GB2075966A - Nitride bonded refractory shapes - Google Patents

Abstract

A method of making nitride bonded refractory shapes, comprises forming a particulate mix which comprises, by weight, from 1 to 25% of silicon, from 1 to 5% of crude clay, and the balance, a brickmaking size-graded refractory aggregate pressing the mix into refractory shapes, and firing the shapes at an elevated temperature in a nitriding atmosphere for a time sufficient to form the nitride bond. The refractory aggregate may be mullite, fireclay, alumina, or spinel.

Description

SPECIFICATION Nitride bonded refractory shapes and their production This invention is concerned with nitride bonded refractory shapes and with a method of making them.

Considerable effort has been directed to the development of ceramic articles containing 80% or 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 as to the use 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 silicon nitride, the instability of certain oxynitrides at high temperature, and the tendency of possible starting materials, such as aluminium nitride and magnesium nitride, to hydrolyse.

We have now found that these disadvantages can be reduced or avoided by forming such nitride phases in situ by the use of a single elemental powder, that is silicon powder, which can react with gaseous nitrogen to produce a crystalline nitride phase capable of ceramic bonding to relatively inexpensive refractory grains with which the elemental powder is mixed. This approach to the manufacture of nitride bonded refractory shapes greatly lowers the cost of the latter and couples the distinct advantages of nitride compounds with the established advantages of conventional refractory aggregates.

According to the present invention, there is provided a method of making nitride bonded refractory shapes, which comprises forming a particulate mix which comprises, by weight, from 1 to 25% of silicon, from 1 to 5% of crude clay, and the balance, a brickmaking size-graded refractory aggregate, pressing the mix into refractory shapes, and firing the shapes at an elevated temperature in a nitriding atmosphere for a time sufficient to form the nitride bond.

The present invention also comprises a nitride bonded refractory shape made from a batch comprising, by weight, from 1 to 25% of silicon, from 1 to 5% of crude clay and the balance, a refractory aggregate.

The mix or batch used to make the refractory shapes preferably comprises, by weight, from 3 to 20% of silicon and from 1 to 2% of crude clay. The shapes are preferably burned at a temperature of from 1090 to 1 7400C and the nitriding atmosphere is preferably composed of nitrogen, industrial annealing gas, or ammonia. The refractory aggregate is preferably calcined fireclay, fused mullite, synthetic alumina or magnesium aluminate spinel.

In a nitrogen atmosphere at elevated temperatures, silicon undergoes a gas-metal reaction and minute nitride crystals are formed surrounding a core of silicon. By maintaining the firing temperature for a sufficient length of time, drainage of the silicon from the core through the pores of the crystalline mat which surrounds it, allows additional nitriding of the silicon to take place. Towards the and of the firing period, true ceramic bonding is achieved with the coarse refractory grains by virtue of their solubility in the nitride phases. The presence of the crude clay enables good densities to be obtained when pressing the shapes and facilitates formation of beta prime sialon and silicon oxynitride if these phases are desired.

To achieve nitriding successfully and also an economical firing schedule, it is preferred that the starting silicon powder should be as fine as possible.

The silicon powder preferably has an average particle diameter of 6.3 microns or less with 95% of the particles being finerthan 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 should not exceed about 20% of the mix for economic reasons; larger quantities do not result in articles with materially improved physical properties.

In order that the invention may be more fully understood, the following examples are given by way of illustration only.

Examples 1-7 Refractory mixes were formed by mixing silicon powder 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 bricks were then fired in the presence of flowing nitrogen to a temperature of about 2600"F (1427"C) with a holding time of four hours.

The composition of the mixes (in % by weight), the physical properties of the fired refractories and the various bonding phases present therein are shown in Table I below.

STABLE! Example 9 2 3 4 5 6 7 Fused Mullite 78% 75% 98% 96% - - - Calcined Fireciay - - - - 84% - Synthetic Alumina - - - - - 85% Magnesium Aluminate Spinel - - - - - - 84% Silicon 20 25 1 3 13 13 13 Crude Clay 2 2 t 1 3 2 3 Apparent Porosity,% 15.2 15.4 20.O 19.9 15.2 16.0 14.3 Modulus of rupture, psi at room temperature 2550 2020 1700 2020 2720 1940 2480 Modulus of rupture, psi at10920C - - - - 2710 4100 3420 Primary Bonding Phase Beta Beta Silicon Silicon Beta Beta Beta Silicon Silicon Oxy- Oxy- Silicon Silicon Silicon Nitride Nitride nitride nitride Nitride Nitride Nitride Silicon Silicon Silicon Silicon Silicon Oxy- Oxy- Oxy- Oxy- Oxy nitride nitride nitride nitride nitride In the above mixes, the refractory aggregate was sized such that 7 to 20% was retained on a 10 mesh screen, 23 to 36% was -10+28 mesh, 15 to 19% was -28+65 mesh, 7 to 10% was -65+200 mesh, and 30 to 35% passed a 200 mesh screen. All these mesh sizes are those of the Tyler standard series.

The refractory aggregates and the crude clay used in the examples had the approximate chemical analysis shown in Table II below.

TABLEII Magnesium Calcined Crude Fused Synthetic Aluminate Fireclay Clay Mullite Alumina Spinel SiO2 47.3% 62.9% 22.9 0.1% 0.2% A1203 49.2 33.5 76.4 99.6 69.0 TiO2 2.4 21 0.1 0.01 0.04 Fe203 1.0 1.0 0.3 0.2 0.09 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 the chemical analyses are based on an oxide analysis.

Claims (11)

1. A method of making nitride bonded refractory shapes, which comprises forming a particulate mix which comprises, by weight, from 1 to 25% of silicon, from 1 to 5% of crude clay, and the balance, a brickmaking size-graded refractory aggregate, pressing the mix into refractory shapes, and firing the shapes at an elevated temperature in a nitriding atmosphere for a time sufficient to form the nitride bond.

2. A method according to claim 1, in which the mix comprises, by weight, from 3 to 20% of silicon and from 1 to 2% of crude clay.

3. A method according to claim 1 or 2, in which the refractory aggregate is calcined fireclay, fused mullite, synthetic alumina or magnesium aluminate spinel.

4. A method according to any of claims 1 to 3, in which the shapes are fired at a temperature of from X090 to 1750 C.

5. A method according to any of claims 1 to 4, in which the nitriding atmosphere is composed of nitrogen, industrial anneating gas or ammonia.

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

7. A refractory shape according to claim 6, in which the batch comprises, by weight, from 3 to 20% of silicon and from 1 to 2% of crude clay.

8. A refractory shape according to claim 6 or7, in which the refractory aggregate is calcined fireclay, fused mullite, synthetic alumina or magnesium aluminate spinel.

9. A refractory shape according to any of claims 6 to 8, in which the nitride bond comprises at least one of beta sialon, silicon oxynitride, beta silicon nitride and alpha silicon nitride.

10. A method of making nitride bonded refractory shapes substantially as herein described in any of the Examples.

11. Nitride bonded refractory shapes substantially as herein described in any of the Examples.