CA1202333A - Refractory material - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

There is provided a refractory mix comprising an acid stabilized aqueous zirconia sol and an active refractory material in an amount effective to gel said sol.

Description

REFRACTORY MATERI AL

Field of Invention This invention relates to the use of a relatively stable acidic aqueous colloidal zirconia sol as the bonding medium for specific refractories.

Back~round and Prior _ A procedure that is well-known has been used in the past for making cexamic shapes, namely mixing a binder and a gelling agent with a refractory and allowing the mix to chemically set or gel to form a bond and then firing the body. Typically many shapes have been made using sodium silicate r potassium silicate, colloidal silica, and hydrolyzed ethyl silicate as bonds. However, to obtain the greatest refractoriness of a body, a bond leaving a residue of a more refractory o~ide is preferable. For example, alumina and zirconia produce high temperature bonds for reractories.
U.S. 4,025,350 shows the use of an aqueous solution of a zirconium salt with a gelling inducing agent and a gelling delaying agent and a refractory powder to form a refractory article. This composition requires additional gelling agents for control thereby increasing costs and control problems. Also the by-products of the ~elat.ion of the zirconium salt would need to be eliminated from the .~

3~3 refractory during firing. There is also an added cost of the zirconium salt versus the oxide.
U.S. 4,201,594 describes ~he binding of refractory materials using zirconium salts and incorporating gelling agents and gel delaying agents. For the same reasons these compositions are less than desirable.
U.S. 2,984,576 describes an unfired mixture of a refractory material bonded with a zirconia or hafnia sol in which the percent of solids in the dispersed phase is at least 30%. This patent does not describe the specific refractories useful with the present stable acidic zirconia sol but on}y as a bond for a variety of refractories.
U.S. 3,758,316 describes the process for producing a refractory from a refractory powder and a binder precursor which would include colloidal zirconia, but also requires the addition of a gelling agent.

Brief Summary of Invention The basic principle of the present invention is to make a refractory mix comprisiny a refractory material and a stable acidic zirconia sol having a fine particle size and acidic pH. The refractory is composed of an active portion and, if desired, a relatively inert portionO

Detailed Description of Invention One would expect that highly refractory materials would 3~3 be relatively inert to the zirconia sol. Howe~er, i~ has been found that a number of refractories are not totally inert to the sol and actually react with the sol to cause yelation of the sol. Very rapid gels or slow ~els can be produced depending upon the particular type of active refractory, its particle size distribution, and its percentage in the refractory mix. Some examples of active re~ractories which will cause gelation with the zirconia sol are alkali and alkaline earth metal aluminates, silicates, zirconates, stannates, titanates, zirconium sllicates and oxides. Specific examples include calcined maynesium oxide, electrically ~used magnes.ium oxide, calcium oxide, electrically ~used calcium oxide, mono calcium aluminate, calcium aluminate cements, fused cordierite, high alkali glasses, magnesium aluminate, magnesium aluminum silicate, magnesium zirconate, magnesium silicate, magnesium zirconium silicate, magnesium ferrite, magnesium titanate, magnesium stannate, calcium zirconate, calcium silicate, calclum zirconium silicate, calcium titanate, calcium stannate, barium zirconate, barium aluminum silicate, barium aluminate, barium zirconium silicate, barium stannate, barium titanate, barium silicate, strontium zirconate, strontium stannate, strontium zirconium silicate, strontium silicate, strontium aluminum silicate, strontium titanate, electrically fused calcium oxide stabilized zirconia, electrically ~used magnesium oxide stabilized zirconia, iron chromite, Zeolex 23, wollastonite, bentonite, strontium aluminate, forsterite, calcium aluminum silicate, ~luorspar, ,, 3~33 fluorbarite, lithlum zirconate, li.thium aluminate, lithium silicate, lithium aluminum silicate, lithium titanate, lithium zirconium silicate, and other refractory materials which are reactive with the zirconia solO Some relatively non-reacting refractory materials are monoclinic zirconia, hafnia, alumina, bauxite, mullite, sillimanite, zircon, ceria, thoria, silioon nitride, silica and other minerals which do not contain any large amounts in their structure of the alkaline and alkaline earth metallic oxides or impurities present that may react with the sol.
~ t is also possible to use this system as a bond or various fibers made from aluminosilicates, low alkali glasses, alumina, zirconia, silica, and various organic fibers such as cotton, rayon, nylon, other synthetic fibers.
The aqueous zirconia sols used in the examples given in this specification are acidic in nature ranging in pH from about 0.3 to 6Ø The particle size of the zirconia particle is generally small, on the order of 25 millimicrons and smaller. The sol is stabilized by acids such as nit.ric, hydrochloric, acidic, etc. The gelling action of the sol with the "active" refractory is believed to he due to a reactio~ of the acid with the "active" refractory, producing a "salt", which rPaction raises the pH thereby lowering the sol stability. Also, the salt formed possibly catalyzes the gelling of the sol. This gelling action bonds the refrac-tory into a strong body.
Several ~actors govern the characteristics of the refractory body bonded with the zirconia sol. The type of acid in the sol, the particle size and age o~ the 501, the percentage of zirconia in the sol, the percentage and type of "active" refractory in the mix, its particle size distribution, temperature, and mixing conditions.
The listing of potential "active" refractories shows the presence in many cases of an alkaline or alkaline earth type oxide present in the structure of the refractory or that the "active" re~ractory is subject to reaction with an acid. The presence of such "active" refractories, serving to react with the sol not only causes gelation but also might serve as sinteriny aids Eor certain refra~tory systems. The comparative scratch hardness of bonded refractory shapes after firing serves as a measure of sintering action by the "active" refractory.
One procedure for utilizing this invention is to produce cast refractory shapes by mixing the zirconia sol with at least one "active" refractory. The balance of the refractory may include a relatively inert refractory.
In some instances, depending upon the nature of the active refractory, the total refractory may be of the active type.
In other instances, the "active" refractory may be a very minor portion of the total refractory in the mix. Particle size distribution and chemical nature of the active refractory are two of the major factors in determining the amount of "active" refractory constituent.
Various refractory shapes can be cast using this inven-tion to produce practical products, such as metal melting crucibles, boats, tundishes, pouring ladles, pouring cups, tubes, rods, slabs, bricks, saggers, kiln ~urniture, kiln car tops, open hearth door facings, kiln parts, pouri~g nozzles, furnace liners, and others. Such m:ixtures can also be used to cast dental and jewelry molds ~or metal casting.
In particular, some of these mixes are especially suitable for molds Eor casting superalloys, stainless steels, niobium, tantalum, titanium, and molybdenum. By selection of a high temperature inert refractory, or low~activity 'lactive" reEractory, such as zirconia, hafnia, ceria, alum.ina, yttria, lanthana, a Eoundry mold can be produced having an extremely high PCE value ancl havlnc~
low reactivity to some oE the above-mentioned reactiv~
metals.
If desirable, pressing mixes can be made which will "set" or "gel" in predetermined times in order that a refractory shape may be made by pressing and then become set or gelled.
Thin or thick Eilms may be made ~rom mixes which will "set" or "gel" in predetermined t.i.mes in order tha-t a re~ractory shape may be made by pressing and then become set or gelled.
Thin or thick Eilms may be made from mixes which may be cast on a belt or ~orm and then becoming gelled or set.
Coatings may be dipped or sprayed on to a form or shape, and then allowed to gel.
Mixes according to this invention may be Eormed into shapes by injecting molding. Present ceramic injection molding techniques usually call Eor various temporary bonds i~

1~2333 for the refractory body to allow for ease of molding.
Examples are costly waxes, resins, plastics, etc. These organic materials are burned out without leaving a high temperature bond, and shrinkage occurs during loss of organic material. The present invention provides a "green"
bond and a fired bond in the refractory body. This technique can be used to mold various-intricate shapes such as spindles, nozzles, ceramic cores ~or metal castings, ceramic turbine blades and vanes, shell mold parts for metal casting, and various other shapes as desirecl ~ primary application ~or this invention is to make cast refractory bodies which will set ox gel.l at controlled times. A proportion of "active" refractory may be adjusted according to the set time required for the mass. This percentage varies with the particular "active" refractory.
The resulting refractory mix can be then mixed with a suitable amount of the zirconia sol to a heavy pouring consistency and poured or cast into a mold form and al]owed to set. Particle size distribution oE the refractory ~ix may be varied according to the desired results, strength, settling within the mold, and gel times. It is usually advantageous to allow adequate time for satisfactory mixing of the refractory before casting into a mold. This depends upon the size of the mold and the equipment used to handle the mix. If a small volume hand mix is used, mixing can usually be carried out in a very short period of time such as one to two minutes and then the mix can be adjusted to gel or set very rapidly. I prefer a relatively fast gel 3~3 time of 5 to 30 minutes for relatively fast production of shapes. It may be desirable to remove bubbles from the mix and to incorporate suitable wetting and defoaming agents to make a relatively bubble-ree or void-free mass. Time may be needed to completely wet in the mass and to deair before casting can be made. Ideally, gelation should occur ~s soon as practical after pouring.
To illustrate this invention, the data in Table 1 shows the percentage of active refractory that might be mixed with an inert reractory, such as tabular alumina, tc~ produce specific set or gel t.imes. The refractory is mixed Witll the zirconia sol containing 20% ZrO2 and having a pH of 0.6.
The alumina portion was composed of 50% 325 mesh and finer tabular alumina and 50% 60 mesh and finer tabular alumina as supplied by Alcoa. The active refractory percentage is calculated on the basis of the total amount of refractory used for the final mix.
The samples indicated in Table 1 all had good green strength and when fired separately to 1200E', 1800F, 2000F, and 2500F had excellent fired strengths.
Another series of similar experiments to those in Table 1 were carried out according to Table 2 in which the tabular alumina refractory base was 25% 325 mesh and finer and 75%
60 mesh and finer. This Table shows the gel times for the various mixes using the active refractory. These were mixed with the same 7irconia sol as was used in Table 1. After gelling these samples had excellent green strength and after firing to the same temperature condi.tions had excellent li~

fired strength. In all cases, the stxength at 2500F was greater than that fired to temperatures below 2500F.
Some unique characteristics were noted about the compositions described in Table ~. ~ series of test specimens approximatel~ thick, 1" wide and 2.375"
long were prepared in a mold using the same compositions as prepared in Table 1. They were allowed to set after gelation for 30 minutes and then removed from the mold.
After removing from the mold, the specimen was set out in the air to air dry overnight and then oven dried for 4 hours at 120C to remove all the water Erom the shape and then placed into a dessicator ~or cooling. It was then removed and immediately measured. It was noted that all specimens showed some shrinkage Erom the mold dimension on the order of about one-half to one percent. After the specimens were dried, they were then heated to a temperature of 1200F and maintained at that temperature for 2 hours and then allowed to cool to room temperature and remeasured. After measur-ing, the specimens were then reheated to 1800F and held Eor

2 hours at temperature, cooled, and then remeasured. This same heating was carried out separately at 2000F and 2500F, after which time measurements were made on the specimens. It was noted that on many specimens some very small to fairly sizeable permanent expansion occurred after cooling. The data in Table 2 shows the permanent expansion obtained on a number of the specimens cast. The negative value indicates shrinkage. The remainder of the figures indicate permanent expansion.

~2~2333 It can be observed from this Table that some substan-tial expansions occur on certain specimens. These expan-sions are not necessarily related to the proportion of active refractory but are definitely attributed to the presence of the active refractory. Each composition pro-bably acts in a different manner and produces different reaction products which govern the amount of expansion obtainable. This may be a means for minimizing shrinkage during firing of refractory bodies utilizing this zirconia sol bonded system. Normally when considerable sintering occurs on firing a refractory to a high temperature, considerable shrinkage occurs with the sintering. It should be noted that several compositions in the tabulation show relatively low shrinkage even when ~ired at 2500F. Table 3 shows a similar series of measurements made on specimens using the tabular alumina refractory containing 2S% 325 mesh and finer and 75% 60 mesh and finer ~article sizes with the corresponding "active" refractory.
The following are examples of other refractory mixes used with the acid stabilized zirconia sol and illustrating the use of "active" refractories.
EXAMPLE I
Composition:

Electrically fused calcium oxide stabilized zirconium oxide - 325 mesh 30 grams E'used Magnesium Oxide -325 mesh 1 gram Tabular alumina 60 mesh and finer 150 grams ~2~333 Tabular alumina - 28 + 48 mesh 120 grams This refractory composition was mixed wi~h 35 ml acid stabilized zirconia sol containing 20% ZrO2. I~ was then poured into a rubber mold. The gel time was determined to be approximately 5 minutes. After 30 minutes, the sample was removed from the mold and by means of a diamond saw was cut into test specimens for modulus of rupture measurements.
Unfired strength of this mix was approximately 57 psi.
Samples were fired to 2500Fr held for two hours and cooled to room temperature, and modulus of rupture was determined as 575 psi. A similar firing to 2700F for two hours and then cooling showed a modulus of rupture of 910 psi. A
firing to 2900F for two hours and cooled showed a modulus of rupture of 1888 psi.
EXAMPLE II
Composition:
Tabular alumina - 325 mesh240 grams Electrically fused magnesium oxide 2 grams This was mixed with 45 ml of the same zirconia sol as in Example 1. The gel time on this mix was approximately 4-1/2 minutes. The green modulus of rupture was not deter-mined but specimens fired to 2000F for two hours and cooled showed a modulus of rupture of 234 psi. Firing to 2500F
for two hours and cooled showed the modulus of rupture to be 1164 psi. Firing to 2700F for two hours and cooling showed a modulus of rupture of 2995 psi. A specimen fired to 2900F for two hours showed a modulus of rupture of 5674 psi .

r ~ 3 3 3 EXAMPLES III
Composition:

FF zirconium oxide t calcium stabilized, 325 mesh 170 grams - 50 + 100 mesh - 325 mesh 160 grams - 12 + 35 mesh - 325 mesh 80 grams This refractory composition was mlxed with 30 ml of the zirconia sol used in Example I. The gel time was 8 minutes.
The modulus of rupture measurements after firing specimens to the particular temperatures for two hours and testing after cooling are as follows:

Modulus of Rupture pounds ~ _nch Unfired 278 2700~F 2019 Test specimens from Examples I, II, and III were also measured before fi.ring and after each firing and showed the following percentage permanent expansion (-~) or shrinkage ( ):
Firing _ Examples Temperature F III III

2000 +0.08 -0.09 -0.11 2500 +0.2g -0.46 -0.51 2700 ~0.40 -1.60 -0.50 The development of some permanPnt exparlsiorl could be helpful in eliminati.ng or minimizing settling and drying shrinkage on some compositions, thereby increasing dimen-sional accuracy in making shapes.
The following is an example of typical shell mold system possible by the use of this invention:
Composition:

Electrically fused calcium oxide stabilized zirconium oxide2000 grams Zirconia sol containing 20~ ZrO2 500 ~rams Concentrated hydrochloric acid 17 ml.
Wetting agent - Sterox NJ15 drops This slurry was prepared to a viscosity of 3~ seconds as measured by the Zahn #4 cup. Sheets of wax, approxi.-mately l/8" thick and 2 1/2" wide by 5 l/2" long were dipped into this slurry and immediately stuccoed while wet with a - 50 + 100 mesh ~irconia of the same composition as used in the slurry. After dipping several specimens, the slurry was diluted with the zirconia sol to a viscosity of 15 seconds and a further dip was applied after the first dip had dried overnight. While the second coating was still wet, it was stuccoed with a relatively coarse zirconia granule of a - 12 + 35 mesh of the same composition as the material in the slurry. This was repeated for additional coatings and a final seal was applied, making a total of 6 stucco layers and 7 slurry layers. Two dips were applied per day through the final dip. The dipped specimens were then allowed to dry for 2 days and the wax was melted out. The specimens were then cut into strips 1" wide, dried, and then tested for unfired strength. Six specim~ns were tested giving an average modulus of rupture value of 500 psi. Additional specimens were fired for 2 hours to various temperatures beginning at 2000F and cooled back to room temperature and tested. The MOR after firing to 2000F was 220 psi. The MOR after firing to 2200F and cooling to room temperature was 300 psi. The MOR increased to 1200 psi after firing to 2500F. This indicated a substantial strength was obtainable on a shell mold composition utilizing this invention.

3~

Table 1 Type of Active Wt. % Active Sample Refractory RefractorY Gel Time 1 Calcium Aluminate Cement 5.0 Immed.
2 Calcium Aluminate Cement 1.0 8 min.
3 Calcium Aluminate Cemen~ 2.0 4 min.

4 Calcium Aluminate Cement 0.5 45 min.
Magnesium Zirconate 1.0 6 min.
6 Magnesium Zircona~e 0.5 2 hr. -~
7 Magnesium Zirconium Silicate 1.0 Overnight 8 Magnesium Zirconium Silicate 5.0 55 min.
9 Magnesium Zirconium Silicate 7.5 12 min.
MagnesiumlZirconium Silicate 10.0 10 min.
11 MgO T-139 - 325 Mesh 1.0 90 sec.
12 MgO T-139 - 325 Mesh 0.8 2 min.
13 MgO T-139 - 325 Mesh 0.6 4 min.
14 MgO T-139 - 325 Mesh 0.4 10 min.
Calcium Zirconium Silicate 1.0 Overnight 16 Calcium Zirconium Silicate 5.0 20 min.
17 Calcium Zirconium Silicate 3.0 28 min.
18 Calciurn Zirconium Silicate 7.5 7 min.
19 Calcium Zirconate lo0 Overn:lght Calcium Zi.rcona~e 5.0 1 hr.
21 Calcium Zirconate 7.5 90 sec.
22 Calcium Zirconate 10.0 Immed.
23 CaO 1.0 Instant 24 CaO 0.1 1 hr. +
CaO 0.25 60 sec.
26 CaO 0.5 Instant 27 Iron Chromite 1.0 Overnight 28 Iron Chromite 5.0 30 sec.
29 Iron Chromite 3.0 Overn.ight Iron Chromite 4.0 2 hrs.
31 Iron Chromite 5.0 9 min.
32 Iron Chro~lte 6.0 5 min.
33 Zeolex 23 1.0 Overnight 34 Zeole~ 23 5.0 Instant Zeolex 23 2.0 8 min.
36 Zeolex 23 3.0 Lnstant 37 Winco Cordierite 3- 200 Mesh 1.0 Overnight 38 Winco Cordierite - 200 Mesh 5.0 1 hr. +
39 Winco Cordierite - 200 Mesh 7.0 12-15 min.
Winco Cordierite - 200 Mesh 8.0 8 min.
41 Wollastonite 1.0 7-11 min.
1. Manuactured by CoE~ Minerals, King of Prussia, Pa.
2. Trademark of J.M. Huber Corp., Baltimore, Mdo 3. Manufactured by Winco Minerals, E. Aurora, N.Y.

.~

Table 2 _.
Permanent Expansion in Thousandths ol Inch at Firing Temperature el Time 1200F 1800F 2000F 2500F
3 4 min. .010 .003 .001- .005-2 8 min. .002 .001- .003 .004-6 min. .006 .004 .003 .006-7 Overnight .004 .003 .004 .009-10 min. .008 .001 .001- .037-11 1 1/2 min. .005 .004 .011 .011-12 2 min. .008 .003 .002 .016-13 4 min. .002 .000 .002- .020-14 10 min. .002 .005 .003 .011-lG Overnight .0:1.6 .01S .016 .013-18 7 min. .006 .009 .005 .026-17 28 min. .005 .006 .004 .022-19 Overnight .004 .003- .001- .007-27 Overnight .008 .010 .000 .012-2 hrs. .002 .005 .009 .002-31 9 min. .003 .001~ .008 .010-37 Overnight .002 .004 .002- .013-39 15 min. .001 .005 .007 .024-8 min. .004 .004 .021-41 11 min. .005- .005 .004 .025-24 60 min. ~ .004 .006 .015-

Claims (19)

I claim:

1. A refractory mix comprising: an acid stabilized aqueous zirconia sol, and an active refractory material in an amount effec-tive to gel said sol, with or without an inert refractory material.

2. A refractory mix comprising an acid stabilized aqueous zirconia sol and an active refractory material selected from the group consisting of alkali and alkaline earth aluminates, silicates, zir-conates, stannates, titanates, zirconium silicates and oxides, with or without an inert refractory material.

3. A method of making an unfired refractory shape comprising mixing an acid stabilized zirconia sol with a refractory material, said refractory material comprising an active refractory with or without a relatively inert refractory, said active refractory being present in an amount effective to gel said sol, forming the mix into a shape, and allowing the sol to gel.

4. A method of making an unfired refractory shape according to claim 3 wherein the active refractory material is selected from the group consisting of alkali and alkaline earth aluminates, silicates, zirconates, stannates, titanates, zirconium silicates and oxides, with or without an inert refractory material.

5. The method of making a fired refractory shape comprising mixing an acid stabilized zirconia sol, with a refractory material, said refractory material comprising an active refractory with or with-out a relatively inert refractory, said active refractory being pre-sent in an amount effective to gel said sol, and forming the mix into a shape, drying the shape and firing to a temperature sufficient to harden the shape.

6. The method of making a fired refractory shape according to claim 4 which further comprises drying the shape and then firing to a temperature sufficient to harden the shape.

7. A method of making a refractory shape according to claim 3 wherein the mix is of a pourable consistency and the shape is formed by pouring the mix into a mold, allowing the mix to harden, and removing the hardened shape from the mold.

8. The method of making a refractory shape according to claim 3 wherein the mix is made to a consistency suitable for injection molding, then adding the mix to an injection molding means whereby said mix is then injected into a closed mold, allowing the mix to gel and removing the shape from the mold.

9. The method of making a refractory shape according to claim 3, comprising placing the mix into a mold form, molding the mix into final shape, allowing the mix to gel and removing the gelled refractory shape from the mold.

10. The refractory shape produced by the method of claim 3.

11. The refractory shape produced by the method of claim 4.

12. A method of making a refractory casting mold comprising the steps of:
a) making a mixture of an acid-stabilized, aqueous, colloidal zirconia sol and a refractory material, said refractory material comprising an active refractory material capable of gelling said sol after a suitable non-gel working period;
b) applying said mixture to a pattern mold;
c) allowing said mixture to gel on said pattern; and d) removing the pattern from the gelled mixture.

13. The method according to claim 12, wherein the gelled mixture of step d) is dried and is heated to a suitable temperature to receive molten metal.

14. The method according to claim 12, additionally comprising after the step of applying said mixture to said pattern mold, sprinkling coarse refractory over the coated mold, allowing the resultant coating to gel, applying a second coating of said mixture to said coated pattern mold, sprinkling coarse refractory on said second coating, allowing said second coating to gel, and repeating the coating and sprinkling process to form a refractory shell of sufficient thickness for metal casting over the pattern, removing the pattern mold from the resultant shell, drying and heating said shell to a suitable temperature to form a shell mold for receiving molten metal.

15. The method of making a metal casting comprising forming a metal casting mold according to claim 13 and pouring metal into said mold.

16. The method of making a metal casting comprising forming a metal casting mold according to claim 14 and pouring metal into said mold.

17. The method of making a metal casting mold according to claim 14, wherein the refractory from the first coating of said mixture is different from the refractory on the second coating of said mixture.

18. The method of making a refractory casting mold according to claim 12, wherein the active refractory material is selected from the group consisting of alkali and alkaline earth aluminates, silicates, zirconates, stannates, titanates, zirconium silicates and oxides.

19, The method of making a refractory casting mold according to claim 12, wherein the active refractory material is selected from the group consisting of calcined magnesium oxide, electrically fused magnesium oxide, calcium oxide, electrically fused calcium oxide, mono calcium aluminate, calcium aluminate cement, fused cordierite, high alkali glass, magnesium aluminate, magnesium aluminum silicate, magnesium zirconate, magnesium silicate, magnesium zirconium silicate, magnesium ferrite, magnesium titanate, magnesium stannate, calcium zirconate, calcium silicate, calcium zirconium silicate, calcium titanate, calcium stannate, barium zirconate, barium aluminum silicate, barium aluminate, barium zirconium silicate, barium stannate, barium titanate, barium silicate, strontium zirconate, strontium stannate, strontium zirconium silicate, strontium silicate, strontium aluminum silicate, strontium titanate, electrically fused calcium oxide stabilized zirconia, electrically fused magnesium oxide stabilized zirconia, iron chromite, wollastonite, bentonite, strontium aluminate, forsterite, calcium aluminum silicate, fluorspar, fluorbarite, lithium zirconate, lithium aluminate, lithium silicate, lithium aluminum silicate, lithium titanate or lithium zirconium silicate.