Sprayable insulating liner compositions for metal vessels

Sprayable aqueous insulating liner compositions for metal casting vessels such as permanent refractory lined tundishes are disclosed that contain a water insoluble and hydration resistant particulate refractory grain containing MgO, organic binder and fibers, with the composition being substantially free of water soluble alkali and alkaline earth metal oxides that would migrate during drying with the water to degrade the permanent refractory lining.

BACKGROUND OF THE INVENTION 
Expendable liners have been employed to protect the permanent refractory 
lining of a metal carrying vessel such as a ladle or tundish. These 
expendable liners are usually fabricated from a composition having heat 
insulating properties either in the form of premolded boards or gunnable 
materials. Recently, the technology in this area has shifted from 
performed boards or gunnable compositions to sprayable insulating 
compositions that consist essentially of a particulate refractory material 
premixed with a binder and sprayed in the form of an aqueous mixture onto 
the permanent refractory surface of a metal carrying vessel like a 
tundish. The sprayable compositions are designed so that they can be 
sprayed into either a warm or a cold tundish. The sprayable compositions 
are based on magnesite refractory grain so that long casting times with 
corrosive steels and slags are possible. In addition to the magnesite 
refractory grain, the sprayable insulating mixes contain additives to 
control the density of the sprayed mixture and binders to produce a strong 
coating after drying. The sprayable mixture usually contains about 25% 
water, thus there is no dust during spraying and virtually zero rebound. 
The sprayable compositions can be sprayed at very high application rates 
usually on the order of about 6,000 to 9,000 pounds per hour. Typically, 
about the same weight of material is used to spray a tundish as the weight 
of tundish boards required to line the same tundish. 
One of the essential features of a sprayed tundish coating compared to a 
gunnable tundish liner is lower thermal conductivity which reduces energy 
losses from the steel; thus allowing reduced metal temperature in the 
ladle and basic oxygen furnace. This extends the useful life of the 
refractories in those vessels. The low thermal conductivity and high 
thermal expansion properties of the sprayed coating are essential so that 
the tundish may be easily stripped of these expendable coatings. The low 
thermal conductivity produces significantly lower temperatures behind the 
hot face of the sprayed coating and at the interface with the permanent 
tundish refractory liner. Maintaining low temperatures behind the hot face 
and at the permanent refractory interface prevents reaction and/or 
sintering and thus gives a friable material layer behind the hot face that 
provides easy stripping of the remaining skull after casting. Thus, 
sprayed liners reduce wear on the permanent tundish refractory liner. 
One of the current problems with sprayable insulating liner compositions is 
their tendency to degrade the permanent refractory liner and cause liquid 
formation at operating temperatures which in turn causes sticking of the 
skull. In other instances, even if the skull does not stick to the 
permanent refractory surface, compositions cause contamination of the 
permanent refractory surface thus producing spalling due to the increase 
in thermal expansion of the surface compared to the interior of the 
refractory. Sticking of the skull to the refractory causes some of the 
permanent refractory to be pulled out of the metal vessel when the skull 
is mechanically removed. 
SUMMARY OF THE INVENTION 
This invention is directed to an aqueous sprayable insulating liner 
composition for lining a permanent refractory surface on the inside of a 
metal carrying vessel. The sprayed liner is expendable and protects the 
permanent refractory surface during a pouring or casting cycle. The 
permanent refractory liner is usually made of refractory brick or 
refractory castable concrete compositions. 
The composition that is adapted for admixture with water comprises a water 
insoluble and hydration resistant particulate refractory grain, or grain 
mixture, consisting essentially of at least about 50% by weight of MgO, an 
organic binder and fibrous materials in amounts effective to bind said 
particulate refractory material together in an essentially dry state after 
the aqueous composition has been sprayed onto the permanent refractory 
surface and dried. It has been found that the sprayable composition must 
be substantially free of water soluble alkali and alkaline earth metal 
oxides. These oxides have been found to migrate with the water to a 
permanent refractory surface and thereby detrimentally affect the surface 
by causing the problems heretofore mentioned in the background of this 
invention. 
In particular, this invention is predicated in part upon the discovery that 
soluble alkali and alkaline earth metal oxides have been found to 
transport to, and concentrate at, the permanent refractory surface. These 
materials then degrade the permanent refractory and cause liquid formation 
at operating temperatures which in turn can cause sticking of the skull. 
Even if the skull does not stick, the contamination of the permanent 
refractory surface by alkali and alkaline earth metal oxides can produce 
spalling due to the increase in the thermal expansion of the surface 
compared to the interior. Therefore, it is critical to the success of this 
invention that all components of the sprayable insulating liner 
composition and aqueous mixture including a particulate refractory, and 
organic and inorganic binders and fibers, all must be chosen to eliminate 
amounts of soluble alkali and alkaline earth metal oxides that cause the 
problems aforementioned. Repeated use of sprayable aqueous compositions in 
the same tundish, increases the deposition of alkali or alkaline 
components on the permanent refractories; with the resulting sticking of 
the skulls and damage to the permanent refractories. Both alkali and 
alkaline earth metal oxides, for example, Na.sub.2 O, K.sub.2 O and CaO, 
quickly degrade the usual 50 to 70% alumina (Al.sub.2 O.sub.3) brick and 
castable compositions of the permanent tundish linings. It has been found 
and phase diagrams predict that as these metal oxides build up on the high 
alumina refractory with each successive spraying and drying cycle, more 
and more liquid formation occurs at lower and lower temperatures. Thus, 
multiple spray-use cycles build up the alkali, or other flux, content at 
the surface of the permanent refractory to the point where the permanent 
refractory is degraded and sticking and/or spalling occurs. 
Therefore, in a preferred form the insulating liner composition of this 
invention is adapted for admixture with water and spraying onto a 
permanent refractory surface on the inside of a tundish. The water 
insoluble and hydration resistant particulate refractory grain containing 
MgO is preferably supplied by a material selected from the group 
consisting of dead burned magnesite, used alone or mixed with olivine, 
magnesite-chrome prereacted grain and/or chrome ore. The organic binder is 
generally in an amount of from about 1 to about 10% by weight and the 
fiber content is present in an amount from about 0.3 to about 5% by 
weight. Fiber is preferably a combination of inorganic fiber and cellulose 
fibers, and mixtures thereof. Where mixtures are employed, the inorganic 
fibers are in an amount of up to about 2% by weight and the cellulose 
fibers are in an amount of up to about 3% by weight. 
The method of spraying the insulating liner composition is achieved by 
mixing with water the particulate refractory material along with the 
organic binder and fibrous material, with said binders present in an 
amount effective to bind the particular refractory material together in an 
essentially dry state on the surface of the permanent refractory liner. 
Usually sprayable compositions containing about 20 to about 30% by water 
are employed. Machines are used to automatically mix the dry insulating 
tundish mix with the correct amount of water and to deliver the slurry to 
the gun for spraying. The machines or conditions for operating them form 
no essential part of this invention and reference is made to the 
literature such as "Spray Lining Tundishes--a Vital Cog in Revitalization 
of Steelmaking", S. Lasday, Industrial Heating, pp. 30-33, April (1989), 
that is incorporated herein by reference. The mixed slurry is held in a 
hopper of the machine and is pumped through a hose to the gun as the 
operator calls for mix. At the gun, compressed air is added to propel the 
premixed slurry onto the permanent refractory tundish lining. The slurry 
can be sprayed onto the walls at rates of up to 9,000 pounds per hour. 
Generally, a thickness of about 1.25 to 1.75 inches of insulating slurry 
is applied, depending upon the length of casting and the location in the 
tundish. Thus, the slurry spray can be applied at rates of up to about 960 
to 1200 square feet per hour for about one 1.25 inch thick lining. This 
makes the rate of application very rapid and a typical bloom or slab 
caster tundish can be sprayed in 20 to 40 minutes. 
For rapid drying, the sprayed tundish can be heated at up to 1500.degree. 
F. to dry out the slurry coating. Two hours at 750.degree. F. is normally 
used to dry out a 1.5 inch thick coating when the tundish walls are cold 
when sprayed. Less drying time is required if the tundish walls are warm 
when sprayed. However, the maximum wall temperature during spraying should 
not exceed 200.degree. F. or problems may occur with the slurry not 
sticking to the wall due to water vaporization as the slurry strikes the 
hot wall. After drying, the tundish can be cooled and used either cold or 
preheated. The sprayed tundish should be used within a few days to prevent 
the insulating magnesite lining from picking up water from the moisture in 
the air. The tundish can be redried if needed. Care should be taken when 
cooling the sprayed tundish after drying at high temperatures. High 
density refractories in the tundish, such as dams, weirs and pouring pans 
can be cracked by thermal shock during rapid cooling. 
The procedure for determining amounts of soluble sodium oxide and calcium 
oxide involves extraction of a 120-140 grams sample of dry sprayable 
material with 250 mls of distilled water for a period of two hours using a 
Soxhlet extraction apparatus. The filtrate is diluted to one liter. Sodium 
and calcium ion concentration of the diluted filtrate is then determined 
by selective ion electrodes. It has been found that the soluble alkali and 
alkaline earth metal oxides should not be contained in the sprayable 
solids at levels of more than about 0.10% by weight of the dry material. 
More preferably, the tolerable amounts of these oxides are closer to 
levels of about 0.01% by weight or less. 
DETAILED DESCRIPTION OF THE INVENTION 
By way of illustrating and providing a better appreciation of the present 
invention, the following detailed description and examples are given 
concerning the sprayable refractory insulating liners of the invention and 
their properties. All of these materials must be carefully selected to be 
substantially free of amounts of water soluble alkali and alkaline earth 
metal oxides that degrade the permanent refractory liner. 
In a preferred embodiment, the sprayable refractory insulating liners are 
formed by about 75% to about 98.5% by weight of a water insoluble and 
hydration resistant particulate refractory grain, or grain mixture, 
containing at least about 50% by weight MgO and about 1% to about 15% 
organic and inorganic binders and fibrous material. The MgO refractory 
grain, also known as magnesium oxide, dead burned magnesite, periclase 
grain or magnesia, can be derived from any suitable sources and especially 
from sources, such as natural, seawater or brine magnesite, or any other 
suitable sources, or mixtures thereof. The magnesite or periclase grain, 
however, preferably is of the type commonly referred to as dead burned 
magnesite or dead burned periclase. By "dead burned" magnesite or 
periclase is meant magnesite or periclase fired to high temperatures to 
produce a water insoluble and hydration resistant particulate refractory 
grain consisting essentially of well-sintered low porosity periclase 
crystals and this grain structure distinguishes it from the more reactive 
lower temperature calcined caustic magnesites. Nevertheless, it should be 
understood that it is preferred that the MgO refractory grain content, 
whether derived from natural, seawater or brine magnesite, periclase 
grain, or other suitable sources, should be substantially pure. By 
"substantially pure", it means containing at least about 80% MgO by weight 
on the basis of an oxide analysis, with the remainder, if any, being only 
minor amounts of incidental impurities essentially excluding soluble 
alkali and alkaline earth oxides. Generally, the best results are achieved 
when the particle size distribution of the MgO refractory grain is below 
20 mesh as such grain sizes will be understood to a person of ordinary 
skill in the art. 
In accordance with other aspects of the invention, the use of a blend of 
different magnesite sources for MgO refractory grain can be used to 
achieve the results. The blend should be of sources for MgO refractory 
grain having an MgO content so that the average MgO content of all the 
refractory grain is at least about 50%. Further, sources of MgO refractory 
grain should be selected on the basis of their low tendency to hydrate due 
to their high dead burning temperatures and as a result of their 
composition. Other suitable refractory grain may also be added in 
acceptable amounts to the particulate refractory MgO such as zircon and 
alumina, as taught in U.S. Pats. Nos. 4,696,455 and 4,703,022. 
In carrying out the invention, the binder component may be derived from any 
suitable binder or mixtures of binders of those known in the refractory 
making and allied industries of organic and inorganic binders. Preheat or 
heating conditions cause some organic binders incorporated within the 
liners to first carbonize to produce a carbon bond and then to eventually 
burn out, for instance, starting at the hot face and sometimes throughout 
the entire sprayed thickness, of course, depending upon preheat time, 
temperature and spray coating thickness. Nonetheless, up until the point 
of burnout, the organic binders and the carbon bond produced serve to hold 
or bind the other materials together and comprise by weight of the liner 
from about 1.0% to about 10%. Samples of organic binders suitable to be 
employed in the liners of the present invention include, but are not 
limited to, starches, cereals, natural or synthetic resins, such as amino 
resins, phenolic resins or mixtures thereof. More particularly, powdered 
forms of the phenol-formaldehyde and urea-formaldehyde resins are best 
suited for use and most preferably is the phenolformaldehyde resin. It 
should be appreciated that when the phenol-formaldehyde resin is employed, 
a catalyst such as hexamethylenetetraamine, also known as HMTA, should be 
added in sufficient amounts to polymerize the phenolformaldehyde resin to 
bond the refractory grains for making a rigid structure suitable for use 
as a liner. 
In addition to providing binding support prior to the burnout of organic 
binder, the fibers serves to control the viscosity of the sprayed material 
to prevent it from slumping off near vertical walls after spraying and 
during drying. The inorganic fibrous material also serves to stick or hold 
the particulate refractory component together during preheat or heat 
conditions particularly after the organic binder and organic fiber has 
been consumed or burnt out. In further keeping with the invention, as to 
the fibers, the following are preferred: inorganic fibrous materials such 
as rockwool, slag wool, glass wool, chopped continuous fiberglass 
filament, refractory aluminum silicate fibers, and organic fibrous 
materials such as cellulosic materials derived from paper, synthetic 
organic fibers or the like, and particularly paper. These fibers generally 
serve to reinforce the sprayed material so that it stays in place when 
sprayed and during drying, preheating and while casting metal. The use of 
such fibers in the sprayable insulating mix also assists in producing a 
dried spray material with a low bulk density, whereby the heat-insulating 
effect thereof is improved. As noted above, the fibers represent by weight 
of the liner from 0% to about 10% and preferably up to about 5%. 
In accordance with the present invention, the sprayable refractory 
insulating liner compositions contain about 20-30% by weight water to 
enable them to be pumped through rubber hose and to be sprayed when the 
slurry is mixed with compressed air in a spray gun. The sprayed liners are 
suitable for forming linings for casting vessels, such as hot tops, 
ladles, tundishes, troughs and pipes etc., which are intended to contain 
molten metals such as ferrous alloy metals. Preferably, the liners of this 
invention are employed in tundishes and the following examples illustrate 
the practice of the invention.

EXAMPLES 
The following represents two preferred compositions A and B for 
manufacturing a sprayable refractory insulating liner in accordance with 
this invention. These compositions essentially are free of water soluble 
alkali and alkaline earth metal oxides, i.e., contain less than 0.10% by 
weight, or about 0.01% by weight of such oxides. 
______________________________________ 
Approx. 
Composition Ingredient % 
______________________________________ 
A Phenol-formaldehyde resin 
2.00 
Hexamethylenetetraamine 
0.13 
Paper 0.80 
Fiberglass 0.80 
Magnesite: about 90% MgO 
64.18 
Olivine 32.09 
100.00% 
B Phenol-formaldehyde resin 
2.00 
Hexamethylenetetraamine 
0.13 
Paper 0.80 
Fiberglass 0.80 
Magnesite: about 90% MgO 
32.09 
Reacted Magnesite-Chrome 
32.09 
Grain 
Olivine 32.09 
100.00% 
______________________________________ 
The compositions A or B are blended with about 20-30% water and sprayed 
under conditions of spray rate and air pressure as will be understood to 
one of skill in the art using a machine of the type referred to in the 
Industrial Heating article referred to above. 
After spraying at rates of about 960 to 1200 square feet per hour to 
provide about 1.25 inch thick lining into a tundish, the coating is dried 
at about 750.degree. F. Tundishes having a permanent refractory lining are 
repeatedly sprayed with compositions like A and B above without any 
detrimental effects on the permanent refractory of the type noted in the 
background of this invention. Wherefore, it will be understood to persons 
of skill in this art that this invention provides advantages in overcoming 
prior art problems and variations may be made without departing from the 
scope hereof.