reproduce lining geometry offers improvements in steel quality and better erosion resistance resulting in the potential to increase sequence lengths.


Merits:

  • no joints
  • no sand
  • low H risk (when hot)
  • low inventory
  • less labour intensive
  • reduced tundish preparing time
  • low washout risk
  • easy deskull
  • uniform liner
  • clean environment friendly application
  • high sequence possible
  • OH and S benefits
  • easy, quick installation
  • improved steel cleanliness because of lining integrity

Demerits:

·        investment in equipments

·        H risk (when cold)

·        lower insulation

·        crainage dependence

Sequence ‘Continuous Casting’ is increasing and creating a demand for higher performance of refractories. Refractories for continuous casting that are exposed to molten steel, are not only subject to heavy corrosion and abrasion by molten steel, but also have a large effect on quality of the steel and the yield points. So, while there are advantages even in bricked and gunning systems, the disadvantages outweigh the merits. Similarly although there are some demerits in all the systems of board, spray or dry lining, the advantages seem to be more in these systems. Making a choice appears to be difficult amongst the three systems with merits and demerits being almost equally balanced. Therefore, recourse must be taken of other factors like those of steel plant operations, quality of steel, etc when trying to decide between board, spray and dry linings.