Nitride bonded oxide refractories

This invention relates to a method for producing nitride bonded refractory shapes in which the bonding matrix is formed in situ. The method comprises forming a batch including a coarser portion selected from the group calcined and fused aggregates of alumina, aluminosilicate, and magnesium aluminate spinel and a finer portion consisting essentially of finely divided silicon metal and alumina as well as fines of the above mentioned refractory aggregate needed to achieve the desired screen analysis. The silicon metal and alumina react in the nitriding atmosphere to form a low porosity matrix generally comprising silicon oxynitride with corundum distributed therethrough.

DESCRIPTION 
1. Technical Field 
This invention relates to nitride bonded oxide refractories and the method 
of making the same. 
2. Background Art 
Nitride bonding of silicon carbide grain is well established as shown, for 
example, in U.S. Pat. No. 2,752,258. There has been considerable recent 
interest in the nitride bonding of oxide refractory aggregates. Use of 
both aluminum metal and silicon metal in the fine portion of the batches 
that are shaped and heated in a nitriding atmosphere are suggested, for 
example, in published Japanese Patent Application No. 1977-1051157 to Ueno 
and Katsura and U.S. Pat. No. 4,243,621 to Mori, Ogawa and Takai. The Ueno 
et al. application discloses the inclusion of fine alumina, silica or 
aluminosilicates in the batch. Mori et al. discloses as essential silica 
powder in the batch and optionally alumina powder. U.S. Pat. No. 3,991,166 
to Jack and Wilson discloses a method of producing a ceramic material by 
nitriding a mixture of silicon and aluminum powders in which the atomic 
ratio of silicon to aluminum is not less than 1:3. 
While the general approach to nitride bonding of oxide refractories has 
been investigated and explained in the documents noted above, there has 
remained a need for a practical and less expensive process for making 
nitride bonded oxide refractories with excellent bulk properties.

DISCLOSURE OF THE INVENTION 
It is an object of this invention to provide a practical and less expensive 
process for the manufacture of nitride bonded oxide refractory with 
excellent bulk properties. It is a further object of this invention to 
provide a refractory composition especially suitable for the containment 
of molten aluminum. It is yet another object of this invention to provide 
a refractory composition suitable for confining the contents of a 
non-ferrous containing electrolysis cell. It is a still further object of 
this invention to provide a refractory composition suitable for slide 
gates used in the teeming of molten steel. 
Briefly according to this invention, there is provided a method of 
producing nitride bonded refractory shapes in which the bonding matrix is 
formed in situ. The method comprises first forming a brickmaking size 
graded batch from refractory oxide aggregates including a coarser portion 
selected from the group: calcined and fused aggregates of alumina, 
aluminosilicate, and magnesium aluminate spinel. The batch also includes a 
finer portion consisting essentially of finely divided silicon metals in 
an amount between about three and twenty percent by weight of the entire 
batch and alumina in an amount between three and twenty percent by weight 
of the entire batch. The ratio of fine alumina to fine silicon metal 
should preferably be at least 1:4. The batch is mixed and tempered with a 
temporary binder and then formed into a shape by conventional brickmaking 
or shapemaking techniques including, for example, power pressing. After 
drying, the shapes are heated in a nitriding atmosphere until 
substantially all the silicon metal is reacted with nitrogen. Examination 
of the matrix of compositions made according to this invention with an 
electron microprobe demonstrate that the elements aluminum, oxygen, 
silicon, and nitrogen are distributed more or less uniformly throughout 
with portions higher in aluminum and oxygen and other portions higher in 
silicon and nitrogen. X-ray diffraction anaylsis of the bonding phases 
identifies silicon oxynitride, .beta.' sialon and corundum. 
The calcined and fused aggregates included in the coarser portion of the 
batch according to this invention should have low iron oxide, chrome 
oxide, and lime (CaO) contents. Iron and chrome oxides are reduced to 
metals under nitriding conditions. Also, under nitriding conditions, the 
lime migrates into the matrix and upon reheating under oxidizing 
conditions causes the brick to exhibit a bubbling phenomenon. The lime, 
iron oxide, and chromium oxide content of the aggregate should each 
preferably be less than about one percent by weight and the lower the 
better. 
Refractory compositions prepared according to the process disclosed herein 
are especially suitable for certain highly critical applications. 
BEST MODE FOR CARRYING OUT THE INVENTION 
Nitride bonded aluminosilicate refractory shapes were prepared from the 
batches set forth in Table I. Compositions 1 and 2 are accordingto the 
teachings of the invention being disclosed and claimed. By way of 
comparison, compositions C1, C2, and C3 illustrate other ways to achieve a 
nitride bond. 
TABLE I 
______________________________________ 
Example: 1 2 C1 C2 C3 
______________________________________ 
Mix (weight percent, calcined 
88% 78% 72% 69% 84% 
aluminosilicate (about 50% 
Al.sub.2 O.sub.3)): 
Silica powder (SiO.sub.2): 
-- -- 13 13 -- 
Fine alumina (Al.sub.2 O.sub.3): 
4 8 -- -- -- 
Raw clay: 2 2 2 -- 3 
Aluminum metal: -- -- 13 13 -- 
Silicon metal: 6 12 -- 5 -- 
______________________________________ 
Each of the mixes set forth in Table I was sized and graded to form a 
typical brickmaking batch. The batches were blended with binders and 
pressed into shapes. The forming techniques were approximately the same 
for all mixes but certainly not precisely the same; for example, Examples 
1 and 2 were formed on an impact press at 80, psi being hammered for ten 
seconds. Mix C2 was likewise formed, except that the hammering time was 
twenty seconds. 
After drying, the shapes were heated in a nitriding atmosphere until the 
metals in the batch were substantially entirely reacted with nitrogen 
present in the heating atmosphere and/or oxygen borrowed from other batch 
ingredients. Properties of the nitride bonded shapes are set forth in 
Table II. The results under the column head C3' are for another 
manufacture of Example C3. 
TABLE II 
__________________________________________________________________________ 
Example 1 2 C1 C2 C3 C3' 
__________________________________________________________________________ 
Bulk density, pcf: 
146 148 150 148 152 143 
Apparent Porosity, %: 
18.2 18.2 23 22.2 15.2 15 
Nitrogen Content (N), %: 
3.76 7.2 5.8 5.9 N.D. 7.3 
Modulus of Rupture 
psi (Av 2): 
At Room Temperature: 
2500 3500 2700 2320 2720 1900 
At 2000.degree. F. (1093.degree. C.): 
3510 4540 3000 2480 2710 3500 
At 2700.degree. F. (1482.degree. C.): 
720 2290 1100 N.D. N.D. 2200 
X-ray Diffraction of 
Si.sub.2 ON.sub.2 
Si.sub.2 ON.sub.2 
corundum 
.beta.'Sialon 
Bonding Phase: corundum 
.beta.'Sialon 
Si.sub.2 ON.sub.2 
__________________________________________________________________________ 
The data in Table II establishes that each example has adequate physical 
properties but the hot strength as measured by modulus of rupture was 
markedly superior for those (Examples 1 and 2) according to this 
invention. Use of aluminum metal only (Example C1), use of a mixture of 
aluminum metal and silicon metal (Example C2), or use of silicon metal 
without alumina (Example C3) resulted in lower hot strenghts. The 
mineralogical characteristic of the bonding matrix differed from example 
to example and comparative example to comparative example depending upon 
the batch ingredients. Simply stated, those examples according to this 
invention have more alumina (corundum) dispersed in the nitride bonding 
matrix. Of course, the total amount of nitride bonding as well as the 
characteristics of the bonding matrix must be considered in interpreting 
these results. The nitrogen content may be taken as an approximate 
indication of the amount of nitride bonding. Example 2 was prepared from a 
batch having twice as much silicon metal as Example 1 and the nitrogen 
content of Example 2 is about double that of Example 1. The strength at 
all temperatures reflects the increased amount of bonding. 
Examples 1 and 2 are especially suitable for use in applications such as 
the non-ferrous electrolysis process for the manufacture of non-ferrous 
metals. 
Much has been made in the prior art of the .beta.' sialon phase as a 
desirable bonding phase. While a certain amount of .beta.' sialon may 
exist in compositions according to this invention, the bonding phases 
identified by X-ray diffraction are primarily silicon oxynitride and 
corundum. The temperatures of the nitriding step and the ratio of 
ingredients are critical for the development of the .beta.' sialon phase. 
Typically temperatures on the order of 1700.degree. to 2000.degree. C. are 
required and no more than about sixty percent alumina in the silicon 
nitride can be tolerated. 
Nitride bonded fused magnesium spinel grain brick were prepared from the 
batches set forth in Table III. Examples 3, 4 and 5 are according to the 
teachings of this invention. Examples C4, C5, and C6 are comparative 
examples illustrating other ways in which nitride bonding can be achieved. 
TABLE III 
______________________________________ 
Example: 3 4 5 C4 C5 C6 
______________________________________ 
Fused Grain 
(90% (70% (60% (70% (70% (70% 
Type: Al.sub.2 O.sub.3) 
Al.sub.2 O.sub.3) 
Al.sub.2 O.sub.3) 
Al.sub.2 O.sub.3) 
Al.sub.2 O.sub.3) 
Al.sub.2 O.sub.3) 
Mix (weight 
86.7% 86.7% 86.7% 
87% 84% 72% 
percent) 
Fused Spinel 
Grain: 
Silica -- -- -- -- -- 13 
Powder: 
Fine 7.3 7.3 7.3 -- -- -- 
Alumina: 
Raw Clay: 
-- -- -- -- 3 2 
Aluminum -- -- -- -- -- 13 
Metal: 
Silicon Metal: 
6.0 6.0 6.0 13 13 -- 
______________________________________ 
The batches described in Table III were made into shapes. Examples 4 and 
C4, C5, and C6 were power pressed whereas Examples 3 and 5 were impact 
pressed with a twenty second hammer time. After shaping and drying, the 
mixes were heated in a nitriding atmosphere. The properties of the 
resulting nitride bonding shapes are set forth in Table IV. 
TABLE IV 
__________________________________________________________________________ 
Example: 3 4 5 C4 C5 C6 
__________________________________________________________________________ 
Bulk density, pcf: 
190 185 188 172 176 173 
Apparent porosity, %: 
15.4 15 13.2 18.2 14.3 17.9 
Nitrogen content (N), %: 
4 3.5 3.3 8.15 7.6 5.84 
Modulus of Rupture, 
psi (Av 2) 
At Room temperature: 
N.D. 2980 2650 1490 2480 3060 
At 2000.degree. F. (1093.degree. C.): 
N.D. 5457 3231 3080 3420 3890 
At 2700.degree. F. (1482.degree. C.): 
920 (melted) 
(melted) 
(body 
(body 
1090 
glazed) 
glazed) 
X-ray diffraction analysis 
Corundum 
N.D. N.D. .beta.Si.sub.3 N.sub.4 
.beta.Si.sub.3 N.sub.4 
* 
Phases other than 
.beta.'Sialon .alpha.Si.sub.3 N.sub.4 
Si.sub.2 ON.sub.2 
Spinel (Decreasing 
Intensity): 
__________________________________________________________________________ 
*Mg-containing .beta.' sialon, 12H magnesium sialon polytype, corundum 
The data in Table IV shows the nitride bonded shapes made with fused 
magnesium aluminate spinel grain of various compositions can be provided 
with excellent hot strength as measured by modulus of rupture at 
2000.degree. F. The grains having the highest alumina content provided the 
best properties to the nitride bonded brick. Magnesium aluminate by itself 
has excellent compatability with molten aluminum and the nitride bonded 
shapes made therefrom have properties suited for applications in the 
manufacture of aluminum. Magnesium aluminate spinel has a known tendency 
to creep under load at elevated temperatures. The nitride bonded matrix 
diminishes this drawback. Moreover, the thermal shock resistance and 
resistance to alkali attack of fused spinel brick as made according to 
this invention are superior. 
The strengths at 2700.degree. F. set forth in Table IV are of interest but 
are not particularly relevant to the use of such compositions in the 
manufacture of aluminum wherein temperatures are not generally in excess 
of 2000.degree. F. 
Nitride bonded high alumina shapes were prepared from batches set forth in 
Table V. 
TABLE V 
______________________________________ 
Example 6 7 8 
______________________________________ 
Mix (weight percent) 
20% 
Calcined aluminosilicate 
(about 70% Al.sub.2 O.sub.3): 
Calcined Bauxite 20% 
(South American): 
Synthetic alumina: 
65 85 65 
Silicon: 13 13 13 
Raw clay: 2 2 2 
______________________________________ 
The batches were mixed with a binder and made into plates (about 
9.times.4.5.times.1.375 inches) on a bumping press. After drying, they 
were heated in a nitriding atmosphere to form nitride bonded shapes. The 
purpose of making plates was to prepare a shape for use as a slide gate. 
The properties of the plates of Examples 6, 7, 8 are set forth in Table VI. 
TABLE VI 
______________________________________ 
Example 6 7 8 
______________________________________ 
Bulk density, pcf: 
178 188 184 
Apparent porosity, % 
17.8 17.2 17.7 
Nitrogen (N), %: 
7.08 6.96 6.84 
Modulus of Ruputure psi 
(1 .times. 3/4 .times. 6" bars) 
At Room Temperature: 
2780 2220 2990 
At 2000.degree. F. (1093.degree. C.): 
5430 4620 4710 
At 2700.degree. F. (1482.degree. C.): 
3610 3490 2770 
______________________________________ 
These nitride bonded high alumina compositions have not strengths as shown 
in Table VI approaching that of nitride bonded silicon carbide. They also 
have excellent thermal shock resistance and thus are suitable compositions 
for slide gates used to gate the flow of molten metal during teeming. 
Slide gates have been fabricated from Example 7 and have been impregnated 
with petroleum pitch followed by coking at 2000.degree. F. There was no 
indication of any reaction between the carbon and the nitride phases. 
In the foregoing mixes, the refractory aggregate was sized to form a 
brickmaking batch, for example, such that seven to twenty percent was 
retained on a ten mesh screen; about twenty-three to thirty-six percent 
was minus ten on twenty-eight mesh; about fifteen to nineteen percent was 
minus twenty-eight on sixty-five mesh; about seven to ten percent was 
minus sixty-five on two hundred mesh; and about thirty to thirty-five 
percent passed the two hundred mesh screen. All the above mesh sizes are 
based on the Tyler Standard Series. 
The refractory aggregates used in the examples have the approximate 
chemical analyses as set forth below: 
______________________________________ 
Calcined Crude Synthetic 
Aluminosilicate Clay Alumina 
______________________________________ 
SiO.sub.2 
47.3% 62.9% 0.1% 
Al.sub.2 O.sub.3 
49.2 33.5 99.6 
TiO.sub.2 
2.4 2.1 0.01 
Fe.sub.2 O.sub.3 
1.0 1.0 0.2 
CaO 0.02 0.2 0.04 
MgO 0.04 0.3 0.04 
Alk. 0.08 0.5 0.05 
______________________________________ 
Spinal Grains for Examples 3, 4, 5 and C4, C5, C6 
______________________________________ 
SiO.sub.2 
0.11% 0.25% 0.40% 0.2% 
Al.sub.2 O.sub.3 
89.2 69.6 57.5 69.0 
TiO.sub.2 
0.01 0.02 0.03 0.04 
Fe.sub.2 O.sub.3 
0.21 0.17 0.21 0.09 
CaO 0.10 0.19 0.40 0.54 
MgO 9.93 29.7 41.4 30.1 
Alk. 0.44 0.04 0.02 N.D. 
______________________________________ 
All of the chemical analyses are based on an oxide analysis. 
In the examples, the batches were tempered with a solution of Dextrin or 
lignin liquor and water which provided a temporary binder. The bricks were 
typically nitrided in the presence of flowing nitrogen at a temperature of 
about 2600.degree. F. (1420.degree. C.) with the hold time of about four 
hours. 
To successfully achieve nitriding and also an economical firing schedule, 
it is preferred that the starting silicon 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 ninety-five percent of the 
particles being finer than 30 microns. 
It is preferred that the reactive material not exceed twenty percent of the 
mix for economic reasons. Also, larger quantities do not result in 
articles with materially improved physical properties. 
Having thus defined my invention with the detail and particularity required 
by the Patent Laws, what is desired protected by Letters Patent is set 
forth in the following claims.