5-April-2009
GUNNING or GUNNABLE
REFRACTORIES - Principles & Procedure
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.
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
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.
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:
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.
The
most commonly faced problems of using a gunning machine or in the process of
gunning are -
- 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.
