Coating compositions for liners of molten metal troughs and ladles

A composition for coating the refractory liners of ladles and troughs for pouring and/or conveying molten metals comprising an aqueous dispersion containing finely divided zirconia, a colloidal silica, finely divided mica, finely divided zircon, finely divided bentonite and, optionally, an inert coloring agent. The coating composition is capable of re-bonding fractured liners and the resulting coating prolongs the effective life of the liner and exhibits superior reflectivity, causing the molten metal to retain its heat which facilitates better pouring.

BACKGROUND OF THE INVENTION 
This invention relates to coatings for the liners of ladles, troughs and 
the like used in handling molten metals. 
Ladles and troughs for pouring and/or conveying molten metals usually have 
a refractory lining which must be repaired or replaced periodically. 
Coating compositions capable of providing a protective overcoating for 
such linings are desirable for the purpose of prolonging life of the 
liners. 
Coating compositions containing refractory ingredients have been used to 
coat the molding surfaces of metal molds used for casting metal parts. 
Examples of such mold coating compositions are disclosed in U.S. Pat. Nos. 
1,886,249 (Bensing), 2,544,598 (Kalina), 3,243,397 (Herkimer et al.), 
3,436,235 (Baer), and 3,447,936 (Ornitz). Examples of other compositions 
containing refractory ingredients and used for different metal molding 
applications are disclosed in U.S. Pat. Nos. 3,059,296 (North) and 
3,859,153 (Beyer et al.). None of these compositions are particularly 
effective as a protective overcoating for refractory liners of ladles and 
troughs. 
SUMMARY OF THE INVENTION 
One of the principal objects of the invention is to provide a composition 
for coating the refractory liners of ladles and troughs for pouring and/or 
conveying molten metals and producing a protective overcoating capable of 
withstanding repeated pourings of molten metal at elevated temperatures up 
to 3,800.degree. F. and higher and thereby prolong the effective life of 
the liner. 
Another of the principal objects of the invention is to provide such a 
coating composition which is capable of re-bonding fractured liners and 
producing a smooth-surfaced overcoating which minimizes the retention of 
slag and/or metal when the ladle or trough is emptied. 
A further of the principal objects of the invention is to provide such a 
coating composition capable of producing a protective overcoating having 
superior heat reflection properties, causing the molten metal to retain 
its heat for a longer time during transit. 
Other objects, aspects and advantages of the invention would become 
apparent to those skilled in the art upon reviewing the following detailed 
description and the appended claims. 
The coating composition provided by the invention is an aqueous suspension 
containing about 20 to about 50 weight % finely divided zirconia, about 25 
to about 40 weight % colloidal silica, about 5 to about 15 weight % finely 
divided mica, about 5 to about 25 weight % finely divided zircon, about 
0.5 to about 5 weight % finely divided bentonite, 0 to about 3 weight % of 
an inert coloring agent, and about 5 to about 20 weight % water. 
DESCRIPTION OF THE PREFERRED EMBODIMENT 
The coating composition of the invention can be applied over refractory 
liners of ladles and troughs for pouring and/or conveying a variety of 
molten metals including cast iron, gray iron, steel, brass, bronze, 
copper, aluminum and stainless steel. The coating composition is quick 
setting and is capable of bonding fractured liners. The resulting 
overcoating is capable of providing temperature protection after repeated 
pours of molten metal at temperatures of up to 3,800.degree. F. and 
higher, thereby extending the effective life of the liner for several 
weeks in some cases. The overcoating has a unique ability to reflect heat, 
causing the molten metal to retain its heat for a longer time which means 
that good pouring can be obtained with less reheating. The surface of the 
overcoating can be smoothed during the application to a degree which 
minimizes the retention of slag and/or metal when the ladle or trough is 
emptied. 
While the theory by which the coating composition of the invention 
functions to provide the above and other advantageous properties is not 
fully understood, some observations have been made. Unless noted 
otherwise, the weight percentages disclosed below are based on the total 
weight of all the ingredients in the coating composition. 
Zirconia provides the coating composition with refractory properties and 
enhances the heat insulating properties of the resulting overcoating so it 
can provide temperature protection when exposed to molten metals at 
temperatures up to 3,800.degree. F. and higher. The zirconia is in finely 
divided or comminuted form, preferably at least the majority of particles 
being -325 mesh. The amount of zirconia is about 20 to about 50, 
preferably about 25 to about 35, weight %. 
To enhance the smoothability of the overcoating surface and maintain high 
heat insulative properties and still minimize costs, a mixture of a lower 
purity zirconia, such as Insuloxide (contains approximately 95% ZrO.sub.2) 
marketed by NL Industries of Highstown, N.J., and a finer, higher purity 
zirconia, such as Electrically Fused Zirconia Oxide 441 (contains 
approximately 99% ZrO.sub.2) marketed by TAM Ceramics of Niagra Falls, 
N.Y., is preferred. When such a mixture is used, the amount of lower 
purity zirconia can be about 5 to about 20, preferably about 7 to about 
17, weight % and the amount of the higher purity zirconia can be about 15 
to about 30, preferably about 17 to about 25 weight %. 
Finely divided zircon (zirconium silicate) particles, i.e., at least a 
majority -325 mesh, usually are rounded and polished and, therefore, 
further enhance the smoothability of the overcoating. Zircon also enhances 
the heat insulating properties and compressive strengths of the 
overcoating. The amount of zircon used is about 5 to about 25, preferably 
about 10 to about 20, weight %. 
Mica, while considerably less less expensive than zirconia and zircon, 
enhances the heat insulation properties of the overcoating because of a 
low thermal conductivity and improves the reflectivity of radiant heat. 
The amount of mica used is about 5 to about 15, preferably about 7 to 
about 12, weight %. 
The colloidal silica serves primarily as a binder for the refractory 
ingredients of the coating composition and promotes adherence of the 
overcoating to the liner surfaces. It has been found that the coating 
composition can be used to bond fractured refractory liners and repair 
chips because of an unique ability to bond to itself. Various colloidal 
silica compositions conventionally used in investment casting and the like 
can be employed. Representative suitable commercially-available colloidal 
silica compositions include the Ludox aquasols marketed by DuPont, 
particularly Ludox HS-40%, the Syton aquasols marketed by Monsanto 
Chemical and the Nalcoag aquasols marketed by Nalco Chemical. The amount 
of colloidal silica used is about 25 to about 40, preferably about 27 to 
about 35, weight %. Coating compositions containing colloidal silica 
compositions less than about 25 weight % generally do not bond adequately 
to refractory liners. 
Bentonite serves primarily as an aid in maintaining the refractory 
ingredients suspended in the water carrier. Excessive amounts of bentonite 
can unduly reduce the compressive strength and heat resistance of the 
resulting overcoating. The amount of bentonite used is about 0.5 to about 
5, preferably about 1 to about 3, weight %. 
The inclusion of an inert coloring agent, such as W-4123 Phthalocyanine 
Blue (an aqueous suspension of phthalocyanine blue containing 35-37 weight 
& solids) marketed by Harshaw Chemical, provides the coating composition 
with a distinctive color which permits the applicator to more easily 
determine surface smoothness of the overcoating during application. When 
used, the amount of the coloring agent can be up to about 3 weight % and 
preferably is about 0.01 to about 2 weight %. The color can be of a shade, 
such as light blue, which brightens the interior of the ladle or trough 
and makes it still easier for the applicator to detect rough surface areas 
which might promote slag or metal deposits. Other suitable coloring agents 
can be used, such as W-3247 Burnt Umber (an aqueous dispersion of burnt 
umber containing 50-52 weight % solids) marketed by Harshaw Chemical. 
The amount of water used in the coating composition depends on the 
particular coating technique employed. In any event, the amount of water 
used is about 5 to about 20, about 7 to about 15, weight %. If the amount 
of water is above about 20 weight % the coating composition is too fluid 
or runny to obtain an overcoating of the desired thickness. On the other 
hand, if the amount of water is below about 5 %, the coating composition 
becomes so thick it cannot be conveniently applied as a uniform, 
smooth-surfaced overcoating. 
The coating composition can be prepared by any suitable procedure whereby 
the ingredients are uniformly dispersed throughout. For example, all the 
liquid-containing ingredients, including the collateral silica, the 
coloring agent and water, can be added to a high speed blender and 
pre-mixed. The solid ingredients can be added to the pre-blended liquid 
ingredients in any sequence and the resultant mixture blended for a time 
sufficient to provide the desired dispersion. 
The coating composition can be applied in any suitable manner, such as 
brushing, wiping, troweling or spraying, capable of providing a smooth 
overcoating of substantially uniform thickness over the surface of the 
liner. Generally, an overcoating thickness on the order of about 3/8 to 
1/2 inch is sufficient. The overcoating preferably should be built up more 
in the areas bearing the force of the pouring molten metal. 
The coating composition preferably is applied over refractory liners at a 
temperature of about 100.degree. to about 450.degree. F. The coating 
composition can be applied while the ladle or trough is still warm from 
use or after the ladle or trough had been preheated. In any case, the 
temperature of the liner should not exceed 450.degree. F. After a buildup 
of about 3/8 to about 1/2 inch, the overcoating is heated to a temperature 
up to 450.degree. F. for at least 30 minutes to remove or "bake out" all, 
or substantially all, the moisture prior to pouring a molten metal into 
the ladle or trough. 
If the coatings are applied too thick or dried too rapidly, superficial 
surface cracks may occur. This can be corrected by simply applying a thin 
finish coat to fill the cracks. As alluded to above, the coating 
composition can be used to re-bond fractured liners. This is accomplished 
by working the coating composition into the cracks. Additional layers of 
the coating composition can be applied periodically over an existing 
overcoating, after removal of slag and/or metal deposits, to further 
increase the effective life of the liner. 
Without further elaboration, it is believed that one skilled in the art 
can, using the preceeding description, utilize the present invention to 
its fullest extent. The following example is presented to exemplify a 
preferred embodiment of the invention and should not be construed as a 
limitation thereof.

EXAMPLE 
A coating composition having the following composition has been found to 
yield excellent results when applied to the refractory liners of ladles 
and troughs used in pouring and/or conveying a variety of molten metals: 
______________________________________ 
Ingredient Weight, lbs. 
Weight % 
______________________________________ 
Low purity zirconia (1) 
90 10.99 
High purity zirconia (2) 
154 18.80 
Colloidal silica (3) 
225 31.13 
Mica 95 11.60 
Zircon (4) 131 16.00 
Bentonite 13.75 1.68 
Colorint agent (5) 
0.25 0.03 
Water 80 9.77 
819.0 100.0 
______________________________________ 
Notes: 
(1) Insuloxide (94.72% ZrO.sub.2, 2.18 max. % retained on 325 mesh sieve) 
marketed by NL Industries. 
(2) Electrically Fused Zirconium Oxide 441 (98.63% ZrO.sub.2, 1.5% 
retained on 325 mesh sieve) marketed by TAM Ceramics. 
(3) Ludox HS40% (aqueous colloidal dispersion, 40 weight % silica as 
SiO.sub.2) marketed by DuPont. 
(4) Zircon G (98% zirconium silicate) marketed by National Lead. 
(5) W4123 Phthalocyanine Blue (aqueous dispersion, 34-37 weight % solids) 
marketed by Harshaw Chemical. 
The resulting suspension has a light blue color, has a Baume' density of 
about 79, does not freeze at temperatures above about 0.degree. F. and has 
been found to remain stable after several months of on-shelf storage. The 
resultant coating is non-flammable and provides excellent temperature 
protection for liners exposed to molten metal at temperatures as high has 
3,400.degree.-3,800.degree. F. In many cases, the effective life of liners 
for ladles and troughs can be prolonged 2-5 weeks with a resultant saving 
in material and labor costs. An overcoating of the coating composition has 
a superior ability to reflect heat, causing the molten metal to retain its 
heat for a longer time during transit and thereby assuring better pouring 
with less energy for reheating. 
From the foregoing description, one skilled in the art can easily ascertain 
the essential characteristics of the invention and, without departing from 
the spirit and scope thereof, can make various changes and modifications 
to adapt it to various usages and conditions.