Solid material comminution or disintegration – Processes – With classifying or separating of material
Reexamination Certificate
1997-12-29
2002-01-29
Husar, John M. (Department: 3725)
Solid material comminution or disintegration
Processes
With classifying or separating of material
C241S024120, C241S024130, C241S024140
Reexamination Certificate
active
06341739
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to the processing of ceramic materials and, in particular, concerns a process for producing improved ceramic materials comprising nitride-containing materials.
Nitride-containing ceramics which are known as ‘advanced ceramics’ are used in a wide range of applications, for example in industrial wear parts and bearings, refractories, welding components and molten metal handling materials, cutting tools for metal turning, dies for metal extrusion and wire pulling, military applications and body armour, electronics and composite materials. In aggressive and high temperature environments, the corrosion resistance, strength, toughness and wear resistance of these advanced ceramics offer considerable advantages over the sophisticated metal alloys currently in use.
The process of making metal nitrides by the carbothermal reduction and subsequent or simultaneous nitriding of appropriate metal oxides is well known. A variety of metal or semi-metal nitrides can be made in this way, including silicon nitride, aluminium nitride, boron nitride and titanium nitride. By way of example, an appropriate metal oxide may be mixed with a suitable amount of carbon and the mixture heated to a temperature in the range of from 1300 to 1600° C. in a flowing stream of nitrogen gas. Oxygen is removed from the oxide as carbon monoxide, and is replaced by nitrogen, with the result that the oxide is partially or fully converted to the nitride.
Nitride-containing ceramic materials of special interest on account of their superior refractory and mechanical properties are the family of silicon aluminium oxynitride materials, which are known collectively as “sialons”. The term “sialon” which is now widely used to identify this type of material is derived from the chemical symbols Si, Al, O and N of its constituent elements. Sialons are believed to be solid solutions of aluminium oxide in silicon nitride, and those which have the most desirable properties generally have a chemical composition which can be represented by the formula:
Si
6−z
Al
z
O
z
N
8−z
where z is greater than zero and less than or equal to 4.2.
Materials of this type are generally prepared by intimately mixing silicon nitride, alumina and aluminium nitride in appropriate proportions, and causing them to react together at a high temperature in an inert atmosphere, which may conveniently be nitrogen gas. Some types of sialon can be made by carbothermal reduction and nitriding of an aluminosilicate. Thus sialon with the chemical composition Si
3
Al
3
O
3
N
5
can be made from kaolinite, which is the principal constituent of many clays.
Most natural aluminosilicates, including kaolin clay, usually contain small amounts or iron, either combined with the aluminosilicate, or as the free oxide or oxyhydroxide. It is well known that the carbothermal reduction and nitriding siliceous materials can be catalysed by the presence of iron oxides in the feed material. The conversion of silica to silicon nitride, and the conversion of kaolin to sialon are both catalysed in this way. If insufficient iron is present naturally, extra iron can be added, usually in the form of the oxide. In the presence of free silica, and in the strongly reducing conditions prevailing during the carbothermal reaction, the iron oxide is converted to ferrosilicon. Ferrosilicon is liquid at temperatures above 1250° C., and it is generally accepted that the presence of this liquid phase greatly enhances the reduction and nitriding of the remaining silica or aluminosilicate.
However, it has not been widely appreciated that the ferrosilicon can have a deleterious effect on the mechanical and/or refractory properties of the resulting nitride-containing ceramic after sintering.
SUMMARY OF THE INVENTION
According to the present invention there is provided a process for treating a nitride-containing ceramic material which includes (1) comminuting the material to produce a particulate ceramic material having a particle size distribution such that at least 40% by weight of the particles have an equivalent spherical diameter (esd) smaller than 2 &mgr;m; and (2) applying a ferrosilicon separation step comprising one or both of the following steps:
a) subjecting the particulate ceramic material produced in step 1 to differential sedimentation in a liquid medium to produce substantial separation of a light fraction from a heavy fraction, ferrosilicon required to be separated being included in the heavy fraction, and refined particulate ceramic material being included in the light fraction;
b) subjecting a suspension of the particulate ceramic material to magnetic separation to produce substantial separation of magnetic particulate material from non-magnetic particulate material, refined particulate ceramic material being included in the non-magnetic material.
DESCRIPTION OF THE INVENTION
As described earlier, the present invention involves in the treatment of a nitride-containing ceramic material, eg. a sialon, by a comminution step followed by one or more ferrosilicon removal steps selected from steps a) and b) defined earlier.
Comminution processes are known per se. Comminution of advanced ceramic materials is described for example in Hoyer, J. L., “Turbomilling: a processing technique for advanced ceramics”, Materials and Manufacturing Processes, 9 (4), pages 623-636, 1994.
We prefer to comminute the ceramic material in step (1) by grinding in an aqueous medium using a hard particulate grinding medium.
In step 2 (a) the differential sedimentation is desirably carried out in the presence of a dispersing agent. Preferably, the liquid medium is an aqueous medium.
In step 2 (b) the suspension may be formed in an aqueous medium. The magnetic separation may be effected by application of a magnetic field having an intensity of at least 0.05 tesla in the region of the suspension to be treated.
The nitride-containing ceramic material may be, for example, silicon nitride which is prepared by carbothermal reduction of a silica, or a &bgr;′-sialon which is prepared by carbothermal reduction of an aluminosilicate material. In the case of &bgr;′-sialon, this may advantageously be prepared in accordance with the method which is described in EP-A-0723932 in which a reaction mixture comprising from 70% to 90% by weight of a hydrous or calcined natural aluminosilicate material, such as a kaolin clay, and from 30% to 10% by weight of a carbonaceous material is calcined at a temperature in the range of from 1300° C. to 1600° C. in a current of nitrogen gas in an enclosed furnace, wherein the particles of the reaction mixture are maintained in substantially continuous motion relative to one another and to the nitrogen gas and the residence time of the reaction mixture in the furnace is not greater than 3 hours.
Where the comminution step (1) in the method according to the first aspect comprises a wet grinding step such a step may be carried out as follows. The ceramic material is suspended in water to form a suspension containing at least 10% by dry weight of the ceramic material. If the suspension contains more than about 40% by dry weight of the ceramic material, a dispersing agent for the ceramic material is preferably dissolved in the water. The dispersing agent is preferably free of alkali metal cations, since these can cause fluxing of the ceramic material. The dispersing agent may be, for example, ammonia or an ammonium salt of a polycarboxylic acid. A particularly suitable dispersing agent is one which can be substantially completely removed from the ceramic material after the treatment in accordance with the invention. Especially preferred is ammonia solution, which is added in a quantity such as to maintain the pH of the suspension at a value of at least 8.0. The ammonium salt of a polycarboxylic acid, if used, is preferably present in an amount of from 0.1 to 1.0% by weight, based on the weight of dry ceramic material.
The hard particulate grinding medium used in the comminution step (where a wet grinding step) should have
Finnegan, Henderson Farabow, Garrett and Dunner L.L.P.
Husar John M.
Imerys Minerals Limited
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