CA1156685A

CA1156685A - Refractory

Info

Publication number: CA1156685A
Application number: CA000375717A
Authority: CA
Inventors: Howard M. Winkelbauer; Daniel R. Petrak; Thomas R. Kleeb; Ke-Chin Wang
Original assignee: Dresser Industries Inc
Current assignee: Dresser Industries Inc
Priority date: 1980-05-14
Filing date: 1981-04-16
Publication date: 1983-11-08
Also published as: BR8102978A; IT1170943B; IT8148390A0; DE3119425A1; JPS577870A; ZA812816B; GB2075966B; GB2075966A

Abstract

REFRACTORY
Abstract of the Disclosure A means for producing nitride bonded refractory shapes, in situ, by mixing a silicon metal powder, crude clay and a refractory aggregate. The mixes are pressed into shapes and burned at elevated temperatures in a nitriding atmosphere to form the bond.

Description

1 ~B68~

REFRACTOR~
Considerable effort has been directed to the ~ development o~ ceramic articles containing 8n% and more of ; silicon nitride, silicon oxynitride and/or sialon. These articles consist predominantly of single phase nitrides and display good thermal shock resistance, strength and corrosion resistance. Little information exists in the utilization of these nitride phases as the bonding agent in ;;
conventional refractories. Several limiting factors which have retarded large scale development of nitride bonded refractories include the high cost of purchased silicon nitride, the instability of certain oxynitrides at high -temperature, and the hydrolizing tendency of possible starting materials, such as, aluminum nitride and magnesium nitride. To overcome these obstacles, it would be 15 advantageous to form, in situ, nitride phases by the -~
addition of a single metallic metal powder which can react with gaseous nitrogen to produce a crystalline nitride phase capable of ceramic bonding to relatively inexpensive refractory grains. This approach will greatly lower the cost of nitride articles and couple the distinct advantages of nitride compounds to the established advantages of ~;
conventional refractory grains.
It is an object of the present invention to produce nitride bonded refractories with improved physical properties compared to refractories made with the addition of two or more reactive metal powders. `

., ~
,:

. . : . ~ : .. . ..
, - ~ ; .
: ~ .

Another object of the invention is to join nitride phases such as silicon oxynitride with conventional refractory grains which are typically bonded by oxides which can be readily decomposed by certain metals to provide properties, such as, non-wetability by molten metals, resistance to chlorine attack and low thermal expansion.
A further object of the invention is to provide nitride bonded refractories having improved porosity and relatively good room temperature and elevated temperature strength.
In accordance with the present invention, there is provided a method for producing nitride bonded refractory shapes in situ. A mixture is prepared comprising about 1 to 25~, by weight, silicon, about 1 to 5% crude clay, and the balance a refractory brick making size graded refractory aggregate. The mixes are pressed into refractory shapes and burned at elevated temperatures in a nitriding atmosphere to form the nitride bond.
In a preferred embodiment, the silicon comprises about 3 to 20~ and the crude clay comprises about 1 to 2~, by weight, of the mix. The shapes are preferably burned at a temperature between about 1090 and 1750C and the nitriding atmosphere is composed of either gaseous nitrogen, industrial annealing gas, or ammonia gas. The refractory aggregate is preferably selected from calcined fireclay, fused mullite, synthetic alumina and magnesium aluminate spinel.
In a nitrogen atmosphere, at elevated temperatures, silicon undergoes a gas-me~al reaction and forms minute crystals surrounding the metal nucleus.
Maintaining a hold during the firing process ensures drainage of the metal from the nucleus through the pores of the crystalline mat which allows additional nitridization of the metal. During the end of the hold period, true ceramic bonding is achieved with the coarse refractory grains by virtue of their solubility in the nitride phases. The presence of crude clay facilitates density at the press and facilitates formation of beta prime sialon and silicon ~r~'' oxynitride if these phases are desired.

-: :
.

1 ~56~

To successfully achieve nitridization and also an economical firing schedule, it is preEerred that khe starting metal powder be as fine as possible. Generally, the silicon powder should have an average particle diameter of about 6~3 microns or less with 95% of the particles being finer than 30 microns. Any type of crude clay may be used.
The sizing of the crude clay should be balanced between coarse and fine.
It is also preferred that the reactive material not exceed about 20% of the mix for economic reasons. Also, larger quantities do not result in articles with materially improved physical properties.
In the following examples, illustrated below, silicon powder was mixed with crude clay and either calcined fireclay, fused mullite, synthetic alumina or magnesium aluminate spinel. A solution of dextrin and/or lignin liquor and water was used as a temporary binder. The mixes were formed into shapes by power pressing to about 18,000 psi. The brick were then fired in the presence of flowing nitrogen to a temperature of about 2600F with a holding time of four hours. The overall results indicated that the mixes made with only a silicon metal addition were strong and had relatively low porosity. The various bonding phases are also shown in Table I.

' 1~5~

o\O f~ o o o ~ o ,~ ~ ~~ ,~ ~ ,~ ~, z ~n o ~ a) ~ a o\ o o o o ~r o ~-,i t) ,i I I 0~ I r~ ~D ~ r~~-ri S~ r~
~r1 ~1 ~~ r-l ~I r~

0\o ~ O O O ~ O
r~ ~ ~~ r~ ~ r~
(L)-r~ rl 'ri X rl m~nz u~o ~

0\O a~ o o ~
~ ~) r1~ rl ~ ~ I I I ~ O
H ''I X~i U~ O
o\ O O O ~
.OD r~ O -~ ~ri F~ ~ a~ I I I o 1~ 1 -,i I h ~ V~ 0~ ~ ' E~ .
0~o ~r o O ~:1 0 ~1 In ~ ~ ~ C)-rl O -r r` I I I ~ In o I ~ ,1 5~ ,i I
~i rl Z U~ O :~
o~O t`l o O ~ O ~1 r l ~I ~ r-l ~) rl ~1 ~
a) ,~-,l rl ~C-ri mu~ z; u,o r r~-ri rl aP ~

r-l j~ ~ ~ ~tH ' ~ ~1 ` O
o ~ ~ ~ o ~ c~ m ~ ~ r~ rl h O ~ 1 O O
~ U~r~ r~ ri ~1 H :~ ~ ~ nl rl S~ Q~ O O
h C ) cn o u~ o u~

. , .~

:.

1 ~ 8 5 In the above mixes, the refractory aggregate was sized such that about 7 to 20% was retained on a 10 mesh screen, about 23 to 36% was -10~28 mesh, about 15 to 19% was -28+65 mesh, about 7 to 10% was -65~200 mesh and about 30 to 5 35% passed a 200 mesh screen. All of the above mesh sizes are based upon the Tyler standard series.
As to the raw materials used above, the aluminum powder was pure aluminum metal, and the silica analyzed in excess of 98% SiO2. The refractory aggregate used in the 10 examples have the approximate chemical analysis as shown in Table II below.
TABLE II
Magnesium Calcined Crude Fused Synthetic Aluminate Fireclay ~_ Mullite Alumina Spinel SiO2 47.3% 62.9% 22.9 0.1% 0.2%
A12O3 49.2 33.576.4 99.6 69.0 Tio2 2.4 2.10.1 0.01 0.04 Fe2O3 1.0 1.00.3 0.2 0.09 20 CaO 0.02 0.2 - 0.04 0.54 MgO 0.04 0.3 - 0.04 30.1 Alk. 0.08 0.5 0.35 0~05 All of the chemical analyses are based on an oxide analysis.

,;
: ~
- . ~
, .~ , ., . .~ ',

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method for producing nitride bonded refractory shapes, in situ, comprising mixing, by weight, from about 1 to 25% silicon, about 1 to 5% crude clay, and the balance a refractory brickmaking size graded refractory aggregate, pressing said mixes into refractory shapes, and burning the shapes at an elevated temperature in a nitriding atmosphere for a time sufficient to form the nitride bond.

2. Method of claim 1 in which the refractory aggregate is selected from the group consisting of calcined fireclay, fused mullitel synthetic alumina and magnesium aluminate spinel.

3. Method of claim 1 in which the silicon comprises, by weight, about 3 to 20% and the crude clay comprises, by weight, about 1 to 2% of the mix.

4. Method of claim 1 in which the shapes are burned at a temperature between about 1090 and 1750°C.

5. Method of claim 1 in which the nitriding atmosphere is selected from the group consisting of gaseous nitrogen, industrial annealing gas and ammonia gas.

6. A nitride bonded refractory shape made from a batch consisting essentially, by weight, of about 1 to 25 silicon, about 1 to 5% crude clay and the balance a refractory aggregate.

7. Shape of claim 6, in which the refractory aggregate is selected from the group consisting of calcined fireclay, fused mullite, synthetic alumina and magneium aluminate spinel.

8. Shape of claim 6 in which the silicon comprises, by weight, about 3 to 20% and the crude clay comprises, by weight, about 1 to 2% of the batch.

9. Shape of claim 6 in which the nitride bond is at least one bond selected from the group consisting of beta sialon, silicon oxynitride, beta silicon nitride and alpha silicon nitride.

10. A refractory batch for producing nitride bonded refractory shapes, consisting essentially, by weight, of between about 1 to 25% silicon, between about l to 5%
crude clay and the balance a refractory aggregate.

11. Batch of claim 10 in which the refractory aggregate is selected from the group consisting of calcined fireclay, fused mullite, synthetic alumina and magnesium aluminate spinel.

12. Batch of claim 11 in which the aluminum com-prises, by weight, between about 3 to 20% and the crude clay comprises, by weight, between about 1 to 2% of the batch.