Refractory insulating products having controlled porosity and the preparing thereof by the wet method

Refractory insulating products having controlled porosity, a high resistance to thermal shock, erosion and corrosion by liquid metals such as aluminium, obtained by the wet method, containing (a) an aluminous cement (b) a refractory aggregate, and (c) more than 10% by weight, in relation to the refractory aggregate, of polycrystalline ceramic fibers with at least 70% alumina and at most 30% silica.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention concerns insulating refractory products having 
controlled porosity as well as the preparation thereof by the wet method. 
2. Description of the Prior Art 
It is known that the manufacturing of devices for casting, purifying and 
handling molten metals and more particularly corrosive molten metals such 
as aluminum, is conditioned by the perfectioning of refractory materials 
capable simutaneously of bearing the high temperature at which these 
molten metals are kept, of withstanding effectively their very corrosive 
action and lastly of bearing the thermal shocks to which the walls of 
these devices are subjected. During the last few years, various efforts 
have been made with a view to perfecting such materials. In general, these 
efforts have made it possible to obtain refractory products of the ceramic 
type which withstand well the aggressiveness of metals. But in numerous 
cases, these products are not very resistant to thermal shocks. Now, it is 
known that the resistance of a material to thermal shocks varies in an 
inverse proportion to the modulus of elasticity of the material, which 
itself depends directly on its porosity. It therefore appeared that a 
method enabling the controlling of the porosity of a refractory product 
during the manufacturing thereof gives the means of defining its 
resistance to thermal shocks. 
Very recently, a process enabling the controlling of the porosity of a 
product of the type comprising ceramic substances obtained by sintering 
has been perfected. It consists more particularly in mixing a powder of 
oxydes of Zirconium, Hafnium, Yttrium, Lanthanum, Beryllium and Calcium, 
with granulated substances resulting from the crushing of a dense sintered 
product obtained from these same powders, then in sintering that mixture 
at a relatively low temperature in a fluoride of a metal of group I of the 
periodic classification of elements. The porosity depends hence on the 
proportion of granulated products incorporated with the powder. The 
results obtained are very satisfactory but the method proves to be 
expensive and the industrial application thereof is probably fairly 
limited. 
Moreover, processes using the wet process seemed not to have any future. 
Indeed, when concrete is made by the adding of a hydraulic binding agent, 
there is no mastery of the porosity obtained. By vibrating the product 
placed in a mould for a more or less long period, the porosity thereof may 
at the most be decreased but not increased. 
Indeed, processes for manufacturing refractory products formed by mixing in 
a stirring machine products such as tabular alumina and aluminous cement 
by the damp method are known, but in these known methods, the product thus 
obtained is fired at a temperature in the order of 1700.degree. C., 
leading to a great sintering of the product. That process therefore leads 
to a refractory ceramic substance which does not withstand well repeated 
thermal shocks. 
Other manufacturing processes using the wet process, with the same 
refractory products and effecting the firing at lower temperatures, are 
also known, but in these processes, the sintering of the product is made 
easier by incorporating a flux such a bentonite which increases, indeed, 
the cohesion of the grains of the refractory product but which reduces, at 
the same time, the porosity of the finished product and, subsequently its 
resistance to thermal shocks, lastly, by the incorporating of the flux, a 
product having a poor resistance to erosion and corrosion by reactive 
metals such as aluminum is formed. 
There were also proposed in U.S. Pat. Nos. 3,294,562 and 3,269,849 
refractory compositions to be used in contact with molten nonferrous 
metals of melting points below about 1000.degree. C., such as aluminium 
and aluminium base alloys, consisting essentially of 40 to 60% by weight 
of asbestos fibers, 30 to 20% by weight of calcium aluminate cement, 30 to 
20% by weight of a fibrous material selected from the group consisting of 
fibrous alumina-silica which is refractory and substantially inert toward 
molten aluminium and fibrous potassium titanate, pressed under a high 
pressure and fired at temperatures from about 1400.degree. F. to 
2000.degree. F. The disclosed alumina-silica fibers were either fibers 
with substantially equal amounts by weight of alumina and silica, such as 
the Fiberfrax and Kaowool fibers, or fibers of substantially pure silica, 
such as the Refrasil material. However, the asbestos fibers become brittle 
upon dehydration between 400.degree. C. and 600.degree. C., and the 
alumina-silica fibers have a high silica content and are therefore subject 
to corrosion by molten aluminium, which progressively reduces their silica 
content into silicium, and moreover such compositions have a relatively 
low resistance to fracture under thermal shock, so that they withstand a 
relatively low number of successive immersions into, and emersions from, 
molten aluminium, and cannot be used in devices to be submitted to contact 
with molten aluminum during long service periods, such as pumps for molten 
aluminum. 
The search for refractory products simultaneously meeting the requirements 
of resistance to corrosion and erosion by molten aluminum and aluminum 
base alloys and of resistance to thermal shock, and capable, moreover, of 
being obtained on an industrial seale has led us to define a new product 
as refractory as ceramic substances but withstanding repeated thermal 
shocks and very long contact with molten aluminum. 
SUMMARY OF THE INVENTION 
The object of the invention is a refractory obtained by firing at a 
temperature below the sintering temperature a mixture of a refractory 
aluminous cement and of ceramic mixture of a) a refractory aggregate 
selected from the group consisting of tabular alumina, corindon, 
stabilized zirconia and calcium zirconate and b) refractory ceramic 
polycrystalline fibers comprising at least 70% alumina and at most 30% 
silica, the proportion of ceramic fibers being higher than 10% of the 
amount of refractory aggregate, and the proportion of the ceramic mixture 
being between 10 and 70% of the total mixture. 
This product, having a low apparent density comprised between 1 and 1.6, 
and a porosity between 60% and 75%, is therefore very light. It is 
obviously much more porous than the ceramic substances obtained by 
sintering the same compounds; nevertheless, it is in the form of a strong 
solid substance having mechanical properties quite sufficient for making 
various devices for casting and pumping molten aluminum and aluminum base 
alloys. Due to its chemical constitution, it withstands the chemical 
action of molten aluminum as well as ceramic substances obtained by 
sintering of the same materials, but it has a very much greater resistance 
to repeated thermal shocks. 
Moreover, the products thus obtained are excellent thermal insulants, since 
their coefficient of heat conduction lies between 0.1 and 0.5 Kcal/m/sq 
m/.degree. C./hour. 
Lastly, these products, although fired at a temperature lower than their 
sintering temperature, are nevertheless perfectly chemically and 
dimensionally stable. 
The process described hereinbelow in its more general form is a wet process 
which makes possible to obtain these refractory products, using 
exclusively usual industrial products easily available in trade. 
That process comprises the following operations: 
forming a dry mixture of refractory aluminous cement and aggregate; 
dry mixing the polycrystalline fibers with the mixture of refractory cement 
and aggregate, in such a ratio that the proportion of fibers is higher 
than 10% of the amount of aggregate; 
dispersing the latter mixture in a certain quantity of water; 
emulsifying the dispersed product by adding 0.5% to 3%, in relation to the 
weight of water, of a foaming agent stable in a medium of a pH greeter 
than 3, and a stabilizing agent; 
casting and drying the product in a mould having lubricated walls; 
firing the product thus obtained at a temperature below its sintering 
temperature. 
The porosity of the final product is directly a function of the 
water/cement ratio. The proportion of water to be mixed to the dry 
cement-aggregate-fibers mixture is therefore to be strictly defined 
according to the porosity to be obtained. 
The preparation of the intermediate emulsified product can also be effected 
by preparing separately the dry mixture cement-aggregate-fibers, and 
preparing on the other hand emulsified pure water, then dispersing the dry 
mixture in the emulsified water. 
It can also be effected by adding simultaneously the components of the 
mixture into the water to which the foaming agent has been already added. 
The refractory cement used basically consists of calcium aluminate. 
The refractory aggregates to be used are, separately or in mixture, tabular 
alumina, corindon, stabilized zirconia or calcium zirconate. The 
refractory fibers preferably contain about 85% alumina and about 15% 
silica. 
To obtain the foam in which the dispersing is effected, it is possible to 
take any foaming agent except these which are stable exclusively in a very 
acid medium (pH &lt;3). 
There can be used as stabilizing agent an organic product stable in an 
alkaline medium (pH 10) having a reticulated structure with large meshing, 
such as a soluble resin or a polysaccharide or a polysulphate.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
A dry mixture of polycrystalline fibers containing about 85% alumina and 
14.5% silica, with minor amounts of iron sesquioride and sodium oxide, is 
formed with a previous mixture of refractory aluminous cement and tabular 
alumina. For instance, 8 kg of the aluminous cement Lafarge Secar 250 with 
73% alumina and 27% calcium oxide, and 6 Kg of alumina type T60 passing 
through sieve 22 (US Standard 100 mesh), manufactured by Alcoa, are placed 
in a Moritz mixer. The mixing is continued for an hour 6 Kg of the 
refractory fibers manufactured by the French Company SEPR under the 
tradename Fibral are added, and the whole is mixed again for an hour. 
The product obtained is placed in a Moritz turbo- milling machine, 
consisting of a closed enclosure of a capacity of 50 liters, provided with 
a blade rotating at a speed adjustable between 0 and 1000 rpm, having a 
double casing allowing the coaling of the mixture by a flow of water. 10 
Kg of water are added, then 50g of foaming agent W53FL of Zimmer & 
Schwartz, and 20g of a stabilizing agent made up by a polysaccharide sold 
under the trade name "Zusoplatz PSI" by Zimmer & Schwartz. 
The emulsion is obtained after 2 minutes. It is cast in a steel mold 
previously lubricated with Johnson No. 103 polish. Then, after 24 hours of 
setting, the article is stripped, dried and heated in a furnace in a damp 
atmosphere according to the following program. 
Drying by heating from ambient temperature to 150.degree. C. at a rate of 
10.degree. C. per hour with stages of 10 hours at 60.degree. C., at 
95.degree. C. and of 20 hours at 150.degree. C. After drying, firing by 
heating at a rate of 10.degree. C. per hour up to 500.degree. C., with a 
stage of 10 hours at 300.degree. C., then heating at a rate of 50.degree. 
C. per hour up to 800.degree. C. The latter temperature is very much below 
the sintering temperature of the product. 
The refractory product thus obtained has an apparent density of about 1.1 
and a porosity of 72.5%. Its mechanical properties are the following: 
Resistance to breaking: .sigma. = 40 Kg/cm.sup.2 
Dilatation coefficient: .alpha. = 5.10.sup.-6 
Young's modulus: E = 2.10.sup.5 Kg/cm.sup.2 
so that the temperature difference above which fracture will occur as a 
result of thermal shock (Kingery, Introduction to Ceramics, 1960, p. 
635-636) 
EQU Tf = k(.SIGMA./.alpha..multidot.E) = 0,4 k 
That temperature difference is about 5 to 10 times higher than that 
obtained with a similar products with refractory fibers of about 50% by 
weight alumina and 50% silica. 
Moreover, pumps for molten aluminum, with inner parts in contact with the 
molten aluminum made up with this refractory, have withstood 2000 hours of 
operation without any substantial crack or spalling, erosion or corrosion 
of the refractory. 
The refractory products according to the invention are therefore highly 
valuable for equipment for treating molding and pumping corrosive molten 
metals such as aluminum, aluminum-base alloys, zinc and zinc-base alloys.