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Showing posts with label Gunning. Show all posts
Showing posts with label Gunning. Show all posts

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

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5-April-2009
In this article we will try to understand the basics of Gunning and Spraying, two frequently used methods for enhancing the refractory lining life of a furnace or refractory lining repair and maintenance. Also, differences between Gunning and Spraying, preferential situations for either of these two methods in EAF, BOF, melting furnaces, steel ladles, tundish lining, etc.
There are instances when steel plants are found switching over from one method to other for refractory lining repair and maintenance depending upon their perceived and actual benefits obtained. But well-documented published data of such experiences, which can be of immense help for others, are either scanty or sparsely available. In the article A Comparative Evaluation of Different Types of Tundish Lining Refractories based on actual experiences in Steel Plants it has been tried to put together some such experiences made by others and were presented in some recently held different seminars and conferences on Refractories. 

GUNNING or GUNNABLE REFRACTORIES - Principles & Procedure

Industry Guru - image of Gunning Maching for Gunnable Refractories
The gunning repair is a well-proven procedure by which refractory material can be applied quickly and cheaply. Initially these were alumino-silicate based and later converted to basic type magnesite based to assist with metallurgical practice.
Gunning or gunnable refractories are used for the hot repair of ladles and melting furnaces as well as relining or cold repair of the back lining. There are two basic methods: dry and wet gunning. With dry gunning the material is discharged from the machine with a maximum of 5% moisture and then fed to the gunning nozzle by an air stream where the required water, typically 5–10%, is added. With wet gunning the gunning material is moistened with water in a mixer and then pumped through a hose by means of an eccentric screw or a piston pump. At the end of the line the material is dispersed with compressed air and, if necessary, an additional liquid bonding/hardening agent can be added (shotcreting). Wet gunning has the problem that the machine or the gunning hose may get clogged by already moistened refractory material, particularly when not in continuous use; hence the process has to be carefully controlled. Also, the equipment requires more intensive cleaning and is not considered an efficient operation for applications less than 400 kg. With dry gunning, blockages in the conveying hose can be blown free by compressed air only. For optimum gunning and refractory performance the gunning material is essentially the same as the original refractory (plus binder material), but with a size distribution of 4 mm maximum. The applied thickness is typically in the range 10–30 mm and a vessel can normally be used again after 3–5 minutes.
Mixing of Water and Gunnable Refractory Materials
Three essential requirements for a good gunning repair of refractory lining are optimal moistening, homogeneous mixing of the gunning material with water, and a high quality gunning machine that guarantees even conveying. With pre-moistening a share of the gunning water is added some meters away from the gunning nozzle and relies on the turbulence within the conveyance for premixing. This approach, however, is very susceptible to operational problems. For instance adding too much water in advance may lead to clogging, particularly with quick binding systems where hardening starts inside the hose. Too much water reduces refractory quality and hence refractory lifetime. Also, the water can dissociate in the liquid steel to hydrogen, which can be detrimental to some steel grades (micro-cracks). With standard mixing heads (for dry gunning) the water is jetted through radial borings and, in order to compensate for inadequate initial moistening, the operator often works with a surplus of water so that the dust is minimized. This, however, often leads to use of incorrect water/ cement ratio, resulting in reduced refractory durability. Improvements can be achieved by use of pressure increasing pumps so that the water jet becomes sharper and more readily reaches the centre of the nozzle cross-section. Optimum nozzle-vessel distance is quite varied; between 40 and 1,000 mm depending on application.
Standard gunning refractories are magnesia, alumina or silica based refractory materials normally a monolithic applied on the areas that encounter severe wear out such as trunnions, scrap impact area and the slag line. These materials differ in price so the optimum choice depends on many factors such as steel grade, slag composition and the vessel used. A shooter type of gun is used for the gunning process to encounter hostile environment of the process. Consumption of gunned refractory is also extremely varied, ranging from 0.2-2 kg per ton steel depending on conditions of the vessel and environment.
Also read: Nomenclatures and Terminologies of Monolithic Refractory Products based on their setting or installation methods  


DIFFERENCE BETWEEN GUNNING AND SPRAYING

The principles, procedure and differences between Gunning and Spraying can be understood by the flow diagram below:

https://www.industry.guru - Gunning vs Spraying of refractories depicted through flow diagram
Gunning materials or gunnable refractories, refractory castables are transported through flexible hoses to the installation position, where the materials are wetted and projected through a handheld nozzle at the target area. Dry gunning allows operating farther away from the feed station, though wet gunning or shotcreting offers a faster output rate.
Since homogeneous mixing is possible in spraying (before the product is applied), the incorporation of special chemical additives can help to improve thermal stability properties of the lining and also impart good flexibility.   
https://www.industry.guru - Refractory Spraying machins image

The most commonly faced problems of using a gunning machine or in the process of gunning are -
  • Dust formation during gunning
  • High rebound losses leading to wastage and high consumption of material
  • Difficulties in applying variable thickness leading to metal penetration and insufficient permanent refractory lives, and
  • Difficult deskulling.
Whereas when the material is applied through spraying it has the following benefit -
  • No dust formation during application
  • No rebound loss hence minimal loss of material
  • The lining thickness was better controllable, thus increasing permanent refractory lives
  • Deskulling was better.

Nomenclatures and Terminologies of Monolithic Refractory Products based on their setting or installation methods

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The following terminologies which have been given for different type of Monolithic Refractory products are based on and therefore, can be related to their lining process or installation methods. Read our previous article

Castables

These are dry mix hydraulic compositions of graded refractory aggregates with a suitable bonding material, Fillers & (/or) special Additives. On addition of the prescribed quantity of water to these compositions, a wet concrete-like mass is produced which forms useful castables. Ceramic bond is developed when the same is fired at a certain high temperature.
Type of Castables -

  • Conventional Castable.
  • Low Cement Castables (LCC).
  • Ultra Low Cement Castable (ULCC).
  • No Cement Castable (NCC).
  • Light Weight Castables.
  • Self Flow Castables (SFC).
  • Insulating Castable.

Coating Mass

These are the Refractory materials especially made for coating on the working surface of the lining having a thin thickness.

Fettling Mass

It is a material which is deposited on the worn out areas of banks and bottom of Electric Arc Furnace (EAF) in hot condition. These are normally applied by shoveling and by using a Fettling Machine.

Gunning Compound or Gunning Mass

Gunning Masses are also unshaped refractory materials (Products) made after blending graded refractory grains with suitable bonding agents and are installed with a Gunning Machine.

Type of Gunning Compounds -

  • Dry Gunning
  • Shortcreting
  • Flame Gunning

Injection Mass

These are the refractory materials that can be injected into a furnace in the slurry state.

Mortar

Powdered or finely ground refractory preparation which becomes plastic and trowel able when tempered with water and is suitable for laying and bonding refractory bricks.

Patching Mass

These are powdered refractory materials mixed with water and/or added with a binder, and with adequate plasticity that can be patched in a place.

Plastic Refractories / Mass / Plastics

Refractory materials tempered with water and/or added with a binder, which has adequate plasticity to be rammed into a place.

Ramming Mass

Ramming Masses are the ready-mix of refractory materials usually tempered with water, that can not be extruded but that has suitable properties to permit ramming into a place to form a monolithic structure.

Type of Ramming Masses -

  • Dry Ramming Mass
  • Wet Ramming Mass

Spraying Mass

These are refractory mix consisting of fines and additives which are installed with a Spraying Machine (can also be applied with a Gunning Machine).

Related articles

Type of Monolithic Refractories - Monolithic Refractory Products

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What are Monolithic Refractories?

Monolithic refractories or Monolithics are those, which have no definite shape. In other words these are the refractories which can be moulded or can be given any shape as per requirement. Because of this reason, these products are also known as the ready-refractories and can be used as such on the site to repair or build a furnace. Monolithics consists of graded Refractory aggregates, Binders, Fillers & (/or) special Additives for modification of properties.

Type or Classification of Monolithics:

The classification and nomenclatures of monolithics have been done on the basis of more than a single criterion.

Based on Physical State

1. Powdered state - Castable, Mortar and Ramming Mass.
2. Mud state - Plastics.
3. Paste state - Coating Mass.

Based on Setting Method

1. Hydraulic setting - Castables.
2. Air setting - Mortars, Plastics.
3. Chemical setting - Plastics, Ramming Masses.
4. Ceramic (Heat) setting - Mortars, Plastics, Ramming Masses.

Based on Installation Method

1. Trawling and Pouring - Castable, Mortar.
2. Ramming - Ramming Mass.
3. Gunning - Gunning compound, Castable.
4. Vibrating - Castables.

The Terminologies of the different type of Monolithic Refractories will be discussed in another article: